Methyl Matters: Molecular Regulation of the Soft-to-Rigid Transition of Shape Memory Hydrogels for Programmable Deformation

Abstract

The properties of intelligent polymeric materials are essentially determined by their chemical structures. Shape memory hydrogels that utilize hydrophobic aggregation at elevated temperatures to stabilize temporary shapes, the association of polymer networks is similar to that of thermoresponsive polymers. To realize a precise control on the shape memory function of these materials and broaden their practical applications, it is essential to build up a structure–property relationship on a molecular level. In this study, we fabricated a series of poly(acrylic acid–methacrylic acid)/Ca(CH3COO)2 shape memory hydrogels with cooling-induced shape transformation behavior. The results showed that the incorporation of α-methyl groups would restrict the hydrophobic aggregation of hydrogel networks, and the soft-to-stiff transition temperature of the hydrogels would shift to elevated temperatures with the increasing density of α-methyl groups. Therefore, the shape recovery process could be regulated by adjusting the association degree of the polymer networks. Finally, the hydrogels were explored as smart capsules with sequential drug delivery capacity and as dredging devices capable of removing thrombus. This study reveals the influence of α-methyl groups on the association dynamics of the poly(acrylic acid–methacrylic acid)/Ca(CH3COO)2 hydrogels, which would promote the design of novel shape memory polymers guided by the relationship between chemical structure and properties.