By now you have probably guessed that a polyimide is a polymer that
contains an imide group.
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.
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
Another interesting property of polyimides which makes
them excellent for use in construction and transportation
industries is they burn.
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:
Polymer Science Learning Center
Department of Polymer Science
The University of Southern Mississippi