Nafion molecular structure

Nearly all of the commercially available membranes are based on Nafion. Nafion also has the largest body of literature devoted to its study because of its demonstrated industrial importance and availability. Nafion composite systems also have already become significant in both industrial and academic research. In composite structures, Nafion can be impregnated into an inert Teflon-like matrix (i.e. W. L. Gore membranes ), or inorganic additives can be added to a supporting Nafion matrix for improved physical or electrochemical properties (i.e. lon-omem °). Some critical aspects of Nation s molecular structure and physical properties will be briefly highlighted to provide a baseline for comparison with the other alternative materials discussed in this review. 

Figure 1.5. Molecular structure ofNafion 117 [12]. (Reprinted from Electrochimica Acta, 46(10-11), Haubold HG, Vad Th, Jungbluth H, Hiller P, Nano structure of NAFION a SAXS study, 1559-63, 2001, with permission from Elsevier.)...

Fig. 3.2 Molecular structure of Nafion membranes. The values of x, y, and z coefficients vary with the manufacturer...

Hopfinger, A.J. Mauritz, K.A. Hora, C.J., Prediction of the molecular structure of Nafion under different physicochemical conditions, presented at the Electrochem. Soc. 

A general formalism is presented to describe the structural organization of ionomers under different physicochemical conditions. The theory is applied specifically to Nafion. Resultant predicted properties are compared with experimental findings. Preliminary application of the predicted ionomeric molecular structure of Nafion to modeling ion transport through Nafion chlor-alkali separators is discussed and evaluated. 

A pseudo-quantitative application of the theoretical formalism has been made for Nafion. The values for the requisite molecular parameters were estimated from a combination of experimental bulk thermodynamic data and molecular structure calculations using both molecular and quantum mechanics (23,24). A constraint was imposed in the development of the structural formalism. The model was constructed so that the predicted structural information could be used in a computer simulation of ion transport through an ionomer, that is, modeling the ionomer as a permselective membrane. 

Qualitative Molecular Structure. The general theory has been applied to Nafion ionomers whose general repeat structure is given as ... 

The molecular structure of a conventional polymer used for a PFSA membrane is shown in Fig. 1. Membranes registered as Nafion (DuPont), Flemion , (Asahi Glass), and Aciplex (Asahi Chemical) have been commercialized for brine electrolysis and they are used in the form of alkali metal salt. Figure 4 shows a schematic illustration of a membrane for chlor-alkali electrolysis. The PFSA layer is laminated with a thin perfluorocarboxylic acid layer, and both sides of the composite membrane are hydrophilized to avoid the sticking of evolved hydrogen and chlorine. The membrane is reinforced with PTFE cloth. The technology was applied to PEFC membranes with thickness of over 50 xm [14]. 

The upper part of Fig. 2 shows the molecular structure of the perfluorinated Nafion cation exchange membranes with sulfonic acid as well as with carboxylic acid groups as fixed ions. These are covalently bonded at the end of side chains of the PTFE (polytetrafiuoroethylene) polymer backbone. The polymer has excellent chemical and thermal stability, similar to PTFE [9]. 

Because of the molecular structural complexity and the diversity of segmental motions in Nafion ionomers, explanations in the limited reports on DSC data for Nafion membranes have been rather vague and inconsistent. In contrast, explanation of TG data on Nafion seems explicit. The thermal stability of Nafion membranes has been widely investigated by several groups (Wilkie et al., 1991 Tiwari et al, 1998 Lage et al., 2004a,b). Perhaps the most cited paper about Nafion thermal durability is of Wilkie et al. (1991). To study the interaction of poly(methyl methacrylate) and Nafion, Wilkie et al. 

FIG U RE 1.15 Nafion molecular aggregated structure in alcohol/water solution. (Reproduced with permission from Szajdzinska-Pietek, E. and Schlick, S., Langmuir, 10, 2188, 1994.)... 

Mauritz summarized a number of molecular models of ionomer structure, including those pertaining to Nafion, that had had been formulated up to 1996. Within the context of the title of this review, it should be appreciated that the results contribute to the state of understanding only if they are verifiable by careful experimentation. To be sure, theoretical predictions are welcome in the design of experiments and pointing the way toward useful applications. 

None of the models address the question of how the main chains are packed, and details of crystallinity are neither factored into nor predicted by mathematical models of the structure and properties of Nafion. Chains packed in crystalline arrays are usually considered to be rigid within the context of certain properties for example, with regard to diffusion, crystallites are viewed as impenetrable obstacles. F NMR studies indicate otherwise. Molecular motions that do not significantly alter symmetry can in fact occur in polymer crystals. It would seem, for example, that the response of the Nafion structure to applied stress would depend on the flexibility of the polymer backbone, a certain fraction of which is incorporated in crystalline regions. On the other hand. Starkweather showed that the crystallinity and swelling of Nafion are not correlated. 

At the time of this writing, it must be conceded that there have been no fundamental principles-based mathematical model for Nafion that has predicted significantly new phenomena or caused property improvements in a significant way. Models that capture the essence of percolation behavior ignore chemical identity. The more ab initio methods that do embrace chemical structure are limited by the number of molecular fragments that the computer can accommodate. Other models are semiempirical in nature, which limits their predictive flexibility. Nonetheless, the diversity of these interesting approaches offers structural perspectives that can serve as guides toward further experimental inquiry. 

As noted in the Molecular Simulation of Structure and Properties section, there have been no fundamental principle-based mathematical models for Nafion that have predicted new phenomena or caused property improvements in a significant way. This is due to a number of limitations inherent in one or the other of the various schemes. These shortcomings include an inability to sufficiently account for chemical identity, an inability to simulate and predict the long-range structure as would be probed by SAXS or TEM, and the failure to simulate structure over different hierarchy levels. Certainly, advances in this important research front will emerge and be combined with advances in experimentally derived information to yield a much deeper state of understanding of Nafion. 

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