Manipulating the Mechanical Response of Hydrophobically Cross-Linked Hydrogels with Ionic Associations

To prevent brittle failure, tough hydrogels rely on energy dissipation, which can be manifested through sacrificial covalent bonds or reversible, noncovalent cross-links. However, these noncovalent cross-links tend to lead to significant creep during deformation due to rearrangements of the effective cross-links. Here, the influence of ionic associations as a secondary network in noncovalently cross-linked hydrogels is examined using a terpolymer of hydroxyethyl acrylate (HEA), 2-(N-ethylperfluorooctane-sulfonamido)ethyl methacrylate (FOSM), and zinc diacrylate (ZnA). Despite the solubility of HEA–ZnA copolymers in water, the incorporation of ionic moieties that contain stoichiometric quantities of zinc into a network cross-linked by hydrophobic associations significantly increased the effective cross-link density. The terpolymer-based hydrogel contained ≈90% of the water of a HEA–FOSM copolymer hydrogel with the same FOSM content, but the storage modulus was nearly an order of magnitude larger than for t...