Computer simulation studies of anisotropic systems. XXVI. Liquid crystal dimers: A generic model

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A generic model has been proposed with which to understand the unusual properties of liquid crystal dimers, in particular their behavior at the nematic–isotropic transition. In the model the many conformational states available to real dimers are replaced by just two, a linear and a bent conformer which are allowed to interconvert. The nematic behavior of the model has been explored with the aid of the molecular field approximation and found to be in good qualitative agreement with that of real dimers, thus providing a valuable aid to our understanding of them. To see to what extent the use of the molecular field approximation influences the predictions we have undertaken a Monte Carlo computer simulation study of the generic model. To facilitate the simulations the particles have been confined to the sites of a simple cubic lattice with the anisotropic interactions being restricted to nearest neighbors and having the form −(ε/4)P2(cos β), where β is the angle between the anisometric groups. The other advantage of this model is that for the linear conformer alone it is identical to the Lebwohl–Lasher model for nematics which has been studied in depth and so allows the influence of the presence of the interconverting bent conformer on the liquid crystal behavior to be readily discerned. We have also extended the discrete generic model, with just two interconverting conformers, to a continuous model in which the torsional angle defining the conformational state varies continuously from 180° (linear) to 0° (bent); this makes the model far more general and more analogous to real dimers. A wide range of properties have been determined from the computer simulations; these include the second rank orientational order parameter for the anisometric groups, the singlet orientational distribution function for these, the second rank short range orientational order parameter, the internal order parameters and the distribution function for the torsional angle. Armed with this wealth of information we are able to make a searching test of the molecular field theories for the two models as well as commenting on the behavior of real liquid crystal dimers.