DIELECTRIC STUDIES ON LIQUID-CRYSTALS UNDER HIGH-PRESSURE .5. STATIC PERMITTIVITY AND LOW-FREQUENCY RELAXATION PROCESS IN 4-N-HEPTYL-4'-CYANOBIPHENYL (7CB)

Dielectric studies have been performed on 4-n-heptyl-4'-cyanobiphenyl, 7CB, between 305 and 341 K and at elevated pressures up to 150 MPa. F rom measurements of the dielectric relaxation (up to 13 MHz) we calcul ate the activation enthalpy (decreasing with increasing pressure) and the activation volume (decreasing with increasing temperature) in the nematic state. The low-frequency process connectcd with molecular rota tions about the short molecular axis was analysed in terms of the Mart in-Meier-Saupe theory, yielding q almost-equal-to 5 kJ/mol for the nem atic potential. The dielectric behaviour is similar to previous result s for 5CB and 6CB.