Stability and Anomalous Aggregation in Suspensions of Polymer-Grafted Nanoparticles

Polymer-grafted nanoparticles (PGNPs) are a class of hybrid materials with properties intermediate between those of polymer and colloidal systems. Here, we assess how polymer conformation and solvent interactions govern structural stability, phase transitions, and aggregation kinetics in suspensions of gold nanoparticles (AuNPs) grafted with polystyrene of varying molecular weights. When suspended in cyclohexane, the PGNPs exhibit a two-step transition as a function of temperature in which the polymer corona contraction precedes aggregation below the upper critical solution temperature. Time-dependent dynamic light scattering measurements reveal that this aggregation follows a diffusion-limited aggregation process in which the hydrodynamic size increases with time according to a power law with an exponent α ≈ 1/3. This exponent is significantly lower than that observed for colloidal systems. We attribute this discrepancy to a combination of long-range interactions and the viscoelasticity of the grafted layer facilitating rearrangements within the aggregate. Our findings provide quantitative measurements of PGNP phase behavior by quantifying the rate of aggregation, combining polymer thermodynamics and colloidal physics.