Cross-link (sulfur bridge)

A cross-link, also called a sulfur bridge, is a bond of sulfur atoms formed between two neighbouring rubber chains during vulcanisation. It ties the chains together so they can no longer slide freely past one another. Malay: ikatan silang (jambatan sulfur) · Chinese: 交联（硫桥）.

In unvulcanised rubber the long poly(isoprene) chains are held to one another only by weak forces, so they slip and the rubber deforms. When rubber is heated with sulfur, sulfur atoms attach at the carbon–carbon double bonds on two different chains and form a bridge between them. Each cross-link acts like a rung joining two chains, and many cross-links turn a set of loose chains into a connected three-dimensional network. This is why the number of cross-links controls how hard and how elastic the rubber becomes: a few cross-links give a springy elastic band, while many give hard tyre rubber.

Example. In vulcanised rubber, sulfur cross-links join the chains at intervals. When the rubber is stretched the chains uncoil, but the cross-links pull them back, so the rubber returns to its original shape instead of staying stretched.

Confusion to avoid. Do not confuse the cross-link with the polymer chain itself. The chain is the long backbone of carbon atoms; the cross-link is a short sulfur bridge running sideways between two chains. Cross-links are not present in ordinary addition polymers like polythene, which is why polythene cannot be vulcanised.

In a diagram, cross-links are drawn as short lines of sulfur atoms linking two long chains, and a common exam task is to add them to a sketch of rubber chains or to explain how they raise the elasticity limit. Remember that too many cross-links make the rubber hard and brittle rather than springy, so the amount of sulfur is chosen to suit the use.

Cross-links are the reason vulcanisation works, so understanding them explains the whole improvement in rubber and answers the “why” questions on rubber in SPM Chemistry.