Dr H. Letheby was the one to first initiate the study of conducting polymers, who was a known professor teaching at the College of London Hospital. In 1862, he first attempted to analyze the behaviors of the chemical reaction and to select them accordingly. His study of electropolymerized aniline sulfate was published in the Journal of the Chemical Society. The results showed the aniline sulphate turning to a bluish black solid layer, formed on a platinum electrode after its electropolymerization (1).
It was between 1907 and 1911 that the Nobel Laureate Richard Willstatter characterized the oligomeric oxidation products of aniline through his methodic way of research (2).
By using a starter Al(Et)3/Ti(OPr)4, the polymerization of acetylene in hexane rendered a polyacetylene for the first time in 1958 in the form of a polymer with a highly crystalline and a heavy molecular weight produced by Natta et al. The method did not attract any attention owing to the highly air sensitive, infusible and the insoluble properties of the compound obtained(3).
The concept of iodine doping was established in the same time period of 1960's by the Czechoslovak researchers for polyaniline. This resulted in the conductivity of 1 S/cm for Polyaniline-iodine complexes.
The attention of researchers was diverted towards organic conductors, in spite of the discovery of the inorganic explosive polysulfurnitride (SN)x in the 1970's, with its additional properties of conductivity at low temperatures. A silvery film of polyacetylene was created in 1974 by the usage of Ziegler-Natta catalyst which was not conductive although it was found to be the closest to metals in terms of appearance (5).
The electronic conductivity of polyaniline was reaffirmed in 1974, a few years before the publishing of the polyacetylene's progress. The paper demonstrated interesting results of a parallel study with another conductive polymer also known as polypyrrole. The conductivity obtained was ranging from 5 to 30 S/cm (6).
One of the important moments was the publishing of the doped polyacetylene in 1977. The modification of the polyacetylene film via a partial oxidation treatment with the oxidizing agents such as the halogens and the AsF5 was obtained by MacDiarmid, Shirakawa and Heeger which resulted in the film becoming conductive (5).
Molecular Orbital and Molecular Structure of Conducting Polymers
All the valence electrons are utilized in covalent σ-bonds of the saturated polymers such as polyethylene. The material will show typical insulating properties owing to the gap between the valance band and the conduction band. Along the polymer's supporting structure, a π system is formed in the conjugated polymers (8). A restricted alternation of double and single bonds is required in the conjugated bonds which is also apparent in the conducting polymer's structure.
The carbon atoms create 3 σ-bonds with the adjacent atoms and the remaining p orbitals which is also explained as the pz orbitals. Nitrogen atoms are found to be involved in the conjugation path such as the polyaniline in some of the conjugated polymers (11).
High energy orbitals are created as a result of this configuration in which the electrons are loosely bonded to their respective atoms. The conducting, semiconducting or the insulating properties of the material are determined by the distance lying between the highest occupied molecular orbital (HOMO) and LUMO( the lowest unoccupied molecular orbital). The carrier movement or the jump from the HOMO to the LUMO creates the conduction mechanism. This becomes convenient if the distance between the HUMO and the LUMO energy is small.
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The sp2 hybridized linear carbon chains partially determines the conductivity of polyacetylene, which is the simplest form of semiconducting and metallic organic polymer. Six electrons are present outside the nucleus of the carbon atom, out of which 4 are valence electrons such as 2s and the 2p electrons, which takes part in the chemical bonds. The 1s and 2s orbitals of the carbon atoms are filled and the 2p orbitals are filled as well with 2 electrons in case of free space or where there is an existence of spherically symmetric distances. The creation of 3 sp2 and 1 p orbital is the result of hybridization. The bonding of 2 out of 3 sp2 orbitals on each carbon atom to another carbon atom adjacent to it and the bonding of last sp2 orbital with hydrogen or any of the side groups is the result. Covalent bonds are created between these atoms which in this case is referred to as 'σ bond'. It has a cylindrical symmetry around the internuclear axis. A 'π bond' [10] is formed by the overlapping of unhybridized p orbitals of the adjacent carbon atoms. The fig 2.1 shows both the σ and π bonds in the conjugated polymer structure.
There is weak interaction between them creating weak bonds of the π electrons which exposes them to the risks of delocalization thus causing electrical conductivity of the polymer (10). According to Hünkel and Bloch's simple free electron molecular orbital model theory, in case of a lengthy molecular chain showing metallic transport properties, the delocalization of π electrons over the entire chain forming a small band gap is seen. A conjugated polymer which has an alternation of double and single bonds, as a result, can be conductive in the right conditions. The delocalized electrons over the conjugated space are evenly distributed creating equivalent bonds (12). Differing bonding lengths of 1.54 nm, double bond: 1.34 nm) [9] are observed under simple conditions. The alternation of double and single bonds can also be observed.
A typical conducting polymer matches an insulator in conductive properties without the need of doping or the orbitals being filled with electrically conductive particles. The transformation of the conductivity of a conjugated polymer from the insulating level to the conducting level is seen through the doping process. By the process of electrochemical oxidation and reduction doping can be obtained or it can also be attained by the direct exposure of the polymer to a doping material. Atomic or molecular species have been used as dopants which are either electron acceptors such as I2, Br2, AsF5, and LiClO4 or electron donors like Li, Na, and K (9, 10).
2.3. Conductor, Semiconductor and Insulator
Solids such as metals have continuous orbitals. The electrons which are nearest to the filled levels can be excited and therefore move to the unoccupied levels without the requirement of any energy. As the temperatures increase the metals lose their conductivity in spite of the electrons being more excited. The electrons and the atoms colliding with each other result in the thermal motion of the atoms. The electrons lose their efficiency in transporting charges owing to the collisions. An energy gap separates the bands for transporting charges in semiconductors and insulators. Certain electrons gain enough energy as the temperature is increased, to move into the next unoccupied orbital. The metal becomes a semiconductor as the electrons are now mobile and create the electrical conductivity of the metal. In case of a large energy gap, the electrons may never be able to reach the conductive bands that will result in zero conductivity, such types of materials are known as insulators. While in semiconductor metals, the increase in the temperatures will also increase the conductivity as more electrons have the ability to reach the valence bands.
2.4Conductivity Mechanism
High electron affiliation or low oxidation potentials are the characteristics of conducting polymers. This means that the polymers can be reduced and doped with electrons donors (n-type) and also have the potential to be oxidized and doped with electron acceptors (p-type). The HOMO-LUMO energy gap which has the semiconductor property of conjugated polymers as compared to conventional inorganic compounds, are owing to the additional charges associated to the creation of new and unfilled electronic energy states existing within the original HOMO-LUMO energy gap.
A dopant's role is either the removal or the addition of electrons. For instance in the case of iodine (I2) an electron will be taken for the formation of I3 - ion. If the relocation of an electron in a semi conductor polymer by moving the electron from top most valence band such as the polyacetylene or polypyrrole, then as per the classical band theory, the creation of the vacancy hole as such will not be delocalized. A radical cation would be obtained in the case of the removal of an electron from a carbon atom. Owing to the Columbo attraction to its opposite ion (I3- ), the localization of the radical cation also known as the polaron takes place which in normal cases would be considered as having low mobility. This would be due to the local change in the geometry's equilibrium of the radical cation to the neutral molecule. The charge is moved along as shown in the fig 2.5 while the mobility of the polaron along the polyacetylene chain may be high. But a high concentration of opposite ions is necessary as the opposite ion (I3 -) to the positive charge is not very mobile, so that the polaron can move in the field of close opposite ions. The polymer which are mobile enough to conduct electric charges have the dopants generate polarons and bipolarons. The ionization of the conjugated polymer chain to a positive polaron (radical cation) is done by the dopants which is taken as an electron acceptor. These will further emerge as bipolarons or in some cases the two polarons may reversibly combine to create a bipolaron (16). Owing to its highly disordered structure the PPy can be given as an example for bipolaron formation. The formation of a bipolaron happens when two polarons are created on one similar chain (see fig 2.5)(17).
3.Poly (3,4-ethylenedioxythiophene) (PEDOT)
3.1. Introduction of PEDOT
The research in 1967 indicates polythiophenes for the first time as a potential conducting polymers. Furan, pyrrole, and thiophene heterocycles acid's catalyzed polymerization was studied by A. G. Davies. In 1982, Tourillon and Garnier first observed true electronic conductivity in polythiophenes. In the presence of perchlorate or tetrafluoroborate opposite ions, Thiophene was electropolymerized on platinum electrodes in acetonitrile. A conductivity of 10-100 S/cm was obtained in spite of the fact that highly conductive polythiophenes were possible from the start with Garnier and Tourillon's fundamental work. It has gained long term stability against air and humidity while it failed to fulfill one of the requirements expected of as a truly conductive polymer not as a semiconductor. In 1930's was initiated the EDOT (3, 4-ethylenedioxythiophene) chemistry when the corresponding 2, 5-dicarboxylic acid esters were synthesized. Thus biheterocyclic EDOT system consisting of one 1, 4-dioxane ring and one thiophene moiety was explained which was annelated over the carbon single ([c]-) bond of the thiophene.
The good chemical properties, high conductivity and good electro optical properties, the PEDOT has emerged as a promising compound (22). It has been used as an antistatic coating, photovoltaic technology, electroluminescent devices and biomedical sensors, etc (23).
Electrochemical polymerization of PEDOT
PEDOT was fabricated for the first time by Jonas et al (24) through anodical polymerization where the products showed high conductivity with enhanced chemical and thermal stability as compared to other polythiophenes. The polymerization occurs at the electrode in an electrochemical oxidative polymerization of a monomeric precursor of the conductive polymer. The ionic dopants in the process of polymerization, forming the electrolyte are induced in the polymer (25). The electrochemical polymerization is utilized in the case of EDOT monomer to create highly transmissive sky-blue, doped PEDOT film at the anode. The compatibility provided by the broad range of electrolyte solutions creates high stability of PEDOT films in different electrolyte solutions(26)
Flexible PEDOT films were obtained, by Yamato et al in the presence of polyanions in the electrolyte solution, with an electrical conductivity as high as 400 S/cm. Several electrolytes were used in this research such as sulfonated poly (β-hydroxy-ethers) (S-PHE), 1,3 bis(4-t-butylphenoxy)-2-propysulfate (BPS), poly (4-styrene sulfonate) (PSS), sulfated poly (β-hydroxy-ethers) having trifluoromethyl groups (S-PHEF), sulfonated poly (β-hydroxy-ethers) (S-PHE) and sulfonated poly (butadiene). However PEDOT film was deposited on the anode, in the presence of S-HPE, S-PBD and S-PHEF electrolytes.
Numerous advantages are present in the electropolymerization technique such as:
Materials required in small quantities.
Characterizations and speedy analysis.
Control of reactions is accurate.
While the disadvantages are:
In convenience in conducting standard analytical procedures owing to the small quantity of resultant with insoluble property.
The electrical conductivity can be increased to a large extent through the electrochemical polymerization while the product obtained has poor transparency and the process must be applied on conducting substrates which will restrict the application purposes (28).
3.2.2 Oxidative chemical polymerization
The deposition of a conjugated polymer by in situ oxidative polymerization on the surface is seen. Electrochemical polymerization has limited applications as compared to the chemical oxidation, which is more versatile. By coating the surface with a mixture of monomer and oxidant, chemical oxidation can be carried out with a mixture of the monomer and an oxidant where it enables the spreading of the mixture onto the surface first, owing to the suppression of the spontaneous reaction. These mixtures will have flexibility in their designing of the coating process with the separate application of the monomer and oxidant. These mixtures have a restricted pot life (29).
A PEDOT with black, insoluble and infusible properties is obtained with the utilization of oxidizing agents such as the FeCl3 in the oxidative chemical polymerization (21). De Leeuw et al. [20 used iron(III) tosylate (FeIII(OTs)3) as oxidizing agents in the presence of imidazole as a base leading to the conductivity of PEDOT of 550 S/cm.(30).
The classification of the reaction of EDOT with Iron (III) tosylate leading to in situ polymerization of PEDOT by Kirchmeyer and Reuter (31) into two categories were as:
The monomer subjected to Oxidative polymerization to form neutral polythiophene.
The neutral polymer subjected to oxidative doping to conductive polycation.
The addition of Lewis acids or protic acids to the polymerization system will cause catalysis of the equilibrium reaction of EDOT to the relevant dimeric and trimeric compounds in the absence of further oxidation, (see fig 2.9)(31).
Vapor Phase Polymerization
Mohammadi et al(32) did the initial experiments named as Chemical Vapor Deposition (CVD) process for polypyrrole polymerization. The oxidants used were FeCl3 and H2O2. In 2003 to 2005, J Kim et al first attempted to establish a new route for the highly conductive PEDOT layers with the process of vapor phase polymerization (VPP) (24). The evaporation of the EDOT and its polymerization on the substrate were done in the polymerization chamber where the deposition of the oxidant iron(III)-tosylate was found by bubbling the various types of gases like nitrogen, air and argon through the EDOT reservoir. FeIII tosylate was used as an oxidant and pyridine as a base-inhibitor, through the VPP process. By using oxidant, e.g., a butanol or ethanol solution of FeIII tosylate mixed with pyridine, the substrate coated with PEDOT film was covered. In a chamber flushed with air, nitrogen or argon gas, the EDOT monomers were heated into vapor phase. The polymerization process began and formed the PEDOT coating, once they react with oxidizing agents on the substrates.
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To prove the concept of using a base inhibitor, pyridine, Winther-Jensen et al conducted the experiments. The compound was applied to PET and Pt coated PET substrates after mixing the ferric tosylate solution with pyridine in the respective molar ratio of 1:0.5. Winther et al investigated some of the liquids such as pyridine, pyrazine and quinoline. The materials and Fe (II) were void of any sort of crystal formation. The VPP of EDOT was best suited to pyridine (pKa 5.14, boiling point 115 C°) as the base, owing to quinoline's low vapor pressure and the insufficient base characteristic of pyrazine [24]. Winther-Jensen et al measured the conductivity of PEDOT films as a function of temperature. A similar behavior as the semi conductors was observed as with the increase in temperature on which the conductivity also increased.
Winther-Jensen et al studied the stability of conductive PEDOT layers in air and aqueous solutions. The creation of PEDOT layer on glass substrates was conducted in this experiment, and the examination carried out in the environments mentioned earlier. Till the constant point a speedy reduction in the water conductivity was seen. A slow paced decrease in the conductivity of air as compared to the water environment was observed. The pH level determines the conductivity of PEDOT. Lower pH values resulted in highest conductivities. The product's acidity was observed in the range of pH 1, during the base inhibited VPP of EDOT. In water of pH7 the long term stability is lower as compared to air which takes more time to reach equilibrium with carbon dioxide (34).
There is no aqueous transport medium in VPP process. During the PEDOT layer formation no agglomeration was seen. The need for dispersants and stabilizer additives is negated in high processability. The process of polymerization is fast and simple (35).
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