FREE RADICAL ELIMINATION IN IRRADIATED UHMWPE THROUGH CRYSTAL MOBILITY IN PHASE TRANSITION TO THE HEXAGONAL PHASE.

Ultra-high molecular weight polyethylene (UHMWPE) is radiation cross-linked to decrease wear in total joint applications. Irradiation decreases the strength of UHMWPE and introduces residual free radicals, which can cause oxidation in the long-term. We advanced a method eliminating the free radicals without a reduction in strength. UHMWPE exhibits a hexagonal phase at high pressure and temperature, where chain mobility in the crystalline phase is increased, leading to the formation of extended chain crystals. We hypothesized that the increased chain mobility during transformation from the orthorhombic to hexagonal phase could be used to eliminate the residual free radicals in irradiated UHMWPE. We eliminated the free radicals in 25-, 65- and 100-kGy irradiated UHMWPE and these materials did not show oxidation after accelerated aging. The ultimate tensile strength and work to failure of 25 and 65-kGy irradiated UHMWPEs were improved significantly while that of 100-kGy irradiated UHMWPE was lower compared to irradiated UHMWPE melted at ambient pressure.