Measuring water hydrogen bonding distributions in proton exchange membranes using linear Fourier Transform Infrared spectroscopy

This paper describes a linear Fourier Transform Infrared (FTIR) spectroscopic method for measuring the hydrogen bonding distribution of water sorbed in proton exchange membranes. This information on the distribution of the microenvironments of water in an ionic polymer is critical to understanding the effects of different acidic groups on the proton conductivity of proton exchange membranes at low relative humidity. The OD stretch of dilute HOD in H2O has a single, well-defined vibrational band. When HOD in dilute H2O is sorbed into a proton exchange membrane, the OD stretch peak appears to shift based on the microenvironment that water encounters within the nanophase separated structure of the material. This peak shift is a signature of different hydrogen bonding populations within the membrane, which can be deconvoluted rigorously for dilute HOD in H2O compared to only qualitative observations that can be made with pure D2O or H2O. The theory and experimental practice of determining the hydrogen bonding distribution of water in a range of proton exchange membranes bearing aromatic sulfonate and perfluorosulfonate groups using this OD stretch technique are discussed.