Interaction potential in nematogenic 6CHBT.
Pressure-volume-temperature (PVT) measurements were carried out on the nematic liquid crystal 4(trans-4'-n-hexylcyclohexyl)isothiocyanatobenzene (6CHBT). In combination with previous dielectric relaxation measurements at elevated pressure and new measurements extending to GHz frequencies, the characteristics of the anisotropic interaction potential were determined. The thermodynamic potential parameter, gamma, which measures the variation of the interaction energy with volume, equals 5.03 +/- 0.06, with a barrier height equal to approximately 7 kJ/mol. Thus, there is a low potential barrier to reorientations of the 6CHBT molecule about its short axis; however, the retarding potential is strongly volume-dependent. The longitudinal reorientational times determined for various thermodynamic conditions superpose using a scaling exponent equal to gamma within the experimental error. It then follows, as found recently for other liquid crystals, that this relaxation time must be constant along the pressure-dependent clearing line. Thus, the control parameter (e.g., Gibbs free energy) governing the competition between the anisotropic energy and the entropy must also govern the rotational dynamics in the ordered phase of this liquid crystal.
Chemistry Division, Code 6120, Naval Research Laboratory, Washington, DC 20375-5342, USA.
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Bogoslovov
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Roland
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Czub
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Urban