The term elastomer is often used interchangeably with the term rubber, and is preferred when referring to vulcanisates. They are amorphous polymers existing above their glass transition temperature, so that considerable segmental motion is possible. At ambient temperatures rubbers are thus relatively soft (E~3MPa) and deformable. Their primary uses are for seals, adhesives and molded flexible parts.
Elastomers are usually thermosets (requiring vulcanization) but may also be thermoplastic. The long polymer chains cross-link during curing and account for the flexible nature of the material. The molecular structure of elastomers can be imagined as a 'spaghetti and meatball' structure, with the meatballs signifying cross-links.
Elastomeric behaviour can be explained further by thinking about entropy. Entropy is fundamentally a measure of disorder. In all natural processes, the entropy of the universe increases. Consequently, gasses difuse, heat disapates, and in this case, molecular structures become disorganised. When an elastomer is stretched or pulled, these disorganised chains of molecules straighten up. This is an unnatural condition, and so when the pull/stretching is stopped, the entropy increases as the material returns to its original state.
Examples of elastomers;