The role of the blend interface type on morphology in cocontinuous polymer blends

Three different categories of blend interfaces are examined systematically in order to isolate the role of the interface in the development of cocontinuous morphologies during melt mixing. They are Type I, compatible binary blends based on high-density polyethylene (HDPE)/styrene-ethylene-butylene-styrene (SEBS) and HDPE/styrene-ethylene-butylene (SEB); Type II, an incompatible binary system comprised of HDPE/polystyrene (PS); and Type III, compatible ternary systems comprised of HDPE/PS compatibilized by SEBS in one case and by SEB in another. The Type I and Type III systems represent conventional approaches to preparing blend systems of low interfacial tension. The cocontinuous morphology is analyzed using three techniques: microscopy/image analysis, solvent extraction/gravimetric analysis, and BET characterization of surface area and pore size. A mechanism for the formation of dual-phase continuity based on droplet and fiber lifetimes during melt mixing has been proposed. For the Type I compatible binary systems continuity development and microstructural features are dominated by thread-thread coalescence. In the Type II incompatible HDPE/PS binary system and Type III compatibilized ternary systems, continuity development and microstructural features are controlled by droplet-droplet coalescence and reduced droplet-droplet coalescence, respectively. In the latter case, the generation of fresh interface during droplet deformation results in a system that is only partially emulsified. The relative presence of fibers or droplets during dynamic mixing is analyzed quantitatively using a matrix dissolution/image analysis technique. A thread frequency ratio (TFR) is proposed as a basic general parameter to classify the relative presence of fibers to droplets during mixing and hence the type of continuity development for a given system.