Adsorbed polymer exchange

We are interested in the application of polymers as adsorbents, ion exchangers, fuel cells, and permeable materials. In this regard, the first resins with some of these properties were obtained by D Aleleio in 1944 based on the copolymerization of styrene and divinylbenzene. Unfunctionalized polystyrene resins cross-linked with divinylbenzene (Amberlite) are widely applied as adsorbents [191,192], In addition, the polystyrene-divinylbenzene resins functionalized with sulfuric acid (sulfonation) to create negatively charged sulfonic sites are applied as cation exchangers, and treated by chloromethylation followed by animation produce anionic resins [193,194],... 

This paper proposes a phenomenological analysis, based on laboratory experimental work, of the effects of adsorption properties on pol3nner slug propagation. The adsorption properties studied include kinetic aspects, i.e. instantaneous adsorption, reorganization of macromolecules inside adsorbed layer, exchanges between free and adsorbed polymer, desorption as well as properties at thermodynamic equilibrium which can be described by a partially reversible adsorption isotherm. The conditions for hydro-dynamic retention are also discussed. In addition, an analysis of the effects of polymer polydispersity on each of these adsorption phenomena shows that these effects cannot be neglected in a predictive simulator. 

It is also possible to study exchange kinetics of the adsorbed polymers using these techniques. In the case of IR spectroscopy, exchange between deuterated and hydrogenated polymer chains at the surface can be studied as a fimction of time. From the rate of change in the specific absorption bands one can estimate the time constant for exchanging a chain in the solution with one on the surface. In the same way it is possible to study the kinetics of one species displacing another adsorbed... 

Arup K. Sengupta (water and wastewater treatment preparation, characterization and innovative use of novel adsorbents, ion exchangers, reactive polymers and specialty membrane in separation and control hybrid separation processes), Department of Civil and Environmental Engineering Department of Chemical Engineering, Lehigh University (LU), Bethlehem, PA... 

Competitive rates of adsorption provide information not only on the kinetics of the adsorption process, but also on the adsorbed polymer conformation. The overall rate of adsorption of a pol3rmer molecule is probably comprised of two separate rates the rate of initial attachment and the rate of reorientation of an attached molecule until it achieves its equilibrium conformation. The rates of desorption and exchange can also yield information on the adsorbed conformation since removal of a molecule from the surface is a function of the number of attached segments. 

New data, obtained using the technique of neutron reflectivity, have been recently reported. A remarkable difference in the time-scale for center of mass diffusion was found between adsorbed polymer chains (PMMA) and those existing entirely in a melt. Because the adsorbed chains are mixed intimately with chains from the bulk melt, it is plausible that surface exchange on the segmental scale would be quite rapid but that the requisite simultaneous loss of every seg-... 

Recently, several other groups have tried to model the kinetics of polymer adsorption (and exchange) by taking into account diffusion and some aspects of reconformation of the adsorbed polymer at the surface [22-24]. 

The time scale for the kineties of polymer adsorption, as found in experiments, ranges from seconds to days or even weeks. As it is not possible to establish the rate of desorption by simply lowering the solution concentration, an alternative, and fmitful, method to study the dynamies of the adsorbed polymer is to measure the exchange against other polymerie or monomerie molecules. 

Although the macroscopic curvature of the surface can have a small effect on the equilibrium properties of the adsorbed polymer layer, it will have a strong effeet if the kinetics of polymer adsorption are dominated by diffusion. Examples of frequently applied well-defined geometries were presented in flie previous section. Wang et al. [34] have studied exchange kinetics in a spherical geometry. It was shown that the influence of bulk diffusion is reduced with decreasing radius of the adsorbent particle. For slow processes the macroscopic curvature will not be very important, and powders with poorly defined macroscopic curvatures ean still be used. 

Desorption of an adsorbed polymer by flushing with solvent proceeds particularly slowly and, in practice, it is impossible to desorb a significant amount in this way. However, we have seen that desorption of a polymer can be accomplished under the influence of an external shear force. In addition, desorption may be caused by competition with a polymeric or a monomeric displacer. In this section we will discuss exchange by a chemically identical polymer or by a polymer differing in size or chemical composition. 

The above examples are ample evidence of the exchange that occurs between uncharged macromolecules in the adsorbed and free state. Unlike these, polyelectrolytes at low ionic strengths do not exchange, even when the more weakly adsorbing polymer is on the surface and a more strongly adsorbing one is in solution. This is discussed further in the section on polyelectrolyte adsorption. 

In contrast to uncharged macromolecules, polyelectrolyte adsorbed on crystals does not exchange freely with non-adsorbed polyelectrolyte in the absence of added electrolyte. This is a result of the electrical repulsion from the adsorbed polymer preventing the close approach of unadsorbed polymers, thus giving a form of kinetic equilibrium rather than thermodynamic equilibrium. 

A logical division is made for the adsorption of nonelectrolytes according to whether they are in dilute or concentrated solution. In dilute solutions, the treatment is very similar to that for gas adsorption, whereas in concentrated binary mixtures the role of the solvent becomes more explicit. An important class of adsorbed materials, self-assembling monolayers, are briefly reviewed along with an overview of the essential features of polymer adsorption. The adsorption of electrolytes is treated briefly, mainly in terms of the exchange of components in an electrical double layer. 

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