MECHANISMS OF PHOTOOXIDATION OF POLY(METHYL METHACRYLATE)

An analytical and kinetic study of PMMA photo-oxidation at long wavelength (lambda > 300 nm) and short wavelength (lambda = 254 nm) is reported. PMMA samples of various origins have been exposed in the form of thin films (40-100-mu-m) and of thick samples (3-6 mm). The photo-oxidation, which proceeds only for a limited period of time, is essentially controlled by external contaminants photo-oxidation of which induces the PMMA oxidation. Specific PMMA oxidation products are observed, after complete dehydration, as alcoholic groups which are hydrogen-bonded to ester groups (at 3550 cm-1) and as self-associated alcoholic groups (at 3320 cm-1). There is no relationship between the photo-oxidation rate and the residual monomer concentration, the monomer being photo-oxidized without any induction effect. In thick samples, which have been photo-oxidized at long wavelength, spatial heterogeneities can be observed using micro-FTIR spectrophotometric technique. Superficial layers (up to 500-mu-m) are extensively oxidized as opposed to the sample core. Spatial heterogeneities must be accounted by oxygen diffusion and by a complementary diffusion of contaminants. Microspectrophotometric determinations on PMMA samples after 15 years weathering reveal the same changes as in artificial photo-ageing.