From rod-like to disc-like Gay-Berne biaxial nematics and back

By means of Monte Carlo simulations at constant pressure and temperature, the phase behaviour of pure systems of soft biaxial Gay-Berne ellipsoids, prototypical models for thermotropic mesogens, was determined as a function of temperature and three different pressures. To reduce the number of free shape and energy parameters to only one, the medium ellipsoid axis sigma(x), particles possessing identical total volume and length sigma(z) were studied, with interactions parameterised in terms of products of axes lengths. The resulting phase diagrams present a variety of crystalline and liquid crystalline phases characterised by different biaxiality and preferential axis alignment, with GB particles behaving like rods for more elongated shapes, and like discs for more squashed ones. In agreement with previous simulation and theoretical studies using simpler models, the transition between rod to disc behaviour (and back) occurs approximately when the so-called dual-shape condition is reached, i.e. when sigma(x) = root sigma(y)sigma(z). This shape ideally identifies also the position of a Landau point, in which at a specific temperature, isotropic, calamitic nematic, discotic nematic, and biaxial nematic may coexist, and hence a direct isotropic to biaxial nematic transition could in principle occur. We did not observe such event in our simulations. [GRAPHICS] .