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.

The work explains the mechanism of the improvement of oxygen barrier performance of cold-crystalized polylactide (PLA) modified with low molecular weight compounds (triethyl citrate, TEC; glycerol, Gly; and polyethylene glycol, PEG). It was found that the incorporation of small amounts of modifier (0.5−1.5 wt %) into the PLA matrix greatly reduces the oxygen transport properties. On the other hand, a further increase in the modifier content in PLA blends led to a deterioration of these properties. Based on the thermal analysis, it was shown that the modifier present in the PLA matrix did not affect the degree of crystallinity. Positron annihilation lifetime spectroscopy (PALS) and density measurements were used to correlate the observed changes in the parameters of oxygen permeability with the molecular packing in the interlamellar amorphous region. Overall, it was found that the nanostructure of the amorphous phase, largely dependent on the content and compatibility of the modifier with the polymer matrix, is of key importance in the barrier properties of PLA/modifier systems.