On the derivation of the nematic order parameter from the dielectric relaxation times

Abstract

The dielectric relaxation times characterizing the rotations around the molecular short axis in the nematic (τ_‖) and isotropic (τ_is) phases of several liquid crystalline cyanophenyl compounds are re-analysed with the aim of obtaining the retardation factor g_‖ and then the nematic potential q and the order parameter S. Two theoretical approaches are employed, those of Meier and Saupe and of Coffey etal., which lead to substantially different S values. The calculated order parameters as functions of temperature and pressure are compared with those measured by several other experimental techniques. On this basis it is concluded that the Coffey etal. theory better relates the relaxation times with the main parameters characterizing the nematic state (nematic potential, order parameter). The quantities obtained are used to check some theoretical predictions, especially at the isochoric condition.