中文摘要：

双网络水凝胶（DN凝胶）是由具有很强的结构非对称性的两种聚合物形成的特殊的聚合物互穿网络。相对于单一聚合物网络（sN凝胶）而言，DN凝胶的机械强度和韧性都有惊人的提高，其拉伸断裂应力和断裂应变分别能达到1～10MPa和1000％-2000％、韧性（撕裂能）可以达到10^2～10^3J·m^-2。DN凝胶性能的提高，主要是由强对称结构的第一重网络（刚而脆）与第二重网络（软而韧）相互缠结和互穿的结果。对DN凝胶断裂过程和增韧本质的理解是设计下一代具有理想机械性能的DN凝胶的关键所在。一些DN凝胶表现出大的滞后、屈服、细颈和软化现象，而这些现象用经典的Lake—Thomas断裂理论是没有办法解释的。根据DN凝胶的滞后和细颈等实验现象，Brown和Tanaka等提出了“破坏区”理论来解释DN凝胶超乎寻常的高韧性。最近，龚剑萍等提出的“牺牲键”理论也已经很好地应用于设计和制备具有新型微纳米结构的高韧性DN凝胶。本文着重阐释了DN凝胶的增韧机理，总结了这一领域的最新研究成果，并讨论各种因素对凝胶韧性的影响，最后对DN凝胶增韧机理存在的问题和研究方向进行了展望。

英文摘要：

Double network hydrogels （DN gels） are unique interpenetrating polymer networks consisting of two kinds of polymer networks with strong asymmetric structure. Compared to single network hydrogels, DN gels exhibit extremely high strength （fracture tensile stress of 1-10 MPa and strain of 1000%-2000% ） and toughness （tearing fracture energy of 10^2 -10^3 J· m^-2）, due to their contrasting network structures where the first, brittle polyelectrolyte network is strongly entangled and interpenetrated with the second, soft, neutral polymer network. Fundamental understanding of the fracture process and toughening mechanisms of DN gels is critical for rational design of the next-generation of tough DN gels with desirable mechanical properties. Some DN gels illustratelarge hysteresis, yielding/necking and softening phenomena, which can＇t be well interpreted by classical Lake- Thomas theory. Based on these experimental facts, Brown and Tanaka had suggested a ＂Damage Zone＂ model to explain the extraordinary high toughness of DN gels. Recently, ＂sacrificial bonds＂ theory, which proposed by Gong＇s group, has been well applied to design and prepare high toughness DN gels with novel nano-/ microstructures. In the present of review, we focus on the toughening mechanisms of DN gels. The latest finding in this field are summarized, and the effect factors on toughness are discussed. In the end, the problems and research directions of the mechanisms of DN gels are pointed out.