Microstructural Evolution of Nafion During Uniaxial Deformation Monitored by X-ray Scattering

Fuel cells enable direct chemical to electrical conversion of fuel to electricity, providing an efficient and clean process. Proton Exchange Membrane Fuel Cells (PEMFC), in which protons from hydrogen or methane cross a membrane to react with oxygen producing electricity, are the preferred transportable fuel cell. Nafion is the membrane of choice for Proton Exchange Membrane Fuel Cells (PEMFC) because its unique microstructure allows rapid transport of protons in a hydrated environment while maintaining mechanical integrity. The teflon-like backbone is hydrophobic while the sulfonated side chains are hydrophilic. In the presence of water molecules this causes the side chains to aggregate into clusters which contain most of the water while the backbone remains relatively dry. Extensive studies have been conducted in order to deduce the size and shape of the microstructural features in order to gain insight into its superior electrochemical and mechanical characteristics, and how they can be further improved (e.g. [1-7]). The shape of these regions (sphere, cylinders, ribbons etc) and their evolution with deformation is still a matter of debate. Here the microstructural evolution is monitored during uniaxial tensile testing via small and wide angle x-ray scattering. Two dimensional scattering profiles are recorded along with stress and strain as a function of time for monotonic, cyclic, and stress relaxation loading histories. These profiles are then reduced to amplitude, location, and orientation for each of the major structural peaks. The scattering data are interpreted in conjunction with existing literature in order to understand the rate and deformation dependent microstructural evolution and its relation to the rate dependent elastic-plastic stress-strain behavior exhibited by Nafion.