Plastic Deformation of Polypropylene Studied by Positron Annihilation Lifetime Spectroscopy

In this work, positron annihilation lifetime spec-troscopy (PALS) was used for the first time to study the plastic deformation of polypropylene. An appropriate methodological approach was proposed in order to analyze the mechanical response of polypropylene in a way similar to the in situ conditions. It was demonstrated that the intermediate lifetime expressed by the tau 2 (mean positron lifetime) and o-2 (dispersion) values gives us information about the crystalline component while the longest lifetime was expressed by the tau 3 (mean ortho-positronium lifetime) and o-3 (dispersion) about the "porosity" of the amorphous phase. Then, the influence of the polypropylene microstructure (thickness/perfection of lamellar crystals) and the cavitation phenomenon (cavitating/non-cavitating material) on the deformation process in the context of PALS data were analyzed/discussed. Among others, the process of relative slips of crystalline blocks within individual lamellae was detected even at very low strains. The simultaneous use of PALS and volume strain measurements confirmed the deformation-induced changes in the shape of free volume pores of the amorphous phase from isotropic (spherical) to anisotropic (ellipsoidal). The use of PALS and X-ray measurements also allowed us to estimate the initial shape of cavitation pores characterized by an aspect ratio exceeding 50.