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Researers Develop New Superstrong, Flexible Polymers
September 25, 2017

Plastic materials, for better or worse, have become so pervasive in modern society that it seems virtually anything could be manufactured from them. And according to information from Columbia University’s School of Engineering and Applied Science (Columbia Engineering), that may well be a closer reality than previously imagined.

Researchers at Columbia Engineering have demonstrated for the first time a new technique to create a composite material that has extraordinary mechanical properties, including great strength and resilience. Drawing inspiration from the mother of pearl of oyster shells, the research team discovered that by changing the crystallization speed of a polymer initially well-mixed with nanoparticles, they could control how the nanoparticles self-assemble into structures at three very different length scale regimes. This multiscale ordering can make the base material almost an order of magnitude stiffer while still retaining the desired deformability and lightweight behavior of the polymeric materials. Funded by the National Science Foundation (NSF), the study, “Tunable Multiscale Nanoparticle Ordering by Polymer Crystallization,” led by Sanat Kumar, Bykhovsky Professor of Chemical Engineering, was published recently online in “ACS Central Science”.

Figure illustrates that polymer crystallization speed can be used to control the spatial distribution of nanoparticles. Impurities (here, the nanoparticles) will become engulfed by the crystal if it grows too rapidly. However, when the rate slows, the crystal will expel the defects. Image courtesy of Sanat Kumar/Columbia Engineering.

“Essentially, we have created a one-step method to build a composite material that is significantly stronger than its host material,” says Prof. Kumar, an expert in polymer dynamics and self-assembly. “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.”

About 75 percent of commercially used polymers, including polyethylene used for packaging and polypropylene for bottles, are semi-crystalline. Their molecular structure prohibits them from being used for many advanced applications such as automobile or aircraft parts because of their low mechanical strength. However, researchers have known for decades that varying nanoparticle dispersion in polymer, metal, and ceramic matrices can dramatically improve material properties.

“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,” explained Dan Zhao, Prof. Kumar’s Ph.D. student and first author on this paper. “We addressed this challenge through the controlled, multiscale assembly of nanoparticles by leveraging the kinetics of polymer crystallization.”

Prof. Kumar’s group, experts in tuning the structure and therefore the properties of polymer nanocomposites, found that, by mixing nanoparticles in a solution of polymers (polyethylene oxide) and changing the crystallization speed by varying the degree of sub-cooling (namely how far below the melting point the crystallization was conducted), they could control how the nanoparticles self-assembled into three different scale regimes: nano, micro, and macro-meter. Each nanoparticle was evenly swathed by the polymers and evenly spaced before the crystallization process began. The nanoparticles then assembled into sheets (10-100 nm) and the sheets into aggregates on the microscale when the polymer was crystallized.

“This controlled self-assembly is important because it improves the stiffness of the materials while keeping them tough,” according to Prof. Kumar. “And the materials retain the low density of the pure semi-crystalline 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.”

Going forward, Prof. Kumar’s team plans to examine the fundamentals that enables particles to move toward certain regions of the system, and to develop methods to speed up the kinetics of particle ordering. With a better understanding in hand, they plan to explore other application-driven polymer/particle systems, such as polylactide/nanoparticle systems that can be engineered as next-generation biodegradable and sustainable polymer nanocomposites, and polyethylene/silica, which is used in car bumpers, buildings, and bridges.

“The potential of replacing structural materials with these new composites could have a profound effect on sustainable materials as well as our nation’s’ infrastructure,” Prof. Kumar noted.

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