Origins of Suppressed Swelling and Increased Mechanical Properties of Ultrathin Nafion

We have previously shown1,2 that there is a suppression in both swelling and water diffusivity when Nafion is confined to thin and ultrathin films. We have also shown that the modulus of ultrathin Nafion films increases upon confinement3,4, as well as upon thermal annealing. Interestingly, we also observe a decrease in swelling and an increase in modulus for ultrathin films aged in high humidity environments. Similar behavior has been shown to occur in bulk Nafion films.5,6 Some researchers suggest that the root cause of such a decrease in swelling and increase in modulus is due to the formation of sulfonic anhydrides, while others suggest that it is due to internal hydrogen bonding of dehydrated sulfonic acids. In this talk, we will present our measurements results on thin and ultrathin Nafion films. We apply polarization-modulation infrared reflection-absorption spectroscopy to study the kinetics of dehydration and formation of sulfonate species in thin Nafion films. We also employ cantilever bending during humidity-induced swelling to probe the mechanical properties of these films. Grazing-incidence small-angle x-ray scattering is used to elucidate changes in morphology upon annealing and/or aging. By combing the results from these techniques, we can paint a more holistic picture of the origins of suppressed swelling and increased mechanical properties in thin and ultrathin Nafion films. S. A. Eastman, S. Kim, K. A. Page, B. W. Rowe, S. Kang, C. L. Soles, and K. G. Yager. Effect of Confinement on Structure, Water Solubility, and Water Transport in Nafion Thin Films, Macromolecules, 45, 7920-7930 (2016). E. M. Davis, C. M. Stafford, and K. A. Page. Elucidating Water Transport Mechanisms in Nafion Thin Films, ACS Macro Letters, 3, 1029-1035 (2014). K. A. Page, J. W. Shin, S. A. Eastman, B. W. Rowe, S. Kim, A. Kusoglu, K. G. Yager, and G. R. Stafford. In Situ Method for Measuring the Mechanical Properties of Nafion Thin Films during Hydration Cycles, ACS Applied Materials and Interfaces, 7, 17874-17883 (2015). K. A. Page, A. Kusoglu, C. M. Stafford, S. Kim, R. J. Kline, and A. Z. Weber. Confinement-Driven Increase in Ionomer Thin-Film Modulus, Nano Letters, 14, 2299-2304 (2014). F. M. Collette, C. Lorentz, G. Gebel, F. Thominette. Hygrothermal aging of Nafion, Journal of Membrane Science, 330, 21-29 (2009). S. Shi, T. J. Dursch, C. Blake, R. Mukundan, R. L. Borup, A. Z. Weber, and A. Kusoglu. Impact of Hygrothermal Aging on Structure/Function Relationship of Perfluorosulfonic-Acid Membranes, Journal of Polymer Science B: Polymer Physics, 54, 570-581 (2016).