DYNAMIC-MECHANICAL AND DIELECTRIC-RELAXATION IN A SERIES OF MAIN-CHAIN THERMOTROPIC LIQUID-CRYSTALLINE POLYESTERS

The effects of structural changes on the frequency dependent dynamic m echanical (d.m.a.) and dielectric relaxation behaviour in a series of wholly aromatic thermotropic main chain liquid crystalline (LC) polyes ters were investigated. The polymers were side-group substituted poly( hydroquinone-terephthalates), modified with biphenols and 4-hydroxyben zoic acid. The side groups, R, were varied as R = methyl, phenyl, or t -butyl. Data were also obtained for Vectra A950. These LC polymers exh ibit three prominent relaxation processes. Some of the LC polymers in this series are 'non-crystalline' with strong and narrow glass transit ions which are shown to vary as much as 80 degrees C depending on the choice of substitution of the hydroquinone group. Tertiary butyl subst itution on the hydroquinone units was found to lead to the highest gla ss transition (ca 185 degrees C). Another common feature of most of th ese LCPs is a sharp but weak second 'glass transition' observed at low er temperatures than the main glass transition. This is attributed to motions of non-substituted aromatic ester species including 4-hydroxyb enzoic acid (HBA) due to their relatively low barriers to rotation. Co mparisons are made with the wholly aromatic, HBA-rich, LC polymer, Vec tra(R) Three of the LC polymers exhibit essentially the same subglass gamma relaxation at ca 60 degrees C (1Hz) which is attributed to local motion involving the non-substituted aromatic units such as HBA. Addi ng methyl side group substituents to the or tho position of the biphen ol group restricts the subglass gamma process in a kinetic sense, shif ting the process to higher temperatures. The effect of substitution of the hydroquinone group on the gamma relaxation was also systematicall y investigated. Copyright (C) 1996 Elsevier Science Ltd.