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.