Nafion cluster morphology

Mauritz and Rogers constructed a water vapor sorption isotherm model for ionomers with inherently clustered morphologies, as applied to Nafion mono-... 

Nafion based on ion-cluster morphology and spectroscopic information. ... 

Figure 7 shows the concentration of fixed ions of Nafion. The concentration of fixed ions increases drastically with electrolyte concentration because of the ion-cluster morphology and the highly dehydrated state of the membrane in concentrated electrolytes, as well as the mass-action effect in counterion activity. Kimoto has recently shown that in concentrated alkaline solutions the effective concentration of fixed ions is less than half of the value calculated from Eq. (8). This decrease is due to the ion-pair formation and strong association in the ion cluster. 

For ETFE- -PSSA membranes with the same lEC, water uptake is higher than MeOH uptake of the membrane, but for Nation and S-SEBS membranes, MeOH uptake of membrane is always higher than water uptake. Chemical structure and morphology of membranes affect the solvent absorption. Nafion is considered to consist of ionic clusters that are separated from the polymer phase. For grafted polymers, heterogeneity exists to some extent due to the hydrophobic base polymer however, a regular clustered structure, as in the case of Nafion, has not been proposed for these materials. 

Based on this new model for the morphology of Nafion, the dimensional variations of the scattering entities with water content were used in simple space filling calculations to estimate the cluster diameter, the number of sulfonate sites per cluster, and the number of water molecules per cluster. The results of these model calculations showed that, for a given equivalent weight, the cluster diameter. 

Figure 1. Cluster-network model for the morphology of hydrated Nafion. (Adapted with permission from ref 16. Copyright 1983 Elsevier.)...

Elliott and co-workers performed a detailed SAXS investigation of the morphology of Nafion membranes that were subjected to uniaxial and biaxial deformation. For as-received membranes, manufactured by Du Pont using an extrusion process, the cluster reflection was shown to exhibit a limited degree of arching in the direction perpendicular to the machine direction. Upon uniaxial extension, this arching was observed to increase in a manner consistent with previous studies. This arching was rationalized on... 

The original cluster-network model proposed by Gierke et al. (also referred to as the cluster-channel model) has been the most widely referenced model in the history of perfluorosulfonate ionomers. Despite the very large number of papers and reports that have strictly relied on this model to explain a wide variety of physical properties and other characteristics of Nafion, this model was never meant to be a definitive description of the actual morphology of Nafion, and the authors recognized that further experimental work would be required to completely define the nature of ionic clustering in these iono-mers. For example, the paracrystalline, cubic lattice... 

Of course, this equation and its theoretical underpinnings does not constitute a model as such and certainly does not address the structural specifics of Nafion, so that it is of no predictive value, as experimental data must be collected beforehand. On the other hand, the results of this study clearly elucidate the percolative nature of the ensemble of contiguous ion-conductive clusters. Since the time of this study, the notion of extended water structures or aggregated clusters has been reinforced to a degree by the morphological studies mentioned above. 

The morphology of the water clusters and their connectivity can be better understood with a visual aid as shown in Fig. 5 and 6 where the snapshots of hydrated Nafion (Fig. 5) and SSC (Fig. 6) PFSA membrane are presented with the ionomer rendered invisible at ). = 4.4 and 9.6 respectively. The snapshots for both the membranes clearly indicate the presence of small clusters whose connectivity is poor at low water content and the clusters appears to have grown in size and more densely packed with better chWiel networks when the water content is increased. However, it is difficult, based solely on snapshots, to discern subtle differences in the size, shape and connectivity of the aqueous phase as a function of side chain length. In order to analyze these differences, we must invoke a more statistical characterization of the stmcture. 

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