Fatigue Fracture of Self-Recovery Hydrogels

Abstract

Hydrogels of superior mechanical behavior are under intense development for many applications. Some of these hydrogels can recover their stress-stretch curves after many loading cycles. These hydrogels are called self-recovery hydrogels, or even fatigue-free hydrogels. Such a hydrogel typically contains a covalent polymer network, together with some non-covalent, reversible interactions. Here we show that self-recovery hydrogels are still susceptible to fatigue fracture. We study a hydrogel containing both covalently crosslinked polyacrylamide and uncrosslinked polyvinyl alcohol. For a sample without pre-cut crack, the stress-stretch curve recovers after thousands of loading cycles. For a sample with a pre-cut crack, however, the crack extends cycle by cycle. The threshold for fatigue fracture depends on the covalent network, but negligibly on non-covalent interactions. Above the threshold, the non-covalent interactions slow down the extension of the crack under cyclic loads.