conducting polymers

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CONDUCTING POLYMERS

Transcript of conducting polymers

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CONDUCTING POLYMERS

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Introduction to Polymers

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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

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Organic polymers - few examples

Polyvinyl Chloride (PVC) (-C2H3Cl-)n

Vinyl chloride n(C2H3Cl)

Polyethylene (-C2H4-)n

Ethylene n(C2H4)

Polymer obtainedMonomer unit

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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

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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

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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

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Classification of conducting polymers

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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

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• Synthesized by

Dehydrohalogenations of vinyl chlorides:

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

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Precursor routes: Durham route

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

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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

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Travel of a soliton by bipolaron mechanism

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Contrast between isomers of polyacetylene

170`C10^-7trans

-77`C10^-13 cis

structureObtainabletemperature

Conductivity(siemens/cm)

isomer

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Reasons of trans’ stability

Two fold degeneracy

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

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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

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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(+)

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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

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Doping with Iodine

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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

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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)

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Polypyrrole

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

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Structure

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Disadvantages of polypyrrole

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

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Various Applications

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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

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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

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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

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Biocompatible Polymers

• Artificial nerves• Brain cells

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Batteries

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

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Displays

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

time

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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

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Current Status

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Problem areas

• Reproducibility

• Stability

• Difficulty to process

• Short life span

• High cost

• Difficult to fabricate in labs

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New Developments

• Application to ‘Smart Structures’

• Conducting polymer nanowires