Nafion ionic clusters

Based on GebeTs calculations for Nafion (where lEC = 0.91 meq/g),i isolated spheres of ionic clusters in the dry state have diameters of 15 A and an intercluster spacing of 27 A. Because the spheres are isolated, proton transport through the membrane is severely impeded and thus low levels of conductivity are observed for a dry membrane. As water content increases, the isolated ionic clusters begin to swell until, at X, > 0.2, the percolation threshold is reached. This significant point represents the point at which connections or channels are now formed between the previously isolated ionic clusters and leads to a concomitant sharp increase in the observed level of proton conductivity. 

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. 

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... 

Nafion (17) is a perfluorinated polymer related to teflon (polytetrafluoroethylene). An electrode is conveniently coated by allowing an ethanolic solution of the polymer to evaporate. The film produced is stable, rather more so in fact than other polymer films, e.g. polyvinylpyridine (see Section 57.3.2.2). At the microscopic level the polymer separates into two phases, the bulk polymer and the lower density ionic cluster phase. Diffusion of ions can occur quite freely for example, the diffusion coefficient of Na+ in Nafion (MW 1200) is only slightly less than in water.44... 

The proton exchange membranes (PEMs) used in PEM fuel cells consist of two domains the hydrophobic polymer backbone domain and the hydrophilic ionic cluster domain. The most commonly used membrane is Nafion , produced by DuPont. 

Fig. 7. A schematic view of Nafion membrane showing the microheterogeneous environment. A hydrophobic fluorocarbon phase B hydrophilic sulfonate ionic clusters C interfacial region formed between A and B and Ru adsorbed ruthenium complex water oxidation catalyst...

Hopfinger and Mauritz and Hopfinger also presented a general formalism to describe the structural organization of Nafion membranes under different physicochemical conditions. It was assumed that ionic clustering does not exist in the dry polymer. This assumption is applicable to the perfluorinated carboxylic acid polymer" but not the perfluorosulfonate polymers." They consider the balance in energy between the elastic deformation of the matrix and the various molecular interactions that exist in the polymer. 

Pineri et al have performed a series of investi-gations " " " on the structural properties of Nafion polymers and have derived a three-phase model in which fluorocarbon crystallites, ionic hydrophilic clusters, and an amorphous hydro-phobic region of lower ionic content coexist. The microcrystallites can be eliminated by heating the Nafion-Na sample to 330°C, and then quenching it to room temperature." They showed that at high water content (>15wt%) the quenched Nafion membranes are essentially two-phase systems." These phases are rather well separated and most of the water is in the ionic clusters. Samples of lower water content (8 wt %) show deviations from two-phase behavior." ... 

Let us now concentrate on these last membranes, which have a relatively homogeneous ionic site distribution at a pm scale. Nafion 1200 equivalent weights have been studied here. We already know that there are small microcrystallites with a PTFE structure. The question now concerns the distribution of SO3H outside of these crystals do we have a statistical distribution of them or do we have some ionic clusters with a larger ionic concentration This question will be developed in the next chapter. 

Journal of the Electrochemical Society Figure 3. Three region structural model for Nafion A, fluorocarbon B, interfacial zone C, ionic clusters (11). 

Gierke, T.D. "Ionic Clustering in Nafion Perfluorosulfonic Acid Membranes and its Relationship to Hydroxyl Rejection and Chlor-Alkali Efficiency", presented at the 152nd National Meeting of the Electrochemical Society, Atlanta, Ga.,... 

Preliminary results of the MIR investigation of the molecular nature of the ionic cluster regions of Nafion perfluorosul-fonate membranes have been presented. An advantage of the MIR probe is the ability to monitor short-range dynamic aspects of the ionic-hydrate molecular architecture for equilibrium states. It has also been demonstrated how the resultant concepts can be translated into a molecular-based model that may be significant in providing a base for mathematical representations of various macroscopic physical properties. 

As in other ionomers, the ion exchange sites in "Nafion" membranes are observed to aggregate and form clusters. Ionic clustering in "Nafion" membranes has been indicated by a variety of physical studies including dielectric relaxation (1), small angle x-ray scattering (1-4), neutron scattering (4), electron micro-... 

In a later chapter we will propose a model of ionic clustering which we believe is an essential agreement with these results (20). The real strength of the model, however, is that it can be used to understand and describe ion transport through "Nafion" perfluor inated membranes 03, 25, 26). 

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