conducting polymers


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conducting polymers

  2. 2. Introduction to Polymers
  3. 3. Polymer basics <ul><ul><ul><li>Long chain like molecular structure where repeated molecular units are connected by covalent bonds </li></ul></ul></ul><ul><ul><ul><li>Polymers used as insulators eg. polyethylene </li></ul></ul></ul><ul><ul><ul><li>Variation in crystallization and orientation results in vast morphologies of polymers today </li></ul></ul></ul><ul><ul><ul><li>Properties of polymers: </li></ul></ul></ul><ul><ul><ul><li>- good chemical resistivity at room temperature </li></ul></ul></ul><ul><ul><ul><li>- low density and Young’s modulus </li></ul></ul></ul><ul><ul><ul><li>- brittleness at low temperatures </li></ul></ul></ul><ul><ul><ul><li>- can be stretched to form films </li></ul></ul></ul>
  4. 4. Organic polymers - few examples Polyvinyl Chloride (PVC) (-C 2 H 3 Cl-) n Vinyl chloride n(C 2 H 3 Cl) Polyethylene (-C 2 H 4 -) n Ethylene n(C 2 H 4 ) Polymer obtained Monomer unit
  5. 5. Classification based on temperature <ul><ul><ul><li>Two types - thermoplastic and thermosetting </li></ul></ul></ul><ul><ul><ul><li>Thermoplastic - soft and deformable upon heating , heating process is reversible , eg : linear polymers like PVC </li></ul></ul></ul><ul><ul><ul><li>Thermosetting - becomes hard and rigid upon heating , heating process is irreversible , eg : network polymers like phenol formaldehyde </li></ul></ul></ul>H OH + H OH O CH 2 + H 2 O
  6. 6. Discovery of conducting polymers <ul><ul><ul><li>Discovered in the late seventies (1977) by Alan Heegar , Dr. Hideki Shirakawa and Alan Macdiarmid </li></ul></ul></ul><ul><ul><ul><li>Before that polymers were used as insulators in the electronic industry </li></ul></ul></ul><ul><ul><ul><li>Advantages over conductors </li></ul></ul></ul><ul><ul><ul><li>Chemical - ion transport possible , redox behavior , catalytic properties, electrochemical effects, Photoactivity, Junction effects </li></ul></ul></ul><ul><ul><ul><li>Mechanical - light weight , flexible , non metallic surface properties </li></ul></ul></ul>
  7. 7. Conductivity <ul><li>Polymers become conducting upon doping </li></ul><ul><li>Polymer becomes electronically charged </li></ul><ul><li>Polymer chains generate charge carriers </li></ul><ul><li>Concentration of dopant causes certain electrons to become unpaired </li></ul><ul><li>Formation of polarons and bipolarons </li></ul><ul><li>They have extended p-orbital system </li></ul>
  8. 8. Classification of conducting polymers
  9. 9. Electron-conducting polymers Polyacetylene <ul><li>First conducting polymer to be synthesized </li></ul><ul><li>Best defined system </li></ul><ul><li>Reaction conditions allow to control the morphology of the polymer to be obtained as gel, powder, spongy mass or a film </li></ul><ul><li>Doped with iodine </li></ul><ul><li>Inherent insolubility and infusibility impose barriers to the processing of the polymer </li></ul>
  10. 10. <ul><li>Synthesized by </li></ul><ul><li>Dehydrohalogenations of vinyl chlorides: </li></ul><ul><li>Polymers prepared by this route have short conjugation length, structural defects and crosslinks </li></ul>
  11. 11. <ul><li>Precursor routes: Durham route </li></ul><ul><li>Polymers prepared by this route are continuous solid films, have controlled morphology range and can be stretched prior to conversion </li></ul>
  12. 12. Conduction mechanism <ul><li>R and L forms are interconverted through a charge carrier soliton </li></ul><ul><li>Soliton is a mobile, charged or a neutral defect or a kink in the polymer chain </li></ul><ul><li>It propagates down the polymer chain </li></ul><ul><li>For short chains Kivelson mechanism is involved </li></ul>
  13. 13. Travel of a soliton by bipolaron mechanism
  14. 14. Contrast between isomers of polyacetylene 170`C 10^-7 trans -77`C 10^-13 cis structure Obtainable temperature Conductivity (siemens/cm) isomer
  15. 15. Reasons of trans’ stability <ul><ul><ul><li>Two fold degeneracy </li></ul></ul></ul><ul><ul><ul><li>SOLITON formation due to symmetry </li></ul></ul></ul><ul><ul><ul><li>An unpaired electron at each end of an inverted sequence of double bonds </li></ul></ul></ul>
  16. 16. Stability(contd.) <ul><ul><ul><li>SOLITONS - Responsible for higher conductivity </li></ul></ul></ul><ul><ul><ul><li>Double bond next to a SOLITON may switch over to give rise a moving SOLITON which leads to conduction </li></ul></ul></ul><ul><ul><ul><li>In presence of many SOLITONS , their sphere of influence overlaps leading to conduction like metals </li></ul></ul></ul>
  17. 17. Doping in polyacetylene <ul><ul><ul><li>Amount of dopant used is significantly higher </li></ul></ul></ul><ul><ul><ul><li>Doped polyacetylene is always in tans form </li></ul></ul></ul><ul><ul><ul><li>Neutral polyacetylene can be doped in two ways </li></ul></ul></ul><ul><ul><ul><li>p type doping : oxidation with anions eg : ClO 4 (-) </li></ul></ul></ul><ul><ul><ul><li>n type doping : reduction with cations eg : Na(+) </li></ul></ul></ul>- e + ClO 4 (-) + ClO 4 (-) + e + Na(+) (-) Na(+)
  18. 18. Method of doping <ul><ul><ul><li>Chemical oxidants : iodine , nitronium species , transition metal salts </li></ul></ul></ul><ul><ul><ul><li>Chemical reducing agents : sodium naphthamide </li></ul></ul></ul><ul><ul><ul><li>Electrochemical methods : used dopants ClO4(-) , BF4(-) and other complex species </li></ul></ul></ul>
  19. 19. Doping with Iodine
  20. 20. Effect of dopant <ul><ul><ul><li>Conductivity - increases upto a certain doping level </li></ul></ul></ul><ul><ul><ul><li>Stability - decreases </li></ul></ul></ul><ul><ul><ul><li>Morphology : due to presence of charges shape will not be retained - reason why doped polyacetylene is always trans </li></ul></ul></ul>
  21. 21. Plot of conductivity vs doping <ul><ul><ul><li>Conductivity increases upto a certain doping level </li></ul></ul></ul>200 100 0.0 0.1 0.2 Doping level (dopant/CH unit) Conductivity (S/cm)
  22. 22. Polypyrrole <ul><ul><ul><li>Hetero atomic polymers </li></ul></ul></ul><ul><ul><ul><li>More stable </li></ul></ul></ul><ul><ul><ul><li>Easy to prepare </li></ul></ul></ul><ul><ul><ul><li>Greater opportunity to functionalize </li></ul></ul></ul>
  23. 23. Structure
  24. 24. Disadvantages of polypyrrole <ul><ul><ul><li>High cost </li></ul></ul></ul><ul><ul><ul><li>Difficult in processing </li></ul></ul></ul><ul><ul><ul><li>Lack of mechanical stability after doping </li></ul></ul></ul><ul><ul><ul><li>Difficult to fabricate </li></ul></ul></ul>
  25. 25. Various Applications
  26. 26. Coatings <ul><ul><ul><li>Prevents buildup of static charge in insulators </li></ul></ul></ul><ul><ul><ul><li>Absorbs the harmful radiation from electrical appliances which are harmful to the nearby appliances </li></ul></ul></ul><ul><ul><ul><li>Polymerization of conducting plastics used in circuit boards </li></ul></ul></ul>
  27. 27. Sensors(to gases and solns.) <ul><ul><ul><li>Polypyrroles can detect NO2 and NH3 gases by changing its conductivity </li></ul></ul></ul><ul><ul><ul><li>Biosensor : polymerization of polyacetylene in presence of enzyme glucose oxidase and suitable redox mediator like triiodide will give rise to a polymer which acts as glucose sensor </li></ul></ul></ul>
  28. 28. Polymeric Ferroelectric RAM(PFRAM) <ul><ul><ul><li>Uses polymer ferroelectric material </li></ul></ul></ul><ul><ul><ul><li>Dipole is used to store data </li></ul></ul></ul><ul><ul><ul><li>Provides low cost per bit with high chip capacity </li></ul></ul></ul><ul><ul><ul><li>Low power consumption </li></ul></ul></ul><ul><ul><ul><li>No power required in stand by mode </li></ul></ul></ul><ul><ul><ul><li>Isn’t a fast access memory </li></ul></ul></ul>
  29. 29. Biocompatible Polymers <ul><ul><ul><ul><ul><li>Artificial nerves </li></ul></ul></ul></ul></ul><ul><ul><ul><ul><ul><li>Brain cells </li></ul></ul></ul></ul></ul>
  30. 30. Batteries <ul><ul><ul><li>Light weight </li></ul></ul></ul><ul><ul><ul><li>Rechargeable </li></ul></ul></ul><ul><ul><ul><li>Example - Polypyrrole - Li & Polyaniline - Li </li></ul></ul></ul>
  31. 31. Displays <ul><li>Flat panels </li></ul><ul><li>Related problems : low life time & long switching time </li></ul>
  32. 32. Conductive Adhesive <ul><li>Monomers are placed between two conducting plates and it allows it to polymerize </li></ul><ul><li>Conducting objects can be stuck together yet allowing electric current to pass through the bonds </li></ul>
  33. 33. Current Status
  34. 34. Problem areas <ul><li>Reproducibility </li></ul><ul><li>Stability </li></ul><ul><li>Difficulty to process </li></ul><ul><li>Short life span </li></ul><ul><li>High cost </li></ul><ul><li>Difficult to fabricate in labs </li></ul>
  35. 35. New Developments <ul><li>Application to ‘Smart Structures’ </li></ul><ul><li>Conducting polymer nanowires </li></ul>