Spherical nanoparticles (NPs) uniformly grafted with polymer chains have recently been shown to assemble into anisotropic phases like strings and sheets. Here we investigated the underlying basis for anisotropic interactions between polymer-grafted NPs in a polymer matrix by computing via molecular dynamics simulations the potential of mean force (PMF), and its three-body contribution, for a test NP interacting with a NP-dimer along a set of reaction coordinates differing in their orientation with respect to the dimer axis. The polymer-mediated portions of the PMF and of the three-body contribution were both found to be highly repulsive and anisotropic with the degree of repulsion rising with increasing angular deviation from the dimer axis. The anisotropy was shown to arise from the expulsion of polymer grafts from in between the dimer NPs which leads to a gradient in the graft segmental density around the dimer from its contact point to its poles. This effect produces a concomitant gradient in steric repulsion between test and dimer NP grafts, a significant portion of which is however negated by an opposing gradient in depletion attraction between NPs due to the matrix. The anisotropy in the polymer-mediated PMF was observed to be particularly strong for NP–polymer systems with long grafts, high grafting densities, and short matrix chains. The overall PMFs allowed us to compute the free energies of formation of two- and three-particle clusters, yielding a phase diagram in graft length–grafting density parameter space analogous to that observed experimentally for the dispersed, stringlike, and sheetlike phases of NPs. The PMFs also revealed possible existence of a stable dimer phase that remains to be tested experimentally. Taken together, this study illustrates how the deformability of NP grafts can introduce novel anisotropic interactions between otherwise isotropic NPs with far-reaching consequences in NP assembly.
We propose a strategy for assembling spherical nanoparticles (NPs) into anisotropic architectures in a polymer matrix. The approach takes advantage of the interfacial tension between two mutually immiscible polymers forming a bilayer and differences in the compatibility of the two polymer layers with polymer grafts on particles to trap NPs within two-dimensional planes parallel to the interface. The ability to precisely tune the location of the entrapment planes via the NP grafting density, and to trap multiple interacting particles within distinct planes, can then be used to assemble NPs into unconventional arrangements near the interface. We carry out molecular dynamics simulations of polymer-grafted NPs in a polymer bilayer to demonstrate the viability of the proposed approach in both trapping NPs at tunable distances from the interface and assembling them into a variety of unusual nanostructures. We illustrate the assembly of NP clusters, such as dimers with tunable tilt relative to the interface and trimers with tunable bending angle, as well as anisotropic macroscopic phases, including serpentine and branched structures, ridged hexagonal monolayers, and square-ordered bilayers. We also develop a theoretical model to predict the preferred positions and free energies of NPs trapped at or near the interface that could help guide the design of polymer-grafted NPs for achieving target NP architectures. Overall, this work suggests that interfacial assembly of NPs could be a promising approach for fabricating next-generation polymer nanocomposites with potential applications in plasmonics, electronics, optics, and catalysis where precise arrangement of polymer-embedded NPs is required for function.
Self-assembly of faceted nanoparticles is a promising route for fabricating nanomaterials; however, achieving low-dimensional assemblies of particles with tunable orientations is challenging. Here, we demonstrate that trapping surface-functionalized faceted nanoparticles at fluid–fluid interfaces is a viable approach for controlling particle orientation and facilitating their assembly into unique one- and two-dimensional superstructures. Using molecular dynamics simulations of polymer-grafted nanocubes in a polymer bilayer along with a particle-orientation classification method we developed, we show that the nanocubes can be induced into face-up, edge-up, or vertex-up orientations by tuning the graft density and differences in their miscibility with the two polymer layers. The orientational preference of the nanocubes is found to be governed by an interplay between the interfacial area occluded by the particle, the difference in interactions of the grafts with the two layers, and the stretching and intercalation of grafts at the interface. The resulting orientationally constrained nanocubes are then shown to assemble into a variety of unusual architectures, such as rectilinear strings, close-packed sheets, bilayer ribbons, and perforated sheets, which are difficult to obtain using other assembly methods. Our work thus demonstrates a versatile strategy for assembling freestanding arrays of faceted nanoparticles with possible applications in plasmonics, optics, catalysis, and membranes, where precise control over particle orientation and position is required.
We investigated the dynamics of polymer-grafted gold nanoparticles loaded into polymer melts using X-ray photon correlation spectroscopy. For low molecular weight host matrix polymer chains, normal isotropic diffusion of the gold nanoparticles is observed. For larger molecular weights, anomalous diffusion of the nanoparticles is observed that can be described by ballistic motion and generalized Levy walks, similar to those often used to discuss the dynamics of jammed systems. 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. Molecular dynamics simulations of a single gold nanoparticle diffusing in a partially aligned polymer network semi-quantitatively reproduce the experimental results to a remarkable degree. The results help to showcase how nanoparticles can under certain circumstances move rapidly in polymer networks.
The shape of Ag2O crystals that evolve from edge and corner‐truncated cubes into rhombicuboctahedrons, then to hexapods can be conducted precisely by simply controlling the amount of AgNO3, NH4NO3 and NaOH precursors. The edge and corner‐truncated cube crystals possess the highest photocatalytic activity, followed by the rhombicuboctahedrons, then the hexapods. The photocatalytic activity of the Ag2O crystals depends greatly on the type of the exposed facets. The {1 0 0} facets exhibit the highest photocatalytic activity in methyl orange solution under full‐spectrum light irradiation, followed by the {1 1 0} facets, then the {1 1 1} facets. The {1 0 0} facets are also demonstrated to show intense susceptibility toward etching by NH3.
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