Ionomers

Keywords
electrolyte, ion, random coil.


No, a polyelectrolyte is not something you lose when you sweat, nor do you replenish them by drinking a neon-colored sports drink. Those are just plain old electrolytes. But they are related to polyelectrolytes. Here's how: Electrolytes are compounds that like to fall apart. Put them in water, and they just break apart into positive and negative ions. Want an example? Good ol' table salt, NaCl, is an electrolyte. Put it in water and it splits into positive sodium ions and negative chloride ions.

Polyelectrolytes are polymers that do the same thing. They fall apart in water. Want to see one? How about polyacrylic acid? Put this stuff in water and the acid hydrogens split off with the water molecules and form H3O+ ions, like this:

And of course, the polymer is left with a bunch of negatively charged groups hanging from it in return.

So what?

Here's what. All those negative charges are going to repel each other. This does weird things to the polymer chain. Normal uncharged polymer molecules, when they're in solution, tend to be tangled up in what bigshot scientists call a random coil.

But when the polymer chain is covered with negative charges (which repel each other), the polymer can't be bunched in on itself. So the chain stretches out, like this:

This makes the solution (remember we're talking about polyelectrolytes in solution) more viscous. Think about it. When the polyelectrolyte chain stretches out it takes up more space, and is more effective at resisting the flow of the solvent molecules around it. So the solution becomes thick and syrupy.

But there's a way you can stop this from happening. If one takes a solution of a polyelectrolyte in water, and throws in a lot of salt, something fun will happen. The NaCl will separate into Na+ and Cl- ions. In the case of a negatively charged polyelectrolyte like poly(acrylic acid), the positively charged Na+ ions will get in-between the negative charges on the polymer, and cancel them out in effect. When this happens, the polymer chain collapses back into random coil again.

What? You don't believe me? Remember I said this was something you could do. And now I'm going to tell you how. Take some hair gel and put a big glob of it in a bowl. Now take a salt shaker, and pour on the salt like you were trying send your grandmother's blood pressure through the roof. When you do this, the gel will collapse into a pretty boring ordinary liquid. This is how dramatic the change in viscosity is when the polymer chains collapse!

What Are These Things Good For?

Hair gel, first of all. Remember, we were just talking about that. Another thing is baby diapers (and adult diapers, too, come to think of it). Our good friend poly(acrylic acid) has an unusual property in that it can absorb many times its own weight in water, or other liquids likely to be found around and in diapers. And that all has to do with the fact that these are "Polyelectrolytes!"

The Best of Both Worlds

Sometimes electrolyte monomers are copolymerized with nonionic monomers. Just a little bit of electrolytic monomer (less than 15%) can change a polymer a lot. We call these copolymers ionomers. Want to know more? Then visit the Ionomer Page.

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