H double- and zero-quantum filtered NMR spectroscopy for probing the environments of water in Nafion

Multiple quantum ²H NMR spectroscopy is used to study the structure and dynamics of [D.sub.2]O in Nafion membranes as a function of membrane hydration. By employing both double- and zero-quantum filtered experiments, residual quadrupolar coupling constants and [T.sub.2] relaxation values are obtained. The residual couplings vary from 240 to 20 Hz and the [T.sub.2] values range from 20 to 180 µs, with the high hydration values having smaller couplings and longer [T.sub.2] values. Analysis of the data using a water-exchange model suggests that the changes in parameters arise from a change in the fraction of time water spends in the anisotropic environments and not from changes in the order parameters that characterize the anisotropic sites. It has been found that a two-site model is needed to accurately fit the spectra above a hydration level of 10 [D.sub.2]O per sulfonate, with the second site having a negligible residual quadrupolar coupling. The data supports a model with two different hydration layers at high hydration and can be understood in terms of the recently proposed parallel- channel model for Nafion hydration.