T. Bruckert et al., DIFFERENTIAL THERMAL-ANALYSIS (DTA) AND DIELECTRIC STUDIES ON 4-(TRANS-4-PENTYL-CYCLOHEXYL)-BENZONITRILE (PCH-5) UNDER HIGH-PRESSURE, Molecular crystals and liquid crystals science and technology. Section A, Molecular crystals and liquid crystals, 262, 1995, pp. 209-220
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.