Friday, October 10, 2008

A polymer that has is all backwards

Poly(N-isopropylacrylamide), often abreviated as PNIPAM or similarly, is a water soluble polymer that doesn't act the way most polymers do. As backed up by your everyday experiences, solubility generally increases as the temperature increases. That is not so with PNIPAM. It is readily soluble at room temperature, but increase the mix to about 33 oC will cause the polymer to settle out. This is called a lower solution critical temperature (LCST). I suppose that having a name for this implies that it is something that is not so uncommon, or at least something that could be theorized. And yes, upper solution critical temperatures exist. They are very common, as that is the lowest temperature at which a material is soluble in a solvent.

Polymer solutions are not very useful in that state. You can remove the solvent (in this case water) and work with the dried polymer, but to do so would leave you without that neat LCST. So the way to have your polymer solution and play with it too is to crosslink the polymer. When this is done, the PNIPAM soaks up water like a sponge. Or at least until the temperature is raised. Once it hits the LCST, the water is expelled.

This is more a lab curiosity than anything important.

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