Oyster shell-inspired composites show that order leads to strength

Shellfish are a slightly surprising inspiration for many engineering innovations. The ancient invertebrates, one of the oldest forms of life on the planet, have evolved many tricks to get themselves through life which human ingenuity has not been able to match and seeks to copy; adhesives, pigments and water filtration systems have all been inspired by our shell-covered friends.

The most obvious feature of these organisms – their shells – is also an inspiration for engineers. A team at Columbia University in New York City, working with materials scientists from Rensselaer Polytechnic University in New York State, the University of South Carolina and the Leon Brillouin Laboratory in France (part of the French National Laboratory network), has now unveiled a technique that it claims can endow a blend of a polymer with strength and toughness inspired by nacre, the smooth, incredibly tough yet flexible iridescent material that lines oyster and other shells.

The key to the technique is controlling the speed of crystallisation of a polymer which is initially well-mixed with nanocrystals of different sizes, explained Sanat Kumar, a professor of chemical engineering at Columbia and director of the research, in a paper published online in ACS Central Science. “Essentially, we have created a one-step method to build a composite material that is significantly stronger than its host material,” he said.

The toughness and strength of nacre is derived from its structure. The substance is 95 per cent an inorganic material called aragonite and 5 per cent a flexible biopolymer called chitin. The aragonite is distributed in a form similar to bricks in a wall, held together by 10nm-thick layers of crystalline chitin acting like mortar, at a variety of scales from micrometre scale and larger. This structure, and the hierarchy of sizes of inorganic particles in it, is the key to the substance’s toughness, which is orders of magnitude greater than that of chitin alone.

“While achieving the spontaneous assembly of nanoparticles into a hierarchy of scales in a polymer host has been a ‘holy grail’ in nanoscience, until now there has been no established method to achieve this goal,” says Dan Zhao, Kumar’s PhD student and first author on the paper. “We addressed this challenge through the controlled, multiscale assembly of nanoparticles by leveraging the kinetics of polymer crystallisation.”

The key to this control was temperature. Kumar and Zhao’s team mixed nanoparticles of silica with a lamellar (scale-like) shape with polyethylene oxide, a polymer of low intrinsic strength. By varying the degree of sub-cooling — how far below the melting point of the polymer that its crystallisation was conducted — they found that they could also control how the nanoparticles self-assembled into three different scale regimes: nano-, micro- and macro-metre. Each nanocrystal was evenly covered with molten polymer before the crystallisation began. They assembled into sheets of size 10-100nm, and the sheets into aggregates on the 1-10µm scale, during crystallisation.

“This controlled self-assembly is important because it improves the stiffness of the materials while keeping them tough,” says Kumar. “And the materials retain the low density of the pure semicrystalline polymer so that we can keep the weight of a structural component low, a property that is critical to applications such as cars and planes, where weight is a critical consideration. With our versatile approach, we can vary either the particle or the polymer to achieve some specific material behavior or device performance.”

Kumar has high hopes. “Our technique may improve the mechanical and potentially other physical properties of commercially relevant plastic materials, with applications in automobiles, protective coatings, and food/beverage packaging, things we use every day. And, looking further ahead, we may also be able to produce interesting electronic or optical properties of the nanocomposite materials, potentially enabling the fabrication of new materials and functional devices that can be used in structural applications such as buildings, but with the ability to monitor their health in situ.”

In the next phase of the research, Kumar and team plan to study the factor that influence how the particles move to different areas in the blend and hope to speed up the ordering of the particles, which currently takes a few days. They are also hoping to study other particle-polymer systems, including those using biodegradable polylactide, and blends of silica and polyethylene that could be used in infrastructure and automotive components such as bumpers.