Danilcuzk, Marek, Andrew Perkowski, and Shulamith Schlick
The fragmentation of Nafion membranes and model compounds upon attack by hydroxyl radicals has been investigated in our laboratory by ESR spectroscopy, using direct ESR detection, or by spin trapping, with 5,5-dimethylpyrroline-N-oxide (DMPO) and methyl-nitroso-propane (MNP) as the major spin traps used.1,2
We present a comparison of the stability of 3M membrane (EW 850) and Nafion (EW 1100), based on the relative intensity of the DMPO adducts of carbon-centered radicals, DMPO/CCR, which were detected when the membrane solutions were exposed to HO• radicals. The hydroxyl radicals were generated by UV-irradiation of hydrogen peroxide.3
It is important to notice that the hyperfine splittings from 14N and Hβ nuclei of the adducts are different for the two membranes: For Nafion aN=15.9 G, aHβ=23.5G, and for 3M aN=15.7 G, aHβ=18.7G, possibly indicating different fragmentation mechanisms. The relative intensity of CCR adduct from the 3M membrane is significantly lower than that expected from the polymer concentration in the mixture, indicating a higher stability to attack by hydroxyl radicals, for the experimental conditions used in our laboratory.
A better understanding of the membrane stability can be obtained by quantitative measurements of the reaction rates between the membranes and HO• radicals, as well as of trapping rates of membrane-derived radicals by DMPO. The rate constants of the reaction of hydroxyl radicals with the membranes in aqueous solutions were determined using the competition kinetics method,4,5 by monitoring the formation of the DMPO/OH adduct in the solution, in the absence and in the presence of different membrane concentrations. The major conclusion is that the 3M membrane is significantly more stable compared to Nafion when exposed to HO• radical attack: The rate constants for the reaction of hydroxyl radicals with the membranes is one order of magnitude slower for 3M membrane compared to Nafion.