Title page
FUSED FILAMENT FABRICATION TO MANUFACTURE
THREE- AND FOUR-DIMENSIONAL OBJECTS MADE OF
SHAPE MEMORY POLYMERS
vorgelegt von
M. Sc.
Dilip Chalissery
ORCID: 0000-0001-7152-6438
an der Fakultät III - Prozesswissenschaften
der Technischen Universität Berlin
zur Erlangung des akademischen Grades
Doktor der Ingenieurwissenschaften
– Dr.-Ing. –
genehmigte Dissertation
Promotionsausschuss:
Vorsitzender: Prof. Dr. Alexander Gurlo
Gutachter: Prof. Dr. -Ing. Dietmar Auhl
Gutachter: Prof. Dr. rer. nat. Alexander Böker
Gutachter: Dr. rer. nat. Thorsten Pretsch
Tag der wissenschaftlichen Aussprache: 24. Januar 2023
Berlin 2023
Enjoy each step, for in every step there is something to learn.
-John P Strelecky
There are things to do that no one has attempted before
and things to create that no one has created before.
With the new findings and technology,
one may not set out to change the world,
but we change the world one step at a time.
... to my loved ones and family
I
Abstract
II | Abstract
Abstract
Programmable materials can perform specific tasks with the function of stimuli, like
temperature, where the programming of material is understood as the programming of a
functionality. The internal structure of the programmable materials enables reversible
material properties, behavior, or shape changes according to a program. As the
programmable materials require neither control electronics nor technical devices or cables,
the self-sufficient behavior makes them fulfill sensor and actuator functionality. Shape
memory polymers (SMPs) are smart materials that qualify as functional base materials to
design programmable materials. SMPs can retain an imposed, temporary shape after
thermomechanical treatment, also called programming. The initial, permanent shape can be
recovered when applying an external stimulus like heat. In the last decade, thermoplastic
polyurethanes (TPUs) belonged to the most researched SMPs. The thermoplastic nature of
TPUs permits them to be molded using classical melt-based processing techniques like
extrusion, injection molding, etc. Additive manufacturing (AM), alias three-dimensional (3D)
printing, is an effective layer-by-layer technique to process thermoplastic polymers into 3D
objects. Amidst various AM technologies, fused filament fabrication (FFF) is a hot-melt
extrusion-based 3D printing process and is widely prevalent. The doctoral thesis aims to utilize
self-synthesized and commercially available TPUs for FFF and to specifically influence the
printing technology to produce either non-thermoresponsive or thermoresponsive objects or
structures and open up new material and system functionalities.
The primary hurdle of this doctoral study was to process SMPs using a standard
commercially available FFF machine. Motivated by the fact that previously presented
manufacturing processes of thermoresponsive quick response (QR) codes were too time-
consuming for production, QR codes were initially developed as anti-counterfeiting
technology. The work introduces a novel manufacturing method for the same, thereby also
addressing the AM of TPU-based SMP using standard FFF machines. Following this, the layer
deposition pattern of tensile bars of TPU with shape memory properties was modified to
achieve printing either in vertical or horizontal orientation. After processing commercially
available polyester urethane and characterization, the mechanical and shape memory
properties of the 3D printed samples were studied, and the results were compared to its
injection-molded analogs and other materials manufactured via FFF. The results showed that
the direction of loading and printing pattern orientation could be utilized to control the shape
recovery stress and mechanical properties. Subsequently, the filigree printing and the
smallest structure that can be obtained from FFF were explored by printing Arial fonts of the
letter “A” in different sizes. Afterward, the potential application of SMP as thermally
activatable and de-activatable gears and innovative smart keyboard keys was developed by
utilizing the one-way (1W) shape memory effect (SME).
The second part of the work concentrates on four-dimensional (4D) printing employing FFF
that enables the production of thermoresponsive objects directly in AM process. The work
III
presents a facile FFF printing strategy for commercially available polylactic acid (PLA) material
and an in-house synthesized thermoplastic polyether urethane to obtain highly shrinkable
objects, which allowed to show how to achieve precise control over the shapes after printing
and heating. Later, the thermoresponsiveness after 4D printing of other objects in the form
of solid cuboid, hollow cuboid, and hollow cylinder, with heights along the z-axis bigger than
30 mm, was explored. One of the applications of the developed highly shrinkable objects is
active assembly. The concept is demonstrated by developing a lightweight, hands-free door
opener for healthcare applications to counteract the spread of smear infections. After
triggering the 4D effect for assembly, the device can be disassembled by heating the TPU over
its glass transition temperature once it reaches its end-of-use. After removal from the door
handle, the device can be mechanically recycled, and the material can be reused for 4D-
printing. Successively, the know-how of 4D-printing was applied to address novel applications
in active assembly, disassembly, programming tools, and as thermally deactivating gear.
Thirdly, FFF was utilized to produce elements that can undergo thermomechanical
treatment to develop thermoresponsive two-way (2W) actuating objects that can bridge the
gap toward manufacturing programmable materials. After developing a poly(1,10-decylene
adipate) (PDA) based polyester urethane, processing it via FFF and thermomechanical
treatment, a novel approach was developed to identify the ideal actuating temperature
conditions using dynamic mechanical analysis. Once characterized, the polymer was found to
actuate reliably under stress-free conditions by expanding on cooling and shrinking on
heating with a maximum thermoreversible strain of 16%. Later, allowing the 2W
programmed TPU-based SMP to actuate in its ideal actuating temperature range between
15 °C and 64 °C for 100 heating-cooling cycles revealed that the reversible strain change
stabilizes after about 25 cycles at 12%. The developed actuating elements were then
integrated into a mechanical linkage system to form a thermally activatable gripper. This way,
a hen’s egg could be picked up, safely transported, and deposited, qualifying for soft robotic
purposes. Further, actuating elements were combined with two types of unit cells to obtain
programmable materials that can actuate on temperature variation. Afterward, the
development of programmable actuating structures using TPU with a 2W-SME was studied.
The primary aim was to enable a better actuation for physically crosslinked SMPs under
stress-free conditions. Therefore, a novel gear design was first developed and processed using
PDA-based TPU as functional base material. After thermomechanical treatment, the
programmable gear actuated efficiently between two metastable states with a reversibility
length change of 42%. In order to prove that the thermoreversible actuation can be
reproduced in other structural motifs, another actuating element was developed and
programmed similarly. Once allowed to actuate under the same condition as the
programmable gear, the element showed a reversible length change of 44%.
Lastly, the work develops a novel FFF approach for semicrystalline SMPs to directly obtain
objects in their thermoresponsive state, which can arbitrarily actuate between two
metastable states by varying the temperature. Here, evidence is shown of how
semicrystalline TPU is suitable for 4D-printing. During FFF, a cold air stream was used to cool
IV | Abstract
the SMP strongly. Upon its removal from the build platform, it can be activated under stress-
free conditions by shrinking on heating to 62 °C and expanding on cooling to 15 °C with a
maximum thermoreversible strain of 7%. Later, a 4D printed actuator was integrated into a
lever mechanism qualified to witness highly complex shape changes. Subsequently, self-
sufficient actuators in the form of a cylinder were fabricated, and their actuation behavior
was studied. The work concludes that the functional integration of SMP via FFF to achieve
2W-SME by an “in-situ” programming method is a promising step to produce inherent
thermoresponsive programmable materials.
V
Zusammenfassung
Programmierbare Materialien können bestimmte Funktionen in Reaktion auf Stimuli, wie
z. B. die Temperatur ausführen können, wobei die Programmierung eines Materials als
Programmierung einer Funktion angesehen wird. Die interne Struktur der programmierbaren
Materialien ermöglicht reversible Materialeigenschaften, Verhaltens- oder Formänderungen
nach einem Programm. Aufgrund der Tatsache, dass die programmierbaren Materialien
weder Steuerelektronik noch technische Geräte oder Kabel benötigen, erfüllen sie durch ihr
autarkes Verhalten sowohl Sensor- als auch Aktorfunktionen. Formgedächtnispolymere (FGP)
sind smarte Materialien, die sich als funktionelle Basismaterialien für die Entwicklung
programmierbarer Materialien eignen. FGPs können nach einer thermomechanischen
Behandlung, auch Programmierung bezeichnet, eine vorgegebene, temporäre Form
beibehalten. Die ursprüngliche, permanente Form kann wiederhergestellt werden, wenn ein
externer Stimulus, wie z. B. Wärme, angewendet wird. In den letzten zehn Jahren gehörten
thermoplastische Polyurethane (TPUs) zu den am meisten erforschten FGPs. Die
thermoplastische Natur von TPUs ermöglicht ihre Verarbeitung mit klassischen
schmelzbasierten Verfahren wie Extrusion, Spritzguss usw. Die additive Fertigung (AM), auch
als dreidimensionaler Druck (3D) bezeichnet, ist eine effektive Technik zur Verarbeitung
thermoplastischer Polymere zu 3D-Objekten. Unter den verschiedenen AM-Technologien ist
die die am häufigsten verwendete Fused Filament Fabrication (FFF), ein 3D-Druckverfahren,
das auf der Schmelzextrusion basiert. Ziel der Dissertation ist es, selbst synthetisierte und
kommerziell erhältliche TPUs mittels FFF für die Herstellung von nicht-thermoresponsiven
oder thermoresponsiven Objekten oder Strukturen zu nutzen, um neue Material- und
Systemfunktionalitäten zu erschließen.
Die erste Herausforderung dieser Dissertation bestand darin, FGP mit einer kommerziellen
FFF-Maschine zu verarbeiten. Motiviert durch die Tatsache, dass die bisher vorgestellten
Herstellungsverfahren für thermoresponsive Quick-Response-Codes (QR-Codes) zu
zeitaufwendig für die Produktion waren, wo sie ursprünglich als Fälschungsschutztechnologie
entwickelt wurden. Die Arbeit beginnt mit der Einführung eines neuartigen
Herstellungsprozesses für QR-codeträger, der sich auch mit dem AM von TPU-basierten FGP
auf Standard-FFF-Maschinen befasst. Anschließend wurde das Schichtablagemuster der
Zugstäbe aus TPU mit Formgedächtniseigenschaften so modifiziert, dass sie sowohl in
vertikaler als auch in horizontaler Orientierung gedruckt werden können. Nach der
Verarbeitung von kommerziell verfügbarem Polyester-Urethan und der Charakterisierung
wurden die mechanischen und Formgedächtniseigenschaften der 3D-gedruckten Proben
untersucht und die Ergebnisse mit ihren spritzgegossenen Analoga und anderen mit FFF
hergestellten Materialien verglichen. Die Ergebnisse zeigten, dass die Belastungsrichtung und
die Orientierung des Druckablagemusters zur Steuerung der Formrückstellungsspannung und
der mechanischen Eigenschaften verwendet werden können. Anschließend wurden der
filigrane Druck und die kleinste Struktur, die mit FFF hergestellt werden kann, durch den Druck
VI | Abstract
von Arial-Schriften des Buchstabens "A" in verschiedenen Größen untersucht. Danach wurde
die potenzielle Anwendung von FGP als thermisch aktivierbare und deaktivierbare Zahnräder
und innovative smarte Tastaturtasten durch Nutzung des Ein-Weg (1W)
Formgedächtniseffekts (FGE) entwickelt.
Der zweite Teil der Arbeit befasst sich mit dem vierdimensionalen (4D) Druck mittels FFF,
der die Herstellung von thermoresponsiven Objekten direkt im AM-Prozess ermöglicht. Diese
Arbeit stellt eine einfache FFF-Druckstrategie für kommerziell verfügbares Polymilchsäure
(PLA)-Material und ein in-house synthetisiertes thermoplastisches Polyether-Urethan vor, um
hoch schrumpfbare Objekte zu erhalten. Dadurch wird gezeigt, wie man die Formen sowohl
nach dem Druck als auch nach dem Erhitzen genau kontrollieren kann. Anschließend wurde
die Thermorepsonsivität nach dem 4D-Druck von anderen Objekten in Form von Vollquadern,
Hohlquadern und Hohlzylindern mit Höhen von mehr als 30 mm entlang der z-Achse
untersucht. Eine der Anwendungen der entwickelten hochschrumpfbaren Objekte ist die
aktive Montage. Das Konzept wird anhand der Entwicklung eines leichten, handfreien
Türöffners für Anwendungen im Gesundheitswesen demonstriert, um die Verbreitung von
Schmierinfektionen zu verhindern. Nach dem Auslösen des 4D Effekts für den Zusammenbau
kann das Gerät zerlegt werden, indem das TPU über seine Glasübergangstemperatur erhitzt
wird, sobald es sein End-of-Use erreicht hat. Nach dem Entfernen des Türgriffs kann das
Produkt mechanisch recycelt und das Material für den 4D-Druck wiederverwendet werden.
Das Know-how des 4D-Drucks wurde anschließend für neuartige Anwendungen in den
Bereichen aktive Montage, Demontage, Programmierwerkzeuge und thermische
Deaktivierung von Zahnrädern eingesetzt.
Drittens wurden mit Hilfe der FFF Elemente hergestellt, die einer thermomechanischen
Behandlung unterzogen werden können, um thermoresponsiv aktivierende Zwei-Wege (2W)
Objekte zu entwickeln, die die Lücke zur Herstellung programmierbarer Materialien schließen
können. Nach der Entwicklung eines Poly(1,10-decylenadipat)-basierten Polyester-Urethans
(PDA), dessen Verarbeitung durch FFF und thermomechanische Behandlung, wird ein neuer
Ansatz entwickelt, um die idealen Aktivierungstemperaturen mittels dynamisch-
mechanischer Analyse zu bestimmen. Die Charakterisierung des Polymers ergab, dass es sich
unter spannungsfreien Bedingungen zuverlässig aktiviert, indem es sich beim Abkühlen
ausdehnt und beim Erwärmen schrumpft, wobei die maximale thermoreversible Dehnung
16% beträgt. Dann wurde das 2W programmierte TPU-basierte FGP in seinem idealen
Temperaturbereich zwischen 15 °C und 64 °C für 100 Heiz-Kühl-Zyklen aktiviert und die
reversible Dehnungsänderung stabilisierte sich nach etwa 25 Zyklen bei 12 %. Die
entwickelten Aktorelemente wurden dann in ein mechanisches Gelenksystem integriert, um
ein thermisch aktivierbares Greifersystem zu realisieren. Auf diese Weise könnte ein Hühnerei
aufgenommen, sicher transportiert und abgelegt werden, so dass es sich für den Einsatz in
der Soft-Robotik eignet. Die Aktuatoren wurden mit zwei verschiedenen Einheitszellen
kombiniert, um programmierbare Materialien zu schaffen, die abhängig von
Temperaturänderungen aktiviert werden können. Daraufhin wurde die Entwicklung
programmierbarer Aktorstrukturen unter Verwendung von TPU mit einer 2W-FGE erforscht.
VII
Das Hauptziel bestand darin, eine bessere Aktivierung von physikalisch vernetztem FGP unter
spannungsfreien Bedingungen zu ermöglichen. Daher wurde zunächst ein neuartiges
Zahnraddesign entwickelt und unter Verwendung von PDA-basiertem TPU als funktionellem
Basismaterial hergestellt. Nach der thermomechanischen Behandlung konnte das
programmierbare Zahnrad effizient zwischen zwei metastabilen Zuständen mit einer
reversiblen Längenänderung von 42% aktiviert werden. Um zu beweisen, dass die
thermoreversible Aktivierung auch in anderen Strukturmotiven reproduzierbar ist, wurde ein
weiteres Aktorelement entwickelt und in ähnlicher Weise programmiert. Bei Aktivierung
unter den gleichen Bedingungen wie das programmierbare Zahnrad zeigte das Element eine
reversible Längenänderung von 44%.
Schließlich wurde in der Arbeit ein neuartiger FFF-Ansatz für teilkristalline FGPs entwickelt,
um Objekte direkt in ihrem thermoresponsiven Zustand zu erzeugen, die durch Variation der
Temperatur beliebig zwischen zwei metastabilen Zuständen umschalten können. Hier wurde
der Beweis erbracht, wie teilkristallines TPU für den 4D-Druck verwendet werden kann.
Während der FFF wurde Luft verwendet, um das FGP stark zu kühlen. Nach der Entnahme aus
der Bauplattform kann es unter spannungsfreien Bedingungen zuverlässig aktiviert werden,
indem es sich bei Erwärmung auf 62 °C zusammenzieht und bei Abkühlung auf 15 °C ausdehnt,
mit einer maximalen thermoreversiblen Dehnung von 7 %. Danach wurde ein 4D-gedruckter
Aktor in einen Hebelmechanismus integriert, der sich für hochkomplexe Formänderungen
qualifiziert. Nachfolgend wurden autarke Aktoren in Form eines Zylinders hergestellt und
deren Aktivierungsverhalten untersucht. Die Arbeit kommt zu dem Schluss, dass die
funktionale Integration von FGP über FFF zur Erreichung von 2W-FGE durch eine "in-situ"-
Programmiermethode ein vielversprechender Schritt zur Herstellung inhärent
thermoresponsiver programmierbarer Materialien ist.
IX
Acknowledgment
X | Acknowledgment
Acknowledgment
This thesis has been made possible by the support, mentoring, and friendship of several
people to whom I would like to express my gratitude.
First, I express my most profound appreciation to my supervisor Dr. Thorsten Pretsch for
offering me the position to pursue my doctoral work in his group at Fraunhofer-Institut für
Angewandte Polymerforschung (IAP). Also, for the continuous support, never-ending help,
invaluable feedback, and advice on all concerns. I am immensely grateful for his professional
and scientific support.
Furthermore, I would like to thank Prof. Dr. Alexander Böker, Prof. Dr. Dietmar Auhl, and
Prof. Dr. André Laschewsky for reviewing my thesis. My deepest gratitude goes to Prof. Dr.
Alexander Böker and Prof. Dr. Dietmar Auhl for undertaking this thesis's academic
responsibility, encouragement, and support.
I am also grateful to the Fraunhofer High-Performance Center for Functional Integration in
Materials (project 630039), Fraunhofer Cluster of Excellence "Programmable Materials"
(projects 63500, 630527, 630507, PSP elements 40-01922-2500-00002 and 40-03420-2500-
00003), and future cluster candidate Additive Manufacturing Cluster Berlin-Brandenburg
(AMBER, 03ZK102AC) for funding my research projects. I am thankful to Covestro Deutschland
AG for kindly providing Desmopan® DP 2795A SMP, as it was used as the model material for
various contexts in this work.
My thanks and appreciations also go to my colleagues, ex-colleagues, and students of the
Shape Memory Polymer group of Fraunhofer IAP. First, I express my gratitude to my
officemate Dennis Schönfeld, for lively chats on SMPs in general, visions, suggestions,
opinions, and ideas of products and mechanisms, and for synthesizing many SMPs on
demand. A special mention goes to Tobias Rümmler, who could transform any ideas and
models into reality, also for his continuous support in carrying out the extrusion of SMPs and
other polymers, conducting dynamic mechanical analysis, and for his constant support for the
last five years. I want to thank Dr. Mario Walter for his support, the lively talks on SMPs, and
his capability to synthesize novel SMPs, as this was the functional base material for developing
new shape memory effects. Further, I wish to thank the ex-colleagues, Dr. Fabian Friess and
Dr. Benjamin Heyne, and all the interns and students who accompanied me during my Ph.D.
work and provided me with various time-consuming measurements: Nishith Puvati, Yu Yu
Chen, Yim Yam Chan, Vincent Scholz, Julia Friederike Kubitz, Saskia Wendland, Nour Adilien,
Fernanda Alvarado Galindo, and Harish Babu Eppa.
Additionally, I would like to thank all the colleagues and students of Fraunhofer IAP for
their daily support, coffee talks, and many entertaining conversations over lunch -as simple
as that- their company and for making my time in Fraunhofer IAP a lot lighter, easier, and
lively. Furthermore, I would like to extend my thanks to the colleagues of the division of
Synthesis- and Polymer Technology and the team members of Strategy and Marketing of
Fraunhofer IAP for their immense support on many occasions. Additionally, I would like to
XI
thank the other doctoral students, group leaders, scientific researchers, and lab assistants for
their general support, helpful scientific discussions, and for making my working time
enjoyable.
Last but not least, I want to thank my family and friends for supporting me throughout my
Ph.D. thesis, particularly my parents, brother, sister-in-law, and parents-in-law for the
extended support, encouragement, and understanding. Also, my cats Finnchen and Mickey,
for their effective distractions when further work on the manuscript was pointless anyway
and for offering the lovely environment. Above all, my wife Anne-Kathrin for her love,
patience, trust, faithful support, and continuous encouragement.
XIII
Preface
XIV | Preface
Preface
This dissertation is a cumulative work based on four peer-reviewed published articles and
four sub-chapters (non-published). The articles are presented separately as independent sub-
chapters with their own introduction, experimental section, results, discussion, conclusions,
and references. The formatting of already published manuscripts was modified to ensure
consistency throughout the dissertation. The thesis also contains a general abstract,
introduction, motivation, discussion, and conclusion. The following four published articles
and sub-chapters are enclosed in this thesis:
Chapter 3: One-Way Shape Memory Effect of Objects Programmed After Fused Filament
Fabrication
Chapter 3.1: Additive Manufacturing of Information Carriers Based on Shape
Memory Polyester Urethane. Chalissery, D.; Pretsch, T.; Staub, S.; Andrä, H. Polymers
2019, 11, 1005. https://doi.org/10.3390/polym11061005
Chapter 3.2: Influence of Print Orientation on Shape Memory- and Mechanical-
Properties After Fused Filament Fabrication
Chapter 3.3: Fused Filament Fabrication of Filigree Objects With Shape Memory
Properties
Chapter 4: Four-Dimensional (4D) Printing Via Fused Filament Fabrication
Chapter 4.1: Highly Shrinkable Objects as Obtained from 4D-printing. Chalissery, D.,
Schönfeld, D., Walter, M., Shklyar, I., Andrae, H., Schwörer, C., Amann, T., Weisheit, L.
and Pretsch, T. (2022). Macromol. Mater. Eng., 307: 2100619.
https://doi.org/10.1002/mame.202100619
Chapter 4.2: Potential Applications of 4D-Printed Objects
Chapter 5: Ex-Situ Programming of Objects Manufactured via Fused Filament Fabrication to
Attain Two-Way Shape Memory Effect
Chapter 5.1: Actuating Shape Memory Polymer for Thermoresponsive Soft Robotic
Gripper and Programmable Materials. Schönfeld, D.; Chalissery, D.; Wenz, F.; Specht,
M.; Eberl, C.; Pretsch, T.. Molecules 2021, 26, 522.
https://doi.org/10.3390/molecules26030522.
Chapter 5.2: Programmable Materials
Chapter 6: Fused Filament Fabrication of Actuating Objects. Chalissery, D., Schönfeld, D.,
Walter, M., Ziervogel, F., Pretsch, T., Macromol. Mater. Eng. 2022, 2200214.
https://doi.org/10.1002/mame.202200214.
Table of Contents
Table of Contents
Table of Contents
TITLE PAGE ............................................................................................................................................... III
ABSTRACT .................................................................................................................................................. I
ZUSAMMENFASSUNG ................................................................................................................................ V
ACKNOWLEDGMENT ................................................................................................................................ IX
PREFACE ................................................................................................................................................. XIII
TABLE OF CONTENTS ...................................................................................................................................
CHAPTER 1: INTRODUCTION .......................................................................................................................1
1.1. POLYMERS .............................................................................................................................................2
1.1.1. Molecular Architecture of Polymers .............................................................................................. 2
1.1.2. States of Matter and Thermal Transitions ..................................................................................... 3
1.1.3. Thermoplastics ............................................................................................................................... 5
1.1.4. Shape Memory Polymers ............................................................................................................. 11
1.1.5. Molecular Mechanism of Shape Memory Effect .......................................................................... 12
1.2. SHAPE MEMORY EFFECT OF OBJECTS MANUFACTURED VIA FUSED FILAMENT FABRICATION .................................... 13
1.2.1. One-Way Shape Memory Effect ................................................................................................... 14
1.2.2. Two-Way Shape Memory Effect .................................................................................................. 15
1.2.3. Four Dimensional (4D)-Printing ................................................................................................... 16
1.2.4. Two-Way Four-Dimensional Printing ........................................................................................... 18
1.3. REFERENCES ......................................................................................................................................... 19
CHAPTER 2: MOTIVATION ........................................................................................................................ 29
REFERENCES ................................................................................................................................................ 32
CHAPTER 3: ONE-WAY SHAPE MEMORY EFFECT OF OBJECTS PROGRAMMED AFTER FUSED FILAMENT
FABRICATION ........................................................................................................................................... 35
CHAPTER 3.1: ADDITIVE MANUFACTURING OF INFORMATION CARRIERS BASED ON SHAPE MEMORY POLYESTER URETHANE36
CHAPTER 3.2: INFLUENCE OF PRINT ORIENTATION ON SHAPE MEMORY- AND MECHANICAL- PROPERTIES AFTER FUSED
FILAMENT FABRICATION ............................................................................................................................... 61
CHAPTER 3.3: FUSED FILAMENT FABRICATION OF FILIGREE OBJECTS WITH SHAPE MEMORY PROPERTIES ........................ 72
CHAPTER 4: FOUR-DIMENSIONAL (4D) PRINTING VIA FUSED FILAMENT FABRICATION ............................ 85
CHAPTER 4.1: HIGHLY SHRINKABLE OBJECTS AS OBTAINED FROM 4D-PRINTING ......................................................... 86
CHAPTER 4.2: POTENTIAL APPLICATIONS OF 4D-PRINTED OBJECTS ....................................................................... 117
CHAPTER 5: EX-SITU PROGRAMMING OF OBJECTS MANUFACTURED VIA FUSED FILAMENT FABRICATION
TO ATTAIN TWO-WAY SHAPE MEMORY EFFECT ..................................................................................... 131
CHAPTER 5.1: ACTUATING SHAPE MEMORY POLYMER FOR THERMORESPONSIVE SOFT ROBOTIC GRIPPER AND
PROGRAMMABLE MATERIALS ..................................................................................................................... 132
CHAPTER 5.2: PROGRAMMABLE MATERIALS .................................................................................................... 159
CHAPTER 6: FUSED FILAMENT FABRICATION OF ACTUATING OBJECTS ................................................... 172
6.1. Introduction .................................................................................................................................. 175
6.2. Results and discussion................................................................................................................... 177
6.3. Conclusions ................................................................................................................................... 187
6.4. Experimental Section .................................................................................................................... 188
6.5. References ..................................................................................................................................... 192
CHAPTER 7: DISCUSSION ........................................................................................................................ 194
References............................................................................................................................................ 201
CHAPTER 8: CONCLUSION AND OUTLOOK .............................................................................................. 205
APPENDIX ............................................................................................................................................... 209
Journals ................................................................................................................................................ 210
Patents ................................................................................................................................................. 210
Presentations ....................................................................................................................................... 211
Permanent Database: .......................................................................................................................... 212
LIST OF ABBREVIATIONS AND SYMBOLS ................................................................................................. 214
Chapter 1:
Introduction
2 | Introduction
Chapter 1: Introduction
1.1. Polymers
Polymers have gained wide popularity in the last decades. They are multifaceted materials
employed in diverse areas of our daily lives, like car key housing, credit cards, clothing made
from synthetic fibers, and car parts like bumpers, wheel caps, instrument panels, and
dashboards. Polymers are materials made from repeating building blocks called
monomers. [1–4]. Polymers can be categorized into two groups based on their origin: natural
and synthetic. Natural polymers are found in nature [5,6], like silk, wool, DNA, cellulose, and
proteins. In turn, synthetic polymers, such as high-density polyethylene (HDPE), are used in
the manufacture of bottles and are produced in the synthesis process by scientists or
engineers [7–9].
Polymers can be classified into thermoplastics, elastomers, and thermosets, depending on
the structure and nature of cross-links. In the case of thermoplastic polymers and elastomers,
the physical cross-linking gives thermoplastic material properties. In contrast, elastomers and
thermosets consist of extensive cross-linking between polymer chains to produce an infusible
and insoluble polymer network. The thermoplastics can be further subdivided into
amorphous and semicrystalline polymers based on bulk state, and the polymer can be again
subcategorized as homopolymer and copolymer based on monomer composition.
1.1.1. Molecular Architecture of Polymers
Polymers composed of one monomer species are called homopolymers [10]. Examples
include polypropylene [11], polyvinyl acetate [12], and polystyrene [12,13]. On the other
hand, polymers whose backbone chain is built up by more than one monomer species are
called copolymers [10]. Here, varying the monomer fractions is an appropriate tool to control
the material's physical, chemical, and processing properties. According to the arrangement
of the monomers, the skeletal structure of polymers is divided into linear, branched, and
network structures [4,14–16] (Figure 1.1).
Figure 1.1. Schematic representation of the different types of polymers.
| 3
The macromolecules in linear polymers form a long chain in which the monomers are
attached end to end. The chains are neither straight nor stiff but flexible. Thus, the
macromolecules can twist, bend and become entangled [17]. Generally, the degree of
entanglement impacts the physical properties of a polymer, and the higher the number of
entanglements per chain, the higher the polymer toughness, strength, and glass transition
temperature (Tg). In branched polymers, the macromolecules have several short or long
branched side chains attached to the main chain [18]. The ability of chains to slide past one
another is affected by branching, thereby influencing chain entanglement. While for network-
structured polymers, the macromolecules form three-dimensional (3D) structures. These
polymers consist of a network macromolecule or an assembly of interacting macromolecules.
Such polymers are usually chemically cross-linked, where the polymer's phase transition
temperatures, rigidity, and crystallinity are adjusted using the chain length, cross-link density,
degree of cross-linking, and nature of the cross-links [3,18]. The network polymers containing
long, flexible branches connected to only a few sites along the chains exhibit elastic
properties, while the network polymers which contain dense networks are generally rigid.
1.1.2. States of Matter and Thermal Transitions
Polymers' bulk state, sometimes called the condensed or solid state, can include crystalline
and amorphous domains [19,20]. Semicrystalline polymers are often opaque because the
crystallites may scatter light, while amorphous polymers are generally glass-like or
transparent.
Polymers like linear homopolymers and block-copolymers may crystallize when their chain
adapts to regular arrangements. This behavior is mainly observed for macromolecules with
lower molecular weight. On the contrary, branched and long-chain polymers have a low
tendency to crystallize because of their high degree of entanglement. This is particularly
evident when the polymer melt is very viscous, and the individual chains do not flow easily.
Polymers typically do not form flawless crystalline materials, whereas they form
semicrystalline polymers consisting of amorphous regions and crystalline domains. Due to the
strong intermolecular interaction forces associated with close chain packing of the
crystallites, semicrystalline polymers generally exhibit high toughness. X-radiation (X-ray)
diffraction [21,22], density measurements [16], and differential scanning calorimetry
(DSC) [20] are usually used to determine the degree of crystallinity.
In a DSC measurement, the crystallites melt when heated and are formed when cooled.
Thus, they exhibit a so-called first-order transition. The melting of crystallites and the
crystallization of amorphous segments can be observed with discontinuity, especially in the
heat flow-temperature diagram. The melting is an endothermic process, while the
crystallization is an exothermic process. The associated heat transitions represent the melting
enthalpy ΔHm and the crystallization enthalpy ΔHc. In contrast, the respective transition
temperatures are the Tm when the polymer is heated and the crystallization temperature Tc
when it is cooled (Figure 1.2). Crystallization can also be induced by severe deformation such
4 | Introduction
as stretching or compression, also known as strain-induced crystallization [23–25]. Here, the
chains line up to crystallize on deformation. Such crystallites are usually harder to break
because of their pronounced reinforcing effects.
Figure 1.2. A typical DSC thermogram of a semicrystalline polymer showing a crystalline melting
transition on heating (red region) and the associated crystallization transition on cooling (blue region).
Branched and long-chain linear macromolecules often tend to be amorphous, where the
atoms rotate around the axis of the covalent bond, and the polymer chain adapts to diverse
configurations. Generally, a randomly coiled and entangled state is the most likely
arrangement observed for a single macromolecule [17]. Due to entropic forces, disordered
chains are more probable than stretched, ordered ones. The glass transition, also called glass-
fluid or glass-rubber transition, is considered the thermal transition for amorphous polymers.
The amorphous domains of the polymer become rigid and brittle on vitrification caused by
the transition from the viscous liquid state into the glassy state on supercooling. Vitrification
reduces the micro-Brownian motion of the (network) chains [26,27], and the polymer is in a
permanent non-equilibrium state. The phase transition of polymers is commonly determined
by dynamic mechanical analysis (DMA) and DSC. In a DMA measurement, the loss modulus
E´´ is a measure of the viscous response of a polymer, and the storage modulus is the energy
stored in the material. In other words, it measures the sample's elastic behavior. The
transition of the polymer's bulk from a glassy to rubbery state is often determined by the ratio
between loss modulus E´´ and storage modulus E´, where the temperature at the peak of the
tanδ curve is frequently used to determine the Tg [28–30]. As the temperature rises above
the glass transition temperature, the viscosity of the polymer increases uniformly, and
Young’s modulus decreases significantly [20,31]. This is particularly visible in a stepwise
decrease in the evolution of the storage modulus E´ (Figure 1.3).
Differential scanning calorimetry is used to study the thermal properties of a polymer. In
the DSC experiment, the Tg of the polymer can be seen by the drastic shift of the baseline,
indicating a change in the heat capacity Cp (Figure 1.4). The midpoint of the transition
temperature from the glassy to the rubbery state is often considered the Tg [20].
T [°C]
Heat flow [J .g—1]
∆HC
∆Hm
TC
Tm
endothermic
exothermic
| 5
Figure 1.3. A typical DMA measurement of an amorphous polymer is measured during heating from a
lower temperature. The evolution of the storage modulus (E´) is represented with the red line and the
tanδ with the green line.
Figure 1.4. A typical DSC measurement of an amorphous polymer shows a second-order glass transition
during heating.
Although the DSC and DMA methods can be used to determine the phase transition
temperature of a polymer, the latter is considered to be the more sensitive method for
determining the Tg, since the accuracy for determining the Tg using DMA is higher by a factor
of about 1000 than the DSC technique [32,33].
1.1.3. Thermoplastics
Thermoplastic polymers consist of linear or branched macromolecules forming physically
cross-linked polymers. Physical cross-linking occurs through interchain hydrogen bonding,
dipole-dipole interactions, entanglements, and van der Waals forces. [17]. Such interactions
support the crystal structure formation and hold the polymer segments together [34].
Processing techniques like extrusion [35–37], injection molding [38–40], additive
manufacturing [41], and blow molding [42,43] help to mold and remold thermoplastics into
virtually any shape.
T [°C]
E‘ [MPa]
Glassy state Glass
transition Rubbery plateau Viscous
flow
tan δ
Tgtan δ
E‘
6 | Introduction
In the case of thermoplastic elastomers, the netpoints are formed by reversible cross-
links [44,45]. These get molten on heating and reform the cross-links on successive cooling.
Systems like these exhibit a phase-segregated microstructure as they are typically composed
of thermodynamically incompatible blocks [46–48]. The netpoints, also known as hard
segments, have the highest thermal transition temperature (Tperm) and serve as physical cross-
links and reinforcing fillers. Thereby, they supply the mechanical strength to the polymer.
Above Tperm, the polymer behaves as a homogenous viscous melt and can be processed as
described above. The so-called soft segment has a lower thermal transition temperature and
acts as a mobile (switching) phase. It supplies elastic properties to the polymer.
Thermoplastics' and thermoplastic elastomers' thermal properties and functional
performance can be tailored by varying polymer synthesis and manufacturing strategies. The
vital parameters of the polymers depend on polymer architecture, including the molecular
weight of the segments [49–52], cross-link density [53], and microphase separation [54,55].
Thermoplastic polyurethanes (TPUs) are a well-known example of phase-segregated
polymers. TPUs are commonly synthesized from three building blocks: polyisocyanates,
polyols, and chain extenders (Figure 1.5) [44,45,56].
Figure 1.5. Schematic representation of a repeated hard segment and soft segment of a typical TPU.
The chain extender is made up of a short-chain low mol mass diol or diamines such as
ethylene diamine [57], 1,4-butanediol (1,4-BD) [58], 1,6-hexanediol [59], or ethylene
glycol [60]. In turn, the hard segment is made from chain extender and aromatic or aliphatic
diisocyanate such as 2,4-toluene diisocyanate [61], 4,4’-methylene diphenyl diisocyanate
(MDI) [62], 1,4-cyclohexane diisocyanate [63], or hexamethylene diisocyanate [64]. Finally,
the polyol, which makes up the soft segment, is usually a long-chain difunctional, hydroxy-
terminated oligoester or -ether. Some examples of polyester polyols are poly(1,10-decylene
adipate) [65], poly(1,4-butylene adipate) (PBA) [66], while polypropylene glycol (PPG) is an
example of a polyether polyol [67].
Thermoplastic polyurethanes are synthesized either by the prepolymer [68–72] or the one-
shot method [71–77]. In the case of the prepolymer method, the polymerization is initiated
by the reaction of a stoichiometric excess of a diisocyanate with linear polyols with a molecular
weight of 50 to 5000 g mol–1, thereby forming an intermediate polymer called prepolymer
[76] (Figure 1.6). Next, the final high mol mass polymer is formed by the reaction of the
prepolymer with a diol chain extender. Figure 1.6 shows an example of the synthesis of a
polyester-based TPU, while for the synthesis of polyether-based TPU, a hydroxy-terminated
oligoether would be used instead of the oligoester.
| 7
Figure 1.6. Synthesis of a TPU in the form of a polyester urethane via prepolymer method using (a)
polycondensation reaction and (b) polyaddition reaction.
In the case of the one-shot method, the polymer is synthesized in a single step, where the
three building blocks are added simultaneously. In both methods, other substances like
additives or components can be supplied during the processing, such as flame retardants [78–
80], dyes [81–83], pigments [84,85], stabilizers against hydrolysis [86], and radiation [87,88]
or reinforcing fibers [89–91]. The urethane bonds connected through the chain extender
molecules show high polarity because of significant hydrogen bonding. Due to this strong
intermolecular interaction, the hard segments are formed and embedded in the amorphous
or semicrystalline matrix of the elastic soft segments. After polymer synthesis, the TPUs can
be processed into final or intermediate products, as described above.
8 | Introduction
1.1.3.1. Extrusion
Polymer extrusion is a high-profile manufacturing process. Extrusion processing is a
technique for continuously shaping a fluidized polymer through an appropriately shaped die,
followed by the solidification of the extrudate [92,93]. The main parts of an extruder are the
plasticator (screw), barrel, motor, and nozzle (Figure 1.7).
Figure 1.7. Schematic diagram of an extruder machine, showing individual parts.
The raw thermoplastic polymer in pellets, granules, flakes, powders, or a combination is
used for extrusion. Additional to the raw material, additives such as pigments, dyes,
reinforced materials, or other materials, either in liquid or pellet form, are often used to
achieve specific characteristics or properties for the final product. The raw material and
additives are fed to the hopper with or without a feeding mechanism. The feeding can be
achieved using different feeding techniques, like starvation or flood feed techniques. As the
material enters the feed throat through the hopper near the rear end of the barrel, it comes
in contact with the screw. The rotating screw forces the plastic resin forward through the
barrel. The screw system is typically categorized by three zones: the feed zone, where solid
pellets are loaded; the transition zone, where mechanical working and heating fluidize the
polymer; and the melt-metering zone, where the molten polymer is driven to the extrusion
die. Each barrel section has a heating element with an independent PID controller
(proportional-integral-derivative) to control the heating according to the temperature profile
precisely. The extra heat is provided to the polymer via intense pressure and friction
generated within the barrel and the screw rotation as the raw material moves forward. The
plastic resin is gradually melted as it is pushed through the barrel, lowering the risk of
overheating, which may cause degradation of the polymer. The molten polymer is then forced
into a die, which imposes the intended final geometry on the extruded material and allows it
to solidify through cooling to maintain the shape. A crucial challenge for extrusion systems is
the phenomenon of die swell, which refers to the increase in the cross-section area observed
when the extrudate leaves the restriction of the die. For a given polymer choice, this effect is
a function of flow resistance, thereof, the melt temperature. Typical design measures used to
stabilize the geometry of the extruded material include the manipulation of the extrusion-die
geometry and associated material, as well as active cooling by forced air convection or water
| 9
quenching. Extrusion processing is customarily used to produce pipe/tubing [94], filaments
[65,95,96], window frames [94], plastic films and sheeting [97], thermoplastic coatings
[98,99], etc.
1.1.3.2. Injection Molding
Injection molding is another polymer processing technique used to produce parts by
injecting material into a mold cavity [100–102]. The injection molding process starts with
feeding polymers with or without additives in the form of granulates through a hopper to a
barrel heated to a sufficient temperature for melting the polymer. The molten polymer is
mixed and carried to the nozzle using a screw. Then the molten polymer will be injected with
high pressure into a mold cavity. Later, the product is set to cool, which helps the solidification
process while adapting to the cavity configuration. Subsequently, the plates are moved apart,
and the product is ejected or removed from the mold, primarily using an ejection pin.
Injection molding is a helpful technique for the cost-effective production of complex three-
dimensional parts. The primary advantage of the processing technique is that the mold cavity
can be so designed that there are a specific number of cavities to enable the production of a
maximum number of products simultaneously out of the same mold in one cycle and
therefore used for mass production.
1.1.3.3. Additive Manufacturing
Additive manufacturing (AM), alias 3D printing, is a rapid prototyping technique [103–107].
The 3D printing technique is a computer numerical controlled (CNC) operation in which a
computer controls the parameters for each layer to form a 3D object by adding layer by layer.
This way, 3D printed objects are created by adding material layers one after the other. Almost
any shape or geometry of the object can be produced from a digital model or other electronic
data files, such as standard triangle language (STL) or an additive manufacturing file (AMF).
In 1981, Hideo Kodama of Nagoya Municipal Industrial Research Institute published the
first account of a working photopolymer-based rapid prototyping system [108]. Since then, it
has marked the development of 3D printing machines. Additive manufacturing has been
successfully applied in various fields, like aerospace and biomedical fields [103]. Due to its
high customizability and design-on-demand, it has shown its importance in product
development for developing visual and functional prototypes and modeling [109–111]. 3D
printing technology is also widely used for small or medium-scale production [112,113].
The process of 3D printing is discussed in the following. The initial step of the 3D printing
process is modeling. Here, the 3D model is designed with computer-aided designing (CAD) or
3D modeling software; a 3D scanner, digital camera, or photogrammetry software can also
be used to develop 3D models. The models created with the CAD software generally have the
least possible errors and are the best source for modeling. Once the model is created, it is
exported into specific file extensions like .STL or .AMF.
10 | Introduction
The second step of the process is carried out with slicing software. The software converts
the 3D model with predefined parameters, such as printing or movement speed, layer height,
geometrical coordinates, nozzle-, bed temperature, and other variables with which the object
should be manufactured. The information is then exported into a file with a 3D printer-
compatible file extension like “.gcode” for fabricating the 3D model. The resulting file is a
plain-text file with a series of GCODE and MCODE commands with a list of the complete X-,
Y- , and Z-axis coordinates and machine parameters required for printing the 3D model.
The final step of the 3D printing process is carried out by transferring the GCODE file to the
3D printer. The 3D printer reads and processes the file accordingly and creates the 3D object
layer by layer.
1.1.3.4. Fused Filament Fabrication
Fused filament fabrication (FFF) is a widely popular and most used technique among
different AM techniques [109,114–118], like stereolithography (SLA), multi-jet fusion (MJF),
selective laser sintering (SLS), and big area additive manufacturing (BAAM). The FFF uses the
raw material in the form of a filament. Commercially available standard FFF printers
prerequisite the filaments in diameter of either 2.85 mm or 1.75 mm with significantly lower
tolerances, depending upon manufacturing requirements. Figure 1.8 shows a typical FFF
printer.
Figure 1.8. The schematic representation of a 3D printer that works according to the principle of FFF
and the individual parts of the print head (blue color).
During the printing process, the material is unwounded from the spool to fabricate the
part. A torque and pinch system is typically used to feed the filament to the printer’s extruder
precisely. The filament from the feeding system is pushed to the printer’s extruder head
directly or through a Bowden tube. A typical FFF 3D printer’s extruder head consists of a heat
sink, a heater block with a thermistor, a heating cartridge, and a nozzle (Figure 1.8). The
filament enters the heater block via the heat sink from the cold end to ensure that the
filament 1
filament 2
printing platform
print head
z-axis
heat sink
liquidifier
(termistor and
heater block)
nozzle
| 11
filament gets molten only at the region of the heater block (hot end). This helps maintain a
constant printing pressure and uniform polymer extrusion throughout the printing process.
Once the material enters the heater block, the block heats and melts the filament to a usable
temperature. The molten filament is then forced out through the nozzle opening and extrudes
the material into thin strands. The extruded material is laid over the model or builds platform
as predetermined by the slicer program, where the print head and/or build platform are
moved to the exact location (X, Y, or Z) for placing the molten material. The 3D printer then
builds the model by layer-by-layer addition of material and thereby builds the physical object
[109,114–118].
1.1.4. Shape Memory Polymers
Shape memory polymers (SMPs) are smart materials that can retain a temporary shape
attained through a thermomechanical treatment called “programming.” The SMP recovers its
permanent shape once exposed to an external stimulus by triggering the shape memory
effect (SME) [119–123] (Figure 1.9). The external stimuli can be temperature [123],
light [124], infrared radiation or laser light [125–128], magnetic field [129–132], electric
field [133,134], or others [123,135–138].
Figure 1.9. Permanent shape, programmed shape, triggering of the SME when heated above the Tm of
the soft segments (switching temperature) and recovered shape of an additively manufactured man
(top and middle row, front and left view respectively) and duck (bottom row).
The SME is independent of a specific material property of single polymers but results from
a combination of the polymer’s morphology and structure in cooperation with the applied
programming and processing technology. Among different SMPs, thermally switchable SMPs
are the most common and studied polymers [135,139,140]. Based on the network structure,
SMPs can be categorized as physically [119,120,141–143] and chemically cross-linked
12 | Introduction
SMPs [119,120,144,145]. While, according to the thermal transition temperature (Ttrans) of
the particular switching segments, the SMPs Ttrans can be either the Tm or Tg. The
thermoresponsive SMEs of SMPs may be triggered either via direct or indirect heating. For
indirect heating, the polymer is doped with suitable fillers, which enable selective heating of
the polymer by irradiation with light [146,147], magnetic fields [132,148–150], or electric
current [151–153].
Amidst other SMPs, TPUs have proven their potential [51,86,154–165], primarily due to
their adjustable phase separation, strong deformability, reprocessing capability, and
recyclability. The TPUs exhibiting shape memory properties consist of switching segments,
also known as soft segments, and crosslinks or netpoints, making the hard segments. The
former is used to maintain the temporary shape, while the latter determines the permanent
shape and is used to recover its initial shape. A typical programming procedure of
thermoresponsive SMPs includes either applying a deformation above the phase transition
temperature (Ttrans), so-called “hot programming,” or deforming the sample at a low
temperature, known as the “cold programming.” Subsequently, for the hot programming
scenario, while holding the deformed shape, the SMP would be cooled below the fixation
temperature (Tfix), i.e., below its Tg or offset of crystallization transition temperature (Tc) of
the soft segment. After a sufficient cooling time, the external load would be removed, and
the temperature would be raised to room temperature to complete the programming steps.
Afterward, the new temporary shape remains stable until the ambient temperature of the
polymer is raised over Ttrans. Consequently, the SME is triggered, and the polymer recovers its
original shape. The SMP can be brought into a subsequent temporary shape by performing
the programming step again. This new temporary shape need not be the first temporary
shape.
1.1.5. Molecular Mechanism of Shape Memory Effect
The polymer chains of thermoplastic SMPs always prefer a randomly coiled arrangement
in their permanent shape. In the case of semicrystalline SMPs, heating above the Ttrans of the
soft segment makes them elastic during programming. Later, stretching the polymer
increases the distance between the netpoints of the hard segment and orients the network
structure. The new temporary shape is fixed by introducing reversible physical cross-links in
the form of crystallites by cooling below the Tfix. The freshly formed crystals stabilize the new
shape and restrict the macromolecules from spontaneously returning to the random coil
structure. The temporary shape remains stable until the SME is triggered. Heating the
material above Ttrans again cleaves the physical cross-links by melting the crystallites in the
switching phase. The associated gain in entropy is the thermodynamic reason for the polymer
to recover its random coiled structure, enabling the SMP to recover its permanent shape
(Figure 1.10).
| 13
Figure 1.10. Molecular mechanism of a physically cross-linked SMP representing permanent shape,
programming of a temporary shape, and shape recovery. Green blocks represent the netpoints formed
by hard segments, and blue lines represent soft segments.
In the case of amorphous SMPs, the polymer can be easily deformed elastically in its
rubbery state once heated above its Tg. Later, the fixation of the temporary shape is
accomplished by kinetically freezing the motion of the polymer chains by cooling below its Tg.
The programmed shape remains stable as the polymeric chains do not possess sufficient
thermal energy to retract to their preferred orientation in the glassy state. The almost
permanent shape is later recovered when thermal energy is brought into the system via
heating above Tg.
1.2. Shape Memory Effect of Objects Manufactured via Fused Filament Fabrication
Fused filament fabrication (FFF) with polymers can be subdivided into five categories based
on the SMEs that can be harnessed from the additively manufactured object (Figure 1.11).
Additive manufacturing with polymers generally creates non-responsive objects that are
static. However, the FFF with thermoresponsive SMPs helps to produce 3D objects that can
exhibit dual shape one-way (1W) SME, one-way 1W-SME (four-dimensional (4D) printing),
two-way (2W)-SME, and 2W-4D SME. The different thermoresponsive SMEs achieved
employing FFF will be described in more detail in the following.
Permanent/
Recovered shape
Deformed shape
Heating
Cooling
Temporary shape
(T > Ttrans)(T > Ttrans)
(T < Tfix)
(T < Ttrans)(T > Ttrans)
Shape recovery
14 | Introduction
Figure 1.11. Classification of the fused filament fabrication (FFF) process based on the responsiveness
of the printed object.
1.2.1. One-Way Shape Memory Effect
Dual-shape 1W-SME is the most prominent and widely employed SME. Following a
programming step, the SMP can take a temporary shape and remain stable at room
temperature [95,166–169]. The permanent shape of the SMP is recovered under stress-free
conditions once the SMP is heated above its Ttrans of the soft segment [95,167,170] (Figure
1.12). An additional programming step has to be undergone after each triggering of the 1W-
SME to restore the thermoresponsive state for enabling the next shape memory cycle.
Figure 1.12. Programming and recovery step of an additively manufactured object to achieve 1W-SME.
The shape memory properties of a polymer are generally quantified in cyclic thermo-
mechanical measurements (CTMs) [119,123,171]. The measurements are carried out using a
3D printing of
thermoresponsive (one-way) object
Programming
Permanent
shape
Temporary
shape
Static
structure 3D printing of
non-responsive object
Fused filament
fabrication
Temporary
shape
Permanent
shape
Heating
4D printing of
thermoresponsive (one-way) object
Permanent
shape Programming
3D printing of
thermoresponsive
(two-way) object
Bistable
state -I
Heating
Cooling
Bistable
state -II
Bistable
state -I
Heating
Cooling
Bistable
state -II
Shape A Shape B
Shape A Shape B Shape C
Shape A Shape B
Shape A Shape B
Shape A
Two-way 4D printing of
thermoresponsive (two-way) object
Heating
unloading
Recovered shape
(Shape A)
Method I:
stress recovery
Method II:
free strain recovery
Permanent shape
(shape A)
T>Ttrans
Programming
T< Ttrans
Temporary shape
(shape B, after
unloading)
T<Tfix T>Ttrans
T< Ttrans
Heating Cooling Heating
| 15
tensile testing machine equipped with an environmental chamber. During a CTM study, the
temperature (T), strain (ε), and stress (σ) are generally closely watched. At the same time, the
respective heating or cooling rates, deformation, and loading or unloading rates can be
adjusted to develop different programming procedures. Generally, a dog-bone-shaped
tensile bar (EN ISO 527-2:1996) [172] is used to perform CTMs.
Additive manufacturing is an established method to produce 3D objects from
SMPs [95,166–170,173,174]. The SMPs processed utilizing FFF can be programmed as
described in sections 1.1.4 and 1.1.5 to show 1W shape memory properties. Among other
AM techniques, FFF has shown its significance in processing SMPs, primarily due to the
adjustable horizontal and vertical resolution, good printing results [95,169], and the ability to
adjust mechanically and shape memory properties by varying geometrical parameters like
print orientation and infill percentage [167,168].
1.2.2. Two-Way Shape Memory Effect
Without further programming steps, the SMPs cannot return to their temporary shape on
heating or cooling after triggering their 1W-SME and the complete recovery of their original
shape (1W-SME). On the other hand, the 2W-SME using SMP can actuate between two
programmable metastable states [65,175–181]. Generally, some semicrystalline SMPs like
TPU have shown this switching behavior for single-material systems. The 2W-SME was initially
observed in chemically cross-linked poly(cyclooctene) [175] and was later transferred to
physically cross-linked polymers. However, the initially discovered 2W-SME was achieved
while applying an external load permanently. It was recently shown that it is possible to
actuate a semicrystalline SMP just by heating and cooling without needing any external load
[66,182,183].
The programming of a reversible 2W-SME is almost identical to a 1W-SME. After heating
the polymer to a temperature of T > Tm, the soft segment becomes completely amorphous.
The material is then deformed from its permanent form (shape A) into a predetermined form,
shape B, using an external force (Figure 1.13). This deformation induces the polymer chains
to achieve a highly oriented molecular structure within the polymer network of the soft
segment. Subsequently, the fixation of the temporary shape is achieved by cooling under
stress to a temperature of Tc, below the offset of the soft segmental crystallization
temperature, thereby storing the temporary shape (shape B). The applied stress can be
removed after crystallization without causing the chain segments to recoil. The
macromolecular alignment is inextricably linked to the macroscale deformation of the
material, and thus, bespoke geometries can be realized by simply varying the applied force
during programming.
The hard segments are responsible for determining the shape-shifting geometry during
actuation. Here, the soft segments provide the driving force for actuation because their chain
segments undergo conformational dis- and reorientation on heating and cooling,
respectively. In detail, on heating to an intermediate temperature Tmax (maximum actuation
16 | Introduction
temperature) < Tm, which is between the onset and offset of the melt transition temperature,
the melting-induced contraction (MIC) of the actuation domains generates a first bistable
state- I (shape C). The soft segmental chains responsible for actuation remain in a partially
oriented conformation, constrained by the deformed crystalline geometry determining units.
Upon cooling to Tc, the recrystallization of the soft segments drives in achieving the second
bistable state-II (shape D). Cycling between Tc and Tmax allows to switch between the two
metastable states (shape C and shape D) reversibly and can be repeated hundreds of times
[65]. Moreover, the SMP can be further reprogrammed by heating to Tm. The original shape
of the material is recovered, and later, a completely new shape can be defined by repeating
the programming procedure.
Figure 1.13. Schematic representation of programming and actuation of an additively manufactured
object to achieve 2W-SME.
1.2.3. Four Dimensional (4D)-Printing
3D printing with SMPs produces non-responsive objects, which always requires an
additional programming step to make them thermoresponsive. In the case of 4D-printing, the
printed objects are obtained directly in their temporary shape and determine the shape after
heating above the switching temperature [67]. In 2013, Skylar Tibbits first introduced the
concept of 4D-printing, where the printed objects can transform their shape directly after
removal from the print platform when appropriate stimuli are applied, without needing a
| 17
further post-treatment step [184]. To demonstrate the 4D-printing concept, Skylar Tibbits
utilized a multi-material combinational approach, where a hydrogel was used as an active
material and a static, rigid polymer as a passive material to achieve different degrees of
bending or shape transformations [184,185]. Later, the 4D-printing technology was
transferred to the FFF and was achieved using a single SMP [67,186–194]. It is worth
mentioning that on using 4D-printing, the objects are manufactured directly in their
thermoresponsive state and are thus available for immediate
use [67,186,188,190,191,193,194] (Figure 1.14). This way, the need for classical 1W-SME
programming can be eliminated for certain SMPs.
Figure 1.14. Schematic representation of 4D-printing and shape transformation of an additively
manufactured object to achieve one-time thermoresponsiveness.
In the case of 4D-printing via FFF with SMPs, specific printing parameters like nozzle
temperature, printing pressure, and speed are precisely selected for attaining individual
polymer chains to assume a highly oriented state. At the same time, the printing pattern is
adjusted to achieve specific bending or shrinkage behavior [67,186,195]. The basic idea
behind the 4D-printing technique is that an extruded polymer strand laid on the printing
platform or a top polymer layer is rapidly cooled below the polymer’s glass transition
temperature so that the individual polymer chains are oriented in the direction of nozzle
movement. Later, they are not allowed to relax on quick vitrification, thus storing the highly
oriented states efficiently.
Figure 1.15 shows an illustration of the 4D-printing process. In detail, the polymer filament
is first heated to a melt temperature (Tperm) above the material's Tg. The molten filament is
then extruded through the nozzle and laid on the print platform, also denoted as the print
bed. The selection of specific printing parameters causes the polymer chains to orientate in
the direction of nozzle movement [186]. The SMP is then allowed to cool quickly below its Tg
once laid on the printing platform or the previous layer (below) [67,196,197]. The vitrification
freezes the oriented chain formation caused by the printing to be stored in the material. The
printed shape (Shape A) remains stable until the ambient temperature is raised above the
switching temperature of the SMP, most commonly the glass transition temperature [67].
Once heated, the polymer releases internal stresses by molecular motion to gain an
entropically more favorable random coil chain conformation, culminating in shrinkage along
the direction of nozzle movement to achieve its permanent shape (Shape B) [196,197].
Temporary shape
after 4D-printing
(shape A)
Permanent shape
(shape B)
Heating Cooling
18 | Introduction
Figure 1.15. Schematic representation of the 4D-printing process and thermal triggering of the 4D effect
on a printed object.
1.2.4. Two-Way Four-Dimensional Printing
The 4D-printing of actuating objects is a novel area of research. The underlying concept
was developed to eliminate the time-consuming classical programming of SMPs to witness
2W-SMEs, directly after the 4D-printing. In other words, an object is fabricated/ additively
manufactured directly in a first bistable state-I after printing, from which the shape can be
switched to a second bistable state-II only by applying an appropriate stimulus. The whole
process is reversible (Figure 1.16).
Figure 1.16. Schematic representation of 2W-4D-printing of an actuating object.
The development and innovation of materials and AM technology has led to objects that
actuate immediately after printing. The 2W-4D-printing technique can be achieved through
gradient- or bilayer-structures or core-shell embedded structures [198]. The bilayer- or
gradient- structure exploits composite AM technology [199–205]. Here, the fabricated
composite layers consist of materials with antagonistic or reversible physical properties, like
shape changes associated with water absorption/desorption or temperature changes, to
T<Tg
T<Tg
T>Tg
4D-printed object
(temporary shape) Permanent shape at 23°C
attained after heating above Tg
Filament
Extruder
Print bed
Nozzle
Heater block
Heat sink
Bistable state-I
(shape A, after
2W-4D printing)
Bistable state-II
(shape B)
Stimulus 2
Stimulus 1
| 19
induce stimulus-governed shape changes. On the contrary, the core-shell embedded
structure focuses on manufacturing elastomeric material composites containing magnetic
particles [206–209]. When applying a specific magnetic field, their shape can be changed in a
controllable manner; as soon as the magnetic field is switched off, the initial shape is
recovered due to the elastomeric properties of the matrix material.
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| 29
Chapter 2:
Motivation
30 | Motivation
Chapter 2: Motivation
Shape memory polymers (SMPs) are smart materials that can promptly recover their
original shapes from their temporary shapes in stress-free conditions upon exposure to
external stimuli. The SMP takes the temporary shape through a thermomechanical treatment
called programming [1,2]. Among various stimuli-responsive SMPs, thermoplastic
polyurethanes (TPU) with shape memory properties have shown superiority due to their
phase segregation and modifiable material properties, like thermal and mechanical behavior.
Today, thermally switchable polymers are the most common and studied SMPs [3–5]. A
significant advantage is that direct or indirect heating can trigger the shape memory effect
(SME). This flexibility makes such materials easy to use and self-sufficient to a certain degree.
On the other hand, additive manufacturing (AM), alias three-dimensional (3D) printing, has
been developed as a processing technique for thermoplastic polymers, which allows the
building of 3D objects from polymers by adding material in a layer-by-layer approach [6–10].
Among various other AM techniques, fused filament fabrication (FFF) is widely used [11–14].
Among various SMP materials, TPUs are the most used and researched technology
platform, likewise the FFF technique in AM. Nevertheless, until the beginning of this doctoral
study in 2017, only four scientific publications were presented, where TPU with shape
memory properties was processed via FFF [15–18]. Furthermore, the presented contributions
could only additively manufacture TPUs using custom-made or modified AM machines. The
doctoral work focuses on utilizing the FFF technique to process TPU-based SMP to uplift and
bring the best of two worlds. The primary goal of the work is to bring the best of the two
technologies to explore and achieve different SMEs, where the additively manufactured TPU
objects are thermomechanically treated after processing to exhibit one-way (1W) SME and
two-way (2W) SME. While on the other hand, the four-dimensional (4D) printing technique is
utilized to achieve “in-situ” programming of TPU with shape memory properties, where the
3D printed models can exhibit thermoresponsive properties directly following AM for
triggering one-time 1W-SME and 2W-4D printed SME.
The initial scientific hurdle of the work was to enable the processing of TPU with shape
memory properties by employing a commercially available standard FFF printer, as this can
reduce the processing complexity, attain standardization, and ease the manufacturing of 3D
models using standard slicer software. In the last decade, quick response (QR) code carriers
were developed using TPU-based SMP as functional base material, which has the potential
application as anti-counterfeiting technology [19] or for supervising cold chains [20].
Nevertheless, the previously presented manufacturing processes are complex, time-
intensive, and tedious, including SMP plaque manufacturing via injection molding, “guest
diffusion” for a surface-specific coloration, and laser engraving of computer-generated two-
dimensional codes onto the polymer surface [19]. An alternative method involves coating the
SMP, code engraving onto the top layer, and laser cutting/ punching [21,22]. These complex
manufacturing processes of information carriers motivated me to research and evaluate the
| 31
FFF technique for easing information carriers' production. As well as to study the influence
on the print quality along XY- and Z- planes and, thereby, the build time for the production of
information carriers using different nozzles. As the functionality and durability of 3D printed
models are the primary functions of the SMP information carriers, the SMEs and
programming techniques were essential to evaluate. After evaluating the influence of print
quality from employing different nozzle sizes, the printability of Arial fonts and filigree
structures was explored. To utilize the full potential of TPU with shape memory properties
fabricated via FFF for developing novel applications that can be addressed with FFF.
Once the 1W-SME of 3D printed TPU-SMP was realized, it was clear that the main
disadvantage of such systems is the need for programming, which is a time-, energy- and cost-
intensive process. Here, the 4D-printing technique that Skylar Tibbits first presented in 2013
can overcome the drawbacks mentioned above; the technique showed that the 3D printed
objects could transform their shape directly after removal from the print platform on
exposure to a distinct stimulus. Later, researchers learned how to attain “in-situ”
programming or 4D-printing using FFF [23]. Even though a few similar kinds of research were
carried out to harness 4D-printing effects, the 4D effect diminished with increasing layers, and
the printed objects could not attain uniform shrinking. In other words, on increasing the
object's height (z<5 mm) [24–26], the 4D effect weakened drastically, which could be caused
by the selection of suboptimal printing parameters. Additionally, there were very few
applications developed utilizing the 4D-printing technique. Here, it was essential to determine
whether preferred orientations can also be introduced into printed objects in higher layers,
which significantly pushes the limits of what is technically feasible. Furthermore, to explore
and understand the potential applications of 4D printed parts for active assembly, active
disassembly, and others.
On the other hand, scientists have learned how to transfer semicrystalline polymers into
two metastable states in the last few years, where SMPs can be switched back and forth by
varying the temperature [27–34]. However, several materials were developed and studied for
2W-SME in previous contributions; 3D printing has not been used to manufacture SMPs that
can later actuate.
In parallel, programmable materials are currently the subject of intensive research [34–
41]. The programmable material utilizes external structure, molecular structure, and material
functionality advantageously for achieving specific shape changes or property changes to
perform specific tasks. An essential advantage of programmable materials is that they are not
dependent on a constant power supply, making them highly resistant to failure and
characterized as self-sufficient material behavior, which makes them fulfill both sensor and
actuator functionality [34–37,39–44].
From previous contributions, it was clear that scientists could implement 2W-SMEs for
physically cross-linked TPU-SMP [39,45,46]. Nevertheless, actuation achieved for such
polymers under constant strain or stress-free conditions was lower than for their chemically
cross-linked counterparts [32,47–51]. One of the reasons could be selecting an inappropriate
temperature range for actuation, as there were no proper temperature screening
32 | Motivation
characterization methods. Therefore, a new characterization technique was essential for
identifying the ideal actuation scenarios. Furthermore, the previous contributions showed
that the 2W-SME could grab or lift smaller objects, like screws and nuts [52]. However, the
scalability of the effect was unknown, and it was not identified how to magnify the 2W-SME
and the combination of 2W-SME programmed polymer with metamaterials for the
development of programmable materials using TPU-based SMP. Another question was
whether an SMP could exhibit bidirectional actuation when programmed by bending on
utilizing programmable material structured TPU-SMP.
The previous investigations on 2W-SMEs revealed that the programming method required
for the SMP to achieve the desired actuation is tedious, complex, and non-economical.
Alternatively, it was apparent from previous works of literature that even though researchers
could attain 2W-SMEs directly after 3D printing without needing a further programming step,
there were many drawbacks to the presented solutions. Some drawbacks are that the 2W-
SME was achieved only with composite materials, such as two- or multilayered material
systems [46] or embedded materials [53–56], where one material provides the elastic
property and the other for changing or fixing temporary shape. Moreover, such systems'
complicated recycling ability and detachability at their end-of-life or end-of-use scenarios
make them significantly less unattractive for a sustainable ecosystem. There are also no
proposed solutions or technological approaches for a single material system to exhibit 2W-
SME directly after its manufacturing.
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| 35
Chapter 3:
One-Way Shape Memory Effect of
Objects Programmed After Fused
Filament Fabrication
36 | Chapter 3.1
Chapter 3.1: Additive Manufacturing of
Information Carriers Based on Shape
Memory Polyester Urethane
| 37
Chapter 3.1: Additive Manufacturing of Information Carriers
Based on Shape Memory Polyester Urethane
The original article Chalissery, D.; Pretsch, T.; Staub, S.; Andrä, H. Additive Manufacturing
of Information Carriers Based on Shape Memory Polyester Urethane. Polymers 2019, 11,
1005 and graphical abstract are published in MDPI polymers and available at
https://doi.org/10.3390/polym11061005.
Figure 3.1.0. The graphical abstract of the article “Additive Manufacturing of Information Carriers Based
on Shape Memory Polyester Urethane” is published in MDPI Polymers 2019, 11, 1005.
Contribution
My contribution: The concept idea of additive manufacturing of information carriers,
utilization of different nozzle sizes, the idea of the whole manuscript, design development,
conducting experiments, formal analysis, investigation, methodology, validation,
visualization, preparation of images (including graphical abstract) and writing—original draft.
Not included in my contribution: Congruence analysis.
Pretsch, T.: Conceptualization of the manuscript, funding acquisition, project administration,
supervision, writing—review and editing
Staub, S.: Congruence analysis: formal analysis, investigation, methodology, visualization, and
writing—original draft.
Andrä, H.: Congruence analysis: funding acquisition, project administration,
conceptualization and formal analysis
38 | Chapter 3.1
Chapter 3.1: Additive Manufacturing of Information Carriers
Based on Shape Memory Polyester Urethane
Dilip Chalissery 1, Thorsten Pretsch 1,*, Sarah Staub 2 and Heiko Andrä 2
1 Fraunhofer Institute for Applied Polymer Research IAP, Geiselbergstr. 69, 14476
Potsdam, Germany; [email protected]
2 Fraunhofer Institute for Industrial Mathematics ITWM, Fraunhofer-Platz 1, 67663
[email protected] (H.A.)
* Correspondence: thorsten.pretsch@iap.fraunhofer.de; Tel.: +49-(0)-331/568-1414
Received: 5 April 2019; Accepted: 4 June 2019; Published: 5 June 2019
3.1.0. Abstract: Shape memory polymers (SMPs) are stimuli-responsive materials, which
are able to retain an imposed, temporary shape and recover the initial, permanent shape
through an external stimulus like heat. In this work, a novel manufacturing method is
introduced for thermoresponsive quick response (QR) code carriers, which originally were
developed as anticounterfeiting technology. Motivated by the fact that earlier manufacturing
processes were sometimes too time-consuming for production, filaments of a polyester
urethane (PEU) with and without dye were extruded and processed into QR code carriers
using fused filament fabrication (FFF). Once programmed, the distinct shape memory
properties enabled a heating-initiated switching from non-decodable to machine-readable
QR codes. The results demonstrate that FFF constitutes a promising additive manufacturing
technology to create complex, filigree structures with adjustable horizontal and vertical print
resolution and, thus, an excellent basis to realize further technically demanding application
concepts for shape memory polymers.
Keywords: additive manufacturing; 3D printing; shape memory polymer; fused filament
fabrication; QR code carrier; thermoplastic polyurethane; filigree structures
3.1.1. Introduction
Additive manufacturing (AM) alias three-dimensional (3D) printing is increasingly gaining
importance, especially because of the rapid availability and the infinite design variety of print
objects. Within the commercially established AM technologies, fused filament fabrication
(FFF), which is a hot-melt extrusion-based 3D printing process, is widely used [1–3]. It requires
a virtual 3D model and appropriate slicing software to convert the model into thin layers and
gain the essential printing instructions. After melting in the extruder nozzle, the polymer
strand is deposited layer-by-layer on the building platform of a 3D printer by moving the
nozzle along a pre-calculated path. Once deposited, the polymer hardens immediately in the
desired arrangement of polymer strands, which set the final shape of an object. Since FFF is
| 39
an extrusion-based technique, it easily gives access to new thermoplastic materials provided
they can be processed with filaments that meet the requirements of a 3D printer. To date,
many thermoplastic materials have been investigated via FFF, at which special attention was
devoted to polylactic acid (PLA), acrylonitrile-butadiene-styrene copolymers (ABS),
polycarbonates, and polyamides [3]. Other indispensable polymer-based AM methods
include stereolithography (SLA), multi-jet fusion (MJF), selective laser sintering (SLS), and big
area additive manufacturing (BAAM).
Basically, the prospect of developing new applications for 3D printing improves as new
functional materials are developed [4,5]. In general, many shape memory polymers (SMPs)
are thermoresponsive thermoplastics. They are able to retain an imposed, temporary shape
after programming and to recover the initial, permanent shape upon exposure to an external
stimulus like heat [6–11]. Today, thermoplastic polyurethanes belong to the most intensively
studied shape memory polymers [12–25]. Intriguingly, there are only a few publications in
which FFF has been employed as a printing method for thermoresponsive polyurethane-
based SMPs, the majority of them concentrating on polyether urethanes. Obviously good
printing results could be achieved by Hendrikson et al. [26]. who demonstrated that scaffolds
can be produced via FFF from the polyether urethane DiAPLEX® MM 3520 from SMP
Technologies Inc. The scaffolds were characterized by a fiber spacing of 982 ± 11 µm, a fiber
diameter of 171 ± 5 µm, and a layer height of 154 ± 2 µm. In another work, Raasch et al.
reported on the extrusion of the thermoplastic polyether urethane DiAPLEX® MM 4520 from
the same company and used the obtained filaments to manufacture specimens out of them;
the 3D objects were later examined by three-point bend tests to study the influence of
annealing upon shape memory behavior [27]. In a work by Yang et al., the same material was
extruded and FFF used to print parts with shape memory properties [28]. Villacres et al.
fabricated tensile bars of DiAPLEX® MM 4520 and proved how to influence the mechanical
properties by varying geometrical parameters like print orientation and infill percentage [29].
The apparently only work on extrusion-based AM of polyester urethane so far has been
reported by Monzón et al. [30], who employed a custom-made 3D printer to produce parts
of Desmopan® DP 2795A SMP from Covestro Deutschland AG. The setting of the individual
layer height was selected to be 400 µm; the stress recovery behavior of programmed parts
was studied and there was a potential seen to be used as mechanical actuators.
Until today, plenty of applications have been suggested for SMPs [31–40]. One of these
applications is switchable information carriers [41–43]. According to the underlying concept
of SMP TagnologiesTM, e.g., a quick response (QR) code, which can be considered as an
example for a complex two-dimensional structure, is contained in the surface of an SMP. After
fabrication, the code can be converted from machine-readable to unreadable by
programming. Upon thermally triggering the shape memory effect, the QR code returns into
the machine-readable state. The special material behavior of information release on demand
can be helpful to verify and identify counterfeit products [41] or to supervise cold chains [44].
In the past, the preparation of information carriers turned out to be labor-intensive, since
several manufacturing steps had to be passed through. In fact, once an SMP was processed
40 | Chapter 3.1
via, e.g., injection molding, “guest diffusion” had to be applied to achieve a surface-specific
coloration and laser treatment to generate a two-dimensional code in the polymer surface,
before the information carrier could be obtained in its final shape by going through a cutting
process [41]. Alternatively, an SMP can be coated and a code engraved into the resulting top
layer, followed, e.g., by laser cutting [45,46]. Since the procedures are partly very complex,
the primary goal of this work includes the introduction of an easier approach to fabricate QR
code carriers. To keep it as simple as possible, the same polyester urethane (PEU), which was
employed as base material in earlier generations of information carriers, was used. On the
way to the production of QR code carriers, the individual steps of filament manufacturing and
processing via FFF were examined, before appropriate programming paths were explored and
the functionality of the QR code carriers was evaluated. Finally, the results of 3D printing were
considered against the background of other technologies used in additive manufacturing.
3.1.2. Experimental Section
Material: The polyester urethane (PEU) Desmopan® DP 2795A SMP from Covestro
Deutschland AG (Leverkusen, Germany) was chosen as model compound and used as
received in the form of a granulate. The hard segment of the PEU is composed of 4,4’-
methylenediphenly diisocyanate and a 1,4-butanediol chain extender. The soft segment is
based on poly(1,4-butylene adipate) (PBA). Further information regarding the thermal and
mechanical properties of the PEU is given in previous publications [25,46,47].
Extrusion: The PEU granulate was dried at 110 ◦C in a Binder vacuum drying chamber
VDL 53 from Binder GmbH (Tuttlingen, Germany) in order to remove water and avoid bubble
formation when extruding filaments at a later stage. The thermal pre-treatment was finalized
after 150 min. Subsequently, the pellets were fed into an extrusion line to produce filaments
(Figure 3.1.1).
Figure 3.1.1. Technical drawing of an extrusion line as used for the production of PEU filaments: Material
feeding system (A), twin screw extruder (B), conveyor belt (C), water bath (D), and filament winding
machine (E). The extrudate is drawn in red.
The individual units of the extrusion line were put together in such a way that it included
a volumetric material feeding system Color-exact 1000 from Plastic Recycling Machinery
(Zhangjiagang City, China), a Leistritz twin screw extruder MICRO 18 GL from Leistritz AG
| 41
(Nürnberg, Germany), characterized by seven heating zones and a screw length of 600 mm, a
conveyor belt, a water bath and a filament winder from Brabender GmbH and Co. KG
(Duisburg, Germany). The temperature of the individual heating zones of the extruder was
180, 185, 190, 195, 200, 190, and 190 ◦C. The screw speed of the extruder was set to 77 rpm.
Initially, the PEU granulate was processed without additives. In another experiment 0.5 wt. %
of Irgazin® Red DPP BO from Kremer Pigmente GmbH and Co. KG (Aichstetten, Germany) were
added to obtain a red filament. To evaluate the quality of the filaments, the evolution of
filament diameter was manually detected at regular intervals using a Vernier caliper from
Fowler High Precision (Auburndale, FL, USA).
Virtual Design: The bar code generator goQR.me [48] was used to create a QR code (Reed-
Solomon error correction, error correction level H) with the encoded information “Fraunhofer
IAP” (Figure 3.1.2) [49].
Figure 3.1.2. Technical drawing of a QR code, which was used as structural motif for the production of
information carriers. All data are provided in mm.
The code was saved in the .jpeg image format and used as starting point to build a virtual
information carrier by means of the vector-oriented drawing program AutoCAD from
Autodesk, Inc. (San Rafael, CA, USA) (Figure 3.1.3) [50].
Figure 3.1.3. Technical drawing of a virtual QR code carrier including a substrate layer (gray color) and a
structural QR code elevation (black color): Top view (a), isometric view (b), front view (c), and left view
(d). All data are provided in mm.
42 | Chapter 3.1
The edge length of the QR code was set to 25 mm (Figures 3.1.2 and 3.1.3a). For the
substrate layer a dimensioning of 30 mm × 40 mm was selected. As can be seen in
Figure 3.1.3b–d, the QR code carrier was built up by two structural units including a substrate
layer with a height of 180 µm and a structural QR code elevation with a height of 190 µm. The
design of the QR code carrier as provided by Figure 3.1.3 was used for most of the
experiments, which are described in this contribution. The only exception was an approach
in which the target height of the substrate was reduced to 15 µm. For convenience, a
terminology for the different types of QR code carriers and the associated print settings is
introduced in Fused Filament fabrication Section. After finalizing the design, the 3D models
were imported into the slicer program Cura 3.3.1 from Ultimaker B. V. (Watermolen, The
Netherlands) [51]. As a result, numerically controlled codes, also denoted as G-codes, were
generated, containing the instructions for the 3D printer in the form of printing paths,
information with reference to the amount of extruded material and the spatially resolved
printing parameters. Finally, the codes were transferred to the 3D printer.
Fused Filament Fabrication (3D Printing): Fused filament fabrication was used to produce
QR code carriers with differing technical specifications. The experiments were carried out
with the commercially available 3D printer Ultimaker 3 from Ultimaker B. V. (Geldermalsen,
The Netherlands). The manufacturer provides an XYZ resolution for Ultimaker 3 of 12.5, 12.5,
and 2.5 µm [52], defining the smallest movement that the 3D printer can make with regard
to the XY plane and in the Z direction. To calibrate the print bed, the Ultimaker build plate
manual leveling calibration method was carried out before the beginning of each experiment.
Therefore, a calibration card characterized by a thickness of about 170 µm was used. The
process included a rough leveling of the build plate followed by a fine leveling. The fine
leveling was achieved with the calibration card, at which the knurled nut was adjusted at the
rear center, front left, and front right of the build plate until slight friction occurred, when
sliding the card between built plate and print head.
Basically, the same design of QR code carriers was used as introduced in Section 3.1.2.3.
The two print heads of the FFF-printer were either equipped with two nozzles having different
diameters of 100 and 400 µm, respectively, or the same diameter of 400 µm. For simplicity,
the following terminology is introduced, pointing out the most relevant variations when
producing QR code carriers:
Type 1: The substrate was printed with non-dyed PEU using a 400 µm nozzle and a target
layer thickness of 180 µm. The elevation was built from red PEU with a 100 µm nozzle.
Type 2: In analogy to the type 1 QR code carrier, the substrate was printed with non-dyed
PEU using a 400 µm nozzle. Again, a target layer thickness of 180 µm was selected for the
substrate, but the elevation was built with red PEU employing a 400 µm nozzle.
Type 3: Similar as in the previous cases, the substrate was printed with non-dyed PEU using
a 400 µm nozzle, but this time a reduced target layer thickness of 15 µm was selected. The
elevation was built with red PEU using a 400 µm nozzle.
The most relevant settings for the 3D printing processes are listed in Table 3.1.1.
| 43
Table 3.1.1. Printing instructions for Ultimaker 3 to produce the three different prototypes of QR code
carriers based on PEU.
Specifications
Substrate
(Non-Dyed PEU)
Elevation
(Red PEU)
Type of QR code carrier
1, 2
3
1
2, 3
Diameter of the nozzle (µm)
400
400
100
400
Temperature of the nozzle (°C)
225
225
190
190
Speed of print head (mm × s−1)
50
50
4
7
Build rate (ml × h−1)
34.2
34.2
5.4
22.8
Build platform temperature (°C)
23
23
23
23
Number of layers
1
1
3
1
Layer height (µm)
180
15
63
190
Characterization of Thermal Properties: Dynamic mechanical analysis (DMA) was used to
investigate the thermomechanical properties of the PEU. The experiments were carried out
on two samples including a cylindrical granulate grain, having a diameter of 1.8 mm and a
length of 4.86 mm, and a sample of a 3D printed substrate of a type 2 QR code carrier having
the size of 5.15 mm × 3.8 mm × 0.19 mm. The measurements were conducted with a Q800
DMA from TA Instruments (New Castle, DE, USA) at a frequency of 10 Hz. At first, the sample
was heated to 100 ◦C, before it was cooled to −100 ◦C to finalize the first heating-cooling cycle.
Adjacently, the measurement cycle was repeated once more. For all experiments, heating
and cooling rates of 3 ◦C·min−1 were selected and the holding time at the highest and lowest
temperature was set to 10 min. The storage modulus (E´), loss factor (tan δ) and the glass
transition temperature (Tg) were determined for the second heating.
The phase transition behavior of the PEU was also studied by differential scanning
calorimetry (DSC) using a Q100 DSC from TA Instruments (New Castle, DE, USA). The
measurements were performed on a granulate grain, a piece of the filament and a sample of
the 3D printed substrate of a type 2 QR code carrier. In any case, the sample weight was
approximately 5 mg. In the experiments a sample was first cooled to −90 °C, before it was
heated to 100 °C and cooled back to −90 °C, which finalized the measurement. Cooling and
heating were carried out with a rate of 10 °C x min−1. The temperature holding time was 10
min at −90 and 100 °C, respectively.
Characterization of Print Quality: Topography measurements were performed on QR code
carriers using a FocusCam LV150 confocal microscope from Confovis GmbH (Jena, Germany),
which was equipped with an objective lens of 5×/0.15 N.A. Any time, the sample was
illuminated with a ring light. The data recorded by the focus variation microscope was
evaluated with the software MountainsMap® imaging topography 7.4 from Digital Surf
(Besançon, France) [53]. The development of the surface profile with regard to a scanned
cuboid including its surrounding was exemplarily determined for type 1 and type 2 QR code
carriers. For a detailed analysis, a line was inserted along the mid-perpendicular through the
44 | Chapter 3.1
cuboid. The cuboid was characterized with a step measurement to determine its height and
width.
Further microscopic investigations were carried out with the microscope Axio Scope.A1
from Carl Zeiss Microscopy GmbH (Jena, Germany) using the imaging software Zen 2.3 lite
also from Carl Zeiss Microscopy GmbH [54]. The experiments were conducted to evaluate the
resolution of the QR code in the XY-plane and to estimate the layer thickness of QR code
carriers and thus the Z-parameter. In the latter case, a cut was made with a scalpel along the
mid-perpendicular through the abovementioned cuboid.
The printing results were also mathematically investigated. For this purpose, QR code
carriers were scanned in a first step as gray value images with a resolution of 600 dpi and then
loaded into the software tool ImageJ developed by Wayne Rasband (Bethesda, MD, USA) [55].
With assistance of this tool the images were cropped to the dimension of the original QR code
and scaled to the corresponding resolution. The brightness and contrast were adjusted such
that the influence of reflections and possible shadows was minimized. Then, the gray value
images were binarized by the automatic binarization function in ImageJ. In a next step, the
binarized images of the printing results were inverted. Thus, those areas where there was
only the substrate of the QR code carrier were marked in black while the printed elevation
parts were marked white. Adjacently, the inverse images of the printing results as well as the
original QR code were imported into the software tool Paraview from Kitware, Inc. (Saratoga
County, NY, USA) [56] and exported into the vtk format. The software Paraview allows
mathematical operations on the values of images. The binary values of the printing results
and the QR code were added up in each pixel. Due to the applied inversion for the printing
results, three different gray values V were obtained in the summation (Equation 3.1.1):
𝑉= {0,𝑒𝑟𝑟𝑎𝑛𝑒𝑜𝑢𝑠𝑙𝑦 𝑛𝑜𝑡 𝑓𝑖𝑙𝑙𝑒𝑑 (𝑏𝑙𝑢𝑒)
255,𝑐𝑜𝑛𝑔𝑟𝑢𝑒𝑛𝑡 (𝑔𝑟𝑒𝑒𝑛)
510,𝑖𝑟𝑟𝑒𝑔𝑢𝑙𝑎𝑟𝑙𝑦 𝑓𝑖𝑙𝑙𝑒𝑑 (𝑟𝑒𝑑) (3.1.1)
Based on these values, the print quality of the different prototypes was evaluated as a
percentage p by means of Equation 3.1.2:
𝑝=# pixels of certain gray value
# pixels (3.1.2)
Programming and Characterization of Shape Memory Properties: The programming of QR
code carriers was carried out with an MTS Criterion universal testing machine from MTS
Systems Corporation (Eden Prairie, MN, USA). The device was operated with a temperature
chamber, which was controlled by a Eurotherm temperature controller unit. Two heating
elements were located at the back of the chamber. Liquid nitrogen from a Dewar vessel was
fed into the chamber under a pressure of 1.3 bar as an essential prerequisite for cooling. At
the beginning of programming, a QR code carrier was clamped with a length of 25 mm,
corresponding to the edge length of the QR code, in the pneumatic grips of the universal
testing machine, the chamber was heated to 60 ◦C and a maximum force Fmax of either 5 or
| 45
25 N was applied using a loading rate of 300 mm x min−1. The maximum distance between
the outer sides of the QR code was immediately determined by means of a Vernier caliper
from Fowler High Precision. The QR code carrier was then cooled to –15 ◦C, whereby the
clamping distance was kept constant. After 10 min, the sample was unloaded and the
chamber was heated to 23 ◦C.
A ZTNG-100B heating plate from Dr. Neumann Peltier-Technik GmbH (Neuried, Germany)
was used to investigate the thermoresponsiveness of the programmed QR code carriers.
Therefore, the temperature was gradually raised from 23 to 60 ◦C and images of the sample
were taken in regular time intervals during shape recovery. After finalizing the experiment,
the congruence of the QR code pattern with regard to the permanent and the recovered
shape was determined and used to evaluate shape recoverability. In this connection, a similar
approach was followed as described in Programming and characterization of shape memory
properties. Section, but this time gray value images were generated for the QR code carrier
in its permanent and recovered shape. The gray value image of the permanent shape was
regarded as the standard with which the recovered shape was compared. Therefore, the
binarized gray value image of the recovered shape was inverted and added to the image
containing the information of the permanent shape. The resulting gray values V were
evaluated such that in the case of consistent pixels the areas were considered to be congruent
while, for nonexistent pixels, the areas were regarded as incongruent (Equation 3.1.3):
𝑉= {255,𝑐𝑜𝑛𝑔𝑟𝑢𝑒𝑛𝑡 (𝑔𝑟𝑒𝑒𝑛)
𝑒𝑙𝑠𝑒,𝑖𝑛𝑐𝑜𝑛𝑔𝑟𝑢𝑒𝑛𝑡 (𝑟𝑒𝑑) (3.1.3)
By analogy with the above procedure, the percentage was determined again according to
Equation 3.1.2, but this time it was the measure of shape recoverability.
A multiple cycle experiment was carried out with the MTS Criterion universal testing
machine, which was equipped with a temperature chamber. For loading, a type 2 QR code
carrier was clamped with a length of 25 mm, corresponding to the edge length of the QR code,
in the pneumatic grips of the universal testing machine, heated to 60 ◦C, deformed with a rate
of 300 mm x min−1 to a maximum clamping distance of 55 mm, before unloading was carried
out at the same temperature with a rate of 150 mm x min−1. In total, 20 cycles of loading and
unloading were conducted. In the 21st cycle, the sample was loaded and the imposed shape
was fixed by cooling to −15 ◦C. After unloading, the temperature was raised to 23 ◦C and the
machine readability of the QR code was checked. The programmed QR code carrier was
adjacently heated to 60 ◦C where, again, the readability of the QR code was investigated. To
characterize the boundary between substrate and elevation, another programming was
accomplished. In this 22nd cycle, a cut was made with a scalpel along the mid-perpendicular
through the abovementioned cuboid and investigated by means of light microscopy. The
sample was finally heated to 60 ◦C and a microscopic investigation was carried out with the
microscope Axio Scope.A1, which was equipped with an objective lens of 20× and 40×
magnification. Following other programming scenarios, a QR code carrier was folded in the
46 | Chapter 3.1
middle or rolled up at 60 ◦C, before it was cooled under load to −15 ◦C. Afterwards, the
thermoresponsiveness was again followed on the heating plate when triggering the shape
memory effect. Independent of the programming technique applied, the machine readability
of QR codes was checked with a Samsung Galaxy S8 smartphone from Samsung Electronics
(Seoul, South Korea), which was equipped with the software “Optical Reader” version 4.4.07
also from Samsung Electronics Co., Ltd [57].
3.1.3. Results and Discussion
The melt extrusion of the physically cross-linked PEU block copolymer led to the
production of a whitish filament whose color can be traced back to the presence of crystals
from poly(1,4-butylene adipate) (PBA); the proof will be given below in a DSC measurement.
In another experiment, 0.5 wt. % of Irgazin® Red DPP BO was added in the course of PEU
extrusion so that a red filament could also be obtained. It is noteworthy that the two filaments
had a homogenous diameter of 2.85 ± 0.07 mm, regardless of whether the dye was added or
not (Figure 3.1.4).
Figure 3.1.4. Evolution of filament diameter over time when extruding PEU. The development in
measured values is also representative for an experiment in which Irgazin® Red DPP BO was added during
extrusion of PEU.
Before starting with the 3D printing experiments, the design of the QR code carriers was
developed (Figure 3.1.3). The objects were sliced to obtain the essential printing instructions.
In a next step, a dual extrusion FFF process was established, in which the already obtained
filaments were reprocessed to build up QR code carriers, characterized by a whitish substrate
and a red QR code elevation. The most relevant settings for the 3D printing processes are
provided in Table 3.1.1.
For the production of a type 1 QR code carrier, a single-layer substrate with a target height
of 180 µm was printed, using the white filament and a nozzle with a diameter of 400 µm. In
contrast, the QR code elevation having a virtual height of 190 µm was then built up in three
layers by melting the red filament in the 100 µm nozzle and placing the resulting strands on
| 47
the substrate. The printing results are portrayed in Figure 3.1.5 together with their
microscopic characterization.
Figure 3.1.5. Type 1 QR code carrier as investigated by light and confocal microscopy including an
evaluation of print quality: Top view and inset exhibiting a randomly selected cuboid (a), surface
topography of the cuboid and its surrounding (b), superposition with a virtual QR code having a
transparency of 60% (c), result of a mathematic calculation to determine the congruence of the virtual
QR code with the physical print object: consistent print areas (green color), irregularly filled areas (red
color) and unfilled print areas (blue color) (d), side view of a cut through the cuboid and the substrate
(e), and the evolution of layer thickness Z with regard to the cuboid and its surrounding (f).
The obtained type 1 QR code carrier exhibited a good spatial resolution with respect to the
XY level as exemplified by the presence of finely resolved rectangles (Figure 3.1.5a). In order
to better assess the print quality with regard to the smallest structural unit of the QR code
pattern, a cuboid of the finished part with a virtual edge length of 1.21 mm was
microscopically examined (Figure 3.1.5a,b). Here, an edge length of approximately 1.25 mm
could be determined. This value exceeded the one of our CAD model by 40 µm corresponding
to 3.2% of the object dimension (Figure 3.1.3a). Basically, a deviation from the technical
specification was anticipated due to slight fluctuations in filament diameter (Figure 3.1.4) and
minor differences in the print bed height resulting from the calibration [58]. However, in the
XY plane the print quality of the overall QR code pattern was pretty good as supported by the
48 | Chapter 3.1
result of a superposition experiment, in which the virtual QR code was put with a
transparency of 60% over the printing pattern (Figure 3.1.5c). In addition, a congruence
measurement was carried out, subtracting the overhanging regions of the QR code elevation
from the black regions of the virtual code. The result gave that 90.7% of the code areas were
congruent, 8.1% were irregularly filled with red PEU and 1.2% were erroneously not filled
(Figure 3.1.5d). Next, the resolution in the Z-direction was closely investigated for the same
cuboid and its nearest surrounding. The substrate of the QR code carrier had a thickness of
about 160 µm (Figure 3.1.5e). The averaged profile height of the elevation was determined
to be approximately 145 µm, corresponding to a mean layer height of about 48 µm
(Figure 3.1.5f). The layer thickness was slightly below the target value, presumably due to
deficits in calibration accuracy. The production of the QR code elevation took 17 min,
culminating for the whole QR code carrier in a production time of 25 min. For a faster
production, the 100 µm nozzle was replaced by a 400 µm nozzle and the technical parameters
were adjusted accordingly (see Table 3.1.1). As a result, a type 2 QR code carrier was obtained
and examined microscopically (Figure 3.1.6).
Figure 3.1.6. Type 2 QR code carrier as investigated by light and confocal microscopy including an
evaluation of print quality: Top view and inset exhibiting a randomly selected cuboid (a), surface
topography of the cuboid and its surrounding (b), superposition with a virtual QR code having a
transparency of 60% (c), result of a mathematic calculation to determine the congruence of the virtual
| 49
QR code with the physical print object: consistent print areas (green color) and irregularly filled areas
(red color) (d), side view of a cut through the cuboid and the substrate (e), and the evolution of layer
thickness Z with regard to the cuboid and its surrounding (f).
This time, the presence of more imperfect rectangles could be witnessed in the QR code
pattern (Figure 3.1.6a,b). Once more, the cuboid was studied, which was located at the same
position of the QR code as in the preceding case (Figure 3.1.5a), in order to get a first
impression about the precision in the XY printing plane. Here, a drastically increased edge
length was determined as documented by a value of about 1.52 mm (Figure 3.1.6a),
exceeding the virtual dimensions of this element by 26% (Figure 3.1.3a). The fact that the
horizontal print resolution substantially deteriorated in the whole QR code area was
confirmed by another superimposition experiment. As visible to the naked eye, the printed
regions generously overlapped the black areas of the virtual QR code pattern (Figure 3.1.6c).
Against this background, another mathematic calculation was carried out. It turned out that
77.4% of the code areas were congruent, whereas 22.6% of those code areas, in which no
printing was desired, were covered with red PEU (Figure 3.1.6d). However, compared to the
type 1 QR code carrier, the same substrate thickness could be verified as expected, but better
control over the vertical print resolution could be achieved as indicated by an average profile
height of 175 µm (Figure 3.1.6e,f). Furthermore, the production time of the QR code elevation
could be drastically reduced to 3 min and 30 s so that the printing of the entire QR code carrier
was finalized after 11 min and 30 s.
Despite the abovementioned dimensional inaccuracies in the 3D printed objects, the QR
codes enabled an error-free decoding with a standard smartphone, independent of which
technical equipment and parameter settings were used for printing. This clearly shows that
the surface contrast was sufficiently high as ensured by the processing of the differently
colored filaments.
To investigate the influence of reprocessing via extrusion and FFF on the viscoelastic
properties of the PEU, dynamic mechanical analyses were conducted. Therefore, the raw
material in the form of a granulate grain was studied and compared with the
thermomechanical behavior of a sample, which was taken from the 3D printed substrate of a
type 2 QR code carrier. The associated temperature-dependent evolution in storage modulus
E’ and in tan δ is provided by Figure 3.1.7.
In both cases, E’ exhibits a two-step decrease in the DMA measurement as characteristic
for physically cross-linked PEU [22,59–61]. The investigation of the granulate grain reveals a
strong drop in E’, starting at −51 ◦C and indicating the presence of a glass transition. The tan δ
peak is located at about −18 ◦C. Upon further heating, a weaker decline in E’ takes place, which
can be associated with the melting of PBA crystals as earlier verified for the same
material [44]. The 3D printed sample shows a similarly pronounced drop in E’, starting again
at approximately −50 ◦C, and a tan δ peak at −20 ◦C, which is in accordance with the thermal
behavior of the granulate grain. In contrast, the decline in storage modulus associated with
PBA melting is slightly extended toward higher temperatures. This could be related to an
orientation effect as supported by reprocessing via FFF, favoring the formation of PBA crystals
50 | Chapter 3.1
with higher temperature stability. In other words, the conditions under which parts of the
PBA phase of PEU crystallized were expected to be more favorable for the 3D printed sample.
To take another look at this, DSC measurements were carried out (Figure 3.1.8).
Figure 3.1.7. Thermal and mechanical properties of PEU as determined by DMA: Evolution of storage
modulus E’ (solid line) and tan δ (dashed line) at the second heating of a granulate grain (red color) and
the sample of the substrate of a type 2 QR code carrier as manufactured via FFF (blue color).
Figure 3.1.8. DSC thermograms of PEU: Thermal behavior of a granulate grain (red color), a piece of
filament (green color) and a sample from the substrate of a type 2 QR code carrier as obtained via FFF
(blue color). The thermograms are exhibited for the second heating and cooling. The individual
enthalpies of melting are 25.9 J x g−1 (granulate grain), 21.1 J x g−1 (filament) and 25.1 J x g−1 (3D printed
sample), the enthalpies of crystallization are −25.7 J x g−1 (granulate grain), −21.0 J x g−1 (filament), and
−25.0 J x g−1 (3D printed sample).
The DSC cooling trace of the 3D printed sample shows an exothermic signal at about 7 ◦C
associated with the recrystallization of the PBA phase [25]. Compared with the thermal
| 51
behavior of the granulate grain, the peak crystallization temperature increased by about
15 ◦C. This observation can be taken as further hint that strand deposition in course of 3D
printing favored a better alignment of polymer chains, thus facilitating the recrystallization of
PBA. In a third DSC measurement, the filament of PEU was investigated. Here, another
exothermic signal associated with PBA crystallization appeared on cooling, justifying the
whitish color of the filament. In this case, the peak crystallization temperature was closer to
the one of the granulate grain. In turn, the DSC heating traces of the three samples show the
presence of two phase transitions. The first one is located at around −50 ◦C and, thus, close
to the point at which E’ started to drop in the DMA measurement. It is related to the glass
transition temperature of the PBA phase, while the endothermic signal in between 20 and
50 ◦C with a maximum at around 40 ◦C can be assigned to the melting of PBA crystallites [25].
Here, the same trend as in the DMA measurement could be verified, but the melting peak
temperature of PBA only increased by 2 ◦C for the 3D printed sample compared with the
granulate grain. Beyond that, the melting behavior of the crystalline PBA phase appeared to
be similar for the filament and the granulate grain. Most importantly, when considering both
the DMA and DSC data, no further evidence was found that two-step processing, including
extrusion and FFF, had a significant impact on the thermal properties of the PEU.
In a next step, the shape memory properties of a type 2 QR code carrier were investigated
(Figure 3.1.9).
Figure 3.1.9. Type 2 QR code carrier: Permanent shape after 3D printing (a), temporary shape as
obtained after programming (Fmax = 5 N) (b), and recovered shape after heating to 60 ◦C (c). To visualize
shape recoverability, the image of the permanent shape was converted to black-and-white and
superimposed with a transparency of 60% on the image of the recovered shape (d). The result of a
mathematical calculation comparing the permanent shape with the recovered shape: congruent areas
(green color) and incongruent areas (red color) (e).
Therefore, the additively manufactured QR code carrier (Figure 3.1.9a) was heated to
60 ◦C, at which the melting of the PBA phase was completed. Subsequently, a tensile force
Fmax of 5 N was applied, whereupon a maximum distance length of 55 mm between the outer
sides of the QR code was detected. The elongated QR code carrier was fixed by cooling below
the crystallization temperature of the PBA phase and unloaded (Figure 3.1.9b). Due to
changes in the design of the QR code carrier and in particular because of the much smaller
52 | Chapter 3.1
structural thickness of only 160 µm, a significantly lower deformation force was required to
achieve a similar QR code distortion in the programmed shape compared to an earlier
generation of QR code carriers, which was characterized by a thickness of 2 mm and required
a tensile force Fmax of 48 N [41]. Intriguingly, the bonding was strong enough to withstand a
removal of the QR code elevation from the substrate in the course of deformation. The
programmed shape of the QR code carrier, which was stable at 23 ◦C, was characterized by
the largest distance length between the outer sides of the QR code of 54 mm, speaking for
the excellent shape fixity of the polymer. Due to its drastic distortion the code was no longer
machine-readable. Upon triggering the shape memory effect, the QR code pattern almost
completely returned to the original shape (Figure 3.1.9c), which was accompanied with the
restoration of machine readability. For a more detailed study, another image analysis was
carried out. Herein, the superimposed QR codes of the original shape and the recovered
shape turned out to be almost identical (Figure 3.1.9d). The distinct shape recoverability was
evidenced by another mathematic calculation, unveiling that 87.8% of the code areas of the
permanent shape and the recovered shape were congruent (Figure 3.1.9e). Overall, the
pronounced shape memory properties, which were detected in the first experimental series,
raised the question if type 2 QR code carriers are able to resist even stronger deformations.
To find out the answer, a similar programming experiment as described above was
performed, but this time Fmax was raised to 25 N (Figure 3.1.10).
Figure 3.1.10. Type 2 QR code carrier: Permanent shape after 3D printing (a), the temporary shape as
obtained after programming (Fmax = 25 N) (b), and the recovered shape after heating to 60 ◦C (c). To
visualize shape recoverability, the image of the permanent shape was converted to black-and-white and
superimposed with a transparency of 60% on the image of the recovered shape (d). The result of a
mathematical calculation comparing the permanent shape with the recovered shape: congruent areas
(green color) and incongruent areas (red color) (e).
As a matter of fact, the QR code carrier produced by FFF (Figure 3.1.10a) was elongated so
that the outer sides of the QR code had a maximum distance of about 155 mm. After cooling
below the crystallization temperature of the PBA phase and unloading, the temperature was
raised to 23 ◦C. Here the new, even more strongly deformed shape proved to be stable
(Figure 3.1.10b). The distance between the outer sides of the elongated QR code measured
153 mm in tensile direction which, again, revealed the excellent shape fixity of the polymer.
| 53
It is remarkable that even in this case the QR code became machine-readable again after
triggering the shape memory effect (Figure 3.1.10c), which demonstrates that the concept of
information release on demand was still working. Apparently, the decoding algorithm of the
smartphone was able to compensate the residual distortion. The discrepancy between the
QR code pattern of the permanent shape and the recovered shape can be clearly seen in the
corresponding superimposed images (Figure 3.1.10d). As quantified in one further
mathematical calculation, 72.7% of the code areas were congruent (Figure 3.1.10e).
Compared to the previous case, a weakening of shape recoverability was expected due to the
stronger deformation applied. It can be assumed that this phenomenon of growing residuals
with increasing elongation can be traced back to the flow of amorphous segments in the
polymer [62].
To determine the degree of deformation, at which the QR code was no longer machine-
readable, another type 2 QR code carrier was deformed at 60 ◦C with a rate of 0.5 mm x min−1
while the machine readability of the QR code was regularly checked. It turned out that the
QR code became unreadable as soon as a distance length of 30 mm between the outer sides
of the QR code was exceeded.
In an attempt to study the reliability of shape memory properties, a type 2 QR carrier was
exposed to a multiple cycle experiment (Figure 3.1.11).
Figure 3.1.11. Type 2 QR code carrier: Permanent shape after 3D printing (a), initial clamping distance =
25 mm), temporary shape as obtained after 20 loading-unloading cycles (maximum clamping distance =
55 mm) at 60 ◦C, followed by programming (b), and the recovered shape after heating to 60 ◦C in the
21st cycle (c). Microscopic investigation of a cut through the cuboid and the substrate as examined in the
22nd cycle for the programmed shape (d) and the recovered shape (e); the insets show an enlarged view
of the boundary between the substrate (below) and the elevation (above).
The additively manufactured QR code carrier (Figure 3.1.11a) was loaded to a clamping
distance of 55 mm and unloaded twenty times, before a loading at 60 ◦C and unloading at
−15 ◦C was accomplished. In this 21st cycle, the QR code was non-decodable at 23 ◦C and
characterized by a maximum distance length of 55 mm between its outer sides
(Figure 3.1.11b). Triggering the shape memory effect by reheating to 60 ◦C resulted in shape
recovery as accompanied with the restoration of the machine-readable code, characterized
by a maximum edge length of 26.8 mm (Figure 3.1.11c). This unequivocally demonstrates the
reliability of the concept of switchable information carriers. In the ensuing 22nd cycle, the
54 | Chapter 3.1
thermomechanical treatment of the previous cycle was repeated, but neither micro cracks
nor delamination could be microscopically detected at the boundary between the substrate
and the elevation both for the programmed shape (Figure 3.1.11d) and for the recovered
shape (Figure 3.1.11e). This finding indicates good layer coalescence. As expected, the
triggering of the shape memory effect led to an increase in layer thickness. In fact, a recovery
from 105 to 155 µm for the substrate and from 120 to 165 µm for the elevation could be
verified.
Next, the deformation scenarios for the programming of QR code carriers were expanded
toward rolling and bending, before the respective thermoresponsivity was studied
(Figure 3.1.12).
Figure 3.1.12. Thermoresponsiveness of type 2 QR code carriers, which were deformed in a folding
approach (a) and a rolling approach (b): Programmed shapes (left), sequential shape recovery when
placed on a 60 ◦C hot heating plate (images 2–4) and recovered shapes (images 5–6).
Therefore, two of our type 2 QR code carriers were heated to 60 ◦C, at which the PBA phase
of the PEU was completely amorphous. The first sample was folded in the middle
(Figure 3.1.12a), the other was rolled up (Figure 3.1.12b). The fixation of the resulting
temporary shapes was then achieved on cooling below the crystallization temperature of the
PBA phase. After unloading, the QR code carriers were placed on a heating plate, which had
a temperature of 60 ◦C. In both cases it took about 10 s to finalize shape recovery, which again
was accompanied with the restauration of the machine-readable QR codes, thus
demonstrating that the concept is not restricted to deformation scenarios like elongation or
compression [41,63].
Following another design approach, the dimensions of the QR code carrier were altered by
drastically reducing the target substrate thickness from 180 to 15 µm. As a result, a type 3 QR
code carrier was obtained (Figure 3.1.13).
The production time of the type 3 QR code carrier was about 11 min 30 s, corresponding
to the processing time of the type 2 QR code carriers. To illustrate the low thickness, a 50
euro cent coin having a thickness of 2 mm was placed next to it (Figure 3.1.13a). As
determined in a microscopic measurement, the thickness of the PEU substrate varied from
about 7 to 10 µm (Figure 3.1.13b).
| 55
Figure 3.1.13. Type 3 QR code carrier: Illustration of size and thickness in comparison with a 50 euro
cent coin, which is characterized by a height of 2 mm (a) and image of a light microscopic investigation
to estimate the thickness of the QR code carrier (b).
It is also worth mentioning that the weight of all QR code carriers described herein was
significantly lower compared to earlier generations of prototypes, which were obtained by
other processing techniques [41,45,46]. In direct comparison with each other, the introduced
type 1 and type 2 QR code carriers were approximately weighing 340 mg while the weight of
the type 3 QR code carrier was 100 mg and, thus, significantly lower, qualifying it for
applications, in which the costs for transport must be kept under control.
For the purpose of comprehensive consideration, the printing results described herein
were compared with those of other printing materials, which were processed by FFF, and
additionally with the results of other 3D printing techniques. For convenience, the same
approach was followed as by Quinlan et al. [64], who compared polymer-based processes like
fused filament fabrication (FFF), stereolithography (SLA), big area additive manufacturing
(BAAM), multi-jet fusion (MJF) and selective laser sintering (SLS) with particular emphasis on
build rate and layer thickness, the latter of which can be considered as a measure of Z-
direction accuracy. The corresponding results are supplied in Figure 3.1.14.
Figure 3.1.14. Build rate versus layer thickness for common additive manufacturing processes. The initial
data was extracted from Quinlan et al [64]. The red stars represent data points for the QR code elevation
as part of the type 1 QR code carrier (1) and the type 2 QR code carrier (2), while the remaining data
points refer to the substrate of the type 1 and type 2 QR code carrier (3) and the type 3 QR code
carrier (4).
56 | Chapter 3.1
It can be clearly seen that SLA, BAAM, MJF, and SLS provide higher build rates compared
with FFF. In turn, FFF makes it particularly possible to control the layer thickness, namely, the
Z-parameter, over quite a wide range as apparent for printing materials like ABS and PLA.
However, the data points introduced for the presented QR code carriers do also cover a broad
area, which in parts overlaps with the already existing data for FFF. Due to the printing result
of the thin layer as evident for the substrate of the type 3 QR code carrier, a data point
emerges, defining the lowest value for Z. Interestingly, this reasonably good print resolution
could neither be achieved by other groups, working on shape memory polyurethanes using
extrusion-based AM techniques [26–30,65,66] nor by other researchers who utilized those
3D printing techniques, which were described by Quinlan et al. [64]. Admittedly, two-photon
lithography (2PL) is another AM technology, which was not included in our considerations,
but allows obtaining 3D objects, which are characterized by even smaller layer thicknesses of
0.2 to 0.3 µm [67]. Although being particularly advantageous in resolution, the good print
results of 2PL are at the expense of the build rate. Therefore, a compromise is needed, which
seems to be achievable by FFF, well-balancing the build rate with print resolution and, thus,
qualifying it as promising technology to obtain shape memory polymers in entirely new
shapes.
3.1.4. Conclusions
Fused filament fabrication is a suitable technique to produce bicolored additively
manufactured QR code carriers in a dual extrusion process as demonstrated for a polyester
urethane, which was used as model compound. The print resolution both in the XY-plane with
regard to the QR code pattern and in Z-direction with reference to the layer height could be
controlled by the experimental setup and the print instructions. This way, filigree, well-
resolved structures could be obtained. The objects were able to resist strong deformations
and characterized by distinct shape memory properties. Even in a multiple cycle experiment
no major damage could be witnessed for the print objects. The use of congruence
measurements has proven to be a valuable tool to determine the printing accuracy and shape
recoverability. Although a higher resolution of the QR code pattern was achieved when using
a setup with a 100 µm nozzle, with extending the production time, FFF seems to be a practical
method in this scenario as well, which may give access to other technically demanding
objects. The main advantages of the new manufacturing process for QR code carriers are that
polymer extrusion can be easily controlled, a significantly lower amount of base material is
needed, facilitating the fabrication of very thin layers with a thickness below 10 µm, and the
use of solvents can be avoided. The latter is of ecological importance. All these aspects
emphasize that the novel production process for QR code carriers is not only attractive for
research purposes, but also from an economic point of view, not least because the material
could be qualified for processing with a commercially available 3D printer. Therefore, FFF
could turn out as an enabling technology to realize applications for SMPs in fields like
counterfeit-proof marking of goods at risk of plagiarism and supervision of cold chains. Future
| 57
challenges consist in shortening the production time without compromising on resolution and
using the dimension of time to autonomously manipulate 3D printed objects, which is also
known as 4D-printing, thus eliminating the need for programming.
Author Contributions: Conceptualization: T.P. and H.A.; formal analysis: D.C., S.S., and H.A.;
funding acquisition: T.P. and H.A.; investigation: D.C. and S.S.; methodology: D.C. and S.S.;
project administration: T.P. and H.A.; supervision: T.P.; validation: D.C.; visualization: D.C. and
S.S.; writing—original draft: D.C., T.P., and S.S.; writing—review and editing: T.P.
Funding: This research was funded by Fraunhofer High Performance Center for Functional
Integration in Materials, grant number 630039, and by Fraunhofer Excellence Cluster
“Programmable Materials”, grant number 630527.
Acknowledgments: This work was supported as Fraunhofer High Performance Center for
Functional Integration in Materials (project 630039). T.P., S.S., and H.A. also acknowledge
support by the Fraunhofer Excellence Cluster “Programmable Materials” under project
630527. T.P. wishes to thank the European Regional Development Fund for financing a large
part of the laboratory equipment (project 85007031). The authors thank Chris Eberl
(Fraunhofer IWM) for fruitful discussions on two-photon lithography. Tobias Rümmler is
kindly acknowledged for carrying out the DMA measurements and Katrin Hohmann for light
microscopic investigations.
Conflicts of Interest: The authors declare no conflict of interest.
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© 2019 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article
distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license
(http://creativecommons.org/licenses/by/4.0/).
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Chapter 3.2:
Influence of Print Orientation on Shape
Memory- and Mechanical- Properties
After Fused Filament Fabrication
62 | Chapter 3.2
Chapter 3.2: Influence of Print Orientation on Shape
Memory- and Mechanical- Properties After Fused Filament
Fabrication
3.2.1. Introduction
Shape memory polymers (SMPs) are smart materials that can retain a temporary shape
attained through a thermomechanical treatment, also known as programming [1–6]. In the
case of thermoresponsive SMPs, the exposure to an external stimulus, like heat, recovers the
initial, permanent shape [7–9]. The permanent shape of the SMP is determined by
conventional processing techniques, like injection molding, extrusion, and additive
manufacturing (AM) [10–16]. In the last few decades, thermoplastic polyurethanes belonged
to the most researched SMPs [17–30].
Fused filament fabrication (FFF) is widely used amidst various AM methods due to its cost-
effectiveness and short manufacturing times [31–33]. Recently, researchers were able to
process SMPs using FFF [6,34–40]. For instance, our group utilized the dual-material printing
technique to manufacture high-contrast information carriers with quick response (QR)
machine-readable codes employing non-dyed and dyed thermoplastic polyester urethane
(PEU) [6]. Our group also showed that a small cuboidal elevation part of the QR code carrier
was found to have exceeded the CAD model by just 40 µm on utilizing a 100 µm nozzle,
thereby demonstrating a higher print resolution. In contrast, Villacres et al. used the
amorphous polyurethane DiAPLEX® MM 4520 as base material to study the influence of
geometrical parameters like print orientation and infill percentage upon the mechanical
properties of tensile bars obtained from FFF [38]. The contribution showed that the higher
the infill percentage, the higher the elastic modulus. On the other hand, the ultimate tensile
strength was highest when the print orientation was parallel to the direction of tensile
stretching. Similarly, the group studied the bending-related shape memory behavior of
samples with different print orientations [41]. The results demonstrated that the shape
recovery force of the samples was highest when the loading direction was in the direction of
the printing pattern.
Here the investigation focuses on how far the mechanical and shape recovery behavior of
FFF printed samples made from semicrystalline PEUs can be controlled by print orientation
when applying a tensile deformation during programming. The selection of the commercially
available semicrystalline PEU Desmopan® DP 2795A SMP was primarily due to its excellent
shape memory properties. Moreover, the ability to be processed via extrusion and
FFF (Chapter 3.1) [6]. After processing, the obtained filaments were used for FFF to fabricate
tensile bars of either horizontal or vertical strand orientation. Once characterized, the results
were compared with standard tensile bars punched out from injection molded plaques.
| 63
3.2.2. Results and Discussion
The Desmopan® DP 2795A SMP is a poly(1,4-butylene adipate) (PBA)-based polyester
urethane (PEU) that showed good printing results upon processing via FFF in our previous
work (Chapter 3.1) [6,30]. The PBA phase exhibits a melting transition (Tm) peak at ≈ 42 °C in
the DSC thermogram, with the onset and offset of the melting temperature at about 33 °C
and 55 °C, respectively [6]. As the melting transition is above room temperature, the PEU can
be applied advantageously in systems like automotive transmissions, in-door safety
appliances, and others. As the quality of filament is a crucial requirement for achieving
seamless print results, the primary requirement is to have a homogenous filament with the
appropriate diameter and low error tolerance. For this reason, extrusion processing is first
carried out to produce virgin PEU filaments, which are later characterized by a diameter of
2.85 ± 0.05 mm. Subsequently, the tensile bars were designed and sliced using the printing
parameters from our previous literature (Chapter 3.1) [6] (Table 3.2.2) and modified by
RepetierHost software [42]. The GCODE was modified to manipulate the strand deposition
printing pattern to obtain either horizontally or vertically oriented tensile bars. The altered
GCODE of the tensile bars was simulated for visualization, and the errors were rectified.
Afterward, the files were used for FFF. The resulting three-dimensional (3D) printed parts and
their quality control was investigated using confocal microscopy, as depicted in Figure 3.2.1.
Figure 3.2.1. Tensile bars of PEU are printed in horizontal- (topmost) and vertical- (middle) orientation.
The inset (bottom) shows the enlarged image of a vertically oriented tensile bar (middle) as investigated
with a confocal microscope (red box).
From Figure 3.2.1, it is noticeable to mention that the printed parts showed very high
dimensional accuracy compared to their virtual design. In order to evaluate the quality of the
strand deposition, the surface of the tensile bar (red box) was closely examined using a
confocal microscope (Figure 3.2.1 bottom image). A closer look at the tensile bar showed that
64 | Chapter 3.2
each deposited strand has a width of about 400 μm. The width of the strand directly coincides
with the diameter of the nozzle opening, and it can also be seen that the strands were free
from air bubbles. Furthermore, no surface damage or distortion was observed. This indicates
that the selected printing parameters were ideally suited.
Once the excellent quality of the 3D-printed parts was proven, a mechanical
characterization was carried out on the tensile bars, printed in the horizontal or vertical
orientation, and compared to the behavior of injection-molded specimens (Figure 3.2.2).
Figure 3.2.2. Mechanical characterization of PEU showing the evolution of engineering stress-strain for
specimens, which were punched out from an injection molded plaque (dark blue), and of three tensile
bars, each additively manufactured in horizontal (blue) and vertical preferential orientation (violet). The
tensile experiments were carried out at 23 °C with an initial strain rate of 1%·min–1 until 5% of strain was
reached and continued with 2000%·min–1 until rupture occurred.
From the engineering stress-strain diagram, the averaged Young’s modulus of the tensile
bars was determined. Among the three specimens, the injection-molded plaque-punched
tensile bars showed the highest Young’s modulus of 56.3 ± 18.2 MPa, followed by the vertical
tensile bars with 26.5 ± 2.0 MPa, and the horizontal tensile bars with 19.6 ± 14.4 MPa. In the
case of the yield point, the strain value was observed to be 21.5 ± 2.5%, 27.1 ± 4.2%, and 29.4
± 0.9%, with corresponding stresses at 9.0 ± 1.2 MPa, 5.2 ± 0.6 MPa, and 4.1 ± 0.5 MPa, for
injection molded plaque punched, horizontally and vertically printed tensile bars, respectively
(Figure 3.2.2). In any case, further increase of the load resulted in necking and strain softening
first, followed by strain hardening as accompanied by an increase in stress, culminating in
sample rupture at strains of 1262.4 ± 83.4%, 922.6 ± 52.9%, and 635.0 ± 48.9% for injection-
molded specimens, horizontal and vertical tensile bars, respectively. The material behavior
can be explained by the coexistence of two types of PBA segments, at which, depending on
temperature, one part was highly flexible and amorphous. At the same time, the other was
more rigid and crystalline [27]. A progressive conversion from amorphous to crystalline
segments occurred during deformation. The assumption was supported by a whitish coloring
of the tensile bars and is associated with the crystallization of hitherto amorphous PBA. The
| 65
mechanical properties of the 3D printed samples were inferior compared to the injection-
molded ones, which was expected due to the layer-by-layer manufacturing method [43–45].
The FFF-processed semicrystalline PEU behaved similarly to other horizontal and vertically
oriented tensile bars, where higher mechanical properties were observed for horizontal
orientation than the vertically oriented tensile bars [44]. Some of FFF processed materials in
vertical and horizontal orientations are acrylonitrile butadiene styrene (ABS) [46,47], ABS-
carbon nanotubes composite [48], polycarbonate-ABS blends [45], polypropylene [43,49,50],
and polyethylene glycol diamines [51].
Having the good mechanical properties of the PEU in mind, a method for programming and
triggering of one-way (1W) shape memory effect (SME) was developed in line with the
thermal properties of the PEU (see chapter 3.1) [6]. For this purpose, cyclic thermomechanical
measurements (CTMs) were conducted (Figure 3.2.3). The shape memory properties were
analyzed using Equations 3.2.1, 3.2.2, and 3.2.3, summarized in Table 3.2.1.
Figure 3.2.3. Cyclic thermomechanical measurements of tensile bars were obtained by punching from
an injection molded plaque (dark green and dark blue), FFF in a horizontal direction (light green and
blue), and FFF in a vertical direction (cyan and violet). Evolution of stress (σ), strain (ε), and temperature
(T in °C, red line) against time for the first (b) and fourth measurement cycle (b).
From Figure 3.2.3 and Table 3.2.1, it can be deduced that the shape memory properties of
injection molded or differently additively manufactured tensile bars are very similar. In detail,
the strain fixity ratio was most pronounced for the 3D printed samples, with about 98% for
horizontal and vertical samples. In contrast, the injection-molded sample had a respective
value of about 97%. The strain recovery ratio increased from ≈96% to ≈99% from the first to
the fourth cycle for all the investigated samples. In contrast, the total strain recovery ratio
decreased from 75% to 71% from the first to the fourth cycle. The main difference among the
samples was the recovery stresses when triggering the SME; namely, 1.5 MPa for an injection
molded tensile bar, 1.25 MPa for an additively manufactured horizontal tensile bar, and
1 MPa for an additively manufactured vertical tensile bar. From this, it is understood that the
mechanical properties and the recovery stresses followed the same trend. The evolution in
recovery stress of the FFF samples is in line with the one verified by Villacres et al. [41]. The
66 | Chapter 3.2
recovery stress and tensile strength are highest when the direction of strand orientation is
parallel to the direction of the load acting [44]. From this, it is clear that it can alter the
properties by introducing preferred orientations. By changing the design, it is now necessary
to make the best possible use of the orientation effects to open up new applications.
Table 3.2.1. Overview of the shape memory properties quantified in cyclic thermomechanical
measurements for different tensile bars.
Sample
Shape memory properties
Cycle
1
2
3
4
[%]
Injection molded
Strain fixity ratio
96.9
97.0
97.4
97.6
Strain recovery ratio
95.9
98.1
98.6
98.9
Total strain recovery ratio
74.2
72.8
71.8
71.5
Horizontally
printed
Strain fixity ratio
97.9
98.2
98.0
98.2
Strain recovery ratio
94.0
97.8
98.8
98.9
Total strain recovery ratio
74.0
72.4
71.5
71.2
Vertically printed
Strain fixity ratio
98.0
98.2
98.1
98.1
Strain recovery ratio
96.1
98.5
98.6
98.9
Total strain recovery ratio
75.1
72.5
71.5
71.3
3.2.3. Conclusions
Chapter 3.2 shows that the FFF is a powerful AM technique to gain control over mechanical
properties and, more importantly, shape recovery stress without using another shape
memory polymer. The study also unravels that the recovery stress and tensile strength are
highest when the direction of strand orientation is parallel to the direction of loading in the
course of programming and weakest in the perpendicular direction. From this work, it was
also understood that the orientation effects brought via FFF for altering shape recovery
stresses and mechanical properties for semicrystalline SMPs, where likewise the amorphous
SMPs. This ability to manipulate the mechanical properties and shape recovery stresses may
be helpful for product developers and researchers to locally optimize or customize parts or
products according to the respective application or requirement.
3.2.4. Experimental Section
Materials: The polyester urethane (PEU) Desmopan® DP 2795A SMP from Covestro
Deutschland AG (Leverkusen, Germany) was chosen as the model compound and used as
received in the form of granules. Further information regarding the PEU's material, thermal
and mechanical properties is given in previous publications [30,52,53].
Extrusion: The PEU granulate was dried at 110 ◦C in a Binder vacuum drying chamber VDL
53 from Binder GmbH (Tuttlingen, Germany) to remove water and avoid bubble formation
| 67
when extruding filaments at a later stage. The thermal pre-treatment was finalized after 150
min. Subsequently, the pellets were fed into an extrusion line to produce filaments-
The individual units of the extrusion line were put together. They were volumetric material
feeding system Color-exact 1000 from Plastic Recycling Machinery (Zhangjiagang City, China),
a Leistritz twin screw extruder MICRO 18 GL from Leistritz AG (Nürnberg, Germany),
characterized by seven heating zones, and a screw length of 600 mm, a conveyor belt, a water
bath and a filament winder from Brabender GmbH and Co. KG (Duisburg, Germany). The
temperatures of the individual heating zones of the extruder were 180, 185, 190, 195, 200,
190, and 190 ◦C. The screw speed of the extruder was set to 77 rpm. The PEU granulates were
processed without additives. The filament diameter evolution was manually detected at
regular intervals using a Vernier caliper from Fowler High Precision (Auburndale, FL, USA) to
evaluate the filaments' quality.
Virtual Design: The virtual designs of the type 5B tensile bars [54], which were used for the
evaluation of the mechanical and shape memory properties, were designed by using AutoCAD
from Autodesk, Inc. (San Rafael, CA, USA).
Fused Filament Fabrication (3D Printing): Fused filament fabrication was used to produce
the 3D printed models. The experiments were carried out using the commercially available
3D printer Ultimaker 3 from Ultimaker B. V. (Geldermalsen, The Netherlands). The
manufacturer provides an XYZ resolution for Ultimaker 3 of 12.5, 12.5, and 2.5 µm, defining
the slightest movement the 3D printer can make concerning the XY plane and in the Z
direction. The Ultimaker build plate manual leveling calibration method was carried out
before the beginning of each experiment to calibrate the print bed. Therefore, a calibration
card with a thickness of about 170 µm was used. The process included a rough leveling of the
build plate followed by a fine leveling. The calibration card achieved fine leveling, at which
the knurled nut was adjusted at the rear center, front left, and front right of the build plate
until slight friction occurred when sliding the card between the built plate and print head.
The most relevant settings for the 3D printing processes are listed in Table 3.2.2.
Table 3.2.2. Printing instructions for Ultimaker 3 to produce the different prototypes of 3D printed
objects.
Characterization of Mechanical Properties: The mechanical behavior of the PEU was
investigated in tensile tests using the universal testing machine Criterion Model 43 from MTS
Systems Corporation (Eden Prairie, MN, USA). The device was equipped with a 500 N load
Specifications
Values
Diameter of the nozzle (µm)
400
Temperature of the nozzle (◦C)
225
Speed of print head (mm × s−1)
50
Build platform temperature (◦C)
60
Layer height (µm)
150
68 | Chapter 3.2
cell. As obtained from FFF, the measurements were carried out on dog-bone-shaped tensile
bars of type 5B [54]. While stretching, the velocity of 1% x min−1 was kept constant until a
total strain of 5% was achieved to enable a more precise determination of Young’s modulus
before the specimen was further elongated with a velocity of 100% x min−1 until failure
occurred. Every tensile test was carried out three times at ambient temperature.
Shape Memory Effect (SME): The programming of the tensile bars was carried out with an
MTS Criterion universal testing machine from MTS Systems Corporation (Eden Prairie, MN,
USA). The device was operated with a temperature chamber controlled by a Eurotherm
temperature controller unit. Two heating elements were located at the back of the chamber.
Liquid nitrogen from a Dewar vessel was fed into the chamber under a pressure of 1.3 bar as
an essential prerequisite for cooling. At the beginning of programming, the tensile bars were
clamped with a clamp length of 10 mm. The chamber was then heated to 60 °C, and a tensile
strain (𝜀𝑚) of 100% was applied for tensile bars using a loading rate of 100 mm x min−1. The
tensile bars were cooled to –15 °C, while the clamp distance remained constant. After 10 min,
the sample was unloaded, and the chamber was heated to 23 °C. While maintaining the new
strain constant (𝜀𝑢), the tensile bars were heated to 60 °C to trigger the SME and held for 10
min before unloading the generated recovery stress. Once cooled back to 23°C and held for
5 min, the CTM measurement was completed by measuring the recovered specimen
strain (𝜀𝑝). The CTM was repeated for four more cycles. A heating and cooling rate of
5 °C x min−1 was used for the whole experiment.
The shape memory properties were determined with the following formulas:
Strain fixity ratio,𝑅𝑓(N)=𝜀𝑢(𝑁)
𝜀𝑚× 100% 𝐸𝑞𝑢𝑎𝑡𝑖𝑜𝑛 3.2.1
Strain recovery ratio,𝑅𝑟(N)=𝜀𝑚−𝜀𝑝(𝑁)
𝜀𝑚−𝜀𝑝(𝑁−1)×100% 𝐸𝑞𝑢𝑎𝑡𝑖𝑜𝑛 3.2.2
Total strain recovery ratio,𝑅𝑟,𝑡𝑜𝑡(N)=𝜀𝑚−𝜀𝑝(𝑁)
𝜀𝑚×100% 𝐸𝑞𝑢𝑎𝑡𝑖𝑜𝑛 3.2.3
Where, 𝜀𝑢 = the fixed strain after programming
𝜀𝑝= the residual strain after shape recovery
N = the number of cycles
Characterization of Print Quality: Topography measurements were performed on tensile
bars using a FocusCam LV150 confocal microscope from Confovis GmbH (Jena, Germany),
equipped with an objective lens of 5×/0.15 N.A. Any time, the sample was illuminated with a
ring light. The data recorded by the focus variation microscope was evaluated with the
software MountainsMap® imaging topography 7.4 from Digital Surf (Besançon, France) [55].
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72 | Chapter 3.3
Chapter 3.3: Fused Filament Fabrication of
Filigree Objects With Shape Memory
Properties
| 73
Chapter 3.3: Fused Filament Fabrication of Filigree Objects
With Shape Memory Properties
3.3.1. Introduction
Shape memory polymers (SMPs) are smart materials that stabilize a temporary shape
through a thermomechanical treatment, also called programming [1–6]. The programmed
shape remains stable until the one-way (1W) shape memory effect (SME) is triggered, after
which the polymer almost completely returns to the permanent shape [1]. The shape memory
effect is generally triggered by heat [1,4,7–12]. Thermoplastic polyurethane (TPU) with shape
memory properties has shown its superiority among other SMPs due to their modifiable
thermal properties and phase segregation.
Additive manufacturing (AM), alias three-dimensional (3D) printing, is a rapid prototyping
technique. That is gaining increasing importance in prototyping, material testing, and small-
scale production due to its design freedom, cost-effectiveness, and short manufacturing time.
Among the other established AM technologies, fused filament fabrication (FFF) is a widely
used AM process and is a melt extrusion-based 3D printing process [13–15]. Later, scientists
learned how to process SMPs via FFF [6,16–22]. In the case of filigree printing using TPU-SMP
with FFF printers, our group employed a commercially available polyester urethane (PEU)
Desmopan® DP 2795A SMP along with a 400 µm and 100 µm nozzle on a standard FFF printer
to produce high resolution machine-readable quick response (QR) code carriers [6]. On
employing the 100 µm nozzle for printing QR code elevations, the code’s small cuboidal cell
was found to have high dimensional accuracy and exceeded the CAD model by just 40 µm.
Comparing the print quality results of extrusion-based AM techniques to process TPU with
shape memory properties, our group discloses superior print results to other working
groups [19–21,23–27]. The achievement is primarily from using a smaller nozzle opening and
highly refined printing parameters like lower printing speed, enabling excellent print quality.
The ability to precisely control the printing parameters in XY- and Z- planes proved that the
FFF could achieve excellent printing resolution in printing TPU- SMP, even compared to other
AM techniques [6]. This has directly opened up the possibility of fabricating filigree structures
of TPU with shape memory properties utilizing FFF. Nonetheless, exploring the lowest printing
limits for manufacturing filigree structures using FFF is of scientific interest to enable the
manufacturing of miniature objects.
The work utilizes a commercially available PEU Desmopan® DP 2795A SMP in the form of
filament processed in Chapter 3.1 [6] and Chapter 3.2 to address the points mentioned above.
The PEU filaments were later processed using a commercially available FFF 3D printer to
produce filigree structures. The Arial fonts of “A” in sizes 3, 4, and 10 were first printed to
analyze the printability of smaller structures. Later, the filigree-printed letters were used to
develop smart keyboard keys and shape memory gears, which can activate or deactivate
transmission systems on demand.
74 | Chapter 3.3
3.3.2. Results and Discussion
The commercially available poly(1,4-butylene adipate) (PBA)-based polyester urethane
(PEU), Desmopan® DP 2795A SMP from Covestro Deutschland AG (Leverkusen, Germany),
was chosen as the functional base material. The PEU's primary selection criteria were its good
processability via FFF and its excellent shape memory properties. The virgin and red PEU
filaments processed into filaments in Chapter 3.1 [6] and Chapter 3.2 were again selected for
this work. Further, a black-colored PEU filament was extruded using graphite powder as an
additive to dye the filament during extrusion.
The soft segmental PBA phase of the PEU exhibits a melting transition (Tm) onset and offset
at about 33 °C and 55 °C, with a peak at ≈ 42 °C, as observed in a DSC measurement [6]. For
3D printing filigree structures to understand and study the printing of the least possible
printing structures using FFF, the letters in Arial font size 3, 4, and 10 on top of a thin substrate
were designed using AutoCAD software. Our QR code carrier contribution documented that
a 100 µm nozzle, a printing speed of 4 mm x s−1, and a nozzle temperature of 190 °C can
process PEU to achieve high printing resolution [6]. This motivated me to take the same dual
extrusion setup along with the respective printing parameters for this study.
In order to achieve contrast structures of Arial fonts to distinguish the structures clearly,
the virgin PEU and red PEU were selected for FFF. Here, the elevations of Arial fonts were
processed using red PEU filament atop a virgin PEU substrate. The font dimension and FFF
printing results are presented in Figure 3.3.1.
Figure 3.3.1. Additive manufacturing of Arial fonts using FFF. The elevations were made from a red PEU
printed atop a virgin PEU substrate. Arial fonts differ in size (topmost row), the respective printing results
(middle row), and the inset exhibiting the light microscopic images of smaller fonts. (all the dimensions
are in mm)
| 75
From Figure 3.3.1., the excellent printing quality and the legibility of the Arial fonts can be
observed by the naked eye. The font size 3 was the least possible size to be printed (middle
row of Figure 3.3.1). The excellent print quality was confirmed by light microscopic images
(bottom row images of Figure 3.3.1). It can be observed that the lower fonts tend to have
edges with rounded corners. The roundness is supposed to be due to a round nozzle opening
and the nozzle movements to build the letter “A.” Once 3D scanned using a confocal
microscope, the characterization revealed a good surface quality of the Arial fonts (Figure
3.3.2a and b). Additionally, the imaging processing tool found that the elevation had a height
of about 65 µm (Figure 3.3.2a and b), which coincides with the given layer height parameter
during 3D printing. Here, the dimensions of the letter “A” along the X and Y- printing plane
were characterized to be about 1.1 mm (Figure 3.3.2a and b), which almost coincides with
the virtual dimensions of the Arial font size 3 (Figure 3.3.1, top row).
Figure 3.3.2. Confocal microscopic images of FFF manufactured Arial “A” in font size three are
represented in (a) isometric view and (b) top view. (all the dimensions are in mm)
As the materials used for producing the 3D objects (Figure 3.3.1, middle row) were the
same red PEU and virgin PEU as in our previous literature [6], likewise the QR code carriers,
the 3D models could also be programmed.
The filigree printing was further used to develop smart keyboard keys with Arial letters “A”
or “B” in font size 10. After CAD modeling (Figure 3.3.3 a), red-colored Desmopan® 9370AU
and grey-colored polylactic acid (PLA) substrates were printed, and atop the letters “A” and
“B” were printed using black PEU (Figure 3.3.3 b and c, respectively). The programmability
and the SME of the filigree structures were later investigated. The results are presented in
Figures 3.3.3 b1, c1, and c2.
From Figure 3.3.3. b and c, the good printing quality of the filigree letters can be deduced
even when printing at increased elevations. On heating to 60 °C, applying a compression load
and cooling to –15 °C permitted the letters “A” and “B” to fix the programmed shape (second
image in Figure 3.3.3. b1 and c1&2). The structure's height was observed to reduce and
flatten onto the top surface of the substrate due to the thermomechanical treatment. The
76 | Chapter 3.3
structures were visually in a non-readable state, which can be noticed clearly in the second
image of Figure 3.3.3. c2. This shape remains stable until the SME is triggered. The initial
shape was almost completely recovered by raising the temperature to 60 °C (rightmost image
in Figure 3.3.3. b1, c1, and c2). The developed SMP letters may be used in security labels,
text-based cold chain indicators, information carriers, or entry tickets.
Figure 3.3.3. Additively manufactured computer keys exhibiting the letters Arial “A” and “B,” made from
black PEU, in font size ten and the corresponding CAD designs (a), letter “A” printed atop a substrate of
red Desmopan® 9370AU (b), letter “B” printed atop a substrate of grey PLA (c). From left to right, the
permanent shape, programmed shape, shape recovery, and recovered shape of the letter “A” in front
view (b1) and “B” (c1(front view) and c2 (top view)) are depicted (all the dimensions are in mm).
Next, the filigree printing of PEUs was extended to develop shape memory gears.
Mechanical transmission systems may generate heat during operation. Once overheated, this
can negatively affect and damage the machine's electronic or other parts and lead to high
repair costs. Such damages can be prevented using PEU as intermediate gear to cut off the
force transmission and stop overheating. A PEU may help to perform preventive maintenance
and save repair costs. Once the overheating problem has been solved, interchanging the PEU
gear or reprogramming it can prepare the system for further operation.
For this, novel gears from PEU were designed and developed to demonstrate their
technical suitability. First, the programming possibilities of the gear were explored (Figure
3.3.4).
The urge for new gear designs emerged since conventional designs are solidly built and
cannot provide enough space for the tooth of the gear to change its shape. For this, a standard
gear design was modified by introducing hollow oval-shaped structures above the root of
each tooth to facilitate the programmability of individual tooths (leftmost image of Figure
3.3.4). For thermomechanical treatment of PEU, the gear was first heated to 60 °C, and the
tooth was stretched and blocked/ fixed using a template before cooling to –15 °C [6] (Figure
3.2.5 a and b). After holding the programmed shape isothermally at –15°C for 15 min, the
template was removed, and the PEU gear was brought to room temperature ( 23 °C), resulting
| 77
in successful programming, as evidenced by a 3 mm extension in length for every single tooth
(middle image of Figure 3.2.4). The programmed shape of the gear qualifies it as a novel force
transfer system. Once the temperature exceeded the Tm of the PEU (42 °C) [6], here at 60 °C,
the almost permanent shape was recovered (rightmost image of Figure 3.2.4).
Figure 3.3.4. Schematic drawings of the permanent, temporary, and recovered shape of a gear (top row)
and the associated additively manufactured objects made from PEU (bottom row) (all the dimensions
are in mm).
Figure 3.3.5. Programming of an additively manufactured gear from PEU after heating to 60°C for 10
min, stretching the tooth, and fixing with the help of a template tool (brownish color) in the perspectives
of top view (a) and isometric view (b).
The programmed gear was employed as an intermediate between an input and an output
gear to demonstrate the practicability of PEU shape memory gears (Figure 3.3.6).
The PEU gear was developed and designed this way so that its temporary shape makes
contact with the input and output gear (green highlighted in the top image of Figure 3.3.6).
This ensured a continuous force transfer from the input to the output gear with minimal
transmission loss. The temporary shape remained stable until the temperature exceeded the
PBA soft segment offset melting transition temperature (~60 °C) [6]. Resulting in recovering
the permanent shape of the gear (red highlighted gear in the bottom image of Figure 3.3.6),
thereby the contact of the PEU gear with the input and output gear was interrupted due to
the SME.
Another gear design was developed in a different approach to enable a force transfer
above Tm (Figure 3.3.7).
78 | Chapter 3.3
Figure 3.3.6. Force transfer system with a shape memory gear in its center position. The temporary
shape is highlighted in green color (top image), and the recovered shape is highlighted in red color
(bottom image).
Figure 3.3.7. Schematic drawing (top row) and the respective additively manufactured gears (bottom
row). From left to right, the permanent, programmed, and recovered shapes are exhibited (all the
dimensions are in mm).
The new gear design was conceived by introducing hollow-shaped structures below the
gear's root circle (leftmost image of Figure 3.3.7). Heating the SMP to 60 °C, compressing the
teeth towards the hollow region, and cooling the compressed shape to –15 °C completed the
programming [6] (middle image of Figure 3.3.7). A smaller outer diameter than the
permanent shape characterized the resulting temporary shape. The discrepancy was about 5
mm. Upon heating the gear above 60 °C (above Tm of the PBA soft segment of the PEU) [6],
the permanent shape was almost recovered (rightmost image of Figure 3.3.7). The
| 79
practicability of the gear to activate the system once heated is evaluated next. The resulting
shape changes are shown in Figure 3.3.8.
After assembling the programmed gear between the input and output gears, the system
can undergo an operation (green highlighted in the top image of Figure 3.3.8). Since the teeth
of the temporary shape remained inside the hollow structures, the force transmission from
the input gear was not in contact with the PEU and then to the output gear. When triggering
the SME, the permanent shape of the gear was recovered, thereby resulting in the teeth'
extension—allowing the PEU gear to engage with the input and output gear to enable the
power transmission (red highlighted gear in the bottom image of Figure 3.3.8).
Figure 3.3.8. Force transfer system with a shape memory gear in its center position. The temporary
shape is highlighted in green color (top image), and the recovered shape is highlighted in red color
(bottom image)
3.3.3. Conclusions
Chapter 3.3 explores miniature structures obtained from 3D printing using FFF along with
an SMP and a 100 µm nozzle. The work shows the possibility of 3D printing filigree structures
such as Arial fonts of the letter “A” in sizes 3, 4, and 10. Since the printing material was the
same PEU used in our previous QR code carriers contribution in Chapter 3.1 [6], it is possible
to program it into a different shape and recover the original shape by heating above the soft
segment's melting transition temperature. This work further utilizes filigree printing to
develop smart keyboard keys, where PEU was used to print the letters “A” or “B” in font size
ten atop a non-thermoresponsive substrate. The shape memory keyboard keys have a
potential application for switching between two scripts, like braille and standard (QWERTY)
80 | Chapter 3.3
keyboards. Here, for the better applicability of smart keyboard keys, the 1W-SME of letters
has to be later extended to a two-way SME to transfer them into two metastable states so
that the tedious 1W-SME reprogramming process can be eliminated. The introduction of
heating and cooling elements for the SMP letters can further help to trigger the 2W-SME on
command. Printing letters from SMP can also be used to develop text-based information
carriers, cold-chain indicators, or even security labels for fighting against product piracy.
Furthermore, novel mechanical force transmission systems were introduced, where the
design of toothed gears facilitates thermoresponsiveness attained through
thermomechanical treatment. The developed gear permitted the shape recovery to be used
advantageously for starting or stopping a force transfer system. Here, the structural design of
the object was crucial to realizing the desired functionality. One of the potential applications
of the developed SMP-toothed gear is a preventive system to break or start a process to avoid
overheating or ensure the preferred temperature is reached. The work thereby shows that
FFF is a powerful AM technique to achieve good print quality and functional objects with a
shape memory effect to address specific applications.
3.3.4. Experimental Section
Materials: The virgin PEU Desmopan® DP 2795A SMP from Covestro Deutschland AG
(Leverkusen, Germany) is used as the functional base material for this study. The red PEU was
attained using 0.5 wt. % of Irgazin® Red DPP BO additive and virgin PEU filaments processed
in the previous literature is again in the study [6]. Further information regarding the PEU's
thermal, mechanical, thermomechanical, and shape memory properties is given in previous
publications [6,28–30]. The graphite powder was bought from Kremer Pigmente GmbH, and
Co (Aichstetten, Germany) and was used as received. The red ether-based thermoplastic
polyurethane elastomer Desmopan® 9370AU was supplied by Covestro Deutschland AG
(Leverkusen, Germany). The grey PLA filaments were bought from Prusa Research A.S.
(Prague, Czech Republic).
Extrusion: The PEU granulates are dried at 110 ◦C in a Binder vacuum drying chamber
VDL 53 from Binder GmbH (Tuttlingen, Germany) to remove water and avoid bubble
formation when extruding filaments. The drying was finalized after 150 min. Subsequently,
the pellets were fed into an extrusion line to produce filaments.
The individual units of the extrusion line were put together in such a way that it included
a volumetric material feeding system Color-exact 1000 from Plastic Recycling Machinery
(Zhangjiagang City, China), a Leistritz twin screw extruder MICRO 18 GL from Leistritz AG
(Nürnberg, Germany), characterized by seven heating zones and a screw length of 600 mm, a
conveyor belt, a water bath and a filament winder from Brabender GmbH and Co. KG
(Duisburg, Germany). In any case, the temperature of the individual heating zones of the
extruder was 180, 185, 190, 195, 200, 190, and 190 ◦C. The screw speed of the extruder was
set to 77 rpm. The PEU granulate was processed with additives of 0.35 wt.% of graphite
powder to obtain a black-colored PEU filament. Whereas the Desmopan® 9370AU was
| 81
extruded into filaments without additives. To evaluate the filaments' quality, the filament
diameter evolution was manually detected at regular intervals using a Vernier caliper from
Fowler High Precision (Auburndale, FL, USA).
Virtual Design: The virtual designs of the models were designed using AutoCAD from
Autodesk, Inc. (San Rafael, CA, USA). These include Arial fonts of the letter “A” in sizes 3, 4,
and 10 with a substrate to test the printing of the least possible filigree structures, letterings
“A” and “B” with a substrate in font size 10 to demonstrate the shape memory effects of
filigree printing structures, and the gears to activate and deactivate a transmission system.
The CAD models were exported as .STL files.
Fused Filament Fabrication (3D Printing): Fused filament fabrication was used to produce
the 3D printed models. The experiments were carried out using the commercially available
3D printer Ultimaker 3 from Ultimaker B. V. (Geldermalsen, The Netherlands). The
manufacturer provides an XYZ resolution for Ultimaker 3 of 12.5, 12.5, and 2.5 µm, defining
the slightest movement that the 3D printer can make concerning the XY plane and in the Z
direction. The Ultimaker build plate manual leveling calibration method was carried out
before the beginning of each experiment to calibrate the print bed. Therefore, a calibration
card with a thickness of about 170 µm was used. The process included a rough leveling of the
build plate followed by a fine leveling. Fine leveling was achieved with the calibration card, at
which the knurled nut was adjusted at the rear center, front left, and front right of the build
plate until slight friction occurred when sliding the card between the build plate and print
head.
The two print heads of the FFF-printer were either equipped with two nozzles having
different diameters of 100 and 400 µm or the same diameter of 400 µm. For simplicity, the
following terminology is introduced, pointing out the most relevant variations when
manufacturing 3D printed objects:
• Type 1: The 3D model of the gears was printed using a 400 µm nozzle and a target
layer thickness of 180 µm.
• Type 2: The substrate of the letters was printed with non-dyed PEU using a 400 µm
nozzle and a target layer thickness of 180 µm. The elevation was built from red PEU with a
100 µm nozzle. Using these settings, the Arial font of the letter “A” in sizes 3, 4, and 10 were
printed, where the red PEU makes the elevation with a target layer thickness of 63 µm and
virgin PEU as the substrate. In the case of the lettering “A” and “B” along with a substrate,
the substrates were printed using red Desmopan® 9370AU or grey PLA. While the elevation
of the letter “A” and “B” with black PEU.
The most relevant settings for the 3D printing process are listed in Table 3.3.1.
Shape Memory Effect (SME): The programming of the letterings “A” and “B” on the
substrates were carried out with an MTS Criterion universal testing machine from MTS
Systems Corporation (Eden Prairie, MN, USA). The device was operated with a temperature
chamber controlled by a Eurotherm temperature controller unit. Two heating elements were
located at the back of the chamber. Liquid nitrogen from a Dewar vessel was fed into the
chamber under a pressure of 1.3 bar as an essential prerequisite for cooling.
82 | Chapter 3.3
Table 3.3.1. Printing instructions for Ultimaker 3 to produce the different demonstrators.
At the beginning of the programming, the demonstrators were placed between
compression clamps. After heating to 60 °C and a holding time of 10 min, a compression force
was applied till the upper clamp touched the substrate of the model. After cooling to –15 °C
and holding the temperature isothermally for 10 min, the load was removed. The
temperature was brought back to 23 °C and held for 5 min. After that, the 1W-SME was
triggered by heating again to 60 °C with an additional isothermal holding time of 10 min.
During 1W-SME, the letters could recover freely/without an external load or by so-called free-
strain conditions. Later, the temperature was brought back to 23 °C and held for 5 min,
completing the first shape memory cycle to repeat the procedure. A loading rate of 100 mm
x min−1, while a heating and cooling rate of 5 °C x min−1 were used for the whole experiment.
In the case of the PEU gears, the demonstrators were first heated to 60 °C and held
isothermally for 10 min in a UF110 heating chamber from Memmert GmbH + Co. KG
(Schwabach, Germany). Afterward, the gears were deformed according to the use-
case/application described above and cooled to –15 °C using a freezer. After 10 min, the load
was removed, and the models were brought back to 23°C (room temperature). Then, the
programmed gear was mounted as an intermediate gear between the input and output gear.
The programmed gear was heated to 60 °C using a heating gun from Conrad Electronic
(Hirschau, Germany) to demonstrate the SME.
Characterization of Print Quality: Topography measurements were performed on the
filigree printed Arial letter “A” with font size three, using a FocusCam LV150 confocal
microscope from Confovis GmbH (Jena, Germany), which was equipped with an objective lens
of 5×/0.15 N.A. The sample was illuminated with a ring light. The data recorded by the focus
variation microscope was evaluated with the software MountainsMap® imaging
topography 7.4 from Digital Surf (Besançon, France) [31]. The development of the surface
profile surrounding was exemplarily determined.
Further microscopic investigations were carried out with the microscope Axio Scope.A1
from Carl Zeiss Microscopy GmbH (Jena, Germany) using the imaging software Zen 2.3 lite
also from Carl Zeiss Microscopy GmbH [32]. The experiments were conducted to evaluate the
resolution of the Arial fonts three and four in the XY-plane.
Specifications
Substrate/ Model
(Non-Dyed PEU)
Elevation
(Red or
black PEU)
Desmopan®
9370AU / PLA
3D Model - Type
1
2
2
Diameter of the nozzle (µm)
400
100
400
Temperature of the nozzle (◦C)
225
190
200
Speed of print head (mm × s−1)
50
4
50
Build platform temperature (◦C)
23
23
60
Layer height (µm)
180
63
150
| 83
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| 85
Chapter 4:
Four-Dimensional (4D) Printing Via
Fused Filament Fabrication
86 | Chapter 4.1
Chapter 4.1: Highly Shrinkable Objects as
Obtained from 4D-printing
| 87
Chapter 4.1: Highly Shrinkable Objects as Obtained from 4D-
printing
The original article Chalissery, D., Schönfeld, D., Walter, M., Shklyar, I., Andrae, H.,
Schwörer, C., Amann, T., Weisheit, L. and Pretsch, T. (2022), Highly Shrinkable Objects as
Obtained from 4D-printing. Macromol. Mater. Eng. 2100619 and graphical abstract are
published in Wiley Macromolecular Materials and Engineering and available at
https://doi.org/10.1002/mame.202100619.
Figure 4.1.0.0. The table of content image of the article “Highly Shrinkable Objects as Obtained from
4D-printing” is published in Wiley Macromolecular Materials and Engineering 2100619.
Contribution
My contribution: The concept idea of 4D-printing and hands-free door opener.
Visualization, preparation of images (including table of content image and cover image,
excluding Figure 4.1.1 and Figure S4.1.1.), design development and design iteration (every
individual design of hands-free door opener), filament extrusion and re-extrusion, mechanical
recycling, 4D-printing, characterization using tensile test, DSC and DMA, and writing—original
draft. Project lead of the article.
Not included in my contribution: Synthesis, titration and FT-IR, simulation of door handle
design and tribological analysis
Schönfeld, D.: Synthesized the polyether urethane (PEU) for the work, carried out FT-IR,
thermal analysis (DSC) and material analysis of PEU (directly after synthesis). Conducted
mechanical recycling, extrusion and re-extrusion. Writing—original draft (synthesis),
preparation and visualization of Figure 4.1.1 and Figure S4.1.1.
Walter, M.: Developed the PEU, conducted first validation, and investigation of PEU.
Shklyar, I.: Conducted simulation of different door handle designs, and prepared images of
simulation. Writing—original draft (simulation), preparation and visualization of Figure 4.1.9
and Figure 4.1.12.
88 | Chapter 4.1
Andrä, H.: Funding acquisition, conceptualization and formal analysis of the of simulation
analysis.
Schwörer, C.: Formal analysis, validation, investigation, and methodology of the tribological
tests.
Amann, T.: Funding acquisition of tribological tests, preparation and visualization of Figure
4.1.11 and Figure S4.1.14.
Weisheit, L.: Funding acquisition and conceptualization of end-of-life concept.
Pretsch, T.: Conceptualization of the manuscript, funding acquisition, project administration,
supervision, writing—review and editing
Figure 4.1.0.1. Suggested cover image of “Highly Shrinkable Objects as Obtained from 4D-printing” (not
published).
| 89
Chapter 4.1. Highly Shrinkable Objects as Obtained from 4D-
printing
Dilip Chalissery1, Dennis Schönfeld1, Mario Walter1, Inga Shklyar2, Heiko Andrae2, Christoph
Schwörer3, Tobias Amann3, Linda Weisheit4, and Thorsten Pretsch1*
1 Fraunhofer Institute for Applied Polymer Research IAP, Geiselbergstraße 69, Potsdam
14476, Germany E-mail: dilip.chalissery@iap.fraunhofer.de;
2 Fraunhofer Institute for Industrial Mathematics ITWM, Fraunhofer-Platz 1, 67663
Kaiserslautern, Germany;
3 Fraunhofer Institute for Mechanics of Materials IWM, Wöhlerstraße 11, Freiburg 79108,
Germany
4 Fraunhofer Institute for Machine Tools and Forming Technology IWU, Nöthnitzer Straße
44, Dresden 01187, Germany
* Correspondence: [email protected]; Tel.: +49-(0)-331/568-1414
4.1.0. Abstract: 4D-printing of shape memory polymers enables the production of
thermoresponsive objects. In this contribution, a facile printing strategy is followed for an in-
house synthesized thermoplastic poly(ether urethane). Processing by means of fused filament
fabrication, in which the difference between nozzle temperature and material-specific glass
transition temperature of the polymer is kept as low as possible, allows to obtain highly
shrinkable objects whose shape and thermoresponsiveness can be precisely controlled. The
effectiveness of the method also applies to the printing material polylactic acid. One possible
application lies in highly shrinkable objects for assembly purposes. As proof-of-concept,
lightweight hands-free door openers for healthcare applications are functionally simulated
and developed. Once printed, such devices shrink when heated to fit on door handles,
allowing an easy assembly. At the end-of-use, a heating-initiated disassembling and
mechanical recycling are proposed. In perspective, a reuse of the materials in 4D-printing can
contribute to the emergence of a circular economy for such highly functional materials.
4.1.1. Introduction
Shape memory polymers (SMPs) can retain a temporary shape after a thermomechanical
treatment, also denoted as “programming.” When exposed to heat, the one-way shape
memory effect is triggered and the polymer almost completely returns to its permanent
shape.[1–5] The advantageous shape-memory behavior of polymers has already been utilized,
among others, in connection with the development of biomedical devices,[6–13] the
counterfeit-proof marking of goods susceptible to plagiarism,[14–19] and for active
90 | Chapter 4.1
assembly[20– 23] and disassembly,[22,24–28] which both requires a rethinking of classical design
processes.
Additive manufacturing (AM) alias 3D printing has gained considerable attention and
enthusiasm as an innovative manufacturing technology.[29] Amidst different known 3D
printing methods, fused filament fabrication (FFF) is a widely used technique.[30] A few studies
have been reporting on FFF with SMPs, in particular with thermoplastic polyurethanes
(TPUs).[19,31–35] Most importantly, in the past few years, it has become known how to
implement internal stresses during AM, which is the so-called “4D-printing,”[36,37] addressing
time evolving structural functions as unattainable by conventional 3D printing.[38–40] The main
advantage of such function integration is that the finished objects can be removed directly
from the printer without the need for an additional thermomechanical treatment. Thus, it is
an important step to reduce the effort required in production of thermoresponsive objects.
Today, there are only a few contributions on the 4D-printing of TPUs,[41] as researchers
have primarily concentrated on other materials like polylactic acid (PLA). For instance, a more
drastic 4D shrinkage behavior was verified for PLA, when printing with a lower nozzle
temperature,[42] building up structures of lower layer height,[42] and/or using higher printing
speeds.[43,44] All these approaches have in common that polymer relaxation on the print
platform can efficiently be inhibited. With regard to shape memory TPU, Bodaghi et al.[36]
unveiled that a decrease in nozzle temperature and an increase in printing speed enhance the
4D bending effect of a straight printed beam. Similarly, Hu et al.[45] studied the bending
behavior caused by the 4D-printing of a polyurethane-based SMP. Once again, a stronger
shrinkage was associated with a higher printing speed. Although it is evident in the literature
that thermoresponsiveness of PLA and TPU can be controlled by 4D-printing when selecting
appropriate printing parameters, the number of TPU materials which can be used for this
purpose still appears to be limited.[36,37,45] Another weakness is that the geometrical
complexity and the extent of the 4D effects are restricted.
Here we propose the synthesis of a poly(ether urethane) (PEU) with promising thermal,
mechanical, and shape memory properties and suitable for processing via extrusion and FFF
to obtain objects with thermally inducible strong shrinkage. Based on the findings from
previous works, we demonstrate that reduced energy consumption during printing is a key to
witness the presumably most efficient storage and release of strain so far. Furthermore, we
give evidence how to transfer our findings to the printing material PLA and suggest opening
up applications in the field of assembly. This includes “hands-free door openers,” which can
ensure that in the current corona pandemic door handles no longer need to be touched by
hand. To keep raw material consumption as low as possible, a lightweight construction was
developed using finite element analysis. In this regard, both the outer shape and core
structure were optimized for the occurrence of compressive loads. The fabrication of hands-
free door openers unveiled that shrinking on demand can advantageously be used for
assembly, while the temperature-sensitive mechanical stiffness of the polymer later enables
a disassembling as required at the end-of-use. To address the reusability, hands-free door
openers made from PEU were mechanically recycled and the material was re-extruded,
| 91
before being subjected to one further 4D reprinting. In this way, we pursued a holistic
approach—from the molecule to the demonstrator, its assembly to disassembly, recycling
and functional reuse.
4.1.2. Results and Discussion
PEUs are attractive polymeric materials due to their pronounced shape memory
properties, thermoplastic nature, which is a prerequisite for processing via FFF, and the
possibility of precisely setting a glass transition at temperatures relevant to a desired
application.[46–48] For instance, PEUs with a glass transition temperature Tg higher than
room temperature can be obtained by polymer synthesis when soft segments of lower
molecular weight are used or when the weight content of aromatic hard segments is
increased.[46,47] Against this background, we selected a polypropylene glycol (PPG) with a
low molecular weight of about 430 g x mol−1.Theunderlyingsynthesis approach followed the
prepolymer method.[49] First, PPG was brought to reaction with 4,4′diphenylmethane
diisocyanate (MDI) to build up an isocyanate end capped prepolymer, before the chain
extender 1,4-butanediol (BD) was added, which resulted in the formation of a PEU (Figure
4.1.1).
Figure 4.1.1. Synthesis of polypropylene glycol (PPG)-based poly(ether urethane) (PEU) via polyaddition
reaction using the prepolymer method.
92 | Chapter 4.1
From a structural point of view, the so-called soft segment was composed of the synthesis
building block PPG while the hard segment was obtained from the reaction of MDI and BD. In
order to obtain a PEU characterized by a good mechanical strength under room temperature
conditions, the hard segment content was selected to be 60 wt.%.
The completeness of the polyaddition reaction was verified by Fourier-transform infrared
spectroscopy (FTIR) spectroscopy (Figure S4.1.1, Supporting Information). In this connection,
the characteristic, PEU-related vibrational modes were detected. The reaction was mostly
complete since only a very weak signal at 2270 cm−1 associated with the presence of freely
available isocyanate appeared in the spectrum. Afterward, the PEU was melt extruded into a
filament as essential for additive manufacturing via FFF. For this purpose, the same extrusion
line was used as recently reported.[19,49] The obtained filament had a smooth surface and a
diameter of 2.85 ± 0.10 mm. The narrow tolerance range ensured that an important
processing criterion was fulfilled.[19] Afterward, tensile bars and cuboidal specimens were
additively manufactured and their mechanical properties characterized (Figure 4.1.2).
Figure 4.1.2. Engineering stress–strain diagram for PPG-based PEU as determined in tensile tests: a)
engineering stress–strain curves and b) the associated deformation behavior. The experiments were
carried out at 23 °C with an initial clamping distance of 10 mm.
From the stress–strain relationship, an averaged Young’s modulus of 1101.1 ± 80.7 MPa
was determined. Compared to other literature on PEU elastomers,[50] the high Young’s
modulus was expected due to the relatively high hard segment content and the fact that the
material was in its glassy state at 23 °C as will be demonstrated below. In all measurements a
yield-point was detected at a strain value of 11% ± 1%, corresponding to a stress of
63.6 ± 2.7 MPa (Figure 4.1.2a). Further increasing the load resulted in necking (Figure 4.1.2b,
images 3– 5) as known from other polyurethane-based networks[49,51–53] and strain softening,
| 93
followed by weak strain hardening as accompanied by an increase in stress, culminating in
specimen rupture at strains of 195% ± 6%. The deformation-related material behavior can be
explained with the presence of flexible amorphous PPG segments, which in parts may have
been transformed during stretching into stiffer crystalline segments. In another tensile test,
the Poisson’s ratio of the material was determined to be 0.36, which is a typical value for
tough and rigid thermoplastic materials.[54] Furthermore, the density of the PEU was specified
to be 1170 kg m−3; the value is in accordance with the one of other PEUs.[55]
Differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA) were used
to investigate the thermal behavior of the PEU and to determine the temperature
dependence of the mechanical properties (Figure 4.1.3).
Figure 4.1.3. Thermal and thermomechanical properties of PPG-based PEU as investigated by a) DSC
(second heating and cooling at temperature rates of 10 °C x min−1) and b) DMA (temperature
dependence of storage modulus E’, solid line, and loss factor tan
𝛿
, dashed dotted line, at a heating rate
of 3 °C x min−1).
In the respective DSC thermogram a thermal transition between 40 and 60 °C could be
detected. The temperature associated with the inflection point was about 55 °C (Figure
4.1.3a) and assigned to the glass transition temperature of the PEU. The results were
confirmed in the DMA measurement (Figure 4.1.3b). Here, the stiffness as exemplified by the
evolution of the storage modulus E′ exhibited a one-step decrease corresponding to a drop
by more than two orders of magnitude between 40 and 70 °C when passing through the glass
transition (Figure 4.1.3b, solid line). Thus, a drastic softening occurred. The tan
𝛿
peak, which
is often used to determine Tg in urethane-based polymers[18] was located at about 58 °C
(Figure 4.1.3b, dashed dotted line). In comparison with the DSC results, Tg is commonly a few
degrees higher in the DMA due to differences in testing procedures and chain dynamics under
the respective conditions.[56]
After finalizing the characterization of the PEU, three FFF approaches were followed. First,
a 3D printing scenario was considered in which common print settings were used to obtain
almost non-thermoresponsive objects from PEU.[32] This was realized by choosing a
comparatively high nozzle temperature TN and by heating the print bed at the same time.
Furthermore, a 4D-printing scenario was selected that aimed at the implementation of
94 | Chapter 4.1
stresses by enabling faster cooling of the PEU on the print bed by lowering TN and turning off
the heating element of the build platform. As a final case, 4D-printing of PLA was considered
and the same principle as for PEU was applied. The exact print settings are listed in Table
4.1.1, along with the print results and evaluation of thermoresponsiveness, as visualized in
Figure 4.1.4.
Figure 4.1.4. Printing results and 4D effects after a) classical 3D printing with PEU, b–d) 4D-printing with
PEU, and e,f 4D-printing with PLA. The underlying printing conditions are specified in Table 4.1.1. The
states of the samples are exhibited after printing (left, nozzle temperature TN) and after heating to 75 °C
right).
Table 4.1.1. Selection of printing materials, printing scenarios and parameters (print platform
temperature Tp, nozzle temperature TN, and printing speed Sp = 50 mm x s−1) used for FFF and results of
the characterization of the samples before and after triggering the 4D effects regarding the amount of
maximum thermally shrinkable strain
𝜖
pre −
𝜖
, arc measure
𝜃
, and radius of curvature r.
Printing materials/
printing scenarios
Printing parameters
Characterization of 4D effects
Tp
(°C)
TN
(°C)
εpre – ε
(%)
θ
(°)
r
(mm)
PEU, 3D printing
65
200
2.2
≈ 0
∞
PEU, 4D-printing
23
180
41.7
129.8
7.3
PEU, 4D-printing
23
165
50.9
105.3
6.5
PEU, 4D-printing
23
150
58.7
≈ 0
∞
PLA, 4D-printing
23
190
17.8
18.7
140.9
PLA, 4D-printing
23
180
21.0
30.6
81.8
It is noticeable that all additively manufactured samples had a length of about 40 mm
(Figure 4.1.4a–f). Even when selecting a lower TN of 150 °C, the adhesion between the
individual layers was strong enough in the PEU. When heated to 75 °C, the PEU samples
behaved differently: After 3D printing of PEU (Table 4.1.1), almost no thermoresponsiveness
was detected; the 4D effect was very weak (Figure 4.1.4a). Apparently, stress relaxation
during printing was supported by the elevated build platform temperature. In contrast, a
more rapid cooling of polymer strands after leaving the nozzle and thus a quicker vitrification
| 95
could be achieved by 4D-printing (Table 4.1.1). As a matter of fact, stress relaxation was only
permitted to a minor extent and distinct 4D effects could be witnessed (Figure 4.1.4b–d).
Intriguingly, almost linear shrinkage occurred when the lowest TN was used for printing and
the PEU was heated to 75 °C (Figure 4.1.4d). We assume that the stress stored in every layer
was almost the same, which helped the printed part to shrink uniformly without bending. As
a result, triggering of the 4D effect gave a very short sample with a length of 16.6 mm (Figure
4.1.4d); in relation to the original size of the object, a shrinkage of 59% was detected (Table
4.1.1).
When systematically raising TN, a clear trend toward decreasing shrinkage behavior was
verified (Figure 4.1.4b,c). Apparently, a partial triggering of the 4D effect occurred when
printing the first layers, as hotter layers were laid on top of them. Furthermore, the samples
were curved as evidenced by values for arc measure 𝜃 > 85°, while the corresponding radius
of curvature r increased (Table 4.1.1). This allowed a very high degree of control over the
shrinkage behavior and shows the possibility to control not only the degree of shrinkage but
also the shape transformation. Thus, the PEU followed the same physical laws as known from
thermoplastics such as other poly(ether urethanes),[36,45] PLA,[42,43,44,57,58] acrylonitrile
butadiene styrene (ABS),[44] and high impact polystyrene (HIPS).[44]
The results of 4D-printing with PLA (Table 4.1.1) are exhibited in Figure 4.1.4e,f. In this
case, the shrinkage of PLA was smaller than the 4D effect of PEU (Figure 4.1.4b–d). Anyway,
further printing tests were carried out with different 3D printers. In each case, the 4D effects
could be reproduced regardless of the 3D printer and material used and were therefore
printer independent.
For a more detailed insight into the evolution of strain upon heating, DMA measurements
were carried out on 3D and 4D-printed samples (Figure 4.1.5).
Heating to 75 °C and thus above the glass transition temperature of the PEU resulted in
strain decreases of ≈−3.9% when applying 3D printing (Figure 4.1.5a) and of −64.1% after 4D-
printing (Figure 4.1.5b). Upon 4D-printing with PLA a strain decreases of −18.7% was detected
(Figure 4.1.5c). The reason for the even higher release of strain compared with the
measurement above (Figure 4.1.4d) was that the material was allowed to relax 15 min longer
at 75 °C. Returning to PEU, it can be clearly seen in Figure 4.1.5b that the polymer started
shrinking upon heating to about 40 °C, which almost corresponds to the onset temperature
of the glass transition determined in the DSC measurement (Figure 4.1.3a). It can also be
deduced that the state of the PEU as achieved after heating is stable at ambient temperature,
which also applies to PLA (Figure 4.1.5c).
To elucidate the effect of 4D-printing on the thermomechanical properties, the respective
PEU sample was investigated by DMA (Figure S4.1.2, Supporting Information). In comparison
with
3D printed PEU (Figure 4.1.3b), the 4D-printed sample had an increased storage modulus
at lower temperatures together with a slightly higher value for the tan
𝛿
signal. Latter is
assumed to be due to the more rapid cooling of the material under its Tg in a higher oriented
state, which obviously contributed to an accelerated vitrification of the PEU.
96 | Chapter 4.1
Figure 4.1.5. DMA, illustrating the evolution of strain
𝜖
(blue, solid line) and length L (black, dashed line)
on variation of temperature T (red, solid line). The samples investigated were a) 3D printed PEU, b) 4D-
printed PEU (TN = 150 °C), and c) 4D-printed PLA (TN = 180 °C).
Next, the temperature distribution during 3D and 4D-printing of PEU was followed by in-
situ thermal imaging (Figure 4.1.6); a more detailed evaluation is included in Table S4.1.1 in
the Supporting Information.
As expected, when selecting a TN of 200 °C and a build platform temperature of 65 °C, the
latter temperature was approximately maintained by the PEU after strand deposition so that
the material was almost not able to cool below its Tg (Figure 4.1.6a and Table S4.1.1,
Supporting Information). The choice of a lower TN of 150 °C combined with a build platform
temperature of 23 °C in turn allowed the individual strands of PEU to cool rapidly to ambient
temperature on the print bed (Figure 4.1.6b and Table S4.1.1, Supporting Information), thus
reducing polymer stress relaxation to a minimum and later enabling a pronounced 4D effect.
As most crucial for our approach, the PEU only took about 10 ms to cool below its Tg. Another
aspect is that the topmost layer’s temperature increased from the first layer to about the
sixth layer and then remained almost constant (Table S4.1.1, Supporting Information), which
later supported a uniform shrinkage behavior of the 4D-printed sample.
In a progressive approach, we discovered that the chosen 4D-printing method is not
limited to the production of smaller samples. In fact, three objects made from PEU with
heights strongly exceeding 5 mm could be fabricated (Figure 4.1.7).
| 97
Figure 4.1.6. In situ thermal imaging to supervise PEU during a, top) 3D printing and below) 4D-printing.
On the left side, randomly selected top layers are exhibited while the strand is being deposited. The
enlarged images on the right side show the temperature distribution in layer number 13 at five randomly
selected positions. A printing speed of 50 mm x s−1 and a TN of a) either 200 °C or b) 150 °C were used.
Upon heating to 75 °C for 15 min, the length of the samples decreased in the x-, y-printing
plane while the height z increased substantially (Figure 4.1.7). In detail, the thin solid cuboid
sample shrank by about −53% along the x-axis and expanded by about +33% and +85% along
the y- and z-axis, respectively (Figure 4.1.7a). The hollow cuboid sample also showed a similar
dimensional change on heating with about −47%, −47%, and +85% along the x-, y-, and z-axis
(Figure 4.1.7b). The third print object, namely the hollow cylinder (Figure 4.1.7c), was also
able to uniformly shrink in the printing plane by about −55% and expanded along the z-axis
by about +79%. This demonstrates that another massive expansion could be achieved,
although a different sample geometry was used. As a result, the inner diameter decreased
from 13.5 to 3.9 mm. In other words, drastic almost uniform shrinkage behavior could be
verified in every single case.
In Table 4.1.2 our results are compared with previous works on 4D-printing using PLA,
HIPS, ABS, and TPU. The PEU examined here exhibits by far the greatest 4D effect. Although
a systematic is not necessarily obvious, which is in particular because of the different printing
parameters and discrepancies in the height of the objects, we assume that more pronounced
4D effects can essentially be attributed to the fact that the temperature difference between
the nozzle and the material-specific glass transition temperature (TN − Tg) is as low as possible.
This seems particularly logical in view that the polymer must be cooled as quickly as possible
below its Tg after leaving the nozzle in order to fix the orientation of polymer chains without
98 | Chapter 4.1
strong stress relaxation. In the current work, the temperature delta TN −Tg is with 90 °C for
PEU by far the lowest compared to the other scientific contributions on TPU. In the case of
PLA, it is also obvious that the magnitude of the 4D effect is the strongest for higher objects
that could be proven so far, even drastically exceeding the value for
𝜖
pre −
𝜖
as published by
Rajkumar et al.[44]
Figure 4.1.7. Samples of 4D-printed PEU (Table 4.1.1, TN = 160 °C) consisting of a a) solid cuboid, b)
hollow cuboid, and c) hollow cylinder. The objects were built up by 187, 200, and 213 layers,
respectively. From left to right in every image series, a vector diagram, the states after 4D-printing and
after heating to 75 °C for 15 min are shown together with the inset focusing on the cross-section. The
parameters of the samples were specified in terms of their x, y, and z coordinates same as the wall
thickness t and outer diameter d.
As we now had the right materials and high degree of control over their
thermoresponsiveness, the potential application as hands-free door opener moved into our
focus. Therefore, an initial rough design was developed for an elliptical profiled door handle
(Figure 4.1.8).
To keep the complexity as small as possible, the entire system was designed from one
material. In a next step, the structure was analyzed, and a numerical shape optimization was
carried out. Finally, the ribbing in the core was optimized. The objective functions in the
structural optimization steps are the compliance minimization and the reduction of stresses
under the constraint of easy printability. The same pressure load on the top surface of the
handle was assumed in the finite element simulation for all variants in the successive
| 99
optimization procedure. Different intermediate designs were made for PEU and PLA (Figure
4.1.9).
Table 4.1.2. Overview on thermal properties, processing temperatures, and the amount of the
maximum thermally shrinkable strain
𝜖
pre −
𝜖
for 4D-printed objects built up from thermoplastic polymers
via FFF.
Material
Literature
TN
[°C]
Tg
[°C]
Δ(TN –
Tg)
[°C]
Tp
[°C]
Sp
[mm
· s– 1]
εpre – ε
[%]
PLA
Zhang et
al.[186]
230
60-
65
170
90
10-150
22.7
(z = 0.6 mm)
van
Manen et
al.[178]
210
60-
65
150
60
60
38
(z = 0.05 mm)
Rajkumar
et al.[185]
220
60
160
60
30-120
14
(z = 2 mm)
10
(z = 5 mm)
current
work
190/
180
60
130
23
70
21.0
(z = 2 mm)
HIPS
Rajkumar
et al.[185]
260
100
160
100
30-120
27
(z = 2 mm)
20
(z = 5 mm)
ABS
Rajkumar
et al.[185]
240
110
130
110
30-120
20
(z = 2 mm)
14
(z = 5 mm)
TPU
Bodaghi
et al.[180]
210/
230
60
150/1
70
24
30-40
37.3
(z = 1 mm)
Hu et
al.[179]
233
60
173
24
20-50
26.7
(z = 1 mm)
current
work
180-
150
60
120-
90
23
50
63.1 (z = 2 mm)
–53 (x), +33 (y), and +85
(z)
(z = 28 mm, solid
cuboid)
–47 (x), –47 (y), and +85
(z)
(z = 30 mm, hollow
cuboid)
–55 (x and y), and +79
(z)
(z = 32 mm, hollow
cylinder)
100 | Chapter 4.1
Figure 4.1.8. Technical drawing of a 4D-printable hands-free door opener in the perspectives a) top view,
b) isometric view, c) front view, and d) right view. All data are provided in mm.
Figure 4.1.9. Simulation of hands-free door openers using ANSYS software. Simulation results for PEU:
a) Design 1, b) design 2, and c) design 3 with an infill of 100%. Design 2 with d) 90° grid infill (design 2.1)
and e) 45° grid infill (design 2.2), design 3 with f) 90° grid infill (design 4.1), g) 45° grid infill (design 3.2),
and for PLA h) 45° grid infill (design 3.2) (volume V, max. deformation Umax, and max. von Mises
equivalent stress
𝜎
max).
The rough design of the hands-free door opener (Figure 4.1.8) was used as starting point
for our simulation (Figure 4.1.9a). To optimize the mechanical behavior, two further designs
were developed based on an infill of 100% (Figure 4.1.9b,c). To keep the consumption of
printing material as low as possible, further modifications were made by introducing support
structures as included in Figure 4.1.9d–g. In the respective simulations, a load of 0.5 MPa was
distributed over the topmost surface of the hands-free door openers, which measured about
2070 mm2 in surface area, corresponding to a resulting force of 1035 N. The associated
deformation behavior as well as the maximum von Mises equivalent stress largely depended
on the respective design. In this context, the design shown in Figure 4.1.9g proved to be
particularly advantageous. Here, the comparatively lowest maximum stress was detected in
the material, which means that the virtual design, when also considering potential material
savings, was best able to uniformly distribute the applied load as evidenced by a maximum
stress of 111.2 MPa. Due to the material-efficient production and optimum stress-loading
capability, the corresponding standard triangle language (STL) file was chosen for the 4D-
printing of demonstrators using the optimized printing parameters for PEU (Table 4.1.1,
| 101
TN = 160 °C). Taking into account the respective deformation behavior of PLA (Figure 4.1.9h),
the same design was selected for 4D-printing with PLA (Table 4.1.1).
In the next step, the practicability of our demonstrator was investigated starting with PEU
as print material (Figure 4.1.10).
Therefore, the hands-free door opener was pushed onto a door handle (Figure 4.1.10a,b)
and the polymer was localized heated to 100 °C (Figure 4.1.10c–e; more detailed information
is supplied in Figure S4.1.3, Supporting Information). Due to the triggering of the 4D effect,
the door opener connected with the door handle. Once cooled to 23 °C, an opening of the
door with a forearm or an elbow became possible without the hand coming into contact with
the door handle (Figure 4.1.10f,g), thus supporting the underlying hygiene concept.
Figure 4.1.10. Assembly and usage of a 4D-printed hands-free door opener made from PEU. An elliptical
profiled door handle served as a counterpart. a) Step one: Placement on the door handle, b) enlarged
image of (a), blue box, c) step two: triggering of the 4D effect with a heating gun, d) fixation as achieved
by shrinking the hands-free door opener, e) enlarged image of (d), red box, f) step three: two overlapping
images of the system in use and g) opening of the door.
To illustrate the good application perspective in the current corona situation, the print
material was exchanged by commercially widely available PLA and further demonstrators
were fabricated (Figure S4.1.4, Supporting Information). Compared to the demonstrator
made of PEU, the 4D shrinkage behavior was not as pronounced here when a shrinkage
temperature of 100 °C was selected, which caused the fastener to slip off the door handle.
This was traced back to the weaker 4D effect of PLA (Figure 4.1.4), but could be avoided when
increasing the localized temperature up to 140 °C, whereby a considerably improved shape
fit between the attachment and the door handle could be achieved. Due to the high shrinkage
temperature selected, we assume that in parts a physical melting of PLA could have occurred.
Tribological tests were conducted to measure the breakaway torque of the door openers
after their attachment to a rotating oval shaft. In Figure 4.1.11 can be seen that the
attachments made from PLA showed higher breakaway torque values of about 1.5 N m
102 | Chapter 4.1
compared to those manufactured from PEU (≈0.75 N m). Beyond that, after the initial relative
movement between the polymer and the shaft, a higher torque had to be applied when
selecting PLA as base material to move the opener further against the shaft. This can be
explained with the in parts physical melting and solidifying of PLA since a significantly higher
temperature was used to trigger the 4D effect. Anyway, the behavior of both PLA and PEU
emphasizes the good suitability of the hands-free door openers investigated.
In a next step, a disassembling of 4D-printed hands-free door openers made from PEU and
PLA was carried out (Figure S4.1.5, Supporting Information). Upon heating, the materials
became soft, where upon the attachment could be easily removed. In other words, it was the
material stiffness, which could be thermoreversibly changed according to a program that has
been defined by the molecular structure, which opened an advantage for the outlined
application at its end-of-use. The thermoplastic nature of the two polymers allowed for a
mechanical recycling as exemplarily done for PEU. Therefore, the disassembled door handles
were collected, granulated, and re-extruded (Figure S4.1.6, Supporting Information). After
the first cycle consisting of mechanical recycling and reprocessing of PEU, the glass transition
temperature remained almost identical while the mechanical properties were hardly affected
(compare Figure 4.1.3b and Figure S4.1.7, Supporting Information). By contrast, Tg and the
Young’s modulus of PEU tended to decrease at lower temperatures after conducting a fifth
recycling step (Figure S7, Supporting Information), which may be associated with the
degradation effects. For this reason, varying thermomechanical properties should be taken
into account when reusing PEU in 4D-printing processes. Alternatively, the usage of stabilizers
may contribute to a further enhancement of technological maturity. However, 4D re-
printability was proven for one-time recycled PEU as evidenced by a maximum thermally
shrinkable strain
𝜖
pre −
𝜖
of 57%.
Figure 4.1.11. Measurement of the torque via the rotation angle for 4D-printed PEU- and PLA-based
hands-free door openers, which in course of assembling were heated to 90 °C (PEU) and 140 °C (PLA),
respectively. a) Torque over the complete rotation angles of 90°. b) Enlarged section of the torque within
the first 5°.
| 103
4.1.3. Conclusions
In this contribution, we introduce a new type of PEU as suitable for additive manufacturing
via FFF. If the difference between the nozzle temperature and the material-specific glass
transition temperature of the polymer is kept as small as possible in the printing process,
objects are obtained whose shape and thermoresponsiveness can be precisely controlled.
Most remarkably, the results demonstrate 4D effects of a previously unknown degree. The
transfer of the findings from the FFF process to PLA as a printing material indicates that the
suggested 4D-printing method can be applied to other thermoplastics, too. Since heating of
the printing bed can be avoided, the FFF process consumes less energy compared with the
standard settings used in 3D printing. 4D-printing is generally attractive because it does not
require any energy-consuming thermomechanical treatment of a shape memory polymer
afterward—the thermoresponsive object can be taken directly from the printer. Therefore, a
wide range of potential applications based on highly shrinkable objects is foreseeable. In a
first step, hands-free door openers for the health sector were developed. The door handle
attachments have the potential to support the containment of viruses, like in the current
severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) pandemic, as well as smear
infections. A sustainable “product” design is now available. The developed lightweight
constructionischaracterizedbyhighmechanicalload-bearingcapability at optimized printing
material consumption. In principle, other shapes of door handles can be addressed by minor
design adjustments. As a result of shrinking hands-free door openers onto door handles,
permanent mounting is possible; no additional fastening elements are required as in other
systems.[59–62] In addition, our concept allows an uncomplicated disassembly at the end-of-
use due to the thermally switchable mechanical stability, which we consider as programmable
mechanical stiffness. As our demonstrators are made in every single case from one
thermoplastic material, a mechanical recycling is possible. Exactly this step allows for a
reprinting with PEU, whose one-time recycled sample also exhibited a high degree of
thermally inducible shrinkage. Although this can be understood as a step toward laying the
foundations for a circular economy for highly functional 4D materials, further research efforts
are needed to stabilize the functional properties in the long term and thus enable safe
reusability. It is the combination of technological approaches like functional design
development, 4Dprinting, assembly, disassembly and mechanical recycling that should be
considered from the beginning of the development of a demonstrator to the end of its use.
This illustrates the great application potential of programmable materials, with which they
can fulfill the function of products or their components and thus make valuable contributions
in the future.
4.1.4. Experimental Section
Materials: Desmophen 1262 BD was supplied by Covestro Deutschland AG (Leverkusen,
Germany). The polymer is a linear PPG exhibiting a molecular weight of about 430 g x mol−1
104 | Chapter 4.1
(acid value ≤ 0.1 mg KOH x g−1 and hydroxyl value = 260 ± 10 mg KOH x g−1, Covestro AG,
Desmophen 1262 BD, Technical Data Sheet 2016[63]) and was used as soft segment
component for PEU synthesis. Titanium(IV)bis(acetylacetonate) diisopropoxide was obtained
from Merck (Darmstadt, Germany), 4,4‘-methylene diphenyl diisocyanate (MDI) was
purchased from Fisher Scientific (Schwerte, Germany), BD, and a sodium aluminosilicate
molecular sieve (Zeolite 4Å) were obtained from Alfa Aesar (Kandel, Germany). To absorb
water prior synthesis, PPG-diol and BD were stored over molecular sieve for at least one day
at 23 °C.
Grey PLA filaments were bought from Prusa Research A.S. (Prague, Czech Republic) while
commercially available neon-yellow, blue, and orange PLA filaments were obtained from
Filamentworld (Ulm, Germany). In any case, the samples were characterized by a glass
transition temperature Tg of about 60 °C.
Synthesis of Thermoplastic Polyurethane: The polypropylene glycol based thermoplastic
polyurethane (PPG-PEU) was synthesized using the prepolymer method. The ratio of the
reactants was chosen so that the hard segment content was ≈60 wt.%. Therefore, the molar
ratio of the reactants PPG, MDI and BD was set to 1: 2.17: 1.16. The reaction was carried out
with a slight excess of isocyanate (NCO/OH = 1.005). First, the PPG-diol was heated under
nitrogen flow and stirring to 125 °C. Then, two droplets of a catalyst solution consisting of
PPG-diol and 5 wt.% of titanium (IV) bis(acetylacetonate) diisopropoxide were added. After a
few seconds, MDI was added, and the mixture was further heated to 155 °C and held there
under continuous stirring for 5 min. Afterward the obtained prepolymer was brought to
reaction with BD, serving as chain extender. In parallel, the stirring speed was raised. As soon
as the viscosity increased significantly, the reaction was stopped, and the melt was poured
onto a plate covered with a thin foil made of polytetrafluoroethylene. Finally, the obtained
PEU was cured in an oven at 80 °C for 120 min.
Extrusion: The synthesized PEU was ground with a cutting mill of the type M 50/80 from
Hellweg Maschinenbau (Roetgen, Germany). The obtained flakes were dried at 110 °C for 150
min in a vacuum drying chamber VDL 53 from Binder GmbH (Tuttlingen, Germany).
Subsequently, the flakes were fed into an extruder to produce filaments with a diameter of
2.85 mm. The individual units of the extrusion line were put together in such a way that it
included the volumetric material feeding system Color-exact 1000 from Plastic Recycling
Machinery (Tranekær, Denmark), a Leistritz twin screw extruder MICRO 18 GL from Leistritz
AG (Nürnberg, Germany), characterized by seven heating zones and a screw length of 600
mm, a conveyor belt, and a filament winder from Brabender GmbH and Co. KG (Duisburg,
Germany). The temperatures of the individual heating zones of the extruder were 200, 200,
202, 203, 190, 180, and 180 °C.
Determination of Density: The density of the PEU was determined for a piece of filament
by means of the buoyancy method following the Archimedes principle. Therefore, a KERN
YDB-03 attachment was used for the analytical balance KERN AJE from KERN and SOHN GmbH
(Balingen, Germany). At 23 °C, ≈0.7 g of the PEU were weighed out both in air as well as in
≈300 mL of demineralized water. The density
𝜌
PEU was calculated according to Equation 4.1.1,
| 105
considering the determined masses mair and mwater, respectively, and the density of
demineralized water
𝜌
water at 23 °C.
𝜌𝑃𝐸𝑈=𝑚𝑎𝑖𝑟
𝑚𝑎𝑖𝑟 − 𝑚𝑤𝑎𝑡𝑒𝑟×𝜌𝑤𝑎𝑡𝑒𝑟 (4.1.1)
Virtual Design, Simulation, and Fused Filament Fabrication: Tinkercad is an online 3D
modeling computer-aided design (CAD) program.[64] It was used for the virtual design of the
samples described in this work. These include type 5B tensile bars[65] as used for the
evaluation of the mechanical properties and cuboids with dimensions of 35 mm × 6 mm ×
3 mm for thermal investigations. Solid cuboids measuring either 40 mm × 2 mm × 2 mm or
40 mm × 4 mm × 28 mm were designed to study the 4d shrinkage behavior same as a hollow
cuboid having a dimension of 16.5 mm × 16.5 mm × 30 mm with a wall thickness of 1.5 mm
and a hollow cylinder with an outer diameter of 16.5 mm and a height of 30 mm at a wall
thickness of 1.5 mm. Furthermore, demonstrators for hands-free door openers were
designed. The CAD models were exported as STL files.
The initial design of a door handle attachment is exhibited in Figure 4.1.12 and was
imported into the finite element analysis software ANSYS,[66] which was used for design
optimization and simulation.
Figure 4.1.12. Mesh and boundary conditions for the stress–strain analysis of a door handle attachment
using ANSYS. a) Hexahedral finite element mesh and b) definition of the boundary condition, A = applied
load and B = fixed bearing.
In the first step, a mesh was generated for the CAD geometry (STL format) consisting of
26931 hexahedral elements and 123592 nodes (Figure 4.1.12a). To evaluate the stress
distribution, a pressure load of 0.5 MPa was applied on the top surface (A in Figure 4.1.12b)
while at the same time it was assumed that the elliptical hollow part of the door opener was
attached to a rigid rod (B in Figure 4.1.12b). The stress distribution and the material required
for the fabrication of the design were analyzed. The design was optimized in two steps. In the
first step, the topology optimization tool of ANSYS was applied to optimize the outer shape.
Then, in the second optimization step, several designs for the inner rib structure were studied
and compared manually.
The following parameters were considered in the finite element analysis of the door handle
attachment: the density of the PEU, the Young’s modulus, and the Poisson’s ratio. In case of
PLA, a Young’s modulus of 3500 MPa, a density of 1240 kg m−3 and a Poisson’s ratio of 0.36
106 | Chapter 4.1
were extracted from literature[67,68] to carry out the structural analysis within the design
optimization loop.
After finalizing the design of the hands-free door opener and cuboidal samples, the 3D
models in STL format were imported into the slicer program Cura 4.8.0.[69] The cuboidal
samples were first processed with standard 3D printing parameters. In case of 4D-printing,
the TP and TN were varied while keeping Sp = 50 mm x s−1 constant for both PEU and PLA.
Additionally, the layer height of 0.15 mm and a nozzle diameter of 400 μm were kept the
same throughout the study. Subsequently, additive manufacturing was carried out to
fabricate the abovementioned samples in three different printing scenarios (Table 4.1.1). All
printed objects were produced via FFF using the commercially available 3D printers Ultimaker
S5 and Ultimaker 3 from Ultimaker B.V. (Utrecht, the Netherlands), Roboze One Xtreme from
Roboze S.P.A. (Bari, Italy), PYOT OneProfessional from PYOT Labs GmbH (Berlin, Germany),
and Original PRUSA I3 MK2 from Prusa Research A.S. (Prague, Czech Republic). At the
beginning of every printing process, a thin layer of Magigoo adhesive from Thought3D Ltd.[70]
was applied to the building platform to achieve a good adhesion of the printed objects. As a
starting point for conducting the experiments, print settings were chosen that were based on
those from a previous work.[19]
Characterization of Mechanical Properties: The mechanical behavior of the synthesized
PEU was investigated in tensile tests using the universal testing machine Criterion Model 43
from MTS Systems Corporation (Eden Prairie, MN, USA). The device was equipped with a
500 N load cell. The measurements were carried out on dog-bone shaped tensile bars of
type 5B[65] as obtained from FFF. While stretching, the velocity of 1% x min−1 was kept constant
until a total strain of 5% was achieved in order to enable a more precise determination of the
Young’s modulus and the Poisson’s ratio, before the specimen was further elongated with a
velocity of 100% x min−1 until failure occurred. Every tensile test was carried out three times
at ambient temperature.
The Poisson’s ratio v of the PEU was characterized in tensile tests on type 5B tensile bars.[65]
For this purpose, a Canon M50 camera was used to take pictures of the sample every second
as the stretching progressed. The images were then imported into GOM Correlate 2020 from
GOM GmbH (Braunschweig, Germany). The 2 mm width of the tensile bar was used to
calibrate the reference length. After selecting the front face of the tensile bar using the
surface component tool, the axial strain
𝜖
axial and the transverse strain
𝜖
trans were determined
for strains between 2% and 6%. The Poisson’s ratio v was then calculated according to
Equation 4.1.2. 𝑣= 𝜀𝑡𝑟𝑎𝑛𝑠
𝜀𝑎𝑥𝑖𝑎𝑙 (4.1.2)
Characterization of Thermal and Thermomechanical Properties: The phase transitions of
the PEU or PLA were characterized by DSC using a Q100 DSC from TA Instruments (New Castle,
DE, USA). The experiments were conducted on dried pieces of filament made from PEU and
PLA and on the respective recycled samples. In any case, the samples were characterized by
a weight of about 5 mg. The samples were first heated from 0 to 100 °C before being cooled
back to 0 °C. The whole thermal cycle was repeated to finalize the measurement. Both for
| 107
cooling and heating, a rate of 10 °C x min−1 was applied. The temperature holding time at the
minimum and maximum temperature was 2 min. The glass transition temperature was
determined for the second heating as the temperature at half the step height between the
tangents of the baseline using the standard analyzing software of the calorimeter.
The thermomechanical properties of PEU and PLA were studied on both 3D and 4D-printed
samples of PEU and on 4D-printed samples of PLA. Additionally, mechanically recycled
samples of PEU were investigated by DMA. The experiments were carried out with a Q800
DMA from TA Instruments (New Castle, DE, USA) using single cantilever clamps on
multifrequency–strain mode. A frequency of 10 Hz, a static force of 0.1 N, and an oscillating
amplitude of 10 μm were selected. Additively manufactured cuboidal samples with
dimensions of 35 mm × 6 mm × 3 mm were clamped with a length of 17 mm in the specimen
holder. At first, every sample was cooled to 0 °C and held there for 5 min, before it was heated
to 100 °C with a rate of 3 °C x min−1 and held there once again for 5 min.
Characterization of 4D Effects: The additively manufactured samples (Table 4.1.1) were
placed for either 5 or 15 min at 75 °C in a UF110 heating chamber from Memmert GmbH + Co.
KG (Schwabach, Germany). The dimensions of the samples before and after heating were
measured with a Vernier caliper from Fowler High Precision GmbH (Massachusetts, USA).
In detail, the strain before heating
𝜖
pre and the strain
𝜖
after triggering of the 4D effect were
determined. In those cases, in which samples have bent through, the radius of curvature r,
and arc measure
𝜃
were determined. Therefore, the sample before and after heating was
photographed using a Canon M50, and the images were imported into CorelDraw 2019[71] and
evaluated with a dimensioning tool as illustrated in Figure 4.1.13.
A Q800 DMA from TA Instruments (New Castle, DE, USA) was utilized to investigate the
heating-initiated shrinking behavior of PEU and PLA. For this purpose, cuboid samples with a
dimension of 40 mm × 2 mm × 2 mm according to 3D printing and 4D-printing of PEU
(TN = 150 °C, Table 4.1.1) and 4D-printing with PLA (TN = 180 °C, Table 4.1.1) were
manufactured and fixed in the film tension clamps of the DMA device. The 4D effect was
adjacently studied under stress-free conditions by following changes in strain. For this
purpose, every sample was first kept at 23 °C for 5 min, before it was heated to 75 °C and the
temperature was held constant for 15 min. Afterward, the samples were cooled back to 23 °C
and kept there for 5 min.
Figure 4.1.13. Schematic representation of a curved 4D-printed object after heating (final length l, radius
of curvature r, and arc measure
𝜃
).
108 | Chapter 4.1
In Situ Thermal Imaging of FFF Printing Process: In situ thermal imaging was done with a
VarioCAM High Definition from InfraTec GmbH (Dresden, Germany) to study the FFF printing
processes (Table 4.1.1). In course of additive manufacturing the thermal images were
recorded with a frequency of 15 Hz and evaluated with the software IRBIS 4.1[72] from InfraTec
GmbH. The temperature of the PEU was measured at five different points on the topmost
layer of every sample investigated.
Assembly and Disassembly of Hands-Free Door Openers: The handsfree door opener was
either additively manufactured from self-synthesized PEU or from commercially available PLA
(Filamentworld-Ulm, Germany and Prusa Research A.S.-Prague, Czech Republic) using the
abovementioned 4D-printing scenarios (Table 4.1.1, PEU: TN = 160 °C; PLA: TN = 190 °C). After
inserting the attachment to the end of the door handle on an exemplary selected door, the
elliptical part of the attachment was heated to 100 °C in case of PEU and to either 100 or
140 °C in case of PLA using a heating gun from Conrad Electronic (Hirschau, Germany). Upon
triggering the 4D effect, the materials became soft and the attachment was manually placed
in the right position of the door handle. The attachment was later allowed to cool to 23 °C,
which took ≈20 min. To investigate the disassembly process, hands-free door openers were
heated to 100 °C in case of PEU and to 140 °C in case of PLA. As the materials became soft,
the attachments could be removed from the door handle without residue.
Tribological Characterization: The tribological properties of the handsfree door openers
were investigated by means of a two-column testing system (Co. Instron, E10000, torque:
max. 100 Nm, normal load: max. 10 kN). With this electrically controlled universal testing
machine, the load on the hands-free door opener was simulated as closely as possible to the
application by a rotational movement (Figure 4.1.14).
Figure 4.1.14. Mechanical characterization of the hands-free door openers using a two-column testing
system.
The hands-free door opener was attached to the rotating oval shaft by heating with an air
gun (Co. Steinel, HG2310LCD) whose temperature was set to 140 °C for PLA and 90 °C for PEU.
After 5 min, the polymers were cooled back to 23 °C and stored there for 15 min. Adjacently,
the experiment started. The test was carried out in a path-controlled manner at a speed of
0.5° x s−1. As a result, the torque was specified via the deflection angle. This allowed to
determine the breakaway torque, i.e., the initial slip between the shaft and the polymer, and
the further deformation at larger deflection angles.
| 109
Mechanical Recycling: Disassembled hands-free door openers made from PEU were
collected and shredded separately with a cutting mill type M 50/80 from Hellweg
Maschinenbau (Roetgen, Germany) to obtain small granules. The shredded material was
subsequently dried, before it was extruded to filament. The PEU was examined by DMA after
passing through one and five mechanical recycling and re-extrusion steps, respectively.
The one-time recycled filament was used for 4D reprinting and had a dimension of 40 mm x
2 mm x 2 mm (PEU: TN = 150 °C, and PLA: TN = 180 °C, Table 4.1.1). The 4D shrinkage behavior
was studied.
Supporting Information: Supporting Information is available from the Wiley Online Library or
from the author.
Acknowledgements: This work was supported by Fraunhofer Cluster of Excellence
“Programmable Materials” under project 630507. T.P. wishes to thank the European Regional
Development Fund for financing a large part of the laboratory equipment (project 85007031),
Fraunhofer High Performance Center for Functional Integration in Materials for the funding
of some of the 3D printers (project 630505) and the future cluster candidate Additive
Manufacturing Cluster Berlin-Brandenburg (AMBER), Bundesministerium für Bildung und
Forschung, (BMBF project 03ZK102AC) for the conceptual development of end-of-use ideas.
Tobias Rümmler is kindly acknowledged for determining the density of the polymer and Nishit
Puvati for his contribution in designing some of the images.
Conflict of Interest: The authors declare no conflict of interest.
Author Contributions: The author contribution is as follows: Conceptualization (T.P., L.W.);
methodology (D.C., D.S., M.W.); software (D.C., I.S., H.A.); validation (M.W., D.C., D.S., C.S.,
I.S.); formal analysis (D.C., D.S., C.S., I.S.); investigation (D.C., M.W., D.S.); writing—original
draft preparation (D.C., D.S.); writing—review and editing (T.P.); visualization (D.C., D.S., C.S.,
I.S.); supervision (T.P.); project administration (T.P.); funding acquisition (T.P., T.A., L.W.,
H.A.). All authors have read and agreed to the published version of the paper.
Data Availability Statement: The data that support the findings of this study are available on
request from the corresponding author.
Keywords: 4D-printing, additive manufacturing, device design, healthcare, mechanical
properties, shape memory polymers
Received: August 19, 2021; Revised: September 23, 2021; Published online: October 13, 2021
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4.1.6. Supporting Information
4.1.6.1. Fourier-transform infrared (FTIR) spectroscopy
In the FT-IR spectrum of the synthesized PEU a very weak signal occurred at about
2270 cm–1, indicating that only few free isocyanate groups were available.[1; 2] This speaks for
the almost complete reaction. At the same time, vibration modes are present as characteristic
for the formation of a polyether urethane (Figure S4.1.1).
112 | Chapter 4.1
A more detailed analysis of the FT-IR spectrum shows overlapping absorbances between
2970 cm–1 and 2852 cm–1, which can be assigned to asymmetric and symmetric stretching
vibrations of CH2 alkyl entities together with CH3 entities of the PPG side chain.[3; 4] The band
at 1065 cm–1 can be assigned to corresponding ν[C–O-C] ether stretching vibrations of the
PPG soft segment.[3; 5] The stretching vibrations ν[N–H] at 3313 cm–1, ν[C=O] in the carbonyl
stretching region at 1703 cm–1, as well as an amide peak (ν[C=N] + δ[N–H]) at 1531 cm–1 are
indicating a formation of the hard segment, since the signals can be attributed to urethane
species.[3; 6; 7] Consequently, the reaction seemed to be successful.
Figure S4.1.1. FT-IR spectrum of polypropylene glycol (PPG)-based polyether urethane (PEU), including
the assignment of vibration modes and the specification of their wavenumbers.
Figure S4.1.2. Thermomechanical properties of PPG-based PEU as determined by DMA for a 4D-printed
sample (Table 4.1.1, TN = 150 °C). The evolution of storage modulus E’ (solid line) and loss factor tan δ
(dashed dotted line) is considered for a heating rate of 3 °C · min–1.
Table S4.1.1. Temperature variation during 3D printing and 4D-printing of PEU in the first 13 layers at
different measuring points as defined in Figure 4.1.6. As a starting point for the coloring, a DMA-related
| 113
Tg of .54.4 °C was assumed. Temperature values above are marked in red, values below in blue
(measurement accuracy: ± 1°C).
Scenario A
Measurement
points
1
2
3
4
5
Layer
1
104
57
50
50
49
2
107
68
57
54
50
3
130
70
64
63
65
4
143
79
76
73
65
5
145
79
76
73
65
6
149
79
75
74
66
7
153
77
75
74
69
8
156
79
77
74
64
9
165
79
77
75
64
10
151
80
76
75
65
11
149
80
76
73
69
12
151
80
78
73
68
13
152
82
78
76
68
Scenario B
Measurement
points
1
2
3
4
5
Layer
1
77
44
39
36
35
2
97
46
40
38
35
3
99
47
39
38
36
4
99
47
40
37
37
5
100
48
40
37
37
6
101
49
40
37
37
7
102
51
41
37
38
8
104
51
41
40
38
9
102
49
42
41
40
10
100
51
43
40
40
11
98
51
44
40
39
12
103
49
44
40
38
13
102
51
45
40
39
T > Tg (PEU)
Tg (PEU) =54.5 °C
T < Tg (PEU)
4.1.6.2. Thermal imaging of FFF printing process
Table S4.1.1. Temperature variation during 3D printing and 4D-printing of PEU in the first
13 layers at different measuring points as defined in Figure 4.1.6. As a starting point for the
114 | Chapter 4.1
coloring, a DMA-related Tg of .54.4 °C was assumed. Temperature values above are marked
in red, values below in blue (measurement accuracy: ± 1°C).
Figure S4.1.3. Thermoresponsiveness of a 4D-printed hands-free door opener made from PEU: (a) As
obtained after FFF, (b) as usable in the application after 5 min of localized heating of the elliptical part
by means of a heating gun, T = ~100 °C and (c) after heating the whole object for 15 min in a temperature
chamber at 75 °C.
The thermoresponsiveness of the hands-free door opener was examined after 4D-printing
(Figure S4.1.4). In agreement with the material behavior of PEU (Figure 4.1.10), an initial
localized heating to 100 °C at the position of the ring was conducted for 5 min with a heating
gun. As a result, a partial shrinkage along the print direction and an expansion along the z-
axis could be achieved. Heating of the whole object in a temperature chamber at 75 °C for 15
min allowed the PEU to almost completely release the 4D effect as exemplified by a shrinkage
of about –47% along the print direction and a massive expansion along the z-axis by about
58%. This behavior illustrates that an individualized further shrinking can be conducted after
assembly.
| 115
Figure S4.1.4. Assembly and usage of a 4D-printed hands-free door opener made from PLA. (a) The 4D-
printed attachment is placed on an elliptical profiled door handle. (b) Fixation as achieved by shrinking
the elliptical part of the door opener. (c) System ready to use.
Figure S4.1.5. Disassembling of a 4D-printed PEU hands-free door opener. (a) Heating to 100 °C with a
heating gun and (b-d) removing the attachment from the door handle. When using PLA as print material,
exactly the same steps were followed.
Figure S4.1.6. Recycling of PEU at its end-of-use. The material is (a) collected, (b) granulated and (c)
extruded to filament.
116 | Chapter 4.1
Figure S4.1.7. Influence of mechanical recycling and re-extrusion upon the thermomechanical
properties of PPG-based PEU as determined by DMA. The evolution of storage modulus E’ (solid line)
and loss factor tan δ (dashed dotted line) is considered after the first recycling (green color) and the fifth
recycling (blue color). A heating rate of 3 °C · min–1 was selected.
4.1.6.3. References
1. Tereshatov, V.V., Slobodinyuk, A.I., Makarova, M.A., Vnutskikh, Z.A., Pinchuk, A.V., and Senichev, V.Y., Russ
J Appl Chem, Vol. 89, 943–948, 2016.
2. Klinedinst, D.B., Yilgör, I., Yilgör, E., Zhang, M., and Wilkes, G.L., Polymer, Vol. 53, 5358–5366, 2012.
3. Tan, C., Tirri, T., and Wilen, C.-E., Polymers, Vol. 9, 2017.
4. Erdem, A., J. Appl. Polym. Sci., Vol. 138, 49997, 2021.
5. Zhang, Y., Qi, Y.-h., and Zhang, Z.-p., J Polym Res, Vol. 22, 2015.
6. Pretsch, T., Jakob, I., and Müller, W., Polymer Degradation and Stability, Vol. 94, 61–73, 2009.
7. Piril Ertem, S., Yilgor, E., Kosak, C., Wilkes, G.L., Zhang, M., and Yilgor, I., Polymer, Vol. 53, 4614–4622, 2012.
| 117
Chapter 4.2: Potential
Applications of 4D-Printed Objects
118 | Chapter 4.2
Chapter 4.2: Potential Applications of 4D-Printed Objects
4.2.1. Introduction
Four-dimensional (4D) printing is a facile additive manufacturing (AM) technique to obtain,
e.g., thermoresponsive objects made from shape memory polymer (SMP) [1–5]. The 4D-
printing technique was first introduced by Skylar Tibbits [6], combining a hydrogel as an active
material with a static, rigid polymer as a passive material to achieve different degrees of
bending or shape transformations [7,8]. Later, scientists transferred the basic principles of
4D-printing to the AM technology “fused filament fabrication” (FFF) [1,2,4,9–14]. In FFF, the
choice of specific printing parameters like nozzle temperature, printing pressure, and speed
are known to affect the print results, thus gaining control over the orientation of polymer
chains in the direction of the nozzle movement or printing pattern [4,10,15]. The basic idea is
that an extruded polymer strand, laid on the printing platform or a top polymer layer, is
rapidly cooled below the polymer’s glass transition temperature so that the individual
polymer chains retain highly oriented states. After 4D-printing, the object remains stable until
the ambient temperature is raised above the switching temperature of the SMP, most
commonly the glass transition temperature [4]. Once heated above the glass transition
temperature (Tg), the polymer releases internal stresses by molecular motion, culminating in
macroscopical shrinkage along the direction of the nozzle movement [14]. My recent study
showed that when selecting a lower nozzle temperature and keeping the difference between
the nozzle temperature and glass transition temperature to its minimum, internal stresses
can be stored and released upon heating, resulting in a significant thermoresponsiveness,
here also denoted as a 4D effect [4]. Until recently, the 4D effect was restricted to objects of
smaller height (z < 5 mm), and with increasing layer height, the 4d effect decreases in the z-
direction [1,2,4,10,14,16]. This can be explained by the occurrence of pronounced relaxation
effects and was one of the main reasons for developing novel applications based on 4D-
printing more difficult.
Against this background, our group presented an approach to realize the 4D-printing of
objects with a more significant height (z > 30mm) recently (Chapter 4.1) [4]. To show that the
range of applications is far from exhausted, virtual models of different demonstrators were
created using computer-aided design (CAD), and the same polypropylene glycol (PPG)–based
polyether urethane as in Chapter 4.1 was used as 4D-printing material for this study [4]. In a
final step, the thermoresponsiveness of 4D-printed objects was evaluated in the context of
new applications.
4.2.2. Results and Discussion
The polypropylene glycol (PPG)-based polyether urethane (PEU), which was used in the
previous Chapter 4.1 [4], was again employed in the form of filaments. In order to achieve
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the desired 4d effect for demonstrators, the same printing parameters were chosen. The
selection was due to the excellent 4D effect exhibited by the material using FFF processing
and, most importantly, the proven possibility to build higher objects (z > 5 mm). The glass
transition temperature of the PEU is close to 60 °C (Figure 4.1.3b in Chapter 4.1) [4]. Further,
the application potential of the 4D-printing technology was investigated. Firstly, the feasibility
of producing active fasteners using 4D-printing was looked into.
Fasteners are widely used in the manufacturing and construction industry and assembly
and conjunction lines. They mechanically join or attach several objects. Even though most
fasteners are produced according to international standards or norms, the need for a suitable
tool to mechanically assemble elements is unavoidable. Here, active assembly techniques are
proposed for fasteners using the 4D-printing technique (Figure 4.2.1).
Figure 4.2.1. CAD of a fastener pin and a round disc (a) and its additively manufactured analogues made
from PET-G (b, left) and PEU (b, right). The round disc was obtained from 4D-printing.
The fastener pin and its compatible round disk were created using CAD (Figure 4.2.1 a).
Once modeled, the fastener pin was additively manufactured using black polyethylene
terephthalate glycol (PET-G). In contrast, the round disk was 4D-printed using the PEU
filament (Figure 4.2.1 b). The individual printing parameters are provided in Table 4.2.1. Once
the 3D printing was complete, active assembly was carried out (Figure 4.2.2).
Figure 4.2.2. A one-time active assembly is enabled by heating the 4D-printed round disk made from
PPG-based PEU. A poly(methyl methacrylate) sheet with an opening in the middle (a), fastener pin, and
round disk were placed together with the sheet (b) and assembled by heating the round disk to 75 °C
(c).
120 | Chapter 4.2
The 4D effect is later triggered by heating the round disk to 75 °C, above the PEU's glass
transition temperature (Tg). This temperature increase led to the shrinkage of the round disk,
thereby connecting the fastener pin, which is inserted through the hole of a poly(methyl
methacrylate) sheet. The shrinkage of a round disk from planar to conical shape was achieved
from the full annulus or the continuous concentric printing pattern. This is achieved due to
the higher shrinkage effect incorporated into the outer circumference than the inner, where
the outer circumference attains a longer strand deposition length and a more substantial
shrinkage effect than the inner. A similar transformation of round disks from two-dimensional
to three-dimensional shapes has been recently reported by Goo et al. [17] and Hu et al. [18].
However, due to the shape change, a tight connection of the three individual parts was
achieved, which turned out to be stable under room temperature. In order to remove and
dismantle the system, the PEU was reheated to 75 °C. Thereupon softens the PEU, allowing
the individual parts to recover.
The accuracy of the fit is an essential criterion for fasteners. For another similar sample, no
visual damage or deformation occurred to the shrunk disk when pulling the round disk apart
from the poly(methyl methacrylate) (PMMA) sheet with a force of 50 N. However, the
fastener pin failed by ripping off its base. A further advantage of using the 4D-printed active
assembly is that the only tool required for different-sized objects is a heating gun or a heating
chamber. The use of smart fixtures could be beneficial in the interior of space stations or
space habitats, or even for furniture, due to its general usability, simplicity, and easy
adaptation to other geometric requirements in a wide variety of situations.
Interestingly, the concept can also be applied in another context, namely in closures that
can be actively assembled one-time (Figure 4.2.3).
Figure 4.2.3. The concept for one-time active assembly of differently sized closures. CAD of closures
with an outer diameter of 30 mm (a, left) and 12 mm (b, left), closures after 4D-printing (2nd image from
left) together with red natural rubber tubes (3rd image from left) with outer diameters of rubber tubes
of 17 mm (a) and 10 mm (b), and the individual systems before and after heating to 75 °C (image on the
right).
The tapered cone pin design of the closure acts as a positioning pin. Due to the tapering, a
single design fits various tubes with varying inner diameters (leftmost image of Figure 4.2.3.
a and b). After 4D-printing, the closure presented good visual accuracy compared to the
virtual design (first and the second image from the left in Figure 4.2.3. a and b). After placing
| 121
the closure onto the tube opening and heating it to 75 °C for 5 min, the 4D effect was
triggered. The closure shrank and snugly fit over the tube (rightmost images of Figure 4.2.3.
a and b). Once again, due to the continuous concentric strand deposition in the circular
printing pattern, the closure shrank uniformly along the printing plane (XY-plane). It is worth
mentioning that the smaller closure (Figure 4.2.3. b) showed almost a similar 4D effect as the
more oversized closure (Figure 4.2.3. a), even though the smaller closure was a miniaturized
filigree version of the bigger one. Interestingly, it could be observed that even after choosing
a closure with a bigger outer diameter of 30 mm (leftmost image Figure 4.2.3. a), which is
twice the outer diameter of the tubing, the closure shrank by about 37%, reaching an outer
diameter of 18.9 mm (rightmost image Figure 4.2.3. a).
In order to quantify the resistance of the closure system to compressed air, the closed tube
was subjected to different air pressures (Figure 4.2.4.).
The pressure was gradually increased from 0 to 10 bar (Figure 4.2.4). When reaching 8 bar,
the closure and tubing were immersed in a water bath. No signs of air bubbles or leakage
from the closure could be witnessed; thus, the closure was still tightly connected to the tube.
The tube burst and failed on further increase the air pressure to about 10 bar (rightmost
image of Figure 4.2.4 a and b). Immersing the still closed part of the tube again in water
showed no signs of air bubbles.
To further investigate the quality of sealing achieved by the 4D effect, the closure with the
tubing was connected to a water tap offering a water pressure of approximately 4 bar. The
experiment also revealed no visible spilling, leakage, or occurrence of moisture on the surface
of the closure. One can see from these that the 4D-printed objects from PPG-PEU can
withstand enormous forces after releasing their 4D effect. For this reason, the concept may
be helpful in many industries wherever efficient sealing solutions are sought.
Subsequently, the concept was extended to connecting two tubes (Figure 4.2.5.).
Figure 4.2.4. The pressure resistance of 4D-printed closures is tightly connected to red natural rubber
tubes with an outer diameter of 30 mm (a) and 12 mm (b), respectively. After the test, the assembled
setup for pneumatic testing states and enlarged images of the blue box in the middle are exhibited from
left to right.
122 | Chapter 4.2
Figure 4.2.5. One-time active assembly: Connecting disk for round tubes or objects. CAD design (a),
round disk after 4D-printing (b), two tubes with an outer diameter of 4 mm (c), and system after heating
to 75 °C.
The round disk developed for the active assembly of fasteners (right image of Figure 4.2.1)
was again used for this purpose (Figure 4.2.5 a and b). The tubes had an outer diameter of 4
mm (Figure 4.2.5 c). The round disk was placed at the contact point of the two tubes and was
heated to 75 °C. As a result, the connecting disk shrank and firmly wrapped onto the surface
of both tubes. After triggering the 4D effect, the round disk changed its shape into a hollow
cylindrical shape, and a uniform shrinking force was exerted on both ends of the tubes,
allowing them to connect and hold the tubes together. Connecting the extended hose to a
water pipe with a pressure of 4 bar and an air stream with a pressure of 10 bar for at least 5
min showed that no leaks could be detected.
In another approach, a flower was developed using the PEU that closes upon heating.
(Figure 4.2.6). The 4D-printing of hydrogel-based flowers inspired the 4D-printing of a flower
following the FFF concept by Gladman et al. [15] and Shiblee et al. [19].
Figure 4.2.6. One-time self-closing flower made of 4D-printed PEU. The four superimposed images
(above) exhibit the different stages when heating the flower from 23 °C to 75 °C (below, from left to
right).
| 123
The flower-shaped structure consisted of a flat thin oval-shaped substrate and a
continuous spline printed on top. The printing path of the oval-shaped substrate was selected
such that the strand deposition took place in a concentric oval printing pattern along the
longer axis of the oval. This printing pattern allowed the structure to shrink along the oval's
longer axis and expand on the oval's shorter axis. At the same time, the continuous spline,
running over the oval-shaped substrate, was formed by laying down a single continuous
strand in a layer-by-layer manner. Thereby, no neighboring deposited strands heated up,
which avoided the relaxation of the polymer. Thereby implies that the 4D effect of the spline
was comparatively more pronounced than for the oval-shaped substrate. The difference in
the 4D shrinkage effect along the longer axis of the petal made each petal bend and shrink
simultaneously, resulting in a petal closing movement of a flower. This biomimicry effect
caused by 4D-printing could be helpful in applications such as automotive, where individual
parts should be assembled.
In the next step, active disassembly as facilitated by 4D-printing was investigated. For this
purpose, a two-component system was designed and later manufactured via FFF using the
printing parameters in Table 4.2.1 (Figure 4.2.7.). In detail, a thermoresponsive round disk
with a protrusion along the outer diameter was 4D-printed from PEU (white part). At the same
time, the red-colored block was printed from Desmopan® 9370AU with a grooved hole
exhibiting a maximum diameter of 19 mm.
Figure 4.2.7. One-time active disassembly of a two-component system made from 4D-printed PEU
(white) and its non-thermoresponsive counterpart made of red-colored Desmopan® 9370AU. The
images show the system in its assembled state (a), the states when raising the temperature to 75 °C (b
and c), and the disassembled state (d). The thermoresponsive disk is shown separately in its states
before (e) and after heating (f) (all the dimensions are in mm).
As the printing pattern of the round disked PEU determines the 4D effect, it was printed
with a grid printing pattern consisting of alternating layers of horizontal and vertical strand
orientation along with two circled outer shells for achieving shrinkage of the disk and thereby
disassembly of the object. The grooved hole of the red block and the protrusion of the round
disk (white part) realizes an easy assembly by a snap-fit mechanism. In other words, the 4D-
printed PEU part (white part) was inserted into the groove of the red component (Figure 4.2.7
a). The assembled system can be used as long as the internal stresses are stored in the PEU.
124 | Chapter 4.2
The assembled system showed good mechanical resilience even after placing a load weighing
500 g on top of the 4D-printed element. The system remained intact until the 4D effect was
triggered. Once the temperature was raised to 75 °C, the PEU shrank significantly (Figure
4.2.7. b and c), after which the middle part could be removed without effort (Figure 4.2.7. d).
Due to the PEU’s alternating vertical and horizontal printing pattern, the round disk shrank
along the printing plane (XY-plane) and expanded from 5 mm to about 10 mm along the
direction of layer thickness and height (Z-plane). The states before and after heating the 4D-
printed part are shown in Figures 4.2.7 e and f. On re-heating the shrunken part (Figure 4.2.7
f) to 75 °C, applying a compressive load of 50 N, and cooling to 23 °C, a thermoresponsive
shape could be re-obtained similar to the initial shape (Figure 4.2.7 e). This implies that it can
be used again in another life cycle.
The active disassembly concept enabled by 4D-printing was further extended using snap-
fit joining mechanisms. A three-component system was used to demonstrate the working
principle (Figure 4.2.8). The system's components were produced from two three-
dimensional (3D) printed blocks with socket extensions and one 4D-printed block with ball
extensions. The component blocks were first 3D-printed using yellow polylactic acid (PLA)
filament (Figure 4.2.8a) and black PET-G filament (Figure 4.2.8c), while the PEU (Figure
4.2.8b) was 4D-printed using the printing parameters from Table 4.2.1.
Figure 4.2.8. Snap-fit joining mechanism and one-time active disassembly as enabled by 4D-printing.
The first component was made from PLA (a), the second component from 4d-printed PEU (b), and the
third component from PET-G (c). The assembled state of the system is shown before (d) and after
heating to 75 °C (e) to visualize active disassembly.
After additively manufacturing the different blocks, the design of the components allowed
to snap-fit the individual members by connecting the ball and socket joint [20–22]. Once
assembled, the whole system can be employed, e.g., until it reaches a point where a repair is
required, or the end-of-use or end-of-life is reached. The system can then be actively
disassembled by heating above the glass transition temperature of the PEU. Once heated to
75 °C, the ball mechanism or 4D-printed PEU shrinks and elongates due to the 4D effect. The
main reason for the elongation and shrinkage was the choice of the concentric circular
printing pattern of the balls along the printing plane (XY-plane), where the strands in the form
| 125
of concentric circles shrink along the print direction and expand in the direction of the layer
height. Once the balls shrink and elongate, the ball-socket snap-fit mechanism no longer
functions, allowing the system to disassemble actively, thereby enabling the separation of
mechanically connected parts. Later, the system can be repaired or recycled.
In Chapter 3.2, a mechanical gear was introduced to deactivate or disconnect a
transmission system by heating above the soft segmental melting transition temperature of
the employed polyurethane. However, the most significant disadvantage of the gear was the
complex programming. In order to overcome the tedious thermomechanical step, a CAD
design of a 4D-printable gear was developed and printed with the PEU (Figure 4.2.9).
Figure 4.2.9. Thermoresponsive gear as created by CAD (a) and after 4D-printing using the PEU, the base
material (b).
After identifying an appropriate design, the model was 4D-printed via FFF. The gear was
assembled intermediate between an input and an output gear (Figure 4.2.10).
Figure 4.2.10. One-time self-deactivating mechanical system triggered by heat consisting of an output
gear made from poly(methyl methacrylate) (left), an intermediate gear made from 4D-printed PEU
(middle), and an input gear again made from poly(methyl methacrylate) (right). Overlapping images
when triggering the 4D effect (top) and states after assembly and during heating to 75 °C (below, from
left to right).
Once the gear was assembled into the system, it responded to heat with a one-time change
of shape. In detail, as soon as the temperature rose above Tg of the PEU, the 4D effect of the
126 | Chapter 4.2
PEU was triggered, resulting in shrinkage of the teeth and the body of the gear. The teeth of
the gear shrank uniformly along the print direction, as achieved through the straight-lined
printing pattern. The strand deposition movement was from the gear's root to the top land
and back to the gear root. In Figure 4.2.10, slight bending of the PEU gear could be observed
during heating. This occurred because the gear was heated with a heat gun, and the heat
transfer was not uniformly attained throughout the gear (from top to bottom). This
temperature gradient caused the bending. However, the substantial reduction in gear size
broke the contact between the input and output gear and thus interrupted the transmission
system. A temperature-induced interruption of a process can bring significant advantages, for
example, in industrial safety appliances or systems to prevent overheating. At the same time,
the risk of damage to a gearbox, transmission system, or neighboring electronics can also be
avoided. Once again, replacing the 4D-printed object with a new thermoresponsive gear after
reprocessing or reprogramming is a prerequisite to enter one further life cycle to allow the
system to continue operating.
To further explore the potential of 4D-printed objects, the technique was used to program
shape memory polymer foam. To demonstrate the concept, a cylindrical element of the PEU
measuring 20 mm in height and 52 mm in diameter was designed, 4D-printed, and assembled
with a sample of an in-house synthesized polyester urethane urea (PEUU) foam (Figure
4.2.11).
Figure 4.2.11. Polyester urethane urea foam, 4D-printed programming cylinder (a), and both of them in
an assembled state (b) (all the dimensions are in mm).
Next, the deformation of the PEUU foam was carried out by triggering the 4D effect of the
PEU cylinder (Figure 4.2.12).
For developing an effective thermomechanical treatment to program the PEUU foam later,
it is necessary to understand the thermal properties of the PEUU foam. The melting transition
of the PEUU foam ranges from 20 °C to 65 °C with a maximum of about 57 °C. At the same
time, the crystallization transition extends from 45 °C to 0 °C with a peak at 37 °C [23]. On
heating the system to 75 °C and holding the temperature for 15 min, the PEUU foam softened,
allowing the system to continue operating. The cylindrical element shrank along the
continuous concentric circular printing pattern and compressed the foam. Remarkably, the
foam could uniformly shrink and close the star-shaped opening (Figure 4.2.12). Subsequently,
the temperature was brought to –15 °C to crystallize the soft segment of the PEUU foam and
| 127
was held isothermally for 15 min to fix the new temporary shape of the PEUU foam. Heating
to 23 °C finalized the programming of the foam, and the PEU cylinder was separated from the
PEUU foam. One potential application of the programmed foam could be a thermally
switchable air permeability system.
Figure 4.2.12. Thermomechanical treatment of PEUU foam with a star-shaped cutout using a 4D-printed
cylinder made of PEU. The temperature was systematically raised from 23 °C to 75 °C (from left to right).
In a final approach, the 4D-printed cylindrical element and the PEUU foam were combined
to explore one further application (Figure 4.2.13).
After placing the PEUU foam inside the 4D-printed cylinder, the setup was mounted over
a rod, holding a weight of 100 g. Upon raising the temperature to 75 °C, the 4D-printed
cylinder shrank, thus compressing the PEUU foam and fixing the rod in its center position. It
demonstrates that the system can grab or hold objects for, e.g., transporting purposes.
Beyond that, it may even stop rotating objects under certain conditions and can be used, e.g.,
in mechanical machines, biomedical devices, or the aerospace industry.
Figure 4.2.13. 4D-printed cylindrical element with PEUU foam before (a) and after triggering the 4D
effect to grab and hold an object (b).
4.2.3. Conclusions
Chapter 4.2 presents some application ideas as enabled by 4D-printing with SMPs. These
include elements for active assembly and disassembly, concepts for end-of-life components,
safety components, and elements that can program other SMPs. The approaches emphasize
that 4D-printing eliminates the need for the classical time-consuming thermomechanical
128 | Chapter 4.2
treatments to attain a thermoresponsive state for some SMPs. This is a significant step to save
energy, time, and cost. The work also shows that thermoresponsiveness can be efficiently
maintained in objects. With the right or appropriate selection of printing patterns, the
direction of the 4D effect can be controlled. It is noteworthy that the 4D-printed parts exhibit
good mechanical stability and could be assembled or disassembled when triggering the 4D
effect. The ability of the PPG-based PEU 4D-printed parts to withstand heavy loads or pressure
makes the material, as well as the 4D-printing technique, attractive for industries like
aerospace, mechanical, and marine. Most importantly, the development of active
disassembly concepts showed that objects could be quickly assembled using mechanical
joining mechanisms like snap-fitting. At the same time, the triggering of the 4D effect initiates
active disassembly and separates the assembled components. This brings significant
advantages in terms of sustainability: The separation of materials by their types enables
effective recycling at the end-of-life of a system. In this way, 4D-printing is a promising
technology that may assist in developing efficient and sustainable solutions.
4.2.4. Experimental Section
Material: The polypropylene glycol (PPG) based thermoplastic polyurethane (PEU), which
was synthesized and developed in the previous literature, is again used in this work in the
form of filaments (Chapter 4.1) [4]. A filament of black polyethylene terephthalate glycol
(PET-G) and yellow polylactic acid (PLA) was purchased from FilamentWorld (Neu-Ulm,
Germany). The ether-based thermoplastic polyurethane elastomer Desmopan® 9370AU was
supplied by Covestro Deutschland AG (Leverkusen, Germany). A polyester urethane urea
(PEUU) foam which was synthesized and developed in previous literature by Walter et al., is
used in this work to demonstrate programmable devices [23].
Virtual Design and Fused Filament Fabrication: Tinkercad is an online 3D modeling
computer-aided design (CAD) program [24]. It was used for designing and developing the
virtual design of the samples described in this work. These include a fastener, closure,
connecting disc, two- and three-component system, gear, and programming cylinder. The
CAD models were exported as STL files.
After finalizing the designs, the 3D models in STL format were imported into the slicer
software Cura 4.8.0. [25]. In the case of 4D-printing, the printing parameters were selected in
accordance with previous literature [4]. All printing parameters of PEU, black polyethylene
terephthalate glycol (PET-G), yellow polylactic acid, and red Desmopan® 9370AU are
summarized in Table 4.2.1.
All printed objects were produced via FFF using the commercially available 3D printer,
using commercially available 3D printers Ultimaker S5 and Ultimaker 3 from Ultimaker B.V.
(Utrecht, the Netherlands). At the beginning of every printing process, a thin layer of Magigoo
adhesive from Thought3D Ltd. [26] was applied to the building platform to ensure good
adhesion between the build platform and printed objects.
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Table 4.2.1. Printing parameters selected for FFF with different materials.
Printing parameter
3D printing
PEU
(4D-printing)
PET-G
Desmopan®
9370AU
PLA
Diameter of the nozzle (µm)
400
400
400
400
Temperature of the nozzle (°C)
235
225
200
150
Speed of print head (mm × s–1)
50
25
40
50
Build platform temperature (°C)
85
60
65
23
Layer height (mm)
0.15
0.15
0.15
0.15
Characterization of 4D Effects: The triggering of the 4D effect was achieved by a heating
gun from Conrad Electronic (Hirschau, Germany) or in the heating chamber UF110 from
Memmert GmbH + Co. KG (Schwabach, Germany). In any case, the samples were heated to
75 °C within a time of 5 min, else otherwise specified. The dimensions of the samples before
and after heating were measured with a Vernier caliper from Fowler High Precision GmbH
(Massachusetts, USA).
The mechanical behavior of the shrunk 4D disk fastener was investigated with the universal
testing machine Criterion Model 43 from MTS (Eden Prairie, MN, USA). The device was
equipped with a 500 N load cell. The measurements were conducted on the poly(methyl
methacrylate) sheets fastened with a fastener pin and a 4D-printed round disk. The
poly(methyl methacrylate) sheets were fixed on the lower clamp, while the shrunk 4D-printed
disk was clamped on the top clamp. Afterward, the parts were pulled apart by stretching with
a velocity of 100%·min–1 until rupture occurred, while the corresponding load required was
measured.
The thermomechanical treatment of the PEUU foam with the 4D-printed programming
device was carried out with an MTS Criterion universal testing machine from MTS Systems
Corporation (Eden Prairie, MN, USA). The device was operated with a temperature chamber
controlled by a Eurotherm temperature controller unit. Two heating elements were located
at the back of the chamber. Liquid nitrogen from a Dewar vessel was fed into the chamber
under a pressure of 1.3 bar as an essential prerequisite for cooling. The assembled setup was
placed on a platform at the beginning of the programming. The chamber was heated to 75 °C
and held isothermally for 15 min. The PEU cylinder around the PEUU foam was cooled to
– 15 °C. After an isothermal holding time of 15 min, the chamber was heated to 23 °C and
held isothermally for another 5 min. A heating and cooling rate of 5 °C x min−1 was used for
the whole experiment.
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| 131
Chapter 5: Ex-Situ Programming of Objects
Manufactured via Fused Filament
Fabrication to Attain Two-Way Shape
Memory Effect
132 | Chapter 5.1
Chapter 5.1: Actuating Shape Memory
Polymer for Thermoresponsive Soft Robotic
Gripper and Programmable Materials
| 133
Chapter 5.1: Actuating Shape Memory Polymer for
Thermoresponsive Soft Robotic Gripper and Programmable
Materials
The original article Schönfeld, D.; Chalissery, D.; Wenz, F.; Specht, M.; Eberl, C.; Pretsch, T.
Actuating Shape Memory Polymer for Thermoresponsive Soft Robotic Gripper and
Programmable Materials. Molecules 2021, 26, 522 and graphical abstract are published in
MDPI Molecules and are available at https://doi.org/10.3390/molecules26030522.
Figure 5.1.0. The graphical abstract of the article “Actuating Shape Memory Polymer for
Thermoresponsive Soft Robotic Gripper and Programmable Materials” is published in MDPI Molecules
2021, 26, 522.
Contribution
My contribution: Conceptualization of two-way shape memory effect (2W-SME) screening
characterization measurement, the combination of actuating element with linkage
mechanism and unit cell (Figure 5.1.13b and c). Concept development, design,
manufacturing, and actuation of the soft gripper. Design development of actuating elements.
3D printing of both unit cells and actuating elements. 2W actuation analysis of unit cell (Figure
5.1.14). Visualization and preparation of images (including videos and graphical abstract,
excluding Figures 5.1.1, 5.1.2, 5.1.3, 5.1.8, 5.1.11c, and 5.1.12)
Equal/ shared contribution of Schönfeld, D. and me: Carried out DSC, thermomechanical
analysis. (DMA), tensile tests, and filament extrusion. Development of 2W programming
134 | Chapter 5.1
procedure. Contributed equally in methodology, validation, formal analysis, investigation,
writing—original draft preparation,
Not included in my contribution: Synthesis, titration and FT-IR, 2W-SME durability
experiment, and design development of the unit cells.
Schönfeld, D.: Conceptualization of novel polyester urethane (PEU) for 2W-SME and
combination of actuating element with unit cell (Figure 5.1.11 c). Conducted the durability
experiment of 2W actuating element. 2W actuation analysis of unit cell (Figure 5.1.12).
Synthesis and development of PEU. Carried out FT-IR. Visualization and preparation of images
(Figures 5.1.1, 5.1.2, 5.1.3, 5.1.8, 5.1.11c, and 5.1.12). Project lead of the article.
Wenz, F.: Design development and writing—original draft preparation of unit cell in Figure
5.1.11a.
Specht, M.: Design development and writing—original draft preparation of unit cell in Figure
5.1.13a.
Eberl, C.: Project administration, writing—review and editing of unit cell.
Pretsch, T.: Conceptualization of the manuscript, funding acquisition, project administration,
supervision, writing—review and editing
| 135
Chapter 5.1: Actuating Shape Memory Polymer for
Thermoresponsive Soft Robotic Gripper and Programmable
Materials
Dennis Schönfeld 1, Dilip Chalissery 1, Franziska Wenz 2,3, Marius Specht 2,3, Chris Eberl2,3
and Thorsten Pretsch 1,*
1 Fraunhofer Institute for Applied Polymer Research IAP, Geiselbergstr. 69, 14476 Potsdam,
[email protected] (D.C.)
2 Fraunhofer Institute for Mechanics of Materials IWM, Wöhlerstr. 11, 79108 Freiburg,
[email protected] (C.E.)
3 Department of Microsystems Engineering IMTEK, University of Freiburg, Georges-Koehler-
Allee 078, 79110 Freiburg, Germany
* Correspondence: [email protected]; Tel.: +49-(0)-331/568-1414.
5.1.0. Abstract: For soft robotics and programmable metamaterials, novel approaches
are required enabling the design of highly integrated thermoresponsive actuating systems. In
the concept presented here, the necessary functional component was obtained by polymer
syntheses. First, poly(1,10-decylene adipate) diol (PDA) with a number average molecular
weight Mn of 3290 g·mol–1 was synthesized from 1,10-decanediol and adipic acid. Afterward,
the PDA was brought to reaction with 4,4′-diphenylmethane diisocyanate and 1,4-butanediol.
The resulting polyester urethane (PEU) was processed to the filament, and samples were
additively manufactured by fused-filament fabrication. After thermomechanical treatment,
the PEU reliably actuated under stress-free conditions by expanding on cooling and shrinking
on heating with a maximum thermoreversible strain of 16.1%. Actuation stabilized at 12.2%,
as verified in a measurement comprising 100 heating-cooling cycles. By adding an actuator
element to a gripper system, a hen’s egg could be picked up, safely transported and
deposited. Finally, one actuator element each was built into two types of unit cells for
programmable materials, thus enabling the design of temperature-dependent behavior. The
approaches are expected to open up new opportunities, e.g., in the fields of soft robotics and
shape morphing.
Keywords: additive manufacturing; soft robotics; actuation; programmable materials;
polyester urethane; shape morphing; unit cell
136 | Chapter 5.1
5.1.1. Introduction
Soft robotics, as well as the still novel metamaterials [1,2] or programmable materials [3],
require compliant actuator materials. Shape memory polymers (SMPs) perfectly fulfill this
criterion. SMPs are stimuli-responsive materials, which are able to fix a temporary shape after
a thermomechanical treatment, also denoted as “programming”. The temporary shape will
be stable until the one-way shape memory effect (1W SME) is triggered, whereupon the
polymer almost completely returns into the permanent shape [4]. Shape recovery is an
entropically driven process. It is based on the phenomenon of entropy elasticity; the theory
of rubber elasticity provides the basis [5]. Most commonly, the shape memory effect is
triggered by heat [4,6–12]. Alternatively, switching can be realized for materials equipped
with appropriate fillers by indirect heating when applying an electric [13,14] or a magnetic
field [15,16] or, e.g., by illumination with near-infrared light in the case of SMPs with
photoresponsive fillers [17,18].
In the past few years, it has become known how to transfer semicrystalline SMPs into two
meta stable states, between which they can be switched back and forth virtually as often as
desired by varying the temperature. The driving forces for the so-called two-way shape
memory effect (2W-SME) in polymers are phase transitions between crystalline and
amorphous phases, supported by entropy elasticity as discovered by Mather’s group for
cross-linked poly(cyclooctene) films in the presence of an external load [19]. Later, the
programming technology was further developed and transferred to other polymer systems
so that actuation in the stress-free state became possible [20–23]. As known from
semicrystalline polyester urethane (PEU), actuation can also be very pronounced and
complies with the same physical principles [24]. The fact that polymers react to temperature
changes in their surroundings makes them interesting candidates for applications in which
complex switching and control electronics are to be avoided [25]. For example, in the field of
soft robotics, a specific subfield of robotics that deals with the construction of robots from
highly compliant materials [26–28], first steps were taken to demonstrate the attractiveness
of SMPs [29–32]. Anyway, there are only a few concepts in this respect, and the developments
are primarily based on the 1W SME [33–35]. On the other side, an example for the 2W-SME
was provided by Behl et al.[20], who employed chemically cross-linked poly(ω-
pentadecalactone) and poly(ε-caprolactone)-based polyester urethane to fabricate a gripper,
which after programming opened at 50 °C and closed at 0 °C. The system was able to grab
and release a small coin upon cooling and heating. In another work, Zhou et al.[36] introduced
a gripper from a chemically cross-linked poly(octylene adipate) and demonstrated how
programming enables the lifting and depositing of a small screw at temperatures between
10 °C and 36 °C. In both cases, the essential advantage of the two-way shape memory effect
becomes clear: autonomous motion. However, both studies focus mainly on the handling of
small and simple objects. They have in common that the selection of the lower actuation
temperature necessitates active cooling since it is below room temperature. In addition, the
complexity of actuation is not really significant.
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To address these problems, this contribution reports on the synthesis of a PEU with
promising thermal and mechanical properties. Furthermore, we describe a route of how to
identify pronounced actuation and used thermomechanical treatment to facilitate reliable
actuator functionality. On this basis, a novel system was built in which the actuator transfers
its motion to the stiff, mobile components of a gripper, thus opening the door to grab and
release bigger and more complex objects.
In order to pursue a new direction, a coupling of actuator elements with the elastic parts
of a mechanical unit cell or with a unit cell, which was completely built from one elastomer,
was realized. It will be demonstrated that such actuating unit cells can change their states as
a function of temperature, thus symbiotically combining actuation with unit cell
functionalities. The obtained shape morphing unit cells are considered as the first step toward
the production of novel, thermoresponsive metamaterials, which are assembled out of
periodically repeated unit cells that determine their physical properties [37]. In essence, it
will be demonstrated that the ability of the polymer to transfer its movements makes it
possible to produce completely new systems with programmable property profiles.
5.1.2. Results and Discussion
Poly(1,10-decylene adipate) diol (PDA) is a promising building block both in the chemistry
of polyester urethanes and polyester urethane ureas [25]. It was synthesized in the present
work in a polycondensation reaction from 1,10-decanediol and adipic acid (Figure 5.1.1).
Figure 5.1.1. Synthesis of poly(1,10-decylene adipate) diol via polycondensation reaction.
The number average molecular weight Mn of the PDA was determined to ~3290 g·mol–1,
and the calorimetric properties were characterized. The obtained polyester exhibits a melting
transition spreading from 55 °C to 77 °C with a peak temperature of 71 °C and a crystallization
transition ranging from 62 °C to 43 °C with a peak temperature located at 58 °C (Figure 5.1.2).
Thus, both phase transition temperatures were well above room temperature. As expected,
the assigned phase transitions were in good agreement with those of other aliphatic
polyesters [38].
Following the prepolymer method [39–41], the freshly synthesized PDA was brought to
reaction with 4,4′-diphenylmethane diisocyanate (4,4′-MDI) in order to build up an
138 | Chapter 5.1
isocyanate-endcapped prepolymer, before the chain extender 1,4-butanediol (BD) was finally
added. This resulted in the formation of a polyester urethane (PEU, Figure 5.1.3).
Figure 5.1.2. Differential scanning calorimetry (DSC) thermogram of poly(1,10-decylene adipate) diol
showing the second heating and cooling with temperature rates of 10 °C·min–1. The enthalpies of
melting ΔHm (red dashed area) and crystallization ΔHc (blue dashed area) are included.
Figure 5.1.3. Synthesis of poly(1,10-decylene adipate) diol (PDA)-based polyester urethane (PEU) via
polyaddition reaction using the prepolymer method.
Herein, it is obvious that the so-called soft segment is composed of the synthesis building
block PDA while the hard segment was obtained from the reaction of MDI and BD. Fourier-
transform infrared (FT-IR) spectroscopy was used to verify the completeness of the
polyaddition reaction. Since the characteristic vibrational modes for polyester urethane were
visible and, in addition, only a very weak signal associated with freely available isocyanate
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appeared in the spectrum, the reaction was mostly complete. A detailed analysis can be found
in the supplementary material (Figure S5.1.16.).
Once characterized, the PEU was melt-extruded into a filament as essential for further
processing via fused filament fabrication (FFF). For this purpose, the same extrusion line was
used as reported recently [42]. The obtained filament had a homogenous diameter of 2.85 ±
0.08 mm, so that an important attribute for further processing was fulfilled. Tensile bars of
type 5B according to ISO 527-2:1996 [43] were then additively manufactured, and their
mechanical behavior was determined in tensile tests at ambient temperature (Figure 5.1.4).
From the stress–strain relationship, an averaged Young’s modulus of 258.7 ± 10.2 MPa was
determined. In all three measurements, a yield-point occurred at a strain value of 17 ± 3%,
corresponding to a stress of 7.1 ± 1.4 MPa (Figure 5.1.4a). Further increasing the load resulted
in necking (Figure 5.1.4b, images 2–4) as already verified for similar materials [44–46] and
strain softening first, followed by strain hardening as accompanied by an increase in stress,
culminating in specimen rupture at strains of 1405 ± 83%. The material behavior can be
explained with the coexistence of two types of PDA segments, at which one part was highly
flexible and amorphous while the other one was rigid and crystalline. In the course of
deformation, a progressive conversion from amorphous to crystalline segments seemed to
occur. The assumption was supported by a whitish coloring of the tensile bar as associated
with a crystallization process (Figure 5.1.4b, image 4).
Figure 5.1.4. Mechanical characterization of PDA-based PEU using tensile tests: engineering stress–
strain curves (a) and the associated deformation behavior (b). The experiments were carried out on
tensile bars at 23 °C with an initial strain rate of 1%·min–1 until 5% of strain were reached and continued
with 2000%·min–1 until rupture occurred.
140 | Chapter 5.1
Having the good mechanical properties of the PEU in mind, differential scanning
calorimetry (DSC) and dynamic mechanical analysis (DMA) were used to study the thermal
phase transition behavior (Figure 5.1.5). This ensured that later thermomechanical tests were
performed at appropriate temperatures. The calorimetric properties of the PEU were
characterized by a broad melting transition between 29 °C and 72 °C and a crystallization
transition spreading from 52 °C to 24 °C (Figure 5.1.5a), which were assigned to the phase
transitions of PDA. In comparison with the measurement data provided for pure PDA (Figure
5.1.2), an extension of both phase transitions toward lower temperatures could be verified
together with lower enthalpies of melting and crystallization. An inhibited crystallization
behavior was expected because of the lower content of polyester polyol and phase
segregation effects [47]. In the network structure of the PEU, the hard segments were
efficiently acting as net points and were thus reducing the crystallinity of the soft segment as
verified earlier for similar PEUs [48,49]. Apart from that, the elastic behavior as exemplified
by the evolution of the storage modulus E’ exhibited a two-step decrease in the DMA (Figure
5.1.5b). This observation can be traced back to the consecutive devitrification and melting of
the PDA phase. Indeed, such behavior is commonly observed for semicrystalline PEUs [50–
52]. The presence of the hard segments acting as netpoints ensured that the PEU was still
characterized by dimensional stability at temperatures exceeding the melting transition of
the PDA phase at approximately 65 °C. In turn, the loss modulus E´´, which is a measure for
the viscous response of a polymer, exhibited a broad signal with a maximum at about −14 °C,
which declined upon heating until a plateau formed at temperatures above 65 °C. The tan δ
peak was located at 7 °C (Figure 5.1.5b). It is defined as the ratio between loss modulus E´´
and storage modulus E´ and is often used to determine the glass transition temperature Tg in
urethane-based polymers [53,54].
In the next step, a method for programming the 2W-SME was developed in line with the
thermal and mechanical properties of the PEU. Figure 5.1.6 shows the evolution of the stress–
strain progression for the thermomechanical treatment of a type 5B tensile bar [43], which
was obtained from FFF.
In accordance with the thermal behavior of the PEU, the tensile bar was heated to 75 °C
and kept there for 20 min in order to ensure that all PDA crystals were molten before the
polymer was elongated. In comparison to the stress–strain behavior verified in Figure 5.1.4,
no yield point could be detected during elongation (Figure 5.1.6), and a significantly lower
Young’s modulus of 1.4 ± 0.1 MPa was determined. A similar temperature dependence in
stress–strain behavior is known from other physically cross-linked polymers like poly(1,4-
butylene adipate)-based PEU [46]. However, to assure that the polymer chains were aligned
in a highly oriented state, the strain was kept constant at a value of 700%, and the sample
was slowly cooled to 23 °C in order to enable the extensive crystallization of the PDA phase
before unloading was carried out. Although not verified in another experiment, the
programming of the 2W-SME appeared to be successful even at a much shorter crystallization
time. This finding can be deduced from the inset of Figure 5.1.6, which shows that most of
the stress relaxation took place immediately after stretching and that the recorded stress did
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not change significantly after a holding time of 100 min. Upon unloading, the polymer
stabilized at a strain of 695%, which was close to the maximum strain applied.
Figure 5.1.5. Thermal and thermomechanical properties of PDA-based PEU as determined by DSC (a,
second heating and cooling with temperature rates of 10 °C·min–1, the enthalpies of melting ΔHm, red
dashed area, and crystallization ΔHc, blue dashed area, are included) and DMA (b, the temperature
dependence of storage modulus E’, loss modulus E’’ and loss factor tan δ at a heating rate of 3 °C·min– 1).
Figure 5.1.6. Stress–strain behavior of PDA-based PEU when programming the 2 W SME. The individual
steps consisted of elongation at Td = 75 °C (red color, strain rate = 300%·min–1) followed by slow cooling
and unloading at 23 °C (blue color, unloading rate = 1 N·min–1). The inset shows the evolution of stress
over time for a longer period before unloading was carried out.
Afterward, a DMA measurement was carried out on a sample, which was
thermomechanically treated as reported before (Figure 5.1.6). The aim was to identify an
ideal scenario for stress-free actuation by systematically varying the considered temperature
range. The results are supplied in Figure 5.1.7.
142 | Chapter 5.1
Figure 5.1.7. Influence of the selection of Tmax on actuation of PDA-based PEU under stress-free
conditions: (a) evolution of strain ε and sample length L (differently colored according to the
temperature intervals investigated) and temperature T with measuring time t for a sample deformed at
Td = 75°; (b) strain–temperature relationship to the experiment shown in (a) using the same color codes;
(c) evolution of thermoreversible strain εrev depending on Tmax for two deformation temperatures, the
values for εrev are averaged for the second and third cycle.
Thermoreversible strain changes, here also denoted as actuation, could be detected in
every single measurement cycle, even when continuously raising the upper temperature Tmax
from 30 °C to 75 °C while keeping the lower temperature at 15 °C (Figure 5.1.7a,b). Apart
from entropy elasticity, the phenomena of melting-induced contraction (MIC) and
crystallization-induced elongation (CIE) were probably the main driving forces for
actuation [19]. In fact, only a small quantity of crystallizable segments was present when
selecting a lower maximum temperature Tmax because a larger part of the PDA phase was still
in a crystalline state, resulting in weak elongation on cooling and weak contraction on heating,
which can particularly be seen in the strain-temperature diagram of Figure 5.1.7b (black
lines). By contrast, the successive increase in Tmax led first of all to an increase in the
proportion of crystallizable segments because more PDA crystals were molten. In parallel, a
hysteresis behavior was observed, and actuation substantially increased. The hysteresis was
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first observed when selecting a Tmax of 55 °C (Figure 5.1.7b, dark green color). Further
increasing Tmax to 65 °C culminated in the most pronounced hysteresis (Figure 5.1.7b, orange
color) with a maximum change in thermoreversible strain εrev of 16.1%. This becomes
particularly clear in the associated εrev/Tmax diagram (Figure 5.1.7c, solid line). Interestingly,
the value for Tmax corresponded exactly with the melting peak temperature of the PDA phase
in the DSC measurement (Figure 5.1.5a). The further increase of Tmax gave a decrease in
actuation since the systematic melting of PDA crystals resulted in strain recovery of the PEU
and thus a lower overall strain. In other words, the elongation at the beginning of each cooling
step was gradually shifted to smaller values. Obviously, under these conditions, highly
oriented crystals serving as netpoints were molten and could therefore no longer support the
structural integrity associated with the respective morphological states of the polymer.
Following a different approach, the programming route was modified by raising the
deformation temperature Td to 85 °C, which was even further above the offset melting
temperature of the crystalline PDA phase (Figure 5.1.5a). This time, a shift in the temperature
range, in which actuation took place, could be witnessed (Figure 5.1.7c, dotted line). Indeed,
the temperature region of actuation was raised together with the temperature, at which the
maximum actuation occurred. Especially, in this case, we assume that PDA crystallites of
higher temperature stability could be introduced in the course of deformation or directly
afterward. These crystals obviously ensured the stability of the actuating polymer at elevated
temperatures, namely in actuation states up to a temperature of about 80 °C. The rise in
maximum actuation temperature at higher deformation temperature indicates that the
temperature window of actuation can be adjusted by varying the deformation temperature
and thus the programming conditions.
To investigate the durability of actuation under stress-free conditions, a sample of PEU was
programmed (Td = 75°) as described above and subjected to 100 heating-cooling cycles in the
DMA with maximum and minimum temperatures of 64 °C and 15 °C, respectively (Figure
5.1.8).
Figure 5.1.8. DMA measurement to determine the durability of actuation for PDA-based PEU: evolution
of (a) nominal strain ε with time t and (b) thermoreversible strain εrev with cycle number N.
144 | Chapter 5.1
In the beginning, the development of strain over time exhibited a strong drop in strain
(Figure 5.1.8a), which was attributed to the melting of highly oriented PDA crystals. In
particular, in the first five cycles, the actuation showed a drop (Figure 5.1.8b), which was
presumably caused by rearrangements of polymer chains [55]. A more stable actuation was
then observed; after about 25 cycles of heating and cooling, the actuation was nearly the
same. Apparently, the PEU formed two temperature-bistable states, which differed in their
elongation. In the end, εrev approached an almost constant value of 12%.
To take advantage of actuation, a suitable design for an actuator element made of PEU was
developed, and possibilities for implementation into a gripping technology were explored
using the linkage mechanism of Gholaminezhad et al. [56]. The corresponding computer-
aided design (CAD) drawing of the gripper is shown in Figure 5.1.9. An important aspect of
the design was that the base material of the gripper exhibited good mechanical stability,
especially in the temperature range of actuation. Therefore polyethylene terephthalate glycol
(PET-G) was selected as a rigid base material (Figure 5.1.9a–g) [57], while the centerpiece of
the gripper, namely the actuator element, was made of thermomechanically pretreated PEU
(Td = 75 °C, Figure 5.1.9h). The design of the gripper was chosen so that one millimeter of
actuation was able to trigger a sixteen-fold increase in distance of the two gripper arms
(Figure 5.1.9e). The diameter of the gripper element (Figure 5.1.9g) was set to 35 mm in order
to enable that an object with the size of a hen´s egg could be reliably gripped. All parts
exhibited in Figure 5.1.9 were manufactured via FFF. After rapid prototyping, the moving
parts were assembled (Figure 5.1.9i,j), and care was taken to avoid friction between the
individual components.
The main part of the gripper (exhibited in Figure 5.1.9a) was constructed to hold the parts
of the system either directly or indirectly. Once the actuator element with the PEU being in
its low-temperature stable state was installed between the upper actuator holder and the
lower actuator- and linkage holder (shown in Figure 5.1.9b,c), the actuation capability was
studied (Figure 5.1.10). In this particular case, the minimum temperature was increased from
15 °C to 23 °C compared to the previous experiments in order to demonstrate a higher degree
of practical suitability while the maximum temperature was kept constant at 64 °C.
As expected, the movement of the PEU could be transferred into motion perpendicular to
the actuation direction in the form of a thermoreversible opening and closing of the clamps.
Taking advantage of this behavior, a hen´s egg could be picked up, transported and deposited
without causing any damage (Figure 5.1.10, s5.1.1.movie in supplementary material).
In detail, raising the temperature to 64 °C resulted in the contraction of the PEU, which
pulled the lower actuator- and linkage holder (exhibited in Figure 5.1.9c) and the linkage bar
(exhibited in Figure 5.1.9d) upwards, thereby opening the gripper arms (shown in Figure
5.1.9e) with the attached egg holder (shown in Figure 5.1.9g). At this time, the gripper system
was positioned near to the egg (second image in Figure 5.1.10). On cooling to 23 °C, the
expansion of the PEU pushed the lower actuator and linkage holder (Figure 5.1.9c) and the
linkage bar (Figure 5.1.9d) downwards, resulting in a closing of the gripper arms and the
gripping of the egg. At this moment (middle of Figure 5.1.10), the object could be lifted at
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ambient temperature and taken to the desired location for further handling. On heating back
to 64 °C, the gripper arms opened again, thus enabling the safe unloading of the fragile object
(right side of Figure 5.1.10). Basically, the gripper system should be capable of repeated
gripping and opening in a large number of cycles as indicated by the durability measurement
(Figure 5.1.8).
Figure 5.1.9. Technical drawing of a gripper: (a) main part; (b) upper actuator holder; (c) lower actuator-
and linkage holder; (d) linkage bar; (e) gripper arm; (f) gripper holding part; (g) egg holder; (h) actuator
element; (i) top view of assembled gripper arrangement; and (j) isometric view of gripper system. All
data are provided in mm.
Figure 5.1.10. Gripper with PEU actuator element in operation, enabling the transport of a hen´s egg
with a height of 55.5 mm, a width of 39 mm and a weight of 58 g. The lifting and lowering of the whole
gripper were done manually.
146 | Chapter 5.1
In a fundamental approach, PEU actuator elements were integrated into distinct unit cells
to obtain thermoresponsive programmable materials. Regarding the first concept, an
actuator element made of PEU was fixed with glue on the surface of a unit cell to investigate
to what extent its motion can be transferred to the cell. The unit cell was designed to allow
larger deformation amplitudes between the temperature-dependent states of the PEU
(Figure 5.1.11).
Figure 5.1.11. Unit cell consisting of an elastomeric material (Desmopan® 9370AU, red color) and a stiff
material (PET-G, black color): Technical drawing (left, all data are provided in mm.), additively
manufactured unit cell (in the middle) and state after installing the actuator element (whitish color,
right). Centimeter paper was used as a base in the second and third image.
The cell was built up by the same PET-G as used for most parts of the gripper (black color)
and by an elastic base material, here another thermoplastic polyurethane (red color), to
which the PEU was attached with adhesive. The cell consisted of two center beams (black
color), providing stability for outside buckling beams (red color). The cross beams were
connecting all elements. This configuration of buckling beams can be used to provide
metastable states, either being popped out or switched through. In this example, the beams
were designed. Hence, in combination with the PEU actuator, the switched through the state
could be reached at elevated temperature. Basically, such unit cells can be employed to
reversibly absorb energy [58] or to implement mechanical memory behavior in materials [3].
Upon heating and cooling between 23 °C and 64 °C, the actuator element was able to
transfer its motion to the unit cell, resulting in a change of its structure with regard to the
elastomer parts (Figure 5.1.12).
Figure 5.1.12. Thermoresponsiveness of a unit cell with a PEU actuator element. Centimeter paper was
used as a base.
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The PEU contracted on heating to 64 °C. As a result, the flexible parts of the unit cell were
pulled together, which resulted in the formation of a second state whose stability was defined
by the length of the actuator element (Figure 5.1.12, middle image). Cooling back to 23 °C
was accompanied by the expansion of the PEU, leading to the return of the cell into its initial
state (Figure 5.1.12, right image). This stretch-dominated approach makes clear that the
movement of the actuator element could easily be transferred to the elastic parts of a cell,
while the stiff parts of the cell remained unaffected. Thus, it could be demonstrated that
combining a PEU actuator element with a unit cell allows switching back and forth between
mechanical states, which differ in the shape of the elastic material.
As the last example, a macroscopic prototype of a unit cell for programmable wetting was
produced. Normally such cells are roughly 100 µm high to work properly [59]. Nevertheless,
a macroscopic prototype can be easily tested and scaled-down in a possible next step. The
selected unit cell consisted of an outer cage providing potential contact to neighboring cells
and mechanical stability for the inner mechanism. The outer cage contained a hole through
which the spike can be pushed, leading to a strongly reduced contact area between the unit
cell and, e.g., a drop of liquid on top (not shown here). The inner mechanisms allowed again
two metastable states, similar to the previous case, with the spike being in or out. Therefore,
a three-dimensional unit cell was manufactured from the same elastic base material as used
above to autonomously open or close a hole in its surface depending on temperature (Figure
5.1.13). To achieve this, thermomechanically treated PEU was bent manually and attached to
the unit cell with glue (Figure 5.1.13, right).
Figure 5.1.13. Unit cell with switchable surface topography. Technical drawing (left, all data are provided
in mm.), unit cell after additive manufacturing with Desmopan® 9370AU (middle) and unit cell as
assembled with the PEU actuator element (right). Centimeter paper was used as a base in the second
and third image.
Figure 5.1.14 shows the actuation of the unit cell upon twofold heating and cooling. First,
the increase in temperature from 23 °C to 64 °C led to the contraction of the PEU, thus
inducing an external strain on the unit cell and lifting the spike through the opening at the
top plane. In parallel, the unit cell itself also changed its structure by broadening. In turn,
cooling to 23 °C resulted in the expansion of the PEU actuator element, which led to the
relaxation of the unit cell and thereby the retraction of the spike and the thinning of the cell.
Following this bend-dominated approach, a proof of principle for the ability of the PEU to
148 | Chapter 5.1
alter the structure of a complex unit cell and thereby the surface of its upper site could be
achieved. A time-lapse video of unit cell actuation is provided as supplementary material
(s5.1.2.movie). The precise opening and closing of the hole in the surface were verified for in
total four thermal cycles. However, due to the good durability of actuation (Figure 5.1.8), we
assume that also, in this case, considerably more cycles can be run through.
Figure 5.1.14. Thermoresponsiveness of a unit cell with a PEU actuator element. The system was cycled
twice between 23 °C and 64 °C (upper row: front view, centimeter paper was used as background paper;
lower row: isometric view, the white box was drawn in to illustrate the shape change in the surface).
5.1.3. Materials and Methods
Materials: 1,10-Decanediol, 4,4′-methylene diphenyl diisocyanate (4,4′-MDI) and
titanium(IV) isopropoxide (TTIP) were purchased from Fisher Scientific (Schwerte, Germany).
For titration tests, acetic anhydride, methanol and potassium hydroxide solution in methanol
with concentrations of 0.5 mol–1 and 0.1 mol–1 were purchased from Merck (Darmstadt,
Germany). N-Methyl-2-pyrrolidone (2-NMP), chloroform and 4-dimethylaminopyridine (4-
DMAP) were bought from Carl Roth (Karlsruhe, Germany). Adipic acid, 1,4-butanediol and a
molecular sieve (4 Å) were obtained from Alfa Aesar (Kandel, Germany). A filament from
polyethylene terephthalate glycol (PET-G) was purchased from FilamentWorld (Neu-Ulm,
Germany). The ether-based thermoplastic polyurethane elastomer Desmopan® 9370AU was
supplied by Covestro Deutschland AG (Leverkusen, Germany). The polymer is characterized
by a Shore A hardness of 70 [60] and was used as a flexible component in our unit cell
approaches.
Synthesis of Polyester Diol: 1,10-Decanediol and adipic acid were mixed at a molar ratio of
1.1:1 and heated in a three-necked round-bottomed flask, which was equipped with a
mechanical stirrer, nitrogen gas inlet and distillation condenser. All reactants were molten at
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about 150 °C while titanium(IV) isopropoxide was added under stirring. Adjacently, the
mixture was heated to 190 °C. After a remarkable decrease in distillation temperature, the
mixture was further heated to 210 °C, whereupon the pressure was reduced to approximately
20 mbar. After two hours of continuous stirring, the melt was poured into a can. The obtained
poly(1,10-decylene adipate) diol (PDA) solidified and was analyzed before PEU synthesis was
carried out.
Titration: Titration was used to determine both the acid value and hydroxyl value and thus
the number average molecular weight Mn of PDA. Therefore, a TitroLine 7000 from SI
Analytics (Mainz, Germany) was employed. The procedure was executed in compliance with
DIN EN ISO 2114 and 4629-2 [61,62]. To determine the acid value, a sample of PDA was
dissolved in a mixture of chloroform/methanol with a volume ratio of 5: 1. The solution was
titrated against a potassium hydroxide solution in methanol, having a concentration of
0.1 mol–1. For the determination of the hydroxyl value, another sample of PDA was dissolved
in chloroform. After adding acetic anhydride diluted in 2-NMP as well as 4-DMAP diluted in
2- NMP, the solution was heated and kept under stirring at 60 °C for 15 min. Thereafter,
deionized water was added. After 12 min, the sample solution was titrated against a
potassium hydroxide solution in methanol, having a concentration of 0.5 mol–1.
Synthesis of Polyester Urethane: A polyester urethane (PEU) was synthesized using the
prepolymer method. In order to obtain a PEU with approximately 15% of hard segment
content, the molar ratio of the reactants was set to 1:1.98:0.97with regard to PDA, 4,4′-MDI
and 1,4-butanediol, respectively. The reaction was carried out with a slight excess of
isocyanate (NCO/OH = 1.005). Overnight, PDA was dried in a glass reactor in a vacuum oven
at 90 °C. The following day, it was heated under nitrogen flow and stirring to 120 °C.
Adjacently, isocyanate was added, and the mixture was continuously stirred for 90 min. The
obtained prepolymer was directly converted to PEU by adding 1,4-butanediol, serving as a
chain extender. In parallel, the stirring speed was raised. As the viscosity increased
significantly, the reaction was stopped, and the polymer melt was poured onto a plate
covered with polytetrafluoroethylene. Finally, the PEU was cured in an oven for 120 min at
80 °C.
Fourier-Transform Infrared Spectroscopy: The synthesized PEU was investigated by
Fourier-transform infrared (FT-IR) spectroscopy. The measurements were carried out with a
Nicolet Nexus 470/670/870 FT-IR spectrometer from Thermo Fisher Scientific (Madison, WI,
USA). The spectrometer was equipped with an attenuated total reflectance (ATR) device. In
total, 40 scans with a spectral resolution of 2 cm–1 were averaged to give the spectrum from
4000 cm–1 to 650 cm–1.
Extrusion: The synthesized PEU was ground with a cutting mill type M 50/80 from Hellweg
Maschinenbau (Roetgen, Germany). The obtained flakes were dried at 110 °C for 150 min in
a vacuum drying chamber VDL 53 from Binder GmbH (Tuttlingen, Germany). Subsequently,
the flakes were fed into an extrusion line to produce filaments. A schematic drawing of the
extrusion line is shown in Figure 5.1.15.
150 | Chapter 5.1
Figure 5.1.15. Technical drawing of an extrusion line as used for the production of PEU filaments:
material feeding system (A), twin-screw extruder (B), conveyor belt (C) and filament winding machine
(D). The extrudate is drawn in gray color.
The individual units of the extrusion line were put together in such a way that it included
the volumetric material feeding system Color-exact 1000 from Plastic Recycling Machinery
(Tranekær, Denmark), a Leistritz twin screw extruder MICRO 18 GL from Leistritz AG
(Nürnberg, Germany), characterized by seven heating zones and a screw length of 600 mm, a
conveyor belt, and a filament winder from Brabender GmbH and Co. KG (Duisburg, Germany).
The temperatures of the individual heating zones of the extruder were 170 °C, 175 °C, 180 °C,
185 °C, 195 °C, 190 °C and 190 °C. To evaluate the quality of the filaments, the evolution in
diameter was manually detected at regular intervals using a vernier caliper from Fowler High
Precision (Newton, MA, USA).
Virtual Design and Fused Filament Fabrication: The online 3D modeling tinkercad.com [63],
which is a web-based computer-aided design (CAD) program, was used for virtual
construction. The developed CAD models were exported as standard triangle language (STL)
files and later used for slicing.
After finalizing the designs of the tensile bar, actuator element, gripper and unit cells,
Cura 3.6.1 [64] was used as a slicer program to generate numerically controlled codes, also
denoted as G-codes. The 3D models were imported into the slicer program, and the models
were sliced into layers according to the predefined printing parameters (Table 5.1.1).
Table 5.1.1. Printing parameters selected for additive manufacturing with different
materials.
PET-G
PEU
Desmopan® 9370AU
Diameter of the nozzle (µm)
400
400
400
Temperature of the nozzle (°C)
235
208
225
Speed of print head (mm·s–1)
50
15
25
Build platform temperature (°C)
85
75
60
Layer height (mm)
0.1
0.1
0.1
The specimens, as well as the actuator element for the gripper and for the different types
of unit cells, were printed with the synthesized PDA-based PEU. Polyethylene terephthalate
| 151
glycol (PET-G) was used to produce the rigid parts of the robotic gripper and one of the unit
cells, while the thermoplastic polyurethane elastomer Desmopan® 9370AU served as the
elastic base material in both types of unit cells. The most relevant settings for additive
manufacturing are listed in Table 5.1.1. To start additive manufacturing, the generated G-
codes were transferred to the 3D printer. All 3D printed objects were produced by fused
filament fabrication (FFF) using the commercially available 3D printer Ultimaker 3 from
Ultimaker B.V. (Utrecht, The Netherlands).
Tensile Tests: The mechanical behavior of the synthesized PEU was investigated with the
universal testing machine Criterion Model 43 from MTS (Eden Prairie, MN, USA). The device
was equipped with a 500 N-load cell. The measurements were performed in compliance with
DIN EN ISO 527 [43] by using dog-bone shaped tensile bars of type 5B as obtained from FFF.
In the course of stretching, the velocity of 1%·min–1 was kept constant until a total strain of
5% was achieved before the specimen was further elongated with a velocity of 2000%·min–1
until rupture occurred. The Young’s modulus, which is defined as the slope of stress-strain
evolution between two stress-strain points during elastic loading, was determined by linear
regression for strain values below 2%. Every tensile test was repeated three times at ambient
temperature.
Characterization of Thermal Properties: The phase transition behavior of PDA was
characterized by differential scanning calorimetry (DSC) using a Q100 DSC from TA
Instruments (New Castle, DE, USA). The measurements were conducted both with the
synthesized PDA and PEU. In the case of the latter, the center part of an additively
manufactured type 5B tensile bar [43] was examined. In general, samples with a weight of 5
mg were investigated.
The synthesized PDA was initially cooled to −30 °C before it was heated to 110 °C and
cooled back to–30 °C. The thermal cycle was repeated to finalize the measurement. In
contrast, a sample of PEU was thermally cycled between −50 °C and 225 °C. For cooling and
heating, a rate of 10 °C·min– 1 was applied. The temperature holding time at the minimum
and maximum temperatures were 2 min.
Characterization of Thermomechanical Properties: The thermomechanical properties of
the PEU were studied by dynamic mechanical analysis (DMA). The experiments were carried
out with a Q800 DMA from TA Instruments (New Castle, DE, USA) using film tension clamps
on multi-frequency–strain mode. A frequency of 10 Hz, a static force of 0.1 N and an
oscillating amplitude of 10 µm were selected to investigate the center part of an additively
manufactured type 5B tensile bar [43]. At first, the sample was cooled to −80 °C and held
there for 5 min, before it was heated to 100 °C with a rate of 3 °C·min–1. In parallel, the
evolution in storage modulus E´ and loss factor tan δ was determined.
Programming and Characterization of 2 W-SME: The programming of PEU samples was
conducted with an MTS Criterion universal testing machine (model 43) from MTS Systems
Corporation (Eden Prairie, MN, USA). The device was equipped with a 500 N-load cell and was
operated with a temperature chamber, which was controlled by a Eurotherm temperature
controller unit. Two heating elements were located at the back of the chamber. Liquid
152 | Chapter 5.1
nitrogen from a Dewar’s vessel was fed into the chamber under a pressure of 1.3 bar as an
essential prerequisite for cooling. At the beginning of thermomechanical treatment, the
thermochamber was preheated to the deformation temperature Td, which was either 75 °C
or 85 °C, and the specimen was fixed in the pneumatic clamps of the universal testing machine
using a clamping air pressure of 0.4 bar. After 20 min at Td, the specimen was elongated to a
maximum tensile strain of 700% using a rate of 300%·min–1. In the next step, the heating
elements of the temperature chamber were switched off, whereupon the specimen cooled
down slowly to 23 °C while the clamping distance was kept constant. After unloading with a
rate of 1 N·min–1, the sample was removed from the clamps.
The Q800 DMA from TA Instruments (New Castle, DE, USA) was utilized to investigate the
two-way shape memory properties of the PEU. For this purpose, a cuboid centerpiece of a
specimen, which was thermomechanically treated as described above (Td = 75 °C) and
characterized by a dimensioning of 25 mm × 0.9 mm × 0.8 mm, was cut out and fixed in the
film tension clamps of the DMA device. The actuation of the PEU was adjacently studied under
stress-free conditions by cycling the temperature. Against this background, a first test series
was run, in which the maximum temperature Tmax was systematically varied, and a constant
minimum temperature Tmin of 15 °C was selected. For this purpose, a sample, which was
thermomechanically treated as described above (Td = 75 °C), was heated from 23 °C to
Tmax = 30 °C and held there for 15 min, before it was cooled to 15 °C, at which the temperature
was kept for another 15 min. The heating and cooling were carried out three times.
Afterward, the three cycles were repeated for each temperature Tmax between 30 °C and 75 °C
with an increment of 5 °C. Heating and cooling rates of 5 °C·min–1 were used for all
experiments conducted.
In another approach, a cutout of a specimen, which was deformed at 85 °C, was
investigated under the same conditions, but with maximum temperatures Tmax between 30 °C
and 90 °C.
In a durability experiment, a cutout of a thermomechanically treated PEU specimen
(Td = 75 °C) was studied under stress-free conditions by cycling the temperature between
15 °C and 64 °C. This time, actuation was investigated in 100 thermal cycles.
The reversible strain εrev is the key parameter when studying the actuation of polymers. It
can be defined according to Equation 5.1.1):
𝜀𝑟𝑒𝑣(𝑁)= 𝑙𝑙𝑜𝑤(𝑁)−𝑙𝑚𝑎𝑥(𝑁)
𝑙𝑚𝑎𝑥(𝑁) × 100%
(5.1.1)
Herein, llow (N) and lmax (N) are the lengths of the specimen in the Nth cycle of actuation at
the respective temperatures Tlow and Tmax.
Demonstrator Development: Samples of PEU were additively manufactured by means of
FFF, characterized by a size of 28 mm × 7 mm × 5 mm. Subsequently, they were
thermomechanically treated with the procedure described in section 5.1.3.11 (Td = 75 °C) to
obtain the desired actuator elements. The actuation of the gripper was studied in the
temperature chamber of our MTS Criterion universal testing machine (model 43) from MTS
Systems Corporation (Eden Prairie, MN, USA). Solvent-free superglue from Pattex [65] was
| 153
used to attach the actuator elements to the unit cells. For functional testing, the unit cells
were characterized similarly as the gripper in the temperature chamber of our universal
testing machine. To better visualize actuation, the unit cells were placed on a platform, which
was lined with centimeter paper. Both in case of the gripper and the unit cells, the
temperature was cycled in between 23 °C and 64 °C with heating and cooling rates of
5 °C·min–1.
5.1.4. Conclusions
A novel polyester urethane (PEU) was synthesized. After processing and
thermomechanical treatment, thermoreversible shape changes could be witnessed. The PEU
was used as an actuator element in a gripper, which was designed to precisely convert the
comparatively small change in the shape of a few millimeters into a macroscopically well
visible and technically relevant motion. The cautious gripping, holding and releasing of a hen’s
egg qualified the gripper for applications in soft robotics. Compared to grippers made entirely
of shape memory polymer, the introduced concept has the advantage that the materials that
come into contact with an object to be gripped can be freely selected according to the design
of the gripper and that predefined movements can be carried out. Hence, in the end, a high
degree of system control is possible. Future developments in gripper design are able to
expand the range of possibilities, e.g., to grip even more challenging and bigger objects. The
design space thus created allows the production of completely new systems with
programmable gripping, holding and releasing properties.
Moreover, the implementation of the PEU actuator into macroscopic unit cells with elastic
components led to programmable materials, which moved autonomously as a function of
temperature. It is precisely this behavior that can initiate a paradigm shift in the future, in
which the programming of material is understood as the programming of a functionality. The
internal structure of materials is such that the material properties and behavior change
reversibly according to a program. This is achieved by programming the reaction of the
material to temperature signals into the material structure. In this way, completely new
components with specific properties can be produced, which can be used in a wide variety of
contexts. Considering that such concepts require neither control electronics nor cables or
other technical devices, the self-sufficient material behavior is all the more promising.
Supplementary Materials: The following materials are available online, Videos: s5.1.1.movie:
Actuation of the PEU-based gripper system (Figure 5.1.10) and s5.1.2.movie: Actuation of
PEU to achieve unit cell morphing (Figure 5.1.14). Figure S5.1.16: FT-IR spectrum of PDA-
based PEU.
Author Contributions: Conceptualization, T.P., D.C., and D.S.; methodology, D.S., D.C., F.W.,
and M.S.; validation, D.S. and D.C.; formal analysis, D.C. and D.S.; investigation, D.S. and D.C.;
154 | Chapter 5.1
writing—original draft preparation, D.S., D.C., F.W. and M.S.; writing—review and editing, T.P.
and C.E.; visualization, D.C. and D.S.; supervision, T.P.; project administration, T.P. and C.E.
Funding: This research was funded by Fraunhofer Excellence Cluster “Programmable
Materials”, grant number 630500.
Acknowledgments: This work was supported as Fraunhofer Excellence Cluster
“Programmable Materials” under project 630500. M.S. and C.E. acknowledge financial
support from Cluster of Excellence livMatS, University of Freiburg. T.P. wishes to thank the
European Regional Development Fund for financing a large part of the laboratory equipment
(project 85007031). Tobias Rümmler is kindly acknowledged for delivering his ideas for
demonstrator development and carrying out the DMA measurements.
Conflicts of Interest: The authors declare no conflicts of interest.
Sample Availability: Samples of the compounds are provided by the authors in special cases.
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62. 2016. Deutsches Institut für Normung e.V. Bindemittel für Beschichtungsstoffe - Bestimmung der
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November 2020).
5.1.6. Supplementary Material
5.1.6.1. Fourier-Transform Infrared (FT-IR) Spectroscopy
Normally, a distinct signal at about 2270 cm–1 occurs in the FT-IR spectrum of
polyurethanes in case of an incomplete reaction as associated with freely available isocyanate
groups [1,2]. In the present case, the reaction seemed to be almost complete because the
signal at 2283 cm–1 was very weak, while at the same time, vibration modes were present as
characteristic for the formation of a polyester urethane (Figure S5.1.16) [3].
Figure S5.1.16. FT-IR spectrum of poly(1,10-decylene adipate) diol (PDA)-based polyester urethane
(PEU), including the assignment of vibration modes and the specification of their wavenumbers. The
inset highlights the carbonyl stretching vibration region.
In fact, a more detailed analysis of the FT-IR spectrum showed distinct absorbances at
2916 cm–1 and 2848 cm–1, which can be assigned to the asymmetric and symmetric stretching
vibrations of CH2 entities [2,4,5]. Two overlapping bands are present in the carbonyl
stretching region – one dominating peak apparently centered at 1730 cm–1 and a broad
shoulder at 1703 cm–1. The two signals are ascribed to the stretching vibrations of free (non-
158 | Chapter 5.1
hydrogen-bonded) and hydrogen-bonded carbonyl, respectively [3,6–8]. The band at
1171 cm–1 was assigned to the corresponding ν[C–O] stretching vibrations [4,9,10]. The
stretching vibration ν[N–H] at 3300 cm–1 and an amide peak (ν[C=N] + δ[N–H]) at 1531 cm–1
were attributed to vibrations associated with the hard segments of the PEU [11,12].
5.1.6.2. References
1. Sáenz-Pérez, M.; Lizundia, E.; Laza, J.M.; García-Barrasa, J.; Vilas, J.L.; León, L.M. Methylene diphenyl
diisocyanate (MDI) and toluene diisocyanate (TDI) based polyurethanes: thermal, shape-memory and
mechanical behavior. Rsc Adv. 2016, 6, 69094–69102, doi:10.1039/c6ra13492k.
2. Tan, C.; Tirri, T.; Wilen, C.-E. Investigation on the Influence of Chain Extenders on the Performance of
OneComponent Moisture-Curable Polyurethane Adhesives. Polym. (Basel) 2017, 9,
doi:10.3390/polym9050184.
3. Pretsch, T.; Jakob, I.; Müller, W. Hydrolytic degradation and functional stability of a segmented shape
memory poly(ester urethane). Polym. Degrad. Stab. 2009, 94, 61–73,
doi:10.1016/j.polymdegradstab.2008.10.012.
4. Panwiriyarat, W.; Tanrattanakul, V.; Pilard, J.-F.; Pasetto, P.; Khaokong, C. Effect of the diisocyanate
structure and the molecular weight of diols on bio-based polyurethanes. J. Appl. Polym. Sci. 2013, 130,
453–462, doi:10.1002/app.39170.
5. Lei, W.; Fang, C.; Zhou, X.; Cheng, Y.; Yang, R.; Liu, D. Morphology and thermal properties of polyurethane
elastomer based on representative structural chain extenders. Thermochim. Acta 2017, 653, 116–125,
doi:10.1016/j.tca.2017.04.008.
6. Seymour, R.W.; Estes, G.M.; Cooper, S.L. Infrared Studies of Segmented Polyurethan Elastomers. I.
Hydrogen Bonding. Macromolecules 1970, 3, 579–583, doi:10.1021/ma60017a021.
7. Lee, H.S.; Wang, Y.K.; Hsu, S.L. Spectroscopic analysis of phase separation behavior of model
polyurethanes. Macromolecules 1987, 20, 2089–2095, doi:10.1021/ma00175a008.
8. Teo, L.-S.; Chen, C.-Y.; Kuo, J.-F. Fourier Transform Infrared Spectroscopy Study on Effects of Temperature
on Hydrogen Bonding in Amine-Containing Polyurethanes and Poly(urethane−urea)s. Macromolecules
1997, 30, 1793–1799, doi:10.1021/ma961035f.
9. Yen, M.-S.; Cheng, K.-L. Synthesis and physical properties of H 12 MDI-based polyurethane resins. J Polym.
Res. 1996, 3, 115–123, doi:10.1007/BF01492902.
10. Peruzzo, P.J.; Anbinder, P.S.; Pardini, O.R.; Vega, J.R.; Amalvy, J.I. Influence of diisocyanate structure on the
morphology and properties of waterborne polyurethane-acrylates. Polym. J. 2012, 44, 232–239,
doi:10.1038/pj.2011.111.
11. Schoonover, J.R.; Thompson, D.G.; Osborn, J.C.; ORLER, E.B.; Wrobleski, D.A. Infrared linear dichroism study
of a hydrolytically degraded poly(ester urethane). In Polymer degradation and stability, 2001; pp. 87–96,
ISBN 01413910.
12. Schoonover, J.R.; Steckle, W.P.; Cox, J.D.; Johnston, C.T.; Wang, Y.; Gillikin, A.M.; Palmer, R.A.
Humiditydependent dynamic infrared linear dichroism study of a poly(ester urethane). Spectrochim. Acta
Part A: Mol. Biomol. Spectrosc. 2007, 67, 208–213, doi:10.1016/j.saa.2006.07.015.
| 159
Chapter 5.2: Programmable Materials
160 | Chapter 5.2
Chapter 5.2: Programmable Materials
Abstract
Thermoresponsive programmable materials can perform sensory and actuator tasks as
soon as their ambient temperature changes. Shape memory polymers (SMPs) are smart
materials that are suitable as functional base materials for the development of
thermoresponsive programmable materials. The examples considered herein include a gear-
like object and a unit cell, which differ in their required thermomechanical treatment. Besides
the comparable temperature guidance, in the case of the gear-like structure, a deformation
by compression was followed, and a deformation applied by stretching was carried out in the
case of the unit cell. The polymer used to design the programmable materials was a self-
synthesized poly(1,10-decylene adipate) diol-based polyester urethane (PEU). After
processing into filaments, programmable materials were additively manufactured by fused
filament fabrication. Once thermomechanically treated, the unit cell actuated under a weak
external stress of 0.01 N at temperatures between 23 °C and 58 °C. In the case of the “gear,”
a maximum thermoreversible change in height ΔH/H0 of about 42% in relation to its external
dimensions was detected. The object is a first step towards the development of systems for
overheating protection or process regulation, which could allow the force transmission to be
switched from "On" to "Off" and vice versa according to a temperature program. In terms of
thermoreversible length changes ΔL/L0, the unit cell even proved to be slightly stronger in
actuation with regard to linear motion.
5.2.1. Introduction
Shape memory polymers (SMPs) are smart materials that can fix a temporary shape after
a thermomechanical treatment, also denoted as "programming." When applying a suitable
stimulus, SMPs can almost completely recover the initial shape. In other words, the so-called
"one-way (1W) shape memory effect (SME)" is triggered [1–6]. Shape recovery is an
entropically driven process that is based on entropy elasticity according to the theory of
rubber elasticity [7]. Among stimulus-responsive SMPs that have been investigated so far,
thermoresponsive SMPs are the most widely investigated [8–10]. After triggering the 1W-
SME, an SMP requires another thermomechanical treatment to become thermoresponsive
again. By contrast, in the case of the so-called "two-way (2W) SME", an SMP is able to switch
between two metastable states without the need for further programming steps [11–17]. For
SMPs, the 2W-SME has been investigated on semicrystalline polymers since these materials
fulfill the necessary morphological requirements [17,18]. Here, the main driving forces are
caused by the crystallization of polymer chains and the melting of the associated crystallites,
the same as entropy elasticity [12,19,20]. In particular, phase-segregated thermoplastic
polyurethanes (TPUs), which are built up of hard and soft segments, have proven their
| 161
potential over other thermoplastic and chemically cross-linked SMPs [21–34], due to the
capability of reprocessing and mechanical recycling. In the last decade, scientists unveiled
how to implement the 2W-SME into physically cross-linked TPUs [11,20,31]. For instance,
Schönfeld et al. developed a poly(1,10-decylene adipate) (PDA)-based polyester urethane
(PEU) and processed the PEU via the additive manufacturing technique "fused filament
fabrication" (FFF). As a result, objects could be obtained, which were programmed by
significant tensile deformation at 75 °C, cooling to 23 °C while holding the elongated shape,
followed by unloading. Adjacently, the object shrank on heating and expanded on cooling
with a maximum thermoreversible strain of 16% [20]. A few years earlier, Bothe and Pretsch
applied substantial tensile deformation to achieve deformation-induced crystallization in the
soft segment phase of a commercially available PEU and witnessed a maximum actuation of
17% upon temperature cycling under free-strain conditions [14]. Likewise, in other
contributions, an actuation of physically cross-linked PEUs with a maximum of 21% was
verified [35–39].
Despite the steadily growing number of publications on 2W SMPs in the last few years, the
question arises to what extent actuation can be realized by more complex motion sequences
or even by the parallel motion of several parts within a single component. The answer is the
design and construction of programmable materials from polymers and harnessing the 2W-
SME with the adaption of thermomechanical treatment. Programmable materials are
currently the subject of intensive research [20,40–46]. When material functionality meets
structural design, new types of material behaviors can be realized. Advantageously,
programmable materials are not dependent on a constant power supply. They are essentially
characterized by self-sufficient material behavior. Thus, they entail an all-in-one sensor,
actuator, and controlling unit functionality [45,47]. Here, it is presented how to take
advantage of thermoresponsive programmable materials in the context of a gearbox. The
idea behind is to provide one day overheating protection; in other words, the force
transmission in gear units is switched from "On" to "Off" and vice versa as a result of heating
and cooling. Taking a first step into this direction, the same PDA-based PEU, which was
recently developed and characterized by promising actuation under stress-free conditions
[20] was selected as the base material. After developing a gear-like design and rapid
prototyping via FFF, the object was thermomechanically treated, and actuation was studied
while systematically varying the temperature. Following the same idea of transferring a
programmable material into bistable states by means of thermomechanical treatment, a
second demonstrator was built, and its actuation behavior was investigated. In this context,
it will be shown that the design of the programmable material leads to the realization of a
considerably larger change in length compared to the pure base material in linear form.
5.2.2. Result and Discussion
For this work, a poly(1,10-decylene adipate) (PDA)-based polyester urethane (PEU) which
was developed in our previous work [20] was selected as functional base material. In the
162 | Chapter 5.2
phase segregated PEU, the PDA served as the soft segment and exhibited a broad melting
transition, ranging from 29 °C to 72 °C, with a maximum located at 65 °C, while a
crystallization transition was found between 52 °C and 24 °C with a crystallization peak at
45 °C. Due to the verified actuation capability under stress-free actuation conditions [20], PEU
in the form of filaments was selected as the base material for the production of
demonstrators using fused filament fabrication (FFF).
Next, the model of an object characterized by a gear-like design (Figure 5.2.1) was
developed.
Figure 5.2.1. Technical drawing for a programmable gear in its (a) isometric view, (b) front view including
the dimensions of the original shape, and (c-g) the potential shape modifications due to stepwise loading
with regard to individual displacements (all the dimensions are in mm).
The object was designed in such a way that a compressive load applied during
thermomechanical treatment would shift the out-of-plane parts resulting in an entirely flat
structure (Figure 5.2.1). Specifically, the gear consisted of three inverted "V" shaped
structures in each line (Figure 5.2.1. b). The structures were designed to translate any vertical
motion directly into horizontal motion. When the gear was compressed from top to bottom,
the "V"-shaped structures bent to form a flat shape, increasing the outer diameter of the gear
(Figure 5.2.1. b-g). This change in diameter could possibly establish contact with similarly
designed objects in a kind of force transmission system.
The next step was to develop a method for thermomechanically treatment, which later
enabled the stress-free actuation of the gear. Here an approach of compression bending was
followed by means of a dynamic mechanical analysis (DMA) device, which was also used to
identify the ideal actuation conditions. It is noteworthy that this approach did not allow the
characterization of length changes with regard to the diameter of the object. In detail, the
thermomechanical treatment consisted of heating to 75 °C, applying a compressive force of
17 N, ensuring that only compression bending and no pressure crushing occurred, cooling to
15 °C, and holding the temperature constant for 30 min, followed by unloading. In response
| 163
to the applied compressive force, the height of the object was reduced from 7 mm to 2 mm,
at which the latter corresponded to the thickness of the “gear tooth.” Upon temperature
cycling, thermoreversible changes in shape and, in particular, the height of the object, also
denoted as actuation could be detected in every single measurement cycle, even when the
upper-temperature Tmax was continuously increased from 55 °C to 70 °C while the lower
temperature of 15 °C was left unchanged (Figure 5.2.2a). Here, actuation was enabled by
heating-induced melting of the PDA soft segment, leading to a movement out of the plane of
the flat gear and the cooling-induced crystallization of the PDA soft segment, resulting in the
movement back into the plane of the flat gear, along the direction of the compressive force
applied during thermomechanical treatment (Figure 5.2.2b).
Only a small quantity of crystallizable segments was present when selecting a lower
maximum temperature Tmax because a larger part of the PDA phase was still in a crystalline
state [20], resulting in weak elongation on heating and weak contraction on cooling, as can
be seen in the ΔH/H0—temperature diagram of Figure 5.2.2c, at which the ratio of ΔH/H0
describes the evolution of changes in the height of the object under a constant external load
of 0.01 N. By contrast, the increase in Tmax can be associated with an increase in the
proportion of crystallizable segments because more PDA crystals were molten [20]. At 58 °C,
the most pronounced actuation could be observed, characterized by a thermoreversible
change in the height of 42% (averaged for three cycles), which can be seen in the associated
ΔH/H0-temperature diagram (Figure 5.2.2c). The respective schematic bistable states of the
object at 58 °C and 15 °C are illustrated in Figures 5.2.2d and e. During actuation, a change in
vertical height of 1.5 mm was accompanied by a change in diameter of 4.4 mm. The further
increase of Tmax (> 58 °C) gave a decrease in actuation due to the systematic melting of PDA
crystals [20], which resulted in the steady recovery of the original shape and thus a lower
overall change in height ΔH/H0. In other words, the elongation at the beginning of each
cooling step was gradually shifted to higher values and recovered large parts of the initial
shape of the object. Obviously, under these conditions, stress-induced oriented PDA crystals
serving as physical netpoints were molten and no longer available to support the structural
integrity associated with the respective metastable states of the polymer. In this context, it
will be shown that the design of the programmable material leads to the realization of a
considerably larger change in length compared to the pure base material in linear form.
Conceptually, the object has its potential application to be used wherever the change in
shape can assure or interrupt force transmission and thus, e.g., prevent systems from
overheating.
To ensure that the developed compression-related programming method can also be used
to obtain bistable states in other programmable materials and thus to verify distinct
actuation, another programmable material was designed (Figure 5.2.3a-d). Additionally, a
simulation was run on the actuator element to understand the shape change during
compressive deformation in the course of thermomechanical treatment (Figure 5.2.3e and
f).
164 | Chapter 5.2
Figure 5.2.2. Influence of the selection of Tmax on the actuation of a programmable gear made from PDA-
based PEU. After thermomechanical treatment, a weak external load of 0.01 N was applied: Evolution
of (a) changes in object heigth ΔH/H0, sample height H (red and blue color for heating and cooling,
respectively) and temperature T (bottom graph) over measurement time t, (b) object height upon
temperature cycling between 15 °C and 58 °C, and (c) ΔH/H0 depending on Tmax (the values are averaged
for the second and third thermal cycle). The corresponding bistable states of the gear in (d) its first
bistable state characterized by a small outer diameter at Tmax = 58 °C and (e) its second bistable and
mostly expanded state, characterized by a maximum outer diameter at Tlow = 15 °C (all the dimensions
are in mm).
| 165
Figure 5.2.3. Technical drawing of an actuating element in the perspectives (a) front view, (b) right view,
(c) top view and (d) isometric view. The von Mises stress distribution on (e) before and (f) after
deformation (all dimensions are in mm).
The actuator element was conceptualized in such a way that the two opposite beams
protrude slightly in the middle (Figure 5.2.3 a-d). Due to the slight protrusion, the beams pull
out when the upper and lower parts of the actuation element are pressed and reach the
maximum bending for each beam segment, as the simulation shows (Figure 5.2.3e and f).
After three-dimensional (3D) printing (Figure 5.2.4a), the actuator element was found to
have excellent dimensional accuracy when compared to the CAD model (Figure 5.2.3a). Once
programmed (Figure 5.2.4b), the new shape of the actuating element was stable at room
temperature (23 °C) and showed an overall height of 23.8 mm. Subsequently, the 2W-SME
was examined. On raising the temperature to 58 °C, the actuating element partially returned
to its original shape to attain its second bistable state by expanding into the opposite direction
of deformation, resulting in a total height of about 36 mm (Figure 5.2.3c). Thus, a change in
length ΔL/L0 of about 51% was determined. Once cooled back to 23 °C, the initial bistable
state was achieved due to contraction of the actuating element in the direction of
deformation (Figure 5.2.4d). This resulted in a length of the bistable state of 24.6mm,
corresponding to a change in ΔL/L0 of about 46%. The actuation was proven on repeating the
heating and cooling cycle four more times (Figure 5.2.4e and f). This unveils that the actuation
achieved through compression bending programming for both the programmable gear and
the actuating element is almost identical and can be reproduced.
166 | Chapter 5.2
Figure 5.2.4. Actuating element made from PDA based PEU in its (a) permanent shape as obtained after
fused filament fabrication, (b) programmed shape and (c-f) during switching between its bistable states
when cycling the temperature. In the background centimeter paper can be seen (all the dimensions are
in mm).
Thus, the path taken here leads to significantly higher actuation compared to solid
materials, as the programmable materials approach combines the functionality of external
structure with the molecular structure [11,20,31, 35–39 ]. More precisely, the pure solid
material, programmed using tensile stretching in our previous work using the same PEU,
showed a maximum actuation of about 16%.[20] Comparing the actuation from our previous
literature with the current work, the added programmable material design for the same
material largely increased the actuation to about 46%. In perspective, the higher actuating
length from comparatively smaller-sized elements can open the door to other applications.
Beyond that, the reduction in the size of the actuating element may contribute to reducing
the complexity and size proportions of the previously presented actuating systems. For
example, a PEU actuation element connected to a gripper, as presented in one of our previous
works [20], could be produced in smaller sizes using programmable materials and also reliably
actuate.
5.2.3. Conclusions
Using an in-house developed polyester urethane, two types of programmable materials
were additively manufactured via fused filament fabrication. First of all, a gear-like object
turned out as fruitful example for a programmable material, which after thermomechanical
treatment was capable of actuating between two bistable states under almost stress-free
conditions. In this regard, increases in the object’s height were reliably transferred into
decreases in the object’s diameter and vice versa. Remarkably, an average thermoreversible
change in height ΔH/H0 of 42% could be witnessed. Theoretically, the developed
| 167
programmable gear can be used to enable or disable a force transmission, which could
protect a system from overheating. Transferring the thermomechanical treatment to a unit
cell allowed witnessing almost identical thermoreversible changes in the object’s length
and/or height. In any case, the main driving forces of actuation were the melting and
crystallization of the soft segment. Such programmable materials may replace earlier
generations of actuating elements and could, for instance, reduce the complexity and size of
grippers. This work thereby unveils that thermomechanically treated programmable
materials are caplable of both drastic and complex shape changes, which cannot be witnessed
when using classical design approaches for 2W-SMPs. In perspective, the unit cell can be
incorporated with embedded wires for heating purposes. This could be done through adapted
printing processes, as this creates possibilities for the local heating of polymers including
complex motion and in addition control over mechanical properties and thereby even expand
the existing potentials.
5.2.4. Experimental Section
Material: The same poly(1,10-decylene adipate) (PDA) based polyester urethane (PEU)
which was synthesized recently [20], is again used as the base material for this work. The
material was used in the form of filament for 3D printing. For more details about the thermal,
mechanical and thermomechanical properties, the characterization details can be obtained
from the previous literature [20].
Virtual Design and Fused Filament Fabrication: The AutoCAD software from Autodesk, Inc.
(San Rafael, CA, USA) was used to design the programmable material in the form of a gear,
while the programmable material in the form of an actuating element was designed and
simulated using Solidworks from Dassault Systèmes (Vélizy-Villacoublay, Île-de-France,
France). The developed CAD models were exported as standard triangle language (STL) files
and later used for slicing. After finalizing the designs of the gear and actuating element, Cura
3.6.1 [48] was used as a slicer program to generate numerically controlled codes, also
denoted as G-codes. The 3D models were imported into the slicer program, and the models
were sliced into layers according to the predefined printing parameters (Table 5.2.1). The
most relevant settings for additive manufacturing (AM) are listed in Table 5.2.1. To start AM,
the generated G-codes were transferred to the 3D printer. All 3D printed objects were
produced by fused filament fabrication (FFF) using the commercially available 3D printer
Ultimaker 3 from Ultimaker B.V. (Utrecht, The Netherlands).
Table 5.2.1. Printing parameters selected for additive manufacturing.
Printing parameters
PEU
Diameter of the nozzle (µm)
400
Temperature of the nozzle (°C)
208
Speed of print head (mm × s–1)
15
Build platform temperature (°C)
75
Layer height (mm)
0.1
168 | Chapter 5.2
Programming and Characterization of 2W-SME: The programmable materials made from
PDA-based PEU were both thermomechanically treated (programmed) and the actuation
analyzed using a Q800 DMA from TA Instruments (New Castle, DE, USA). In order to
characterize the 2W-SME, the gear-like object was initially placed between two compression
clamps. The chamber was heated to 75 °C at the beginning of thermomechanical treatment.
After holding the temperature at 75 °C constant for 30 min, the object was compressed with
a force of 17 N using a loading rate of 1 N·min–1. Then, the deformed object was cooled to
15 °C (Tmin) while maintaining the compressive force. The temperature Tmin was held
isothermally for 30 min. Subsequently, the applied force was unloaded with a rate of 1
N·min– 1, and the object was heated to 23 °C and held at that temperature for another 5 min
to mark the end of programming. The actuation of the programmable material was studied
under a constant external load of 0.01 N by cycling the temperature. In detail, the sample was
heated from 23 °C to Tmax = 55 °C and held there for 30 min, before it was cooled to 15 °C, at
which the temperature was kept for another 30 min. Heating and cooling were carried out
three times. Afterward, Tmax was increased from 55 °C to 70 °C with an increment of 2.5 °C
and for each increment, the three temperature cycles were repeated. Heating and cooling
rates of 5 °C·min–1 were used for the whole experiment.
In case of the unit cell serving as actuating element, the thermomechanical treatment of
samples was conducted with an MTS Criterion universal testing machine (model 43) from MTS
Systems Corporation (Eden Prairie, MN, USA). The device was equipped with a 500 N load cell
and was operated with a temperature chamber, which was controlled by a Eurotherm
temperature controller unit. Two heating elements were located at the back of the chamber.
Liquid nitrogen from a Dewar's vessel was fed into the chamber under a pressure of 1.3 bar
as an essential prerequisite for cooling. At the beginning of thermomechanical treatment, the
thermochamber was preheated to 75 °C, and the unit cell was fixed in the pneumatic clamps
of the universal testing machine using a clamping air pressure of 0.4 bar. After 30 min at 75 °C,
the unit cell was compressed in such a way that the top part touched the bottom using a
loading rate of 1 N·min–1. In the next step, the temperature was brought to 15 °C and held
there for 30 min to fix the new shape while maintaining the compressive load. Then, the unit
cell was unloaded with a rate of 1 N·min–1. The temperature was then heated to 23°C, and
the unit cell was removed from the clamps. The 2W-SME of the programmable material was
subsequently characterized by unclamping only from the upper clamp and cycling the
temperature between 58 °C (Tmax) and 23 °C (Tmin).
The thermoreversible changes in height ΔH/H0 and length ΔL/L0 are key parameters when
studying the actuation of polymers. They can be defined according to Equation 5.2.1 and
Equation 5.2.2:
∆𝐻
𝐻0= ℎ𝑚𝑖𝑛(𝑁)−ℎ𝑚𝑎𝑥(𝑁)
ℎ𝑚𝑎𝑥(𝑁) × 100%
(5.2.1)
∆𝐿
𝐿0= 𝑙𝑚𝑖𝑛(𝑁)−𝑙𝑚𝑎𝑥(𝑁)
𝑙𝑚𝑎𝑥(𝑁) × 100%
(5.2.2)
| 169
In the present case, hmin(N) and hmax(N) are the heights of a programmable material in the
Nth cycle of actuation at the respective temperatures Tmin and Tmax. While, lmin(N) and lmax(N)
are the lengths of the programmable material in the Nth cycle of actuation at the respective
temperatures Tmin and Tmax.
Simulation: The simulation of the programmable material in the sense of the actuating
element was conducted using Solidworks from Dassault Systèmes (Vélizy-Villacoublay, Île-de-
France, France). After CAD modelling, the simulation was carried out using a nonlinear-static
study, where the bottom part of the sample (orange arrows in Figure 5.2.3e and f) was fixed
while a pressure of 10 N s–2 was applied on its top part (green arrows in Figure 5.2.3e and f).
After simulation, the von Mises stresses were analyzed.
5.2.5. References
1. Liu, C.; Qin, H.; Mather, P.T. Review of progress in shape-memory polymers. J. Mater. Chem. 2007, 17,
1543, doi:10.1039/b615954k.
2. Dietsch, B.; Tong, T. A review-: Features and benefits of shape memory polymers (smps). Journal of
advanced materials 2007, 39, 3–12.
3. Ratna, D.; Karger-Kocsis, J. Recent advances in shape memory polymers and composites: a review. J Mater
Sci 2008, 43, 254–269, doi:10.1007/s10853-007-2176-7.
4. Pretsch, T. Review on the Functional Determinants and Durability of Shape Memory Polymers. Polymers
2010, 2, 120–158, doi:10.3390/polym2030120.
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172 | Chapter 6
Chapter 6: Fused Filament Fabrication
of Actuating Objects
| 173
Chapter 6: Fused Filament Fabrication of Actuating Objects
The original article Chalissery, D., Schönfeld, D., Walter, M., Ziervogel, F., Pretsch, T., Fused
Filament Fabrication of Actuating Objects. Macromol. Mater. Eng. 2022, 2200214 and
graphical abstract are published in Wiley Macromolecular Materials and Engineering and
available at https://doi.org/10.1002/mame.202200214.
Figure 6.0.0 The table of content image of the article “Fused Filament Fabrication of Actuating Objects”
is published in Wiley Macromolecular Materials and Engineering 2200214.
Contribution
My contribution: The concept idea of two-way four dimensional printing and its potential
applications. The idea of the whole manuscript, design development, conducting
experiments, formal analysis, investigation, methodology, validation, visualization, filament
extrusion, 4D-printing, characterization: DSC and DMA, preparation of all the images
(including table of content image and cover image), videos, writing—original draft and project
lead.
Not included in my contribution: Polymer-synthesis.
Schönfeld, D.: Synthesis of polymer and carried out differential scanning calorimetry of
synthesized polymer (Figures 6.1a).
Walter, M.: Developed the polymer.
Ziervogel, F.: Concept development of wire integration for future scope.
Pretsch, T.: Funding acquisition, project administration, supervision, writing—review and
editing
174 | Chapter 6
Figure 6.0.1. Cover image of “Fused Filament Fabrication of Actuating Objects” published in Wiley
Macromolecular materials and engineering, Volume 307, Issue 10 October 202. © 2022 Chalissery et al.
Macromolecular Materials and Engineering published by Wiley-VCH GmbH.
| 175
Chapter 6: Fused Filament Fabrication of Actuating Objects
Dilip Chalissery1, Dennis Schönfeld1, Mario Walter1, Fabian Ziervogel2, and Thorsten
Pretsch1*
1 Fraunhofer Institute for Applied Polymer Research IAP, Geiselbergstraße 69, Potsdam
14476, Germany E-mail: dilip.chalissery@iap.fraunhofer.de;
2 Fraunhofer Institute for Machine Tools and Forming Technology IWU, Nöthnitzer Straße
44, Dresden 01187, Germany
* Correspondence: thorst[email protected]; Tel.: +49-(0)-331/568-1414
6.0. Abstract: Thermoresponsive objects can be manufactured from shape memory
polymers (SMPs) via fused filament fabrication (FFF). Here a new technological approach to
obtain thermally actuating objects using an in-house synthesized, phase segregated
poly(ester urethane) (PEU) is introduced. Under almost stress-free conditions of a dynamic
mechanical analysis, cuboid objects obtained from FFF shrank when heated to 62 °C and
expanded when cooled to 15 °C with a maximum thermoreversible strain of 7.2%. Actuation
can be traced back to the phenomena of melting-induced contraction and crystallization-
induced elongation of the PEU's soft segment, supported by internal stresses as implemented
in course of FFF. To translate small changes in shape to a next larger scale, an artificial
butterfly is developed in which the movements of two actuator elements are transferred to
the wings with the aid of a lever concept. Following a different approach, additive
manufacturing of cylindrical samples implied application potential as a self-sufficient gripper,
enabling a programmable material behavior in the sense of temperature-controlled gripping,
transport, and release of exemplarily selected smooth surfaced objects in the form of vials.
Keywords: shape memory polymer, actuation, additive manufacturing, two-way 4D
printing, gripper, programmable materials
6.1. Introduction
Shape memory polymers (SMPs) can maintain a temporary shape after a
thermomechanical treatment, also known as “programming”. The temporary shape remains
stable until the “one-way shape memory effect” (1W SME) is triggered,[1-6] whereupon the
polymer returns into its almost permanent shape. Shape recovery is an entropically driven
process; it is based on the theory of rubber elasticity.[7]
In the context of additive manufacturing (AM), the so-called “four-dimensional (4D)-
printing” makes it possible to control the shape of an SMP both before and after heating
above the material-specific switching temperature.[8] This means that in certain cases the
classic programming of the 1W SME can be omitted. In other words, 4D-printed objects are
thermoresponsive and thus available for immediate use. Today, there are only a few
176 | Chapter 6
contributions on 4D-printing primarily focusing on AM via fused filament fabrication (FFF),
which makes it clear that the technology is still in its infancy. For instance, researchers carried
out 4D-printing with “glass transition temperature-based” polymers like poly(ether
urethanes),[9-11] polylactic acid (PLA),[12-16] acrylonitrile butadiene styrene (ABS),[13] and high
impact polystyrene (HIPS).[13] Until recently, researchers focused on the implementation of
thermoresponsiveness in objects with smaller layer heights (z < 5 mm), where the degree of
shrinkage becomes weaker from layer to layer.[8, 14] In a recent study, some of us employed
an amorphous poly(ether urethane) and PLA for 4D-printing and showed that the
technological key to realize distinct shrinkage is keeping the difference between the nozzle
temperature and the glass transition temperature (Tg) of the polymers to a minimum possible
value. This allows a rapid vitrification of the polymers once deposited on a printing bed.[8] All
these approaches have in common is that the underlying technique of 4D-printing can be
controlled by appropriate AM parameters, ensuring that polymer relaxation is avoided as far
as possible. For this reason, 4D-printing can be understood as an “in situ” programming
technology, which does not necessitate a thermomechanical treatment in the aftermath of
production.
In the last few years, researchers have learned how to program semicrystalline SMPs in
order to transform them into metastable states, between which they can be switched back
and forth by varying the temperature.[17-20] The underlying material behavior is the so-called
“two-way shape memory effect” (2W-SME). Beyond entropy elasticity, the main driving
forces are crystallization-induced elongation (CIE) and melting-induced contraction (MIC) as
caused by the phase transitions in the polymer matrix.[20, 21] The fundamental understanding
of the 2W-SME was originally developed for a semicrystalline polymer network by
Chung et al., who investigated cross-linked poly(cyclooctene) films under constant external
loads.[22] Later on, the same working principle and thus the general validity of the approach
was verified for other semicrystalline polymers under externally imposed stresses including,
i.e., poly(ethylene-co-vinyl acetate) cross-linked by dicumyl peroxide,[23] poly(ε-
caprolactone)-based polymer systems,[24-26] phase segregated poly(ester urethanes) (PEUs)
with crystallizable segments of poly(1,4-butylene adipate),[27] polymer networks with
crystallizable segments of poly(ω-pentadecalactone),[28] poly(ε-caprolactone)-co-
poly(ethylene glycol) foams [29] and polyester-based poly(urethane urea) foam.[30]
Conversely, stress-free actuation can be achieved for phase segregated, semicrystalline
PEU by programming the 2W-SME. The essential steps include the application of relatively
high strains at temperatures above the offset melting transition temperature of the soft
segment, followed by cooling below the corresponding offset crystallization transition
temperature while retaining the deformation and a final step of unloading.[31, 32] Although
even other programming methods and concepts were applied to SMPs to realize stress-free
bidirectional actuation, [33, 34] the necessary thermomechanical treatment is still a time-,
energy-, and thus a cost-intensive process which stands in the way of economic exploitation.
The maturation of AM technology in recent years has led to the development of objects
that actuate immediately after printing.[35] Today, two-way (2W) 4D-printing approaches can
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be subdivided as follows: core-shell embedded structures and bilayer or gradient
structures.[36] The first ones focused on the manufacturing of elastomeric material
composites containing magnetic particles.[37-40] When applying a specific magnetic field, their
shape can be changed in a controllable manner; as soon as the magnetic field is switched off,
the initial shape is recovered due to the elastomeric properties of the matrix material. By
contrast, the concept of bilayer or gradient structures is based on composite AM
technology.[41-47] In this case, composite layers are fabricated consisting of a material with
antagonistic properties, e.g. reversible physical properties associated with water
absorption/desorption or temperature changes, to realize stimulus-governed shape changes.
However, the main disadvantage is the need for two entirely different stimuli like water and
heat. To make matters worse, the effort associated with recycling in the sense of a recovering
the individual materials at the end of their life cycle is very high. One in all, there seem to be
no technological approaches so far, considering a single material system, which exhibits
bidirectional actuation immediately after AM.
Here we demonstrate how to obtain thermally actuating objects from a semicrystalline
SMP by means of FFF. To gain the greatest possible control over material functionality, a
poly(1,10-decylene adipate) (PDA)-based PEU was selected, as it was previously identified as
a promising material with regard to 2W-SMEs under stress-free actuation conditions.[32]
Exploiting a lever mechanism coupled with two PEU actuators turned out as an appropriate
strategy to translate small changes in shape to a next larger scale. Beyond that, we describe
how to design and manufacture structures, like cylindrical actuating elements which are able
to grab, transport, and release smooth surfaced objects in the form of vials. In this way, we
provide first examples to open a new design space for actuators made from programmable
materials.[30, 32, 48-53]
6.2. Results and discussion
Poly(1,10-decylene adipate) diol (PDA) is an attractive building block in the chemistry of
PEUs, which may exhibit pronounced two-way shape memory properties.[32] In order to
synthesize the hydroxyl-terminated PDA oligoester, 1,10-decanediol and adipic acid were
brought to reaction. Subsequent to this polycondensation, further reaction with 4,4′-
diphenylmethane diisocyanate (4,4′-MDI) was carried out to build up an isocyanate-
endcapped prepolymer, before 1,4-butanediol (BD), serving as a chain extender, was finally
added. This led to the formation of the desired PEU.
After polymer synthesis, the PDA-based PEU was extruded into filament. The filament was
characterized by a uniform diameter of 2.85 ± 0.05 mm. The thermal properties of the PEU
raw material were analyzed by means of differential scanning calorimetry (DSC, Figure 6.1a).
Thermomechanical investigations were carried out by dynamic mechanical analysis (DMA,
Figure 6.1b) on a cuboid sample consisting of 20 individual layers with dimensions of 40 mm ×
2 mm × 2 mm and fabricated by FFF (“3D-printing”; the underlying print settings are
summarized in Table 6.1.).
178 | Chapter 6
Figure 6.1. Thermal and thermomechanical properties of PDA-based PEU as determined by a) DSC
subsequent to the synthesis, second heating and cooling with temperature rates of 10 °C x min−1. The
enthalpies of melting ΔHm, red dashed area, and crystallization ΔHc, blue dashed area, are included. b)
DMA of a cuboid sample obtained after FFF exhibiting the temperature dependence of storage modulus
E’, and loss factor tan δ at a heating rate of 3 °C x min−1.
The thermal properties of the PDA-based PEU were characterized by a broad melt
transition temperature in between 30 and 71 °C with a maximum at about 63 °C, while the
crystallization transition spread from 53 to 19 °C and exhibited a peak at 38 °C (Figure 6.1a).
With regard to the DMA (Figure 6.1b) a similar evolution was detected for the storage
modulus E’ and tan δ curve as in our previous work.[32] In fact, the storage modulus E’ declined
upon heating in two steps as associated with the devitrification and melting of the PDA phase.
At temperatures of about 71 °C and thus above the melting transition of the PDA phase, the
hard segments of the PEU ensured that the polymer still had a sufficiently high degree of
dimensional stability. The evolution of tan δ is often used to determine the glass transition
temperature Tg in urethane-based polymers.[54-56] Here, the tan δ peak was located at ≈6 °C.
After characterizing the PEU, an FFF 4D-printing approach was followed in accordance with
the above-studied thermal properties of the material. The approach is aimed at realizing a
rapid vitrification. For this reason, the FFF printer was modified with a cooling unit, which
continuously applied compressed nitrogen with a stream temperature (Ts) of about −5 °C as
soon as the polymer strand left the nozzle (Figure 6.2).
Figure 6.2. Schematic drawing of a fused filament fabrication printer modified with a hose to apply a
cold stream of compressed nitrogen to polymer strands as soon as they left the nozzle.
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In course of additive manufacturing, the efficiency of the cooling unit was verified by
means of in situ thermal imaging for an exemplarily selected cuboid sample (Figure 6.3).
Figure 6.3. In situ thermal imaging to supervise the temperature distribution in PDA-based PEU a) during
FFF and b) schematic representation of the process at five randomly selected measurement points in
the top layer during strand deposition.
It was found that the temperature of each freshly deposited polymer strand fell quickly
below 0 °C (Figure 6.3a). In any case, temperatures of 11 °C, −1 °C, −4 °C, −5 °C, and –5 °C
were recorded at the individual measurement points 1 to 5 (Figure 6.3b). Taking into account
the printing speed and the distance of the individual measurement points, it can be concluded
that the use of the external cooling unit permitted the PEU to cool below its Tg in ≈150 ms.
Most importantly, after leaving the nozzle the rapid cooling of polymer strands seemed to
support a quick crystallization and vitrification of the PDA phase. The former is indicated by a
whitish coloring of the cuboid sample while the latter is implied by the temperature
distribution in the freshly deposited polymer strands (Figure 6.3a).
Further cuboid samples of PEU were fabricated via FFF with a printing speed SP of
8 mm x s−1 and a nozzle temperature TN of 170 °C under permanent exposure to a compressed
nitrogen stream with a temperature TS of about −5 °C. Hereafter it will be demonstrated that
these objects exhibited both the so-called “one-way (1W) 4D effect” and the “two-way (2W)
4D effect”. In any case, the underlying print settings were the same and will be referred to as
“2W 4D-printing” in the following (compare Table 6.1).
First, the print result and stability of another cuboid sample were investigated at 23 °C,
before the responsiveness on heating to 75 °C was studied with respect to the 1W 4D effect
to evaluate the efficiency of the printing process for the implementation and subsequent
release of internal stresses (Figure 6.4).
Figure 6.4. Cuboid sample of PDA-based PEU: a) Immediately after FFF using the method of 2W 4D-
printing (Table 6.1), and b) after temperature conditioning for 5 min at 75 °C to trigger the 1W 4D effect
(all the dimensions are in cm).
180 | Chapter 6
The sample remained stable after printing (Figure 6.4a) even when raising the temperature
to 23 °C and thus above the glass transition temperature of the PEU (Figure 6.1b). Upon
heating to 75 °C under stress-free conditions and holding that temperature for 5 min,
significant shrinkage accompanied by a bending of the sample occurred. After shrinkage an
arc length of 30.5 mm was measured; in parallel, an arc measure of 134.8° and a radius of
curvature of 13 mm were verified (Figure 6.4b). The bending behavior of the sample can be
explained by the fact that every single hot molten top layer favored the relaxation of the layer
below in course of FFF. As a result, a gradient in stress distribution presumably formed. This
phenomenon is well-known from other studies when utilizing the same printing method. For
instance, similar shrinkage and bending behavior was observed after FFF 4D-printing of
objects made of poly(ether urethanes),[9, 10] PLA,[13-16] and ABS.[13]
Subsequently, two types of cuboid samples of PEU were additively manufactured
according to the object dimensions from Figure 6.4a. In the first case, the abovementioned
2W 4D-printing method (Table 6.1) was used again. In the other case, TN was increased to
208 °C and TS to 75 °C while SP remained unchanged, using the print settings of “3D-printing”
(Table 6.1). Later, the thermoresponsiveness of the samples was studied by means of DMA
(Figure 6.5).
Figure 6.5. DMA measurement protocols of PDA-based PEU, exhibiting the evolution of strain ε (solid
blue line) when varying the temperature T (dashed red line). The samples were obtained by FFF via a)
3D-printing and b) 2W 4D-printing (Table 6.1).
In contrast to the first study of thermoresponsiveness, in which no fixation of the sample
was made (Figure 6.4b), this time the clamping conditions, where the distance between the
clamping units of the DMA was 20 mm, ensured that no bending occurred during heating. In
the case of the 3D-printed sample, heating to 75 °C and holding that temperature constant
resulted in a slight decrease in strain of ≈1.6% (Figure 6.5a). By contrast, slight expansion
followed by strong shrinkage of ≈34% occurred after 2W 4D-printing (Figure 6.5b). Here, the
initial increase in sample length by 1.7% during heating from 23 to 53 °C can be associated
with the recrystallization of the PDA phase, since some of the polymer chains may have
vitrified as a consequence of the rapid cooling before, so that they did not have sufficient
time to crystallize. Further heating initiated strong shrinkage and thus the 1W 4D effect. After
cooling back to 23 °C, the shrunken sample remained almost stable.
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In the next step, the focus of the investigations was directed away from 1W 4D effects and
toward 2W 4D effects. From a conceptual point of view it was evident from Figure 6.5b that
a too high maximum temperature, as selected with 75 °C, would lead to the complete melting
of the PDA soft segment (compare Figure 6.1a) and thus no movement of the sample in the
ensuing cooling contrary to the reduction of strain. For this reason, the maximum
temperature was limited to a lower temperature value of 64 °C in the following DMA
measurement. The measurement was carried out under almost stress-free conditions – only
a weak load of 0.001 N was applied – on a 2W 4D-printed cuboid sample (Figure 6.4a and
Table 6.1) to investigate both the actuation capability and identify the optimal temperature
conditions. Again, the same clamping conditions as used for the other DMA were selected to
avoid any bending in the first heating. The results are exhibited in (Figure 6.6).
Figure 6.6. Actuation of a 2W 4D-printed sample of PDA-based PEU (Figure 6.4a and Table 6.1) under a
weak load of 0.001 N: a) Evolution of strain ε and sample length L (differently colored according to the
temperature intervals investigated) and temperature T with measuring time t; b) relationship between
strain and temperature for the experiment shown in (a) using the same color codes and c) evolution of
thermoreversible strain εrev depending on Tmax, the values for εrev are averaged for the second and third
cycle.
182 | Chapter 6
Remarkably, thermoreversible changes in strain could be verified in every single
measurement cycle, even when continuously raising the upper temperature Tmax from 56 to
64 °C, while keeping the lower temperature Tmin at 15 °C (Figure 6.6a,b). As known from
similar semicrystalline polymeric materials,[20, 32] the driving force for actuation is – beyond
entropy elasticity – melting-induced contraction (MIC) and crystallization-induced elongation
(CIE). When selecting a lower Tmax, obviously only a small quantity of PDA crystals were
molten, resulting in weak PDA crystallization and sample elongation on cooling and weak PDA
melting and sample contraction on heating as can particularly be seen in the respective strain-
temperature diagram in Figure6. 6b (black lines). By contrast, the steady increase in Tmax
enhanced polymer shrinkage in the first heating due to a more pronounced release of internal
stress introduced in the course of additive manufacturing and stored by rapid cooling. With
ongoing measurement, a directed crystallization of the PDA phase seemed to occur during
cooling (Figure 6.6b); in parallel, the proportion of crystallizable segments increased, since
more PDA crystals were melted in the previous heating, favoring more pronounced polymer
expansion on cooling. At the same time, a hysteresis behavior was detected. Actuation
substantially increased when selecting a Tmax of 58 °C (Figure 6.6b, red color) and even got
stronger at Tmax = 60 °C (Figure 6.6b, blue color). Further increasing Tmax to 62 °C culminated
in the most pronounced actuation (Figure 6.6b, green color) with a maximum change in
thermoreversible strain εrev of 7.2% and an average value for εrev of 7.0%. This becomes
particularly clear in the associated εrev /Tmax diagram (Figure 6.6c, green point). Interestingly,
the value for Tmax corresponded precisely with the melting peak temperature (second
heating) of the FFF-printed PDA-based PEU in our DSC measurements (Figure 6.1a). Upon
further increasing Tmax, actuation decreased (Figure 6.6b, orange) since the systematic
melting of PDA crystals resulted in strain recovery of the PEU and thus a lower overall strain
as caused by a reduction of internal stresses implemented in course of additive manufacturing
and stored by rapid cooling. In other words, the overall elongation at the beginning of each
cooling step was gradually reduced. Above all, a new type of functional integration using 2W
4D-printing could be realized due to the thermoreversible shape changes detected. In
comparison with our previous work on thermomechanically treated PDA-based PEU,[32] the
values for εrev were found to be lower here. This is not surprising, as in the other scenario a
strong deformation, measuring 700% in strain, was applied during programming to achieve
strain-induced PDA crystallization. Advantageously, in our present approach, an actuator can
be manufactured within a short time of 5 min. This is in sharp contrast to the other route of
processing and programming, which roughly takes 245 min.[32]
The durability of almost stress-free actuation was investigated in another DMA
measurement. A 2W 4D-printed cuboid sample of PEU (Figure 6.4a and Table 6.1) was
subjected to 100 heating-cooling cycles with maximum and minimum temperatures of 62 °C
and –15 °C, respectively (Figure 6.7).
In the beginning, a strong drop in strain was observed (Figure 6.7a). This was attributed to
the melting of highly oriented PDA crystals and the associated release of internal stresses. In
the first three cycles, actuation decreased (Figure 6.7b), which was presumably caused by the
| 183
rearrangement of polymer chains.[57] More stable actuation was detected after 20 cycles of
heating and cooling. Apparently, the PDA-based PEU formed temperature-bistable states,
differing in elongation. In the end, εrev approached an almost constant value of 5.7%, thus
qualifying it as a reliable actuator. In addition, we would like to emphasize that the clamping
conditions in the DMA assured once more that no bending effects were observed.
Figure 6.7. DMA measurement to determine the durability of actuation of 2W 4D-printed PEU
(Table 6.1) under a weak load of 0.001 N: Evolution of a) nominal strain ε with time t in the actuation
measurement (red) including the measurement protocol of an isothermal strain measurement (green)
and b) thermoreversible strain εrev with cycle number N.
To closely examine the temperature resistance of 2W 4D-printed PEU (Figure 6.4a and
Table 6.1), a final DMA measurement was conducted. This time, a cuboid sample was heated
to 62 °C and kept there for 70 h without external load. In the course of the experiment, the
strain of the sample decreased sharply at first and later gradually to a value of −22%
(Figure 6.7a, green). It is noteworthy that the length of the sample largely coincided with the
length of the sample from the actuation measurement at 62 °C (Figure 6.7a, red).
As the 2W 4D-printed PEU showed promising actuation even in multiple cycles, some
potential applications were explored. For a start, a lever mechanism was developed for an
artificial butterfly in order to transfer the actuation in a targeted manner to its wings. In this
regard, the concept was inspired by our recently published work on a gripper system
containing embedded actuating elements, whose linkage mechanism allowed to magnify the
2W-SME.[32] The underlying scalable vector graphics of the lever mechanism is shown
together with the demonstrator in Figure 6.8.
After finalizing the design, the individual parts (Figure 6.8a-p) were laser cut from 5 mm
thick poly(methyl methacrylate) sheets (parts in Figure 6.8a–e,h–p), while the left and right
wing (Figure 6.8f,g) were laser cut from a thin, rigid sheet of paper. The design was such that
one millimeter of actuation of the 2W 4D-printed cuboid sample (Figure 6.4a and Table 6.1)
can cause a wing to rotate by 10°, resulting in lateral differences of 11 mm between its first
and second position (Figure 6.8q). After cutting out the parts, the artificial butterfly was
assembled (Figure 6.9).
184 | Chapter 6
Figure 6.8. Technical drawing of the individual components of a butterfly: a) Head; b) front base; c) front
alignment part; d) right wing actuator holder; e) right fulcrum; f) left wing; g) right wing; h) left wing
actuator holder; i) left fulcrum; j) rear alignment part; k) rear base; l) tail; m) base; n) intermediate base;
o) two base actuator fixing part and p) base actuator holder. q) Illustration of the leverage effect.
Figure 6.9. Butterfly demonstrator in the perspectives a) front view, b) right view, c) top view, and d)
isometric view.
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Two 2W 4D-printed actuators were then manually inserted, each responsible to move one
wing, by gluing their ends in the upper area of the butterfly and fixing the other ends with the
aid of two screws, whereupon the system was ready to use. Since the fixing length of every
actuator was about 3 cm, a larger change in total length could be expected compared to the
DMA measurement, in which the sample was clamped at a length of 2 cm (compare
Figure 6.6a). For functional proof-of-principle, the temperature was cycled three times
between 23 and 62 °C while the movements of the butterfly were followed (Figure 6.10,
s6.1.movie in Supporting Information).
Figure 6.10. Artificially generated wing beat of a butterfly, enabled by two actuators from 2W 4D-printed
PDA-based PEU: a) Four superimposed images of the setup during heating from 23 °C (blue) to 62 °C
(red coloring was accomplished with an image editing software)[58]; b) shapes at 23 °C and c) at 62 °C.
Figure 6.10 proves that the actuation of the PEU of roughly 3 mm could be transferred to
the butterfly's wings as verified by four superimposed images taken at different temperatures
(Figure 6.10a). In fact, the wings of the butterfly hung down at first (Figure 6.10b), but when
raising the temperature to 62 °C a slight expansion and an associated bending of the actuators
occurred coupled with a slight lowering of the wings, followed by the contraction and
straightening of the PEU. In this way, the longer sides of the wing actuator holders
186 | Chapter 6
(Figures 6.8d,h) were pulled, which in turn caused the shorter ends to be lifted pressing
against the fulcrums (Figures 6.8e,i). The wings attached in between (Figures 6.8f,g) followed
the movement of the fulcrums and were lifted (Figure 6.10c, s6.1.movie in Supporting
Information). Thus, the thermoresponsive behavior of the two actuators during the first
heating was in good agreement with the behavior determined in the DMA (Figure 6.5b). Upon
cooling to 23 °C, the expansion of the actuators pushed the wing actuator holders
(Figures 6.8d,h) upward, causing the fulcrums (Figures 6.8e,i) to lower while the actuators
finally bend through. In parallel, the wings returned almost to their original position
(Figure 6.10b, s6.1.movie in Supporting Information). The experiment was repeated twice, in
which the actuation proved to be stable.
In a progressive approach, two hollow cylinders were 2W 4D-printed using the same
printing equipment and parameters as selected before (Table 6.1) and their actuation
capability was explored (Figure 6.11, s6.2.movie and s6.3.movie in supplementary material).
Figure 6.11. 2W 4D-printed cylindrical actuators of PDA-based PEU in operation: Gripping, lifting,
releasing of (top row) an empty brownish vial (15 g in weight) and (bottom row) a transparent vial filled
with red liquid (total weight of 30 g) in two consecutive thermal cycles.
The image series in Figure 6.11 clearly show that the hollow cylinders actuated when
cycling the temperature. At the beginning, the cylinders had an inner diameter ø of ≈30 mm
and an inner circumference of 94.2 mm at 23 °C, thus exceeding the dimensions of both the
brownish vial (outer ø of 27 mm and circumference of 86.4 mm) and the transparent vial
(outer ø of 25 mm and circumference of 78.5 mm). As a result, the cylinders were able to slide
over both vials. When raising the temperature to 62 °C, the PEU shrank and initiated the
gripping. At this temperature, the degree of shrinkage was so pronounced that a lifting of
each vial became possible. Cooling to 23 °C expanded the cylindrical elements, so that the
vials could be released. The overall process was repeated ten times for both cases, thus
demonstrating the reliable functionality of the actuators. It is obvious that a different
bidirectional actuation behavior has been demonstrated than for the other actuator
| 187
elements (Figure 6.11). The reasons are that the actuators were not fixed at the beginning of
the experiment and that the printing paths were different. Obviously selecting a circular
printing path as done for 2W 4D-printing of the cylinders favored a more uniform distribution
of internal stresses and thus a more homogenous shrinkage and expansion behavior of
printed objects. Thus, the path of strand deposition can be understood as another key
element to gain control over thermoresponsiveness of 2W 4D-printed objects. In a final
experiment, the vials could be gripped one after the other with an actuator, which makes it
clear that the actuator is even capable of adapting to different shapes.
6.3. Conclusions
Here we introduce a 2W 4D-printing approach based on FFF to obtain thermoresponsive
actuators from an in-house synthesized PEU. The actuation could be attributed to the
phenomena of melting-induced contraction and crystallization-induced elongation of the
PEU's switching segment as supported by internal stresses implemented in the course of
additive manufacturing. Beyond a 1W 4D effect, actuation could be witnessed under different
conditions. The movements of the polymer followed a program in terms of shrinkage on
heating and expansion on cooling under the clamped, but almost stress-free conditions of
samples in the DMA. Similarly, straightening and contraction on heating and expansion and
bending on cooling in the clamped situation of an artificial butterfly were witnessed.
Following the concept of a gripper, contraction in the sense of circular shrinkage on heating
and circular expansion corresponding to a widening on cooling could be achieved. All this
makes it clear that the material behavior in the actuation depends on the prevailing test
conditions or installation situation as well as on the sample geometry and the printing paths
during additive manufacturing. Having this in mind, the introduced technology may pave the
way for the production of actuators that enable even more complex system functionalities
when taking into account design-related boundary conditions.
Most importantly, the introduced 2W 4D-printing approach may save time and costs when
considering scenarios, where minor changes in sample size are desired. The reason is that a
thermomechanical treatment of a shape memory polymer is no longer needed. Provided that
no severe degradation has occurred, the thermoplastic nature of the PEU implies that even a
mechanical recycling is possible at the end-of-life.
In the future, the printing technology can contribute to qualify programmable materials
for new applications. In perspective, FFF printing of fiber-reinforced polymers can even
expand the existing potentials, as this creates possibilities for the local heating of polymers
including complex motion and in addition control over mechanical properties.[59-62] All this
can further strengthen the potential of programmable materials and the underlying
manufacturing technology.
188 | Chapter 6
6.4. Experimental Section
Materials: 1,10-Decanediol, 4,4′-methylene diphenyl diisocyanate (4,4′-MDI), and
titanium(IV) isopropoxide (TTIP) were purchased from Fisher Scientific (Schwerte, Germany).
For titration tests, acetic anhydride, methanol, and potassium hydroxide solution in methanol
with concentrations of 0.5 mol−1 and 0.1 mol−1 were purchased from Merck (Darmstadt,
Germany). N-Methyl-2-pyrrolidone (2-NMP), chloroform, and 4-dimethylaminopyridine (4-
DMAP) were bought from Carl Roth (Karlsruhe, Germany). Adipic acid, 1,4-butanediol, and a
molecular sieve (4 Å) were obtained from Alfa Aesar (Kandel, Germany).
Synthesis of polyester diol: 1,10-Decanediol and adipic acid were mixed at a molar ratio
of 1.1:1 and heated in a three-necked round-bottomed flask, which was equipped with a
mechanical stirrer, nitrogen gas inlet, and distillation condenser. All reactants were molten at
about 150 °C while titanium(IV) isopropoxide was added under stirring. Adjacently, the
mixture was heated to 190 °C. After a remarkable decrease in distillation temperature, the
mixture was further heated to 210 °C, whereupon the pressure was reduced to approximately
20 mbar. After two hours of continuous stirring, the melt was poured into a can. The obtained
poly(1,10-decylene adipate) diol (PDA) solidified and was analyzed prior to the synthesis of
PEU.
Titration: Titration was used to determine both the acid value and hydroxyl value and thus
the number average molecular weight Mn of PDA. Therefore, a TitroLine 7000 from SI
Analytics (Mainz, Germany) was employed. The procedure was executed in compliance with
DIN EN ISO 2114[63] and DIN EN ISO 4629-2.[64] To determine the acid value, a sample of PDA
was dissolved in a mixture of chloroform/methanol with a volume ratio of 5:1. The solution
was titrated against a potassium hydroxide solution in methanol, having a concentration of
0.1 mol−1. For the determination of the hydroxyl value, another sample of PDA was dissolved
in chloroform. After adding acetic anhydride diluted in 2-NMP as well as 4-DMAP diluted in
2-NMP, the solution was heated and kept under stirring at 60 °C for 15 min. Thereafter,
deionized water was added. After 12 min, the sample solution was titrated against a
potassium hydroxide solution in methanol, having a concentration of 0.5 mol−1.
Synthesis of polyester urethane (PEU): A PEU was synthesized using the prepolymer
method. In order to obtain a PDA-based PEU with approximately 15% of hard segment
content, the molar ratio of the reactants was set to 1:1.98:0.97 with regard to PDA, 4,4′-MDI,
and 1,4-butanediol, respectively. The reaction was carried out with a slight excess of
isocyanate (NCO/OH = 1.005). Overnight, PDA was dried in a glass reactor in a vacuum oven
at 90 °C. The following day, it was heated under nitrogen flow and stirred to 120 °C.
Adjacently, isocyanate was added, and the mixture was continuously stirred for 90 min. The
obtained prepolymer was directly converted to PEU by adding 1,4-butanediol, serving as a
chain extender. In parallel, the stirring speed was raised. As the viscosity increased
significantly, the reaction was stopped, and the polymer melt was poured onto a plate
covered with polytetrafluoroethylene. Finally, the PDA-based PEU was cured in an oven for
120 min at 80 °C.
| 189
Extrusion: The synthesized PEU was ground with a cutting mill type M 50/80 from Hellweg
Maschinenbau (Roetgen, Germany). The obtained flakes were dried at 110 °C for 150 min in
a vacuum drying chamber VDL 53 from Binder GmbH (Tuttlingen, Germany). Subsequently,
the flakes were fed into an extrusion line to produce filaments. The same extrusion line as
previously used was employed in this work.[6, 8, 32] The individual units of the extrusion line
were put together in such a way that it included the volumetric material feeding system Color-
exact 1000 from Plastic Recycling Machinery (Tranekær, Denmark), a Leistritz twin screw
extruder MICRO 18 GL from Leistritz AG (Nürnberg, Germany), characterized by seven heating
zones and a screw length of 600 mm, a conveyor belt, and a filament winder from Brabender
GmbH and Co. KG (Duisburg, Germany). The temperatures of the individual heating zones of
the extruder were 170, 175, 180, 185, 195, 190, and 190 °C. To evaluate the quality of the
filaments, the evolution in diameter was manually detected at regular intervals using a vernier
caliper from Fowler High Precision (Newton, MA, USA).
Virtual design and fused filament fabrication: The AutoCAD from Autodesk, Inc. (San
Rafael, CA, USA) was used for the virtual construction of 3D models, like cuboid sample with
dimensions 40 mm × 2 mm × 2 mm and a hollow cylinder with an outer diameter of 30 mm,
a height of 20 mm and a thickness of 1 mm. The developed CAD models were then exported
as standard triangle language (STL) files and later used for slicing. After finalizing the designs,
Cura 3.6.1[65] was used as a slicer program to generate numerically controlled codes, also
denoted as G-codes. The 3D models were imported into the slicer program and were sliced
into layers according to the predefined printing parameters (Table 6.1). The most relevant
settings for 3D-printing and 2W 4D-printing via fused filament fabrication (FFF) are listed in
Table 6.1. To start additive manufacturing, the generated G-codes were transferred to the
3D-printer. All printed objects were produced by FFF using the commercially available 3D-
printer Ultimaker three from Ultimaker B.V. (Utrecht, The Netherlands). In the case of 2W 4D-
printing with the PDA-based PEU, the air chiller system from TA Instruments (New Castle, DE,
USA) featuring a multi-stage cascading compressor system was used to generate a cold
stream of compressed nitrogen, which was brought to the print head of the 3D-printer using
a Teflon tube, as shown in Figure 6.2. For 2W 4D-printing, the air chiller system was
connected to a nitrogen gas system of the house, and the pressure was regulated to 1.5 bar.
Additionally, the 3D-printer was equipped with a top cover and a door to restrict the escape
of cold air.
Table 6.1. Selected printing equipment and parameters for additive manufacturing of objects from PDA-
based PEU using 3D- and 2W 4D-printing
Printing parameters
3D-printing
4D-printing
Diameter of the nozzle (µm)
400
400
Temperature of the nozzle (TN, °C)
208
170
Speed of print head (SP, mm × s–1)
8
8
Nitrogen stream temperature (TS,°C)
75
–5
Layer height (mm)
0.1
0.1
190 | Chapter 6
In-situ thermal imaging: In situ thermal imaging was applied during FFF using a Variocam
HD from InfraTec GmbH (Dresden, Germany) to study the temperature distribution in freshly
deposited polymer strands at five different measurement points (Table 6.1). In course of
additive manufacturing the thermal images were recorded with a frequency of 15 Hz and
evaluated with the software IRBIS 3.1[66] from InfraTec GmbH.
Characterization of thermal properties: The PEU was investigated with regard to the phase
transition behavior of PDA by differential scanning calorimetry (DSC) using a Q100 DSC from
TA Instruments (New Castle, DE, USA). The measurement was conducted on a freshly
synthesized sample with a weight of 5 mg. The sample was thermally cycled between −70 and
100 °C. For cooling and heating, a rate of 10 °C x min−1 was applied. The temperature holding
time at the minimum and maximum temperature was 2 min.
Characterization of thermomechanical properties: The thermomechanical properties of the
PEU were studied by dynamic mechanical analysis (DMA). The experiments were carried out
with a Q800 DMA from TA Instruments (New Castle, DE, USA) using film tension clamps on
multi-frequency–strain mode. A frequency of 10 Hz, a static force of 0.1 N and an oscillating
amplitude of 10 µm were selected to investigate the center part of a 3D-printed additively
manufactured cuboid with dimensions of 40 mm × 2 mm × 2 mm (Table 6.1). At first, the
sample was cooled to −80 °C and held there for 5 min, before it was heated to 100 °C with a
rate of 3 °C x min−1. In parallel, the evolution in storage modulus E´ and loss factor tan δ was
determined.
Characterization of actuation under stress-free conditions: The actuation of 2W 4D-printed
cuboid samples made from PDA-based PEU (Table 6.1) was studied by means of DMA
immediately after additive manufacturing. Against this background, a first test series was run,
in which the maximum temperature Tmax was systematically varied, and a constant minimum
temperature Tmin of 15 °C was selected. First, the sample was clamped with a distance of
20 mm in the holders of the DMA and a weak load of 0.001 N was applied, so that almost
stress-free conditions prevailed. Adjacently, the sample was heated from 23 °C to Tmax = 56 °C
and held there for 15 min, before it was cooled to 15 °C, at which the temperature was kept
for another 15 min. Heating and cooling were carried out three times. Afterwards, the three
cycles were repeated for different temperatures Tmax of 58, 60, 62, and 64 °C. Heating and
cooling rates of 5 °C x min−1 were selected in all experiments conducted.
In a durability experiment, a cuboid sample of 4D-printed PDA-based PEU (Figure 6.4a and
Table 6.1) was studied in the DMA under a weak load of 0.001 N by cycling the temperature
between 15 and 62 °C. In the measurement, heating and cooling rates of 5 °C x min−1 were
used. This time, actuation was investigated in 100 thermal cycles.
The thermoreversible strain εrev is the key parameter when studying the actuation of
polymers. It can be defined according to Equation 6.1:
𝜀𝑟𝑒𝑣(𝑁)= 𝑙𝑇𝑙𝑜𝑤(𝑁)−𝑙𝑇𝑚𝑎𝑥(𝑁)
𝑙𝑇𝑚𝑎𝑥(𝑁) × 100%
(6.1)
Herein, lTlow (N) and lTmax (N) are the lengths of the sample in the Nth cycle of actuation at
the respective temperatures Tlow and Tmax.
| 191
In a temperature resistance measurement, a cuboid sample of 2W 4D-printed PDA-based
PEU (Figure 6.4a and Table 6.1) was clamped in the DMA and heated to 62 °C with at a rate
of 5 °C x min−1. Adjacently, the temperature was held constant for 4200 min while the
evolution of strain was recorded under stress-free conditions.
Demonstrator development: The actuation of a sample made from a 2W 4D-printed PDA-
based PEU was examined with an MTS Criterion universal testing machine (UTM) (model 43)
from MTS Systems Corporation (Eden Prairie, MN, USA). The device was operated with a
temperature chamber, which was controlled by a Eurotherm temperature controller unit.
Two heating elements were located at the back of the chamber. Liquid nitrogen from a
Dewar's vessel was fed into the chamber under a pressure of 1.3 bar as an essential
prerequisite for cooling. At first, the 2W 4D-printed hollow cylinder with an outer diameter
of 30 mm, a height of 20 mm, and a thickness of 1 mm, was glued to a bar made of
poly(methyl methacrylate) with dimensions 40 mm × 20 mm. The other end of the bar was
then attached to an L-shaped bracket, and the bracket was attached to the top clamp of the
UTM. To better visualize actuation, a brownish and a colorless bottle of which the latter was
filled with a red liquid were placed on a platform, which was lined with centimeter paper.
While, in the case of the artificial butterfly, individual parts were first laser cut using Epilog
Zing 16 laser Engraver 30 W with suction exhaust from Epilog Laser United Kingdom (Kenn,
Clevedon, United Kingdom). The parts and 2W 4D-printed cuboid PDA-based PEU samples
(Table 6.1) were then assembled. Afterwards, the temperature was cycled in between 23 and
62 °C with heating and cooling rates of 10 °C x min−1.
Supporting Information: Supporting Information is available from the Wiley Online Library or
from the author.
Acknowledgements: This work was supported by the Fraunhofer Cluster of Excellence
“Programmable Materials” under project PSP elements 40-01922-2500-00002 and 40-03420-
2500-00003. The working group of Fraunhofer IAP wishes to thank the European Regional
Development Fund for financing a large part of the laboratory equipment (project 85007031)
and the Fraunhofer High-Performance Center “Integration of Biological and Physical-Chemical
Material Functions” in Potsdam-Golm for the funding of some of the FFF printers (project
630505). Tobias Rümmler is kindly acknowledged for crafting the artificial butterfly and
supporting DC during demonstrator filming.
Conflict of Interest: There are no conflicts to declare.
Data Availability Statement: The data that support the findings of this study are available
from the corresponding author upon reasonable request.
Received: March 25, 2022; Revised: May 23, 2022
Published online: (First published: 13 July 2022)
192 | Chapter 6
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194 | Discussion
Chapter 7:
Discussion
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Chapter 7: Discussion
Before the beginning of this doctoral work in 2017, there were only a few scientific
research works on fused filament fabrication (FFF) with thermoplastic polyurethanes (TPU)
with shape memory properties [1–4]. This doctoral thesis focuses on processing TPU-based
shape memory polymer (SMP) with commercially available FFF machines to reduce the
complexity of additive manufacturing (AM) and ease the slicing of three-dimensional (3D)
models using standard slicer software. The primary goal of this doctoral study is to utilize FFF
3D printers to manufacture TPU-SMP objects. For exhibiting shape memory properties on
thermomechanical treatment (“ex-situ” programming) or even as a result of printing (“in-situ”
programming).
In 2012, Pretsch and his group introduced quick response (QR) code carriers made from
TPU with shape memory properties, which can be both employed as anti-counterfeiting
technology [5] and for supervising cold chains [6]. However, the production of information
carriers was labor-intensive, where, at first, the TPU-SMP plaques were produced using an
injection molding technique. Surface-specific coloration via “guest diffusion” and engraving
of two-dimensional computer-generated codes onto the surface of the polymer [5], or – as
later introduced – applying coating and code engraving on the surface, punching or laser
cutting [7,8]. In the first part of the doctoral study, information carriers using the QR code
technique are manufactured via FFF from a dyed and a non-dyed TPU-SMP to establish a new
production route [9]. The developed manufacturing process eases production and allows
exchanging of QR codes with less effort. Since the codes can be easily imported into the slicer
software, rapid prototyping is only associated with little effort. An essential understanding
extracted from this approach is that using a 100 µm nozzle qualifies to manufacture
information carriers with higher print quality in 25 min. Here, on employing a 100 µm nozzle,
one of the small cuboidal elevations of the QR code was characterized using a microscope
and found to have exceeded the virtual 3D model by just 40 µm, thereby revealing the high
print resolution and less dimensional tolerance. On the other hand, using a 400 µm nozzle for
FFF printing shows slightly compromised print quality. However, it permits to finalization of
the manufacturing of machine-readable information carriers in 11 min and 30 s.
Furthermore, by reducing the substrate height of the information carriers from 180 µm to
9 µm, it is possible to significantly reduce the weight of the QR code from 340 mg to about
100 mg, which qualifies the manufacturing of lightweight QR codes. Most importantly, the
shape memory properties of the QR codes reveal that the information carriers are durable
and can be bent or rolled in a thermomechanical treatment step in contrast to the classical
programming method, which includes tensile deformation, compressive deformation, or
indentation [10]. In any case, the QR codes may recover their permanent shape in less than
10 s, proving the high sensitivity toward heating, primarily achieved using a thinner base
substrate. The development presented here may be considered the first step toward mass
production as this may reduce material costs, manufacturing time, and logistics costs. The
196 | Discussion
work also compares the build rate versus layer thickness of different AM techniques like fused
filament fabrication (FFF), stereolithography (SLA), big area additive manufacturing (BAAM),
multi-jet fusion (MJF), and selective laser sintering (SLS). Here it becomes clear that even
though FFF has lower build rates than other AM methods, the FFF technique can control the
layer thickness over a wide range, achieving the lowest value for Z (layer thickness) [11]. The
excellent print resolution presented via FFF is also superior to the results from other working
groups using extrusion-based AM techniques to process TPU with shape memory properties
[1–4,12–15]. This good print quality was achieved primarily from the precise selection of
printing parameters in the slicer software, such as lower printing speed and lower nozzle
temperature, along with a smaller nozzle opening (100 µm). The extruded polymer strand
could be precisely controlled during deposition with minor errors and stringing.
In a different approach, efforts are taken to control the printing pattern of objects made
from TPU-SMP to understand how print-orientation in semicrystalline polyester urethanes
(PEUs) affects the mechanical and shape memory properties. The study was conducted
because few scientific publications have previously studied the influence of printing patterns
of SMP 3D models manufactured via FFF. The studies were primarily carried out on
amorphous SMPs [12,16]. Here, it was unknown how semicrystalline SMP properties vary
using different printing patterns. Specifically when applying a tensile strain during
thermomechanical treatment. This work observed that injection-molded tensile bars exhibit
superior mechanical properties compared to 3D-printed samples [17–19]. Most importantly,
the properties of the semicrystalline PEU are analogous to other thermoplastics, which are
processed via FFF in horizontal and vertical orientations. The ultimate tensile strength was
highest when the tensile strain acted parallel to the print orientation [18]. The other FFF
thermoplastic materials that were used to perform similar studies are acrylonitrile butadiene
styrene (ABS) [20,21], ABS-carbon nanotubes composite [22], polycarbonate-ABS blends [19],
polypropylene [17,23,24], and polyethylene glycol diamines [25]. Astonishingly, the
characterization reveals that the recovery strain remains unaffected for print orientation
patterns or injection molded tensile bars. Nevertheless, when measuring the recovery
stresses during the triggering of the shape memory effect (SME) under constant strain
conditions, the injection-molded sample shows the highest recovery stress, followed by the
horizontally printed and then vertically printed FFF-PEU sample. Comparing the results of the
work to other scientific contributions, the evolution in recovery stresses is in line with the one
verified by Villacres et al. [16], where it is deduced that the recovery stress and tensile
strength are the highest once the load direction is parallel to the strand orientation for both
semicrystalline and amorphous materials [18]. In this way, the print orientation of AM objects
can be locally controlled to set the mechanical and shape memory properties. Without the
need to restart the material or product development from the polymer synthesis or add
additives during processing.
The ability to print high-resolution small cuboidal elevations as part of the QR codes atop
a thin film of PEU using a 100 µm nozzle in the first part of the study (Chapter 3.1) has directly
opened up the bridge to fabricate filigree structures of TPU-SMP with FFF. Here, exploring the
| 197
lowest printing limits of manufacturing filigree structures is of scientific interest to enable the
manufacturing of miniature objects in high resolution using FFF. For this, Arial fonts of the
letter “A” in sizes 3, 4, and 10 are printed using FFF with differently colored TPU-based SMP
filaments. Closely studying the print quality under a microscope reveals the high-quality
printability of filigree structures. The Arial font “A” in size three exceeded the longitudinal and
transverse dimension of about 100 µm compared to its virtual design. It is also apparent at
this point that the miniature printing exceeds the print resolution of the previous extrusion-
based AM contributions [1–4,11–15], which was again attained due to the careful selection
of printing parameters and printing setups, as explained earlier. Further, the Arial font size
ten is used to print texts that can be read with the naked eye. The developed SMP letters have
application potential as privacy etiquettes, security labels, text-based cold chain indicators,
information carriers, admission tickets, or entry tickets. Additionally, smart keyboard keys
were developed, where the fonts on the keys can change their shape and height from a non-
legible to readable form when triggering the SME. The shape memory keyboard keys may be
employed in bilingual keyboards such as Braille and QWERTY layouts.
Shape memory gears are one further example of filigree printing. They can activate or
deactivate transmission systems on demand. This application of SMP has potential
applicability in mechanical and automotive industries, where an overheating of a system can
be prevented by triggering the shape memory effect and thus disconnecting the transmission.
In turn, activating a transmission system can regulate the flow of fluids, e.g., in the coffee
industry or biomedical applications.
The second part of the doctoral work focuses on four-dimensional (4D)-printing, where
internal stresses are implemented in SMPs during AM to manufacture thermoresponsive
objects [26–28]. The key benefit of this functional integration is that the objects do not
require additional thermomechanical treatment. Thus, it is a crucial step to reduce the effort
required to produce thermoresponsive objects. The concept of 4D-printing was introduced in
a TED talk in 2013 by Skylar Tibbits on multi-material printing using a poly jet printer [29,30].
Later, 4D-printing was realized via FFF with a single material [26–28,31–37]. Until the first half
of 2021, researchers focused on implementing thermoresponsiveness in objects with smaller
layer heights ( z < 5 mm), where the degree of shrinkage gets weaker from layer to layer
[26,28,32,33,36,37]. Furthermore, there were only a few contributions to 4D-printing utilizing
TPU-based SMP as functional base material [28].
This work aims at investigating new materials suitable for 4D-printing. In the first step, a
novel polypropylene glycol (PPG)-based polyether urethane is investigated as 4D-printing
material. Here, the work reveals that using a lower nozzle temperature and keeping the
difference between the nozzle and glass transition temperature of the TPU-SMP to its
minimum can effectively store internal stresses and supports strong “4D effects” in the sense
of sample shrinkage upon heating. This allowed us to deduce the key for 4D-printing to avoid
bending and enable uniform shrinkage, allowing us to build larger objects up to 32 mm on the
z-axis without compromising the 4D shrinkage effect. From a processing point of view, it is
advantageous when the glass transition temperature of the printing material exceeds
198 | Discussion
ambient temperature, which helps to store more significant internal stresses. The work
further advances by demonstrating one of the highest 4D shrinkage effects achieved with FFF
for a rectangular cuboid with a height and width of 2 mm and length of 40 mm, for which a
strain shrinkage of about 63% is witnessed [26–28,31–37]. One of the successes of the work
is transferring the know-how of 4D-printing to other materials like polylactic acid (PLA), where
the ability to store maximum “4D effects” allowed to achieve the strongest 4D-shrinkage
behavior, among other contributions [32,37,38]. Transferring the results to more complex
objects is proven for a hollow cylinder and solid- and hollow cuboids with a total height of 32
mm. Finally, heat-shrinkable hands-free door openers are developed, whose design follows a
lightweight construction and allows optimum distribution of stresses upon loading. The
respective dimensions in the x-,y-, and z-axis are 105 mm × 18 mm × 30 mm and were able to
change to about 55 mm × 9 mm × 48 mm after heating to 75 °C and thereby triggering the 4D
effect. The uniform 4D shrinkage effect may help researchers develop new thermoresponsive
products of even larger sizes. The design freedom of the thermoresponsiveness indicates that
it will not always be necessary to change the material composition to address innovations in
the field of 4D-printing. The 4D-printing technique is then used to develop demonstrators for
active assembly that is directly programmed during the printing process. The active assembly
concept was first introduced by Chiodo et al. in 1999 [39]. This way, active thermoresponsive
fixtures, closures, connecting elements, and grippers can be produced without requiring
further programming step.
Additionally, the proof-of-concept for active disassembly and end-of-life technologies is
also shown, in which the separation of two and three-component systems is facilitated. The
objects are capable of self-disassembling upon heating. The active assembly and disassembly
of parts have application potential in parts for furniture or construction, manufacturing or
production industries, or even inside space stations, space habitats, or environments where
the human body has restricted access. For space habitats and technologies, it would be
necessary to meet space material requirements by adjusting the material formulation as it is
a preliminary requirement of materials to be used for space applications. Another application
is an active-deactivating gear. Once the ambient temperature exceeds a critical value, power
transmission can be switched off, thus interrupting the transmission system to reduce the risk
of damaging any components or neighboring electronics. Such systems may be implemented
in industrial safety systems to avoid overheating. Later, replacing the 4D-printed object with
a new gear after reprocessing or reprogramming is a prerequisite to entering the next life
cycle. Finally, the 4D-printing technique is used to develop programming tools for the
thermomechanical treatment of other SMPs. The 4D-printed tools can also be used with foam
to grip or hold objects during transport.
The objects programmed via classical one-way (1W) programming (“ex-situ”) or 4D-
printing (“in-situ”) always require reprogramming or reprocessing to make them
thermoresponsive again. To enable switching between two metastable states, scientists and
researchers have learned how to attain with semicrystalline SMPs, where the shapes can be
reversibly switched as often as desired by varying the stimuli. The so-called two-way (2W)
| 199
SME was first observed in 2008 for cross-linked poly(cyclooctene) films in the presence of an
external load by Chung et al. [40]. The programming technology was later transferred to other
polymer systems, so actuation in the stress-free state became possible [41–47]. Although
scientists could transfer the 2W-SME to physically cross-linked TPU with shape memory
properties under constant strain and stress-free conditions, their overall thermoreversible
actuation was lower than their chemically cross-linked counterparts [41,48–52].
Additionally, SMPs were employed in soft robotics to demonstrate the thermoreversible
actuation of grippers [53–56]. Although the contributions presented triggering of the 2W-
SME could grab or lift smaller objects like screws and nuts [57], the scalability of the 2W-SME
to transport larger seized objects remained unknown. Furthermore, it was unclear how to
amplify the effect and combinate 2W-programmed SMP with other polymers to develop
programmable materials. These points are addressed in the third part of this doctoral work
using an in-house synthesized poly(1,10-decylene adipate) diol (PDA) based polyester
urethane. After identifying a suitable programming method, a novel characterization
technique is developed to identify the ideal actuation temperatures for the 2W-SME at which
a maximum actuation occurs. Here, stable shrinkage on heating and expansion on cooling,
exemplified by averaged strain changes of about 16%, is verified for a thermomechanically
pretreated TPU-SMP. Combining a TPU-based SMP actuator with a mechanical linkage system
allows for magnifying the 2W-SME. For instance, in a soft robotic gripper system, the 2W-SME
of an actuator can transfer and increase its motion sixteen-fold for the gripper arms, thus
achieving an opening and closing of the arms to grab and release bigger and more complex
objects. On the other hand, coupling the SMP actuator with an elastic unit cell leads to the
developing of novel programmable materials. In this way, the 2W-SME is directly transferred
to the unit cell to achieve a thermoreversible shape switch, accompanied by the opening and
closing of a hole in the surface. Unit cells with thermally switchable openings can potentially
enable a programmable heat transfer. The self-sufficient material behavior is desirable for
applications that do not require a power supply, control technology, or similar.
However, these studies show that the actuation of physically cross-linked PEUs could
achieve a maximum of only 21% [41,58–62], and a significant actuation can be extracted from
the SMP only with the combination of a mechanical linkage [47]. It is essential to understand
if an SMP-based programmable material structure can exhibit bidirectional actuation, in
which non-linear and more complex motion sequences characterize the movements. For this,
efforts are taken to develop a 2W programmable gear. Once programmed and characterized,
the results revealed that a reversible change in length ΔL/L0 of about 45% could be achieved.
Since the programming procedure includes compressional bending, the results cannot be
compared with those of other contributions because the deformation in the programming
course was realized by tensile stretching.
Furthermore, an actuating element is developed to ensure that the strong 2W-SME
achieved with the gear can be transferred and achieved for another actuating element when
programmed similarly. After thermomechanically treating the actuating element similar to
the 2W programmable gear, almost similar actuation is detected by varying the temperature
200 | Discussion
between 23 °C and 58 °C. Excellent actuation is achieved due to the employment of
programmable materials as actuating elements, where the external and molecular structures
are synergetically utilized during actuation. At the same time, previously presented
contributions focused only on the molecular structure for 2W-SME.
From the previous section of this work, it becomes apparent that SMPs require a complex
thermomechanical treatment that includes time-, energy-, and cost-intensive steps. Here, the
maturation of AM technology in recent years has led to the development of objects that
actuate immediately after AM [63]. Up to 2022, the 2W-4D printing technique was attained
either by using core-shell embedded structures or bilayer or gradient structures.[46] The
core-shell embedded structures utilized the manufacturing of elastomeric material
composites containing magnetic particles[64–67]. Under an external magnetic field, the
material composite could change its shape controllably. Once the magnetic field was turned
off, the original shape was recovered due to the elastomeric properties of the matrix material.
In contrast, the bilayer or gradient structures were based on composite AM technology.[68–
74], where composite layers were selected with antagonistic or reversible physical properties,
e.g., associated with water absorption/desorption or temperature changes, to realize
stimulus-governed shape changes. Nevertheless, the main drawback of such systems is the
need for two entirely different stimuli, like water and heat. Moreover, the effort to recover
and recycle individual materials at the end of their life cycle was very high. In short, there
were no technological approaches for a single material system that exhibited bidirectional
actuation immediately after AM. Motivated by this, the heart of the work is to develop a novel
additive manufacturing technique for semicrystalline SMPs, which allows programming the
2W-SME using a single material. To pursue this, a TPU-SMP, which was built up from soft
segments of poly(1,10-decylene adipate) diol (PDA) and hard segments of 4,4′-
diphenylmethane diisocyanate (4,4′-MDI) and 1,4-butanediol (BD) were selected. For
attaining 4D-printing and, thereby, functional integration. For this, an external cooling unit is
combined with the lower nozzle temperature, allowing the SMP to store internal stresses in
its oriented state. Modifying a standard printer with an external cooling air unit ensures the
polymer quickly cools below its glass transition temperature, thereby achieving rapid
vitrification of the switching segment. The work thereby shows one of the first attempts to
4D-print a bidirectionally actuating semicrystalline SMP utilizing FFF. After a temperature
screening and extrapolating the ideal actuation temperature range for attaining the most
pronounced 2W-SME, it is observed that the polymer can arbitrarily actuate between two
shapes under stress-free conditions. This way, new generations of programmable materials
or actuating elements can be manufactured without needing a subsequent
thermomechanical treatment.
The work further shows an actuating hollow cylinder fabricated via the “in-situ” 2W-4D
printing technique. After printing, the developed cylinder could contract and expand its
circumference on heating and cooling. The actuation is strong enough to lift smooth surfaced
objects and release them on demand. Such actuating elements can play a role in soft robotics,
e.g., in grippers for transportation or even in smart etiquettes, which can actively assemble
| 201
or disassemble at the end of life. Assuming that the high material requirements in space
technologies are met, the extent of actuation may be leveraged by specific mechanisms to
qualify such systems for space-deployable structures.
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Chapter 8: Conclusion and Outlook
206 | Conclusion and Outlook
Chapter 8: Conclusion and Outlook
This dissertation aims at utilizing fused filament fabrication (FFF) to enable various shape
memory effects (SMEs) for shape memory polymers (SMPs), making it a powerful tool to
qualify both commercial and novel shape memory polyurethanes for new applications.
The doctoral work shows the processing of SMPs as differently colored filaments to build
up high-contrast three-dimensional (3D) objects. By using different-sized nozzles, it is
experimentally proven that the print resolution in the XY- plane and Z-direction concerning
the layer height can be controlled by the experimental setup same as by the print instructions.
Most importantly, additively manufactured quick response (QR) codes are characterized by
distinct shape memory properties after thermomechanical treatment. The primary
advantages of the developed additive manufacturing (AM) process are the quick availability
of QR codes, control over the print resolution, the possibility to fabricate QR codes even with
substrate thicknesses below 10 µm, and avoiding using solvents during production.
Further understanding of the ability to manipulate the print orientation helps to control
the mechanical and shape memory properties of SMPs. This is particularly important when
tuning or adjusting the material properties of specific parts of an object without changing the
material itself by modifying the polymer synthesis. Extending the filigree FFF printing
approach to the fabrication of miniature objects like letters with Arial font sizes 3, 4, and 10
allows for identifying the lower boundaries of FFF, which helps to expand the application
platform for FFF 3D printers. The development of filigree printing demonstrated shape
memory keyboard keys, which may be employed in bilingual keyboards like Braille and those
with QWERTY layout. Here, for better applicability of smart keyboard keys, the one-way (1W)
SME of font scripts has to be later extended to a two-way (2W) SME to transfer them into two
metastable states. The introduction of heating and cooling elements for the SMP letters can
further trigger the 2W-SME on command. Further, developing SMP gears for activating or
deactivating a transmission gear is a significant step towards addressing potential applications
in many industries and mechanical systems to activate or deactivate transmission systems on
demand.
Taking a step further, FFF is first utilized to achieve “in-situ” programming and thus the
implementation of the one-way SME. Such one-way four-dimensional (4D)-printed objects
exhibit thermoresponsive behavior directly after printing. Here, the challenges were to
enable uniform shrinkage of objects and to build up homogenous and thermoresponsive
objects of larger size. This is achieved when printing with a lower nozzle temperature, thereby
keeping the difference between the nozzle and glass transition temperature of the SMP as
small as possible. Transferring the know-how to polylactic acid (PLA) material shows that the
findings can be transferred to other thermoplastics is possible. The understanding of the work
enables the development of novel applications for 4D-printing, including active assembly,
disassembly, deactivating gear, and programming devices that can program other SMPs or
for holding or gripping complex objects.
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The work further shows the processing of a semicrystalline poly(1,10-decylene adipate)
diol (PDA) based polyester urethane via FFF and its thermomechanical treatment to achieve
2W-SME. The novel temperature screening characterization method allows us to identify the
optimum actuation range and conditions. Further, it is noteworthy to mention that the
combination of actuating elements with a mechanical linkage system allows multiplying the
actuation by about sixteen-fold of the materials 2W-SME. This permitted the development of
grippers, especially for soft robotics. Moreover, the combination of actuating elements with
deformable structures allows new approaches in the field of thermoresponsive
programmable materials.
Furthermore, the work put effort into combining programmable materials with SMPs,
where the former is manufactured using the latter. Once thermomechanically treated and
investigated in a stress-free state by varying temperatures, thermoreversible length changes
ΔL/L0 of about 45% were witnessed. The developed programmable gear can actively engage
and/or disengage a force transmission system at specific temperatures. The work further
shows that other actuator designs can obtain more complex motion sequences. The more
significant reversible length change ΔL/L0 was achieved from a relatively smaller actuating
element to be used as an alternative actuating element for the previously presented
actuation grippers.
Last but not least, the ability to process the same semicrystalline PDA-based SMP permits
the accomplishment of two-way 4D-printing, where objects actuate directly after AM when
changing the temperature. This is particularly beneficial because no further
thermomechanical treatment is necessary. This approach creates many opportunities in the
future for the mechanical, aerospace, and biomedical industries, where moving parts can be
manufactured with fewer parts, thereby reducing the probability of failures. Even though the
thermoreversible actuation was comparatively lower than in the case of “ex-situ” 2W-
programmed samples, the extent of actuation may be large enough to address new
applications, e.g., as temperature-surface morphing, biomedical grippers, or sensory
applications.
In the future, one-way 4D-printing using FFF could turn out as a seminal technology for
SMPs in fields like the counterfeit-proof marking of goods at risk of plagiarism or the
supervision of cold chains. Future challenges include shortening the production time without
compromising on resolution. In the case of 2W-4D printing, the material platform will have to
be drastically expanded to eliminate the need for an external cooling unit. This would open
up a wide variety of applications, including sustainable energy objects or parts that may
contribute to the introduction of a circular plastics/materials economy, where one
metastable state allows active assembly and the other active disassembly. This permits the
parts to be recovered separately for effective recycling for the next life cycle. The
improvement of the future 3D printers would be to include multi-extrusion heads that work
parallelly in a single-build platform to realize large-scale manufacturing. Artificial intelligence
and monitoring devices could be used to observe real-time printing conditions and forecast
and adapt the printing parameter to achieve failure-free printed parts. In the case of slicer
208 | Conclusion and Outlook
software, the software should be improved to implement user-friendly options, e.g., the
printing patterns or print orientation can be easily modified for each or selected layer during
the slicing. In addition, it can run a simulation study to understand the thermal, mechanical,
and thermomechanical behavior and predict and optimize the printing parameter to achieve
desired 4D-shape changes on appropriate stimulus. Another critical step is evaluating how
similar results can be achieved using other AM techniques. A further significant technological
step forward would be the transfer of the 2W-4D printing to other melt-based processing
methods like injection molding to realize mass production of thermoresponsive objects.
| 209
Appendix
210 | Appendix
Appendix
Journals
1. Chalissery, D.; Pretsch, T.; Staub, S.; Andrä, H. Additive Manufacturing of Information
Carriers Based on Shape Memory Polyester Urethane. Polymers 2019, 11, 1005.
https://doi.org/10.3390/polym11061005
2. Chalissery, D; Pretsch, T; Staub, S; Kasack, K; Andrä, H; 3D-Druck von QR-Codes mit
Formgedächtniseigenschaften. In Druckspiegel (Nr.11-12), pp. 34–37.
3. Schönfeld, D.; Chalissery, D.; Wenz, F.; Specht, M.; Eberl, C.; Pretsch, T. Actuating
Shape Memory Polymer for Thermoresponsive Soft Robotic Gripper and
Programmable Materials. Molecules 2021, 26, 522.
https://doi.org/10.3390/molecules26030522
4. Chalissery, D., Schönfeld, D., Walter, M., Shklyar, I., Andrae, H., Schwörer, C., Amann,
T., Weisheit, L. and Pretsch, T. (2022), Highly Shrinkable Objects as Obtained from 4D-
printing. Macromol. Mater. Eng., 307: 2100619.
https://doi.org/10.1002/mame.202100619
5. Chalissery, D., Schönfeld, D., Walter, M., Ziervogel, F., Pretsch, T., Fused Filament
Fabrication of Actuating Objects. Macromol. Mater. Eng. 2022, 2200214.
https://doi.org/10.1002/mame.202200214.
Patents
1. DE102018003273A1 Verfahren zur Herstellung von komplexen Strukturen aus
thermoplastischen Polymeren und Polymer-Formteile mit solchermaßen
hergestellten, komplexen Strukturen; Pretsch, Thorsten; Chalissery, Dilip
2. DE102019007939A1 Verfahren zur Herstellung von Polymer-Formteilen aus
thermoplastischen Polymeren mit Formgedächtniseigenschaften und/oder mit
thermoresponsiven Eigenschaften, insbesondere mittels 4D-Druck, sowie
solchermaßen hergestelltes Polymer-Formteil; Walter, Mario; Pretsch, Thorsten;
Chalissery, Dilip; Frieß, Fabian; Rümmler, Tobias; Schönfeld, Dennis
3. DE102022124851.8 Verfahren zur Programmierung von Polymer-Formteilen aus
Polymeren mit Formgedächtniseigenschaften oder mit thermoresponsiven
Eigenschaften; Pretsch, Thorsten; Chalissery, Dilip; Ziervogel, Fabian
4. DE102022126382.7 Lagenförmiges Verbundmaterial mit Formgedächtnispolymeren
und Verfahren zu seiner Herstellung; Pretsch, Thorsten; Chalissery, Dilip; Ziervogel,
Fabian
| 211
Presentations
1. Thermoresponsive Programmable Materials, International Conference on
Programmable Materials, 12.7.2022, Dilip Chalissery, Dennis Schönfeld, Mario
Walter, Heiko Andrae, Franziska Wenz, Chris Eberl, Linda Weisheit, Fabian Ziervogel,
Sarah C. L. Fischer, Thorsten Pretsch, Berlin (Deutschland)
2. Formgedächtnispolymere – neue Möglichkeiten mit 4D-Druck, KeyNote Lecture, 9.
Mitteldeutsches Forum 3D-Druck in der Anwendung, 29.6.2022, Dilip Chalissery,
Dennis Schönfeld, Mario Walter, Inga Shklyar, Heiko Andrä, Christoph Schwörer,
Tobias Amann, Linda Weisheit, Thorsten Pretsch, Jena (Deutschland)
3. Recognizing the Potential of 4D-Printing, 2nd Edition of International Conference on
Materials Science and Engineering, 28.3.2022, Dilip Chalissery, Dennis Schönfeld,
Mario Walter, Inga Shklyar, Heiko Andrä, Christoph Schwörer, Tobias Amann, Linda
Weisheit, Thorsten Pretsch, Virtuelle Veranstaltung (Indien)
4. Additive Manufacturing of thermoresponsive Objects, KeyNote Lecture, Global
Scientific Guild Conference: Global Webinar on 3D Printing and Addtive Modeling
25.2.2022, Dilip Chalissery, Dennis Schönfeld, Mario Walter, Inga Shklyar, Heiko
Andrä, Christoph Schwörer, Tobias Amann, Linda Weisheit, Thorsten Pretsch, Virtuelle
Veranstaltung (Indien)
5. Funktionsintegration mit Additiver Fertigung, 4. Veranstaltung - Virtuelles
Werkstattgespräch, 19.01.2022, Dilip Chalissery, Dennis Schönfeld, Mario Walter,
Inga Shklyar, Heiko Andrä, Christoph Schwörer, Tobias Amann, Linda Weisheit,
Thorsten Pretsch, Virtuelle Veranstaltung (Deutschland)
6. Presentation titled “Addive Manufacturing of thermoresponsive Objects” at Global
Scientific Guild Conference on 18-20 March 2022 (Online-event); Chalissery, Dilip;
Schönfeld, Dennis; Walter, Mario; Shkylar, Inga; Andrä, Heiko; Schwörer, Christoph;
Amann, Tobias; Weisheit, Linda; Pretsch, Thorsten
7. Presentation titled “Funktionsintegration mit Additiver Fertigung“ at 4th Virtual
Workshop “Additive Fertigung” on 19 Januar 2022; Pretsch, Thorsten; Chalissery,
Dilip; Schönfeld, Dennis; Walter, Mario; Shklyar, Inga; Andrä, Heiko; Schwörer,
Christoph; Amann, Tobias; Weisheit, Linda
8. Presentation titled “Additive Fertigung mit Formgedächtnispolymeren” at 4th Status-
Workshop of Fraunhofer High-Performance Center for Functional Integration in
Materials, Phase III & KickOff zum Joint Lab BioF, 30. September 2021, (Online event);
Pretsch, Thorsten; Chalissery, Dilip; Schönfeld, Dennis; Walter, Mario; Shkylar, Inga;
Andrä, Heiko; Schwörer, Christoph; Amann, Tobias; Weisheit, Linda
9. Presentation titled “Shape Memory Polymers in Transition to Programmable
Materials” at 2nd Advanced Chemistry World Congress - Latest Global Innovations and
Market Insights in Chemistry, 14-15 June 2021, Berlin, Germany; Pretsch, Thorsten;
Schönfeld, Dennis; Chalissery, Dilip; Walter, Mario; Köbler, Jonathan; Andrä, Heiko;
Wenz, Franziska; Eberl, Chris
212 | Appendix
10. Presentation titled “Design for Recycling mit Formgedächtnispolymeren” at VDI -
Arbeitskreis Kunststofftechnik, 25. November 2020; Pretsch, Thorsten; Chalissery,
Dilip; Schönfeld, Dennis; Walter, Mario; Wafzig, Florian
11. Presentation titled “Formgedächtnispolymere für Soft Robotics” at WerkstoffWoche
2019, Symposium “Soft Robotik, mechanische und funktionalisierte Materialien”, 18.-
20. September 2019, Dresden; Walter, Mario; Frieß, Fabian; Rümmler, Tobias;
Chalissery, Dilip; Pretsch, Thorsten
12. Presentation titled “Additive Fertigung von Informationsträgern mit
Formgedächtniseigenschaften” at WerkstoffWoche 2019, Symposium “Additive
Fertigung”, 18.-20. September 2019, Dresden; Chalissery, Dilip; Rümmler, Tobias;
Pretsch, Thorsten
13. Presentation titled “Additive Manufacturing of complex filigree-structured Objects
based on Shape-Memory Polymer” at Elmia Subcontractor Trade Show, 13.–16.
November 2018, Jönköping/Sweden; Pretsch, Thorsten; Chalissery, Dilip
14. Industry Workshop: High-Performance Center »Integration of Biological and Physical-
Chemical Material Functions« 3D-Printing of Shape-Memory Polymers; 17 July 2018;
Dilip Chalissery, Dr. Thorsten Pretsch
15. Bericht zum Status des Projektes LIM-5: Formgedächtnispolymere für den 3D-Druck;
17 May 2018; Dr. Thorsten Pretsch, Dilip Chalissery, Dennis Schönfeld
16. Bericht zum Status des Projektes LIM-5: Integration von
Formgedächtnisfunktionalitäten in polymere 3D-Druckmaterialien; 20 November
2018; Dr. Thorsten Pretsch, Dilip Chalissery, Harish Babu Eppa, Dr. Mario Walter, Dr.
Fabian Frieß, Tobias Rümmler
17. Vorstellung der Projektskizze: Funktionsintegration durch Additive Fertigung von
Objekten mit Formgedächtniseigenschaften; 16 May 2019; Dr. Thorsten Pretsch, Dilip
Chalissery, Dr. Mario Walter, Dennis Schönfeld
18. AMBER - AM Cluster Berlin-Brandenburg Kickoff meeting and virtuell industry
Workshop on 28 May 2020 and 10 June 2020, respectively; Prof. Dr. Aleksander Gurlo,
Dr. Stephan Schröder, Dr. Siegfried Behrendt, Dr. Jens Kurreck, Ben Jastram, Dr.
Dietmar Göhlich, Dr. Mario Birkholz, Dr. Jens Günster, Dr. Dietmar Stephan, Dr.
Mathias Czasny, David Karl, Dr. Thorsten Pretsch, Dilip Chalissery
Permanent Database:
https://publica.fraunhofer.de/entities/person/ab790674-007c-41bf-8b52-
aca0a75b0f10/publications
| 213
List of Abbreviations and Symbols
214 | Appendix
List of Abbreviations and Symbols
1W
One-way
2W
Two-way
2-NMP
N-Methyl-2-pyrrolidone
2PL
Two-photon lithography
3D
Three-dimensional
4D
Four-dimensional
ABS
Acrylonitrile butadiene
styrene
AM
Additive manufacturing
AMF
Additive manufacturing file
BAAM
Big area additive
manufacturing
BD
Butanediol
CAD
Computer-aided design
CIE
Crystallization-induced
elongation
CNC
Computer numerical
controlled
CTM
Cyclic thermo-mechanical
measurements
DMA
Dynamic mechanical analysis
DSC
Differential scanning
calorimetry
FFF
Fused filament fabrication
FGE
Formgedächtniseffekt
FGP
Formgedächtnispolymere
FTIR
Fourier-transform infrared
spectroscopy
HIPS
High impact polystyrene
MDI
Methylene diphenyl
diisocyanate
MIC
Melting-induced contraction
MJF
Multi-jet fusion
PBA
Poly(butylene adipate)
PDA
Poly(decylene adipate)
PET-G
Polyethylene terephthalate
glycol
PEU
Poly ester/ether urethane
PEUU
Polyester urethane urea
PLA
Polylactic acid
PMMA
Poly(methyl methacrylate)
PPG
Polypropylene glycol
QR
Quick response
SLA
Stereolithography
SLS
Selective laser sintering
SME
Shape memory effect
SMP
Shape memory polymer
STL
Standard triangle language
TPU
Thermoplastic polyurethane
TTIP
Titanium(IV) isopropoxide
X-ray
X-radiation
E´
Storage modulus
E´´
Loss modulus
L
Length
SP
Speed of print head
T
Temperature
Tc
Crystallization transition
temperature
Td
Deformation temperature
Tfix
Fixation temperature
Tg
Glass transition temperature
Tm
Melt transition temperature
Tmax
Maximum actuation
temperature
TN
Nozzle temperature
TP
Print platform temperature
Tperm
Melt temperature
Ttrans
Phase transition temperature
ΔHc
Crystallization enthalpy
ΔHm
Melting enthalphy
ε
Strain
σ
Stress
𝜌
Density
