Polyme 226 (2021) 123812
A ailable online 5 May 2021
0032-3861/© 2021 The Au ho (s). Published by Else ie L d. This is an open access a icle unde he CC BY license (h p://c ea i ecommons.o g/licenses/by/4.0/).
Nanos uc u al o ganiza ion o hin ilms p epa ed by sequen ial
dip-coa ing deposi ion o poly(bu ylene succina e), poly(
ε
-cap olac one)
and hei copolyes e s (PBS- an-PCL)
Ma io I ´
an Pe˜
nas
a
,
b
, Connie Ocando
b
, E is Peno -Chang
c
, Ma yam Sa a i
b
,
Tibe io A. Ezque a
d
, Es he Rebolla
e
, Au o a Nogales
d
, Rebeca He n´
andez
a
,
*
,
Alejand o J. Mülle
b
,
c
,
,
**
a
Ins i u o de Ciencia y Tecnología de Políme os, ICTP-CSIC, Juan de la Cie a, 3, 28006, Mad id, Spain
b
POLYMAT y Depa amen o de Políme os y Ma e iales A anzados: Física, Química y Tecnología, Facul ad de Química, Uni e sidad del País Vasco UPV/EHU, Paseo
Manuel de La dizabal 3, 20018, Donos ia-San Sebas i´
an, Spain
c
G upo de Políme os I USB, Depa amen o de Ciencias de los Ma e iales, Uni e sidad Sim´
on Bolí a , 1080A, Ca acas, Venezuela
d
Ins i u o de Es uc u a de la Ma e ia, IEM-CSIC, Se ano 121, 28006, Mad id, Spain
e
Ins i u o de Química Física Rocasolano, IQFR-CSIC, Se ano 119, 28006, Mad id, Spain
IKERBASQUE, Basque Founda ion o Science, Bilbao, Spain
ARTICLE INFO
Keywo ds:
Poly(
ε
-cap olac one)
poly(bu ylene succina e)
Random copolyes e s
Thin- ilms
Laye -by-laye dip-coa ing
s-SNOM
nano-FTIR
ABSTRACT
In his wo k, we p epa e and cha ac e ize mul iphasic hin ilms con aining poly(
ε
-cap olac one), PCL, poly
(bu ylene succina e), PBS, and a poly (bu ylene succina e- an-ϵ-cap olac one) (PBS- an-PCL) andom copo-
lyes e . To ha aim, hin ilms we e p epa ed by sequen ial dipping o a silicon subs a e in o chlo o o m so-
lu ions o he espec i e polyme s. The p epa a ion me hod esul ed in ilms wi h a ying composi ions o PCL
and PBS componen s depending on he ini ial concen a ion o he dipping solu ions and he numbe o dipping
s eps employed o he p epa a ion o he samples. A omic o ce mic oscopy (AFM), g azing incidence X- ay
sca e ing a wide angle (GIWAXS) and sca e ing- ype scanning nea - ield op ical mic oscopy (s-SNOM) and
Fou ie ans o m in a ed nanospec oscopy (nano-FTIR) we e employed o cha ac e ize he ilms ob ained. As
chlo o o m can dissol e all componen s, he inal composi ion o he ilm was always ich in he las deposi ed
laye componen . The hin ilms ob ained we e semic ys alline wi h a complex axiali ic o dend i ic mo phology
o he dominan componen ( ha one deposi ed las ) wi h aces o he o he componen s, whose p esence and
loca ion was e ealed by s-SNOM/nano-FTIR.
1. In oduc ion
Poly(ϵ-cap olac one) (PCL) and poly(bu ylene succina e) (PBS) a e
ully biodeg adable alipha ic polyes e s. They ha e a ac ed a g ea
deal o a en ion o he de elopmen o biodeg adable packaging and
biomedical applica ions, such as implan de ices, issue sca olds and
wound d essings [1]. The blending o biodeg adable polyes e s is
conside ed one o he mos p omising ways o ob ain sus ainable ma-
e ials wi h enhanced and ailo ed p ope ies. PCL/PBS blends ha e
been p epa ed by mel blending o solu ion mixing me hods a emp ing
o imp o e he p ope ies o he indi idual polyme s. Rega dless o he
p epa a ion me hod used, PCL/PBS blends a e immiscible, as e idenced
by composi ion independen T
g
s and a biphasic mel , which leads o poo
in e acial adhesion and mac ophase sepa a ion [2]. Ne e heless,
ea lie epo s on PBS/PCL blends ha e indica ed ha a no iceable
imp o emen in he mechanical p ope ies o PBS/PCL blends could be
ob ained. These esul s we e explained by conside ing possible in-
e ac ions be ween he es e g oups o he nea polyme s h ough
hyd ogen bonds [3]. Al hough bulk p ope ies o hese wo polyme s
and hei blends ha e been ex ensi ely s udied [4] [–] [7], no much
* Co esponding au ho . Ins i u o de Ciencia y Tecnología de Políme os, ICTP-CSIC, Juan de la Cie a, 3, 28006, Mad id, Spain.
** Co esponding au ho . POLYMAT y Depa amen o de Políme os y Ma e iales A anzados: Física, Química y Tecnología, Facul ad de Química, Uni e sidad del
País Vasco UPV/EHU, Paseo Manuel de La dizabal 3, 20018, Donos ia-San Sebas i´
an, Spain.
E-mail add esses: [email p o ec ed] (R. He n´
andez), [email p o ec ed] (A.J. Mülle ).
Con en s lis s a ailable a ScienceDi ec
Polyme
jou nal homepage: www.else ie .com/loca e/polyme
h ps://doi.o g/10.1016/j.polyme .2021.123812
Recei ed 30 Janua y 2021; Recei ed in e ised o m 19 Ap il 2021; Accep ed 21 Ap il 2021
Polyme 226 (2021) 123812
2
in o ma ion abou he s uc u e and mo phology o hei hin and ul-
a hin ilms has been epo ed.
Thin polyme ilms ob ained om deposi ion o polyme blends in
solu ion has been widely epo ed in li e a u e as a bo om up me hod o
p oduce pa e ned su aces a ising om di e en phenomena such as
dewe ing and phase sepa a ion [8]. The p epa a ion o hin polyme
ilms h ough sequen ial deposi ion o polyme aqueous solu ions,
known as laye -by-laye (LbL) me hod has been widely in es iga ed o
achie e nanos uc u ed ilms and coa ings h ough he inco po a ion o
successi e polyme laye s ha in e ac wi h each o he elec os a ically,
H-bonded o co alen ly [9] [–] [11]. In con as , he p epa a ion o hin
polyme ilms h ough sequen ial deposi ion o di e en polyme s in
o ganic solu ions is less add essed in li e a u e. Such me hod o p epa-
a ion allows o deposi i s a polyme laye on a subs a e o ha e an
o de ed s uc u e ollowed by he deposi ion o a second laye o a
di e en polyme (o o ganic molecule) in o de o minimize hei
in e mixing. Such app oach has been employed mos ly o he p epa-
a ion o hin ilms applied as elec onic de ices and ene gy con e sion
se -ups [12] [–] [14].
The mo phology o polyme hin ilms deposi ed by spin-coa ing is
mos ly d i en by he composi ion o he polyme solu ion and pos -
ea men s such as he mal o sol en annealing a e employed as a
mean o modula e hei inal mo phology. Du ing he annealing p ocess,
su ace seg ega ion is induced by p e e en ial we ing o one componen
in polyme blends and block copolyme hin ilms [15]. This phenom-
enon known as su ace we ing induced su ace seg ega ion esul s in
changes o he su ace chemical composi ion o hin ilms and in luences
c ys alliza ion o c ys alline block copolyme hin ilms and blends [16]
[–] [19]. On he o he hand, o hin ilms ob ained by dip-coa ing, he
inal ilm mo phology depends o a high ex en on he expe imen al
condi ions, such as wi hd awal speed, na u e o he sol en , solu ion
concen a ion and geome y o he ese oi and i can be modula ed
wi hou he need o addi ional pos annealing echniques [20–22].
Wi hin his con ex , dip-coa ing cons i u es one e ec i e p ocess o he
ab ica ion o hin polyme ilms, wi h ex ensi e applica ion in
small-scale ab ica ion o academic s udies [23]. The me hod is based
on he deposi ion o hin ilms ia p ecision imme sion and wi hd awal
o a subs a e in o a ese oi con aining a polyme solu ion. Speci -
ically, dip-coa ing has been ex ensi ely epo ed o p epa e PCL hin
ilms ha can be employed as coa ings o deg ada ion p e en ion in
biodeg adable magnesium alloys and blend coa ings wi h PEG o hei
applica ion as bioma e ials [24,25]. To he bes o ou knowledge, he
p epa a ion and cha ac e iza ion o PCL/PBS hin polyme ilms ha e
no been desc ibed in he li e a u e.
In his s udy, hin ilms o PCL, PBS and a andom copolyes e (PBS-
an-PCL) we e p epa ed by sequen ial dipping s eps o a silicon subs a e
in o chlo o o m solu ions o he espec i e polyme s, p e iously syn-
hesized by some o us [26]. The use o poly (bu ylene succina e- -
an-ϵ-cap olac one) (PBS- an-PCL) copolyme laye s oge he wi h PCL
and/o PBS is expec ed o imp o e miscibili y, in e acial adhesion and
he esul an mechanical p ope ies o PCL/PBS blends as p e iously
epo ed [27]. A nea ly symme ic copolyme was chosen o his s udy,
as his has a be e chance o in e ac ing wi h nea PCL and nea PBS,
hence i would ha e a be e chance o p omo e adhesion be ween a
laye o nea PBS and a laye o nea PCL. Copolyme composi ions
di e en o app oxima ely 50/50 would be mo e compa ible wi h ei he
PBS o PCL depending on he composi ion. The esul ing mo phology
was obse ed by a omic o ce mic oscopy (AFM), and i was ela ed o
he hickness and composi ion o he ilms. The p esence o PCL o PBS
c ys als was explo ed by g azing incidence X- ay sca e ing a wide
angle (GIWAXS). P e ious s udies ca ied ou on blend hin ilms p e-
pa ed by dip-coa ing ha e employed selec i e dissolu ion o one o he
polyme s in o de o be able o assign he polyme phases obse ed in
AFM images [22]. Fo he ilms unde s udy he e, i is no possible o
selec i ely elimina e one polyme as he h ee polyme s dissol e in
common sol en s. The e o e, sca e ing- ype scanning nea - ield op ical
mic oscopy (s-SNOM) and Fou ie ans o m in a ed nanospec oscopy
(nano-FTIR) we e employed in selec ed samples o image he local dis-
ibu ion o he h ee polyme s in he ilms unde s udy. The in o ma ion
ob ained allows us o co ela e he dip-coa ing p ocessing employed o
he p epa a ion o he samples o he s uc u e and mo phology
exhibi ed by PCL/PBS hin ilms.
2. Expe imen al pa
2.1. Ma e ials
Poly (bu ylene succina e) (PBS), poly (ϵ-cap olac one) (PCL) and a
poly (bu ylene succina e- an-ϵ-cap olac one) andom copolyes e ,
designa ed as COPOL, we e syn hesized acco ding o a me hod epo ed
elsewhe e [26]. Table 1 epo s he a e age molecula weigh and
composi ion o he polyme s unde s udy.
2.2. Thin ilms p epa a ion
Solu ions o PCL, PBS and COPOL we e p epa ed by dissol ing he
app op ia e mass o polyme in CHCl
3
o ob ain concen a ions o 1 mg/
mL and 2 mg/mL. Thin ilms o PCL, PBS, and COPOL we e p epa ed by
dip-coa ing silicon wa e s (40 ×9 ×0.5 mm
3
), p e iously washed ou
om impu i ies wi h a pi anha solu ion (60:40 H
2
SO
4
:H
2
O
2
), in o he
co esponding CHCl
3
solu ions du ing 2 min. The ilm d ying ime o
p is ine hin ilms o PCL, PBS, and COPOL was de e mined by isuali-
za ion o he heigh posi ion o he d ying line a e wi hd awing sample,
loca ed by a sha p colou (see supplemen a y ideo). The e apo a ion
speed, calcula ed by conside ing ha he solu ion heigh was main-
ained a 30 mm, was ~3 mm s
−1
in all cases. I is impo an o no e ha
concen a ions o he polyme s in he dipping solu ions highe han 2
mg/mL led o he o ma ion o s ipes on he su ace o he ilm,
pe pendicula ly o he wi hd awal di ec ion (see Figu e S1, in he Sup-
plemen a y In o ma ion). This phenomenon, named “s ick-slip mo ion”
has been obse ed and desc ibed o ilms p epa ed by dip-coa ing, and
i is he esul o he deposi ion o high concen a ions o he polyme in
he uppe pa o he meniscus gi ing ise o he o ma ion o local
hickness he e ogenei ies [22].
Fo he p epa a ion o ilms con aining PBS, PCL, and COPOL, Si
wa e s we e sequen ially imme sed in CHCl
3
solu ions o PCL, COPOL,
and PBS a 1 and 2 mg/mL (se ies I and II, espec i ely). A schema ic
ep esen a ion o he p ocess employed o he p epa a ion o he
Table 1
Molecula cha ac e iza ion da a
a
.
Polyme Composi ion (BS/CL mol/mol) M
w
(g/mol) M
w
/M
n
PBS (100/0) 21,470 2.9
PCL (0/100) 17,400 3.2
COPOL (51/49) 23,500 3.1
a
Da a aken om Re . [26].
Scheme 1. Schema ic ep esen a ion o he me hod o p epa a ion o he ilms.
M.I. Pe˜
nas e al.
Polyme 226 (2021) 123812
3
samples is shown in Scheme 1. The imme sion ime was 2 min wi h an
ai -d ying s ep o 20 s in be ween dipping s eps.
Films co esponding o se ies I we e 3-laye ilms p epa ed om
CHCl
3
solu ions o PCL, COPOL, and PBS a 1 mg/mL. The di e ence
among hem was he o de employed o he deposi ion o he polyme s.
Fo he ilm designa ed as 3-laye (PBS)_1, he las deposi ed laye was
PBS (i.e., he laye s we e deposi ed in he ollowing o de : PCL, COPOL
and PBS), and o he ilm designa ed as 3-laye (PCL)_1, he las
deposi ed laye was PCL (i.e., he laye s we e deposi ed in he ollowing
o de : PBS, COPOL and PCL).
Films co esponding o se ies II we e p epa ed om CHCl
3
solu ions
o PCL, COPOL, and PBS a 2 mg/mL. Two ilms we e p epa ed wi h
di e en numbe o laye s (3 and 13) in o de o de e mine he e ec o
he numbe o laye s on he mo phology o he esul ing ilms. In bo h
cases, he las deposi ed laye was PBS. The 3-laye ilm was designa ed
as 3-laye (PBS)_2, whe eas he 13-laye ilm was designa ed as 13-laye
(PBS)_2.
2.3. Nuclea magne ic esonance (NMR)
Thin polyme ilms we e dissol ed in 1 mL o deu e a ed chlo o o m
and
1
H NMR spec a we e aken in a Va ian Sys em 500 MHz NMR
equipmen .
2.4. A omic o ce mic oscopy (AFM)
The mo phology and hickness o he laye s we e obse ed by AFM
(Mul imode Scanning P obe Mic oscope, Veeco/B uke ), employing a di
NanoScope IVa Con olle wi h he con en ional heigh mode ( apping
mode, no mal AFM) a a nominal o ce cons an o 42 N/m and 320 kHz
esonan equency. The AFM samples (9 ×9 ×0.5 mm
3
) we e cu om
he p e iously p epa ed ilms on silicon wa e subs a es. Film hick-
nesses we e measu ed by AFM by he sc a ch me hod and he esul s
we e compa ed o hose ob ained om he analysis o he AFM heigh
p o iles. WSxM5.0 so wa e was employed o he isualiza ion and
analysis o he AFM images.
2.5. X- ay sca e ing
G azing incidence X- ay sca e ing a wide angle (GIWAXS) was
measu ed using synch o on adia ion a he NCD-SWEET beamline a
he ALBA Synch o on (Ce danyola del Vall`
es, Ba celona, Spain). The
sample was inclined o achie e di e en incidence angles, anging om
0.1◦ o 0.3◦be ween he sample su ace and he X- ay beam. The X- ay
wa eleng h used was 1 Å. The sca e ing in ensi y was collec ed by a
Rayonix de ec o o 960 ×2880 pixels (pixel size 88.54
μ
m), placed a
145.6 mm om he sample. Pa e ns acqui ed wi h an exposi ion ime o
5 s we e co ec ed o backg ound sca e ing and analyzed by he Fi 2D
so wa e [28].
2.6. Sca e ing- ype scanning nea - ield op ical mic oscopy (s-SNOM)
and ou ie ans o m in a ed nanospec oscopy (nano-FTIR)
s-SNOM, based on he AFM echnique, is ca ied ou wi h a me al-
lized ip which is illumina ed wi h a monoch oma ic in a ed lase a-
dia ion concen a ed a he e ex o he ip, ac ing as an an enna. s-
SNOM yields in a ed ampli ude and phase in a ed images a nanoscale
esolu ion and allows o ob ain maps o he chemical p ope ies o he
su ace o he sample. Nano-FTIR is based on s-SNOM, whe e he ip is
illumina ed wi h a b oadband in a ed adia ion. The ip-sca e ed ligh
is eco ded wi h an asymme ic Fou ie ans o m spec ome e a a
ixed sample posi ion, yielding ampli ude- and phase- esol ed in a ed
spec a [29] [–] [31].
A comme cial s-SNOM/nano-FTIR se up (NeaSNOM, Neaspec
GmbH, Ge many) was employed o measu ing s-SNOM phase images
and nano-FTIR phase spec a using gold-coa ed comme cial Si ips
(Nanosenso s, PPP-NCSTAu) wi h a mechanical esonance equency
~135 kHz o apping mode a omic o ce mic oscope (AFM). s-SNOM
imaging was collec ed di ec ly on hin ilm samples p epa ed in silicon
wa e s by means o a mic oscope equipped wi h a MIRca lase (MIRca
Mid-IR lase , Dayligh Solu ions, USA), a a lase powe o 3 mW, and a
apping ampli ude o 50 nm. The acquisi ion ime o one s-SNOM image
was 11 min. Fo nano-FTIR, a b oadband in a ed lase con inuum was
employed wi h an a e age ou pu powe o ~600
μ
W co e ing a spec al
ange om 2200 o 650 cm
−1
. All nano-FTIR spec a we e eco ded wi h
a spec al esolu ion o 17 cm
−1
and a apping ampli ude o 80 nm.
Repo ed nano-FTIR spec a a e a e ages o 10 ull spec a.
3. Resul s and discussion
3.1. De e mina ion o he composi ion o he ilms
The p ocess o sequen ial dip coa ing employed o p oducing he
ilms akes place h ough sequen ial imme sion o a silicon subs a e
on o chlo o o m solu ions o each o he polyme s as shown in Scheme 1.
Gi en he ac ha chlo o o m is a sol en o PCL, PBS and COPOL, as a
i s s ep, we se o de e mine he (BS/CL) a io o all he samples unde
s udy. Fo compa ison, he spec a co esponding o nea PBS, PCL and
COPOL (see chemical s uc u es in Figu e S2, in he Supplemen a y In-
o ma ion) a e also included in Fig. 1 and he peaks a e assigned ac-
co ding o li e a u e [26]. The iple signal cen e ed a 4.06 ppm can be
assigned o he me hylene p o on esonance o CH
2
(7) in PCL and he
mul iple signal cen e ed a 4.12 ppm co esponds o he p o on eso-
nances o CH
2
(1,4) in PBS. Fo COPOL, i was no possible o ob ain
sepa a e signals co esponding o PBS and PCL sequences wi hin his
ppm egion. In he ilms, he p esence o a esonance peak a 4.08 ppm
ha can be assigned o COPOL (ma ked wi h an a ow in Fig. 1) is
obse ed, mos no ably o ilms co esponding o se ies II, which con-
i ms he p esence o he h ee componen s o all he samples unde
s udy. Ne e heless, a quan i a i e de e mina ion is no possible due o
he o e lapping o he NMR signals o he COPOL wi h hose co e-
sponding o he p is ine polyme s, PBS and PCL.
Fig. 1.
1
H NMR spec a in he 4.00–4.20 ppm egion o ilms co esponding o
se ies I, 3-laye (PBS)_1 and 3-laye (PCL)_1, and se ies II, 3-laye (PBS)_2 and
13-laye (PBS)_2. Fo compa ison, he spec a co esponding o nea PBS, PCL
and COPOL a e also included in he igu e.
M.I. Pe˜
nas e al.
Polyme 226 (2021) 123812
4
3.2. S udy o he mo phology and c ys allini y
Fig. 2 shows AFM heigh images o p is ine ilms ob ained om
CHCl
3
solu ions o PBS and PCL and ilms co esponding o se ies I
p epa ed a polyme concen a ions o 1 mg/mL, 3-laye (PBS)_1 and 3-
laye (PCL)_1. All he samples unde s udy a e ul a hin ilms wi h
hicknesses below 10 nm as de e mined by AFM by he sc a ch me hod.
The esul s we e shown o ma ch he dep h p o ile analyzed o each o
he samples, bo h analysis a e shown in he supplemen a y in o ma ion
(Figu e S3, in he Supplemen a y In o ma ion).
The mo phology o PCL in ul a- hin ilms has been epo ed in he
li e a u e [32,33], howe e , o he bes o ou knowledge, he e a e no
s udies ega ding he mo phology o PBS in ul a- hin ilms. Films ob-
ained om CHCl
3
solu ions o PBS a 1 mg/mL (Fig. 2a) exhibi a
semi-c ys alline mo phology ich in edge-on lamellae adia ing om
cen al nuclei ha esemble sphe uli ic/axiali ic supe s uc u es,
whe eas he mo phology exhibi ed by PCL hin ilms is dend i ic
(Fig. 2b). In all cases, he ela i ely s aigh bounda ies be ween su-
pe s uc u al s uc u es (sphe uli es, axiali es o dend i es), which a e
caused by he impinging o hese s uc u es as hey g ow adially, can be
clea ly obse ed in Fig. 2. The mo phology obse ed in Fig. 2b o PCL
( hickness~8 nm) is consis en wi h ha epo ed o spin-coa ed PCL
ilms om PCL- oluene solu ion o which dense-b anching mo phology
(DBM) and dend i es we e obse ed when <2Rg (a hicknesses below
12 nm), ela ed o he di usion-limi ed agg ega ion (DLA) p ocess [32].
The mo phology co esponding o he COPOL sample consis s o
PBS- ich phase sphe uli ic/axiali ic c ys als (see Figu e S4, in he Sup-
plemen a y In o ma ion). Ex ensi e p e ious s udies by WAXS and DSC
ha e shown ha in his isodimo phic copolyme sample, only he
PBS- ich phase can c ys allize [25].
Bo h ilms o se ies I (3-laye ilms desc ibed in Table 2) ha e
hicknesses ha a e compa able wi hin he expe imen al e o and a e
also simila o hose o nea componen s single laye ilms, ~7.5 nm o
he ilms o se ies I and 5.7 and 8.5 nm o nea PBS and PCL espec i ely
(a 1 mg/mL). This is as a esul o he pa ial dissolu ion o he ilms in
CHCl
3
du ing he consecu i e deposi ion s eps, since chlo o o m dis-
sol es bo h PCL and PBS. S ill, he p esence o he h ee polyme s could
be asce ained h ough
1
H NMR as shown in Fig. 1. In gene al e ms, he
mo phology obse ed by AFM on he ilm su aces is de e mined by he
las deposi ed polyme laye which con i ms he sequen ial deposi ion o
he polyme s on o he silicon subs a e. The e o e, ilms ha we e
coa ed wi h PBS as las laye show a simila sphe uli ic/axiali ic
mo phology o ha exhibi ed by nea PBS ilms (single laye ) dip-coa ed
om chlo o o m solu ion. Co espondingly, hose in which he las
applied laye was PCL display he dend i ic mo phology obse ed in
nea PCL ilms (single laye ). The mo phology obse ed o he 3-laye
(PCL)_1 sample is analogous o ha epo ed o PS/PCL blend ilms
wi h ilm hickness o abou 15 nm, o which he c ys alliza ion o PCL
leads o he o ma ion o PCL dend i ic c ys als [19]. I is impo an o
no e ha Fig. 2c shows he de elopmen o a new c ys alline
mo phology wi h espec o he mo phology obse ed o p is ine PBS
ilms (Fig. 2a). On op o he sphe uli ic s uc u e, we obse e cu ed
s uc u es (highligh ed wi h a squa e in Fig. 2c). These cu ed c ys als
ha e been p e iously epo ed o PLLA ul a hin ilms and ela ed o
Fig. 2. Tapping mode AFM heigh images o a) PBS ilms, b) PCL ilms c) 3-laye (PBS)_1 and d) 3-laye (PCL)_1 p epa ed by dip-coa ing om CHCl
3
solu ions a 1
mg/mL. Black a ows ma k he s aigh bounda ies obse ed be ween supe s uc u al s uc u es (sphe uli es, axiali es o dend i es) which a e caused by he
impinging o hese s uc u es as hey g ow adially. The black squa e in Fig. 2c ma ks he occu ence o new c ys alline mo phologies as explained in he ex .
Table 2
Nomencla u e employed o all he samples unde s udy.
Sample C (mg/mL)
a
Numbe o laye s Las deposi ed laye
Se ies I
3-laye (PBS)_1 1 3 PBS
3-laye (PCL)_1 1 3 PCL
Se ies II
3-laye (PBS)_2 2 3 PBS
13-laye (PBS)_2 2 13 PBS
a
Concen a ion o he dipping solu ions.
M.I. Pe˜
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Polyme 226 (2021) 123812
5
edge-on lamellae [34,35], in e y hin ilms ( hicknesses a ound and
below 10 nm). A s udy o he c ys al uni cell pa ame e s and chain
o ien a ion o hin ilms o linea alipha ic polyes e s e ealed ha
lamella c ys al edge-on mo phology seems o be he p e e ed con ig-
u a ion o alipha ic polyes e s. This is ela ed o he es ablishmen o a
molecula in e ac ion o he ca bonyl g oups o he polyes e s, lying on
he bc ace pa allel o he subs a e plane, wi h he ou e silicon oxide
laye o he subs a e ha con ols he chain ixa ion o he subs a e
[36].
The c ys alliza ion o he ilms was u he in es iga ed by G azing
Incidence X Ray Di ac ion analysis and he di ac og ams ob ained
using GIXRD a 0.1◦incidence angle a e shown in Fig. 3. Fo he p is ine
PCL ilm (1 mg/mL), wi h hickness as low as 8.5 nm, he cha ac e is ic
di ac ions o (110), (111) and (200) planes a e clea ly dis inguished in
Fig. 3a, [37]. Fo he p is ine PBS ilm (1 mg/mL), he GIWAXS di -
ac og am in Fig. 3b showed no di ac ion peaks e en hough a semi-
c ys alline mo phology was obse ed o his ilm in AFM images
(Fig. 2a). In con as , a b oad peak a 15.5 nm
−1
is obse ed o he PBS
ilm p epa ed om chlo o o m solu ions o PBS a 2 mg/mL (Fig. 3c),
which is a ibu ed o he di ac ion o he (021) plane. No e ha he
peak co esponding o he di ac ion o he (110) plane a 16.0 nm
−1
, is
also included wi hin his b oad peak [38,39].
Rega ding he esul s ob ained o blend hin ilms, only he 3-laye
(PCL)_1 sample showed he cha ac e is ic di ac ion peaks o PCL,
placed as he ou e mos laye o his sample (Fig. 3d). Fo his sample,
he di ac ion peaks a e shi ed o he le wi h espec o he di ac ion
peaks obse ed o he p is ine PCL ilm (Fig. 3a). This migh be a esul
o he di e en sp eading o c ys alli es on he illumina ed a ea [40]. As
shown in Fig. 2, he dis ibu ion o c ys als appea s o be mo e homo-
geneous in he case o he PCL ilm (Fig. 2b) compa ed o he dis ibu ion
o c ys als obse ed o sample 3-laye PCL (Fig. 2d). This migh esul in
con ibu ions in di e en posi ions o he di ac ion cen e s which
causes a shi ing o he le o he di ac ion peaks co esponding o he
sample 3-laye PCL (Fig. 3d) wi h espec o he sample PCL (Fig. 3a). In
con as , nei he he sample 3-laye (PBS)_1 (Fig. 3e) no he samples
co esponding o se ies II, o which he ou e mos laye was PBS,
showed di ac ion peaks in GIWAXS expe imen s ( esul s no shown).
This is an appa en inconsis ency wi h he AFM esul s shown in Fig. 2
ha clea ly show an sphe uli ic/axiali ic s uc u e o p is ine PBS ilms
as well as o ilms whose las laye is PBS. In o de o explain hese
esul s i is impo an o ake in o accoun he p ocess o p epa a ion o
he samples h ough dip-coa ing. Fo his p ocess, he e apo a ion a e
o he sol en plays an impo an ole on he c ys alliza ion o he
polyme s. Sphe uli e/axiali e g ow h is he consequence o he
Fig. 3. In ensi y as a unc ion o he sca e ing ec o q as calcula ed om he
co esponding GIWAXS pa e ns (shown in he Supplemen a y In o ma ion,
Figu e S5). a) PCL 1 mg/mL, b) PBS 1 mg/mL, c) PBS 2 mg/mL, d) 3-laye
(PCL)_1, e) 3-laye (PBS)_1.
Fig. 4. Nano-FTIR spec a o p is ine ilms o a) PBS, b) COPOL and c) PCL.
M.I. Pe˜
nas e al.
Polyme 226 (2021) 123812
6
mac omolecules di usion owa ds he c ys al in e ace. Capilla y low
occu ing du ing he dip-coa ing p ocess b ings abou a cons an supply
o polyme solu ion ha con ains he polyme ha can c ys allize [41].
In he case o he ilms unde s udy he e, as he e apo a ion a e is high
(chlo o o m is a sol en wi h a high apo p essu e [42] and hence,
capilla y low is high, he apid polyme di usion gi es ise o comple e
sphe uli es/axiali es due o a cons an polyme supply. S ill, he deg ee
o c ys alliza ion o he PBS on he hin ilms unde s udy is oo low o be
de ec ed h ough GIWAXS expe imen s. Such low deg ee o c ys allini y
could be a ibu ed o slow c ys alliza ion kine ics o PBS ilms cas ed
om dilu ed chlo o o m solu ions. PBS ilms unde s udy he e p esen
hicknesses below 10 nm. As i has been epo ed in li e a u e, polyme
chains in ul a hin ilms usually c ys allize slowly in p oximi y o in-
e aces [35].
3.3. Nanoscale chemical cha ac e iza ion by s-SNOM and nano-FTIR
echniques
The echnique o in a ed nea - ield mic oscopy (IR s-SNOM) and
nanospec oscopy (nano-FTIR) allows o in a ed imaging and spec-
oscopy wi h nanoscale spa ial esolu ion [43,44]. I has ecen ly been
employed o in es iga e submic ome e chemical dis ibu ion and
c ys alliza ion o polyme blend ilms, as an example cas ed om
PCL/PEG blends in THF [45]. In o de o shed u he ligh on o he
chemical dis ibu ion o he h ee polyme s, PBS, PCL and COPOL, on o
he hin ilms p epa ed h ough sequen ial dip-coa ing, s-SNOM phase
images and nano-FTIR phase spec a we e eco ded o samples
co esponding o se ies II, 3-laye (PBS)_2 and 13_laye (PBS)_2. The
p obing dep h o he nano-FTIR echnique is up o 100 nm [46,47],
which is ela ed o he oscilla ion ampli ude o he can ile e a a ixed
poin on he sample. Taking in o accoun ha he ilms unde s udy
p esen hicknesses in he ange om 10 o 25 nm, he nano-FTIR esul s
p o ide in o ma ion abou he whole sample. By using his echnique, i
is possible o ob ain IR abso p ion spec a ha co ela e o bulk ans-
mission in a ed spec oscopy (e.g., FT-IR) as bo h echniques measu e
he amoun o abso bed ligh [48].
As a i s s ep, he main FTIR bands co esponding o he indi idual
polyme s we e iden i ied om nano-FTIR spec a o p is ine hin ilms o
PCL, PBS and COPOL. Fig. 4 shows ep esen a i e spec a in he 1900-
900 cm
−1
ange, in which i is possible o obse e a la ge numbe o
abso p ion bands and band o e laps (see he chemical s uc u es in
Figu e S2 in he Supplemen a y In o ma ion). All h ee spec a show a
C
–
–
O s e ching band (a 1734 cm
−1
) and a C–O–C s e ching band (a
1174 cm
−1
o PBS and COPOL and a 1190 cm
−1
o PCL). Howe e , he
band a 1335 cm
−1
band, assigned o he C–H asymme ic angula
de o ma ion, is s ong in he PBS spec um and absen in he PCL
spec um. In addi ion, he 1241 cm
−1
band, assigned o he asymme ic
s e ching ib a ion o C–O–C g oup is e y s ong in he PCL spec um
and absen in he PBS spec um. Addi ional FTIR bands loca ed a 1295
and 1367 cm
−1
a e clea ly isible in he FTIR spec um co esponding o
PCL which can be assigned o C–O and C–C s e ching and C
–
–
O
s e ching in he c ys alline phase, espec i ely [49,50]. PBS hin ilms
unde s udy p esen a low c ys allini y as demons a ed h ough
GIWAXS esul s, his was u he demons a ed h ough analysis o he
Fig. 5. a) AFM heigh image (le ) and mechanical phase image ( igh ) co esponding o sample 3-laye (PBS)_2 and nea - ield phase (i.e. abso p ion) images a b)
1335 cm
−1
and c) 1740 cm
−1
. The ed squa e in Fig. 5a ma ks he egion employed o ca y ou he analysis h ough nano-FTIR spec oscopy (shown in Fig. 6). (Fo
in e p e a ion o he e e ences o colou in his igu e legend, he eade is e e ed o he Web e sion o his a icle.)
M.I. Pe˜
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Polyme 226 (2021) 123812
7
ca bonyl egion in he nano-FTIR spec a shown in he Supplemen a y
In o ma ion, Figu e S6. Acco ding o li e a u e, he peak in he ca bonyl
egion in FTIR can be decon olu ed o h ee abso p ion bands ha can
be assigned o he s e ching mode o C
–
–
O g oups in he c ys alline
phase (1714 cm
−1
), in he igid amo phous ac ion (1720 cm
−1
) and in
he ee amo phous ac ion (1736 cm
−1
) [51]. The nano-FTIR spec um
co esponding o PBS ilm showed a b oad peak in he ca bonyl egion
wi h he highes in ensi y loca ed a 1730 cm
−1
, which suppo s he low
deg ee o c ys allini y o he PBS ilm unde s udy.
The abso p ion bands obse ed o COPOL a e o e lapped o hose
obse ed in nea PBS and nea PCL. Howe e , he in ensi y a io o he
bands a 1335 cm
−1
and 1241 cm
−1
inc eases om ~2 in he spec a
co esponding o COPOL o ~8 in he spec a co esponding o PBS, a
esul ha e lec s he p esence o BS and CL sequences wi hin he
COPOL sample.
AFM heigh and mechanical phase images co esponding o sample
3-laye (PBS)_2 a e shown in Fig. 5a. As can be obse ed, PBS c ys alline
supe s uc u es ( esembling sphe uli es o axiali es) composed o PBS
edge-on lamella c ys als g owing om cen al nuclei a e seen. The
polygonal bounda ies be ween hese supe s uc u es (p oduced by hei
impinging as hey g ow adially) a e e y clea in Fig. 5a. We assume
ha he lamellae obse ed a e PBS, as PBS was he las ma e ial o be
deposi ed in he ilm. This was con i med by he s ong con as
obse ed o he abso p ion images a 1335 cm
−1
(Fig. 5b) and a 1740
cm-1 (Fig. 5c), which a e cha ac e is ic in a ed abso p ion bands o
PBS (see Fig. 4). A composi ional map co esponding o he sample 3-
laye (PBS)_2 is shown in Figu e S7 as suppo ing in o ma ion. The
image shows mainly blue a eas whe e abso p ion is highes a 1740
cm
−1
co esponding o he ca bonyl egion o he h ee polyme s, PBS,
PCL and COPOL and pink a eas whe e abso p ion is highes a 1335
cm
−1
, which has been iden i ied as a cha ac e is ic in a ed abso p ion
band o PBS. F om his image, i is no possible o asce ain he p esence
o PCL wi hin he sample. Hence, o u he in es iga e he polyme s’
chemical dis ibu ion, se e al nano-FTIR spec a we e eco ded a
de e mined posi ions wi hin he egion ma ked by he ed squa e in he
AFM heigh image in Fig. 5a. An enla gemen o his egion is shown in
Fig. 6a, and he co esponding abso p ion image a 1335 cm
−1
(Fig. 6b).
Se e al nano-FTIR spec a we e eco ded a di e en posi ions ma ked
wi h ed, black, and blue ci cles. The eco ded indi idual nano-FTIR
spec a show high quali y and ep oducibili y ( esul s no shown).
The black ci cles co espond o spo s ha showed high abso p ion a
1335 cm
−1
, and he ed ci cles co espond o posi ions ha showed high
abso p ion a 1740 cm
−1
(see Fig. 5c). The blue ci cles co espond o
posi ions wi h ela i ely low abso p ion a bo h IR equencies. The
co esponding a e age nano-FTIR spec a depic ed in Fig. 6c show he
cha ac e is ic C–O s e ching band a 1734 cm
−1
and he C–O–C
s e ching band a 1174 cm
−1
ha is o e lapped o PBS, PCL, and
COPOL. In addi ion, he h ee spec a show a band loca ed a ~1335
cm
−1
cha ac e is ic o PBS, as shown in Fig. 4. A b oad band cen e ed a
~1268 cm
−1
is also obse ed o he h ee samples. The in ensi y a ios
Fig. 6. a) AFM heigh image and b) abso p ion image a 1335 cm
−1
co esponding o sample 3-laye (PBS)_2 and c) Nano-FTIR spec a ob ained by a e aging spec a
eco ded a he posi ions ma ked by co esponding blue, ed and black colo s in panel b. (Fo in e p e a ion o he e e ences o colou in his igu e legend, he
eade is e e ed o he Web e sion o his a icle.)
M.I. Pe˜
nas e al.
Polyme 226 (2021) 123812
8
be ween he peak a 1268 cm
−1
and he peak a 1335 cm
−1
e eal di -
e ences in he in a ed abso p ion in di e en posi ions o he 3-laye
(PBS)_2 sample and hus in he chemical dis ibu ion wi hin he sam-
ple. The I
1335
/I
1268
is ~5 o he a e age spec um co esponding o
posi ions ma ked wi h black, whe eas I
1335
/I
1268
o he a e age spec a
ob ained om posi ions ma ked wi h blue and ed colou dec eases o
~2 o bo h cases, which e eals egions wi h highe ela i e con en o
CL sequences.
We now u n ou a en ion o he esul s ob ained o he 13-laye
(PBS)_2 sample o which he o al numbe o laye s was inc eased
Fig. 7. a) AFM heigh image (le ) and mechanical phase image ( igh ) co esponding o sample 13-laye (PBS)_2, abso p ion images a b) 1335 cm
−1
and c) 1733
cm
−1
, and d) Nano-FTIR spec a ob ained by a e aging spec a eco ded a he posi ions ma ked by co esponding black, ed and blue colo s in panel b. (Fo
in e p e a ion o he e e ences o colou in his igu e legend, he eade is e e ed o he Web e sion o his a icle.)
M.I. Pe˜
nas e al.
Polyme 226 (2021) 123812
9
om 3 o 13. The mo phology obse ed o his sample (Fig. 7a) is
simila o he one desc ibed o sample 3-laye (PBS)_2, see Fig. 5a. Tha
is, he numbe o dipping s eps employed o he p epa a ion o he
sample does no seem o ha e an in luence on he ob ained mo phology.
The p esence o small holes on he 13-laye (PBS)_2 sample can be
a ibu ed o he apid e apo a ion o chlo o o m du ing ilm p epa a-
ion by dip-coa ing. As i is well known, when he sol en is apidly
e apo a ing, he sol en - ich ilms do no ha e enough ime o le el and
heal su ace oughness c ea ed by Ma angoni ins abili ies. This e ec
and he esul ing mo phology could be uned h ough he con ol o he
e apo a ion a e du ing ilm p epa a ion [42]. As in he case o he
3-laye (PBS)_2 sample, he abso p ion images aken a 1335 cm
−1
and
1733 cm
−1
(Fig. 7b and c, espec i ely) showed high con as , which is
mo e clea ly obse ed o he image aken a 1335 cm
−1
. This is
consis en wi h he ac ha he c ys alline mo phology can be a ib-
u ed o he las deposi ed polyme , ha is, o PBS.
The polyme dis ibu ion was u he analyzed by eco ding se e al
nano-FTIR spec a a di e en posi ions ma ked by colou ed ci cles in
he AFM heigh image (Fig. 7a, le ) and he absop ion image aken a
1335 cm
−1
(Fig. 7b). Red ci cles co espond o loca ions ou side he
holes and black ci cles co espond o loca ions wi hin he holes, as seen
in he AFM opog aphy image. Bo h ed and black spec a shown in
Fig. 7c i nicely o he e e ence spec um o PBS in ag eemen wi h he
ac ha he mo phology obse ed is mos ly due o PBS. In bo h cases,
I
1335
/I
1268
is ~3, which indica es a simila chemical composi ion
ega ding BS/CL sequences. A close examina ion o he a e age nano-
FTIR spec a ob ained om egions ma ked wi h blue do s, which co -
esponds o posi ions wi h ela i ely low abso p ion a he IR equency
o 1335 cm
−1
, allows o clea ly dis inguish a no iceable inc ease in he
in ensi y o he band loca ed a 1268 cm
−1
wi h espec o he band
loca ed a 1335 cm
−1
, which is much mo e e iden in he case o he
a e age spec um ob ained om egions ma ked wi h blue ci cles. Fo
hese egions, he band loca ed a 1365 cm
−1
, cha ac e is ic o PCL, is
also clea ly isible.
The esul s show ha he inc ease in he numbe o deposi ed laye s
gi e ise o almos isola ed PCL nanodomains wi hin a ma ix p edom-
inan ly cons i u ed o PBS. Thus i poin s o mo e seg ega ion be ween
BS and CL sequences o he 13-laye (PBS)_2 sample as compa ed o he
3-laye (PBS)_2 sample. The inal mo phology ob ained o hese hin
ilms could be compa ed o ha o an immiscible blend o PCL/PBS as
p e iously epo ed in li e a u e. This could be a ibu ed o he ac ha
he epe i i e dipping s eps employed o p epa e he sample leads o he
deposi ion o mixed solu ions o he h ee polyme s due o he pa ial
dissolu ion o he ilms du ing he dipping p ocedu e.
4. Conclusions
In his s udy, sequen ial dip-coa ing o silicon subs a es in chlo o-
o m solu ions o PCL, PBS, and a poly (bu ylene succina e- an-ϵ-cap-
olac one) (PBS- an-PCL) copolyme has been employed o ob ain hin
polyme ilms wi h a a ying numbe o laye s (3 and 13). The hickness
ob ained o he ilms was, in all cases, in he ange 10–25 nm ega dless
o he numbe o laye s employed o hei p epa a ion, which poin s o
pa ial dissolu ion o he ilms occu ing du ing he p ocess o p epa-
a ion. The c ys alline mo phology obse ed co esponds o ha o he
las deposi ed polyme , as e ealed by AFM mic oscopy. In he case o
ilms o which he las deposi ed laye was PCL, di ac ion peaks co -
esponding o he semic ys alline s uc u e o PCL we e de ec ed by
GIWAXS expe imen s. On he o he hand, o ilms whose inal laye was
PBS, e en i sphe uli ic/axiali ic o ma ion could be obse ed by AFM,
no di ac ion peaks we e ob ained in GIWAXS.
Fo ilms whose inal laye was PBS, nano-FTIR spec oscopy p o-
ided he main e idences o de e mine he nanoscale o ganiza ion o BS
and CL sequences on 3-laye ilms and 13-laye ilms. The esul s showed
ha bo h samples p esen a he e ogeneously mixed chemical composi-
ion wi h nanodomain egions o a ying PBS and PCL con en .
Howe e , in he case o 13-laye ilms, he analysis o he a e age
spec a co esponding o di e en egions o he sample ilms e ealed
he p esence o almos seg ega ed PCL nanodomains wi hin a PBS ma-
ix. The esul s could be a ibu ed o he ac ha he pa ial dissolu ion
o he ilm occu ing du ing he sequen ial dipping s eps migh gi e ise
o he deposi ion o a mixed solu ion o he h ee polyme s esul ing in a
mo phology ha is eminiscen o ha exhibi ed by immiscible blends.
Decla a ion o compe ing in e es
The au ho s decla e ha hey ha e no known compe ing inancial
in e es s o pe sonal ela ionships ha could ha e appea ed o in luence
he wo k epo ed in his pape .
Acknowledgemen s
The au ho s acknowledge Pila Posadas o AFM measu emen s and
Iban Amenaba (Ex e nal Se ices Depa men , CIC NanoGUNE San
Sebas i´
an) o nano-FTIR measu emen s. This s udy was suppo ed by
Spanish Minis y o Science, Inno a ion and Uni e si ies (MAT2017-
83014-C2-1-P, MAT2017-83014-C2-2-P, PID2019-107514 GB-I00/AEI/
10.13039/501100011033 and PID2019-106125 GB-I00/AEI/
10.13039/501100011033). We also acknowledge unding by Basque
Go e nmen h ough g an IT1309-19. The au ho s acknowledge
beam ime access a ALBA BL11 ia p oposal 2018072905 and he
assis ance o JC Ma inez du ing he measu emen s. We would also like
o hank he inancial suppo p o ided by he BIODEST p ojec ; his
p ojec has ecei ed unding om he Eu opean Union’s Ho izon 2020
esea ch and inno a ion p og amme unde he Ma ie Skłodowska-Cu ie
g an ag eemen No. 778092. CO wan s o acknowledge “Dipu aci´
on
Fo al de Gipuzkoa” in he amewo k p og am “Fellows Gipuzkoa de
A acci´
on y Re enci´
on de Talen o”. RH is membe o he In e disci-
plina y Pla o m o Sus ainable Plas ics owa ds a Ci cula Economy
(SusPlas ) om he Spanish Na ional Resea ch Council (CSIC).
Appendix A. Supplemen a y da a
Supplemen a y da a o his a icle can be ound online a h ps://doi.
o g/10.1016/j.polyme .2021.123812.
Da a a ailabili y s a emen
The aw/p ocessed da a equi ed o ep oduce hese indings canno
be sha ed a his ime due o echnical o ime limi a ions.
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