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Ionic Cluster Morphology of Sulfonated Polyarylenethioethersulfone Copolymer Membranes for Fuel Cell Application

Mitra Yoonessi1, Michael F. Durstock1, and Richard A. Vaia2. (1) Materials Directorate, Wright Patterson Air Force Research Laboratory, 2941 Hobson way, Bldg 654, WPAFB, OH 45433, (2) Materials and Manufacturing Directorate, Wright Patterson Air Force Research Laboratory, 2941 Hobson way, Bldg 654, WPAFB, OH 45433

Highly sulfonated endcapped polyarylenethioethersulfone copolymer (SPTES 50) with outstanding thermal-mechanical properties and good electrochemical characteristics is an excellent candidate for future hydrogen fuel cell membranes. Below 80o C, the membrane electrode assembly formed from SPTES 50 performs comparable to Nafion 117 (prepared with the same method). However from 80-120 o C, SPTES 50 maintains its performance whereas that of Nafion 117 deteriorates.

Microstructure and morphology of the polymer film has direct influence on the proton conducting mechanisms and ultimate performance of the membrane. Small angle neutron scattering (SANS) from fully-hydrated SPTES 50 revealed that the hydrated ionic clusters range from 4 to 14 nms, increasing in size with temperature and water content. Overall, morphology of the hydrated film could be best described by the presence of polydisperse domains of water embedded within the polymer matrix, where the domains exhibit liquid-like local ordering in terms of a Percus Yevick hard sphere potential interference function. The average radius of the clusters predicted by this model is in excellent agreement with the average radius obtained from direct Porod and Invariant analysis as well as transmission electron microscopy. Models with more refined structural detail, such as the core-shell or depleted zone model where the contrast variation due to the presence of sulfonic groups at the water-polymer interface are also taken into consideration, were also examined. Insufficient features in the experimental data though inhibited differentiation between these descriptions. These models, with reasonable approximation of the physical properties of the interface, were also in good agreement with the direct Porod and Invariant analysis.

Finally, inclusion of a Debye Bueche correlation function to account for large scale structure (>60 nm) provided description of the over 6-orders of magnitude of experimental intensity. Overall, quantifying information regarding the impact of temperature and hydration on the ionic clusters, including average radius, polydispersity, area per sulfonic group, number of sulfonic groups per cluster, and correlation lengths of aggregates, have been obtained from SANS experiments..