弹性体Elastomer

柔性长链在松弛状态下处于无规卷曲、构象熵最大；拉伸使链伸直、熵减小，产生恢复力。交联点防止链之间滑移，保证形变可逆。这种熵弹性使橡胶具有特殊的热力学行为：拉伸的橡胶带受热会收缩（古-焦耳效应），快速拉伸时会发热。橡胶弹性理论由库恩、詹姆斯-古斯和弗洛里在 1930–40 年代建立。

Relaxed flexible chains are randomly coiled with maximum conformational entropy; stretching straightens them, lowering entropy and producing a restoring force, while crosslinks stop chains slipping past one another so the deformation is reversible. This entropic elasticity gives rubber unusual thermodynamics: a stretched band contracts when heated (the Gough–Joule effect) and warms when stretched quickly. The theory of rubber elasticity was developed by Kuhn, James and Guth, and Flory in the 1930s and 1940s.

天然橡胶之外，1909 年霍夫曼合成了甲基橡胶，1931 年杜邦推出氯丁橡胶，1930 年代德国开发丁苯橡胶和丁腈橡胶。此后出现了顺丁橡胶、乙丙橡胶（EPDM）、丁基橡胶、硅橡胶、氟橡胶和聚氨酯弹性体。热塑性弹性体（如 SBS）靠物理交联，可像塑料一样熔融加工和回收。

Besides natural rubber, Hofmann synthesised methyl rubber in 1909, DuPont launched neoprene in 1931, and Germany developed styrene–butadiene and nitrile rubbers in the 1930s. Polybutadiene, ethylene–propylene (EPDM), butyl, silicone, fluoroelastomers and polyurethane elastomers followed. Thermoplastic elastomers such as SBS rely on physical crosslinks and can be melt-processed and recycled like plastics.

弹性体约 60% 用于轮胎，其余用于密封件、胶管、传送带、鞋材、减震件、医用手套和电线绝缘。不同品种各有专长：丁腈橡胶耐油，氟橡胶耐高温和化学品，乙丙橡胶耐候耐臭氧，丁基橡胶气密性好，硅橡胶耐高低温且生理惰性。

About 60% of elastomers go into tyres, the rest into seals, hoses, belts, footwear, vibration mounts, medical gloves and cable insulation. Each has its strength: nitrile resists oil, fluoroelastomers resist heat and chemicals, EPDM resists weather and ozone, butyl is gas-tight, and silicone tolerates heat and cold and is biologically inert.