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October 04, 2007

Superstrength polymers using layer by layer assembly

Superstrong transparent plastic has been made using superior layering technique.

Before this process load transfer has been quite ineffective at loadings more than 5-10%.
After that the mechanical properties of regular composites actually start degrading with increasing portion of inorganic filler. Kotov's findings indicate that organization of the composite and tuning of molecular interactions between clay and polymer can give almost ideal stress transfer at loadings as high as 50-60%. This can be achieved with a technique called layer-by-layer (LBL) assembly, which makes clay sheets to become oriented parallel to the substrate. Kotov explains that the LBL process is based on sequential adsorption of nanometer-thick monolayers of oppositely charged compounds (e.g. polyelectrolytes, charged nanoparticles, biological macromolecules, etc.) to form a multilayered structure with nanometer-level control over the architecture. Kotov points out that all polymer chains poorly bound to clay sheet are removed during their LBL assembly process.

"We solved the problem of load transfer quite well for clay sheets" says Kotov. "This is very encouraging. Potentially it can be solved for carbon nanotubes and other nanostructures as well." Specific applications for these clay composites are, of course, in military vehicles but also in aerospace and the automotive industry. Basically, these materials could be considered for components in unmanned aerial vehicles and electronic devices that are exposed to extreme performance conditions. "The future directions of this research will be increasing not only the strength and stiffness but also the strain of the composites, which will yield the toughness" says Kotov. "Having high strains would truly result in 'plastic steel.' We are also working on the better understanding the nanomechanics of these materials.

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