Stimuli-responsive dendronized polymeric hydrogels through Schiff-base chemistry showing remarkable topological effects

Abstract

Hydrogels with dual-responsive and self-healing properties were prepared through Schiff-base chemistry from oligoethylene glycol (OEG)-based dendronized copolymers. These copolymers were comprised of 88–94% dendritic OEG components to provide thermoresponsiveness and 6–12% amino moieties for crosslinking with dialdehydes. To examine the architectural effect of the dendritic polymers on the formation and performance of the hydrogels, copolymers with linear OEG pendants of similar compositions were prepared. These hydrogels inherited the thermoresponsive properties from their parent OEG-based dendronized copolymers, and their thermally induced phase transition behavior as well as reversible sol–gel transitions mediated by pH were examined. Moreover, the dynamic covalent nature of the imine bond affords the hydrogels with adaptive self-healing properties without applying any extra stimulus. Most interestingly, by virtue of the enhanced Schiff-base reaction through either thermally induced polymer aggregation or freezing of polymer aqueous solutions, hydrogels were also prepared above the polymers’ cloud points (T_cs) or below the freezing points (T_fs) of the aqueous solutions, respectively. Hydrogels with different mechanical properties and porous structures were obtained accordingly, and exhibited different thermally induced shrinking behavior. The present work not only provides a convenient strategy for the preparation of novel hydrogels with unique multi-responsive and self-healing ability from dendritic polymers with the aid of Schiff-base chemistry, but also proves that crowding and thickness effects from dendronized polymers can make a significant contribution to the formation of hydrogels and lead to the enhancement of their mechanical strength.