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A review of experimental techniques to produce a nacre-like structure

This article has been downloaded from IOPscience. Please scroll down to see the full text article. 2012 Bioinspir. Biomim. 7 031001 (http://iopscience.iop.org/1748-3190/7/3/031001) View the table of contents for this issue, or go to the journal homepage for more

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IOP PUBLISHING

BIOINSPIRATION & BIOMIMETICS

doi:10.1088/1748-3182/7/3/031001

Bioinspir. Biomim. 7 (2012) 031001 (23pp)

TOPICAL REVIEW

A review of experimental techniques to produce a nacre-like structure I Corni 1 , T J Harvey 1 , J A Wharton 1 , K R Stokes 1,2 , F C Walsh 1 and R J K Wood 1 1

National Centre for Advanced Tribology at Southampton (nCATS), Engineering Sciences, University of Southampton, Highfield, Southampton, SO17 1BJ, UK 2 Physical Sciences Department, Defence Science and Technology Laboratory, Porton Down, Salisbury, Wiltshire, SP4 0JQ, UK E-mail: [email protected]

Received 2 December 2011 Accepted for publication 12 March 2012 Published 25 April 2012 Online at stacks.iop.org/BB/7/031001 Abstract The performance of man-made materials can be improved by exploring new structures inspired by the architecture of biological materials. Natural materials, such as nacre (mother-of-pearl), can have outstanding mechanical properties due to their complicated architecture and hierarchical structure at the nano-, micro- and meso-levels which have evolved over millions of years. This review describes the numerous experimental methods explored to date to produce composites with structures and mechanical properties similar to those of natural nacre. The materials produced have sizes ranging from nanometres to centimetres, processing times varying from a few minutes to several months and a different range of mechanical properties that render them suitable for various applications. For the first time, these techniques have been divided into those producing bulk materials, coatings and free-standing films. This is due to the fact that the material’s application strongly depends on its dimensions and different results have been reported by applying the same technique to produce materials with different sizes. The limitations and capabilities of these methodologies have been also described. (Some figures may appear in colour only in the online journal)

1. Introduction

new structures, new methodologies and new materials with enhanced properties. The inspiration for some of these studies has often been taken from the architecture of biological materials that combine compounds with poor macroscale mechanical properties, producing composites with properties that are an order of magnitude higher than expected by the ‘rule of mixtures’. On the other hand, man-made materials present properties that are always controlled by this law and are, therefore, intermediate to those of its components. The enhanced mechanical properties of biological materials, compared to those achieved by their correspondent synthetic materials, is surprising, especially considering that they are produced at ambient temperatures and pressures using a relatively limited number of compounds. Their extraordinary

Steel and other alloys have been extensively utilized to produce strong and flaw-tolerant materials for structural applications. Polymers tend to be flaw-tolerant but deform readily even when low stresses are applied. Ceramics are strong but poorly resistant to surface flaws and cracks; they are also brittle, rendering them unsuitable for structural applications. For these reasons, many attempts have been made to increase the toughness and overcome the brittleness of ceramic materials by adding fibres/whiskers or particles, or by introducing into the composite weak interfaces that would deflect the crack growth, but a universal solution has not been achieved [1, 2]. The continuous need in modern technology to improve the performance of materials has driven researchers to explore 1748-3182/12/031001+23$33.00

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Table 1. Summary of the mechanical properties of natural nacre. Properties of natural nacre Young’s modulus (GPa) Tensile strength (MPa) Ultimate tensile strength (MPa) Compressive strength (MPa) Flexural strength (MPa) Fracture strength (MPa) Work of fracture (J · m−2) Fracture toughness (MPa · m1/2) Module of rupture (MPa)

60–70 (bivalve mollusc Pinctada) [4] 140–170 (bivalve mollusc Pinctada) [4] 130 [40] 540 (red abalone) [4] 135 [22], 190 [55] 185 ± 20 (red abalone), 56–116 (bivalves, gastropods and cephalopods) [4] 350 to 1240 (bivalve mollusc Pinctada) [4] 8 ± 3 (red abalone) [4] 110–185 [35]

properties, such as the toughness of spider’s silk, the adhesion capabilities of a gecko’s feet and the light weight and strength of bamboo, are due to their organized structure and composition (i.e. in the gecko the extreme contact area between the feet and the surface; the spider’s silk has a different composition and mechanical properties depending on the function needed) and millions of years of evolution [3, 4]. Since the beginning of the 20th century, an increasing interest has been dedicated to studying and understanding the structure of natural materials and the reasons behind their outstanding mechanical properties and capabilities, e.g. adaptability to changing conditions, multifunctionality of the same material, miniaturization, and self-healing [3–14]. From these studies, it has emerged that the superior mechanical properties observed in natural materials are due to their complicated architecture and a hierarchical structure at the nano-, micro- and meso-levels. This branch of science is known as biomimetics and strictly connects biology, physics, chemistry and material science. The main aim of biomimetics is to understand why biological materials present these excellent properties and to determine what experimental techniques could transfer to synthetic materials the structural design and the mechanical properties observed in nature; this is achieved by inserting in man-made materials mechanisms that take place on a multiple of length scales and by introducing a complex interplay between morphology, surface structure, physical and chemical properties of the material. Further, it must be taken into consideration that the total replication of the structure of natural materials would not be very useful. In fact, in engineering the choice of materials is much wider compared to nature and not every single microstructural feature observed in natural materials may have a purpose and some of them might be very difficult to replicate with no real improvement in properties [3, 4, 8, 15–19]. Amongst the many natural materials studied, nacre, also known as mother-of-pearl, has attracted the most interest in biomimetics, largely due to its exceptional mechanical properties (see table 1). In nature, nacre is found in the inner layer of the structure of some shells and it has been developed through evolution to protect the soft body of the mollusc against attack from predators, debris and rocks moved by the current. Natural nacre (consisting of 95 wt% aragonite, a crystal form of calcium carbonate (CaCO3), and 5 wt% biological macromolecules) presents a three orders

of magnitude higher toughness than its main constituents. The outstanding strength, toughness, stiffness and impact resistance of this material are due to its robust ‘bricks and mortar’ nanostructure of alternating layers of protein (10– 50 nm thick) and aragonite tablets (200–900 nm thick) with a diameter of around 5–8 μm. The proteinaceous layers are a complex arrangement of biopolymer organized into numerous layers that also play a fundamental role in the formation of the aragonite tablets arresting the growth in different directions. The crystalline orthorhombic aragonite tablets are the rigid building blocks of the structure and present all the same crystallographic orientation, due to the inter-tile connections through mineral bridges across the porous polymeric layers. In more detail, the observed improvement in mechanical properties, compared to its constituents, could be further explained considering that: • The polymeric layers act as a strong viscoelastic adhesive that increases the tensile strength of the composite, thanks to the stretching of the molecular chains attached to neighbouring tiles. • The sliding of contiguous tiles is made more difficult by the nanoasperities observed on the tile surfaces and by the need to rupture the connection bridges between tiles, which increases the friction between them. • The greater thickness of the tiles at their edges (low-angle dovetails) is also fundamental as this locks the tiles in place when they are pulled apart; in fact under tensile deformation the tablets start to slide and as they do, it becomes more difficult for them to slide any further, other tiles start sliding, spreading the inelastic deformation over a larger volume. All these mechanisms contribute to the high tensile strength of nacre in the direction along the tablets, with a 1% strain at failure, observed by applying a tensile stress of 60 MPa [3–6, 12–14, 18]. The complex hierarchical structure of nacre is shown in figure 1. It demonstrates three levels of hierarchy: the structure made of the polymer matrix and aragonite tablets with a characteristic waviness (first row) (see figure 1(e)); the interface between the tablets and the organic material, nanoasperities and mineral connections between tiles (second row) (see figure 1( f )), and finally the tablets themselves made of aragonite nanograins connected by a three-dimensional network of organic material (third row). More comprehensive 2

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Topical Review

(a)

(c)

(b)

(d )

(e)

(f )

Figure 1. The structure of nacre at different length scales: (a) the inside of a red abalone shell, (b) a schematic of the ‘bricks and mortar’ structure, (c) the view from above, (d) the fractured surface, (e) a TEM micrograph showing the CaCO3 tablet waviness [12] and ( f ) the nanoasperities on the tile surfaces and some mineral bridges between the tiles [4].

details of the structure of nacre and its mechanical properties have been extensively reviewed elsewhere [3–6, 12–14, 18]. The remarkable mechanical properties observed in nacre are also found in other biological materials. For example, bone is also made of ceramic (hydroxyapatite) and polymer (collagen) like nacre, but it shows a completely different structure: compact and porous. The porous part has different functions, beyond reducing the structural weight, and notwithstanding the porosity the bones are strong because they are shaped in a way to provide strength where required. Dentin is the internal part of the teeth, which is even tougher than the external enamel, it is also made of ceramic (45 vol.% hydroxyapatite), protein (30 vol.% collagen) and water (25 vol.%) with a structure consisting of tubules with a 1 μm diameter surrounded by randomly arranged hydroxyapatite crystals (0.5–1 μm diameter). These tubules are then surrounded by a collagen–hydroxyapatite composite. Wood presents a completely different composition compared to these materials, being made of cellulose (a high molecular weight polysaccharide), but still its exceptional mechanical properties (the stiffness and strength per unit weight are similar to those of steel) are principally due to its hierarchical structure [4].

Although substantial progress has been achieved in the understanding of the mechanical properties and the architecture typical of natural structures, the development of experimental techniques to produce artificial composites with structures inspired by nature is still a challenging goal. While other reviews [3–6, 12, 14] mainly focus on the structure of nacre and give just a brief description of some of the methodologies explored to reproduce it, in this paper we review, for the first time in great detail, the experimental methods applied to date to reproduce the structure of nacre. We give particular attention to the mechanical properties achieved, where they have been measured and reported by the authors. Since the practical applications of a material are strongly dependent on it being a film or a bulk material and different results are achieved by applying the same technique to produce materials with different sizes, the techniques explored by numerous research groups have been divided into those producing bulk and free-standing materials (see section 2) and those producing coatings (see section 3) with a nacre-like structure, as schematically summarized in figure 2. Beyond describing the state of the art development of this field, the aim of this review is also to underline the advantages and limitations of the different techniques applied and to suggest some critical areas for further work. 3

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Figure 2. A schematic of the main techniques explored to produce nacre-like materials as macroscopic composites, free-standing films and coatings.

2. Methods to produce bulk materials with nacre-like structures

scaffold. The samples are freeze-dried to eliminate the water, fired at 400 ◦ C to remove the organic additives and sintered at 1500 ◦ C to densify the ceramic lamellae and produce 1 μm thick ceramic bridges connecting them [22]. This ceramic scaffold could also be employed to produce a ‘bricks-and-mortar’ structure by infiltrating it with a polymer and applying pressure in the direction perpendicular to the lamellae. The polymer is fundamental to let the ceramic flow, to obtain a dense structure and to hold the composite together. The polymer is then removed with thermal treatment and the structure is densified. The samples are finally isostatically pressed and sintered to promote the formation of submicrometre bridges between the ‘bricks’ providing strong and stiff links between the ceramic layers. The ceramic ‘bricks’ are 5–10 μm thick and 20–100 μm long [20, 22]. Both lamellar and ‘bricks-and-mortar’ ceramic scaffolds are then infiltrated with polymethyl methacrylate (PMMA) by a free-radical polymerization, to produce Al2O3–PMMA hybrid materials (as shown in figure 3(a) and (b)). In this ‘bricks-andmortar’ structure the polymer layers have an average thickness of ∼1–2 μm, achieving in some areas a sub-micrometre thickness similar to that of nacre [20, 22]. The ‘bricks-andmortar’ structure mimics nacre on numerous length scales (a high ceramic content (80 vol.%), thin polymeric layers the characteristic surface roughness (see figure 3(c)) and the high density of thin ceramic bridges between ‘bricks’), but it could be improved further by decreasing the structural size by an order of magnitude in order to achieve the dimensions typical of the natural material.

Depending on the final application for which the material has been designed, it might be useful to produce either a coating or a free-standing material with a nacre-like structure. The following sections review the experimental methods that have been reported in the literature to produce macroscopic bulk materials and free-standing thin composite materials with structures and mechanical properties similar to those characteristic of nacre. 2.1. Macroscopic nacre-like composite materials 2.1.1. Freeze casting. A recently reported method to mimic the hierarchical structure of nacre, producing hybrid materials several centimetres in size with either a lamellar or a ‘bricksand-mortar’ structure, is freeze casting (or ice templating) employed by Tomsia et al [20–22]. This technique is inspired by the expulsion of solutes from the water and their entrapment in the space between ice crystals during the formation of seaice. Tomsia et al have prepared large porous ceramic scaffolds starting from a water-based suspension containing 50 wt% of sub-micron alumina Al2O3 particles and organic additives to (i) disperse the powder, (ii) ensure the integrity of the ceramic scaffold and (iii) control the ice crystal structure ensuring the formation of thin ceramic lamellae with a typical microscopic roughness. The suspension is then cooled to −80 ◦ C; in these conditions the lamellar ice grows directionally expelling the Al2O3 particles and produces a template for the ceramic 4

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and natural nacre (135 MPa). The crack-initiation fracture toughness values of both structures were almost double than expected using the ‘rule of mixtures’. The fracture toughness as a function of crack extension for both composites exceeded that of natural nacre [20, 22]. Both the flexural strength and the crack-initiation fracture toughness further increased by decreasing the thickness of the ceramic lamellae, controlled by changing the cooling rate, the particle size, the additives and the suspension concentration [22–24], or by chemically grafting a methacrylate group onto the Al2O3 surfaces before infiltrating the PMMA to encourage stronger covalent bonding between the two phases [20, 22]. They [20, 22] also observed that the chemical grafting increased the strength of the ‘bricksand-mortar’ structure by 80% and the toughness by 70% thanks to the stronger interface that allows the organic layer to behave as a viscoelastic glue that permits and limits the interfacial sliding in a similar fashion to the mechanism reported for natural nacre. It was also observed that the microstructural damage of the two artificial composites was not localized but widely distributed (on the millimetre scale), e.g. the inelastic deformation within the polymer, multiple microcracks within the ceramic layers outside the main crack path, as also observed in nacre. The damage ahead of the main growing crack produces elastic bridges (that are different from the ceramic bridges) that effectively bridge the crack and bear the load to slow down the crack propagation. In particular, in the ‘bricksand-mortar’ material the crack does not propagate through the ceramic ‘bricks’ but it deviates going around them, resulting in the pull-out of the ‘bricks’ from the structure, as also observed in natural nacre and clearly shown in figure 4 [20, 22]. Between the lamellar and the ‘brick-and-mortar’ structures, the latter is the one that best mimics nacre. In this structure the polymeric phase serves as a stress-relieving lubricant that controls the sliding of the load-bearing ceramic blocks providing an impressive combination of high strength and high toughness. It is also expected that to best mimic the structure of nacre, the amount of polymer in the composite should be reduced from 20 vol.% to 5 wt% [20, 22]. The same freeze-casting methodology has also been successfully applied to produce Al2O3/Al, Al2O3/epoxy, hydroxyapatite/Al, hydroxyapatite/epoxy [15] and Al2O3/Al-Si [25] composites, demonstrating the versatility of this technique. The composites containing 36 vol.% of Al2O3 and 64 vol.% of Al-Si presented a flexural strength close to 300 MPa and a fracture toughness higher than 40 MPa · m1/2: both values are considerably higher than those of natural nacre [25]. Summarizing the observations above, freeze casting produces a ‘bricks-and-mortar’ structure very similar to that of nacre, even if the dimensions and the polymeric content are much higher. Moreover, the mechanical properties obtained are reported to be better than expected by the ‘rule of mixtures’, making this technique very promising for the production of nacre-like bulk materials. This methodology should easily be scaled-up to fabricate samples bigger than a few cm; the only limitation could be the difficulty in uniformly infiltrating the polymer into the ceramic scaffold without creating porosities

(a)

(b)

(c )

Figure 3. (a) The Al2O3/PMMA lamellar composite structure produced by freeze casting a ceramic suspension and polymer infiltration (lighter phase: ceramic; dark phase: polymer); (b) the ‘bricks-and-mortar’ structure made by pressing and sintering the lamellar scaffolds (ceramic content up to 80 vol.%); (c) the roughness of Al2O3 lamellae produced by freeze casting a ceramic suspension containing sucrose [22].

The material fabricated with this technique produces stress–strain curves from the bending test which are very similar to those of nacre, with an inelastic deformation higher that 1% before failure. The flexural strengths were 120 and 210 MPa for the lamellae and the ‘bricks-andmortar’ structures, comparable to those of alumina (320 MPa) 5

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(a)

(b)

Figure 4. A comparison between the toughening mechanism in (a) artificial (Al2O3–PMMA) ‘bricks-and-mortar’ structure and (b) natural nacre [22].

alumina interface due to debonding between the two materials. The flexural moduli and strength of the pure epoxy were 2.78 GPa and 122 MPa; these two values considerably improved with the addition of Al2O3 flakes, reaching 19–23 GPa and 133–155 MPa, respectively. The flexural strength is very similar to that measured for natural nacre (135 MPa) [20, 22]. The hardness of the HASC composites was 85 HV5, much higher than that of pure resin (15 HV5) and comparable with that of low carbon steel. The mechanical properties of this composite could be further improved by increasing the bonding between the two phases by, for example, functionalizing the alumina flakes or introducing nanoasperities on their surface to produce a mechanical keying between the two phases. Considering that both hot-pressing and slip casting are widely employed industrial techniques, after optimizing the interfacial bonding between the two phases, this new methodology might be expected to be easily applied within an industrial context to fabricate nacre-like bulk materials.

Figure 5. A cross section of the alumina flakes–epoxy composite produced by HASC [16].

if the samples are much larger in the direction parallel to the lamellae.

2.1.3. Biomineralization or bottom-up approach. In situ biomineralization is an experimental technique in which the crystallization of minerals or mineralization takes place in vitro from supersaturated solutions in the presence of organic macromolecules that influence the size, type and morphology of the crystals and act as templates for their nucleation. The macromolecules can accelerate or inhibit the growth of the crystals depending on their concentration, molar mass and functional groups. In order to prepare the composite, the macromolecule solution is added to a supersaturated solution of the inorganic constituents and the mixture is left to perform. This technique is inspired by nature, where well-defined and ordered hierarchical structures form with a bottom-up approach, in which an organic phase is the template for the nucleation and growth of inorganic crystals from a solution. In order to apply this technique, it is fundamental to choose the most appropriate organic template to form a desired inorganic layer and to understand how the organic layer influences the nucleation, the orientation and the shape of the growing inorganic layer [6, 26]. This process has been widely studied to understand the underlying fundamental processes and then

2.1.2. Hot-press assisted slip casting. Another method to produce nacre-like bulk composites is hot-press assisted slip casting (HASC), that has been applied to produce Al2O3 flake (10 μm in diameter and 350 nm thick)–epoxy composites [16]. The composite is prepared by manually mixing the two components and pouring the mixture onto a porous mould within a steel die; the system is then hot-pressed at 150 ◦ C for 30 min to cure the epoxy matrix. The applied pressure forces the liquid matrix to flow through the mould, decreasing the matrix content in the composite and aligning the flakes perpendicularly to the pressure direction. The final composite contains up to 60 vol.% of highly oriented Al2O3 flakes and has a microstructure similar to that of natural nacre, as shown in figure 5; the Al2O3 flakes have a thickness comparable to that of the aragonite platelets and a diameter that is slightly bigger, but the thickness of the resin layers is surely higher than that of the polymeric layers in nacre due to the high resin content in the composite (40 vol.% instead of 5 wt%). A three-point bending test on this composite has demonstrated that fractures evolved mainly along the epoxy– 6

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Some of the nanoplates were inter-connected by mineral bridges, similar to those observed in nacre and employed to align neighbouring plates. These ZnO microcrystals could be employed to produce ZnO-polymer composites with a nacrelike structure. Gehrke et al [29] prepared artificial nacre by applying a retrosynthetic approach using calcium chloride, ammonium carbonate and the insoluble organic matrix from natural nacre. The organic matrix from nacre was demineralized using a 10% acetic acid solution and applying the gas diffusion method. Then two flasks, one containing the organic matrix, calcium chloride and polyaspartic acid and the other one containing ammonium carbonate, were placed in a closed chamber at room temperature for 24 h maintaining their contents separate. Amorphous calcium carbonate nanoparticles were formed and stabilized by polyaspartic acid, producing a nacre-like structure. The platelets had a thickness of 100–400 nm and a width of 1.6 μm, slightly smaller than those found in natural nacre, while the polymeric layers were very similar to those observed in nacre. The smaller tablet size and their different composition (calcite instead of aragonite) is due to the higher nucleation rate induced by the modified organic phase, probably because of the degradation of part of the protein during the demineralization process. The mechanical properties of this material were evaluated by nanoindentation, observing that the stiffness varied in different areas of the sample, due to the different degree of mineralization: the maximum value for the modulus was 37.7 GPa with an average of 16.1 GPa, much lower than the value measured in nacre (53.6 GPa). From the results reported, it seems that biomineralization is a successful method to produce bulk materials with a nacrelike structure; but the drawbacks of this technique are that it is very slow, requiring a day or more to produce microscale materials, and that the mechanical properties achieved are not as good as would be expected from their structure. More studies are therefore needed to produce more homogeneous bulk materials with improved mechanical properties.

(a)

(b)

Figure 6. Micrographs showing (a) the top and (b) the cross section of a ZnO microcrystal. (Reprinted with permission from [28] Copyright 2009 American Chemical Society.)

extensively applied to develop new materials as described below and in other sections of this review. Oaki and Imai [27] applied this technique to produce an organized nacre-like architecture starting from potassium sulphate (K2SO4) and poly(acrylic acid) (PAA). An aqueous solution of PAA was added to a K2SO4 solution and the mixture was left at 25 ◦ C for several days; the crystal growth proceeded as the water evaporated. Using this process, a layered morphology was obtained consisting of units with a thickness of about 0.5–1.0 μm, containing an oriented assembly of smaller units with a diameter of around 20 nm. Using this experimental approach, a hierarchy similar to this characteristic of nacre was reproduced by combining inorganic crystals and organic polymers. Tseng et al [28] produced zinc oxide (ZnO) hierarchical crystal structures. The ZnO was produced from a chemical reaction between zinc nitrate and hexamethylenetetramine at 80 ◦ C for 21 h in the presence of various amounts of gelatin that controlled the nucleation and determined the crystal growth orientation. The ZnO microcrystals presented a well-defined hexagonal shape (see figure 6(a)) consisting of a stack of wellaligned and oriented nanoplates made up of self-organized ZnO nanoparticles (see figure 6(b)). The hierarchical structure of the composite is similar to that of natural nacre with three levels of hierarchies: ZnO nanoparticles, ZnO nanoplates and hexagonal single crystal made of aligned nanoplates.

2.1.4. Extrusion and roll compaction. Wang et al [2, 30] applied extrusion and roll compaction followed by hot-pressed sintering to prepare a fibrous monolithic silicon nitride– boron nitride (Si3N4–BN) composite similar to bamboo and a laminated Si3N4–BN composite similar to natural nacre (see figure 7). The mechanical properties of both composites were improved by adding silicon carbide (SiC) whiskers to the Si3N4 matrix and Al2O3 or Si3N4 to the BN layers to adjust the bonding strength between the layers. Using this technique very high values of fracture toughness (24 and 28 MPa · m1/2) and work of fracture (4000 J m−2 instead of 100 J m−2 for monolithic Si3N4) were obtained for both composites. Load– displacement curves showed that the two composites gave a non-brittle failure mechanism while the Si3N4 bulk material failed catastrophically under the same conditions. The high values of fracture toughness and work of fracture of these composites were due to the multi-level toughening mechanism at different length scales: weak interfaces, whiskers and 7

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[31, 32]. These two composites do not really mimic the structure of nacre, apart from the load transfer to the platelets and the pull-out fracture mechanism, demonstrating that the mechanical properties of a biomimetic composite could be improved even without replication of all the characteristic features typical of nacre. This methodology is fast, simple and versatile and therefore it could be easily scaled-up and applied to a variety of polymers and platelets combinations to produce composites with tailored mechanical properties [31, 32]. 2.1.6. Other methods. Clegg et al [33] produced bulk composites by pressing and sintering 200 μm thin SiC sheets coated with graphite. The graphite provided appropriate interfacial properties and introduced into the composite weak interfaces to improve the brittleness of the ceramic. The weak interfaces between the ceramic sheets deflected the cracks and increased the toughness and the composite work of fracture (that is the area under the stress–strain curve and is strongly influenced by gradual and graceful fracture of the sample) by between 4 and 100 times compared to monolithic SiC. The composite fracture toughness was much lower than that observed in natural materials but higher than that of monolithic SiC. Overall, this technique improves the properties of SiC by introducing weak interfaces, like in nacre, but without mimicking other features and the length-scale typical of nacre. Akkapeddi et al [34] prepared polyamide-6 nanocomposites (PA-6/NC) by melting the polymer in the presence of chemically modified montmorillonite (MMT) clay platelets (1 nm thick and with a diameter varying from 30 nm to several microns) followed by a twin-screw extrusion with the addition of short and long glass fibres to the composite. The PA-6/NC presented well-dispersed and aligned MMT platelets when a low MMT concentration was employed. The addition of glass fibres produced a composite with a unique combination of nano-, micro- and macro-scale reinforcements, that in a way could be considered inspired by the hierarchical structure of natural nacre, even if the ratio between the organic and the inorganic phase and the complicate microarchitectures that make nacre so extraordinary were not reproduced. The high surface/volume ratio of the MMT platelets produced a high reinforcement efficiency, even with 2–5 wt% concentration in the composite, obtaining a high flexural modulus and strength. The glass fibre addition increased the flexural modulus of the composite, compared to the PA-6/NC (from 4.7 to 7 GPa). More research has to be applied to this methodology to increase further the inorganic content in order to obtain a structure more similar to that of nacre. Almqvist et al [35] produced, using different techniques, microlaminate composite materials made of 90 wt% oriented magnesium silicate (talc) tablets and 10 wt% polyvinylacetate (PVA) or epoxy silane. They prepared a suspension of talc and PVA or epoxy silane in a water–ethanol mixture and then applied to it seven methods to produce a composite material. Centrifugation was not a successful method; in fact it separated the inorganic and organic components and was not able to orient the talc tablets. In shearing between cylinders, the mixture was placed in a plastic tube and rotated at a

Figure 7. A cross section of the microstructure of the laminated Si3N4–BN composite showing a fracture mechanism similar to that of nacre [2].

elongated grains in the ceramic matrix, partly inspired by the hierarchical structure observed in nacre and other natural materials. These results demonstrate that this technique can be employed to produce bulk materials with a structure and mechanical properties similar to those of natural nacre; figure 7 shows that the fracture mechanism of this composite is similar to that observed in nacre and reported in figure 4(b). 2.1.5. Gel-casting and hot-pressing. Bonderer et al [31, 32] produced platelet-reinforced polymer matrix composites by using gel-casting and hot-pressing. Sub-micrometre Al2O3 platelets were dispersed in a polypropylene [31] or polyurethane solution [32]; during cooling/evaporation the polymer–platelet solution densified forming a gel in which the polymer surrounded the individual platelets. By repeating the solvent evaporation a composite with horizontally oriented platelets was produced. Then the dried composite was hot-pressed to additionally increase the platelet orientation and to close potential pores in the polymeric matrix. The polypropylene–Al2O3 composite contained a maximum of 0.5 vol.% Al2O3, while the polyurethane–Al2O3 composite presented a maximum of 0.73 vol.% Al2O3; both values are much lower than the amounts reported in natural nacre (95 wt% of inorganic phase). The maximum yield strength and elastic modulus for the polypropylene–Al2O3 composites were 82% and 13 times higher than those of the pure polymer [31]. The ultimate tensile strength of polyurethane– Al2O3 composites decreased with increasing platelet content and the elastic modulus was more than 100 times higher than the value for the pure polymer [32]. The mechanical properties of the two composites improved significantly due to the load transfer to the platelets, even if their concentration was low. The composites with a low alumina concentration showed a pull-out fracture mechanism that allowed a plastic deformation of the composite before fracture, while at a higher platelet concentration their misalignment and the voids in the composite produced a decrease in the mechanical properties 8

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but without duplicating the hardening behaviour typical of nacre [14, 37]. Considering the results obtained by this model, more computational studies are needed to understand the effect that different morphological parameters play on the composite mechanical properties and more research is required to reproduce experimentally the nacre structure, since a lack of reproducing the waviness, the mineral bridges and the volume fraction typical of nacre has been observed in the methodologies currently employed [36]. Similar studies were also carried out by Barthelat et al [18, 19] who employed a computer numericaly controlled machine to produce PMMA tablets 1 mm thick and 2.8 or 5.6 mm long with 5◦ angular waviness on both surfaces. These two kinds of tablets were organized to produce an overlap between them and kept in place using transverse fasteners, obtaining an increase in strain from 2% to 10% under tension compared with pure PMMA; the tensile strength of the material could be further modified by changing the tightening of the fasteners. This material reproduces, for the first time, all the key features of natural nacre: the organization and waviness of the tablets, the sliding and locking of the tablets under tension, thanks to the dovetails that produce strain hardening and spread the deformation. The dry friction between the tablets transmits the shear stresses between them, replacing the function of the organic material in natural nacre. This methodology therefore shows that it is possible to reproduce the main features of natural nacre at the millimetre scale [18, 19]. A Finite Element Modelling was applied to this material [19] gaining a further understanding of the fundamental mechanisms that generate the mechanical properties of the composite; a further optimization of composition and structure should produce a material with exceptional stiffness, toughness and hardness. Among all the techniques reviewed to produce nacre-like bulk materials, the most promising seems to be freeze casting as it partially mimics the structure of nacre and produces, in a short time, bulk materials with superior mechanical properties. Another interesting method is biomineralization that copies the natural process but produces materials with very poor mechanical properties. More research should also be carried out to further explore the capabilities of hot-press assisted slip casting, extrusion and roll-compaction and the more recent work to produce millimetre size nacre-like material, as they all showed potential in producing composites similar to natural nacre and improved mechanical properties.

speed lower that 1 rpm for several hours until completely solidified; this method was able to orient the talc tablets in the composite. In sedimentation, the suspension was poured into a plastic container and left undisturbed for one day. In dipping, a glass slide was dipped into the suspension and left to dry under a heat lamp; the process was then repeated several times. In shearing plates, the mixture was sheared by hand between two glass slides. In spinning cylinder and spinning plate, the suspension was continuously dripped onto a spinning cylinder or a rotating glass plate. Comparing all these methodologies, the spinning and shearing plates seemed to be the best for orienting the talc tablets. It was also observed that in addition to the physical method used to produce the composite, the surface chemistry of the tablets influenced their degree of orientation. Between the two polymers employed, the epoxysilane seemed to be more effective in orienting the tablets for all the methods applied; this is because the silane altered the surface chemistry of the talc, enhancing its wettability. All these composites were fairly brittle, with a modulus of rupture (MOR) of 6.4 MPa for the samples prepared using the spinning cylinder method, and lower values for all the other samples. Between all the samples, those produced by centrifugation were the weakest with a MOR an order of magnitude lower. The composites with the epoxy silane were more elastic but with a lower MOR compared to the composites with PVA. The MOR of natural nacre was measured to be between 110 and 185 MPa, much higher than the values observed in these experimental materials; the lower values of MOR obtained could be explained by considering the lower degree of order in these composites compared to nacre and the weak binding strength between the two components. Therefore, it is recognized that in order to produce high strength laminate materials, it is essential to align the inorganic platelets and to wet them to increase the bond to each other and to the organic matrix [35]. Not much information on the structures of these composites has been reported by the authors. Rim et al [36] developed a Finite Element computational model to study the structure and mechanical properties of nacre taking into consideration its key morphological features, such as tablet brittleness, polymer ductility, tablet waviness and bridges between tablets. They identified, theoretically, an optimal geometry to increase the energy dissipation with a deformation mechanism similar to that of nacre, taking into consideration the structure morphology and the properties of the materials employed. Macroscopic specimens, based on the computational optimized results, were experimentally manufactured using fused deposition moulding with acrylonitrile butadiene styrene (ABS) tablets and epoxy [14, 37]. The tablets presented a characteristic waviness and were arranged in columns in order to have some overlap between them. The dovetail overlap length and dovetail angle were varied and it was noticed that rather small variations in the dovetail angle produced considerable variations in strength and energy dissipation; thus demonstrating that an accurate control of the geometry is necessary in order to obtain optimal performances and that the optimal geometry corresponds to that of natural nacre. This composite was able to reproduce the sliding of the tablets with some locking between them

2.2. Thin nacre-like free-standing composites 2.2.1. Layer-by-layer self-assembly. In the layer-by-layer (LBL) self-assembly process a substrate is immersed sequentially into two dilute solutions of oppositely charged materials that are adsorbed onto the substrate, producing nanometre thick monolayers interacting with each other through electrostatic attractions. One or more washing steps are carried out after the adsorption of one layer to avoid contamination between the two solutions. The adsorption time for each layer varies from a few seconds to hours, and depends on the concentration of the solution and the molar mass of the solute. The process is repeated until a multilayered structure 9

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Figure 8. A cross section of an MMT-PDADMAC multilayer film deposited by LBL technique. (Reprinted by permission from Macmillan Publishers Ltd: Nature Materials [40] copyright (2003).)

is obtained [38]. This technique is simple and allows the production of complex multilayer structures with an accurate control of the composition and thickness, plus it can be applied to different substrates with different sizes and it is versatile being applicable to a wide range of materials, e.g. biomolecules, especially since it is performed under mild conditions. The only drawback is the requirement of repeating the deposition steps numerous times in order to achieve a film with a practical thickness; therefore this technique could be time-consuming. The coating could then be detached from the substrate to produce a free-standing film. More information on fabrication techniques and the potential applications of free-standing films deposited applying the layer-by-layer technique, has been reviewed elsewhere [39]. Tang et al [40] applied the LBL method to produce MMT clay and poly(diallydimethylammonium) chloride (PDADMAC) multilayer films. In the adsorption step the clay platelets oriented themselves parallel to the substrate surface and in the rinsing step all the irregularly adsorbed and loosely attached platelets were removed; this automatic arrangement ensured a high degree of order in the multilayer structure. The strong bond between the layers was assured by the PDADMAC that interacted with the MMT with attractive electrostatic and van der Waals interactions and acted as a glue between the clay platelets that overlapped each other. A cross section of a dense and uniformly thick film with 100 MMT–PDADMAC layers is shown in figure 8. The film thickness could be adjusted by increasing the number of deposition steps/layers, e.g. 1.2 ± 0.1 μm for 50 layers, 2.4 ± 0.15 μm for 100 layers and 4.9 ± 0.4 μm for 200 layers. All the free-standing composites present an initial plastic deformation under a stress lower than 10 MPa and suddenly harden when further stretched, similar to the behaviour observed in nacre. The toughness and structural strength observed in these structures could be explained by the inelastic processes occurring at the organic–inorganic interfaces due to the strong electrostatic interactions, the extensive van der Waals forces and the ionic interactions within the polyelectrolyte chains; all these interactions need to be broken during stretching. The ultimate tensile stress, ultimate tensile strain and Young’s modulus of the composites increase with their thickness. The thicker composites presented an ultimate tensile stress of 109 ± 8 MPa, just slightly lower than the value of 130 MPa measured for nacre. The Young’s modulus was 11 ± 2 GPa

lower than the 64 GPa observed in nacre; this could be explained by considering the relatively high organic content in the artificial composite and the flatness of the clay platelets compared to the rough aragonite ‘bricks’ in nacre that provide additional friction during sliding [40]. The same technique has been also employed by Podsiadlo et al [41] to produce PVA and MMT clay platelets nanocomposites with a well-defined layered architecture and an average bilayer thickness of 5 nm. Even if the PVA is uncharged, it still produces stronger composites compared to those obtained using other polymers interacting electrostatically with the negatively charged clay. This is explained by considering that the PVA–MMT couple has two outstanding properties: the high effectiveness of the hydrogen bonding and the cross-linking between PVA and MMT that maximizes the interactions between them and constrains the movement of the polymer chains, producing an efficient load transfer between the PVA and the rigid MMT platelets, improving the mechanical properties of the composite overall. After the LBL assembly, the bonds and load transfer in the film were further strengthened by applying a cross-linking agent, glutaraldehyde (GA). The mechanical properties of the composite were assessed by a microtensile test. For the MMT/PVA composite not treated with GA, the strength was four times higher and the modulus was almost an order of magnitude higher than that of pure PVA. These results are particularly surprising, considering that the maximum amount of clay added homogeneously into the composite is just 10 wt%. Higher clay concentrations marginally increase or decrease the strength and stiffness of the composite due to particle agglomeration. The GA cross-linking treatment increased the strength, the stiffness and the brittleness of both PVA and PVA/MMT composites. The ultimate tensile strength of the crossed-linked composite reached values of 480 MPa, three times and ten times higher than that of uncross-linked material and pure PVA, respectively; this value is also higher than that reported for natural nacre. The modulus of the PVA/MMT composite treated with GA reached 125 GPa; one order of magnitude higher than that of uncross-linked composites and two orders of magnitude higher than that of pure PVA [41]. The same group [42] also demonstrated the versatility of the LBL method, producing PDADMAC–MMT biocompatible starch-coated silver nanoparticle multilayered films with 10

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(a)

(b)

(c)

(d )

Figure 9. SEM micrographs of the cross section of a DAR/PAA and CaCO3 multilayer laminate structure. (Reprinted with permission from [43]: copyright 2007 American Chemical Society.)

antimicrobial properties. These results demonstrated that the layer-by-layer self-assembly technique could be applied for the production of composites with a wide range of function. The MMT–polymer structures described above [40–42] reproduce the strong linkage between the ‘bricks’ and ‘mortar’ typical of nacre; the main differences with nacre are the very small thickness of the ‘brick’ layers (a few nm instead of a few hundred) and the high amount of polymer in the composite (90 wt%, instead of 5 wt%). Wei et al [43] employed an LBL assembly and the diffusion method to deposit a polycationic diazoresins (DAR)/poly(acrylic acid) (PAA) and calcium carbonate multilayer film, which could be made free-standing. The DAR/PAA multilayer film was deposited on silicon by dipping the substrate alternatively into PAA and DAR solutions obtaining a coating with a known thickness and composition; the films were then exposed to UV light to covalently bond the two polymers. The organic DAR/PAA film was then inserted in a dessicator containing two flasks: one with a CaCl2 solution and the other with (NH4)2CO3; a CaCO3 film formed on the organic film thanks to the slow diffusion of CO2 decomposed from the (NH4)2CO3. The CaCO3 layer thickness could be varied by changing the diffusion time of the CO2. The procedure was then repeated to produce a multilayer structure, such as those shown in figure 9. This technique can clearly mimic the ‘bricks-and-mortar’ structure and the tablet’s roughness typical of nacre; it is also versatile, allowing for the control of the thicknesses of both the polymeric and the inorganic layers. Yao et al [44] prepared organic–inorganic hybrid films containing micro- or nano-platelets of layered double hydroxides (LDHs): Cu-NO3, Co-Al-CO3, and Eu-Cl functionalized with hydrophobic amine-terminated silane species and chitosan as the organic components. The platelets were transferred by dip coating to a glass substrate and a chitosan layer was applied by spin coating; strong hydrogen

bonds formed between the two materials. By repeating these two steps many times, multilayer films with thicknesses of a few tens of μm were produced; these coatings could also be peeled off the substrate, producing free-standing coloured and transparent films. The Cu-NO3-chitosan films have a tensile strength of 160 MPa which is eight times higher than that of the pure chitosan films (produced with the same methodology) and similar to that typical of nacre. The authors did not reported the ratio between LDHs and chitosan in the composites but were able to produce a ‘brick-and-mortar’ structure similar to that of nacre, even if it did not reproduce any of the micro-features. Bonderer et al [1] went beyond producing bulk materials, as described in section 2.1.5, depositing an alumina platelets and chitosan layered composite by sequential deposition. The Al2O3 platelets were surface modified, adding hydrophobic amine silane species to increase their ability to be adsorbed at the air/water interface and improve their adhesion to the chitosan through hydrogen bonds. An ethanolic suspension of Al2O3 platelets was applied over a water surface, and upon sonication, a perfectly oriented monolayer of platelets was achieved at the interface; this 2D structure was then transferred onto a substrate by dip-coating. The organic layer was deposited by spin-coating a chitosan solution, with the possibility of controlling the film’s thickness by varying the solution’s concentration. These two steps were then repeated one after the other to produce a free-standing multilayered film with a total thickness of a few tens of microns. In films with an Al2O3 content lower than 20 vol.%, the platelets were well-aligned; voids and misalignment were observed at higher Al2O3 concentrations. The tensile stress and the elastic modulus of the chitosan were 50 MPa and 2 GPa, respectively; these increased to 300 MPa and 10 GPa by adding 15 vol.% of Al2O3. This considerable increase in the mechanical properties was explained by taking into consideration the extensive plastic flow of the chitosan and the load transfer to the 11

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Al2O3 platelets that take place through the strong Al2O3– chitosan interface. The tensile stress values of this composite were higher than those reported for nacre (150 MPa), but the elastic modulus was around ten times lower than that of nacre. This ‘bricks-and-mortar’ structure fails to reproduce the complex microarchitecture of nacre and presents a much lower ceramic content (less than 20 vol.% instead of 95 wt%), which cannot really be increased further without affecting the mechanical properties. However, the composite’s mechanical properties are considerably increased compared to those of the matrix, demonstrating that the load transfer to the ‘bricks’ is extremely important and that this methodology is potentially useful even if the structures produced are not nacre-like. 2.2.2. Biomineralization. The biomineralization or templateinhibition technique has been already described in section 2.1.3 for the production of bulk materials; this technique can also be applied to produce free-standing films, with the mineral deposition taking place from a solution onto a solid substrate and a subsequent peel-off of the coating from the substrate. Kato et al [45] studied the effect that soluble and solid macromolecules play in the crystallization of CaCO3 on glass substrates from a supersaturated calcium bicarbonate solution. They observed that poly(glutamic acid) (PGA) produced a vaterite crystal, a crystalline form of CaCO3, but that there was no crystal growth when PAA or poly(allylamine) (PAAm) were employed. It is interesting to note that both PGA and PAA contain a carboxylic acid group in their structure; therefore, the different effects observed in the crystallization of CaCO3 must also be influenced by the polymer backbone structure. They [45] also studied the effect that poly(ethylene-co-acrylic acid) (PEAA) and chitosan as solid substrates played on the CaCO3 crystallization in the presence of soluble additives. Rhombohedral calcite crystals were formed on both substrates without using any additives. The CaCO3 crystallization was inhibited on PEAA using PAA and PAAm, while thin CaCO3 films were obtained on a chitosan matrix using PAA and PGA, It is interesting to note that in the presence of chitosan, PAA and PGA behave in the same way while without chitosan they had opposite behaviours. In another study, the same group [46] applied the same methodology to produce thin CaCO3 films on chitin fibres using PAA, poly (L-aspartate) (PAsp) and poly(L-glutamate) (PGlu) as soluble additives. They were also able to produce aragonite–chitosan multilayer composites by combining spincoating and crystallization in the presence of MgCl2 and PAsp [47]. By repeating the deposition of CaCO3 by thin-film crystallization and the deposition of chitin or chitosan layers by spin-coating a few times, they demonstrate [48] that it is possible to produce organic/inorganic multilayer composites by LBL, as shown in figure 10, even if the structure of nacre was not reproduced. They [49] also demonstrated for the first time that aragonite thin films could be deposited from a calcium carbonate solution using only synthetic macromolecules, e.g. poly(vinyl alcohol) as a substrate and PAA as an additive. In another study, Kato et al [50] successfully prepared, for the first time, calcium carbonate films with periodic and regular surface-relief structures in the sub-micrometre length on a

Figure 10. A cross section of a multilayer film of CaCO3 and chitosan deposited in the presence of PAA [48].

thin matrix of cholesterol modified pullulan from an aqueous solution containing PAA. Tarasevich et al [51] deposited thin, dense and nanocrystalline films of iron hydroxide onto surfaces modified with organic molecules (sulfonated polystyrene or sulfonated self-assembling monolayers (SAM)). This process was based on heterogeneous nucleation and growth from aqueous solutions and involved the study of the fundamental variables influencing these two processes. The deposition amounts varied with the supersaturation of the solution that depended on the pH value and the temperature. Nucleation started by binding cationic species to the sulfonate sites; the growth was mainly due to hydrolysis and condensation reactions. They also showed that by creating a pattern with non-sulfonated and sulfonated alternating regions, it was possible to deposit the iron hydroxide only on the regions containing the sulfonate sites. This clearly demonstrated that the sulfonate sites have a considerable effect in promoting the deposition. Xu et al [52] produced macroscopic calcium carbonate films with a uniform thickness, using the self-organization of porphyrin as a template for the mineral deposition of CaCO3 at the air/Ca(HCO3)2 solution interface. This process produced mixed amorphous-crystalline CaCO3 films with inclusions of organic materials and thicknesses between 400 and 600 nm, depending on the conditions employed. The results achieved by this group present a new approach for the production of inorganic films and demonstrate the mutual interactions between the template, the inhibitor and the biomineral. They also demonstrate that the same method could be applied to synthesise highly crystalline carbonate apatite thin freestanding films [53]. All these studies are very interesting as they provide a deeper understanding of the biomineralization process, but the materials produced do not mimic the structure and 12

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features typical of natural nacre, while bulk materials produced by biomineralization do, as reported in section 2.1.3. The mechanical properties of these materials have not been tested, probably because the authors were more interested in understanding the process instead of producing materials with practical applications. Much more research is needed to make biomineralization a practical method for the production of nacre-like free-standing film. 2.2.3. Centrifugation. Chen et al [54] produced MMT/polyimide (PI) composites by centrifugating a suspension containing MMT platelets with adsorbed imide monomers at 3000 rpm. During the deposition, the MMT platelets oriented themselves parallel to a glass substrate to minimize their potential energy, and coatings with thickness over 100 μm were obtained in minutes. The coatings were then heat-treated to facilitate polymerization of the imide monomers, obtaining strong interactions between the MMT and the PI; the polymeric layers were 20 wt% of the whole composite and had thicknesses between 1.8 and 2 nm. The structure obtained is similar to that produced when applying layer-by-layer self-assembly (described in section 2.2.1); this methodology is much better as it can produce thicker composites in a shorter time. The free-standing composite presented an ultimate tensile stress of 80 MPa and a Young’s modulus of 7–9 GPa are lower than those of natural nacre which probably due to the high organic content in the composite. The hardness of the composites was around 1.5 GPa, almost ten times higher than that of pure PI and seven times higher than that of pure MMT.

Figure 11. A cross section of a free-standing coating produced by evaporation of an aqueous suspension containing MMT and HMPP or (SPA) [55].

2.2.5. Paper-making method. Walther et al [56] produced nacre-like sub-millimetre films using a paper-making method (see figure 12). MMT nano clay platelets in water were individually coated with a PDADMAC polymer and then forced to self-assemble by filtration, heating and pressing (paper-making process). Different counter-ions were employed to control the interactions, the molecular structure and the attractions between the MMT and the polymer in the composite. The use of different counter-ions resulted in different mechanical properties. The highest Young’s modulus and ultimate tensile strength were 32.9 GPa and 151 MPa, respectively; 200 times and 10 times higher than that of the pure polymer. The Young’s modulus of the composite was 11 times higher than that of pure MMT. This methodology is a fast and simple way to produce nacre-like films with a thickness of less than 1 mm with outstanding mechanical properties; it is also ready for scale-up and to be applied in real applications. The ratio between the MMT and PDADMAC in the composite has not been mentioned by the authors, but from the cross-section in figure 12 it seems that the MMT concentration is very high; even if the microarchitecture typical of nacre has not been reproduced (e.g. nanoasperities, connection bridges between the tablets) the composite structure, with the platelets being individually coated, is quite similar to that of natural nacre.

2.2.4. Evaporation. Bennadji-Gridi et al [55] prepared nacre-like films by progressively evaporating an aqueous suspension of MMT and sodium hexametaphosphate (HMPP) or sodium polyacrylate (SPA) on a substrate. Using this method, they obtained a well-textured sample with the platelets strongly overlapping each other and all oriented with their faces parallel to the substrate and an amount of polymer between 2 and 10 wt%, as shown in figure 11. The ratio between the two components and the structure obtained are quite similar to the ‘brick-and-mortar’ structure of nacre, even if in a much smaller scale and without reproducing the features typical of nacre. The mechanical properties of the free-standing films were measured with a three point flexion apparatus; the thinner samples had a bending strength of 112 MPa, similar to that reported by Tang et al [40] (109 ± 8 MPa) using the LBL method, while the thickest samples (>140 μm) gave lower values (45 MPa) because of the numerous cracks perpendicular to the film surface which were generated during the water evaporation. The values obtained are lower than those reported for natural nacre (190 MPa) probably due to the lack of mineral bonds between the two phases. The bending strength of the composite decreased by increasing the evaporation temperature, maintaining the polymer concentration in the initial suspension and an enhanced increasing of the polymer content in the suspension, because at high concentrations the polymer acts as a binder.

2.2.6. Other methods. Yao et al [57] fabricated chitosan– MMT nanocomposite films, taking advantage of the strong electrostatic and hydrogen-bonding interactions between the MMT and the chitosan. Chitosan and MMT mixed in solution for 12 h produces MMT–chitosan building blocks that are dispersed in distilled water and aligned by vacuumfiltration or water-evaporation driven by the chitosan–chitosan interactions. This structure contains between 24 and 35 wt% of chitosan and the MMT tablets are well-aligned, producing a ‘bricks-and-mortar’ structure similar to that of nacre. The Young’s modulus and the ultimate tensile strength of these 13

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Figure 12. SEM cross-section of the MMT–polymer composite produced by the paper-making method showing the alignment of the platelets [56].

ordered multilayered structure with controllable thickness. The nanoplatelets in the composite were aligned parallel to the filter. The composite was then dried in ambient conditions and removed from the filter membrane to produce selfstanding films. This methodology is easily scalable and the nanocomposites showed tensile strengths (between 70 and 122 MPa depending on the polymer content) and toughnesses higher than those of nacre. The same group [60] also reported the formation of nacre-like free-standing films by electrophoretic deposition (EPD) (technique described in section 3.1). A film was obtained by applying an electric field to a suspension of hexagonal-shape gibbsite Al(OH)3 nanoplatelets with thicknesses of around 10 nm and diameters of around 180 nm; the platelets were deposited with their larger surface parallel to the electrode. The film thickness was uniform and could be varied by changing the bath concentration. The interstitials between the ceramic platelets were then infiltrated by spin coating with ethoxylated trimethylolpropane triacrylate (ETPTA) monomers followed by UV polymerization, obtaining a homogeneous and transparent nanocomposite with a ‘brickand-mortar’ structure. The gibbsite–ETPTA composites with 50 vol.% of ceramic present a two times higher strength and a three times higher modulus compared with the pure ETPTA; this increase in the mechanical properties was mainly due to the covalent linkage between the ceramic platelets and the polymeric matrix. Considering the versatility of EPD and its ability to quickly deposit nanocomposites on large-areas, this is a promising methodology to produce free-standing nanocomposites with a nacre-like structure; it presents a widerange of mechanical properties.

composites are 3–5 times (6.8–10.7 GPa instead of 1.6 GPa) and 2–3 times higher (80–110 MPa instead of 38 MPa) than the properties of the films made by simply mixing MMT and chitosan. Liu et al [58] developed several methodologies to produce MMT clay and polyethylene oxide (PEO) composites that mimic nacre. The first technique consisted of sedimenting MMT films on thin glass plates for 1–2 months at room temperature or for 5 days at 60 ◦ C, obtaining 50 μm thick wellordered clay films. In the second method the MMT suspension was centrifuged at 50 rps (revolution per second) for 1 h, obtaining compact sediments but with a reduced alignment compared to the sedimented samples. The third methodology employed a controlled rate slip casting on a disk of plaster; this technique produced films in 2 days that were considerably aligned with a thickness of 250 μm. In the fourth method, a MMT suspension was filtered on a nitrocellulose membrane for 2 days, obtaining well-aligned films with a thickness of several micrometres; the drawback of this methodology was the fact that the membrane blocked quickly and therefore it was difficult to achieve high thicknesses. The last method consisted of applying electrophoretic deposition to a 5 wt% aqueous MMT suspension; a clay film was produced on the anode but was of a low quality due to the oxygen gas evolution released during the deposition, and this was due to the simultaneous water electrolysis. Among all these techniques, the slip casting was the most effective for the production of the MMT structures. Once the clay structure was produced by slip casting, the polar PEO, that is easily accepted by the MMT platelets, was introduced into the film by placing a PEO solution on the top of the film, producing a non-uniform distribution of the polymer in the composite. A different methodology was then applied to produce clay–PEO composites: a clay–PEO suspension was prepared before slip casting, obtaining a layered structure in which 10–50 platelets formed each clay layer, but there were still some gaps not filled by the polymer. Further studies are therefore needed to improve these techniques in order to fill the inter-stack spaces and to individually coat the MMT platelets, without considerably increasing the polymer content in the composite. Wei-Han et al [59] produced MMT nanoplatelets-polyvinyl alcohol nanocomposites with a ‘brick-and-mortar’ structure by a simple vacuum filtration. The MMT platelets were mixed with poly-vinyl alcohol in water, producing a colloidal suspension that was vacuum filtered to produce an

3. Methods to produce thin films with a nacre-like structure 3.1. Electrophoretic deposition Electrophoretic deposition (EDP) is a low-cost electrochemical method which is employed to produce coatings and freestanding objects. The versatility of this technique allows the application to numerous materials and the deposition of composite materials, layered materials, functionally graded materials and nanomaterials. EPD occurs in two steps. In the first step, an electric field is applied between two electrodes and the charged particles in suspension move towards the opposite charged electrode (electrophoresis). In the second step, 14

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the particles accumulate at the electrode and create a relatively compact and homogeneous film (deposition). Therefore, in order to apply this technique, it is fundamental to produce a stable suspension containing freely moving charged particles [61, 62]. This technique has been widely applied to manufacture nacre-like thin films; the results obtained are reported below. Lin et al [63] produced a gibbsite nanoplatelets and polyethylenimine (PEI) nacre-like structure, applying a difference of potential to a suspension in a parallel plate cell. In these conditions, both particles were attracted to the cathode forming a PEI–gibbsite nano-composite, with the nanoplatelets parallel to the electrode’s surface due to the electrical forces involved in the process, and just a 3 wt% of polymer in the composite. Nanoindentation was performed on these coatings: showing a reduced modulus of between 2.20 and 5.17 GPa, depending on the contact depth (the contact depth was less than 10% of the coating thickness). By increasing the contact depth, the reduced modulus of PEI–gibbsite composite was 0.4 GPa lower than that of pure gibbsite films as a consequence of the presence of the soft PEI between the hard gibbsite layers. These results were counter to what was expected and are probably due to the lack of interactions between PEI and gibbsite, but the authors did not comment on this. In another study, the same group [64] mimicked the surface roughness typical of nacre applying by sol–gel a thin layer of silica on the gibbsite nanoplatelets. PEI was used to reduce the severe cracking of the applied coating and to change the surface charge of the coated nanoplatelets. A photocurable polymer was then infiltrated in the space between the nanoplatelet multilayers to produce the composite after photopolymerization. The coatings could be made selfstanding and were highly transparent. Their tensile strength was almost threefold (30 MPa) and their toughness was oneorder-of-magnitude higher than those of the pure polymer. MMT platelets and an acrylic anodic electrophoretic resin (AAER) or epoxy resin were used to produce nanolaminated composite films; both polymers were added onto the MMT platelet surface, obtaining MMT–AAER and MMT–epoxy composite platelets. EPD of the composite particles produced films with thicknesses from nm to 20 μm with the MMT platelets overlapping each other and bonded together due to the polymer layers between them [65]. In this structure the polymer layer had an average thickness of 2 nm and the inorganic layers were a few nm thick; both much thinner compared to the nacre structure (10–50 nm for the polymer and 200–900 nm for the aragonite). In order to reproduce the micro–nanocomposite structure typical of natural nacre, more studies are needed changing the scale of the two starting materials and also using a more rigid inorganic phase with some asperities like those of the aragonite tablets. The reduced modulus of the film increased from 2.9 ± 0.4 GPa to 5.0 ± 1.0 GPa by adding 8 wt% of AAER; this increase was mainly attributed to the ‘brick-and-mortar’ structure of the composite. The reduced Young’s modulus of the composite cannot directly be compared with that of natural nacre (48.5 GPa) because of the different testing methods employed. Further studies are needed to determine which polymer would be the best to improve the composite mechanical properties [65].

Figure 13. A cross section of the MMT/polyacrylamide multilayer structure deposited by EPD [66].

A polyacrylamide/MMT multi-layer composite was prepared by EPD, starting from an aqueous suspension of acrylamide–MMT; the monomers were then polymerized under UV light, obtaining the structure shown in figure 13. The composite contained 5 wt% of polymer and 95 wt% of ceramic as in natural nacre [66]. The Young’s modulus of the composite was about 16.92 GPa, much higher than the Young’s modulus of pure MMT coatings (2.92 GPa) and of the hybrid clay–polyelectrolyte composites produced by Tang et al (11 ± 2 GPa) [40]. The hardness was 0.95 GPa, higher than the value obtained for pure MMT (0.20 GPa) films prepared in the same conditions. The composite films were more flexible and rigid compared to the pure MMT films, thanks to the ‘bricks-and-mortar’ structure that can be stretched during deformation adsorbing energy, and the interlocking of the inorganic platelets, as observed in natural nacre [66]. 3.2. Biomineralization or self-assembly The biomineralization process has already been explained in section 2.1.3 for the fabrication of nacre-like bulk materials; its application to produce free-standing films is discussed in section 2.2.2. Hence this section will only cover the research reported in the literature for the deposition of films. Aksay et al [67] applied supramolecular self-assembly to produce, in two steps, a silica-based continuous thin film of meso-structured organic/inorganic nano composites, with the main aim of understanding the templating interactions taking place at the organic/inorganic interface, that are the basis for biomineralization. In the first step, micellar structures of the surfactant are self-assembled from a dilute aqueous acidic solution at the water/substrate interface. The first layer of surfactant molecules is adsorbed onto the substrate through electrostatic and/or van der Waals forces and then the film is left to grow for 24 h. This obtains tubular-like structures parallel to the substrate surface, with the porous 15

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Topical Review

nanostructure depending on the interactions between the surfactant and the surface. In the second step, a mesoscopic silica film condensates within the surfactant layers, producing an inorganic–organic nano composite. On the freshly formed silica interface, more layers of surfactant were adsorbed and the film continued to grow away from the substrate’s surface. It is important to underline that the growing tubules present strains due to the epitaxial mismatch between the first layer of surfactant molecules and the periodic structure of the substrate and, as the film grows thicker (
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