Deswelling

Direct mechanical methods can be used to determine the swelling pressure of hydrogels, e.g., by means of devices in the form of a cylindrical chamber equipped with a piston in which the gel contacts the solution through a porous membrane. This technique allows measuring very low pressure (of the order of 0.1-10 kPa) and makes it possible to analyze the SAH with swelling up to 700 ml g-1 [102, 103]. Among others, the method of osmotic deswelling is to be mentioned [104]. 

Osmotic deswelling experiments, performed with a series of PAAm [20] and PVA [112] gels have revealed a correlation between their sensitivity to the external pressure and the equilibrium swelling degree in the absence of external forces. This fact is illustrated below ... 

The swelling pressure or osmotic deswelling data can be, therefore, described as the functions of n(w) by either of the theories [115]. This description can be then applied to determining the network parameters (see, for example, Ref. [22]). On the other hand, the swelling pressure which is directly connected with the chemical potential of water in the gel ... 

Figure 6 shows the shear modulus values for a series of neutral PAAm gels at different stages of deswelling [20]. The slopes of the dotted lines describing the deswelling of each sample are about 0.334, which perfectly agrees with the theory. 

The swelling pattern considered above allows us to understand the peculiarities of the behavior of SAH and the effects encountered during their application. The kinetic aspects of swelling seem to be as important as the thermodynamics of this process. Therefore, we shall touch upon some problems concerning the kinetics of hydrogel swelling and deswelling. 

Therefore, the SAH swelling and deswelling rates can be quantitatively characterized by the time t which for a given hydrogel type is determined mainly by the gel particle size. The gel instability, both mechanical and thermodynamical, constitutes an additional complication [128 -130]. 

Figure 11 The partitioning behavior of norethindrone in poly(/V-isopropylacrylamide) gel. This gel substantially deswells at 34°C. (Adapted from Ref. 176.)...

Figure 13 Typical swelling and deswelling rates of cross-linked poly(acryloyl pyrroli-dine-co-styrene) between 27°C and 37°C. AS15 ( ) AS20 (A). The numbers indicate the content of styrene in the feed composition in moles during polymerization. Membrane thickness is 0.5 mm in the dried state. (From Ref. 34.)...

The samples were weighed before swelling and after deswelling. 

Desulfogypsum, 4 591-593, 595 Desulfurization, 1 650 10 785 Desulfurizing reagents, calcium carbide application, 4 549-550 Deswelling process, for solvent removal, 28104... 

Baking of hides involves degradation of some elastin and keratin, removal of hair residues and deswelling of collagen. A variety of proteases are used by... 

Figures 3 and 4 show the kinetics of water deswelling of the NA-100 and NA-95 gels. It can be seen that as little as 5% AAm has a significant effect on both the rate and extent of deswelling at temperatures between ca. 34 and 40 . This is the region where collapse of the poly(NIPAAm) gel would be occurring due to the 31-33 LCST of poly(NIPAAm). The LCST of a copolymer of 95% NIPAAm/5% AAm has an LCST around 34-38 , as estimated from Fig. 2. Relatively rapid collapse of this gel would be expected only above 40 , as seen in Fig. 4.

Figure 2. Deswelling ratios for NIPAAm-co-AAm hydrogels at different temperatures (24 hr. equilibration at each temperature).

Kwon and coworkers described solid polyelectrolyte complex systems which dissolve rapidly in response to small electric currents. The solid doses were based on poly(ethyl oxazoline) and poly(methacrylic acid) with a repeating unit stoichiometry of 1 1. Insulin was released in response to slight electric currents due to electrically induced polymer dissolution [380]. In similar work Kwon and coworkers [381] studied release of edrophonium chloride and hydrocortisone from poly(2-acrylamido-2-methylpropane sulfonate-co-n-butyl methacrylate). An on/oflf mechanism of the edrophonium chloride release was observed and was attributed to ion exchange of solute and hydroxonium ion. The cationic solute release was assisted by electrostatic forces, whereas release of the neutral hydrocortisone solute was only affected by swelling and deswelling. 

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