T. Bruckert et al., DIFFERENTIAL THERMAL-ANALYSIS (DTA) AND DIELECTRIC STUDIES OF 4-(TRANS-4-HEPTYL-CYCLOHEXYL)-BENZONITRILE (7PCH) UNDER HIGH-PRESSURE, Zeitschrift fur Naturforschung. A, A journal of physical sciences, 50(11), 1995, pp. 977-983
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
.