Letter pubs.acs.org/NanoLett
Confinement-Driven Increase in Ionomer Thin-Film Modulus Kirt A. Page,*,† Ahmet Kusoglu,*,‡ Christopher M. Stafford,† Sangcheol Kim,† R. Joseph Kline,† and Adam Z. Weber‡ †
Materials Science and Engineering Division, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, United States ‡ Environmental Energy Technologies Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States S Supporting Information *
ABSTRACT: 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−property 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 one particular 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-induced stiffening through the application of a simple theoretical framework based on self-consistent micromechanics. This framework can be applied to other polymer systems and assumes that as the polymer is confined the mechanical response becomes dominated by the modulus of individual polymer chains. KEYWORDS: Nafion, ionomers, membranes, fuel cells, thin films, modulus, stiffness, confinement order-of-magnitude decrease in ionic conductivity.15,16 The higher activation energy of thin films suggests intrinsic changes in morphology and other fundamental materials properties.15,17 Grazing-incidence small-angle X-ray scattering (GISAXS) studies of Nafion thin films suggest thickness and substrate/ film interactions control the reorganization and alignment of the phase-separated nanostructure,12,14,18 thereby altering the resistance to water transport.14,19,20 These findings indicate a bulk-to-thin-film transition in the transport properties and nanostructure of PFSA, yet a transition in mechanical properties remains to be observed. In this work, we demonstrate the confinement-driven change in mechanical properties of ionomer films and its role in the mechanical/ chemical equilibrium of the ionomer with water vapor in an effort to elucidate the molecular origins of the observed bulkto-thin-film transition in materials properties. The elastic modulus of polymer thin films on a substrate can be determined from its buckling response using an experimental technique21 that employs uniaxial stretching or compression of the substrate above a critical strain at which the
Because of their high conductivity, wide electrochemical window and good thermomechanical stability, perfluorosulfonic-acid (PFSA) membranes (e.g., Nafion) are commonly used as the solid-electrolyte in energy storage and conversion devices, such as polymer−electrolyte fuel cells,1−3 redox flow batteries,4 solar-fuel generators,5 and ionic-polymer metal composites.6 In the hydrated state, PFSA’s hydrophilic sulfonic-acid groups are solvated by water, forming an ionrich water network composed of (2 to 6) nm domains in a matrix of the polytetrafluoroethylene (PTFE) backbone.1,7−10 This nanophase-separated morphology is governed by the quasi-equilibrium between the chemical energies for solvation and water uptake and the mechanical energy associated with the hydrophobic backbone deformation.9 In addition, for various applications ionomers occur in a vast thickness range from a few nanometers in porous electrodes to tens of micrometers as separators.2,3,11 Optimization of membrane properties and the development of alternative materials relies on understanding how the structure is related to the chemical and mechanical properties, especially when confined to the nanoscale. It has been shown that when Nafion films are confined to less than (50 to 100) nm thickness on a substrate, the properties deviate from that of bulk membranes in terms of slower water diffusion,12 reduced water uptake,12−15 and an © 2014 American Chemical Society
Received: December 11, 2013 Revised: April 20, 2014 Published: April 28, 2014 2299
dx.doi.org/10.1021/nl501233g | Nano Lett. 2014, 14, 2299−2304
Nano Letters
Letter
Figure 1. (a) Methodology21 used to induce buckling deformation of thin film by compressing at small strains of
