Nematic-isotropic phase transition in polar liquid crystals. 1. Statistical theory

The theory of transition from the nematic to the isotropic phase for liquid crystals in the system of rodlike particles with large longitudinal dipole s has been developed with due regard for the equilibrium between monomers a nd antiparallel molecular pairs-dimers. The order parameters of monomers an d dimers are determined as well as the dimer fraction. It is shown that, in accordance with the results obtained earlier, for low values of dipole mom ents, the temperature of the nematic-isotropic phase transition increases w ith an increase of the dipole moment. However, for large dipoles, the trans ition temperature starts decreasing with an increase of the dipole moment b ecause of higher dimer concentration. This provides the interpretation of t he recent computer simulation, which showed a destabilization of the nemati c phase in the system of rigid rods with pronounced central dipoles. The te mperature dependence of the dimer fraction is also studied. The qualitative relation between the sign of the jump in the dimer fraction at the transit ion point and the effect of dimerization on the transition temperature are established. (C) 2000 MAIK "Nauka/Interperiodica".