Plastic Deformation of Polypropylene Studied by Positron Annihilation Lifetime Spectroscopy

Abstract

In this work, positron annihilation lifetime spectroscopy (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 τ2 (mean positron lifetime) and σ2 (dispersion) values gives us information about the crystalline component while the longest lifetime was expressed by the τ3 (mean ortho-positronium lifetime) and σ3 (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.

Evolution of the Deformation‐ and Flow‐Induced Crystallization and Characterization of the Microstructure of a Single Spherulite, Lamella, and Chain of Isotactic Polypropylene

“…With in‐situ WAXS, crystal slips and crystal‐crystal transitions with specific planes were observed, which provide direct experimental evidence on the crystal slip model. [ 231–233 ] Combining WAXS and SAXS, researchers corroborated the occurrence of homogeneous and localized crystal slip and a companion martensitic phase transition in PE. [ 76,217,234,235 ] Together with TEM, crystalline slip processes were analyzed during texturing of PE, proving that the investigation of single‐crystal‐textured samples was a better method to reveal the mechanisms of crystal slip.…”

Evolution of free volume elements in amorphous polymers undergoing uniaxial deformation: a molecular dynamics simulations study

“…Huang et al recently performed in situ positron annihilation lifetime spectroscopy (PALS) experiments to observe free volume evolution in polyethylene through active mechanical deformation on the minute scale, 34 improving the throughput of this free volume analysis technique as well as circumventing chain relaxation effects over PALS's traditional, much longer length scales. [35][36][37] However, to gain a molecularlevel insight about membrane mechanical response and the underlying physics on shorter timescales, molecular dynamics (MD) simulations provide a useful tool.…”

Minute-Scale Evolution of Free-Volume Holes in Polyethylenes during the Continuous Stretching Process Observed by In Situ Positron Annihilation Lifetime Experiments

“…This means that the average size of free-volume holes in amorphous regions was smaller than that of undeformed sample. They also tested polypropylenes using the same techniques in 2022 and found that as the local strain increased, the o-Ps lifetime decreased and then increased at approximately 0.03 strain but was always smaller than that of the unstretched sample . In all previous studies, the PALS measurements were performed after the samples were stretched to a certain strain, and it took several hours to collect one spectrum due to the limitation of low count rate of less than 300 counts per second (cps).…”

“…They also tested polypropylenes using the same techniques in 2022 and found that as the local strain increased, the o-Ps lifetime decreased and then increased at approximately 0.03 strain but was always smaller than that of the unstretched sample. 25 In all previous studies, the PALS measurements were performed after the samples were stretched to a certain strain, and it took several hours to collect one spectrum due to the limitation of low count rate of less than 300 counts per second (cps). In this case, it is difficult to probe the change in microstructure of a sample during continuous stretching process.…”