Electric Field Effects on the Optical Rotatory Power of a Compensated Cholesteric Liquid Crystal

Abstract

By studying the optical rotatory power (OR) of a compensated cholesteric helical structure, it is possible to observe both thermally induced helical inversion and electric field perturbations of the helix. For a cholesteryl chloride–cholesteryl myristate 1.75:1 mixture, the OR changes sign at T = T( nematic ) = 42° increasing to infinity prior to the inversion. Upon application of a dcelectric field parallel to the helix axis, the OR decreases continuously and reversibly by a factor of 4 in the field range 0.25F u <F<F u , where F u denotes the critical field required to induce a nematic transition. This effect cannot be explained as a macroscopic reordering of the material, but is rather due to a decrease in pitch accompanying a conical helical perturbation. This perturbation arises from bending forces tending to align the dipole moments of cholesteryl chloride in the field direction. When a dc field is applied perpendicular to the helix axis, the pitch diverges logarithmically to infinity as F approaches the critical value for inducing a transition.