Viscosity Coefficients of Some Nematic Liquid Crystals

Abstract

The viscosity coefficients η₁, η₂ and η₃ of 4-n-pentyl-4′-cyanobiphenyl (5CB), 4-n-octyl-4′-cyanobiphenyl (8 CB), 4-n-pentyloxy-4′-cyanobiphenyl (50 CB), and 4-n-octyloxy-4′-cyanobiphenyl (8OCB) have been determined over their nematic phases. Using literature data for the temperature dependence of the order parameter S, given for 5CB, and 8CB, a relation between the anisotropy of viscosity coefficients and parameter S is discussed. Two models; first the Imura-Okano model, and the second, assumping a dependence between the term (η_i/η_iso − 1) and the order parameter, are analysed. For the second assumption much higher values of the correlation coefficients have been obtained. On the base of our own and Skarp's data for 5CB, it has been proved that the assumption of α₃ ≅ 0 made for the elongated molecules is correct. Following this some anomalies observed for 8CB are discussed. The linear dependence obtained between the viscosity coefficients and the order parameter is analogous to the equations obtained by the author for the anisotropy of other transport properties, i.e. diffusivity of impurities and heat conduction, for the same liquid crystals.