Tuning non-ergodic aging in hydrogels using environment-dependent interfacial chemistry*

Hydrogels are polymeric soft materials that can be engineered to suit a multitude of applications that exploit their tunable mechanochemical properties. Dynamic hydrogels formed of polymer-nanoparticle assemblies employing noncovalent, physically crosslinked networks dominated by either enthalpic or entropic effects enable unique rheological and stimuli-responsive properties. As opposed to enthalpy-driven dynamic networks that soften with temperature, entropic interactions result in largely temperature-independent mechanical properties. By engineering interfacial polymer-nanoparticle interactions, we induce a dynamic-to-covalent transition in entropic hydrogels that leads to non-ergodic aging in the microstructure. This transition is tuned by varying temperature and formulation environment such as pH, which allows for multivalent tunability in properties. These hydrogels can thus be designed to exhibit either temperature-independent metastable dynamic crosslinking or time-dependent stiffening based on formulation and storage conditions. Such robust materials with versatile and adaptable properties can be utilized in applications such as wildfire suppression, surgical adhesive films, and depot-forming injectable drug delivery systems.