Anomalous Dynamics of Concentrated Silica-PNIPAm Nanogels.

Abstract

We present the structure and dynamics of highly concentrated core-shell nanoparticles composed of a silica core and a poly(N-isoproylacrylamide) (PNIPAm) shell suspended in water. With X-ray photon correlation spectroscopy we are able to follow dynamical changes over the volume phase transition of PNIPAm at LCST = 32°C. On raising the temperature beyond LCST, the structural relaxation times continue to decrease. The effect is accompanied by a transition from stretched to compressed exponential shape of the intensity autocorrelation function. Upon further heating we find a sudden slowing down for the particles in their collapsed state. The q-dependence of the relaxation time shows an anomalous change from τc ∝ q-3 to τc ∝ q-1. Small angle x-ray scattering data indicate a temperature-induced transition from repulsive to attractive forces. Our results indicate a temperature-induced phase transition from a colloidal liquid with polymer-driven dynamics towards a colloidal gel.