| MadSci Network: Chemistry |
These links provide some basic information on rubber; a search on polyisoprene rubber will return many more sites.
Rubber information: http://en.wikipedia.org/wiki/Natural_rubber
Types of rubber and examples of molecular structures: http://www.xamplified.com/rubber/
Natural rubber is cis-polyisoprene with lesser amounts of many other compounds from the rubber tree latex. The features of this polymer include coiling of the polymer chain that allow it to be stretched, a low crystallinity that maks it soft, and one double bond per repeating unit that makes it susceptable to oxidation by oxygen and ozone. To improve these properties, many additives are added to natural rubber, and many synthetic rubbers have been invented. Generally, rubber formulations are somewhat complicated and are proprietary. There are books on the subject and the above two links give a decent amount of information.
Reaction of the rubber polymer, either at the double bond or at the allylic hydrogens, with ozone or oxygen or other oxidizing agents results in breaking the polymer chain. This weakens the bulk material and with time destroys the material. This weakening is noticeable in rubber articles such as rubberbands that have been around a while and have become brittle and cracked and have lost their ability to stretch. Most formulations contain antioxidants to slow these processes.
Compressed and stretched rubbers store and lose energy from stretching the coiled polymers, forcing the polymer strands together, and the slow formation of crystal domains in the stressed material. This is also described in the links above. This is noticeable in a rubber band that has been stretched for a time in use and when removed seems to have lost its stretchiness and recovers it after a while. This behavior is usually reversible, and while it affects the usefulness of the rubber is not chemical degradation.
Try the links in the MadSci Library for more information on Chemistry.