DIELECTRIC STUDIES ON LIQUID-CRYSTALS UNDER HIGH-PRESSURE .6. LOW-FREQUENCY RELAXATION PROCESS IN THE NEMATIC AND SMECTIC A PHASE OF 4-N-OCTYL-4'-CYANOBIPHENYL (8CB)

Dielectric relaxation studies of 4-n-octyl-4'-cyanobiphenyl (8CB) were performed in the pressure range 0.1-120 MPa, the frequency range 1 kH z-13 MHz and the temperature range 295-331 K. The dielectric behaviour of 8CB in the nematic phase is similar to that of 5CB, 6CB and 7CB. I n particular, the activation enthalpy decreases with increasing pressu re and the activation volume decreases with increasing temperature in the nematic phase. In the smectic A phase of 8CB both these parameters are markedly lower and show opposite pressure and temperature depende ncies. We tentatively interpret these effects as a result of breaking of the dipole-dipole correlations which manifests itself differently i n these liquid crystalline phases.