Degradation of Polymer Coating Systems Studied by Positron Annihilation Spectroscopy. 3. Wavelength Dependence of UV Irradiation Effect

Abstract

Photodegradation of a polyurethane coating induced by accelerated UV irradiation is studied using three different light sources: 340 nm UVA, 313 nm UVB, and Xe lamps. Doppler-broadened energy spectra (DBES) and positron annihilation lifetime (PAL) are measured as a function of incident positron energy (0-30 keV) and of irradiation time (0-800 h). The photodegradation of the coating is characterized in terms of subnanometer defect changes. Significant variations of S parameter, orthopositronium lifetime, intensity, and lifetime distribution are observed as a function of depth and of exposure time. These results indicate a decrease of free volume and holes at the atomic level during the degradation process. The UV irradiation at 313 nm shows a significant effect compared with that at 340 nm on the photodegradation:(1) the magnitude of S reduction increases a factor of 2 near the surface; (2) the photodegradation rate increases ∼25 times near the surface; (3) the irradiation effect attenuates with depth, and a larger extinction coefficient is observed with shorter wavelength UV irradiation. The observed effect by Xe light irradiation is the strongest among these three light sources. Enhanced degradation at shorter wavelengths is also indicated by increased free radical production, as observed by electron spin resonance spectroscopy.

Life science research using positron annihilation spectroscopy: UV-irradiated mouse skin

“…1 that the S parameter decreases as a function of UV irradiation is also similar to that observed in polymeric coatings (Cao et al, 1998a(Cao et al, , 1999b. In polymeric UV studies, we interpreted it to be a result of photodegradation where irradiation breaks chemical bonds of organic molecules and forms free radicals.…”

Stabiliser distribution and efficiency examined by depth profiling polypropylene using a positron beam

“…From approximately 2-3 keV onwards, the S-parameter approaches a plateau. This result is a reminiscent of other works on polymer coatings [11,[20][21][22] and semiconductors and is a function of three parameters, namely the positron annihilations in the bulk state, the positron annihilations in the surface state and the diffusion of positrons and Ps into the vacuum. Fig.…”

“…Numerous subsequent PALS and DBES experiments using both the sandwich geometry [3,9,[14][15][16][17] and a positron beam [8,11,[18][19][20][21][22][23] have been performed to detect degradation in polymeric coatings. In general, decreases in the S-parameter from DBES experiments, as a result of degradation, have been attributed to a loss in the free volume of the material [24], an increase in the percentage of crystallinity [14,15] and a reduction in Ps formation probability (due to the presence of carbonyl groups [15,16] or free radicals [22]).…”

Deterioration of a polyurethane coating studied by positron annihilation spectroscopy: Correlation with surface properties

“…In our recent studies12–15 we investigated the photodegradation problem by a molecular and nanoscopic approach. We used an innovative, nondestructive technique—positron annihilation spectroscopy (PAS)16—to investigate the properties of subnanometer defects in polymeric coating systems generated during photodegradation processes induced by UV irradiation.…”