FAST-NEUTRON IRRADIATION EFFECTS ON POLYMERS .1. DEGRADATION OF POLY(METHYL METHACRYLATE)

Irradiation effects of fast neutrons on poly(methyl methacrylate) were compared with those of 60Co γ rays from the viewpoint of linear energy transfer (LET). The G value of main-chain scissions, G(S), and the change in the molecular weight distribution of irradiated polymers were determined by means of gel-permeation chromatography. The LET dependence of G(S) is definite for in-vacuo irradiations [G-γvacuo(S) = 1.54 Ω 0.11 for 60Co γ rays (LET: ca. 0.02 eV/A) and Gnvacuo(S) = 1.04 ± 0.13 for fast neutrons (LET: ca. 3.7 eV/A)], but the dependence is not so definite for in-air irradiations [Gγair(S) = 0.73 ± 0.06 and Gnair(S) = 0.66 ± 0.10]. The plot of the Mw/Mn ratio vs. the number of main-chain scissions deviates from that predicted by the random-degradation theory. The deviation is more distinct for fast-neutron than for γ-ray irradiations. The reduced G(S) value and the enhanced deviation from the random degradation at high LET are attributable to the increased inhomogeneity of microscopic energy deposition. A parameter expressing the microscopic inhomogeneity, ΔD/x, is introduced in the computer simulation of the nonrandom degradation and is adjusted so as to give the best fit between the observed and simulated plots of the Mw/Mn change. The computer simulation gives the ΔD/x value of 1.54 and 0.52 Mrad for fast-neutron and -ray irradiations, respectively, indicating that the ΔD/x ratio (ca. 3) is strikingly small compared with the LET ratio (ca. 180). © 1979, American Chemical Society. All rights reserved.