Dipole Relaxation in a Liquid Crystal

Abstract

The real and imaginary parts of the dielectric constant of a 1.75:1.00 mixture of cholesteryl chloride and cholesteryl myristate have been measured in the frequency range 1 kHz–10MHz. Two modes of molecular rotation are found to contribute to dipole relaxation: rotation of cholesteryl chloride around a short molecular axis (process $R_1$ ) and rotation of cholesteryl myristate around a long molecular axis ${\mathrm{(R}}_2)$ . The relaxation frequency associated with $R_1$ changes by not more than a factor of 2 when going from the cholesteric phase to the isotropic melt or to a field‐induced aligned nematic phase. This indicates that the potential barrier hindering molecular rotation is determined by short‐range order rather than long‐range order. From the magnitude of the dielectric increment, conclusions are drawn regarding the molecular arrangement in the various phases. The temperature dependence of the relaxation frequency is analyzed in terms of absolute rate theory.