Characterization of photodegradation of a polyurethane coating by positron annihilation spectroscopy: correlation with cross-link density

Photodegradation of a polyurethane coating by Xe arc-lamp irradiation as a function of time is studied by positron annihilation spectroscopy. Doppler broadening energy spectra (DBES) of annihilation irradiation is measured as a function of incident positron energy (0–30 keV). A significant decrease in the S-defect parameter from DBES is observed as a function of irradiation time. This is interpreted as a loss of free-volume and holes during the photodegradation process. The cross-link density in the same system is measured by a solvent (methylene chloride) swelling method and is found to increase as a function of irradiation time. A direct correlation between the increase in cross-link density and the decrease in the S-defect parameter from the DBES data is observed. These results are used to discuss the photodegradation process in terms of chemical and physical changes due to UV-irradiation in protective polymeric systems.