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 combinations.

Electrical resistivity ,defined as follows, is an intrinsic property , inverse of which called conductivity. ρ =R A / L ...whereA=cross sectional area L=length of the objectσ =1/ ρ ...soσ =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 = n μ e • 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 n-doped through reaction with either an oxidant or a reductant. p-doping 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 polymers and inorganic semiconductors :

• 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