Differential Thermal Analysis (DTA) and Dielectric Studies of 4 -(trans-4-Heptyl-Cyclohexyl)-benzonitrile (7PCH) under High Pressure

Abstract

Abstract With the aid of DTA the phase diagram of 4-(trans-4-heptyl-cyclohexyl)-benzonitrile, 7PCH, was redetermined. Dielectric studies on the static and complex permittivity have been performed on 7PCH in the pressure range 0.1 -220 MPa, the frequency range 1 kHz-13 MHz and the temperature range 290-380 K. For the slow relaxation process in the nematic phase, characterized by the relaxa­tion time tII , we have calculated the activation volume, Δ ≠ V II, of about 80 to 60 cm3/mol (decreasing with increasing temperature), and the activation enthalpy, Δ ≠ H II, of approximately 70 kJ/mol. The isochoric activation energy, Δ ≠ U II, amounts to about 36 kJ/mol. The pressure dependence of Δ ≠ H II is less pronounced compared with previous findings for 5PCH and 7CB. Estimates of t0 (extrapo­lated from the relaxation time in the isotropic phase) allow the calculation of the retardation factor g II = tII/t0 that is analysed in terms of theories of Martin-Meier-Saupe and Coffey in order to evalu­ate the nematic potential q. Using data for the order parameter 5 from NMR studies it was possible to determine the strength of the interaction potential in the nematic phase and to check the relation q~S which is valid at p = const., but not at V = const. The results are compared with recent findings for other homologous series.