EFFECT OF THE THICKNESS OF ALIGNMENT LAYER ON THE LOW-FREQUENCY DIELECTRIC-DISPERSION OF A NEMATIC LIQUID-CRYSTAL

The low-frequency dispersion of the complex dielectric constant of a n ematic liquid crystal was measured as a function of the thickness of t he polyimide alignment layer at 25 and 50 degrees C. The dielectric re laxation strength of the low-frequency dispersion was strongly depende nt on the thickness of the alignment layer and decreased with increasi ng thickness. A simple model assuming series combination of the alignm ent layer and liquid crystal phase was proposed to explain the frequen cy dependence of equivalent parallel capacitance of the liquid crystal cell. The experimental dispersion curves were excellently fitted by t he equation based on the proposed model. This made possible the accura te determination of both the capacitive and conductive components of t he alignment layer and the liquid crystal phase. The capacitive compon ent of the alignment layer was proportional to the reciprocal of the t hickness. The result indicated that the low-frequency dielectric dispe rsion of the cell is due to the Maxwell-Wagner effect of the alignment layer and the liquid crystal phase having different values of conduct ance. This made it possible to explain completely the quantitative asp ect of the ionic effect in the nematic liquid crystal cell.