Phase behaviors of a binary blend of weakly charged polyanions and polycations without salt*

A Landau free energy density is obtained for a salt-free binary blend of weakly charged polyanions and polycations in order to investigate their phase behaviors. The free energy functional is first formulated through the combination of the Gaussian thread approach and a molecular equation of state based on liquid state theory. The series expansion of the free energy functional in constituent density fluctuations up to quartic order yields the desired Landau free energy density. It is shown that long-ranged Coulomb interactions between charged monomers make the blend microphase separate. Various situations are covered for the blend at selected charged monomer fractions with disparity in self dispersion interaction and chain sizes. The effective second-order vertex coefficient is used to probe blend stability. The critical behavior and equilibrium phase diagram containing classical nanostructures for the blend are considered in the mean-field picture. In addition, we discuss the effects of osmotic pressure on the phase boundaries of the blend. Thermo and baroresponsive properties of the blend are also to be investigated.