What Makes Rubber Stretch and Snap Back?

College Students: We are a group of students from college arguing among ourselves as to why rubber stretches and snaps back. We would love to hear your explanation in the simplest terms.

John Woon (Senior Latex Consultant): Sometimes it is difficult to explain how things behave in simple ways. But I'd try. There are more than one ways to explain this. Here is one:

All rubbers or rather elastomers, either natural or man-made (i.e.synthetic) have something in common - that is the ability to stretch (or elongate) and the ability to recover after the force in stretching is removed. The reason why they do so relates to the term "entropy". This is the Second Law of Thermodynamics (part of the study of physics) which states that a system would move from a state of order to a state of disorder. Entropy is "disorder". Rubbers are long chain polymer molecules and when you leave them alone, the molecules tends to tangle up in a random disordered mass like a plate of noodles. We describe the molecules as having a high degree of entropy. When the rubber is stretched, the molecules align and pack themselves together forming crystals, hence the term, crystallization of rubber. In other words the rubber molecules are "ordered" and have low degree of entropy when stretched.

When you let go of the rubber, it snaps back to its original shape (or close to it) because the Second Law of Thermodynamics says the molecules must "move from a state of order to a state of disorder."

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