It is easy to understand the self-assembly of particles with anisotropic shapes or interactions (for example, cobalt nanoparticles or proteins) into highly extended structures. However, there is no experimentally established strategy for creating a range of anisotropic structures from common spherical nanoparticles. We demonstrate that spherical nanoparticles uniformly grafted with macromolecules ('nanoparticle amphiphiles') robustly self-assemble into a variety of anisotropic superstructures when they are dispersed in the corresponding homopolymer matrix. Theory and simulations suggest that this self-assembly reflects a balance between the energy gain when particle cores approach and the entropy of distorting the grafted polymers. The effectively directional nature of the particle interactions is thus a many-body emergent property. Our experiments demonstrate that this approach to nanoparticle self-assembly enables considerable control for the creation of polymer nanocomposites with enhanced mechanical properties. Grafted nanoparticles are thus versatile building blocks for creating tunable and functional particle superstructures with significant practical applications.

We critically explore the role of particle dispersion on the melt state mechanical properties of nanocomposites formed by mixing polystyrene homopolymers with polystyrene grafted silica nanoparticles. We selected this system since we previously showed that nanoparticle spatial distribution can be controlled through judicious choices of the brush and matrix parameters. Here we focus on the temporal evolution of the nanoparticle self-assembly dispersion state and its effect on mechanical reinforcement using rheology, electron microscopy, and the measurement of nanoscale particle dynamics using X-ray photon correlation spectroscopy. Nanoscale and macroscopic experiments show that a composite with percolating sheets of particles displays "gel-like" or solid-like mechanical behavior at lower particle loadings than one with uniform particle dispersion. This conclusion allows us to conjecture that mechanical reinforcement is primarily controlled by interparticle interactions (including those facilitated by the grafted chains) and that the matrix plays a relatively minor role. This statement has far-reaching consequences on the design of polymer nanocomposites with desired properties.

No abstract

Hydrophobic iron oxide nanoparticles grafted with hydrophobic polymer chains of varying molecular weights and graft densities are synthesized to underpin the role of brush entanglement and dipolar forces on creating nanostructures. Grafting density on magnetic nanoparticles is controlled in graftingto method by changing the concentration of functionalized polymer in solution. The grafting density and brush length have varied systemically to observe the changes in nanostructures. Bridging between grafted chains and dipolar forces become effective only at low grafting density and result in long chains of particles. We demonstrate experimentally that structural transition of magnetic nanoparticles is controlled with the balance between grafted chain entanglements and dipolar forces.

We present soft, layered nanocomposites that exhibit controlled swelling anisotropy and spatially specific shape reconfigurations in response to light irradiation. The use of gold nanoparticles grafted with a temperature-responsive polymer (poly(N-isopropylacrylamide), PNIPAM) with layer-by-layer (LbL) assembly allowed placement of plasmonic structures within specific regions in the film, while exposure to light caused localized material deswelling by a photothermal mechanism. By layering PNIPAM-grafted gold nanoparticles in between nonresponsive polymer stacks, we have achieved zero Poisson's ratio materials that exhibit reversible, light-induced unidirectional shape changes. In addition, we report rheological properties of these LbL assemblies in their equilibrium swollen states. Moreover, incorporation of dissimilar plasmonic nanostructures (solid gold nanoparticles and nanoshells) within different material strata enabled controlled shrinkage of specific regions of hydrogels at specific excitation wavelengths. The approach is applicable to a wide range of metal nanoparticles and temperature-responsive polymers and affords many advanced build-in options useful in optically manipulated functional devices, including precise control of plasmonic layer thickness, tunability of shape variations to the excitation wavelength, and programmable spatial control of optical response.

We have recently shown that silica nanoparticles grafted with polystyrene chains behave akin to block copolymers due to the “dislike” between the nanoparticles and the grafts. These decorated nanoparticles, thus, self-assemble into various morphologies, from well-dispersed nanoparticles to anisotropic superstructures, when they are placed in homopolystyrene matrices of different molecular masses. Here, we consider a slightly different case, where the grafted chains and the matrix (both PMMA) are strongly attracted to the silica nanoparticle surface. We then conjecture that these systems show phase mixing or demixing depending on the miscibility between the brush and matrix chains (“autophobic dewetting”). At 15 mass % particle loading, composites created using the same grafted nanoparticle, but with two different matrices, yield well dispersed nanoparticles or nanoparticle “agglomerates”, respectively. Rheology experiments show that the composites display solid-like behavior only when the particles are aggregated. As deduced in previous work, this difference in behavior is attributed to the presence of percolating particle clusters in the agglomerated samples which allows for stress propagation through the system. Going further, we compare the local mobility of matrix and grafted segments of both composites using quasi-elastic neutron scattering experiments. For the liquid-like system, the mean square displacements of the grafted chains and matrix chains, the particle structuring and mechanical response are all unaffected by annealing time. In contrast, in the reinforced case, only the local matrix motion is unaffected by time. Since the particle clustering and solid-like mechanical reinforcement increase with increasing time, we conclude that mechanical reinforcement in polymer nanocomposites is purely based on the nanoparticles, with essentially no “interference” from the matrix. In conjunction with other results in the literature, we then surmise that mechanical reinforcement is caused by the bridging of particles by the grafted polymer layers and not due to the formation of “glassy” polymer layers on the nanoparticles.

We compare the rheological behavior of three classes of polymer nanocomposites (PNCs) to understand the role of particle shape and interactions on mechanical reinforcement. The first two correspond to favorably interacting composites formed by mixing poly(2-vinylpyridine) with either fumed silica nanoparticles (NPs) or colloidal spherical silica NPs. We show that fumed silica NPs readily form a percolated network at low NP volume fractions. We deduce that the NPs act as network junctions with the effectively irreversibly bound polymer chains serving as the connecting bridges. By comparing with colloidal spherical silica, which has a significantly higher percolation threshold, we conclude that the fractal shape of the fumed silica is responsible for its unusually low percolation threshold. The third system corresponds to polystyrene grafted colloidal silica nanoparticles (PGNPs) in a polystyrene matrix. These PNCs have an even lower percolation threshold probably because the grafted chains increase the effective volume fraction of the NPs. When we take these different thickness of the polymer layers in the two cases into account (i.e., grafted layer vs adsorbed layer thickness), the percolation threshold for the fumed and the grafted system occurs at similar effective loadings, but the NP network with fumed silica has a higher low-frequency plateau modulus than that formed with the PGNPs. These findings can be reconciled by the fact that the fumed silica NPs are composed of fused entities, thus ensuring that they have a higher modulus than the PGNPs where the modulus is largely attributed to interactions between the grafts. Our results systematically stress the important role of the nanofiller shape and connectivity on the mechanical reinforcement of PNCs.

Dynamics of the interphase region between matrix and bound polymers on nanoparticles is important to understand the macroscopic rheological properties of nanocomposites. Here, we present neutron scattering investigations on nanocomposites with dynamically asymmetric interphases formed by a high-glass transition temperature polymer, poly(methyl methacrylate), adsorbed on nanoparticles and a low-glass transition temperature miscible matrix, poly(ethylene oxide). By taking advantage of selective isotope labeling of the chains, we studied the role of interfacial polymer on segmental and collective dynamics of the matrix chains from subnanoseconds to 100 nanoseconds. Our results show that the Rouse relaxation remains unchanged in a weakly attractive composite system while the dynamics significantly slows down in a strongly attractive composite. More importantly, the chains disentangle with a remarkable increase of the reptation tube size when the bound polymer is vitreous. The glassy and rubbery states of the bound polymer as temperature changes underpin the macroscopic stiffening of nanocomposites.