MEAM Seminar: “Mechanical Behavior of Self-healing Hydrogels with Chemical and Physical Cross-links: Theory and Experiments”

In recent years polymer chemists have made tremendous strides in the synthesis of biocompatible, tough, self-healing hydrogels. However, there are not many comprehensive mechanical models that capture the observed time dependent mechanical behavior of these gels (especially fracture) to the underlying, rate dependent bond breaking and reformation processes. In this talk I will summarize some of the progress we have made on two systems; the first is a Poly(vinylalcohol) (PVA) hydrogel chemically crosslinked by glutaraldehyde and physically crosslinked by Borax ions. The second is a chemically crosslinked Polyampholyte gel (c-PA) synthesized by random copolymerization of cationic and anionic monomers at a high concentration around the charge balanced point. We formulated a 3D, large deformation viscoelastic constitutive model based on breaking and healing kinetics of physical cross-links. We demonstrate this model accurately captures the rate dependent behavior of these gels under complex loading histories. We studied the asymptotic structure of the crack tip fields. We develop a finite element model to numerically study the stress and deformation fields near the tip of a stationary crack in single edge cracked specimens. The theoretical and finite element results (3D and 2D plane stress) agree remarkably well with experimentally observed crack opening profiles in the PVA gel system. The model parameters are extracted from experiments using a newly developed fitting method based on Meta-modeling and Neutral network. We carried out relaxation experiments on two different types of samples to study the nonlinear viscoelastic behavior of the c-PA system. A simple model is used to explain this behavior. Our modelling and experimental efforts can lead to better understanding of the complex load transfer process near the crack tip and time dependent delayed fracture of gels and elastomers.