The rotational viscosity coefficient ${\gamma }_1$ of $4-n$-pentyl-$4^{\prime }$-cyanobiphenyl in the nematic phase is investigated by combination of existing statistical-mechanical approaches (SMAs), based on a rotational diffusion model and computer simulation technique. The SMAs rest on a model in which it is assumed that the reorientation of an individual molecule is a stochastic Brownian motion in a certain potential of mean torque. According to the SMAs, ${\gamma }_1$ is found to be a function of temperature, density, rotational diffusion coefficient, and a number of order parameters (OPs). The diffusion coefficient and the OPs were 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 calculated and experimental values of ${\gamma }_1$ is obtained. It is shown that near the clearing point ${\gamma }_1$ is proportional to $P_2^2$, where $P_2$ is the second-rank OP. This limiting value of ${\gamma }_1$ is in agreement with mean-field theory.

• Received 3 November 1998

DOI:https://doi.org/10.1103/PhysRevE.59.6802