High pressure dielectric studies of 8CB in the isotropic, nematic, and smectic A phases

The static and complex permittivity of 4-n-octyl-4'-cyanobiphenyl (8CB) has been measured for the isotropic, nematic and smectic A phases as functions of temperature and pressure. The ranges of 297 - 361 K, 0.1 - 220 MPa, and 0.1 - 13 MHz, were covered. Only the parallel component of the complex permittivity, epsilon*(f) = epsilon'(f) - i epsilon "(f), was measured. The relaxation times tau(parallel to)(p,T) as well as tau(is)(p,T) were analysed at constant temperature, pressure and volume yielding the activation volume, Delta(not equal)V(T), activation enthalpy Delta(not equal)H(p), and activation energy Delta(not equal)U(V), respectively. All activation parameters calculated for the smectic A phase of 8CB are smaller than those,obtained for the nematic phase. The activation energy constitutes approximately half of the activation enthalpy value in all three phases studied. The pressure study allowed to calculate the pressure dependence of the retardation factor g(parallel to)(p,T), from which the nematic potential q(p, T) can be derived. Using the relationships between g(parallel to) and q/RT proposed by Kalmykov and Coffey, the order parameter < P-2(p,T)> was calculated as a function of pressure.