Extended Debye theory for dielectric relaxations in nematic liquid crystals

Abstract

A theory of orientation polarization in nematic liquid crystals is developed for a molecular dipole moment parallel to the long axis of the molecule. The theory is essentially an extension of the Debye model for normal polar liquids to take into account the strong intermolecular potential which is responsible for the nematic state. It is shown in detail that the relaxation time depends strongly on the nematic potential and also on whether the polarization takes place parallel or perpendicular to the symmetry axis of the crystal. In the first case, the relaxation time increases from its value in the isotropic liquid phase by a “retardation factor” which may amount to several orders of magnitude, depending on the strength of the nematic potential. In the second case, the relaxation time decreases slightly. The calculated retardation factor is smaller than the experimental one. It is suggested that this discrepancy is due to the neglect of molecular short-range order.