For the last century, technology has blossomed in an age of plastics. We drive cars with plastic parts, we wear eyeglasses with plastic lenses, and we sip mineral water from plastic bottles. Plastic cell phones connect us to family and friends, and plastic keys typed these very words. Plastics may now be entering additional avenues of technological greatness based on one of their newer properties–electrical conductance.
First discovered in the 1970s, conducting polymers have made it into a few small-scale commercial applications, such as antistatic coatings on photographic film and light emitting diodes in a display of maintenance information on an electric razor.
Now, however, a range of development efforts aims to put conducting polymers to use in products as diverse as paper-thin televisions and sensors for chemical-weapons detectors. Meanwhile, polymer scientists are doing fundamental research, seeking ever more conductive plastic materials.
One major thrust of this work is to figure out how to make existing conducting plastics more orderly on the molecular level. Disorder limits the polymers’ conductivity and can hinder their performance in electronic devices. In two recent reports centered on the leading conducting polymer, researchers describe progress in the quest for more order.
Plastics that conduct
Before the 1970s, plastics’ closest association with electricity was as the insulation around electrical wires. The discovery of conducting polymers has been regarded as so important that it was recognized with the 2000 Nobel Prize in Chemistry (SN: 10/14/00, p. 247). Three researchers who had worked together at the University of Pennsylvania–Alan Heeger of the University of California, Santa Barbara, Alan MacDiarmid of the University of Pennsylvania, and Hideki Shirakawa of the University of Tsukuba in Japan–shared that award.
A theme in the development of conducting polymers has been chemists’ ingeniously capitalizing on mistakes. Both luck and insight played roles in the discovery of the first conducting polymer, a form of the material called polyacetylene. As its name implies, this polymer molecule is built from smaller molecules of acetylene, the substance that burns in welders’ torches.
While trying to make ordinary polyacetylene, a researcher visiting Shirakawa’s laboratory in Japan accidentally added 1,000 times the usual amount of polymerization catalyst to a vessel containing acetylene. Instead of yielding what looked like a typical plastic, the reaction produced a shiny, metallic-looking material. Working on a hunch they had developed from experiments on inorganic materials that conduct electricity, the three future Nobel laureates added small amounts of bromine or iodine gas to remove some electrons from the plastic. Called doping, this process afforded the strange polyacetylene’s remaining electrons enough freedom to move rapidly up and down the polymer’s molecular chains.
But polyacetylene has one important flaw: It decomposes quickly in air.
Researchers, however, soon formulated other electrically conductive plastics. One of the best studied, most stable, and most commercially important of these is a class of polymers called polythiophenes, whose members are made up of repeating units called thiophenes. It’s “the conducting polymer of choice,” says materials scientist George Malliaras of Cornell University.
However, while polythiophenes have many superior properties, researchers can’t easily align the molecules within a sample, which limits current flow.
“The properties of the materials are definitely limited by disorder,” says Heeger, who in 1990 cofounded the company UNIAX, which was purchased by DuPont in 2000, to commercialize conducting polymers.
A different type of accident contributed to the discovery of a way to make a well-ordered, conducting polythiophene. In May 2000, Hong Meng, a student working in chemist Fred Wudl’s laboratory at the University of California, Los Angeles, made a sample of a thiophene monomer known as 2,5-dibromo-3,4-ethylenedioxythiophene and sealed it in a jar. In March 2002, when Meng retrieved the jar, he discovered that the white crystalline powder he’d prepared now looked like shiny, black crystals.
Because Wudl’s lab studies conducting polymers, it has a rule that any metallic-appearing material that a researcher makes or finds must be tested for electrical conductivity.
As it turned out, the Wudl team discovered that Meng’s material–a polymer that formed in the jar when the stored monomers linked up–conducted electricity better than commercially available versions of the same polythiophene.
The transformation of monomer powder into a solid polymer material had not been seen before in a polythiophene, says Wudl. He suspected that this so-called solid-state transformation might have created a polymer in a highly ordered, defect-free, crystalline form–locking in the regimented orientation of the original monomer components. This kind of organization doesn’t appear in polythiophenes created via the standard procedure of mixing catalysts and other additives with monomers in solution.
What had caused the transformation in the storage jar? Was it light? Heat? In laboratory experiments, Meng, Wudl, and their coworker Dmitrii Perepichka found that they could polymerize the monomer in a solid-state reaction simply by heating it. And the reaction didn’t need to take 2 years. It could be achieved in just a day, or even several hours, by heating the material to 60C or 80C, respectively, the researchers reported in the Feb. 10 Angewandte Chemie International Edition. That’s well below the monomer’s melting temperature of 96C.
To conduct electricity, a polymer needs to be doped so that electrons can move freely. As it happened, Meng’s 2-year reaction on the UCLA shelf had itself taken care of this doping. Each monomer contained two bromine atoms, and during the material’s polymerization, some carbon-bromine bonds broke. This liberated bromine gas had doped the polymer, the researchers found.
In subsequent experiments, the team added steps that remove the bromine dopant and replace it with iodine. This increases the polymer’s conductivity.
While this conducting plastic looks crystalline to the naked eye, experiments revealed that it’s not really crystalline nor as highly ordered as solid-state reactions might be able to produce, says Wudl. Now, he says, his lab and others will try to use the solid-state synthesis to create even more highly ordered polythiophenes.
If a solid-state reaction can produce crystals of polythiophenes, Malliaras comments, researchers will have the opportunity to examine them in ways that will provide a better fundamental understanding of the materials. “If you can enhance the properties of the conducting polymer, you might be able to enhance the properties of the devices” that you make of it, he says.
With an approach that seems the opposite of accidental discovery, Samuel Stupp’s lab at Northwestern University in Evanston, Ill., is also searching for better conducting polythiophenes. Stupp and James Hulvat, also at Northwestern, have created a novel template for organizing thiophene monomers into more highly aligned arrangements. Made of liquid crystal, this template holds the monomers in place while they polymerize.
Liquid crystals are fluid materials that nonetheless contain particles arranged in a very uniform structure. The liquid crystals chosen by Stupp and Hulvat are gels made of tiny cylinders, just 3 nanometers wide, that in water assemble into a honeycomb pattern. The inside of the cylinders are water-avoiding, as are monomers of 3,4-ethylenedioxythiophene. When the researchers mixed the gel and the monomers, the monomers sequestered themselves within the dry interiors of the cylinders.
Hulvat and Stupp then used an electric field to polymerize the molecules inside the cylinders.
This procedure resulted in the formation of polythiophene molecules, all lined up in the same direction. After the scientists washed away the liquid crystal, they were left with a polymer film that retained the nanoscale and microscale structure of the liquid-crystal cylinders, says Stupp.
Hulvat and Stupp described these results in the Feb. 17 Angewandte Chemie International Edition. In further experiments, preliminary tests of conductivity supported the researchers’ expectations. The more regularly oriented polymer structure conducted electricity better than less regularly structured versions of the same polymer.
Moreover, light-emitting diodes containing the highly aligned material performed better than light-emitting diodes using the disordered material, says Stupp.
Using liquid crystals as a template is clever and promising, says Heeger. After all, “we cannot reach in there and pull on each molecule and align each one separately,” he says.
Hulvat and Stupp say that by using different liquid-crystal templates, they expect to achieve a wide variety of molecular orientations. No single orientation will be the best choice for every application.
As scientists wield ever more refined control over the structures of conducting polymers, these materials may extend the Plastics Age into the indefinite future.
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