Swelling water uptake

Swelling water uptake, electric conductivity, and transport number of the membranes are measured as a function of the ion-exchange capacity (lEC). lEC has been estimated in terms of... 

Earoongsarng, D. Peck, G.E. The swelling, water uptake of tablets III. Moisture sorption behavior of tablet disinte-grants. Drug Dev. Ind. Pharm. 1994, 20, 779-798. 

Keywords Neutron reflectometry Polyelectrolytes Polyelectrolyte multilayers Swelling Water uptake... 

The anomalous increase of the water uptake observed in Fig. 10 when approaching equilibrium at 60 °C has been associated to the damage. The abrupt upturn of the sorption curve may be explained considering a possible crazing of the low crosslinked internodular matrix induced by the differential swelling stresses that can arise, at high water contents, between areas of different crosslinking density. 

Hydrophilic polymers (Table 5) provide a matrix which is comparable to an aqueous environment. Ions can diffuse quite freely, but the possible water uptake (10-1000%) can cause significant swelling of the polymer. Swelling of the matrix affects the optical properties of the sensors and, consequently, the signal changes. Immobilization of the indicator chemistry usually is achieved via covalent bonding to the polymer. 

Water uptake and swelling of reinforced PEMs have also been studied. It has been found that water uptake and water flux of Nafion/porous PTFE PEMs increases with increased ionomer loading and that the rate of water uptake as a function of temperature is actually higher for the reinforced membrane in comparison to Nation 112.2 However, it was also seen that the actual water uptake of a similar reinforced membrane was about half as much as a Nation 1135 sample with a commensurate lower degree of swelling.2 ... 

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. ... 

Nevertheless, Equation (6.17) can serve as a qualitative tool for explaining membrane swelling upon water uptake. Under equilibrium conditions, the elastic pressure in Equation (6.15) increases with increasing vapor pressure due to Equation (6.14). Overall, by invoking Equations (6.14)-(6.17), we obtain a relation between external conditions (Ps and P ") and X ... 

Equation (6.20) determines the maximum degree of swelling and the maximum pore radius of a liquid-equilibrated membrane. This relation suggests that the external gas pressure over the bulk water phase, which is in direct contact with the membrane, controls membrane swelling. The observa-hon of different water uptake by vapor-equilibrated and by liquid water-equilibrated PEMs, denoted as Schroeder s paradox, is thus not paradoxical because an obvious disparity in the external conditions that control water uptake and swelling lies at its root cause. 

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... 

In general, pores swell nonuniformly. As a simplification, fhe random network was assumed to consist of fwo types of pores. In fhis fwo-stafe model, nonswollen or "dry" pores (referred to later as "red" pores) permit only a small residual conductance due to tightly bound surface water, which solvates the charged surface groups. Swollen or "wet" pores (referred to later as "blue" pores) contain extra water in the bulk, allowing them to promote the high bulk-like conductance. Water uptake by the membrane corresponds to the swelling of wef pores and to the increase of their relative fraction. 

A rigid microporous morphology, which does not reorganize upon water uptake, corresponds to a simple linear relation x w) = yw. In this limiting case, the model resembles the archetypal problem of percolation in bicontin-uous random media. Due to deviations of swelling from fhis law, universal percolation exponents in relations between conductivity and water content are not warranted. 

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