The Temperature Dependence of the Dielectric and Conductivity Anisotropies of Several Liquid Crystalline Materials

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Abstract
The temperature dependence of the conductivity and dielectric anisotropies are reported for several liquid crystalline materials of varying structure. The types of compounds studied are Schiff bases, p-phenyl benzoates, and phenyl-p-benzoyloxy benzoates, having either carbonate or alkyl terminal substituents. The anisotropies are computed from the bulk conductance and capacitance measured both parallel and perpendicular to an external magnetic field. The sign and magnitude of the anistropies are sensitive to changes in molecular structure as well as to phase transitions among the mesophases. Changes in sign of the conductivity anisotropy, dielectric anisotropy, or both, were found to occur with changing temperature in the alkoxy-carbonate Schiff bases, whereas no such anomaly was observed for the carbonate-alkoxy homologs. In such anisotropy sign reversals, there occurs a depression in ∊‖ which is frequency-independent from 10–105 Hz. A sign reversal in the dielectric anisotropy also occurs in the phenyl-p-benzoyloxy benzonates, because of an unusually low frequency dielectric loss in ‖. The loss appears to occur as a single relaxation process with a characteristic frequency of 10 kHz at about 50°C. The dielectric behavior of the p-phenyl benzoates shows a nearly constant anisotropy throughout their nematic range.