Theoretical investigations of rotational phenomena and dielectric properties in a nematic liquid crystal

The molecular dynamics (MD) simulation, based on a realistic atom-atom inte raction potential. was performed on 3-n-pentyl-4'-cyanobiphenyl (5CB) in th e nematic phase. The rotational viscosity coefficients (RVCs) gamma (i), (i = 1, 2) and the ratio of the RVCs lambda = -gamma (2)/gamma (1) were inves tigated. Furthermore, static and frequency-dependent dielectric constants e psilon (parallel to) and epsilon (perpendicular to) were calculated using p arameters obtained from the MD simulation. Time correlation functions were computed and used to determine the rotational diffusion coefficient, D-perp endicular to. The RVCs and lambda were evaluated using the existing statist ical-mechanical approach (SMA), based on a rotational diffusion model. The SMA rests on a model in which it is assumed that the reorientation of an in dividual molecule is a stochastic Brownian motion in a certain potential of mean torque. According to the SMA, gamma (i) are dependent on the orientat ional order and rotational diffusion coefficients. The former was character ized using: i) orientational distribution function (ODF), and ii) a set of order parameters, both derived from analyses of the MD trajectory. A reason able agreement between the calculated and experimental values of gamma (i) and lambda was obtained.