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Water sorption pore swelling

The theory of bundle formation in the section Aggregation Prenomena in Solutions of Charged Polymers provides sizes, as well as electrostatic and elastic properties of ionomer bundles. The theory of water sorption and swelling, described in this section, gives a statistical distribution of pore size and local stress in pores. The merging point of both theories is a theory of fracture formation in charged polymer... [Pg.120]

In general, pores swell nonuniformly, as seen in the section Water Sorption and Swelling of PEMs. As a simplification, the random network was assumed to consist of two types of pores. Nonswollen or dry pores (referred to as red pores) permit only a small residual conductance resulting from tightly bound surface water. Swollen or wet pores (referred to as blue pores) contain extra water with high bulklike conductance. Water uptake corresponds to the swelling of wet pores and to the increase of their relative fraction. In this model, proton transport in the PEM is mapped as a percolation problem, wherein randomly distributed sites represent pores of variable size and conductance. The distinction of red and blue pores accounts for variations of proton transport properties due to different water environments at the microscopic scale, as discussed in the section Water in PEMs Classification Schemes. ... [Pg.147]

The water content is the state variable of PEMs. Water uptake from a vapor or liquid water reservoir results in a characteristic vapor sorption isotherm. This isotherm can be described theoretically under a premise that the mechanism of water uptake is sufficiently understood. The main assumption is a distinction between surface water and bulk water. The former is chemisorbed at pore walls and it strongly interacts with sulfonate anions. Weakly bound bulk-like water equilibrates with the nanoporous PEM through the interplay of capillary, osmotic, and elastic forces, as discussed in the section Water Sorption and Swelling of PEMs in Chapter 2. Given the amounts and random distribution of water, effective transport properties of the PEM can be calculated. Applicable approaches in theory and simulation are rooted in the theory of random heterogeneous media. They involve, for instance, effective medium theory, percolation theory, or random network simulations. [Pg.366]

While several simplifying assumptions needed to be made so as to derive an analytical model, the model captures all relevant physical processes. Specifically, it employed thermodynamic equilibrium conditions for temperature, pressure, and chemical potential to derive the equation of state for water sorption by a single cylindrical PEM pore. This equation of state yields the pore radius or a volumetric pore swelling parameter as a function of environmental conditions. Constitutive relations for elastic modulus, dielectric constant, and wall charge density must be specified for the considered microscopic domain. In order to treat ensemble effects in equilibrium water sorption, dispersion in the aforementioned materials properties is accounted for. [Pg.101]

SAXS, SANS, porosimetry, and water sorption studies provide ample evidence for the dispersion in pore size and the evolution of the pore size distribution in the PEM upon water uptake. The changes in the pore space morphology upon water uptake translate into variations in transport properties of the PEM, as is well known (Eikerling et al., 1997, 2007a, 2008 Kreuer et al., 2004). There is, however, uncertainty regarding the mechanism of these macroscopic swelling phenomena. [Pg.111]

Different values of (Jq correspond to different equilibrium values of r)c, as illustrated in Figure 2.22. Statistical spatial fluctuations of gq give rise to the evolution of the pore radius distribution (PRD) upon water sorption. The PRD evolution is influenced as well by dispersions in elastic and dielectric properties. Larger values of G will give smaller equilibrium pore radii due to the stronger elastic forces that constrain the swelling. Below, only the effect of fluctuations in gq is evaluated. [Pg.112]

Under equilibration in a saturated vapor atmosphere, a determines the maximal values of radius, swelling parameter and liquid pressure of swollen pores. A typical set of parameters gives= 21 nm, = 105 and P —67 atm. For this case, water sorption from vapor would level off at 0.95. The leveling-off of inte-... [Pg.115]

To begin, it is essential to rationalize the equilibration of water within the membrane at AP = 0, APs = 0, j = 0, and = 0. The suggested scenario of membrane swelling is based on the interplay of capillary forces and polymer elasticity. In order to justify a scenario based on capillary condensation, isopiestic vapor sorption isotherms for Nafioni in Figure 6.9(a) are compared with data on pore size distributions in Figure 6.9(b) obtained by standard porosimetry.i In Figure 6.9(a), a simple fit function. [Pg.373]

The first step corresponds to uptake of water by solvation by the ions in the membrane, whereas the second step corresponds to water that fills the pores and swells the polymer. It is important to notice that the resulting water uptake from the fully saturated vapor phase (with anio = 1) is significantly lower than that from the liquid phase (also aH2o = 1)/ that is, A. = 14 vs A = 22, respectively. This phenomenon was first reported in 1903 by Schroeder, and is therefore called Schroeder s paradox [1]. A possible explanation of this difference in uptake from vapor and liquid phases is that sorption from the vapor phase involves condensation of water inside the polymer, most probably on the strongly hydrophobic polymer backbone, and the resulting uptake is lower than if sorption and imbibition occurred directly from the liquid phase [1]. [Pg.77]

It was found that the liquid uptakes of both m branes iuCTeased linearly with increasing methanol concentration. For example, the liquid uptake of Nafion 115 increased from 34.3 wt% in 0 M methanol solution to 58.6 wt% in 10 M methanol solution, while that of the composite membrane increased from 28.3 to 37.5 wt% in the corresponding methanol solution. This suggests that Nafion 115 absorbed more liquids and swelled more seriously than ZrP/Nafion 115 in high concentration methanol solution. It is considered that the mechanical stability for ZrP/Nafion 115 was improved due to the incorporation of ZrP. The reason may be as follows when Nafion 115 was soaked into the methanol solution, methanol molecules could diffuse easily into the ion clusters within Nafion 115 like water molecules. However, for ZrP/Nafion 115, ZrP in the pores possibly occupied an amorphous region that was originally filled with water and methanol molecules in Nafion 115, resulting in the decrease in free volume, cluster size, and the percent sorption capacity of methanol. [Pg.434]


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See also in sourсe #XX -- [ Pg.108 , Pg.115 ]




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