A molecular dynamics study of a steric multipole model of liquid crystal molecular geometry

Abstract

We report results from a series of molecular dynamics simulations designed to study the phase behaviour of model rod-like liquid crystal molecules with different geometries interacting via the Gay-Berne potential. Following the classification of molecular geometry in terms of a multipole expansion in steric asymmetry, two models have been studied in detail: a zigzag model defined as a steric quadrupole and a triangle model defined as a longitudinal steric dipole, and comparison has been made with a cylindrical model. Results from the NVE ensemble indicate that the model steric quadrupole delays the temperature of onset of the nematic phase. Extensive simulations in the NPT ensemble demonstrate a similar trend in the temperature of onset of the smectic B phase, with a lower temperature of onset observed with the steric quadrupole than the steric dipole. Local antiparallel steric ordering within a layer was observed with the model steric dipole in the crystal B phase but not with the model steric quadrupole. This structure is in agreement with experimental results and with the prediction of the generalized molecular asymmetry model. The steric quadrupole demonstrated a rippled structure through the smectic B phase, increasing in amplitude and wavelength sufficiently to tilt molecules along a wave with respect to the system director as the system was cooled. This structure was almost absent in the final crystal structure simulated. The ensemble also allowed comparison with experiment and agreement, scaled with respect to the single-site Gay-Berne mesogen, was found to be good.