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-(CH2)5-COO-C6H4-OOC-C6H4-X (where X = F, Cl, Br, I) has been investigated by means of broadband dielectric relaxation spectroscopy (DRS, 10-2-106 Hz) and thermally stimulated depolarization current (TSDC; -180 to 30 C) techniques. The process, with a significant contribution being provided by the segmental main-chain motions, has been studied in detail. The temperature dependence of the relaxation rate of the approximately process shows a crossover at a characteristic temperature Tc from the Arrhenius type at high temperatures to the Vogel-Tammann-Fulcher-Hesse type at lower temperatures. The magnitude and the shape of the relaxation show characteristic changes in the liquid-crystalline phases, in particular close to Tc. These results are discussed in the framework of cooperativity and of the influence of the liquid-crystalline layers.