交联Cross-link

化学交联由共价键形成，如橡胶的硫桥、环氧树脂的胺-环氧加成、过氧化物交联聚乙烯（PEX 管）和辐射交联。物理交联靠结晶区、玻璃态微区（SBS 中的聚苯乙烯相）、氢键或离子簇形成，加热可逆。交联密度用交联点之间的平均分子量 M_c 表示：M_c 越小，网络越紧密，材料越硬越脆。交联网络在良溶剂中只溶胀不溶解，溶胀度可用弗洛里-雷纳方程计算交联密度。

Chemical crosslinks are covalent bonds, such as sulfur bridges in rubber, amine–epoxide links in epoxies, peroxide-crosslinked polyethylene (PEX pipe) and radiation crosslinking. Physical crosslinks come from crystallites, glassy domains (the polystyrene phase in SBS), hydrogen bonds or ionic clusters and are thermally reversible. Crosslink density is expressed as the average molar mass between crosslinks, M_c: the smaller M_c, the tighter the network and the harder and more brittle the material. A network swells but does not dissolve in a good solvent, and the Flory–Rehner equation relates the degree of swelling to crosslink density.

1941 年弗洛里和斯托克迈尔提出凝胶化理论：在多官能团体系中，当反应程度达到临界值 p_c = 1/[r(f−1)]^½ 时，体系中突然出现贯穿整个样品的无限网络（凝胶），黏度急剧上升。这一理论可预测热固性树脂的适用期和固化时间，也被推广到渗流理论。

In 1941 Flory and Stockmayer formulated gelation theory: in a multifunctional system, at a critical conversion p_c = 1/[r(f−1)]^½ an infinite network spanning the sample (the gel) suddenly appears and viscosity rises steeply. The theory predicts the pot life and cure time of thermosets and was later generalised as percolation theory.

蛋白质中半胱氨酸形成的二硫键是天然交联，烫发就是先还原二硫键、塑形后再氧化重建。胶原蛋白中的赖氨酸交联赋予组织强度。高吸水性树脂（尿不湿中的轻度交联聚丙烯酸钠）、软性隐形眼镜（交联水凝胶）、牙科填充树脂都依赖交联结构。

Disulfide bonds between cysteines are natural crosslinks in proteins; a perm first reduces them and, once the hair is shaped, reoxidises them. Lysine-derived crosslinks give collagen its strength. Superabsorbent polymers (lightly crosslinked sodium polyacrylate in nappies), soft contact lenses (crosslinked hydrogels) and dental filling resins all depend on crosslinked structures.