D.E. Thomas

Structural changes of ultra-high molecular weight polyethylene exposed to X-ray flux in X-ray photoelectron spectroscopy detected by valence band and electron spin resonance spectroscopy

The effect of X-ray flux in an X-ray photoelectron spectroscopy (XPS) instrument on the chemical structure of ultra-high molecular weight polyethylene (UHMWPE) has been examined. The UHMWPE samples were exposed in vacuum to radiation from both a standard (non-monochromatic) source and a monochromatic source. For samples exposed to the standard source for up to 5 h, we observed very little change in the core level spectra but observed significant changes in the valence band (VB) spectra. We also observed the production of free radicals with an electron spin resonance (ESR) spectrometer which confirm radiation-induced structural changes and which correspond to the VB spectral changes. For samples exposed to the monochromatic source for up to 18 h, we see changes similar to the standard source, and very little free radical production compared to the standard source. Our results show: (1) that structural changes occur in polyethylene under X-irradiation with energies as low as those in the XPS. These structural changes are initially free radicals and lead to structural changes. (2) the structural changes cause very small changes in core level spectra, and (3) structural changes cause relatively large, easily identifiable VB spectral changes, which increase along with the free radical concentration as a function of exposure time. VB spectra can be an important indicator of radiation damage in purely hydrocarbon polymers.

Electron spin resonance study of oxidation in X-irradiated poly(ester urethane) containing nitroplasticizer

Abstract The effect of oxidation on X-irradiated Estane®5703 containing nitroplasticizer (NP) has been examined by electron spin resonance (ESR) spectroscopy, and the results are compared to similar data previously obtained on pristine Estane®5703. Although both specimens exhibit similar spectra immediately following X-irradiation, their decay upon exposure to air is quite different. The free radical concentration of the pristine specimen continuously decreases with time whereas the NP sample exhibits an initial decrease followed by a significant increase due to the growth of a newly-formed radical. Terminal species of the pristine and NP-Estane®5703 samples are identified as peroxy and nitroxide radicals, respectively. Hyperfine coupling constants and g-values are extracted for the nitroxide radical and a tentative model is proposed to explain the reaction pathway leading to its production.

Investigation of long-lived free radicals in shelf-aged UHMWPE tibial liners

UHMWPE used in total joint arthroplasty has commonly been sterilized by gamma radiation. Irradiation and subsequent aging in air reportedly cause extensive oxidation and, thus, adversely affect this material. Long-lived free radicals in gamma-sterilized/shelf-aged UHMWPE tibial inserts were investigated in this study. Significant concentrations of peroxy and primary radicals were found in the surface layer and the core region, respectively. Based on this study, it is expected that ir radiation of UHMWPE in an inert environment would result in long-lived primary radicals, which could be a potential cause of long-term oxidation.

A study of free radicals in irradiated/aged UHMWPE materials

UHMWPE bar stock, resin powder and fibers that were irradiated with an electron beam and then stored in air for three years were investigated for long-lived free radicals. Significant concentrations of free radicals were found in all UHMWPE materials. Peroxy radicals are predominant in the UHMWPE materials that are highly accessible to oxygen, while more primary radicals remain in the UHMWPE materials that are isolated from oxygen. Based on this study, it is expected that irradiation of UHMWPE in an inert environment would result in long-lived primary radicals, which could be a potential cause of long-term oxidation. Furthermore, a highly crystalline UHMWPE may be more susceptible to long-term oxidation as more free radicals can be trapped in crystalline regions.