Rubber

1. Natural rubber

Rubber is a natural, elastic polymer — an elastomer. In elastomers, the individual polymer chains are held together only by weak intermolecular forces, which is exactly what lets the material be stretched; a small number of crosslinks between chains then pull it back to its original shape once the stretching force is released.

Natural rubber is harvested from rubber latex, a colloidal dispersion of rubber in water tapped from rubber trees grown in India, Sri Lanka, Indonesia, Malaysia, and South America.

Chemically, natural rubber is a linear polymer of isoprene (2-methyl-1,3-butadiene, $H_2C{=}C(CH_3){-}CH{=}CH_2$), and because every double bond in the chain has a cis configuration, it is called cis-1,4-polyisoprene. This consistent cis geometry gives the chain a naturally coiled shape, held together by weak van der Waals forces between chains — and it is precisely this coiled structure that lets natural rubber stretch like a spring and snap back, i.e. behave elastically.

Vulcanisation

On its own, natural rubber has real weaknesses: it turns soft above 335 K, goes brittle below 283 K, absorbs water readily, dissolves in non-polar solvents, and has poor resistance to oxidising agents. Vulcanisation fixes this — raw rubber is heated with sulphur and a suitable additive at 373–415 K. The sulphur atoms form crosslinks at the reactive C=C sites along the chains, stiffening and toughening the rubber. For tyre rubber specifically, about 5% sulphur is used as the crosslinking agent.

2. Synthetic rubbers

A synthetic rubber is any vulcanisable, rubber-like polymer that can be stretched to roughly twice its length and springs back once released. Synthetic rubbers are either homopolymers of 1,3-butadiene derivatives, or copolymers of 1,3-butadiene (or a derivative) with another unsaturated monomer.

• Neoprene (polychloroprene) — made by free-radical polymerisation of chloroprene (2-chloro-1,3-butadiene):

It resists vegetable and mineral oils well, and is used for conveyor belts, gaskets, and hoses. …