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由两个具有明显差异和互穿的聚合物网络组成的双网络水凝胶，被认为是最坚韧的软湿材料。双网络水凝胶具有独特的网络结构和有效的能量耗散模式，使其具有极高的机械强度和韧性。尽管实验在双网络水凝胶设计、合成和机械性能的证实上已有了很多进展，但在分子水平上对其网络结构、相互作用、力学关系的认识仍嫌不足。同时，水凝胶中不均匀双网络结构的分子建模和模拟也极具挑战性。
来自美国阿克伦大学化学生物分子与腐蚀工程系的郑洁团队，开发了一种“随机游动反应聚合”算法，可以模拟自由基聚合过程，并计算从单体构建物理化学连接的琼脂/PAM双网络水凝胶的过程，能在更大程度上贴近网络结构和动力学、机械应力-应变行为和水动力学的实验性质。该研究同时作了非平衡操纵的分子动力学模拟，以研究琼脂/ PAM双网络水凝胶在松弛和变形状态下与结构有关的力学和断裂行为。结果表明，琼脂/ PAM水凝胶的能量耗散归因于双网络中的琼脂链的拉出以及双网络结构之间和内部大量氢键的破坏，从而解释了琼脂/ PAM水凝胶在机械上的不同力学增强作用。该计算模拟平台为面向双网状水凝胶网络结构和断裂的多尺度聚合提供了计算框架，为解决水凝胶中不均匀双网络结构的分子建模和模拟提供了新思路，从而为该方向的研究提供了强大的模拟工具。
该文近期发表于npj Computational Materials 7: 39(2021)，英文标题与摘要如下，点击左下角“阅读原文”可以自由获取论文PDF。
A multiscale polymerization framework towards network structure and fracture of double-network hydrogels
Mingzhen Zhang, Dong Zhang, Hong Chen, Yanxian Zhang, Yonglan Liu, Baiping Ren & Jie Zheng
Double-network (DN) hydrogels, consisting of two contrasting and interpenetrating polymer networks, are considered as perhaps the toughest soft-wet materials. Current knowledge of DN gels from synthesis methods to toughening mechanisms almost exclusively comes from chemically-linked DN hydrogels by experiments. Molecular modeling and simulations of inhomogeneous DN structure in hydrogels have proved to be extremely challenging. Herein, we developed a new multiscale simulation platform to computationally investigate the early fracture of physically-chemically linked agar/polyacrylamide (agar/PAM) DN hydrogels at a long timescale. A “random walk reactive polymerization” (RWRP) was developed to mimic a radical polymerization process, which enables to construct a physically-chemically linked agar/PAM DN hydrogel from monomers, while conventional and steered MD simulations were conducted to examine the structural-dependent energy dissipation and fracture behaviors at the relax and deformation states. Collective simulation results revealed that energy dissipation of agar/PAM hydrogels was attributed to a combination of the pulling out of agar chains from the DNs, the disruption of massive hydrogen bonds between and within DN structures, and the strong association of water molecules with both networks, thus explaining a different mechanical enhancement of agar/PAM hydrogels. This computational work provided atomic details of network structure, dynamics, solvation, and interactions of a hybrid DN hydrogel, and a different structural-dependent energy dissipation mode and fracture behavior of a hybrid DN hydrogel, which help to design tough hydrogels with new network structures and efficient energy dissipation modes. Additionally, the RWRP algorithm can be generally applied to construct the radical polymerization-produced hydrogels, elastomers, and polymers.
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