Rotational viscosity in a nematic liquid crystal: A theoretical treatment and molecular dynamics simulation

The rotational viscosity coefficient gamma(1) of 4-n-pentyl-4'-cyanobipheny l in the nematic phase is investigated by combination of existing statistic al-mechanical approaches (SMAs), based on a rotational diffusion model and computer simulation technique. The SMAs rest on a model in which it is assu med that the reorientation of an individual molecule is a stochastic Browni an motion in a certain potential of mean torque. According to the SMAs, gam ma(1) is found to be a function of temperature, density, rotational diffusi on coefficient, and a number of order parameters (OPs). The diffusion coeff icient and the OPs Hen obtained from an analysis of a trajectory generated in a molecular dynamics simulation using realistic atom-atom interactions. In addition, a set of experimentally determined diffusion coefficients and OPs was used for evaluation of gamma(1). Reasonable agreement between calcu lated and experimental values of gamma(1) is obtained. It is shown that nea r the clearing point gamma(1) is proportional to (P) over bar(2)(2) where ( P) over bar(2) is the second-rank OP. This limiting value of gamma(1) is in agreement with mean-field theory.