Photodegradation (PD) in polymers occurs through a series of photochemical reactions: oxidation, chain scission, and crosslinking. The PD mechanisms in phenolic thermoset polymers were poorly comprehended owing to the heterogeneity in photochemical reactions created by decay of the radiation intensity as well as reactant concentration with depth. We overcome these limitations by characterizing the photochemical reactions in thin Epoxy Polymer (EP) films (~750 nm thick) coated on quartz substrates. Further, a fundamental kinetics model has been developed in terms of coupled and nonlinear differential equations to elucidate the complex sequence of associated photochemical reactions. This model shows an excellent agreement with data systematically recorded by UV-Vis as well as FTIR spectroscopies. Additionally, one of the model parameters, namely crosslinking density, again shows excellent agreement with elastic modulus acquired by nanoindentation. Our analysis establishes that the PD is mainly driven by UVA excitation of the carbonyl groups in the EP network (i.e., n?p* transitions). Carbonyl excitation results in either carbonyl dissociation or energy transfer to O2/H2O for the creation of reactive oxygen species (ROS). Subsequently, radicals from dissociation reactions as well as ROS induce a second round of PD reactions including oxidation (e.g., H-abstraction and creation of new carbonyls), crosslinking and chain scission.

Photodegradation in epoxy polymers

248th National Meeting (2014)

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