DIFFERENTIAL THERMAL-ANALYSIS (DTA) AND DIELECTRIC STUDIES ON 4-(TRANS-4-PENTYL-CYCLOHEXYL)-BENZONITRILE (PCH-5) UNDER HIGH-PRESSURE

The phase diagram of 4-(trans-4-pentyl-cyclohexyl)-benzonitrile, PCH-5 , was obtained by DTA measurements up to a temperature of 370 K and up to pressures of 260 MPa. Dielectric studies on the static and complex permittivity have been performed on PCH-5 in the pressure range 0.1 - 140 MPa, the frequency range 1 kHz - 13 MHz and the temperature range 311 - 350 K. From the relaxation times in the nematic phase, tau(para llel to), we have calculated the activation volume, Delta double dagge r V-parallel to, of about 70 cm(3)/mol (decreasing with increasing tem perature), and the activation, enthalpy, Delta double dagger H-paralle l to, of approximately 70 kJ/mol. The pressure dependence of Delta H-p arallel to is weaker than that found recently for 5CB. We have also ca lculated the (isochoric) activation energy of about 40 kJ/mol. The low -frequency relaxation process connected with molecular rotations about the short molecular axis was analysed in terms of the Martin-Meier-Sa upe theory, yielding q similar or equal to 10 kJ/mol for the nematic p otential. The results are compared with recent high pressure results o n 4-n-alkyl-4'-cyanobiphenyls.