Yellowing of plastics under sunlight is the most common sign of their degradation, which is propelled by a series of photochemical reactions -- oxidation, dissociation, crosslinking, and chain scission. The mechanism of polymer photodegradation (PD) has been poorly comprehended owing to the heterogeneity in photochemical reactions created by decay of radiation intensity as well as reactant concentration with depth. In the present work, we overcome these limitations by analyzing the discoloration of epoxy thermoset thin films (~ 650 nm) under UV exposure by optical absorption spectroscopy. To reveal the mechanistic steps of degradation, a fundamental kinetic model has been formulated that encompasses the kinetics of these 4 photochemical reactions. This model exhibits an excellent agreement with the data systematically recorded by UV-Vis as well as FTIR spectroscopies. Our analysis establishes that the polymer PD is autocatalytic under sunlight, as new C=O sites are generated by UVA excited C=O moieties (i.e., n→π* transitions) through creation of singlet oxygen (1O2) due to excited energy transfer to ground state triplet oxygen (3O2). These electronically excited C=O moieties also undergo dissociation creating free radical sites, which undergo either crosslinking or chain scission. It is also demonstrated that the rate of PD is increased either by incorporation intrinsic C=O sites or non-C=O sensitizers. Additionally, the kinetics model is employed to elucidate the propagation of PD reactions along the thickness that are difficult to acquire experimentally.

Polymer photodegradation: Autocatalytic under sunlight

251st National Meeting (2016)

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