PREPARATION AND PROPERTIES OF A SERIES OF THERMOTROPIC LIQUID-CRYSTALLINE COPOLYESTERS

A series of thermotropic liquid crystalline copolyesters were prepared by melt polymerization using p‐hydroxybenzoic acid (HBA) and terephthalic acid (TPA) as mesogenic monomers, poly(ethylene terephthalate) (PET) to give a flexible linkage, and 1,4‐dihydroxynaphthalene (1,4‐DHN) as a dissymmetrical monomer. The composition of these monomers was varied as a means of manipulating processing temperature, morphology, and properties. Copolyesters with inherent viscosities near 0.7 that were melt processible in the temperature range of 200–300°C were obtained. The glass transition, thermal degradation, and melting temperatures, and crystal size all increased with increasing mesogenic unit content. The heterogeneous structure consisting of a PET‐rich phase and a phase rich in mesogenic units was observed by SEM after chemical etching and by X‐ray diffraction analysis. Highly anisotropic melts were observed indicating the presence of a nematic mesophase. The thermotropic LC copolyesters exhibit unusually well developed and highly oriented fibrillar structure at the neck area when injection molded. The orientation developed during processing is very similar to those of short‐fiber‐filled polymer composites. The LC copolyesters commonly have three relaxations in the temperature range of −100–200°C. By dynamic mechanical thermal analysis at 10 Hz, the relaxation temperatures were found to increase with increasing content of mesogenic units and of 1,4‐DHN. For the temperature range of 25–120°C, the storage modulus of one of the members of this series were compared with Vectra (Celanese) and with Eastman PHB80, which have almost the same mesogenic contents. This new copolyester maintained its modulus until 80°C, while the modulus of Vectra decreased in two steps; a small depression at 40°C and then a large reduction at 85°C. PHB80 shows a large single transition at 50°C. Copyright © 1990 John Wiley & Sons, Inc.