Hydrogen-Bonding Toughened Hydrogels and Emerging CO2 -Responsive Shape Memory Effect

A double hydrogen bonding (DHB) hydrogel is constructed by copolymerization of 2-vinyl-4,6-diamino-1,3,5-triazine (hydrophobic hydrogen bonding monomer) and N,N-dimethylacrylamide (hydrophilic hydrogen bonding monomer) with polyethylene glycol diacrylates. The DHB hydrogels demonstrate tunable robust mechanical properties by varying the ratio of hydrogen bonding monomer or crosslinker. Importantly, because of synergistic energy dissipating mechanism of strong diaminotriazine (DAT) hydrogen bonding and weak amide hydrogen bonding, the DHB hydrogels exhibit high toughness (up to 2.32 kJ m−2), meanwhile maintaining 0.7 MPa tensile strength, 130% elongation at break, and 8.3 MPa compressive strength. Moreover, rehydration can help to recover the mechanical properties of the cyclic loaded–unloaded gels. Attractively, the DHB hydrogels are responsive to CO2 in water, and demonstrate unprecedented CO2-triggered shape memory behavior owing to the reversible destruction and reconstruction of DAT hydrogen bonding upon passing and degassing CO2 without introducing external acid. The CO2 triggering mechanism may point out a new approach to fabricate shape memory hydrogels.