Temperature-induced orientational phase transitions in freely-suspended nematic films

Abstract

We report observations of a temperature-induced orientational transition in a freely suspended nematic film of the liquid crystal 4-ethyl-2-fluoro-4′-[2-(trans-4-n-pentylcyclohexyl)-ethyl]-biphenyl (I52). The transition is from a high-symmetry state, where the molecules are perpendicular to the film boundaries (homeotropic alignment), to a lower-symmetry state, where the molecules are inclined (tilted alignment), and occurs as the temperature of the sample is raised. A previous study by Faetti and Fronzoni (1978) discovered a similar transition in freely suspended nematic films of N-(p-methoxybenzylidene)-p-n-butylaniline, except that it occurred as the temperature was lowered, a result that we confirm here. In order to explain both orientational transitions and the temperature-independent alignment of other nematics, we have generalized a model by Parsons (1978) that is based on competition between polar and quadrupolar contributions to the surface free energy. By considering the effects of smectic ordering and ionic impurities, we can account for all of the various observations.