Radiation degradation of polymethacrylates. Dose rate and medium effects

Abstract

Abstractγ‐Irradiations of polymethyl methacrylate (PMMA) powders confirm observations in literature that air retards the radiation‐induced molecular weight lowering of this polymer. The energy absorptions per scission Ed are 91 e.v. in air at 0.117 Mreps/hr., 76 e.v. in air at 0.70 Mrep/hr., and 56 e.v. in vacuum at 0.117 Mrep/hr. γ‐Radiation degradation of PMMA solutions (5 gm. PMMA/100 ml. solvent) in butanone and in ethyl acetate gave apparent energies per scission of 63 e.v. and 26–30 e.v., respectively. The latter low value is not understood. Poly‐n‐butyl methacrylate (PNBMA) displays considerable sensitivity to air and dose‐rate effects during γ‐irradiation. Apparent energy absorptions per main‐chain scission observed for γ‐irradiated PNBMA powder are: Ed = 454 e.v. in air at 0.117 Mrep/hr. and Ed = 226 e.v. in vacuum at 0.117 Mrep/hr. The latter value is far above the Ed (146 e.v.) observed for electron‐irradiated PNBMA film. To decrease possible chain recombination and/or polymer–polymer crosslinking reactions degrassed solutions of 4.5, 10.0, and 55 wt.‐% PNBMA in ethyl acetate were γ‐irradiated. Apparent Ed values of 74 e.v., 74 e. v., and 226 e. v., respectively were found. Thus, the more dilute solutions give PNBMA scission energies near that of bulk PMMA. The 55 wt.‐% PNBMA solution gives a scission energy indistinguishable from that of bulk PNBMA under the same irradiation conditions. Competitive crosslinking and main‐chain scission are postulated for electron‐irradiated poly‐n‐alkyl methacrylates. The energy per main‐chain scission is assumed constant for all the homologes. The ratio of crosslinked units to chain scissions, α/β, is then found to increase linearly with (n−1), where n is the number of carbon atoms in n‐alkyl groups of the side chain. This scheme yields a consistent explanation of the radiation response of the lower members of the poly‐n‐alkyl methacrylate homologous series. A comparison of light‐scattering Mw and solution viscosity (Mw)v molecular weight changes did not reveal the concurrent crosslinking in PNBMA films undergoing chain scission by 1 m.e.v.‐peak electron irradiation. Some inquiry is made into the theoretical sensitivity of this comparison method.