Phase behavior of ternary homopolymer/diblock blends: Microphase unbinding in the symmetric system

We consider the phase behavior of a symmetric AB diblock, blended with corresponding A and B homopolymers, all of equal chain length. Evaluating the phase diagram in mean-field theory at all compositions of the ternary blend, we find no hexagonal and cubic phases. However, up to three distinct lamellar phases can coexist, one being symmetric and the other two being asymmetric. For strong incompatibility, added homopolymer eventually phase separates from the microstructure. For weak segregation, the symmetric lamellar phase at moderate dilution breaks up into two coexisting lamellar phases, each asymmetrically swollen by either A or B homopolymer. These phases can be swollen indefinitely, finally unbinding into the disordered phase. As a consequence, one expects to find lamellar order at much lower diblock concentrations toward the binary sides of the phase diagram than on the isopleth.