Fatigue has long been studied for many materials, but many aspects are not well understood. Our recent study of the distinct roles of crosslinks and entanglements in the synthesis-property relation of a polymer network under monotonic load leads to a fundamental question: how do crosslinks and entanglements affect the synthesis-property relation of a polymer network under cyclic load? Here we study the relation of synthesis and fatigue property of elastic soft materials without precut cracks. We prepare polyacrylamide hydrogels by free radical polymerization as a model system, and swell the hydrogels in water to equilibrium or to a certain amount of polymer content. The synthesis parameters include the crosslinker-to-monomer molar ratio and the water-to-monomer molar ratio in the precursor, as well as the polymer content in the hydrogel. Three series of hydrogels are prepared. For each hydrogel, the stress-stretch curve under cyclic stretch of various amplitudes and the number of cycles to rupture are measured, giving four properties: fatigue life, endurance stretch, endurance stress, and endurance work. When the crosslinker-to-monomer molar ratio in the precursor is high, the degree of network imperfection on average is low. When the water-to-monomer molar ratio in the precursor is low, the number of entanglements per polymer segment on average is large. We show that crosslinks decrease the susceptibility to fatigue and entanglements increase the endurance stress. By contrast, both crosslinks and entanglements negligibly affect the endurance stretch and the endurance work.

中文翻译：

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弹性软材料合成与疲劳性能的关系


长期以来，人们一直对许多材料进行疲劳研究，但许多方面尚不清楚。我们最近对单调载荷下交联和纠缠在聚合物网络合成-性能关系中的不同作用的研究引出了一个基本问题：交联和纠缠如何影响循环载荷下聚合物网络的合成-性能关系？在这里，我们研究了无预切割裂纹的弹性软材料的合成与疲劳性能的关系。我们通过自由基聚合作为模型系统制备聚丙烯酰胺水凝胶，并将水凝胶在水中膨胀至平衡或达到一定量的聚合物含量。合成参数包括前驱体中的交联剂与单体的摩尔比和水与单体的摩尔比，以及水凝胶中的聚合物含量。制备了三个系列的水凝胶。对于每种水凝胶，测量了不同振幅的循环拉伸下的应力-拉伸曲线和破裂的循环次数，给出了四个属性：疲劳寿命、耐力拉伸、耐力应力和耐力功。当前驱体中的交联剂与单体摩尔比较高时，平均网络缺陷程度较低。当前驱体中的水与单体摩尔比较低时，平均每个聚合物链段的缠结数量很大。我们表明，交联降低了对疲劳的敏感性，而缠结增加了耐久应力。相比之下，交联和缠结对耐力拉伸和耐力功的影响可以忽略不计。