Effect of the Thickness of Alignment Layer on the Low-Frequency Dielectric Dispersion of a Nematic Liquid Crystal

Abstract

The low-frequency dispersion of the complex dielectric constant of a nematic liquid crystal was measured as a function of the thickness of the polyimide alignment layer at 25 and 50° C. The dielectric relaxation strength of the low-frequency dispersion was strongly dependent on the thickness of the alignment layer and decreased with increasing thickness. A simple model assuming series combination of the alignment layer and liquid crystal phase was proposed to explain the frequency dependence of equivalent parallel capacitance of the liquid crystal cell. The experimental dispersion curves were excellently fitted by the equation based on the proposed model. This made possible the accurate determination of both the capacitive and conductive components of the alignment layer and the liquid crystal phase. The capacitive component of the alignment layer was proportional to the reciprocal of the thickness. The result indicated that the low-frequency dielectric dispersion of the cell is due to the Maxwell-Wagner effect of the alignment layer and the liquid crystal phase having different values of conductance. This made it possible to explain completely the quantitative aspect of the ionic effect in the nematic liquid crystal cell.