DIAMAGNETIC SUSCEPTIBILITY OF LIQUID-CRYSTALLINE COPOLYESTERS - STUDYOF MOLECULAR RELAXATION

The diamagnetic susceptibility of a series of random copolyesters, P(H B/HN), of 4-hydroxybenzoic acid (HBA) with 2-hydroxy-6-naphtoic acid ( HNA) has been investigated, both, as a function of composition and tem perature. It is shown that the molecular susceptibility of the uniaxia lly oriented samples linearly decreases with HE content. Results are d iscussed in terms of the increase in magnetic anisotropy occurring wit h the introduction of the HN groups. Furthermore, the temperature depe ndence of the specific susceptibility has been investigated. The diama gnetic susceptibility of these copolymers rises at the beginning of th e solid-liquid crystalline transition. Above this temperature, the spe cific susceptibility exhibits a conspicuous time-dependent behaviour w hich is a function of the temperature at the mesophase. After long sto rage times at the mesophase, and upon cooling at room temperature, the susceptibility shows values which approach those of a macroscopically isotropic state, i.e., a state with no overall preferred orientation. X-ray diffraction measurements confirm a relaxation mechanism of the molecular orientation occurring at temperatures above the melting poin t. A similar relaxation process is observed for samples with an initia l planar orientation. (C) 1996 John Wiley & Sons, Inc.