Determination of the Maier-Saupe strength parameter from dielectric relaxation experiments: a molecular dynamics simulation study

Molecular dynamics simulations have been performed to investigate the rotat ional motion in the nematic and isotropic phases of a model mesogenic syste m in which the interactions between the molecules are represented by the Ga y-Berne potential. First-rank end-over-end rotational relaxation times, ana logous to those measured using dielectric relaxation spectroscopy for real mesogens with a longitudinal electric dipole, have been determined as a fun ction of temperature and density. The relaxation times at temperatures thro ughout the nematic region are found to be larger than the values extrapolat ed from the isotropic phase to the same temperature. The simulation results are compared with the extended Debye theory for dielectric relaxation in t he nematic phase. This relates the reduction in the relaxation rate to the retardation factor which depends on the Maier-Saupe strength parameter, and in turn is defined uniquely by the second-rank orientational order paramet er. The simulations indicate that the retardation factor at constant streng th parameter is density dependent, a feature neglected in the relaxation th eory. We compare the simulation results where possible with experiment.