CONDUCTING POLYMERS

Introduction to Polymers

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

Organic polymers - few examples

Polyvinyl Chloride (PVC) (-C2H3Cl-)n

Vinyl chloride n(C2H3Cl)

Polyethylene (-C2H4-)n

Ethylene n(C2H4)

Polymer obtainedMonomer unit

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

HOH

+

HOH

O

CH2

+ H2O

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

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

Classification of conducting polymers

Electron-conducting polymersPolyacetylene

• 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

• Synthesized by

Dehydrohalogenations of vinyl chlorides:

Polymers prepared by this route have short conjugation length, structural defects and crosslinks

Precursor routes: Durham route

Polymers prepared by this route are continuous solid films, have controlled morphology range and can be stretched prior to conversion

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

Travel of a soliton by bipolaron mechanism

Contrast between isomers of polyacetylene

170`C10^-7trans

-77`C10^-13 cis

structureObtainabletemperature

Conductivity(siemens/cm)

isomer

Reasons of trans’ stability

Two fold degeneracy

SOLITON formation due to symmetryAn unpaired electron at each end of an inverted sequence of double bonds

Stability(contd.)

SOLITONS - Responsible for higher conductivityDouble 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

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(+)

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

Doping with Iodine

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

Plot of conductivity vs doping

Conductivity increases upto a certain doping level

200

100

0.0 0.1 0.2Doping level (dopant/CH unit)

Conductivity(S/cm)

Polypyrrole

•Hetero atomic polymers •More stable•Easy to prepare•Greater opportunity to functionalize

Structure

Disadvantages of polypyrrole

•High cost •Difficult in processing •Lack of mechanical stability after doping•Difficult to fabricate

Various Applications

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

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

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

Biocompatible Polymers

• Artificial nerves• Brain cells

Batteries

• Light weight• Rechargeable• Example - Polypyrrole - Li & Polyaniline - Li

Displays

• Flat panels• Related problems : low life time & long switching

time

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

Current Status

Problem areas

• Reproducibility

• Stability

• Difficulty to process

• Short life span

• High cost

• Difficult to fabricate in labs

New Developments

• Application to ‘Smart Structures’

• Conducting polymer nanowires