conducting polymers

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)
(-C2H3Cl-)n
Vinyl chloride
n(C2H3Cl)
Polyethylene
(-C2H4-)n
Ethylene
n(C2H4)
Polymer obtainedMonomer 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
CH2
+ H2O

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`C10^-7trans
-77`C10^-13 cis
structureObtainable
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 : ClO4(-)
n type doping : reduction with cations eg : Na(+)
- e
+ ClO4(-) +
ClO4(-)
+ 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