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
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
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
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 : 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
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)
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
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
