Solvent Effects on Nafion Dispersion Structure Probed By Infrared Spectroscopy

The structure of Nafion dispersed in water and ethanol solutions was investigated using infrared spectroscopy. Since the properties of dispersing fluids can affect the mechanical strength of solution-cast Nafion thin films, 1 it was of interest to gain insights into the structure and the vibrational spectral properties of the ionomer in the dispersion phase. To minimize deleterious effects of optical dispersion, 2 traditional transmission infrared spectra were recorded from both dispersions as well as solvent-free thin films. Subsequently, transitions from the dispersion, through the gel-phase and dried film were followed as a function of time during film formation using the attenuated total reflection (ATR) infrared sampling technique. The vibrational spectra of thin Nafion films prepared from dispersions in ethanol solvent and water solvent were similar, except the former had a pronounced shoulder near 1125 cm -1 and sharper 983 cm -1 peak, both of which can be ascribed to sidechain vibrations. The enrichment of trace water in the ethanol solvent during evaporative drying was quantified through analysis of the 2750 cm -1 – 3500 cm -1 spectral region using partial least square (PLS) modeling. In the gel-phase, water was detected above 50 mol%, indicating water enrichment should be considered in developing models for solvent effects on Nafion film formation. Kim, Y. S.; Welch, C. F.; Hjelm, R. P.; Mack, N. H.; Labouriau, A.; Orler, E. B. Origin of toughness in dispersion-cast Nafion membranes Macromolecules 2015 , 48 , 2161-2172. Kollath, V. O.; Liang, Y.; Mayer, F. D.; Ma, X.; Korzeniewski, C.; Karan, K. Model-based analyses of confined polymer electrolyte nanothin films experimentally probed by polarized ATR−FTIR spectroscopy J. Phys. Chem. C 2018 , 122 , 9578−9585. Figure 1