Cluster water uptake

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. 

This oversimplified random network model proved to be rather useful for understanding water fluxes and proton transport properties of PEMs in fuel cells. - - - It helped rationalize the percolation transition in proton conductivity upon water uptake as a continuous reorganization of the cluster network due to swelling and merging of individual clusters and the emergence of new necks linking them. ... 

The effective conductivity of the membrane depends on its random heterogeneous morphology—namely, the size distribution and connectivity of fhe proton-bearing aqueous pafhways. On fhe basis of the cluster network model, a random network model of microporous PEMs was developed in Eikerling ef al. If included effecfs of varying connectivity of the pore network and of swelling of pores upon water uptake. The model was applied to exploring the dependence of membrane conductivity on water content and... 

The intramolecular correlations of O—H at r= 1.00 A and of H—H at r = 1.56 A are identical, and the second peaks show small differences, perhaps arising from interactions between water molecules and sulfonate groups, although it would seem that water coordination around Ni would have to be considered, as well. From this, the authors concluded that the structure of water in the clusters is essentially that of water in the bulk state. This would seem reasonable considering that the water uptake of these samples corresponded to an average of 21 H2O molecules per —SOa" group. 

The latter authors found that a reversible water isothenn was obtained after the low-temperature (i.e. 40°C) evacuation of a carbon cloth, which had been activated by oxidative HNO, treatment. The molecular sieve character of this material was reduced by evacuation at 400°C and this also led to the appearance of hysteresis in the water vapour isothenn. Barton and Koresh (1983) conclude that such hysteresis is mainly due to the concentration of surface oxides which dictate the adsorption value at which the change from cluster adsorption to a continuous adsorbed phase takes place . The relationship between the adsorption of water and the surface concentration of chemisorbed oxygen was first established by Walker and Janov (1968). Bansal et al. (1978a,b) also investigated the influence of the surface oxygen on the adsorption of water they concluded that at p/p° < 0.5 the level of water uptake is determined by the concentration of surface oxygen-containing structures. 

Application of the percolation theory allows explanation of the changes in the release and hydration kinetics of swellable matrix-type controlled delivery systems. According to this theory, the critical points observed in dissolution and water uptake studies can be attributed to the excipient percolation threshold. Knowledge of these thresholds is important in order to optimize the design of swellable matrix tablets. Above the excipient percolation threshold an infinite cluster of this component is formed which is able to control the hydration and release rate. Below this threshold the excipient does not percolate the system and drug release is not controlled. 

An adequate structure of polymer molecules promotes the advantageous phase separation into hydrophobic and hydrophilic domains upon water uptake. The most notable class of membranes based on this principle are the perfluorosulfonic acid ionomers (PFSI), Nafion [26] and similar membranes [27]. In these membranes, perfluorosulfonate side chains, terminated with hydrophilic —SO3H groups, are attached to a hydrophobic fluorocarbon backbone. The tendency of ionic groups to aggregate into ion clusters due to the amphiphilic nature of the ionomer leads to the formation of basic aqueous units. At sufficient humidity these units first get connected by narrow channels and then may even fuse to provide continuous aqueous pathways [28]. 

Since the product of flow rate, time and concentration equal the input mass, a constant input concentration permits the calculation of mass from either time or retention volume. Empty columns provide an essentially constant ratio of input mass to time at constant flow rate with the concentration of water vapor fixed by the temperature of the carrier gas saturated with water vapor (100Z RH or Aw of 1 see Figure 6). This state can be achieved with substrates that do not dissolve in water when saturated (for example, starches and many proteins), or when the relative humidity is constant but insufficient to allow uptake to produce a highly multilayered or clustered water state in the substrate equivalent to a continuous water phase or solution. This condition requires a source of humidified gas as in (7.). The sorption isotherm equation is then given by... 

Ways to increase membrane durability have been examined by various researchers across the country. Maurtiz et al. examined the use of metal-oxide metal particles to increase the properties of the membrane. A titanium isopropoxide (Figure 11.4) addition to Nation membranes generates quasi-network particles this improves membrane modulus and dimensional stability [17], In addition, the titanium matrix reduces fuel crossover and minimizes chemical degradation. Table 11.2 shows the increase in modulus along with stress and strain and stress changes after the addition of the titanium matrix [17], With a 20% load of the titanium matrix, performance criteria remain comparable. Acid functionality remains intact however, water uptake is reduced as volume inside clusters is occupied. Conductivity is reduced due to chain mobility [17],... 

The structural parameters D, Xc, size of the ionic cluster, and spherulite size as well as amount of water uptake were investigated as a function of the E.W. and of the functional groups. 

One can control an equilibrium amount of water uptake by the membranes and also the size of the ionic clusters by changing relative humidity of the membranes or by immersing the membranes in aqueous solutions of sodium chloride with different concentrations (11.). Figure 13 shows change of SAXS profiles upon immersing the sodium-sulfonated (a) and -carboxylated membranes having 1100 E.W. (b) into the aqueous sodium-chloride solutions with various concentrations. 

Chemical potential and water content, X, can be related through an uptake isotherm. Uptake isotherms of k as a function of water-vapor activity or relative humidity, such as that given in Figure 5.1, are prevalent in the literature [4, 6, 42, 43]. They have been used in almost every model that deals with vapor-equilibrated membranes and treats the membrane as a single phase [1]. As discussed in the proposed physical model, the water uptake is described by the hydration of the sulfonic acid sites in the membrane clusters and a balance between osmotic, elastic, and electrostatic forces. The approach taken here is to calculate the isotherms using the chemical model of Meyers and Newman [5] with some modifications [39]. 

The physical model can be used to describe trends seen in experimental data. For example, the interconnectivity of the cluster network is predicted to have a profound effect on a membrane s transport properties. The percolation threshold for conductivity should increase when the clusters become smaller, which could be due to a stiflfer and/or more crystalline polymer matrix. These smaller clusters would also mean that the membrane would exhibit lower electro-osmotic coefficients, larger liquid water uptakes, and a greater dependence of the various properties on water content than in Nafion . In fact, these predictions are what is seen in such systems as sulfonated polyetherketones [19, 72] and Dow membranes [73, 74] or when the equivalent weight [22] or drying temperature [4, 6] of Nafion is increased. 

Advancement in computer simulation founded on the molecular orbital method is expected to contribute to deeper understanding of the formation of ionic clusters, proton conduction, chemical deterioration of membranes, and water uptake over 100 °C, etc. 

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