Conductive polymers, with their wide-ranging physical and electrical properties, are used in applications from organic transistors, coatings for fuel cells, smart textiles and electromagnetic shielding. However, the process for making conductive polymer composites such as electrically-conductive cotton, wool or nylon is difficult since the mechanical properties, or its strength, are weakened during preparation. A new method of crafting the fibres could open up normally flimsy materials, such as cotton, to conduct electricity in technologies normally reserved for stronger fibers. The process could also make conductive polymer composites less expensive to prepare with fewer harmful environmental side-effects. By using liquid salts during formation instead of harsh chemicals, fibers that conduct electricity can be strengthened, according to a patent issued to a team of researchers at The University of Alabama. The UA researchers worked with polypyrrole, a particularly useful conductive polymer that can be difficult to bind to fibers. To turn the base chemical, pyrrole, into a polymer that can conduct electricity, polypyrrole, it is put through a chemical process using methanol and the iron-containing ferric chloride. Methanol, sometimes called wood alcohol, is a volatile organic compound that is highly toxic to humans.
Polypyrrole made through this method, though, does not stick well to fibers such as cotton. To bind the fibers with the polypyrrole they are dipped in an acidic solvent that degrades the fiber to increase the surface area so the polypyrrole can stick. This degradation, though, weakens the fiber. The patented method developed at UA would retain much of the fibers strength by using ionic liquids, which are liquid salts at or near room temperature with low volatility that carry an electric charge. The ionic liquids do not degrade the fibers as much, and the process creates nanostructures that result in a stronger composite material. It also makes the composite better at conducting electricity.
The inventors of the patent are Dr. Scott Spear, a research engineer with UA's Alabama Innovation and Mentoring of Entrepreneurs, known as AIME; Dr. Anwarul Haque, associate professor of aerospace engineering and mechanics; Dr. Robin Rogers, the Robert Ramsay Chair of Chemistry at UA and director of UA's Center for Green Manufacturing; Dr. Rachel Frazier, a research engineer at AIME, and Dr. Dan Daly, director of AIME.
The ionic liquids cannot only be used to fabricate conductive polymer composites, but can be used in place of other solvents to create the polypyrrole. The process is potentially cheaper and environmentally cleaner since using ionic liquids results in much less harmful by-products from the chemical reaction. The process does away with the use of methanol through the novel use of iconic liquids, which, by their very nature, have a low volatility that essentially eliminates environmental release pathways exhibited by methanol.
The patented process could impact what are known as smart textiles, clothing often with traditional electronic features woven into the fabric. The UA-developed method, though, could make it easier for the clothing itself to transmit the electric signals. Smart textiles could be employed in protective clothing, medical textiles and other applications foreseen in military, sports, medical, industrial as well as consumer products. Conductive cotton fiber represents an important component in the development of smart materials for a variety of military, industrial and commercial applications.
Polymer materials are usually thermal insulators. Researchers have developed a thermal interface material able to conduct heat 20 times better than the original polymer by harnessing an electropolymerization process to produce aligned arrays of polymer nanofibers. The modified material can reliably operate at temperatures of up to 200 degrees Celsius. The new thermal interface material could be used to draw heat away from electronic devices in servers, automobiles, high-brightness LEDs and certain mobile devices. The material is fabricated on heat sinks and heat spreaders and adheres well to devices, potentially avoiding the reliability challenges caused by differential expansion in other thermally-conducting materials. Amorphous polymer materials are poor thermal conductors because their disordered state limits the transfer of heat-conducting phonons. That transfer can be improved by creating aligned crystalline structures in the polymers, but those structures – formed through a fiber drawing processes – can leave the material brittle and easily fractured as devices expand and contract during heating and cooling cycles.
The new interface material is produced from a conjugated polymer, polythiophene, in which aligned polymer chains in nanofibers facilitate the transfer of phonons – but without the brittleness associated with crystalline structures, Cola explained. Formation of the nanofibers produces an amorphous material with thermal conductivity of up to 4.4 watts per meter Kelvin at room temperature. Baratunde Cola, an assistant professor in the George W. Woodruff School of Mechanical Engineering at the Georgia Institute of Technology says, "A material like this, which could also offer higher reliability, could be attractive for addressing thermal management issues. This material could ultimately allow us to design electronic systems in different ways". The material has been tested up to 200 degrees Celsius, a temperature that could make it useful for applications in vehicles. Solder materials have been used for thermal interfaces between chips and heat sinks, but may not be reliable when operated close to their reflow temperatures.
The research, which was supported by the National Science Foundation, involved researchers from the Georgia Institute of Technology, University of Texas at Austin, and the Raytheon Company. Virendra Singh, a research scientist in the Woodruff School, and Thomas Bougher, a Ph.D. student in the Woodruff School, are the paper's co-first authors.
"Polymers aren't typically thought of for these applications because they normally degrade at such a low temperature," Cola explained, "but these conjugated polymers are already used in solar cells and electronic devices, and can also work as thermal materials. We are taking advantage of the fact that they have a higher thermal stability because the bonding is stronger than in typical polymers."
The structures are grown in a multi-step process that begins with an alumina template containing tiny pores covered by an electrolyte containing monomer precursors. When an electrical potential is applied to the template, electrodes at the base of each pore attract the monomers and begin forming hollow nanofibers. The amount of current applied and the growth time control the length of the fibers and the thickness of their walls, while the pore size controls the diameter. Fiber diameters range from 18 to 300 nanometers, depending on the pore template. After formation of the monomer chains, the nanofibers are cross-linked with an electropolymerization process, and the template removed. The resulting structure can be attached to electronic devices through the application of a liquid such as water or a solvent, which spreads the fibers and creates adhesion through capillary action and van der Waals forces. Though the technique still requires further development and is not fully understood theoretically, Cola believes it could be scaled up for manufacturing and commercialization. The new material could allow reliable thermal interfaces as thin as three microns – compared to as much as 50 to 75 microns with conventional materials.
A patent application has been filed on the material. Cola has formed a startup company, Carbice Nanotechnologies, to commercialize thermal interface technologies. It is a member of Georgia Tech's VentureLab program.