ELECTRON-SPIN ECHO STUDIES OF FREE-RADICALS IN IRRADIATED POLYMERS

Studies of the utilization of the electron spin ecbo method for elucidating the nature and behavior of free radicals in gamma- irradiated polyethylene and poly(methyl methacrylate) are reviewed to demonstrate the importance and usefulness of this method generally for studying the effects of radiation on polymers. The paramagnetic relaxation was found to be largely due to the dipolar interaction between radicals and generally follow non-exponential kinetics. The dependence of the relaxations on the molecular motion and, therefore, on the site of the radicals is utilized for the relaxation-resolved electron spin resonance measurement. This new technique was able to discriminate the overlapped ESR spectra due to coexisting free radicals from each other and facilitated the elucidation of the behavior of the radicals. The dependence of the relaxations on the radiation dose or on the thermal annealing of radicals gave information on the local spatial distribution of the radicals, or the structure of a radiation induced spur. The radicals were found to be generated and trapped pairwisely in a spur. The average separation distance between the radicals in a pair was estimated to be 2.6 and 3.1 nm for polyethylene irradiated at 77 K and poly(methyl methacrylate) irradiated at room temperature. These electron spin echo results were discussed in terms of radiation-induced reactions leading to the crosslinking or the degradation of the polymers.