language: en

High Pressure Dielectric Studies of 8CB in the Isotropic, Nematic, and Smectic A Phases

Phineus Markwick, StanisJaw Urban^, and Albert Würflinger^
Department of Physics, University of Edinburgh,
U. K.
^ Institute of Physics, Jagellonian University,
Reymonta 4, 30-059 Krakow, Poland
^ Institute of Physical Chemistry II, Ruhr University,
D-44780 Bochum, Germany

[Reprint requests to Prof. A. W., Fax: -»49 234 7094 183,]

E-mail: Albert.Wuerflinger.ruhr-uni-bochum.de Z. Naturforsch. 54 a, 275-280 (1999);

[received March 13, 1999]

The static and complex permittivity of 4-«-octyl-4'-cyanobiphenyl (8CB) has been measured for the isotropic, nemadc and smectic A phases as funcdons of temperature and pressure. The ranges of 297 - 361 K, 0.1 - 220 MPa, and 0.1 -13 MHz, were covered. Only die parallel component of the complex permittivity, = €'{f) — i£"(f)y was measured. The relaxation times r\\(p, T) as well as Ti^ip.T) were analysed at constant temperature, pressure and volume^ yielding the acdvadon volume, A^V(T), acdvation enthalpy A^/f(p), and activadon energy AW{V\ respectively. All acdvation parameters calculated for die smectic A phase of 8CB are smaller than those obtained for the nematic phase. The activadon energy consdtutes approximately half of the activadon enthalpy value in all diree phases studied. The pressure study allowed to calculate the pressure dependence of the retardation factor g^(p,T), from which die nemadc potendal q(p,T) can be derived. Using the reladonships between g\\ and q/RT proposed by Kalmykov and Coffey, the order parameter {Piip, T)) was calculated as a funcdon of pressure.

Key words: Liquid Crystals; 8CB; High Pressure; Dielectric; Thermodynamics; Order Parameter.

1. Introduction