Hierarchically ordered ABA triblock copolymer with large difference in glass transition temperatures of the two blocks

Article abstract of DOI:10.1002/pola.27175

Rod-coil-rod triblock copolymers, poly[4′-(methoxy)-2-vinylbiphenyl- 4-methyl ether] -blockpolydimethylsiloxane-block-poly[4′-(methoxy)-2- vinylbiphenyl-4-methyl ether] (PMVBP-b-PDMS-b-PMVBP), were synthesized by nitroxide-mediated radical polymerization,

Full text of DOI:10.1002/pola.27175

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Hierarchically Ordered ABA Triblock Copolymer with Large Difference
in Glass Transition Temperatures of the Two Blocks
Chang-Feng Li,1,2 Qi-Kai Zhang,2y Hai-Jian Tian,2 Chun-Peng Chai,1 Long-Cheng Gao,3
Zhihao Shen,2 Xing-He Fan,2 Yun-Jun Luo1
1School of Materials Science & Engineering, Beijing Institute of Technology, Beijing 100081, China
2Beijing National Laboratory for Molecular Sciences, Key Laboratory of Polymer Chemistry and Physics of Ministry of Education,
College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China
3Key Laboratory of Bio-Inspired Smart Interfacial Science and Technology of Ministry of Education, School of Chemistry and
Environment, Beihang University, Beijing 100191, China
†Joint first authors.
Correspondence to: X.-H. Fan (E-mail: fanxh@pku.edu.cn) or C.-P. Chai (E-mail: chaicp@bit.edu.cn)
Received 10 January 2014; accepted 15 March 2014; published online 3 April 2014
DOI: 10.1002/pola.27175
ABSTRACT: Rod-coil-rod triblock copolymers, poly[40-(methoxy)-
2-vinylbiphenyl-4-methyl ether] -blockpolydimethylsiloxane-
matography. Differential scanning calorimetry results demon-
strate that the triblock copolymers exhibit two glass transition
temperatures (2120 ꢀC and 160 ꢀC) that are markedly different. As
revealed by small-angle X-ray scattering and transmission elec-
tron microscopy experiments, the triblock copolymers self-
block-poly[40-(methoxy)-2-vinylbiphenyl-4-methyl ether] (PMVBP-
b-PDMS-b-PMVBP), were synthesized by nitroxide-mediated
radical polymerization, and their hierarchically ordered structures
were studied. A difunctional TEMPO macroinitiator was first syn-
thesized by the hydrosilylation reaction between hydride-
endcapped PDMS and allylic alkoxyamine. The polymerization of
the monomer MVBP was triggered by the macroinitiator, result-
ing in a series of triblock copolymers. The molecular weights and
polydispersity indexes were determined by gel permeation chro-
assemble into hierarchical structures with the liquid crystalline
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phase of PMVBP in the lamellar structures. 2014 Wiley Periodi-
cals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014, 52, 1737–1744
KEYWORDS: hierarchical structure; lamellar; liquid-crystalline
polymers; phase separation; triblock polymer
Mesogen-jacketed LC polymers (MJLCPs)15,16 are one kind of
semirigid polymers. They have bulky side groups laterally
attached to the polymer backbone with a short or no spacer,
leading to properties similar to semirigid main-chain LC
polymers.17,18 The length of the rod-like MJLCP chain can be
controlled by the degree of polymerization through living
free radical polymerization,19 and MJLCPs can serve as rod
blocks in BCPs. Poly(2,5-bis-[4-methoxyphenyl]oxycarbonyl)-
styrene) (PMPCS),20 a typical MJLCP, has been used in a vari-
ety of rod-coil BCPs, including the first MJLCP-coil diblock
copolymer PMPCS-b-PS.21,22 Rod-coil BCPs containing PMPCS
can form hierarchical structures, for example, a “bilayer
smectic A” structure was observed with the rigid PMPCS col-
umns aligned parallel to the lamellar normal.23 ABA-type
rod-coil-rod triblock copolymer PMPCS-b-PIB-b-PMPCS,
which was synthesized with the combination of living cati-
onic polymerization and atom transfer radical polymeriza-
tion,24 forms hierarchical structures when the temperature is
INTRODUCTION Because of unfavorable mixing enthalpy and
weak mixing entropy, block copolymers (BCPs) with two
incompatible blocks undergo microphase separation and
self-assemble into tunable nanostructures including spheres,
cylinders, bicontinuous gyroid, and lamellae.1,2 The proper-
ties of BCPs resulting from their microphase-separated nano-
structures make them useful in electronics,3,4 solar cells,5
and biomedical applications.6–8 Hierarchical assemblies9–11
that optimize material structures12 can be obtained by orga-
nization with precision at different length scales ranging
from molecular level (nanometer) to macroscopic sizes
(micrometer).2 For BCPs, rod-coil ones can form hierarchical
structures13 owing to the parallel arrangement of the rod-
like block which exhibits liquid crystalline (LC) phases such
as a nematic phase. For example, Segalman and coworkers
reported the hierarchical structure combining a 1-nm LC
phase and a 10-nm BCP microphase-separated structure
formed by a BCP with a main-chain LC polymer.14
Additional Supporting Information may be found in the online version of this article.
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SCHEME 1 The NMRP Polymerization Route for PMVBP-b-PDMS-b-PMVBP.
ꢀ
increased to 160 C at which the PMPCS block enters into
the LC phase.25 However, some MJLCP-based rod-coil BCP
systems studied previously have glass transition tempera-
tures (Tg’s) that are not dramatically different, which may
limit their applications in certain fields. A BCP with a large
difference in Tg values can expand the temperature range for
maintaining the microstructures and have a larger applica-
tion window. For instance, a BCP with greatly different Tg
values is potentially useful for damping applications.26 A tri-
block copolymer with a large difference in Tg values have
the potential as a thermoplastic elastomer.27,28
as received. Triethylamine (98%, J&K) and tetrahydrofuran
(THF) (A.R., Beijing Chemical Reagents Co.) were used after dis-
tilled. Chlorobenzene was purified by washing with concen-
trated sulfuric acid to remove residual thiophenes, followed by
washing with a 5% sodium carbonate solution and with water,
dried with anhydrous calcium chloride, and finally distilled.
Characterization Techniques
1H/13C NMR spectra were recorded on a Bruker 400-MHz
spectrometer with tetramethylsilane as the internal standard
and CDCl3 as the solvent. Gel permeation chromatography
(GPC) measurements were performed at 35 ꢀC on a Waters
2414 instrument, using THF as the eluent at a flow rate of 1.0
mL/min. The GPC data were calibrated with polystyrene (PS)
standards.
Recently, a new MJLCP, poly[40-(methoxy)-2-vinylbiphenyl-4-
methyl ether] (PMVBP), has been synthesized, and its Tg is
ꢀ
about 200 C.29 Because polꢀydimethylsiloxane (PDMS) pos-
sesses a Tg as low as 2125 C, rod-coil-rod triblock copoly-
mers containing PDMS and PMVBP (PMVBP-b-PDMS-b-
PMVBP) will have a remarkable difference (about 300 ꢀC)
in Tg values. Therefore, rod-coil-rod PMVBP-b-PDMS-b-
PMVBP triblock copolymers are designed and synthesized
in this work. The triblock copolymers are synthesized via
nitroxide-mediated living free radical polymerization
(NMRP) using a difunctional PDMS macroinitiator end-
capped by nitroxide at both terminals.30 This is the first
Differential scanning calorimetry (DSC) experiments were
performed on a TA 2010 calorimeter with a programmed
heating procedure in nitrogen. The temperature and heat
flow were calibrated with benzoic acid and indium. The sam-
ples were encapsulated in hermetically sealed aluminum
pans with a typical sample weight of ꢁ10 mg. The heating
ꢀ
experiments were performed at a rate of 20 C/min.
X-ray scattering experiments were performed with a high-
flux small-angle X-ray scattering (SAXS) instrument (SAXSess,
Anton Paar) equipped with Kratky block-collimation system
and a Philips PW 3830 sealed-tube X-ray generator (Cu Ka).
The scattering pattern was recorded on an imaging plate
with a pixel size of 42.3 3 42.3 mm2 which is extended to
the high-angle region, and thus the small- and wide-angle
scattering can be recorded simultaneously. The peak posi-
tions were calibrated with silver behenate. After background
subtraction, desmearing was performed according to the
Lake’s method. A temperature control unit (Anton Paar TCS
MJLCP-based triblock copolymer with distinctly different
Tg’s (2120 and 160 C). The hierarchical structures of these
triblock copolymers have also been studied. Such ABA BCPs
have the potential as thermoplastic LC elastomers.
ꢀ
EXPERIMENTAL
Materials
Hydride-endcapped PDMS (H-PDMS-H, Gelest), platinum(0)-1,3-
divinyl-1,1,3,3-tetramethyldisiloxane complex solution in xylenes
(ꢁ2% Pt, Alrich), and acryloyl chloride (95%, TCI) were used
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FIGURE 2 GPC curves of the macroinitiator and the triblock
copolymers. [Color figure can be viewed in the online issue,
which is available at wileyonlinelibrary.com.]
FIGURE 1 FT-IR spectra of PDMS and the TEMPO-PDMS-
TEMPO macroinitiator. [Color figure can be viewed in the
online issue, which is available at wileyonlinelibrary.com.]
pale yellow liquid after purification by a column of silica gel
using dichloromethane and petroleum ether mixtures (1:2,
v:v) as the eluent (39% yield).
300) in conjunction with the SAXSess was utilized to study
the structures at various temperatures. The microphase-
separated structures of the BCP samples were further exam-
ined with transmission electron microscopy (TEM) using a
Hitachi H-800 electron microscope. To prepare the TEM sam-
ples, the triblock copolymer films were embedded in epoxy,
and ultrathin sections were cut from the thin film on an
ultramicrotome.
Synthesis of Acrylate 3
Compound 2 (1.50 g, 5.41 mmol) was mixed with 2 equiv. of
triethylamine in THF (100 mL). The solution was stirred in an
ice/water bath, and acryloyl chloride (1.47 g, 16.2 mmol) was
added dropwise slowly via a constant pressure dropping fun-
nel, the solution was stirred in an ice/water bath for 2 h then
at ambient temperature for 24 h, during which period the
solution became ꢀturbid. After removal of THF on a rotary
evaporator at 35 C, the residue was dissolved in CH2Cl2 (100
mL), washed with a saturated NaHCO3 (5 3 100 mL) and
deionized water (3 3 100 mL), and dried by anhydrous mag-
nesium sulfate for 5 h. CH2Cl2 was removed under reduced
pressure at 35 ꢀC, and the residue was purified by a column of
silica gel using dichloromethane and petroleum ether mixtures
(1:2, v:v) as the eluent to give a pale yellow oil (57% yield).
Synthesis of the Triblock Copolymers
The triblock copolymers are synthesized as shown in Scheme
1. The synthetic details are described as follows.
Synthesis of Hawker’s Initiator, 2
Compound 1 was synthesized according to the procedure
described in literature.31 For purification, 1 was first filtered
over a column of silica gel using dichloromethane and petro-
leum ether mixtures (1:2, v:v) as the eluent to give a pale
yellow oil, and then it was recrystallized from 2-propanol to
yield 1 as a white solid (53% yield). Compound 2 was
obtained from 1 (14.0 g, 36.8 mmol) by hydrolysis with 2
mol/L KOH (80 mL) under reflux conditions in ethanol (180
mL) for 3 h according to literatures.31,32 Compound 2 is a
Synthesis of the Macroinitiator, TEMPO-Terminated
PDMS (TEMPO-PDMS-TEMPO)
Hydride-endcapped PDMS (6.26 g, 0.224 mmol) was dis-
solved in THF (60 mL) in a pressure bottle. Then 2.24 equiv.
TABLE 1 MWs, Polydispersity Indexes and fPDMS and Tg Values of the Macroinitiator and the Triblock Copolymers, along with
Nanostructural Characteristics of the Triblock Copolymers from SAXS Results
nano-
ꢀ
ꢀ
Mn (3 104 g/mol)
Mn (3 104 g/mol)
PDIa
fPDMS (%)
Tg1 ( C)
Tg2 ( C)
qd (nm21
)
dd (nm)
structure
a
b
b
c
c
Sample
MI
P1
P2
P3
2.80
6.85
6.11
4.91
2.80
10.9
9.00
7.80
1.30
1.38
1.27
1.27
100
2125
2119
2122
2124
–
–
–
–
27.8
33.5
38.4
173
161
159
0.110
0.151
0.183
57.1
41.6
34.3
LAM
LAM
LAM
a
c
Determined by GPC using linear polystyrene standards.
Determined by 1H NMR on the basis of the Mn of the MI from GPC.
Determined by DSC.
b
d
Determined by SAXS.
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FIGURE 5 SAXS profiles of the triblock copolymers after
annealing. [Color figure can be viewed in the online issue,
which is available at wileyonlinelibrary.com.]
FIGURE 3 1H NMR spectrum of sample P3 in CDC13. [Color fig-
ure can be viewed in the online issue, which is available at
wileyonlinelibrary.com.]
Fractionation of the Macroinitiator TEMPO-PDMS-TEMPO
TEMPO-PDMS-TEMPO synthesized from H-PDMS-H had a
broad and asymmetric molecular weight (MW) distribution
with a polydispersity index (PDI) of 1.76. To synthesize
structurally well-defined triblock copolymers, it was neces-
sary to fractionate the macroiniator. With THF as the good
solvent and methanol as the poor solvent, the fractionation
was conducted by gradual precipitation. The raw material
was first dissolved in THF, and then methanol was added
dropwise, leading to the precipitation of macromolecules
with high MWs. The final product used as the macroinitiator
has a relatively narrow MW distribution as demonstrated by
GPC (Supporting Information Fig. S1 and Table S1).
(based on the estimated moles of Si–H in 4) of 3 (0.331 g,
1.00 mmol) was added, and the mixture was stirred until 3
was completely dissolved, followed by adding 2 drops (10
mL) of platinum catalyst [platinum(0)-1,3-divinyl-1,1,3,3-tet-
ramethyldisiloxane complex solution in xylenes] to the pres-
sure bottle. The bottle was sealed tightly, and the solution
was stirred in an oil bath at 60 ꢀC for 48 h. Upon cooling,
the solution was diluted with THF and filtered over a plug of
deactivated charcoal and neutral alumina to remove particu-
lates and for decolorization. The polymer was then precipi-
tated from THF with methanol, and the recovered polymer
was placed on a vacuum line to remove residual solvents. A
viscous, slightly murky oil was obtained in a 80–85% yield.
Synthesis of the Monomer PMVBP
The monomer MVBP was synthesized similar to the proce-
dure described preciously.29,33 3-Methoxybenzaldehyde was
used to replace 5-bromo-2-hydrocybenzaldehyde and react
with 4-methoxyphenylboronic acid, and then Suzuki cross-
coupling reaction was utilized to obtain the target product
MVBP. The structure of the monomer was characterized by
1H/13C NMR (CDCl3) (Supporting Information Fig. S2). MS
(HR-ESI): m/z 241.12166 [M1H]1. Anal. Calcd. for C16H16O2:
C, 79.97; H, 6.71; and O, 13.32. Found: C, 80.08; H, 6.85; and
O, 13.07.
Synthesis of the Triblock Copolymers PMVBP-b-PDMS-b-
PMVBP by NMRP
In a typical experiment for polymerization, the monomer
MVBP (0.500 g, 2.08 mmol), TEMPO-PDMS-TEMPO (0.278 g,
9.64 mmol), and chlorobenzene (2.51 mL) were charged into
a polymerization tube. After stirring and degassing by three
freeze2pump2thaw cycles, the tube was sealed under vac-
uum and subsequently immerged into an oil bath thermo-
stated at 125 ꢀC for 12 h. It was then quenched in liquid
nitrogen and taken out to ambient condition. The BCP was
FIGURE 4 DSC thermograms of the triblock copolymers during
second heating at a rate of 20 ꢀC/min under a nitrogen atmos-
phere. [Color figure can be viewed in the online issue, which is
available at wileyonlinelibrary.com.]
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FIGURE 6 TEM micrographs of the triblock copolymers after
annealing.
precipitated in a large volume of methanol and dried in vac-
uum overnight.
Density Measurements of the Homopolymer PMVBP
To calculate the volume fraction of PDMS (fPDMS) in the tri-
block copolymer, the density of PMVBP should be measured.
The densities of PMVBP homopolymers were measured using
the suspension method with potassium bromide solutions.34
The final density value is 1.08 g/cm3, the average of the
results from three measurements.
FIGURE 7 WAXS profiles of an as-cast film of sample P2 dur-
ing the first heating (a) and the first cooling (b). [Color figure
can be viewed in the online issue, which is available at
wileyonlinelibrary.com.]
RESULTS AND DISCUSSION
Synthesis and Characterization of PMVBP-b-PDMS-b-
PMVBP Triblock Copolymers 1
were characterized by GPC and 1H NMR. The MWs range
from 4.91 3 104 to 6.85 3 104 g/mol from GPC results. The
volume fraction of PDMS (fPDMS) is in the range of 27.8–
38.4% calculated from the 1H NMR results of the triblock
copolymers using densities of 0.971 g/cm3 for PDMS (pro-
vided by supplier) and 1.08 g/cm3 for PMVBP. Figure 3 is
the H NMR of sample P3. All results confirm that these tri-
block copolymers have well-defined chemical structures with
symmetric and relatively narrow MW distributions.
As shown in Scheme 1, a series of PMVBP-b-PDMS-b-PMVBP
triblock copolymers with different MWs were synthesized
through NMRP initiated by the same PDMS macroinitiator
prepared from commercially available H-PDMS-H. The mac-
roinitiator, TEMPO-PDMS-TEMPO, was synthesized through
the reaction between H-PDMS-H and allyl ester and was then
fractionated to yield a relatively low PDI. Confirmation of
hydrosilylation was made by FT-IR. The SiAH vibration band,
at 2128 cm21, was absent from the spectrum of the macroi-
nitiator upon completion of the reaction, as shown in Figure
1. GPC results demonstrate that the number-average MW
(Mn) and the PDI of the macroinitiator TEMPO-PDMS-TEMPO
are 2.8 3 104 g/mol and 1.30, respectively, as shown in Fig-
ure 2 and Table 1.
1
Thermal Properties of PMVBP-b-PDMS-b-PMVBP
Triblock Copolymers
As shown in Table 1, Tg values of the triblock copolymers
were determined by DSC. To erase thermal history, the sam-
ꢀ
PMVBP-b-PDMS-b-PMVBP triblock copolymers were synthe-
sized using TEMPO-PDMS-TEMPO as macroinitiator, chloro-
benzene as solvent by changing the ratio of macroinitiator to
monomer. All PMVBP-b-PDMS-b-PMVBP triblock copolymers
ples were heated from ambient temperature to 270 C at a
ꢀ
rate of 20 C/min, followed by a cooling process using liquid
nitrogen manually to 2150ꢀ ꢀC. The DSC experiments were
ꢀ
then performed from 2150 C to 270 C at a heating rate of
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ꢀ
and the other around 160 C as the Tg of the PMVBP block,
as shown in Table 1. Because of the plasticizing effect of the
low-Tg PDMS in the center of the triblock copolymer,35,36
resulting in a more flexible environment,37 the Tg of PMVBP
in the triblock copolymers is lower than that of theꢀ PMVBP
homopolymer.29 The endothermic peak around 250 C is the
melting of PDMS.38 Tg values of these samples are given in
ꢀ
ꢀ
Table 1. Tg’s of 2120 C and 160 C, with a very large differ-
ence, are observed in the BCPs.
Phase Structures of PMVBP-b-PDMS-b-PMVBP Triblock
Copolymers in Bulk
The microphase separation of the originally amorphous BCPs
FIGURE 8 TEM micrographs of the triblock copolymer P3 with
PMVBP in the amorphous state (a) and in the LC phase (b).
usually occurs above the Tg’s of the constituting blocks. The
ꢀ
Tg of PDMS is known to be low at ꢀabout 2125 C, while the
ꢀ
20 C/min. Two distinct glass transitions are clearly detected
Tg of PMVBP is high at about 200 C. Therefore, the samples
ꢀ
for all samples as shown in Figure 4, which indicates the
immiscibility of the two different blocks. The transition
around 2120 ꢀC is assigned as the Tg of the PDMS block,
were annealed at 220 C, higher than the Tg of PMVBP. How-
ever, oxidation occurs in such an annealing process. Conse-
quently, a solvent-thermal annealing process was used. In a
typical annealing process, the as-casting film of the sample
ꢀ
was annealed in the saturated vapor of toluenꢀe at 80 C for
48 h, and then dried in a vacuum oven at 60 C for 24 h to
remove the residual solvent.
SAXS experiments on the samples were performed at ambi-
ent temperature. The SAXS profiles are shown in Figure 5,
and the primary peak positions (q*’s) and the corresponding
d-spacing values of the diffraction peaks are shown in Table
1. The two peaks in the scattering profiles of all the four
samples have the same scattering vector ratio of 1:2, charac-
teristic of lamellar structures. The primary reflections are at
q* 5 0.110, 0.151, and 0.183 nm21, corresponding to d-spac-
ing values of 57.1, 41.6, and 34.3 nm, respectively. The
d-spacing increases with increasing MW, as expected.
TEM results corroborate with the SAXS results. As shown in
Figure 6, lamellar structures are observed. In the TEM
micrographs, the black component is PDMS because it has a
higher electron density than PMVBP. The lamellar spacings
from TEM results are in good agreement with those from
SAXS measurements.
The mesophase properties of the BCPs were investigated by
SAXS at elevated temperatures. With sample P2 as an exam-
ple, Figure 7 shows two sets of WAXS powder patterns dur-
ing the first heating (part a) and thꢀe first cooling (part b).
When the temperature is below 180 C, only a broad scatter-
ing halo is observed at q ꢂ 5 nm21, which arises from the
PMVBP block. As the temperature is increased to above
ꢀ
250 C, a sharp and intense diffraction peak appears to the
right of the halo. The q value is 5.29 nm21, which corre-
sponds to a d-spacing of 1.19 nm (d 5 2p/q). As in our pre-
vious study, the PMVBP homopolymer enters into the LC
ꢀ
phase at 260 C, and the rigid PMVBP columns order into a
hexagonal columnar phase.29 Thus, the diameter of the
PMVBP column can be calculated to be 1.37 nm from the q
value, 5.29 nm21. Such a diameter is close to the simulated
length of the side chain (about 1.32 nm). When the tempera-
ture is decreased, the peak does not disappear, indicating
FIGURE 9 SAXS (a) and WAXS (b) profiles of the triblock
copolymer samples at ambient temperature. [Color figure can
be viewed in the online issue, which is available at wileyonline-
library.com.]
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TABLE 2 fPDMS and Different d-Spacing Values of the Triblock Copolymers
a
b
b
c
Sample
fPDMS (%)
d1 (nm)
d2 (nm)
dcoil (nm)
drod (nm)
dPMVBP (nm)
MI
P1
P2
P3
100
–
27.8
33.5
38.4
57.1
41.6
34.3
77.5
66.1
64.1
21.5
22.1
24.6
56.0
44.0
39.5
42.2
32.3
26.0
a
c
Determined by 1H NMR on the basis of the Mn of the MI from GPC.
Determined by SAXS, with d1 and d2 representing the d-spacing val-
Simulated values.
b
ues of the lamellar structure before and after the formation of the LC
phase of PMVBP, respectively.
that the LC phase of PMVBP retains, as in the
homopolymers.29
calculated to be 22.1 nm, 44.0 nm using the fPDMS value,
respectively. Assuming a zig-zag conformation of the PMVBP
backbone,23 from the dPMVBP of 32.3 nm and drod of 44.0 nm,
it can be concluded that the PMVBP rods are partially inter-
digitated within the PMVBP microdomains. It should be
noted that the d-spacing of the sample P3 increases almost
two times after PMVBP enters into the LC phase, possibly
due to the less degree of interdigitation of the PMVBP rods
according to the dPMVBP of 26.0 nm and drod of 39.5 nm. The
hierarchical structures of the PMVBP-b-PDMS-b-PMVBP rod-
coil-rod triblock copolymers are schematically illustrated in
Figure 10.
After the samples are thermally annealed above the tempera-
ture at which the LC phase of PMVBP starts to develop and
then cooled to ambient temperature, TEM experiments were
also performed. P3 was taken as an example. As shown in
Figure 8, part “a” was obtained from the solvent-annealed
sample (with PMVBP in the amorphous state), and part “b”
was from the sample thermally annealed at high temperature
for PMVBP to develop the LC phase and then cooled to ambi-
ent temperature. The TEM results also demonstrate the sig-
nificant increase in the d-spacing when PMVBP develops into
the LC phase.
CONCLUSIONS
In summary, a series of rod-coil-rod triblock copolymers are
successfully synthesized via NMRP using a difunctional
PDMS macroinitiator endcapped by nitroxide at both termi-
nals. Structures of the BCPs of high MWs and relatively low
PDIs are confirmed by GPC and 1H NMR. DSC results show
Interestingly, as shown by the SAXS results in Figure 9,
when the samples are heated above the temperature at
which the LC phase starts to develop and are then cooled to
ambient temperature, the lamellar structures remain, with
an increase in the d-spacing. This can be mainly attributed
to the increase in the chain length of PMVBP caused by the
conformational change from a wormlike chain to a rod-like
one following the phase transition from the amorphous state
to the LC phase.25,39 The inconsistency of nonequilibrium
morphologies and close-equilibrium morphologies obtained
from solvent annealing and thermal annealing, respectively,
may also cause the difference in lamellar spacing. The
d-spacing values are summarized in the Table 2. Taking P2
as an example, with the consideration of the shared interfa-
ces,40 the thicknesses of coil and rod sublayers (after PMVBP
develops into the LC phase), the dcoil and drod values can be
that the triblock copolymers have two widely separated glass
ꢀ
transitions at about 2125 and 160 C. Results from variable-
temperature SAXS experiments confirm that the PMVBP
blocks in the BCPs start to enter into the LC phase at about
250 ꢀC. And the LC phase of PMVBP retains upon cooling.
When the volume fraction of PDMS is between 27.8 and
38.4%, SAXS and TEM results demonstrate that the triblock
copolymers form lamellar structures, and the d-spacing val-
ues range from 34.3 to 57.1 nm and are dependent on the
MWs. Upon heating to the temperature at which the LC
phase of PMVBP forms, the PMVBP blocks become LC within
the lamellar structures of the BCPs, resulting in hierarchi-
cally ordered structures. These triblock copolymers are
potentially useful as thermoplastic LC elastomers owing to
their distinctly different glass transition temperatures. We
are currently investigating such a possibility.
FIGURE 10 Schematic representation of the hierarchical struc-
tures formed by PMVBP-b-PDMS-b-PMVBP. [Color figure can
be viewed in the online issue, which is available at wileyonline-
library.com.]
CHART 1 Chemical Structure of MVBP
WWW.MATERIALSVIEWS.COM
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