Influence of liquid crystalline formation on the phase behavior of side-chain liquid crystalline block copolymers

Article abstract of DOI:10.1016/j.polymer.2015.01.077

A series of narrowly distributed diblock copolymers composed of amorphous components and side-chain liquid crystalline (LC) polymer, poly (butyl acrylate)-block-poly [8-(4-cyano-4′-biphenyl)-1-octanoyl acrylate] (PBA-b-PCBOA), with side-chain LC block weight fraction (f_w,PCBOA) ranging from 25% to 87%, were synthesized by atom transfer radical polymerization (ATRP). Their thermal property, LC behavior, bulk phase behavior and thin film morphology were studied by differential scanning calorimetry (DSC), polarizing optical microscope (POM), small-angle X-ray scattering (SAXS) techniques and atomic force microscopy (AFM), respectively. The results show the diblock copolymers with different composition could present sphere, lamellar and cylinder morphologies before the order-disorder transition. As the mesogen units self-organize to form smectic phase, the lamellar morphology dominates during the majority LC block weight fraction range (f_w,PCBOA = 46%-77%) to minimized the surface energy. Interestingly, for spin-coated thin film, the lamella phase separation size decreased with increasing annealing time. For the copolymer with f_w,PCBOA of 67%, a thermoreversible order-order transition (OOT) and lamella to lamella/modified layer (L/ML), were observed.

Full text of DOI:10.1016/j.polymer.2015.01.077

Polymer 61 (2015) 147e154
Contents lists available at ScienceDirect
Polymer
journal homepage: www.elsevier.com/locate/polymer
Influence of liquid crystalline formation on the phase behavior of
side-chain liquid crystalline block copolymers
Liangliang Ji, Xiaofang Chen, Yanhong Wu, Xiaoming Yang*, Yingfeng Tu*
Suzhou Key Laboratory of Macromolecular Design and Precision Synthesis, Jiangsu Key Laboratory of Advanced Functional Polymer Design and Application,
College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, China
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of narrowly distributed diblock copolymers composed of amorphous components and side-chain
liquid crystalline (LC) polymer, poly (butyl acrylate)-block-poly [8-(4-cyano-40-biphenyl)-1-octanoyl
acrylate] (PBA-b-PCBOA), with side-chain LC block weight fraction (fw,PCBOA) ranging from 25% to 87%,
were synthesized by atom transfer radical polymerization (ATRP). Their thermal property, LC behavior,
bulk phase behavior and thin film morphology were studied by differential scanning calorimetry (DSC),
polarizing optical microscope (POM), small-angle X-ray scattering (SAXS) techniques and atomic force
microscopy (AFM), respectively. The results show the diblock copolymers with different composition
could present sphere, lamellar and cylinder morphologies before the orderedisorder transition. As the
mesogen units self-organize to form smectic phase, the lamellar morphology dominates during the
majority LC block weight fraction range (fw,PCBOA ¼ 46%e77%) to minimized the surface energy. Inter-
estingly, for spin-coated thin film, the lamella phase separation size decreased with increasing annealing
time. For the copolymer with fw,PCBOA of 67%, a thermoreversible ordereorder transition (OOT) and
lamella to lamella/modified layer (L/ML), were observed.
Received 1 November 2014
Received in revised form
23 January 2015
Accepted 29 January 2015
Available online 9 February 2015
Keywords:
Liquid crystalline block copolymers
Self-assembly
Surface morphology
© 2015 Elsevier Ltd. All rights reserved.
1. Introduction
separation structure and about 1 nm of LC mesogen structure are
formed in LCBCPs [14e18]. For example, the LC phase transitions of
The bulk phase separation of block copolymers (BCPs) has been
investigated intensively in the last decades because of the diverse
nanostructures formed, such as spheres, cylinders, and lamellas
[1e5]. The self-assembled morphologies are determined by the
the LC blocks can trigger orderedisorder transitions (ODTs) or
ordereorder transitions (OOTs) of the nanophase-separated struc-
tures of BCPs, due to the cooperatively orientation in a preferred
direction [19e21]. The OOT corresponds to a thermotropic phase
change in microphase morphology induced simply by the change in
volume fraction (f) of the individual block and cN, where c is the
€
FloryeHuggins interaction parameter and N is the total degree of
polymerization of the copolymers [6e8]. Among BCPs, liquid
crystalline block copolymers (LCBCPs) have been extensively
studied in recent years owing to their organization at different
scales to yield structure-within-structure hierarchical structures
and offering an opportunity to investigate the interplay between
the molecular level LC orders and the microphase-separated
domain morphologies [9,10]. As a result of the strong phase sepa-
ration between the LC blocks to coil blocks, the asymmetry phase
diagram and new morphologies have been observed [11e13]. In
addition, hierarchical structures with several 10 nm of phase-
annealing temperatures. Sanger et al. reported an OOT in the
morphology of a block copolymer accompanied by the loss of the
anisotropic phase structure above the isotropization temperature
of the LC block [19]. In this study, the OOT of LCBCPs triggered by LC
blocks is investigated.
It has been reported that the region of stability of the lamellar
phase is enhanced in certain side-chain LCBCPs compared to
coilecoil diblocks, where it is shifted by composition range [22]. For
BCPs with side-chain LC and amorphous blocks, at high volume
fractions of the LC block, hexagonally packed cylinders by amor-
phous blocks have been observed inside a continuous matrix of the
blocks with the LC side groups [23,24]. With decreased volume
fraction of the LC block, lamellar structures, and LC cylindrical
microdomains with the mesogens parallel to the axis of cylinders
have been observed [25]. The thermal-dynamic stable phase of
* Corresponding authors.
E-mail addresses: yangxiaoming@suda.edu.cn (X. Yang), tuyingfeng@suda.edu.
cn (Y. Tu).
http://dx.doi.org/10.1016/j.polymer.2015.01.077
0032-3861/© 2015 Elsevier Ltd. All rights reserved.

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L. Ji et al. / Polymer 61 (2015) 147e154
BCPs with side-chain LC and amorphous blocks have attracted great
research interests since the orientation of the LC mesogen groups
will exhibit rich order structures and show strong interplay be-
tween orders of different scales. Potemkin et al. developed a theory
of microphase separation in a melt of smectic BCPs with side-chain
LC and amorphous blocks using the strong segregation approxi-
mation. They predicted thermodynamic stability of two types of
cylindrical structures having amorphous and liquid crystalline
cores, and four types of lamellar structures [26]. Hammond et al.
used the side-chain LC content as a tool to tune the interfacial in-
teractions of the LC mesophase, which allows for the orientation
and ordering of the morphologies to be manipulated, includes
perpendicular and parallel cylinder morphologies [27]. On the
other hand, the phase-separation structure affected by the dynamic
process of LC formation on microphase separation morphology is
important and need further investigation.
Scheme 1. The synthetic route of the side-chain LC monomer and the diblock co-
polymers by ATRP.
To study the influences of LC formation on BCPs self-assembly at
different annealing conditions, herein we report the controllable
synthesis of a series of the diblock copolymers composed of
amorphous components and side-chain LC, poly (butyl acrylate)-
block-poly [8-(4-cyano-40-biphenyl)-1-octanoyl acrylate] (PBA-b-
PCBOA). PBA was selected as amorphous block component while
PCBOA as side-chain liquid crystalline block due to their relatively
low Tgs, which enables the observation of phase transitions near
room temperature (RT). The thermal properties and LC behaviors of
these block copolymers as a function of weight fraction of LC blocks
were investigated by differential scanning calorimetry (DSC) and
polarizing optical microscope (POM), while the phase behaviors
and surface morphologies were studied by small angle X-ray scat-
tering (SAXS) and atomic force microscopy (AFM).
The 1H NMR spectrum was recorded on an INOVA 400 MHz
nuclear magnetic resonance instrument using CDCl3 as the solvent
and tetramethylsilane (TMS) as the internal standard, with the
solution concentration of 0.01 g/mL.
DSC analysis was performed using a TA Q100 instrument under
a nitrogen atmosphere over a temperature range ꢂ20~180 ꢁC with a
scanning rate of 10 ꢁC minꢂ1. The first cooling and second heating
scans were used to determine the glass transition and liquid crys-
talline transition peaks. Typically, about 5 mg of the powdered
sample was encapsulated in a sealed aluminum pan with an
identical empty pan as the reference.
The phase transitions and LC textures were also investigated by
POM (Olympus Corporation, BX51-P), which was coupled with a
computer-controlled video camera. A dual hot stage (Linkam
THMS600) was used for controlling the temperature.
2. Experimental section
SAXS measurements were performed at an X-ray scattering in-
strument (SAXSess mc2, Anton Paar) equipped with line collima-
2.1. Materials
tion and
a 2200 W sealed-tube X-ray generator (Cu-Ka,
l
¼ 0.154 nm). Before measurement, samples were annealed under
n-Butyl acrylate (BA) (SigmaeAldrich Chemical Co.) was puri-
fied by washing with 5% sodium hydroxide solution, then with
distilled water, dried over anhydrous sodium sulfate overnight, and
distilled at reduced pressure over CaH2 and stored in a refrigerator.
CuBr (Aldrich, 98%) was washed with glacial acetic acid, ethanol
and ether to remove traces of CuBr2. 4-Cyano-40-hydroxybiphenyl,
N,N0,N00,N00-pentamethyldiethylenetriamine (PMDETA), octane-1,8-
diol were obtained from Aldrich and used without further purifi-
cation. All other chemical reagents were purified according to
standard procedures before use.
vacuum for 72 h at 80 ꢁC to have good phase segregation. Samples
were kept under vacuum during irradiation, and the irradiation
time was 20 min at certain temperature, while imaging plate (IP)
was used to record the scattering pattern. Silver behenate was used
as the calibration substance to calibrate the peak positions. Anton
Paar TCS 300 (20 ꢁCe300 ꢁC) was used as temperature control unit
conjunction with SAXSess mc2 to study the molecular arrangement
at various temperatures, with a 5 min pre-equilibration delay prior
to data collection at a given temperature (typical exposure time
~1 s). The scattering vector was defined as q ¼ (4
p/l) sin (q/2),
where and are the scattering angle and the wavelength of the X-
q
l
2.2. Synthesis of diblock copolymers
ray, respectively.
The surface morphologies of thin films were measured by AFM
with a tapping mode at RT (Veeco Instruments Inc., Nanoscope IV).
Solutions of copolymers in toluene (4 wt%) were spin-coated at
2500 rpm onto precleaned silicon wafers. The film thickness of
about 100 nm was estimated with a surface profiler. After the
solvent was removed under vacuum overnight at RT, the films
were annealed at a given temperature for some time and slowly
cooled to RT.
The synthesis of PBA-b-PCBOA diblock copolymers is shown in
Scheme 1, using sequential ATRP. The LC monomer CBOA was
synthesized according to the procedure reported by Kasko et al.
[28,29]. Detailed information on the synthesis and characterization
is given in the Supporting information (see Fig. S1 and S2).
2.3. Characterization
The gel permeation chromatography (GPC) measurements were
performed on a modular system comprising a Waters 1515 pump,
Waters 717 plus autosampler, and 2414 refractive index detector
with three 300 mm (length) ꢀ 7.5 mm (inner diameter) columns
3. Results and discussion
Up to date, as one of the controlled radical polymerization
methods, ATRP has proved to be an efficient technique to synthe-
size different types of topological copolymers with complex
macromolecular architectures and controlled molecular weights
(MWs). Herein, the PBA-Br was used as macroinitiator to prepare
the LCBCPs. The general synthetic procedure of the PBA-b-PCBOA is
with a particle size of 5 mm (PL gel mixed-C, Polymer Laboratories).
THF was used as an eluent at a flow rate of 0.60 mL/min at 35 ꢁC.
Calibration was made against standard monodisperse linear poly-
methyl methacrylate (PMMA).

L. Ji et al. / Polymer 61 (2015) 147e154
149
Table 1
In general, for all the sets of copolymers, the DSC curves also
exhibit two transition peaks, similar to that of the homopolymer. It
should be noted that the Tisos of all PBA-b-PCBOA copolymers are
lower than that of the PCBOA homopolymer, similar to other re-
ports about the LCBCPs [35,36]. In two series of samples respec-
tively, the Tisos decrease with the decrease of LC content. But in the
different series, BC-57 has higher Tiso than BC-67. This is because of
the higher degree of polymerization of LC block in BC-57 (PBA69-b-
PCBOA32) than in BC-67 (PBA31-b-PCBOA22). The isotropization
transition peaks of the LCBCPs are much broader than that of the LC
homopolymer. This observation might be related to a relatively
poorly defined interface between the smectic blocks and the
amorphous blocks, where the amorphous blocks play as “impu-
rities” toward the smectic mesophase. Upon heating, the two
blocks become more incompatible and the poorly ordered LC
interfacial area becomes larger. This also results in the wide iso-
tropization transition range observed in the DSC curve and the
depression of the isotropization temperature related to the LC ho-
mopolymer. Higher LC content in the LCBCPs increases the enthalpy
of LC isotropization transitions: for example, the isotropization
transitions are more pronounced in BC-87 as compared to BC-80.
The individual mesomorphic properties of the homopolymer
and copolymers respectively were examined by POM (Fig. 2). The
focal conic texture, which belongs to typical SmA phase, was
observed for homopolymer. The similar texture was also found by
others [37,38]. For the PBA-b-PCBOA, the mesophases seem to be
preserved in the LC block, as suggested by the DSC curves where the
same number of peaks was detected. Also in the POM photographs,
PBA-b-PCBOA samples show birefringence texture originating from
LC phase. It's interesting that a defective focal conic texture is
formed for BC-87, according to high LC weight fraction. Otherwise,
the textures of the other PBA-b-PCBOA samples are poorly devel-
oped. For example, the sample BC-46 shows obvious birefringence
but obscure texture. Furthermore, BC-25 shows both low birefrin-
gence. The birefringence in the LC phase decreases with decreasing
LC weight fraction, indicating that the LC mesophase becomes less
organized due to broken textures originating from less LC content
and phase separation. Other samples' POM pictures are shown in
Fig. S3. The destabilization of smectic phases observed in LCBCPs
may result from an entropic frustration arising from confinement
effect of conformationally asymmetric diblocks [39].
Characterization data of PBA macroinitiators, LCBCPs and PCBOA.
fw,PCBOA (%)c
a
b
a
Sample Sample
PBA69
BC-25
BC-33
BC-46
BC-57
Mn,GPC
Mn,NMR
Mw/Mn
8.80 ꢀ 103
1.12
PBA69-b-PCBOA8
9.50 ꢀ 103 1.18 ꢀ 104 1.32
PBA69-b-PCBOA12 9.60 ꢀ 103 1.33 ꢀ 104 1.29
PBA69-b-PCBOA20 1.02 ꢀ 104 1.63 ꢀ 104 1.32
PBA69-b-PCBOA32 1.07 ꢀ 104 2.09 ꢀ 104 1.33
25.5
33.9
46.1
57.8
PBA31
4.00 ꢀ 103
1.10
BC-67
BC-77
BC-80
BC-87
PBA31-b-PCBOA22 9.60 ꢀ 103 1.23 ꢀ 104 1.29
PBA31-b-PCBOA36 9.90 ꢀ 103 1.75 ꢀ 104 1.28
PBA31-b-PCBOA42 1.01 ꢀ 104 1.98 ꢀ 104 1.30
PBA31-b-PCBOA72 1.12 ꢀ 104 3.11 ꢀ 104 1.28
67.3
77.6
80.1
87.3
100
PCBOA
7.90 ꢀ 103
1.25
a
Molecular weights (g/mol) and their distribution Mw/Mn were evaluated by GPC
with poly(methyl methacrylate) as standards.
b
Number average molecular weights (g/mol) of the LC block were evaluated by
1H NMR.
c
Data in parentheses are block weight ratios of PCBOA as calculated by corre-
sponding degree of polymerization.
shown in Scheme 1. Two types of macroinitiator (Mn are 8800 and
4000 g/mol) were synthesized, and PBA-b-PCBOA copolymers were
synthesized using PBA-Br as macroinitiators and CBOA as mono-
mer. By varying the LC monomer to macroinitiator molar ratio ([M]/
[I]) from 50/1 to 100/1 and polymerization time, a series of LCBCPs
with different composition were obtained (shown in Table 1).
The chain-end estimation method by 1H NMR to determine the
number-average molecular weights of polymers is well established
[30e34]. The detailed procedure is shown in the supporting in-
formation (Fig. S2). The characteristics data of the diblock co-
polymers are summarized in Table 1. Hereafter, BC-x will be used to
represent LCBCPs, where B represents PBA, C represents PCBOA and
x represents the weight percentage (wt%) of LC block in the
copolymers.
The successful preparation of a series of well-defined PBA-b-
PCBOA copolymers with LC weight fraction (fw,PCBOA) ranging from
25% to 87% offers an ideal platform for systematically investigation
of the thermal behaviors and the self-assembled suprastructures of
the block copolymers.
Fig. 1 depicts a set of DSC traces for the LC homopolymer and the
diblock copolymers recorded upon the second heating (A) and the
first cooling (B) runs at a rate of 10 ꢁC/min. All the samples inves-
tigated exhibited quite good reproducible thermograms in the DSC
measurements. For homopolymer, two transition temperatures are
observed in the DSC thermogram as follows: 22.2 ꢁC and 123.5 ꢁC,
which are assigned to the Tg and the isotropization transition
temperature (Tiso) respectively [33,34].
The morphology of the bulk films was investigated with 1D
SAXS, as shown in Fig. 3. All samples were annealed in vacuum at
80 ꢁC higher than their Tg values of the PCBOA blocks but lower
than the Tiso values of the PCBOA blocks of LCBCPs. Typical SAXS
profiles of all copolymers with various fw,PCBOA recorded at RT are
shown in Fig. 3. All the SAXS curves have peaks originating from
Fig. 1. DSC curves of PCBOA and PBA-b-PCBOA copolymers at (A) second heating and (B) first cooling with a scanning rate of 10 ꢁC/min.

150
L. Ji et al. / Polymer 61 (2015) 147e154
Fig. 2. Representative POM images of PCBOA and PBA-b-PCBOA copolymers under crossed polarizers. Samples were annealed from an isotropic state at a rate of 1 ꢁC/h. The scale bar
represents 50 m.
m
microphase separation structure at around q ¼ 0.2e1.0 nmꢂ1
,
SAXS curves, indicating less ordered structure formed. By
combining with the results from AFM image (Fig. 4A), and also from
the calculated weight fraction of LC blocks, we believe the phase
separation morphology is PBA spheres in continuous PCBOA phase.
The reason for the less ordered phase separation structure formed
is due to the highest molecular weights of this block copolymer. The
well-ordered phase separation morphology is suppressed due to
increased chain entanglement and high bulk viscosity, and less
although the intensity of the scattering peak for BC-87 is very weak.
Similar to Shiomi's observation on their LC polymers with similar
structure to ours, it's difficult to observe scattering peaks from LC
mesogens [40].
The observed microphase-separated morphology is a function
of the LC weight fraction. For sample BC-87, only the 1st peak is
pffiffiffi
observed while the peaks at 3q or 2q can not be found from the
Fig. 3. (A) 1D SAXS of LCBCPs recorded at RT after annealing at 80 ꢁC in vacuum for 72 h. Data are plotted as the logarithm of the relative scattered intensity log I(q) vs the scattering
factor q. (B) Experimental phase diagram of PBA-b-PCBOA diblock copolymers. S: sphere, C: cylinder, L: lamella.

L. Ji et al. / Polymer 61 (2015) 147e154
151
Fig. 4. AFM phase images of spin-coated LCBCPs films on silicon substrates after annealing at 80 ꢁC in vacuum for 72 h. (A) BC-87, (B) BC-80, (C) BC-77. Their schematic models of the
phase structure in LC state are shown as: (D) sphere, (E) cylinder, (F) lamella.
ordered structure is formed. As a result, only 1st order peak is
observed from SAXS. At lower LC fractions, for BC-80, primary
15 nm, close to 14.5 nm estimated from SAXS. Because PBA has low
Tg, it's difficult to observe obvious phase images by AFM for BCPs
with low LC contents, i.e., BC-25 for high viscosity of the sample.
The AFM measurements can provide the surface morphology of
polymers. However, this technique can hardly tell the underneath
morphology due to its working method. For example, the laid down
cylinder A in continuous B structure and the lamella A in lamella B
structure should provide the similar AFM results [44e46]. In
addition, for AFM measurement, the film thickness is usually
around 100 nm. At this condition, the phase separation morphology
in the film may be different as in the bulk [47]. Thus AFM results are
usually used as a supplemental technique to better illustrate the
phase separation morphology but not the critical data to determine
the morphology. Usually, SAXS data are used as the crucial data to
deduce the phase separation morphology from the higher ordered
reflection peaks. In our case, SAXS data show clearly that BC-80
scattering and its higher order reflections were observed with the
pffiffiffi
scattering vector in the ratio of 1 : 3 : 2, suggesting the micro-
phase separation structure of PBA cylinder within LC matrix, with
calculated d-spacing value of 15.0 nm. The lower LC content poly-
mer (i.e., BC-77) exhibits primary scattering and its obscure higher
order reflections in the ratio of 1:2. Combining AFM phase images,
it can be concluded that a lamellar morphology with d-spacing
value of 14.4 nm is formed. Meanwhile, polymers with even lower
LC content (i.e., BC-67, BC-57, and BC-46) also form lamellar mor-
phologies, similar to BC-77. For BC-33, primary scattering and its
pffiffiffi
higher order reflections are in the ratio of 1 : 3 : 2, which suggests
PCBOA cylinder within PBA matrix. The profiles of BC-25 have the
pffiffiffi pffiffiffi
scattering vector ratio of 1: 2: 3:2, a characteristic of body-
centered cubic structure with corresponding d-spacing of 22.8 nm.
The phase diagram of PBA-b-PCBOA diblock copolymers is
summarized in Fig. 3B. It's obvious that the lamellar morphology
dominates during the majority LC block weight fraction range
(fw,PCBOA ¼ 46%e77%). Zhou et al. found that in rodecoil BCPs, the
lamellar phase was observed with the supramolecular PMPCS rods
aligning parallel to the lamellar normal [15]. Shen et al. reported
that, for rodecoil BCPs, because long rods prefer a flat intermaterial
dividing surface (IMDS), lamellar phases are stable in a wide range
of composition even in some rodecoil BCPs with high composi-
tional asymmetry [17]. Compared to those mesogen-jacketed
rodecoil diblock copolymers, our block copolymers are similar to
traditional side-chain LC copolymers where the smectic LC order
contributes to stable lamellar structures [39].
forms cylindrical morphology due to the ratio of scattering peak
pffiffiffi
position in the order of 1 : 3 : 2 , while BC-77 and BC-67 (Fig. 5D)
form lamellar morphology due to the ratio of scattering peak po-
sition in the order of 1:2. As pointed above and in the mentioned
references, the reason that BC-87 shows similar morphology as BC-
67 is that the cylinders are laid down in the film, and provides
similar results from AFM. The chain conformations of PCBOA seg-
ments, the phases of the PCBOA matrix, and the nanophase-
separated structures of LCBCPs with different LC contents are
schematically shown in Fig. 4DeF.
For the presence of LC block, lamellar morphology dominated
the main range. It has been shown that the lamellar morphology
can also stabilize LC phases. BCP thin films are of particular interest
because of the possibility of obtaining two-dimensional patterns
with very high regularity [47,48]. It is interesting to investigate how
thermal annealing condition affects the microphase separation of
BCP since the LC can promote the microphase separation of BCP.
The change of surface morphology from spin-coated BC-67 BCP
thin films as a function of annealing time was investigated. After
thermal annealing for 12 h and then slowly cooled to RT, an obvious
lamellar morphology with average interlamellar spacing of 20 nm
was observed, as shown in Fig. 5A. The continuity and ordering of
self-assembled lamellar morphology are not satisfied for shorter
annealing time. With increasing thermal annealing time, the
Fig. 4 shows AFM phase images of the spin-coated and annealed
films. The lighter and darker regions in the AFM images can be
assigned to LC blocks and PBA blocks, respectively. As shown in
Fig. 4A, a nonperiodic array of PBA spheres inside a continuous LC
matrix was observed for BC-87. For BC-80, cylinders oriented par-
allel to the substrate were found in the films by combining with the
SAXS results (Fig. 4B) [7,9,41e43]. Based on SAXS result, PBA
cylinder-to-cylinder distance can be estimated as d0 ¼ 17.3 nm
[using d0 ¼ (4/3)1/2d], which is close to the d-spacing value of 17 nm
estimated from AFM phase images. The lamellar morphology was
observed obviously for BC-77 in Fig. 4C. The lamellar distance is

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L. Ji et al. / Polymer 61 (2015) 147e154
Fig. 5. AFM phase images of spin-coated BC-67 films on silicon substrates after annealing at 80 ꢁC for different time. (A) 12 h, (B) 24 h, (C) 48 h and (D) 72 h. AFM phase profiles of A
and D were shown in (E) and (F), respectively.
lamellar phase separation size reduced. When annealed for 72 h,
the lamella phase separation size reduced to about 16 nm, as
inferred through AFM imaging from the presence of a typical stri-
ped pattern (Fig. 5D and F). This phenomenon can be attributed to
slow cooperative motion among mesomorphic units [49,50]. When
annealed for enough time, self-assembly morphology induced by
the formation of LC phase will develop to a better structure, that is
an ordered lamellar structure with smaller interlamellar distance.
Furthermore, another thermal annealing condition can produce
lamellar morphology with the smallest interlamellar spacing. For
BC-67 after quenching from the isotropic phase to RT, a relatively
disorder morphology was obtained, which is not a stable thermo-
dynamic state of microphase-separated morphology. Fig. 6B shows
the AFM image of BC-67 cooling slowly (0.4 ꢁC minꢂ1) from the
isotropic phase to RT. It can be seen that the mesomorphic units
have enough time and appropriate temperature to undergo LC
cooperative motion, so the self-assembly morphology can develop
well with interlamellar spacing of 14 nm, as shown in Fig. 6C. The
possible reason is that the mesomorphic units align most closely
during cooling process.
The scattering vector positions of BC-77 annealed at different
temperatures are almost the same within the range of errors
(Fig. 7A). At temperatures higher than 100 ꢁC, the lamellar reflec-
tion becomes less intense, suggesting an ODT. As the PCBOA blocks
transform from the rod-like to the coil chain, the rearrangement of
the polymer chains and the instability of microphase separation
takes place during the transition process of the nanostructure. The
isotropization temperature of BC-77 is nearly within experimental
error of its ODT temperature. This correlation confirms our hy-
pothesis that the isotropization of the LC domain can trigger the
ODT [51]. This phenomenon can be explained on the basis of dif-
ferences in smectic elastic free energies above and below the iso-
tropization temperature.
For BC-67, when the temperature is increased to be higher than
the 40 ꢁC, a new peak with a higher q value appears in the ratio of
1:1.2 as shown in Fig. 7B, indicating the formation of a new struc-
ture. The strong first-order scattering vector, L, at q ¼ 0.43 nmꢂ1
corresponds to a periodicity of 14.6 nm. The other, less intense
reflections are due to the presence of cylinders and appear starting
at q ¼ 0.55 nmꢂ1 in a 1:2 scattering ratio of q, implying that the
cylinders are arranged in a modified layer (ML) fashion [22,39]. The
morphology is predominately lamellar with a small fraction
contribution from cylinders. Upon cooling (see Fig. S4), the shoul-
der of the first-order L peak appears, thus confirming that at lower
temperatures, dual L/ML morphology is one of thermal equilibrium.
It's obvious that the mobility and phase behavior of LCBCPs may
change the kinetic pathway of the self-assembly process in a su-
pramolecular system. In-situ SAXS experiments were performed to
determine the influence of temperature on the nanophase-
separated structures of BC-77 and BC-67, as shown in Fig. 7.
Fig. 6. AFM phase images of spin-coated BC-67 films on silicon substrates after annealing for different conditions: (A) at 140 ꢁC above Tiso for 2 h and quenched at RT, (B) at 140 ꢁ
above Tiso for 2 h and slowly cooled (0.4 ꢁC minꢂ1) from the isotropic phase to the mesomorphic phase (T ¼ 40 ꢁC). (C) AFM phase profiles of image B.
C

L. Ji et al. / Polymer 61 (2015) 147e154
153
Fig. 7. Temperature-dependent 1D SAXS profiles of (A) BC-77, (B) BC-67 during heating. Data are plotted as the logarithm of the relative scattered intensity log I(q) vs the scattering
factor q. (C) Schematic model of ordereorder transition for BC-67. L: lamella, ML: modified layer.
This experiment proves that this sample undergoes a thermor-
eversible OOT at 30e90 ꢁC from a purely lamellar morphology to a
predominately lamellar morphology with cylindrical morphology,
as shown in Fig. 7C. With further increase of temperature, the in-
tensity of the diffraction peak of the new structure increases. As the
temperature is further increased to above 100 ꢁC, the diffraction
peaks of the new structure disappear, and the intensities of
diffraction peaks attributed to the lamellar reflection decrease,
indicative of an ODT.
Appendix A. Supplementary data
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.polymer.2015.01.077.
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