Electric dipoles and phase stability in nematic liquid crystals

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Abstract
A theory for the nematic-isotropic (N-I) phase transition of prolate uniaxial molecules with longitudinal dipole moments is presented. The theory is based on the variational cluster expansion, truncated after the two-molecule term, and is implemented for polar hard spherocylinders with and without attractions, and for polar linear arrays of Lennard-Jones interactions centre. We find that the dipole interactions substantially shift the N-I transition temperature and strongly promote antiparallel molecular association, but have a weak effect on the order parameters, the pressure, and the N-I coexistence densities. The effect of dipoles on phase stability is very sensitive to their position within 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 constant density. The theory does not predict a stable ferroelectric nematic phase.