|MadSci Network: Physics|
Jennifer, We tried stretching [by hand] various types of rubber bands and found that room T, freezer chilled, and hot water heated bands stretched almost the same amount and that the differences were more dependent on the person stretching the bands than on the temperature. This is a very hard experiment to quantify and do reproducibly. Also if you stretch a rubber band and hold it against your cheek you will notice it got warm. This means that it is difficult to do the experiment at a constant temperature [isothermally is the big word to describe such conditions]. If you stretch a rubber band quickly, thereby increasing its energy content and causing changes in molecular orientations, any heat produced does not equilibrate with the surroundings[this is called an adiabatic process] and the temperature changes. Therefore to study the stretching one would have to use a device which measures force as a function of displacement [The Instron Corp. makes such machines] The process has to be carried out at a slow enough speed to maintain temperature equilibrium with the surroundings. I can't answer your question without quantitative reproducible data. Also the chemical handbooks show rubbers to have a positive coeffecient of expansion, you heat them they expand. Most rubber materials contain oil- like plasticizers which can leach out at higher temperatures causing an apparent shrinkage and certain materials will crosslink[vulcanize] on heating again causing contraction. Natural rubber softens on heating and probably does stretch more and recover less at a higher temperature. I don't think many things are made of totally uncrosslinked natural rubber tho. Again this can get complicated and the material has to be studied carefully. Mechanical properties of rubber materials are notoriously difficult to measure accurately. A given material usually has a range of acceptable values rather than a definite value of its various properties. You picked interesting but difficult material to study.
Try the links in the MadSci Library for more information on Physics.