Nematic-isotropic transition in some lattice models for rigid cores having semiflexible tails: Segmental Lennard-Jones interactions

Abstract

Two average-environment simple cubic lattice models---a refined model and a simple model, both having site-site (segmental) pair Lennard-Jones (LJ) interactions---for molecules composed of rigid cores having semiflexible tails are presented. The calculated values of the following properties at the nematic-isotropic transition for rigid rods of varying length are compared with relevant experimental data for PAA ($p$-azoxyanisole, or 4,4\ensuremath{'}-dimethoxyazoxybenzene): temperature, core orientational order parameter, nematic density and volume, relative density change, and relative entropy change. The temperature change as a function of volume change at constant order parameter is also discussed. In general, both LJ models give considerably better quantitative agreement with experiment, especially for the temperature and the relative density change, than do the earlier lattice models with hard repulsions, with or without constant segmental pair interaction energies. In most aspects, these LJ models give good quantitative agreement with experiment. These LJ models elucidate the importance of realistic intermolecular potentials, especially the role of soft repulsions, in describing an order-disorder transition between two condensed phases.