Conducting polymers were discovered in the late 1970s and represent an important class of organic polymers that can conduct electricity. They become conductive through a process called doping, where their polymer chains take on charges. This allows for charge carriers called polarons and bipolarons to form, making the material conductive. Conducting polymers have advantages over traditional conductors including light weight, flexibility, and potential applications in areas like sensors, batteries, and displays. However, issues remain around their reproducibility, stability, processing difficulties, and high costs.

1. CONDUCTING POLYMERS

2. Introduction to Polymers

3. Polymer basics Long chain like molecular structure where repeated molecular units are connected by covalent bonds Polymers used as insulators eg. polyethylene Variation in crystallization and orientation results in vast morphologies of polymers today Properties of polymers: - good chemical resistivity at room temperature - low density and Young’s modulus - brittleness at low temperatures - can be stretched to form films

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. Classification based on temperature Two types - thermoplastic and thermosetting Thermoplastic - soft and deformable upon heating , heating process is reversible , eg : linear polymers like PVC Thermosetting - becomes hard and rigid upon heating , heating process is irreversible , eg : network polymers like phenol formaldehyde H OH + H OH O CH 2 + H 2 O

6. Discovery of conducting polymers Discovered in the late seventies (1977) by Alan Heegar , Dr. Hideki Shirakawa and Alan Macdiarmid Before that polymers were used as insulators in the electronic industry Advantages over conductors Chemical - ion transport possible , redox behavior , catalytic properties, electrochemical effects, Photoactivity, Junction effects Mechanical - light weight , flexible , non metallic surface properties

7. Conductivity Polymers become conducting upon doping Polymer becomes electronically charged Polymer chains generate charge carriers Concentration of dopant causes certain electrons to become unpaired Formation of polarons and bipolarons They have extended p-orbital system

8. Classification of conducting polymers

9. Electron-conducting polymers Polyacetylene First conducting polymer to be synthesized Best defined system Reaction conditions allow to control the morphology of the polymer to be obtained as gel, powder, spongy mass or a film Doped with iodine Inherent insolubility and infusibility impose barriers to the processing of the polymer

10. Synthesized by Dehydrohalogenations of vinyl chlorides: Polymers prepared by this route have short conjugation length, structural defects and crosslinks

11. Precursor routes: Durham route Polymers prepared by this route are continuous solid films, have controlled morphology range and can be stretched prior to conversion

12. Conduction mechanism R and L forms are interconverted through a charge carrier soliton Soliton is a mobile, charged or a neutral defect or a kink in the polymer chain It propagates down the polymer chain For short chains Kivelson mechanism is involved

13. Travel of a soliton by bipolaron mechanism

14. Contrast between isomers of polyacetylene 170`C 10^-7 trans -77`C 10^-13 cis structure Obtainable temperature Conductivity (siemens/cm) isomer

15. Reasons of trans’ stability Two fold degeneracy SOLITON formation due to symmetry An unpaired electron at each end of an inverted sequence of double bonds

16. Stability(contd.) SOLITONS - Responsible for higher conductivity Double bond next to a SOLITON may switch over to give rise a moving SOLITON which leads to conduction In presence of many SOLITONS , their sphere of influence overlaps leading to conduction like metals

17. Doping in polyacetylene Amount of dopant used is significantly higher Doped polyacetylene is always in tans form Neutral polyacetylene can be doped in two ways p type doping : oxidation with anions eg : ClO 4 (-) n type doping : reduction with cations eg : Na(+) - e + ClO 4 (-) + ClO 4 (-) + e + Na(+) (-) Na(+)

18. Method of doping Chemical oxidants : iodine , nitronium species , transition metal salts Chemical reducing agents : sodium naphthamide Electrochemical methods : used dopants ClO4(-) , BF4(-) and other complex species

19. Doping with Iodine

20. Effect of dopant Conductivity - increases upto a certain doping level Stability - decreases Morphology : due to presence of charges shape will not be retained - reason why doped polyacetylene is always trans

21. Plot of conductivity vs doping Conductivity increases upto a certain doping level 200 100 0.0 0.1 0.2 Doping level (dopant/CH unit) Conductivity (S/cm)

22. Polypyrrole Hetero atomic polymers More stable Easy to prepare Greater opportunity to functionalize

23. Structure

24. Disadvantages of polypyrrole High cost Difficult in processing Lack of mechanical stability after doping Difficult to fabricate

25. Various Applications

26. Coatings Prevents buildup of static charge in insulators Absorbs the harmful radiation from electrical appliances which are harmful to the nearby appliances Polymerization of conducting plastics used in circuit boards

27. Sensors(to gases and solns.) Polypyrroles can detect NO2 and NH3 gases by changing its conductivity 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

28. Polymeric Ferroelectric RAM(PFRAM) Uses polymer ferroelectric material Dipole is used to store data Provides low cost per bit with high chip capacity Low power consumption No power required in stand by mode Isn’t a fast access memory

29. Biocompatible Polymers Artificial nerves Brain cells

30. Batteries Light weight Rechargeable Example - Polypyrrole - Li & Polyaniline - Li

31. Displays Flat panels Related problems : low life time & long switching time

32. Conductive Adhesive Monomers are placed between two conducting plates and it allows it to polymerize Conducting objects can be stuck together yet allowing electric current to pass through the bonds

33. Current Status

34. Problem areas Reproducibility Stability Difficulty to process Short life span High cost Difficult to fabricate in labs

35. New Developments Application to ‘Smart Structures’ Conducting polymer nanowires