Chains of natural rubber are very long and have few if any cross-links, and so the material is a thermoplastic, becoming soft and sticky in the summer and hard and brittle in the winter. These disadvantages were overcome in 1839 by a discovery made accidentally by Charles Goodyear in Woburn, Massachusetts. The story goes that after many years of experimenting, he had spilt or accidentally placed a mixture of rubber, sulfur and lead oxide on a hot stove. The rubber was no longer sticky but had been converted to a tough, elastic substance stable to heat and cold. It also did not dissolve in the solvents that dissolved natural rubber. He had invented the process now known as vulcanization.
Vulcanization is a chemical reaction between sulfur and rubber resulting in cross-links being formed between the rubber polymer chains. Notice here:
that there are double bonds present in the polymer molecule. You should remember that double bonds provide a major route to the formation of polymers, so it should not be a surprise to find that these double bonds can serve to provide covalent links between the chains.
We can form covalent bonds between the polymer molecules, and if we do this the material will become much more rigid because the chains are no longer free to move apart. The more cross-links between chains, the more rigid the rubber until eventually the polymer is so cross-linked that it is no longer rubbery because there is no flexibility of the chains between the cross-links. Goodyear’s vulcanization process produces a controlled amount of cross-linking. The sulfur reacts with the double bonds and forms sulfur bridges as cross-links between the chains, resulting in a huge three-dimensional network as you can see here.
(a) Unvulcanized natural rubber molecules have few if any cross-links.
(b) Vulcanized rubber has a network structure with cross-links.
(c) Vulcanized rubber on stretching.
The covalent cross-links survive the stretching and help the molecules to spring back once the tension has been relaxed. This type of network molecular structure lies behind the explanation of why rubber is rubbery.
Many synthetic and natural fibres can be stretched, as we can see in the making of polymer fibres:
The main difference between rubber and other fibres is that rubber goes back to its previous shape and size. We can see that stretching a fibre aligns the polymer chains. In fibres, this alignment allows forces such as those of hydrogen bonding between chains to have an increased effect, and they will be strong enough to hold the fibres in their stretched, aligned position. In rubbery polymers (elastomers), we find there are large and bulky groups along the chains, and these prevent the chains from packing together so closely. As the chains are further apart, there are not the same forces between them to keep them in the uncoiled arrangement. This weak interaction between polymer molecules is not enough to keep the rubber in its stretched position so it reverts to the original coiled state.