Conductive Polymers
Introduction
Materials can be divided into conductors, semiconductors and
insulators according to their electrical conductivity. Conductivity
borderlines between conductors, semiconductors and insulators
are fluent and not clearly defined. An overview with typical,
widely accepted ranges of conductivity for these three, are not
sharply separated, the different conductivity of materials is given
in Fig. (1).The most essential property that distinguishes
polymers from metals is electrical conductivity. The electrical
conductivity for metals is very high and is generally in the order
of 104 – 106 S/ cm (good conductors such as copper and silver
have conductivities close to 106 S/cm while for good insulator
such as quartz the conductivity as low as 10-18 s/cm ,
Semiconductors have conductivity ranging between conductors
and insulators .
Fig(1) : The conductivities of different materials
Polymers have traditionally been considered good electrical insulators and a
variety of their applications have relied on this property.Common polymers
such as teflon and polystyrene have conductivity value about 10-18 S/cm..
Due to the presence of partially filled energy bands, conductors have high
electrical conductivity ,in the same time energy bands of insulators and
semiconductors, are either completely filled or completely empty and so they
cannot
be
electrical
conductors
.
The gap between the highest filled energy level(valence band) and lowest
unfilled energy level(the conduction band) is called band gap (Eg).
There is no band gap in metals i.e., Eg=0 eV; large band gab in insulators and
intermediate band gab in semiconductors . The different band gabs in metals,
insulators and semiconductors are indicated in fig (2) , . In a semiconductor
there is a filled valence band and an empty conduction band at 0 K. Most
conventional polymers as an example of insulators have full valence bands
and empty conduction bands, which are separated from each other by a wide
energy gab.
Fig (2): Band gab in metals , semiconductors and insulators .
• Conductive Polymers :
•
Intrinsically conducting polymers are organic polymers that conduct
electricity .During the last years , The electrically conducting
polymers is called synthetic metals to signify their organic
characteristic and metal-like properties. due to combining the
electrical properties of a semiconductor and metals as well as the
advantages of conventional polymers such as easy synthesizing ,
preparation and fabrication ,greater workability, light weight,
resistance to corrosion and chemical attack , low cost and their
properties can be tailored to the required applications .
• Conductive polymers firstly
characterized
by controllable
conductivity and have special electrical and optical properties
comparable to those of metals and inorganic semiconductors . The
unique electronic structure is responsible for their , high electron
affinity and so there is no wander that the conductive polymers are
called as the materials of the 21s century . Fig(3) shows some of these
polymers. There will be important to mention that being a multi-phase
system in nature will result homogeneity lacking and reproducibility
has been an inherent weakness for conductively filled polymers. And
so, controlling the quality of dispersion to obtain homogeneous
conducting polymer composites is critically important.
fig (3) :Examples of conductive polymers
What is •
conductivity?
• Conductivity can be defined as the ability of the material to
pass the electrical current. In metals, electrones are free to
carry charge and the impedance to flow of charge is mainly
due to the electrons "bumping" in to each other. Insulators
are opposite to metals have tightly bound electrons and no
electron flow occurs so they offer high resistance to charge
flow. For conductance free electrons are needed .This can
be explained simply by Ohms Law.
• V= IR
• Where R is the resistance, I is the current and V is the
voltage present in the material. The conductivity depends
on the following :
• 1) number of charge carriers in the material
• 2) the mobility of charge carreir. The flowing charges
include either electrons , ions , charged holes ,and their
Electrical resistivity ,defined as follows, is an intrinsic
property , inverse of which called conductivity.
ρ =R A / L ...where
A=cross sectional area
L=length of the object
σ =1/ ρ ...so
σ =L / R A
• Conductivity depends on the number density
of charge carriers (number of electrons n) and
how fast they can move in the material
(mobility μ):
• σ=nμe
• where -e is the electron charge.
• Conductivity mechanism:
• The ground state of a carbon atom has six electrons arranged as 1s2
2s2 2p2,thats leads to four electrons in the outer electronic orbital.
In the presence of other atoms like( Hydrogen , Oxgen ... etc) these
levels may be hybridized either into sp, sp2 or sp3 orbitals,
Diamond for example had carbon atoms of sp3 hybridized and
containing only σ bonds, it is an insulater whereas graphite and
polyacetylene the carbon atoms exhibit sp2 hybridization and both
have mobile electrons. Graphite is known to be a conductor while
doped polyacetylene shows highly anisotropic metallic conductivity
.
• Certain classes of polymers exhibit semiconducting behavior and so
the conductivity of such polymers is the result of several processes.
These polymers is called conjugated polymers as they possess an
extended π -conjugation along the polymer backbone. For example,
in traditional polymers such as polyethylene, the valence electrons
are bound in sp3 hybridized covalent bonds. Such sigma-bonding
electrons have low mobility and so this will reduce the conductivity
largly ,while Conducting polymers possess a high degree of
conjugation along the polymer chain leading to increase the
electrical conductivity as it allows transferring the electrons (or
positive charges) along the polymer backbone effectively.
Conjugated double bonds in a molecule mean alternative single and
double bonds thus the electrons will be delocalized over the system
completely and so could shared by many atoms. This means that
the delocalized electrons can move around the whole system.
fig (4:a ): π and σ bonds in Ethylene molecule
• The semiconducting behavior of organic materials, including
π-conjugated polymers, stems from the sp2pz hybridized of
the carbon atoms. The valence electrons of carbon atoms in
ethylene molecules having conjugated π-electron systems are
sp2-hybridized .The characteristic spatial electron distribution
leads to an overlap of the pz as well as of the sp2 orbitals of
neighboring carbon atoms and thus result molecular π and σ
bonds, respectively . as shown in Figure (4).
• The key property of a conductive polymers is the presence of
conjugated double bonds through the backbone of the
polymer. In the case of conjugation, the bonds between the
carbon atoms are alternately single and double. Every bond
contains a localized sigma (σ) bond which forms a strong
chemical bond. and also contains a less strongly localized pi
(π) bond which is weaker. However, conjugation is not enough
to make the polymer material conductive. In addition – and
this is what the dopant does – charge carriers in the form of
extra electrons or ”holes” have to be injected into the material.
sigma (σ) bond can be defined as a covalent bond formed by orbitals
overlapping along the bond axis end to end while pi (π) bond can be
defined as a covalent bond formed by the parallel (side by side)
overlap of two p orbitals . pi (π) bond form the side by side overlap of
adjacent p orbitals above and below the sigma plane. While the (σ)
electrons are fixed and immobile due to the formation of covalent
bonds between the carbon atoms, the remaining pi (π) electrons can be
easily delocalized upon doping . The σ-bonds will hold the structure
of molecules together while the π-bonds are responsible for their
semiconducting properties .The presence of the
extended π
conjugation however, imparts the necessary mobility to the charges
created which are formed on the polymer backbone (by the process
of doping) and make them electrically conductive.
fig (4:b): simplified schematic of a conjugated backbone chain
containing alternating single and double bonds
• The common polymers mainly consists of σ bands and the
hybridization of each atom of carbon is sp3. The high energy gap (E
gap > 6 eV) between the bonding band and antibonding band makes
these materials act as insulators. On the other side , the conductive
polymers consists of atoms of carbon hybridized sp2, that form three
σ bonds, and a pz orbital that allows a π overlapping with the pz
orbital of the adjacent carbon .
• The presence of these conjugated double bonds produces two bands,
that similarly to the metals can be called valence band and
conduction band. The energy band that results from the bonding
orbitals of a molecule is known as the valence band, while the
conduction band is as a result of the antibonding orbitals and the
width of individual bands across the range of energy levels is called
band width . The metals have high conductivity due to the free
movement of electrons through their structure and polymers are
considered as electronically conductive because they possess not only
charge carriers but also an orbital system that allows the charge
carriers to move.
Generally if the valance band was half filled ,comprised from a
continuous delocalized π-system, this considered as an ideal
condition for conductivity. The π- conjugated polymer could
reduce its energy efficiently by bond alteration which mean (
alternating single and double bonds ) and so introduce a band
width of about 1.5 eV making the conjugated polymers good
semiconductor . It has been found that conjugation alone is not
enough to make the polymer conductive or semi conductive .
Dopants as well as the formation of charge carriers in the polymer
chains are also essential features leading to exhibit conductive
properties.
• Doping:
Doping is a process by which the polymer is either oxidized or reduced to create
charge carriers.The concept of doping is the unique, central, underlying, and
unifying theme which distinguishes conducting polymers from all other types of
polymers. During the doping process, an organic polymer, either an insulator or
semiconductor having a small conductivity, typically in the range 10-10 to 10-5
Scm-1, is converted into a polymer which is in the metallic conducting regime (1
to 104 Scm-1) . The conjugated polymers in the undoped, original state are
semiconductors or insulators due to the large energy gap more than(2 eV) .
Therefore, undoped conjugated polymers, such as polythiophenes , polyacetylenes
have a low electrical conductivity of around 10-10 to 10-8 S/cm. Even at a very low
level of doping less than (1 %) , electrical conductivity will increase several times
of around 10-1 S/cm. Subsequent doping of the conducting polymers cause
saturation in the conductivity at values around 103-104 S/cm for different polymers.
conductive.
• Conducting polymers usually on two types either p-doped or ndoped through reaction with either an oxidant or a reductant. pdoping is partial oxidation of the polymer chain with electron
acceptors (e.g. I2, AsF5 ) , while n-doping is partial reduction with
electron donors (e.g. Na, K), of the π-backbone of the polymer. The
dopants either removes (p-doping) or adds (n-doping) electrons to
the polymer, which adds extra holes or electrons to the polymer
chain, these charge carrier move freely along the polymer and thus
cause the polymer to become conductive.
• The movement of charge carriers along the conjugated backbone
produces electrical conductivity. The smaller distance between the
conducting band and valence band (band gap) the higher
conductivity state, Dopant, oxidation level , doping percentage ,
synthesis method and also temperature are important factors on
the band gap and so the conductivity of the conductive polymers .
• Difference between conductive
inorganic semiconductors :
polymers
and
• The conducting polymers are different form inorganic semi–
conductors in two important structural categories :
• 1) Polymers are molecular in structure and lack the long-range
order .This molecular behavior makes the electronic motion
through the individual macromolecules one dimensional .
• 2) Doping in polymers is a charge transfer reaction and so the way
to induce conductivity in both are different. In the doping of
inorganic semiconductors; the dopant species occupies positions
within the lattice of the host material leading to a large-scale
change in the conductivity of the doped material compared to the
undoped material and lead to electron rich or electron deficient
sites without charge transfer occurring between the two sites,
where as the doping reaction of polymer lead to partial oxidation or
reduction of polymer. The doping in conjugated polymers is
interstitial whereas in inorganic semiconductors the doping is
substitutional. In the doped state, the backbone of a conducting
polymer consists of highly delocalized π electrons.
• In the inorganic semiconductor removing or adding electrons can
be done by several ways, for example by photo excitation or by
adding impurities (dopants). Electrons and holes are the charge
carriers responsible for conductivity in these materials. If the
impurity added an electron to the conduction band (CB), this is
called n-type doping. On the contrary, if the impurity removed an
electron from the valence band (VB), producing a hole. This hole is
positive, it interspersed across the material with unique electrical
properties and this is called p-type doping.
fig (6): show the difference between the doping mechanism in inorganic
semiconductors and conductive polymers
• In the conductive polymers , conductivity is associated with
spinless charge carriers. Starting from the band theory , it is
possible to define two quantities: the ionization energy and the
electronic affinity . The ionization energy is the energy necessary
for removing an electron from the valence band. While the
electron affinity is the energy necessary for capturing an electron
in the conduction band. Generally, conducting organic polymers
are characterized by small ionization energy and large electronic
affinity, that easily leading to the oxidization (n-type doping) or
the reduction (p-type doping) the system.
• because the inorganic semiconductors are strict, they maintain their
structure , in the same time, conducting organic polymers are
characterized by low coordination and high flexibility and ability to
structural distortions. For these reasons, adding or removing the
charges will lead to distortion which is considered to be
energetically favored due to allowing the stabilization of the
charges.
• Dopands :
• Dopands can be diffiened as oxidizing or reducing agents that
effect strongly on the conductivity phenemina of conductive
polymers . These dopands create postively or negatively charge
carriers in polymers.
• The first case: Polymer + Dopant → [Polymer +- Dopant -]
•
(Acceptor) charge transfer complex
• This is called Oxidation process ( p- doping) and the dopands
in this process is called (acceptors) . I2, Br2, AsF5.....etc are
good examples for this type of dopands .
• The second case: Polymer + Dopant → [Polymer- - Dopant+]
•
(Donor) charge transfer complex
• This is called reduction process (n- doping) and the dopands is
called (donors), for example Na, Li, K....etc.
• Types of dopands agents :
• Dopands may be neutral molecules and chemical compounds
or inorganic salts that can easily form ions, organic and
polymeric dopants. The nature of dopants is very impotant
factor in the stability of conductive polymers .
• Dopants could be classified as:
• 1- Neutral dopants: They are dopants converted into negative
or positive ions with or without chemical modifications during
the process of doping for example I2, Br2, AsF2, Na, K,
H2SO4, FeCl3 etc.
• 2- Ionic dopants: These dopands are oxidized or reduced via
electron transfer with the polymer and the counter ion remains
with the polymer to make the system neutral, for example
LiClO4, FeClO4, CF3SO3Na, BuNClO4 etc.
• 3- Organic dopants: These are anionic dopants confederated
into polymers from aqueous electrolytes during anodic
deposition of the polymer for example CF3COOH, CF3SO3Na
• 4- Polymeric dopants: PVS, PPS.
• Different methods of doping :
• As indicated in fig (7) ,there are several methods of conductive polymer
doping .
Fig ( 7 ) : Different methods for doping conducting polymers
• Synthesis and processing of conductive polymers:
• 1) chemical synthesis:
• During this process the monomer solution is mixed with
an oxidizing agent
for example
(ferric chloride,
ammonium persulfate) . This process produces a powder
or a thick film of the polymer, and also used for the bulk
production, that makes this method the choice for
commercial applications . chemical synthesis is applied for
all types of conductive polymers, including the polymers
that is not able to be synthesized by the other methods
Unfortunately, the conductivity of the polymers by using
this process is
lower than their electrochemically
synthesized counterparts .
• Also the conductivity of the produced polymers is known
to be very effected to the type of the solvent and the
oxidant, the reagents concentration , reaction time,
temperature, stirring rate, etc., these factors made
chemical synthesis a difficult process .
• 2)Electrochemical polymerization :
• This process occurs by applying an electrical current
through electrodes placed into the solution which contain the
polymeric monomer , the solvent and the doping agent .This
method characterize by high controlling on thickness
deposition and morphology of the produced thin film (down
to 20 nm)
• The electrical current lead to deposit and oxidize the
monomer to on the positively charged working electrode
producing insoluble polymer chains .The properties of the
produced film will be effected by the deposition charge and
time, the temperature, the solvent, the doping agent and the
electrode system. This process is used for synthesis of the
polymer due to condition : if the monomer can undergo
oxidation process in the presence of an electrical potential
• 3- Composites:
• One way to compensate for the shortcomings of a conductive
• polymer is to use it together with another polymer,
combining the positive qualities of both materials
• 4- Electrospinning:
• Electrospinning is a versatile process that allows the
production of nano- and micrometer-scale fibres from a wide
range of polymers During electrospinning, a high-voltage
electrostatic field is used to draw a jet from a polymer
solution . As this jet travels toward a collector electrode, the
solvent evaporates and a polymer fibre is formed
• 5- Hydrogels
• Conductive polymers have also been successfully
polymerized inside hydrogel networks . This allows the
creations of electroactive hydrogels, which combine the
redox switching capabilities of conductive polymers with the
fast ion mobility and biocompatibility of hydrogels