Radiation Chemistry of Polyethylene. VIII. Kinetics of Growth and Decay of Polyene and Polyenyl Free Radical Groups

Abstract

Free radicals produced as intermediates during the radiolysis of polyethylene have been most frequently studied by ESR techniques. We have now used ultraviolet absorption spectroscopy because the uv absorption bands of the allyl, dienyl, and trienyl conjugated free radicals are well separated on the wavelength scale and do not overlap as do the ESR signals from these intermediates. Stable polyenes are also produced by the radiolysis and these groups can also be studied quantitatively by ultraviolet spectroscopy. In contrast to the diene groups whose concentration grows linearly with the dose, the allyl and dienyl free radicals as well as conjugated trienes grow with increasing rates as the irradiation proceeds. In fact, a statistical calculation of the triene growth reaction rate constant in terms of the diene concentration and a probability factor based on the vinylene growth rate has been quite successful. From this result and others it is concluded that trienes and allyl, dienyl, and trienyl free radicals are not primary products of the irradiation. The effect of ultraviolet light on the allyl and dienyl free radicals has also been studied. In agreement with others we find that uv light decreases the allyl free radical concentration; however, the dienyl and trienyl concentrations increase. On standing after the uv irradiation the allyl groups recover practically their initial abundance—but the dienyl and trienyl free radical concentrations continue to grow. Various mechanisms are discussed.