Measurements of free radical in vitamin E-doped ultra-high molecular weight polyethylene: Dependence on materials processing and irradiation environments

Ultra-high molecular weight polyethylene (UHMWPE), doped with vitamin E (α-tocopherol (α-T)), was irradiated with gamma rays in nitrogen (N2) or air, and the resulting free radicals were detected in air using an electron spin resonance (ESR) technique. Two groups of samples were investigated. In one group, samples were prepared from blends of α-T (20 wt%) and UHMWPE powder (PPE-α-T) and, in the other, from compression molded blocks (CMPE-α-T). The CMPE-α-T blocks contained 0% (control), 0.5%, 1.0%, 10.0%, 15.0%, 20.0% and 25.0% α-T by weight.When irradiation was performed in air, the ESR spectrum of powder samples showed the presence of only vitamin E radical (tocopheroxyl, α-T-O), and there was no detectable signal due to PE radicals (alkyl/allyl). Most likely, all PE radicals were quenched by vitamin E during irradiation in air. However, when irradiation was performed in N2, composite ESR spectra showed the presence of both PE and α-T-O radicals. Compared to the control (PPE, 0% α-T) PE radicals in PPE-20% α-T were found to be significantly reduced or quenched by α-T. The presence of α-T in powder samples, however, did not affect the long-term (71 days in this study) oxidation behavior of the PE radicals.Compression molded samples, with and without α-T, produced identical ESR spectra irrespective of their irradiation environment N2 or air. However, radical concentration, measured immediately after irradiation, was found to be an order of magnitude less in CMPE-α-T than in the control (CMPE-0% α-T). They also evidenced identical structural changes in the respective ESR spectra during subsequent oxidation for 24 days in open air. These observations suggest that α-T can effectively quench a significant fraction of PE radicals during irradiation, but has no measurable effect on subsequent reactions. No significant difference was found in the ESR spectra of samples containing different α-T concentration.