The influence of association and isomerization on re-entrant nematic and smectic-A polymorphism

The effect of chemical equilibria on the phase behaviour of mesogenic molecules which exhibit nematic and smectic A phases is investigated by using a molecular field approach, which is an extension of the McMillan theory. The particular reactions of isomerization and dimerization are treated in detail, for two species in equilibrium, taken as uniaxial particles experiencing different potentials of mean force and mean torque. The change of the equilibrium between the two species in the various phases is described in terms of an equilibrium constant for the isotropic phase. Lowering of the temperature below the first smectic phase can give rise to re-entrant nematics when (i) the isomerization equilibrium is shifted in favour of the most elongated molecules, or (ii) dimer dissociation increases the concentration of the species with lower translational order. In both cases, nematic and smectic A re-entrant phases are predicted under physically reasonable conditions.