The Collapse of Polymer Networks

Suppose we have a piece of polymer network, swollen because it is in a good solvent. Let s look at one of the subchains (that is, a part of a chain between two adjacent cross-links, see Section 7.5). Naturally, it tends to take the shape of a loose polymer coil typical of a good solvent. Now, say the solvent becomes worse. The subchains will shrink, which leads to the shrinking of the whole network. If the temperature drops below the 6 point, each of the subchains will undergo a coil-globule transition. As a result, the entire network will rapidly collapse. Unlike single molecule case, the gel collapse is easy to observe by the naked eye, as illustrated by the cartoon 

the counterions move freely inside the network, but are not allowed outside. You could say that the shell of the network (i.e., its outside surface) stops them. Evidently then, a crowd of counterions exert some pressure on the shell . This pressure favors stretching the network in all directions. We are going to show now that this is exactly what makes the collapse so different from what you might expect. 

The free energy of the whole network is the sum of the free energies of all the individual subchains. The free energy of each subchain, in its turn, consists of entropy and energy terms, Ues a) and U a) (Equation 

The collapse of polymer networks has recently attracted a lot of attention. This boom is partially due to some important applications, which all stem from the fact that you need only slightly change the quality of the solvent to make the network collapse rapidly. It is especially useful that the collapse is very sensitive to the presence of charged monomers and counterions in the solution. Thus collapsing networks can be adapt to detect small ion impurities in a solution, as well as to clear the impurities away. Besides all this, the collapse of networks can also serve as a good model for some other processes in biology (e.g., in the vitreous body in the eye). 

Tanaka s group at MIT have assembled a sort of a large collection  

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This is exactly the phenomenon called the collapse of polymer networks. It was discovered by T. Tanaka (1946-2000) and his colleagues at the Massachusetts Institute of Technology (MIT) in 1978. They used networks of polyacrylamide diluted in a mixture of acetone and water. In these experiments, the temperature was not varied. To make the solvent worse, they just poured some extra acetone into the solution. (This worked because acetone, in contrast to water, is a bad solvent for polyacrylamide.) Figure C9.7 gives an idea of what was found. It sketches how the size of the network depends on the acetone concentration. You can see that if you dump 42% of acetone, the network collapses suddenly. Its volume drops by a factor of nearly 20. 

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