Hydrogels with Superior Mechanical Properties from the Synergistic Effect in Hydrophobic–Hydrophilic Copolymers

The mechanical properties (e.g., modulus and strength) of conventional tough hydrogels are considerably inferior to those required for practical load-bearing applications. However, developing hydrogels with a combination of superior mechanical properties such as stiffness, strength, and toughness is very challenging. Herein, we propose a new design strategy based on the synergistic effect of hydrophobic/hydrophilic components to fabricate novel hydrogels with superior mechanical properties. Both hydrophobic and hydrophilic monomers were integrated into hydrogels using a simple two-step fabrication process, which led to the formation of a randomly distributed interconnected 3D structure of hard-phase and soft-phase regions comprising hydrophobic-rich and hydrophilic-rich components, respectively. The 3D structure of hard-phase regions imparted exceptional stiffness and strength, whereas soft-phase regions provided sacrificial bonds to achieve stretchability and toughness. Different hydrophobic monomers with widely variable extents of hydrophobicity were used. The obtained results indicated that hydrogels based on benzene-containing hydrophobic monomers and exhibiting a higher-than-ambient glass transition temperature can show superior mechanical properties. Highly water-stable physical hydrogels with water contents of 50–60 wt.% and exceptionally high Young’s modulus (150–280 MPa), tensile strength (7–17 MPa), and fracture energy (6680–7450 J/m2) were developed; these values are far superior to those of single-component hydrogels and tough hydrogels reported to date. This combination of superior properties, reported here for the first time, is expected to significantly broaden the application of hydrogels. The proposed strategy is quite general and offers further opportunities to develop diverse ultra-stiff, strong, and tough functional hydrogels using suitable monomer combinations.