Assessment of residual radicals in γ-sterilized shelf-aged UHMWPE stabilized with α-tocopherol

Abstract

In this study, we investigated the effects of α-tocopherol (vitamin E, (α-T)) on residual free radicals in ultra-high molecular weight polyethylene (UHMWPE). More specifically, its effects on the carbon-centered polyenyl radical R1 [(–˙CH–[CHCH–]m–) with m > 3] and the oxygen-centered di-enyl or tri-enyl radical R2 in shelf-aged γ-irradiated UHMWPE stabilized (blended) with α-T were examined. For the UHMWPE/α-T blends, the α-T concentrations were 5000 ppm, 10,000 ppm, 100,000 ppm, and 150,000 ppm; the blends were γ-irradiated (∼30-kGy) at room temperature (23 °C) and shelf-aged under light for five years in air. Electron spin resonance (ESR) power saturation techniques had been employed at room temperature (23 °C), which revealed that α-T was more effective in reducing residual radicals when irradiation was performed in air as compared to a nitrogen environment. While the total quantities of residual radicals reduced for the α-T concentration of up to 10,000 ppm, unexpected increasing trends in radicals were observed for 100,000 ppm and 150,000 ppm of α-T concentrations. The average crystallite sizes were found to increase from approximately 40 nm to 65 nm at L (110) and from approximately 28 nm to 42 nm at L (200), as suggested by wide angle X-ray scattering (WAXS) data, which indicated that total amount of residual radicals had strong dependence on the average crystallite sizes, as opposite to % crystallinity of these UHMWPE/α-T blends. In addition, behavior of oxygen-centered di-enyle or tri-enyl radicals (R2) was investigated using Raman spectroscopic methods and it was reported experimentally for the first time, to our knowledge, that radical R2 resides at the interphase region of UHWMPE and has direct proportion with % interphase contents of UHMWPE. Our result may be relevant when considering the long-term resistance of UHMWPE/α-T blends to oxidation processes.