Confinement-Driven Increase in Ionomer Thin-Film Modulus
Kirt A. Page, Ahmet Kusoglu, Christopher M. Stafford, Sangcheol Kim, Regis J. Kline, Adam Weber
Ion-conductive polymers, or ionomers, are critical materials for a wide range of electrochemical technologies. For optimizing the complex, heterogeneous structures in which they occur, there is a need to elucidate the governing structure/function relationships, especially at nanoscale dimensions where interfacial interactions dominate the overall materials response due to confinement effects. It is widely acknowledged that polymer physical behavior can be drastically altered from the bulk when under confinement and the literature is replete with examples thereof. However, there is a deficit in the understanding of ionomers when confined to the nanoscale, although it is apparent from literature that confinement can influence ionomer properties. Herein we show that as an ionomer, Nafion, is confined to thin films, there is a drastic increase in the modulus over the bulk value, and we demonstrate that this stiffening can explain previously observed deviations in materials properties such as water transport and uptake upon confinement. Moreover, we provide insight into the underlying confinement effects, with regards to mechanical properties, that occur within both Nafion and traditional polymers through the application of a simple theoretical framework based on self-consistent micromechanics, which assumes that as the polymer is confined the mechanical response becomes dominated by the modulus of individual polymer chains.