Cross-Linker Hydrophobicity Determines Microgel Nanostructures

In aqueous precipitation polymerization, which enables the synthesis of uniformly sized microgels, the monomers are dissolved and the polymer precipitates, leading to nucleation and growth process driven by hydrophobic interactions. Therefore, the (in)compatibility of chemical species used with aqueous solvents is expected to influence the nanostructure of the resulting microgels. In this study, several cross-linkers differing in the number of ethylene glycol units─hence varying in their compatibility with aqueous solvents─were used to synthesize thermoresponsive poly(N-isopropylacrylamide)-based microgels via aqueous precipitation polymerization. The structure and thermoresponsiveness of the resulting microgels were evaluated using dynamic light scattering and temperature-controllable high-speed atomic force microscopy. While microgels cross-linked with hydrophilic cross-linkers exhibited a hierarchically heterogeneous nanostructure, microgels cross-linked with hydrophobic cross-linker showed significant changes in nanostructure with increasing cross-linker raito, changing from a core-shell structure to a spherical structure with rough surface. Kinetic analysis of polymerization revealed that increasing the amount of hydrophobic cross-linker led to a polymerization mechanism other than precipitation polymerization. Similar nanostructures and thermoresponsiveness were also observed with different types of hydrophobic cross-linker, suggesting the universality of these features. In an era where cross-linkers are no longer limited to maintaining structure but are also expected to provide functionality, this study offers guidance for designing microgels with hydrophobic cross-linkers.