Molecular dynamics of main-chain liquid crystalline polymers

Main-chain liquid crystalline polymers (LCPs) present interesting conformational and dynamic properties due to the connectivity of the mesogens via flexible linkages. Over a number of years we have investigated the static properties of LCPs and here we present a study of their dynamic behaviour. The technique of quasielastic neutron scattering has been applied to investigate the molecular motion in liquid crystalline (LC) polyesters containing 4,4'-dihydroxy,a,a'-dimethyl-benzalazine as the mesogenic unit and LC polyethers of 4,4'-dihydroxy,a-methylstilbene. The motion of the LCPs is studied in a wide temperature range extending across the glass transition (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 from the molecular motion of the flexible spacers. The dynamics of the polyesters below T_m has been studied in detail and our results indicate that, contrary to expectations, the motion of the flexible and rigid parts of the polymers are very similar. In addition, molecular motion below T_m does not appear to vary considerably from polymer to polymer.