chapter 1 introduction to conducting...

Chapter 1 Introduction to Conducting Polymers

This chapter deals with the historical background and begins with some basic information about conducting polymers. A survey of important research works in the field of conducting polymers, in particular polyaniline is presented. This includes the synthesis, processing, optical, structural, morphological properties charge creation and its transport and device applications, etc. The chapter ends by highlighting objectives of the present investigations

Chapter 1: Introduction to conducting Polymers

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CHAPTER 1

INTRODUCTION TO CONDUCTING POLYMERS

1.1 INTRODUCTION Polymer is a high molecular weight generally organic compound formed by chemical reaction (polymerization) of monomers [1] or the continued reaction between lower molecular weight polymers or oligomers. It can be natural or synthetic. Natural polymers include proteins cellulose (polymer of sugar molecules) and (polymer of amino acids).Synthetic polymers are man made materials they include insulating polymers as well as intrinsically conducting polymers. There are many examples of synthetic polymers: (i) insulating polymers (IPs)- polytetrafluoroethylene (PTFE or Teflon), polyvinyl chloride (PVC), Nylons, Polyethylene terephthalate, polythene, rubbers, etc., and (ii) intrinsically conducting polymers (ICPs): polyaniline (PANI), polyacetylene (PA), polyparaphenylene (PPP), polypyrrole (PPy), polythiophenes (PTs), polyparaphenylenevinylene (PPV) etc. The best known property of these intrinsically conducting polymers is their ability to conduct electric current after being partly oxidized or reduced [2]. The evolution of conducting polymers began in 1975 with the discovery of a linear conjugated organic polymer polyacetylene by Hideki Shirakawa. However, its quick degradation marred its applications [3]. This gave rise to a surge of activity directed towards the exploration, synthesis and characterization of this class of materials, also known as ‘Synthetic Metals or conjugated polymers’ [4]. In recognition of this path-breaking discovery Hideki Shirakawa, Alan G. MacDiarmid, and Alan J. Heeger were jointly awarded the 2000 Nobel Prize in Chemistry. 1.1.1. Conducting Polymers In 1977 Heeger, MacDiarmid and Shirakawa showed that PA, which is the simplest polyconjugated system, can be made conductive by reaction with bromine or iodine vapors. Spectroscopic studies, revealed that this reaction is redox in nature. These redox reactions transform neutral polymer chains into polycarbocations with simultaneous insertion of the corresponding number of Br3- or I3- anions between the polymer chains and hence neutralize the positive charge generated on the polymer


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chain. This important discovery initiated an extensive and systematic research of various aspects in the chemistry and physics of conjugated polymers both in their neutral (undoped) and charged (doped) states. In most of the cases, polymers are insulators in their neutral state and they become conducting only after introduction of electron acceptors/donors by a process known as ‘doping’. The conductivity of these polymers can be tuned by chemical manipulation of various factors such as the nature of the dopant, the degree of doping and blending with other polymers. The development of polymers with their respective conductivity range comparable with semiconductors and metals have been presented in Fig. 1.1.

Figure 1.1: Conductivity of some metals, semiconductors and doped conducting polymers

Conducting polymer is highly delocalized π-electron system with alternate single and double bonds in the polymer backbone. The π-conjugation of the polymer chain generates high energy occupied molecular orbitals (HOMO) and low energy unoccupied molecular orbitals (LUMO) leading to a system that can be readily oxidized or reduced [5]. Undoped conjugated polymers are semiconductors with band gaps ranging from 1 to 4eV, therefore their room temperature conductivities are very low, typically of the order


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of 10−8 S/cm or lower[6]. During the doping process, an undoped polymer having low conductivity, typically in the range of 10−10 to 10−5 S/cm, is converted to doped polymer, which is in a ‘metallic’ conducting regime (1 to 104 S/cm). The highest value reported till date has been obtained for iodine- doped polyacetylene (PA) (>105 S/cm). The predicted theoretical value of conductivity is about 2 × 107S/cm for doped PA which is more than that of copper [3]. Conductivity of other conjugated polymers reaches up to 103 S/cm [7-10] as shown in Fig 1.1. Recent advances in the field of conducting polymers have led to a variety of materials with great potential for commercial applications such as rechargeable batteries [10-12], light emitting diodes (LEDs) [13-15], field-effect transistors (FETs) [16], solar cells [17-19], EMI shielding [20-24], electrostatic charge dissipation [25,26], electrochromic devices [27-31], supercapacitors [32- 34], artificial muscles [35-37], corrosion control [38-40] and sensors [41-43], etc. Fig. 1.2 shows the chemical structure of some of the important conducting polymers in their neutral insulating state.

Figure 1.2: Chemical structure of some undoped conjugated polymers


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These polymers have many versatile applications. In the present thesis, the use of conducting polymers in electrostatic charge dissipation and electrochromic device applications has been studied and discussed. 1.2 ORIGIN OF CONDUCTIVITY: A CONSEQUENCE OF DOPING The conductivities of the undoped electronic polymers vary from insulating to conducting rigime through the method of doping, with the increase of conductivity as the doping level increases [45-47,48-50]. Both n-type (electron donating) and p-type (electron accepting) dopants; n-type → Na, K, Li, Ca and p-type → I2, PF6, BF4, ClO4-, AsF6, CSA and FeCl3, etc. dopants have been utilized to induce an insulator-to-metal transition in electronically conducting polymers. The doping procedures are different from conventional ion implantation techniques used for ppm level doping in conventional semiconductors. The doping process for these polymers is carried out typically by electrochemical means or by exposing the films or powders to vapours or solutions of the dopants[36]. In some cases, the polymer and the dopant are dissolved in the same solvent before forming the film or the powder [50]. In the conventional inorganic semiconductor the subsitutional doping occurs in which each atom has covalent bonds to its neighbours (coordination number 4,6 etc) in a three dimensionally. Solid structure and conductivity can be describe as excitations of electrons or holes. While in electronic polymers, the atomic or molecular dopant ion is placed interstitially between the chains and donate or accept charges from the polymer backbone [44,51,52]. Vander waal’s forces are attractive between the chains, at least in nonpolar systems. Even if conduction exists in practice between the chains, this type of conductor is classified as “quasi-one dimensional” (quasi-1D). In these the interatomic spacing is atoms seems to be more easily influenced than in the solid structure of conventional semiconductors [44,45]. There exists a rich variety of these structures occurring for different dopant levels and variation in the processing routes along with a wide range of degree of local order [53,54]. Due to doping the effect of negative or positive charges initially added to the polymer chain, does not immediately fill the rigid conduction band (lowest unoccupied molecular orbital or LUMO level) or valence band (highest occupied molecular orbital or HOMO level), causing metallic behavior. The strong coupling between the electrons and


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the phonons (vibrations) causes distortion of the bond lengths in the vicinity of the doped charges [50]. This causes as a consequence of doping defect states originate. For the simplest conducting polymer, i.e. for undoped trans PA these defects have been termed as “solitons” [55]. The concept of soliton has been derived from quantum mechanics to solve the nonlinear differential equations rather than numerical approximation analytically and to associate new quasi-particles with exact solutions. The essential feature of polyacetyline system is that the alternate bond should be double bond, if the system is disturbed at the one site, the whole system is affected. Theoretically it has been shown that the perturbation of the conjugation propagates like solitary waves in water. Also conjugation leads to interesting symmetry and bears some analogy to the behavior of elementary particles therefore, these conjugational defects can be termed as “solitons”. There are a number of ways to represent this phenomenon, first from the chemistry point of view and secondly from the viewpoint of solid state Physics. The chemist looks at a conjugated system as an alternating single and double bond while the physicists see a charge density wave because a double bond contains more electrons than a single bond. Therefore, the electron density oscillates periodically along the polymer chain. Precisely this phenomenon of charge density wave is termed as bond order wave in PA. The charge density wave of the electron modifies the lattice of positive ions. To establish the local charge neutrality, the ions move to region of higher electron density i.e. in to the regions of the double bonds. This makes the double bond shorter than the single bonds. Hence a distortion in the lattice occurs which is called a perierls distortion. A soliton can be viewed as excitation of the radical from one potential well to another well of the same energy (Fig.1.3 degenerate polyacetylene). The change in the band structure arising from the peierls transition is shown in Fig. 1.4, in semiconductor physics, the lower level is called valence band and the upper empty band, the conduction band. In chemistry the terms π and π* band are common, the former contains the bonding and the latter the antibonding states. HOMO is at the edge of the valence band and at the edge of the conduction band is the LUMO.


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Figure 1.3: Energetically equivalent forms of polyacetylene

Figure 1.4: Top: schematic illustration of the geometric structure of a neutral solitons on a trans-polyacetylene chain. Bottom: band structure for a trans-polyacetylene chain containing a neutral soliton, positively charged soliton and negatively charged soliton. The energy gap is a consequence of pattern of alternating single (long) and double (short) bonds [48-50] with an additional contribution due to coulomb repulsion [56,57]. The interchange between short and long bonds results in an equivalent (degenerate) ground state stabilized by electron-phonon interaction [55,56,57] poly (1,6 heptadiyne) and the PNB oxidation state of PAN[58-60] and their derivatives also have degenerate ground state i.e. single and double bonds (benzenoid and quinoid


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rings for PNB polymer) can be interchanged without affecting the ground state energy. Photoexcitation also leads to generation of neutral solitons in polymers having degenerate ground state [83-84] and neutral excitations in polymers having non degenerate ground state [84-91] which is important in case of polymer based light emitting diodes (LED). At high doping levels of trans PA, the solitons energy levels essentially overlap the filled valence and empty conduction bands, leading to a conducting polymer [92,93] for non-degenerate polymers, high doping levels results in polaron interaction to form, “polaron lattice” partially filled bands [94-96]. Some models suggest equilibrium between polarons and bipolarons [73,77,80]. In contrast to the n- and p- type doping process applicable to PA, PPY, PT and LEB, etc, for PANI-EB form, the conductivity varies with the proton (H+ ion) doping level (protonic acid doping) in the protonation process, there is no addition or removal of electrons to form the conducting state [94] Polyparaphenylene, polyaniline, polypyrrole, polythiophene. Polyfuran, etc., have nondegenerate ground state[35]. The two resonance forms of polyaniline benzoid and quinoid are not energetically equal. The quinoid form has energy state greater than benzoid form by 0.4 eV per ring[35]. So the solitons cannot be the expected solution of the wave equation in these polymers[29,36]. Here polarons and bibolaarons are considered to be the primary charge species in these conducting polymers created during doping. Because of the lower ionization potential and high electron affinity on doping quionid structure for polyaniline is expected. The schematic representation of polaron and bipolaron is shown in Fig.1.5. In solid state physics, the concept of polaron is very familiar. It consists of an electron or hole localized to a deformed region of lattice. It is the coulomb interaction between the electron and ions which is responsible for lattice distortion and breaking translational symmetry and results in a localized wave equation occupied by electron and the electron is said to be self trapped. The similar situation exists in conjugated polymers. In this case lattice deformation consists of a tendency for the double bonds to become longer, more like single bonds and vice versa. Hence two localized states are formed, one split off from top of valence band and onefrom the bottom of conduction band[36].


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Figure 1.5: Schematic representation of polaron and bipolaron on polyaniline chain

If the defect is formed by removal of electron, then the lower state is singly occupied and is called hole polaron. If the electron is injected then it results in a single occupation of upper level. In either case there is a net charge of one unit and the odd electron contributes to spin 1/2. In case of acceptor doping or oxidation as in case of polypyrrole, further charge transfer from polymer chain with one hole polaron, may proceed either by formation of another polaron or removing single electron in the lower polaron level, thereby forming doubly charged spinless species which is called a bipolaron. Formation of polarons and bipolarons leads to the new energy levels in the band gap. Both inorganic acids such as hydrochloric acid (HCl) and organic acids such as camphor sulphonic acid are effective with the organic sulphonic acid leading to solubility in a wide variety of organic solvents such as chloroform and m-cresol [97,98]. The protonic acid may be also covalently bound to the polymer backbone as has been achieved in water soluble sulphonated polyaniline [99]. Similar electronic behavior has been observed for protonic acid doped PANI as is the case for other non-degenerate ground state system [94,95,100-103] in which the polarons are formed at low doping levels and for doping in to highly conducting state. A polaron lattice

Chapter 1:

(partially filled energy band)[94,95,104in less orderd region of the doped polymer[105

1.3 POLYANILINEPolyaniline was first prepared in 1862, by H. Letheby of the college by anodic oxidation of as a heat resistant paint, among other things. Its structure was not describof the 20th century, when it was noted that it exists incolours. It was in 1968 that M. Jozefowicz discovered that the conductivity of the polymer increased by orders of magnitude the discovering polyacetyline and subsequently other polymers 1970’s it has become next interestingly studied conducting polymer. Since intensively, as it is one of the most versatile conducting polymers up to approximately 80in a film or powder form emeraldinesalt and pernigranilinestates. The chemical structure of there forms is shown in

Introduction to conducting Polymers

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partially filled energy band)[94,95,104] form polaron pairs or bipolarons are formed d region of the doped polymer[105].

POLYANILINE Polyaniline was first prepared in 1862, by H. Letheby of the college by anodic oxidation of aniline in sulfuric acid. ‘Aniline black’ as it was called, was used as a heat resistant paint, among other things. Its structure was not describ

hen it was noted that it exists in different forms, which had different colours. It was in 1968 that M. Jozefowicz discovered that the conductivity of the polymer increased by orders of magnitude with the decrease of pH of dopant acid. After the discovering polyacetyline and subsequently other polymers 1970’s it has become next interestingly studied conducting polymer. Since 1980’s polyaniline has been studied

t is one of the most versatile conducting polymers [100to approximately 80oC, and can be prepared by chemical or electrochemical oxidation,

form and has four known states, leucoemeraldine, emeraldinebase, inesalt and pernigraniline only emeraldinesalt is conductingThe chemical structure of there forms is shown in Fig.1.6

Figure 1.6: Different state of polyaniline

Introduction to conducting Polymers

] form polaron pairs or bipolarons are formed

Polyaniline was first prepared in 1862, by H. Letheby of the college of London Hospital aniline in sulfuric acid. ‘Aniline black’ as it was called, was used

as a heat resistant paint, among other things. Its structure was not described until the turn different forms, which had different

colours. It was in 1968 that M. Jozefowicz discovered that the conductivity of the pH of dopant acid. After

the discovering polyacetyline and subsequently other polymers 1970’s it has become next 1980’s polyaniline has been studied

[100-103]. It is stable or electrochemical oxidation,

leucoemeraldine, emeraldinebase, only emeraldinesalt is conducting, out of these four


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1.4 DIFFERENT METHODS FOR THE SYNTHESIS OF POLYANILINE In the available literature reviewed for this study, different ways to produce PANI have been demonstrated, including chemical, electrochemical, template, enzymatic, plasma, photo, and a number of other special methods. The different synthesis methods used to produce PANI are detailed in the next section. 1.4.1 Chemical Synthesis Chemical polymerization may be subdivided into heterophase, solution, interfacial, seeding, metathesis, self-assembling, and sonochemical polymerizations. Chemical synthesis has the advantage of being a simple process capable of producing bulk quantities of ICPs on a batch basis [106-109]. To date it has been the major commercial method of producing such materials. Chemical polymerization is typically carried out with relatively strong chemical oxidants like ammonium peroxydisulfate [110], ferric ions [111], tetrabutyl ammonium persulphate [112], bezoyl peroxide [113], chloroauric acid [114], or hydrogen peroxide [115]. These oxidants are able to oxidize the monomers in solution, leading to the formation of cation radicals. These cation radicals further react with other monomer units yielding oligomers or insoluble polymer. The different types of chemical polymerizations are discussed below. (a) Heterophase Polymerization The heterophase polymerization is used to produce high quality polymers with specially tailored properties from a small to a large volume scale [116]. It includes different methods of polymerization such as precipitation, suspension, emulsion, dispersion, reverse micelle and inverse polyymerizations. In the case of suspension and emulsion polymerization methods, the monomer should be sparingly soluble in water (as it has to form a separate phase) and form spherical droplets, whose size is controlled by a proper choice of the dispersing technique (such as stirring, ultrasonic treatment or homogenization). These droplets are stabilized in an aqueous media through the addition of a surface active agent (stabilizer). The emulsions can be of two types: “direct”, oil in water (o/w); and “inverse”, water in oil (w/o). The selection depends on the chosen emulsifier, the water to oil ratio, and the temperature of the


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polymerization. The emulsion is subdivided into microemulsion and miniemulsion depending upon the droplet size and stability and the amount of surfactant used. To prepare macro emulsion and miniemulsion, a degree of shear force is required, whereas the microemulsion is formed spontaneously. Synthesis of polyaniline colloidal dispersion is by the oxidative polymerization of aniline in an acidic aqueous medium and is obtained as a precipitate [117,118]. By adding polymer such as poly(N-vinylpyrrolidone) (PVP) to the reaction mixture, colloidal PANI particles are formed instead of precipitation. Such a process is known as dispersion polymerization. In this process: (a) the monomer is soluble in the reaction medium; (b) the produced polymer is insoluble under the same conditions; and (c) its macroscopic precipitation is prevented by the presence of the so-called steric stabilizer. The colloidal PANI particles have a typical average size of a few tens to hundreds of nanometers and are thus often regarded as nanocolloids. The shape of the particles may be spherical, globular, granular, cylindrical or branched dendritic structures [119,120]. (b) Direct and Inverse Emulsion Polymerization In the emulsion polymerization the monomer is dispersed in an aqueous phase to form a uniform emulsion. The emulsion is stabilized by a surfactant and the polymerization reaction is carried out [121]. For the synthesis of PANI by emulsion polymerization, aniline along with a protonic acid and an oxidant are combined with a mixture of water and a nonpolar or weakly polar solvent, like xylene, chloroform, or toluene (sparingly soluble or insoluble in water). The inverse emulsion polymerization process involves the formation of an aqueous solution of the monomer aniline, which is emulsified in a non polar organic solvent, like chloroform, isooctane, toluene, or in a mixture of solvents. The polymerization is then initiated with an oil soluble initiator such as ammonium persulfate (APS), benzoyl peroxide, etc. The reaction carried out in such a heterogeneous phase has several distinct advantages compared to the other techniques. The physical state of the inverse emulsion system makes it easier to control the process. Thermal and viscosity problems are much less significant here than those in bulk polymerization. The product of an emulsion polymerization can be


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used in many cases directly without further separations. During the polymerization, both a high molecular weight and a high reaction rate can be simultaneously achieved. During the course of the reaction, PANI remains as a soluble component in the organic phase. At the end of polymerization the organic phase is separated and washed repeatedly with distilled water. The solution is then treated with anhydrous sodium sulfate or other suitable chemicals to remove the excess water. The viscous organic solution is then added to acetone or other suitable solvent in order to break the emulsion and precipitate the PANI salt [122,123]. (c) Reversed Micelle Polymerization A transparent and homogeneous reversed micelle solution (A) is prepared by dissolving forge the dodecyl benzene sulfonic acid (DBSA) and an aqueous APS solution in the isooctane through vigorous stirring. The aniline is dissolved in isooctane to form a homogeneous solution (B); the ethanol is added to this system with thorough stirring. The reaction is initiated through the drop-wise addition of solution (B) into the reversed micelle solution (A). At the end of the reaction a dark green colored DBSA-PANI suspension is obtained. The suspended precipitate is then filtered, followed by washing with methanol and de-ionized water to remove impurities such as APS, free DBSA and un-reacted aniline. The particles of the DBSA-PANI synthesized through the reversed micelle process can attain a size within the nanoscale, with a needle-like shape [124]. (d) Interfacial Polymerization When polymerization reaction is carried out in the interfaces of two immiscible solvents it is known as interfacial polymerization. PANI has been synthesized by an interfacial polymerization technique using a mixture of two immiscible solvents such as water and chloroform in the presence of different acids acting as dopants. The reaction is initiated by an oxidizing agent such as APS, H2O2, and so on at room temperature or at any preferable temperature in the presence or absence of a surfactant. The final product is isolated by centrifugation [125, 126].


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(e) Seeding Polymerization The seeding polymerization is a typical template approach where a foreign material is used as a seed and the polymerization reaction is carried out in the presence of this seed. PANI nanofibers can be synthesized from aniline in the presence of some PANI powder acting as a “seed” using different acid dopants and different solvents [127]. The morphology of these nanofibers depends upon the type of acid used, the acid concentration, type of seed, the solvent used, and the relative amount of seed with respect to solvent. Among these reaction conditions, the influence of monomer concentration and solvent type was found to be pronounced in the morphology of the resultant PANI. (f) Metathesis Polymerization When the p-dichlorobenzene is heated at 220°C for 12 h with the sodium amide in an organic medium such as benzene, a metathesis reaction takes place giving rise to the formation of PANI [128]. This is another route for the synthesis of PANI for which aniline monomer is not required. 1.4.2 Sonochemical Synthesis The sonochemical process is initiated with the drop wise addition of an acidic APS solution to an acidic aniline solution, similar to that of conventional PANI synthesis. However, the polymerization is accomplished with the aid of ultrasonic irradiation. Jing et al. synthesized PANI nanofibers with high polymer yields using this technique [125,126]. At the higher concentrations of aniline and APS, three possible competitive reactions may be operative within the system: (i) The continuous formation of primary PANI nanofibers; (ii) The conversion of the primary nanofibers into thicker fibers with uneven surfaces; and (iii) The growth and agglomeration of the thicker fibers into irregular particles. In the conventional method of preparation, irregular PANI particles are obtained because of the simultaneous occurrence of reactions (ii) and (iii). While during the sonochemical synthesis, the further growth and agglomeration of the primary nanofibers are effectively prevented (even if more aniline and APS were added into the system), following the formation of more primary PANI nanofibers [129,130].


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1.4.3 Electrochemical Synthesis The electrochemical polymerization reaction provides a better method of polymerization with a fine control of the initiation and termination steps of the reaction. The electrochemical reactions are often much cleaner, and the PANI obtained is expected to be in a relatively purer form (as no additional chemicals such as surfactant, oxidant, and so on are used here) compared to that obtained from chemical polymerization. Furthermore, the use of limited chemicals reduces the problem of pollution. Electrochemical methods are generally employed for the polymerization of aniline under: (i) a constant current (galvanostatic); (ii) a constant potential (potentiostatic); and (iii) a potential scanning/cycling or sweeping. The first method essentially consists of a two-electrode assembly dipped in an electrolyte solution containing the monomer, and a specified level of current is passed to form PANI film on the surface of a platinum foil electrode. The polymerization of aniline at a constant potential produces polymer powder which adheres weakly on the electrode [131]. On the other hand, the electro-oxidation of aniline by continuous cycling between the predetermined potentials produces an even polymeric film which firmly adheres on the electrode surface [132,133]. This thin film of PANI can then either be reduced or oxidized to control conductivity. Even thicker film can be produced and peeled off from the electrode surface to yield a free-standing, electrically conducting film. The anodic oxidation of aniline is generally carried out on an inert electrode. Though the usual anode material is platinum or conducting glass, many metals such as Fe, Cu, Au, vitreous carbon and stainless steel [134-141] have also been used. 1.4.4 Template Synthesis One of the most effective and simple techniques of nanostructure formation is template synthesis. In this synthesis the desired material with the required shape is synthesized within the pores of a template and the template is then dissolved, leaving the material with the shape of the pores of the template. The template method has been used both for chemical and electrochemical polymerization in order to obtain conducting polymer nanotubes [142]. In the self-templating electrochemical synthesis


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of PANI nanowires aluminum is used as the template. The aluminum foil is first anodized followed by removal of aluminum oxide removed from the aluminum substrate through chemical etching. This gives rise to a pre structured from aluminum. A constant current density is then applied on the pre-structured aluminum in the mixture of acidic aqueous aniline solution. PANI nanowires are built in porous alumina. Finally, a barrier is dissolved in phosphoric acid. The morphology of the pores formed depends on the anodization process [143]. One of the interesting and useful features of this method is its effectiveness in the preparation of one-dimensional microstructure or nanostructured PANI with a controllable diameter and length [144]. 1.4.5 Enzymatic Synthesis Enzymatic polymerization of aniline to PANI has attracted significant attention because it is carried out under milder conditions, in comparison to those used for chemical polymerization. Horseradish peroxidase (HRP) and soybean peroxidase (SBP) are oxidoreductase enzymes capable of oxidizing aromatic amines in the presence of hydrogen peroxide [145-147]. 1.5 CHARACTERIZATION TECHNIQUES 1.5.1 Electron Spin Resonance (ESR) ESR works on the principle of absorption spectroscopy in which radiation of microwave frequency are absorbed by a paramagnetic substance to induce transition between magnetic energy levels of electron with unpaired spins. The magnetic energy level splitting is done by applying a static magnetic field. This is a phenomenon shown by ions, atoms and molecules having odd number of electrons. ESR is a complementary technique and its good resolution allows discrimination between unpaired electrons present in different environments. ESR spectral peculiarities such as multiplicity, position, shape, linewidth and its symmetry [365] give information about the static and dynamic characteristics of paramagnetic spin. ESR spectroscopy has played an important role for investigation of magnetic properties of conjugated polymers. The explanation of magnetic properties of


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undoped conducting polymers is fairly satisfactory but this situation is not completely clear in case of doped polymers. ESR results obtained for various conducting polymers show that there is no universal behavior of spin density, linewidth and symmetry upon doping. The nature of ESR signal of the conducting polymers crucially depend on the nature of the dopant species used. PA does not exhibit any ESR signal on doping with I2 whereas strong and narrow signal is clearly seen on perchlorate doping [365]. Perchlorate doped polypyrrole exhibit a strong and narrow ESR signal, although absence of ESR signal has been observed in some tetrafluoroborate polypyrrole [366]. 1.5.2 X-Ray Diffraction (XRD) Studies It is one of the most widely used techniques available for the study of structure of polymers. This technique is most suitable for the study of degree of crystallinity in a semicrystalline polymer. Amorophous materials produce abroad diffuse scattering while crystalline materials given sharp and well defined peaks. Low angle X-ray diffraction is suitable for study of thin films whereas wide angle X-ray diffraction (XRD) is used for thick films and powdered samples. Due to the presence of partial crystallinity in conducting polymers, the observed XRD patterns become much complicated. The Shirakawa polyacetylene having high degree of crystallinity (75-90%) gives sharp Debye Scherrer rings and remain largely crystalline even after p or n doping [367]. Polyaniline is another conducting polymer having partial crystallinity which is again dependent upon the method of synthesis. Polypyrrole has been considered as an amorphous material [368] but some reports give the evidence if its semicrystalline nature[369]. 1.5.3 Fourier Transform Infrared (FTIR) Spectroscopy Infrared spectroscopy is one of the spectroscopic techniques uniquely used to characterize the material by frequencies and intensities of various vibration bands exhibited by the material. It can be used to identify and quantify a particular substance in an unknown sample. IR spectroscopic analysis has been proved to be a versatile analytical tool for getting an insight about the synthesis and characterization of


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organic conducting polymers. This technique has been used to select a precursor that does affect the final product in the synthesis of PPV. This has also shown that this precursor polymer is air sensitive and prone to oxidation while fully converted films of PPV are air stable and have no aerial oxidation [370]. This technique has also been used to investigate conducting polymer blends of PT and PPY with polycarbazole [371]. It has been shown that there is a peak broading associated with C=O stretching vibration at 1770 cm-1 arising as a result of increased loading of conducting polymer in order to quantify the oxidation states of PANI, this technique is of great importance. The intensity ratio of the peaks at 1600/1500 cm-1 is a qualitative measure of the oxidation state of PANI. The very small intensity ratio of these peaks in the LEB form of PANI has been found to be consistent with this conclusion. This technique has been used to monitor the extent of cross-linking as a result of annealing on the polymeric backbone of PANI[372]. 1.5.4 Thermal Analysis The use of thermal analytical techniques such as Differential Scanning Caloriemetry (DSC), Thermogravimetric Analysis (TGA) and Dynamic Mechanical Thermal Analysis (DMTA). etc, as a method for material characterization have been in extensive use for the field of conducting polymers. These techniques provide information about phase transitions like glass transitions, melting/softening and thermal relaxation associated with a particular physic-chemical change[373]. TGA reveals sufficient information about stability including degradation kinetics and cross-linking in case of conducting polymers. 1.5.5 Ultraviolet-visible (UV-vis) Spectroscopy Optical studies of conducting polymers have been useful in formulating the defect states which govern their other properties such as electrical transport. The history of conducting polymers originated from PA has evolved an extensive field of theoretical and experimental research in this regard. Band structure calculations can theoretically predict the energetic of the defect formed on these polymer chains and the changes in the band structure upon doping. A most common technique based on valence effective


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Hamiltonian (VEH) using STO-3G optimized geometrics has been shown to properly predict the band gap and band width in hydrocarbon as well as nitrogen containing conducting polymers[374]. The use of this technique has fully explained the evolution of the absorption spectra of PPY and PT upon doping. It explains well, the initial formation of polaron and then bipolaron formation at for fully doped PPY in agreement with its ESR result [375]. The band gap of the EB form of PANI has been calculated using optical spectra. Based on these studies, the attempts for the theoretical design of a conducting polymer having a narrow band gap have been successful with report of a conducting polymer, polyisothianopthene having a band gap of ~1.1 Ev [376]. Electronic absorption peaks of some important conducting polymers are given in table 1.1 Table 1.1: Electronic absorption peaks of some important conducting polymers

Conducting polymers Energy (eV) of maximum absorption Reference

Undoped state Doped state Polypyrrole 3.2 0.7, 2.1, 3.2 374

Polythiophene 2.6 0.7, 1.7, 2.6 375 Polyaniline 2.0, 3.8 1.5, 2.9, 3.8 376

Polyacetylene 1.9 0.7, 1.9 1 1.5.6 Nuclear Magnetic Resonance In this branch of spectroscopy, radio waves induce transitions between magnetic energy levels and nuclei of molecule. These magnetic energy levels are created by keeping nuclei in a magnetic field. It is an important tool for polymer characterization. Progress in solid state NMR spectroscopy not only allows the determination of chemical composition but also opens up new possibilities for studying typical material properties like molecular dynamics, chain alignment and phase separation. Various types of nuclear resonance method based on proton, carbon-13, nitrogen-15, and fluorine-19 have been used for characterization of conducting polymers [377].


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1.6. MECHANISM OF CHARGE TRANSPORT IN CONDUCTING POLYMERS The conjugation arising due to chemical unsaturation of the carbon atom in these systems is responsible for charge transport [,55,61,159]. The organic conducting polymers, the electrical conductivity of which can be varied up to the metallic state by doping in the range more than ten orders of magnitude, is the most interesting class of 1D materials [51]. In contrast to the usual semiconductors, a charge is transferred by the nonlinear topological excitations (defects) formed in the chain as a result of Peierls instability [167] namely solitons in trans-PA, and polarons or bipolarons in PPP and other conducting polymers [168]. The specific nature of such carriers is responsible for unusual charge transport behaviour of these polymers. The polyacetylene, the simplest conducting polymer was studied thoroughly[148-150,161,169,170]. In this experimental results of temperature, pressure and frequency of dependence of electrical conductivity on lightly doped trans-PA were explained by a model proposed by Kivelson[171], assuming inter-chain transport as charge hopping between neutral and charged solition states at iso-energetic levels. This model was successfully used by Epstein [169] to explain the charge transport of lightly doped trans-PA samples. At higher doping levels these were replaced by tunneling or hopping between neighbouring highly conducting islands [172] in the framework of Sheng’s [173] and Mott’s VRH models [174]. The highest RT conductivity was 105 S/cm for I2 doped and stretched oriented trans-PA [148,149] which is one or two orders of magnitude lower than that predicted by Kivelson & Heegar [175] for a metal-like cluster in the polymer. PPY, PT, polyfuran (PF), polyindole, polycarbazole, etc. have a 5-membered ring structure with one heteroatom and have been termed as polyheterocyclics whereas systems like polypyrene, PPP, PPV and PAN, etc. belong to another class of conducting polymers known as polyaromatics. All these conducting polymers have intrinsic topological defects introduced during polymerization and their ground states are nondegenerate [61] in contrast with PA. Therefore, they are not expected to accommodate single solitons. Interestingly, they have been named as polarons and bipolarons. A polaron has been understood to comprise of a bound state of neutral soliton and a charged soliton whereas bipolaron is a bound state of two polarons. The


20

concentration of these topological defects can be varied by an external species called dopants [55]. Bredas et al. have shown [176] that soliton-antisolitons pair in the form of polarons and bipolarons could be stabilized in doped PPP. Kuivalanien et al. [177] have tested the applicability of these on a variety of doped PPP. The variable range hopping (VRH) has been shown applicable to explain the conducting properties of a wide range of moderately and highly doped PPP like systems[162,163,177-180]. In contrast with trans-PA and PPP like conducting polymers, the chains of PANI contain nitrogen heteroatoms involved in a conjugation [178]. Moreover benzene rings of PANI can rotate or flip, modulating strong electron-phonon interactions [179]. As a result somewhat in magnetic and charge transport properties of PANI are somewhat different as compared with other conducting polymers. More recently, the charge transport of substituted PANIs, its copolymers and blends have been the subject of considerable interest. 1.6.1 DC Conductivity The study of disorder which controls the transport properties of most real materials is strongly correlated with the localization length of the charge carriers. In this regard, the first hand information about the characteristic energies of the system is given by the dc conductivity measurements and their temperature variation, while the ac response of the system is related to characteristic lengths and/or times. This kind of work has been done in multitude of systems [181-184] such as amorphous and vitreous materials [51, 174, 181-185] in the form of glasses [174], ceramics, nano-structures [185] and recently conducting polymers [51]. In the context of the conducting polymers, with the advancement of synthesis techniques their electrical conductivity has increased many forms [148-150, 151-153]. Many predicted applications of synthetic materials have practically realized with unexpected growth. But a thorough understanding of the conduction mechanism and identification of the charge carriers is still necessary for their use on a large scale. In order to understand the electrical properties of these systems resistivity measurements as a function of temperature (R-T) provide significant information. These measurements have shown that though their conductivity at room temperature may be as high or comparable to that of a


21

metal but it decreases with a decrease in temperature for most of the conducting polymers. This indicates that the underlying conduction mechanism is due to hopping of charge carriers typical of a semiconductor. In order to understand the electrical conduction, many models have been reported [159,186-188]. Bloor and Movaghar [159] have suggested that the electrical properties of conducting polymers are influenced by conjugation i.e. chemical unsaturation of the carbon atoms in the polymer chain. Sun and coworkers [186] and later Kevelson [187] describe the observed spinless conduction in trans-PA, but none of existing model can give a satisfactory picture for conduction mechanism. Over the last few years, for the polymers like PPY, high and low mobility inorganic semiconductors and compound semiconductors [174] can be explained by polaronic hopping conduction. Based on polaronic hopping conduction, Singh et al. have explained the temperature dependence of dc conductivity of PPY, poly(N-methyl pyrrole) and its copolymers; poly(N-methyl pyrrole pyrrole) [160] by Mott’s 3D VRH model where in the multi-phonon processes are replaced by the processes in which the only contribution to the jump frequency of the polaron is due to the single optical phonon absorption and emission. For most of the conducting polymers, it is well established that, although the conductivities are high, mobilities are low, and it is generally considered that transport occurs in states which are localized [189-191]. The localization is considered to result from combined effect of disorder and formation of polaron [174]. The models for variable range hopping (VRH) have been used extensive to describe the charge transport at low temperature, Most widely the plots of logσdc versus T-m have been used where m is a parameter which straightens out the curves. When m=1, it may be due to conduction mechanism activated over a gap, a barrier or a mobility edge; m=1/4 is considered to be representative of 3D VRH, while m=1/2 indicates a 1D VRH[132] or due to charging energy limited tunneling (CLET) conduction in granular metals developed by Abeles et al. [192]. In case of PANI in which conductivity can be varied reversibly by a process of protonation and oxidation, several models have been used to describe the temperature variation of dc conductivity [193-196]. The conductivity of PANI increases by several orders of magnitude by doping with aqueous protonic acid [193]. A salient feature of


22

electrical conductivity in PANI is the active role of protons that have led to the proposal for a new mechanism of charge transport in PANI known as “proton exchange assisted conduction of electron (PEACE)” mechanism [194]. A model based on Granular Metal Island[195] has been proposed to explain the charge transport in PANI which states that the undoped PANI is highly amorphous. As a consequence, of doping a small three dimensional metallic region (2000A) is created just like an island in an insulating sea of polymeric matrix. It has been suggested that electrical conduction in PANI takes by hopping between metallic islands which may be 1D or 3D in variable range[196]. In case of HCl and CSA doped PANI, T-1/2 dependence of conductivity has been described in terms of quasi-1D VRH [154, 178, 180, 197]. Charge transport in PANI doped with para-toluene sulphonic acid and sulpho salicyclic acid has been explained by CELT conduction model [198] similar to the variation obtained for the salt form of HCL doped PANI [195]. Fluctuation induced tunneling (FIT) conduction model has been suggested for T-1/2 dependence as 3D VRH in presence of Coulomb interactions[173]. In such cases one should observe, in addition to the T-1/2 behaviour over a large temperature range, a crossover to T-1/4 at low temperatures and a variation > T-1/2 at higher temperatures [199]. This result in the build up of a coulomb pseudo-gap at the Fermi level. The order of magnitude of this gap is given by the charging energy evoked by Abeles et al. in granular metal [192]. At higher temperatures, the blends of PANI/PMMA and PANI/PVC have shown a T-1/2 dependence of conductivity, in the millikelvin range, obeying the FIT conduction whereas PANI/polyethylene terephthalate (PETG) blends exhibit CELT conduction [200]. This suggests that the method of synthesis and the insulating polymer used for blending play a vital role in deciding the morophology and conduction mechanism of a polymer blend. The role of structural order/disorder can be predicted by these measurements. It has been shown that PAN/PMMA blend shows a smaller decrease of conductivity than PAN/PETG, which has been described in terms of a model involving metallic conduction in addition to tunneling between metallic particles [201]. The measurements of samples doped near the metal-insulator (M-I) transition offer more. Research in latest studies on PANI protonated by camphor sulphonic acid


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(CSA) and PPY doped with PF6 down to 25K in magnetic field have been reported [156]. The role of order/disorder can be predicted by these measurements which have been used to characterize the disorder phenomena in PF6 doped polypyrrole, CSA doped PANI and I2 doped polyalkylthiophene close to metal-insulator boundary in which the ratio ρr is given as ρr= ρ1.4 K/ ρ300 K. If ρr<102 at low temperature, it follows Mott’s VRH, for102< ρr <103 it shows crossover from VRH to Efros-Skhlovskii VRH with small coulomb gap and ρr <103, with two distinct phenomena for homogenous Mott’s VRH and for inhomogenous metallic island as granular system [202]. Conductivity investigations of CSA doped Polyaniline [203] as a function of doping level have shown the onset of metallic conductivity at 30% doping level, whereas at 60% doping level, a metallic behavior up to a temperature of 135 K has been noticed. A model based on tunneling and metallic transport has been proposed to explain the observed phenomena[203]. It has been shown that the transport properties can be controlled both by solvent as well as dopant [204]. Only ferric chloride doped PANI shows increase in σdc with temperature up to 220 K [205] and then it decreases monotonically up to 15 K. Many models have been suggested to explain the conduction mechanism in conducting polymers in both undoped and doped states [53, 206-208]. It is 3D at low temperatures and 1D at high temperatures [209]. The appearance of metallic properties of pure CSA doped PANI such as positive temperature dependence of conductivity, linear temperature dependence of thermoelectric power and a frequency independent ac conductivity have been reported. These properties have been retained in the blends of PANI-CSA/PMMA down to percolation threshold of the volume fraction (fc). A low percolation threshold ~0.003 has been attributed due to phase separated morphology and fibrillar geometry of the percolating object[210]. At low temperatures, the hopping transport through the conducting polymer network depends upon the volume fraction of the PANI-CSA in blend and its network structure. Blending of PANI with nanoconducting and conducting host not only aids the study of the role of disorder but also facilitates its comparison with other conducting


24

polymers. As accepted PANI blending with PET (polyester) [211] reduces its conductivity but unexpectedly blending with PMMA and PVC has been ascribed to the lessening of insulating barriers around PANI particles. These results are similar to PA described by series combination of quasi-1D particles havemetallic conductivity and tunnelling (between small metallic particles) for the blends and lightly doped PA and between extended metallic region in highly conducting PA[212]. It has been possible via improved chemistry to attain the maximum conductivity [157] in oriented PANI ~104 S/cm, having a similar temperature dependence to that of highly doped polyacetylene, but having a strongly temperature dependent anisotropy below a characteristic temperature ~150 K. Nanocomposites of carbon nanotubes with polyaniline (PANI) constitute promising conducting nanomaterials, due to their ease of synthesis, electrical conductivity, and environmental stability. It was found the electrical conductivities of the MWCNT-PANI nanocomposites increased when the PANI/MWCNT ratio was decreased [378]. Polyaniline–TiO2 composites/hybrids are highly tunable materials with unique electrical properties. The electrical conductivity, of polyaniline is enhanced by the addition of TiO2. The electrical conductivity of the composites are reported to increase initially with TiO2 content and then decrease on excess of TiO2. This initial increase is attributed to the well-extended conjugation conformation of doped polyaniline and decrease on excess concentration of TiO2 which attributes to the particle blockade of conducting path. The electrochemical method is also difficult due to the insulating nature of the fabric. Very recently, polyaniline–ZnO coatings on fabrics for conductive network was studied and reported by Zhao et al. [379]. A comparison of PANI-CSA and H2SO4 doped PANI [212] has shown that for PANI-CSA the counter ion induced processibility results in the improvement in material quality due to free charge carriers in the partially filled conduction band, leading to more metallic behaviour with extended electronic states near EF making it a disordered metal on the metal-insulator boundary, in contrast with PANI doped with H2SO4 in which states near Fermi energy (EF) are localized, making it Fermi glass [212]. Recently the VRH model has been used to explain the charge conduction


25

mechanism for nano-composites of polymeric materials [213]. Recently direct current conductivity as a function of temperature for polyaniline and polyaniline–PbO composites was studied in the temperature range from 300C to 1600C polyaniline. At higher temperatures, it was found that conductivity increases because of the polarons hopping from one localised state to another. This is attributed to the temperaturedependence of the conductivity in the composites, which exhibits typical semiconductor behaviour [380]. 1.6.2 AC Conductivity The temperature dependence of dc conductivity of conjugated polymers provides the first hand information about the mechanism of charge transport. However, low frequency conductivity and dielectric relaxation measurements in the typical frequency range have proven to be valuable in providing the additional information [174, 214-216] on the mechanism of charge transport that dc conductivity alone do not provide. So the proper understanding of the mechanism of ac conduction is necessary in order to arrive at a comprehensive picture of charge transport in conducting polymers. For this reason, the low frequency measurements have been extensively used in many research areas [174, 181, 216-228] in condensed matter physics, material science, glass science and polymer science. The low frequency ac conduction relaxation study is a subject which is inter-disciplinary in nature. It has become a major tool for study of mechanism of conductions in polymers [214-216], glasses [174,181, 217-222], liquid [219], ceramic [221, 223-226] and solid electrolytes [229]. In the solid state physics, there is a tradition of using low frequency conductivity relaxation (often referred to ac conductivity) to study the electronic hopping conduction [174] in the doped semiconductors. The work was started after the first report of low frequency hopping conductivity in n-type silicon by Pollak and Geballe [217] nearly 45 years ago and is being continued with vigour by various worker [151, 166, 230-234], till date. The emergence [44] of conducting polymers as a new class of electronic materials has offered a new concept. Various complex functions have been used to characterize the electrical properties of π-conjugated polymers such as Z*[166,235-241], electric modulus (M*) [242-246], admittance (Y*) [247] and capacitance (C*) or dielectric permittivity (ε') [233, 243, 248-249]. Epstein and co-workers [164,165, 250-253] have done extensive studies on frequency dependent


26

conductivity of polyacetylene. The low frequency conductivity relaxation measurements gives valuable information regarding the mechanism of charge transport in polyacetylene. In the inter-soliton hopping model proposed by Kivelson [171], a much more stronger dependence of ac conductivity on temperature has been predicted as compared to the temperature dependence of ac conductivity in VRH. On the basis of the conductivity relaxation, conductivity of polyacetylene films have been analyzed by using a model of multiple conductivity relaxation model [151]. Since other conjugated polymers such as PANI, PPY, PT, and PPP, etc. have non-degenerate ground state as does trans-PA, the single soliton excitations are not stable in the above mentioned systems [171]. It has been shown that soliton-antisoliton pairs in the form of polarons and bipolarons are stable in these systems with slightly non-degenerate ground state [78]. Several doubts have been raised by Emin and Ngai [242] on the procedure adopted by Kivelson [171] in formulating his model. It has been concluded [216] that none of the existing models are satisfactory for trans-PA. Chrobozek and Sommerfield [254,255] have given another model based on the equivalent circuit approach to the theory of ac and dc hopping conductivity. The ac conductivity and dielectric relaxation mechanism in the PPY family of polymers has been proposed to explain the ac conductivity data. The model considers contribution from two mechanisms of charge transport in which one may be associated with the hopping of the charge carriers excited to the localized band tails giving rise to dielectric loss peaks and the other is due to the hopping of the charge carriers near the Fermi level giving rise to linear frequency dependence of conductivity [232-233]. The complex conductivity σ* of lightly doped poly(3-methyl thiophene) has been reported [248] and at low temperature the ac conductivity data could well be described by the relation σac = Aωs and has been attributed [174] to a classical hopping between localized states close to Fermi level.

Emin and Nagai have introduced a method of analysis of ac conductivity using dielectric modulus approach [218]. Zuo et al. [166] have studied the ac conductivity of emeraldine form of polyaniline. These results are inconsistent with


27

Kiveson type [165] inter-soliton hopping between polarons and bipolarons sites [177, 256] applied to other systems like PPP and PPY having non-degenerate ground state. They proposed that the dominant mechanism in the unprotonated regions of emeraldine base polymer involves pairwise hopping of charge among polarons and bipolarons sites [166]. In this report the ac conductivity data of lightly doped samples has been interpreted as a response of two components having one contribution from the polymeric backbone and the other from the oscillations of the isolated polarons and bipolarons between their pinning centers. Microwave transport in the emeraldine form of PANI has been reported [256,257] and the analysis of the microwave data supports the increased delocalization of charge, reflecting the formation of textured metallic islands. Wang et. al. [180] have studied the electron localization and charge transport in poly(o-toluidine) and concluded that their experimental results could not be explained by charging energy tunnelling model for granular metals and three dimensional VRH model with a coulomb gap. The ac conductivity of thermally dedoped PANI [236] is ensured by three distinct mechanisms of charge transport i.e. within a chain of polymer, between neighbouring chains and between different clusters. Also the ac conductivity of polyemeraldine base has been explained in terms of the polaron hopping model by other workers [258]. The microwave dielectric behaviour of HCl doped polyaniline and poly(o-toluidine)[259] fibres have been understood in the frame work of interrupted metallic strand’s island model by assuming the size of the metallic island within individual increases linearly with temperature, with the largest effects in the samples stretched at the lowest temperature. The highly conducting states of polyacetylene, polypyrrole and polyaniline display [156] a unified electronic response (dielectric constant and conductivity) as a function of frequency from microwave region (10 GHZ) through to all electron plasma frequency. This common behaviour is propsed to be a consequence of the inhomogeneous crystalline order in these materials leading to the three dimensional metallic domains or islands separated by disordered regions and interfiber links. In the H3PO4 doped PANI, Rouloeu et al. [230] have reported that the ac conductivity σac varies as ωs with s ~0.6. they have used Mott’s model for hopping conductivity and found that this value of s yields a reasonable estimation of hopping frequency Vph = 6.92 × 1013 sec-1. The dielectric constant and ac


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conductivity of polyaniline derivatives [260] has been explained by Mott’s VRH model and pinning model approach. Low frequency measurement has been used to study the influence of residual water retained in the EB form of PANI [234] on its properties such as dielectric loss, ac conductivity and complex electric modulus. The characteristic time of the electrical relaxation with corresponding reduction in the electrical conductivity due to change in water and aniline ratio has been noticed. This effect has been found comparable to acid doping of PANI causing partial protonation. The ac conduction of conducting polymer blends of PANI [261] have suggested a Debye type relaxation as a result of the effective medium approximation in a cluster of conducting inclusions in an insulating matrix while on the contrary, Krinichuyi et al. have shown [262] that charge carriers hop between polaron states according to Kivelson’s theory. Polyaniline/nano-titanium dioxide composites (PANI/n-TiO2) were prepared using α-dextrose as surfactant and ammonium per sulfate as an oxidant. Ac conductivity, permittivity, and tangent loss studies on these samples suggest that these composites may be well suited for gas sensor application[381]. The ac conductivity of PANI nanofiber-PMMA composite exhibits the power-law behaviour. The temperature dependence of s-parameter suggests that the charge transfer process in the material giving rise to ac conductivity is dominated by correlated barrier hopping mechanism [382]. Polyaniline-yttrium trioxide (PAni-Y2O3) composite exihibited the huge dielectric constant, high ac conductivity, and very high tan delta values, thus can be potential candidate for absorbent panels, decoupling capacitors and EMI shielding applications [383]. Recently, electrical conductivity of PANI/c-MWCNT (3 wt%) nanocomposite recorded about three orders of magnitude higher than that of pure PANI. In addition, the increase in the conductivity with respect to temperature revealed the semiconductor behavior of the nanocomposites and dielectric permittivity of the nanocomposites enhanced from 3.34 to 10.2 at 1 kHz as the c-MWCNT concentration increased from 0.5 to 3 wt%. The loss tangent was still low (<0.025) even at a higher concentration of c-MWCNT, i.e., 3 wt%. The response of permittivity to temperature is increased with increasing the c-MWCNT concentration in the nanocomposite [384].


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1.7 APPLICATIONS OF CONDUCTING POLYMERS The greatest advantage of organic conducting polymers over inorganic materials is their architectural flexibility and can be easily shaped as per device requirement. Their environmental stability, light weight, and ease of fabrication make them most fascinating material for device applications. They have found their uses in light weight rechargeable batteries, remotely readable indicator devices, electromagnetic interface shielding, electrostatic charge dissipation, electroluminescent devices, sensors, electrochromic displays, solid state devices, artificial muscles, integrated circuits and many more. 1.7.1 Light Weight Rechargeable Polymer Batteries

The reversible process of chemical-electrochemical process occurs in secondary batteries therefore it has the major advantage of rechargeability over a primary battery. In charging and discharging process a secondary battery acts as an electrolytic and voltage device respectively. The factor such as cyclability, energy-density, and stability are important from the practical point of view for the success of the rechargeable battery. Techagumpuch, Nalwa and Miyata [263] have reviewed the suitability of conducting polymers for battery applications and suggested that polypyrrole and polyaniline are most suitable for this purpose. Latter, rechargeable thin film batteries of polypyrrole and polyaniline had also been reported[263]. Sivakumar et al have shown that poly(N-methylaniline) can be used as a cathode active material in aqueous rechargeable batteries[264]. Polyaniline and polypyrrole can be used as oxygen reversible electrodes [265]. Recently new energy industry including electric vehicles and large-scale energy storage in smart grids requires energy storage systems of good safety, high reliability, high energy density and low cost. Here a coated Li metal is used as anode for an aqueous rechargeable lithium battery (ARLB) combining LiMn2O4 as cathode and 0.5 mol l−1 Li2SO4 aqueous solution as electrolyte. Due to the “cross-over” effect of Li+ ions in the coating, this ARLB delivers an output voltage of about 4.0 V, a big breakthrough of the theoretic stable window of water, 1.229 V. It has excellent cycling with Coulomb efficiency of


30

100% except in the first cycle. Its energy density can be 446 Wh kg−1, about 80% higher than that for traditional lithium ion battery. Its power efficiency can be above 95%. Furthermore, its cost is low and safety is much reliable. It provides another chemistry for post lithium ion batteries[385]. 1.7.2 Remotely Readable Indicator Devices The unstability of polyacetylene in air has always hindered its electronic device applications. However, this drawback of polyacetylene proved to be a boon in remotely readable indicator devices such as valve position in includes a wireless identifier device, which are designed on the basis of the change in electrical conductivity of the conjugated polymers due to environmental degradation. Recently in 2013 in a patent claim by Michael Wildie Mccarty [389] remotely readable valve position indicators and related methods are described. 1.7.3 Electrostatic Charge Dissipation The generation of electricity is a surface phenomenon which is caused by the transfer of electrons between two surfaces brought into contact. Plastics accumulate high surface charges that attract dust and dirt particles due to its insulating properties. Incorporation of conducting charge carriers causes easy dissipation of static charges, for which they are known as antistatic agents. For any material to be used as antistatic agents, the two most important requirements [266] are the material should have conductivity (10-4-10-8 Ohm-

1cm-1) and it should have ability to decay electrostatic charges within 2 seconds. Recently copolymers of aniline, o-methoxy aniline and o-ethoxy aniline with m-amino benzenesulfonic acid have been used for antistatic application [386]. 1.7.4 Electromagnetic Interference (EMI) Shielding and Microwave Absorption Electromagnetic interference causes an undesired response, malfunctioning or degradation in the performance of an electronic equipment directly or indirectly. Plastics are transparent to electromagnetic waves while metals either absorb or reflect


31

them and protect the electronic equipments. Metals are best shielding materials, however, have very high corrosion rate [267]. Coating of conducting polymers on insulating surfaces or conducting polymers embedded in an insulating polymer matrix are usefull, candidates for technological application as EMI shields [268]. EMI shielding effectiveness can be measured in decibels (dB) and in radio frequency range it varies from 50-90 dB. Milliken research laboratory (USA) had developed a process of grafting of conducting polymers on fabrics which gives a shielding effectiveness of 40dB in the frequency region 100 KHz to 1GHz [269]. It has been observed that shielding efficiency (SE) values of the PANI- coated transparent thin films has been found to increase from 14.28% to 52.67% with the increase of the thickness of films from 229 nm to 823 nm [270]. The electromagnetic interference shielding effectiveness of conducting polyaniline (PANI)-ABS composites was studied at 101 GHz and shielding effectiveness of 60 dB has been achieved [271]. 1.7.5 Electrochromic Displays The change in color of a material induced by the passage of charge is defined as electrochromism. Conducting polymers are an important class of electrochromic materials which show color as a function of change in potential. For the fabrication of an electrochromic display device, the three important parameters are range of color change, electrochromic switching time and switching life cycle. Polypyrrole and its N substituted derivatives have been first to be reported to show electrochromism[272]. It is known that almost all the conducting polymers like polythiophene, polyparaphnylene, polyparaphenylene-vinylene, polyfuran, polyazulene, polycarbazole, polyquinoline, polyisothianapthalene and polyaniline show electrochromism [273-276]. Recently, much interest of researchers is in the increase of the efficiency and properties of electrochromic devices by making some blends with other polymers [277-279]. Recently copolymer of o-methoxy aniline with m-amino benzenesulfonic acid [387] has been used for electrochromic devices as shown in Fig. 1.7.


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Figure 1.7: Electrochromic devices of copolymer [387].

1.7.6 Sensors Conducting polymers, such as polypyrrole (PPy), polyaniline (Pani), polythiophene (PTh) and their derivatives, have been used as the active layers of gas sensors since early 1980s [280]. In comparison with most of the commercially available sensors, based usually on metal oxides and operated at high temperatures, the sensors made of conducting polymers have many improved characteristics. They have high sensitivities and short response time; especially, these featuers are ensured at room temperature. Conducting polymers are easy to be synthesized through chemical or electrochemical process, and their molecular chain structure can be modified conveniently by copolymerization or structural derivations. Furthermore, conducting polymers have good mechanical properties, which allow a facile fabrication of sensors. As a result, more and more attentions have been paid to the sensors fabricated from conducting polymers [281], and a lot of related articles were published. The importance of conducting polymers in applications as sensors arises due to the ease of their synthesis and ability to incorporate sensing elements in the polymeric network [282]. It has been shown by Messiac et.al. [283] that the resistance of polypyrrole deposited on filter paper is highly sensitive to presence of ammonia. This effect has led to the development of a sensor based on electrochemically polymerized doped PPY films on inter digited gold electrodes screen printed on aluminum substrate. This type of sensors is sensitive to ammonia and nitrogen oxide gas. Now, the attention is shifting towards the conducting polymer nanofiber and nanocomposites sensors because of their large surface to volume ration they show fast and better response to gasses and chemicals [284, 285]. In a recent report, Hsp90-


33

functionalized polypyrrole nanotube FET has been used for anti-cancer agent detection sensors and force sensors[286,388].

1.7.7 Electroluminescent Devices A typical organic light emitting diode (OLED) is (Fig. 1.8) composed of a layer of organic materials situated between two electrodes, the anode and cathode, all deposited on a substrate. The organic molecules are electrically conductive as a result of delocalization of π electrons caused by conjugation over all or part of the molecule. Originally, the most basic polymer OLEDs consist of a single organic layer. The first light-emitting device synthesized by J.H. Burroughes et. al., which involved a single layer of poly(p-phenylene vinylene)[287]. However, multilayer OLEDs can be fabricated with two or more layers for to improving their device efficiency, as well as conductive properties. Different materials may be chosen to aid charge injection at electrodes by providing a more gradual electronic profile,[288] or block a charge from reaching the opposite electrode and being wasted.[289] Many modern OLEDs incorporate a simple bilayer structure, consisting of a conductive layer and an emissive layer. Indium tin oxide (ITO) is commonly used as the anode material. It is transparent to visible light and has a high work function which promotes injection of holes in to the HOMO level of the organic layer. A typical conductive layer may consist of PEDOT: PSS [290] as the HOMO level of this material generally lies between the work function of ITO and the HOMO of other commonly used polymers, reducing the energy barriers for hole injection. Metals such as barium and calcium are often used as the cathode due to their low work function which promote injection of electrons into the LUMO of the organic layer.[291] Such metals are reactive, so require a capping layer of aluminum to avoid degradation. Now the OLEDs have been commercialized. OLEDs are used in television screens, computer monitors, small, portable system screens such as mobile phones and watches, advertising, information and indication. In October 2008, Sony published results of research it carried out with the Max Planck institute tuebingen over the possibility of mass-market bending displays, which could replace rigid LCDs and plasma screens. Eventually, bendable, transparent OLED screens could be stacked to produce 3D images with much greater contrast ratios and viewing angles than existing products.[292] Sony exhibited a 24.5” prototype OLED 3D television during the consumer electronics show in Delhi January 2010 [293].


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Figure 1.8: A typical organic light emitting diode (OLED)[289].

1.7.8 Solar Cells Organic, hybrid and photoelectrochemical (fig. 1.9.) solar cells have been the alternatives for conventional silicon solar cells [294]. 4th generation or the present generation organic solar cells mainly consist of two organic organic materials an electron-donating material and an electron accepting material to make a percolating structure with interpenetrating networks. The realization of the photo induced charge transfer from the conjugated polymer to the fullerene derivatives led the development “bulk hetrojunction” organic solar cells.

Figure 1.9: The solar cell sandwich form[294].

A hybrid solar cell consists of a combination of both organic and inorganic materials therefore, combines the unique properties of inorganic semiconductors. Bulk hetrojunction solar cell of poly(3-hexylthiophene) and C60 with high efficiencies of 2.2% under white light and 6.5% under monochromatic light have


35

been obtained in a cell. The efficiencies were markedly dependent on composite ratios and annealing temperatures, and energy. Recent device efficiencies reported by mukesh kumar et.al. is 8.5%[390]. 1.7.9 Artificial Muscles The most outstanding feature of conducting polymers is drastic enhancement of electroconductivity upon oxidation and reduction. Upon oxidization and reduction, the conducting polymers change physical properties as swelling or shrinking[295]. The variation of the dimension is induced by an electrochemical cycle, which is called electrochemo mechanical deformation (ECMD) and can be utilized as soft actuators and artificial muscles[300,301]. The soft actuator based on the conducting polymer has attracted much attention recently. Many authors have reported the (ECMD) in conducting polymers, such as polyaniline (PANI) [296], poly(o-methoxyaniline) (PmAn) [296,297], poly(3-alkylthiophene) [298] and polypyrroles (PPY) [299]. Mutlu et al. have been able to make artificial muscles with adjustable stiffness and have shown that the larger the radius of the contact surface is, the higher is the stiffness of the polymer actuators[302-304]. 1.8 OBJECTIVE OF RESEARCH The objective of this research is to synthesize conducting polymers polyaniline and its copolymers with 3-aminobenzene sulfonic acid for their application as antistatic materials and electrochromic devices. Polymerization was also carried using substituted anilines like o-ethoxy aniline, and o-methoxy aniline with 3-aminobenzene sulfonic acid and to see how the substituted anilines affect the solubility and proceassability without the use of external dopant. The aim was to determine the effect of external dopant like p-toluene sulfonic acid (PTSA) on the polymerization of aniline in the presence of 3-aminobenzene sulfonic acid (ABSA). ABSA not only acts as a dopant and a co-monomer but also plays an important role in improving the solubility and processability of polyaniline. For improving the processability, the polymers so designed were blended with conventional polymer low-density polyethylene (LDPE) so that the resultant blend which possesses both electrical and


36

mechanical properties can be used for electrostatic charge dissipation (ESD) application. Hence, the aim of my work was to synthesize. The copolymers of self-doped poly(3-ABSA-co-AA), poly(3-ABSA-co-OMAA), poly(3-ABSA-co-OEAA) and PTSA-doped poly(3-ABSA-co-AA), poly(3-ABSA-co-OMAA), poly(3-ABSA-co-OEAA), and to study the antistatic performance of films containing these materials. Electrochemical polymerizations, of these polymers were also carried to study the growth behaviour of conducting polymers, to determine the redox behaviour and to use the thin films deposited on electrode surface for their application in electrochromic devices, when the potential of the electrode is switched between reduced and oxidized states.


37

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