Anisotropic steric effects and negative〈P4〉in nematic liquid crystals

Abstract

Starting with a model intermolecular potential that includes $\hat{\Omega }·\hat{r}$ terms to account for anisotropic steric interactions, we carry out a statistical mechanical calculation in the cell approximation, and apply it specifically to the isotropic-nematic transition of methoxybenzylidene butylaniline (MBBA). The potential is determined to fit the experimental transition temperature $T_{\mathrm{IN}}$ and the discontinuity in the orientational order parameter $〈P_2〉$ at transition. $〈P_2〉$ and $〈P_4〉$ are then calculated from solving a set of coupled self-consistency equations for the orientational and spatial parts of the distribution function, as are other phase-transition properties. There are improvements over model calculations which do not account for anisotropic steric effects, but the improvements are generally less than significant. The most striking result is that a stable nematic phase requires $〈P_4〉$ to be negative at and near the transition. It is a theoretical result qualitatively consistent with experimental data but has not been attained until now.