Far-infrared studies on Nafion and perfluoroimide acid (PFIA) and their alkali salts

The terahertz/far-infrared spectra (<300 cm−1) of perfluorinated sulfonic acid (Nafion NR211 polymer) and perfluoroimide acid (PFIA polymer) and their alkali (M+) salts have been analyzed and the results are presented. Pronounced features in the spectra of these ionomers that correlate systematically with the corresponding cation mass are reported and from their spectral position the force constants are derived. The average vibrational force constants for Nafion/M+ and PFIA/M+ are found to be 54 ± 7 and 39 ± 4 N/m, respectively. Such terahertz/far-infrared signatures probe the detailed structure of the Nafion/M+ and PFIA/M+ ionic clusters and, in turn, provide benchmarks for elucidating the ionomer “water channels” or water molecules located in the ionomer–water interface upon hydration. Qualitative trends in the vibrational energies of Nafion and PFIA can be explained by consideration of electronic and/or structural (ionic domain-size) effects.