A molecular dynamics simulation study of the phase behavior of an ensemble of rigid bead-necklace molecules

Abstract

We have performed molecular dynamics simulation investigations of the phase behavior of an ensemble of rigid molecules interacting via a soft-core repulsive potential. The system consisted of 600 bead-necklace molecules each composed of 11 interaction centers (beads). The system manifested two liquid crystalline phases, a nematic phase and a smectic A phase. Initial points on the isotropic–nematic and nematic–smectic A (P,T) phase coexistence curves were established through rigorous calculation of chemical potentials. The Gibbs–Duhem integration method was subsequently employed to trace the isotropic–nematic and nematic–smectic A coexistence curves over a wide range of pressure and temperature. This simple model was found to capture qualitatively many of the features of the phase behavior of real thermotropic liquid crystals. The isotropic–nematic transition was found to be weakly first order. The enthalpy, entropy, and density of transition for the isotropic-nematic increased with increasing temperature, while the temperature (pressure) range over which the nematic phase is stable increased with increasing pressure (temperature). The nematic–smectic A transition was also found to be weakly first order and was accompanied an increase in the orientational order parameter and a decrease in the thermal fluctuations of the orientational order parameter, indicating coupling between positional order and thermal fluctuations of orientational order. However, contrary to mean-field predictions and the observed behavior for some real thermotropic liquid crystalline materials, the enthalpy of the nematic–smectic A transition was found to increase with increasing temperature/pressure.