Synthesis and properties of mesogen-jacketed liquid crystalline polymers containing biphenyl mesogen with asymmetric substitutions

Article abstract of DOI:10.1002/pola.24757

This work focuses on the design, synthesis, and characterization of a series of mesogen-jacketed liquid crystalline polymers (MJLCPs), poly(alkyl 4′-(octyloxy)-2-vinylbiphenyl-4-carboxylate) (pVBP(m,8), m = 1, 2, 4, 6, 8, 10, 12). For the first time, we r

Full text of DOI:10.1002/pola.24757

Synthesis and Properties of Mesogen-Jacketed Liquid Crystalline
Polymers Containing Biphenyl Mesogen with Asymmetric Substitutions
LANYING ZHANG, HONGLIANG WU, ZHIHAO SHEN, XINGHE FAN, QIFENG ZHOU
Beijing 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
Received 1 February 2011; accepted 26 April 2011
DOI: 10.1002/pola.24757
Published online 23 May 2011 in Wiley Online Library (wileyonlinelibrary.com).
ABSTRACT: This work focuses on the design, synthesis, and
mer pVBP(1,8) was not liquid crystalline, columnar liquid
crystalline phases were observed for all other pVBP(m,8) (m
¼ 2, 4, 6, 8, 10, 12) polymers. Polymer pVBP(8,8) showed a
tetragonal columnar nematic liquid crystalline phase, and the
other LC polymers exhibited columnar nematic phases. In
additions, the smaller the difference in the lengths of the ter-
characterization of a series of mesogen-jacketed liquid crystal-
0
line polymers (MJLCPs), poly(alkyl 4 -(octyloxy)-2-vinylbi-
phenyl-4-carboxylate) (pVBP(m,8), m ¼ 1, 2, 4, 6, 8, 10, 12).
For the first time, we realized asymmetric substitutions in
the mesogens of MJLCPs. The polymers obtained by conven-
tional free radical polymerization were investigated in detail
by a combination of various techniques, such as differential
scanning calorimetry, wide-angle X-ray diffraction, and
polarized light microscopy. Our results showed that all the
polymers were thermally stable, and their glass transition
temperatures decreased when m increased. The liquid crys-
talline (LC) phases that developed at high temperatures and
disappeared at low temperatures were strongly dependent
minal alkyls, the easier the development of the liquid crystal-
line phase. Birefringence measurements showed that
solution-cast polymer films exhibited moderately high posi-
tive birefringence values, indicating potential applications as
optical compensation films for liquid crystal displays. VC 2011
Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 49:
3207–3217, 2011
0
on the difference in lengths of alkyl groups on the 4 and 4
KEYWORDS: liquid-crystalline polymers (LCP); phase behavior;
substitution positions of the side-chain biphenyl. While poly-
supramolecular structures
INTRODUCTION The design and synthesis of soft matter is a
hot topic of research and is very important for functional
materials science and life science. Liquid crystalline poly-
mers (LCPs), especially side-chain liquid crystalline polymers
MJLCPs exhibit properties similar to those of main-chain
LCPs (MCLCPs), such as large persistence length in good sol-
5
vents and banded texture of the sheared liquid crystalline
6
(
(
LC) phase. One-dimensional (1D) and two-dimensional
2D) wide-angle X-ray diffraction (WAXD) investigations
(
SCLCPs), which combine the ordered structures of liquid
combined with polarized light microscopy (PLM) and differ-
ential scanning calorimetry (DSC) results have revealed that
crystals and the excellent properties of polymers, have
attracted considerable attention and become one of the most
important research fields in polymer chemistry and physics
due to their outstanding applications in engineering plastics,
one of the most common phase structures of MJLCPs is the
7
columnar nematic (U
N
) phase. Higher-ordered LC phases,
8
1
such as hexagonal columnar (U ) phase, hexatic columnar
H
nematic (UHN) phase, and smectic A or smectic C phase,
optic and electro-optic devices, and so on. For SCLCPs, the
9
10
11
mesogens can be either terminally or laterally attached to
the polymer backbone. According to Finkelmann’s principle,
flexible spacers are needed to decouple motions of the main
have also been observed in some systems.
Since the first publication on MJLCPs, up to date, a rich vari-
ety of MJLCPs with different structures and unique proper-
ties have been rationally designed and synthesized. Among
them, there are three series of MJLCPs (Chart 1), which are
2
chain and the mesogenic side groups. However, as proposed
by Zhou, mesogen-jacketed liquid crystalline polymers
(
MJLCPs), which are a special class of SCLCPs with their
8
7
mesogenic units attached laterally to the main chain through
based on 2-vinylterephthalic acid with alkyl or aryl ester,
3
4
12
13
a short spacer or only one covalent CAC bond, can also
form stable liquid crystalline phases. The bulky and rigid
mesogenic units directly linking to the polymer backbone
force the main chain to take an extended chain conformation.
2-vinyl-terphenyl with alkyl ester or alkyl ether, and 2(or
3)-vinylbiphenyl-4,4 -dicarboxylic acid with aryl.
[di(alkyl) vinylterephthalates] with suitable tail lengths can
0
11
Poly
8
form stable long-range-ordered UH phases, and the polymers
Additional Supporting Information may be found in the online version of this article. Correspondence to: Z. Shen (E-mail: zshen@pku.edu.cn) or X. Fan
E-mail: fanxh@pku.edu.cn)
Journal of Polymer Science Part A: Polymer Chemistry, Vol. 49, 3207–3217 (2011) V
(
C
2011 Wiley Periodicals, Inc.
SYNTHESIS AND PROPERTIES OF MJLCPs, ZHANG ET AL.
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JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
challenging. At present, commercial polymeric optical com-
pensators with positive birefringence require expensive post-
3
1
32
33
processing such as rubbing, stretching, light irradiation,
3
4
or application of a high electric field, all of which will
unavoidably increase the cost. We have recently reported
solution-cast MJLCP films with strikingly high positive bire-
fringence values of up to 0.0350 without any postprocessing,
which hold tremendous promise for application in optical
3
5
compensators. However, their usually poor film-forming
ability and high cost in synthesis limit widespread applica-
tions of these polymers.
Herein, we present the first example of MJLCPs based on
biphenyl mesogenic cores with asymmetric substitutions of
0
alkyl tails with different lengths. A series of poly(alkyl 4 -
(
1
octyloxy)-2-vinylbiphenyl-4-carboxylate) (pVBP(m,8), m ¼
, 2, 4, 6, 8, 10, 12) were successfully synthesized by con-
ventional free radical polymerization in solution. The result-
ant polymers are therefore a model system to investigate the
effect of the difference in lengths of the two terminal alkyl
groups on the LC phase structure and phase transition.
CHART 1 Chemical structures of the three series of mesogen-
jacketed liquid crystalline polymers.
EXPERIMENTAL
Materials
0
based on 5-terphenyl, poly[2,5-bis(4 -alkoxycarbonylphenyl)-
Methyl 4-bromo-3-methylbenzoate (98%, Alfa Aesar), 4-
(octyloxy)phenylboronic acid (99.6%, Alfa Aesar), triphenyl-
phosphine (99%, Alfa Aesar), 40% formaldehyde (AR, Beijing
Yi Li Chemical), N,N -dicyclohexyl carbodiimide (DCC, 95%,
Sinopharm Chemical Reagent), 4-dimethylaminopyridine
1
3
styrene]s, with longer alkoxy tails (m ¼ 3ꢀ12) exhibit UN
phases at high temperatures. On the other hand, whether
polymers based on the biphenyl mesogenic core can still
self-assemble into stable LC phases is a subject that has not
been fully investigated, although biphenyl itself as a meso-
genic group has long been widely used as the liquid crystal-
or building blocks in main-
and main-chain/side-chain LCPs.
Percec et al. synthesized jacketed LCPs, poly(p-phenylene)s
PPPs) containing 4-cyano-4 -biphenylyl mesogenic side
0
(DMAP, 99%, ACRO), and N-bromosuccinimide (NBS, 99%,
Aldrich) were used as received without further purification.
Azobisisobutyronitrile (AIBN) was recrystallized from etha-
nol before use. Benzoyl peroxide (BPO) was recrystallized
from chloroform and methanol. Chlorobenzene was washed
1
4–16
line core of small molecules
1
7
18–23
24
chain, side-chain,
2
5
0
(
2 4 3
with H SO , NaHCO , and distilled water and then distilled
groups with spacers of different lengths, and found the first
examples of the NII phase. They also designed and synthe-
sized a series of self-organized helical dendronized, there-
fore, also jacketed, polyphenylacetylenes (PPAs) exhibiting a
hexagonal columnar lattice with an internal order and a hex-
from calcium hydride. Tetrahydrofuran (THF, AR, Beijing
Chemical) was refluxed over sodium and distilled before use.
Dichloromethane (AR, Beijing Chemical) was dried over mag-
nesium sulfate anhydrous.
2
6,27
Monomer Synthesis
agonal columnar liquid crystalline phase.
Furthermore,
The synthetic route of monomers is represented in Scheme 1.
The experimental details are described as follows.
the alkyl tails are all the same for the MJLCPs synthesized
previously, with more attention paid to the effect of the tail
length, but the influence of the relative length of the termi-
nal alkyl groups on the phase structures has not been stud-
ied. Although Thayumanavan and coworkers have recently
reported the self-assembling behaviors in solution of some
polystyrene-based amphiphilic homopolymers containing
two benzene rings connected by methoxy in the side chain,
their bulk liquid crystalline properties have not been
Synthesis of 4-Bromo-2-Vinylbenzoic Acid
Methyl 4-bromo-3-methylbenzoate (8.030 g, 0.035 mol), NBS
(
9.362 g, 0.053 mol), and BPO (0.1697 g, 0.701 mmol) were
dissolved in 160 mL of CCl and refluxed for 6 h. The float-
4
ing succinimide was filtered off, and the solvent was evapo-
rated. Then the residue was boiled with 9.197 g (0.035 mol)
of triphenylphosphine in 180 mL of acetone for 6 h until
white phosphonium salt precipitated. NaOH aqueous solution
(5 M, 250 mL) was added slowly to 190 mL of 40% formal-
dehyde containing 9.715 g (0.017 mol) of the above phos-
phonium salt obtained, and the mixture was stirred for 48 h
at ambient temperature. After the floating substance was fil-
trated off, the resultant crude product was washed by 5 M
NaOH aqueous solution twice. HCl (12 M) was added to the
filtrate until the pH of the solution was less than 1, and then
2
8,29
examined.
Liquid crystal displays (LCDs) play important roles in our
modern life. Polymers have been utilized as optical compen-
sators for improved viewing angles in LCDs, owing to their
3
0
unique optical properties, easy processing, and low cost
for device integration, and high birefringence, either positive
or negative, of the polymeric flexible thin film is preferred.
Obtaining polymer films with highly positive birefringence is
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ARTICLE
8
e).
.181ꢀ8.184 (d, 1H, Ar), 13.048 (s, 1H, COOH). MS: 352 (m/
0
Synthesis of Methyl 4 -(Octyloxy)-2-Vinylbiphenyl-4-
Carboxylate (mVBP(1,8))
0
4
-(Octyloxy)-2-vinylbiphenyl-4-carboxylic acid (1 g, 2.841
mmol) in 60 mL of methanol with 5 drops of sulfuric acid
98%) was refluxed for 24 h. After methanol was evaporated
under reduced pressure, the crude product was dissolved
in CH Cl and extracted with saturated solution of NaHCO
(
2
2
3
.
The combined organic layer was dried by magnesium sulfate
anhydrous. Then the product was obtained after the solvent
1
was removed. Yield: 98%.
H NMR (d, ppm, CDCl3):
0
1
3
2
.875ꢀ0.909 (t, 3H, terminal CH ), 1.254ꢀ1.376, 1.454ꢀ1.492,
3
.768ꢀ1.821 (m, 12H, CH ), 3.935 (s, 3H, COOCH ),
2
3
.970ꢀ4.003 (t, 2H, OCH ), 5.240ꢀ5.268, 5.772ꢀ5.816 (d,
2
H, ¼¼ CH ), 6.696ꢀ6.767 (q, 1H, CH ¼¼ ), 6.931ꢀ6.960 (m, 2H,
2
Ar), 7.245ꢀ7.273 (m, 2H, Ar), 7.320ꢀ7.340 (m, 1H, Ar),
13
7
.930ꢀ7.954 (m, 1H, Ar), 8.287ꢀ8.291 (d, 1H, Ar). C NMR
(d, ppm, CDCl ): 167.047, 158.814, 144.863, 136.001, 135.384,
3
131.870, 130.732, 130.192, 128.752, 128.433, 127.318,
115.638, 114.135, 68.059, 52.123, 31.807, 29.355, 29.266,
29.236, 26.059, 22.650, 14.092. MS: 366 (m/e).
0
Synthesis of Butyl 4 -(Octyloxy)-2-Vinylbiphenyl-4-
Carboxylate (mVBP(4,8))
0
4
0
0
-(Octyloxy)-2-vinylbiphenyl-4-carboxylic acid (0.500 g,
.001 mol), n-butanol (0.526 g, 0.007 mol), DCC (0.293 g,
.001 mol), and DMAP (0.052 g, 0.426 mmol) were dissolved
SCHEME 1 Synthetic route of styrene liquid crystalline
monomers.
in dichloromethane. The solution was stirred at ambient
temperature for about 24 h. The precipitate was filtered and
washed with dichloromethane for several times. After evapo-
ration of the solvent, the ester obtained was purified by
silica gel column chromatography with dichloromethane as
the eluent to yield 0.208 g (51%) of the product.
1
white product appeared. Yield: 60%. H NMR (d, ppm,
DMSOꢀd ): 5.508ꢀ5.536 (d, 1H, ¼¼ CHH), 5.901ꢀ5.944 (d,
6
1
2
H, ¼¼ CHH), 6.961ꢀ7.032 (q, 1H, CH ¼¼ ), 7.738ꢀ7.787 (m,
H, Ar), 8.153ꢀ8.156 (d, 1H, Ar). MS: 226 (m/e).
Synthesis of Pd(PPh3)4
1
Pd(PPh3)4 was prepared according to the method reported
H NMR (d, ppm, CDCl ): 0.878ꢀ0.911 (t, 3H, terminal CH ),
3
3
3
6
in literature.
0.978ꢀ1.015 (t, 3H, terminal CH ), 1.298ꢀ1.356,
3
1
2
5
6
.398ꢀ1.478, 1.789ꢀ1.826 (m, 16H, CH ), 3.984ꢀ4.017 (t,
2
0
Synthesis of 4 -(Octyloxy)-2-Vinylbiphenyl-4-Carboxylic
Acid (mVBP(0,8))
In this step, a typical Suzuki cross-coupling reaction was
used. 4-Bromo-2-vinylbenzoic acid (7.762 g, 0.034 mol), 4-
octyloxy)phenylboronic acid (9.240 g, 0.037 mol), Pd(PPh3)4
0.126 g, 1.109 mol), and potassium carbonate (K CO , 9.274
g, 0.067 mol) were mixed in a 250-mL three-necked flask
under an argon atmosphere. Then 160 mL of acetonitrile/
H, OCH
2
), 4.341ꢀ4.374 (t, 2H, COOCH
2
), 5.247ꢀ5.276,
.775ꢀ5.819 (d, 2H, ¼¼ CH ), 6.701ꢀ6.773 (q, 1H, CH ¼¼ ),
2
.941ꢀ6.963 (m, 2H, Ar), 7.255ꢀ7.282 (m, 2H, Ar),
7
.328ꢀ7.348 (m, 1H, Ar), 7.940ꢀ7.964 (m, 1H, Ar),
(
13
8
1
1
6
2
.288ꢀ8.291 (d, 1H, Ar). C NMR (d, ppm, CDCl ): 166.507,
3
(
2
3
58.743, 144.689, 135.885, 135.367, 131.832, 130.663,
30.072, 129.074, 128.338, 127.210, 115.502, 114.053,
7.095, 64.775, 31.762, 30.751, 29.312, 29.219, 29.197,
6.017, 22.605, 19.227, 14.043, 13.720. MS: 408 (m/e).
ꢁ
water (3/1) mixture was added and refluxed at 80 C for
4
0 h. The mixture was cooled to ambient temperature and
filtrated to remove the floating substance. HCl (50 mL,
2 M) was added to the filtrate, and then a white solid
Syntheses of other monomers can be found in the Support-
ing Information.
1
appeared. The mixture was filtered, and the obtained white
solid was washed with acetonitrile twice. Yield: 80%.
Polymer Synthesis
All polymers were obtained via conventional free radical
polymerization in solution, as shown in Scheme 2. A typical
polymerization procedure is summarized as the following.
About 0.538 g of mVBP(6,8), 15 lL of 0.106 M AIBN/chloro-
benzene solution, and 1.244 g of chlorobenzene were trans-
ferred into a polymerization tube. After three freeze-pump-
thaw cycles, the tube was sealed under vacuum.
1
H NMR (d, ppm, DMSOꢀd ): 0.837ꢀ0.871 (t, 3H, terminal
6
CH ), 1.257ꢀ1.307, 1.393ꢀ1.413, 1.699ꢀ1.736 (m, 12H,
3
(CH
2
)
6
), 3.976ꢀ4.009 (t, 2H, OCH ), 5.282ꢀ5.311,
2
5
6
7
.791ꢀ5.835 (d, 2H, ¼¼ CH ), 6.627ꢀ6.671 (q, 1H, CH ¼¼ ),
2
.999ꢀ7.020 (d, 2H, Ar), 7.244ꢀ7.265 (d, 2H, Ar),
.367ꢀ7.387 (d, 1H, Ar), 7.873ꢀ7.897 (m, 1H, Ar),
SYNTHESIS AND PROPERTIES OF MJLCPs, ZHANG ET AL.
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JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
SCHEME 2 Chemical structures and synthesis of the polymers.
ꢁ
Polymerization was carried out in an oil bath at 60ꢀ65
C
lecular weight distributions of the polymers. The typical GPC
traces of all the polymers are shown in Supporting Informa-
tion (Fig. S1). The apparent number average molecular
for 48 h. It was stopped by dipping the tube in ice/water,
and the tube was broken. The mixture was diluted with
about 10 mL of THF and precipitated into methanol twice to
yield a white product. After purification, the polymer was
dried under vacuum for 2 d.
Characterization
1
H NMR spectra, mass spectra (MS), gel permeation chro-
matographic (GPC) measurements, thermogravimetric analy-
ses (TGA), DSC, PLM, and WAXD (1D and 2D) experiments
were performed according to the procedures described pre-
3
7
viously, except that 2D WAXD patterns at various tempera-
tures were recorded utilizing a hot stage set up on the dif-
ꢁ
fractometer with
a
temperature accuracy of 61 C.
Birefringence (Dn) values of the monomers were estimated
3
8
by the host-guest method in literature. Birefringence values
of the free-standing polymer films were measured by a
3
9
prism coupler (Model 2010, from Metricon) at 633 nm.
Sample preparation followed the procedures in the
3
5
literature.
RESULTS AND DISCUSSION
Synthesis and Characterization of the
Monomers and Polymers
As shown in Scheme 1, the monomers could be successfully
synthesized through a multistep reaction route. The struc-
tures of the monomers and precursors were confirmed by
1
13
H NMR, C NMR, and mass spectrometry.
1
13
Figure 1 presents the H (part a) and C (part b) NMR spec-
1
tra (CDCl ꢀd) of the monomer mVBP(1,8) and the H NMR
3
spectrum (part c) of the corresponding polymer pVBP(1,8).
The spectrum of mVBP(1,8) showed the representative
resonances of the vinyl group at 5.24ꢀ5.82 and 6.70ꢀ6.77
ppm. After polymerization, these signals completely disap-
peared. The resonance peaks of the spectrum of pVBP(1,8)
were broad due to the slower motion of the protons, which
was common for polymers. All other monomers and poly-
mers were characterized similarly, and the results were in
accordance with the desired structures. The polymers were
completely soluble in common organic solvents, such as
dichloromethane, chloroform, acetone, THF, chlorobenzene,
and so on.
1
13
0
FIGURE 1 H (a) and C (b) NMR spectra of methyl 4 -(octy-
1
loxy)-2-vinylbiphenyl-4-carboxylate, mVBP(1,8) and
H NMR
0
The preliminary characterization results of the polymers are
summarized in Table 1. GPC analysis was performed to
determine the apparent molecular weights (MWs) and mo-
spectrum of poly{methyl 4 -(octyloxy)-2-vinylbiphenyl-4-carbox-
ylate}, pVBP(1,8) (c). [Color figure can be viewed in the online
issue, which is available at wileyonlinelibrary.com.]
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ARTICLE
7
,11
reported previously.
The T ’s of the polymers gradually
g
TABLE 1 Molecular Weights, Polydispersity Indexes, 5%
Weight Loss Temperatures, Glass Transition Temperatures,
and Liquid Crystallinity of the Polymers
decreased with increasing m due to enhanced internal plasti-
cization. Because the DSC method was not sensitive enough
to determine the phase transitions of these polymers except
for glass transitions, PLM and WAXD techniques were
utilized.
M
n
T
d
ꢁ
T
g
ꢁ
Liquid
4
a
a
b
c
d
Sample
(ꢂ10 g/mol)
PDI
( C)
( C)
Crystallinity
Birefringence of the polymers was observed under PLM with
pVBP(1,8)
pVBP(2,8)
pVBP(4,8)
pVBP(6,8)
pVBP(8,8)
10.8
12.0
15.1
10.5
9.2
1.34 392
1.31 381
1.16 385
1.37 396
1.46 369
1.27 369
1.38 392
126
95
76
64
57
44
42
No
the films cast from CHCl solutions and slowly dried at ambi-
3
Yes
Yes
Yes
Yes
Yes
Yes
ent temperature. At ambient temperature, all the polymers
exhibited only very weak birefringence, presumably due to
the existence of certain alignment of the polymer chains dur-
ing precipitation. Upon heating, the polymers behaved differ-
ently depending on the length of the alkyl ester. Polymer
pVBP(1,8), which had the shortest alkyl ester tails, displayed
pVBP(10,8) 11.8
pVBP(12,8) 10.4
ꢁ
no discernible change even when heated up to 250 C. For
a
n
The apparent M and PDI values were measured by GPC using PS
polymer pVBP(2,8), obvious birefringence could be observed
when the sample was heated to a temperature higher than
Tg. A typical texture, which was schlieren-like, developed
during further heating is shown in Figure 2(a). However,
when m was more than 2 (i.e., m ¼ 4, 6, 8, 10, 12), distinct
texture [as shown in Fig. 2(b), taking pVBP(8,8) as an exam-
ple] that was quite different from that of pVBP(2,8) could be
standards.
b
The temperatures at 5% weight loss of the samples under nitrogen
ꢁ
were measured by TGA heating experiments at a rate of 20 C/min.
c
The glass transition temperatures of the samples under a nitrogen
ꢁ
atmosphere were measured by DSC at a scanning rate of 10 C/min.
d
Determined by the combination of PLM and 1D WAXD observations.
4
weights (M
n
’s) of all the polymers were about 10 ꢂ 10 g/
g
observed when the temperature was much higher than T .
mol and polydispersity indexes (PDIs) were below 1.50,
which showed good polymerizability of the monomers. All
polymers were quite thermally stable under the nitrogen
atmosphere and their 5% weight loss temperatures (T ’s)
were above 368 C as shown in Table 1.
When shearing was applied to these samples in their LC
states, a banded texture formed, with the shear direction
parallel to the normal of the bands [Fig. 2(c), taking
pVBP(10,8) as an example]. The banded texture is usually
regarded as the characteristic texture for rod-like molecules,
d
ꢁ
4
0
such as main-chain liquid-crystalline polymers. In addition,
the banded texture remained unchanged upon the isothermal
process. This confirmed again that structurally side-chain-
type MJLCPs displayed some properties represented by rigid
Phase Transitions and Phase Structures of the
Monomers and Polymers
All the monomers were light yellow liquid at ambient tem-
perature, and no mesophases were detected in PLM experi-
ments at higher temperatures.
6
or semirigid MCLCPs.
The phase transitions of the polymers were further verified
by variable-temperature, 1D WAXD experiments. Sets of 1D
WAXD profiles of the as-cast sample of pVBP(1,8) recorded
during the first heating and subsequent cooling processes
are presented in Figure 3. The as-cast amorphous sample
demonstrated two scattering halos in a low-angle region of
The phase transitions of the polymers were first examined
by DSC experiments. To eliminate the complex thermal his-
tory of the samples generated during solvent evaporation
and drying, the second DSC heating traces were used to
determine the Tg values. All the DSC curves showed only
glass transitions (listed in Table 1), and no other transition
processes were observed, as found for most other MJLCPs
ꢁ
ꢁ
2ꢀ7.5 and a high-angle region of 15ꢀ25 at low tempera-
tures. The scattering halo in the low-angle region increased
ꢁ
ꢁ
FIGURE 2 Polarized light microscope images of solution-cast films of pVBP(2,8) taken at 230 C (a), pVBP(8,8) taken at 210 C (b),
ꢁ
and mechanically sheared solution-cast film of pVBP(10,8) taken at 250 C (c).
SYNTHESIS AND PROPERTIES OF MJLCPs, ZHANG ET AL.
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JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
the d-spacing of the low-angle peak/halo with respect to
temperature during the first cooling and the second heating
for pVBP(2,8) on the basis of the results in parts a and b of
FIGURE 3 1D WAXD powder patterns of pVBP(1,8) during heat-
ing (a) and cooling (b) processes under a nitrogen atmosphere.
[
Color figure can be viewed in the online issue, which is avail-
able at wileyonlinelibrary.com.]
in intensity with increasing temperature, but remained as a
broad scattering for the entire temperature region. Combin-
ing with the PLM results, we concluded that this polymer
was not liquid crystalline.
Parts a and b of Figure 4 describe the structurally sensitive
1D WAXD patterns of pVBP(2,8). In order to eliminate ther-
mal history, we collected the data of the first cooling and the
subsequent, second heating. Upon the first cooling, as shown
in Figure 4(a), the intensity of the low-angle peak/halo
decreased, and the peak/halo became broader. And a reverse
trend was observed during heating, as shown in Figure 4(b).
Due to the lack of sharp change in either the intensity or the
breadth of this low-angle peak/halo, it was difficult to judge
from these two figures whether there were phase transitions
during cooling and heating. Figure 4(c) shows the change in
FIGURE 4 1D WAXD powder patterns of pVBP(2,8) during the first
cooling (a) and the second heating (b) processes under a nitrogen
atmosphere and the d-spacing of the low-angle peak/halo as a
function of temperature (c) during the first cooling and the second
heating as shown in (a) and (b). [Color figure can be viewed in the
online issue, which is available at wileyonlinelibrary.com.]
3212
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ARTICLE
Figure 4. For the first cooling curve, the first inflection at
ꢁ
around 100 C could be attributed to the glass transition, a
second-order-like transition. The d-spacing remained almost
ꢁ
unchanged until a sharp jump occurred at about 150 C, fol-
lowed by a plateau, which indicated a first-order transition.
For the second heating curve, similar changes were also
ꢁ
observed, although the jump at about 175 C was less promi-
nent than that in the first cooling curve, indicating an enan-
tiotropic phase behavior of pVBP(2,8). Due to the lack of
higher-order diffractions, the more ordered high-temperature
phase could be a low-ordered LC phase. The structural char-
acteristics (the jacketed structure and the simulated length,
2.10 nm, of the bulky side group being not far away from
the d-spacing, 1.81 nm, of the low-angle peak at high tem-
peratures) of the polymer and the schlieren-like texture
shown in Figure 2(a) indicated that the LC phase was prob-
ably a U . Similar to the cases of typical MJLCPs, the low-
N
angle peak corresponded to the columnar diameter, which
was determined mainly by the length of the side-chain meso-
gen. Furthermore, because the LC phase formed at higher
temperatures transformed into the amorphous phase during
cooling, this polymer showed a re-entrant phase behavior,
which could be due to entropic reasons, similar to some
FIGURE 5 2D WAXD patterns of a sheared pVBP(2,8) sample
recorded with the X-ray incident beam along Z (a), Y (b), and X
9
ꢁ
other MJLCPs reported previously.
(c) directions at 200 C; the shearing geometry (d). [Color figure
can be viewed in the online issue, which is available at
wileyonlinelibrary.com.]
Because the 1D WAXD patterns lack dimensionality, we
employed 2D WAXD to identify the phase structure of the
sample pVBP(2,8). The sample was oriented by mechanical
shearing at 160 C during cooling from higher temperatures
and annealed for a short time. Due to the enantiotropic na-
ture of the LC phase, we conducted variable-temperature, 2D
ꢁ
sity of the scattering halo increased slightly in Figure 6(a).
ꢁ
When the temperature reached 160 C which was much
higher than the T of the polymer, a sharp diffraction peak at
g
ꢁ
WAXD experiments. Parts a, b, and c of Figure 5 shows 2D
a 2h of 4.72 (d ¼ 1.87 nm) developed, indicating the exis-
ꢁ
WAXD patterns of the oriented, annealed sample at 200
C
tence of an ordered phase, possibly an LC phase. Combining
with the PLM results, we could conclude that it was truly an
LC phase. On the other hand, the scattering halo in the wide-
angle region remained as a halo which moved to higher
angles with increasing temperature due to thermal expan-
sion. No higher-order diffractions of the low-angle peak
could be observed in the 1D WAXD experiments, even when
we attempted with more sample amounts or longer exposure
times. Upon cooling, the sharp peak which originated from
through three orthogonal directions. The patterns in parts a
and b of Figure 5 were virtually identical when the shearing
direction was along the equatorial direction. A pair of sharp
arcs could be observed in the low-angle region on the merid-
ian, indicating the existence of ordered structures on the
nanometer scale with lattice planes oriented primarily paral-
lel to the equatorial direction. In addition, the scattering halo
ꢁ
at a 2h of about 20 in the high-angle region was more or
ꢁ
less concentrated on the equator with rather broad azi-
muthal distributions, indicative of subnanometer structures
with a short-range order existing mainly along the meridian
direction. On the other hand, a sharp low-angle ring and a
diffuse ring-like high-angle halo are shown in Figure 5(c)
with the X-ray beam parallel to the shearing direction, dem-
onstrating a rotational isotropy with the shear direction as
the axis. These three patterns served as additional evidence
to support that the high-temperature LC phase of pVBP(2,8)
was a UN.
the LC phase disappeared at temperatures below 120 C,
and the low-angle scattering halo reappeared [Fig. 6(b)],
which suggested that the ordered structure formed at high
temperatures transformed into the low-temperature amor-
phous phase. Therefore, pVBP(8,8) also exhibited an enantio-
tropic, re-entrant phase behavior.
ꢁ
1
D WAXD patterns of all the pVBP(m,8) polymers at 250 C
are presented in Figure 7 and the corresponding d-spacing
values of the low-angle peak/halo are summarized in Table
2. Table 2 also lists the calculated lengths of the side groups
For the polymers pVBP(m,8) (m ¼ 4, 6, 8, 10, 12), their 1D
WAXD patterns were similar. With pVBP(8,8) as an example,
parts a and b of Figure 6 show the 1D WAXD patterns of the
sample during the first heating and the subsequent cooling.
The isotropic sample prepared by solution-casting from a
THF solution rendered an amorphous halo in the low-angle
region at low temperatures. Upon the first heating, the inten-
of the polymers along with the temperatures at which the
LC phases started to develop. As shown in Figure 7, only an
amorphous halo could be observed in the high-angle region
for all the polymers. In the low-angle region, the pattern of
pVBP(1,8) showed
a scattering halo. The pattern of
pVBP(2,8) exhibited a relatively broad diffraction peak, and
those of the other pVBP(m,8) (m ¼ 4, 6, 8, 10, 12) samples
SYNTHESIS AND PROPERTIES OF MJLCPs, ZHANG ET AL.
3213

JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
FIGURE 7 Set of 1D WAXD patterns of all the polymers
ꢁ
recorded at 250 C. [Color figure can be viewed in the online
issue, which is available at wileyonlinelibrary.com.]
The phase transition temperatures, obtained from 1D WAXD
measurement, of all the polymers are summarized in Table
2
. They were strongly dependent on the difference in lengths
0
of alkyls on the 4 and 4 positions of biphenyl in the meso-
gen. For the pVBP(m,8) (m ¼ 2, 4, 6, 8, 10, 12) polymers,
the transition temperature was the lowest for pVBP(8,8),
and, with the exception of pVBP(2,8), it generally increased
with the increase or decrease of the alkyl ester tail, which
indicated that the smaller the difference in the lengths of the
terminal alkyl groups, the easier the formation of the liquid
crystalline phase. The terminal alkyl groups in the mesogens
of MJLCPs reported previously were the same, and this phe-
nomenon has not been reported before.
FIGURE 6 1D WAXD powder patterns of pVBP(8,8) during the
first heating (a) and the first cooling (b) processes under a
nitrogen atmosphere. [Color figure can be viewed in the online
issue, which is available at wileyonlinelibrary.com.]
Variable-temperature, 2D WAXD experiments were carried
out to further identify the phase structures of pVBP(m,8) (m
¼
4, 6, 8, 10, 12) samples. These experiments demonstrated
different 2D WAXD patterns, although the phase transition
had a sharp diffraction peak, revealing the presence of or-
dered structures, possibly columnar phases for these jack-
eted polymers. The d-spacing values of the low-angle peak/
halo for the polymers were comparable to the calculated
lengths of the side chains under the assumption that the n-
alkyl tails were in an all-trans conformation, indicating that
the low-angle peak/halo was associated with the size of the
side groups, which affected the phase structure. On the other
hand, the d-spacing values were all smaller than the calcu-
lated side-chain lengths, indicating that the n-alkyl tails
adopted a partially interdigitated packing. The more the dif-
ference between the experimental d-spacing value and the
calculated side-group length for each polymer, the more
degree of interdigitation. For the LC pVBP(m,8) (m ¼ 2, 4, 6,
TABLE 2 d-Spacing Values, Calculated Side-Group Lengths,
and LC-Forming Temperatures of the pVBP(m,8) Polymers
a
b
ꢁ
c
Sample
d (nm)
L (nm)
T
LC ( C)
pVBP(1,8)
pVBP(2,8)
pVBP(4,8)
pVBP(6,8)
pVBP(8,8)
pVBP(10,8)
pVBP(12,8)
a
1.86
1.81
1.75
1.83
1.92
2.00
2.05
ꢁ
1.96
2.10
2.22
2.36
2.52
2.66
2.82
–
180
250
180
160
200
210
d-Spacing values at 250 C obtained by 1D WAXD.
b
Calculated length of the side groups from the ether end to the ester
8
, 10, 12), when m increased, this difference became larger,
end with the alkyl chains in all-trans conformation.
suggesting that the alkyl tails were packed in a more interdi-
gitated manner with increasing m.
c
Temperature at which the LC phase was formed on the basis of 1D
WAXD experiments.
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ARTICLE
like LCPs from ionic complexation of dendronized polymers
4
1,42
and lipids
have been reported to self-organize into
tetragonal columnar mesophases, phases more ordered than
UTN, by tuning the generation and lipid chain length. Upon
cooling, this UTN phase disappeared, and the sample became
amorphous, in agreement with the 1D WAXD results, which
confirmed that pVBP(8,8) also exhibited a re-entrant phase
behavior.
Dependence of the Phase Structure on the
Difference of Alkyl-Tail Lengths
In this series of pVBP(m,8) polymers, two different types of
LC phases and three types of phase transitions were
observed, depending on the length of the terminal alkyl ester
group, as shown in Figure 10. The polymer with the shortest
methyl ester group was not liquid crystalline probably
because the two ends on the side-chain biphenyl were so
imbalanced that the polymer could not form a rod-like
supramolecular mesogen. The longer alkyl tail in the ester
side with increasing m increased the possibility of the forma-
tion of the supramolecular mesogen, and the pVBP(m,8)
polymers could form liquid crystalline phases. They formed
UN phases when m was different from the number of carbon
FIGURE 8 2D WAXD patterns of a sheared pVBP(6,8) sample
recorded with the X-ray incident beam along Z (a), Y (b), and X
ꢁ
(
c) directions at 200 C; the shearing geometry (d). [Color figure
can be viewed in the online issue, which is available at
wileyonlinelibrary.com.]
behaviors and textures of the aforementioned polymers from
1D WAXD and PLM experiments appeared to be similar.
A film sample of pVBP(6,8) was mechanically sheared at 145
ꢁ
C and annealed for several hours. Parts a, b, and c of Figure
8
show the 2D WAXD patterns with the X-ray incident beam
perpendicular (along Y or Z direction, with the X direction as
the equatorial direction) and parallel (along X direction) to
ꢁ
the shear direction at 200 C. The similarity of these
patterns to those in Figure 5 suggested that the pVBP(6,8)
sample also developed a UN phase at high temperatures.
Similar patterns were also observed with other pVBP(m,8)
(
m ¼ 4, 10, 12) polymers, indicative of UN phases for these
polymers.
In contrast, the 2D WAXD patterns (Fig. 9) of a pVBP(8,8)
ꢁ
film sample mechanically sheared at 135 C and annealed
for several hours were different from those of pVBP(2,8) and
pVBP(6,8); in particular, the difference was in the pattern
obtained with the X-ray incident beam along the shear direc-
tion. Parts a and b of Figure 9 was similar to Figures 5(a,b)
and 8(a,b). On the other hand, Figure 9c shows an interest-
ing pattern. The low-angle diffraction exhibited an ‘‘X’’ pat-
tern. The azimuthal scan of the low-angle diffraction arcs
o
displayed four maxima which were separated by 90 from
the adjacent ones [Fig. 9(d)], indicating a fourfold symmetry.
There were no higher-order diffractions discernible in parts
a, b, and c of Figure 9. Therefore, the pVBP(8,8) sample
developed a columnar nematic phase with a local fourfold
symmetry. Such a mesophase could be termed as a tetrago-
nal columnar nematic (UTN) phase, which, to the best of our
knowledge, has not been reported before for MJLCPs. Comb-
FIGURE 9 2D WAXD patterns of a sheared pVBP(8,8) sample
recorded with the X-ray incident beam along Z (a), Y (b), and X
ꢁ
(c) directions at 180 C; azimuthal scan (d) of (c); shearing
geometry (e). [Color figure can be viewed in the online issue,
which is available at wileyonlinelibrary.com.]
SYNTHESIS AND PROPERTIES OF MJLCPs, ZHANG ET AL.
3215

JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
FIGURE 10 Schematic drawing of the thermotropic phase behavior of the polymers. [Color figure can be viewed in the online
issue, which is available at wileyonlinelibrary.com.]
atoms in the alkyl ether tail (m ¼ 2, 4, 6, 10, 12). The more
symmetric sample pVBP(8,8) formed a more ordered liquid
crystalline phase, UTN, a columnar nematic phase with a
local square lattice (a ¼ 1.92 nm).
ent on the length of the alkyl ester end group, but the trend
3
5
was opposite to that in our previous report. The discrep-
ancy could be attributed to the fact that the birefringence
value was mainly determined by the effective orientation of
the rigid side chains. The improved orientation of the rigid
side chains resulted in higher birefringence values with
increasing m, although the underlying reason could not be
determined. Our values were still lower than 0.02, a value
regarded as viable for industrial applications, but by adjust-
ing the length of terminal alkyl ester groups or by adding
cyano or isothiocyanate functional groups to extend the
length of the rigid side chain, we might obtain polymer films
with higher birefringence values exceeding 0.02 without los-
ing the excellent film-forming ability. Thus, we might utilize
these films as optical compensation films. Related work is
currently in progress.
Optical Anisotropies of the Polymer Films
The polymers obtained in this work were easy to be proc-
essed into flexible freestanding films without the tendency of
breaking or cracking (see Supporting Information, Fig. S2).
The monomer precursor, mVBP(0,8), showed moderate Dn
value of 0.2113, while the films of some of these polymers
that we prepared possessed relatively high positive birefrin-
gence values (Table 3). The birefringence value was depend-
TABLE 3 Refractive Indices and Birefringence Values of Some
a
Polymer Films
a
Polymer
Structure
n
//
n
?
Dn
CONCLUSIONS
pVBP(2,8)
1.5613 1.5684 0.0071
In summary, we successfully synthesized a novel series of
0
MJLCPs of poly(alkyl 4 -(octyloxy)-2-vinylbiphenyl-4-carboxy-
late) pVBP(m,8) (m ¼ 1, 2, 4, 6, 8, 10, 12), with biphenyl-
based side chains containing alkyl groups of different lengths
in the two ends of biphenyl via conventional free radical
polymerization. All the polymers were thermally stable, and
pVBP(4,8)
1.5523 1.5622 0.0099
1.5368 1.5516 0.0148
pVBP(10,8)
their T values decreased with increasing tail length due to
g
increased internal plasticization. While pVBP(1,8) showed no
liquid crystallinity, other pVBP(m,8) polymers (m ¼ 2, 4, 6,
8
, 10, 12) all exhibited columnar phases at high tempera-
a
39
Measured according to the procedure described in literature.
tures, and the LC phases transformed into the amorphous
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WILEYONLINELIBRARY.COM/JOURNAL/JPOLA

ARTICLE
state upon cooling. The asymmetric substitution of the tails
in the side-chain biphenyl affected the phase structures of
these MJLCPs. The more symmetric polymer pVBP(8,8)
developed a more ordered columnar phase (UTN), while
other LC pVBP(m,8) polymers formed UN phases. Further-
more, the smaller the difference in the lengths of the alkyl
tails in the side chains, the easier the formation of the LC
phase. The polymers possessed good film-forming ability,
and the films we tested had relatively high positive birefrin-
gence values, indicating their potential applications as optical
compensation films in LCDs.
17 Fernandez-Blazquez, J. P.; Bello, A.; Perez, E. Macromole-
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SYNTHESIS AND PROPERTIES OF MJLCPs, ZHANG ET AL.
3217