Using Fast-Reactive Cross-Linkers to Modulate the Internal Structure of Thermoresponsive Microgels

The internal architecture of poly(N-isopropylacrylamide) (PNIPAM) microgels, which switches from fuzzy-sphere to star-like structures when the standard N,N′-methylenebis(acrylamide) (BIS) cross-linker is replaced with ethylene glycol dimethacrylate (EGDMA), critically determines their interactions and swelling behavior. Here, we systematically investigate the role of the surfactant and cross-linker content in modulating the internal structure of the microgels using dynamic light scattering, small-angle X-ray scattering, and monomer-resolved numerical simulations. We reveal that the presence of the surfactant is crucial for obtaining the star-like architecture, and that the transition from the star-like regime to a more core-dominated structure occurs above a threshold EGDMA concentration. Monomer-resolved simulations capture how the role of surfactant differs between EGDMA-cross-linked and BIS-cross-linked microgels. Our findings establish a direct synthesis–structure relationship, providing a clear guidance for the rational design of soft, star-like microgels with ultrasoft interactions, strengthening the connection between microgels and model star polymers.