Positron Annihilation Lifetime Spectroscopic Analysis of Plastic Deformation of High-Density Polyethylene

Abstract

In this paper, for the first time, positron annihilation lifetime spectroscopy, after an appropriate adaptation, was used to analyze the plastic deformation process of high-density polyethylene as a representative semicrystalline polymer. It was shown that the average size of the free volume pores of the amorphous phase in the studied strain range decreased in comparison with the undeformed polyethylene, even after the initiation of the cavitation phenomenon due to highly anisotropic, ellipsoidal shape of cavities with the aspect ratio amounting to ∼45. The mean positron lifetime was practically constant in the analyzed range of strains even after activation of the micromechanisms of plastic deformation of crystals due to the mutually compensating effects activated in the crystalline phase. A clear change of the dispersion of positron lifetime as a function of local strain was observed. This effect was correlated with the reduction of the crystallites length by the relative displacement (slips) of adjacent crystalline blocks within individual lamellae.

Plastic Deformation of Polypropylene Studied by Positron Annihilation Lifetime Spectroscopy

“…In our latest work, an adaptation of the PALS technique for in situ analysis of plastic deformation of high density polyethylene was performed. 46 In the nutshell: two samples were deformed to the identical value of strain, fixed in the specially designed frames, removed from the tensile machine, and then placed between two detectors used in PALS measurements. Each time, fresh samples were stretched to the selected strain value to collect the appropriate PALS data.…”

“…More methodological details were presented elsewhere. 46 A similar set of frames/optimalization process was used/performed in the current work.…”

“…The relaxation processes of the amorphous or crystalline component affect the intensity of the cavitation. 46 Therefore, to optimize the tightening torque of the screws in frames, the healing process of cavitation pores in samples deformed to a local strain of 0.2, monitored with the use of the SAXS technique, was carried out. The SAXS patterns were also compared with the data collected in situ during deformation (synchrotron beamline, Petra III, DESY) for the same polypropylene deformed to the LS = 0.2.…”

“…54,55 All details of the deconvolution process were described elsewhere. 46 2.8. Positron Annihilation Lifetime Spectroscopy.…”

“…All details of the local and volume strain calculations were described elsewhere. 46 2.4. Differential Scanning Calorimetry.…”