Grafting of α-tocopherol upon γ-irradiation in UHMWPE probed by model hydrocarbons

Today, UHMWPE implants are stabilized with alpha-tocopherol and cross-linked by irradiation in order to reduce wear. Little is known about the structural transformation of the antioxidant alpha-tocopherol upon irradiation. In the present investigation, the major irradiation reaction products of alpha-tocopherol dissolved at 0.1 wt.% in liquid model hydrocarbons were characterized spectroscopically and by independent synthesis. We observed only a single product group, namely phenolic alkyl ethers formed by radical recombination of a phenoxyl radical with a secondary alkyl radical. The irradiation dose is the parameter which controls the amount of consumption of alpha-tocopherol. At a dose of 27.5 kGy, 31-34% of alpha-tocopherol was transformed into the corresponding ether, while at 97.9 kGy, the degree of transformation was 68-76%. The observed ether formation in the liquid model hydrocarbons explains two significant observations for the alpha-tocopherol stabilized polymers, namely depletion of the alpha-tocopherol's phenol group upon irradiation and "grafting", i.e. formation of a chemical bond between the polymer and its antioxidant. (C) 2012 Elsevier Ltd. All rights reserved.