We quantify the interfacial structure and corresponding water transport kinetics in thin films of Nafion using neutron reflectivity (NR) and quartz-crystal microbalance (QCM) by integrating these measurements with in-situ, controlled relative humidity chambers. Rigorous fitting of the NR profiles under humidified conditions reveals that a hydrophilic organosilicate substrate induces an interfacial layering of the water transport channels in Nafion parallel to the substrate whereas hydrophobic analogs does not trigger this interfacial ordering. The interfacial layering on the hydrophilic substrate leads to an excess in the total water mass uptake that is verified by QCM measurements. The excess of water in thin Nafion films are quantitatively consistent with the segregation amounts and length scales quantified by NR. However, there is little difference in the transport kinetics between the hydrophilic and hydrophobic substrates, implying that the preferential orientation of the water transport domains parallel to the hydrophilic substrate has little effect on the transport kinetics of water ingress/egress out of the film.
Interfacial structure, water transport kinetics, thin film, surface energy, reflectivity