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

We have performed molecular dynamics simulation investigations of the phase behavior of an ensemble of rigid molecules interacting via a soft-core rep ulsive potential. The system consisted of 600 bead-necklace molecules each composed of 11 interaction centers (beads). The system manifested two liqui d crystalline phases, a nematic phase and a smectic A phase. Initial points on the isotropic-nematic and nematic-smectic A (P,T) phase coexistence cur ves were established through rigorous calculation of chemical potentials. T he Gibbs-Duhem integration method was subsequently employed to trace the is otropic-nematic and nematic-smectic A coexistence curves over a wide range of pressure and temperature. This simple model was found to capture qualita tively many of the features of the phase behavior of real thermotropic liqu id crystals. The isotropic-nematic transition was found to be weakly first order. The enthalpy, entropy, and density of transition for the isotropic-n ematic increased with increasing temperature, while the temperature (pressu re) 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 orientati onal order parameter, indicating coupling between positional order and ther mal fluctuations of orientational order. However, contrary to mean-field pr edictions and the observed behavior for some real thermotropic liquid cryst alline materials, the enthalpy of the nematic-smectic A transition was foun d to increase with increasing temperature/pressure. (C) 2001 American Insti tute of Physics.