Adhesion of statistical and blocky ethylene-octene copolymers to polypropylene

This study examined the effect of chain microstructure on the adhesion of ethylene-octene copolymers to polypropylene. The copolymers were candidates for compatibilization of polypropylene (PP) and high density polyethylene (HDPE) blends, and included a blocky copolymer, a statistical copolymer that had the same composition as the soft segment of the blocky copolymer, and a statistical copolymer that had the same comonomer content and crystallinity as the blocky copolymer. The compatibilized melt blend was modeled by a microlayered, one-dimensional structure consisting of alternating layers of PP and HDPE, each separated by a thin tie-layer. The microlayered structure made it possible to directly measure the adhesion using the T-peel test. Infrared analysis of matching peel fracture surfaces established that fracture occurred adhesively at the interface between PP and the tie-layer. Direct observation of the damage zone at the crack tip and microscopic examination of the fracture surfaces revealed that the tie-layer was highly deformed before final separation occurred at the interface. The substantially higher delamination toughness of the blocky copolymer compared to the statistical copolymers could be accounted for by considering both interspherulitic mechanically interlocking influxes and intraspherulitic entrapment of interdiffused tie-layer chains. The blocky copolymer also retained delamination toughness to a higher temperature due to the greater stability of lamellar crystals compared to fringed micellar crystals.