No abstract

No 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.

No abstract

No abstract

Polylactide/triethyl citrate (PLA/TEC) systems were prepared in two ways by introducing TEC to solidified polymer matrix (SS) and by blending in a molten state (MS) to investigate the effectiveness of the plasticization process after solidification of polylactide. The plasticization processes, independent of the way of introducing the TEC into PLA matrix, leads to systems characterized by similar stability, morphology, and properties. Some differences in mechanical properties between MS and SS systems result primarily from the difference in the degree of crystallinity/crystal thickness of the PLA matrix itself. Based on the presented results, it was concluded that the plasticization process after solidification of polylactide is an alternative to the conventional method of modification-blending in a molten state. Then, this new approach to plasticization process was utilized for the interpretation of thermal properties of PLA and PLA/TEC systems. It turned out that double melting peak observed at differential scanning calorimetry (DSC) thermograms does not result from the melting of a double population of crystals with different lamellar thickness, or the melting of both the α′ and α crystalline phase (commonly used explanations in literature), but is associated with the improvement of perfection of crystalline structure of PLA during heating process.

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.

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.