The Nobel Prize in Chemistry 2000

Hideki Shirakawa

discovery and development of conductive polymers.

Until about 30 years ago all carbon based polymers were rigidly regarded as insulators. The notion that plastics could be made to conduct electricity would have been considered to be absurd. Indeed, plastics have been extensively utilized by the electronics industry for this very property. They are used as inactive packaging and insulating material. This very narrow perspective is rapidly changing as a new class of polymers known as intrinsically conductive polymers or electroactive polymers are being discovered. Although this class of polymer is in its infancy, much like the plastic industry was between the 1930's and 50's, the potential uses of these polymers are quite significant1.

The first conducting plastics were discovered by accident at the Plastics Research Laboratory of BASF in Germany. They were attempting the oxidative coupling of aromatic compounds. When they made polyphenylene and polythiophene they found that they showed electrical conductivities of up to 0.1 s cm-1. Since then other conducting polymers have been discovered. A Logarithmic conductivity ladder of some of these polymers are shown below 2.

There are two main groups of applications for these polymers. The first group utilizes their conductivity as its main property. The second group utilizes their electroactivity. The extended p -systems of conjugated polymer are highly susceptible to chemical or electrochemical oxidation or reduction. These alter the electrical and optical properties of the polymer, and by controlling this oxidation and reduction, it is possible to precisely control these properties. Since these reactions are often reversible, it is possible to systematically control the electrical and optical properties with a great deal of precision. It is even possible to switch from a conducting state to an insulating state. The two groups of applications are shown below:

Group 1 Group 2 Electrostatic materials Molecular electronics Conducting adhesives Electrical displays Electromagnetic shielding Chemical, biochemical and thermal sensors Printed circuit boards Rechargeable batteries and solid electrolytes Artificial nerves Drug release systems Antistatic clothing Optical computers Piezoceramics Ion exchange membranes Active electronics (diodes, transistors) Electromechanical actuators Aircraft structures 'Smart' structures Switches

GROUP 1 - CONDUCTIVITY:

These applications uses just the polymer's conductivity. The polymers are used because of either their light weight, biological compatibility for ease of manufacturing or cost.

By coating an insulator with a very thin layer of conducting polymer it is possible to prevent the buildup of static electricity. This is particularly important where such a discharge is undesirable. Such a discharge can be dangerous in an environment with flammable gasses and liquids and also in the explosives industry. In the computer industry the sudden discharge of static electricity can damage microcircuits. This has become particularly acute in recent years with the development of modern integrated circuits. To increase speed and reduce power consumption, junctions and connecting lines are finer and closer together. The resulting integrated circuits are more sensitive and can be easily damaged by static discharge at a very low voltage. By modifying the thermoplastic used by adding a conducting plastic into the resin results in a plastic that can be used for the protection against electrostatic discharge3.

By placing monomer between two conducting surfaces and allowing it to polymerise it is possible to stick them together. This is a conductive adhesive and is used to stick conducting objects together and allow an electric current to pass through them.

Many electrical devices, particularly computers, generate electromagnetic radiation, often radio and microwave frequencies. This can cause malfunctions in nearby electrical devices. The plastic casing used in many of these devices are transparent to such radiation. By coating the inside of the plastic casing with a conductive surface this radiation can be absorbed. This can best be achieved by using conducting plastics. This is cheap, easy to apply and can be used with a wide range of resins. The final finish generally has good adhesion, gives a good coverage, thermally expands approximately the same as the polymer it is coating, needs just one step and gives a good thickness 4 .

Many electrical appliances use printed circuit boards. These are copper coated epoxy-resins. The copper is selectively etched to produce conducting lines used to connect various devices. These devices are placed in holes cut into the resin. In order to get a good connection the holes need to be lined with a conductor. Copper has been used but the coating method, electroless copper plating, has several problems. It is an expensive multistage process, the copper plating is not very selective and the adhesion is generally poor. This process is being replaced by the polymerisation of a conducting plastic. If the board is etched with potassium permanganate solution a thin layer of manganese dioxide is produced only on the surface of the resin. This will then initiate polymerisation of a suitable monomer to produce a layer of conducting polymer. This is much cheaper, easy and quick to do, is very selective and has good adhesion5 .

Due to the biocompatability of some conducting polymers they may be used to transport small electrical signals through the body, i.e. act as artificial nerves. Perhaps modifications to the brain might eventually be contemplated6.

Weight is at a premium for aircraft and spacecraft. The use of polymers with a density of about 1 g cm-1 rather than 10 g cm -1 for metals is attractive. Moreover, the power ratio of the internal combustion engine is about 676.6 watts per kilogramme. This compares to 33.8 watts per kilogramme for a battery-electric motor combination. A drop in magnitude of weight could give similar ratios to the internal combustion engine 6. Modern planes are often made with light weight composites. This makes them vulnerable to damage from lightning bolts. By coating aircraft with a conducting polymer the electricity can be directed away from the vulnerable internals of the aircraft.

GROUP TWO: ELECTROACTIVE:

Molecular electronics are electronic structures assembled atom by atom. One proposal for this method involves conducting polymers. A possible example is a modified polyacetylene with an electron accepting group at one end and a withdrawing group at the other. A short section of the chain is saturated in order to decouple the functional groups. This section is known as a 'spacer' or a 'modulable barrier'. This can be used to create a logic device. There are two inputs, one light pulse which excites one end and another which excites the modulable barrier. There is one output, a light pulse to see if the other end has become excited. To use this there must be a great deal of redundancy to compensate for switching 'errors' 7 .

Depending on the conducting polymer chosen, the doped and undoped states can be either colourless or intensely coloured. However, the colour of the doped state is greatly redshifted from that of the undoped state. The colour of this state can be altered by using dopant ions that absorb in visible light. Because conducting polymers are intensely coloured, only a very thin layer is required for devices with a high contrast and large viewing angle. Unlike liquid crystal displays, the image formed by redox of a conducting polymer can have a high stability even in the absence of an applied field. The switching time achieved with such systems has been as low as 100 ms but a time of about 2 ms is more common. The cycle lifetime is generally about 106 cycles. Experiments are being done to try to increase cycle lifetime to above 107 cycles8.

The chemical properties of conducting polymers make them very useful for use in sensors. This utilizes the ability of such materials to change their electrical properties during reaction with various redox agents (dopants) or via their instability to moisture and heat.

An example of this is the development of gas sensors. It has been shown that polypyrrole behaves as a quasi 'p' type material. Its resistance increases in the presence of a reducing gas such as ammonia, and decreases in the presence of an oxidizing gas such as nitrogen dioxide. The gases cause a change in the near surface charge carrier (here electron holes) density by reacting with surface adsorbed oxygen ions9. Another type of sensor developed is a biosensor. This utilizes the ability of triiodide to oxidize polyacetylene as a means to measure glucose concentration. Glucose is oxidized with oxygen with the help of glucose oxidase. This produces hydrogen peroxide which oxidizes iodide ions to form triiodide ions. Hence, conductivity is proportional to the peroxide concentration which is proportional to the glucose concentration10.

Probably the most publicized and promising of the current applications are light weight rechargeable batteries. Some prototype cells are comparable to, or better than nickel-cadmium cells now on the market. The polymer battery, such as a polypyrrole- lithium cell operates by the oxidation and reduction of the polymer backbone. During charging the polymer oxidizes anions in the electrolyte enter the porous polymer to balance the charge created Simultaneously, lithium ions in electrolyte are electrodeposited at the lithium surface. During discharging electrons are removed from the lithium, causing lithium ions to reenter the electrolyte and to pass through the load and into the oxidized polymer. The positive sites on the polymer are reduced, releasing the charge-balancing anions back to the electrolyte. This process can be repeated about as often as a typical secondary battery cell11.

Conducting polymers can be used to directly convert electrical energy into mechanical energy. This utilizes large changes in size undergone during the doping and dedoping of many conducting polymers. This can be as large as 10%. Electrochemical actuators can function by using changes in a dimension of a conducting polymer, changes in the relative dimensions of a conducting polymer and a counter electrode and changes in total volume of a conducting polymer electrode, electrolyte and counter electrode. The method of doping and dedoping is very similar as that used in rechargeable batteries discussed above. What is required are the anodic strip and the cathodic strip changing size at different rates during charging and discharging. The applications of this include microtweezers, microvalves, micropositioners for microscopic optical elements, and actuators for micromechanical sorting (such as the sorting of biological cells)

One of the most futuristic applications for conducting polymers are 'smart' structures. These are items which alter themselves to make themselves better. An example is a golf club which adapt in real time to a persons tendency to slice or undercut their shots. A more realizable application is vibration control13. Smart skis have recently been developed which do not vibrate during skiing. This is achieved by using the force of the vibration to apply a force opposite to the vibration14 . Other applications of smart structures include active suspension systems on cars, trucks and train; traffic control in tunnels and on roads and bridges; damage assessment on boats; automatic damping of buildings and programmable floors for robotics and AGV's13.

Much research will be needed before many of the above applications will become a reality. The stability and processibility both need to be substantially improved if they are to be used in the market place. The cost of such polymers must also be substantially lowered. However, one must consider that, although conventional polymers were synthesized and studied in laboratories around the world, they did not become widespread until years of research and development had been done. In a way, conducting polymers are at the same stage of development as their insulating brothers were some 50 years ago. Regardless of the practical application that are eventually developed for them, they will certainly challenge researchers in the years to come with new and unexpected phenomena. Only time will tell whether the impact of these novel plastics will be as large as their insulating relatives.

For the ten years from the third grade of elementary school to the end of high school, I lived in the small city of Takayama,a town of less than sixty thousand, located in the middle of Honshu, Japan. Even though it was far away from Japan's principal cities, Takayama has been called a "little Kyoto" because of the similarity of its landform to Kyoto, the city sits in a basin surrounded by mountains with a river flowing through it, and because of its long-established cultural heritage and tradition. In this small town, rich in natural beauty, I spent my days enthusiastically collecting insects and plants, and making radios. My affinity for science was awakened and grew during in these ten years.

Long after I became a polymer scientist, I occasionally remembered a short composition I had written during my last year in junior high school. At that time students compiled a commemorative collection of compositions describing our future dreams. As I recalled, I wrote something about my wish to be a scientist in the future and to conduct research on plastics useful for ordinary people. I cannot be sure what I wrote exactly because I lost the book of essays during repeated moves afterwards. I had long regretted this loss because I wanted to know more about why and how a junior high school boy decided on a future research career in plastics.

Much to my surprise, I found that the full composition I had lost was printed in every Japanese newspaper the day after the Royal Swedish Academy of Sciences announced its award of the Nobel Prize in Chemistry for 2000 to two friends and myself. After 45 years, I could finally read the complete composition again. I was deeply impressed with the great power of the Nobel Prize.

I was born in Tokyo in August 1936, the third child of Hatsutarou, a medical doctor, and Fuyuno, a daughter of a chief priest of a Buddhist temple. After me, a sister and a brother were born, joining my elder brother, my elder sister and me. After I was born, my family moved many times, following my father's work, but we finally settled in Takayama, my mother's hometown, in 1944 during the confusion toward the end of the war.

My higher education began when I entered Tokyo Institute of Technology in April 1957. In March 1966, I completed my doctoral course and received the degree of Doctor of Engineering. In the same year, I married Chiyoko Shibuya, and we were later blessed with two sons, Chihiro and Yasuki.

There were three specific fields I wished to study at university. One was polymer chemistry, just as I had written in my junior high school composition. The other possibilities were horticulture and electronics. I had decided to major in polymer chemistry only if I successfully passed the entrance examination for Tokyo Institute of Technology. In April 1957, after entering Tokyo Institute of Technology, I mainly studied applied chemistry during my undergraduate career. In Japanese universities, an undergraduate major in an science course has to belong to one of the laboratories in his department during his final year in order to work on a graduation thesis. I was interested in synthesizing new polymers, so I applied to a laboratory conducting synthesis research. But since there were too many applicants who wanted to enter into the laboratory I had chosen, I had to switch to a laboratory working on polymer physics. Initially I was reluctant to work in this field, but actually, I realize that my experiences in this laboratory were of great importance to me when I worked with polyacetylene later on.

I finally began working on polymer synthesis, my original interest, in my graduate program, but I started the work on polyacetylene, the work for which I now share the Nobel Prize, just after I received my doctorate and I became a research associate in April 1966. The initial purpose of this study was to determine the polymierization mechanism of polyacetylene using the Ziegler-Natta catalysts. In the fall of 1967, only a short time after we started polyacetylene film through an unforeseeable experimental failure.

With the conventional method of polymerization, chemists had obtained polyacetylene in the form of black powder; however, one day, when a visiting scientist tried to make polyacetylene in the usual way, he only produced some ragged pieces of a film. In order to clarify the reason for the failure, I inspected various polymerization conditions again and again. I finally found that the concentration of the catalyst was the decisive factor for making the film. In any chemical reaction, a very small quantity of the catalyst, about mmol would be sufficient, but the result I got was for a quantity of mol, a thousand times higher than I had intended. It was an extraordinary unit for a catalyst. I might have missed the "m" for "mmol" in my experimental instructions, or the visitor might have misread it. For whatever reason, he had added the catalyst of some molar quantities in the reaction vessel. The catalyst concentration of a thousand-fold higher than I had planned apparently accelerated the rate of the polymerization reaction about a thousand times. Roughly speaking, as soon as acetylene gas was put into the catalyst, the reaction occurred so quickly that the gas was just polymerized on the surface of the catalyst as a thin film.

But we noticed another important factor besides the concentration of the catalyst. Polyacetylene has a property of being insoluble in any solvent, a property which contributed to the formation of the film. Even more surprising, when we observed the film through a transmission electron microscope, we saw that the film was composed of long entangled micro-fibers of polyacetylene. These two properties are essential for the formation of any film, and they were inherent in polyacetylene.

One more important factor contributed to the formation of the film was the Ziegler-Natta catalyst we had used. Most of the Ziegler-Natta catalysts tend to form precipitates which give an inhomogeneous solution. From such an inhomogeneous catalyst, it is very difficult to form polyacetylene film. But the Ziegler-Natta catalyst we had used in our experiment was a unique one. It had good solubility in organic solvents to give a homogeneous solution and it also had high activity to give a high molecular weight and crystalline form of polyacetylene. I could say that nature had prepared us for the way to make polyacetylene film. Later, through the measurements of various absorption spectra of this thin film, we determined the molecular structure of polyacetylene, and thus, we fulfilled the initial purpose of our work.

By chance, this glittering, silvery film, caught the eyes of Professor Alan G. MacDiarmid, one of the co-recipients of the prize, and he invited me to work with him in the U.S.A. In September 1976, I went to the University of Pennsylvania, where Professor Alan J. Heeger, another co-recipient, was also working, and I spent one year there.

I still vividly remember the day of November 23, 1976. With Dr. C.K. Chiang, a postdoctoral fellow who was working under Professor Heeger, I was measuring the electric conductivity of polyacetylene by the four-probe method, adding bromine. At exactly the moment we added bromine, the conductivity jumped so rapidly that he couldn't switch the range of the electrometers. Actually, the conductivity was ten million times higher than before adding bromine. This day marked the first time we observed the doping effect, although it was a pity that the expensive equipment was broken. The discovery of chemical doping is one of the representative results of our collaboration in this period.

After returning to Japan, I continued to work on polyacetylene. What I did first was to shed light on the chemical reaction associated with the doping phenomena. In cooperation with many co-workers, I investigated various spectra of the doped polyacetylene films: infrared absorption, Raman scattering, ultraviolet-visible absorption, the M?ssbauer effect, and EXAFS. As a result, we found that the emergence of electrical conductivity on the doped polyacetylene was due to the creation of carbocations or positively charged solitons associated with withdrawing of p electrons from polyacetylene by the dopant when iodine was used as an acceptor dopant.

In November 1979, I moved from Tokyo Institute of Technology to the Institute of Materials Science, University of Tsukuba, where I was appointed Associate Professor. In October 1982, I was promoted to full professor and worked on polyacetylene and other conducting polymers. Since my retirement from University of Tsukuba at the end of March 2000, I have withdrawn from scientific research and other educational activities.

Let me mention two of my major contributions to polyacetylene research during my time at Tsukuba. One is the preparation of oriented films. The significance of polyacetylene being a typical quasi-one dimensional material was recognized very early. In this sense, an oriented film was indispensable to study the intrinsic one-dimensional properties. The polyacetylene films synthesized until then were an isotropic material in which the fibrils were entangled in three-dimensional disorder. I came up with the idea to directly synthesize the uniaxially oriented films by using liquid crystal as a solvent. The same idea was proposed by a scientist from a company. We found that an equimolar mixture of nematic liquid crystals bearing a phenylcyclohexyl moiety was useful for that purpose. We succeeded in simultaneously polymerizing acetylene and synthesizing uniaxially oriented polyacetylene films by orienting the catalyst solution of liquid crystal solvent under flow condition or magnetic field. Further development of this technique enabled us to synthesize helical polyacetylene that consists of clockwise or counterclockwise helical structure of fibrils, by use of chiral nematic liquid crystals. The chiral helicity of the films may be useful for electromagnetic and optical applications.

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