Star-Like Thermoresponsive Microgels as an Emerging Class of Soft Nanocolloids

We provide experimental and numerical evidence of an emerging class of soft nanocolloids: star-like microgels with thermoresponsive character. This is achieved by using the standard precipitation polymerization synthesis of poly(N-isopropylacrylamide) (PNIPAM) microgels and replacing the usually employed cross-linking agent, N,N'-methylenebisacrylamide (BIS), with ethylene glycol dimethacrylate (EGDMA). The fast reactivity of EGDMA, combined with its strong tendency to self-bind, produces colloidal networks with a central, cross-linker-rich core, surrounded by a corona of long, cross-linker-free arms. These star-like microgels fully retain PNIPAM thermoresponsivity and undergo a volume phase transition at a temperature of ∼32 °C that is very sharp compared to standard PNIPAM-BIS microgels, independently of the cross-linker content. Dynamic light scattering and small-angle X-ray scattering experiments are compared to extensive simulation results, based on ideal star polymers as well as on state-of-the-art monomer-resolved simulations, offering microscopic evidence of the star-like internal structure of PNIPAM-EGDMA microgels. This can be described by an appropriate model for the form factors combining star and microgel features. The present work thus bridges the fields of star polymers and microgels, providing the former with the ability to respond to temperature via a facile synthetic route that can be routinely employed, opening the way to exploit these soft particles for a variety of fundamental studies and applicative purposes.