Abstract:
Hydrogel is a kind of polymer network swelling in water, which has structural similarity with biotissues. Traditional hydrogels are mechanically weak and brittle, due to their inherent heterogeneous microstructures and high water contents, which severely limit their applications. In recent years, researchers have been committed to improving the mechanical properties of hydrogels and have developed many strong and tough hydrogels, such as double-network (DN) hydrogels, dynamic bond hydrogels, hydrogels with rich entanglements, and fiber reinforced soft composites. In this review, we first summarize the development of hydrogels for enhancing mechanical properties and then introduce the relationship between stiffness and toughness of hydrogels. After that, several typical tough and strong hydrogels are selected to introduce their deformation and fracture behaviors, which give a preliminary understanding of the energy dissipation mechanisms of different hydrogels. The mechanisms mentioned in this review include the breakage and reformation of physical bonds, the enhanced crack resistance due to phase-separated structure, the bulk viscoelastic energy dissipation, the near-crack dissipation and the fiber pullout and rupture. Then, the characteristics of these strong and tough hydrogels are summarized as follows: DN hydrogels dissipate energy through chain scission but soften after large deformation; dynamic bond hydrogels show self-healing behavior and large bulk hysteresis; hydrogels with rich entanglements have dense entanglements but sparse cross-links and low hysteresis; fiber reinforced soft composites have multi-scale energy dissipation and very large process zones.