Gel collapse

The upper sketch portrays gel collapse, as evidenced by the abrupt increase in volume fraction of polymer in the gel as it shrinks when the temperature drops to a critical value [4]. This syneresis and reswelling can be exploited by harnessing the mechanical motion in a variety of devices. The lower sketch shows the corresponding situation as the pressure of a gas is increased to a critical value that causes condensation to the liquid state. 

There are parallels and differences in the case of the condensation of gases, as is illustrated by the p-V isotherm shown in the lower part of Fig. 1.34. Here, an increase in pressure causes a discontinuous decrease in volume to that of the liquid. Again, there are large changes in dimensions upon condensation of a gas to the much denser liquid phase, but the isotropic nature of the phases makes this much more limited with regard to possible mechanical applications. 

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Figure 5 A gel exuding solvent upon decrease in temperature, with the shrinkage ( syneresis ) generally described as gel collapse .

Saito and co-workers [31]. It is interesting to observe that gels swell at lower temperatures and collapse at higher temperatures. This temperature dependence, which is opposite to the transition induced by van der Waals interaction, is due to the hydrophobic interaction of the polymer network and water. At higher temperatures the polymer network shrinks and becomes more ordered, but the water molecules excluded from the polymer network become less ordered. As a whole, the gel collapse amounts to a higher entropy of the entire gel system, as should be. Detailed theory and experiments have been carried out in the literature [26-28]... 

In addition to the experimental investigations, the phenomenon of the gel collapse was intensively studied theoretically. Tanaka and coworkers from Massachusetts Institute of Technology gave the first theoretical description of the collapse of charged networks in the absence of salt [7]. Further theoretical studies in this field were made by M. Ilavsky at the Institute of Macro-molecular Chemistry in Prague [18] and also at the Moscow State University [19, 20]. 

Obtaining the gel collapse in aqueous medium has been attempted by exchanging some of the counter ions with hydrophobic ions of a surfactant which are capable of forming micelles in solution. This was reported for the first time in Refs. [63-65], A theoretical analysis was developed in Refs. [38, 39],... 

It is well known that there are numerous factors that influence the gel collapse (charge density, topological structure of a network, medium composition, etc.). The same factors are effective in the case under study as well (see... 

The investigation of the collapse phenomenon have shown that the topological structure of the network plays an essential role in the process of gel collapse [42, 43]. In order to check the influence of the topology of a network on the equilibrium properties of the network-surfactant complexes, a set of experiments with gels differing in the number of crosslinks or in the conditions of synthesis have been performed. It has been shown that tie decrease of crosslink density or concentration of monomers in the polymerization mixture results in a sharper gel collapse. 

Consider the gel collapse in more detail. As hydrogen ions diffuse into the gel, they will rapidly react to form neutral carboxylic acid groups. Thus, a nonionic shell of collapsing gel will develop around a still-swollen ionized core. The diffusion of ions occurs freely in this nonionic shell, so that the collapse is limited by the Fickian diffusion of water out of the gel. We have confirmed this Fickian behavior by measuring the collapse of cylindrical gel samples of differing radii and ionic compositions [5]. The data for the fractional approach to equilibrium fall on a single curve against [Dt/R2]1/2, where D is the diffusion coefficient and R is the initial radius of the gel cylinder. 

When a gel collapses, it does not regain its original crystalline appearance groups of plates conglomerate, but the structure as a whole breaks up and does not decrease significantly in volume. The most reliable indication of collapse is the appearance of the metallic sheen of the crystal, but this too can be hard to spot, and so the errors are larger in Table 5.5c. This method, however, was the only one for which more than one sample under each set of r,c conditions was used, and so the errors in Table 5.5c do contain an indication of the level of sample-to-sample variation. In any case, this is the most likely source of variation in the results, rather than experimental error,... 

FIGURE 10.5 Schematic illustration of the swelling of n-butylammonium vermiculite in a 0.1 M n-butylammonium chloride solution (a) represents the n-butylammonium vermiculite crystal id =2 nm) prior to swelling, (b) the gel id =12 nm) formed by a homogeneous sixfold expansion in the range 0°C < T < 14°C, and (c) the tactoid formed when the gel collapses at T < 0°C or T > 14°C. In (c), the dashed line represents the fact that the tactoid structure occupies roughly the same volume as the gel structure. 

High release rate due to formation of voids at temperatures higher than the lower gel collapsing temperature... 

Fig. 6 Schematic diagram of the structure of the particle for positively thermosensitive release of drug utilizing gel-collapsing behavior of poly(ACisopropylacrylamide).

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