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

The effect of X-ray flux in an X-ray photoelectron spectroscopy (XPS) instr ument on the chemical structure of ultra-high molecular weight polyethylene (UHMWPE) has been examined. The UHMWPE samples were exposed in vacuum to r adiation from both a standard (non-monochromatic) source and a monochromati c source. For samples exposed to the standard source for up to 5 h, we obse rved very little change in the core level spectra bur observed significant changes in the valence band (VB) spectra. We also observed the production o f free radicals with an electron spin resonance (ESR) spectrometer which co nfirm radiation-induced structural changes and which correspond to the VB s pectral 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 r adical production compared to the standard source. Oar results show: (1) th at structural changes occur in polyethylene under X-irradiation with energi es as low as those in the XPS. These structural changes are initially free radicals and lead to structural changes. (2) the structural changes cause v ery small changes in core level spectra, and (3) structural changes cause r elatively large, easily identifiable VB spectral changes, which increase al ong with the free radical concentration as a function of exposure time. VB spectra can be an important indicator of radiation damage in purely hydroca rbon polymers. (C) 2000 Elsevier Science B.V. All rights reserved.