The determination of nitroxide radical distributions in polymers by EPR imaging

The concentration of nitroxide radicals formed by photooxidation of one of four different hindered amine stabilisers (HAS) in polypropylene plaques 2 mm thick subjected to continuous irradiation for periods ranging from 1 to 6 months, has been measured by electron paramagnetic resonance (EPR) spectroscopy. The spatial distribution of these radicals across the polymeric plaques has also been determined either by cutting thin slices of the plaque and then measuring the nitroxide concentration within each slice, or by using an X-band EPR imaging technique (EPRI) which provides the radical concentration profile along the magnetic field direction. Experiments on samples containing two UV absorbers, beside the HAS stabilisers, are also reported. It is shown that the changes of the nitroxide distribution inside the polypropylene plaques observed by increasing the irradiation time, follow some interesting trends peculiar to the additives contained in the polymer. It seems that the EPRI technique may provide useful information on the mechanism of stabilisation by HAS of polymers subjected to photo-degradation. (C) 1996 Elsevier Science Limited