Question:
What happens when you heat a rubber band that is being stretched by having it hold up a weight?
Does it become:
You will be given about 15 minutes to discuss the problem within your group. A demonstration experiment will than be attempted. You will have the rest of the period to continue your discussion. You may hand in written analysis either at the end of the session, or at the start of the Thursday session at which the problem will be discussed. Be prepared to present your ideas! You should consider whether any observed effect is the result of entropic or enthalpic effects.
Considerations and hints:
Rubber is poly-isoprene, a branched-chain polyethylene. It might be simplest to ignore the chain branching, and consider only the polyethylene polymer chains.
Would it help to think about what might happen to the polymer chains when the rubber band is stretched?
Recall that, in the simple RIS (``rotatomer isomeric states'') model, butane can exist as either a trans, t, or one of two gauche, g+ or g-, conformers. The g+ and g- states have an energy that is 2000 J/mol higher that of the trans state. This leads to a 3-dimensional picture. For the purposes of argument, would it help to make a simpler, one-dimensional model?