Synthesis of mesogen-jacketed liquid crystalline polymers with long symmetry mesogenic core containing two biphenyls

Article abstract of DOI:10.1016/j.polymer.2013.01.048

A series of mesogen-jacketed liquid crystalline polymers, poly {2,5-bis[(4-alkoxybiphenyl)oxycarbonyl]-styrenes} (P-Cm, m = 8, 10, 12, 14 and 18) were designed and successfully synthesized via free radical polymerization. The chemical structures of the monomers were confirmed by ¹H NMR, ¹³C NMR and Mass Spectrometry. The molecular characterizations of the polymers were performed with ¹H NMR, GPC and TGA. The phase structures and transitions of the polymers were investigated by the combination of techniques including DSC, POM, temperature-variable FT-IR spectroscopy, 1D WAXD and SAXS. The experimental results suggest that the polymers with symmetry mesogenic core containing two biphenyls can develop into a well-defined smectic A (S_A) phase. This implies that MJLCPs molecules shape can be modulated from rod-like to ribbon-like by simply changing the length of the mesogenic group.

Full text of DOI:10.1016/j.polymer.2013.01.048

Polymer 54 (2013) 1794e1802
Contents lists available at SciVerse ScienceDirect
Polymer
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Synthesis of mesogen-jacketed liquid crystalline polymers with long symmetry
mesogenic core containing two biphenyls
*
Sheng Chen, Hang Luo, 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
Article history:
A series of mesogen-jacketed liquid crystalline polymers, poly {2,5-bis[(4-alkoxybiphenyl)oxycarbonyl]-
styrenes} (P-Cm, m ¼ 8, 10, 12, 14 and 18) were designed and successfully synthesized via free radical
polymerization. The chemical structures of the monomers were confirmed by 1H NMR, 13C NMR and
Mass Spectrometry. The molecular characterizations of the polymers were performed with 1H NMR, GPC
and TGA. The phase structures and transitions of the polymers were investigated by the combination of
techniques including DSC, POM, temperature-variable FT-IR spectroscopy, 1D WAXD and SAXS. The
experimental results suggest that the polymers with symmetry mesogenic core containing two biphenyls
can develop into a well-defined smectic A (SA) phase. This implies that MJLCPs molecules shape can be
modulated from rod-like to ribbon-like by simply changing the length of the mesogenic group.
Ó 2013 Elsevier Ltd. All rights reserved.
Received 28 September 2012
Received in revised form
16 January 2013
Accepted 31 January 2013
Available online 8 February 2013
Keywords:
Liquid crystalline polymers
Phase behavior
Structureeproperty relation
1. Introduction
between the polymer main chain and the highly crowded, rigid,
and bulky side groups. So, the LC phase structures are found to be
Liquid-crystalline polymers (LCPs) have attracted long standing
attention for their potential applications in many fields, including
optical data storage, optic, electro-optic, nonlinear optic devices,
photomechanical and so on [1e7]. Generally speaking, LCPs have
two main categories: main-chain liquid crystalline polymers
(MCLCPs) with mesogen units located in the main chain [8e10] and
side-chain liquid crystalline polymers (SCLCPs) with mesogens
attached to the main chain as side groups [11,12]. For SCLCPs, Fin-
kelmann pointed out that the flexible spacer inserted between
main chain and mesogenic side groups was indispensable in order
for a SC-LCP to achieve LC phase since it decoupled the interaction
of side chain and polymer backbone, which disrupted the ordered
packing of the side mesogens [13]. However, as proposed by Zhou
et al., in 1987, mesogen jacketed liquid crystal polymers (MJLCPs),
which were a special class of SCLCPs with their mesogenic units
attached laterally to the main chain through a short spacer or with
a single covalent bond, could also form stable LC phases [14e25].
Unlike the conventional SCLCPs whose backbones usually take
a random-coil chain conformation, MJLCPs are somewhat rigid and
exhibit some features of MCLCPs because the strong coupling
columnar nematic phase and hexagonal columnar phase in which
each cylinder is formed by a single MJLCPs chain molecule.
However, the rod-like chain shall not be the only molecular
shape of MJLCPs. Smectic phases have been found in laterally
attached side-chain LC polymers by changing several variables,
including the length or type of the terminal flexible substituent,
chemical structures of mesogens, hydrogen bonding and the elec-
trostatic interactions. For example, Padma Gopalan has reported
the design, synthesis and characterization of MJLCPs with semi-
fluorinated mesogenic groups (see Scheme 1(a)) [26]. Authors
pointed out that the polymers could form SA phase because of the
microphase separation. Through changing chemical structures of
mesogens, Chen et al. have researched the phase behavior of MJLCPs
with unbalanced mesogenic core and different terminal flexible
substituents (see Scheme 1(b)) [27].The results showed that well-
defined SA phase was formed when the carbon number for the
flexible substituents were shorter than four (butoxy groups);
whereas the one reached four, well-defined SC phase was observed.
Chai et al. also have found that the MJLCPs containing the
1,3,4-oxadiazole unit exhibited SA phase (see Scheme 1(c)) [28].
Cheng et al. have found that the polymers contained amide linkages
in the side groups formed SA phase due to hydrogen bonding (see
Scheme 1(d)) [29]. Meantime, when the terminal groups were
replaced by ionic groups, the polymers still presented SA phase
because of the electrostatic interactions (see Scheme 1(e)) [30].
* Corresponding author. Tel.: þ86 731 58293377; fax: þ86 731 58293264.
E-mail addresses: hailiangzhang@xtu.edu.cn, zhanghangliang@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.2013.01.048

S. Chen et al. / Polymer 54 (2013) 1794e1802
1795
Scheme 1. Chemical structures of MJLCPs synthesized previously.
In our previous studies, we have systematically researched the
influence of alkoxy tail length on the self-organization of poly {2,5-
bis[(4-alkoxyphenyl)oxycarbonyl]styrenes} (P-OCm, m is the num-
ber of the carbons in the alkoxy groups, m ¼ 1, 2, 4, 6, 8, 10, 12, 14, 16
and 18) based on 2-vinylterephthalic acid and 4-alkoxyphenyl (see
Scheme 2(a)) [31].The experiments results demonstrated that the
polymers could form smectic phase, isotropic phase and columnar
phase when m ꢀ 10. The attachment of long flexible alkyl side chains
to the polymer backbone can lead to layered structures due to the
unfavorable interaction between mesogens and the apolar alkyl side
chains, and it was considered that the smectic phase was proposed
to form by the driving force of the enthalpy. Interesting point arises
concerning the properties of MJLCPs when 4-alkoxyphenol was
replaced by 4-alkoxybiphenol. One hand, compared with the rigid of
core of the P-OCm, the new MJLCP (poly{2,5-bis[(4-alkoxybiphenyl)
oxycarbonyl]styrene}, P-Cm) has a larger aspect ratio, leading to the
stronger interaction between the side on rigid mesogens. The other
hand, the phase behaviors of P-OCm were strongly alkyl length
dependent due to the competing of the steric effect, the microphase
separation and the driving force of the entropy. Of particular interest
is to ask in whether the alkyl tails length has also influence on the
phase behaviors of P-Cm. So, in this paper, we reported the synthesis
and characterization of a series of vinyl monomers with different
numbers of alkoxy terminal groups and their corresponding ho-
mopolymers. The chemical structures of polymers are shown in
Scheme 2(b). However, the monomers M-Cm (m < 8) were found to
be difficultly dissolved in common organic solvents or be poly-
merized to a sufficiently high Mn. Our experimental results proved
that the polymers could develop into a stable SA phase, whatever the
alkyl tail length is short or long. Therefore, through studying the
phase behavior of these polymers, we can deeply understand the
relationship between a simple structural variable of the monomer
and the change in the phase structure of MJLCPs.
2. Experimental section
2.1. Materials
The precursor 2-vinylterephthalic acid (VTA) was synthesized
according to previous paper [25]. Chlorobenzene (Acros, 99%) was
purified by washing with concentrated sulfuric acid to remove re-
sidual 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), 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 monomers
The synthetic route of monomers of 2,5-bis [(4-alkoxybiphenyl)
oxycarbonyl]-styrenes (M-Cm, m ¼ 8, 10, 12, 14, 18) is shown in
Scheme 3. The experimental details are described as follows using
2,5-bis[(4-octanoxybiphenyl)oxycarbonyl]-styrene (M-C8) as an
example.
2.2.1. Synthesis of 4-octanoxybiphenol
4, 40-biphenol (20 g, 0.1074 mol), octyl bromide (3.5 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,
Scheme 2. Chemical structures of P-OCm (a) and P-Cm (b).

1796
S. Chen et al. / Polymer 54 (2013) 1794e1802
Scheme 3. Scheme of the synthesis of monomers and corresponding polymers.
CH2Cl2). The final product of 4-octanoxybiphenol was white pow-
der with a yield of 3 g. 1H NMR (400 HMz, CDCl3):
15, alkoxy H), 3.97e4.04 (t, 2H, eOCH2e), 6.89e6.94 (m, 4H, AreH),
7.42e7.47(m, 4H, AreH).
Ar0eHeAr), 7.28e7.33(m, 4H, 4H, Ar0eAreH), 7.53e7.55(4H, Ar0e
HeAr), 7.56e7.65 (m, 4H, Ar0eAreH and 1H, eCH]), 8.25e8.51
d
¼ 0.86e1.83 (m,
(m, 3H, AreH). 13C NMR (CDCl3):
d
¼ 14.10 (eCH3), 22.70 (eCH2e),
26.12 (eCH2e), 29.37e29.62 (eCH2e), 31.94 (eCH2e), 68.24 (e
OCH2e), 114.99 (]CH2), 121.78 (aromatic C ortho to CeO), 127.8
(middle aromatic C), 128.14e128.74 (side aromatic C), 129.17 (mid-
dle aromatic C), 131.03 (middle aromatic C), 132.08e132.65 (side
aromatic C), 133.30 (middle aromatic C), 134.73 (eHC]CH2), 139.14
(aromatic CeHC]CH2), 149.66e149.77 (side aromatic CeO), 159.00
(side aromatic CeO), 164.30e165.08 (C]O). Mass Spectrometry
(MS) (m/z) [M] Calcd for C54H64O6, 808.47; found, 807.544 þ 1.
2.2.2. Synthesis of 5-bis[(4-octanoxybiphenyl)oxycarbonyl]-styrene
(M-C8)
VTA (5.2 mmol, 1 g) was added into 30 ml dried thionyl chloride
in a 100 ml three-necked flask, then, the mixture was refluxed.
After VTA was dissolved absolutely, the 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-octanox-
ybiphenol (13.0 mmol, 3.86 g) and 4-(dimethylamino)pyridine
(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
2.2.3.2. 2,5-bis[(4-dodecyloxybiphenyl)oxycarbonyl]styrene (M-C12).
1H NMR (400 HMz, CDCl3):
d
¼ 0.89e1.87 (m, 46H, alkoxy H), 4.01e
4.04 (t, 4H, eOCH2e), 5.52e5.91 (2d, 2H, ]CH2), 6.99e7.02 (m, 4H,
Ar0eHeAr), 7.28e7.33(m, 4H, 4H, Ar0eAreH), 7.53e7.55(4H, Ar0e
HeAr), 7.56e7.65 (m, 4H, Ar0eAreH and 1H, eCH]), 8.23e8.50
(m, 3H, AreH). 13C NMR (CDCl3):
d
¼ 14.12 (eCH3), 22.73 (eCH2e),
26.14 (eCH2e), 29.40e29.67 (eCH2e), 31.98 (eCH2e), 68.25 (e
OCH2e), 115.02 (]CH2), 121.79 (aromatic C ortho to CeO), 127.78
(middle aromatic C), 128.13e128.73 (side aromatic C), 129.17 (mid-
dle aromatic C), 131.04 (middle aromatic C), 132.12e132.66 (side
aromatic C), 133.32 (middle aromatic C), 134.72 (eHC]CH2), 139.12
(aromatic CeHC]CH2), 149.70e149.81 (side aromatic CeO), 159.03
(side aromatic CeO), 164.26e165.06 (C]O). Mass Spectrometry
(MS) (m/z) [M] Calcd for C58H72O6, 864.53; found, 863.315 þ 1.
as eluant. 1H NMR (400 HMz, CDCl3):
d
¼ 0.90e1.87 (m, 30H, alkoxy
H), 4.01e4.04 (t, 4H, eOCH2e), 5.52e5.91 (2d, 2H, ]CH2), 6.91e
7.01 (m, 4H, Ar0eHeAr), 7.28e7.33(m, 4H, 4H, Ar0eAreH), 7.53e
7.55(4H, Ar0eHeAr), 7.56e7.65 (m, 4H, Ar0eAreH and 1H, e
CH]), 8.23e8.50 (m, 3H, AreH). 13C NMR (CDCl3):
d
¼ 14.0.3 (e
2.2.3.3. 2,5-bis[(4-tetradecyloxybiphenyl)oxycarbonyl]styrene
C14). 1H NMR (400 HMz, CDCl3):
(M-
CH3), 22.64 (eCH2e), 26.09 (eCH2e), 29.23e29.37 (eCH2e), 31.82
(eCH2e), 68.24 (eOCH2e), 114.98 (]CH2), 121.75 (aromatic C ortho
to CeO), 127.81 (middle aromatic C), 128.13e128.72 (side aromatic
C), 129.18 (middle aromatic C), 131.00 (middle aromatic C), 132.12e
132.67 (side aromatic C),133.29 (middle aromatic C), 134.71 (eHC]
CH2), 139.12 (aromatic CeHC]CH2), 149.66e149.77 (side aromatic
CeO), 158.98 (side aromatic CeO), 164.33e165.11 (C]O). Mass
Spectrometry (MS) (m/z) [M] Calcd for C50H56O6, 752.41; found,
751.228 þ 1.
d
¼ 0.88e1.87 (m, 54H, alkoxy H),
4.01e4.04 (t, 4H, eOCH2e), 5.52e5.91 (2d, 2H, ]CH2), 6.99e7.01
(m, 4H, Ar0eHeAr), 7.28e7.33(m, 4H, 4H, Ar0eAreH), 7.53e
7.55(4H, Ar0eHeAr), 7.56e7.65 (m, 4H, Ar0eAreH and 1H, eCH]),
8.23e8.50 (m, 3H, AreH). 13C NMR (CDCl3):
d
¼ 14.07 (eCH3), 22.67
(eCH2e), 26.15 (eCH2e), 29.35e29.60 (eCH2e), 31.92 (eCH2e),
68.23 (eOCH2e), 114.95 (]CH2), 121.75 (aromatic C ortho to CeO),
127.74 (middle aromatic C), 128.10e128.71 (side aromatic C),
129.14 (middle aromatic C), 131.01 (middle aromatic C), 132.14e
132.62 (side aromatic C), 133.31 (middle aromatic C), 134.71 (e
HC]CH2), 139.08 (aromatic CeHC]CH2), 149.70e149.79 (side ar-
omatic CeO), 159.11 (side aromatic CeO), 164.25e165.04 (C]O).
Mass Spectrometry (MS) (m/z) [M] Calcd for C62H80O6, 920.6; found,
919.431 þ 1.
2.2.3. Synthesis of 2,5-bis [(4-alkoxybiphenyl)oxycarbonyl]-styrenes
(M-Cm, m ¼ 10, 12, 14, 18)
All the other monomers were synthesized and characterized
similarly. The characterization data of M-Cm (m ¼ 10, 12, 14 and 18)
were as follows:
2.2.3.4. 2,5-bis[(4-octadecaoxybiphenyl)oxycarbonyl]-styrene
C18). 1H NMR (400 HMz, CDCl3):
(M-
2.2.3.1. 2,5-bis[(4-decyloxybiphenyl)oxycarbonyl]styrene
1H NMR (400 HMz, CDCl3):
¼ 0.87e1.87 (m, 38H, alkoxy H), 4.01e
4.04 (t, 4H, eOCH2e), 5.52e5.91 (2d, 2H, ]CH2), 6.99e7.01 (m, 4H,
(M-C10).
d
¼ 0.86e1.84 (m, 70H, alkoxy H),
d
3.96e4.02 (t, 4H, eOCH2e), 5.51e5.90 (2d, 2H, ]CH2), 6.91e7.00
(m, 4H, AreAr0eH), 7.28e7.33(m, 4H, AreHeAr0), 7.51e7.65 (m,

S. Chen et al. / Polymer 54 (2013) 1794e1802
1797
4H, AreAr0eH and m, 4H, AreHeAr0 and 1H, eCH]), 8.23e8.50 (m,
3H, AreH). 13C NMR (CDCl3):
¼ 14.06 (eCH3), 22.67 (eCH2e),
d
26.09(eCH2e), 29.35e29.70 (eCH2e), 31.93 (eCH2e), 68.24 (e
OCH2e), 114.99 (]CH2), 121.75 (aromatic C ortho to CeO), 127.65
(middle aromatic C), 128.13e128.75 (side aromatic C), 129.15
(middle aromatic C), 131.04 (middle aromatic C), 132.15e132.67
(side aromatic C), 133.34 (middle aromatic C), 134.75 (eHC]CH2),
139.15 (aromatic CeHC]CH2), 149.64e149.72 (side aromatic Ce
O), 159.11 (side aromatic CeO), 164.28e165.04 (C]O). Mass Spec-
trometry (MS) (m/z) [M] Calcd for C70H96O6, 1032.72; found,
1031.571 þ 1.
2.3. Polymerization
All the polymers were synthesized by conventional solution
radical polymerization (see Scheme 3). For example, 0.752 g
(1 mmol) of M-C8, 100 mL of chlorobenzene solution of 0.01 mmol
AIBN, 2 ml of chlorobenzene and a magnetic stir bar were added
into a polymerization tube, the tube was purged with nitrogen and
subjected to four freezeethaw cycles to remove any dissolved ox-
ygen and sealed off under vacuum. Polymerization was carried out
at 80 ꢁC for 24 h. The tube was then opened, and the reaction
mixture was diluted with 20 ml THF. The resultant polymer was
precipitated and washed with acetone. To eliminate the unreacted
monomers completely, the purification was repeated, until no peak
was observed at the elution time of monomer in gel permeation
chromatography (GPC) measurement.
2.4. 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 pol-
ydispersity index (PDI ¼ Mw/Mn) were measured on a GPC (WA-
TERS 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.
TGA was performed on a TA-SDT 2960 instrument at a heating
rate of 20 ꢁC/min in nitrogen atmosphere.
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.
Fig. 1. 1H NMR spectrums of the monomer M-C8 (a) and the polymer P-C8 (b) in CDCl3.
FTIR spectra in KBr pellets were recorded on a PE Spectrum One
FTIR spectrophotometer.
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
Table 1
Molecular characteristics of polymers.
with a 3 kW ceramic tube as the X-ray source (Cu Ka) and an
Sample
Mn(ꢂ105)a
PDIa
Tmb
Tgb
Tdc
Liquid
X’celerator detector. The sample stage was set horizontally. The
reflection peak positions were calibrated with silicon powder
crystallinityd
P-C8
2.47
1.75
1.52
1.60
1.87
2.4
2.2
2.5
2.2
2.0
e
225.9
222.6
221.3
211.9
192.4
424
424
426
421
414
Yes
Yes
Yes
Yes
Yes
(2q q
> 15ꢁ) and silver behenate (2 < 10ꢁ). A temperature control
P-C10
P-C12
P-C14
P-C18
e
unit (Paar Physica TCU 100) in conjunction with the diffractometer
was utilized to study the structure evolutions as a function of
temperature.
Small-angle X-ray scattering (SAXS) experiments were per-
formed using a high-flux SAXS instrument (SAXSess, Anton Paar)
equipped with Kratky block-collimation system and a Philips
PW3830 sealed-tube X-ray generator (Cu KR). The wavelength was
0.1542 nm. A highly sensitive SAXS imaging plate which was
264.5 mm away from the sample was used to collect the signal in
ꢃ4.8
23.2
52.7
a
Relative Mn and polydispersity (PDI) were measured by GPC using PS standards.
The melting temperatures and the glass transition temperatures were measured
b
by DSC at a heating rate of 10 ꢁC/min under nitrogen atmosphere during the second
heating process.
c
The temperatures at 5% weight loss of the samples under nitrogen [Td(N2)] were
measured by TGA heating experiments at a rate of 20 ꢁC/min.
d
The liquid crystallinity was observed by POM.

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S. Chen et al. / Polymer 54 (2013) 1794e1802
Table 2
Table 1. At the same time, the apparent number-average molecular
Liquid crystallinities of the monomers.
weights (Mn’s) of the polymers determined by GPC were higher
than 1.5 ꢂ 105 g/mol, demonstrating good polymerizability of the
monomers.
Monomers
Phase transitions (ꢁC)
First cooling
Second heating
M-C8
N 97.2 S 87.6 Cr 2.4
N 96.57 S 89.4 Cr 2.2
N 95.4 Cr
N 98.3 Cr
N 95.1 Cr
Cr 98.6 S 118.09 N
Cr1 105.6 Cr2 110.5 S 124.9 N
Cr1 109.7 Cr2 125.2 N
Cr1 114.1 Cr2 120.8 N
Cr 111.1 N
M-C10
M-C12
M-C14
M-C18
3.2. Mesomorphic properties of the monomers
The phase behaviors of the monomers M-Cm (m ¼ 8, 10, 12, 14
and 18) were examined by the conventional analyses including
DSC and POM. In order to avoid the thermal polymerization, the
detection temperature of DSC and POM was below 200 ꢁC. The
thermograms of M-Cm on both first cooling scans and the sec-
ond heating are illustrated in supporting information Fig. S1, and
their phase transition temperatures are collected in Table 2. The
results showed that all the monomers presented crystal phase in
low temperature (Fig. 2(a)) and formed nematic phase in high
temperature (Fig. 2(c)). Moreover, nematic phase didn’t dis-
appear above 200 ꢁC under POM. In addition, in the intermediate
temperature, the M-C8 and M-C10 exhibited smectic phase
(Fig. 2(b)).
Cr: crystalline; N: nematic; and S: smectic phase.
vacuum. Samples were placed in between aluminum foils which
were folded and sandwiched in a steel sample holder. After back-
ground subtraction, desmearing was performed according to the
Lake’s method. A temperature control unit (Anton Paar TCS300) in
conjunction with the SAXSess was utilized to study the structure at
various temperatures.
3. Results and discussion
3.1. Synthesis and characterization of monomers and polymers
3.3. Phase transitions and phase structures of the polymers
All the monomers could be easily polymerized via free radical
polymerization. Herein, we use P-8C as an example to elucidate
the process. Part (a) and (b) of Fig. 1 give the 1H NMR spectra
(CDClꢃ3 d) of the monomer M-C8 and the polymer PeC8, respec-
tively. The M-C8 showed the characteristic resonances of the vinyl
group at 5.52e5.91 and 7.51e7.60 ppm. After polymerization,
these signals disappeared completely. The chemical shifts of PeC8
were quite broad and consistent with the expected polymer
structure. In order to remove the influence of Mn on the LC be-
haviors, we synthesized a series of polymers with the similar Mn.
The molecular characterizations of the polymers are summarized
Fig. 3 shows that all polymers have good thermal stability, i.e.,
the temperatures at 5% weight loss of the samples under nitrogen
were about 410 ꢁC measured by TGA at a rate of 20 ꢁC/min (see
Table 1), indicating the strong interaction among the biphenyl
moieties. Fig. 4 shows the second heating DSC curves of P-Cm
(m ¼ 8, 10, 12, 14 and 18) at a rate of 10 ꢁC/min under nitrogen
atmosphere after eliminating the thermal history. As can be seen,
the glass transitions (Tg) of all polymers can be observed, and were
basically decreased with the increasing of the length of alkyl
because of the intrinsic plastification of the alkoxy tails (listed in
Fig. 2. Representative POM images of the texture of M-C8 at (a) 80 ꢁC, (b) 95 ꢁC and (c) 140 ꢁC (magnification: ꢂ200).

S. Chen et al. / Polymer 54 (2013) 1794e1802
1799
Fig. 3. TGA diagrams of polymers in N2 at a rate of 20 ꢁC/min.
Table 1). Contrast to the Tg of P-OCm, the Tg of P-Cm is much higher
because of the bulkier structure of mesogens. It also indicates that
side chains have the strong interaction, hindering the movement of
main chains. Beside Tg, the melting points of the alkoxy tails can be
seen for P-C12, P-C14 and P-C18. After Tg, no more transition peaks
were seen. The phenomenon is similar to some other MJLCPs sys-
tems [27].
In order to further elucidate the phase transition and macro-
molecular chain mobility in the polymers, VT-FTIR experiments
were carried out. It is well known the rocking vibration at ca.
720 cmꢃ1 and scissoring vibration at ca. 1470 cmꢃ1 are taken into
account as the characteristic bands of methylene group, which are
sensitive to the conformational variation as well as the environ-
mental alternation [32]. The rocking vibrational FTIR spectra of the
P-C18 are shown in Fig. 5 (a). In heating process, the intensity of
rocking band at 720 cmꢃ1 decreased with temperature increasing.
Fig. 5. Temperature-variable FT-IR spectra of (a) methylene rocking band for the
sample P-C18 and (b) methylene scissoring band for the sample P-C18. Heating pro-
cess, from top to bottom: 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, 100,
105, 110 and 120 ꢁC.
Moreover, the sudden changed emerges at 40e75 ꢁC, which is
consistent with DSC results, indicating that crystalline structure
of alkoxy side chain was transformed to disordered state. Fig. 5 (b)
shows the VT-FTIR spectra of PeC18 in the range of 1490e
1450 cmꢃ1. The peaks around 1466 cmꢃ1 and 1457 cmꢃ1 corre-
spond to the trans and gauche conformational structure of meth-
ylene group (CH2), respectively [32]. It can be seen that the
intensity of trans conformational band at 1466 cmꢃ1 decreases with
increasing temperature, while the intensity of gauche conforma-
tional band at 1457 cmꢃ1 increases. Therefore we can make a con-
clusion that the conformation of alkoxy side chain in P-C18
experiences a variation process from stable structure (trans) to
metastable structure (gauche) during the heating process.
Birefringence of the polymers is observed by POM. All samples
were cast from THF solution and slowly dried at room temperature,
then slowly heated. The POM images of polymers were shown in
Fig. 6 when heating to 150 ꢁC, showing that all polymers formed the
LC phase. Especially, for P-C18, the needle-like texture was
observed, indicating the polymer formed smectic phase. For this, it
may be that the polymers with longer alkyl group had the lower
Tg and good solubility and easily formed the thin film when
the samples were cast from THF solution. Moreover, the liquid
crystalline birefringence did not disappear until the samples were
Fig. 4. DSC curves of P-Cm during the second heating scan at a rate of 10 ꢁC/min under
nitrogen atmosphere.

1800
S. Chen et al. / Polymer 54 (2013) 1794e1802
Fig. 6. Representative POM images of the texture of P-C8 (a), P-C10 (b), P-C12 (c), P-C14 (d) and P-C18 (e) at 150 ꢁC (Magnification: ꢂ200).
decomposed. When cooled to room temperature from 310 ꢁC, the
birefringence of the sample remained the same, implying that the
ordered structure kept unchanged upon cooling.
the thermal expansion. In the wide-angle region of 2q
> 10ꢁ, only
an amorphous halo around 20ꢁ could be recognized. This reflected
that no long range ordered structure formed via molecular packing
was detected over the entire temperature region studied. From the
1D WXRD results, the homopolymers formed the stable LC phase.
Moreover, we presume that the structure of the LC phase is layer SA
phase based on two points. On the one hand, calculated length of
the mesogenic units is 4.58 nm when the alkane takes trans con-
formation (the length of side chain will increase 0.25 nm with
increasing per two methylene). In our experiment data, the max-
imum d-spacing value is 4.47 nm, which is close to the calculated
length (4.58 nm). On the other hand, the side chains have the
strong interactions between biphenyl. Meantime, the long flexible
alkoxy tails are immiscible with the rigid core and need larger
space in packing. So the polymers form layer SA. In addition, no
noticeable change could be observed during the second heating
and cooling processes in 1D-WAXD experiments, implying that
there was no isotropic phase for P-C8 in the temperature region
from 50 to 250 ꢁC.
In order to study the phase transitions and the phase structures
of P-Cm’s at different temperatures, 1D WAXD experiments were
performed. About 60 mg of the polymer was added into an alu-
minum foil substrate. In order to be consistent with the DSC re-
sults, the samples were heated to 250 ꢁC, and then slowly cooled to
the room temperature. The temperature was not higher than
280 ꢁC during the experiments for all samples to avoid thermal
degradation. Fig. 7(a) and (b) illustrates the temperature-variable
1D WAXD patterns of PeC8 from 50 to 250 ꢁC and from 250 to
50 ꢁC in the 2 region from 2.5ꢁ to 30ꢁ. At low angle, one narrow
q
reflection peak was observed at 50 ꢁC, which was lower than the Tg
of the homopolymer (226 ꢁC), indicating the ordered structures on
the nanometer scale formed (2
q
¼ 1.98ꢁ, which a d-spacing value of
4.47 nm). The intensity of the halo basically kept the same with the
temperature elevated, i.e., 250 ꢁC, which was higher than the Tg of
the polymer, only the peak shifted slightly to high 2q value due to

S. Chen et al. / Polymer 54 (2013) 1794e1802
1801
Table 3
The 2q, d-spacing values and calculated length of the mesogenic units.
Sample
2
q
(ꢁ)a
d-spacing
(nm)a
Calculated length of the
mesogenic units (nm)b
P-C8
1.98
1.85
1.80
1.72
1.59
4.47
4.78
4.94
5.16
5.56
4.58
5.04
5.54
6.04
7.04
P-C10
P-C12
P-C14
P-C18
a
Obtained from the two-dimensional WAXD experiments.
Assuming the n-alkoxy tails in the side chains have an all-trans conformation.
b
All other P-Cm (m ¼ 10, 12, 14 and 18) showed similar results in
1D WAXD experiments, indicating that all polymers formed the SA
phase (see Fig. S2 in Supporting information). The d-spacing
values are shown in Table 3. Compared to the theoretic value, the
scale of d-spacing is smaller when m ꢀ 12. For this point, it may be
caused by two facts. One was, it was impossibility to take trans
conformation for all alkane, leading to the increase of theoretical
memsogen unit length. Another one, the polymers formed layer
structure where the alkyl tails were interdigitated with each
other, further leading to the decrease of the d-spacing. Meantime,
the 2q of the polymer is close to the minimum detection value,
leading no other peaks to be found. In order to further elucidate
the phase structure, we performed 1D SAXS experiment. The
samples were heated to 250 ꢁC, then, slowly cooled to the room
temperature. Fig. 8 shows the temperature-variable SAXS profiles
of PeC18 from 25 to 200 ꢁC. As can be seen from it, a first-order
sharp peak is centered at q* ¼ 1.04 nmꢃ1 and a higher-order dif-
fraction peak appear at 2q* ¼ 2.11 nmꢃ1. The ratio of the scattering
vectors of the two peaks q*:2q* was 1:2, indicating a smectic
structure with a periodicity of 5.60 nm (d100 ¼ 2
p/q*) for P-C18,
which is consistent with the 1D WAXD results (5.56 nm). Similar
results were obtained from different samples in P-Cm (m ¼ 8, 10,
12, and 14).
Fig. 7. 1D WAXD patterns of the P-C8 during (a) the second heating process and (b) the
second cooling process.
Fig. 8. SAXS profiles of P-C18 recorded during the second heating process with in-
tensity in log scale.

1802
S. Chen et al. / Polymer 54 (2013) 1794e1802
Chen Xiao-Fang of Peking University for their help with 1D WAXD
and SAXS experiments measurements.
Appendix A. Supplementary data
Supplementary data related to this article can be found at
http://dx.doi.org/10.1016/j.polymer.2013.01.048.
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This research was financially supported by the Innovation Plat-
form Open Foundation of University of Hunan Province (10k067),
the National Nature Science Foundation of China (51073131,
21174117, 21104062). The authors thank Prof. Chen Er-Qiang and