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

► Introduction of HDPE can slightly toughen PP at no expense of its stiffness. ► Further annealing dramatically improved the toughness of PP blended with HDPE. ► Blending with HDPE and further annealing treatment has significant synergistic effect. ► The synergistic effect is largely associated with microstructure changes upon annealing.