materials
Article
Boehmite Nanofillers in Epoxy Oligosiloxane Resins:
Influencing the Curing Process by Complex Physical
and Chemical Interactions
Ievgeniia Topolniak 1,*, Vasile-Dan Hodoroaba 1, Dietmar Pfeifer 1, Ulrike Braun 1and
Heinz Sturm 1,2,*
1Federal Institute for Material Research and Testing (BAM), 12205 Berlin, Germany;
2Institute Machine Tools and Factory Management, Technical University Berlin, 10587 Berlin, Germany
*Correspondence: [email protected] (I.T.); [email protected] (H.S.)
Received: 22 April 2019; Accepted: 6 May 2019; Published: 9 May 2019
Abstract:
In this work, a novel boehmite (BA)-embedded organic/inorganic nanocomposite coating
based on cycloaliphatic epoxy oligosiloxane (CEOS) resin was fabricated applying UV-induced
cationic polymerization. The main changes of the material behavior caused by the nanofiller were
investigated with regard to its photocuring kinetics, thermal stability, and glass transition. The role of
the particle surface was of particular interest, thus, unmodified nanoparticles (HP14) and particles
modified with p-toluenesulfonic acid (OS1) were incorporated into a CEOS matrix in the concentration
range of 1–10 wt.%. Resulting nanocomposites exhibited improved thermal properties, with the
glass transition temperature (T
g
) being shifted from 30
◦
C for unfilled CEOS to 54
◦
C (2 wt.% HP14)
and 73
◦
C (2 wt.% OS1) for filled CEOS. Additionally, TGA analysis showed increased thermal
stability of samples filled with nanoparticles. An attractive interaction between boehmite and CEOS
matrix influenced the curing. Real-time infrared spectroscopy (RT-IR) experiments demonstrated
that the epoxide conversion rate of nanocomposites was slightly increased compared to neat resin.
The beneficial role of the BA can be explained by the participation of hydroxyl groups at the particle
surface in photopolymerization processes and by the complementary contribution of p-toluenesulfonic
acid surface modifier and water molecules introduced into the system with nanoparticles.
Keywords:
boehmite; nanocomposite; cationic photocuring; cycloaliphatic epoxy oligosiloxane;
epoxy conversion degree; real-time infrared spectroscopy
1. Introduction
Organic–inorganic nanostructured materials such as nanocomposites and hybrid materials have
attracted growing interest during the last decade, particularly due to their extraordinary properties
resulting from the structure of alternating organic and inorganic components. A relevant difference
between hybrids and nanocomposites is the fact that in the former ones the inorganic phase is formed
in situ, for instance by a sol-gel process, while nanocomposites are usually produced by dispersing
inorganic particles with at least one dimension less than 100 nm in a polymer matrix.
The use of inorganic filler in the nanoscale range is a well-known approach to enhance the specific
properties of polymers, e.g., thermal, electrical, optical, mechanical or barrier properties [
1
,
2
]. The effect
of a filler on the resultant nanocomposite depends on different factors including the particle shape,
size, loading, surface properties, agglomeration, and dispersion in the matrix. Therefore, it is quite
often hard to predict an overall change in polymeric material upon nanofiller inclusion.
Boehmite alumina (BA) shows promising results as a nanofiller. It was shown that incorporation
of these inorganic nanoparticles into polymers resulted in modification of their characteristics,
Materials 2019,12, 1513; doi:10.3390/ma12091513 www.mdpi.com/journal/materials
Materials 2019,12, 1513 2 of 18
in particular, surface hardness [
3
], fire retardancy [
4
], electrical [
5
], thermal [
5
], mechanical [
6
],
and barrier properties [
7
]. Boehmite is an aluminum oxyhydroxide containing the crystalline part
known as
γ
-AlO(OH) along with a pseudo-boehmite part which includes some water molecules [
8
].
The surface of BA can be easily modified enhancing the dispersion of the particles in various polymers.
In combination with ease of particle customization, this sparked a significant amount of research
performed on the integration of BA filler in thermoplastics as well as in thermosets [7].
Another path to achieve superior material properties is by implementing hybrid materials where
the advantageous properties of the polymeric materials and the inorganic structures are combined.
In these materials, organic and inorganic segments are covalently bonded and interpenetrate each other
on a few nanometers to a few micrometer ranges [
9
]. Among this class of materials, siloxane-based
hybrids have several advantages for material design. They are easy to synthesize or to modify with a
broad variety of commercially available precursors, easy to process, are non-toxic materials exhibiting
good transparency and superior mechanical properties which can be tuned between those of glasses
and those of polymers [10].
Applying the photocuring technology to produce hybrid coating offers numerous benefits covering
a considerable sector of industrial production. One of the promising areas in the photocuring industry is
cationic photopolymerization. In contrast to free-radical polymerization, cationic photopolymerization
is not inhibited by oxygen and exhibits a low degree of shrinkage. Furthermore, a very long lifetime of
active species allows dark-curing and thermal post-curing after UV light initiation [
11
–
13
]. However,
one of the major limitations of this process is low reactivity of such epoxies, especially compared
with acrylate-based hybrids. Cycloaliphatic epoxy formulations are the most reactive, and therefore,
extensively applied in the production of cationic UV-cured coatings [
14
]. Nevertheless, extremely
high crosslinking density of this resin leads to the poor toughness of cured films. To overcome this
drawback and to achieve superior performance of cycloaliphatic epoxy resins, the introduction of
oligomers and pre-polymers into the resin network can be applied [15,16].
Ultraviolet-curable cycloaliphatic epoxy oligosiloxane (CEOS) resin synthesized by simple
non-hydrolytic sol-gel reaction [
17
] was shown to be a material with superior properties. On one side,
implemented cycloaliphatic epoxy groups make it easy to process thin films at ambient temperatures
in a short time-frame. At the same time, CEOS displays good thermal stability, high refractive index,
low permeability, and was reported to be a good candidate for the encapsulation of flexible organic
electronics [
18
]. Nevertheless, nowadays, multifunctionality in a material formulation is essential.
For instance, coatings do not only play a protection role, but also deliver supplementary functions
including, but not limited to high adhesion and stability, chemical and scratch resistance, and high
durability [
19
]. Therefore, further reinforcement of the CEOS properties holds a great interest for
advanced material applications. It was shown that embedding of silica nanoparticles leads to enhanced
barrier properties of CEOS films [
18
]. However, the nature of the enhancement of its properties with
different nanofillers has not been fully established. The boehmite nanofillers carry potential due to the
versatility of particle morphology and surface properties. As a result, incorporation of well-tailored
BA into the CEOS matrix could yield a flexible, transparent resin layer with advanced functional
characteristics, which could be advantageous in applications where short-time processing at ambient
temperatures is required. This reflects the potential of this material in fields like microfluidics, organic
electronics or 3D micro-structuring.
The current study aims at investigating the preparation and characterization of a novel
UV-cured boehmite-embedded cycloaliphatic epoxy oligosiloxane resin films. To determine the
role of particle–matrix interphase in nanocomposite properties unmodified and surface modified
(with p-toluenesulfonic acid) boehmite nanoparticles were applied. The effect of surface modification
was verified by scanning electron microscopy (SEM) operated in transmission mode where the
particles’ distribution was evaluated. The curing behavior was studied by means of real-time infrared
spectroscopy (RT-IR) and the thermal behavior of the cured networks were analyzed with differential
scanning calorimetry (DSC) and thermogravimetric analysis (TGA). The suggested analytical approach
Materials 2019,12, 1513 3 of 18
allowed to correlate the evolution of curing degree, thermal events, and presence/absence of filler
interaction with polymeric network with respect to BA loading, filler distribution, and presence
of surface modifier. Consequently, possible hypotheses of boehmite influence on CEOS behavior
were discussed.
2. Materials and Methods
2.1. Materials
Commercially available boehmite alumina nanoparticles (BA) without surface modifier
(DISPERAL
®
, HP14) and modified with p-toluenesulfonic acid (DISPERAL
®
, OS1) were supplied
by Sasol, Hamburg, Germany. Selected particle parameters are presented in Table 1. The chemical
structures can be found elsewhere [20] (Supplementary Materials Figure S1).
Table 1. Physio-chemical properties of employed boehmite (BA) nanoparticles [21].
DISPERAL®HP14 DISPERAL®OS1
Mean crystallite size (120) [nm] 14 10
Surface area 1(m2/g) 160 240
Loose bulk density (g/cm3)400–600 400–600
Surface treatment - p-toluenesulfonic acid
1Calculated according to Brunauer–Emmett–Teller (BET) theory, activation at 550 ◦C for 3 h.
Cycloaliphatic epoxy oligosiloxane (CEOS) was synthesized via non-hydrolytic sol-gel reaction
and used as a UV curable matrix. The synthesis procedure is described below. Tetrahydropyran (THF,
99.9% for HPLC, Chemsolute, Berlin, Germany) was used as a solvent for nanocomposite preparation
without further purification.
2.2. Synthesis of Cycloaliphatic Epoxy Oligosiloxane (CEOS)
The CEOS was synthesized by a condensation reaction between 2-(3,4-epoxycyclohexyl)
ethyltrimethoxysilane (ECTS, 99.8%, Sigma–Aldrich, St. Louis, MO, USA) and diphenylsilanediol
(DPSD, 98%, Alfa Aesar, Tewksbury, MA, USA) in the presence of barium hydroxide monohydrate
(Honeywell, Düsseldorf, Germany) as a catalyst. The schematic representation of reaction is shown
in Scheme 1. For the sake of achieving high condensation yield, different ECTS:DPSD molar ratios
were investigated. The amount of added catalyst was 0.2 mol % equivalents of ECTS in the mixture.
The DPSD was continuously added to the ECTS in the flask under N
2
atmosphere while stirring the
solution with a magnetic stirrer for 2 h at 80
◦
C and additional 2 h at room temperature to complete
the reaction. After the reaction ended, the flask was vacuum heated to remove methanol formed as a
by-product during the condensation. Ba(OH)
2·
H
2
O was removed using 0.45-
µ
m pore-sized PTFE filter.
The detailed synthesis procedure is also described in detail elsewhere [
17
]. As a result, a colorless,
viscous solution of CEOS was obtained.
Materials 2019, 12, x FOR PEER REVIEW 3 of 18
scanning calorimetry (DSC) and thermogravimetric analysis (TGA). The suggested analytical
approach allowed to correlate the evolution of curing degree, thermal events, and presence/absence
of filler interaction with polymeric network with respect to BA loading, filler distribution, and
presence of surface modifier. Consequently, possible hypotheses of boehmite influence on CEOS
behavior were discussed.
2. Materials and Methods
2.1. Materials
Commercially available boehmite alumina nanoparticles (BA) without surface modifier
(DISPERAL®, HP14) and modified with p-toluenesulfonic acid (DISPERAL®, OS1) were supplied by
Sasol, Hamburg, Germany. Selected particle parameters are presented in Table 1. The chemical
structures can be found elsewhere [20] (Supplementary Materials Figure S1).
Table 1. Physio-chemical properties of employed boehmite (BA) nanoparticles [21].
DISPERAL® HP14 DISPERAL® OS1
Mean crystallite size (120) [nm] 14 10
Surface area 1 (m2/g) 160 240
Loose bulk density (g/cm3) 400–600 400–600
Surface treatment - p-toluenesulfonic acid
1 Calculated according to Brunauer–Emmett–Teller (BET) theory, activation at 550 °C for 3 hours.
Cycloaliphatic epoxy oligosiloxane (CEOS) was synthesized via non-hydrolytic sol-gel reaction
and used as a UV curable matrix. The synthesis procedure is described below. Tetrahydropyran
(THF, 99.9% for HPLC, Chemsolute, Berlin, Germany) was used as a solvent for nanocomposite
preparation without further purification.
2.2. Synthesis of Cycloaliphatic Epoxy Oligosiloxane (CEOS)
The CEOS was synthesized by a condensation reaction between 2-(3,4-epoxycyclohexyl)
ethyltrimethoxysilane (ECTS, 99.8%, Sigma–Aldrich, St. Louis, MO, USA) and diphenylsilanediol
(DPSD, 98%, Alfa Aesar, Tewksbury, MA, USA) in the presence of barium hydroxide monohydrate
(Honeywell, Düsseldorf, Germany) as a catalyst. The schematic representation of reaction is shown
in Scheme 1. For the sake of achieving high condensation yield, different ECTS:DPSD molar ratios
were investigated. The amount of added catalyst was 0.2 mol % equivalents of ECTS in the mixture.
The DPSD was continuously added to the ECTS in the flask under N2 atmosphere while stirring the
solution with a magnetic stirrer for 2 hours at 80 °C and additional 2 h at room temperature to
complete the reaction. After the reaction ended, the flask was vacuum heated to remove methanol
formed as a by-product during the condensation. Ba(OH)2‧H2O was removed using 0.45-μm pore-
sized PTFE filter. The detailed synthesis procedure is also described in detail elsewhere [17]. As a
result, a colorless, viscous solution of CEOS was obtained.
Scheme 1. Synthesis of cycloaliphatic epoxy oligosiloxane (CEOS) by a sol-gel condensation.
Scheme 1. Synthesis of cycloaliphatic epoxy oligosiloxane (CEOS) by a sol-gel condensation.
Materials 2019,12, 1513 4 of 18
Triarylsulfonium hexafluorophosphate salt (50 % in propylene carbonate, Sigma-Aldrich,
Steinheim, Germany) was added to CEOS as a cationic photoinitiator (PI) to enable further UV
photocuring reaction. The concentration of PI was fixed at 2 wt.% with regard to CEOS amount.
2.3. Nanocomposite Preparation and Film Curing
Boehmite suspension in THF was placed in an ultrasonic bath for 30 min and then treated by
an ultrasonic homogenizer (Sonopuls HP4200, Bandelin, Berlin, Germany) for 15 min immediately
preceding the mixing with CEOS/PI solution. To tailor the final viscosity, THF was added to CEOS
prior to mixing with the nanoparticles. After sonication, aliquots of BA suspensions were added to
CEOS solution so that formulations with filler loadings between 1 and 10 wt.% in the final cured
solid-state hybrid were achieved. The nanocomposite solutions were sonicated for 15 min with
ultrasonic homogenizer to ensure homogeneous distribution of the particles. Next, depending on
the further characterization method, films with different thickness/area size were obtained by either
spin-coating or bar-coating techniques. Due to the different sample sizes required, two different UV
light sources were used for CEOS curing. A set of Hg–Xe lamp Hamamatsu LC8 (Hamamatsu, Iwata,
Japan) with an optical waveguide and A9616-05 bandpass filter (Schott, Mainz, Germany) was used
for irradiating small sample areas (UV dose of 0.12 mJ/cm
2
/sec) as it was needed for the monitoring of
curing kinetic or to prepare the samples for the BA distribution investigations. To obtain large-area
samples, the Vacuum-UV-Exposure-Box-1 (Gie-Tec, Eiterfeld, Germany) equipped with low-pressure
Hg-vapor gas-discharge lamps was used to expose CEOS and nanocomposite films (UV dose of
2.2 mJ/cm2/sec for 15 min). These samples were subject of the thermal analysis.
2.4. Characterization
The condensation reaction between DPSD and ECTS was verified by
29
Si and
13
C nuclear magnetic
resonance (NMR) where the spectra of CEOS solution in chloroform-d were recorded with FT 600 MHz
(Bruker, Karlsruhe, Germany) spectrometer. Tetramethylsilane was used as an internal reference.
Differential scanning calorimetry (DSC) analysis was performed on a DSC 7020 (Hitachi,
Tokyo, Japan) instrument to determine the glass transition temperature (T
g
) of CEOS and various
CEOS/BA-cured films. The films of different nanocomposite formulations were obtained by exposing
bar-coated films to UV light for 15 min using the exposure box. The samples of the weight around
2.5 mg were heated in a sealed Al pan at the ramp of 10
◦
C/min under N
2
atmosphere. The scanning
range was between 50
◦
C and 250
◦
C. In a DSC scan, T
g
was detected as a temperature at a half-height
of heat flow step.
Thermogravimetric analysis (TGA) was carried out with 10 mg of samples using a TGA/SDTA 851
(Mettler Toledo, Greifensee, Switzerland) unit at the heating rate of 10
◦
C/min under N
2
atmosphere.
The same sample batch as that analyzed by DSC was investigated by TGA.
Boehmite nanoparticles and their distribution in the CEOS films were investigated by SEM using
a Zeiss Supra 40 microscope (Carl Zeiss, Oberkochen, Germany) equipped with a high-resolution
cathode of type Schottky–Feld emitter and conventional secondary electron (SE) and In-Lens secondary
electron (In-Lens) detectors. For better observation of the particles within the sample volume, SEM was
operated in the transmission mode, i.e., the so-called T-SEM, whereby a dedicated sample holder was
used [
22
]. Hence, the superior material contrast of the T-SEM operation mode could be exploited for
the observation of the particles in the polymeric matrix. Boehmite suspension in THF was deposited on
the substrate and observed after the solvent was evaporated. The samples for T-SEM were prepared by
curing the spin-coated thin films of CEOS and its nanocomposites on the KBr-based substrate followed
by dissolving the substrate in Millipore purified water. Resultant free-standing CEOS-based films
were deposited on transmission electron microscopy (Plano, Wetzlar, Germany) grid with the support
carbon film.
Photocuring kinetic was followed by RT-IR spectroscopy conducted in transmission mode using
a Nicolet 8700 FTIR spectrometer (Thermo Electron Corporation, Madison, WI, USA). The samples
Materials 2019,12, 1513 5 of 18
were prepared by spin-coating on the KBr substrates from CEOS solutions in THF and its mixtures
with BA nanoparticles. As a result, films with thicknesses ranging from 10–20
µ
m were obtained.
The samples were placed perpendicularly to the IR beam direction. The mid-IR spectra were collected
in the 650–4000 cm
−1
wavelength range at a resolution of 4 cm
−1
with the acquisition time of 5.5 s.
The total time of UV exposure was 30 min. Additionally, the IR spectra of the samples were measured
after 24 h of keeping them in the darkness to evaluate the impact of post-curing processes. To reduce
variability, all RT-IR experiments were performed on the same day and under the same conditions.
Experiments were performed at least in triplicate for each formulation.
The conversion of epoxy groups αep was calculated by following equation:
αep =1−"(Aepoxy/Aaromatic)t
(Aepoxy/Aaromatic)t=0#×100% (1)
where A
epoxy
,the peak area of absorption band of epoxy group at 885 cm
−1
, is normalized by the
peak area of Si–Ph group located at 1430 cm
−1
[
23
], abbreviated as A
aromatic
. The peak of Si–Ph was
chosen as a reference due to the assumption that the number of Si–Ph bonds remains constant during
the polymerization.
3. Results and Discussion
3.1. Characterization of Cycloaliphatic Epoxy Oligosiloxane (CEOS)
The molecular structure of synthesized CEOS was characterized by
13
C NMR,
29
Si NMR, and FTIR
spectroscopy. Figure 1displays
29
Si NMR spectra of CEOS with different ECTS:DPSD adduct ratios.
From the CEOS structure, Si atoms can be assigned to DPSD and ECTS parts, denoted as D
n
and T
n
,
respectively (Figure 1b). The superscript indicates the number of siloxane bonds of Si atom.
Materials 2019, 12, x FOR PEER REVIEW 5 of 18
with BA nanoparticles. As a result, films with thicknesses ranging from 10–20 μm were obtained. The
samples were placed perpendicularly to the IR beam direction. The mid-IR spectra were collected in
the 650–4000 cm−1 wavelength range at a resolution of 4 cm−1 with the acquisition time of 5.5 s. The
total time of UV exposure was 30 min. Additionally, the IR spectra of the samples were measured
after 24 h of keeping them in the darkness to evaluate the impact of post-curing processes. To reduce
variability, all RT-IR experiments were performed on the same day and under the same conditions.
Experiments were performed at least in triplicate for each formulation.
The conversion of epoxy groups αep was calculated by following equation:
𝛼 =1−⎣
⎢
⎢
⎡
𝐴
𝐴
𝐴
𝐴
⎦
⎥
⎥
⎤
×100% (1)
where Aepoxy, the peak area of absorption band of epoxy group at 885 cm−1, is normalized by the
peak area of Si–Ph group located at 1430 cm−1 [23], abbreviated as Aaromatic. The peak of Si–Ph was
chosen as a reference due to the assumption that the number of Si–Ph bonds remains constant during
the polymerization.
3. Results and Discussion
3.1. Characterization of Cycloaliphatic Epoxy Oligosiloxane (CEOS)
The molecular structure of synthesized CEOS was characterized by 13C NMR, 29Si NMR, and
FTIR spectroscopy. Figure 1 displays 29Si NMR spectra of CEOS with different ECTS:DPSD adduct
ratios. From the CEOS structure, Si atoms can be assigned to DPSD and ECTS parts, denoted as Dn
and Tn, respectively (Figure 1b). The superscript indicates the number of siloxane bonds of Si atom.
Figure 1. (a) 29Si NMR spectra of CEOS synthesized at different ECTS:DPSD molar ratios; (b)
Representation of Si atoms with different bond states (R: phenyl; R': 2(3,4-epoxycyclohexyl)ethyl).
As one can see in Figure 1b, D and T peaks located at −29.7 ppm and −41.2 ppm, respectively,
represent product of DPSD dimerization and unreacted ECTS molecules [17]. Their absence in CEOS
with the highest DPSD amount signify that the reaction had successfully proceeded, enabling the
formation of epoxy-contained oligosiloxane. Furthermore, the increase of DPSD concentration leads
to more condensed structure as indicated by higher intensity of D2, T2, and T3 peaks compared to
peaks of Si with only one siloxane bond formed (D1, T1). This observation is in good agreement with
previous investigations of CEOS synthesis [17,24]. Due to higher reaction yield, ECTS:DPSD molar
ratio of adducts was chosen as 2:3 for further nanocomposite processing. It is important to mention
that epoxy groups remained stable during condensation reaction as no opening of oxirane ring was
Figure 1.
(
a
)
29
Si NMR spectra of CEOS synthesized at different ECTS:DPSD molar ratios;
(
b
) Representation of Si atoms with different bond states (R: phenyl; R
0
: 2(3,4-epoxycyclohexyl)ethyl).
As one can see in Figure 1b, D and T peaks located at
−
29.7 ppm and
−
41.2 ppm, respectively,
represent product of DPSD dimerization and unreacted ECTS molecules [
17
]. Their absence in CEOS
with the highest DPSD amount signify that the reaction had successfully proceeded, enabling the
formation of epoxy-contained oligosiloxane. Furthermore, the increase of DPSD concentration leads
to more condensed structure as indicated by higher intensity of D
2
, T
2
, and T
3
peaks compared to
peaks of Si with only one siloxane bond formed (D
1
, T
1
). This observation is in good agreement with
previous investigations of CEOS synthesis [
17
,
24
]. Due to higher reaction yield, ECTS:DPSD molar
ratio of adducts was chosen as 2:3 for further nanocomposite processing. It is important to mention
that epoxy groups remained stable during condensation reaction as no opening of oxirane ring was
Materials 2019,12, 1513 6 of 18
observed at
13
C NMR spectra (Supplementary Materials Figure S2), where all CH–O signals appear at
~52 ppm (oxirane site) but not at 55–70 ppm (opened oxirane ring).
The FTIR spectrum of neat CEOS before curing was recorded and compared with unmodified
boehmite particles (HP14) and resultant CEOS/HP14 mixture with 10 wt.% nanoparticle loading
(Figure 2). Characteristic IR bands of CEOS and BA are summarized in Supplementary Materials
Table S1.
Materials 2019, 12, x FOR PEER REVIEW 6 of 18
observed at 13C NMR spectra (Supplementary Materials Figure S2), where all CH–O signals appear
at ~52 ppm (oxirane site) but not at 55–70 ppm (opened oxirane ring).
The FTIR spectrum of neat CEOS before curing was recorded and compared with unmodified
boehmite particles (HP14) and resultant CEOS/HP14 mixture with 10 wt.% nanoparticle loading
(Figure 2). Characteristic IR bands of CEOS and BA are summarized in Supplementary Materials
Table S1.
Figure 2. FTIR spectra of neat CEOS, unmodified BA nanoparticles HP14, and CEOS/HP14-10%
before curing.
As expected from the synthesis route, FTIR spectrum of CEOS thin film did not exhibit any Si–
OH peak in the range between 3700 and 3200 cm−1 [23]. Neither hydroxyl groups were observed,
which confirms that the oxirane ring had not been opened during the synthesis. Newly formed Si–
O–Si bonds produced an absorption in the range of 1040–1090 cm−1. Nevertheless, it should be noted
that the peak observed at 2840 cm−1 represents residual Si–OMe groups which can be also confirmed
by the presence of a T2 structure reported by 29Si NMR (Figure 1). The vibration of the epoxy ring,
detected at 885 cm−1, was not affected by BA addition.
Typical spectrum of boehmite, as shown in Figure 2, displays a sharp peak at 1064 cm−1 and
shoulder bands at 3100 and 3290 cm−1, which were assigned to bending and stretching vibrations of
OH groups in BA phase, respectively. A peak at 1640 cm−1 was typical for OH vibration in water
molecules indicating the presence of physically adsorbed water in BA powder. The presence of water
can be corroborated by a shoulder at around 3500 cm−1. The spectrum of organophilic boehmite
particles OS1 modified with p-toluenesulfonic acid (PTSA) exhibits the similar bands as those of
unmodified HP14 with additional peaks in the range of 1010–1170 cm−1 associated with PTSA [25]
(Supplementary Materials Figure S3). It was observed that the FTIR spectrum of nanocomposite
displayed the combination of both CEOS and BA. Despite an overlap of most vibration bands of the
two components, no formation of new peaks or peak shifts signifying chemical reaction was observed
after mixing CEOS and BA.
3.2. Thermal Analysis of Cured CEOS and CEOS/BA Nanocomposites
Differential scanning calorimetry (DSC) was applied to investigate the thermal characteristics of
neat CEOS and CEOS/BA nanocomposites. Figure 3 displays the thermal behavior of CEOS at first
and second heating and cooling scans. The glass transition temperature (Tg), appearing as an
endothermic step change, can be detected at 22 °C and 30 °C for the first and second heating,
respectively. At the same time, an endothermic relaxation peak was observed during glass transition
at the first heating cycle. This relaxation corresponds to a rearrangement in the molecules to release
Figure 2.
FTIR spectra of neat CEOS, unmodified BA nanoparticles HP14, and CEOS/HP14-10%
before curing.
As expected from the synthesis route, FTIR spectrum of CEOS thin film did not exhibit any Si–OH
peak in the range between 3700 and 3200 cm
−1
[
23
]. Neither hydroxyl groups were observed, which
confirms that the oxirane ring had not been opened during the synthesis. Newly formed Si–O–Si
bonds produced an absorption in the range of 1040–1090 cm
−1
. Nevertheless, it should be noted that
the peak observed at 2840 cm
−1
represents residual Si–OMe groups which can be also confirmed by the
presence of a T
2
structure reported by
29
Si NMR (Figure 1). The vibration of the epoxy ring, detected at
885 cm−1, was not affected by BA addition.
Typical spectrum of boehmite, as shown in Figure 2, displays a sharp peak at 1064 cm
−1
and
shoulder bands at 3100 and 3290 cm
−1
, which were assigned to bending and stretching vibrations of
OH groups in BA phase, respectively. A peak at 1640 cm
−1
was typical for OH vibration in water
molecules indicating the presence of physically adsorbed water in BA powder. The presence of water
can be corroborated by a shoulder at around 3500 cm
−1
. The spectrum of organophilic boehmite
particles OS1 modified with p-toluenesulfonic acid (PTSA) exhibits the similar bands as those of
unmodified HP14 with additional peaks in the range of 1010–1170 cm
−1
associated with PTSA [
25
]
(Supplementary Materials Figure S3). It was observed that the FTIR spectrum of nanocomposite
displayed the combination of both CEOS and BA. Despite an overlap of most vibration bands of the
two components, no formation of new peaks or peak shifts signifying chemical reaction was observed
after mixing CEOS and BA.
3.2. Thermal Analysis of Cured CEOS and CEOS/BA Nanocomposites
Differential scanning calorimetry (DSC) was applied to investigate the thermal characteristics of
neat CEOS and CEOS/BA nanocomposites. Figure 3displays the thermal behavior of CEOS at first and
second heating and cooling scans. The glass transition temperature (T
g
), appearing as an endothermic
step change, can be detected at 22
◦
C and 30
◦
C for the first and second heating, respectively. At the
same time, an endothermic relaxation peak was observed during glass transition at the first heating
cycle. This relaxation corresponds to a rearrangement in the molecules to release the stress frozen at
Materials 2019,12, 1513 7 of 18
temperatures below T
g
[
26
]. Therefore, it is no longer observed on cooling or second heating scans.
In this study, only the second heating scan was considered for further analysis.
Materials 2019, 12, x FOR PEER REVIEW 7 of 18
the stress frozen at temperatures below Tg [26]. Therefore, it is no longer observed on cooling or
second heating scans. In this study, only the second heating scan was considered for further analysis.
It was reported that an increase in Tg was detected with an increase of the curing degree of
thermosets [27]. Based on that, the glass transition temperature can be considered highly dependent
on the curing process. In contrast to gelation (liquid-to-rubber), vitrification (rubber-to-glass) is a
reversible phenomenon and can be eliminated by increasing the curing temperature above Tg.
However, photocuring, which is usually carried out at ambient temperatures, suffers from early
vitrification resulting in hindered monomer crosslinking and low overall curing degree. This effect
can be recognized in this experiment. As the CEOS resin was photocured at room temperature, the
low value of detected Tg represents the described limitation due to an early approach to a glassy state.
Figure 3. DSC thermogram of neat CEOS hybrid.
Analyzing the second heating scan of neat CEOS and its nanocomposites (Supplementary
Materials Figure S4–7), the results of the thermogram analysis have been summarized in Table 2. One
can see that the glass transition shifts to higher temperatures with particle addition for all
nanocomposite formulations. It indicates the hindrance effect of BA on segmental motions of CEOS
network, what usually results from the attractive nature of nanoparticle–polymer interactions.
However, thermal behavior of nanocomposites differs depending on the used filler. Higher
enhancement of Tg was observed for CEOS/OS1 formulations, where boehmite surface was modified
with p-toluenesulfonic acid. At the same time, in contrast to CEOS/HP14, the glass transition step
ΔCp of CEOS/OS1 films becomes less pronounced with an increase of the filler amount as can be
observed from thermograms (Supplementary Materials Figure S4–7) and ΔCp values in Table 2. The
highest Tg rise was detected for CEOS/HP14-2% and CEOS/OS1-2% reaching values of 54 °C and 73
°C, compared to neat polymer, respectively.
Table 2. Thermal properties of CEOS and CEOS/BA nanocomposites during second heating run: glass
transition at half height (Tg) and heat capacity (Cp).
Tg (°C) ΔCp (J/g
/
°C)
CEOS 30 ± 2 0.23 ± 0.01
CEOS/HP14-1% 52 ± 3 0.20 ± 0.02
CEOS/HP14-2% 54 ± 2 0.16 ± 0.04
CEOS/HP14-5% 48 ± 2 0.16 ± 0.02
CEOS/HP14-10% 45 ± 3 0.15 ± 0.02
CEOS/OS1-1% 64 ± 5 0.10 ± 0.06
CEOS/OS1-2% 73 ± 4 0.05 ± 0.05
CEOS/OS1-5% 71 ± 3 0.07 ± 0.02
CEOS/OS1-10% 65 ± 4 0.08 ± 0.01
Figure 3. DSC thermogram of neat CEOS hybrid.
It was reported that an increase in T
g
was detected with an increase of the curing degree of
thermosets [
27
]. Based on that, the glass transition temperature can be considered highly dependent
on the curing process. In contrast to gelation (liquid-to-rubber), vitrification (rubber-to-glass) is
a reversible phenomenon and can be eliminated by increasing the curing temperature above T
g
.
However, photocuring, which is usually carried out at ambient temperatures, suffers from early
vitrification resulting in hindered monomer crosslinking and low overall curing degree. This effect can
be recognized in this experiment. As the CEOS resin was photocured at room temperature, the low
value of detected Tgrepresents the described limitation due to an early approach to a glassy state.
Analyzing the second heating scan of neat CEOS and its nanocomposites (Supplementary Materials
Figures S4–S7), the results of the thermogram analysis have been summarized in Table 2. One can
see that the glass transition shifts to higher temperatures with particle addition for all nanocomposite
formulations. It indicates the hindrance effect of BA on segmental motions of CEOS network, what
usually results from the attractive nature of nanoparticle–polymer interactions. However, thermal
behavior of nanocomposites differs depending on the used filler. Higher enhancement of T
g
was
observed for CEOS/OS1 formulations, where boehmite surface was modified with p-toluenesulfonic
acid. At the same time, in contrast to CEOS/HP14, the glass transition step
∆
C
p
of CEOS/OS1 films
becomes less pronounced with an increase of the filler amount as can be observed from thermograms
(Supplementary Materials Figures S4–S7) and
∆
C
p
values in Table 2. The highest T
g
rise was
detected for CEOS/HP14-2% and CEOS/OS1-2% reaching values of 54
◦
C and 73
◦
C, compared to neat
polymer, respectively.
Table 2.
Thermal properties of CEOS and CEOS/BA nanocomposites during second heating run: glass
transition at half height (Tg) and heat capacity (Cp).
Tg(◦C) ∆Cp(J/g/◦C)
CEOS 30 ±2 0.23 ±0.01
CEOS/HP14-1% 52 ±3 0.20 ±0.02
CEOS/HP14-2% 54 ±2 0.16 ±0.04
CEOS/HP14-5% 48 ±2 0.16 ±0.02
CEOS/HP14-10%
45 ±3 0.15 ±0.02
CEOS/OS1-1% 64 ±5 0.10 ±0.06
CEOS/OS1-2% 73 ±4 0.05 ±0.05
CEOS/OS1-5% 71 ±3 0.07 ±0.02
CEOS/OS1-10% 65 ±4 0.08 ±0.01
Materials 2019,12, 1513 8 of 18
To complement thermal analysis, TGA was performed to evaluate the influence of BA on
CEOS stability at high temperatures. Figure 4depicts the normalized TG curves (a,c) and their
derivatives (b,d) for the neat CEOS, the BA nanoparticles and different nanocomposite formulations.
For comparison, the detailed description of TG behavior of boehmite nanoparticles is given in
Supplementary Information (Supplementary Materials Figure S8). First, one can mention the high
thermal stability of the CEOS network. The 5% weight loss temperature of CEOS was detected at
410
◦
C what is similar to the value reported previously [
17
]. As observed in Figure 4a, the temperature
range of decomposition remains almost the same for CEOS/HP14 with 1–5 wt.% filler concentration,
but starts to decrease for film with 10 wt.% of HP14 loading.
Materials 2019, 12, x FOR PEER REVIEW 8 of 18
To complement thermal analysis, TGA was performed to evaluate the influence of BA on CEOS
stability at high temperatures. Figure 4 depicts the normalized TG curves (a,c) and their derivatives
(b,d) for the neat CEOS, the BA nanoparticles and different nanocomposite formulations. For
comparison, the detailed description of TG behavior of boehmite nanoparticles is given in
supplementary information (Supplementary Materials Figure S8). First, one can mention the high
thermal stability of the CEOS network. The 5% weight loss temperature of CEOS was detected at 410
°C what is similar to the value reported previously [17]. As observed in Figure 4a, the temperature
range of decomposition remains almost the same for CEOS/HP14 with 1–5 wt.% filler concentration,
but starts to decrease for film with 10 wt.% of HP14 loading.
Figure 4. (a,c) TG curves of CEOS resin with different loadings of (a) unmodified boehmite, HP14 and
(c) organically-modified boehmite, OS1; (b,d) differential thermal gravimetric (DTG) curves of BA
powder, neat CEOS and CEOS/BA nanocomposites with 1–10 wt.% of BA contents: (b) with
unmodified boehmite, HP14, and (d) with organically-modified boehmite, OS1.
Differential thermogravimetric (DTG) curves (Figure 4b,d) show that the main decomposition
step of CEOS resin starts at 420 °C with the maximum rate located at 480 °C. However, the
dihydroxylation of BA particles (Figure 4b,d), begins already at 380–390 °C. Boehmite presence leads
to the broadening of CEOS degradation peak due to the superposition of both hybrid and filler peaks.
However, the addition of boehmite filler does not postpone the main decomposition event.
On the other hand, the reinforcement of overall temperature resistance of the films with
boehmite addition can be detected. Figure 5 displays the evolution of residual weight detected for
analyzed formulations at 750 °C, after the main decomposition associated with the organic segment
in CEOS has occurred. Comparing these values with those calculated from superposition principle,
Figure 4.
(
a
,
c
) TG curves of CEOS resin with different loadings of (
a
) unmodified boehmite, HP14
and (
c
) organically-modified boehmite, OS1; (
b
,
d
) differential thermal gravimetric (DTG) curves of BA
powder, neat CEOS and CEOS/BA nanocomposites with 1–10 wt.% of BA contents: (
b
) with unmodified
boehmite, HP14, and (d) with organically-modified boehmite, OS1.
Differential thermogravimetric (DTG) curves (Figure 4b,d) show that the main decomposition step
of CEOS resin starts at 420
◦
C with the maximum rate located at 480
◦
C. However, the dihydroxylation
of BA particles (Figure 4b,d), begins already at 380–390
◦
C. Boehmite presence leads to the broadening of
CEOS degradation peak due to the superposition of both hybrid and filler peaks. However, the addition
of boehmite filler does not postpone the main decomposition event.
On the other hand, the reinforcement of overall temperature resistance of the films with boehmite
addition can be detected. Figure 5displays the evolution of residual weight detected for analyzed
formulations at 750
◦
C, after the main decomposition associated with the organic segment in CEOS has
occurred. Comparing these values with those calculated from superposition principle, the experimental
Materials 2019,12, 1513 9 of 18
data are considerably higher. This could indicate the formation of highly stable species as a result of
attractive filler-resin interactions.
Materials 2019, 12, x FOR PEER REVIEW 9 of 18
the experimental data are considerably higher. This could indicate the formation of highly stable
species as a result of attractive filler-resin interactions.
Figure 5. Comparison between expected and measured values of residue weight of CEOs and
CEOS/BA nanocomposites as a function of filler content.
3.3. Boehmite Distribution
The particle morphology and more importantly, their distribution within the epoxy matrix have
a significant impact on the properties of resultant nanocomposite materials. Strong particle
agglomeration and aggregation can lead to inhomogeneity of the material properties and indicates
poor interaction between organic and inorganic components. To ensure the sufficient boehmite
distribution in the hybrid network, nanocomposite films were verified by means of scanning electron
microscopy operated in transmission mode (T-SEM). For the sake of highly sensitive surface analysis,
e.g., particles morphology, SEM with surface-sensitive In-Lens detector was applied. Comparing
boehmite particles without and with surface modifier (Figure 6), one can see that unmodified HP14
and PTSA-modified OS1 exhibit similar morphology. It is known that γ-AlO(OH) has an
orthorhombic unit cell [28,29]. Considering the size of BA crystallites reported in Table 1 (14 nm and
10 nm for HP14 and OS1, respectively), one assumes that the observed 10–50 nm long particles are
mostly aggregates made up of several boehmite single crystallites. Meanwhile, the widths of the
particles differ, being in the ranges of 10–15 nm and 10–20 nm for HP14 and OS1, respectively. This
indicates mostly monocrystallite width. Increased width of OS1 particles can be a result of surface
treatment with PTSA.
Figure 6. 10 kV SEM micrographs of boehmite nanoparticles obtained with In-Lens detector: (a) HP14
deposited on Si wafer; (b) OS1 deposited on carbon film.
While HP14 particles are compatible with polar environment, organophilic OS1 are meant to be
used in medium polar matrices. As expected, this has caused the difference in the filler distribution
Figure 5.
Comparison between expected and measured values of residue weight of CEOs and CEOS/BA
nanocomposites as a function of filler content.
3.3. Boehmite Distribution
The particle morphology and more importantly, their distribution within the epoxy matrix
have a significant impact on the properties of resultant nanocomposite materials. Strong particle
agglomeration and aggregation can lead to inhomogeneity of the material properties and indicates poor
interaction between organic and inorganic components. To ensure the sufficient boehmite distribution
in the hybrid network, nanocomposite films were verified by means of scanning electron microscopy
operated in transmission mode (T-SEM). For the sake of highly sensitive surface analysis, e.g., particles
morphology, SEM with surface-sensitive In-Lens detector was applied. Comparing boehmite particles
without and with surface modifier (Figure 6), one can see that unmodified HP14 and PTSA-modified
OS1 exhibit similar morphology. It is known that
γ
-AlO(OH) has an orthorhombic unit cell [
28
,
29
].
Considering the size of BA crystallites reported in Table 1(14 nm and 10 nm for HP14 and OS1,
respectively), one assumes that the observed 10–50 nm long particles are mostly aggregates made up of
several boehmite single crystallites. Meanwhile, the widths of the particles differ, being in the ranges of
10–15 nm and 10–20 nm for HP14 and OS1, respectively. This indicates mostly monocrystallite width.
Increased width of OS1 particles can be a result of surface treatment with PTSA.
Materials 2019, 12, x FOR PEER REVIEW 9 of 18
the experimental data are considerably higher. This could indicate the formation of highly stable
species as a result of attractive filler-resin interactions.
Figure 5. Comparison between expected and measured values of residue weight of CEOs and
CEOS/BA nanocomposites as a function of filler content.
3.3. Boehmite Distribution
The particle morphology and more importantly, their distribution within the epoxy matrix have
a significant impact on the properties of resultant nanocomposite materials. Strong particle
agglomeration and aggregation can lead to inhomogeneity of the material properties and indicates
poor interaction between organic and inorganic components. To ensure the sufficient boehmite
distribution in the hybrid network, nanocomposite films were verified by means of scanning electron
microscopy operated in transmission mode (T-SEM). For the sake of highly sensitive surface analysis,
e.g., particles morphology, SEM with surface-sensitive In-Lens detector was applied. Comparing
boehmite particles without and with surface modifier (Figure 6), one can see that unmodified HP14
and PTSA-modified OS1 exhibit similar morphology. It is known that γ-AlO(OH) has an
orthorhombic unit cell [28,29]. Considering the size of BA crystallites reported in Table 1 (14 nm and
10 nm for HP14 and OS1, respectively), one assumes that the observed 10–50 nm long particles are
mostly aggregates made up of several boehmite single crystallites. Meanwhile, the widths of the
particles differ, being in the ranges of 10–15 nm and 10–20 nm for HP14 and OS1, respectively. This
indicates mostly monocrystallite width. Increased width of OS1 particles can be a result of surface
treatment with PTSA.
Figure 6. 10 kV SEM micrographs of boehmite nanoparticles obtained with In-Lens detector: (a) HP14
deposited on Si wafer; (b) OS1 deposited on carbon film.
While HP14 particles are compatible with polar environment, organophilic OS1 are meant to be
used in medium polar matrices. As expected, this has caused the difference in the filler distribution
Figure 6.
10 kV SEM micrographs of boehmite nanoparticles obtained with In-Lens detector: (
a
) HP14
deposited on Si wafer; (b) OS1 deposited on carbon film.
Materials 2019,12, 1513 10 of 18
While HP14 particles are compatible with polar environment, organophilic OS1 are meant to be
used in medium polar matrices. As expected, this has caused the difference in the filler distribution in
CEOS films. Figure 7shows that HP14 in general forms agglomerates of considerably larger size than
those of OS1, as a result of different compatibility between the resin and the particles. Nevertheless,
surface modification of boehmite does not have any noticeable impact on the particle dispersion within
agglomerate as it appears to be alike for both HP14 and OS1 (Figure 7b,d).
Materials 2019, 12, x FOR PEER REVIEW 10 of 18
in CEOS films. Figure 7 shows that HP14 in general forms agglomerates of considerably larger size
than those of OS1, as a result of different compatibility between the resin and the particles.
Nevertheless, surface modification of boehmite does not have any noticeable impact on the particle
dispersion within agglomerate as it appears to be alike for both HP14 and OS1 (Figure 7b,d).
Figure 7. 20 kV T-SEM micrographs of (a,b) CEOS/HP14 and (c,d) CEOS/OS1 nanocomposite films
with 5 wt.% of boehmite content.
It is worth noting that boehmite agglomerates appear as clusters of network-like-connected
particles (Figure 8). This agglomerate structure causes the formation of a large-area matrix–boehmite
interface what is a precondition for enhanced resin-filler interaction.
Figure 8. 20 kV SEM micrographs of boehmite agglomerates in the CEOS/OS1 film obtained in the
transmission mode.
One can suspect that better distribution of organically-modified OS1 clusters in the CEOS matrix
leads to a higher degree and better homogeneity of structuration in nanocomposites as well as to
improved interaction with the polymer compared to the unmodified nanoparticles. This is supported
by enhanced effect on T
g
shift for CEOS/OS1 formulations, which indicates the hindrance of the
motions of polymer chain and a decrease of free-volume by well-dispersed OS1 particles.
Figure 7.
20 kV T-SEM micrographs of (
a
,
b
) CEOS/HP14 and (
c
,
d
) CEOS/OS1 nanocomposite films
with 5 wt.% of boehmite content.
It is worth noting that boehmite agglomerates appear as clusters of network-like-connected
particles (Figure 8). This agglomerate structure causes the formation of a large-area matrix–boehmite
interface what is a precondition for enhanced resin-filler interaction.
Materials 2019, 12, x FOR PEER REVIEW 10 of 18
in CEOS films. Figure 7 shows that HP14 in general forms agglomerates of considerably larger size
than those of OS1, as a result of different compatibility between the resin and the particles.
Nevertheless, surface modification of boehmite does not have any noticeable impact on the particle
dispersion within agglomerate as it appears to be alike for both HP14 and OS1 (Figure 7b,d).
Figure 7. 20 kV T-SEM micrographs of (a,b) CEOS/HP14 and (c,d) CEOS/OS1 nanocomposite films
with 5 wt.% of boehmite content.
It is worth noting that boehmite agglomerates appear as clusters of network-like-connected
particles (Figure 8). This agglomerate structure causes the formation of a large-area matrix–boehmite
interface what is a precondition for enhanced resin-filler interaction.
Figure 8. 20 kV SEM micrographs of boehmite agglomerates in the CEOS/OS1 film obtained in the
transmission mode.
One can suspect that better distribution of organically-modified OS1 clusters in the CEOS matrix
leads to a higher degree and better homogeneity of structuration in nanocomposites as well as to
improved interaction with the polymer compared to the unmodified nanoparticles. This is supported
by enhanced effect on T
g
shift for CEOS/OS1 formulations, which indicates the hindrance of the
motions of polymer chain and a decrease of free-volume by well-dispersed OS1 particles.
Figure 8.
20 kV SEM micrographs of boehmite agglomerates in the CEOS/OS1 film obtained in the
transmission mode.
Materials 2019,12, 1513 11 of 18
One can suspect that better distribution of organically-modified OS1 clusters in the CEOS matrix
leads to a higher degree and better homogeneity of structuration in nanocomposites as well as to
improved interaction with the polymer compared to the unmodified nanoparticles. This is supported
by enhanced effect on T
g
shift for CEOS/OS1 formulations, which indicates the hindrance of the motions
of polymer chain and a decrease of free-volume by well-dispersed OS1 particles.
3.4. UV Curing Kinetics
The properties of thermosets strongly depend on the curing parameters such as curing procedure,
degree and depth of curing, post-curing treatments, etc. To better understand the reasons for the
observed thermal behavior of nanocomposites, the photocuring of CEOS and all CEOS/BA formulations
were investigated in situ by means of RT-IR spectroscopy. The thickness of all studied formulations
was kept similar (10–15
µ
m) and measured after light exposure when the solid film was obtained.
In this range of coating thicknesses and since the high-energy UV source was applied, the differences
in penetration depth of the UV light between different samples can be neglected.
Figure 9displays the changes in IR absorption of CEOS occurring upon UV light exposure. Band
area at 1810 cm
−1
decreases during UV irradiation indicating photoinitiator (PI) photodecomposition
(Figure 9b). This is followed by formation of radicals and ionic fragments leading to the generation of
a strong Brönsted acid in the presence of a hydrogen donor [
12
,
30
] (Supplementary Materials Scheme
S1). Formed super acid acts as a primary initiator of ring-opening polymerization of the epoxide.
This process proceeds rapidly through the oxonium ion and can be seen in Figure 9by a decrease of
the characteristic band at 885 cm
−1
from epoxy ring vibrations [
31
,
32
]. It was supported by a decline in
the absorbance at 2980 cm
−1
associated with C–H stretching in the oxirane ring. Moreover, the band
at 3450 cm
−1
rises as a result of the hydroxyl group formation during the ring-opening. After the
exposure was terminated, the crosslinking processes proceeded in the dark as is typical for cationic
polymerization [
11
]. For CEOS, this effect was observed by the absorbance changes displayed by the
FTIR spectrum of the film kept for 24 h in the darkness after being UV exposed (Figure 9a: after UV
exposure +dark-curing).
Materials 2019, 12, x FOR PEER REVIEW 11 of 18
3.4. UV Curing Kinetics
The properties of thermosets strongly depend on the curing parameters such as curing
procedure, degree and depth of curing, post-curing treatments, etc. To better understand the reasons
for the observed thermal behavior of nanocomposites, the photocuring of CEOS and all CEOS/BA
formulations were investigated in situ by means of RT-IR spectroscopy. The thickness of all studied
formulations was kept similar (10–15 μm) and measured after light exposure when the solid film was
obtained. In this range of coating thicknesses and since the high-energy UV source was applied, the
differences in penetration depth of the UV light between different samples can be neglected.
Figure 9 displays the changes in IR absorption of CEOS occurring upon UV light exposure. Band
area at 1810 cm−1 decreases during UV irradiation indicating photoinitiator (PI) photodecomposition
(Figure 9b). This is followed by formation of radicals and ionic fragments leading to the generation
of a strong Brönsted acid in the presence of a hydrogen donor [12,30] (Supplementary Materials
Scheme S1). Formed super acid acts as a primary initiator of ring-opening polymerization of the
epoxide. This process proceeds rapidly through the oxonium ion and can be seen in Figure 9 by a
decrease of the characteristic band at 885 cm−1 from epoxy ring vibrations [31,32]. It was supported
by a decline in the absorbance at 2980 cm−1 associated with C–H stretching in the oxirane ring.
Moreover, the band at 3450 cm−1 rises as a result of the hydroxyl group formation during the ring-
opening. After the exposure was terminated, the crosslinking processes proceeded in the dark as is
typical for cationic polymerization [11]. For CEOS, this effect was observed by the absorbance
changes displayed by the FTIR spectrum of the film kept for 24 h in the darkness after being UV
exposed (Figure 9a: after UV exposure + dark-curing).
Figure 9. (a) FTIR spectra of CEOS before and after 30 min of UV exposure and 24 h of following dark-
curing, (inset: zoomed IR range of C–H stretching of epoxy groups in the fingerprint region); (b)
Evolution of IR absorption of CEOS during UV light exposure: C–H stretching of epoxy groups (885
and 2980 cm−1); formation of hydroxyl groups (3450 cm−1); decomposition of photoinitiator (1810
cm−1).
The extent of photopolymerization in neat CEOS and CEOS/BA nanocomposites is shown in
Figure 10, as the changes in the conversion degree of epoxy groups (αep) versus exposure time. While
the curve slope gives an indication of the polymerization rate, the plateau value identifies the final
conversion efficiency. A similar evolution was observed for bands at 2980 cm−1 and 3450 cm−1 related
to a decrease of C–H stretching in oxirane ring and the formation of hydroxyl groups, respectively
(Supplementary Materials Figure S9, Figure S10).
Figure 9.
(
a
) FTIR spectra of CEOS before and after 30 min of UV exposure and 24 h of following
dark-curing, (inset: zoomed IR range of C–H stretching of epoxy groups in the fingerprint region);
(
b
) Evolution of IR absorption of CEOS during UV light exposure: C–H stretching of epoxy groups
(885 and 2980 cm
−1
); formation of hydroxyl groups (3450 cm
−1
); decomposition of photoinitiator
(1810 cm−1).
The extent of photopolymerization in neat CEOS and CEOS/BA nanocomposites is shown in
Figure 10, as the changes in the conversion degree of epoxy groups (
αep
) versus exposure time. While
the curve slope gives an indication of the polymerization rate, the plateau value identifies the final
conversion efficiency. A similar evolution was observed for bands at 2980 cm
−1
and 3450 cm
−1
related
Materials 2019,12, 1513 12 of 18
to a decrease of C–H stretching in oxirane ring and the formation of hydroxyl groups, respectively
(Supplementary Materials Figures S9 and S10).
Materials 2019, 12, x FOR PEER REVIEW 12 of 18
Figure 10. Photopolymerization profiles of neat CEOS and its nanocomposites with different (a) HP14
and (b) OS1 loadings.
It was observed that two stages in the cationic photopolymerization take place. The first stage at
low-curing times exhibited a fast increase in epoxy group conversion (αep), up to roughly 30%. The
second stage, at higher conversion degrees, represents a decelerating CEOS conversion rate that can
be explained by the formation of a glassy network. It resulted in a significant drop of mobility of
reactive groups due to gelation and vitrification phenomena. Consequently, a large number of epoxy
groups remains trapped within the polymeric network with no possibility to reach a neighboring
oxirane ring. Therefore, one can observe only a 2% increase in epoxy conversion degree during the
last 20 min of UV irradiation.
For better comparison of kinetics in neat CEOS and nanocomposites, the normalized epoxy
conversion curves were calculated (Figure 11).
Figure 11. Normalized conversion degree of neat CEOS and its nanocomposites with different (a)
HP14 and (b) OS1 loadings.
As one can see, the induction period of the kinetics extends with particle loading (Figure 11,
inset). At the same time, vitrification processes during photocuring appear to be somewhat retarded
for the nanocomposites with higher particle content (5–10 wt.%). The inhibition caused by particles
during the induction period can be better identified from the first derivative of changes in epoxy
band ΔA885 (Supplementary Materials Figure S11). However, the possible impact of nanoparticles on
the penetration depth of UV light should also be considered as one of the possible factors influencing
the photopolymerization kinetic.
Figure 10.
Photopolymerization profiles of neat CEOS and its nanocomposites with different (
a
) HP14
and (b) OS1 loadings.
It was observed that two stages in the cationic photopolymerization take place. The first stage
at low-curing times exhibited a fast increase in epoxy group conversion (
αep
), up to roughly 30%.
The second stage, at higher conversion degrees, represents a decelerating CEOS conversion rate that
can be explained by the formation of a glassy network. It resulted in a significant drop of mobility of
reactive groups due to gelation and vitrification phenomena. Consequently, a large number of epoxy
groups remains trapped within the polymeric network with no possibility to reach a neighboring
oxirane ring. Therefore, one can observe only a 2% increase in epoxy conversion degree during the last
20 min of UV irradiation.
For better comparison of kinetics in neat CEOS and nanocomposites, the normalized epoxy
conversion curves were calculated (Figure 11).
Materials 2019, 12, x FOR PEER REVIEW 12 of 18
Figure 10. Photopolymerization profiles of neat CEOS and its nanocomposites with different (a) HP14
and (b) OS1 loadings.
It was observed that two stages in the cationic photopolymerization take place. The first stage at
low-curing times exhibited a fast increase in epoxy group conversion (αep), up to roughly 30%. The
second stage, at higher conversion degrees, represents a decelerating CEOS conversion rate that can
be explained by the formation of a glassy network. It resulted in a significant drop of mobility of
reactive groups due to gelation and vitrification phenomena. Consequently, a large number of epoxy
groups remains trapped within the polymeric network with no possibility to reach a neighboring
oxirane ring. Therefore, one can observe only a 2% increase in epoxy conversion degree during the
last 20 min of UV irradiation.
For better comparison of kinetics in neat CEOS and nanocomposites, the normalized epoxy
conversion curves were calculated (Figure 11).
Figure 11. Normalized conversion degree of neat CEOS and its nanocomposites with different (a)
HP14 and (b) OS1 loadings.
As one can see, the induction period of the kinetics extends with particle loading (Figure 11,
inset). At the same time, vitrification processes during photocuring appear to be somewhat retarded
for the nanocomposites with higher particle content (5–10 wt.%). The inhibition caused by particles
during the induction period can be better identified from the first derivative of changes in epoxy
band ΔA885 (Supplementary Materials Figure S11). However, the possible impact of nanoparticles on
the penetration depth of UV light should also be considered as one of the possible factors influencing
the photopolymerization kinetic.
Figure 11.
Normalized conversion degree of neat CEOS and its nanocomposites with different (
a
) HP14
and (b) OS1 loadings.
As one can see, the induction period of the kinetics extends with particle loading (Figure 11,
inset). At the same time, vitrification processes during photocuring appear to be somewhat retarded
for the nanocomposites with higher particle content (5–10 wt.%). The inhibition caused by particles
during the induction period can be better identified from the first derivative of changes in epoxy band
∆
A
885
(Supplementary Materials Figure S11). However, the possible impact of nanoparticles on the
Materials 2019,12, 1513 13 of 18
penetration depth of UV light should also be considered as one of the possible factors influencing the
photopolymerization kinetic.
While the first stage of CEOS photopolymerization can be considered an autocatalytic (kinetic
controlled), the second stage was controlled by the diffusion occurring in the partially crosslinked
polymer at vitrification. High loadings of boehmite particles seemed to influence this process.
For better understanding of BA’s influence on the network formation of CEOS resin, no additional
exposure to the heat or moisture during post-curing was applied in our experiment, thus excluding
additional influences after UV light exposure was terminated.
Figure 12 depicts the dependence of the final
αep
on the BA content detected right after UV
irradiation and consequently after the following 24 h of exposed samples resting in the dark.
Materials 2019, 12, x FOR PEER REVIEW 13 of 18
While the first stage of CEOS photopolymerization can be considered an autocatalytic (kinetic
controlled), the second stage was controlled by the diffusion occurring in the partially crosslinked
polymer at vitrification. High loadings of boehmite particles seemed to influence this process.
For better understanding of BA’s influence on the network formation of CEOS resin, no
additional exposure to the heat or moisture during post-curing was applied in our experiment, thus
excluding additional influences after UV light exposure was terminated.
Figure 12 depicts the dependence of the final αep on the BA content detected right after UV
irradiation and consequently after the following 24 hours of exposed samples resting in the dark.
Figure 12. Dependence of conversion degree of CEOS on boehmite content in nanocomposites after
30 min of UV exposure and after subsequent 24h dark curing period: (■: CEOS; ▲: CEOS/HP14;
●: CEOS/OS1).
It is known that cationic centers of polymerization are not reactive towards each other and
therefore have much longer lifetimes in contrast to free-radical polymerization. As a result, cationic
polymerization proceeds even after irradiation has been terminated, when the active species are no
longer being created. It can be noticed from Figure 12 that the αep of 33% detected for neat hybrid
after 30 min of UV light exposure remarkably increased up to 51% during dark-curing.
In the case of unmodified boehmite HP14, a statistically significant increase in the
nanocomposite’s αep compared to neat CEOS was observed only at 10 wt.% particle loading. The αep
of 36% and 56% was detected after UV exposure and after dark-curing, respectively.
At the same time, considering the compositions with organically-modified boehmite, the
maximum αep was detected for CEOS/OS1-5% composition with values of 40% and 57% after
irradiation and following post-curing, respectively. These values are by 7% and 6% higher than those
found for the neat CEOS.
As one can see, no reduction in a degree of curing was caused by boehmite embedding, neither
after UV exposure stage nor after post-curing in the dark. At the same time, an increase in conversion
detected after dark-curing is observed for all studied formulations and is in 17–18% range. This
indicates that the differences caused by the two different types of boehmite nanoparticles originate
mainly from the processes occurring during the UV light exposure stage.
3.5. Proposed Curing Mechanism
Summarizing the results obtained within RT-IR experiment, we can conclude that the mobility
restriction of the active sites of the CEOS network appearing during crosslinking/polymerization
limits the final conversion degree of epoxy groups up to roughly only 50%. The incorporation of BA
at 5–10 wt.% to CEOS matrix results in a slight decrease in curing rate during the first stage of
polymerization, before the gelation occurs. However, surprisingly, the presence of BA has led to an
increase in overall αep of the CEOS network due to the contribution of the processes occurring in the
Figure 12.
Dependence of conversion degree of CEOS on boehmite content in nanocomposites after
30 min of UV exposure and after subsequent 24 h dark curing period: (
: CEOS;
N
: CEOS/HP14;
: CEOS/OS1).
It is known that cationic centers of polymerization are not reactive towards each other and
therefore have much longer lifetimes in contrast to free-radical polymerization. As a result, cationic
polymerization proceeds even after irradiation has been terminated, when the active species are no
longer being created. It can be noticed from Figure 12 that the
αep
of 33% detected for neat hybrid after
30 min of UV light exposure remarkably increased up to 51% during dark-curing.
In the case of unmodified boehmite HP14, a statistically significant increase in the nanocomposite’s
αep
compared to neat CEOS was observed only at 10 wt.% particle loading. The
αep
of 36% and 56%
was detected after UV exposure and after dark-curing, respectively.
At the same time, considering the compositions with organically-modified boehmite, the maximum
αep
was detected for CEOS/OS1-5% composition with values of 40% and 57% after irradiation and
following post-curing, respectively. These values are by 7% and 6% higher than those found for the
neat CEOS.
As one can see, no reduction in a degree of curing was caused by boehmite embedding, neither
after UV exposure stage nor after post-curing in the dark. At the same time, an increase in conversion
detected after dark-curing is observed for all studied formulations and is in 17–18% range. This indicates
that the differences caused by the two different types of boehmite nanoparticles originate mainly from
the processes occurring during the UV light exposure stage.
3.5. Proposed Curing Mechanism
Summarizing the results obtained within RT-IR experiment, we can conclude that the mobility
restriction of the active sites of the CEOS network appearing during crosslinking/polymerization
Materials 2019,12, 1513 14 of 18
limits the final conversion degree of epoxy groups up to roughly only 50%. The incorporation of
BA at 5–10 wt.% to CEOS matrix results in a slight decrease in curing rate during the first stage of
polymerization, before the gelation occurs. However, surprisingly, the presence of BA has led to an
increase in overall
αep
of the CEOS network due to the contribution of the processes occurring in
the already somewhat vitrified matrix structure. The observed effect excludes previously detected
phenomena like a light screening effect of boehmite nanoparticles [
33
] or a decrease in mobility of the
reactive species due to the raised viscosity [
5
]. These phenomena were shown to induce an opposite
effect in other cycloaliphatic epoxy systems where a significant drop of epoxy conversion degree was
observed [5,33].
Since CEOS films prepared for DSC and TGA analysis as well as T-SEM imaging were cured using
different irradiation sources and exposure times due to technical reasons, one cannot directly compare
these results to RT-IR curing experiments. Nevertheless, the results from these methods indicate that
BA addition leads to a decrease in mobility of cured CEOS network.
The positive impact of boehmite on the final epoxy conversion that was observed in our
investigations can be explained through the presence of hydroxyl groups located on the particle surface
as they can be involved in the propagation step of epoxy polymerization. Generally, the propagation can
proceed through the subsequent attacks of epoxy groups by oxonium ions as described by active-chain
end (ACE) mechanism. However, it can be shifted to active monomer (AM) mechanism through
alcohol attack and proton-transfer reactions because of the presence of protonic nucleophiles [
34
–
36
].
Both mechanisms are displayed in Scheme 2and described in detail elsewhere [
34
,
37
]. The hydroxyl
groups carried on the boehmite surface can react with oxirane ring via an AM mechanism (Scheme 3)
forming covalent linkages. This fact could explain the significant rise of T
g
and increased thermal
stability of “inorganic” components of nanocomposites (as seen in Figure 5), even when only 1 wt.% of
boehmite was present.
Materials 2019, 12, x FOR PEER REVIEW 14 of 18
already somewhat vitrified matrix structure. The observed effect excludes previously detected
phenomena like a light screening effect of boehmite nanoparticles [33] or a decrease in mobility of
the reactive species due to the raised viscosity [5]. These phenomena were shown to induce an
opposite effect in other cycloaliphatic epoxy systems where a significant drop of epoxy conversion
degree was observed [5,33].
Since CEOS films prepared for DSC and TGA analysis as well as T-SEM imaging were cured
using different irradiation sources and exposure times due to technical reasons, one cannot directly
compare these results to RT-IR curing experiments. Nevertheless, the results from these methods
indicate that BA addition leads to a decrease in mobility of cured CEOS network.
The positive impact of boehmite on the final epoxy conversion that was observed in our
investigations can be explained through the presence of hydroxyl groups located on the particle
surface as they can be involved in the propagation step of epoxy polymerization. Generally, the
propagation can proceed through the subsequent attacks of epoxy groups by oxonium ions as
described by active-chain end (ACE) mechanism. However, it can be shifted to active monomer (AM)
mechanism through alcohol attack and proton-transfer reactions because of the presence of protonic
nucleophiles [34–36]. Both mechanisms are displayed in Scheme 2 and described in detail elsewhere
[34,37]. The hydroxyl groups carried on the boehmite surface can react with oxirane ring via an AM
mechanism (Scheme 3) forming covalent linkages. This fact could explain the significant rise of Tg
and increased thermal stability of “inorganic” components of nanocomposites (as seen in Figure 5),
even when only 1 wt.% of boehmite was present.
The interaction between Al–OH groups and epoxy has been also previously considered [38,39].
The reaction between exposed hydroxyl groups of boehmite and neutral epoxy group should be also
taken into account as another possible reaction mechanism resulting in covalent bonding [39].
Scheme 2. Simplified representation of cationic polymerization of cycloaliphatic epoxy groups: Active
Monomer (AM) and Active-Chain End (ACE) mechanisms.
Scheme 2.
Simplified representation of cationic polymerization of cycloaliphatic epoxy groups: Active
Monomer (AM) and Active-Chain End (ACE) mechanisms.
Materials 2019,12, 1513 15 of 18
Materials 2019, 12, x FOR PEER REVIEW 15 of 18
Scheme 3. Reaction between oxonium ion and Al–OH groups of boehmite via AM mechanism.
As was detected from IR spectra and TGA, both boehmites, OS1, and HP14, contain certain
amounts of physically absorbed water. This nucleophile is known to influence the cationic
polymerization, and therefore, is expected to be another cause boosting the polymerization processes.
Water molecule should be considered during cationic ring-opening polymerization to act as a chain-
transfer agent as for active-chain end (ACE) so for active monomer (AM) mechanisms
(Supplementary Materials Scheme S2), thus, leading to re-initialization [37,40,41]. This was also the
case for the previous studies where a small amount of water present in clays caused enhancement in
the final epoxy group conversion [42].
A higher improvement of different parameters such as epoxy conversion degree and glass
transition temperature were observed with use of organo-modified boehmite (OS1) compared to
unmodified nanoparticles (HP14). This effect can be explained by improved particle spatial
distribution as well as higher dissociation constant of p-toluenesulfonic acid. This modifying agent
can in fact interfere with the photoinitiator producing photons, thus, initiating the epoxy groups
located at the BA-CEOS interphase.
4. Conclusions
Novel boehmite-embedded organic/inorganic nanocomposites based on cycloaliphatic epoxy
oligosiloxane resin were prepared with two different boehmite fillers via UV-induced cationic ring-
opening photopolymerization. Material properties such as curing kinetic, epoxy conversion degree,
particles distribution, glass transition and thermal stability were investigated with regard to various
loading amounts of boehmite nanoparticles.
First, an increase of glass transition temperature was found for all nanocomposites indicating
reduced mobility of chain segments in cured hybrid network. The strong reinforcement effect was
already detected at 1 wt.% boehmite loading. Second, improved thermal stability indicated attractive
particle-resin interactions.
The particles dispersion can be described as a homogeneously distributed network of cluster-
like agglomerates of boehmite. The size of detected agglomerates was generally bigger for
unmodified boehmite, due to the insufficient polymer–particle interaction.
The vitrification of the CEOS network had somewhat retarded with the addition of BA; the
overall epoxy conversion was increased for nanocomposites with 5–10 wt.% of boehmite. These
results are contradictory to the previous studies on the cycloaliphatic epoxy/BA systems where the
decreased monomer mobility due to raised viscosity and/or light screening effect of the boehmite
particles caused the hindering of cross-linking processes in the polymer. However, in our study, the
vitrification appears to be the main limitation factor preventing reaching a fully polymerized
structure.
The elevated αep of CEOS/BA nanocomposites compared to the neat resin film can be explained
by several reasons. First, hydroxyl groups present at boehmite’s surface might react with oxonium
Boehmite
Boehmite
Boehmite
Scheme 3. Reaction between oxonium ion and Al–OH groups of boehmite via AM mechanism.
The interaction between Al–OH groups and epoxy has been also previously considered [
38
,
39
].
The reaction between exposed hydroxyl groups of boehmite and neutral epoxy group should be also
taken into account as another possible reaction mechanism resulting in covalent bonding [39].
As was detected from IR spectra and TGA, both boehmites, OS1, and HP14, contain certain amounts
of physically absorbed water. This nucleophile is known to influence the cationic polymerization, and
therefore, is expected to be another cause boosting the polymerization processes. Water molecule
should be considered during cationic ring-opening polymerization to act as a chain-transfer agent as for
active-chain end (ACE) so for active monomer (AM) mechanisms (Supplementary Materials Scheme
S2), thus, leading to re-initialization [
37
,
40
,
41
]. This was also the case for the previous studies where a
small amount of water present in clays caused enhancement in the final epoxy group conversion [
42
].
A higher improvement of different parameters such as epoxy conversion degree and glass
transition temperature were observed with use of organo-modified boehmite (OS1) compared to
unmodified nanoparticles (HP14). This effect can be explained by improved particle spatial distribution
as well as higher dissociation constant of p-toluenesulfonic acid. This modifying agent can in fact
interfere with the photoinitiator producing photons, thus, initiating the epoxy groups located at the
BA-CEOS interphase.
4. Conclusions
Novel boehmite-embedded organic/inorganic nanocomposites based on cycloaliphatic epoxy
oligosiloxane resin were prepared with two different boehmite fillers via UV-induced cationic
ring-opening photopolymerization. Material properties such as curing kinetic, epoxy conversion
degree, particles distribution, glass transition and thermal stability were investigated with regard to
various loading amounts of boehmite nanoparticles.
First, an increase of glass transition temperature was found for all nanocomposites indicating
reduced mobility of chain segments in cured hybrid network. The strong reinforcement effect was
already detected at 1 wt.% boehmite loading. Second, improved thermal stability indicated attractive
particle-resin interactions.
The particles dispersion can be described as a homogeneously distributed network of cluster-like
agglomerates of boehmite. The size of detected agglomerates was generally bigger for unmodified
boehmite, due to the insufficient polymer–particle interaction.
The vitrification of the CEOS network had somewhat retarded with the addition of BA; the overall
epoxy conversion was increased for nanocomposites with 5–10 wt.% of boehmite. These results are
contradictory to the previous studies on the cycloaliphatic epoxy/BA systems where the decreased
monomer mobility due to raised viscosity and/or light screening effect of the boehmite particles caused
Materials 2019,12, 1513 16 of 18
the hindering of cross-linking processes in the polymer. However, in our study, the vitrification appears
to be the main limitation factor preventing reaching a fully polymerized structure.
The elevated
αep
of CEOS/BA nanocomposites compared to the neat resin film can be explained
by several reasons. First, hydroxyl groups present at boehmite’s surface might react with oxonium
ion via an active monomer (AM) polymerization mechanism. The second reason is the contribution
of chain-transfer reactions initiated by nucleophilic water introduced into the system with boehmite
nanoparticles. Third is the additional proton production induced by the presence of p-toluenesulfonic
acid as a surface modifier.
The impact of particle surface on nanocomposite behavior is evident as it defines the nature of
polymer–particle interactions. With organo-modified boehmite nanoparticles (OS1), we achieved the
nanocomposites with better spatial particle distribution, higher final epoxy group conversion, as well
as higher T
g
values compared to nanocomposite with unmodified particles (HP14). On the one hand,
larger polymer–particle interphase resulted from the better particle–polymer compatibility and particle
distribution. On the other hand, positive contribution to the initiation processes can be explained by
the presence of p-toluenesulfonic acid as well.
Supplementary Materials:
The following are available online at http://www.mdpi.com/1996-1944/12/9/1513/s1,
Figure S1: Structures of (a) unmodified and (b) p-toluenesulfonic acid modified boehmite. Figure S2:
13
C NMR
spectra of CEOS synthesized at different ECTS:DPSD molar ratios. Figure S3: FTIR spectra of boehmite powders:
unmodified, HP14 and surface modified with p-toluenesulfuric acid, OS1. Figure S4: DSC curves at second
heating of CEOS without and with different boehmite loadings: (a) HP14 and (b) OS1. Figure S5: Determination
of the glass transition temperature for CEOS hybrid. Figure S6: Determination of the glass transition temperature
for CEOS/HP14 compositions. Figure S7: Determination of the glass transition temperature for CEOS/HP14
compositions. Figure S8: (a) TG and (b) DTG curves of HP14 and OS1 boehmite powders. Figure S9: Decrease of
C–H stretching of oxirane ring during UV irradiation for CEOS with (a) HP14 and (b) OS1 boehmite nanoparticles.
Figure S10: Formation of hydroxyl groups yielded from oxirane ring opening for CEOS with (a) HP14 and (b) OS1
boehmite nanoparticles. Figure S11: First derivative of curing kinetics of neat CEOS and its nanocomposites with
different (a) HP14 and (b) OS1 loadings. Table S1. Characteristic IR bands of CEOS and BA in middle infrared
region. Scheme S1: (a) Chemical structure and (b) simplified scheme of photodecomposition of arylsulfonium
hexafluorophosphate salt. Scheme S2: Reaction of epoxy with water under acidic conditions.
Author Contributions:
RT-IR, DSC and TGA investigation, I.T.; NMR investigation, D.P.; SEM data curation,
V.-D.H.; writing—original draft preparation, I.T.; writing—review and editing, U.B. and H.S.; supervision, H.S.
Funding: Financial support was received from Adolf–Martens-Fellowship funded by BAM institute.
Acknowledgments:
The authors would like to thank Sigrid Benemann, BAM, Berlin, Division 6.1 for performing
SEM measurements.
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
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