Molecular Mechanisms of Highly Polar Liquid Crystal Dielectric Polarization

Abstract

This article studies the dielectric constants of the nematic ε, ε, and isotropic εis phases of a highly polar liquid crystal of 4-cyanophenyl ether of 4′-n-octyloxybenzoic acid in the megahertz range of electric field frequencies (105-108) Hz, as well as the anisotropy of the dielectric constant Δε = ε − ε. The authors have revealed the relaxation processes and mechanisms responsible for them. In addition, they calculated the relaxation times, dipole polarization activation energy, and deceleration parameters in the nematic phase. The design of a measuring cell is presented, which allows studying the dielectric constant of the liquid crystal at various orientations of the director (provided by the magnetic field) relative to an alternating electric field with small volumes of matter. The results show that the dispersion of the dielectric constant in the investigated frequency range of the electric field is characterized by two relaxation processes with τ ~3 ∙ 10−8 s and (τ)в ~ 1,5 ∙ 10−9 s. The dispersion mechanisms are due to the rotation of polar molecules about their short and long axes respectively. The dispersion of the dielectric constant corresponds to the spectrum of relaxation times. The dispersion mechanisms are associated with two relaxation processes: the precession of molecules along a cone within the spatial angle allowed by the value of the order parameter of the liquid crystal S < 1, and the rotation of molecules around the longitudinal axes. The relaxation times corresponding to these mechanisms are commensurate with each other. The dispersion of the dielectric constant in the isotropic phase is characterized by a relaxation process with τis ~ 6 ∙ 10−9 s. The dispersion mechanism is due to the rotation of molecules around short axes.