Polymers and Composites

Flexible Polymers with a Design Inspired by Oysters

07 June 2017

Illustration of polymer crystallization speed being used to control the spatial distribution of nanoparticles. (Credit: Sanat Kumar/Columbia Engineering)Illustration of polymer crystallization speed being used to control the spatial distribution of nanoparticles. (Credit: Sanat Kumar/Columbia Engineering)Oysters and other mollusks produce an organic-inorganic composite material called “nacre” as an inner shell layer — you might also know it by the name “mother of pearl.” The material has extraordinary mechanical properties, including great strength and resilience.

Why are we talking about oysters? Well, researchers at Columbia Engineering have demonstrated a new technique that takes its inspiration from them. The hierarchical nanoparticle ordering of nacre — a mixture of intercalated brittle platelets and thin layers of elastic biopolymers — strongly improves its mechanical properties. Parallel aragonite layers are held together by a nanoscale crystalline biopolymer layer, forming “bricks” that assemble into “brick-and-mortar” superstructures at the micrometer scale and larger. The researchers found that by changing the crystallization speed of a polymer mixed with nanoparticles, they could control how the nanoparticles assemble themselves into structures — increasing the base material’s stiffness, while still retaining its desired deformability and lightweight behavior.

Polyethylene used for packaging and polypropylene for bottles are semicrystalline — as are around 75 percent of commercially-used polymers overall. These materials have low mechanical strength, making them unsuitable for advanced applications such as automobile fittings (tires, fan belts, bumpers, etc.). The research has the potential to dramatically address that limitation.

Before now, no one has found a way to tune nanoparticle assembly in a crystalline polymer matrix across a hierarchy of multiple scales — something thought of as a “holy grail” in nanoscience. By varying the degree of sub-cooling (namely how far below the melting point the crystallization was conducted) in order to change crystallization speed, the research team could control how the nanoparticles self-assembled into three different scale regimes: nano-, micro-, and macro-meter. Either the particle or the polymer can be varied to achieve a specific material behavior or device performance.

"Essentially, we have created a one-step method to build a composite material that is significantly stronger than its host material," said Sanat Kumar, a professor of chemical engineering who led the research. “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.”

Kumar’s team plans next to explore other application-driven polymer/particle systems, looking toward next-generation biodegradable and sustainable nanocomposites, and polyethylene/silica, used in car bumpers, buildings and bridges. Looking further ahead, Kumar adds, the technique may be applied to electronic or optical properties, potentially enabling the fabrication of new materials and functional devices to be used in structural applications.

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