Temperature Dependence of the Relaxation Rates of <i>α</i> and δ Relaxation in Liquid-Crystalline Side-Group Polymethacrylates

Abstract

Dielectric spectroscopy in the frequency range from 10-2 to 106 Hz and in the temperature range from 190 to 430 K is employed to analyze the temperature dependence of the relaxation rates of the α and δ relaxation of liquid-crystalline polymethacrylates having derivatives of (p-alkoxyphenyl)benzoate as mesogenic units in the side group. Different mesophases were achieved by variation of the spacer length. When a temperature derivative method was applied, it was found that the temperature dependence of the relaxation rate of the dielectric α relaxation in these systems displays two different regions: an Arrhenius-like behavior at high temperatures and a behavior according to the Vogel−Fulcher−Tammann law at low temperatures which is characteristic for the dynamic glass transition in general. The crossover between these two dependencies characterized by a temperature Tdc is shifted to lower temperatures with increasing spacer length, but the ratio of Tdc and the glass transtion temperature is nearly independent of the spacer length. The result is discussed in the framework of the cooperativity of the glass transtion. The ratio of the relaxation rates of the α and δ processes increases with decreasing temperature in the liquid-crystalline mesophase range. This is explained by an increase of local order with decreasing temperature. Close to the glass transiton temperature, the ratio of the relaxation rates of the α and δ processes decreases. This indicates that the dielectric α and δ processes freeze at least together and cannot be regarded further as independent relaxation processes in that temperature regime.