Implications of Supramolecular Crosslinking on Hydrogel Toughening by Directional Freeze-Casting and Salting-Out

Abstract

Dynamic hydrogel crosslinking captures network reorganization and self-healing of natural materials, yet is often accompanied by reduced mechanical properties compared to covalent analogs. Toughening is possible in certain materials with processing by directional freeze-casting and salting-out, producing hierarchically organized networks with directionally enhanced mechanical properties. The implications of including dynamic supramolecular crosslinking alongside such processes are unclear. Here, a supramolecular hydrogel prepared from homoternary crosslinking by pendant guests with a free macrocycle is subsequently processed by directional freeze-casting and salting-out. The resulting hydrogels tolerate multiple cycles of compression. Excitingly, supramolecular affinity dictates the mechanical properties of the bulk hydrogels, with higher affinity interactions producing materials with higher Young's modulus and enhanced toughness under compression. The importance of supramolecular crosslinking is emphasized with a supramolecular complex that is converted in situ into a covalent crosslink. While supramolecular hydrogels do not fracture and spontaneously self-heal when cut, their covalent analogs fracture under moderate strain and do not self-heal. This work shows a molecular-scale origin of bulk hydrogel toughening attributed to affinity and dynamics of supramolecular crosslinking, offering synergy in combination with bulk post-processing techniques to yield materials with enhanced mechanical properties tunable at the molecular scale for the needs of specific applications.