Mesoscale Modeling and Computer Simulation of Tethered Nanoparticle "Shape-Amphiphile" Assemblies.

Abstract

In this dissertation, we explore the use of polymer-tethered nanoparticles as a means to self-assemble highly ordered arrays of nanoparticles and nanometer-sized domains. We perform Brownian dynamics simulations to study the self-assembly of polymer functionalized spherical and rod-like nanoparticles. Immiscibility between tethers and nanoparticles facilitates assembly into highly ordered structures reminiscent of phases formed by surfactants and block copolymers, but with greater complexity. We explore the influence of key factors such as the nanoparticle size and shape, tether architecture, solvent selectivity, and bulk volume fraction on the resulting structures.

In this thesis we perform several studies. First, we explore the phase behavior of mono-tethered nanospheres. Under solvent conditions that are poor for the tethers, we find phase behavior that is similar to surfactants with structures including lamellae, perforated lamellae, hexagonally packed cylinders, and spherical micelles. We report quasicrystalline-like ordering between the spherical micelles and propose an entropic model to explain this behavior. We also explore the phase behavior of a mono-tethered nanosphere system where nanospheres are in poor solvent. We find phases similar to surfactants including lamellae, perforated lamellae, double gyroid, and hexagonally packed cylinders. We see a predominance of icosahedral arrangements of nanospheres in phases with 2D confinement and crystalline packing of nanospheres in structures with 1D confinement. We also compare and contrast the formation of the double gyroid structure for tethered nanospheres and tethered nanorods. We show that the ability of the nanoparticles to locally order into icosahedra (nanospheres) and hexagonally splayed bundles (nanorods) reduces packing frustration making these structures more stable than their block copolymer counterparts.

We also explore the phase behavior of di-tethered nanospheres. We find a complex phase behavior for di-tethered nanospheres similar to triblock copolymers with phases including lamellae, tetragonally packed cylinders, alternating gyroid, and alternating diamond. We also report two novel phases not seen in triblock copolymers: NaCl ordered spherical micelles with a complementary simple cubic network of nanospheres and ZnS ordered micelles with a complementary diamond network of nanospheres.

Throughout this thesis we focus on understanding why these complex structures form and what trends exist.