Anomalous Dynamics of Concentrated Silica-PNIPAm Nanogels

In this study, performed at beamline “8-ID” (APS), we present the dynamics and interaction of highly concentrated colloidal core-shell systems, composed of a silica core and a poly(N-isopropylacrylamide) (PNIPAm) nanogel shell.

Poly(N-isopropylacrylamide) (PNIPAm) nanogels undergo a reversible volume phase transition (VPT) in water at a lower critical solution temperature (LCST) around 32°C. Thus, structure and dynamics have been studied as a function of temperature around the LCST as well as the modifications due to their reversible VPT.. Upon heating from room temperature to 45°C we found three dynamical regimes: At low temperatures the colloids show a sub-diffusive, polymer driven dynamical behavior. Around the LCST the relaxation time decreases rapidly. For T>37°C the dynamics slow down by more than three orders of magnitude and the particle motion becomes hyper-diffusive. Together with the static data, where we found indications of a transition from repulsive to attractive forces during the VPT, our results suggest a transition from a polymer-dominated liquid below the LCST to a jammed state at high temperatures and volume fractions.

J.Phys.Chem.Lett. 10, 5231 (2019)