There is one thing that all rubbers, natural and synthetic, have in common – they are all stretchy. In essence that’s what makes rubber, rubber!
But what exactly makes it stretchy. The answer to this is entropy.
Entropy is a state of 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 disorder. We have all seen this in everyday life. For example a room is easy to make messy but hard to make clean again. It is easy to crash a new car but hard to repair it afterwards.
Entropy is often inconvenient but it is also the thing that makes rubber stretchy. Remember that rubber molecules are polymers and are shaped like very long chains. When a piece of rubber is just sitting there without any strain, the molecules are just tangled up in a random mess, similar to the diagram below:
When molecules are like this we say they have a high degree of entropy. However, when the rubber is stretched, the chains become aligned in one direction, like the diagram below:
When the chains are aligned they are in a state of order. They don’t have as much entropy as they did before the rubber was stretched. Once you let go of the rubber the chains go back to their relaxed state of high entropy and disorder. This is what makes rubber go back to its original shape and size.
You can observe this happening yourself by taking an rubber band with your two hands and stretch it. While it is stretched hold it to your face and you should feel the band become hot. This is because the chains can line up into extremely ordered arrangements called crystals. This is how the rubber molecules are arranged in a crystal:
When molecules form crystals they give off heat hence the reason the rubber band gets hot when stretched. When you let go of the rubber band the polymer molecules break out of the crystals. Whenever molecules break out of crystals they absorb heat hence the reason the rubber band now goes cold.
There is something else that makes rubber stretchy and that is called cross-linking. Most rubber objects are made of some kind of cross-linked rubber. Cross-linking is a way of chemically joining all the polymer chains of a piece of rubber into one giant molecule. Take a look at the picture below and you can see the difference between a polymer that is cross-linked and one that isn’t.
In a piece of cross-linked rubber, the cross-links (shown in red) tie the polymer chains into one specific shape. This means the rubber will hold its shape better. Without cross-links, the rubber might deform after being stretched over and over again.
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