Flaw sensitivity of hydrogels with dynamic covalent bonds

Dynamic covalent bonds endow polymers with unique mechanical properties, such as tunable viscoelastic response and self-healing capability. The study on the fracture of dynamic polymers has recently initiated. However, it is still elusive how dynamic covalent bonds affect the flaw sensitivity of dynamic polymers. Here, we prepare the polyacrylamide hydrogel with dynamic disulfide bonds as the model material. Under UV irradiation, dynamic bond exchange reaction effectively releases the stress concentrated at the crack tip, making the hydrogel less flaw sensitive. Notably, both the rupture stretch and rupture stress of dynamic hydrogels under UV irradiation are much higher than the control group. We further investigate the effect of photoinitiator concentration and dynamic bond content on the flaw sensitivity of dynamic hydrogels. The increase of photoinitiator concentration or dynamic bond content intensifies the dynamic bond exchange reaction, causing more stress relaxation and improved flaw insensitivity of the hydrogel. Moreover, we exploit this flaw insensitivity to tune the crack propagation path in dynamic hydrogel with double edge cracks. The crack propagates in the non-irradiated area, but is retarded in the irradiated area to show asymmetric fracture mode. We hope this work can provide new insights for the design of polymers with dynamic covalent bonds.