Mixing A and B homopolymers with AC diblock copolymers: Phase behavior of asymmetric polymer blends

We use grand-canonical-ensemble self-consistent field theory to study blends of both moderately and strongly segregated homopolymers A and B with some diblock copolymer AC, where C is attracted to B. We derive an analytical condition for the Flory-Huggins interaction parameters, which describes a balanced copolymer surfactant AC. We then calculate ternary phase diagrams mainly for blends containing such balanced surfactants. Among the ordered structures, we generally consider lamellar and hexagonal phases, whereas cubic phases are included in the calculation when the polymer blends are studied far from the Lifshitz point. The resulting phase diagrams are highly asymmetric. In particular we show that even a compositionally symmetric polymer blends - that is with equal concentrations and molecular weights of the two homopolymers and with identical polymerization degrees of the copolymer blocks - may organize into either of the two distinct hexagonal structures, as well as into the lamellar structure. One of these hexagonal phases, with the B-rich matrix, has a rather low content of the stabilizing copolymer and may therefore, under experimental conditions, disorder into a polymeric microemulsion. Overall we conclude that the AC diblock can provide a slightly more efficient compatibilizer than its AB counterpart, provided that the incompatibility of homopolymers A and B is not too strong.