There's one thing that all rubber, natural and synthetic, has in common. All rubber is stretchy. That's what makes it rubber, now isn't it? Just what makes rubber stretchy? The answer has to with entropy. What is entropy? Entropy is disorder. There is an important law of physics, called the Second Law of Thermodynamics which says that a system will move from a state of order to a state of disorder. We've all seen this in everyday life. A room is easy to make messy, but hard to make clean. It's easy to crash a new car, but hard to repair it afterward.
This entropy business is often inconvenient, but entropy is also what makes rubber work. Think about a piece of rubber. Remember that rubber molecules are polymers, that is they are shaped like very long chains. When the piece of rubber is just sitting there, the molecules are just tangled up in a random mess, like this:
When the molecules are like this we say they have a high degree of entropy. But when the rubber is stretched, the chains become aligned in one direction, like this:
When the chains are aligned, they are ordered. That is, they don't have as much entropy as they did before the rubber was stretched. As soon as one lets go of the stretched rubber, those chains are going to try to become disordered again. This means the return to their original tangled mess. This makes the piece of rubber snap back to its original size and shape.
You can observe this happening. That's right you really can. 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:
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.
In a piece of crosslinked rubber, the crosslinks (shown in red) tie the polymer chains into one specific shape. This means the rubber will hold its shape better. Without crosslinks, the rubber might deform after being stretched over and over again.