Deswelling ratios

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

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... 

One important feature of randomly crosslinked chains is that they can be highly strained and/or swollen they recover their initial shape when the stress is interrupted and/or after deswelling. Neither the dependence of the modulus of elasticity, E, upon the variable e nor that of the swelling ratio, Qm, have been exactly predicted, until now. Nevertheless, these two physical quantities as well as the rate %c are functions of the variable, e. 

FIGURE 15.3 Model fitting of the hydrocortisone permeation through a swelling/deswelling copolymer (DMAEMA and AAm (molar ratio 57% DMAEMA, 33% AAm)) gel undergoing step temperature variations. The permeated amount is plotted as a function of time t. The dashed line indicates the temperature profile (From Grassi, M., Yuk, S.H., and Cho, S.H., J. Membr. Sci., 152, 241, 1999. With permission.)... 

It is true that pH and NaCl concentration influence the ionization state of polyions. If we assume that these factors affect the state of ionization of the network in a polyelectrolyte gel and thereby its swelling degree is altered, the ionization state of a PEI gel in a 0.1 M NaCl solution and that in a salt-free solution at pH 10.7 seem to be almost the same, because there is little difference between the d/d ) ratios determined in both solutions. However, a large difference was observed in the deswelling of the gel when increasing the pH and NaCl concentration. At present, to our knowledge, there is no theory that fully accounts for this difference on the basis of a mathematical model. 

If the elastomer is swollen to below the equilibrium swelling so that no deswelling will occur upon deformation, the statistical expression for the shear modulus [equation (6-59)] can be readily modified. We define Vr as the ratio of the unswollen volume to the swollen one, which is identical to the volume fraction of polymer in the mixture. The number of network chains per unit volume then becomes NQVr and the mean square end-to-end distance of the network chain is now r02 / Vr2/3. Equation (6-59) then reads... 

Fig. 19 Thermally induced collapse of poly(A, Af-diethylacrylamide) hydrogel (curve 1) and cryogel samples (curve 2) upon increasing the temperature from room temperatiffe to 60 °C. The variation in the volume of the gel samples is shown as a function of the deswelling time. Synthesis conditions initial monomer concentration 5.66 wt% molar ratio of vinyl to divinyl monomers 200 1 gelation temperature +20 °C (curve 1) and — 10 C (curve 2). (From [172] with permission from Springer)...

The effect of c on the swelling and the deswelling was measured by microscopic observation, and the results were shown as the ratio of the gel volume at time t, K to the volume before the swelling, Vq that is, as specific volume, VJVq, on the left and right sides of Fig. 6. The maximum specific volume, F ax/Fo, of the swelling in water is 284.7 80.7 and this value is greater than that of the maximum specific weight, 221.5 + 10.1,... 

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