DIELECTRIC STUDY OF THE MAIN-CHAIN LIQUID-CRYSTAL POLYESTERS POLY(ALKYL TEREPHTHALOYL-BIS-4-OXYBENZOATE)S AND POLY(ALKYL-4,4'-DIPHENOXY TEREPHTHALATE)S

Two analogous series of thermotropic main-chain polyesters, poly(alkyl -4,4'-diphenoxy terephthalate)s and poly(alkyl terephthaloyl-bis-4-oxy benzoate)s, have been analyzed by dielectric techniques. Analysis of t he dielectric properties as a function of temperature and frequency sh ow three main processes, termed alpha, beta and beta. The alpha trans ition has been associated with the glass transition of the amorphous p hase. The low-temperature processes beta and beta have been related t o the local motion of internal and external polar groups in the mesoge nic unit. The activation energy and dielectric strength values associa ted with these relaxations have been determined, and a very important effect of the crystalline order on these parameters has been observed. The effect of the thermal history on the dielectric behaviour has als o been studied. (C) 1998 Elsevier Science Ltd. All rights reserved.