Kerr-Effect in the Isotropic Liquid Phase of Nematogens^†

Abstract

The existence of strong fluctuations of the orientational order in the isotropic phase of nematics determines the nature of electro-optical effects near to T_c the temperature of the phase transition of the isotropic liquid into the liquid crystalline state. Electric birefringence in the isotropic phase of liquid crystals differing in the value and direction of the dipole moment of the molecules was investigated. The relaxation phenomena manifest using a radio frequency sinusoidal electric field in the isotropic phase of nematogens in the vicinity of T_c are discussed. Two ranges of dispersion of the Kerr effect were detected experimentally: a low frequency range (f = 10⁵ to 10⁶ Hz) and a high frequency range (for f> 10⁶ Hz) The first range has been related to supermolecular relaxation processes and characterizes the rotational mobility of the fluctuation ‘swarm’ (i.e., the nucleus of the mesophase) in the isotropic phase. The second range was due to the relaxation of the dipole orientation of the molecule forming the dielectrically anisotropic ‘swarm'. It was established that the high frequency dependence of the Kerr constant corresponds to the type of dispersion of the dielectric anisotropy of the same substances in the liquid-crystalline state. This dependence has been related to the exclusion of the molecular dipole mechanism of rotation of the polar molecules about the short transverse axis from the polarization of the liquid.