Radical identification during photo-oxidative aging of epoxy resin

Photo-oxidative aging of epoxy resins is widely believed to be governed by radical chain reactions, yet direct, structure-relevant identification of transient radical intermediates remains limited. Here, we combine EPR with spin-trapping technique to directly probe radical generation and location, and identify radical species during photo-oxidation of the epoxy resin. Direct EPR measurement confirms that light irradiation induces a pronounced radical signal in the epoxy matrix, supporting a light-driven initiation process; in situ spin trapping in cyclohexane further shows that the signal intensity of spin-trapping agent adduct increases more rapidly at higher irradiation intensity. Using various solvents to selectively react or quench radicals, we are able to identify key radical intermediates and propose the radical initiation mechanism of epoxy resin during photo-oxidation accordingly. Since radicals are mainly on macromolecular chain, they can live for months, and trapping efficiency is governed by the accessability to spin-trapping agent to radicals in solid crosslinking network. Overall, this work provides direct evidence for radical-initiated epoxy photo-oxidative aging and offers a practical route toward structural identification of radicals in complex crosslinked polymer systems.