Synergistic improvement of toughness of isotactic polypropylene: The introduction of high density polyethylene and annealing treatment

The effects of introduction of high density polyethylene (HDPE) and further annealing treatment on the toughness of isotactic polypropylene (PP) were systemically investigated. The introduction of HDPE can improve the toughness of PP at no expense of the stiffness. For the PP containing 10 wt% of HDPE, the notched Izod impact strength is increased by approximately 16.3%. After annealed at 130 °C for 12 h, the impact strength of PP alone is increased by 336.7% compared to pristine PP, while this value is dramatically increased by 1107% for the PP containing 10 wt% of HDPE, which suggests that the introduction of HDPE and further annealing treatment have significant synergistic effect on the toughness improvement of PP. Characterization of microstructure evolution of PP with and without HDPE upon annealing reveals that the dramatic improvement in toughness of PP may be attributed to at least three aspects: (1) the prominently reduced spherulite size of PP induced by the introduction of HDPE, (2) the changes of crystallinity and molecular mobility upon the annealing treatment, which would promote the cavitation process during deformation, and (3) the enhanced interface in PP/HDPE blend upon the annealing treatment.