Generalization of the sacrificial bond principle for gel and elastomer toughening

Although conventional polymer gels are known as mechanically weak materials, their fracture toughness can be effectively improved by introducing weak and brittle bonds into soft and stretchy polymer networks. This toughening method, denoted as the 'sacrificial bond principle', has been recently proposed by our group. When force is applied to such modified gels with an initial crack, brittle bonds surrounding the crack tip are widely and catastrophically ruptured prior to macroscopic crack propagation. As this extensive brittle bond fracture requires significant energy input, the total energy required for gel fracture is remarkably increased. Since the gel toughness is increased due to sacrificing the introduced brittle bonds, they are termed sacrificial bonds. In this focus review, I describe some extremely tough gels prepared by our group using this principle, e.g., double- or multiple-network gels with high water content featuring covalent sacrificial bonds, self-healing polyampholyte gels containing ionic sacrificial bonds, and PDGI/PAAm gels based on hydrophobic sacrificial bonds exhibiting stress-responsive structural colors.