Role of dynamic bonds on fatigue threshold of tough hydrogels

Abstract

Fatigue threshold (G0) is the energy release rate below which no fatigue crack advances. Understanding the relationship between the static/dynamic structures and G0 of viscoelastic hydrogels is crucial for designing materials exposed to cyclic loading. In this study, we elaborate the role of dynamic bonds in the G0 of tough hydrogels. Using polyampholyte hydrogels possessing abundant ionic dynamic bonds, we can adopt a time-salt superposition principle to access a wide range of time scales that are difficult to access in fatigue tests. G0 was found rate-independent in the elastic regime at a low cyclic strain rate, whereas weak power-law relations between G0 and strain rate were first observed in viscoelastic regimes. The former agrees with the molecular explanation of the Lake-Thomas model, and the latter is related to the self-healing property of dynamic bonds in the viscoelastic regime that protects the permanent bonds from breaking. This work modifies the previous consideration that the dynamic bonds only contribute to the fracture toughness and crack resistance but not to the G0