Surface-Induced Nanostructure and Water Transport of Thin Proton-Conducting Polymer Films

Abstract

We quantify the interfacial nanostructure and corresponding water transport kinetics in thin films of Nafion which are known to show nonbulk like transport properties using neutron reflectivity (NR) and quartz-crystal microbalance (QCM) measurements integrated with in-situ, controlled relative humidity environments. Rigorous fitting of the NR data under humidified conditions reveals that a hydrophilic organosilicate substrate induces an interfacial layering of the water transport domains parallel to the substrate whereas the hydrophobic organosilicate analogue does not trigger this interfacial ordering. The interfacial layering on the hydrophilic substrate is accompanied by an excess in the total mass of water absorption as verified by QCM measurements. The excess water in the thin Nafion films is quantitatively consistent with the segregation amounts and length scales quantified by NR. However, we do not observe strong differences in the water transport kinetics in thin Nafion films where the volume fraction of the materials with the water transport oriented parallel substrate, orthogonal to the primary direction of transport, is on the order of ≈7 vol %; to a first approximation the majority of the transport kinetics are similar on the hydrophilic (oriented) and hydrophobic (disordered) surfaces.