Rate of formation and decay of free radicals in γ‐irradiated polymethyl methacrylate by means of electron spin resonance absorption measurements

Abstract

AbstractThe rate of formation of the free radicals in polymethyl methacrylate by γ‐irradiation, the rate of decay of the free radicals in vacuum and in the presence of air after irradiation, and also the rate of decay of the oxygenated radicals have been measured by means of electron spin resonance absorption measurements. The formation of the free radicals can be expressed in the beginning range of irradiation up to 10 hr. (7.1 × 106 r.) by the equation [R·] = 5.8 × 10−7 tanh (6.4 × 10−5t), where [R·] is the concentration of the free radicals in moles/cm.3 and t is the time of irradiation in seconds. Assuming the rate of formation follows the equation d[R·]/dt = k′I ‐ k[R·]2, where I is the dose rate, the rate constants, k′ and k, of the formation and of the decay of the free radicals are calculated to be k′ = 1.9 × 10−13 moles cm.−3 r−1 and k = 110 moles cm.3 sec.−1, respectively. The G‐value for the radical formation can be estimated to be 0.16 from this k′‐ value. A discussion on this small G‐value obtained is given in the paper, and it is supposed that the G‐value 0.16 may be that for the formation of the long‐living radicals in γ‐irradiated polymethyl methacrylate. The decaying processes of the radicals in vacuum and in the presence of air and those of the oxygenated radicals after irradiation are to be interpreted as reactions of the second order. The rate constants k′s of the decaying processes are estimated and tabulated. Comparing the k values under different conditions among themselves, some interesting features may be pointed out. The k‐value 110 during irradiation is very much larger than that 3.3 after irradiation. The k‐value 1130 in the presence of air is still larger. A discussion on these points is also given. In the case of decay of the radicals in vacuum, log k was plotted against reciprocal of absolute temperature and a straight line was obtained. From the slope, the activation energy was estimated to be 28 kcal./mole. When this value is compared with the activation energy 27 ± 10 kcal./mole estimated by Powles for the motions of chain segments in polymethyl methacrylate, it may be concluded that the motions of the chain segments are of primary importance for the rate determining step of the decaying reaction of the long‐living radicals.