Landau analysis of a salt-free polyanion/polycation blends: a continuum approach

Recently, the combination of a molecular equation-of-state model from liquid state theory and a self-consistent mean-field theory has been made for analyzing the equilibrium phase behaviors of various mixtures containing charged polymer chains in a uniform dielectric medium. Here, we expand the free energy functional as a series in proper order parameters to obtain the 4-field Landau free energy targeting a salt-free blend of weakly charged polyanions and polycations. The ion-ion correlations are shown to convert the polymer blends of Ising class to a microphase separating Brazovskii class. Our theory, which is based on continuum-space integral equation approaches, possesses various features that is missing in the classical Voorn-Overbeek theory such as excluded volume, dispersion interactions, and chain connectivity. In particular, we focus on the systems with disparity in dispersion interactions between the constituents, which affects the critical behavior of the blend. The phase diagrams containing classical morphologies are to be determined in various situations of asymmetric dispersion interactions. It will also be mentioned how this Landau analysis for rather simple blend is generalized to more complicated mixtures with uncharged small components and also salts.