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

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 degrees C up to 38.9 degrees 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.

Automated summary

The collapse dynamics of gold-poly(N-isopropylacrylamide) core-shell microgels were studied using capacitor-discharge temperature-jump spectroscopy. The results showed that the collapse process consists of a fast component and a slow component. The fast component corresponds to the rapid contraction of the loosely cross-linked outer corona of the microgels, while the slow component involves the subsequent rearrangement of the polymer chains. The lifetime of the fast component scales linearly with the change in microgel radius, while the lifetime of the slow component remains constant. The relative contribution of the slow component to the overall change in optical density is largest when the initial and final states are closer to the fully collapsed state of the microgels.