Self-organized polymer membranes find uses encompassing reverse-osmosis, nanofiltration, gas separations, and fuel cells, and are at or approaching the lowest cost technology in these venues. Mobile species within these membranes (ions, water, alcohols, gases) move according to local molecular interactions as well as a hierarchy of structural morphologies that span sub-nm to > 10 mm in length scale. We draw on structural information from scattering and microscopy as well as on theories of porous media, electrolytic transport, and oriented matter, then combine these ideas with transport measurements using NMR. Pulsed-field-gradient (PFG) NMR can track chemically distinct moving species, and one can vary the measurement encoding time to uncover the polymer structures that influence diffusion. This talk will focus on how polymer alignment relates to transport in ionomers, as well as on restricted diffusion that is ion specific. On this last subject, we have probed diffusion over a wide range of diffusion lengths of ionic liquids absorbed into a perfluorosulfonate ionomer. Surprisingly, anions experience diffusive restrictions on the micron scale that are strongly temperature and hydration dependent, while cations experience no restrictions. These new fundamental insights will drive rational membrane materials and device design, and will enable new ionomer membrane applications.

Multiscale transport and collective interactions in ionic liquids and polymers: Vistas from gradient NMR

245th National Meeting (2013)

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