Technology Advances
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TECHNOLOGY ADVANCES
Self-Assembled Nanocomposite Combines Electrical Conductivity and Mechanical Flexibility A novel nanocomposite material that combines the electrical conductivity of metals with the low elastic modulus of elastomers, called Metal Rubber™, has been developed by NanoSonic Inc. The material is a multilayered nanocomposite made by electrostatic self-assembly (ESA) and consists of polymers and metal nanoclusters in which multiple nanocluster layers are deposited on a charged substrate. It may be formed as a conformal coating on a substrate surface or as freestanding, mechanically robust sheet material. Similar to other composite materials, its macroscopic properties are determined by the individual material constituents, their relative mixture percentages, and the way they are combined during production. In the basic ESA process for the selfassembly of polymer molecules a substrate surface is typically cleaned and prepared so the outermost surface layer has a net negative charge. The resulting negatively charged substrate is then dipped into an aqueous solution containing water-soluble cation polymer molecules that have positively charged functional groups fixed to the polymer backbone. Because the polymer chain is flexible, it is free to orient with respect to the underlying substrate so that a relatively low-energy configuration can be achieved. As a result, some of the positively charged functional groups along the polymer chain experience attractive ionic forces toward the negative substrate, and the polymer chain is bent in response to those forces. The net negative charge on the substrate is masked from other positive groups along the polymer chain. Those groups feel a net repulsive force due to the fixed positive functional groups at the substrate surface, so they move away from that surface to form a net positive charge distribution on the surface of the substrate. Since the total polymer layer is neutral, negative charges with relatively loose binding to the polymer network pair up with positive ions. Subsequent polyanion and polycation monolayers are then added to produce the multilayer structure. The properties of the multilayer thin films fabricated by this method are determined by both the properties of the molecules in each monolayer and the physical ordering of the multiple monolayers through the composite multilayer structure. This method can be extended to form thicker materials simply by continuing
MRS BULLETIN • VOLUME 30 • JULY 2005
the described process. Materials as thick as 1 cm have been formed, consisting of as many as 1000 layers. The materials can be removed from the substrate on which they are formed by depositing a chemical release layer prior to the desired molecular layers, then chemically removing this layer to free the material once deposition is completed. For example, gold nanoclusters and polymers have been used to form millimeter-thick coatings on substrates treated with such release layers. An example of such a free-standing, mechanically robust, and electrical
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