Absorption spectrum of gamma-irradiated poly(methylmethacrylate) within the range 245 to 450mμ

Abstract

The absorption spectrum of poly(methylmethacrylate) irradiated with γ-rays at room and liquid nitrogen temperatures has been studied within the range 245 to 450 mμ. The materials used varied considerably in nature and concentration of initiator and chain-transfer agent additives. Most of the samples irradiated at liquid nitrogen temperature showed peak absorption wavelengths very close to those noted after room-temperature irradiation; a minority group showed shorter peak wavelengths, which shifted to the room-temperature values when subsequently warmed to room temperature. A marked dependence of spectral behaviour on both initiator and chain-transfer agent content was noted, indicating that most of the absorbing species originate in the additives; on the other hand, an absorption peak around 252 mμ was found in several samples independent of their additive content, and also independent of the temperature of irradiation. As a strong absorption peaking close to this wavelength was observed in radiation polymerized poly(methylmethacrylate), it is postulated that the species giving rise to this particular absorption is formed on methylmethacrylate monomer residue in the polymer. Evidence is presented to show that the absorbing species are not radicals. It is possible that they are ionic in nature, probably cations. It is postulated that the absorbing species formed on the monomer residue is a carbanion C̈HCHCH which, with appropriate electron pairing, would not give rise to an e.s.r. signal. Of the materials polymerized with the help of additives, those containing fairly large concentrations of mercaptan compounds as chain-transfer agents showed a slowly decaying absorption peaking around 252 mμ at liquid nitrogen temperature and 255 mμ at room temperature; the species giving rise to this absorption is to be distinguished from that formed on monomer residue. Materials containing benzoyl peroxide initiator showed peak absorption values around 280 and 290 mμ, independent of temperature.