Elasticity

Elasticity is the ability of a material to return to its original shape after a stretching force is removed. In the Polymers chapter it describes rubber, and vulcanised rubber has greater elasticity than unvulcanised rubber. Malay: kekenyalan · Chinese: 弹性.

Rubber is elastic because its long poly(isoprene) chains are coiled. When you pull rubber, the coiled chains straighten out; when you let go, they coil up again and the rubber springs back. In unvulcanised rubber the chains can also slide past one another, so some of the stretch is permanent and the elasticity is limited. Vulcanisation adds sulfur cross-links that stop the chains from sliding, so they can only uncoil and recoil. This is why vulcanised rubber returns to shape more completely and can be stretched further before it is permanently deformed.

Example. A rubber band is elastic: stretch it and it lengthens, release it and it returns to its first length. A vulcanised rubber tyre shows the same property under a car’s weight, springing back into shape thousands of times without going slack.

Confusion to avoid. Do not confuse elasticity with plasticity. An elastic material returns to its original shape, but a plastic material keeps the new shape it is forced into. Rubber is elastic; softened polythene is plastic. Take care too not to treat “elastic” as meaning simply “stretchy”, the key is the return to the original shape.

A common exam question is to explain, in terms of chains and cross-links, why vulcanised rubber is more elastic than unvulcanised rubber: the sulfur cross-links stop the chains slipping, so the rubber recovers its shape fully instead of staying stretched. Answering with the chains-and-cross-links reason, not just the word “better”, is what earns the mark.

Elasticity is the property vulcanisation is designed to improve, so it links directly to cross-links and the elasticity limit and appears whenever rubber is tested in SPM Chemistry.