Subsequent but Independent Cavitation Processes in Isotactic Polypropylene during Stretching at Small- and Large-Strain Regimes

Isotactic polypropylene (iPP) samples with different molecular weights were used to investigate the relationship between cavitation processes triggered at small- and large-strain regimes. The cavitation triggered at large strains was observed in all deformation cases regardless of the occurrence of cavitation activated at small strain. The initiation of the large-strain-cavitation process required a critical stress, which was mainly determined by the molecular weight of iPP, because it was a result of the disentanglement of the highly oriented amorphous network initiated by the breakage of interfibrillar load-bearing tie chains. In general, iPP with higher molecular weight required a higher critical stress to trigger this cavitation and presented more intensive whitening. However, the intensity of cavitation activated at small strains decreased with increasing iPP molecular weight. The cavities generated at small strain are therefore supposed to have no contribution to the cavitation initiated at large strain.