Molecular dynamics of main-chain liquid crystalline polymers

Main-chain liquid crystalline polymers (LCPs) present interesting conformat ional and dynamic properties due to the connectivity of the mesogens via fl exible linkages. Over a number of years we have investigated the static pro perties of LCPs and here we present a study of their dynamic behaviour. The technique of quasielastic neutron scattering has been applied to investiga te the molecular motion in liquid crystalline (LC) polyesters containing 4, 4'-dihydroxy,alpha,alpha'-dimethyl-benzalazine as the mesogenic unit and LC polyethers of 4,4'-dihydroxy,alpha-methylstilbene. The motion of the LCPs is studied in a wide temperature range extending across the glass transitio n (T-g) and the melting temperature (T-m). We show that, by using deuterium labelling it is possible to separate the dynamics of the rigid mesogens fr om the molecular motion of the flexible spacers. The dynamics of the polyes ters below T-m has been studied in detail and our results indicate that, co ntrary to expectations, the motion of the flexible and rigid parts of the p olymers are very similar. In addition, molecular motion below T-m does not appear to vary considerably from polymer to polymer. (C) 1999 Elsevier Scie nce B.V. All rights reserved.