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Abstract
A lattice model is proposed to study reentrant isotropic-nematic phase transitions mediated by helix-coil transformations within individual liquid crystal molecules. The system consists of a racemic mixture of molecules in solution with a solvent capable of hydrogen bonding. We first introduce a model to describe helix-coil transformations in the presence of such a solvent. This model displays not only normal helix-coil transformations, but also inverted transformations (i.e. reentrant), as well as reversion to the coiled state at high temperatures. Secondly, we develop a model for the isotropic-nematic phase transition which incorporates intermolecular interactions on the same footing with the intramolecular interactions. Reentrance of the isotropic phase is driven by the inverted helix-coil transformation. Within the nematic phase the effect of induced rigidity is observed. In addition, when solvent-solvent bonding is important in the system, doubly reentrant phase diagrams are predicted. We study our model using renormalization-group and other partial-trace methods.