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.

A functional monomer with a pendant azide moiety, 6-azidohexyl methacrylate (AHMA), was polymerized on the surface of silica nanoparticles via surface-initiated reversible addition−fragmentation chain transfer (RAFT) polymerization with considerable control over the molecular weight and molecular weight distribution. The kinetics of AHMA polymerization mediated by 4-cyanopentanoic acid dithiobenzoate (CPDB) anchored nanoparticles was investigated and compared with that of AHMA polymerization mediated by free CPDB under similar conditions. The subsequent postfunctionalizations of PAHMA-grafted nanoparticles were demonstrated by reacting with various functional alkynes via click reactions. Kinetic studies showed that the reaction of surface-grafted PAHMA with phenylacetylene surface-grafted PAHMA was much faster than that of free PAHMA with phenylacetylene, whereas in the case of high molecular weight alkynes surface-grafted PAHMA showed lower reaction rates as compared to free PAHMA.

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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.