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.