ELECTRIC DIPOLES AND PHASE-STABILITY IN NEMATIC LIQUID-CRYSTALS

A theory for the nematic-isotropic (N-I) phase transition of prolate u niaxial molecules with longitudinal dipole moments is presented. The t heory is based on the variational cluster expansion, truncated after t he two-molecule term, and is implemented for polar hard spherocylinder s with and without attractions, and for polar linear arrays of Lennard -Jones interaction centres. We find that the dipole interactions subst antially shift the N-I transition temperature and strongly promote ant iparallel molecular association, but have a weak effect on the order p arameters, the pressure, and the N-I coexistence densities. The effect of dipoles on phase stability is very sensitive to their position wit hin the molecular frame. Off-centre dipoles are shown to give rise to phase re-entrance according to the sequence N-I-N on heating at consta nt density. The theory does not predict a stable ferroelectric nematic phase.