Fatigue of amorphous hydrogels with dynamic covalent bonds

Hydrogels composed of amorphous polymer networks are widely used as soft and stretchable components in diverse devices and machines. However, existing amorphous hydrogels are susceptible to fatigue fracture. Here, we propose that amorphous hydrogels with dynamic covalent bonds can resist fatigue fracture, so long as the dynamics of bond recovery is faster than the rate of deformation. We examine this hypothesis by performing fatigue tests for polyacrylamide hydrogels crosslinked by covalent C–C bonds and by dynamic covalent siloxane bonds, using N, N’-methylenebis-acrylamide (MBAA) and silanes as the crosslinkers, respectively. Experiments show that the fatigue threshold of MBAA-crosslinked hydrogels is 12.7 J/m, much lower than the toughness, 70.2 J/m, and comparable to the Lake–Thomas prediction, 11.17 J/m, while the silane-crosslinked hydrogels achieve a fatigue threshold of 68.7 J/m, surprisingly close to the toughness, 86.2 J/m, which is reported for the first time in hydrogels with dynamic covalent bonds. We trace the violation of the Lake–Thomas mechanism to the dynamic nature of siloxane bonds. We carry out self-healing tests, self-recovery tests, and fatigue tests for silane-crosslinked hydrogels with different pH values to examine the bond dynamics. We discuss the importance of dynamic covalent bonds in synthesizing fatigue-free hydrogels by constructing a fatigue threshold-toughness Ashby plot. The current work suggests that dynamic covalent bonds pave alternative avenues towards anti-fatigue soft materials with amorphous polymer networks.