MOLECULAR-FIELD THEORY AND MONTE-CARLO SIMULATION OF PHASE-TRANSITION IN SURFACE ALIGNED NEMATIC FILMS

The molecular field theory (MF) of nematic to isotropic (N-I) phase transition in surface-aligned nematic film is proposed by using Lebwohl-Lasher model. The calculated results indicate that the N-I-phase transition will be strongly affected by the interaction between liquid crystal molecules and substrate. The nematic film can be divided into three regions according to their phase transition behavior: the interface, the bulk 2nd the free surface. The interface phase transition depends strongly on the strength of interaction between liquid crystal and substrate, and its phase transition temperature is higher than that of bulk phase transition. Especially, the order parameter P2BAR will become a continuous function of temperature when the anchoring energy is strong enough. In the bulk-region, the N-I phase transition is still of the first order while in the free surface, the N-I phase transition belongs to the typical second order transition behaviour. When the temperature is increased, the N-I phase transition starts in the free surface region and then extends to the bulk and interface regions gradually. All these results are confirmed by Monte Carlo (MC) study and agree with the Landau-de Gennes theory.