Effects of ionizing radiation on epoxy,graphite fiber,and epoxy/graphite fiber composites. Part II: Radical types and radical decay behavior

Electron spin resonance (ESR) investigations of line shapes and radical decay behavior have been made on an epoxy based on tetraglycidyl diaminodiphenyl methane (TGDDM)/diaminodiphenyl sulfone (DDS), T-300 graphite fiber, and T-300/5208 (graphite fiber/epoxy) composites after irradiation with Co₆₀ γ-radiation or 0.5 MeV electrons. Two kinetically distinct radical species are found in the irradiated epoxy as the temperature is raised beyond 120 K following irradiation of samples at 77 K with Co₆₀ γ. One has been termed a fast-decaying species and the other a slow-decaying species. The ratio of fast-decaying/slow-decaying radicals increases as the decay temperature rises. The fast-decaying radicals at room temperature are attributed largely to alkyl type radicals residing in regions of relatively low crosslink density, while the long-lived radicals are attributed to radicals residing in the highly crosslinked regions of the epoxy. A large concentration (ca. 10₂₀ to 10₂₁ spins/g) of unpaired electrons was found in unirradiated graphite fiber which masked the ESR spectral change in irradiated composites.