The Nobel Prize in Chemistry 2000

Alan G. MacDiarmid

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.

I was born a Kiwi (a New Zealander) in Masterton, New Zealand on April 14, 1927, and still am a Kiwi by New Zealand law, although I became a naturalized United States citizen many years ago in order to have the right to vote in US elections and, hence, voice my political opinions in a meaningful way. My father, an engineer, was unemployed for four years during the Great Depression which hit New Zealand rather severely in the early 1930s. Since jobs were believed to be more plentiful in the vicinity of Wellington, the capital city of New Zealand, located at the bottom of the North Island, we moved to Lower Hutt a few miles from Wellington. There my two older brothers and my elder sister were able to find jobs while I and my younger sister were still at primary school.

My mother and father set the stage for nurturing a warm, loving united, mutually supportive family who always pulled together and also helped others outside the family in need when necessary. Although we did not have too much food, my mother was always inviting other, less fortunate people to meals. On such occasions, my older brothers and sister would frequently remind me and my younger sister at meals not to ask for more food by saying to us out loud at the table, "FHB," which meant, "Family Hold Back," i.e., don't eat too much! We had no phone or refrigerator. In one of the houses we lived in Lower Hutt, our hot water came from water pipes embedded in the brick at the back of the open fireplace in the living room. This resulted in our weekly bath night - where the younger children used the bath water from the older children, to which we were allowed to add more hot water if any still remained! For most of my time at primary school, I went to school barefooted, like most of the other kids. The soles of our feet literally became leather!

Even though I have been away from New Zealand for about 50 years, my brothers and sisters and I (my parents passed on several years ago) are still very closely connected to each other. Throughout the decades we have telephoned each other about every ten days and we all keep up to date with what we are each doing. Shortly after learning of my being a recipient of the Nobel Prize I was speaking to one of my brothers in New Zealand by phone and I said how lucky I was to have been raised in a poor family which was also a close loving family. The fact that we were poor made us self reliant and conscious of the value of money. The fact that we were closely knit taught us the important aspects of interpersonal relationships. Everyone expects "the important things" in life that such as birthday and Christmas presents, but it is the "little unimportant" actions which actually are the real important things. These put the flesh on the skeleton of any relationship. Several hundred of these each week - the unimportant, the unexpected, the unnecessary, "the little things", are the things that really count. We are lucky to have been brought up in this environment, but there is a statement on the wall of my study at home in suburban Philadelphia which reads, "I am a very lucky person and the harder I work the luckier I seem to be"!

It is my home life while growing up through high school, which I consider to have been the single most important factor in any success which I may have had in life. As my parents always said, "...an 'A's grade in a class is not a sign of success." Success is knowing that you have done your best and have exploited your God-given or gene-given abilities to the next maximum extent. More than this, no one can do.

Alan (age 10). For a period in grade school, I attended a two-room school in Keri Keri (town population, 600) where most of my school chums were Maori boys and girls from whom I learned so much. During much of my time at grade school I had an early morning, pre-school job delivering milk on my bicycle for Mr. Bradley, who had a few cows in a nearby paddock. My mother was superb - she would get up with me while it was still dark to make me hot tea to send me on my way. When I started high school it was necessary to give up my Milk route. Instead, I delivered the "Evening Post" newspaper on my bicycle after school.

Alan (age 12) with bicycle. When my father retired (on a very small pension) and moved away from Wellington, it was necessary for me to leave Hutt Valley High School after only three years at the age of 16 and take a low-paying, part-time job as "lab boy"/janitor in the chemistry department at Victoria University College, as it was then known. The total student population was 1200; the Chemistry Department had a faculty of 2! I boarded with friends of my parents and, as a part-time student, took only two courses - one in chemistry and one in mathematics. During this time I became a resident at Weir House, the University dormitory for men. This I found to be one of the most enjoyable and maturing times of my life where I made many good friends from the other ninety residents, with some of whom I still keep in close contact. I remained a part-time student throughout my B.Sc. and M.Sc. studies at Victoria University College. After completing my B.Sc. degree I graduated to the position of demonstrator. Since the age of 17 I have supported myself financially, assisted later only by scholarships and fellowships for which I am most grateful.

Alan (age 15) in high school uniform. My interest in chemistry was kindled when I was about ten years old at which time I found one of my father's old chemistry text books dating back to the late 1800's when he was studying engineering. I spent hours pouring over the pages in complete confusion but with burning curiosity! Some clarification of a type occurred when I rode my bicycle to the public library in Lower Hutt and entered the children's section. There, on the right hand side on the bottom shelf, in the new books section, was a book with a bright blue cover. It was called, "The Boy Chemist." I took it out and continually renewed it by borrowing it for over a year and carried out most of the experiments in it. One of my duties as lab boy, when I was not washing dirty labware or sweeping floors, was to prepare demonstration chemicals for Mr. A.D. "Bobbie" Monro, the lecturer in first-year chemistry. On one occasion he asked me to prepare some S4N4 - beautiful bright orange crystals. When it became time for me to start my M.Sc. thesis, I asked Mr. Monro if I could look at some of its chemistry. He agreed. This resulted in my first publication in 1949. Its derivatives were highly colored. Color continued to be one of the driving forces in my future career in chemistry. I love color. Little did I know that thirty years later this was going to be a key factor which would shape my professional life.

In 1950, I had the good fortune to receive a Fullbright fellowship from the U.S. State Department to do a Ph.D. at the University of Wisconsin in the USA where I studied under Professor Norris F. Hall, majoring in Inorganic Chemistry, studying the rate of exchange in 14C-tagged complex metal cyanides. It was at the University of Wisconsin that I became president of the International Club - the largest student organization on campus and had the crucial chance meeting of my life when I met my future wife, Marian Mathieu, at an International Club dance. During this time I was elected by the Department of Chemistry to the position of Knapp Research Fellow and had the privilege of living rent free in the beautiful old ex-governor's mansion on the shores of Lake Mendota.

When I was still at the University of Wisconsin I was successful in obtaining a New Zealand Shell graduate scholarship to study silicon hydrides at Cambridge University, England under the directorship of Professor H.J. Emel?us. It was there that Marian and I were married in the chapel at my college, Sidney Sussex College.

After a brief appointment as a junior faculty member at Queens College of the University of St. Andrews, Scotland, I accepted a junior position on the faculty of the Department of Chemistry at the University of Pennsylvania where I have been for the past 45 years and became father of three daughters and a son and grandparent of nine lovely boys and girls. I grew to love teaching and the stimulation of young fresh inquiring minds. I am still fully engaged in teaching as well as research and indeed have requested to teach a section of first-year chemistry at Penn later this year.

I had the good fortune to meet my future friend and colleague, Professor Alan J. Heeger, Professor of Physics at the University of Pennsylvania. On one occasion he came to my office and informed me that Mort Labes, Professor of Chemistry at Temple University, had published a paper on a highly conducting material. I asked Heeger its formula and he replied, "sss-nnn-ex". Being an inorganic chemist, I wrote down on a piece of paper, "(Sn)x" and said, "Of course you expect it to be conducting, it's a metal!" To which Heeger replied on paper, "No, not (Sn)x, but (SN)x! This was the beginning of our each learning each other's scientific language. I told him that I had made the precursor to (SN)x, i. e. S4N4 during my M.Sc. thesis work in New Zealand. He asked me if I could make some (SN)x - as golden crystals. We were ultimately successful, and co-published many papers together, on this conducting polymer.

When I was a Visiting Professor at Kyoto University in Japan, lecturing on molecular silicon compounds, I visited Tokyo Institute of Technology in 1975 and described our work on (SN)x, Hideki Shirakawa and I met over a cup of green tea after a lecture I gave and as I was showing a sample of our golden (SN)x, he showed me a sample of his silvery (CH)x.

I asked him how he had made this silvery film of polyacetylene and he replied that this occurred because of a misunderstanding between the japanese language and that of a foreign student who had just joined his group. Shirakawa had been polymerizing ordinary acetylene welding gas using a Ziegler-Natta catalyst and had been obtaining a rather uninteresting black-brown powder. He told the new student to repeat this work using a concentration of the catalyst which was milli-molar. A few days later the student came back and said that the stirring bar would not go around in the flask. Shirakawa went to the laboratory and, sure enough, instead of the black brown powder, there were lumps of silvery-pinkish jelly floating around. Shirakawa asked what the student had done and the student replied that he had done exactly as Shirakawa had told him; he had made the catalyst with a concentration of "x-molar"- in other words, he had made the catalyst 1000 times more concentrated than Shirakawa had told him! Shirakawa was most intrigued by this observation, since as all good chemists know, a catalyst should only increase the rate of a chemical reaction and should not alter the nature of the product. This then started Shirakawa investigating this silvery form of polyacetylene. I asked Shirakawa if he could join me for a year at the University of Pennsylvania since I was already interested in conducting materials such as the golden (SN)x films. He stated that he could and when he arrived we tried to make the silvery polyacetylene, (CH)x, more pure and, hence, increase its conductivity. However, we found that the purer we made the (CH)x, by elemental analysis, the lower was its conductivity! Since we had found previously that by adding bromine to the golden (SN)x material, we could increase its conductivity tenfold, we thought that perhaps the impurity in the polyacetylene was acting as a dopant and was actually increasing the conductivity of the polyacetylene, rather than decreasing it. We therefore decided to add some bromine to the silvery (CH)x films and immediately, within a few minutes at room temperature, the conductivity increased many millions of times. We then collaborated with my colleague, Professor Alan Heeger, who was well-versed in the physics of conducting materials. The rest is history! When Alan left Penn almost 10 years ago, my ongoing collaboration with my good friend Professor Art Epstein (Physics Dept, Ohio State Univ.) continued at an even more rapid pace.

free web hits counter

This is my BrainyGoose:
United States, IL, Chicago, English, Italian, Genry, Male, 21-25, bodybulding, swiming.