By Colin Pratt
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
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
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,
a very thin layer is required for devices with a high contrast and
viewing angle. Unlike liquid crystal displays, the image formed by
of a conducting polymer can have a high stability even in the absence
an applied field. The switching time achieved with such systems has
as low as 100 ms but a time of about 2 ms
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)12.
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.
1. Richard B. Kaner, Alan G. MacDiarmid, Scientific
America , February 1988, p60 - 65
2. Herbert Naarmann, Polymers to the Year 2000 and Beyond , John Wiley & Sons, 1993, Chapter 4
3. James Margolis, Conductive Polymers and Plastics, Chapman and Hall, 1989, P. 121
4. James Margolis, Conductive Polymers and Plastics, Chapman and Hall, 1989, P. 120
5. Andrew Barlow, Entone-OMI representative, 1997
6. Luis Alcacer, Conducting Polymers Special Applications , D. Reidel Publishing Company, 1987, p5
7. W. R. Salaneck D. T. Clark E. J. Samuelsen, Science and Application of Conducting Polymers, IOP Publishing, 1991, p135
8. W. R. Salaneck D. T. Clark E. J. Samuelsen, Science and Application of Conducting Polymers, IOP Publishing, 1991, p 55
9. Luis Alcacer, Conducting Polymers Special Applications , D. Reidel Publishing Company, 1987, p192
10. James Margolis, Conductive Polymers and Plastics, Chapman and Hall, 1989, P. 121
11. James Margolis, Conductive Polymers and Plastics, Chapman and Hall, 1989, P. 33
12. W. R. Salaneck D. T. Clark E. J. Samuelsen, Science and Application of Conducting Polymers, IOP Publishing, 1991, p52
13. W. R. Salaneck D. T. Clark E. J. Samuelsen, Science and Application of Conducting Polymers,IOP Publishing, 1991, p168
14. Tomorrow World, BBC TV., 1997