Connecting Bond-breaking and Healing Kinetics to the Deformation and Fracture of Tough Hydrogels

水凝胶是一种软材料，主要是水。人们可以把水凝胶想象成一个三维的“鱼网”，它由明胶分子链(网络)组成，被水吸进网中而膨胀。水凝胶目前有许多应用，如隐形眼镜和作为组织工程的支架材料。未来的潜在应用可能包括人造软骨和软机器人的“肌肉”。简单水凝胶的一个缺点是容易失效，这严重限制了它们在承载部件中的实际应用。近年来，化学家们发明了高度抗断裂的水凝胶。这种新型水凝胶的网络由分子链组成，分子链通过不同类型的连接物(键)相互连接。这些连接器可以是永久性的，也可以是临时的。临时连接件可能会断开并重新安装。这一过程耗散了有助于抵抗断裂的能量，并使凝胶得以愈合。目前，工程师们还没有预测工具来确定由这些水凝胶制成的组件在载荷下如何改变形状以及何时断裂。该项目将开发这样的预测工具。最终产品将是一种定量方法和计算机代码，允许工程师设计和分析由这些水凝胶组成的成分。软材料变形和断裂力学中的两个长期存在的问题是：(1)三维大变形非线性粘弹性；(2)这些三维非线性粘弹性方程的有效积分。这项研究要探索的一个新想法是，非线性粘弹性可以保护裂纹免受高应力的影响，从而提高韧性。该项目将通过以下方式解决这些问题：(A)对一种坚韧的两性水凝胶进行实验，(B)开发将非线性粘弹性行为与键断裂和重塑动力学联系起来的定量模型，(C)开发发现和量化裂纹屏蔽的实验方法，以及(D)开发有效的时间积分方案来求解非线性粘弹性方程。该项目将通过建立一个开发与微结构直接相关并可用于实际有限元模拟的时间相关本构和断裂模型的框架来推动力学领域的发展。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

A hydrogel is a soft material that is mostly water. One can think of a hydrogel as a three dimensional "fishnet" consisting of gelatin molecular chains (network) swollen by water drawn into the net. Hydrogels have many current applications such as contact lenses and as scaffolding materials for tissue engineering. Potential future applications may include artificial cartilage and "muscles" for soft robots. A shortcoming of simple hydrogels is that they are easy to fail; this severely limits their practical use in load bearing components. In recent years, chemists have invented hydrogels that are highly resistant to fracture. The network of this new class of hydrogels consists of molecular chains linked to each other by different types of connectors (bonds). These connectors can be permanent or temporary. Temporary connectors can break and reform. This process dissipates energy which contributes to resistance to fracture and they also allow the gel to heal. Currently, engineers do not have the predictive tools to determine how components made of these hydrogels change shape under load and when they break. This project will develop such predictive tools. The end product will be a quantitative method and a computer code which allows engineers to design and analyze components made of these hydrogels. Two long standing problems in the time dependent mechanics of deformation and fracture of soft materials are: (1) three dimensional large deformation nonlinear viscoelasticity and (2) efficient integration of these three dimensional nonlinear viscoelastic equations. A new idea to be explored by this research is that nonlinear viscoelasticity shields the crack from high stresses and thus enhances toughness. This project will address these issues by: (a) performing experiments on a tough Polyampholyte hydrogel, (b) developing quantitative models relating nonlinear viscoelastic behavior to bond breaking and reformation kinetics, (c) developing experimental methods to discover and to quantify crack shielding and (d) developing efficient time integration schemes to solve the nonlinear viscoelastic equations. The project will advance the field of mechanics by building a framework for development of time dependent constitutive and fracture models that are linked directly to the microstructure and that can be used in practical finite element simulations.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.