Infrared Spectroscopy of Bis[(perfluoroalkyl)sulfonyl] Imide Ionomer Membrane Materials

Structural properties of the proton-exchanged forms of bis[(perfluoroalkyl)sulfonyl] imide (PFSI) ionomer materials were investigated. The hydration and dehydration of samples prepared as thin films and freestanding membrane were probed by applying transmission infrared spectroscopy. Spectral bands were assigned and effects of water incorporation into membrane pores and channels were understood by drawing upon results from related measurements performed on the structurally similar, perfluorosulfonic acid ionomer, Nafion. Both PFSI and Nafion membrane materials display a prominent infrared absorbance band near 1060 cm−1 that arises from a vibrational mode of the ionizable group present on the side chains that extend from the poly(tetrafluoroethylene) backbone on the polymers. The mode can be traced to symmetric stretching of the −SO3− (sulfonate) group in Nafion and to antisymmetric S−N−S stretching within the sulfonyl imide end group (−SO2(N−)SO2CF3) in the PFSI materials. For Nafion samples, the position...