(603f) Water-Polymer Mobility and Distribution in Hydrated Aromatic Ionomer Thin Films

The confined state properties of ionomers are surprisingly different from bulk membranes and are still not well understood. Hydration of fuel cell ionomers in supported thin films leads to complex multimodal interactions among water, polymer chains and substrate and these interactions may lead to interesting mechanical and transport properties. In this work, thin films (~25-250 nm) of sulfonated aromatic ionomer, S-Radel were investigated to understand thickness and hydration effects on density, water-polymer distribution and mechanical properties. The density values obtained from quartz crystal microbalance and spectroscopic ellipsometry showed a similar trend with thickness to those obtained from neutron reflectometry. Thicker films were always more dense and less water-rich at the interface compared to thinner samples. A fluorescent rotor probe was incorporated into the polymer samples to predict the stiffness of the films using time resolved fluorescnece. The presence of voids rationalized greater mobility and lower stiffness in the thinner films in the dry state. When the same sample was hydrated, film density and interfacial water volume fraction significantly increased with the disappearance of voids. In addition, the water distribution near substrate interface in the aromatic s-Radel films was distinctly different from that in fluorocarbon based Nafion films seen in previous work. Antiplasticization, or stiffening of the films with the addition of water, was observed from fluorescence lifetime of rotor probes. The plasticization properties appeared to be controlled by film thickness, film density, and water mobility.