Wholly aromatic thermotropic liquid crystalline polyesters of 2-(α-phenylisopropyl)hydroquinone; copolymers with 1,1′-biphenyl-4,4′-diol and 2,6-dihydroxynaphthalene

Two series of new aromatic copolyesters have been synthesized and their properties including liquid crystallinity have been studied. The first series was synthesized by polymerizing mixtures of diacetates of 2-(α-phenylisopropyl)hydroquinone and 1,1′-biphenyl-4,4′-diol with terephthalic acid, and the second by polymerizing mixtures of diacetates of 2-(α-phenylisopropyl)hydroquinone and 2,6-dihydroxynaphthalene with terephthalic acid. These polyesters were characterized by differential scanning calorimetry, thermogravimetric analysis, wide-angle X-ray diffractomerty, and optical microscopy. The glass transition temperatures of the first series decrease from 167 to 138°C as one increases the content of the 1,1′-biphenyl-4,4′-diol unit to 50 mol %. The T_g values of the second series are slightly higher than those of the first series, and appear to be less dependent on their composition. The degree of crystallinity of the first series decreases rapidly by copolymerization, much faster than that of the second series. The melting points of the first series copolymers are significantly lower than those of the second series. All of the copolyesters reported in this investigation form nematic melts. The initial decomposition temperatures of the copolymers were higher than 450°C. It was confirmed that thermal stability of the homopolyester, PIBPL-1.00, containing the isopropylidene units is significantly improved when compared with that carrying benzylic hydrogens. © 1997 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 35 : 2777–2786, 1997