Conducting Polymers PDF

Conducting Polymers  Conducting Polymers: Introduction The polymers that are used in daily basis are insulators. Polymers were considered to be electrical insulators before the invention of conducting polymers (conjugate polymers). These organic polymers have unique electrical and optica...

Conducting Polymers  Conducting Polymers: Introduction The polymers that are used in daily basis are insulators. Polymers were considered to be electrical insulators before the invention of conducting polymers (conjugate polymers). These organic polymers have unique electrical and optical properties similar to those of inorganic semiconductors and conductors. There are some mechanisms through which electrons can be made available in organic molecules. These materials are sometimes called synthetic metals. 2000 Nobel Prize in Chemistry was awarded jointly to Alan J. Heeger, Alan G. MacDiarmid and Hideki Shirakawa “for the discovery and development of conductive polymers.”  What is electrical conductivity? Conductivity is defined by Ohm’s law: Not all materials obey Ohm’s law. Gas discharges, vacuum tubes, semiconductors and what are termed one-dimensional conductors (e.g., a linear polyene chain) generally all deviate from Ohm’s law.  Conductivity In Ohmic material the resistance is proportional to the length (l) of the sample and inversely proportional to the sample cross-section (A): where ρ is the resistivity measured in Ω cm (in SI units Ω m). Its inverse σ = ρ–1 is the conductivity. The unit of conductance is the Siemens or mho (S = Ω–1). The unit of conductivity is: S m–1. Conductivity depends on the number density of charge carriers (number of electrons, n) and how fast they can move in the material (mobility µ):  Conductivity of Conjugated Polymers Conductivity of conductive polymers compared to those of other materials, from quartz (insulator) to copper (conductor). Polymers may also have conductivities corresponding to those of semiconductors. Electrical Conductivity of Common Conducting Polymers  Temperature Dependence of Conductivity Conductivity depends on temperature: it generally increases with decreasing temperature for “metallic” materials (some of which become superconductive below a certain critical temperature, Tc). However. it generally decreases with lowered temperature for semiconductors and insulators.  What makes a material conductive? Material’s electrical conductivity may depend on direction, and it to be anisotropic. Diamond, which contains only σ-bonds, is an insulator, and its high symmetry gives it isotropic properties. Graphite and polyacetylene both have mobile π-electrons and are, when doped, highly anisotropic metallic conductors.  Explanation by Band Theory The energy gap between the highest occupied, and the lowest unoccupied bands is called the band gap. The lowest unoccupied band is called the conduction band and the highest occupied one the valence band. The conductivity of the metal is due either to only-partly-filled valence or conduction bands, or to the band gap being near zero (or overlap). π - Molecular Orbital Diagram of Ethylene + + + + π - Molecular Orbital Diagram of 1,3-butadiene + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + +  The Effect of Conjugation MO diagram E Ethylene 1,3-butadiene ▪ Due to conjugation the energy gap between HOMO (i.e., valence band) and LUMO (i.e., conduction band), the energy of electronic transition, gets significantly lowered. ▪ For a conjugated polymer, this gap becomes nearly zero (overlap), and thus becomes conductive. 12  Types of Conducting Polymers ▪ Conductive polymers are organic materials, but generally are not thermoplastics, i.e., they are not thermoformable. ▪ They can offer high electrical conductivity but do not show similar mechanical properties to other commercially available polymers. ▪ The electrical properties can be fine-tuned using the methods of organic synthesis, and by advanced dispersion techniques. Types of Conducting Polymers according to their Composition Intrinsically conducting materials are characterized by good electrical conductivity, capability to store charge, capacity to exchange ions, ability to absorb visible radiation, thereby yielding the coloured compounds. These are also X-ray transparent.  Intrinsically Conducting Polymers ▪ A conjugated carbon chain consists of alternating single and double bonds, where the highly delocalized, polarized, and electron-dense π bonds are responsible for its electrical and optical behavior. ▪ Typical conducting polymers include polyacetylene (PA), polyaniline(PANI), polypyrrole (PPy), polythiophene (PTh), poly(para-phenylene) (PPP), poly(phenylenevinylene) (PPV), and polyfuran (PF). ▪ Conducting polymers have backbones of contiguous sp2 hybridized carbon centres. One valence electron on each sp2 hybridized carbon centre resides in a pz orbital, which is orthogonal to the other three σ-bonds. ▪ All the pz orbitals are parallel to each other, as a result they can overlap with each other to form a delocalized set of orbitals. The electrons in these delocalized orbitals have high mobility when the material is “doped” by oxidation, which removes some of these delocalized electrons. ▪ Thus, the conjugated p-orbitals form a one-dimensional electronic band, and the electrons within this band become mobile when it is partially vacant. Examples of (Intrinsically) Conducting Polymers Overlapping parallel pz orbitals in polyacetylene  Extrinsically Conducting Polymers ▪ Those conducting polymers, which owe their conductivity due to the presence of externally added ingredients, are called extrinsically conducting polymers. Extrinsically conducting polymers (ECP’s) are of two types. 1. Conducting Elements Filled Polymers (CEFP): a conducting element is added to the polymer. Therefore, the polymer acts as a binder to hold the conducting elements together in solid entity. Thus, conductivity of these polymers is due to the addition of external ingredients. Upon addition of conducting element, the polymer will have a property of that conducting element, and it will start conducting electricity. ▪ For example, when carbon black or some metal oxides or metal fibres are added, the polymer becomes conductive. ▪ The minimum concentration of conducting filler required to start the conduction is called percolation threshold. 2. Blended conducting polymers: These types of polymers are obtained by blending a conventional polymer with a conducting polymer either physically or chemically. This blend of polymers conduct electricity. Such polymers can be easily processed and possess better physical, chemical and mechanical properties.  Doped Conducting Polymers Conjugated polymers when treated with electron-deficient species (Lewis acid) like FeCl3 or I2 vapour or I2 in CCl4, oxidation takes place, and a positive charge is created in the molecule. Removal of one electron in the π backbone of a conjugated polymer forms a radical cation (polaron), which on losing another electron forms bipolaron. The delocalization of positive charges causes electrical conduction. Lewis acids (FeCl3, AlCl3) are generally used as doping agent. The mobility of a polaron along the polyacetylene chain can be high, leading to metal-like conductivity. The role of the dopant is either to remove (p-doping) or to add electrons (n- doping) to the polymer.  p-Doped Conducting Polymers Radical cation (”polaron”) formed by removal of one electron on the 5th carbon atom of a P-type doping creates positive undecahexaene chain (a → b). The polaron charge. migration shown in c → e.  n-Doped Conducting Polymers In this type of doping some electrons are introduced to the conjugated π-bonds through reduction creating a negative hole or charge inside the polymer. The negative charge can move throughout the polymeric chain, and make it conducting polymer. Lewis bases, Na+C10H8 −, K+C10H8−, etc., are generally used as doping agents. Migration of negative charge in a n-doped conducting polymer  Polyacetylene A variety of methods have been developed to synthesize polyacetylene, from pure acetylene and other monomers. One of the most common methods uses a Ziegler–Natta catalyst, such as Ti(OiPr)4/Al(C2H5)3, with gaseous acetylene. When the synthesis is performed below −78 °C, the cis-form predominates, while above 150 °C the trans-form is favoured. At room temperature, the polymerization yields a ratio of 60:40 cis:trans. Temperature dependence of polymerization  Polyacetylene Polyacetylene can be synthesized by ring-opening metathesis polymerisation (ROMP) from cyclooctatetraene, a material easier to handle than the acetylene monomer. This synthetic route also provides a facile method for adding solubilizing groups to the polymer while maintaining the conjugation. Grubbs route to polyacetylene (metathesis) © Wikipedia Polyacetylene can also be synthesized via electrochemical, and photoelectrochemical processes.  Polythiophene (PTh) Polythiophenes (PTs) are polymerized thiophenes, a sulfur heterocycle. The rings are linked through the 2- and 5-positions. They are insoluble in nature. However, substitutions at 3- and 4-positions can improve their solubility. PTs become conductive when oxidized. The electrical conductivity results from the delocalization of electrons along the polymer backbone. In addition to conductivity, polythiophenes show dramatic colour shifts in response to changes in solvent, temperature, applied potential, and binding to other molecules. Changes in both colour and conductivity are induced by twisting of the polymer backbone, and disrupting conjugation, Due to such properties polythiophenes are used as sensors that can provide a range of optical and electronic responses. Substituted PThs have potential application in field-effect transistors, electroluminescent devices, solar cells, photochemical resists, nonlinear optical devices, batteries, diodes, and chemical sensors, etc. © Wikipedia  Synthesis of Polythiophene (PTh) Chemical oxidation method The oxidative polymerization of thiophenes using ferric chloride proceeds at room temperature in chloroform or CCl4, etc. Mechanism of polymerization follows a radical pathway. Electrochemical synthesis A solution containing thiophene and an electrolyte produces a conductive PTh film on the anode. Electrochemical polymerization is convenient, however, it can produce polymers with undesirable alpha-beta linkages and varying degrees of regioregularity. The degree of polymerization, and quality of the resulting polymer depends upon the electrode material, current density, temperature, solvent, electrolyte, presence of water, and monomer concentration. Oxidation at anode  Doping of Polythiophene A variety of reagents have been used to dope PThs. Iodine and bromine produce highly conductive materials, which are unstable owing to slow evaporation of the halogen. Organic acids, including trifluoroacetic acid, propionic acid, and sulfonic acids etc offers stable doped PThs. Oxidative polymerization with ferric chloride can result in doping by residual catalyst. Removal of two electrons (p-doping) from a PTh chain produces a bipolaron. Q. What are the three methods of production of conductive polymers? And what are their limitations? The chemical, electrochemical, and the photoelectrochemical. The chemical needs a high control in the process, since the reaction is very exothermic, which is releases a big amount of energy. The electrochemical limitation is related to the shape of the polymer that has the shape of the electrode, being necessary the posterior processing. Finally, the photoelectrochemical result a material that does not have good mechanical properties. Q. Why do most of polymers have a poor conductivity? And why do a conductive polymer have a great conductivity? Polymers are organic macromolecules and therefore having covalent bonds, which are directional bonds. Thus, the electrons are locked in these bonds, cannot being drift along the material, resulting in the poor conductivity. Moreover, according to the Band Theory, polymers have the band structure of an insulator, because have a big band gap. When a polymer is doped, it arises charges in the polymer. For polymers with alternating double and single bonds, which was doped, has a movement of charge in the chain by resonance resulting in conductivity.  Doping of Polythiophene As the doping level increases, more charges are formed in the polymer and, thus, results in a greater conductivity.  Poly(3-hexylthiophene) (P3HT) Poly(3-hexylthiophene) (P3HT) is a polymer with chemical formula (C10H14S)n. It is a polythiophene with a short alkyl group on each repeat unit. P3HT is widely useful in organic electronics. Synthesis by Reiki Method