Nafion water absorption

In the experiments, the specimen was stretched and held at a constant length in a water environment. The decay of the stress on the membrane was monitored as a function of time and temperature. Typical results of the underwater stress relaxation studies of Nafion acid (EW=1200) and its sodium and barium salts are displayed in Figure 18. The degree of neutralization of the Na and Ba salt forms are 90% and 77%, respectively. The relaxation behavior appears to be similar for these three samples. The modulus values for Nafion-Na are comparable to those of the corresponding divalent Ba salt at most temperatures, while the moduli for the acid membrane are somewhat lower as a result of higher water absorption. These values are one order of magnitude lower than those in the dry state. 

When the concentration reaches a level of 6.25 M NaOH, the modulus of the Nafion membrane increases noticably while the rate of relaxation appears to slow down. The increase of modulus values may be attributed to the reduced swelling due to decreased water absorption of the membrane with increasing caustic concentration (53-55). The latter observation may be due to the enhancement of the Coulombic interaction between the bound anion and the sodium counterion within ionic region (31, 33). Further increase in NaOH concentration to a level 12.5 M accentuates the above tendency in that the modulus increases remarkably, and the relaxation continues to slow down further. It seems that the ionic interaction within the ionic regions is enhanced at such high levels of caustic concentration due to reduced water absorption. This in turn Immobilizes the chains leading to deceleration of the relaxation process. 

The effect of NaOH concentration on the ion transport and rheological properties of the Nafion ion exchange membranes may be attributable to some variation in the ionic domain structure in the presence of NaOH. Therefore, it is extremely Important to understand the ionic domain structure under these conditions. The anomalous behavior of Na" " ion transport as a function of NaOH concentration is seen more frequently in bilayer Nafion membranes in which one layer is treated with diamine and also in perfluorinated carboxylic ion exchange membranes. Several mechanisms have been proposed to explain their ion transport results including water absorption, transport of hydroxide ion tunneling, ion pairing mechanisms, etc. (54-56). As the ion transport properties are beyond the scope of this review, no detailed discussion will be presented. 

Under-water stress relaxation of the Nafion acid and Nafion-Na samples with an equivalent weight of 1200 was carried out by Kyu and Eisenberg (40). The degree of neutralization of the Nafion-Na was measured to be ca. 80%. Time-temperature superposed master curves of the two systems, reduced to a reference temperature of 50°C, are shown in Figure 6. The under-water stress relaxation behavior of Nafion acid resembles that of Nafion-Na, except for the fact that the elastic modulus is somewhat lower in the acid. This latter feature may be due to the difference in the degree of water absorption of the acid and salt samples (26,31). The swelling of Nafion acid is greater than that of Nafion-Na, which yields a material of lower modulus. 

Figure 5. Comparison of experimentally measured and theoretically predicted water absorption vs. equivalent weight for standard Nafion...

Plots of water absorption and wet density of the sodium sulfonate form of the ionomer as a function of the solution molarity of sodium ions are shown, respectively, in Figures 6 and 7 for 1100- and 1200-equiv wt materials. As with Figure 5, both theoretical predictions and experimental data are presented in these latter figures to demonstrate the ability of the calibrated model to describe quantitatively the equilibrium solution behavior of standard Nafion ionomers. 

At fuel cell start-up, the dry membrane will absorb water from the feed reactants and swell. The linear expansion of Nafion N115 is 10% and 15% from 50% RH at 23°C to water-soaked at 23°C and to water-soaked at 100°C, respectively. The thickness change is 10% and 14% from 50% RH at 23°C to water-soaked at 23°C and to water-soaked at 100°C, respectively. In fuel cells, the membrane is constrained between the bipolar plates. Water absorption by the membranes is accompanied by membrane volume... 

In fuel cells, temperature (X) and water activity (Ow) change in response to the current. These changes in T and can alter the transport properties of the Nafion membrane, which can in turn alter the fuel cell operation. Decreases in water activity cause the proton conductivity to decrease, which reduces the current and further reduces the water activity this feedback mechanism can result in steady-state multiplicity in PEM fuel cells [15,16]. Water absorption and desorption from Nafion causes it to swell and shrink, which can cause mechanical failure of the membrane and alter the interfacial contact between the membrane and the electrode [17, 18]. Choosing and controlling the operating conditions of PEM fuel cells require knowledge of the mechanical and transport properties of Nafion. 

Fig. 4 Free volume of water absorption in 1100 EW H -Nafion as a function of water activity and temperature...

Zhao and Benziger were able to determine the contribution of the tortuous network for water diffusion through Nafion. They determined the effective diffusion coefficient of water with the PGSE NMR method at short delay times, where the diffusion was limited by short-range molecular interactions. They also determined diffusion coefficients at long delay times, where diffusion allowed the molecules to move through the hydrophilic domain network. The ratio of these two diffusion coefficients is the tortuosity of the hydrophilic domains, which is shown in Fig. 10. Tortuosity decreases by more than a factor of ten as water activity increases. The tortuosity follows a similar trend with water activity as the free volume of water absorption. Tortuosity, free volume, and proton conductivity are related to the evolution of the connectivity of the hydrophilic domains. 

Fig. 13 Clustering of hydrophilic domains in Nafion. At low temperatures, the sulfonic acid groups cluster to reduce the repulsive interactions with the PTFE matrix. Entropy drives the sulfonic acid groups to disperse at high temperatures. Water absorption increases the fraction of hydrophilic species inducing microphase separation...

Nation /Silica System Jungetal. [174] have reported the synthesis of a Nafion /silica composite membrane obtained via the sol-gel acid catalysis of tetraethoxysilane (TEOS) in a Nafion 115 membrane (equivalent weight = 1100). The amount of TEOS impregnated in the membrane varied with the reaction time. These authors noted that the water absorption of the composite membrane increased... 

For ETFE- -PSSA membranes with the same lEC, water uptake is higher than MeOH uptake of the membrane, but for Nation and S-SEBS membranes, MeOH uptake of membrane is always higher than water uptake. Chemical structure and morphology of membranes affect the solvent absorption. Nafion is considered to consist of ionic clusters that are separated from the polymer phase. For grafted polymers, heterogeneity exists to some extent due to the hydrophobic base polymer however, a regular clustered structure, as in the case of Nafion, has not been proposed for these materials. 

Figure 7. Solid state UV-vis absorption spectrum of Nafion 417 and Ru(bpy)32-l--modified NPyc catalytic system (psrPyc-Ru(bpy)32+ ) in ambient conditions. In the modification procedure, 1 cm2 test membrane is first dipped in 1 mM Ru(bpy)3 2+ CH3CN H20 (1 1) solution for 30 min followed by water washing.

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