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Report for Chemistry 446, Spring 2002 by Jason Shaw & Derek Marin
Introduction: brief history and mechanism of conductive polymers (polyacetylene)
Discussion: three coating applications of conductive polymers
1. Polyaniline coatings for corrosion resistance
2. Polyaniline coatings for the aerospace industry
Introduction: Brief History and Mechanism of Conductive Polymers (Polyacetylene)
Every person in America grows up learning not to touch
an electrical cord that is frayed or you might receive a shock from the
exposed conductive metal wire. We all know that plastics do not conduct
electricity and can be used to insulate the electrical wire and protect
us from electrical current.
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In the year 200 the Nobel Prize in Chemistry
was awarded to three men from the field of conductive polymers. The award was
given to Alan J. Heeger, Alan G. MacDiarmid and Hideki Shirakawa. Heeger,
currently a professor of Physics at the University of California Santa Barbara
and has done pioneering research in the area of semiconducting and metallic
polymers and was appointed Chief Scientist of UNIAX Corporation in 1999 the
company which he founded in 1990. MacDiarmid, Professor of Chemistry at
the University of Pennsylvania “co-discoverer of the field of conducting polymers,
more commonly known as “synthetic metals,” was the chemist responsible in 1977
for the chemical and electrochemical doping of polyacetylene” (2).
Shirakawa, who retired earlier this spring from the University of Tsukuba and
was also awarded the Order of Culture by the Japanese Government in November
2000. The three men found that a “thin film of polyacetylene could be
oxidized with iodine vapour, increasing its electrical conductivity a billion
times” (1).
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In the early 1970s, Shirakawa synthesized a silvery film, trans-polyacetylene, by accidentally adding “a thousand-fold too much catalyst” to the reaction vessel (1). Shirakawa also learned that he could create pure cis-polyacetylene by doing the same reaction at a different temperature. Around the same time “chemist MacDiarmid and physicist Heeger were experimenting with a metallic-looking film of the inorganic polymer sulphur nitride” (1). Amazingly the two men, Shirakawa and MacDiarmid met at a seminar and discussed their findings. This event led to Shirakawa being invited to the University of Pennsylvania in Philadelphia were he and MacDiarmid modified polyacetylene by oxidation with iodine vapour. Heeger was then asked to look for changes in the optical properties of the oxidation process. Conductivity measurements of iodine –doped trans-polyacetylene were done and the incredible increase of ten million times the original conductivity was discovered.
Conductivity and how it relates to polyacetylene will now be
examined. Conductance is the reciprocal of resistance (R^-1), where R
is defined by Ohm’s Law. The resistance is proportional to the length
(l) of the sample and inversely proportional to the sample cross-section A in
Ohmic materials ( R=pl/A). Resistivity is defined by ( p ), its inverse
is conductivity.
Unfortunately semiconductors, linear polyene chains, like polyacetylene generally deviate from Ohm’s law. Conductivity depends on the number density of charged carriers and how fast they can move in the material; it also depends on the temperature. For semiconductors conductivity generally decreases with lower temperatures. Conductivity of a stretched polymer like polyacetylene depends on direction and may be anistropic (properties such as strength and optical and electrical properties). The conductivity of “stretched oriented polyacetylene is some 100 times higher in the stretched direction than perpendicular to it” (1). Polyacetylene has alternating single and double bonds that give rise to mobile pi electrons that when doped become highly anistropic metallic conductors.
Polyacetylene is a semiconductor on
its own, but when the polymer is doped its conductivity is increased.
Polyacetylene can be doped by oxidation with a halogen (iodine) called
p-doping or by reduction with an alkali metal (sodium) called n-doping.
In oxidation “the iodine molecule attracts an electron from the polyacetylene
chain and becomes iodine three” a negatively charged ion, “the polyacetylene
molecule, now positively charged, is termed a radical cation, or polaron”
(1). The electron is removed from the top of the
valence band of polyacetylene creating a vacancy or hole. Although
the hole created does not delocalize completely. An electron can
be removed locally from one carbon atom and produce a radical cation
For example, undecahexaene chain has one electron removed on the fifth
carbon atom; the polaron then migrates down the chain. Polyacetylene
behaves in this same manner when doped “however, since the counterion (iodine
three) to the positive charge is not very mobile, a high concentration
of counterions is required so that the polaron can move in the field of
close counterions” explaining why excessive doping is needed (1).
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Two other types of polarons may be created, one called a second independent poloran which occurs when a second electron is removed from an already-oxidized section of the polymer or a bipolaron is formed if it is the unpaired electron of the first polaron that is removed. Solitary wave defects or solitons are also created through doping and are important for the conductivity of polyacetylene. Thermal isomerisation of cis-polyacetylene to trans-polyacetylene created defects, “a stable free radical: this is a neutral soliton, which although it can propagate along the chain may not in itself carry any charge” (1).
Discussion: Three Coating Applications of Conducting Polymers
Corrosion is a part of our everyday life; at anytime of the day a person can observe the continual work of painters coating the Golden Gate Bridge in San Francisco to protect its steel structure from corrosion. When metal donates some of its electrons to oxygen it causes the formation of impurities that weaken the structure, this process we call corrosion. Painting a metal with zinc or covering the metal with plates of zinc can delay corrosion. Zinc, being more reactive and a good electron donor reacts with oxygen more readily and the metal underneath is not affected. Unfortunately, coatings and plates of zinc only last so long as demonstrated by the continual painting of the Golden Gate Bridge. All of the problems caused by corrosion can be helped considerably by a wonderful “plastic coating that virtually eliminates rust and corrosion-which could help cars, bridges, pipelines and other metal structures last up to 10 times longer” (3) The coating is polyaniline.
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Coatings like zinc paint create a physical barrier, but polyaniline works completely different. Polyaniline is a “catalysts that mediates the reaction that leads to rust”(3). Polyaniline halts corrosion by “accepting electrons from the metal and in turn, donates them to oxygen” creating a two-step reaction that “forms a layer of pure iron oxide”(3). In the laboratory under controlled conditions “polyaniline prevented rust 10,000 times more effectively than zinc” and in the field polyaniline “proved three to 10 times more effective” reported Bernard Wessling Ph.D., president and managing partner of Ormecon Chemie GmdH & Co in Ammersbek, Germany (3). The polymer coating polyaniline is not a heavy metal and does not propose a threat to human health and “it is also cheaper than zinc”, “Wessling described it as an “organic metal [that could] last forever”(3).
The Ormecon company has developed “a new and efficient method to disperse polyaniline in paints… for the production of industrially applicable and effective corrosion prevention primers”(4). By itself the polyaniline primer is not as sufficient at stopping corrosion, but “complete coating systems composed by the dispersed polyaniline containing primer or a zinc rich primer with the same acrylic top coat were investigated”(4). The experiment examined “the new polyaniline containing primer CORRPASSIV scaled with two different top coats and characterized and compared with top coated samples using no or a conventional zinc-rich primer”(4). During the experiment “measurements were made using scanning Kelvin-probe (SKP), electrochemical impedance spectroscopy (EIS), voltammetry and analysis of iron and zinc content in electrolytes for testing”(4). By contaminating the metal/polymer interface with NaCl “the Volta potential of the polymer coating iron surface and … the delamination of the polymer coatings in the vicinity of a defect on the metal substrate” was mapped using the SKP (4).
Electrochemical impedance spectroscopy was used to evaluate the properties of the coatings and “the corrosion potentials and the corrosion currents were determined by potentiodynamic measurements from the Tafel plots”(4). Finally, “the iron and zinc concentrations of the NaCl solutions for the immersion of the scratched panels for SKP were measured with ASS in order to calculate the corrosion rate of the metals in the scratch”(4). After about two weeks “the CORRPASSIV primer with a two component epoxy top coat compared to a zinc-rich primer with an acrylic top coat showed much better corrosion protection capability”(4). Hopefully Wessling and Ormecon will not sell this coating system to the city of San Francisco so that the painting crew of the Golden Gate Bridge can keep their jobs.
Continued Use of Polyaniline for Corrosion Resistance
Harsh environments can push equipment to their limits. Structures and ground equipment at the John F. Kennedy Space Center (KSC) need to be resistant to hydrochloric acid (HCl) and corrosion. Environmental conditions during a launch “consists of marine, severe solar, and intermittent high acid/elevated temperature” conditions (5). The current zinc coatings provide the necessary protection but do not last under the extremely high concentrations of HCl produced during launch conditions. Researchers at KSC and the Los Alamos National Laboratory (LANL) have focused their efforts on “electrically active polymers” (5).
The researchers objective was to formulate organic coatings “to provide easy application, repair, and long term resistance to the KSC launch environments” (5). Material that were to be researched had to qualify under many conditions including processing, dopability, electrical conductivity, and adhesion to steel. Adhesion to steel was the largest hurdle to be overcome, but was accomplished by blending the conductive polymers with epoxies in the undoped form and then “doping the coated surface to the conductive state” (5). Several polymers were looked at and tested under exposure to salt water and 0.1 M HCl. All of the best results pointed in the direction of polyaniline. The best results were obtained when polyaniline was doped with tetracyanoethylene (TCNE), zinc nitrate, and p-toluene sufonic acid.
Polyaniline in the emeralidine form was made using the standard synthesis method. This method uses the procedure of oxidizing polyaniline in HCl and using ammonium persulfates as the oxidizing agent. The "polyaniline powder was converted to the non-conducting emeralidine base by stirring in an ammonium hydroxide solution"(5).
Corrosion testing for polyaniline consisted of steel coupons coated with the doped TCNE and placed in vials of 3.5% NaCl and vials of 0.1 M HCl. Pictures of the samples were taken before and during the testing which lasted for 12 weeks in some cases. Results for both sets of tests showed that there was “no evidence of corrosion was seen, with the scratched surface still shiny and the edges of the sample still intact and showing no mass loss” (5). Additional tests were conducted near and at the launch site at KSC with results favoring the polyaniline primer as determined by “low-power optical microscopy” (5).
Results of their work “determined that in both laboratory and harsh outdoor corrosion testing environments electrically active polymers provide corrosion protection to steel substrates” (5). Doped polyaniline showed long lasting effectiveness to salt water, aqueous HCl, harsh environments of sea spray and launch conditions. Electrically conductive polymers are finding their way into many of our current coating applications and polyaniline is leading the way.
A house that generates it's own electricity and clothing that can power a lap top computer will soon become a reality. One may ask how this is possible? The answer is polymeric photovoltaics.
While photovoltaic clothing products are almost here the photovoltaic structural coatings are still a ways off. Things are developing quickly in the field. All that remains is to make the coatings more efficient and cheaper to manufacture.
One such individual who is working on this is Greg Van Patten of the Los Alamos National Laboratory. His research is dealing with photovoltaic pigments to augment coatings. He is currently focused on porphyrins, which are chemically similar to chlorophyll.(8)
So how exactly do polymers produce electricity from light? First you need to start off with a conjugated polymer.
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The you dope the polymer to increase its conductivity.
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Now in order for a polymer to be photovoltaic there needs to be a band gap present. This is easy to explain in metals because the band gap is merely the gap between the valence band and the conduction band. A photon knocks an electron from the valence band and into the conduction band and two charge carriers are formed. The hole in the valence band and the electron in the conduction band. In polymers it is a bit more difficult.
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It is the characteristic of the pi bonds which give the conducting polymers their ability to act as photovoltaics. For starters the electrons of the pi bonds are delocalized over the entire polymer. As previously stated this allows the polymeric material to be conducting. However to act as a photovoltaic - or more generally a semiconductor - a small band gap is needed. This is caused by the overlap of the pz orbitals which produces two orbitals - a bonding and an anti-bonding pi. The lower energy bonding pi acts as the valence band and the anti-bonding pi is the conduction band.
Now when a photon strikes the polymer it excites an electron to the conduction band. However unlike in traditional semiconductors where the electron is free to roam, in the polymer it is bound to the gap it left in the valence band. This charge pair is called an exciton. (9)
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Because the exciton is a bound charge pair it can not do work as is. First it must be split. This can be done at interfaces and this is where the electricity comes from.
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We have discussed conductive polymeric coatings on several levels. Polymers contain conjugated double bonds or band gaps that allow them to become conductive when doped. A doped conductive polymer may be used for many applications. We have shown how conductive polymer may be used in paint coatings to resist corrosion much more efficiently then most every other type of coating. We have also shown how conductive may be used as photovoltaics to create coatings that produce electricity.
Overall conductive polymers are very useful in surface applications. The availability and low cost of polymers like polyaniline make the field of conductive polymers a thriving industry. Conductive polymers have found their way into many other fields including the photovoltaics (which converts photons into electricity). Surface chemistry can only gain from the use of conductive polymers.
In the future conductive coatings could find their way into all different kinds of applications. Photovoltaic coatings that gather light and emit electricity could be used to power your house or your vehicle. If your vehicle was painted with a photovoltaic coating it could gather sun light and emit the electricity needed to power itself. This would be environmentally friend and in most cases cheaper than petroleum products. Corrosion is always an ongoing battle and coatings that can resist corrosion are very important. In the near future conductive polymeric coatings will be the most economical way to fight corrosion.
Literature(1) http://www.nobel.se/chemistry/laureates/2000/public.htmlImages(2) http://www.sas.upenn.edu/~macdiarm/Bio-Short.html
(3) http://www.eurekalert.org/pub_releases/2000-08/ACS-Cabt-2008100.php
(4) Posadorfer J., Wessling B., "Experimental Evidence for Passivation by the Organic Metal", Polymer Preprints, 2000, V. 41, issue: 2, pp 1735-1736
(5) Thompson, K.G., Benicewicz, B. C., "Corrosion-Protective Coatings From Electroactive Polymers", Polymer Preprints, 2000, V. 41, issue: 2, pp 1731-1732
(6) Baur, Jeffery W., Nanostructured Organic Photovoltaic Devices Via Electronic Self-Assembly Of Electroactive Polymers and Molecules, (April 2000): n.p. On-line. Internet. 31 Mar. 2000 Available: http://www.eng.uc.edu/~dwschae/seminar/baur.html
(7) Blanch, Smart Bra-Intelligent Fabrics, Innovations: Stories, n. p. On-Line, Internet 16 July 2000, Available: http://www.abc.net.au/ra/elp/innovatn/inots777_c.htm
(8) Editors; Light Harvesting Coatings, Popular Mechanic Web Tech Update, n. p. On-line, Internet aug. 1998. Available: http://www.popularmechanics.com/popmech/sci/tech/9808TUCHRM.html
(9) Wallace, Gordon G.; Dastoor, Paul C.; Officer, David L.; Too, Chee O. Conjugated polymers: New materials for photovoltaics, Chemical Innovation, A.C.S., April 2000, Vol. 30, No. 1, pages 14-22 Available On-line: http://pubs.acs.org/hotartcl/ci/00/apr/0650wallace.html
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