Phase Behavior of Highly Immiscible Polymer Blends Stabilized by a Balanced Block Copolymer Surfactant

The phase behavior of mixtures of polyisobutylene (PIB), polyethylene (PE), and a symmetric polyethylene-block-head-to-head polypropylene copolymer (PE−PP) was studied by transmission electron microscopy (TEM) and small-angle neutron and light scattering. The thermodynamic interactions between PE/PP and PE/PIB are repulsive (Flory−Huggins parameter χ > 0 and decreases with increasing temperature), while those between PP/PIB are attractive (χ < 0 and increases with increasing temperature). When the PE−PP copolymer is added to a 50/50 PE/PIB mixture, the resulting phase diagram in temperature−copolymer composition space exhibits many of the characteristics of “fish-shaped” phase diagrams found in oil/water mixtures stabilized by balanced surfactants. This is due to the interplay between the different χ parameters that characterize the system. Lamellar phases, single droplet microemulsions, and bicontinuous microemulsions were observed. The length scales of these structures and the locations of the phase tra...