Thermally Programmable Off-Equilibrium Pathways in Salt-Doped Polyelectrolyte Hydrogels

Creating lifelike dynamic adaptive performance in synthetic hydrogels is of great interest yet remains a grand challenge. The main difficulty lies in establishing robust off-equilibrium responses to external stimuli, which often necessitates intricate chemical structure designs or complex preparation protocols. Here, we demonstrate an off-equilibrium pathway control in monovalent salt-doped poly(acrylic acid) (PAAc) hydrogels under thermal stimuli, which enables unreported transient turbidity, reminiscent of lifelike dynamic responsive behaviors. The underlying mechanism of such an off-equilibrium phenomenon has been explored by correlating microscopic polyelectrolyte network properties with its macroscopic turbidity evolution. Upon heating, salt-doped PAAc gels absorb water and swell rapidly due to the weakening of ion-mediated attractive electrostatic and hydrophobic interactions. Intriguingly, subsequent rapid cooling leads to the swift recovery of these attractive interactions, resulting in the formation of metastable aggregated structures that kinetically trap water to scatter light, during which the gradual diffusion of the entrapped water molecules causes a dramatically slow shrinking of the gel compared with its swelling process, giving rise to a transient transparent-to-turbid transition. Notably, an elevated temperature leads to decreased asymmetric swelling/shrinking kinetics, providing effective ways to tune its recovery behaviors. Leveraging this mechanism, we demonstrate spatially controlled thermal excitation for programming dynamic optical patterns and rewritable displays. This work not only offers a facile route to biomimetic adaptive hydrogels but also brings deeper insights into the understanding of off-equilibrium dynamics in ion-polyelectrolyte networks.