Morphology and elastomeric properties of isotactic polypropylene/hydrogenated polybutadiene blends

Isotactic polypropylene (PP) was found miscible on a molecular level with hydrogenated polybutadiene (hBR). As there is a possibility of using this blend for designing new types of thermoplastic elastomer (TPE), mechanical properties were measured with the emphasis on elastic behavior. The best elastomeric behavior was found for blends crystallized at room temperature (26 degreesC), while poor elastomeric properties with high modulus were found for blends crystallized at high temperatures (e.g., 120 degreesC). Elastic properties of blends quenched to low temperatures (e.g., 0 degreesC-73 degreesC) were slightly worse. Transmission electron microscopy showed that the blend crystallized at the high temperature had a crosshatch lamella structure, while the blends crystallized at lower temperatures (below room temperature) had only small fragmented lamellar crystallites. The latter structure of PP is very unique and can be found only under special conditions. The essential may be the presence of high molecular weight impurity, which prevents formation of crosshatch lamellar structure at the high supercooling depths. Fragmented crystals then probably function as tie points for amorphous chains so that good elastomeric behavior can be achieved. There is a certain possibility of improvement in elastic properties, by annealing quenched samples at high temperature close to melting point of PP. In this case, the crystallinity and size of lamellae increased, while the fragmented character of lamellae was preserved.