Thermal surface switching and thermo electro-optical properties of nematic liquid crystals

Abstract

In liquid crystal technology, polymeric layers are used to obtain thermally stable planar or homeotropic alignments of nematic liquid crystals (NLC). Here we report a reversible temperature-induced switching with NLCs between these two alignments. The alignment, on a few copolymers, is homeotropic at low temperatures and planar at higher temperatures. The transition from homeotropic to planar occurs in a narrow temperature range. The transition temperature (T _HP) depends on the NLC for a given copolymer. The reduced transition temperatures (T _NP/T _NI), are correlated, for a selected copolymer, with the ratio (k ₃₃/k ₁₁) of the bend to splay elastic constants of NLCs. For a given NLC, the transition temperature (T _HP) can be shifted by the composition of the copolymer. By combining commercial NLC mixtures with an appropriate copolymer, we found transition temperatures T _HP in the range between −30°C and +120°C. Experimental results on the surface switching of NLCs, with positive and negative dielectric anisotropies, on two copolymers are reported. A tentative explanation, based on a conformational change of the surface molecules in interaction with the NLC molecules, is presented.