Toward understanding the morphology-related crack initiation and propagation behavior in polypropylene systems as assessed by the essential work of fracture approach

Fracture mechanical terms related to the crack initiation and propagation, respectively, were assessed by the essential work of fracture (EWF) method for compression-molded polypropylene (PP) blends and elastomeric polypropylenes (ELPPs). The resistance to crack initiation (RCI) was characterized by the essential work of fracture w_e The w_e of the PP blends with 20 vol% ethyiene-co-butylene (EB) copolymers of various butylene (B) contents (58 and 90 wt%) and morphology (EB58 is amorphous, whereas EB90 is semicrystalline) increased with increasing butylene content and thus crystallinity. At the same time, in the slope of the EWF resistance curves, which inform about the resistance to crack propagation (RCP) of the PP/EB blends, an adverse change was observed. For the ELPPs of various crystallinity and morphology, the opposite tendency was found, that is, the RCI decreased, but the RCP increased with increasing crystallinity. The above findings were interpreted by considering the morphology and its rearrangement due to loading. It was suggested that the RCI is improved by the order of the crystalline structure and by the density of tie molecules that transfer the stress from the amorphous toward the crystalline phase. On the other hand, the RCP is controlled by a stress redistribution process that is influenced by possible changes in the local morphology.