Computer simulations of bent-core liquid crystals

Abstract

The phase behavior of model linear and bent-core molecules has been studied using isothermal-isobaric Monte Carlo computer simulations. The molecular model consists of seven Lennard-Jones spheres rigidly arranged in a "V" shape, with external bond angle, gamma. With gamma=0 degrees (linear molecules), we find isotropic, nematic, untilted smectic A, and two layered phases in which the molecules are tilted with respect to the layer normal. The latter two phases correspond to distinct branches in the equation of state, and possess different types of ordering within and between the layers; these phases are tentatively assigned as being smectic B and crystal. Apart from the possible existence of a tilted smectic B, the phase behavior of this system is broadly in line with earlier simulation studies on related linear molecular models. In the gamma=20 degrees system, isotropic, nematic, and tilted smectic- B phases are observed. Interestingly, the range of stability of the nematic phase is enhanced compared to the gamma=0 degrees system. In simulations of the gamma=40 degrees system, nematic phases are absent, and only isotropic and tilted phases are in evidence. The in-layer structure of the tilted phases shows a very clear change from smectic- B to smectic- A ordering upon increasing the temperature. In all instances of a tilted phase, the degree of molecular tilt is in the region of 30+/-5 degrees, with respect to the smectic layer normal, which corresponds closely to typical experimental observations in real bent-core liquid crystals. In our model, the tilt provides efficient packing of the spheres and favorable attractive interactions between molecules. The relevance of the present simulation model to real bent-core liquid crystals is discussed critically.