The chemical effects of ionizing radiations on fluorinated ethers

The state of knowledge on the mechanism of the radiolysis of poly(perfluoroethers) (PFPE) with the poly(trimethylene oxide), poly(propylene oxide) and poly(ethylene-methylene oxide) structures is reviewed and the results of recent investigations performed using EPR, radiothermal luminescence (RTL), electron scavenging, pulse radiolysis and UV-Vis matrix spectroscopy are reported. The radiolytic degradation of the poly(perfluoroethers) is based on excited species formed presumably via the cation-electron geminate recombination. The excited ether moieties undergo homolytic C-O and C-C bond cleavage with formation of a variety of neutral free radicals which by coupling and cage disproportionation accounts for the major chemical changes detected in the PFPE chains. As a consequence of the bond cleavages a linear decrease of the average molecular weights with increasing the radiation dose is observed. The C-F splitting is also detected from the related free radicals and branching units in the chains; however their incidence is of minor importance with respect of chain scissions and it is significant only in the linear PFPEs. The role of ionic intermediates has been analyzed by matrix EPR and UV-Vis spectroscopy, pulse radiolysis, RTL spectroscopy and electron scavenging experiments: the results strongly suggest that ionic reactions are participating to the radiolysis mechanism but the upper limit estimate of their contribution to free radical yields is <20% for the Demnum(TM) and Krytox(TM) and <30% for the more polar Fomblin(TM)Z. An insight of the stability and decomposition modes of the elusive radical-cations and radical-anions of the perfluoroethers associated to the ionic mechanism was obtained through low-temperature electron loss and electron capture selective experiments coupled with matrix EPR spectroscopy and M.O. calculations; the results suggest that both species are intrinsically unstable at 77 K, the favored decomposition modes being the C-O splitting for the anions and the cleavage of the C-C bonds adjacent to oxygen for the cations. (C) 2003 Elsevier B.V. All rights reserved.