Synthesis and thermal properties of poly(2-chloroethyl glycidyl ether)-based side-chain liquid-crystalline polymers

Article abstract of DOI:10.1080/15421406.2010.526560

A novel series of poly(2-chloroethyl glycidyl ether)-based side-chain liquid-crystalline copolymers has been synthesized in which the mesogenic units are biphenyl 4-methoxybiphenyl, 4-methoxyazobenzene, and N-benzylidene-4- methoxyaniline, respectively (corresponding polymers are referred to as PCBH, PCMB, PCMA, and PCBM). The structures of the compounds obtained are characterized by infrared (IR) and 1H nuclear magnetic resonance (NMR). The thermal properties were investigated using polarized optical microscopy, differential scanning calorimetry, and X-ray diffraction analysis. The results indicate that PCMA, PCMB, and PCBM exhibit thermotropic liquid-crystalline behavior and present nematic phases with Schlieren textures. However, PCBH does not display a mesogenic phase. The decomposition temperatures of copolymers begin at 290°C. Copyright Taylor & Francis Group, LLC.

Full text of DOI:10.1080/15421406.2010.526560

Mol. Cryst. Liq. Cryst., Vol. 533: pp. 42–51, 2010
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421406.2010.526560
Synthesis and Thermal Properties of
Poly(2-Chloroethyl Glycidyl Ether)-Based
Side-Chain Liquid-Crystalline Polymers
1
CHANGHAI HE AND CHAOCAN ZHANG
,2
1
1
School of Materials Science and Engineering, Wuhan University
of Technology, Wuhan, China
2
Hubei Light Industry Technology Institute, Wuhan, China
A novel series of poly(2-chloroethyl glycidyl ether)-based side-chain liquid-crystal-
line copolymers has been synthesized in which the mesogenic units are biphenyl
4-methoxybiphenyl, 4-methoxyazobenzene, and N-benzylidene-4-methoxyaniline,
respectively (corresponding polymers are referred to as PCBH, PCMB, PCMA,
and PCBM). The structures of the compounds obtained are characterized by infrared
(
1
IR) and H nuclear magnetic resonance (NMR). The thermal properties were
investigated using polarized optical microscopy, differential scanning calorimetry,
and X-ray diffraction analysis. The results indicate that PCMA, PCMB, and PCBM
exhibit thermotropic liquid-crystalline behavior and present nematic phases with
Schlieren textures. However, PCBH does not display a mesogenic phase. The
ꢀ
decomposition temperatures of copolymers begin at 290 C.
Keywords Azobenzene; biphenyl; liquid-crystalline polymer; N-benzylidene-
4-methoxyaniline; poly(2-chloroethyl glycidyl ether); thermal properties
Introduction
Side-chain liquid-crystalline polymers (SCLCPs) have great potential in a wide range
of applications such as information storage [1], holographic imaging [2–4], nonlinear
optics [5], collecting and extracting of some metal ions [6], which gives us a better
understanding of the self-assembly in molecular systems [7,8].
SCLCPs consist of three structural units: a polymer backbone, a spacer, and a
mesogenic group. The most frequently used mesogens are biphenyl, azobenzene,
Shiff’s base, ester and cyano, nitro, alkyl, or alkoxy groups attached as terminal
units because these groups induce dipole–dipole intermolecular interactions, which
contribute to the overall stability of the mesophase. The spacer connecting the
mesogenic unit to the polymer backbone is regarded as an important constituent,
in order to decouple the motion of the backbone component from the arrangement
of the mesogenic side chains [9]. The length and flexibility of the spacer play a
significant role in determining the thermal stability, phase transition temperatures,
Address correspondence to Chaocan Zhang, School of Materials Science and Engineer-
ing, Wuhan University of Technology, Luoshi Road 122, Wuhan, 430070, China. E-mail:
polymers@whut.edu.cn
42

Side-Chain Liquid-Crystal Polymers
43
and corresponding enthalpy change of the copolymers, especially the order
arrangement of the mesogenic groups to self-assemble from those of the main chain
to adopt a random coil configuration [10].
Polyether is a flexible linear backbone. Polyepichlorohydrin is usually used as
the main chain of polyether-based SCLCPs and it can react with many compounds
containing a mesogenic unit to form liquid-crystalline compounds [11–15]. However,
the disadvantage of polyepichlorohydrin is the absence of on extra spacer. Poly(2-
chloroethyl glycidyl ether) (PCGE) can overcome it for its extra ethoxy unit in side
chain, but few examples have been reported to employ PCGE as the main chain of
SCLCP.
In this work, an attempt is made to synthesize PCGE-based SCLCPs by means
of a nucleophilic substitution reaction of PCGE with sodium phenoxide of a
mesogenic group such as biphenyl, 4-methoxyazobenzene, 4-methoxybiphenyl, and
N-benzylidene-4-methoxyaniline. The study mainly focuses on the synthesis and
thermal behavior of the copolymers and the effect of different mesogenic groups
on thermal properties.
Experimental
Materials
The solution of tributyl aluminum in toluene was provided by Wuhan Organic
Synthesis Limit Co., China. 4-Hydroxylbiphenyl was purchased from Alfa Aesar.
All other chemicals (Shanghai Chemical Regent Co., China) used in this study were
analytical grade. Ethanol was dried over magnesium and distilled. Toluene and
1,4-dioxane were refluxed with sodium and distilled. Other reagents were used
without further purification.
Instruments
Infrared (IR) spectra were recorded using a Nicolet-60SXB Fourier transform
1
infrared (FTIR) spectrometer (Nicolet, USA). H nuclear magnetic resonance
(NMR) (300 MHz) spectra were recorded on a Varian Mercury VX-300 (Varian,
USA) in CDCl solutions, using trimethyl siloxane (TMS) as internal standard.
3
The number-average molecular weight (Mn) of PCGE was determined by gel per-
meation chromatography (GPC) using a system equipped with a Waters 2690D sep-
aration module and a Waters 2410 refractive index detector (Alliance HPLC, USA).
À1
Tetrahydrofuran (THF) was used as the eluent at a flow rate of 0.3 mL min [13].
Waters Millennium Module software was used to calculate the molecular weight on
the basis of a universal calibration curve generated by a polystyrene standard with a
narrow molecular weight distribution [13]. Thermal characterization was done via
differential scanning calorimetry (DSC) on a TA DSC-Q10 instrument (TA, USA).
The optical textures of the mesophases were studied with an Olympus BH-2 polariz-
ing optical microscope (POM, magnification: Â100) fitted with a Linkam THMSE
6
00 heating stage (Olympus, Japan). The X-ray diffraction patterns were obtained
using a D=MAX-IA diffractometer (Rigaku, Tokyo, Japan) with nickel-filtered
ꢀ
À1
Cu Ka radiation, at a scanning rate of 8 min . Thermogravimetric analysis
TGA) was conducted using a STA 449C (NETZSCH, Germany) instrument with
a nitrogen atmosphere.
(

4
4
C. He and C. Zhang
Scheme 1. Reaction scheme of PCGE-based copolymers.
Synthesis
The synthetic route for the preparation of polymers is outlined in Scheme 1.
0
4
-Hydroxyl-4 -methoxybiphenyl (HMB) was prepared according to the literature
ꢀ
16]. Yield: 40.8%; mp: 178–180 C. H NMR (CDCl , d in ppm): 7.48, 6.94 (m, aro-
3
1
[
matic, 8H); 3.89 (s, OCH , 3H).
3
0
4
-Hydroxyl-4 -methoxyazobenzene (HMA) was synthesized according to the
1
ꢀ
literature [13]. Yield: 82%; mp: 140–141 C. H NMR (CDCl , d in ppm): 7.92, 6.9
3
4
(m, aromatic, 8H); 3.91 (s, OCH , 3H).
N-(4-Hydroxylbenzylidene)-4-methoxyaniline (HBM) was prepared according
3
ꢀ
1
to the reported method [17]. Yield: 86%; mp: 172–174 C. H NMR (CDCl , d in
3
ppm): 8.38 (s, -CH=N-, 1H), 7.79, 7.20, 6.91 (m, aromatic, 8H); 3.83 (s, OCH3,
3
H), shown in Fig. 1a.
-Chloroethl glycidy ether was prepared using the procedures described in
2
ꢀ
25
D
the literature [18]. Yield: 47.6%, bp: 60–61 C=2 mmHg, n ¼ 1:4492 (reported
2
5
n
¼ 1:4492).
Polymerization of 2-chloroethyl Glycidy Ether. 2-Chloroethyl glycidyl ether (PCGE;
D
2
0 g) was mixed with dry toluene (20 mL) and tributyl aluminum in toluene (2 mL)
ꢀ
was added. After being stirred at 100 C under a nitrogen stream for 5 h, the
mixture was poured into methanol to precipitate polymer. The resulting polymer
was purified by precipitation from CHCl solution into methanol and dried at
3
ꢀ
0 C in vacuum for 48 h. Yield: 65%. H NMR (CDCl , d in ppm): 3.73–3.53 (m),
3
1
6
as shown in Fig. 1b. M ¼ 2,347, M =M ¼ 2.13.
n
w
n
Substitution of PCGE. According to a similar method described previously [19], The
monomer unit (10 mmol) and sodium ethoxide (10 mmol) were mixed in dry ethanol
(10 mL) and refluxed for 1 h under a nitrogen stream and then the solvent was
removed. PCGE (9 mmol) dissolved in dry 1,4-dioxane (20 mL) and tetrabutylamine
ꢀ
bromoate (1 mmol) were added. The reaction mixture was stirred at 100 C under a
dried nitrogen stream for 54h and centrifuged to remove sodium chloride, and the
filtrate was poured into methanol. The crude polymer was purified by precipitation

Side-Chain Liquid-Crystal Polymers
45
1
3
Figure 1. H NMR spectra of HBM (a), PCGE (b), and PCBM (c) (CDCl , TMS).
from CH Cl into methanol. The removal of the monomer was monitored using
3
thin-layer chromatography. Finally, the obtained copolymer was dried in a vacuum
ꢀ
1
at 50 C for 48h. Their structures were confirmed by IR and H NMR.
ꢁ1
PCBH Yield; 46%; Substitution degree: 87%. IR (KBr, cm ): 3,034 (C-H aromatic),
1
1
4
608 (C=C aromatic). H NMR (CDCl ) d(ppm): 7.49–6.92 (s, aromatic, 9H),
3
.01 (s, PhOCH , 2H), 3.75–3.62 (s, PCGE backbone, 8.3H).
2
ꢁ1
PCMB Yield; 50%; Substitution degree: 89%. IR (KBr, cm ): 3,048 (C-H romatic),
1
1
,610 (C=C aromatic). H NMR (CDCl ) d(ppm): 7.38 (s, aromatic, 4H), 6.93
3
(s, aromatic, 4H), 4.12 (s, PhOCH , 2H), 3.84–3.62 (t, PhOCH , PCGE back-
bone, 11.1H).
2
3
ꢁ
1
PCMA Yield; 51%; Substitution degree: 91%. IR (KBr, cm ): 3,070 (C-H romatic),
1
1
matic, 4H), 6.95 (s, aromatic, 4H), 4.01 (s, PhOCH , 2H), 3.83–3.72 (t, PhOCH ,
,598 (C=C aromatic), 1,499 (-N=N-). H NMR (CDCl ) d(ppm): 7.84 (s, aro-
3
2
3
PCGE backbone, 10.9H).

4
6
C. He and C. Zhang
ꢁ
1
PCBM Yield; 50%; Substitution degree: 90%. IR (KBr, cm ): 3,065 (C-H romatic),
1
1
,610 (C=C aromatic), 1,496 (-C=N-). H NMR (CDCl ) d(ppm): 8.26 (s,
3
-
CH=N-, 1H), 7.71 (s, aromatic, 2H), 7.12 (s, aromatic, 2H), 6.84 (s, aromatic,
4
H), 4.02 (s, PhOCH , 2H), 3.75–3.62 (t, PhOCH , PCGE backbone, 11.0H).
2 3
Result and Discussion
Synthesis
The substitution degree of the mesogenic group depends on the reaction time. For
PCBM, an 8% increase of substitution degree was obtained as the reaction time
increased by 6 h. The substitution degree increased slightly when the reaction time
was over 50 h. Therefore, the reaction time was 54 h for PCBM. The same reaction
time was taken for other copolymers. The substitution degrees of mesogenic groups
of the above-mentioned polymers are close to each other and do not depend on the
mesogenic groups. In addition, more attention should be paid to water in the
synthesis, because sodium ethoxide and sodium phenoxide react with water easily
in solvent and in air and therefore affect the substitution degree and yield.
1
IR and H NMR Characterization
The chemical structures of the synthesized monomers and copolymers were charac-
1
terized by IR spectroscopy and H NMR to confirm the expected reaction routes
and the resulting products.
Figure 2 shows the IR spectra of HBA, PCGE, and PCMA as representative
examples. From the spectra, the characteristic absorption bands at 1,499, 1,598, and
ꢁ
1
3
,070 cm , which are attributed to -N=N-, C=C (aromatic) and C-H (aromatic),
respectively, are observed in the IR spectrum of PCMA. The -OH stretching and
ꢁ
1
bending vibrations at 3,050 cm appears in the spectrum of HBA and disappears
in the spectrum of PCMA. This demonstartes that the HBA has been successfully
grafted on to the main chain of PCGE.
Figure 2. IR spectra of HMA, PCGE, and PCMA.

Side-Chain Liquid-Crystal Polymers
47
1
Figure 1 displays the H NMR spectra of monomer HBM, polymer (PCGE),
and copolymer (PCBM). The structure of monomer HBM can be identified clearly
by corresponding resonance peaks, the chemical shift of the hydrogen in Shiff’s base,
1
especially, is about 8.4 ppm. Comparing the three H NMR spectra, the resonance
peak of H(-CH OAr) shifts from 3.8 ppm (-CH Cl shown in Fig. 1) to 4.2 ppm
2
2
due to the change of chemical environment for the hydrogen; the other protons
do not change apparently in chemical shifts (d). In addition, the shape of peaks in
HBM exhibits a sharp multiplet as in Fig. 1a and become a fewer or a broad singlet
in Fig. 1c, which implies that the compound characterized in Fig. 1c is a copolymer.
The spectroscopic data demonstrate that the structures of all compounds are consist-
1
ent with those expected. The H NMR analysis confirms the attachment of the meso-
genic groups onto the PEGE backbone, which is in agreement with the reaction
1
shown in Scheme 1. The H NMR spectra of PCBH, PCMB, and PCBM are similar
to the model.
Thermal Properties
Thermal properties of the copolymers were examined by DSC measurement, which
ꢁ1
ꢀ
was carried out with heating and cooling rates at 20 C min under a dry nitrogen
purge.
Figure 3a shows DSC trace of PCMA. The DSC traces in the second heating
scan for PCBH, PCMB, and PCBM are shown in Fig. 3b. The thermal data of
the copolymers are listed in Table 1 and were extracted from the second heating
and cooling cycles.
ꢀ
PCMA exhibits two endothermic peaks at about 118.5 C (DH ¼ 28.1 J=g) and
ꢀ
1
33.1 C (DH ¼ 1.9 J=g) in the second heating scan. During the second cooling scan,
ꢀ
on isotropic to a liquid-crystalline phase transition at about 128.4 C (DH ¼ 1.6 J=g)
ꢀ
and the crystallization transition at about 87.3 C (DH ¼ 28.1 J=g) were observed. In
Fig. 3b, PCMB and PCBM show two endothermic peaks, namely, a melting tran-
sition and a clearing transition, in the second heating scan. The results indicate that
PCMA, PCMB, and PCBM possess liquid-crystalline behavior. PCBH does not
Figure 3. DSC thermogram of PCMA (a) and PCMB, PCMB, and PCBM (b) at a heating
ꢁ1
ꢀ
rate 20 C min
.

4
8
C. He and C. Zhang
Table 1. Thermal transition temperature and enthalpy change of PCGE-based
polymers
Phase transition temperature
ꢀ
(
C) and enthalpy change
Substitution
a
degree (%)
b
c
d
e
Sample
Tm (DH)
T (DH)
Tn-k
Ti-n (DH)
i
PCBH
PCMB
PCMA
PCBM
87
89
91
90
51.5 (8.70)
113.8 (27.2)
118.5 (28.1)
101.5 (22.9)
—
—
101.5 (18.7)
87.3 (28.0)
—
128.4 (0.9)
122.0 (1.3)
148.8 (0.8)
133.2 (1.1)
142.1 (0.52)
f
f
98
142
a
1
Determined from H NMR measurements.
Melting temperature.
b
c
Isotropization temperature.
d
Transition from nematic to crystalline.
Transition from isotropic to nematic.
Determined by optical microscopy.
e
f
exhibit a thermotropic liquid-crystalline phase due to its unique endothermic peak
for melting transition and without a clearing transition.
Table 1 summarizes the thermal properties of the synthesized copolymers. In the
cooling scan, the phase transitions from the melting state to the solid state of PCBH
and PCBM have not appeared because, as observed in our experiment, the process is
very slow at ambient temperature and is unable to solidify in minutes even at rate of
ꢀ
ꢁ1
ꢀ
5
C min until ꢁ20 C. On the other hand, the enthalpy changes from nematic to
isotropic phase transition are too small to be found in the curves. This behavior is
similar to that of compounds that have been synthesized previously [20]. Their soli-
difying points, therefore, can only be determined by optical microscopy.
The phase transition temperatures and corresponding enthalpy changes for these
copolymers during cooling are lower and smaller than those obtained during heat-
ing, which should be due to the motion of the flexible main backbone hindering sam-
ple crystallization. The change of melting point and corresponding enthalpy change
do not display apparent rule with different mesogens, which the usual behavior seen
with liquid crystals will be further investigated in future.
Liquid Crystalline Textures
Polarizing optical microscopy (POM) can reveal directly the presence and type of
mesophase in terms of the texture and the temperature of phase transition. In obser-
ꢀ
vation, the samples are heated to approximately 10 C above their clearing points and
ꢁ1
ꢀ
then cooled at 0.2 C min
.
POM of copolymers PCMB, PCMA, and PCBM exhibits a typical Schlieren
texture, which is assigned as a nematic phase as shown in Fig. 4. The textures are
ꢀ
ꢀ
obtained by cooling the isotropic phase to 131 C for PCMB, to 132 C for PCBM,
ꢀ
and to 100 C for PCMA, respectively. PCBH is not mesogenic. Obviously, the tex-
ture is affected greatly by the mesogenic unit. The result shows that the temperature
of the texture developing is approximately consistent with the second transition
measured by DSC.

Side-Chain Liquid-Crystal Polymers
49
Figure 4. POM of copolymer in cooling from isotropic phase (ꢃ100). .
X-Ray Diffraction
X-ray diffraction measurement can further confirm the existence of
a
liquid-crystalline phase and identify the types of mesophase. The diffractograms of
PCMB and PCBM are similar to that of PCMA, shown in Fig. 5 as a representative
example.
Figure 5a displays the diffraction pattern obtained with powder sample at
ambient temperature without special treatment and exhibits some sharp peak reflec-
ꢀ
tions in the wide-angle region (about 2h ꢂ 22 ), indicating the complex structure
similar to a quasi-crystal phase. The diffraction pattern (Fig. 5b), which was
obtained by heating the sample to the liquid-crystalline phase and then quenching
in a liquid nitrogen bath, presents a diffuse, broad halo in the wide-angle region
and no peak was observed in the low-angle region, which implies that the polymer
has no lamellar order in the direction of the molecular long axes and resembles an
amorphous polymer. The result indicates the presence of a nematic mesophase.
Figure 5. Wide-angle X-ray diffractograms of PCMA: (a) before quenching and (b) after
quenching.

5
0
C. He and C. Zhang
Figure 6. TGA curves of copolymers under nitrogen.
Thermal Stability
The thermal degradation properties of the PCGE-based copolymers was investigated
by TGA and the measurement was carried out under a nitrogen atmosphere in the
ꢀ
ꢀ
range of 50 Cꢄ 600 C. The obtained TG curves are shown in Fig. 6, which exhibits
the comparative thermal degradation of resulting polymers. The decomposition of
ꢀ
PCMA and PCBM begins at near 295 C corresponding to the degradation of the azo-
ꢀ
benzene and the Shiff’s base groups. At around 350 C, the cleavage of the PCGE back-
ꢀ
ꢀ
bone (-OCH CH Cl) occurs. The decomposition stage, at around 420 C–440 C, may
2
2
be due to cleavage of the benzene ring and the chain (-OCH CH ). Obviously, the ther-
2
2
mal stability of the copolymers is related to the structure of mesogenic groups.
Conclusion
A series of poly(2-chloroethyl glycidyl ether)-based side-chain liquid-crystalline
copolymers has been synthesized by the nucleophilic substitution reaction of poly(2-
chloroethyl glycidyl ether) with compounds containing a mesogenic group according
to a previously developed synthetic approach. The structures of the synthesized com-
1
pounds were confirmed by IR and H NMR spectroscopy, which give satisfactory
data corresponding to the molecular structures expected. DSC measurement,
POM investigation, and X-ray diffraction studies indicate that the obtained copoly-
mers, except for that bearing a biphenyl group, possess liquid-crystalline properties
and display a nematic phase. All the results demonstrate that poly(2-chloroethyl gly-
cidyl ether) has an ability to act as the main-chain backbone of a side-chain liquid-
crystalline copolymer; nevertheless, its molecular weight is expected to be further
raised in the future.
Acknowledgment
We are pleased to acknowledge support from Professor Cheng Sixue, Wuhan
University, for her experimental assistance and a grant from the National Nature

Side-Chain Liquid-Crystal Polymers
51
Science Foundation (No. 20474049), Specialized Research Fund for the Doctoral
Program of Higher Education (No. 20040497001) of China.
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