Stronger composite materials for use in commercial products inspired by oyster shells are on the way thanks to work at Columbia Engineering. The research could looks to nacre, also known as mother-of-pearl, the tough and iridescent substance that lines the shells of many mollusks as a model for superstrong, flexible polymers.
While the formation of nacre is not yet fully understood it is known to occur quite slowly and now the Columbia team has demonstrated that changing the crystallization speed of a polymer initially well-mixed with nanoparticles can lead to self-assembly with different structural characteristics at three length scales. The team, led by Sanat Kumar, shows that this multiscale ordering can make the base material almost one order of magnitude stiffer than the original material but without reducing flexibility or low density. [S Kumar et al, ACS Central Sci (2017) DOI: 10.1021/acscentsci.7b00157]
"Essentially, we have created a one-step method to build a composite material that is significantly stronger than its host material," Kumar explains. "Our technique may improve the mechanical and potentially other physical properties of commercially relevant plastic materials, with applications in automobiles, protective coatings, and food and beverage packaging, things we use every day." He also suggests that the same techniques might be exploited to make novel materials with useful electronic or optical properties for "smart" nanocomposites.
Of all the commercially available polymers about three quarters are semicrystalline, including the common packaging materials, polyethylene and polypropylene. This means they are low density but also low strength so they cannot be used in automobile fittings, for instance. However, it has been known for more than a century that adding certain types of particle to a material matrix can boost its strength. In nature, the example of nacre is often cited. Nacre is 95 percent inorganic aragonite with 5 percent crystalline polymer, chitin, with a hierarchical nanoparticulate ordering that makes it much stronger than it would otherwise be.
Former team member Dan Zhao explains how the researchers have addressed the challenge of emulating this multiscale assembly to control the kinetics of polymer crystallization and so toughen up their polymer matrices. The group demonstrated proof of principle with a solution of polyethylene oxide to which they added nanoparticles and controlled the rate of crystallization using "sub-cooling". Each nanoparticle is evenly swathed with polymer and evenly separated in the mixture before crystallization begins and they then self-assemble into sheets of 10 to 100 nanometers and the sheets form aggregates on the microscale (1 to 10 micrometers as the polymer crystallizes.
"This controlled self-assembly improves the stiffness of the materials while keeping them tough," Kumar adds. "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."
David Bradley blogs at Sciencebase Science Blog and tweets @sciencebase, he is author of the popular science book "Deceived Wisdom".