Thermodynamics of Polymer Blends Organized by Balanced Block Copolymer Surfactants Studied by Mean-field Theories and Scattering

The phase behavior of multicomponent blends of two immiscible homopolymers (A,B) and an A-C diblock copolymer was studied by scattering experiments and mean field theories. The interactions between the components were tuned to create organized blends with the copolymer serving as a surfactant. The morphology of A/B/A-C blends changed between lamellar phases, microemulsions, homogeneous phases and macrophase- separated states simply by adjusting the temperature. The experimentally determined phase transition temperatures and domain spacings were compared with calculations based on the Random Phase Approximation (RPA) and Self-Consistent Field Theory (SCFT). The only inputs into the calculations were the binary Flory-Huggins interaction parameters (chi) between the three kinds of monomers in our system, and statistical segment lengths. The domain spacing determined from theory was often within 5\% of the experimental values. In a particular range of molecular weights, we find that a critical A/B blend can be organized into a periodic phase by the addition of only 5\% of the diblock copolymer. To our knowledge, all previous experiments that have led to organized critical mixtures have required a significantly larger copolymer concentration.