Water Sorption Properties of Sub-Micron Thin Ionomer Films
Shim, Hyun Ki
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This thesis is focused on the water uptake and swelling properties of sub-micron thin ion conducting polymer (ionomer) films. One motivation for this work stems from the need to resolve the discrepancy between high water uptake and low proton conductivity properties observed in Nafion® thin films on SiO2 substrate. In addition, the effects of film thickness, substrate, ionomer equivalent weight (EW), and annealing on the intrinsic properties of ionomer thin films were studied. Ionomer thin films were characterized using the quartz crystal microbalance (QCM) and ellipsometry for their density, refractive index (RI), water uptake and swelling properties. Water uptake studies showed that the effect of substrate on water uptake of Nafion® thin films were not as strong as expected. Meanwhile, a thickness-dependent water uptake was observed for Nafion® thin films. Similar trends were observed for the two low EW ionomers suggesting similar nano-structure in the thin film form. Interestingly, comparison of water uptake showed that low EW ionomers adsorb comparable or less water than Nafion in the thin film form. A combined QCM and ellipsometer setup was employed for the first time to simultaneously measure the water uptake and swelling, respectively as a function of humidity. All thin films appear to be affected by confinement effect as evident by the swelling dimension and volume of mixing. Thermally annealed (160 oC) Nafion® thin films experienced a significant decrease in water uptake and swelling properties.