Computer simulation of dipolar liquid crystals

Abstract

Abstract The aim of the work presented here is to use computer simulation techniques to examine the effect of dipolar interactions on the structures formed by model mesogenic molecules. The most wide-ranging and substantial part of the work involves an in-depth Monte Carlo (MC) study of the effect of dipolar interactions on the phase behaviour of hard spherocylinders with dipoles in a number of positions and orientations. A reaction-field (RF) method, with a simple self-consistent iterative method to accurately describe the dielectric constant of the surrounding medium, is used to treat of the long-range dipolar interactions. For the system with a central longitudinal dipole the density range of the nematic phase is found to be considerably reduced. The nematic phase is destabilised relative to both the isotropic and the smectic-A phases when compared with the non-polar system, and disappears altogether below the I-N-SmA triple point. No evidence of ferroelectricity is found although some short-range anti-ferroelectric ordering is seen. As for the molecules with central longitudinal dipoles, the smectic-A phase is stabilised for a system with central transverse dipoles with strong nose-to-tail interactions. The densities of the isotropic-nematic transitions are essentially unchanged from that of the nonpolar system. The most interesting features of this system are the domain structures formed by the dipoles within the smectic-A layers. At high temperature the dipoles are orientationally disordered, but as the temperature is lowered the stable structures are rings and chains of dipoles. As for the system with central longitudinal dipoles, the terminal longitudinal dipole is seen to slightly destabilise the nematic phase relative to the isotropic phase. More interestingly, the smectic phase is destabilised, and is only seen at the very highest densities. This is in stark contrast to what is seen for the systems with longitudinal and transverse central dipoles, and appears to be a consequence of the anti- parallel geometry of the dipoles in their minimum- energy conformation.