Polymer-Mediated Assembly of Nanoparticles into Anisotropic Architectures

Polymer-nanoparticle composites have attracted considerable interest over the past few decades. While many traditional applications of composites require the nanoparticles (NPs) to remain well dispersed within the polymer matrix, some of the newer proposed applications rely on higher-order organization of NPs. Self-assembly provides a powerful bottom-up approach for organizing nanoparticles in a highly parallelized fashion. However, directing nanoparticles to self-assemble into anisotropic architectures more complex than the isotropic, close-packed structures or random aggregates observed under equilibrium or non-equilibrium conditions is highly challenging. In this dissertation, I will demonstrate how we have used molecular dynamics simulations to investigate and propose new polymer-mediated strategies for assembling spherical NPs into anisotropic, and often unique, configurations.We first investigated the underlying basis for anisotropic interactions between spherical NPs uniformly grafted with polymer chains, which were recently shown to assemble into anisotropic phases like strings and sheets. The anisotropy was shown to arise from the expulsion of polymer grafts between two contacting NPs, which led to anisotropic graft-mediated steric repulsion felt by a third approaching NP. Our computed phase diagram for formation of isotropic versus anisotropic 3-particle clusters agreed qualitatively with that obtained experimentally for larger aggregates of NPs. Next, we proposed a new strategy for assembling spherical nanoparticles into unique, anisotropic architectures in a polymer matrix. The approach takes advantage of the interfacial tension between two mutually immiscible polymers forming a bilayer to trap NPs within two-dimensional planes parallel to the interface. We demonstrated both trapping NPs at tunable distances from the interface and assembling them into a variety of unconventional nanostructures. We also developed a theoretical model to predict the preferred positions and free energies of NPs. Lastly, we studied the dynamics of polymer-grafted gold nanoparticles loaded into polymer melts. Under certain annealing conditions, the diffusion is one-dimensional and related to the direction of heat flow during annealing and is associated with an dynamic alignment of the host polymer chains. We used molecular dynamics simulations to investigate a single gold nanoparticle diffusing in a partially aligned polymer network which semi-quantitatively reproduce the experimental results to a remarkable degree