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International audience; Wetting of polymer-grafted nanoparticles (NPs) in a polymer nanocomposite (PNC) film is driven by a difference in surface energy between components as well as bulk thermodynamics, namely, the value of the interaction parameter, χ. The interplay between these contributions is investigated in a PNC containing 25 wt % polymethyl methacrylate (PMMA)-grafted silica NPs (PMMA-NPs) in poly(styrene-ran-acrylonitrile) (SAN) upon annealing above the lower critical solution temperature (LCST, 160 °C). Atomic force microscopy (AFM) studies show that the areal density of particles increases rapidly and then approaches 80% of that expected for random close-packed hard spheres. A slightly greater areal density is observed at 190 °C compared to 170 °C. The PMMA-NPs are also shown to prevent dewetting of PNC films under conditions where the analogous polymer blend is unstable. Transmission electron microscopy (TEM) imaging shows that PMMANPs symmetrically wet both interfaces and form columns that span the free surface and substrate interface. Using grazingincidence Rutherford backscattering spectrometry (GI-RBS), the PMMA-NP surface excess (Z*) initially increases rapidly with time and then approaches a constant value at longer times. Consistent with the areal density, Z* is slightly greater at deeper quench depths, which is attributed to the more unfavorable interactions between the PMMA brush and SAN segments. The Z* values at early times are used to determine the PMMA-NP diffusion coefficients, which are significantly larger than theoretical predictions. These studies provide insights into the interplay between wetting and phase separation in PNCs and can be utilized in nanotechnology applications where surface-dependent properties, such as wettability, durability, and friction, are important.

Within ordered poly(styrene-b-methyl methacrylate) (PS-b-PMMA) block copolymer (BCP) lamellae, oriented nanoplates [gadolinium trifluoride doped with ytterbium and erbium, GdF3:Yb/Er (20/2 mol %)] ...

Computationally guided ligand selection for enhanced co-assembly of noncomplementary nanocrystals., The use of nanocrystal (NC) building blocks to create metamaterials is a powerful approach to access emergent materials. Given the immense library of materials choices, progress in this area for anisotropic NCs is limited by the lack of co-assembly design principles. Here, we use a rational design approach to guide the co-assembly of two such anisotropic systems. We modulate the removal of geometrical incompatibilities between NCs by tuning the ligand shell, taking advantage of the lock-and-key motifs between emergent shapes of the ligand coating to subvert phase separation. Using a combination of theory, simulation, and experiments, we use our strategy to achieve co-assembly of a binary system of cubes and triangular plates and a secondary system involving two two-dimensional (2D) nanoplates. This theory-guided approach to NC assembly has the potential to direct materials choices for targeted binary co-assembly.

International audience; In this work, we examine binary and ternary nanocomposites of poly(methyl methacrylate) grafted silica nanoparticles (PMMA-NP), in poly(styrene-ran-acrylonitrile) (SAN), and poly(methyl methacrylate) matrices as a platform to directly probe governing parameters guiding phase behavior and nanoparticle assembly in composite materials. Through the addition of PMMA matrix chains similar in molecular weight to the grafted PMMA chains and significantly smaller than the SAN matrix chains, we observe increased nanoparticle miscibility in off-critical compositions due to interfacial segregation of PMMA matrix chains. A simple interfacial model provides a general guideline for predicting the extent of compatibilization. Further insights on compatibilization behavior are provided by polymer particle pair correlation functions and structure factors obtained using polymer reference interaction site model theory calculations as well as polymer concentration profiles from molecular dynamics simulations. This study serves as a guideline to facilitate PNC processing and design of materials for a broad range of technological applications.

Because of the considerable parameter space, efficient theoretical and simulation methods are required to predict the morphology and guide experiments in polymer nanocomposites (PNCs). Unfortunately, theoretical and simulation methods are restricted in their ability to accurately map to experiments based on necessary approximations and numerical limitations. In this study, we provide direct comparisons of two recently developed coarse-grained approaches for modeling polymer nanocomposites (PNCs): polymer nanocomposite field theory (PNC-FT) and dynamic mean-field theory (DMFT). These methods are uniquely suited to efficiently capture mesoscale phase behavior of PNCs in comparison to other theoretical and simulation frameworks. We demonstrate the ability of both methods to capture macrophase separation and describe the thermodynamics of PNCs. We systematically test how the nanoparticle morphology in PNCs is affected by a uniform probability distribution of grafting sites, common in field-based methods, vers...

Many studies on nanocrystal (NC) self-assembly into ordered superlattices have focused mainly on attractive forces between the NCs, whereas the role of organic ligands on anisotropic NCs is only in its infancy. Herein, we report the use of a series of dendrimer ligands to direct the assembly of nanoplates into 2D and 3D geometries. It was found that the dendrimer-nanoplates consistently form a directionally offset architecture in 3D films. We present a theory to predict ligand surface distribution and Monte Carlo simulation results that characterize the ligand shell around the nanoplates. Bulky dendrimer ligands create a nontrivial corona around the plates that changes with ligand architecture. When this organic-inorganic effective shape is used in conjunction with thermodynamic perturbation theory to predict both lattice morphology and equilibrium relative orientations between NCs, a lock-and-key type of mechanism is found for the 3D assembly. We observe excellent agreement between our experimental results and theoretical model for 2D and 3D geometries, including the percent of offset between the layers of NCs. Such level of theoretical understanding and modeling will help guide future design frameworks to achieve targeted assemblies of NCs.

Polymer nanocomposites (PNCs) that employ diblock copolymers (BCPs) to organize and align anisotropic nanoparticles (NPs) have the potential to facilitate self-assembling hierarchical structures. However, limited studies have been completed to understand the parameters that guide the assembly of nonspherical NPs in BCPs. In this work, we establish a well-defined nanoplate system to investigate the alignment of two-dimensional materials in a lamellar-forming poly(styrene-b-methyl methacrylate) (PS-b-PMMA) BCP with domains oriented parallel to the substrate. Monodisperse gadolinium trifluoride rhombic nanoplates doped with ytterbium and erbium [GdF3:Yb/Er (20/2 mol %)] are synthesized and grafted with phosphoric acid functionalized polyethylene glycol (PEG-PO3H2). Designed with chemical specificity to one block, the nanoplates align in the PMMA domain at low volume fractions (ϕ = 0.0083 and ϕ = 0.017). At these low NP loadings, the BCP lamellae are ordered and induce preferential alignment of the GdF3:Yb/Er...

Controlled chemical transformation of nanostructures is a promising technique to obtain precisely designed novel materials, which are difficult to synthesize otherwise. We report high-temperature vapor-phase anion-exchange reactions to chemically transform II-VI semiconductor nanostructures (100-300 nm length scale) while retaining the single crystallinity, crystal structure, morphology, and even defect distribution of the parent material via atomic templating. The concept of atomic templating is employed to obtain kinetically controlled, thermodynamically metastable structural phases such as zincblende CdSe and CdS from zincblende CdTe upon complete chemical replacement of Te with Se or S. The underlying transformation mechanisms are explained through first-principles density functional theory calculations. Atomic templating is a unique path to independently tune materials' phase and composition at the nanoscale, allowing the synthesis of novel materials.