The rotational diffusion constants $D_{\perp }$ and $D_{\parallel },$ rotational viscosity coefficients ${\gamma }_i\hspace{0.5em}\mathrm{}(i=1,2\mathrm{}),$ the orientational correlation times ${\tau }_{\mathrm{mn}}^L,$ and the dielectric permittivities for nematic liquid crystals (NLCs) are investigated. ${\gamma }_i$ are calculated by a combination of existing statistical-mechanical approach (SMA) and NMR relaxation theory, both based on a rotational diffusion model. In the rotational diffusion model, 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 SMA, ${\gamma }_i$ are found to be a function of temperature, density, rotational diffusion constant for tumbling motions, and the orientational order parameters. The order parameters and rotational diffusion constant are obtained from an analysis of NMR measurements. Reasonable agreement between the calculated and experimental values of ${\gamma }_i$ for 4-n-octyloxy-$4^{\prime }$-cyanobiphenyl (8OCB) is obtained. The orientational correlation times, and the longitudinal and transverse components of the real ${\chi }_j^{\prime }\left(\omega \right)$ and imaginary ${\chi }_j^{″}\left(\omega \right)\hspace{0.5em}(j=\parallel ,\perp )$ parts of the complex susceptibility tensor for 8OCB molecules in the nematic phase are also obtained.

• Received 18 August 2000

DOI:https://doi.org/10.1103/PhysRevE.63.011704