Morphology and thermal properties in the binary blends of poly(propylene-co-ethylene) copolymer and isotactic polypropylene with polyethylene

The morphology and thermal properties of isothermal crystallized binary blends of poly(propylene-co-ethylene) copolymer (PP-co-PE) and isotactic polypropylene (iPP) with low molecular weight polyethylene (PE) were studied with differential scanning calorimeter (DSC), dynamic mechanical analysis (DMA), polarized optical microscopy (POM) and wide-angle X-ray diffraction (WAXD). In PP-co-PE/PE binary blends, however, the connected PE acted as a phase separating agent to promote phase separation for PP-co-PE/PE binary blends during crystallization. Therefore, the thermal properties of PP-co-PE/PE presented double melting peaks of PE and a single melting temperature of PP during melting trace; on the other hand, at cooling trace, the connected PE promoted crystallization rate because of enhanced segmental mobility of PP-co-PE during crystallization. At isothermal crystallization temperature between the melting points of iPP and PE, the binary blend was a crystalline/amorphous system resulting in persistent remarkable molten PE separated domains in the broken iPP spherulite. And then, when temperature was quenched to room temperature, the melted PE separated domains were crystallized that presented a crystalline/crystalline system and formed the intra-spherulite segregation morphology: these PE separated domains/droplet crystals contained mixed diluent PE with connected PE components. On the other hand, in the iPP/PE binary blends, the thermal properties showed only single melting peaks for both PE and iPP. Moreover, the glass transition temperature of iPP shifted to lower temperature with increasing PE content, implying that the diluent PE molecules were miscible with iPP to form two interfibrillar segregation morphologies: iPP-rich and PE-rich spherulites. In this work, therefore, we considered that the connected PE in PP-co-PE functioned as an effective phase separating agent for PP and diluent PE may be due to the miscibility between connected PE and diluent PE larger than that between PP and dispersed PE.