Nematic-isotropic phase transition in polar liquid crystals. 2. Role of dispersion interactions

The present study furthers the development of the theory of the nematic-iso tropic phase transition in the system of rodlike particles with large longi tudinal dipoles. The effect of dispersion interactions between single molec ules (monomers) and molecular pairs (dimers) on the transition temperature is considered. The crucial point of the approach used is existence of dimer s in the system. Dispersion interactions are estimated within the framework of the model in which molecules consist of spherical blocks interacting ac cording to the Lennard-Jones law. The direct pair-correlation functions are approximated by spherical invariants forming a complete system of function s. Then, these correlation functions are varied in order to study the behav ior of the phase-transition temperature depending on the choice of the mole cular model. It is shown that, depending on the molecule shapes, dimers can either destroy or stabilize the nematic order. It is also shown that the p resence of dimers in the system decelerates an increase of the transition t emperature (and, in many instances, can even reduce it) with an increase in the value of the molecular dipole. (C) 2000 MAIK "Nauka/Interperiodica".