Synthesis and liquid crystalline behavior of 2,5-disubstituted styrene-based random copolymers: Effect of difference in length of the rigid core on the mesomorphic behavior of mesogen-jacketed liquid crystalline polymers

Article abstract of DOI:10.1016/j.polymer.2013.04.056

A series of binary copolymers, poly{2,5-bis[(4-octadecyloxyphenyl) oxycarbonyl]-styrene-co-2,5-bis[(4′-octadecyloxybiphenyl)oxycarbonyl] -styrene} (poly(M-OC18-co-M-C18)), were synthesized via free radical polymerization. The random nature of the copolymers was expected on the basis of the assumed similar reactivities because of the analogous monomers. A combination of differential scanning calorimetry (DSC), polarized optical microscopy (POM) and wide-angle X-ray diffraction (WAXD) demonstrated that phase behavior of this series of copolymers were strongly composition dependent. The P-OC18 formed smectic A phase, re-entrant isotropic phase and column nematic phase, while the P-C18 only exhibited the stable smectic A phase. The comparison between P-OC18 and P-C18 indicates that the length of the rigid core is crucial to determine the liquid crystalline (LC) structures. After copolymerization, the glass transition temperature and the phase transition temperature of the copolymers from smectic phase to isotropic phase both increased with the molar fraction of 2,5-bis[(4′-octadecyloxybiphenyl)oxycarbonyl]-styrene (M-C18) in feed. In addition, when the feed was below 0.3, a column nematic phase was observed in high temperature. Namely, simply through copolymerization, we can greatly vary LC behavior of the mesogen-jacketed liquid crystalline polymers.

Full text of DOI:10.1016/j.polymer.2013.04.056

Polymer 54 (2013) 3556e3565
Contents lists available at SciVerse ScienceDirect
Polymer
journal homepage: www.elsevier.com/locate/polymer
Synthesis and liquid crystalline behavior of 2,5-disubstituted styrene-
based random copolymers: Effect of difference in length of the rigid
core on the mesomorphic behavior of mesogen-jacketed liquid
crystalline polymers
*
Sheng Chen, Xiang Shu, He-Lou Xie, Hai-Liang Zhang
Key Laboratory of Polymeric Materials and Application Technology of Hunan Province, Key Laboratory of Advanced Functional Polymer Materials of
Colleges, and Universities of Hunan Province, College of Chemistry, Xiangtan University, Xiangtan 411105, Hunan Province, China
a r t i c l e i n f o
a b s t r a c t
A series of binary copolymers, poly{2,5-bis[(4-octadecyloxyphenyl)oxycarbonyl]-styrene-co-2,5-bis[(40-
octadecyloxybiphenyl)oxycarbonyl]-styrene} (poly(M-OC18-co-M-C18)), were synthesized via free
radical polymerization. The random nature of the copolymers was expected on the basis of the assumed
similar reactivities because of the analogous monomers. A combination of differential scanning calo-
rimetry (DSC), polarized optical microscopy (POM) and wide-angle X-ray diffraction (WAXD) demon-
strated that phase behavior of this series of copolymers were strongly composition dependent. The
P-OC18 formed smectic A phase, re-entrant isotropic phase and column nematic phase, while the P-C18
only exhibited the stable smectic A phase. The comparison between P-OC18 and P-C18 indicates that the
length of the rigid core is crucial to determine the liquid crystalline (LC) structures. After copolymeri-
zation, the glass transition temperature and the phase transition temperature of the copolymers from
smectic phase to isotropic phase both increased with the molar fraction of 2,5-bis[(40-octadecylox-
ybiphenyl)oxycarbonyl]-styrene (M-C18) in feed. In addition, when the feed was below 0.3, a column
nematic phase was observed in high temperature. Namely, simply through copolymerization, we can
greatly vary LC behavior of the mesogen-jacketed liquid crystalline polymers.
Article history:
Received 7 October 2012
Received in revised form
12 April 2013
Accepted 21 April 2013
Available online 14 May 2013
Keywords:
Liquid crystalline polymer
Random copolymerization
Phase behavior
Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction
increasing interest has been devoted to mesogen-jacketed liquid
crystalline polymers (MJLCPs) in which the rodlike mesogens are
Phase structures and transitions of thermotropic liquid crystal-
line (LC) polymers have been one of the important topics to study in
polymer chemistry and physics because LC polymers have a great
potential in functional soft material [1e3]. Therefore, many re-
searchers have investigated the structureeproperty relationship of
side-chain liquid crystalline polymer (SCLCP) or main-chain liquid
crystalline polymer (MCLCP) in order to understand the principles
of structure formation and structure manipulation [4e17].
Depending on the position where the mesogens are linked to the
backbone, the SCLCP can be categorized into either end-on SCLCPs
or side-on SCLCPs. The former has mesogens linked at the end
position, while another has mesogens linked at the waist or mass
center position. For side-on SCLCPs, during the last 20 years,
connected to every other carbon atom of polymer main chain at the
waist position or mass center via a single covalent bond [18,19]
because they display many thermotropic properties characterized
by MCLCPs, although MJLCPs belong to SCLCPs chemically.
Since the concept of MJLCPs was proposed by Zhou et al., in order
to understand the structureeproperty relationships of MJLCPs,
containing glass transition, LC phase, clearing point, chain stiffness
and conformation, one hand, the studies has been focused on the
synthesis and characterization of new polymers with the type of
mesogen and the length of flexible spacer linking the backbone and
the side groups as well as the terminal substituent [20,21]. On the
other hand, through atom transfer radical polymerization (ATRP) or
reversible addition fragmentation chain transfer (RAFT), MJLCPs
with different molecular weights or the block polymers in which
polymer consisted of MJLCPs have been obtained [22e25]. The
research shows that MJLCPs usuallyform columnar phase because of
the strong steric effects, such as columnar nematic, hexagonal
columnar, rectangular columnar [26]. Later, MJLCPs can display
* Corresponding author. Tel.: þ86 731 58293377; fax: þ86 731 58293264.
E-mail addresses: zhanghangliang@xtu.edu.cn, hailiangzhang@xtu.edu.cn
(H.-L. Zhang).
0032-3861/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.polymer.2013.04.056

S. Chen et al. / Polymer 54 (2013) 3556e3565
3557
smectic A and smectic C, resulting from increasing the rigidity of the
mesogen, H-bonding or electrostatic interaction [27e30].
demonstrated by the increased glass transition temperatures,
variation in phase dimensions, and the mesophase depression in
certain copolymers.
For the phase behavior of the SCLCPs, it can be tailored not only
by changing chemical structure, but also by changing the external
conditions, such as, solvent, blends and copolymerization. For
example, Kihara, H et al. have studied the thermodynamic phase
behavior of LC copolymer/liquid crystal blends [31]. The results
showed that the new type of phase diagram was obtained. Cui et al.
have reported that the amorphous azopyridine polymer could
easily be converted into LC polymers through self-assembly with a
series of commercially available, aliphatic and aromatic carboxylic
acids including the acetic acid [32]. Cheng et al. have researched the
synthesis and phase structures of MJLCPs polyelectrolytes and their
ionic complexes [29]. Authors found out that the complexes were
amorphous when the MJLCPs were complexed with dodecyl-
trimethylammonium bromide (C12) because the alkyl tails were
too short to induce the formation of ordered LC structures.
However, by increasing the length of alkyl tails as in cetyl-
trimethylammonium bromide (C16) or by changing the shape of
surfactant as in the fan-shaped amphiphilic molecule with
three C12 tails (fan-shaped 3,4,5-tris(dodecyloxy)benzenamine),
lamellar phases were observed for the complexes. For copolymer-
ization, it represents one of the simplest synthetic techniques to
effective control over the transition temperature and the phase
structure and to widen their application range as high-performance
materials. For example, when 2,5-bis[(4-methoxyphenyl)oxy-
carbonyl]styrenes (MPCS) was copolymerized with non-liquid-
crystalline vinyl monomers, such as styrene (St) and methyl
methacrylate (MMA), mesophase can only be observed when the
molar contents of MPCS in copolymers exceed about 89% and 84%,
respectively [33]. It was speculated that the constitutional disorder,
which diluted the concentration of mesogenic units and disrupted
the interactions between backbone and side groups, was respon-
sible for the destabilization of the mesophase. Tang et al. also have
reported that 2,5-di(n-butoxycarbonyl)styrene was employed to
tailor the mesomorphic property of PMPCS by random radical
polymerization [34]. All the resultant copolymers were able to form
stable mesophase regardless of the compositions. Moreover, the
structure and dimension of the mesophase formed were manipu-
lated through random copolymerization with nonmesogenic
monomer in the whole composition range. Later, the author has
researched that five series of binary copolymers, poly[2,5-di(ROOC)
styrene-co-2,5-di(R0OOC)styrene]s, were synthesized via free
radical polymerization [35]. The mesomorphic behaviors of the
copolymers were found to be strongly affected by the presence of
analogous comonomers differing in alkyl groups only, as
This article presents the mediation of the LC properties of the
random copolymers based on 2,5-bis[(4-octadecyloxyphenyl)oxy-
carbonyl]-styrene (M-OC18) and 2,5-bis[(4-octadecyloxybiphenyl)
oxycarbonyl]-styrene (M-C18). In our previous studies, our group
has proved that the P-OC18 formed the smectic A phase, re-entrant
isotropic phase and columnar phases with the increasing of tem-
perature [36]. The P-C18 presented the stable smectic A phase [37].
When M-OC18 was radically copolymerized with M-C18, the
resultant copolymers displayed the interesting phase behaviors.
The LC behaviors of this series of copolymers were investigated
using DSC, POM and 1D WXRD experiments. This study provided a
unique example of tuning the LC properties of MJLCPs by random
copolymerization. Meantime, it can help us to understand the in-
fluence of thermotropic properties of the corresponding homo-
polymers on the mesomorphic properties of the copolymers.
2. Experimental section
2.1. Materials
The precursor 2-vinylterephthalic acid (VTA) was synthesized
according to previous paper. The p-(n-octadecaoxyl)phenols were
prepared as described by Klarmann et al. Chlorobenzene (Acros,
99%) was purified by washing with concentrated sulfuric acid to
remove residual thiophenes, followed by washing twice with
distilled water, once with 5% sodium carbonate solution, and again
with distilled water before being dried with anhydrous calcium
chloride and then distilled. 4-(dimethylamino)pyridine (DMAP,
99%, ACROS), Hydroquinone and 4, 40-Diphenol (98%, Alfa Aesar)
and the corresponding octadecaoxyl bromides together with other
reagents and solvents were used as received without further
purification.
2.2. Synthesis of 40-octadecyloxybiphenol
4,40-biphenol (20 g, 0.1074 mol), octadecyl bromide (6 g,
0.0182 mol), K2CO3 (14.964 g, 0.1086 mol) and 200 ml were added
into a 500 ml round-bottom flask. The mixture was heated for 12 h
in 55 ꢀC, next the reaction mixture poured into a large amount of
water to precipitate products. The products were purified by pre-
cipitation in water from THF solution three times. At last, the crude
products were purified by column chromatograph (silica gel,
CH2Cl2). The final product of 40-octadecyloxybiphenol was white
Fig. 1. Scheme of the synthesis of monomers M-OC18 and M-C18.

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S. Chen et al. / Polymer 54 (2013) 3556e3565
Fig. 2. 1H NMR spectrums of the monomers M-OC18 and M-C18.
powder with a yield of 5 g. 1H NMR (400 MHz, CDCl3):
1.81 (m, 35, alkoxy H), 3.96e4.01 (t, 2H, eOCH2e), 6.88e6.95 (m,
4H, AreH), 7.41e7.46(m, 4H, AreH).
d
¼ 0.86e
excess thionyl chloride was removed under reduced pressure. The
residue was washed with petroleum ether for three times. The
petroleum ether was removed under the reduced pressure and
afforded yellow liquid. This liquid was dissolved in dried THF to
obtain solution A. 40-octadecyloxybiphenol (13.0 mmol, 4.25 g) and
DMAP (26.0 mmol, 3.18 g) were dissolved in 20 ml dried THF to
obtain solution B. Under intense stirring at 0 ꢀC, the solution A was
slowly added into the solution B over a period of 2 h. The mixture
was further stirred at room temperature for 12 h, and then most of
the THF was distilled off by evaporation under reduced pressure.
After water was added into the residue, the crude solid product was
first collected by filtration and dried under vacuum, and then pu-
rified using column chromatography on silica gel with chloroform
as eluant. The M-OC18 was synthesized according to our work [36].
The characterization data of these monomers were as follows and
2.3. Synthesis of 2,5-bis[(4-octadecyloxyphenyl)oxycarbonyl]-
styrene (M-OC18) and 2,5-bis[(40-octadecyloxybiphenyl)
oxycarbonyl]-styrene (M-C18) monomers
Both M-OC18 and M-C18 were synthesized with similar pro-
cedures. The synthetic routes of the monomers were outlined in
Fig. 1.
The experimental details were described as follows using M-C18
as an example: VTA (5.2 mmol, 1 g) was added into 30 mL dried
thionyl chloride in a 100 mL three-necked flask, and then the
mixture was refluxed. After VTA was dissolved absolutely, the
Fig. 3. The constituent monomeric units of the poly (M-OC18-co-M-C18).

S. Chen et al. / Polymer 54 (2013) 3556e3565
3559
Fig. 2 gives the 1H NMR spectra of the monomers M-C18 and
M-OC18.
Table 1
Summary of molecular weights (Mn) and polydispersities (PDI) of poly (M-OC18-co-
M-C18) system.
M-C18: 1H NMR (400 MHz, CDCl3):
d
¼ 0.86e1.84 (m, 70H,
Mn ꢁ 10ꢂ5b(g/mol)
PDIb
a
Sample
MM-OC18:MM-C18
alkoxy H), 3.96e4.02 (t, 4H, eOCH2e), 5.51e5.90 (2d, 2H, ¼CH2),
6.91e7.00 (m, 4H, AreAr0eH), 7.28e7.33(m, 4H, AreHeAr0), 7.51e
7.65 (m, 4H, AreAr0eH and m, 4H, AreHeAr’ and 1H, eCH¼), 8.23e
8.50 (m, 3H, AreH). Mass Spectrometry (MS) (m/z) [M] Calcd for
C70H96O6, 1032.72; found, 1031.571 þ 1.
P1
P2
P3
P4
P5
P6
P7
P8
100:0
95:5
2.10
2.00
1.95
1.50
1.81
2.10
1.84
2.15
2.47
2.12
2.57
2.13
2.20
2.10
2.19
2.25
2.20
2.32
2.30
2.51
2.37
2.32
2.42
2.12
90:10
80:20
70:30
60:40
50:50
40:60
30:70
20:80
10:90
0:100
M-OC18: 1H NMR (CDCl3):
d
¼ 0.86e1.86 (m, 70H, alkoxy H),
3.97e4.00 (t, 4H, eOCH2e), 5.48e5.87 (2d, 2H, ¼CH2), 6.95e7.17 (m,
8H, AreH), 7.53e7.60 (q, 1H, eCH¼), 8.18e8.44 (m, 3H, AreH). Mass
Spectrometry (MS) (m/z) [M] Calcd for C58H88O6, 880.66; found,
879.465 þ 1.
P9
P10
P11
P12
2.4. Synthesis of copolymers
a
MM-OC18:MM-C18 is the mole fraction of M-OC18 to mole fraction of M-C18 ratios
in the feed.
b
Solution copolymerizations of M-OC18 with M-C18, with
different feed compositions, were carried out in chlorobenzene at
65 ꢀC using AIBN as initiator (0.05%, based on the total molar of
monomers). Typical procedures of polymerization were as follows:
An appropriate amount of AIBN, monomers and chlorobenzene
were mixed in a reaction tube. After three freezeepumpethaw
cycles, the tube was sealed under vacuum and put into a 65 ꢀC oil
bath. Polymerization was carried out at 65 ꢀC for several hours with
constant stirring. After the tube was broken, the mixture was
diluted with THF and then poured into a large amount of acetone to
precipitate the polymer. The polymers were purified by precipita-
tion in acetone from THF solution three times to eliminate the
unreacted monomers completely, then, dried in a vacuum oven
overnight at room temperature.
Mn and polydispersity (PDI) were measured by GPC using PS standards.
2.5. Instruments and measurements
1NMR measurements were performed on a Bruker ARX400 MHz
spectrometer using with CDCl3 as solvent, tetramethylsilane (TMS)
as the internal standard at room temperature.
The apparent number-average molecular weight (Mn) and
polydispersity index (PDI ¼ Mw/Mn) were measured on a GPC
(WATERS 1515) instrument with a set of HT3, HT4 and HT5. The m-
styragel columns used THF as an eluent and the flow rate was
1.0 mL/min at 38 ꢀC. All the GPC data were calibrated with poly-
styrene standards.
DSC traces of all the polymers were obtained using a TA-Q10
DSC instrument. The temperature and heat flow were calibrated
using standard materials (indium and zinc) at a cooling and heating
rates of 10 ꢀC/min. Samples with a typical mass of about 5 mg were
encapsulated in sealed aluminum pans.
LC texture of the polymers was examined under POM (Leica
DM-LM-P) equipped with a Mettler-Toledo hot stage (FP82HT).
One-dimensional (1D) wide-angle X-ray diffraction (WAXD)
experiments were performed on a Philips X’Pert Pro diffractometer
with a 3 kW ceramic tube as the X-ray source (Cu Ka) and an
X’celerator detector. The sample stage was set horizontally. The
reflection peak positions were calibrated with silicon powder
(2q q
> 15ꢀ) and silver behenate (2 < 10ꢀ). A temperature control
unit (Paar Physica TCU 100) in conjunction with the diffractometer
was utilized to study the structure evolutions as a function of
temperature.
3. Results and discussion
3.1. Copolymerization
The copolymerizations of M-OC18 with M-C18 in chlorobenzene
were studied with different monomer to monomer ratios in the
Fig. 4. DSC curves of copolymers during the first cooling scan (a) and the second
heating scan (b) at a rate of 10 ꢀC/min under a N2 atmosphere.

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S. Chen et al. / Polymer 54 (2013) 3556e3565
Table 2
distribution of the two building units along the copolymer back-
Thermotropic properties of poly (M-OC18-co-M-C18).
bone was expected. The copolymerization results of M-OC18 with
M-C18 are summarized in Table 1 with the molar percentage of
M-C18 in feed ranging from 0 to 1. From the Table 1, we can see that
there are nearly the same Mns and high Mns for all samples.
Sample
Tma
Tga
T1b
115
125
132
144
168
195
212
240
255
T2c
P1
P2
P3
P4
P5
P6
P7
P8
48
44
44
43
47
48
49
48
49
49
51
51
91
93
217
250
270
310
330
105
110
118
131
142
161
168
176
187
188
3.2. Phase transitions and phase structures of copolymers
The dependence of the thermal transition temperatures on the
compositions of copolymers poly (M-OC18-co-M-C18) was studied
by DSC experiments. Fig. 4 shows the first cooling and the second
heating DSC curves of copolymers at a rate of 10 ꢀC/min under
nitrogen atmosphere after eliminating the thermal history. Based
on the second heating process, all phase transition temperatures of
copolymers are shown in Table 2. First, as can be seen from Table 2,
the melting temperatures of alkyl side chains (Tm) of P-OC18 and
P-C18 are 48 ꢀC and 50 ꢀC, respectively. Second, a glass transition
process can be observed in all these samples from Fig. 4(b).
Moreover, only single glass transition was identified (see Fig. S1),
indicating the monomers were placed randomly along the main
chain and no phase separation occurred. The glass transition tem-
peratures (Tg) of the copolymers increases from 91 ꢀC to 188 ꢀC
with increasing the content of M-C18 in the feed. Contrast to the
P-OC18, the Tg of P-C18 is higher presumably because of the
difference length of the rigid core and the strong interactions
between biphenyl. This continuous adjustment in Tg is evidence of
molecular-level homogeneity of monomer distribution in polymer
main-chain. Third, after Tg, the DSC traces contain one weak
endothermic peak (except P12). Combined the POM and 1D WAXD
experiment results, this transition peak assigned as the transition
P9
P10
P11
P12
279
299
>350.0
a
The melting temperatures of alkyl side chains (Tm) and the glass transition
temperatures of copolymers (Tg) were measured by DSC at a heating rate of 10 ꢀC/
min under nitrogen atmosphere during the second heating process.
b
The transition temperature from smectic to isotropic phase (T1) measured by
DSC at a heating rate of 10 ꢀC/min under nitrogen atmosphere during the second
heating process.
c
The transition temperature from isotropic phase to liquid crystalline phase (T2)
measured by POM at a heating rate of 10 ꢀC/min.
feed. The constituent monomeric units of the copolymers are as
shown in Fig. 3. GPC analysis was performed to determine Mn and
molecular weight distributions of the polymers. To exclude the
effect of the Mn on the LC behaviors, all copolymers were obtained
with high enough Mns by using rigorously purified monomers and
solvent and relatively high monomer to initiator ratio. Because of
the similarity in the structures of the monomers, the reactivity of
the monomers was assumed to be equal. As a result, a random
Fig. 5. Representative POM images of the texture of P1 maintained at 115 ꢀC (a), 145 ꢀC (b), 250 ꢀC (c) (Magnification: ꢁ200).

S. Chen et al. / Polymer 54 (2013) 3556e3565
3561
temperature from smectic A phase to isotropic phase (T1). Mean-
while, with the increasing of the M-C18 fraction in feed from P1 to
P11, the T1 increased from 115.1 ꢀC to 299 ꢀC, indicated that the
more the longer rigid core (M-C18) copolymers contained, the
more stable smectic A phase became. When cooling, this weak
transition peak can also be seen in Fig. 4(a). Last, after T1, one
transition peak appeared at 217 ꢀC for P1, which was caused by the
phase transition from isotropic phase to column nematic phase (T2)
[36]. For P2eP5, the T2 were difficultly detected by DSC because the
thermal heat was small. However, they were detected by POM and
increased from 250 ꢀC to 330 ꢀC (see Table 2).
To study morphological structures by POM, thin films of the
copolymers were prepared by solution-cast method on clean cover
glass, followed by slowly drying at ambient temperature. And then
the samples were sandwiched in between two cover glasses and,
therefore, were protected from direct contact with the air. On the
basis of the POM observations, copolymers could be divided into
two groups. The first group was P1eP5. Fig. 5 shows the observation
for sample P1. When heating, the sample became soft above the Tg,
then the granular LC texture (Fig. 5(a)) started to disappear when
the temperature reached about 120 ꢀC at which a corresponding
endotherm appeared in DSC curve. Upon heating further, there was
no birefringence to be seen (Fig. 5(b)). However, needle-like texture
was formed when the temperature reached to 217 ꢀC. Continue
heating, no visible change of birefringence was observed before
decomposition (see Fig. 5(c)). The POM results of the P2eP5 were
same to P1 and the representative POM images of them in high
temperature are shown in Fig. 6(a), (b), (c) and (d), respectively. So,
the phase transitions of P1eP5 followed the sequence of smectic A
phase 4 re-entrant isotropic phase 4 columnar phase [36].
The second group was P6eP12. Taking the P6 as an example, the
sand-shape texture was observed when heating and is shown in
Fig. 7(a), suggesting the formation of smectic phase. The texture
disappeared when temperature reached 195 ꢀC. It was consistent
with the DSC result. Upon heating to 350 ꢀC, there was no bire-
fringence to be seen. After cooling, the sand-shape texture reap-
peared. For the copolymers P7eP11, the POM results were same to
P6. The POM images of P9 and P11 are shown in Fig. 7(b) and (c). For
the POM results of P12, the LC birefringence did not disappear
before the polymer decomposed (Fig. 7(d)). So, the phase transi-
tions of P6eP11 followed the sequence of smectic A phase 4
isotropic phase and P12 exhibited the stable smectic A phase [37].
To get more insight into the phase transition temperature and
phase structure, we performed reflection light intensity (birefrin-
gence) measurements. Fig. 7(a) shows the transition temperatures
of P2 which evaluated by DSC and Fig. 7(b) shows the recorded
changes in reflection light intensity for P2 by POM both at a rate of
10 ꢀC/min. It can be seen from Fig. 7(b) that two transition peaks
appear. For the former peak, it appeared at T1, which agreed well
with the transition temperature of this copolymer on DSC. Around
T1, the reflection light intensity sharply decreased to 2.9% from
47.9%, indicating a transition from ordered state to isotropic state.
For another one, it was difficult to be detected by DSC, however, it
was obvious to appear at 250 ꢀC (T2) in which the reflection light
intensity sharply increased to 48.2% from 2.9%, showing a transition
from isotropic state to ordered state. The dynamic process of the
heating and cooling on the texture of P2 is shown in supporting
information. Before, our group has researched the influence of the
heating and cooling rates on the thermal transition temperature
(T1) by DSC. The results showed that the value of T1 was
Fig. 6. Representative POM images of the texture of P2 at 255 ꢀC (a), P3 at 275 ꢀC (b), P4 at 315 ꢀC (c) and P5 at 335 ꢀC (d) (Magnification: ꢁ200).

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S. Chen et al. / Polymer 54 (2013) 3556e3565
Fig. 7. Representative POM images of the texture of P6 at 185 ꢀC (a), P9 at 220 ꢀC (b), P11 at 275 ꢀC (c) and P12 at 300 ꢀC (d) (Magnification: ꢁ200).
independent of them [39]. To further know the phase transition and
information about various phases, the influence of the heating and
cooling rates on the thermal transitions from isotropic phase to
columnar phase were studied by POM. Taking the P2 as an example,
the reflection light intensity curves are shown in Fig. 3. First, for the
transition temperature from isotropic phase to columnar phase
(T2) or from columnar phase to isotropic phase, they were also
independent of the change of heating or cooling rate and basically
kept 250 ꢀC and 210 ꢀC, respectively. Second, when cooling, the
change of intensity was slower than heating process, indicating
after the polymer formed columnar phase, the polymer could keep
the order structure in some degree to reduce the influence of
driving force of the entropy. For the T1 of all copolymers, they were
detected by reflection light intensity measurements, and the results
were shown in Fig. S2, which was consistent with the DSC results.
Fig. 8. (a) Heat flow at a heating rate 10 ꢀC/min in DSC and (b) the change of reflection
Fig. 9. Reflection light intensity curves of heating and subsequent cooling of P2 at
different rates in POM.
light intensity for P2 at a heating rate 10 ꢀC/min in POM.

S. Chen et al. / Polymer 54 (2013) 3556e3565
3563
The phase transitions of the polymers were further verified on
the 1D WAXD instrument. In order to be consistent with the DSC
result, all samples were heated to 250 ꢀC, and then slowly cooled to
the room temperature. Based on the 1D WAXD observations, co-
polymers were also divided into two groups. The first group was
P1eP5. Fig. 8 describes the structurally sensitive 1D WAXD patterns
The reason for the disappearance of the second-order reflections
possibly lies in the electron density profiles normal to the layers
tending to be sinusoidal, which would diminish the higher-order
reflections [38]. When the sample was heated to 140 ꢀC, the
diffraction peaks disappeared and the scattering halo appeared,
indicating that the sample becomes isotropic. Further increasing
the temperature to 260 ꢀC, one peaks appeared about 3.07ꢀ
(d ¼ 2.87 nm), indicating a transition from isotropic to columnar
of the P2 from 35 to 260 ꢀC. Upon the second heating, in the low 2
q
region of 1e3ꢀ (as shown in Figs. 9 and 10(a)), one obvious
diffraction peaks are observed about 1.98ꢀ (d ¼ 4.48 nm), indicating
a smectic structure of the sample although the second-order
diffraction was absence.
phase [36]. In the high 2q
region of 10e35ꢀ, at lower temperature
range the WAXD patterns only rendered one peaks (as shown in
Fig. 10(b)), indicating the typical crystal formed by the long alkoxy
tails.
Fig. 10. 1D WAXD patterns of the P2 during the second heating process of the as-cast
Fig. 11. 1D WAXD patterns of the P-C18 (P12) during the second heating process of the
film ((a) in the low 2
q
and (b) in the high 2
q
).
as-cast film ((a) in the low 2 and (b) in the high 2q).

3564
S. Chen et al. / Polymer 54 (2013) 3556e3565
Table 3
1D WAXD results of the polymers at 50 ꢀC.
Sample
d-spacing (nm)
P1
P2
P7
P9
P12
4.01
4.48
4.85
5.09
5.56
The second group was P6eP12. Before, our group has proved that
P-C18 (P12) formed the stable SA phase, herein we wouldn’t discuss
the experiment results again (see Fig. 11) [37]. Fig. 12(a) and (b)
describe the structurally sensitive 1D WAXD patterns of P7 and P9
from 50 to 265 ꢀC in the 2 region from 2.5ꢀ to 30ꢀ. The 1D WAXD
q
experiment results of them were similar to the P12, and the only
difference was that the sharp diffractions in low angle disappeared
at 220 ꢀC and 265 ꢀC, respectively for P7 and P9, indicating the
polymers entered into isotropic phase from smectic phase, which
was consistent with the POM and DSC results. In addition, as can be
seen from Fig. 12(b), the ratios of the lower diffraction peak angle
(2q) with respect to the higher angle are 1:2, further indicating a
smectic structure of the copolymer. On the basis of the 1D WAXD
studies, the d-spacing values measured at 50 ꢀC are summarized in
Table 3. The d-spacing value of mesophase increased from 4.21 nm
to 5.56 nm, indicating that the more the longer side groups
copolymers contained, the larger the d-spacing values was.
3.3. Roles governing mesophase formation of copolymers
Based on DSC, POM and 1D WAXD experiments results, all co-
polymers showed the smectic phase. The schematic drawing of the
layer structure of copolymers is shown in Fig.13. Comparing with the
phase behavior of P-OC18, P-C18 exhibited the different phase be-
haviors although there was only minute difference in structure
between M-OC18 and M-C18. Combined the microphase separation
with the driving force of the entropy, the P-OC18 can form smectic A
phase, re-entrant isotropic andcolumn nematic phase. The P-C18 can
exhibit a stable smectic phase because of the long symmetry meso-
genic core and strong interactions between biphenyls. After copo-
lymerization, the copolymers presented the interesting phase
behavior. Phase transition behaviors of poly (M-OC18-co-M-C18) are
showninFig.14. As can be seen, all copolymerspresented thesmectic
phase although the smectic phase formation mechanism of P-OC18
was different P-C18. Meanwhile, the T1 increased with the increasing
of M-C18 fraction in feed. However, it was hard for the copolymers to
form the column nematic phase. When the feed was above 0.3, the
copolymers just have exhibited the smectic phase. According to Self-
Compacting Chain Model, the entropy gain of the side groups
obtained from liquidemesophase transition of the copolymer would
be less than that of the homopolymer, resulting in a less tendency for
a copolymer to form a mesomorphic state [35,36,40].
Fig. 12. 1D WAXD patterns of the P7 (a) and P9 (b) during the second heating process of
the as-cast film ((a) in the low 2q and (b) in the high 2q).
Fig. 13. Schematic drawing of the layer structure of the polymers.

S. Chen et al. / Polymer 54 (2013) 3556e3565
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Appendix A. Supplementary data
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.polymer.2013.04.056.
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