Examination of the long-lived, oxygen-induced radicals in irradiated ultra-high molecular weight polyethylene

Electron spin resonance (ESR) measurements were conducted to determine the thermal stability and structure of the long-lived, oxygen-induced radicals in irradiated ultra-high molecular weight polyethylene (UHMWPE). Employing a microwave saturation technique in conjunction with low-temperature (−133 °C) ESR, it was revealed that two most likely radicals, polyenyl (R1) and oxygen-centred di- or tri-enyl (R2) radicals, constitute the so-called oxygen-induced radicals (OIR). The reported single line due to OIR has in fact two resonance structures, one can be detected as an isolated signal (due to polyenyl, R1) at low microwave power (near 0.01 mW) and the other (oxygen-centred di- or tri-enyl) is prominent at high microwave power (>10 mW). The g-values of the radicals R1 and R2 were found to be 2.0044 and 2.0056, respectively. While R1 is a well-defined singlet with ΔHpp = 5.5 G, R2 has weak hyperfine lines (6) with proton coupling constant aH = 4.8 G. Annealing tests near 130 °C and 140 °C further suggest that R1 resides in crystalline regions, and R2 near crystalline surface with a dangling bond or in the voids or imperfections within the crystalline regions favouring oxygen attachment.