Studies of Radiation Effects in Three High Polymers

Three high polymers, Mylar^circler , Ultem^circler, and Kapton^circler, were irradiated to total doses of 1, 5, and 9.5 gigarad using 1-MeV electrons. The glass transition temperatures ({rm T }_{rm g}) of the materials before and after irradiation were measured using an AC electrical dissipation factor technique. From the {rm T}_{rm g} data, it was found that the electron radiation at these total doses results in net chain scissioning in Mylar and net crosslinking in Ultem, while self-mending is predominant in Kapton. The dielectric constant was measured before and after irradiation, but no significant changes due to irradiation were observed. Electron paramagnetic resonance (EPR) was used to determine the total organic radical densities in the materials 0.5 hr after irradiation. Total organic radical densities of approximately 10^{18 } spins/g were recorded in all three materials. Additional postirradiation EPR radical density measurements were made at later times to determine the radical decay rates. The radical decay rates were highly varied and did not conform to first-order or second-order decay kinetics, due to the simultaneous presence of several different radical species. Utilizing the EPR spectra, postirradiation radical identifications were made for all three materials. The predominant radicals in Mylar were a phenyl radical and a carbonyl radical, both resulting from a main chain C-C bond scission. In Ultem, an ethylene radical due to a C-C bond scission in the crosslinked material is observed. In Kapton, phenyl and phenoxyl radicals are present resulting from the scission of an ether linkage. Ketone radicals due to the opening of imide rings are also present in Kapton. At long postirradiation times, peroxy radicals are present in all three materials. DC conductivity was measured before and 0.5 hr after irradiation, as well as at longer postirradiation times. Kapton exhibited a 5-order of magnitude increase in conductivity following irradiation, whereas Mylar and Ultem showed no significant change. A hopping model of conductivity, with radicals as hopping sites, is proposed to relate the postirradiation DC conductivity and total postirradiation organic radical density in Kapton.