Modification of UHMWPE crystalline structure by means of e-beam irradiation and thermal treatment

M. Slouf¹, H. Synková¹, J. Baldrian¹, M. Stephan², H. Dorschner²

¹Institute of Macromolecular Chemistry, Academy of Sciences of the Czech Republic, Heyrovskeho nam. 2, 162 06 Praha 6, Czech Republic

²Leibniz-Institute of Polymer Research Dresden, Hohe Strasse 6, 01069 Dresden, Germany

slouf@imc.cas.cz

Ultra-high molecular weight polyethylene (UHMWPE) is used as a key component of artificial human joints, such as hip and knee, due to its balanced mechanical and friction properties. Nevertheless, the wear of UHMWPE, i.e. the release of microscopic particles from the polymer surface, seems to be the main reason why total joint replacements (TJR) fail. The wear particles move from the joint space to the surroundings of TJR, where they cause inflammatory reactions and osteolysis. In recent years it has been demonstrated that UHMWPE wear resistance can be increased by means radiation-induced crosslinking.

In this study, bulk UHMWPE was irradiated with accelerated electrons (doses from 0 to 100 kGy, dose rates > 25kGy/h) to crosslink the polymer and thermally treated above the melting point (T_m = 140 °C) to eliminate residual macroradicals and to limit oxidative degradation. Level of crosslinking was checked by solubility experiments and extent of oxidation was investigated by spectroscopic methods (IR, EPR). Irradiation and thermal treatment result in considerable changes in both molecular and supermolecular structure of UHMWPE, which influences not only its wear resistance, but also other mechanical properties. We followed the structural changes by small- and wide-angle X-ray scattering (SAXS and WAXS). Supplementary pieces of information were obtained also by differential scanning calorimetry (DSC) and scanning electron microscopy (SEM). We proposed model of supermolecular structure changes after irradiation and/or thermal treatment. The model is based on quite simple assumption that UHMWPE is composed of three phases: crystalline, amorphous and crosslinked.

Acknowledgement: this project was supported through grant 106/04/1118 (Grant Agency of the Czech Republic) and project AVOZ4050913 (Academy of Sciences of the Czech Republic).