Change in Structure of Ultrahigh Molecular Weight Polyethylene Due to Irradiation in Air and in Nitrogen

Abstract

The change in the structure and properties of ultrahigh molecular weight polyethylene (UHMWPE) due to gamma irradiation has been studied in air and in a nitrogen atmosphere at different doses up to 400 kGy. Thermal analysis using differential scanning calorimetry (DSC) and swelling were used to monitor changes in crystallinity and cross-linking. New crystallites with different lamellae thicknesses were formed in samples irradiated in air. The samples irradiated in nitrogen did not show new crystallites unless irradiated at more than 200 kGy. Gel percentage and the Charlesby-Pinner equation were used to confirm the increase in cross-linking in the absence of oxygen.Characterization by all the techniques used indicated that irradiation in air causes the production of new crystallites due to oxidation, by which shorter molecular chains are formed and lamellar thickness increases. A lower degree of oxidation and scission occur in nitrogen, as confirmed by scanning electron microscopy (SEM) analysis, which allowed the determination of fracture shape and morphology. Fewer micro-voids and less brittleness were noticed in an oxygen-free environment. To gain more cross-linking, less production of crystalline regions, and more ductility, it is recommended that UHMWPE be irradiated in the absence of oxygen.

Processing and characterization of polyethylene-based composites

“…In the second heating and cooling in air and in nitrogen, UHMWPE exhibited approximately the tendency to slightly decrease the lamellar thickness because of exceeding the melting point and breaking down and dissolution of the big crystals into smaller ones. 86 The decrease in lamellar thickness in the composites does not affect the crystallinity. There are other factors such as the distribution, length and type of curvature that changes the lamellar thickness of the composites.…”

“…where T m is the melting temperature (the maximum of the endothermic melting peak); T m0 is the equilibrium melting point of a perfect crystalline PE ¼ 145.88C; r c is the crystalline phase density ¼ 1.005 g cm 23 ; d is the fold surface energy ¼ 93 £ 10 27 J cm 22 and DH 0 m is the enthalpy of melting of a perfect crystalline PE ¼ 288.84 J g 21 . 86 Rectangular-shape UHMWPE samples that was irradiated by gamma rays at room temperature to total doses of 5, 25, 50, 100, 200, and 400 kGy in two different environments, air and nitrogen 86 were studied. Increasing irradiation in a nitrogen environment caused an increase of lamellar thicknesses in first heating, while increasing the irradiation in air caused the formation of two or more new lamellae with higher thicknesses and consequently higher melting points.…”

The stabilizing effect of expanded graphite on the artificial aging of shape stabilized phase change materials

“…It is clearly seen that aging lead to the development of macroscopic cracks on the surface of LLDPE, resulting in the loss of mechanical integrity of the sample. This is expected to be due to the oxidation of the samples due to the UV irradiation, as the creation of defects is a common phenomenon in polymers after irradiation [27,28] and after weathering for gamma irradiated samples, in which oxygen enters the amorphous parts in the samples and reacts with free radicals creating cracks [29]. Fig.…”

“…Decrease of onset as well as melting temperature have been observed, which can be caused by photo thermal degradation of LLDPE causing thinning of the lamella. On the other hand, a slight increase of melting enthalpy from 100.3 to 110.9 J/g has resulted due to artificial aging causing more oxidation, scission and increase in the crystalline regions [34].…”

Secondary chain motion and mechanical properties of γ‐irradiated‐regenerated cellulose films

“…γ‐Irradiation can be used for sterilization of the materials for applications related to food packaging or medical devices. Ionizing radiation upon interaction with polymers produces free radicals, ions, and highly reactive excited states; all of which can participate in several possible reactions including chain scission (degradation), cross‐linking, chain aggregation, sterilization, grafting, and oxidation reactions . Radiation‐induced modification of natural polymers includes generation of starch nanoparticles by γ‐irradiation of cassava and waxy maize .…”