Gel deswelling

Figure 38 DIG microscopy images (top) of microgels on a glass substrate. The microgel acts as a microlens by projecting a cross pattern (bottom). The quality of the image improves with the increase in temperature and concomitant gel deswelling. Reprinted with permission from Kim, J. Serpe, M. J. Lyon, L. A. J. Am. Chem. Soc. 2004, 126, 9512-9513.

Poly-N-isopropylacrylamide (NIPAM) solution in water precipitates above 40 °C, so NIP AM gels have provided the most fruitful example of a thermally driven phase change. A swollen cross-linked NIPAM gel deswells at the same temperature, 40 °C. Hompolymer gels show a discontinuous phase transition while copolymer gels with ionizable groups such as acrylic acid can show a continuous transition or critical point behavior. Having resolved the thermodynamics of the system, the kinetics of the phase change become important. 

Gel deswelling method Length of a polyion Poly(acrylic acid)... 

Tables 3 6 give a survey of literature data for the vapor-liquid equilibrium of aqueous solutions of a single polyelectrolyte with various counterions. Abbreviations (shown in Table 2) are used to characterize the polyelectrolyte and the experimental procedures (MO membrane osmometry DMO differential membrane osmometry VO vapor pressure osmometry ISO isopiestic experiments EMF electromotive force measurements including also measurements with ion-selective electrodes as well as titration FPD freezing point depression GDM gel deswelling investigations). Table 3 gives a survey for aqueous solutions of poly(styrene sulfonic acid).

Figure 29 Distribution of water in collapsed PNlPAAm gel, deswelling induced by methanol. Fourier images (slice selection, T2 weighted) of a vertical plane of PNlPAAm gels, (a) Homogeneous gel and (b) sponge-like gel after 3 days in D2O/CH3OD. Reproduced from Arndt, K.-F. ...

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. 

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.)...

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.

The transition from the expanded state to the collapsed one and vice versa is controlled by diffusion of the solvent in the gel [56, 57], It was found [56] that the kinetics of swelling and deswelling of the gel is determined by local motions controlled by the diffusion equation in which the diffusion coefficient is given by the ratio of the bulk modulus to the frictional factor (between network and liquid). Whereas in our samples with a volume 1 cm3, the transition from one to another equilibrium state takes several days, for submicron spheres this time... 

---