Thermoresponsive Supramolecular Hydrogels with High Fracture Toughness

Abstract

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 te...