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 the terpolymer hydrogel. To obtain the same storage modulus, the FOSM content for the copolymer hydrogel would need to be more than doubled, but this hydrogel has almost 40% less water than the terpolymer hydrogel. The terpolymer-based hydrogel exhibited improved creep resistance by increasing the relaxation times through the synergistic effect of hydrophobic and ionic associations. On recovery from creep, the terpolymer-based hydrogel responded primarily elastically. Despite this elastic-like behavior, the terpolymer-based hydrogel can also efficiently self-heal its microstructure. These results illustrate the ability to dramatically alter the mechanical response of hydrogels through ionic associations even when only stoichiometric quantities of Zn²⁺ are present.