Microstructural Evolution of Nafion During Uniaxial Deformation Monitored by X-ray Scattering

INTRODUCTION

Fuel cells enable direct chemical to electrical conversion of fuel to electricity, providing an efficient and clean process. Proton Exchange Membrane Fuel Cells (PEMFC), in which protons from hydrogen or methane cross a membrane to react with oxygen producing electricity, are the preferred transportable fuel cell. Nafion is the membrane of choice for Proton Exchange Membrane Fuel Cells (PEMFC) because its unique microstructure allows rapid transport of protons in a hydrated environment while maintaining mechanical integrity. The teflon-like backbone is hydrophobic while the sulfonated side chains are hydrophilic. In the presence of water molecules this causes the side chains to aggregate into clusters which contain most of the water while the backbone remains relatively dry. Extensive studies have been conducted in order to deduce the size and shape of the microstructural features in order to gain insight into its superior electrochemical and mechanical characteristics, and how they can be further improved (e.g. [1-7]). The shape of these regions (sphere, cylinders, ribbons etc) and their evolution with deformation is still a matter of debate. Here the microstructural evolution is monitored during uniaxial tensile testing via small and wide angle x-ray scattering. Two dimensional scattering profiles are recorded along with stress and strain as a function of time for monotonic, cyclic, and stress relaxation loading histories. These profiles are then reduced to amplitude, location, and orientation for each of the major structural peaks. The scattering data are interpreted in conjunction with existing literature in order to understand the rate and deformation dependent microstructural evolution and its relation to the rate dependent elastic-plastic stress-strain behavior exhibited by Nafion.


EXPERIMENTAL METHODS

Commercial NRE212 films (54^m, dispersion cast, Dupont, Ion Power Inc) were used for the experimental characterization of Nafion. The films were stored in a desiccator cabinet upon removal from the initial packaging to minimize variability in data from aging and humidity effects. The film was cut into tensile specimens using a dogbone shaped die with gauge length of 9.54mm and gauge width 3.14mm. The nominal thickness is 54^m. The thickness of each specimen was determined from the average of three measurements taken along the gauge length with a Mitutoyo micrometer.

SAXS and WAXS experiments were conducted at Argonne National Laboratory Advanced Photon Source at Sector 5 (Dupont-Northwestern-Dow Collaborative Access Team). The hutch was setup for simultaneous collection of SAXS and WAXS by 2-D detectors. The SAXS sensor was configured to collect q-values ranging from 0.007A" 1 to 0.17A"1. The WAXS sensor was configured to collect q-values ranging from 0.5A"1 to 4.5A"1. Scattering data was collected while the specimen was subjected to uniaxial loading via a biactuator servo-hydraulic Instron. All tests were conducted at a nominal strain rate of 0.005s"1. This value was chosen to allow collection of a sufficient scattering intensity without a significant change in strain. A Qimaging Retiga 1300 video extensometer was used to track the local axial deformation in the region through which the X-ray beam passed. The 2-D scattering profiles were reduced to oriented 1 -D profiles of intensity versus scattering vector (q) by integration +/-5° about the stretching direction and +/-5° about the transverse direction. Peak locations and intensities were determined for each 1 -D profile via a Lorentzian fit. The video extensometer images and the Instron force were reduced to true strain and true stress respectively using a constant volume assumption for the membrane.

RESULTS AND DISCUSSION

The two dimensional scattering profiles for both the small angle (SAXS) and wide angle (WAXS) on undeformed Nafion NRE212 are shown in Figure la,b. Darker coloring indicates greater scattering intensity. Both profiles are axisymmetric indicating initial isotropy in the membrane plane. This differs from the scattering exhibited by N117 and other extruded forms of Nafion which show anisotropy in the undeformed state. This initial isotropy will simplify analysis of structural evolution as the membrane undergoes uniaxial deformation. The SAXS and the WAXS each capture two characteristic peaks. Here they will be numbered 1-4 from longest characteristic spacing (smallest q) to shortest characteristic spacing (largest q).

Peak 1, commonly referred to as the matrix peak occurs at q=0.6nm~l corresponding roughly to a spacing of 10.5nm\ peak 2, commonly referred to as the ionomer peak, occurs at q=2.2nm~l corresponding roughly to a spacing of 2.9nm\ peak 3 occurs at q=\2.lnm~l corresponding roughly to a spacing of 0.52nm\ and peak 4 occurs at q=H.9nm’1 corresponding roughly to a spacing of 0.23nm. Each of these peaks evolve anisotropically with applied deformation as shown in Figure lc,d with "SD" indicating the stretching direction. Peaks 1 and 2 move to smaller g-value in the stretching direction and larger g-value in the transverse direction ("TD"). Peaks 3 and 4 remain at the same g-value but have significant intensity evolutions; peak 3 becomes most intense along the transverse direction and peak 4 becomes most intense along the stretching direction.

Two dimensional x-ray scattering profiles for Nafion NRE212 under unaixial tension (a) small angle undeformed (b) wide angle undeformed (c) small angle at 40% strain (d) wide angle at 40% strain

Figure 1 Two dimensional x-ray scattering profiles for Nafion NRE212 under unaixial tension (a) small angle undeformed (b) wide angle undeformed (c) small angle at 40% strain (d) wide angle at 40% strain

Building on the scattering profile analysis of Heijden et. al. [4] and Schmidt-Rohr and Chen[7] the structure of Nafion is taken to be composed of randomly oriented cylinders (ionomer clusters) consisting of aligned backbone strands with the sulfonated side chains pointing inward and of randomly oriented crystalline regions. According to this interpretation peak 1 results from spacing between crystallites, peak 2 results from spacing between ionomer clusters, peak 3 is an inter-strand backbone spacing, and peak 4 is an intra-strand backbone spacing. The deformed scattering images therefore imply that: 1) both the clusters and crystallites are moving farther apart in the stretching direction and closer together in the transverse direction, and 2) the locally aligned backbone regions are rotating to align with the applied deformation. Since peaks 1 and 2 and peaks 3 and 4 seem to have direct correspondence, the analysis will focus on peaks 1 and 4 as these are fully captured by the scattering angles accessible in this setup throughout the deformation process.

The continuous and simultaneous acquisition of scattering profiles, local deformation (converted to strain), and axial force (converted to stress) enables direct assessment of microstructural evolution both with applied strain and with time dependent stress evolution at constant strain. Here we begin with the former, examining whether the clusters and crystallites deform and rotate affinely with monotonic uniaxial tension. Affine displacement of the crystallites is assessed via the peak 1 spacing evolution. This spacing (approximated by 2n/q) is converted to a strain like quantity by normalizing by the value in the undeformed state. The crystallites are shown to move closer together in the transverse direction according to the affine deformation prediction, but to move apart greater than affinely in the stretching direction (Figure 2a). The clusters show similar results at the strains over which they are well tracked (not shown). If the crystallites (and clusters) are assumed to remain rigid, the remaining structure would need to deform greater than affinely in order to accommodate the applied macroscopic strain, the spacing between crystallites would therefore also be expected to increase greater than affinely. Affine rotation of the aligned backbone regions (both in the crystallites and the clusters) is assessed via the peak 4 intensity evolution. This intensity is normalized by its value in the undeformed state to compare with the relative intensity evolution predicted by affine rotation (Figure 2b). For strains below 0.25 the clusters are seen to rotate roughly affinely to align with the stretching direction whereas at strains greater than 0.25 they rotate less than affinely as indicated by both the stretching direction and transverse direction peaks. This strain is well past the yield strain of 0.05 suggesting (1) that yield is only expressed in the amorphous region not detectable via x-ray scattering methods and (2) a second important microstructural event occurs at this strain which allows the amorphous regions to deform independently of crystallite and cluster rotation.

Peak evolution during monotonic uniaxial tension compared to theoretical affine evolution (a) small angle peak compared to affine displacement (b) wide angle peak compared to affine rotation

Figure 2 Peak evolution during monotonic uniaxial tension compared to theoretical affine evolution (a) small angle peak compared to affine displacement (b) wide angle peak compared to affine rotation

Stress relaxation experiments, for which the macroscopic strain is held constant for 180s at two distinct strains, are used to evaluate the microstructural correlation with stress independently of strain. The true stress-true strain curve from this experiment (Figure 3 a) shows the elastic-plastic nature of Nafion as well as the stress relaxation that occurs as the two strains where the 180s hold occurs, one in the elastic region and one in the plastic region just past where the rotational alignment becomes less than affine. For the hold at a strain of 0.02 there is a small but measureable relaxation of the stress and a slight increase in each of the microstructural evolutions (increase defined as positive change for the stretching direction and negative change for the transverse direction), indicating that in this mostly elastic region the microstructure continues to slowly evolve to align with strain even as the stress relaxes (Figure 3b). For the hold at a strain of 0.28 there is a significant stress relaxation, a slight increase in the crystallite spacing, and a noticeable relaxation in the cluster rotation (Figure 3b). This further supports that at this strain the amorphous regions are able to undergo time dependent deformation independent of cluster rotation.

Uniaxial stress relaxation (a) true stress-true strain (b) simultaneous time dependent evolution of stress and microstructural features captured by small and wide angle x-ray scattering

Figure 3 Uniaxial stress relaxation (a) true stress-true strain (b) simultaneous time dependent evolution of stress and microstructural features captured by small and wide angle x-ray scattering

CONCLUSIONS

Small and wide angle x-ray scattering collected during uniaxial stress-strain experiments has been used here to investigate the relationships among stress, strain, and microstructure for Nafion NRE-212. Two peaks each were seen in the SAXS and WAXS scattering profiles with the SAXS peaks taken to indicate spacing between microstructural features (clusters and crystallites) and the WAXS peaks taken to indicate orientation of these same microstructural features. These peaks evolve from isotropic in the undeformed state to anisotropic with applied strain. These evolutions are compared with the corresponding affine deformation predictions to reveal greater than affine displacement at all strains, affine rotation at strains less than 0.25, and less than affine rotation at strains greater than 0.25. This suggests that a second microstructural event occurs well after yield that allows the amorphous regions to deform independently of cluster and crystallite rotations. The stress relaxation data further supports this interpretation, as the orientation microstructural indicators are seen to relax as the stress relaxes at a strain of 0.28 whereas the spacing microstructural indicators continue to increase at this same strain. Since the clusters are responsible for the rapid proton transport, knowledge of their time, strain, and stress dependent orientation could prove useful in mechanically manipulating the membrane to improve electrochemical performance.

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