(284b) Dynamics of Polymer-Grafted Nanoparticles Under Soft Confinement

Polymer-grafted nanoparticles (PGNPs) are broadly useful materials that can enhance the mechanical properties of composite materials, improve biocompatibility of targeted drug delivery methods, and serve as flexible sensors. Controlling the performance of PGNPs in these applications requires fundamental understanding of their structure and dynamics in complex environments that is currently lacking. PGNPs are scientifically interesting due to their finite-sized hard core and flexible corona of grafted polymer. When the grafted polymer is much smaller than the hard core, PGNPs behave as nearly hard spheres. In the opposite limit, when the grafted polymer is much larger than the nanoparticle core, PGNPs would theoretically behave like star or dendritic polymers. In the intermediate regime where the polymer and core are comparably sized, the combination of hard and soft domains makes PGNPs structurally distinct from classical hard spheres or compressible polymer architectures. Although relevant to many applications, the structure and dynamics of these materials is poorly understood, due in part to the difficulty of isolating the behavior of individual components over relevant time and length scales. Here, we develop a novel model system that allows us to measure the nanoscale structure and dynamics of PGNPs using a combination of x-ray and neutron scattering methods.

We graft high molecular weight polystyrene (M_w = 355 kDa, R_g = 21 nm) to the surface of comparably sized silica nanoparticles (R = 24 nm) using a “grafting-to” synthetic route with “click” chemistry. Thus, these PGNPs are in the intermediate regime where both the finite core and polymer chain flexibility affect the controlling physics. We then disperse the PGNPs in semidilute solutions of linear polymer with molecular weights of 138, 656, and 1114 kDa. Exploiting the differences in scattering length densities, we probe the dynamics and structure of PGNPs on the nano- and microscale with complementary x-ray and neutron scattering methods. The PGNPs undergo structural changes like those of other soft colloids, but their dynamics are distinct. In the presence of free polymer, the grafted polymers compress because of an increase in the osmotic pressure of the solution, in agreement with earlier studies on polymers with complex architectures. The dynamics of the grafted polymer are confined by the neighboring polymer chains and become more confined as the grafted corona compresses. Additionally, the dynamics of the PGNPs through the polymer solutions decouple from the bulk solution viscoelasticity despite the PGNP hydrodynamic radius being much larger than that of the free polymer. These novel dynamics illustrate the physical complexity underlying the structure and dynamics of PGNPs, especially when dispersed in structured fluids. With this work, we identify how the structure and dynamics of PGNPs respond to complex environments, such as those found in materials processing and biological applications. The changes in the dynamics of grafted polymers and the polymer-grafted nanoparticles will be important to designing improved composite materials and targeted drug delivery vectors.