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Next-generation structural materials are expected to be lightweight, high-strength and tough composites with embedded functionalities to sense, adapt, self-repair, morph and restore. This Review highlights recent developments and concepts in bioinspired nanocomposites, emphasizing tailoring of the architecture, interphases and confinement to achieve dynamic and synergetic responses. We highlight cornerstone examples from natural materials with unique mechanical property combinations based on relatively simple building blocks produced in aqueous environments under ambient conditions. A particular focus is on structural hierarchies across multiple length scales to achieve multifunctionality and robustness. We further discuss recent advances, trends and emerging opportunities for combining biological and synthetic components, state-of-the-art characterization and modelling approaches to assess the physical principles underlying nature-inspired design and mechanical responses at multiple length scales. These multidisciplinary approaches promote the synergetic enhancement of individual materials properties and an improved predictive and prescriptive design of the next era of structural materials at multilength scales for a wide range of applications.This Review discusses recent progress in bioinspired nanocomposite design, emphasizing the role of hierarchical structuring at distinct length scales to create multifunctional, lightweight and robust structural materials for diverse technological applications.

Eutectic gallium indium (EGaIn) is a liquid metal alloy at room temperature. EGaIn microdroplets can be incorporated into elastomers to fabricate highly stretchable, mechanically robust, soft multifunctional composites with high thermal stability1 and electrical conductivity2–4 that are suitable for applications in soft robotics and self-healing electronics5–7. However, the current methods of preparation rely on mechanical mixing, which may lead to irregularly shaped micrometre-sized droplets and an anisotropic distribution of properties8. Therefore, procedures for the stabilization of sub-micrometre-sized droplets of EGaIn and compatibilization in polymer matrices and solvents have attracted significant attention9–12. Here we report the synthesis of EGaIn nanodroplets stabilized by polymeric ligand encapsulation. We use a surface-initiated atom transfer radical polymerization initiator to covalently functionalize the oxide layer on the surface of the EGaIn nanodroplets13 with poly(methyl methacrylate) (PMMA), poly(n-butyl acrylate) (PBMA), poly(2-dimethylamino)ethyl methacrylate) (PDMAEMA) and poly(n-butyl acrylate-block-methyl methacrylate) (PBA-b-PMMA). These nanodroplets are stable in organic solvents, in water or in polymer matrices up to 50 wt% concentration, enabling direct solution-casting into flexible hybrid materials. The liquid metal can be recovered from dispersion by acid treatment. The nanodroplets show good mechanical, thermal and optical properties, with a remarkable suppression of crystallization and melting temperatures (down to −80 °C from 15 °C).Eutectic Ga-In droplets can be functionalized with various polymers and co-polymers using atom transfer radical polymerization. The droplets are ready for direct solution processing to form liquid-metal nanocomposites for potential applications in soft robotics.

The modification of nanoparticles with polymer ligands has emerged as a versatile approach to control the interactions and organization of nanoparticles in polymer nanocomposite materials. Besides their technological significance, polymer-grafted nanoparticle (PGNP) dispersions have attracted interest as model systems to understand the role of entropy as a driving force for microstructure formation. For instance, densely and sparsely grafted nanoparticles show distinct dispersion and assembly behaviors within polymer matrices due to the entropy variation associated with conformational changes in brush and matrix chains. Here we demonstrate how this entropy change can be harnessed to drive PGNPs into spatially organized domain structures on submicrometer scale within topographically patterned thin films. This selective segregation of PGNPs is induced by the conformational entropy penalty arising from local perturbations of grafted and matrix chains under confinement. The efficiency of this particle segregation process within patterned mesa–trench films can be tuned by changing the relative entropic confinement effects on grafted and matrix chains. The versatility of topographic patterning, combined with the compatibility with a wide range of nanoparticle and polymeric materials, renders SCPINS (soft-confinement pattern-induced nanoparticle segregation) an attractive method for fabricating nanostructured hybrid films with potential applications in nanomaterial-based technologies.

A simple route to synthesize ultra-high molecular weight particle brushes by surface-initiated atom transfer radical polymerization (SI-ATRP) from silica nanoparticles was developed. SiO 2 - g -PMMA and SiO 2 - g -PS particle brushes were prepared with different [SiO 2 –Br] 0 concentration of initiating sites on the surface of the nanoparticles. Ultra-high MW (> 10 6 ) SiO 2 - g -PMMA particle brushes with narrow molecular weight distribution (< 1.3) and different grafting densities were synthesized. The grafting density of SiO 2 - g -PMMA particle brushes decreased with increasing target degree of polymerization. The same conditions were applied to the synthesis of SiO 2 - g -PS particle brushes. However, due to the lower propagation rate constant of styrene, coupling between SiO 2 - g -PS particle brushes occurred and also some fraction of unattached homopolystyrene was generated by the thermal self-initiation of styrene, preventing successful synthesis of ultra-high MW SiO 2 - g -PS particle brushes.

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When different materials are combined to form a heterogeneous structure, the properties of the resulting composite material depend on the properties of the constituent materials, the length scale as well as the chemical and morphological details of the dispersion. Nanocomposites, i.e., heterogeneous materials in which at least one characteristic length scale of the filler material is in the nanometer range, have attracted particular attention and currently represent one of the fastest growing areas in materials science. Inorganic nanoparticle additives are blended with polymers in order to enhance particular optical characteristics, mechanic stability, wear resistance, barrier properties or flame resistance. In many instances the performance of the nanocomposite is intimately related to the location and orientation of the dispersed particle additive. The use of structure-guiding host materials such as block copolymers affords opportunities for controlling the spatial and orientational distribution of filler particles and facilitates more sophisticated property design than classical single-phase host materials. This article reviews recent research in the area of block copolymer/nanoparticle (BCP/NP) composites with particular focus on the relevant parameters that control the structure formation in block copolymer/nanoparticle blends and the structure-property relations of the resulting composite materials. The morphological characteristics of BCP/NP blends are discussed with respect to equilibrium and non-equilibrium conditions and compared to experimental observations in related areas of materials science in order to provide a framework for interpretation. It will be shown that by control of the particle's location within a microstructured host material new properties can emerge that are not inherent in the properties of the constituent materials (and thus are out of the realm of uniform composite materials) but rather are the result of particle-particle interactions that occur due to the morphology of the dispersion. These emergent properties hold the promise to design novel ‘chimera’ composite materials, i.e., materials that combine multiple properties that in uniform distributions would exclude each other.