Synthesis and applications of conducting polymer nanofibers
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on This article focuses on the conducting polymer polyaniline and its potential for use in various applications, ranging from sensors and molecular memory devices to catalysis and actuators. The article presents findings on growing single crystals of the aniline tetramer, the basic building block of polyaniline. Surprisingly, single crystals of tetraaniline can exhibit electrical conductivity comparable to that of the polymer. The first conducting polymer discovered, polyacetylene, can be synthesized in two forms: a cis-isomer and a trans-isomer. When compared to a piece of aluminum foil, this polymer looks metallic (Figure 1). This metallic appearance is what originally excited the imagination of scientists around the world. In fact, both forms of polyacetylene are semiconductors; however, through doping with either electron acceptors (p-type) or electron donors (n-type), polyacetylene becomes a metallic-like conductor. Figure 2 shows an illustration of the structure of polyacetylene showing bond angles and bond distances, with alternating double and single bonds.1 To make a metal, you need two things: a pathway for electricity—this is provided by the π bonds in polyacetylene; and carriers of electricity—which are created by adding dopants. In Figure 2, iodine (I2) has been added, which extracts an electron from the polyacetylene
chain to form I3–, creating a hole that is essentially free to move along the chain. Doped polyacetylene thus becomes one of the most conducting substances known at room temperature. When doped with iodine, the conductivity reaches 10,000 Siemens per centimeter (S cm–1). If you stretch polyacetylene, the conductivity along the stretch direction increases to >100,000 S cm–1. For copper, silver, or gold, the conductivity is ∼650,000 S cm–1, but on a weight basis, polyacetylene is composed of just carbon, hydrogen, and a bit of iodine. Therefore, based on weight, polyacetylene is actually more conducting than most common metals. However, polyacetylene has a problem—it is air-sensitive. Twenty years after I began my PhD, A. MacDiarmid and A. Heeger, along with H. Shirakawa, shared the 2000 Nobel Prize in Chemistry2 for the discovery of conducting polymers. When I was invited back to speak at the University of Pennsylvania for a Nobel Prize symposium, I told the joke, “What’s the only true application for polyacetylene (knowing that it’s air-sensitive)?” The answer is “To produce PhDs.” However, I added that “on this occasion we now know it’s a dual-use material—PhDs and Nobel Prizes.” Hardly anyone studies polyacetylene anymore. What we do study are air-stable conducting polymers, including polypyrrole, polythiophene, and polyaniline. What they all have in
Richard B. Kaner, Department of Chemistry and Biochemistry, Department of Materials Science and Engineering and California NanoSystems Institute, University of California, Los Angeles, USA; [email protected]. doi:10.1557/mrs.2016.213
• VOLUME • OCTOBER • www.mrs.org/bulletin 2016 Materials Research Society MRS to BULLETIN 41 terms 2016 Dow
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