Hydroperoxide formation in irradiated polyethylene

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 intermediate in the oxidation of polyethylene and is believed to result from hydrogen abstraction reactions by peroxy radicals in a polyethylene matrix. During γ 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 irradiation 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

Interaction of oxidation and crosslinking in gamma‐irradiated ultrahigh molecular‐weight polyethylene

“…9) with the oxygen diffusion–control oxidation process cannot adequately explain the subsurface oxidation and its location beneath the surface in the shelf‐aged or clinically retrieved UHMW polyethylene that historically has been gamma sterilized in air. In a concurrent study,35 we found that following irradiation the hydroperoxide concentration at the surface of air‐irradiated specimens first increased (within 3 months) and then slowly decreased with storage time in ambient air (up to 21 months). In contrast, the hydroperoxide below the surface increased with storage time and showed a faster increase than on the surface.…”

“…That is, the concentration of residual free radicals is at its minimum at the surface, gradually increasing below the surface and reaching a plateau at about 1 mm below the surface. Residual free radicals that remain after irradiation react with oxygen that diffuses in during storage in air and form peroxy radicals, which either can combine with each other to form ketones, or can abstract hydrogen atoms from polyethylene to form hydroperoxides 35, 37…”

“…To correlate the hydroperoxide concentration with the oxidation profile induced by artificial aging, 50‐micron‐thick sections were microtomed across the thickness of specimens from Set A2 and Set S. In order to evaluate the reaction of residual‐free radicals with oxygen, one sample from set S was exposed to air for 1 month following the 5 months in storage in a low‐oxygen package. Thin specimens were placed in a glass chamber that had been evacuated to less than 10 −3 Torr for about 4 h, flushed with a mixture of nitric oxide and nitrogen gas (0.2 v/v % NO), and then re‐evacuated in order to completely remove the oxygen 35…”

“…After nitric oxide treatment, the specimens were analyzed with FTIR spectroscopy, as described above. The concentration of hydroperoxide was indirectly inferred from that of the nitrate groups, calculated as the ratio of the peak height at 1632 cm −1 (secondary hydroperoxide derivatives) to that at 2022 cm −1 35…”

Chemical analysis of functionalized polymer surfaces

“…The most important N1s component appears at a BE of 402.3 eV which is between oximes (C N-OH, BE = 400.5 eV [22,23] ) and nitroso groups (C-N O, BE = 403.7 eV). Consequently, the nitrogen group of interest has one bond to oxygen and at least one bond to a carbon atom with one or more bonds to oxygen.…”