Prolonged fracture resistance of hydrogels through spontaneous network reconfiguration

Abstract

In engineering applications, hydrogels are often susceptible to fatigue loads, either static or dynamic. Extensive efforts have been devoted to studying dynamic fatigue facture of hydrogels, which promotes the understanding of underlying mechanisms and facilitates the design and synthesis of fatigue-fracture-resistant hydrogels under cyclic loads. In stark contrast, lifetime of hydrogels under static loading is much less investigated and hydrogels resistant to delayed fracture have not been reported. Here we propose a mechanism against delayed fracture by deconcentrating stress at crack tip through spontaneous network reconfiguration, during which reversible crosslinks dissociate to relieve stress and reassociate to reconstruct the polymer network in a stress-free manner. We validate the proposed mechanism by investigating the delayed fracture behaviors of polyacrylamide hydrogels with reversible and irreversible crosslinks. We show that a hydrogel with reversible crosslinks exhibits a threshold against delayed fracture, > 132 J/m², one order of magnitude higher than that of its counterpart with irreversible crosslinks, ∼13 J/m², which obeys the Lake-Thomas prediction. We provide further validations, including experimental observations on training-enhanced fracture stretch, decreased threshold for delayed fracture at a lower rate of network reconfiguration, prominent stress relaxation, as well as numerical simulations. Evidently, spontaneous network reconfiguration offers an effective way to deconcentrate stress at the crack tip for prolonged resistance to fracture.