PREDICTION OF THE TRANSITIONAL PROPERTIES OF LIQUID-CRYSTAL DIMERS - A MOLECULAR-FIELD CALCULATION BASED ON THE SURFACE TENSOR PARAMETRIZATION

The transitional properties of the liquid crystal dimers formed by two mesogenic groups linked by a flexible chain exhibit a pronounced depe ndence on the number of atoms in the spacer. Here we present the resul ts of a theoretical calculation of the nematic-isotropic transition te mperature, the entropy of transition, and the second rank orientationa l order parameters at the transition for two homologous series of cyan obiphenyl dimers. In one the alkyl spacer is attached directly to the mesogenic groups while in the other the alkyl chain is linked via ethe r units to the two cyanobiphenyl groups. At the heart of the molecular field theory is its parametrization based on the surface tenser appro ach. In this theory the potential of mean torque for each conformer is related to its topology by the surface tenser, which ensures that mol ecules are aligned such that their surfaces overlap to a maximal exten t. The transitional properties predicted by the theory are found to de pend in a sensitive manner on the geometry of the cyanobiphenyl group and that of its link to the alkyl spacer. By using realistic geometrie s and allowing for the various conformational states within the rotati onal isomeric state model the theory is able to account for the behavi or of homologous series of dimers as the spacer length is changed, and for the dependence of the transitional properties on the nature of th e link between the alkyl chain and the mesogenic groups.