Double-network gels are a class of tough soft materials comprising two elastic networks with contrasting structures. The formation of a large internal damage zone ahead of the crack tip by the rupturing of the brittle network accounts for the large crack resistance of the materials. Understanding what determines the damage zone is the central question of the fracture mechanics of double-network gels. In this work, we found that at the onset of crack propagation, the size of necking zone, in which the brittle network breaks into fragments and the stretchable network is highly stretched, distinctly decreases with the increase of the solvent viscosity, resulting in a reduction in the fracture toughness of the material. This is in sharp contrast to the tensile behavior of the material that does not change with the solvent viscosity. This result suggests that the dynamics of stretchable network strands, triggered by the rupture of the brittle network, plays a role. To account for this solvent viscosity effect on the crack initiation, a delayed blunting mechanism regarding the polymer dynamics effect is proposed. The discovery on the role of the polymer dynamic adds an important missing piece to the fracture mechanism of this unique material.

双网络凝胶是一类坚韧的软材料，由两个具有对比结构的弹性网络组成。脆性网状结构的破裂在裂纹尖端前形成较大的内部损伤区，是材料抗裂性大的原因。了解决定损伤区域的因素是双网络凝胶断裂力学的核心问题。在这项工作中，我们发现在裂纹扩展开始时，脆性网络断裂成碎片而可拉伸网络高度拉伸的颈缩区的尺寸随着溶剂粘度的增加而明显减小，导致减小在材料的断裂韧性方面。这与不随溶剂粘度变化的材料的拉伸行为形成鲜明对比。这一结果表明，由脆性网络破裂引发的可拉伸网络链的动力学发挥了作用。为了解释这种溶剂粘度对裂纹萌生的影响，提出了一种关于聚合物动力学效应的延迟钝化机制。关于聚合物动力学作用的发现为这种独特材料的断裂机制增加了一个重要的缺失部分。