Swelling unlimited

What happens upon equilibration with liquid water instead of water vapor According to Equation (6.13), the capillary radius would go to infinity for PVP —> 1. Thus, in terms of external conditions, swelling would be thermodynamically unlimited, corresponding to the formation of an infinitely dilute aqueous solution of ionomer. However, the self-organized polymer is an effectively cross-linked elastic medium. Under liquid-equilibrated conditions, swelling is not controlled by external vapor... 

At pH 7.0, as temperature increased from 4 to 90°C, solubility increased but water absorption increased then decreased. This suggested that water absorption and solubility may be related to a point, perhaps maximum hydration, at which solubility continues to increase and hydration does not. This appears consistent with the statements of Hermansson ( ) that water absorption is the first step in the solvation of polymers, and swelling may be limited or unlimited. 

The situation may be further explained as follows. We consider a gel of a polymer substance free from interlinking. In principle, therefore, this gel may show unlimited swelling (in a suitable solvent, that is). Suppose now that this gel contains m moles of solvent per gram of polymer and is in equilibrium with the solvent vapour through a semipermeable membrane (see Fig. 20). Let p be the pressure in the vapour phase and a that in the gel. The equilibrium requires that the partial Gibbs free energies of the solvent are equal in the two phases go = g o. This represents a relation between p, a and m. Changing these quantities at constant temperature we obtain... 

Cellulose and particularly nitrocellulose represent reversible xerogels showing the phenomena of limited and unlimited swelling respectively. In certain instances sorption of a vapour may give rise to continuous transitions between the xerogel state and the formation of a true solution. 

In similar systems exhibiting unlimited swelling the analogy is more complete (sec the next section). 

The analogy with Fig. 40 is evident and we have here a splendid example of a gradual transition between unlimited and limited swelling, demonstrated with one and the same macromolecular substance. 

Let ns now turn to the analysis of the AF(n) dependence for the case involving a constant degree of dispersion, that is, for r = const [33,69]. This situation does not permit unlimited dispersion down to molecular dimensions and is realistic for a number of real systems. The examples of the latter include coagulates consisting of insoluble particles, disperse system with phase contacts, solids with a microdomain strnctnre, or mnltiphase solids (alloys or composites) with a fine structure of grains. Other examples include swelling and dispersion of clays, and surfactant micelles and polymer macromolecules in particnlar. 

Examples of limited sweUing are gelatine and agar in warm water (e.g. at 25 °C). Examples of unlimited swelling are gum arabic or... 

Brown and Weir [1963] have argued strongly in favor of the identity of these two minerals. Their behavior with a variety of treatments is the same (Table 2). Both are capable of unlimited swelling, separating into double layers held together by interlamellar Na or K ions. 

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