Revealing toughening mechanism for alternating multilayered polypropylene/poly(ethylene-co-octene) sheets

Alternating multilayered (16, 32, and 128 layers) polypropylene/poly(ethylene-co-octene) (PP/POE) sheets were prepared via multilayer co-extrusion. Impact (−20°C), tensile, and dynamic rheological tests were carried out on the prepared multilayered samples. The toughening mechanism for the multilayered samples was revealed via investigating their fracture surface morphology and analyzing the work of fracture. The results showed that the impact energy for the notched multilayered samples is mainly absorbed by the continuous thin ductile POE layers due to cavitation, whereas for the unnotched multilayered samples the continuous thin POE and PP layers both contributed significantly to the total work of fracture due to multiple crazing, cracking and distinct plastic deformation in both PP and POE layers, and thus significantly extended crack propagation paths. Hence, the multilayered samples possessed much higher unnotched impact strength than notched impact strength. Moreover, the multilayered samples exhibited slightly increased notched impact strength and obviously increased unnotched impact strength with increasing layer number. Interestingly, the multilayered samples exhibited lower notch sensitivity than the PP sample. In addition to significantly improved low temperature impact toughness, the multilayered samples maintained the strength and stiffness as well as having superior extensibility to that of the PP sample.