In this work the effects of γ-radiation on monomethoxy-poly(ethylene glycol)s, (MPEG)s, in aqueous solutions were studied. Polymers with two different nominal molecular weights, 2,000 and 5,000, have been used. The influence of dose and dose rate of radiation, initial polymer concentration and pH were investigated both in the absence and presence of oxygen.
The gelation dose dependence on the concentration and molecular weight of polymers irradiated in deaerated solutions at natural pH 6.3 and at pH 8.4, with added bicarbonate ions, indicate that hydroxyl radicals from water radiolysis are the main precursors of macroradicals formed at MPEG molecules. Crosslinking efficiency and stability of MPEG networks formed under these pH conditions are markedly different. The sol fraction is related to the radiation dose according to the Charlesby-Pinner approach. At pH 6.3 degradation led to dissolution of the hydrogels. Permanent, tightly crosslinked networks resulted in deaerated alkaline solutions. The molecular weight between junction points ranged from ~300 to ~2200 g mol-1 and the corresponding mesh size was ~40-~150 Å. In acid deaerated and in oxygen-containing systems at natural pH, the effect of radical formation by γ-radiation was chain scission. The oxidative polymer degradation propagated in a chain-like process.
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