Mechanical enhancement of hydrophobically associating hydrogels by solvent-regulated phase separation

Abstract Hydrogels have a wide range of potential applications due to their unique soft and hydrated structure. Many applications, however, require an efficient combination of mechanical properties including strength, stiffness, toughness, and viscoelasticity, which is rarely achieved. In this work, we report the simultaneously strengthening, stiffening, and toughening of a conventionally weak gel, hydrophobically associating hydrogel, by a simple regulation of external solvent. Phase separation is induced when the hydrogel is immersed into the mixed solvent that has a relatively high concentration of a poor solvent, resulting in the increment of polymer volume fraction in the gel. As a result, strong inter/intra polymer interactions emerge at the nanoscale, which serve the role of sacrificial bonds to dissipate large amounts of energy. The phase-separated, viscoelastic hydrogel exhibits significant enhancements in strength, stiffness, and toughness by up to 270-, 9200-, and 2500-folds, respectively, exceeding a great many tough hydrogels. Periodically changing the solvent enables the reversible phase-separation process, facilitating the regulation of mechanical properties of the hydrogel upon demand. This solvent-regulated phase-separation strategy, due to its facile procedure, repeatability, and high efficiency, offers a universal guideline to toughen weak hydrogels.