Sandra Plaza García. Departamento de Física de Materiales, Facultad de Ciencias Químicas, Universidad del País Vasco (UPV)

Transcription

1 DOPING AND CONDUCTING POLYMERS Sandra Plaza García Departamento de Física de Materiales, Facultad de Ciencias Químicas, Universidad del País Vasco (UPV) Donostia International Physics Center (DIPC)

2 OUTLINE Introduction The Discovery of Conductive Polymers Doping Process Factors that influence the Conductivity Applications Conclusion Bibliography

3 INTRODUCTION We are used to polymers (plastics) being somehow the opposite of metals. They insulate, they do not conduct electricity. Yet Alan J. Heeger, Alan G. MacDiarmid and Hideki Shirakawa have changed this view with their discovery that a polymer, polyacetylene, can be made conductive almost like a metal. Alan J. Heeger Alan G. MacDiarmid Hideki Shirakawa

4 Plastic can, under certain circumstances, be made to behave like a metal - a discovery for which Alan J. Heeger, Alan G. MacDiarmid and Hideki Shirakawa received the Nobel Prize in Chemistry How can plastic become conductive? Plastics are polymers. In becoming electrically conductive, a polymer has to imitate a metal (its electrons need to be free to move and not bound to the atoms). 2 2 Conditions to become conductive: 1st condition the polymer has to consist of alternating single and double bonds, called conjugated double bonds.

5 In this case the polymer is Polyacetylene, prepared through polymerization of the hydrocarbon acetylene, and it has such a structure: Polyacetylene Every bond contains a localised sigma (σ) bond which forms a strong chemical bond. In addition, every double bond also contains a less strongly localised pi (π) bond which is weaker.

6 In polydienes, such trans-polyacetylene, each C is sp2 hybridized this polymer can be treated as a one- dimensional analogue to graphite. However, while in the graphite layers the C-C C bond lenghts are equivalent, in polyacetylene the backbone bond lengths are alternately slightly longer and slightly shorter. This is due to the so-called Peierls distortion.

7 2nd condition the plastic has to be disturbed - either by removing electrons from (oxidation), or inserting them into (reduction), the material. The process is known as doping. Oxidation with halogen (p-doping): [CH] n + 3x/2 I 2 --> [CH] x+ n + x I 3- Reduction with alkali metal (n-doping): [CH] n + x Na --> [CH] x- n + xna +

8 Simulation of a simple model of a doped polymer The pieces cannot move unless there is at least one empty "hole (position where an electron is missing ). In the polymer each piece is an electron that jumps to a hole vacated by another one. This creates a movement along the molecule - an electric current.

9 THE DISCOVERY OF CONDUCTIVE POLYMERS 1958 Preparation of Polyacetylene by Natta and co-workers ( acetylene + hexane + Et 3 Al/Ti(OPr) 4 catalyst ) 1970 Shirakawa and co-workers adapted the method to make well-defined films of polyacetylene. ( Synthesis by controlling the proportions of cis- and trans-isomers in the black polyacetylene film ) Once (by mistake) too much catalyst was added a beautiful silvery film (trans-polyacetylene )

10 On the other hand, MacDiarmid and Heeger were experimenting with a metallic-looking looking film of the inorganic polymer sulphur nitride, (SN)x. MacDiarmid and Shirakawa set about modifying Polyacetylene by oxidation with iodine vapour Change of optical properties. MacDiarmid and Shirakawa in collaboration with Heeger measurement of the conductivity of the iodine-doped doped trans- polyacetylene (increase ten million times) They discovered that a thin film of polyacetylene could be oxidised with iodine vapour, increasing its electrical conductivity and being more conductive than originally. Polyacetylene the first plastic with electrical conductivity.

11 DOPING PROCESS Treatment with halogen was called doping. It consists of atomic pollution with different atoms from those of the original structure altering this way the energetic profile of the bands catching (conductivity type p) or yielding e - (conductivity type n). With doping process you can get conductivity type p (catching e -) conductivity type n (yielding e -) Once doped and turned into conductive polymer a change is produced in the positions of the atoms due to the introduction of load.

12 DOPING - FOR BETTER MOLECULE PERFORMANCE What exactly happened in the polyacetylene films? Doped polyacetylene is, e.g., comparable to good conductors such as copper and silver, whereas in its original form it is a semiconductor. Conductivity of conductive polymers compared to those of other materials, from quartz (insulator) to copper (conductor). Polymers may also have conductivities corresponding to those of semiconductors.

13 FACTORS THAT INFLUENCE THE CONDUCTIVITY Doped Polyacetylene: Conductivity along the chain» Conductivity perpendicular to the chain Conductivity increases if: - chains parallel aligned - purity - presence of doping materials (additives that facilitate the polymer conductivity) - density of charge carriers (number of electrons n) } - mobility (µ) σ = n µ e - T ( metallic materials) - T (semiconductors and insulators)

14 The conductivity of conductive polymers decreases with falling temperature in contrast to the conductivities of typical metals, e.g. silver, which increase with falling temperature.

15 - Polyacetylene: semiconductor if T, conductivity - doped Polyacetylene: conductor if T, conductivity due to the decomposition of the doped one.

16 APPLICATIONS Conductive plastics are used in: anti-static substances for photographic film corrosion inhibitors compact capacitors antistatic coating shields for computer screen against electromagnetic radiation and for "smart" windows that can vary the amount of light they allow to pass and can exclude sunlight Semi-conductive polymers have recently been developed in: transistors light-emitting diodes solar cells displays in mobile telephones and mini-format television screens

17 Shield for computer screen against electromagnetic radiation "smart" windows Photographic Film

18 CONCLUSION The biggest advantage of conductive polymers is their processibility. Polyacetylene (original form) is semiconductor (NOT conductor) conjugated double bonds (bonds not equal). POLYACETYLENE DOPED POLYACETYLENE (Semiconductor) (Conductor) Conductive polymers: plastics & organic polymers mechanical properties (flexibility, toughness, malleability, elasticity, etc.) of plastics + high electrical conductivities of a doped conjugated polymer. Conductive polymers (plastics & organic polymers) mechanical properties high electrical conductivities

19 BIBLIOGRAPHY H. Shirakawa, E.J. Louis, A.G. MacDiarmid, C.K. Chiang and A.J. Heeger, J Chem Soc Chem Comm (1977) 579 T. Ito, H. Shirakawa and S. Ikeda, J.Polym.Sci.,Polym.Chem. Ed. 12 (1974) C.K. Chiang, C.R. Fischer, Y.W. Park, A.J. Heeger, H. Shirakawa, E.J. Louis, S.C. Gau and A.G. MacDiarmid, Phys. Rev. Letters 39 (1977) 1098 C.K. Chiang, M.A. Druy, S.C. Gau, A.J. Heeger, E.J. Louis, A.G. MacDiarmid*, Y.W. Park and H. Shirakawa, J. Am. Chem. Soc. 100 (1978) 1013 Evaristo Riande and Ricardo Díaz-Calleja, Electrical Properties of Polymers ml