A powerful one-step, "chain growth" method should make it easier to design and synthesize a variety of highly conductive polymers for different research and commercial applications, according to a presentation by the method's developer, Carnegie Mellon University chemist Richard McCullough. McCullough, dean of the Mellon College of Science and professor of chemistry, reported his research on March 30, at the 227th annual meeting of the American Chemical Society in Anaheim, Calif.
McCullough has harnessed the chain-growth method to increase the versatility of the conducting polymers, called regioregular polythiophenes. This new method allows scientists to "cap" each conducting polymer with chemical groups that link to other structural polymers. With this research, funded by the National Science Foundation, researchers can form highly conductive nanowire sheets within polymer block or create a plethora of new conducting polymers.
Variations in the chemical "cap" also allow regioregular polythiophene strands to adhere directly to metal, silicon or other industrially important templates used in devices like transistor. They effectively self-assemble into a well-ordered, highly conducting nanoscale layers.
"The chain-growth method eliminates six production steps to create block co-polymer nanowires that conduct electricity a million times better than the all other conducting block copolymers," said McCullough.
Conducting polymers are remarkable materials that possess the electrical properties of metals yet retain the mechanical properties of polymers. In 1992 McCullough was the first to report the synthesis of regioregular polythiophenes, which in 2002 became the basis of a Carnegie Mellon spinout company, Plextronics, Inc.
The current research was conducted, in large part, by postdoctoral research fellows Malika Jeffries-El and Genevieve Sauve.
Block copolymers of regioregular polythiophenes conduct electricity so well due to their uniform composition and neat alignment into nanowires. Impurities and random orientation of polymer strands created by other methods vastly reduces their ability to conduct electricity, according to McCullough.
"A good analogy is a water hose. A bent hose transports water poorly, whereas a straight hose conducts water much more effectively. Likewise, irregularly shaped, disorganized polymers are poor conductors of electricity, whereas straight, stackable regioregular polythiophenes are excellent electrical conductors," said McCullough.
Regioregular polythiophenes have a wide range of potential applications, such as dissipating static electrical charges that build up on coated floors or use in disposable devices called radio frequency identification tags.
The superior conducting performance of regioregular polythiophenes is captured in their structure. Each polymer unit is composed of a chemical ring (thiophene) with a chemical branch on one side. Units are attached head to tail, so that all of the branches line up in one direction, much like feathers. The head-to-tail structure effectively straightens polythiophenes into rods that can be stacked one atop another.
To make a regioregular polythiophene polymer conductive, the scientists incorporate a pinch of a reactive additive to the polymer. This step removes some electrons from the forming polymer, thereby freeing the remaining electrons to move up and down the final polymer.
By attaching normal plastics to the polythiophene backbone, McCullough's team can create nanowire stacks with versatile properties, such as softness and solubility in different fluids used in industrial manufacturing. Because their properties can be varied, regioregular conducting polymers have the widest range of commercial applications compared with any other conducting polymer, he said.