Influence of weak ionic associations on the mechanical properties of hydrogels crosslinked by hydrophobic associations

Since the initial reports of double network hydrogels, a variety of routes to provide energy dissipation in hydrogels have been reported such as the inclusion of physical crosslinks to provide reversible crosslinks. These hydrogels based on physical crosslinks tend to creep due to the reversibility. Inclusion of a second stronger network generally has been used to reduce creep and improve the elastic recovery. Here we take the opposite approach and include weak ionic associations (zinc diacrylate) to manipulate the mechanical behavior of hydrogels formed by hydration of copolymers of hydroxyethyl acrylate (HEA) and 2-(N-ethylperfluorooctane-sulfonamido)ethyl methacrylate (FOSM). The terpolymer-based hydrogel contained >85% 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 for the terpolymer hydrogel. To obtain the same storage modulus, the FOSM content would need to be more than doubled, but this HEA-FOSM copolymer-based hydrogel has almost 40 % less water than the terpolymer hydrogel. These results illustrate the ability to tune the mechanical response of hydrogels through weak ionic associations.