Résumé
We use dynamic light scattering (DLS) and synchrotron small-angle X-ray scattering (SAXS) to investigate the glassy dynamics of concentrated suspensions of thermosensitive Ultra-Low Crosslinked (ULC) poly(N-isopropylacrylamide) (PNIPAM) microgels. We explore states at various effective volume fraction φ, which can be tuned by swelling or deswelling the microgels (by changing temperature T) at fixed mass concentration c, or by varying c at fixed T. Remarkably, we find that the dynamics are governed only by φ(T,c), while the microscopic structure depends on both T and c. Although our microgels are softer than conventionally cross-linked PNIMPAM microgels, we find that they are fragile glass formers, questioning the notion that softer colloids make stronger glasses. Finally, we find a non-conventional dependence of the structural relaxation time on the scattering vector q, which we rationalize by proposing a master curve that collapses data collected at several q and effective volume fractions φ.