Crystallization Behavior of Ultrahigh Molecular Weight Polyethylene as a Function of in Vacuo γ-Irradiation

A comprehensive investigation was carried out to study the effects of in vacuo γ-irradiation on the crystallization behavior of ultrahigh molecular weight polyethylene (UHMWPE) used in joint replacement prostheses. A wide variety of experimental techniques, including differential scanning calorimetry (DSC), wide-angle X-ray diffraction (WAXD), synchrotron small-angle X-ray scattering (SAXS), hot stage optical microscopy (HSOM), and depolarized light scattering (DLS), was used to access information about lamellar and supramolecular morphology, degree of crystallization, and crystallization kinetics during different nonisothermal and isothermal procedures. It was found that network formation prevailed over chain scission processes in the in vacuo irradiated samples, lowering the crystallizability of the polymer due to imposed geometrical constrains. In addition, the physical net of stable entanglements accompanied by the extensive cross-linking produced by γ-irradiation generated quite unique crystallization morphology of UHMWPE. We detected the presence of distinct ordered grains, whose size depended on the irradiation dose, that did not “melt” even after being subjected to a high-temperature heating cycle. At lower temperatures, however, progressive chainlike nucleation proceeded right on the skirt of these domains, maintaining the constant crystallization rate regardless of the degree of undercooling.