Determination of equilibrium pitch of cholesteric phases by isobaric-isothermal Monte Carlo simulation

In canonical Monte Carlo simulations of chiral liquid crystals, generally c holesteric phases with nonequilibrium pitch were formed under periodic boun dary conditions. In order to overcome system-size effects the phase diagram of chiral calamitic liquid crystals, described here by the chiral Gay-Bern e fluid, was now studied by simulations in the isothermal-isobaric ensemble (NpT). In the temperature-chirality parameter plane a rich polymorphism co uld be proven. In the cholesteric phase the ability of the simulation box t o change the dimensions during the simulations enabled the determination of the equilibrium pitch in dependence on the chirality parameter describing the strength of the chiral interaction. Additionally, in dependence on temp erature first-order phase transitions characterized by phase transition ent halpy, entropy and relative volume change have been observed. On heating a cholesteric phase (N*), for the first time by computer simulation of a many -particle system based on model intermolecular interactions a temperature-d riven phase transition to a blue phase could be proven, a characteristic fe ature of many experimental and theoretical phase diagrams of chiral liquid crystals. Additionally, on cooling a cholesteric system a phase with a heli cal superstructure and smectic layers formed perpendicular to the helical a xis, both characteristics of a twist grain boundary (TGB) phase, has been o bserved. This clear evidence shows that a chiral interaction proportional t o the first pseudoscalar term of the expansion in rotational invariants is able to yield the observed phase sequence TGB-like phase, cholesteric phase , blue phase, and isotropic phase in dependence on one scalar parameter giv en here by the temperature, only. (C) 2001 American Institute of Physics.