Ionomers

Keywords
ion


An ionomer, as one might guess from the name, is an ion containing polymer. (An ion, you might recall, is an atom that has an electric charge, either positive [+] or negative [-].) But an ionomer is more than just a polymer with ionic groups. We call any old polymer with ionic groups a polyelectrolyte. But an ionomer is a special kind of polyelectrolyte. First of all, they are copolymer. They contain both nonionic repeat units, and a small amount of ion containing repeat units. How small? The ionic groups make up less than 15% of the polymer.

Wonderful. But you're talking abstractly.
Show me a real life ionomer, why don't you?

If you insist.

One example of an ionomer is poly(ethylene-co-methacrylic acid). This polymer is a sodium or zinc salt (which provides the ions) of copolymers derived from ethylene and methacrylic acid.




The model above is an image of the pdb model of the copolymer.
You can view the 3D image by clicking here or you can just click on the image itself.
Either way, be sure to close the new window that opens up with the 3D model
when you are ready to come back here.


The ionic attractions that result strongly influence the polymer properties. Look at the image above, or better yet, go to the 3D version and move that model around. Zoom in and rotate it so you can see how the two ionic groups ended up near each other after the energy minimization that was done on this short segment of the actual polymer. Also note that the polyethylene segments adopt a linear structure and come near each other, just like in pure PE crystallites. Models certainly help see things better, don't they?

The ionomer above is used as the cover of golf balls. Imagine being hit over and over again with a hammer shaped just like a golf club, and not tearing or breaking (unless, of course, the duffer makes a bad slice and cuts the cover open). Let's take a look at how ionomers work to have such fantastic properties.

In an ionomer, the nonpolar chains are grouped together and the polar ionic groups are attracted to each other. The ionic groups would like to go off into a little corner by themselves, but since they are attached to the polymer chain, they cannot. This allows thermoplastic ionomers to act in ways similar to that of crosslinked polymers or block copolymers. Take a look.

However, ionomers are not crosslinked polymers, and are in fact a type of thermoplastic called a reversible crosslinker. When heated, the ionic groups will lose their attractions for each other and the chains will move around freely. As the temperature increases, the chains move around faster and faster and the groups cannot stay in their clusters. This allows for a polymer with the properties of an elastomer and the processability of a thermoplastic. These ionomers are sometimes known as thermoplastic elastomers. Imagine that!

So far, we've only talked about what are called random ionomers. A random ionomer is one where the ionic groups are attached to the backbone chain at random intervals. However, there are a few other types of ionomers.

Another use for ionomers is that of a semi-permeable membrane. A semi-permeable membrane is a very thin piece of material lets some materials pass through while others remain inside. The membranes made with ionomers are called specifically ion-selective membranes. These ion-selective membranes work by letting water pass through and not the metal ions.

That's great,
but what's it got to do with me?

I'll tell you. Do you like to drink clean water? Sure, we all do. These ion-selective membranes reduce the levels of metal ions, like lead which none of us want to drink, in aqueous waste to limits even below those established by the EPA (Environmental Protection Agency). Polymers working to make the world a cleaner, better place. How 'bout that. These membranes can also be used to recover valuable metals from dilute solutions, remove zinc from textile wastes, and soften brackish water.

One specific ion-selective membrane is a perfluorosulfonate ionomer which DuPont calls Nafion. All perfluorosulfonate ionomers have outstanding chemical and thermal stability and the ability to absorb incredible amounts of water. Nafion membranes can be made into films or tubes and can be used in many caustic and dangerous processes. Some such processes are the production of chlorine, regeneration of spent acids, separations in chemical processing, as well as uses in fuel cells and electrodialysis.

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