Smart hydrogels based on responsive star-block copolymers

Abstract

A series of smart hydrogels based on dual stimuli responsive star-block copolymers responding to pH and temperature were prepared via atom transfer radical polymerization (ATRP) employing the core-first method. They consist of poly(2-(dimethylamino)ethyl methacrylate) (PDMA) inner blocks and outer blocks comprised of poly(di(ethylene glycol) methyl ether methacrylate) (PDEGMA). The aggregation behavior of these block copolymer stars is analyzed by dependence on block length and arm number. The dual stimuli responsiveness of the stars is demonstrated by turbidity as well as dynamic light scattering on dilute aqueous solution, and the gelation behavior of concentrated aqueous solutions is studied by rheology. Above the transition temperature of the PDEGMA outer blocks the stars form flower-like aggregates in dilute solution or free-standing gels at higher concentrations. When the temperature is increased further above the transition temperature of the PDMA inner block, the aggregates start to contract and a weakening was observed for soft gels, whereas for strong gels no influence on the moduli was detected. The behavior is controlled by both concentration and pH value. In addition, we show that the minimum polymer concentration for gel formation can be lowered by quaternizing the inner block of the stars, but a second response to stimuli is lost during the procedure.