Facile access to large-scale, self-assembled, nacre-inspired, high-performance materials with tunable nanoscale periodicities

The Critical Role of Ca2+ in Improving the Transparency and Strength of High-Filler-Content Nanocellulose/Montmorillonite Nanocomposite Films

Strong and transparent nanocellulose/montmorillonite (MMT) nanocomposite films with high filler content (≥50 wt %) are emerging as versatile materials for advanced applications due to their excellent optical, barrier, mechanical, and thermal properties, and environmental friendliness. Nonetheless, these films undergo a notable decline in optical and mechanical properties at high MMT loadings. This study first demonstrates that calcium-ion-induced tactoids are the key factor causing disordered structures in nanocomposite films, leading to the degradation of optical and mechanical properties. We then address this issue by employing a Ca2+ removal strategy─dialysis. Through removing 43% of free Ca2+, simultaneous improvements in both properties are observed. For example, in a nanocomposite film with 70 wt % MMT, light transmittance increases from 75.9 to 91.6%, and the tensile strength rises from 100.4 to 139.4 MPa. This work offers insights into developing strong and transparent nanocomposite films with high MMT contents.

Robust, biodegradable and flame-retardant nanocomposite films based on TEMPO-oxidized cellulose nanofibers and hydroxyapatite nanowires

Flammability is a fatal drawback for sustainable packaging materials made from cellulose and its derivatives. Incorporating inorganic nanomaterials is a viable approach to improve the fire-resistant property. However, due to the aggregation of inorganic fillers and weak interactions between components, incorporating inorganic nanomaterials always had an adverse impact on the mechanical properties and optical transparency of cellulose-based nanocomposites. Herein, we presented a robust, biodegradable, and flame-retardant nanocomposite film composed of TEMPO-oxidized cellulose nanofibers (TOCNFs) and inorganic hydroxyapatite nanowires (HNWs). Both TOCNFs and HNWs possessed one-dimensional microstructure and could form unique organic-inorganic networks microstructure. The organic-inorganic networks interact through physical intertwinement and multiple chemical bonds, endowing nanocomposite film with outstanding mechanical properties. This nanocomposite film showed a tensile strength of 223.68 MPa and Young's modulus of 9.18 GPa, which were superior to most reported cellulose-based nanocomposite. Furthermore, this nanocomposite film demonstrated exceptional thermal stability and flame-retardant feature attributed to the inorganic framework formed by HNWs. This nanocomposite film also possessed a high optical transmittance even when HNWs content reached 30 % and could be decomposed quickly in soil. By employing organic-inorganic interpenetrating network structure design and multiple bonding interaction, cellulose-based nanocomposites can overcome inherent limitations and attain desirable comprehensive properties.

Systematic Study of the Nanostructures of Exfoliated Polymer Nanocomposites

High-performance bioinspired materials have shown rapid development over the last decade. Examples are brick-and-mortar hierarchical structures, which are often achieved via solvent evaporation. Although good properties are claimed, most systems are composed of stacked or intercalated platelets. Exfoliation is a crucial step to give ultimate anisotropic properties, e.g., thermal, mechanical, and barrier properties. We propose a general framework for all the various types of micro-scale structures that should be distinguished for 2D filler nanocomposites. In particular, the exfoliated state is systematically explored by the immobilization of montmorillonite platelets via (gelatin) hydrogelation. Scattering techniques were used to evaluate this strategy at the level of the particle dispersion and the regularity of spatial arrangement. The gelatin/montmorillonite exfoliated nanostructures are fully controlled by the filler volume fraction since the observed gallery d-spacings perfectly fall onto the predicted values. Surprisingly, X-ray analysis also revealed short- and quasi long-range arrangement of the montmorillonite clay at high loading.

A Comparative Study on the Roll-to-Roll Processing of a Silicate-Polyvinyl Alcohol Composite Barrier Lacquer Using Slot-Die and Reverse Gravure Coating Techniques

The integration of platelet-shaped montmorillonite particles to improve the oxygen barrier of polyvinyl-alcohol-based barrier layers is state-of-the-art, but research on roll-to-roll coatings of such composite barrier lacquers has not been widely published. In this study, two different coating techniques, slot-die and reverse gravure, were used on a roll-to-roll scale to apply barrier lacquers comprising polyvinyl alcohol and montmorillonite. The lacquers were analyzed regarding viscosity at certain shear rates and surface energy and the dried coating layers regarding oxygen barrier, surface morphology, and particle orientation. Low permeability coefficients delivering a high oxygen barrier of 0.14 and 0.12 cm3 (STP) 1 μmm2 d bar were achieved for the coating layers with slot-die and reverse gravure coating, respectively. It turned out that the properties of the barrier lacquer need to be adjusted to the coating technique to achieve high oxygen barrier performance. By tailoring the barrier lacquer formulation, the orientation of the platelet-shaped montmorillonite particles can be achieved using both techniques. A low solid content of down to 3 wt% is preferable for the premetered slot-die coating, because it results in low agglomeration quantity in the coating layer. A high solid content of up to 9 wt% is preferable for the self-metered reverse gravure coating to assure a homogeneously coated layer.

Bioinspired Nanoheterogeneous Alternating Multiarched Architecture: Toward a Superior Strength-Toughness Integration

Strength and toughness are at odds with each other in coating design. Constructing strength-toughness-integrated coatings has long been a pursuit in materials design but is a challenge to achieve. Conventional wisdom suggests that growth of coatings is only a uniform cumulative growth on a two-dimensional plane. However, by constructing growth templates and controlling the alternation of heterogeneous materials, it subverts the traditional perception of cumulative growth in planes and creates the fact that the coating grows on a curved surface. Regulating the microstructure of the coating autonomously and matching the strength and toughness of heterogeneous materials, drawing inspiration from the multiarched structure in the nacre of red abalone, are crucial for achieving strength-toughness integration. Herein, we propose a new idea of coating deposition to achieve strength-toughness integration via preconstructing a nanoscale island-like discontinuous seed layer as a template for coating growth and then growing a nanoscale hard/soft heterogeneous multiarched architecture in situ. We refer to this architecture with intrinsic mechanical advantage as the "Nanoheterogeneous Alternating Multiarched" (NHAM) architecture. We design a nacre-like multiarched coating with a strength of 12.42 GPa and a K_IC value of 2.12 MPa·m^1/2, depositing the hard phase (TiSiCN layer) and the soft phase (Ag layer) with the unique NHAM architecture via physical vapor deposition technology, which exhibits a superior improvement in the strength-toughness integration compared to that reported in other studies (increased strength by at least 1 GPa without sacrificing toughness). The NHAM architecture strategy provides a pathway to design strength-toughness-integrated coatings. Two heterogeneous materials with well-matched strength and toughness can be deposited to achieve the NHAM architecture to greatly reflect the effect of strength-toughness integration.

A Constrained Assembly Strategy for High-Strength Natural Nanoclay Film

Developing high-performance materials from existing natural materials is highly desired because of their environmental friendliness and low cost; two-dimensional nanoclay exfoliated from layered silicate minerals is a good building block to construct multilayered macroscopic assemblies for achieving high mechanical and functional properties. Nevertheless, the efforts have been frustrated by insufficient inter-nanosheet stress transfer and nanosheet misalignment caused by capillary force during solution-based spontaneous assembly, degrading the mechanical strength of clay-based materials. Herein, a constrained assembly strategy that is implemented by in-plane stretching a robust water-containing nanoclay network with hydrogen and ionic bonding is developed to adjust the 2D topography of nanosheets within multilayered nanoclay film. In-plane stretching overcomes capillary force during water removal and thus restrains nanosheet conformation transition from nearly flat to wrinkled, leading to a highly aligned multilayered nanostructure with synergistic hydrogen and ionic bonding. It is proved that inter-nanosheet hydrogen and ionic bonding and nanosheet conformation extension generate profound mechanical reinforcement. The tensile strength and modulus of natural nanoclay film reach up to 429.0 MPa and 43.8 GPa and surpass the counterparts fabricated by normal spontaneous assembly. Additionally, improved heat insulation function and good nonflammability are shown for the natural nanoclay film and extend its potential for realistic uses.

Advancing the Mechanical Performance of Glasses: Perspectives and Challenges

Glasses are materials that lack a crystalline microstructure and long-range atomic order. Instead, they feature heterogeneity and disorder on superstructural scales, which have profound consequences for their elastic response, material strength, fracture toughness, and the characteristics of dynamic fracture. These structure-property relations present a rich field of study in fundamental glass physics and are also becoming increasingly important in the design of modern materials with improved mechanical performance. A first step in this direction involves glass-like materials that retain optical transparency and the haptics of classical glass products, while overcoming the limitations of brittleness. Among these, novel types of oxide glasses, hybrid glasses, phase-separated glasses, and bioinspired glass-polymer composites hold significant promise. Such materials are designed from the bottom-up, building on structure-property relations, modeling of stresses and strains at relevant length scales, and machine learning predictions. Their fabrication requires a more scientifically driven approach to materials design and processing, building on the physics of structural disorder and its consequences for structural rearrangements, defect initiation, and dynamic fracture in response to mechanical load. In this article, a perspective is provided on this highly interdisciplinary field of research in terms of its most recent challenges and opportunities.

Nanoscale-Structured Hybrid Bragg Stacks with Orientation- and Composition-Dependent Mechanical and Thermal Transport Properties: Implications for Nacre Mimetics and Heat Management Applications

Layered nanomaterials fascinate researchers for their mechanical, barrier, optical, and transport properties. Nacre is a biological example thereof, combining excellent mechanical properties by aligned submicron inorganic platelets and nanoscale proteinic interlayers. Mimicking nacre with advanced nanosheets requires ultraconfined organic layers aimed at nacre-like high reinforcement fractions. We describe inorganic/polymer hybrid Bragg stacks with one or two fluorohectorite clay layers alternating with one or two poly(ethylene glycol) layers. As indicated by X-ray diffraction, perfect one-dimensional crystallinity allows for homogeneous single-phase materials with up to a 84% clay volume fraction. Brillouin light spectroscopy allows the exploration of ultimate mechanical moduli without disturbance by flaws, suggesting an unprecedentedly high Young's modulus of 162 GPa along the aligned clays, indicating almost ideal reinforcement under these conditions. Importantly, low heat conductivity is observed across films, κ_⊥ = 0.11-0.15 W m^-1 K^-1, with a high anisotropy of κ_∥/κ_⊥ = 28-33. The macroscopic mechanical properties show ductile-to-brittle change with an increase in the clay volume fraction from 54% to 70%. Conceptually, this work reveals the ultimate elastic and thermal properties of aligned layered clay nanocomposites in flaw-tolerant conditions.

Recyclable nanocomposites of well-dispersed 2D layered silicates in cellulose nanofibril (CNF) matrix

Nanocomposites based on components from nature, which can be recycled are of great interest in new materials for sustainable development. The range of properties of nacre-inspired hybrids of 1D cellulose and 2D clay platelets are investigated in nanocomposites with improved nanoparticle dispersion in the starting hydrocolloid mixture. Films with a wide range of compositions are prepared by capillary force assisted physical assembly (vacuum-assisted filtration) of TEMPO-oxidized cellulose nanofibers (TOCN) reinforced by exfoliated nanoclays of three different aspect ratios: saponite, montmorillonite and mica. X-ray diffraction and transmission electron micrographs show almost monolayer dispersion of saponite and montmorillonite and high orientation parallel to the film surface. Films exhibit ultimate strength up to 573 MPa. Young's modulus exceeds 38 GPa even at high MTM contents (40-80 vol%). Optical transmittance, UV-shielding, thermal shielding and fire-retardant properties are measured, found to be very good and are sensitive to the 2D nanoplatelet dispersion.

Preparation and Machine-Learning Methods of Nacre-like Composites from the Self-Assembly of Magnetic Colloids Exposed to Rotating Magnetic Fields

Composite materials designed by nature, such as nacre, can display unique mechanical properties and have therefore been often mimicked by scientists. In this work, we prepared composite materials mimicking the nacre structure in two steps. First, we synthesized a silica gel skeleton with a layered structure using a bottom-up approach by modifying a sol-gel synthesis. Magnetic colloids were added to the sol solution, and a rotating magnetic field was applied during the sol-gel transition. When exposed to a rotating magnetic field, magnetic colloids organize in layers parallel to the plane of rotation of the field and template the growing silica phase, resulting in a layered anisotropic silica network mimicking the nacre's inorganic phase. Heat treatment has been applied to further harden the silica monoliths. The final nacre-inspired composite is created by filling the porous structure with a monomer, leading to a soft elastomer upon polymerization. Compression tests of the platelet-structured composite show that the mechanical properties of the nacre-like composite material far exceed those of nonstructured composite materials with an identical chemical composition. Increased toughness and a nearly 10-fold increase in Young's modulus were achieved. The natural brittleness and low elastic deformation of silica monoliths could be overcome by mimicking the natural architecture of nacre. Pattern recognition obtained with a classification of machine learning algorithms was applied to achieve a better understanding of the physical and chemical parameters that have the highest impact on the mechanical properties of the monoliths. Multivariate statistical analysis was performed to show that the structural control and the heat treatment have a very strong influence on the mechanical properties of the monoliths.

High Elongation and Transparent Nacre-Inspired PVA/MMT Nanocomposites

Currently, high strength nacre-inspired PVA/MMT (polyvinyl alcohol/montmorillonite) nanocomposites with high MMT nanofiller content (50-70 wt%) have been constructed successfully. However, this seriously sacrifices the elongation and reduces the corresponding transparency. In this paper, high elongation and transparent PVA/MMT nanocomposites with high MMT content are prepared by the evaporation-induced assembly with the introduction of the micro-crosslinking. Results demonstrate that the micro-crosslinking can inhibit the formation of rod-shaped arrays, and contribute to a more ordered layered microstructure, where an elongation of 76.2% in 47.8 wt% MMT content nanocomposites is gained, nearly 19 times of that of non-crosslinked nanocomposites (ultimate strain is 4.1%). This provides a potential approach for compromise between high strength and excellent elongation at the same MMT content. Moreover, disappearance of rod-shaped arrays and resultant ordered layered microstructure make eventual films more transparent.

Recyclable and Light-Adaptive Vitrimer-Based Nacre-Mimetic Nanocomposites

Nacre's natural design consists of a perfect hierarchical assembly that resembles a brick-and-mortar structure with synergistic stiffness and toughness. The field of bioinspired materials often provides attractive architecture and engineering pathways which allow to explore outstanding property areas. However, the study of nacre-mimetic materials should not be limited to the design of its architecture but ought to include the understanding, operation, and improvement of internal interactions between their components. Here, we introduce a vitrimer prepolymer system that, once integrated into the nacre-mimetic nanocomposites, cures and cross-links with the presence of Lewis acid catalyst and further manifests associative dynamic exchange reactions. Bond exchanges are controllable by molecular composition and catalyst content and characterized by creep, shear-lag, and shape-locking tests. We exploit the vitrimer properties by laminating ca. 70 films into thick bulk materials, and characterize the flexural resistance and crack propagation. More importantly, we introduce recycling by grinding and hot-pressing. The recycling for highly reinforced nacre-mimetic nanocomposites is critically enabled by the vitrimer chemistry and improves the sustainability of bioinspired nanocomposites in cyclic economy. Finally, we integrate photothermal converters into the structures and use laser irradiation as external trigger to activate the vitrimer exchange reactions.

Polymer-infiltrated nanoplatelet films with nacre-like structure via flow coating and capillary rise infiltration (CaRI)

Alignment of highly anisotropic nanomaterials in a polymer matrix can yield nanocomposites with unique mechanical and transport properties. Conventional methods of nanocomposite film fabrication are not well-suited for manufacturing composites with very high concentrations of anisotropic nanomaterials, potentially limiting the widespread implementation of these useful structures. In this work, we present a scalable approach to fabricate polymer-infiltrated nanoplatelet films (PINFs) based on flow coating and capillary rise infiltration (CaRI) and study the processing-structure-property relationship of these PINFs. We show that films with high aspect ratio (AR) gibbsite (Al (OH)3) nanoplatelets (NPTs) aligned parallel to the substrate can be prepared using a flow coating process. NPTs are highly aligned with a Herman's order parameter of 0.96 and a high packing fraction >80 vol%. Such packings show significantly higher fracture toughness compared to low AR nanoparticle (NP) packings. By depositing NPTs on a polymer film and subsequently annealing the bilayer above the glass transition temperature of the polymer, polymer infiltrates into the tortuous NPT packings though capillarity. We observe larger enhancement in the modulus, hardness and scratch resistance of NPT films upon polymer infiltration compared to NP packings. The excellent mechanical properties of such films benefit from both thermally promoted oxide bridge formation between NPTs as well as polymer infiltration increasing the strength of NPT contacts. Our approach is widely applicable to highly anisotropic nanomaterials and allows the generation of mechanically robust polymer nanocomposite films for a diverse set of applications.

Room-Temperature Phosphorescence Enabled through Nacre-Mimetic Nanocomposite Design

A generic, facile, and waterborne strategy is introduced to fabricate flexible, low-cost nanocomposite films with room-temperature phosphorescence (RTP) by incorporating waterborne RTP polymers into self-assembled bioinspired polymer/nanoclay nanocomposites. The excellent oxygen barrier of the lamellar nanoclay structure suppresses the quenching effect from ambient oxygen (k_q ) and broadens the choice of polymer matrices towards lower glass transition temperature (T_g ), while providing better mechanical properties and processability. Moreover, the oxygen permeation and diffusion inside the films can be fine-tuned by varying the polymer/nanoclay ratio, enabling programmable retention times of the RTP signals, which is exploited for transient information storage and anti-counterfeiting materials. Additionally, anti-interception materials are showcased by tracing the interception-induced oxygen history that interferes with the preset self-erasing time. Merging bioinspired nanocomposite design with RTP materials contributes to overcoming the inherent limitations of molecular design of organic RTP compounds, and allows programmable temporal features to be added into RTP materials by controlled mesostructures. This will assist in paving the way for practical applications of RTP materials as novel anti-counterfeiting materials.

Self-Assembled Bioinspired Nanocomposites

Bioinspired materials engineering impacts the design of advanced functional materials across many domains of sciences from wetting behavior to optical and mechanical materials. In all cases, the advances in understanding how biology uses hierarchical design to create failure and defect-tolerant materials with emergent properties lays the groundwork for engaging into these topics. Biological mechanical materials are particularly inspiring for their unique combinations of stiffness, strength, and toughness together with lightweightness, as assembled and grown in water from a limited set of building blocks at room temperature. Wood, nacre, crustacean cuticles, and spider silk serve as some examples, where the correct arrangement of constituents and balanced molecular energy dissipation mechanisms allows overcoming the shortcomings of the individual components and leads to synergistic materials performance beyond additive behavior. They constitute a paradigm for future structural materials engineering-in the formation process, the use of sustainable building blocks and energy-efficient pathways, as well as in the property profiles-that will in the long term allow for new classes of high-performance and lightweight structural materials needed to promote energy efficiency in mobile technologies.This Account summarizes our efforts of the past decade with respect to designing self-assembling bioinspired materials aiming for both mechanical high-performance structures and new types of multifunctional property profiles. The Account is set out to first give a definition of bioinspired nanocomposite materials and self-assembly therein, followed by an in-depth discussion on the understanding of mechanical performance and rational design to increase the mechanical performance. We place a particular emphasis on materials formed at high fractions of reinforcements and with tailor-made functional polymers using self-assembly to create highly ordered structures and elucidate in detail how the soft polymer phase needs to be designed in terms of thermomechanical properties and sacrificial supramolecular bonds. We focus on nanoscale reinforcements such as nanoclay and nanocellulose that lead to high contents of internal interfaces and intercalated polymer layers that experience nanoconfinement. Both aspects add fundamental challenges for macromolecular design of soft phases using precision polymer synthesis. We build upon those design criteria and further develop the concepts of adaptive bioinspired nanocomposites, whose properties are switchable from the outside using molecularly defined triggers with light. In a last section, we discuss how new types of functional properties, in particular flexible and transparent gas barrier materials or fire barrier materials, can be reached on the basis of the bioinspired nanocomposite design strategies. Additionally, we show new types of self-assembled photonic materials that can even be evolved into self-assembling lasers, hence moving the concept of mechanical nanocomposite design to other functionalities.The comparative discussion of different bioinspired nanocomposite architectures with nematic, fibrillar, and cholesteric structures, as based on different reinforcing nanoparticles, aims for a unified understanding of the design principles and shall aid researchers in the field in the more elaborate design of future bioinspired nanocomposite materials based on molecular control principles. We conclude by addressing challenges, in particular also the need for a transfer from fundamental molecular materials science into scalable engineering materials of technological and societal relevance.

Electromechanical Behaviors of Graphene Reinforced Polymer Composites: A Review

Graphene (including its derivatives)-reinforced polymer composites (GRPCs) have been drawing tremendous attention from academic and industrial communities for developing smart materials and structures. Such interest stems from the excellent combination of the mechanical and electrical properties of these composites while keeping the beneficial intrinsic attributes of the polymers, including flexibility, easy processability, low cost and good biological and chemical compatibility. The electromechanical performances of these GRPCs are of great importance for the design and optimization of engineering structures and components. Extensive work has been devoted to this topic. This paper reviews the recent studies on the electromechanical behaviors of GRPCs. First the methods and techniques to manufacture graphene and GRPCs are introduced, in which the pros and cons of each method are discussed. Then the experimental examination and theoretical modeling on the electromechanical behaviors of the nanocomposites are presented and discussed.

Bioinspired Design and Fabrication of Polymer Composite Films Consisting of a Strong and Stiff Organic Matrix and Microsized Inorganic Platelets

Intensive studies on nacre-inspired composites with exceptional mechanical properties based on an organic/inorganic hierarchical layered structure have been conducted; however, integrating high strength, stiffness, and toughness for engineering materials still remains a challenge. We herein report the design and fabrication of polymer composites through a hydrogel-film casting method that allow for building uniformly layered organic/inorganic microstructure. Alginate (Alg) was used for an organic matrix, whose mechanical properties were controlled by Ca²⁺ cross-linking toward the simultaneously strong, stiff, and tough resultant composite. Alumina (Alu) microplatelets were used for horizontally aligned inorganic phase, and their alignment and interactions with the organic matrix were improved by polyvinylpyrrolidone (PVP) coating on the platelet. The composite film exhibits well-balanced elastic and plastic deformation under tensile stress, leading to high stiffness and toughness, which have not been generally achieved in microplatelet-based composite films developed in previous studies. The synergistic effect of Ca²⁺ cross-linking and PVP-coated Alu platelets on the mechanical properties improved polymer-platelet interfacial interactions, and platelet alignment is clearly demonstrated through mechanical tests and Fourier transform infrared and X-ray diffraction analyses. We further demonstrate that the reinforcing effect of the Alu platelet and PVP-coated platelet on the mechanical properties is dependent on humidity. Such effects are maximized at highly dry conditions, which is consistent with the model estimation. Furthermore, a thick bulk composite was produced by laminating thin films and showed high mechanical properties under flexural stress. Our design and fabrication strategies combined with the understanding of their mechanism yield an alternative approach to produce engineered composite materials.

Bioinspired Environmentally Friendly Amorphous CaCO3-Based Transparent Composites Comprising Cellulose Nanofibers

Amorphous calcium carbonate (ACC) stabilized by acidic macromolecules is a useful material for the development of environmentally friendly composites. In this study, we synthesized transparent and mechanically tough ACC-based composite materials by the incorporation of water-dispersible cellulose derivatives, namely, carboxymethyl cellulose (CMC) and surface-modified crystalline cellulose nanofibers (CNFs). A solution mixing method used in the present work proved to be a powerful and efficient method for the production of mechanically tough and environmentally friendly materials. Molecular-scale interactions between carboxyl groups and Ca2+ ions induce homogeneous dispersion of CNFs in the composites, and this gives composite films with high transparency and high mechanical properties. The composite films of CMC, CNFs, and ACC at the mixture ratios of 40, 40, and 20 wt %, showed high mechanical properties of 15.8 ± 0.93 GPa for the Young's modulus and 268 ± 20 MPa for the tensile strength. These designed materials that are based on ACC may open up new opportunities in many fields in applications that require the use of environmentally friendly, biodegradable, mechanically tough, and transparent composite materials.

Exceptionally Ductile and Tough Biomimetic Artificial Nacre with Gas Barrier Function

Synthetic mimics of natural high-performance structural materials have shown great and partly unforeseen opportunities for the design of multifunctional materials. For nacre-mimetic nanocomposites, it has remained extraordinarily challenging to make ductile materials with high stretchability at high fractions of reinforcements, which is however of crucial importance for flexible barrier materials. Here, highly ductile and tough nacre-mimetic nanocomposites are presented, by implementing weak, but many hydrogen bonds in a ternary nacre-mimetic system consisting of two polymers (poly(vinyl amine) and poly(vinyl alcohol)) and natural nanoclay (montmorillonite) to provide efficient energy dissipation and slippage at high nanoclay content (50 wt%). Tailored interactions enable exceptional combinations of ductility (close to 50% strain) and toughness (up to 27.5 MJ m^-3 ). Extensive stress whitening, a clear sign of high internal dynamics at high internal cohesion, can be observed during mechanical deformation, and the materials can be folded like paper into origami planes without fracture. Overall, the new levels of ductility and toughness are unprecedented in highly reinforced bioinspired nanocomposites and are of critical importance to future applications, e.g., as barrier materials needed for encapsulation and as a printing substrate for flexible organic electronics.

Facile and On-Demand Cross-Linking of Nacre-Mimetic Nanocomposites Using Tailor-Made Polymers with Latent Reactivity

The development of on-demand cross-linking strategies is a key aspect in promoting mechanical properties of high-performance bioinspired nanocomposites. Here, we embed styrene sulfonyl azide groups with latent chemical reactivity into water-soluble copolymers and assemble those with high-aspect-ratio synthetic nanoclays to generate well-defined layered polymer/nanoclay nacre-mimetics. A considerable stiffening and strengthening occurs upon activation of the covalent cross-linking using simple heating. Varying the amount of cross-linkable units allows molecular control of mechanical properties from ductile to stiff and strong. Moreover, the covalent cross-linking enhances the moisture stability of water-borne nacre-mimetics. The strategy is facile and versatile allowing for a transfer into applications.

Silver nanoparticles incorporated konjac glucomannan-montmorillonite nacre-like composite films for antibacterial applications

Artificial nacre-like konjac glucomannan-Montmorillonite (KGM-MTM) composite films with 'brick and mortar' microstructures have been fabricated based on using KGM-MTM hybrid nanosheets as building blocks. In the designed fabrication procedure, we assembled hybrid building blocks with a thin layer of KGM coating on the MTM nanosheets to form KGM-MTM composite film via vacuum filtration. The nacre-like microstructures enhanced the light transmission performance and mechanical properties (Tensile strength: 116 MPa) of KGM-MTM composite films. Additionally, Ag nanoparticles (Ag NPs) can be incorporated into the layered structures of KGM-MTM composite films via an in situ reduced method. It was found that KGM-MTM-Ag composite films significantly suppress bacterial growth, which makes them potentially applicable as antimicrobial films in the biomedical field.

A Hydrogel-Film Casting to Fabricate Platelet-Reinforced Polymer Composite Films Exhibiting Superior Mechanical Properties

The fabrication of mechanically superior polymer composite films with controllable shapes on various scales is difficult. Despite recent research on polymer composites consisting of organic matrices and inorganic materials with layered structures, these films suffer from complex preparations and limited mechanical properties that do not have even integration of high strength, stiffness, and toughness. Herein, a hydrogel-film casting approach to achieve fabrication of simultaneously strong, stiff, and tough polymer composite films with well-defined microstructure, inspired from a layer-by-layer structure of nacre is reported. Ca²⁺ -crosslinked alginate hydrogels incorporated with platelet-like alumina particles are dried to form composite films composed of horizontally aligned alumina platelets and alginate matrix with uniformly layered microstructure. Alumina platelets are evenly distributed parallel without precipitations and contribute to synergistic enhancements of strength, stiffness and toughness in the resultant film. Consequentially, Ca²⁺ -crosslinked alginate/alumina (Ca²⁺ -Alg/Alu) films show exceptional tensile strength (267 MPa), modulus (17.9 GPa), and toughness (3.60 MJ m^-3 ). Furthermore, the hydrogel-film casting allows facile preparation of polymer composite films with controllable shapes and various scales. The results suggest an alternative approach to design and prepare polymer composites with the layer-by-layer structure for superior mechanical properties.

Mesocrystalline calcium silicate hydrate: A bioinspired route toward elastic concrete materials

Calcium silicate hydrate (C-S-H) is the binder in concrete, the most used synthetic material in the world. The main weakness of concrete is the lack of elasticity and poor flexural strength considerably limiting its potential, making reinforcing steel constructions necessary. Although the properties of C-S-H could be significantly improved in organic hybrids, the full potential of this approach could not be reached because of the random C-S-H nanoplatelet structure. Taking inspiration from a sea urchin spine with highly ordered nanoparticles in the biomineral mesocrystal, we report a bioinspired route toward a C-S-H mesocrystal with highly aligned C-S-H nanoplatelets interspaced with a polymeric binder. A material with a bending strength similar to nacre is obtained, outperforming all C-S-H-based materials known to date. This strategy could greatly benefit future construction processes because fracture toughness and elasticity of brittle cementitious materials can be largely enhanced on the nanoscale.

Highly Transparent and Self-Extinguishing Nanofibrillated Cellulose-Monolayer Clay Nanoplatelet Hybrid Films

A viable solution toward "green" optoelectronics is rooted in our ability to fabricate optoelectronics on transparent nanofibrillated cellulose (NFC) film substrates. However, the flammability of transparent NFC film poses a severe fire hazard in optoelectronic devices. Despite many efforts toward enhancing the fire-retardant features of transparent NFC film, making NFC film fire-retardant while maintaining its high transparency (≥90%) remains an ambitious objective. Herein, we combine NFC with NFC-dispersed monolayer clay nanoplatelets as a fire retardant to prepare highly transparent NFC-monolayer clay nanoplatelet hybrid films with a superb self-extinguishing behavior. Homogeneous and stable monolayer clay nanoplatelet dispersion was initially obtained by using NFC as a green dispersing agent with the assistance of ultrasonication and then used to blend with NFC to prepare highly transparent and self-extinguishing hybrid films by a water evaporation-induced self-assembly process. As the content of monolayer clay nanoplatelets increased from 5 wt % to 50 wt %, the obtained hybrid films presented enhanced self-extinguishing behavior (limiting oxygen index sharply increased from 21% to 96.5%) while retaining a ∼90% transparency at 600 nm. More significantly, the underlying mechanisms for the high transparency and excellent self-extinguishing behavior of these hybrid films with a clay nanoplatelet content of over 30 wt % were unveiled by a series of characterizations such as SEM, XRD, TGA, and limiting oxygen index tester. This work offers an alternative environmentally friendly, self-extinguishing, and highly transparent substrate to next-generation optoelectronics, and is aimed at providing a viable solution to environmental concerns that are caused by ever-increasing electronic waste.

Bioinspired Interface Engineering for Moisture Resistance in Nacre-Mimetic Cellulose Nanofibrils/Clay Nanocomposites

The interfacial adhesion design between "mortar" and "bricks" is essential for mechanical and barrier performance of nanocellulose-based nacre-mimetic nanocomposites, especially at high moisture conditions. To address this fundamental challenge, dopamine (DA) has been conjugated to cellulose nanofibrils (CNFs) and subsequently assembled with montmorillonite (MTM) to generate layered nanocomposite films inspired by the strong adhesion of mussel adhesive proteins to inorganic surfaces under water. The selective formation of catechol/metal ion chelation and hydrogen bonding at the interface between MTM platelets and CNFs bearing DA renders transparent films with strong mechanical properties, particularly at high humidity and in wet state. Increasing the amount of conjugated DA on CNFs results in nanocomposites with increased tensile strength and modulus, up to 57.4 MPa and 1.1 GPa, respectively, after the films are swollen in water. The nanocomposites also show excellent gas barrier properties at high relative humidity (95%), complementing the multifunctional property profile.

A General Bioinspired, Metals-Based Synergic Cross-Linking Strategy toward Mechanically Enhanced Materials

Creating lightweight engineering materials combining high strength and great toughness remains a significant challenge. Despite possessing-enhanced strength and stiffness, bioinspired/polymeric materials usually suffer from clearly reduced extensibility and toughness when compared to corresponding bulk polymer materials. Herein, inspired by tiny amounts of various inorganic impurities for mechanical improvement in natural materials, we present a versatile and effective metal ion (Mⁿ⁺)-based synergic cross-linking (MSC) strategy incorporating eight types of metal ions into material bulks that can drastically enhance the tensile strength (∼24.1-70.8%), toughness (∼18.6-110.1%), modulus (∼21.6-66.7%), and hardness (∼6.4-176.5%) of multiple types of pristine materials (from hydrophilic to hydrophobic and from unary to binary). More importantly, we also explore the primarily elastic-plastic deformation mechanism and brittle fracture behavior (indentation strain of >5%) of the synergic cross-linked graphene oxide (Syn-GO) paper by means of in situ nanoindentation SEM. The MSC strategy for mechanically enhanced integration can be readily attributed to the formation of the complicated metals-based cross-linking/complex networks in the interfaces and intermolecules between functional groups of materials and various metal ions that give rise to efficient energy dissipation. This work suggests a promising MSC strategy for designing advanced materials with outstanding mechanical properties by adding low amounts (<1.0 wt %) of synergic metal ions serving as synergic ion-bonding cross-linkers.

Bio-Based Artificial Nacre with Excellent Mechanical and Barrier Properties Realized by a Facile In Situ Reduction and Cross-Linking Reaction

Demands for high strength integrated materials have substantially increased across various kinds of industries. Inspired by the relationship of excellent integration of mechanical properties and hierarchical nano/microscale structure of the natural nacre, a simple and facile method to fabricate high strength integrated artificial nacre based on sodium carboxymethylcellulose (CMC) and borate cross-linked graphene oxide (GO) sheets has been developed. The tensile strength and toughness of cellulose-based hybrid material reached 480.5 ± 13.1 MPa and 11.8 ± 0.4 MJm^-3 by a facile in situ reduction and cross-linking reaction between CMC and GO (0.7%), which are 3.55 and 6.55 times that of natural nacre. This hybrid film exhibits better thermal stability and flame retardancy. More interestingly, the hybrid material showed good water stability compared to that in the original water-soluble CMC. This type of hybrid has great potential applications in aerospace, artificial muscle, and tissue engineering.

Facile synthesis of high strength hot-water wood extract films with oxygen-barrier performance

Biobased nanocomposite films for food packaging with high mechanical strength and good oxygen-barrier performance were developed using a hot-water wood extract (HWE). In this work, a facile approach to produce HWE/montmorillonite (MMT) based nanocomposite films with excellent physical properties is described. The focus of this study was to determine the effects of the MMT content on the structure and mechanical properties of nanocomposites and the effects of carboxymethyl cellulose (CMC) on the physical properties of the HWE-MMT films. The experimental results suggested that the intercalation of HWE and CMC in montmorillonite could produce compact, robust films with a nacre-like structure and multifunctional characteristics. This results of this study showed that the mechanical properties of the film designated F_CMC0.05 (91.5 MPa) were dramatically enhanced because the proportion of HWE, MMT and CMC was 1:1.5:0.05. In addition, the optimized films exhibited an oxygen permeability below 2.0 cm³μm/day·m²·kPa, as well as good thermal stability due to the small amount of CMC. These results provide a comprehensive understanding for further development of high-performance nanocomposites which are based on natural polymers (HWE) and assembled layered clays (MMT). These films offer great potential in the field of sustainable packaging.

Large-scale, thick, self-assembled, nacre-mimetic brick-walls as fire barrier coatings on textiles

Highly loaded polymer/clay nanocomposites with layered structures are emerging as robust fire retardant surface coatings. However, time-intensive sequential deposition processes, e.g. layer-by-layer strategies, hinders obtaining large coating thicknesses and complicates an implementation into existing technologies. Here, we demonstrate a single-step, water-borne approach to prepare thick, self-assembling, hybrid fire barrier coatings of sodium carboxymethyl cellulose (CMC)/montmorillonite (MTM) with well-defined, bioinspired brick-wall nanostructure, and showcase their application on textile. The coating thickness on the textile is tailored using different concentrations of CMC/MTM (1-5 wt%) in the coating bath. While lower concentrations impart conformal coatings of fibers, thicker continuous coatings are obtained on the textile surface from highest concentration. Comprehensive fire barrier and fire retardancy tests elucidate the increasing fire barrier and retardancy properties with increasing coating thickness. The materials are free of halogen and heavy metal atoms, and are sourced from sustainable and partly even renewable building blocks. We further introduce an amphiphobic surface modification on the coating to impart oil and water repellency, as well as self-cleaning features. Hence, our study presents a generic, environmentally friendly, scalable, and one-pot coating approach that can be introduced into existing technologies to prepare bioinspired, thick, fire barrier nanocomposite coatings on diverse surfaces.

Ca2+ Enhanced Nacre-Inspired Montmorillonite-Alginate Film with Superior Mechanical, Transparent, Fire Retardancy, and Shape Memory Properties

Inspired by nacre, this is the first time that using the cross-linking of alginate with Ca ions to fabricate organic-inorganic nacre-inspired films we have successfully prepared a new class of Ca²⁺ ion enhanced montmorillonite (MMT)-alginate (ALG) composites, realizing an optimum combination of high strength (∼280 MPa) and high toughness (∼7.2 MJ m^-3) compared with other MMT based artificial nacre. Furthermore, high temperature performance of the composites (with a maximum strength of ∼170 MPa at 100 °C) along with excellent transmittance, fire retardancy, and unique shape memory response to alcohols could greatly expand the application of the mutilfunctional composites, which are believed to show competitive advantages in transportion, construction, and insulations, protection of a flammable biological material, etc.

Chitosan nanocomposites based on distinct inorganic fillers for biomedical applications

Chitosan (CHI), a biocompatible and biodegradable polysaccharide with the ability to provide a non-protein matrix for tissue growth, is considered to be an ideal material in the biomedical field. However, the lack of good mechanical properties limits its applications. In order to overcome this drawback, CHI has been combined with different polymers and fillers, leading to a variety of chitosan-based nanocomposites. The extensive research on CHI nanocomposites as well as their main biomedical applications are reviewed in this paper. An overview of the different fillers and assembly techniques available to produce CHI nanocomposites is presented. Finally, the properties of such nanocomposites are discussed with particular focus on bone regeneration, drug delivery, wound healing and biosensing applications.

Minimizing Corrosion of Outdoor Metalworks Using Dispersed Chemically Stabilized Nanoclays in Polyvinylidene Fluoride Latex Coatings

Nanoclays are small enough to appear optically transparent, yet they have large surface-to-volume and high aspect ratios that can significantly inhibit water diffusion when incorporated into protective coatings. Clear coatings, which minimally affect the aesthetics of metalworks, are commonly applied to outdoor metalworks, such as sculptures, to prevent and slow corrosion. In recent years, waterborne clear coatings, rather than solvent-based clear coatings, are increasingly used in many applications to reduce the quantity of volatile organic components in the formulation, yet the performance of dry films produced from waterborne colloidal suspensions is generally poorer. In this work, we aim to improve the barrier properties of a highly weatherable waterborne acrylic/polyvinylidene fluoride emulsion by adding a synthetic nanoclay, Laponite, into the formulation. To improve clay-polymer compatibility, the clay was covalently modified using an acetoxy or perfluoroalkyl silane monomer that is reactive with the hydroxyl groups at the edges of the Laponite platelets. Cation exchange on the clay faces using phosphorylcholine was conducted to increase the stability in water and characterized by zeta potential. Resulting changes in barrier properties of the polymer nanocomposite films were characterized by gravimetry, colorimetry, and electrochemical impedance spectroscopy. Surface ablation after accelerated artificial weathering was monitored by attenuated total internal reflectance Fourier transform infrared microspectroscopy and Raman microspectroscopy, thin film X-ray diffraction (TF-XRD) and gloss and thickness measurements. The composite films showed many improved properties: reduced water sensitivity and ultraviolet-induced polymer degradation, which increased the barrier properties and reduced the diffusion constants over both short- and long-term weathering studies compared with films without nanoclays. The diffusion constant measured for the highest performing composite film showed that the performance gap between relevant water- and solvent-borne coatings used to protect outdoor metals was narrowed by half.

Strong and Tough Layered Nanocomposites with Buried Interfaces

In nacre, the excellent mechanical properties of materials are highly dependent on their intricate hierarchical structures. However, strengthening and toughening effects induced by the buried inorganic-organic interfaces actually originate from various minerals/ions with small amounts, and have not drawn enough attention yet. Herein, we present a typical class of artificial nacres, fabricated by graphene oxide (GO) nanosheets, carboxymethylcellulose (CMC) polymer, and multivalent cationic (M(n+)) ions, in which the M(n+) ions cross-linking with plenty of oxygen-containing groups serve as the reinforcing "evocator", working together with other cooperative interactions (e.g., hydrogen (H)-bonding) to strengthen the GO/CMC interfaces. When compared with the pristine GO/CMC paper, the cross-linking strategies dramatically reinforce the mechanical properties of our artificial nacres. This special reinforcing effect opens a promising route to strengthen and toughen materials to be applied in aerospace, tissue engineering, and wearable electronic devices, which also has implication for better understanding of the role of these minerals/ions in natural materials for the mechanical improvement.

Polymer Nanocomposites-A Comparison between Carbon Nanotubes, Graphene, and Clay as Nanofillers

Nanofilled polymeric matrices have demonstrated remarkable mechanical, electrical, and thermal properties. In this article we review the processing of carbon nanotube, graphene, and clay montmorillonite platelet as potential nanofillers to form nanocomposites. The various functionalization techniques of modifying the nanofillers to enable interaction with polymers are summarized. The importance of filler dispersion in the polymeric matrix is highlighted. Finally, the challenges and future outlook for nanofilled polymeric composites are presented.