DIFFERENTIAL THERMAL-ANALYSIS (DTA) AND DIELECTRIC STUDIES OF 4-(TRANS-4-HEPTYL-CYCLOHEXYL)-BENZONITRILE (7PCH) UNDER HIGH-PRESSURE

With the aid of DTA the phase diagram of 4-(trans-4-heptyl-cyclohexyl) -benzonitrile, 7PCH, was redetermined. Dielectric studies on the stati c 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 temperatu re range 290-380 K. For the slow relaxation process in the nematic pha se, characterized by the relaxation time tau(II), we have calculated t he activation volume, Delta(not equal) V-II, of about 80 to 60 cm(3)/m ol (decreasing with increasing temperature), and the activation enthal py, Delta(not equal) H-II, of approximately 70 kJ/mol. The isochoric a ctivation energy, Delta(not equal) U-II, amounts to about 36 kJ/mol. T he pressure dependence of Delta(not equal)H(II) is less pronounced com pared with previous findings for 5PCH and 7CB. Estimates of tau(o) (ex trapolated from the relaxation time in the isotropic phase) allow the calculation of the retardation factor g(II) = tau(II)/tau(o) that is a nalysed in terms of theories of Martin-Meier-Saupe and Coffey in order to evaluate the nematic potential q. Using data for the order paramet er S from NMR studies it was possible to determine the strength of the interaction potential in the nematic phase and to check the relation q similar to S which is valid at p = const., but not at V = const. The results are compared with recent findings for other homologous series .