Ionic clusters polymer

Figure 4. Two representations (on the left) of cation motion in a polymer electrolyte assisted by polymer chain motion only, and two (on the right) showing cation motion taking account of ionic cluster contributions.

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 ionic clusters act as sites of cross-linking at low temperatures. The interchain forces resulting from this ionic bond produces properties normally associated with a cross-linked thermoset polymer. The association in ionomers can be partially overcome through application of heat and pressure allowing processability while truly" cross-linked network polymers cannot be remelted, dissolved or reshaped. Thus, ionomers are often referred to as processable thermosets. 

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... 

Ionomers are copolymers in which a small portion of the repeat units have ionic pendant groups on usually a nonpolar backbone. The ionic groups tend to separate themselves into domains similar to the polystyrene segments in the SBS rubber because they are insoluble in the nonpolar polymer chains. Therefore, these ionic clusters serve as cross-links up to temperatures where they tend to disassociate. Most commercial grades of ionic elastomers are based on ethylene and propylene monomers. 

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. 

G. Xu. High-resolution x-ray-scattering of ionic clusters in perfiuorinated ionomers. Polymer Journal 26, 840-844 1994. 

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." ... 

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