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Posted: Dec 14, 2015

Replicating nacre through nanomimetics

(Nanowerk Spotlight) Materials that have to perform under extreme conditions – jet engines, power turbines, catalytic heat exchangers, military armors, aircraft, spacecraft – require exceptional mechanical properties.
For many years now, the remarkable properties of some natural materials have motivated researchers to synthesize biomimetic nanocomposites that attempt to reproduce Nature’s achievements and to understand the toughening and deformation mechanisms of natural nanocomposite materials. One of the best examples is nacre, the pearly internal layer of many mollusc shells. It has evolved through millions of years to a level of optimization currently achieved in very few engineered composites.
Researchers have approached the preparation of artificial analogs of nacre by using various methods and the resulting materials have captured some of the characteristics of the natural composite – but so far never have fully replicated it.
Nacre has a layered structure composed of approximately 95% calcium carbonate (CaCO3) nanoscale building blocks interfaced by about 5% organics. The typical characteristic of nacre is the 'brick and mortar' arrangement of these materials, which is thought to be crucial to mechanical and other outstanding properties that nacre possesses.
Over the past decades, researchers in this field have devoted significant effort to investigate and mimic such brick and mortar structures hoping that the understanding on formation of nacreous structure and biological mineralization would lead to new advances in materials technology and related applications.
New work coming out of the Imperial College London represents the first successful attempt to mimic the structure of nacre while maintaining the same characteristic geometry, aspect ratio and phase proportions.
The team, led by professors Alexander Bismarck and Milo Shaffer, has published their findings in ACS Applied Materials & Interfaces ("Nacre-nanomimetics: Strong, Stiff, and Plastic").
"Our main insight was to adjust the absolute length scale of the constituent platelets to an intermediate range," Shaffer tells Nanowerk. "Previously, large platelets – the same dimensions as the natural system – have been used, but are difficult to assemble correctly since the packing is poor; too little inorganic reinforcement is included. On the other hand, very thin nanosheets are often used, but they are too similar to the dimensions of the polymer molecules, so that too much polymer is necessarily included. Our intermediate size retains the same shape and aspect ratio as the natural platelets, but scaled to about 20 times smaller size."
He points out that the smaller size makes it possible to form a well-packed, aligned structure with a large inorganic fraction, using self-assembly techniques. In addition to replicating many of the excellent properties of nacre, the smaller size appears to increase plasticity.
Morphology and deformation mechanisms of a nacre-nanomimetic coating deposition on a glass slide
Morphology and deformation mechanisms of a nacre-nanomimetic coating deposition on a glass slide. In situ SEM nanoindentation shows the ability of the mimic to deflect a crack as well as strain harden. (Image: F. De Luca) (click on image to enlarge)
"Most of the research in the field of artificial nacre has been based on the use of nanosheets reinforcement with graphene oxide, LDH and nanoclay, limiting the proportion of the inorganic phase to only 50-70 wt.% – which are the wrong phase proportions," explains Francois De Luca, a PhD student in Shaffer's NanoHAC group and the paper's first author. "Our work shows that good mimics of nacre can be achieved by maintaining the same geometry and proportions, even at a different scale."
The team used 10-20 nm thick layered double hydroxide (LDH) platelets with an aspect ratio similar to the aragonite platelets in nacre and 'glued' them together with a simple organic 'mortar' (PSS). This soft polyelectrolyte is around ten times thinner than the natural biopolymer in order to retain the correct dimensional ratios and phase proportions as natural nacre.
They then used the layer-by-layer (LbL) assembly method to deposit well-controlled alternating layers, allowing the alignment of anisotropic nanoplatelets by simple sequential dipping.
The researchers caution that scaling up their process, or at least speeding it up, to manufacture a large quantity of materials in a short period of time still needs to be investigated.
"Currently, the deposition of our nanostructure is quite time consuming in terms of thickness (15 hours for a micrometer), which we believe could easily be improved," says De Luca. "However, the great advantage of the layer-by-layer assembly is that very large surface areas can be coated as the deposition is self-limited – our simple dipping procedure can easily be scaled up."
Beyond the demonstration of achieving nanomaterials with the combination of high strength and stiffness with large plastic deformation, such biomimetic nanocomposites could be used as protective and/or energy absorbing coatings, potentially also as barrier coating against gas.
The next step for the team will be to design this nature-inspired nanostructure inspired over multiple levels of hierarchy – just like natural nacre.
"Indeed, the platelets contained in nacre are actually nanocomposites – nano-grains of aragonite glued together in a soft biopolymer," De Luca points out. "Therefore, porous nano-reinforcements could be used and filled with a polymer afterwards to achieve another level of hierarchy below the platelet level."
However, the nanoscale of the platelet might already be limiting this research direction. Also, the use of higher aspect ratio platelets could be investigated in order to further improve the mechanical properties of the nanocomposites.
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