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

Abstract

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.

Cavitation‐Induced Stress Whitening in Semi‐Crystalline Polymers

“…Clearly, the relationship between these two cavitation processes activated at different strain regimes requires a thorough investigation. Recent results obtained in P4M1P and iPP indicate that the two cavitation processes are independent although sequential. As schematized in Figure , the cavitation triggered at large strains can occur in the sample no matter whether the small strain cavitation takes place or not.…”

“…Indeed, recent works carried out by Chen et al . and Lyu et al . formulated a first sight on the relationship between cavitation triggered at early and later stages of tensile deformation.…”

“…Both of them stressed that the cavitation starting at small strain did not influence the nucleation and growth of the cavitation at large strain in P4M1P and iPP, respectively. Figure registers the corresponding results revealing the two cavitation processes presented in iPP samples under different situations . At a deformation temperature of 30 °C, iPP190K annealed at 130 °C showed an intensive whitening at small strain while the one annealed at 30 °C exhibited a much weaker whitening at the same stage based on the macroscopic photographs.…”

“…(Selected photographs of iPP190K samples annealed at 30 and 130 °C during deformation were also included.) Reproduced with permission . Copyright 2018, American Chemical Society.…”

“…It raises clearly an interesting question about what can be expected in the sample during stretching if both small and large strain cavitations are initiated subsequently. Indeed, recent works carried out by Chen et al [159] and Lyu et al [160] formulated a first sight on the relationship between cavitation triggered at early and later stages of tensile deformation. Both of them stressed that the cavitation starting at small strain did not influence the nucleation and growth of the cavitation at large strain in P4M1P [159] and iPP, [160] respectively.…”