Abstract
AbstractThe static and complex permittivity of octyloxy‐cyanobiphenyl (8 OCB) has been measured for the smectic, nematic and isotropic phase as a function of temperature and pressure. The main part of the work is concerned with the study of ϵ∥, the component of the permittivity parallel to the director n, and its frequency dependence up to 13 MHz, yielding the dielectric relaxation time τ∥, connected with the rotation of the molecules around their short axes. The temperature and pressure dependence of τ∥ allows to calculate the activation enthalpy, Δ≠H∥, and activation volume, Δ≠V∥, respectively. The activation parameters for the nematic phase (Δ≠H∥ = 75‐55 kJ/mol, decreasing with increasing pressure, Δ≠V∥ = 65‐45 cm3/mol, decreasing with increasing temperature) are similar to previous results for octyl‐cyanobiphenyl (8 CB), however, for the smectic Ad phase (Δ≠H∥ ≈︁ 40 kJ/mol, Δ≠V∥ = 40‐50 cm3/mol) the p, T‐dependencies are opposite to 8 CB. A comparison of the relaxation times for the isotropic and nematic phase enables us to estimate retardation factors and the nematic potential q in the frame of theories of Meier‐Saupe and Coffey et al. With the aid of literature data for the order parameter S the linear relationship q ∼ S was checked.
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