Regulation of the Thermal Reactions of Polyacrylonitrile by γ-Irradiation

Thermal treatment of γ-irradiated polyacrylonitrile was performed to investigate the radiation effects on the thermally activated reactions. It was found that the exothermic reaction and the discoloration of the irradiated samples occurred at much lower temperatures than for nonirradiated ones, and the reactions can be regulated by the preirradiation dose. The structures formed by the thermally activated reactions of the irradiated samples consisted of C N and/or C C conjugation sequences having broad electronic absorption bands up to 500 nm and an emission band at green-blue light region. The preirradiation caused an increase in absorption intensity with no shift in wavelength of the absorption peaks. A spectral shift was observed for the emission of the preirradiated samples, demonstrating an occurrence of the energy transfer among the conjugated unsaturation sequences. Free radical identification by ESR indicated that the radiation effect was not due directly to trapped radicals but caused by the functional groups generated during irradiation, which contributed to the initiation of the thermal reactions. The differential scanning calorimetric and thermogravimetric analyses confirmed that the preirradiation lowered the onset temperature of the reactions and alleviated the heat evolution process.