Polymer solution model with anisotropic phase

We investigate a lattice self-avoiding walk model with directive (hydrogen bond like) interactions between chain segments and isotropic interactions between monomers and solvent molecules. The model, meant to describe a polymer in the dilute solution regime, is investigated in the framework of the Bethe approximation. Directive interactions produce, in addition to the ordinary collapse, a first order transition to an anisotropic (structured) phase. The interplay with solvent interactions modifies the nature of the isotropic phase, which can be either swollen or collapsed. Such an interplay may be an important ingredient for the formation of ordered structures in biopolymers. The relationship with previously investigated self-avoiding walk models is also discussed.