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

The\nproperties of intelligent polymeric materials are essentially\ndetermined by their chemical structures. Shape memory hydrogels that\nutilize hydrophobic aggregation at elevated temperatures to stabilize\ntemporary shapes, the association of polymer networks is similar to\nthat of thermoresponsive polymers. To realize a precise control on\nthe shape memory function of these materials and broaden their practical\napplications, it is essential to build up a structure–property\nrelationship on a molecular level. In this study, we fabricated a\nseries of poly(acrylic acid–methacrylic acid)/Ca(CH<sub>3</sub>COO)<sub>2</sub> shape memory hydrogels with cooling-induced shape\ntransformation behavior. The results showed that the incorporation\nof α-methyl groups would restrict the hydrophobic aggregation\nof hydrogel networks, and the soft-to-stiff transition temperature\nof the hydrogels would shift to elevated temperatures with the increasing\ndensity of α-methyl groups. Therefore, the shape recovery process\ncould be regulated by adjusting the association degree of the polymer\nnetworks. Finally, the hydrogels were explored as smart capsules with\nsequential drug delivery capacity and as dredging devices capable\nof removing thrombus. This study reveals the influence of α-methyl\ngroups on the association dynamics of the poly(acrylic acid–methacrylic\nacid)/Ca(CH<sub>3</sub>COO)<sub>2</sub> hydrogels, which would promote\nthe design of novel shape memory polymers guided by the relationship\nbetween chemical structure and properties.