Nafion solubility parameter

The solubility parameters of insoluble polymers are often determined by swelling the polymer in a series of solvents with different solubility parameter values. Maximum swelling of the polymer then occurs in those solvents whose solubility parameters are closest to that of the polymer. It is interesting to note that swelling data for Nafion, when interpreted in this way. Indicate the presence of two solubility maxima that is, this ionomeric polymer acts as if it had two different solubility parameters (21). This seems reasonable for a polymer that is composed of two different phases neither solubility maximum, however, is very close to the solubility parameter of water. 

The membrane conductivity of Nafion in various protic solvents has been measured in a recent study and the results are given in Table 12. The ratio of the membrane conductivity (k) to the solvent conductivity (k ) is listed in the last column of the table. A plot of the conductivity ratio (/solubility parameter of the solvent is shown in Fig. 22. The membrane conductivity is higher than the solvent conductivity in all solvents except formamide. 

Figure 22. Conductivity ratio (k/k ) of Nafion vs. solubility parameter of solvents. (Ref. 179 reprinted by permission of the publisher. The Electrochemical Society, Inc.)...

The solubility parameter of Nafion membranes has been determined experimentally in a recent study (3) The samples which have been studied have an equivalent weight (EW) of either 1100 or 1200 (weight of polymer per sulfonic acid group). Since the samples are not soluble, the solubility parameter of the polymer can be determined only from the swelling technique (40 ... 

SOLUBILITY PARAMETERS AND MOLAR VOLUMES OF SOLVENTS AND SOLVENT UPTAKE BY NAFION (3). 

Swelling in Pure Solvents. Table I shows the solvent uptake by Nafion-H. Figure 1, a plot of the solvent uptake for the 1200 EW sample against the solvent solubility parameter,, exhibits two distinct swelling envelopes. Apart from the presence of a sharp peak at 9.5 Hb, a broad peak spans from 12.8 to 23.4 Hb. They are denoted as envelopes I and II and have been tentatively ascribed to the organic backbone and the ion clusters of the membrane, respectively (3, 5). 

Figure 1. Solvent uptake of Nafion 120 vs. solubility parameters of solvents (3).

Solubility Parameter of Nafion Calculated from Small s and Hayes Methods (3)... 

It is obvious that Nafion, in contrast to many other polymers, exhibits two solubility parameter values. This feature is uncommon for any other material whose solubility parameter has been reported (2). However, in a recent study (5) on radiation grafted membranes, it is reported that membranes consisting of sulfonic acid groups also show two solubility parameter values. 

Effect of Neutralization. Figure 6 shows the solvent uptake for Nafion in both H and Li form (5). It is clear that the swelling behavior of Nafion in Li form is rather similar to that of the un-neutralized sample. The magnitude of the swelling envelope II decreases as the membrane is converted to Li form. In contrast to this, the swelling peak at low < i of the radiation-grafted sulfonic acid membranes disappears upon neutralization (5). Yeo, et al, have proposed that the disappearance of the dual solubility parameter is related to the decrease of the organic character of the material. 

Figure 7. Upper part conductivity ratio (vm/ve) of Nafion in solvent + 0.05 N LiOH vs. solubility parameter of solvent. aM, membrane conductivity

The majority of studies on the structure and properties of Nafion membranes are very often performed on the dry or humidified samples while many important applications of these materials are in the "wet form. The knowledge pertaining to the interaction between the solvents and the polymer by the use of the solubility parameter should facilitate the understanding of the structure-property-performance relationship. Investigations of ionic transport (13), spectroscopic properties (14) and dielectric loss tangent (12) of the membrane in light of the solubility parameter could prove to be an interesting and perhaps profitable line of inquiry. 

If we assume that the O2 solubility in Nafion is linearly proportional to its pressure, and the parameters for H2 are the same as those for O2, Table 3.15 lists the O2 and H2 crossover current densities at different pressures. 

The solubility parameter (8) of a material is a fundamental property of a material and is a measure of the intermolecular forces in a given substance. Yeo determined the solubility parameter of Naflon membrane experimentally by the swelling technique. " The results indicated that Nafion exhibits two solubility parameter values 821 and 822, which have been tentatively ascribed to the fluorocarbon backbone and the ion clusters of the membranes, respectively. For Nafion 1100,821 is equal to -10.0 and 822 is equal to -16.8. While for Nafion 1200,821 and 822 are -9.6 and -17.3, respectively. 

Solubility Nafion Nafion Solvent parameter acid form Li form... 

In Chapter 1 of this book, the necessary parameters for both RDE/RRDE analysis in ORR study, such as O2 solubility, O2 diffusion coefficient, and the viscosity of the aqueous electrolyte solutions, are discussed in depth in terms of their definitions, theoretical backgroimd, and experimental measurements. The effects of type/concentration of electrolyte, temperature, and pressure on values of these parameters are also discussed. To provide the readers with useful information, the values of these parameters are collected from the literature, and summarized in several tables. In addition, the values of both the O2 solubility and diffusion coefficient in Nafion membranes or ionomers are also listed in the tables. Hopefully, this chapter would be able to serve as a data source for the later chapters of this book, and also the readers could find it useful in their experimental data analysis. 

High proton conductivity and low methanol permeability (PCH3OH) are two of the essential characteristics that a polymer electrolyte membrane should possess to constitute a viable alternative to Nafion in DMFC applications. These parameters may be combined to constitute the p selectivity (or, shortly, selectivity p = log[ff/P]), which is useful for clearly comparing various sPEEK membranes. Indeed, p selectivity is a common metric for evaluating membrane performances and, as reported in Table 3.6, the few data about p selectivity show that the selectivity decreases with increasing DS. Indeed, as stated earlier, for DS >70%, sPEEK becomes more soluble in methanol and, as a consequence, methanol permeability increases determining a depletion of p selectivity. 

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