Morphology and thermal properties of liquid crystalline p-PAEB and its homopolymer

Received Jan. 18, 2006; Supported by the Natural Science Foundation of Hebei Province (B2005000108).

Abstract The unsaturated liquid crystalline (LC) compound of p-phenylene di {4- [2- (allyloxy) ethoxy] benzoate} (p-PAEB) and homopolymer poly(p-PAEB) were synthesized. The liquid crystalline behavior and thermal properties of p-PAEB and poly (p-PAEB) have been studied by polarizing optical microscopic (POM), differential scanning calorimetry (DSC), X-ray diffractometer (XRD), torsional braid analysis (TBA). The results of XRD and POM showed that p-PAEB is smectic texture and the poly(p-PAEB) are nematic texture. The dynamic mechanical properties and glass transition temperature Tg of liquid crystalline polymers network (LCP) were investigated with TBA.
Keywords Unsaturated Liquid crystalline monomer; polymerization; Morphology; Thermal properties

1. INTRODUCTION
The effect of monomer organization on the polymerization and properties of polymers has been investigated from both scientific and industrial points of view. One of the typical monomer organizations is based on the thermotropic liquid crystalline (LC) systems. Thermotropic LC has become important in the field of advanced materials, such as used in electronic devices and high-strength fibers. Recently many studies have been focused on polymer networks with an anisotropic molecular orientation by polymerization of liquid crystalline monomers in a unidirectional orientation ^[1-3]. The macroscopically oriented polymer networks were found to exhibit anisotropic mechanical and optical properties. In particular, densely cross-linked polymer network, which were prepared by bi- or polyfunctional LC monomers, showed excellent transparency and stable anisotropic molecular orientation ^[2]. The lightly cross-linked polymer networks can be shown a reversible phase transition between a liquid-crystalline phase and an isotropic phase ^[4].
    LC monomers having polymerizable groups such as acrylate, methacrylate, vinyl ether, epoxide, and diene derivatives were used in the pursuit ^[4-6]. In this paper, the unsaturated LC monomer p-phenylene di {4- [2- (allyloxy) ethoxy] benzoate} (p-PAEB) was synthesized, the polymerization of p-PAEB was investigated. The morphological structure and thermal properties of p-PAEB and poly (p-PAEB) were examined.

2. EXPERIMENTAL
2.1 Material
Hydroquinone, sulfoxide chloride, allyl 2-hydroxyethyl ether, 4-hydroxy ethyl benzoate, dimethylformamide (DMF) and tetrahydrofuran (THF) were all of analytically pure grades and supplied by the Beijing Chemical Reagent Co. China.
2.2 Synthesis of p-PAEB
The liquid-crystalline monomer p-PAEB was synthesized according to the literature ^[7]. The molecular structure of p-PAEB is as follows:

2.3 Polymerization
The homopolymer was synthesized as the following: 5.18g p-PAEB (0.01mol) and 50ml THF were added into a 100ml four-necked glass flask. The reaction was carried out under a nitrogen atmosphere, after 1%AIBN (mol of monomer) was added, the mixture was heated under reflux for 48h.The formed polymers were isolated by precipitating into a large amount of methanol and purified, then vacuum dried overnight at 50°C.
2.4 Characterization of monomer and polymer
The structure of p-PAEB was examined by means of FTIR (Bio-Rad FTS-40, USA) and ¹H-NMR (Bruker 400k, Switzerland); FTIR spectra of LC monomer and homopolymer were obtained by KBr method. Thermal phase transition behavior was examined by means of differential scanning calorimetry (DSC, Diamond, Perkin-Elmer Co.) and polarizing optical microscopic observation (POM, XPT-7, Nanjing, China). Scanning rate of DSC measurements was 10°C/min, and the peak temperatures obtained were taken as the phase transition temperatures.
    X-ray diffraction patterns were recorded by monitoring the diffraction angle 2q from 1-30° using a Savitzkey-Golay^'s Y-2000 X-ray diffractometer. The diffractometer was equipped with a CuKa (l=0.1542nm) radiation, produced under the conditions of 40 kV and 100 mA.
    Torsional braid analysis (TBA) were performed by using GDP-4 Torsional Braid Analyzer (Jilin Univ.China) at the heating rate of 2°C/min from 20-300°C.

3. RESULTS AND DISCUSSION
3.1 NMR and FTIR analysis
Monomer p-PAEB:¹H-NMR(DMSO), d[ppm]= 3.76-4.26 (m, 12H, 2-CH₂-O-CH₂-CH₂-); 5.16-5.31 (m, 4H, H₂C=CH-); 5.8-5.9(m, 2H, H₂C=CH-); 7.36-8.11 (m, 12H, Ar-H). This assignment is in agreement with the structural compositions of the sample.
    The IR spectra of p-PAEB and homopolymer reveal the presence of characteristic absorptions of components. The absorption peaks at 3073 cm^-1 of p-PAEB can be associated with -C=C- stretching vibration absorption bands, on the other hand, at 3000-2800 cm^-1 (CH₂ stretching), 1750 cm^-1 (carbonyl group vibration), 1620-1436 cm^-1 (aromatic C=C stretching) are consequences of the p-PAEB. Especially, a peak of homopolymer at 3073cm^-1 becomes much lower than that of p-PAEB. It indicated that the p-PAEB has been polymerized.
3.2 Optical texture and X-ray diffraction
The textures of liquid-crystalline p-PAEB is shown in Fig.1, As seen from Fig.1 the liquid-crystalline compound (p-PAEB) under POM reveals fan optical texture of smectic phase^[8], the texture of polymer reveals irregularly nematic schlieren texture^[8] (Fig. 2), which will be clearly identified in the X-ray analysis described.

Figure 1 Texture of LC p-PAEB	Figure 2 Texture of polymer

    To obtain more detailed information on the mesophase structure of the polymer, XRD studies were carried out. The patterns of p-PAEB and poly(p-PAEB) are shown in Fig.3. The diffraction pattern of pure p-PAEB exhibited a high degree of crystallinity and attributed to a smectic mesophase. It was exhibited two narrow peaks in the wide-angle region (2q =20^o), in addition to one sharp peak in the small-angle region (2q =5^o). On the other hand, the X-ray patterns of poly (p-PAEB) showed smaller peak in the small-angle region and the two narrow peaks in the wide-angle region (2q =20^o). These are shown that the degree of crystallinity of poly (p-PAEB) is lower than that of p-PAEB, and showing that the polymerization of monomer p-PAEB has some restrict for its crystallization.
    The small-angle diffraction (2q =5^o) corresponds to the Bragg maximum of d=1.7nm for p-PAEB. The wide-angle diffraction (2q =20^o) corresponds to an intermolecular spacing (or the diffuse diffraction) of  0.446nm in the layer. The results suggest that the layer spacings correspond to single-layer packing.

Figuer 3 WAXD curves of p-PAEB and P-1 (polymer of p-PAEB)

Figure 4 DSC thermograms of monomer p-PAEB and homopolymer

3.3 Mesomorphic properties of p-PAEB and homopolymer
It was found that the LC polymers change to liquid crystal phase when heated to their T_m. The broken focal-conics texture can be observed in the range of melting temperatures (T_m) to isotropization temperatures (T_i, clearing point).
    Fig. 4 shows the DSC thermograms obtained for p-PAEB and its homopolymer. As seen from Fig. 4, in heating DSC scan of p-PAEB, two peaks are observed at T_m=120.5°C, T_i=191.5°C. The mesomorphic state of polymer occurs in the range 77-170°C. The temperatures of T_m and liquid crystal to isotropic transition T_i are lower than the monomer and increased broad. This is due to increase length of flexible chain between the two liquid crystal units after polymerization and the polydispersity of polymeric product ^[9,10].
3.4 Mechanical loss and glass transition temperature T_g
The torsional braid analysis (TBA) was used to determine the mechanical loss and glass transition temperature T_g of the LC polymer network systems. Generally, the T_g of a low cross-linking density resin system is related to the conversion and a soft segment, the conversion depends upon the curing conditions, such as temperature and time. With these curing conditions varying, the T_g of the system will be changed, so that T_g has been used directly as a parameter for conversion of thermosets in the analysis of reaction kinetic models ^[11].
    The TBA spectrum for polymer is shown in Fig. 5, as seen from Fig.5, the maximum loss peak appears at 114°C which is lower than T_m (120.5°C). This result also indicates that the T_g is at about 114°C and macroscopically uniaxial molecular orientation of LC monomer and LC order-distribution is playing a more important role in the glass transition.

Figure 5 TBA spectrum of poly(p-PAEB) schlieren texture

4. CONCLUSIONS
The compound of p-PAEB is smectic LC compound, it has a LC temperature range of 120.5 to 191.5°C. The homopolymer of p-PAEB is nematic schlieren liquid crystalline texture, and the LC temperature range is 77 to 170°C. The glass transition temperature T_g is about at 114°C and lower than T_m (120.5°C) of p-PAEB.

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p-PAEB液晶聚合物的形态与热性能
王勇,  高俊刚，侯桂香，
（河北大学化学与环境科学学院， 保定， 071002）
摘要 合成了液晶p-PAEB和它的均聚合物，用偏光显微镜、DSC、x-射线衍射仪和扭辩分析仪研究了它们的液晶行为和热性能。结果说明单体是近晶型织构，而均聚合物是向列型纹影织构，聚合物有更宽的液晶温度范围。用TBA测定了聚合物的Tg, 为114℃。
关键词 不饱和液晶单体，聚合，形态，热性能