Perfluorosulfonated ion-conductive polymers (PFSA ionomers) serve as amphiphilic solid electrolytes in applications that range from energy storage to space exploration. PFSAs consist of a polytetrafluoroethylene backbone, fluorocarbon sidechain, and terminal sulfonate ion; this results in a hydration induced phase-separated nanomorphology. Hydration promotes conductivity via solvation and transport of exchange cations which charge-balance the sulfonate groups. Despite the ubiquity of PFSA-related studies in the literature, the quantitative effects of water content and counterion identity on the ionomer’s heterogeneous phase-chemistry remains under contention. FTIR spectroscopy is a nondestructive and facile method for studying local changes in ionomer morphology. We have used transmission FTIR to validate the influence of λavg, the ratio of total water molecules to total ion-exchange sites, and cation identity on the mid-infrared vibrational spectra of Nafion®. λavg for all Nafion-M (M is Li, Na, K, Rb, Cs, Be, Mg, Ca, Sr, or Ba) vs. relative humidity (RH) were first determined gravimetrically in a dynamic vapor sorption (DVS) apparatus. IR spectra vs. RH for all Nafion-M were then assigned λavg based on our sorption isotherms. Both gravimetric and spectral measurements were conducted within in-house designed DVS chambers with unprecedented control of temperature and RH. Emphasis will be placed on the changes in the fingerprint region and water vibrational modes in our discussion.

Dependence of humidity and exchange cation on ionomer vibrational modes

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