Miscibility and isothermal crystallisation of polypropylene in polyethylene melts

Abstract

Three blends were made, consisting of 20% polypropylene (PP) homopolymer and 80% high density polyethylene (HDPE), low density polyethylene (LDPE) or linear low density polyethylene (LLDPE). Isothermal crystallisation of PP, at temperatures where the PE remained molten was studied by differential scanning calorimetry (DSC) and hot-stage optical microscopy (HSOM) with polarised light. The resulting semi-crystalline morphology was studied by transmission electron microscopy (TEM). It was observed by HSOM that the PP crystallised as open armed, diffuse spherulites in the PP–LLDPE blend, similar to the crystal morphology observed in miscible blends, while in the PP–LDPE and PP–HDPE blends, PP crystallised in phase separated droplets. The crystallisation rate of PP decreased significantly in the PP–LLDPE blend, but in PP–LDPE, PP–HDPE blends, it was similar to that of the pure PP. The difference in crystallisation rates indicated that PP was much more dispersed in the LLDPE melt, either by dissolution, or greater dispersion of droplets without nuclei, or both. The DSC and HSOM results suggested that the PP was miscible with the LLDPE at elevated temperatures at a PP composition of 20%, while the PP was immiscible with the HDPE and LDPE at these temperatures. However, TEM showed that there was phase separation in the PP–LLDPE (20:80) blend as well, though the droplets were much finer. Nevertheless, the form of spherulitic growth implies a substantial amount of PP dissolved in the molten LLDPE.