Compatibilization of isotactic polypropylene (iPP) and polyethylene (PE) with PP-based olefin block copolymers

The role of entanglements and co-crystallization in the compatibilization of iPP/PE blends with PP-based diblock copolymers (BCs) was investigated. A smart design of experiments was set up, based on the selection of two different BCs, iPP/HDPE and iPP/LDPE blends (30/70 wt%/wt%), and different processing conditions. The selected BCs were a hard-hard copolymer with crystalline iPP and PE blocks (BC1), and a hard-soft copolymer with iPP blocks linked to amorphous EP blocks (BC2). Both BCs significantly improved blend morphology by inducing a fine dispersion of the minority iPP phase in the PE matrix, even at low loads (3–5 wt%). However, they exert different compatibilization effectiveness. In iPP/HDPE blends, trapped entanglements are coupled with well-formed co-crystals of large lamellar thickness, and only 3 wt% of BC1 is sufficient to ensure good ductility and toughness. For iPP/LDPE blends, larger BC1 content (over 5 wt%) was needed due to formation of thinner co-crystals. In contrast, BC2, which does not co-crystallize with PE, only ensures good adhesion in rapidly cooled samples due to the “freezing” of trapped entanglements.