On the Dependence of Impact Behavior on the Crystalline Morphology in Polypropylenes

The correlation between impact resistance and crystalline morphology was studied on specimens prepared from three different polypropylene (PP) homo- and copolymers, obtained by injection molding. The crystalline morphology was varied using three different nucleating agents. Linear elastic fracture mechanics was applied for the description of fracture resistance. The results indicate that morphological characteristics strongly influence the mechanical performance of the polymers. According to Young's theory for yielding, the resistance to crack initiation, represented by K _Ic, correlates with the thickness of the lamellae. This suggests that the onset for yield can be described by a dislocation mechanism. The total energy absorbed during impact, G _Ic, is a complex function of the morphology with no single identifiable morphological characteristic that dominates its value. The heterogeneous, dispersed morphology of the PP block copolymer initiates an additional energy absorption mode compared to the homo-polymer and the random-copolymer.