Rubber

Rubber belongs to the class of substances termed ‘polymers’: high molecular weight compounds, predominantly organic, consisting of  long –chain molecules made up repeating units usually on a back bone of carbon atoms. These high molecular weight polymers have a lower temperature limit to their rubbery state.  At the so called glass transition temperature Tg, there is a fairly abrupt change to a glassy state. Materials in the class of polymers which are, at normal temperatures, plastics, become rubber like as the temperature is raised above their Tg.

In rubbery state, polymers behave in many ways like viscous liquids, because the links in the long chain are freely rotating and enable flow and distortion of material to occur under stress. Because of the chain length and the presence of side groups on the chain, their molecular freedom is restricted and they show both time-dependent viscous and elastic properties, and are said to be viscoelastic.

Mention must be made of the phenomenon of crystallisation, which is more complex in rubbers than in ordinary low molecular weight substances   crystallisation of rubbers takes place by local rearrangement of portions of molecules to form crystallites.