Direct Observation on the Surface Fracture of Ultrathin Film Double-Network Hydrogels

Double-network (DN) hydrogels have attracted much attention in the soft matter community due to their excellent mechanical performance and unique fracture mechanism. On the basis of ultrathin film technique and optical microscope, we here report a method which is quite effective for accessing the fractured microstructure of DN hydrogels formed during the tensile and tearing process. During tensile deformation, fracture of the first network occurs even before yielding occurs. Concomitant with the yielding, wrinkle-like structure, which is due to the fracture of the first network, is clearly observed for the first time. This wrinkle-like structure disappears upon further deformation to strain-hardening region because of the further fracture of the first network. Similar microstructures are observed at the crack tip for samples experienced the tearing test. This work provides direct proofs for the assumption concerning the occurrence of local yielding and the formation of a huge damage zone at the crack tip during the crack propagation process of DN hydrogels.