Influence of the dimerization process on the nematic ordering

A theoretical model taking into account the dimerization influence on the nematic scalar order parameter is presented. At each temperature the nematic liquid crystal is considered as a mixture of closed dimers, open dimers and monomers. By means of simple statistical mechanics methods, the temperature dependence of the equilibrium concentrations of closed dimers, open dimers and monomers are obtained. The nematic order parameters for the open and closed dimers is deduced using an extension of the Maier-Saupe mean field approximation. In our model the effective mean field takes into account the molecular interactions closed dimers-closed dimers, open dimers-open dimers and closed dimers-open dimers. The detectable scalar order parameter of the nematic liquid crystal is then evaluated. In our analysis we assume that the monomers do not contribute to the nematic scalar order parameters of the mixture. Our theoretical predictions for the concentrations of the species forming the mixture, and for its nematic scalar order parameter are in good agreement with the experimental data. (C) 1999 Elsevier Science B.V. All rights reserved.