PHASE INVERSION IN COMPATIBILIZED IMMISCIBLE POLYMER BLENDS WITH EXOTHERMIC INTERFACIAL MIXING

We have investigated the morphology of solvent-cast immiscible polymer blends, emulsified by block copolymers that exhibit exothermic interfacial mixing. By manipulating the balance between core and corona swelling, we generate a variety of new microstructures. Specifically, it is demonstrated that the minor component becomes the continuous phase when the core swelling is sufficiently large by a process of vesicle fusion which produces an interconnected phase-inverted network morphology. This phenomenon was observed in blends of poly(styrene-co-acrylonitrile) (SAN) with polystyrene (PS) emulsified with poly(methyl methacrylate-b-styrene) (PMMA-b-PS). The degree of exothermic mixing between SAN and the PMMA block was varied by changing the AN content of SAN. In blends where SAN is the minor component, the core swelling increases on decreasing AN content, while the corona swelling in the PS major phase is determined by the molecular weight of the block copolymer PS segment relative to that of the matrix. For a particular combination of molecular weights of the blend components, by decreasing the AN content of SAN, we produce microstructural transformations from isolated vesicles to fused vesicles and finally to the phase-inverted network. Exothermic swelling maximizes the interfacial area of the minor phase and tends to yield planar interfaces, without sacrificing high molecular weight. Thus, the minor phase domains are stable against rupture and form the continuous phase. When SAN is the major component, the same tendency leads to a loss of emulsifying effectiveness because of the extremely small particle sizes of the disperse PS phase.