DYNAMIC PROPERTIES OF DISCOTIC LIQUID-CRYSTALS WITH TRIPHENYLENE AND BENZENE CORES, STUDIED BY BROAD-BAND DIELECTRIC-SPECTROSCOPY

Broadband dielectric spectroscopy in the frequency range from 10(-4) t o 10(9) Hz was employed to study the molecular dynamics of various tri phenylene and benzene derivatives in their isotropic and liquid crysta lline phases. For all derivatives, two relaxation processes were detec ted in their columnar mesophases. The beta-process shows an Arrhenius- like temperature dependence with activation energies between 37 and 40 kJ mol-1. This process is assigned to a librational motion of the est er groups in the side chains. The alpha-process follows a Williams-Lan del-Ferry temperature dependence and is correlated to the glass transi tion. This process is assigned to the hindered rotation of the molecul ar discs around the columnar axis. In the isotropic phase, the dielect ric properties of the benzene derivatives are essentially different fr om the triphenylenes, most probably because of the difference in capab ility of conformational rearrangement.