Notes

One way to do this is the take a rubber band with your two hands and stretch it. While its stretched, hold it to your face. It feels hot doesn't it? Now release the tension on the rubber band so it can snap back to its original size. Now hold it to your face. How does it feel now? It should feel cold.

There's a reason for all this. When you pull on the rubber band, the polymer chains become aligned, remember? When the chains align, something can happen. The chains can line up and pack together into extremely ordered arrangements called crystals. This is how the rubber molecules are arranged in a crystal:


rubber molecules in a crystal
When molecules, not just rubber molecules, but any molecules, form crystals, they give off heat. This is why the rubber band feels hot when its stretched. When you let go of the rubber band, the polymer molecules break out of those crystals. Whenever molecules break out of crystals, they absorb heat. That's why the rubber band feels cold when you let go of it.

Crosslinking

There's something else that makes rubber stretchy. That is crosslinking. Most rubber objects are made of some kind of crosslinked rubber.