Influence of different linkage groups in biphenyl mesogenic core on phase behaviors of mesogen-jacketed liquid crystalline polymers

Article abstract of DOI:10.1002/pola.26662

The work focuses on the design, synthesis, and characterization of a series of mesogen-jacketed liquid crystalline polymers (MJLCPs) based on the octyl substituted biphenyl mesogenic core through different linkage groups. The molecular characterizations o

Full text of DOI:10.1002/pola.26662

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Influence of Different Linkage Groups in Biphenyl Mesogenic Core on
Phase Behaviors of Mesogen-Jacketed Liquid Crystalline Polymers
Lan-Ying Zhang, Yang-Fei Zhang
Department of Materials Science and Engineering, College of Engineering, Peking University, Beijing 100871, China
Correspondence to: Y.-F. Zhang (E-mail: zhangyangfei@pku.edu.cn)
Received 6 December 2012; accepted 22 February 2013; published online 29 March 2013
DOI: 10.1002/pola.26662
ABSTRACT: The work focuses on the design, synthesis, and
characterization of a series of mesogen-jacketed liquid crystal-
line polymers (MJLCPs) based on the octyl substituted
biphenyl mesogenic core through different linkage groups. The
molecular characterizations of the polymers obtained by con-
ventional free radical polymerization were performed with
1H NMR, gel permeation chromatography, and thermogravi-
metric analysis. Their thermotropic liquid crystalline (LC)
behaviors were investigated in detail by a combination of vari-
ous techniques, such as polarized light microscopy, differential
scanning calorimetry, and 1D and 2D wide-angle X-ray diffrac-
tion. Our results showed that all the polymers were thermally
stable, and their LC phases were greatly dependent on the link-
ing groups between the biphenyl mesogenic core and terminal
alkyl group substituent. Polymers with ether/ester or ether link-
age group exhibited an unusual phase behavior with tempera-
ture increasing, tetragonal columnar nematic LC phase, or
columnar nematic phase developed at high temperatures for
the polymers transformed into amorphous phase during cool-
ing process, showing a re-entrant phase behaviors. However,
polymers with ester linkage group were not LC with tempera-
ture varied. It is illustrated that subtle changes in the molecular
structure brought about tremendous variation of the LC phase
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properties for MJLCPs. 2013 Wiley Periodicals, Inc. J. Polym.
Sci., Part A: Polym. Chem. 2013, 51, 2545–2554
KEYWORDS: biphenyl core; different linkage group; glass
transition; liquid-crystalline polymers (LCP); phase behavior
In the past few decades, researchers in Zhou’s group have
focused on the design and synthesis of SCLCPs with the grav-
ity centers of mesogen units attached to the backbones via a
very short linkage or only a single covalent carbon–carbon
bond, named mesogen-jacketed LC polymers (MJLCPs).5,6 Dif-
ferent from Finkelmann’s principle, the bulky and rigid meso-
genic units directly linking to the polymer backbone force the
main chain of the MJLCPs to take an extended chain confor-
mation, thus the flexible spacers to decouple the dynamics of
main chain and side groups seemed not necessary.7 The con-
cept of MJLCPs has been confirmed by experimental results
via X-ray diffraction8 and small-angle neutron scattering.9,10
Since the first publication of MJLPCs, up to date, a rich vari-
ety of MJLCPs with different structures and unique properties
have been designed, synthesized, and investigated in detail.
Abundant phase structures such as columnar nematic (UN),5,6
hexagonal columnar (UH),11 hexatic columnar nematic
(UHN),12 rectangular columnar (UR),13 smectic A (SmA),14,15
and C (SmC)16 phase have been reported. Additionally,
adjusting the phase structures and phase behaviors of
MJLCPs by trying to change some variables such as the
molecular weight, rigidity or shape of the mesogens, terminal
INTRODUCTION The relationships between the molecular struc-
tures and phase structures or transitions of thermotropic
liquid-crystalline polymers (LCPs) have been one of the most
important topics for the fundamental research in polymer
chemistry and physics for their applications in the past sev-
eral decades.1 Through designed synthetic strategies, rigid
mesogenic groups (rod, disks, or bent-core) can be incorpo-
rated in LCPs and results in several different molecular
architectures combining with various mesomorphic struc-
tures and phase behaviors. According to the location of the
mesogenic groups in polymers, LCPs can be mainly classified
into two categories: main-chain LCPs with mesogenic units
located in the main chain and side-chain LCPs (SCLCPs) with
mesogens as side groups.2 For both of these two types of
LCPs, to promote the formation of liquid crystalline (LC)
phase and improve the mobility of the mesogenic groups,
flexible spacers were often introduced into their structures.3
Moreover, in the SCLCPs with mesogenic groups either later-
ally or terminally attached to the backbones, the insertion of
flexible spacers with reasonable length are needed to decou-
ple the motions between the main chain and the mesogenic
side groups based on Finkelmann’s principle.4
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acid (99.6%, Alfa Aesar), triphenylphosphine (99%, Alfa Aesar),
40% formaldehyde (AR, Beijing Yi Li Chemical), N,N0-dicyclo-
hexyl carbodiimide (95%, Sinopharm Chemical Reagent),
4-dimethylaminopyridine (99%, ACRO), and N-bromosuccini-
mide (NBS, 99%, Aldrich) were used as received without further
purification. Azobisisobutyronitrile (AIBN) was recrystallized
from ethanol before use. Benzoyl peroxide (BPO) was recrystal-
lized from chloroform and methanol. The catalyst Pd(PPh3)4
was synthesized according to the procedure in the literature.38
The compound of 2(3)-vinylbiphenyl-4,40-dicarboxylic acid was
synthesized using the method previously reported.39 Chloro-
benzene was washed with H2SO4, NaHCO3, and distilled
water and then distilled from calcium hydride. Tetrahydrofu-
ran (THF, AR, Beijing Chemical) was refluxed over sodium
and distilled before use. Dichloromethane (AR, Beijing Chem-
ical) was dried over magnesium sulfate anhydrous. All other
reagents and solvents were used as received from commer-
cial sources.
SCHEME 1 Chemical structures of the designed monomers
Equipments and Measurements
based on biphenyl mesogenic core with different linkage groups.
1H/13C NMR spectra, mass spectra (MS), gel permeation
chromatographic (GPC) measurements, thermogravimetric
analyses (TGAs), differential scanning calorimetry (DSC),
polarized light microscopy (PLM), and wide-angle X-ray dif-
fraction (WAXD) (1D and 2D) experiments were performed
according to the procedures described previously.37
alkyl length, and the molecular architectures have also been
widely investigated, and some interesting results were found.
Other work about jacketed polymers from V. Percec labora-
tory was also widely investigated and reported. A variety of
polymers where the backbones were jacketed with function-
ally modified bulky dendritic or taper-shaped side groups
were designed and synthesized,17–22 and interesting self-
assembly structures23–32 and unusual molecular conforma-
tions33,34 were discovered and descripted in detail. Reviews
on the self-organization of polymers jacketed with self-assem-
bling dendrons are also available.35,36
Synthesis of the Monomers
The synthetic routes of the monomers are shown in Scheme
2, the experimental details are described as follows.
Synthesis of Mono-1
The experimental details of mono-1 synthesis are in accordance
with the literature.37 1H NMR (d, ppm, CDCl3): 0.86920.910 (m,
6H, terminal CH3), 1.25521.357, 1.45921.478, 1.77021.807
(m, 24H, CH2), 3.98424.017 (t, 2H, OCH2), 4.32724.460 (t, 2H,
COOCH2), 5.24625.274, 5.77525.818 (d, 2H, @CH2),
6.70126.773 (q, 1H, CH@), 6.94126.963 (m, 2H, ArAH),
7.25527.282 (m, 2H, ArAH), 7.32927.349 (m, 1H, ArAH),
7.93927.964 (m, 1H, ArAH), 8.28728.291 (d, 1H, ArAH). 13C
NMR (d, ppm, CDCl3): 166.449, 158.736, 144.670, 135.864,
135.372, 131.817, 130.643, 130.049, 129.080, 128.333,
127.207, 115.446, 114.035, 67.918, 65.053, 31.762, 31.731,
29.314, 29.219, 29.199, 29.151, 28.689, 26.018, 25.988, 22.600,
22.586, 14.027. MS: 464 (m/e).
In our previous article,37 we have reported the design, synthe-
sis, and characterization of a series of MJLCPs based on biphenyl
mesogen with asymmetric alkyl substitutions, poly(alkyl
40-(octyloxy)22-vinylbiphenyl-4-carboxylate)
(pVBP(m,8),
m 5 1, 2, 4, 6, 8, 10, and 12) in detail. Unusual LC phase behav-
iors and columnar LC phase structure were found for most of
the polymers, and their phase transformations and phase struc-
tures were greatly dependent on the relative length of the asym-
metric terminal alkyl substitutions. Here, in this article, based
on the above research work studied, we tried to change another
variable—the linkage groups in the mesogenic core so as to fur-
ther investigate the relationship between a single structural
change of the monomers and the variable in the phase behaviors
of MJLCPs. The monomer structures designed are shown in
Scheme 1, we simply chose the commonly used linkage units
such as ether and ester groups between the biphenyl mesogenic
core and the terminal flexible alkyl substituent. To the best of
our knowledge, the first time that the linkage group as a struc-
tural variable was studied in MJLCPs.
Synthesis of 1-Bromo-2-methyl-4-(octyloxy)benzene
4-Bromo-3-methylphenol (5.3 g, 0.0283 mol), K2CO3 (7.821
g, 0.0567 mol), KI (catalytic amount), and acetonitrile (160
mL) were charged in a 250-mL three-necked flask. Then, a
solution of 1-bromo-octane (6.021 g, 0.0312 mol) in acetoni-
trile (10 mL) was added dropwise, and the mixture was
refluxed overnight. Thin-layer chromatography monitored
the reaction process until the raw material was completely
consumed. The mixture was cooled to ambient temperature
and filtrated to remove the floating substance. After it was
filtered, the filtrate was concentrated by a rotary evaporator
to give the crude product. The purified product was obtained
by silica gel column chromatography with petroleum ether
EXPERIMENTAL
Materials
Methyl 4-bromo-3-methylbenzoate (98%, Alfa Aesar), 4-bromo-
3-methylphenol (98%, Alfa Aesar), 4-(octyloxy)phenylboronic
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SCHEME 2 Synthetic routes of the monomers.
as the eluent to yield 8.270 g (98%). 1H NMR (300 MHz,
d, ppm, CDCl3): 7.36027.389 (d, 1H, ArAH), 6.77526.785
(d, 1H, ArAH), 6.54726.617 (m, 1H, ArAH), 3.87723.921
(t, 2H, AOCH2), 2.33422.350 (s, 3H, ACH3), 1.72921.779,
1.28621.434, 0.86220.907 (m, 15H, aliphatic chain). MS
m/z (M1): 298.
necked flask under an argon atmosphere. Then, 160 mL of
acetonitrile/water (3/1) mixture was added and refluxed at
80ꢀC for 40 h. The mixture was cooled to ambient tempera-
ture and filtrated to remove the floating substance. After
evaporation of the solvent, a colorless liquid was obtained
by passing through a silica gel column with dichloromethane
and petroleum ether (v:v, 1:12) as eluent to yield 6.436 g
(91.8%). 1H NMR (300 MHz, d, ppm, CDCl3): 7.21027.234
(m, 2H, ArAH), 7.12027.188 (d, 1H, ArAH), 6.89626.925
(m, 3H, ArAH), 6.79326.802(m, 1H, ArAH), 3.94223.999(m,
4H, AOCH2), 2.245 (s, 1H, AArCH3), 1.76221.823,
1.29621.515, 0.83120.956 (m, 30H, aliphatic chain). MS
m/z (M1): 424.
Synthesis of 2-Methyl-4,4’-bis(octyloxy)biphenyl
In this step, a typical Suzuki cross-coupling reaction was
used. 1-Bromo-2-methyl-4-(octyloxy)benzene (4.520 g, 0.015
mol), 4-(octyloxy)phenylboronic acid (4.168 g, 0.017 mol),
Pd(PPh3)4 (0.062 g, 0.546 mmol), and potassium carbonate
(K2CO3, 4.183 g, 0.030 mol) were mixed in a 250-mL three-
SCHEME 3 Chemical structures and the synthesis of the polymers.
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stirred for 48 h at room temperature. After the floating sub-
stance was filtrated off, the filtrate was washed by dichloro-
methane for several times. Combined the organic phase,
evaporated and purified by column chromatography with pe-
troleum and dichloromethane (v:v, 2:1) as the eluent, a col-
1
orless oily liquid product was obtained. Yield: 65%. H NMR
(300 MHz, d, ppm, CDCl3): 7.22827.253 (m, 3H, ArAH),
7.20627.221 (m, 1H, ArAH), 7.17027.191 (m, 2H, ArAH),
7.13527.142 (m, 1H, ArAH), 6.86926.926 (m, 1H, @CH),
5.64825.695 (d, 1H, @CHH), 5.16025.190 (d, 1H, @CHH),
3.96924.026 (m, 4H, AOCH2), 1.78421.826, 1.46021.492,
1.29721.355, 0.87520.909 (m, 30H, aliphatic chain). 13C
NMR (d, ppm, CDCl3): 163.315, 158.291, 138.611, 136.098,
131.003, 129.711, 128.314, 122.614, 114.911, 114.113,
111.112, 68.790, 31.897, 29.612, 29.311, 25.899, 22.703,
14.111. MS m/z (M1): 436.
Synthesis of Mono-3
The synthesis of mono-3 used the same method as described
for the preparation of mono-1. Yield: 45%. 1H NMR (400
MHz, d, ppm, CDCl3): 8.328 (s, 1H, ArAH), 8.12228.317 (m,
2H, ArAH), 8.09028.101 (m, 1H, ArAH), 7.45727.526 (m,
2H, ArAH), 7.26627.423 (m, 1H, ArAH), 6.63726.659 (m,
1H, @CH), 5.79125.853 (d, 1H, @CHH), 5.27025.310 (d, 1H,
@CHH), 4.32724.381 (m, 4H, AOCH2), 1.74321.813,
1.17721.669, 0.86620.910 (m, 30H, aliphatic chain). 13C
NMR (d, ppm, CDCl3): 165.809, 146.605, 143.735, 138.587,
135.401, 130.100, 127.713, 127.100, 126.789, 126.310,
114.239, 65.210, 31.910, 31.713, 29.234, 25.809, 22.713,
14.098. MS m/z (M1): 492.
Synthesis of Mono-4
The synthesis of mono-1 used the same method as described
for the preparation of mono-3. Yield: 48%. 1H NMR (400
MHz, d, ppm, CDCl3): 8.12728.148 (d, 2H, ArAH),
7.98328.003 (d, 1H, ArAH), 7.79427.798 (s, 1H, ArAH),
7.68527.707 (m, 2H, ArAH), 7.50727.579 (d, 2H, ArAH,
@CH), 5.70025.747 (d, 1H, @CHH), 5.39825.428 (d, 1H,
@CHH), 4.31224.363 (m, 4H, AOCH2), 1.76021.808,
1.43921.473, 1.29321.380, 0.87220.906 (m, 30H, aliphatic
FIGURE 1 1H (a) and 13C (b) NMR spectra of mono-1 and 1H
TABLE 1 Molecular Weights, Polydispersity Indexes, 5%
Weight Loss Temperatures, Glass Transition Temperatures,
and Liquid Crystallinity of the Polymers
NMR spectrum of poly-1 (c).
Synthesis of Mono-2
Mn (3104
Tg
Liquid
Samples of methyl 4-bromo-3-methylbenzoate (5.907 g,
0.014 mol), NBS (3.714 g, 0.021 mol), and BPO (0.067 g,
0.277 mmol) were dissolved in CCl4 (160 mL) and refluxed
for 6 h. The floating succinimide was filtrated off, and the
solvent was evaporated. Then, the residue was boiled with
triphenylphosphine (3.648 g, 0.014 mol) in acetone (180
mL) overnight, and the obtained phosphonium salt was puri-
fied by silica gel column chromatography with dichlorome-
thane and methanol, respectively.
Sample
g/mol)a
PDIa
Td (ꢀC)b
(ꢀC)c
Crystallinityd
Poly-137
Poly-2
Poly-3
Poly-4
9.2
9.7
7.8
16.6
1.46
1.78
1.87
1.97
369
378
387
394
64
68
75
78
Yes
Yes
No
No
a
The apparent Mn and PDI values were measured by GPC using PS
standards.
b
The temperatures at 5% weight loss of the samples under nitrogen
were measured by TGA heating experiments at a rate of 20ꢀC/min.
Then, 5 M NaOH aqueous solution (100 mL) was added
slowly to formaldehyde (40%, 200 mL) containing (10.06 g,
0.0146 mmol) of the phosphonium salt. The mixture was
c
The glass transition temperatures of the samples under a nitrogen
atmosphere were measured by DSC at a scanning rate of 10ꢀC/min.
Determined by the combination of PLM and 1-D WAXD observations.
d
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FIGURE 2 Typical polarized light microscope images of solution-cast films of poly-1 taken at 180ꢀC (a) and poly-2 taken at 220ꢀC
(b).
chain). 13C NMR (d, ppm, CDCl3): 167.891, 165.345, 146.615,
145.801, 145.103, 138.218, 134.541, 130.312, 127.819,
127.098, 127.001, 114.312, 64.819, 31.901, 31.718, 29.314,
25.819, 22.713, 14.123. MS m/z (M1): 492 (m/e).
polymerization was stopped by dipping the tube in ice/
water. After the tube was broken, the solution was diluted
with THF (10 mL) and then added dropwise to vigorously
stirred methanol (300 mL). To eliminate the unreacted
monomer completely, we repeated the precipitation process
three times. After purification, the polymer was dried under
vacuum for 2 d.
Polymerization
As shown in Scheme 3, the polymers were obtained by typi-
cal conventional free-radical polymerization in solution. Tak-
ing the polymerization of poly-2 as an example, the detailed
procedure was carried out as follows. About 0.538 g of
mono-2, 15 lL of 0.106 M AIBN/chlorobenzene solution, and
1.244 g of chlorobenzene were transferred into a polymer-
ization tube with a magnetic stir bar. The reaction mixture
was purged with nitrogen and subjected to three freeze–
pump–thaw cycles and sealed under vacuum. Polymerization
was performed at 60ꢀC for 48 h. Subsequently, the
RESULTS AND DISCUSSION
Synthesis and Characterizations of the Monomers and
Polymers
As shown in Scheme 2, the monomers could be successfully
prepared through a few simple and effective reaction such as
typical Witting reaction, Suzuki coupling reaction, etherifica-
tion, and esterification reaction. The chemical structures of
FIGURE 3 1D WAXD powder patterns of poly-2 during the first cooling (a) and the second heating (b) process under a nitrogen
atmosphere.
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The phase behaviors of all the polymers were investigated
by DSC, PLM, and WAXD. Their thermal properties are listed
in Table 1. First, DSC experiments were conducted. With the
consideration of the effect of thermal history generated dur-
ing solvent evaporation and drying, the second heating
traces were used to determine the Tg values, all the DSC pro-
files showed only glass transitions (the data are listed in
Table 1), and no other transition processes were observed,
as found for most other MJLCPs reported previously.16,37 The
glass transition temperatures of all the polymers were rela-
tively low compared to most other MJLCPs synthesized pre-
viously that have large molecules rigidity, we believe that
the relatively weak rigid biphenyl mesogenic core and the
long flexible terminal alkyl contribute to the result. In addi-
tion, the glass transition temperatures of the polymers with
ester linkage group are about 10ꢀC higher than that of the
polymers with ether linkage group, which is in accordance
with the higher polarity of the ester groups. Due to the
insensitivity of the DSC method to determine the phase tran-
sitions of these polymers except for glass transitions, PLM
and WAXD techniques were utilized to further investigate
their LC phase behavior.
FIGURE 4 2D WAXD patterns of
a sheared poly-2 sample
recorded with the X-ray incident beam along Z (a), Y (b), and X
(c) directions at 200ꢀC; the shearing geometry (d).
the monomers were confirmed by 1H/13C NMR and mass
spectrometry.
Figure 1 shows the 1H (a) and 13C (b) NMR spectra of
mono-1 and the 1H NMR spectrum (c) of the corresponding
polymer poly-1. The mono-1 displays the representative
resonances of the vinyl group at 5.24625.818 and
6.70126.773 ppm. After polymerization, the signals disap-
peared completely and the chemical shifts of the polymer
became quite broad, indicating the successful polymerization.
Other monomers and polymers were characterized in a simi-
lar manner, and the results were in accordance with the
desired structures.
All the polymers were obtained with high yield by conven-
tional free radical polymerization in chlorobenzene with
AIBN as an initiator. The polymers were easily soluble in
common organic solvents such as THF, acetone, chloroform,
dichloromethane, and chlorobenzene. Their preliminary
molecular characterizations are summarized in Table 1. GPC
analysis of the polymers shows that the apparent molecular
weights are about 8 3 104 g/mol or more, with molecular
distribution below 2, confirming their polymeric nature. TGA
results shows that all the polymers were quite thermally sta-
ble under nitrogen atmosphere and their 5% weight loss
temperatures (Td’s) were above 369ꢀC as shown in Table 1.
Phase Transitions and Phase Structures of the
Monomers and Polymers
Similar to mono-1, all other monomers were also pale yellow
liquid at ambient temperature, and no mesophases were
observed in any of the monomers in PLM experiments at
higher temperatures.
FIGURE 5 1D WAXD powder patterns of poly-3 during the first
cooling (a) and the second heating (b) process under a nitro-
gen 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).
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Because the PLM textures were ambiguous for the phase
structure identification of the polymers, variable-temperature
1D and 2D WAXD experiments were performed to further
verify their phase transitions.
Poly-1
The phase transition and phase structure of poly-1 sample
has been detailed investigated and reported in our paper
previously, a more ordered tetragonal columnar nematic
(UTN) phase with a local square lattice (a 5 1.92 nm) devel-
oped at high temperatures, and the LC phase transformed
into the amorphous state upon cooling.37
Poly-2
The phase transition of as-cast poly-2 sample was evidenced
by variable-temperature 1D WAXD patterns. Sets of 1D
WAXD patterns of the sample obtained during the first heat-
ing and the subsequent cooling are shown in Figure 3(a,b),
respectively. The sample prepared by solution casting from a
THF solution was amorphous due to the fast solvent evapo-
ration that froze the chain motion. Upon the first heating in
Figure 3(a),
a
diffraction peak at the lower angle
(2h 5 227.5ꢀ) rapidly developed after 140ꢀC, which was
much higher than the Tg of the polymer, and the peak
slightly shifted to a higher angle, indicating the development
of an ordered phase, probably a LC phase. Combining with
the PLM results, we could conclude that it was truly a LC
phase. On the other hand, the scattering halo in the wide-
angle region remained as a halo which moved to lower
angles with increasing temperature possibly due to the close
molecular arrangement. No higher-order diffractions of the
low-angle peak could be observed in the 1D WAXD experi-
ments, even when we attempted with more sample amounts
or longer exposure times. Upon cooling, the sharp peak
which originated from the LC phase disappeared at tempera-
tures below 120ꢀC, and the low-angle scattering halo reap-
peared [as shown in Fig. 3(b)], which suggested that the
ordered structure formed at high temperatures transformed
into the low-temperature amorphous phase. Therefore, simi-
lar to the poly-1, poly-2 also exhibited an enantiotropic, re-
entrant phase behavior.
FIGURE 6 1D WAXD powder patterns of poly-4 during the first
cooling (a) and the second heating (b) process under a nitro-
gen 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).
Birefringence of the polymers was observed with PLM. The
polymers were cast from CDCl3 solution and dried at room
temperature. At ambient temperature, all the polymers
exhibited only very weak birefringence, probably due to the
existence of certain alignment of the polymer chains during
the sample preparation. Upon heating, the polymers behaved
quite differently depending on the different linkage groups
between the biphenyl and terminal alkyl in the side chain.
Poly-1, which had the ester linkage as one side and ether
linkage group as another, exhibited distinct birefringent tex-
ture [as shown in Fig. 2(a)] when the temperature was
much higher than Tg, as reported in our article previously.37
When cooled to room temperature from 250ꢀC, the birefrin-
gence of the sample remained unchanged. For poly-2 that
had ether bond as the linkage group, obvious birefringence
could be observed when the sample heated to a temperature
higher than Tg, but not a typical texture developed during
further heating process [as displayed in Fig. 2(b)]. While for
the poly-3 and poly-4 that used ester bond as the connecting
group, no discernible change was displayed even heated to
up to 250ꢀC, indicating probably no LC properties developed
during the whole process.
Owing to the dimensionality limit of 1D WAXD and the prop-
erties of the re-entrant phase behavior, varied-temperature
2D WAXD experiments were conducted with mechanically
sheared samples at 135ꢀC. Figure 4(a–c) shows 2D WAXD
patterns of the oriented, annealed sample 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 200ꢀC. The pat-
terns were virtually identical when the shearing direction
was along the equatorial direction [in Fig. 4(a,b)]. A pair of
sharp arcs could be observed in the low-angle region on the
meridian, indicating the existence of an ordered structure on
the nanometer scale with lattice planes oriented primarily
parallel to the equatorial direction. In addition, the scattering
halo at a 2h of about 20ꢀ in the high-angle region was more
or less concentrated on the equator with rather broad
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FIGURE 7 Schematic drawing of the whole phase evolution diagram for all the polymers at different temperatures.
azimuthal distributions, indicative of subnanometer struc-
tures with a short-range order existing mainly along the me-
ridian direction. On the other hand, a sharp low-angle ring
and a diffuse ring-like high-angle halo are shown in Figure
4(c) with the X-ray beam parallel to the shearing direction,
demonstrating a rotational isotropy with the shear direction
as the axis. The 2D WAXD patterns combining the results of
PLM and 1D WAXD support that the high-temperature LC
phase of poly-2 was a UN just like the polymers reported
previously.37
the quantitative changes in the d-spacing of the low-angle
peak/halo with respect to the temperature during the first
heating and first cooling for poly-3 on the basis of the
results in Figure 5(a,b). For the first heating curve, the
d-spacing increased a little with the increasing temperature,
the inflection point at about 802100ꢀC represents the glass
transition and no other sharp changes or jump occurred dur-
ing the whole process, indicating the as-cast sample poly-3 is
amorphous during the whole process40 combining the
results of PLM and 1D WAXD.
Poly-3
Poly-4
The phase transitions of the poly-3 were further verified by
variable-temperature, 1D WAXD experiments. Sets of 1D
WAXD profiles of the as-cast sample of poly-3 recorded dur-
ing the first heating and subsequent cooling processes are
presented in Figure 5. Upon the first heating, as shown in
Figure 5(a), the as-cast amorphous sample demonstrated
two scattering halos in a low-angle region of 227.5ꢀ and a
high-angle region of 15225ꢀ at low temperatures. The scat-
tering halo in the low-angle region increased a little in inten-
sity with increasing temperature, but still kept as a broad
scattering for the whole temperature region. As shown in
Figure 5(b), upon cooling, a reverse trend was observed. Due
to the lack of sharp changes in either the breadth of the
low-angle peak/halo or the intensity, it was very difficult to
judge from the figures whether there were phase transitions
during the heating and cooling process. Figure 5(c) presents
The phase behavior of the sample poly-4 was verified by 1D
WAXD experiment at various temperatures. Figure 6(a,b)
presents the changes of the structurally sensitive 1D WAXD
patterns of poly-4 during heating and subsequent cooling.
Similar to the 1D WAXD result of the sample poly-3, the as-
cast amorphous sample demonstrated two scattering halos
in a low-angle region of 2–5ꢀ and a high-angle region of 15–
25ꢀ at low temperatures. Upon the first heating and subse-
quent cooling, the scattering halo in the low-angle region
changed a little in intensity and breadth with temperature
varied. The d-spacing changes as shown in Figure 6(c) of the
low-angle peak/halo as a function of the temperature during
the first heating and first cooling for poly-4 decreased slowly
below 100ꢀC at first, then deceased sharply after that, indi-
cating that there was no phase transition except glass transi-
tion. Combining the results of PLM, we can conclude that the
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sample poly-4 was amorphous during the whole variable-
temperature process.
DSC, PLM, and WAXD, and dependence of the phase transfor-
mations on their structures was visually found. Polymers
with ether linkage group in the side chain were easier to de-
velop a LC phase, and poly-1 with ether linkage as one side
and ester linkage as the other exhibited a more ordered tet-
ragonal columnar nematic (UTN) phase at high temperatures.
While polymers with ester bond linkage group was not LC
even at high temperatures. We expect that these MJLCPs
with biphenyl mesogenic core will possess more interesting
properties and have potential applications, and our work
will provide more guidance in designing new MJLCPs and
investigating their structure–property relationships.
Dependence of the Mesophase Behaviors of the
Polymers on Molecular Structures
Since the first evolution of MJLCPs reported by Zhou and co-
worker, many kinds of MJLCPs with various structures, mo-
lecular weight, and properties have been designed and
investigated. Furthermore, dependence of the phase behav-
iors on the chemical structures of the polymer repeated
unit40 and rigidity of the side chain41 has been investigated
in detail. Reports have shown that the phase behaviors of
the MJLCPs can be modulated by a few factors such as the
shape of the mesogens,13 the molecular weight,40 and the
terminal alkyl length in the side chain,15,16. But from this
work, we can elucidate that by tuning the linkage groups
between biphenyl mesogenic core and terminal alkyl in the
side chain, their phase behaviors can also be changed greatly.
In this series of four polymers, two different types of LC
phase and three types of phase structures were found,
depending on the different linkage groups in the side chain,
as shown in Figure 7. The poly-1 sample with ether linkage
as one side and ester linkage as the other exhibited unusual
LC phase behaviors with temperature varying, a more or-
dered tetragonal columnar nematic (UTN) phase with a local
square lattice (a 5 1.92 nm) developed at high temperatures,
and the LC phase transformed into the amorphous state
upon cooling. For poly-2 polymer with ether bond as the
connecting group, it also showed unusual LC transformation,
but only nematic phase (UTN) developed in the high temper-
atures. While poly-3 and poly-4 with ester bond linkage
group was not LC even at high temperatures. Thus, we can
conclude that subtle changes in the molecular structure
brought about tremendous variation of the LC phase proper-
ties for MJLCPs. Compared with the similar work reported
by Yin,42 who designed and synthesized a series of MJLCPs
with one benzene ring modifying by different length of alkyl
through different linkage group as the mesogenic core.
Results revealed that ester bond was more beneficial to sta-
bilize the LC phase than ether bond. While for our biphenyl
system, polymers with ether linkage group were easier to
develop a LC phase. Therefore, we believe that the relation-
ship of complex MJLCPs system between chemical structures
and properties still need further deep investigation.
ACKNOWLEDGMENTS
The financial support from the National Natural Science Foun-
dation of China (Grants 51203003 and 11202005) and Beijing
National Science Foundation (No. 3122019) are gratefully
acknowledged.
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