Cavitation in Isotactic Polypropylene at Large Strains during Tensile Deformation at Elevated Temperatures

Two quenched isotactic polypropylene samples with different molecular weight were used to explore the sudden stress whitening activated at large strains during stretching at elevated temperatures via the ultrasmall-angle X-ray scattering technique. This kind of whitening was confirmed to be caused by the creation of cavities within the highly oriented samples. The critical strain for initiating the stress whitening increases with the increase of stretching temperature and molecular weight whereas the critical stress for the stress whitening depends only on molecular weight irrespective of stretching temperature. Thus, this peculiar stress whitening behavior under large strain deformation can be understood as a consequence of disentanglement of the highly oriented amorphous network initiated by the breaking of interfibrillar tie chains. During large strain deformation, two independent processes can occur. Besides the disentanglement induced cavitation, the stacked lamellar structure within microfibrils can be destroyed in the high molecular weight sample. In such case, the critical stress for lamellae destruction is lower than that of breaking of interfibrillar tie chains.