Polyimides

by Gregory Brust
Polyimides are a very interesting group of incredibly strong and astoundingly heat and chemical resistant polymers. Their strength and heat and chemical resistance are so great that these materials often replace glass and metals, such as steel, in many demanding industrial applications. Polyimides are even used in many everyday applications. Who knows, you may even have a few in your house. They are used for the struts and chassis in some cars as well as some parts under-the-hood because they can withstand the intense heat and corrosive lubricants, fuels, and coolants cars require. Do you have polyimides in your kitchen? They are also used in the construction of many appliances as well as microwave cookware and food packaging because of their thermal stability, resistance to oils, greases, and fats (YUM!), and their transparency to microwave radiation. They can also be used in circuit boards, insulation, fibers for protective clothing, composites, and adhesives.

By now you have probably guessed that a polyimide is a polymer that contains an imide group.

Yeah, I figured that out, but what is an imide?

I just knew you were going to ask that. An imide is a group in a molecule that has a general structure (drawn in red) which looks like this:

So if the molecule shown above were to be polymerized the product would be, you guessed it, a polyimide. Polyimides usually take one of two forms. The first of these is a linear structure where the atoms of the imide group are part of a linear chain. The second of these structures is a heterocyclic structure where the imide group is part of a cyclic unit in the polymer chain. Take a look.

Aromatic heterocyclic polyimides, like the one on the left, are typical of most commercial polyimides, such as Ultem from G.E. and DuPont's Kapton, and are therefore the polyimides we will be spending most of our time on. These polymers have such incredible mechanical and thermal properties that they are used in place of metals and glass in many high performance applications in the electronics, automotive, and even the aerospace industries. These properties come from strong intermolecular forces between the polymer chains.

A polymer which contains a charge transfer complex consists of two different types of monomers, a donor and an acceptor. The donor is like a rich man with more money than he knows what to do with. It has plenty of electrons to go around because of its nitrogen groups. The acceptor, then, is like a mooching houseguest. Its carbonyl groups, like our houseguest's many vices of gambling, drinking and such, sucks away its electron density. The donor doesn't mind supporting the acceptor, in fact, with the acceptor around, the donor looks better. Charity looks good in some social circles (and it's good for a tax break too!). So the donor lends some of its electrons to the acceptor, holding them tightly together.

The charge transfer complex works not only between adjacent units in the polymer chain, but also between chains. The chains will stack together like strips of paper, with donors and acceptors paired up.

This charge transfer complex holds the chains together very tightly, not allowing them to move around very much. When things can't move around on the molecular level, they can't move around in the whole material. This is why polyimides are so strong.

The charge transfer complex is so strong, it sometimes becomes necessary to make the polymer a little softer so it is able to be processed. This is accomplished with the bisphenol-A derived linkage shown in the polymer segment below.

Another interesting property of polyimides which makes them excellent for use in construction and transportation industries is they burn.

Why would you want to build a building or a car out of something that burns?

Good question, and I can answer it too. It is not the burning which industry likes, but polyimide's self extinguishing property which builders like. Self extinguishing? Yes, when an aromatic polyimide catches on fire, which by the way is difficult to begin with, a surface char develops which smothers the flame, blocking it from the fuel to burn. Then you just wipe it off, and it's just like the fire never happened. Neat, huh?

Here are some other polymers used as thermosets:

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