Collapse, polymer gels

The 21 -shadowgram snapshots cover 4 h of experiment. In the figure, the swollen states appear brighter than the shrunken states, because the surface of the former is smooth while that of the latter is crumpled. A dark pulse of collapsed polymer gel is seen to propagate from top to bottom, down to a minimal position. The phenomenon repeats a few times. To guide the eyes, the locations of the denser polymer pulses are demarcated by red lines. Behind the dark pulse, the gel reswells... 

T Momii, T Nose. Concentration-dependent collapse of polymer gels in solution of incompatible polymers. Macromolecules 22 1384-1389, 1989. 

Similarly to the van der Waals fluid, polymer gels were found to have a volume-phase transition. In the case of gels, the gas and liquid phases correspond to the swollen and collapsed (shrunken) phases, respectively. The prototype of the free energy expression was given by [9, 10, 18]. 

We present a review of theoretical and experimental results on the swelling behavior and collapse transition in polymer gels obtained by our group at Moscow State University. The main attention is paid to polyelectrolyte networks where the most important factor is additional osmotic pressure created by mobile counter ions. The influence of other factors such as condensation of counter ions, external mechanical force, the mixed nature of low-molecular solvents, interaction of network chains with linear macromolecules and surfactants etc. is also taken into account Experimental results demonstrate a good correlation with theoretical analysis. 

The collapse of a polymer gel in response to a change in environment can be scaled down to the single chain level. A layer of polymer chains grafted to a surface forms a polymer brush, for which the collapse transition can be nicely observed using neutron reflectivity. A pore lined with a responsive polymer brush will form a selective valve for example if the grafted polymer is a weak polybase, in aqueous acidic conditions the brush will be charged and will expand to close the pore, while in basic conditions the brush will be neutral. This principle has been used to create a selective membrane, which shows greatly reduced permeability in acidic conditions. 

Later the theory proposed was successfully developed to describe the collapse of polymer gels in a solution of low-molecular salt and surfactant, in a mixture of two solvents, and in a solution of linear polymer [27,28,32,33], This theory describes the collapse of polymer gels under the action of external uniaxial extension and the collapse transition of polymer gels in nonpolar solvents [27,28,22],... 

Vasilevskaya VV, Khokhlov AR. Swelling and collapse of polymer gel in polymer solutions and melts. Macromolecules 1992 25 384-390. 

Khokhlov AR, Kramarenko EYu. Collapse of a polymer gel induced by complex formation with linear polymers. Makromol. Chem. Theory Simul. 1993 2 169-177. 

Silberberg-Bouhnik M, Ramon O, Ladyzhinski I, Mizrahi S (1995) Osmotic deswelling of weakly charged poly(acrylic acid) solutions and gels. J Polym Sci Polym Phys 33 2269-2279 Tanaka T (1978) Collapse of gels and the critical end point. Phys Rev Lett 40 820-823 Tanaka T (1981) Gels. Sci Am 244 110-123... 

D. Patterson in 1968 based on an analysis of Flory-Rehner theory. It took ten years for the phenomenon to be experimentally observed after prediction. It was found by T. Tanaka that, when a critical amount of an organic solvent was added to a water-swollen poly(acrylamide) gel, the gel collapses. Many gels of synthetic and natural polymers have been studied. Subsequent experiments showed that a volume phase transition (swelling/collapse) could also be brought about by changes in other environmental parameters such as pH, ionic strength, and temperature. 

Figure 11.38 Evolution of the diffusion coefficient along the penetration zone, and collapse of the diffusion coefficient at the polymer-gel interface. ...

For PIPAAm hydrogels, a large temperature increase originating below the polymer transition temperature of 32°C induces an outside-in shrinking response in the gel thermal transfer and polymer mass transfer kinetics compete for the determining of the polymer phase behavior. For this condition, the result is the formation of a dense shrunken collapsed polymer layer at the gel-water interface (a skin layer). These dehydrated polymer skin layers on the surface of shrunken PIPAAm hydrogels prevent even water molecules from... 

Synthetic polymer gels are known to exist in two phases, swollen and collapsed. Polyampholyte gels should show a re-entrant volume transition in response to pH [164]. Tanaka et al. [165] have reported the existence of more than two phases in randomly distributed polyampholytes of MAPTAC and AA. Let us consider the behaviour of a sample containing 65 mol% of AA and 35 mol% of MAPTAC. At neutral pH this sample has a diameter d=do that is referred to as phase 1 (d/do=l) according to the label in the original paper. At pH=8.5 the gel swells discontinuously to phase 2.7 (Fig. 38). If the pH is lowered from 8.5 to 7.0, the gel returns to phase 1 discontinuously. If instead the pH is increased further from 8.5 to 9.8 and goes back from this point the gel collapses into phase 1 at pH=6.4. The same experiments were done from the acidic side. NMR spectra... 

Studies carried out by Yoshida and coworkers have coupled this phenomena with oscillating chemical reactions (such as the Belousov-Zhabotinsky, BZ, reaction) to create conditions where pseudo non-equilibrium systems which maintain rhythmical oscillations can demonstrated, in both quiescent (4) and continuously stirred reactors (5). The ruthenium complex of the BZ reaction was introduced as a functional group into poly(N-isopropyl acrylamide), which is a temperature-sensitive polymer. The ruthenium group plays it s part in the BZ reaction, and the oxidation state of the catalyst changes the collapse temperature of the gel. The result is, at intermediate temperature, a gel whose shape oscillated (by a factor of 2 in volume) in a BZ reaction, providing an elegant demonstration of oscillation in a polymer gel. This system, however, is limited by the concentration of the catalyst which has to remain relatively small, and hence the volume change is small. 

The theory of polymer gel swelling and collapse accounting for the partial binding of counterions with co-ions on network chains was proposed by Khokhlov and Kramarenko. It was suggested that inside a gel, free counterions and ion pairs can coexist. The fraction of counterions that form ion pairs y/ depends strongly on the dielectric constant s of the gel medium which in turn is different for different gel conformations in the swollen state the value of s is close to that of the solvent 0 (for polar solvent, 1)/ while in the collapsed state it is close to that of the dry polymer si ( i is of order 1). A simplest linear dependence of the dielectric constant of the partially swollen gel on the volume fraction of polymer inside the gel O was adopted ... 

The fact that s depends on is central to this theory of polymer gel collapse. 

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