Mechanisms of Crosslinking, Oxidative Degradation and Stabilization of UHMWPE

This chapter describes the chemical reactions that take place in ultra-high molecular weight polyethylene (UHMWPE) during irradiation and subsequent exposure to oxygen. It defines the mechanism of crosslinking of a polymer that is defined as the linking of two or more polymeric chains by means of chemical (covalent) bonds. In this way, the molecular mass increases theoretically up to infinity. Thus, crosslinking results in one long, branched molecule with infinite molecular mass. Crosslinking can be achieved by chemical or by radio-chemical reactions. Chemical crosslinking is generally not used to process UHMWPE for medical applications, and for this reason this analysis focuses on radiation crosslinking. Radical reaction pathways govern the crosslinking and degradative reactions of UHMWPE. For these reactions to occur, macroradicals must be induced in the polymer, for example, by thermal decomposition of hydroperoxides or by high-energy radiation, which leads to homolytic bond scissions with the production of alkyl macroradicals. © 2009 Copyright © 2009 Elsevier Inc. All rights reserved.