Solvent‐Free Fabrication of Robust Physical Hydrogels via Bulk Copolymerization for Underwater Acoustics

Conventional hydrogel preparation typically involves the use of solvents (mostly water), which increases the free volume between polymer chains and weakens their interactions, resulting in mechanically weak hydrogels with limited viscoelasticity. Here, a solvent-free synthesis strategy is presented to produce tough, swelling-resistant physical hydrogels with ultra-wide frequency viscoelasticity (loss factor tanδ > 0.3 at 10^-10-10⁵ Hz). This is achieved through the bulk copolymerization of highly miscible hydrophilic and hydrophobic monomers, forming water-free copolymers that develop into phase-separated hydrogels with dense chain entanglements upon equilibration in water. The resulting hydrogel exhibits significant mechanical enhancements over counterparts prepared through classical micellar copolymerization, with stiffness, strength, and fracture toughness increased by up to 81, 46, and 41 times, respectively. As a proof-of-concept demonstration, the tough hydrogel with excellent swelling resistance is applied as a sound-absorbing coating for underwater vehicles, minimizing acoustic interference during marine research through its strong viscoelasticity and near-perfect acoustic impedance matching with water. Unlike conventional solvent-assisted polymerization methods used for hydrogel preparation, bulk copolymerization offers advantages such as ultra-high monomer concentration, pure products, and straightforward process, providing new insights for developing simple yet tough hydrogels that are not only promising for underwater applications but also conducive to theoretical studies.