Structural, Dielectric, and Rheological Characterization of Thermotropic Liquid Crystalline Copolyesters Based on 4-Hydroxybenzoic Acid, 4,4′-Dihydroxybiphenyl, Terephthalic Acid, and Isophthalic Acid

We have investigated the structures, segmental dynamics, and flow characteristics of two aromatic copolyesters: Xydar SRT-300 (Dartco) comprising 4-hydroxybenzoic acid (HBA), 4,4′-dihydroxy-biphenyl (BP), and terephthalic acid (TA) in a 68/16/16 molar ratio and POB S6635 (Sumitomo) comprising HBA, BP, TA, and a small fraction of isophthalic acid (IA) “kinks” in a 62/19/15/4 ratio. The X-ray powder diffraction measurements indicate the predominance of a high degree of three-dimensional order in the annealed Xydar polymer, with a substantial reduction in the degree of order upon the introduction of a small fraction of kinked comonomer units in the POB polymer. At high temperatures below the transition to nematic melts, both polymers display one sharp X-ray peak characteristic of hexagonal interchain packing order of highly extended chains. The dielectric relaxation results show that the rotational motions of HBA dipoles are as active in the highly (three-dimensionally) ordered annealed Xydar samples as they are in less ordered quenched material. Both Xydar and POB polymers display a low-temperature Arrhenius-type (β) relaxation (ca. 0 °C for Xydar (10 kHz)) which corresponds to localized motions of the HBA dipoles, with the POB copolymer displaying an additional weak WLF-type (α) relaxation at higher temperatures (ca. 140 °C (10 kHz)). The measured relaxation strength for Xydar is much smaller than that of POB and indicates a high degree of conformational order that tends to cancel the dipole vectors of mobile ester groups, in close resemblance to the situation observed for the HBA homopolymer (PHBA) in the smectic E-type structure above 340 °C. In contrast to the Xydar result, the much larger relaxation strength observed for POB across the β transition indicates the absence of such conformational order, the dielectric increment approaching that expected for independent, uncorrected reorientations of the HBA dipoles. Dynamic rheological measurements, limited to the POB polymer, indicate a shear-independent complex viscosity at low frequencies for the melt (370 °C) which is unusual for a nematic melt. Moreover, rheological cooling sweeps exhibit an essentially temperature-independent solidlike behavior throughout the temperature range 350 to 50 °C which appears to be dominated by the rudimentary interchain packing order but completely unaffected by the rapid rotational segmental motions.