|MadSci Network: Chemistry|
Hello and sorry for the delay in answering your question.
KEVLAR® is a very interesting polymer that is a member of the nylon family. It is typically referred to as an aramid (another aramid is NOMEX® which is used in fire-proof clothing). KEVLAR® is made from para-phenylenediamine and terephthaloyl chloride resulting in poly-paraphenylene terephthalamide.
To see the structure of this polymer, check out http://en.wikipedia.org/wiki/Kevlar (a very useful polymer web page that is extremely helpful and easy to use).
KEVLAR® is a very inflexible polymer due to its packing (that is, how the individual polymer chains stack; think of stacked firewood-if the logs are straight with no branches, you can stack it much nicer. If there are branches, nice packing is prevented or at least hindered). KEVLAR® stacks very nicely, so nicely in fact that it is crystalline (chains are highly oriented with respect to each other, that is they are aligned with each other-think of the firewood). This crystallinity makes it very strong, especially when lots of polymer chains are stacked together as happens in a fiber. These fibers are used to make bullet proof vests. In addition to this high orientation, there are also very strong intermolecular interactions among the chains, which works in KEVLAR®'s favor. In other words the chains are attracted to each other and it thus takes a lot of energy to pull them apart. The combination of the high orientation (crystallinity) and intermolecular bonding give KEVLAR® its unique properties, such as being bullet proof. The chains like each other so much, and are stacked together so nicely, not even a bullet can penetrate a material made from KEVLAR®. However, it is interesting to note that a knife or other sharp object may penetrate a bullet proof vest. Can you figure out why?
Excellent question and I hope you continue to ask questions. I highly recommend checking out the web page (address given above) to learn more about polymers.
Try the links in the MadSci Library for more information on Chemistry.