天然橡胶Natural rubber

顺式双键使分子链卷曲、链间作用弱，玻璃化温度约 −70 °C，常温下处于高弹态。拉伸时卷曲的链被拉直、构象熵减小，松开后链自发回到无序卷曲状态——橡胶弹性本质上是熵弹性。拉伸到较大形变时天然橡胶会发生应变诱导结晶，使强度显著提高。反式-1,4-聚异戊二烯（古塔波胶、杜仲胶）链排列规整，常温下是坚硬的塑料状物质。

The cis double bonds coil the chains and weaken interchain forces, giving a glass transition near −70 °C, so the material is rubbery at room temperature. Stretching straightens the coils and lowers conformational entropy, and on release the chains spontaneously return to random coils: rubber elasticity is essentially entropic. At large strains natural rubber undergoes strain-induced crystallisation, which greatly raises its strength. trans-1,4-Polyisoprene (gutta-percha, eucommia gum) packs regularly and is a hard, plastic-like solid at room temperature.

美洲原住民很早就用胶乳制作球和防水物品。1770 年普里斯特利发现它能擦掉铅笔痕，英文名 rubber 由此而来。1839 年固特异发现橡胶与硫磺共热后不再在热天发黏、冷天变脆，发明了硫化法。1860 年威廉斯热解橡胶得到异戊二烯，证明了其组成。1876 年魏克汉把橡胶种子带出巴西，东南亚由此成为主要产区。

Indigenous Americans long used latex to make balls and waterproof goods. In 1770 Priestley found it rubbed out pencil marks, which gave it the English name rubber. In 1839 Goodyear found that heating rubber with sulfur stopped it turning sticky in heat and brittle in cold, inventing vulcanisation. In 1860 Williams pyrolysed rubber to isoprene, establishing its composition. Wickham took rubber seeds out of Brazil in 1876, and South-East Asia became the main producing region.

全球天然橡胶年产约 1400 万吨，约 70% 用于轮胎，其余用于输送带、医用手套和胶管等。由于天然橡胶具有高强度和低生热，载重轮胎和飞机轮胎仍大量使用。齐格勒-纳塔催化剂可合成高顺式聚异戊二烯（异戊橡胶），丁苯橡胶、顺丁橡胶等合成橡胶产量已超过天然橡胶。少数人对胶乳中的蛋白质过敏。

World output is about 14 million tonnes a year, about 70% of it for tyres and the rest for belts, medical gloves and hoses. Its high strength and low heat build-up keep it essential in truck and aircraft tyres. Ziegler–Natta catalysts make high-cis synthetic polyisoprene, and synthetic rubbers such as SBR and polybutadiene now outproduce natural rubber. Some people are allergic to latex proteins.