Molecular mobility in halogen-containing side-chain liquid-crystalline polymers studied by dielectric spectroscopy

Molecular mobility in polymethacrylate-based thermotropic side-chain liquid -crystalline polymers (SCLCPs) of the chemical structure -CH2-C(CH3)-COO-(C H2)(5)-COO-C6H4-OOC-C6H4-X (where X=F, Cl, Br, I) has been investigated by means of broadband dielectric relaxation spectroscopy (DRS, 10(-2)-10(6)Hz) and thermally stimulated depolarization current (TSDC; -180 to 30 degrees C) techniques. The process, with a significant contribution being provided by the segmental main-chain motions, has been studied in detail. The temper ature dependence of the relaxation rate of the alpha process shows a crosso ver at a characteristic temperature T-c from the Arrhenius type at high tem peratures to the Vogel-Tammann-Fulcher-Hesse type at lower temperatures. Th e magnitude and the shape of the alpha relaxation show characteristic chang es in the Liquid-crystalline phases, in particular close to T-c. These resu lts are discussed in the framework of cooperativity and of the influence of the liquid-crystalline layers. (C) 2000 Society of Chemical Industry.