Sulfonate ionic clusters

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...

Such ionic clusters have been studied in ionomers such as Naflon poly(perfluoropropylene oxide sulfonic acid) membranes clusters in ionomers have been reviewed recently by Mauritz and Hopfinger ( 7). These reviewers also quote a model for hydrated Naflon proposed by T. D. Gierke at the October, 1977, meeting of the Electrochemical Society, Atlanta, Georgia, in which aqueous spher-... 

Ion clusters are commonly observed in the ionized forms of the perfluorinated membranes. The size of the clusters appears to be larger for sulfonate than for carboxylate membranes." " The size increases in the order Na, and Cs" and decreases with increasing number of functional groups per chain and with increasing temperature.As in the case of ethylene ionomers, the perfluorinated carboxylic acid membranes do not form ion clusters, at least in the dry state." The electrostatic interaction may be too weak to form ionic clusters. These observations are expected according to the Eisenberg theory (see Section II.2). 

Reference 7 reviews a number of electron microscopy studies of ionomer morphology in the period up to 1979. None of these studies makes a convincing case for the direct imaging of ionic clusters. This is because of the small size of the clusters (less than 5 nm based on scattering studies) and difficulties encountered in sample preparation. The entire problem was reexamined in 1980(21). In this study ionomers based on ethylene-methacrylic acid copolymers, sulfonated polypentenamer, sulfonated polystyrene and sulfonated ethylene-propylene-diene rubber (EPDM) were examined. The transfer theory of imaging was used to interpret the results. Solvent casting was found to produce no useful information about ionic clusters, and microtomed sections showed no distinct domain structure even in ionomers neutralized with cesium. Microtomed sections of sulfonated EPDM, however,... 

Figure 4. Ionic clusters formed in perfluorinated sulfonic acid membrane.

One can control an equilibrium amount of water uptake by the membranes and also the size of the ionic clusters by changing relative humidity of the membranes or by immersing the membranes in aqueous solutions of sodium chloride with different concentrations (11.). Figure 13 shows change of SAXS profiles upon immersing the sodium-sulfonated (a) and -carboxylated membranes having 1100 E.W. (b) into the aqueous sodium-chloride solutions with various concentrations. 

The perfluorinated, carboxylated and sulfonated ionomer membranes form the ionic clusters of a few nm in size, as in the case of the hydrocarbon-based ionomers such as polyethylene,polystyrene and polybutadiene(9). The ionic clusters strongly affect physical properties of the membranes, e.g., the swelling behavior of the membranes (amount of water uptaken by the membranes, W and... 

The model of ionic clustering we believe to be most likely at present is that of an approximately spherical, inverted micellar structure. In this model the absorbed water phase separates into approximately spherical domains, and the ion exchange sites are found near the interface, probably imbedded into the water phase. Such a structure satisfies the strong tendency for the sulfonic acid sites to be hydrated, and at the same time this structure will minimize unfavorable interactions between water and the fluorocarbon matrix. 

The dynamic mechanical and DSC results for sulfonated PP s combine to give strong evidence for the existence of ionic clusters in these materials. In addition they suggest that these clusters are only present above a critical sulfonate concentration of 10 mol %. These conclusions are based on the existence of an ionic-phase a relaxation for materials sulfonated above 10% and the deviation of the Tg vs. % sulfonation plot from typical copolymer-type behavior at this same concentration. 

We have reported previously the sulfonation of a polypentenamer (PP) under reaction conditions that preclude the formation of covalent cross-links (J). The sulfonated materials are isolated in the form of sodium salts to give ion-containing elastomers. The thermal and dynamic mechanical properties of the sulfonated PP s indicate the existence of phase-separated ionic clusters above a sulfonate concentration of roughly 10 mol % (2). It has been shown (J) that the unsaturation in the sulfonated PP s can be removed by a diimide hydrogenation reaction to yield a material that is essentially linear polyethylene with pendant sulfonate groups. In this manner the effect of backbone crystallinity on... 

A plot of the melt viscosity of a typical SPSNa. (1.78 mol% sulfonate) against its Tg value is shown in Figure 6. The viscosity collapse on addition of glycerol causes a sudden drop in Tg, practically eliminating the quasi-cross-linking effect of the ionic clusters. The decrease in Tg upon addition of DOP reflects a classical plasticizing effect on the hydrophobic domains that form the bulk of the polymer41. 

Interestingly, ionic cluster formation was observed in the case of another aromatic ionomer, the Na salt of sulfonated polyether-ketone47,67. 

---