The study presented here is a fundamental investigation into the molecular origins of the thermal transitions and dynamic mechanical relaxations of Nafion membranes as studied by DSC, DMA, variable temperature small-angle X-ray scattering (SAXS), and solid-state 19F NMR spectroscopy. While several studies in the literature have attempted to explain the molecular origins of these thermal transitions and mechanical relaxations, the assignments were based primarily on limited DMA results and have at times been contradictory. In DSC traces of Nafion, the low- and high-temperature endotherms are shown to be dependent on thermal history and are now attributed to melting of relatively small and large crystallites, respectively. DSC analysis of Nafion yields information only on the crystalline nature of this ionomer, and neither of the transitions can be assigned to glass transitions. The intensity of the small-angle ionomer peak at ca. q = 2 nm-1 was monitored as a function of temperature for each alkylammonium neutralized sample. Changes in intensity of the ionomer peak as a function of temperature were shown to correlate well with the α and β relaxations observed in DMA. Variable temperature solid-state 19F NMR techniques were used to investigate the dynamics of the Nafion chains. Spin-diffusion experiments revealed a profound increase in mobility at the onset of the α relaxation. Sideband analysis indicated that the side chain is more mobile than the main chain and that the mobility is greatly affected by the size of the counterion. Molecular level information from this analysis in correlation with SAXS and DMA data supports the assignment of the β relaxation to the genuine T g of Nafion and the α relaxation to the onset of long-range mobility of chains/side chains via a thermally activated destabilization of the electrostatic network.

Fuel cells based on polymer electrolyte membranes (PEM) show promise as a means of energy conversion for a wide range of applications both in the transportation sector and for stationary power production due to their high charge density and low operating temperatures. While the structure and transport of bulk PEMs for fuel cell applications have been studied extensively, much less is known about these materials at interfaces and under confinement, conditions that are highly relevant in the membrane electrode assembly of a working PEM fuel cell. Using X-ray reflectivity, neutron reflectivity, grazing-incidence small-angle X-ray scattering, quartz crystal microbalance, and polarization-modulation infrared reflection–absorption spectroscopy, we have studied the structure, swelling, water solubility, and water transport kinetics as a function of relative humidity for confined polyelectrolyte films thinner than 222 nm. While the humidity-dependent equilibrium swelling ratio, volumetric water fraction, and effective diffusivity are relatively constant for films thicker than ca. 60 nm, we observe measurable suppressions of these properties in films less than ca. 60 nm. These effects occur at length scales that are relevant to transport (ion and water) in the polyelectrolyte binders found in the catalyst layer of the membrane–electrode assembly (MEA) of a functional fuel cell. The thin film methodology and findings presented here provide a platform to quantify and validate models of interfacial impedance used within the fuel cell community and have the potential to lead to improvements in MEA materials, design, and optimization.

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.

The current study uses variable temperature, small-angle X-ray scattering (SAXS) to examine the thermal relaxation behavior of oriented Nafion films as a means to evaluate the morphological stability of this ionomer at elevated temperatures. The SAXS patterns of uniaxially oriented films are characterized by strong equatorial scattering peaks which are attributed to scattering arising from the oriented ionic domains (ionomer peak ca. q ) 2 nm -1 ). The intensity of the equatorial peakssobtained from integration in the azimuthal direction (χ)sand the degree of orientationscalculated using the Hermans orientation function (f)swere monitored as a function of temperature. At lower temperatures, a constant value of f and a correlation between the β relaxation and a slight decrease in the scattering intensity of the equatorial peaks are in agreement with our earlier assignment of the β relaxation to the T g of Nafion. At temperatures in the vicinity of the β relaxation, the static electrostatic network inhibits long-range molecular relaxation and yields a persistent anisotropic morphology. In contrast, significant changes in intensity of the equatorial peaks and values of the orientation parameter at elevated temperatures were shown to correlate well with the R relaxation observed in DMA. At temperatures in the vicinity of the R relaxation, a significant destabilization of the oriented electrostatic network occurs (i.e., through the activation of a dynamic network involving significant ion-hopping processes), thus facilitating the observed relaxation to an isotropic morphology. Therefore, morphological stability in this ionomer is principally governed by the thermal stability of the electrostatic network and not the glass transition.