Thermoresponsive Supramolecular Hydrogels with High Fracture Toughness

Supramolecular hydrogels formed from random copolymers of N-isopropylacrylamide (NIPAM) and 2-(N-ethylperfluorooctanesulfonamido)­ethyl acrylate (FOSA) exhibit a volume phase transition due to a lower critical solution temperature (LCST). The LCST can be tuned between 32 and 5 °C by incorporating up to 14 mol % FOSA in the copolymer. The tensile modulus and strength of the hydrogels increased above the LCST as a consequence of an increase in the effective cross-link density due to the phase separation of water from the hydrogel. Below the LCST, the hydrogels exhibited extraordinary fracture toughness and crack blunting capability. Fracture energies of ∼8000 J/m² were achieved, which is comparable or greater than that of cartilage and what has been previously achieved with synthetic supramolecular hydrogels. These gels exhibited large hysteresis behavior, though unlike tough double network hydrogels, the NIPAM/FOSA hydrogels can fully recover their dimensions (i.e., no permanent set) and properties after tensile deformations as high as 400% strain. Although the recovery of the macroscopic dimensions is relatively quick, the recovery of the microstructure requires times on the order of hours. The excellent energy dissipation behavior and recovery of the hydrogel is due to the reversible nature of the hydrophobic bonds and their aggregation into core–shell nanodomains, ∼6 nm in diameter. Under stress, the hydrophobic FOSA bonds break and the FOSA groups can rearrange either within the nanodomain or pull out of the nanodomain to dissipate energy. The bonds, however, reform when the stress is removed and the nanostructure heals. The hydrogels also exhibit stress-softening behavior (Mullins effect) as a consequence of the kinetics of the reversible structure and properties recovery following a deformation.