Evolution of morphology in UHMWPE following accelerated aging: The effect of heating rates

Accelerated aging methods are used to evaluate the oxidative stability of U HMWPE components for total joint replacements. In this study, we traced the evolution of the crystalline morphology during accelerated thermal aging o f UHMWPE in air with the intent of explaining previous, counterintuitive he ating rate effects. GUR4150HP extruded rod stock material was machined into miniature (0.5 mm thick) specimens that were either gamma irradiated in ai r or in nitrogen (27 +/- 3 kGy) or left unirradiated (control). Accelerated aging in an air furnace (at 80 degrees C, atmospheric pressure) was perfor med on half of the test samples at a heating rate of 0.1 degrees C/min and at 5 degrees C/min for the remaining half. Although the initial heating rat e, as measured by changes in density, did influence the absolute degradatio n rate by up to 214%, the heating rate effect did not appear to influence t he relative ranking of UHMWPE in terms of its oxidative stability. The heat ing rate effect is more consistent with a kinetic mechanism of the oxidatio n process than it is with a previously hypothesized diffusion mechanism. UH MWPE morphology, as characterized using a transmission electron microscope (TEM), demonstrated considerable rearrangement of the crystalline regions a s a result of the accelerated aging. The stacking: of the lamellae observed after: accelerated aging was not consistent with the morphology of natural ly aged UHMWPE components. The observed differences in crystalline morpholo gy likely result from the enhanced mobility of the polymer chains due to th ermal aging and may be analogous to an annealing process. (C) 1999 John Wil ey & Sons, Inc.