Although remarkable success has been achieved to mimic the mechanically excellent structure of nacre in laboratory-scale models, it remains difficult to foresee mainstream applications due to time-consuming sequential depositions or energy-intensive processes. Here, we introduce a surprisingly simple and rapid methodology for large-area, lightweight, and thick nacre-mimetic films and laminates with superior material properties. Nanoclay sheets with soft polymer coatings are used as ideal building blocks with intrinsic hard/soft character. They are forced to rapidly self-assemble into aligned nacre-mimetic films via paper-making, doctor-blading or simple painting, giving rise to strong and thick films with tensile modulus of 45 GPa and strength of 250 MPa, that is, partly exceeding nacre. The concepts are environmentally friendly, energy-efficient, and economic and are ready for scale-up via continuous roll-to-roll processes. Excellent gas barrier properties, optical translucency, and extraordinary shape-persistent fire-resistance are demonstrated. We foresee advanced large-scale biomimetic materials, relevant for lightweight sustainable construction and energy-efficient transportation.

Nacre-mimetics hold great promise as mechanical high-performance and functional materials. Here we demonstrate large progress of mechanical and functional properties of self-assembled polymer/nanoclay nacre-mimetics by using synthetic nanoclays with aspect ratios covering three orders in magnitude (25-3,500). We establish comprehensive relationships among structure formation, nanostructuration, deformation mechanisms and mechanical properties as a function of nanoclay aspect ratio, and by tuning the viscoelastic properties of the soft phase via hydration. Highly ordered, large-scale nacre-mimetics are obtained even for low aspect ratio nanoplatelets and show pronounced inelastic deformation with very high toughness, while those formed by ultralarge nanoplatelets exhibit superb stiffness and strength, previously only reachable for highly crosslinked materials. Regarding functionalities, we report formerly impossible glass-like transparency, and excellent gas barrier considerably exceeding earlier nacre-mimetics based on natural nanoclay. Our study enables rational design of future high-performance nacre-mimetic materials and opens avenues for ecofriendly, transparent, self-standing and strong advanced barrier materials.

Biological materials fascinate us with their ability to withstand extreme mechanical forces under complex conditions. Their excellent performance originates from a multilevel hierarchical structure; understanding these structures is pursued in structural biology and biomechanics research. A common feature in many biological materials with superior mechanical properties is the combination and ordered arrangement of hard and soft building blocks. [1][2][3] Therein, the hard matter serves as the load bearing and reinforcing part, whereas energy can be dissipated into the soft segments. Many of these materials combine good toughness with admirable strength and stiffness. For instance, in nacre, the layered arrangement of platelet-shaped CaCO 3 crystals and proteins into a brick and mortar structure leads to a synergistic performance with respect to the mechanical properties.[5] The Youngs modulus and stress at break can reach 40-70 GPa and 80-135 MPa, respectively. [6][7][8] The material is remarkably tough under wet conditions. Dynamic processes, such as sacrificial (dynamic) bonds and hidden length scales contribute significantly to toughness improvements or the ability of a material to undergo self-healing. Recently, it was shown that infiltration of metal ions drastically increase the toughness of silk dragline or increase stiffness and strength in layer-by-layer (LbL) materials. [9,10] Moreover, modeling by Fratzl and co-workers showed how randomly distributed multivalent binding sites in layered materials can lead to sacrificial bonds and provide shear deformability and larger deformations similar to that found in natural materials.[12] Thus, ionic bonding is a promising tool for tailoring the mechanical properties of biological or biomimetic systems, and to access important features such as sacrificial bonds and hidden length scales.Considering the lightweight character of the mechanically strong and tough biomaterials, a large-scale preparation of biomimetic materials is of preeminent importance for future construction and coating applications. However, this is a major scientific challenge. Various efforts have been undertaken to mimic the layered hard/soft composite structure of nacre by synthetic means. Nacre mimics can be obtained by several sequential approaches, such as layer-by-layer (LbL) [13][14][15][16] and other multilayer deposition strategies, [17] icetemplating and sintering of ceramics, [18,19] uncontrolled cocasting of polymer/clay mixtures, [20][21][22] or processes at interfaces. [23][24][25] Unfortunately, most of the approaches are limited to the structural characterization of the materials at very small scales, and often there have been challenges in even producing large enough specimens for mechanical characterization beyond nanointendation. Using LbL [26] deposition of polymers and nanoclay, the maximum stiffness and strength could even exceed those of natural nacre, [13][14][15][16] thus demonstrating how valuable such layered polymer/clay structures can be. Toughness could be increased...

Although advances have been reported to mimic the mechanically excellent structure of natural nacre, larger-scale applications are still limited due to time and energy-intensive preparation pathways. Herein, we demonstrate that simple high-shear homogenization of dispersions containing biobased high molecular weight sodium carboxymethyl cellulose (700 kg/mol, CMC) and natural sodium montmorillonite (MTM), serving as the soft energy-dissipating phase and reinforcing platelets, respectively, can be used to prepare large-area and thick films with well-aligned hard/soft nacre-mimetic mesostructure. During this process, core-shell nanoplatelets with intrinsic hard/soft structure form, which then self-assemble into a layered nanocomposite during water removal. The nanoscale periodicities of the alternating hard/soft layers can be precisely tuned by changing the ratio of CMC to MTM, which allows studying the evolution of mechanical properties as a function of the lamellar nanoscale periodicity and fractions of hard to soft material. Remarkable mechanical stiffness (25 GPa) and strength (320 MPa) can be obtained placing these materials among the top end of nacre-inspired materials reported so far. Mechanical homogenization also allows direct preparation of concentrated, yet homogeneous, gel-like dispersions of high nanoclay content, suited to doctor-blade large-area and thick films with essentially the same properties as films cast from dilute dispersions. In terms of functional properties, we report high-transparency, shape-persistent fire-blocking and the ability to surface-pattern via inkjet printing. Considering the simple, fully scalable, waterborne preparation pathway, and the use of nature-based components, we foresee applications as ecofriendly, bioinspired materials to promote sustainable engineering materials and novel types of functional barrier coatings and substrates.

Nanofibrillated cellulose (NFC) is a natural fibrillar material with exceptionally high mechanical properties. It has, however, been exceedingly difficult to achieve nanocomposites with drastically improved mechanical properties by dispersing NFC as random networks to polymer matrices, even using compatibilization. We show nanocomposites consisting of aligned assemblies of multilayered graphene and NFC with excellent tensile mechanical properties without any surface treatments. The optimum composition was found at 1.25 wt % graphene multilayers, giving a Young's modulus of 16.9 GPa, ultimate strength of 351 MPa, strain of 12%, and work-of-fracture of 22.3 MJ m(-3). This combines high strength with relatively high toughness and is obtained by direct exfoliation of graphite within aqueous hydrogels of NFC where an optimum sonication power is described. The results suggest the existence of an attractive interaction between multilayered graphene flakes and cellulose. Aligned assemblies are obtained by removal of water by filtration. The concept can be beneficial for applications because it results in high mechanical properties by a simple and environmentally green process.

No abstract

This paper deals, with cationically modified NanoFibrillar Cellulose (cat NFC), obtained by reacting a dissolving pulp with 2,3-epoxypropyl trimethylammonium chloride (EPTMAC). The cat NFC was thoroughly characterized in terms of morphology and physical properties. The dimensions of individual cellulose nanofibrils were determined by atomic force microscopy (AFM) imaging in water and in air. Fibrils as thin as 0.8-1.2 nm were observed in water. The fibril diameter changed upon drying and the average size was further quantified by image analysis. The experiments showed the importance of characterizing nanocellulosic materials in situ before drying. The fibril size in air was confirmed by cryogenic transmission electron microscopy (cryo-TEM), and it was found to be 2.6-3.0 nm. Smooth ultrathin films of cationic NFC were prepared by spincoating on silica substrates. The effect of electrolyte concentration and pH on swelling of the cationic NFC film was studied using a quartz crystal microbalance with dissipation. The results showed that at pH = 8 the cat NFC film was insensitive to electrolyte changes while at pH = 4.5, the water content of the film decreased with increasing ionic strength. The electrophoretic mobility measurements showed a cationic zeta potential for the cat NFC that decreased at increasing pH, verifying the swelling behaviour.

Cellulose nanofibrils show remarkable properties with applications in several fields of materials science, such as for composites, hydrogels, aerogels, foams, and coatings. Cellulose nanofibrils are typically produced by mechanical and enzymatic processing leading to fibrils having a width in the nanometer range and very high aspect ratios. The formation of percolating networks and interactions between fibrils lead to useful properties in for example gel formation and composites. In this work we studied how the residual xylan that is found in cellulose nanofibrils that have been produced from hardwood pulp affects these properties. We used enzymatic hydrolysis to specifically remove xylan and studied rheological properties, morphological features, and properties of paper-like films of cellulose nanofibrils. We found that removal of xylan enhances the formation of fibril networks, resulting in both stiffer gels and stronger films. However xylan also stabilizes the fibrils against flocculation. Also the history of processing of the preparations affects the results significantly.