Temperature dependence of the relaxation rates of alpha and delta relaxation in liquid-crystalline side-group polymethacrylates

Dielectric spectroscopy in the frequency range from 10(-2) to 10(6) Hz and in the temperature range from 190 to 430 K is employed to analyze the tempe rature dependence of the relaxation rates of the alpha and delta relaxation of liquid-crystalline polymethacrylates having derivatives of (p-alkoxyphe nyl)-benzoate as mesogenic units in the side group. Different mesophases we re achieved by variation of the spacer length. When a temperature derivativ e method was applied, it was found that the temperature dependence of the r elaxation rate of the dielectric a relaxation in these systems displays two different regions: an Arrhenius-like behavior at high temperatures and a b ehavior according to the Vogel-Fulcher-Tammann law at low temperatures whic h is characteristic for the dynamic glass transition in general. The crosso ver between these two dependencies characterized by a temperature T-dc is s hifted to lower temperatures with increasing spacer length, but the ratio o f T-dc and the glass transtion temperature is nearly independent of the spa cer length. The result is discussed in the framework of the cooperativity o f the glass transtion. The ratio of the relaxation rates of the alpha and d elta processes increases with decreasing temperature in the liquid-crystall ine mesophase range. This is explained by an increase of local order with d ecreasing temperature. Close to the glass transiton temperature, the ratio of the relaxation rates of the alpha and delta processes decreases. This in dicates that the dielectric alpha and delta processes freeze at least toget her and cannot be regarded further as independent relaxation processes in t hat temperature regime.