Entropy is disorder. Things in our universe like entropy, and tend to become more disordered. That's why keeping your room messy is easier than keeping it neat. This dog is named Entropy, which is appropriate because she runs around like free-range chicken with its head cut off whenever her human lets her in the house. Polymer molecules are the same way. The molecules in a piece of rubber, any kind of rubber, have no order to them. They just wind and tangle around each other in one jumbled mess. They're perfectly happy this way.
But now pull on the piece of rubber, and everything gets upset. The molecules are forced to line up in the direction in which the rubber is being pulled. When the molecules line up like this they become more ordered. If you stretch it far enough the chains will line up straight enough to crystallize. They don't like this. Remember, they like entropy (being disordered).
Now when you let go of this rubber sample you've been stretching, the molecules will quickly go back to their tangled and disordered state. They do this to return to a state of entropy. Remember, they like entropy. When this happens, the sample pops back to its original shape.
To help elastomers bounce back even better it helps to crosslink them. Crosslinking is the forming of covalent links between the different polymer chains, joining them all into a single networked molecule. That's right, most objects made of rubber contain only one molecule! When the polymer chains are joined together like this, it is even harder to pull them out of their original positions, and so it bounces back even better when stretched.
But this makes elastomers hard to recycle. Think about it. How does one melt down one molecule? To make recyclable elastomers we need to find a way to tie the molecules together when the rubber is being used, but one which would allow the chains to separate when being processed. The answer is called a thermoplastic elastomer.
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