HOW COPOLYMERS PROMOTE MIXING OF IMMISCIBLE HOMOPOLYMERS

Copolymers and stirring are commonly used to produce fine dispersions of immiscible homopolymers. Recent experiments call into question the classical view that copolymers promote the mixing by reducing the inte rfacial tension, suggesting rather that copolymers induce repulsive in teractions between droplets and thus inhibit collision-induced coalesc ence events. We present a dynamical theory of the breakup and coalesce nce of polymer droplets in a mixing shear how including hydrodynamic a nd repulsive interactions between droplets. We find that a low surface coverage of copolymers, of the order of one chain per square radius o f gyration, is sufficient to inhibit collisions between submicron-size d droplets, while giving a negligible reduction in interfacial tension . (C) 1996 Society of Rheology.