Aggregation of thermoresponsive core-shell nanoparticles: Influence of particle concentration, dispersant molecular weight and grafting.

Thermoresponsive core-shell nanoparticles represent an interesting class of materials with a triggered solubility transition. However, depending on the system, their aggregation behavior above the lower critical solution temperature (LCST) is ambiguous and obviously linked to a multitude of parameters. The induced aggregation of a set of well characterized poly(N-isopropylacrylamide)-nitrodopamine grafted superparamagnetic iron oxide nanoparticles is investigated with respect to the PNIPAM molecular weight (5-30kDa) and concentration using differential scanning calorimetry and temperature-cycled dynamic light scattering measurements. PNIPAM molecular weight clearly influences the thermoresponsiveness of the material, including LCST, colloidal aggregation and deaggregation as well as transition enthalpy. Furthermore, a strong impact of topology (grafted vs. free polymer chain) on the thermoresponsiveness is observed. Cluster size above the LCST depends on concentration and PNIPAM molecular weight. This makes it reasonable to conduct aggregation experiments under constant (high) particle molar concentration for comparative studies. Similarly, low particle molar concentration is used to elucidate individual particle shell properties by avoiding masking inter-particle effects.