Temperature-Jump Spectroscopy of Gold–Poly(N-isopropylacrylamide) Core–Shell Microgels

Abstract

The collapse dynamics of gold–poly(N-isopropylacrylamide) core–shell microgels were measured by capacitor-discharge temperature-jump spectroscopy. Using a series of temperature jumps from 31 °C up to 38.9 °C, we could monitor a characteristic two-component volume phase transition by changes in optical density that occurred on a time scale of milliseconds. Kinetic data were compared for microgels over a range of polymer shell thicknesses and cross-linker densities. We show that the fast component of the two-component collapse is consistent with the rapid contraction of the loosely cross-linked outer corona of the polymeric microgel, where the polymer density is lowest. The slow component corresponds to subsequent rearrangement of the polymer chains. The lifetime of the fast component scales linearly with the overall change in microgel radius, and the dynamics are consistent with the collapse of long polymer chains in the outer corona. The lifetime of the slow polymer rearrangement is almost constant over all the tested parameters. The relative contribution of the slow component to the overall change in optical density is largest when the initial and final states of the transition are closer to the fully collapsed state of the microgels. The relative contribution of the fast component is largest when the microgel is initially more swollen.