Transport of Nanoparticles through Complex Polymeric Fluids



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Transport of nanoparticles affects applications ranging from targeted drug delivery to enhanced oil recovery to processing of nanocomposite materials. In each of these applications, the nanoparticles must be transported through a complex fluid to reach the desired target, whether that be a cancerous tumor, the oil-water interface, or a polymer melt. For large particles, the surrounding medium is effectively homogeneous across the surface of the particle so that the transport properties can be directly related to the bulk fluid properties. For nanoparticles, however, the particle size is comparable to the length scales of heterogeneities in the fluid so that the particle dynamics decouple from bulk properties and are poorly understood. Here, we experimentally investigate how functionalized nanoparticles transport through polymer solutions as a model system for a complex, heterogeneous material using a combination of optical microscopy and scattering techniques. When the nanoparticles are comparably sized to the dissolved polymer, the nanoparticle dynamics exhibit two qualitatively different behaviors – subdiffusive on short time scales before recovering diffusive dynamics on long time scales. The long-time diffusivities are controlled by the segmental relaxations of the polymer chains over the particle surface, whereas the short time subdiffusive dynamics additionally couple to the center-of-mass dynamics of the polymer chains. In the presence of long-range interparticle interactions, the subdiffusive particle dynamics are slowed over the interparticle distance indicating that the polymer solution provides only a finite energy barrier to nanoparticle transport. This finite energy barrier can be exploited to drive the self-assembly of anisotropic nanoparticles functionalized with short-chain polymers into porous, fractal structures with controllable structural properties. For high molecular weight grafted chains, however, the dynamics of the grafted chains are confined over the distance between the grafted chains at the nanoparticle surface and affect the transport of the polymer-grafted nanoparticle through solutions of free polymer. Using the physics elucidated with this work will grant better control over the transport and dispersion of nanoparticles through complex, heterogeneous materials.



Polymers, Nanoparticles, Dynamics, Scattering