Perfluorinated ionomer structure

The DuPont Nafion materials, both sulfonate and carboxylate varieties, are not entirely unique, as similar perfluorinated ionomers have been developed by others such as the Asahi Chemical Company (commercial name Aciplex) and the Asahi Glass Company (commercial name Flemion). The comonomer chemical structures of and further information on these materials are given in the recent review article by Doyle and Rajendvan. Now commercially unavailable, but once considered a viable alternative, the Dow Chemical Company developed a somewhat similar perfluorinated ionomer that resembled the sulfonate form of Nafion except that the side chain of the former is shorter and contains one ether oxygen, rather than two ether oxygens, that is, —O—... 

Scheme 1. Structure of the perfluorinated ionomer membrane (Nafion ).

Heitner-Wirguin, C. (1996). Recent advances in perfluorinated ionomer membranes Structure, properties and applications, J. Membrane Sci. 120, 1. 

Figure 3.43. Structure of Nafion-115 at ambient humidity derived from small-angle X-ray spectroscopy. The lighter areas are cluster structures in the material. (Reprinted with permission from J. Elliott, S. Hanna, A. Elliott, G. Cooley (2000). Interpretation of the small-angle X-ray scattering from swollen and oriented perfluorinated ionomer membranes, Macromolecules 33, 4161-4171. Copyright American Chemical Society.)...

Pinerii and coworkers, and a few other groups, have used ESR and Mossbauer spectroscopy as well as SANS, extended x-ray absorption fine structure (EX.AFS), and magnetization and susceptibility data to analyze local. struct.ure in perfluorinated ionomer membranes and the distribution of water within them isee, for inst,ance, (61-65) 1. The application of the KNDOR (electron nuclear double resonance) technique to deuteriated methanol-swollen Scunples of these membranes has been reportesd i-ecentiy (66). Photophysical methods have also tef n applied in hydration. si.udies of these membranes (67-69). Finally, some NMR results on the same hydrated perfluorinat,ed ionomer.s well as on hydrated... 

Even less is known about ionomer/plasticizer interactions on a molecular level. A variety of scattering and spectroscopic techniques that can probe this level have been mentioned, but they have been applied primarily to the specific case of water in ionomers, and in particular to hjdrated perfluorinated ionomers. At the least, these studies demonstrate the powerful potential of the techniques to contribute to a more complete understanding of structure-property relationships in plasticizer/ionomer systems. For e.xample, to return to the question of the effect of nonpolar plasticizers on the ionic domains how can the decrease in the ionic transition temperature be reconciled with the apparently minimal effect on the SAXS ionomer peaks and with the ESR studies that indicate (not surprisingly) tiiat these plasticizers have essentially no influence on the local structure of the ions Is it due to their association with the hydrocai bon component of the large aggregates or clusters Or if these entities do not exist, as some researchers postulate, what is the interaction between the nonpolar plasticizer, the hydrocarbon component and the ionic domains These questions are, of course, intimately related to the understanding of ionomer microstructure even in the absence of plasticizer. The interpretation of SAXS data in particular is subject to the choice of model used. 

In scientific terms, the unusual ion-clustered morphology of the perfluorinated ionomer polymers has provoked much interest. Clearly, the microphase-separated structure that is revealed through various types of experiments is strongly related to their unusual transport properties. It is important to refine our understanding of this relationship in order to exploit these materials in various electrochemical applications. 

Membranes can be characterized by their structure and function, that is how they form and how they perform. It is essential that the cation exchange membranes used in chlor-alkali cells have very good chemical stability and good structural properties. The combination of unusual ionic conductivity, high ionic selectivity and resistance to oxidative hydrolysis, make the perfluorinated ionomer materials prime candidates for chlor-alkali membrane cell separators. 

These performance goals have now largely been attained by continued improvements through several generations of materials. Currently, commercial perfluorinated ionomer materials for this application consist of membranes with carboxylate or mixed carboxylate-sulfonate functionality the latter membranes often have layered structures with the carboxylate layer exposed to the caustic catholyte solution. Fabric reinforcement is used in some instances to improve strength. 

Because of the technological importance of the perfluorinated ionomers, as well as the novel structural features encountered in these materials, a wide range of physical and physico-chemical tools have been brought to bear on the problems related to the structure of these polymers. 

Structure of Sulfonated and Carboxylated Perfluorinated Ionomer Membranes... 

The electrochemical (1) and mechanical properties(2,3) of the perfluorinated ionomer membranes as an ion-exchange membrane are obviously influenced by their internal structure of the membranes, especially spatial organization of the ionic sites. In this paper we attempted to carry out very basic studies on the structure of the perfluorinated ionomer membranes in the absence of applied external electric field. Although for practical applications of the membranes it is extremely important to study the structure under... 

E.J. Roche, M. Pineri and R. Duplessix, Phase separation in perfluorosulfonic ionomer membrane, J. Polym. Sci., Polym. Phys. Ed., 1982, 20, 107-116 C. Heitner-Wirguin, Recent advances in perfluorinated ionomer membranes structure, properties and application, J. Membr. Sci., 1996, 120, 1-33 G. Gebel and J. Lambard, Small-angle scattering study of water-swollen perfluorinated ionomer membranes, Macromolecules,... 

M. A. F. Robertson and H. L. Yeager, Structure and properties of perfluorinated ionomers, in lonomers. Synthesis, Structure, Properties and Applications (M. R. Tant, K. A. Mauritz and G. L. Wilkes), Chapman and Hall, London, 1997, p. 290. K. A. Mauritz, Morphological theories, in lonomers. Synthesis, structure, properties and applications (M. R. Tant, K. A. Mauritz, G. L. Wilkes), Chapman and Hall, London, 1997, p. 95. 

Yeo RS, Yeager HL (1980) Structural and transport properties of perfluorinated ion-exchange membranes. In Eisenberg A, Yeager HL (eds) Perfluorinated ionomer membranes. American Chemical Society, Washington, pp 437-505... 

Nafion materials, and more generally perfluorinated ionomers, are particularly suitable for water and brine electrolysis and, to date, no viable alternative has been found for SPE applications. The dissolution of Nafion membranes allows the preparation of material with high porosity and high electroactive area. Such structures are required for the development of high power density SPE fuel cells. In recent work, Aldebert et al. have presented different methods for the preparation of SPE... 

Recently, scientific literature evidenced that different companies developed per-fluorosulfonic ionomers able to sustain 120°C operating conditions for long time. In some examples the relation with linear short side chain structure is evident, most of all 3M [32] and Solvay [33,34], in other cases the polymer structure is not described, like for Asahi Glass [35] or Asahi Kasey [36], Figure 17.7 summarizes the chemical formula of the most common perfluorinated ionomers. 

Nafion ionomer is not the only perfluorosulfonic membrane material to be considered for PEMFCs. There are many other commercial perfluorinated ionomers, such as Asahi Glass (Flemion ), Asahi Kasei (Aciplex ), 3M (3M polymer), and Solvay Solexis (Hyflon ), all of which share some structural similarities with the polytetrafluoroethylene-based Nafion but use different perfluorinated vinyl ethers, as shown in Table 4.1. 

FIG U RE 2.2 Chemical structures of perfluorinated ionomers with sulfonic acid (la = Nafion, Flemion lb = Aciplex 2a = Dow, Hyflon Ion 2b = 3M 2c = Asahi Kasei 3 = Asahi Glass) and bis[(perfluoro)alkyl sulfonyl] groups (4). (Reprinted with permission from Peckham, T. J., Yang, Y, and Holdcroft, S. et al., Proton Exchange Membrane Fuel Cells Materials, Properties and Performance, Wilkinson, D. P. et al., Eds., Figure 3.16, 138, 2010, CRC Press, Boca Raton. Copyright (2010) CRC Press.)... 

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