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

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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.