Effect of Hydrophobic Side Group on Structural Heterogeneities and Mechanical Performance of Gelatin-Based Hydrogen-Bonded Hydrogel

Hydrogen bonds stabilized by hydrophobic side groups have been widely applied to realize high toughness of hydrogels. Herein, we studied the effect of the content of the hydrophobic methyl side group on the structure and mechanical properties of tough hydrogen-bonded gelatin/poly (methacrylic acid–co-acrylic acid) (G/Mx-Ay) hydrogels by alternating the content of methacrylic acid (MAA) in the methacrylic acid–acrylic acid copolymer (MAA-co-AA). The content of MAA did not affect the hydrogen-bond formation between the MAA-co-AA and gelatin but affected the phase-separated structure. With an increase in the MAA content, the local structure of the hydrogels with similar water content transformed from a "network" structure to a "bicontinuous" phase-separated structure and to a phase-separated structure with wide size distribution. The hydrogels with the same structure exhibited similar viscoelastic behaviors, and the G/Mx-Ay hydrogels with "bicontinuous" phase separation exhibited the best mechanical properties with an optimal Young's modulus of ∼7.5 MPa and fracture tensile stress of ∼10 MPa.