Physically Entangled Hydrogels Constructed Through Pre-Stretched Backbone Provide Excellent Comprehensive Mechanical Properties

It is difficult for hydrogels to have both excellent stiffness and toughness in conventional polymer networks. Physically entangled hydrogels provide ideas to solve this problem, but the loss of physical entanglement in the swelling process does not effectively utilize the potential of this approach. In this work, the hydrogel is synthesized including two layers of interpenetrating physically entangled networks, the backbone network and the filler network. According to rubberlike elasticity theory, the backbone network inhibits the swelling of the filler network retaining a large amount of physical entanglements, inheriting and enhancing the advantages of physically entanglement hydrogels. The stiffness of the hydrogel reaches 0.33 MPa, and the fracture toughness reaches 1.93 × 10⁴ J m^-2, which shows significant advantages over other hydrogels. The removable physical entanglements tend to favor an ideal uniform network after being subjected to cyclic loading, and the toughness and fracture strain improve more than 2 times to 4.2 MJ m^-3 and 7.2, respectively. The fatigue threshold reaches 5.41 × 10³ J m^-2, which is five times higher than the fatigue-resistant hydrogels. The present work perfectly inherits the physically entangled network structure while realizing the anti-swelling property, which provides a way to break the toughness-stiffness conflict intractable in hydrogels.