Thermodynamic model for the electro-optical and structural properties of Sm C*_A phase in antiferroelectric liquid crystal MHPOBC

摘要

We construct a phenomenological model to describe the electro-optical and structural properties of the Sm C*_A phase of antiferroelectric liquid crystal MHPOBC. The free-energy density of the system is expanded in terms of three degrees of freedom—tensor orientational order Q_(ij), scalar smectic order Ψ, and polarization vector P and couplings between these order parameters. In this expansion the tilt vector ξ is not incorporated because ξ is governed by Q_(ij) The values of Landau coefficients are determined by comparing with the experimental data of helical pitch, tilt angle and spontaneous polarization of MHPOBC in the Sm C*_A phase. Taking these values we evaluate the temperature variation of the tilt angle, spontaneous polarization and pitch of the Sm C*_A phase for MHPOBC. It is found that the theoretical results agree very well with the experimental data for antiferroic mesogens.