Nanocomposite Research Yields Strong and Stretchy Fibres

28 January 2007

MIT researchers have developed a nanoclay reinforced polyurethane elastomer that is both stretchy and strong.

These polymer nanocomposite materials could be used to strengthen and toughen packaging materials and develop tear-resistant fabrics or biomedical devices. Professor Gareth McKinley, graduate student Shawna Liff and postdoctoral researcher Nitin Kumar worked at MIT's Institute for Soldier Nanotechnologies (ISN) to develop a new method for effectively preparing these materials. The research appears in the January issue of Nature Materials.

The work is based on recent increased understanding of the combined strength and flexibility of spider silk, which lies in the arrangement of the nano-crystalline reinforcement of the silk while it is being produced.

""If you look closely at the structure of spider silk, it is filled with a lot of very small crystals,"" says McKinley, a professor of mechanical engineering. ""It's highly reinforced.""

The silk's strength and flexibility come from this nanoscale crystalline reinforcement and from the way these tiny crystals are oriented towards and strongly adhere to the stretchy protein that forms their surrounding polymeric matrix.

Liff, a Ph.D. student in mechanical engineering, and Kumar, a former MIT postdoctoral associate, teamed up to figure out how to begin to emulate this nano-reinforced structure in a synthetic polymer (A polymer or plastic consists of long chains composed of small repeating molecular units). Numerous earlier unsuccessful attempts, tackling the same issue, relied on heating and mixing molten plastics with reinforcing agents, but Liff and Kumar took a different approach: They focused on reinforcing solutions of a commercial polyurethane elastomer with nanosized clay platelets.

The researchers developed a process to embed the clay platelets in the rubbery polymer--first dissolving them in water, then slowly exchanging water for a solvent that also dissolves polyurethane. They then dissolved the polymer in the new mixture, and finally removed the solvent. The end result is a nanocomposite of stiff clay particles dispersed throughout a stretchy matrix that is now stronger and tougher.

Importantly, the clay platelets are distributed randomly in the material, forming a very disorderly ""jammed"" structure, according to McKinley. Consequently the nanocomposite material is reinforced in every direction and the material exhibits very little distortion even when heated to temperatures above 150 degrees Celsius.

In a Nature Materials commentary that accompanied the research paper, Evangelos Manias, professor of materials science and engineering at the University of Pennsylvania, suggests that ""molecular composites"" such as those developed by the MIT group are especially suitable for new lightweight membranes and gas barriers, because the hard clay structure provides extra mechanical support and prevents degradation of the material even at high temperatures. One possible use for such barriers is in fuel cells.

The research was funded by the US Army through MIT's Institute for Soldier Nanotechnologies and by the National Science Foundation. McKinley's team was assisted by technical staff at the ISN, including research engineer Steven Kooi, who helped prepare special samples for transmission electron microscopy.

This micrograph shows that, upon stretching, the dark, disorganized polymer chains become aligned and brighten in colour due to polymer alignment, while the bright ordered domains become disordered or darken.

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