Crystallization and melting of polyethylene strongly cross-linked in the molten state

We examined the crystallization and melting behavior of highly cross-linked polyethylene (xPE) samples, including structural characterization of the resulting crystalline domains. Using γ-ray irradiation, cross-linking was performed in the molten state and not in the solid state. This approach assured a statistical and homogeneous distribution of the cross-link points. Upon increasing the radiation dose, the molecular weight of the strands connecting the cross-link points reached values even lower than that of the strands between physical entanglements. Results from differential scanning calorimetry as well as small-angle and wide-angle X-ray scattering showed for <i>M</i>_c < <i>ca.</i> 1800 g mol^-1 and <i>M</i>_c > <i>ca.</i> 1800 g mol^-1, respectively, distinctly different variations of the melting temperature, the crystallization rate, the degree of crystallinity and the fraction of non-crystallizable CH₂ units per strand. Both unit cell parameters, <i>a</i> and <i>b</i>, increased with increasing cross-link density, much more than the published changes for corresponding samples cross-linked in the solid state. We conclude that the increase in cross-link density caused the exclusion of a progressively higher number of chemical cross-link points from crystalline domains, which, in turn, generated increasingly higher tensions on all chain segments, thereby reducing their conformational flexibility and chain folding possibility. As a consequence of the respectively induced changes in physical properties especially for our xPE samples with the highest cross-link density, we observed materials with extremely low crystallinity and melting temperatures close to room temperature, transforming polyethylene into a material with rubber-like properties.