Thermal and photolytic degradation of plates of poly(methyl methacrylate) containing monomer

Specimens of 1.5 mm thick absorber-free poly(methyl methacrylate) (PMMA) containing ∼0.6% monomer but no absorber have been photolytically degraded in air at 50°, 85° and 115°C and thermally degraded in air at 115° and 125°C. Specimens were exposed to a simulated solar spectral range. Degradation was followed by gel permeation chromatographic determinations of molecular weight as a function of depth in the specimens. The results show increased photodegradation at the plate faces (back and front) over that occurring in the centres, and a rapidly attained constant amount of degradation for thermal degradation. Degradation mechanisms are proposed. The thermal degradation is ascribed to weak links and unspent initiator. Photolytic initiation is ascribed, at least in part, to degradation of the ester group by wavelengths in the range 300 to 330 nm. The effect of oxygen is to convert alkyl radicals into peroxyl radicals, some of which form alkoxyl radicals which tne undergo β-scission to give in-chain ruptures. Where the oxygen concentration is low, monomer changes non-tertiary alkyl radical sinto tertiary radicals by addition to the monomer double bond. After their peroxidation by molecular oxygen, tertiary radicals react with one another to give alkoxyl radicals and subsequent chain scission rather than undergoing the Russell termination reaction with no chain scission characteristic of non-tertiary peroxyl radicals. The effect of temperature is mostly to decrease the importance of the cage effect and to allow the initial radicals formed to diffuse away from one another. The products of photo-oxidation absorb the shorter (300 to 330 nm) radiation significantly and progressively shield the remainder of the plate as degradation proceeds.