Rotational viscosity, dynamic phenomena, and dielectric properties in a long-chain liquid crystal: NMR study and theoretical treatment

The rotational diffusion constants D_⊥ and D_∥, rotational viscosity coefficients γ_i (i=1,2), the orientational correlation times τ_mn^L, and the dielectric permittivities for nematic liquid crystals (NLCs) are investigated. γ_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, γ_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 γ_i for 4-n-octyloxy-4^′-cyanobiphenyl (8OCB) is obtained. The orientational correlation times, and the longitudinal and transverse components of the real χ_j^′(ω) and imaginary χ_j^″(ω) (j=∥,⊥) parts of the complex susceptibility tensor for 8OCB molecules in the nematic phase are also obtained.