The dipolar relaxation behaviour of a liquid-crystalline side-chain polymer as studied by thermally stimulated discharge currents

Abstract

The various dipolar relaxation mechanisms of a liquid-crystalline side-chain polymer have been studied in detail by the technique of thermally stimulated currents. The α relaxation, or glass transition relaxation, showed a compensation behaviour, i.e. a correlation between the activation enthalpy and the activation entropy of its components. Two sub-glass relaxations were observed. The lower-temperature one shows a small activation enthalpy (5–6 kcal mol −1), and is attributed to slightly hindered internal rotations in the side group. The higher-temperature one, on the other hand, shows a higher activation enthalpy (23 kcal mol −1) and a low positive activation entropy, and is attributed to restricted internal rotations of the side groups around bonds near the main chain, eventually in cooperation with the movements of the main-chain segments. An upper glass relaxation was also observed. This relaxation shows a Williams-Landel-Ferry or Vogel behaviour and high values of its mean activation enthalpy and entropy. The attribution of this discharge at the molecular level is discussed.