M. Paronen, F. Sundholm, D. Ostrovskii, P. Jacobsson, G. Jeschke, E. Rauhala, P. TikkanenPreparation of proton-conducting membranes by direct sulfonation. 1. Effect of radicals and radical decay on the sulfonation of poly(vinyl fluoride) films.Chemistry of Materials. 15(23), 4447-4455 (2003)
The effect of irradiation treatment on the structure and sulfonation reactivity of polyvinyl fluoride (PVF) films was studied mainly with EPR and IR spectroscopy. The main radical species produced by electron irradiation are aliphatic radicals, whereas proton irradiation creates a significant amount of polyenyl radicals. The half-life time of the aliphatic radicals was about 6 h. FTIR study shows that sulfonation reaction of PVF is a single-stage process and thus the formed sulfonic acid structures do not react further and produce new structures. In addition, it is independent of the irradiation treatment. Despite the careful synthesis and removal of O-2 from the sulfonation solution, the most important side reaction produced by the sulfonation is the formation of C=O functionalities. Irradiation treatment increases. the C=O content. Both the sulfonation time and sulfonation reagent concentration demonstrate similar linear correlation with the extent of oxidation. Therefore, the oxidation cannot be decreased by means of optimizing these variables. The only important factor allowing adjustment of the structure of the final membranes is the type of irradiation and radical decay in the case of proton irradiation. Irradiation with either electrons or protons did not increase the ion exchange capacity as measured with a titrimetric method. IR study of the samples showed, however, indication of the formation of derivates of sulfonic acid in the proton-irradiated samples containing radicals.