Hydroperoxide Formation in Irradiated Polyethylene Y.-J. YU, 1 F.-W. SHEN, 2 H. A. MCKELLOP, 2 R. SALOVEY 1 1 Department of Materials Science and Engineering, University of Southern California, Los Angeles, California 90089-1211 2 The J. Vernon Luck Orthopaedic Research Center, Orthopaedic Hospital, and Department of Orthopaedic and Biomedical Engineering, University of Southern California, Los Angeles, California 90089 Received 21 September 1998; accepted 15 April 1999 ABSTRACT: Spectroscopic analysis for hydroperoxide in irradiated ultrahigh molecular weight polyethylene, on the basis of the formation of a nitrate derivative after exposure to dilute nitric oxide, is examined. Hydroperoxide is found to be an important interme- diate in the oxidation of polyethylene and is believed to result from hydrogen abstrac- tion reactions by peroxy radicals in a polyethylene matrix. During  irradiation in air, the rates of bimolecular combination of peroxy radicals on the surface to form ketones or hydrogen abstraction to form hydroperoxides are similar. However, as a result of bimolecular combination, the concentration of peroxy radicals decreases. After irradi- ation and storage in ambient air, isolated peroxy radicals below the polymer surface induce a slow chain reaction leading to a long-term increase in hydroperoxides and carbonyls. Differences in hydroperoxide and oxygen content for samples irradiated in air or vacuum are primarily confined to or near the surface. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 3309 –3316, 1999 Keywords: ultrahigh molecular weight polyethylene (UHMWPE);  irradiation; hy- droperoxide; oxidation; FTIR INTRODUCTION In a recent publication, 1 we suggested a mecha- nism for the formation of a subsurface maximum in carbonyl concentration in polyethylene steril- ized for medical application by  irradiation in air. Although, right after irradiation, oxidative changes are maximal on the surface, subsurface oxidation increases faster than that on the sur- face for years, forming a band of maximum oxi- dation about 1–2 mm below the surface. Follow- ing irradiation in air, a large concentration of peroxy radicals is formed on the surface and bi- molecular reactions of peroxy radicals are com- mon which lead primarily to ketone. The concen- tration of ketone on the polyethylene surface be- comes relatively stable with time. 1 Below the surface of irradiated polyethylene, however, some radiolytic radicals are scavenged by oxygen and form isolated peroxy radicals. Al- though free radicals are always formed in pairs, there are several conditions in which geminate recombination may be avoided. If a carbon– hy- drogen bond is broken as a result of ionizing irra- diation, the highly mobile hydrogen atom often has sufficient energy to escape the proximity of the secondary alkyl radical. The secondary alkyl radical can react with oxygen to produce a sec- ondary peroxy radical. Indeed, the only peroxy species formed in irradiated ordinary linear poly- ethylene exposed to oxygen has been shown to be secondary peroxy radicals. 2 Further, the large concentration of tie mole- cules in ultrahigh molecular weight polyethylene (UHMWPE) results in extensive main chain scis- Correspondence to: R. Salovey Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 37, 3309 –3316 (1999) © 1999 John Wiley & Sons, Inc. CCC 0887-624X/99/373309-08 3309