We investigate the fracture properties of poly(acrylamide-co-1-vinylimidazole) dual crosslink hydrogels [P(AAm-co-VIm)-M²⁺ gels] containing a small fraction of covalent bonds and a majority of dynamic bonds based on metal coordination bonds (Ni²⁺ or Zn²⁺). Unlike a previous study on a different dual crosslink hydrogel system having slower dynamic bonds based on poly(vinylalcohol) and borate ions (PVA-Borax gels), the presence of these faster dynamic coordination bonds has two main effects: They significantly toughen the P(AAm-co-VIm)-M²⁺ gels even at high stretch rates, where the dynamic bonds should in principle behave as covalent bonds at the crack tip, and they toughen the gels at very low stretch rates, where the dynamic bonds are invisible during the loading stage. We propose two additional molecular mechanisms to rationalize this behavior of P(AAm-co-VIm)-M²⁺ gels: we hypothesize that fast exchanging dynamic bonds remain slow compared to the characteristic time of bond scission and are, therefore, able to share the load upon covalent bond scission even at low loading rates. We also argue of the existence of longer-lived clusters of dynamic bonds that introduce a stretch rate-dependent strain hardening in uniaxial tension and stabilize and increase the size of the dissipative zone at the crack tip, thereby introducing a strain-dependent dissipative mechanism.

中文翻译：

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具有永久和瞬时交联的双交联凝胶的断裂：瞬时交联的弛豫时间的影响

我们研究了聚（丙烯酰胺-co-1-乙烯基咪唑）双交联水凝胶 [P(AAm-co-VIm)-M 2+ 凝胶] 的断裂特性，该水凝胶含有一^小部分共价键和大部分基于金属的动态键配位键（Ni ²⁺或 Zn ²⁺）。与先前对基于聚乙烯醇和硼酸盐离子（PVA-硼砂凝胶）的具有较慢动态键的不同双交联水凝胶系统的研究不同，这些较快的动态配位键的存在有两个主要影响：它们显着增强了 P( AAm- co -VIm)-M ²⁺即使在高拉伸速率下也能凝胶化，其中动态键原则上应该在裂纹尖端表现为共价键，并且它们在非常低的拉伸速率下使凝胶变韧，其中动态键在加载阶段是不可见的。我们提出了两个额外的分子机制来合理化 P(AAm- co -VIm)-M ^{2+的这种行为}凝胶：我们假设与键断裂的特征时间相比，快速交换的动态键仍然很慢，因此即使在低负载率下也能够在共价键断裂时分担负载。我们还论证了存在寿命更长的动态键簇，它们在单轴拉伸中引入了与拉伸速率相关的应变硬化，并稳定并增加了裂纹尖端耗散区的大小，从而引入了应变相关的耗散机制。