The effect of oxygen on the photodegradation of a polyurethane coating indu
ced by accelerated UV irradiation was studied with two different light sour
ces: 313-nm UVB and Xe lamps. Doppler-broadened energy spectra (DBES) and p
ositron annihilation lifetimes were measured. as a function of incident pos
itron energy (0-30 keV) and irradiation time (0-100 h). The photodegradatio
n of the coating was characterized in terms of subnanometer defect changes.
Significant variations of the S parameter (a defect parameter from DBES) a
nd the ortho-positronium lifetime and intensity were observed as a function
of oxygen concentration during exposure to UV irradiation. These results s
howed a significant enhancement in photodegradation due to the presence of
oxygen, as indicated by a decrease in free volume and holes at the atomic l
evel. Enhanced degradation in the presence of oxygen, as indicated by incre
ased free-radical production (observed with electron spin resonance spectro
scopy), was also observed. (C) 2001 John Wiley & Sons, Inc.