(381y) Microscopic Diffusion of Organic Molecules in Perfluorosulfonic Acid Polymer Membranes

Perfluorosulfonic acid (PSA) polymer membranes such as commercially available Nafion have high ion permselectivity and can be used as a solid fuel cell electrolyte and solid super-acid catalyst. PSA membranes are also among the most promising candidates for novel applications including optodes for chemical sensing and personal protective clothing. PSA membranes are composed of a hydrophobic backbone with hydrophilic acid head groups which form respective hydrophobic or hydrophilic domains. While the microscale transport of water and some small organic molecules such as methanol has been studied, the microscopic diffusion of larger organic molecules in PSA polymers is not well understood. In the present work, self-diffusion of organic molecules such as acetone and vanillic acid are studied in Nafion on micrometer and submicrometer length scale to address this gap in the current knowledge.

The diffusion studies were performed by multinuclear pulsed field gradient (PFG) NMR at high magnetic fields (14 T and 17.6 T) and large magnetic field gradients (up to 25 T/m). The combined application of C-13 and H-1 PFG NMR for diffusion studies of the same molecules allowed elucidating a possible influence of proton exchange and proton diffusion on the apparent self-diffusivities measured for organic molecules in Nafion by H-1 PFG NMR. The diffusion studies were performed for a broad range of diffusion times and at different temperatures and concentrations of water and organic molecules in Nafion. The diffusion properties of neat Nafion are compared with those of resorcinol- or vanillic acid-functionalized Nafion. These functionalized membranes have been shown to have potential as chemical sensors and improved acid catalysts. The role of the membrane functionalization in the diffusion process in Nafion will be discussed in detail.