Kumada, T., Y. Noda, and N. Ishikawa, Dynamic nuclear polarization of electron-beam irradiated polyethylene by pairs of alkyl free radicals. J. Magn. Reson., 2012. 218(0): p. 59-65.
We compared dynamic nuclear polarization (DNP) behavior of electron-beam irradiated polyethylene (e-PE) with that of 2,2,6,6-tetramethylpiperidine 1-oxyl (TEMPO)-doped polystyrene (T-PS). Growth and decay rates of nuclear polarization Pn of e-PE increased linearly with increasing concentration Ce of free radicals, whereas those of T-PS increased with square of Ce. From these results, we suggest that the pair of alkyl radicals produced by the radiolysis, –CH2–CH2–CH2– → –CH2–CH–CH2– + H, and the subsequent hydrogen abstraction reaction, H + –CH2–CH2–CH2– → H2 + –CH2–CH–CH2– in e-PE plays an important role on DNP by the cross effect, whereas T-PS is polarized by a pair of unspecified TEMPO radicals accidentally located nearby. We propose that higher Pn would be expected by optimizing the inter-radical distance within the pair in e-PE and other irradiated samples.