Resolving light-induced structural rearrangements in responsive microgels

Optically-responsive microgels offer a versatile platform for designing adaptive soft materials with coupled light and thermal responsiveness. Control over the crosslinking degree is particularly appealing as it can regulate not only particle size but also stiffness, thereby enabling remote tuning of key material functionalities. However, the internal structural changes that couple molecular photoresponsive mechanisms to mesoscopic properties remain poorly resolved. Here, we investigate different light-responsive microgels containing covalently incorporated coumarin moieties, which impart optical sensitivity through UV-induced cycloaddition, by combining dynamic light scattering, small-angle neutron scattering, and molecular dynamics simulations. We show that light irradiation alters not only particle size but also the internal polymer density distribution and subsequent thermal response. Before irradiation, the microgels exhibit a star-like architecture with a dense core and extended polymeric arms. After irradiation, the network evolves toward a markedly more compact structure. This transformation cannot be rationalized simply as an equivalent to an increase in crosslinking density during synthesis, as evidenced by the altered thermal response. Our results identify light-induced crosslinking as an effective post-synthetic route to tune both microgel architecture and coupled optical and thermal responsiveness.