Dual Temperature- and pH-Responsive Layered Hydrogels Synthesized via Halogen Bond-Based Solid Phase Radical Polymerization

Stimuli-responsive shape-changing layered hydrogels were, for the first time, prepared via solid-phase polymerization, where halogen bond-based solid-phase radical polymerization was utilized. Monomer cocrystals were assembled to form predetermined layered structures before polymerization, and all layers are polymerized at one time. AB bilayer and ABA and ABC trilayer hydrogel sheets that consisted of temperature- and pH-responsive layers were prepared. The obtained layered sheets were responsive to temperature and pH in dual manners at relatively wide ranges of temperature (5-65 °C) and pH (2.0-11.0). The bilayer sheets exhibited bending upon stimuli. The bending angle was tunable, and the bending direction (negative and positive directions) was also switchable in response to temperature and pH. The trilayer sheets exhibited switchable concave, trapezoid, and convex shape changes with modulated angles, which were unprecedented shape changes. Because of the ease of operation and wide monomer scope (using radical polymerization and halogen bonding), the present method offers a facile and versatile approach to fabricate stimuli-responsive shape-changing hydrogel materials.