Anisotropic Diffusion and Morphology in Perfluorosulfonate Ionomers Investigated by NMR

Anisotropy in ionomer membranes represents a powerful interaction for modulating properties such as mechanical moduli, thermal expansions, and small molecule transport, all tunable via controlled processing. We observe uniform hydrophilic channel alignment in three perfluorosulfonate ionomer membrane types, quantified by 2 H NMR spectroscopy of absorbed D 2 O molecules. Our measurements show biaxial or uniaxial in-plane alignment for extruded membranes, but uniaxial through-plane alignment for dispersion-cast membranes, and further demonstrate affine swelling with both water uptake and thermal expansion. In order to correlate alignment data with a quantity relevant to proton transport, we measure the anisotropy of water self-diffusion using pulsed-field-gradient NMR along different membrane directions. Extruded membranes with stronger alignment exhibit 18% faster in-plane diffusion than through-plane diffusion, while diffusion anisotropy is minimal for weakly aligned membranes. These results should lead to a more quantitative understanding of and control over membrane properties via manipulation of molecular order.