Optical Properties and Swelling of Thin Film Perfluorinated Sulfonic-Acid Ionomer

Perfluorinated sulfonic-acid (PFSA) ionomers are members of a class of ion-conductive polymers that are commonly used in both membranes and catalyst layers in polymer-electrolyte fuel cells. Transport properties of thin film ionomers and their deviation from bulk properties are still unclear. Label free, high-speed Surface Plasmon Resonance imaging (SPRi) is proposed to simultaneously measure refractive index and thickness of a 30 nm thick Nafion film at relative humidity of 3%, 36%, and 93% . The result shows the thickness of the thin film increases with increasing the relative humidity. The refractive index of the thin Nafion film decreases from 1.310±0.003 to 1.228±0.003 when the relative humidity increases from 3% to 93%. The drop in optical properties of 30 nm thick Nafion film, indicates the void part of hydrated Nafion film is filled with a mixture of water and water vapor.