Nafion structural model

As was noted above, the structure and behavior of Nafion polymers were intensively investigated and structural models were suggested. It is obvious that during the transformation from precursor (nonionic form) to ionic form the internal stmcture of the polymer is reorganized into the ordered type. But this order has a random nature, and all earlier investigations were peformed with such randomly ordered films. 

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. 

These structures are fictional in the sense that these sequences do not correspond to the actual statistical polymerization based on the comonomer reactivity ratio, although it was said that the results have significance with respect to Nafion structural optimization and guidance in the search for Nafion replacements. Also, the non-insignificant degree of crystallinity of Nafion was not accounted for in the model. 

Differences in conductivity data of modified perfluorosulfonate membranes can be related to structural differences on the basis of the pore structure models. Smaller equivalent weights (e.g., for Nafion 105, Dow, Membrane C), that is, higher specific ion content, lead to superior performance compared to Nafion 117 due to narrower psds and, thus, more homogeneous water distributions. 

Several structural models of Nafion have been proposed which are based on various transport and spectroscopic properties of the... 

Figure 2. Three-region structural model for Nafion ...

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

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. 

Interesting results have been obtained by a combination of NMR and quasi-elastic incoherent neutron scattering. The presence of one single line in an NMR spectrum, for all the water concentrations, can be interpreted in two ways either we have only one kind of water molecule with a very well defined environment or we have different kinds subject to a fast chemical exchange (t < 10 3 sec.). Two regimes of absorption have been demonstrated and two different motions have been characterized both by NMR spin lattice relation time measurements and quasi-elastic incoherent neutron scattering. From these results and from results obtained on the Nafion salts W a structural model will be proposed (9). 

In principle, using the DPD results [20], it was possible to mimic SAXS and SANS experiments to discriminate the structural models used for interpretation of the experimental data [20]. Also, Hyodo [21] proposed a hierarchical procedure for calculating the electronic states of a hydronium ion in a hydrated Nafion membrane via the mesoscopic structure predicted by DPD [20]. A mixed basis function method was introduced for electronic state calculations in inhomogeneous fields as a combination of Gaussian basis functions and shape functions of the finite element method for expressing electronic wave functions. 

Kong, X. and Schmidt-Rohr, K. (2011) Water-polymer interfacial area in Nafion comparison with structural models. Polymer, 52, 1971-1974. 

FIG U RE 1.12 Layered structure model for Nation membranes. The Nafion perfluorocarbon backbones (O) are in the direction vertical to the paper plane, and the sulfonated side chains ( ) are on the plane surface of the paper and are vertical in direction to the perfluorocarbon backbones. (Reproduced with permission from Starkweather, H.W., Macromolecules, 15, 320, 1982.)... 

FIGURE 1.13 Fibrillar structure model for Nafion membranes. (Reproduced with permission from Rubatat, L. et af. Macromolecules, 37, 7772, 2004.)... 

Nafion. Models of the initial structure of Krytox-Silica as shown in Figure 9.20a and of a composition structure incorporating 5 wt% Krytox-Silica in a Nafion composite polymer consisting of 15 Nafion side chains, 15 hydronium ions, and 1 of Krytox-Silica, as shown in Figure 9.20b, were established. In another system, pure Nafion was modeled without Krytox-Silica (model structure shown in Figure 9.20c). 

Because the reaction in a CL requires three-phase boundaries (or interfaces) among Nafion (for proton transfer), platinum (for catalysis), and carbon (for electron transfer), as well as reacfanf, an optimized CL structure should balance electrochemical activity, gas transport capability, and effective wafer management. These goals are achieved through modeling simulations and experimental investigations, as well as the interplay between modeling and experimental validation. 

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