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Mechanics of a self-healing hydrogel with covalent and physical cross-links : theory and experiments

C.Y. Hui Department of Mechanical and Aerospace Engineering, Field of Theoretical & Applied Mechanics, Ithaca, NY

In recent years polymer chemists have made tremendous strides in the synthesis of biocompatible, tough, self-healing hydrogels. However, there are few comprehensive mechanical models that links 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 a model system ; a Poly(vinylalcohol) (PVA) hydrogel chemically crosslinked by glutaraldehyde and physically crosslinked by Borax ions. The physical bonds in this gel are formed by interaction of borax ions with the OH groups in the PVA chains. 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 this gel under complex loading histories. The asymptotic structure of the crack tip fields is derived in closed form. We establish local correspondence principle between finite strain viscoelasticity and finite strain elasticity. 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. We also carried out Digitial Image Correlation (DIC) to measure the strain field in tension loaded samples containing a central hole, a circular edge notch and a sharp crack. These experiments are modeled using FEM. Again, we found good agreement between FEM and DIC results for all three geometries. Time permitting, I will discuss some limitations of our model and future work.