Computer simulation studies of anisotropic systems - Part XXXI. The continuous generic model for liquid crystal dimers

The essential molecular features responsible for the unusual nematic behaviour of liquid crystal dimers appear to be captured by the continuous version of the generic model. In this, two cylindrically symmetric mesogenic groups are linked by a virtual group to give a butane-like structure where the torsional potential controls the relative amounts of linear and bent conformers. The transitional behaviour of this continuous generic model has been predicted, using molecular field theory, as a function of the energy separation between the cis and trans conformers. The predictions are compared with the behaviour of real liquid crystal dimers. In addition the properties of the model have been evaluated for one parameterisation of the torsional potential, which favours the bent conformers, using a Monte Carlo simulation. Included in the properties determined as a function of temperature are the orientational order parameters for the mesogenic groups, the local orientational order parameter and the conformational order parameters. These results are compared with the predictions of the molecular field theory. The continuous generic model can also be used to describe the behaviour of other systems which include mesogens such as the azobenzenes capable of undergoing photo-induced cis trans isomerisation. We have, therefore, used our model to undertake a preliminary investigation of the quasi-dynamics of the nematic isotropic phase transition resulting from a conformational change. In these and other studies the behaviour of the continuous generic model is seen to depend critically on the synergy between the orientational and conformational order.