Investigation of the oxygen-induced radicals in ultra-high molecular weight polyethylene

Electron spin resonance (ESR) technique was employed for detection of oxygen-induced radicals in x-irradiated ultra-high molecular weight polyethylene (UHMWPE). Initial oxidation was found to occur when allyl radicals were exposed to oxygen. Presumably, peroxy radicals were formed; however, under normal experimental conditions at room temperature, ESR spectra did not show the finger prints of peroxy. When the microwave power was increased from 1 mW to 32 mW and the temperature was decreased to 118 K, the resonance lines due to alkyl and allyl were suppressed (saturated) and those due to peroxy became very clear. The peroxy radical was found to be very short-lived (half-life less than 1 h), and its characteristic g-values were g∥ = 2.032 and g⊥ = 2.003. The long-lived oxygen-induced radical in UHMWPE, which is known to produce a single line in the ESR spectrum (width ∼10 G and g = 2.0015) at room temperature, was found to exhibit hyperfine lines (separation 5 G) at 106 K. Power saturation behavior suggest that there are more than one species present of which one could be oxygen-centered radical.