Dynamics of interacting nanoparticles in complex polymeric solutions

The nanoscale mechanisms underlying the unusual mechanical properties of polymer nanocomposites and other soft composites remain elusive. Because nanoparticles are of comparable size to heterogeneities present in many complex fluids, their dynamics cannot be described through the framework of microrheology but rather decouple from the bulk fluid properties. Indeed, nanoparticles diffuse many orders of magnitude faster than expected when coupled to complex fluid relaxations on similar length scales, or much more slowly than expected when physically obstructed. Interactions between the nanoparticles, arising from high particle densities and/or chemical surface modifications, can further modify their dynamics. To understand the underlying physics of transport in this size regime, we measure the dynamics of nanoparticles and polymers in solution, which are simple models of polymer nanocomposites with well-controlled and tunable heterogeneities, using x-ray photon correlation spectroscopy and neutron spin-echo spectroscopy. Electrostatic charges lead to long-range interactions between the particles in organic solvents without disrupting the structure or dynamics of the surrounding polymer solution. The long-range interparticle interactions slow nanoparticle dynamics across the interparticle distance, even though the nanoparticle dynamics are subdiffusive and coupled to the polymer relaxations. Grafted polymers help to stabilize the nanoparticles in complex fluids and lead to soft physical interactions between the grafted particles and the surrounding polymer chains that alter their transport. Our work illustrates that the particle surface chemistry sensitively modifies the transport of nanoparticles through complex media.