DIELECTRIC STUDIES OF OCTYLOXY-CYANOBIPHENYL (8OCB) UNDER HIGH-PRESSURE

The static and complex permittivity of octyloxy-cyanobiphenyl (8OCB) h as been measured for the smectic, nematic and isotropic phase as a fun ction of temperature and pressure. The main part of the work is concer ned with the study of epsilon(parallel to), the component of the permi ttivity parallel to the director n, and its frequency dependence up to 13 MHz, yielding the dielectric relaxation time tau(parallel to) conn ected with the rotation of the molecules around their short axes. The temperature and pressure dependence of tau(parallel to) allows to calc ulate the activation enthalpy, Delta double dagger H-parallel to and a ctivation volume, Delta double dagger V-parallel to, respectively. The activation parameters for the nematic phase (Delta double dagger H-pa rallel to = 75-55 kJ/mol, decreasing with increasing pressure, Delta d ouble dagger V-parallel to = 65-45 cm(3)/mol, decreasing with increasi ng temperature) are similar to previous results for octyl-cyanobipheny l (8 CB), however, for the smectic A(d) phase (Delta double dagger H-p arallel to approximate to 40 kJ/mol, Delta double dagger V-parallel to = 40-50 cm(3)/mol) the p, T-dependencies are opposite to 8 CB. A comp arison of the ii 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 ai d of literature data for the order parameter S the linear relationship q similar to S was checked.