Correlation of residual radical’s with three phase morphology of UHMWPE: Analysis for the dependence on heat involved during vitamin E diffusion

Ultra-high molecular weight polyethylene (UHMWPE) is often used as a biomaterial component of hip and knee implants. Alpha-tocopherol (vitamin E) is an antioxidant that is sometimes diffused into the UHMWPE during the manufacturing process to help in eliminating free radicals and preventing these implants from oxidation. In order to diffuse vitamin E into the UHMWPE, the UHMWPE is heated while submerged in a hot bath of vitamin E. The aim of this study is to investigate the effect of this heat on polyenyl radicals (single bondradical dotCHsingle bond[CHdouble bond; length as m-dashCHsingle bond]msingle bond) with m > 3, and oxygen-centered dienyl or trienyl radicals (radical dotOCHsingle bond[CHdouble bond; length as m-dashCHsingle bond]msingle bond) with m = 2 or 3] and their correlation with the three phase morphology of irradiated, vitamin E-doped UHMWPE. Free radical analysis via electron spin resonance (ESR) reveal that heat involved during vitamin E diffusion is responsible for the decay of ∼98% of the primary trapped radicals, just as in the control samples without vitamin E. However, for vitamin E-doped UHMWPE, the vitamin E is then present for subsequent protection, and control samples (heated without vitamin E) suffer more during shelf-aging of 6 months. Differential scanning calorimetry (DSC), wide angle X-ray scattering (WAXS), and Raman spectroscopic methods conclude that the % interphase contents of UHMWPE control samples are 9.70 ± 1.00, 6.06 ± 1.20, 2.61 ± 0.80, and 1.50 ± 0.71, while for vitamin E-doped samples are 15.43 ± 1.66, 11.48 ± 1.46, 7.74 ± 1.54, and 5.54 ± 1.31, for sample heated/doped for 10 min, 30 min, 1 h, and 2 h, respectively. Moreover, it is also found that the % interphase contents of UHMWPE are in direct correlation with total amount of residual radicals, and oxygen centered di- or tri-enyl radicals regardless of samples initial treatment before the shelf aging.