(163c) Microscopic Diffusion of a Chemical Warfare Agent Simulant in the Presence of Water in Nafion Membranes By PFG NMR

Perfluorosulfonic acid (PSA) polymer membranes such as the commercially available Nafion are among the most promising materials in a wide variety of potential or current industrial applications including fuel cells, water desalination processes, chemical sensing, and selective capture/immobilization of chemical warfare agents (CWA). Molecular diffusion plays an important role in these applications. However, detailed fundamental understanding of a relationship between diffusion of different types of molecules and structural properties of Nafion membranes is still missing. In the presence of water, the Nafion structure exhibits a hydrophobic semi-crystalline matrix made of a backbone of polytetrafluoroethylene and interfacial perfluoroether regions containing channels available for water diffusion, viz. water channels. In this work we applied pulsed field gradient (PFG) NMR to study diffusion of CWA simulant dimethyl methyl phosphonate (DMMP) and water in Nafion. DMMP molecules have a “Janus” structure which contains a hydrophilic “head” and a hydrophobic “tail”. Our PFG NMR data allowed quantifying self-diffusion of DMMP and water in Nafion for a broad range of molecular displacements. The analysis of the data indicates that DMMP and water diffuse in different regions of the Nafion polymer. Water mostly diffuses in water channels while DMMP diffusion is mostly limited to interfacial perfluoroether regions between these water channels and the semi-crystalline matrix. Such separation of diffusion pathways suggests approaches for a selective capture of CWA (e.g., by reacting with selectively imbibed reagents) while allowing water to diffuse uninhibited. To our knowledge, this is the first work in which the self-diffusion coefficients of large organic molecules have been quantified in Nafion.