Welcome to VIEWCHEM, the Chemistry Animations Channel. Mechanism of Conduction in Conducting Polymers In general, many organic polymers are used as electric insulators. For example, polyethylene, PVC, and Teflon. The main reason for the non-conducting nature of organic polymers is due to the absence of conjugation.
In saturated chemical structures, all the valence electrons are strongly localized. These electrons do not contribute to the electrical conductivity of the material. But due to the extensive research work, scientists are able to synthesize conducting polymers. Conducting polymers are broadly classified into two types.
These are intrinsically conducting polymers and extrinsically conducting polymers. In the intrinsically conducting polymers, the conduction of electricity is due to the extensive conjugation in the backbone of the polymer. Extrinsically conducting polymers are basically blended polymers. The blends are responsible for conduction.
Intrinsically conducting polymers are further classified into two more types. These are conjugated conducting polymers and doped conducting polymers. Extrinsically conducting polymers also classified into two types. These are polymers with conducting elements and polymers with conducting blends. Conjugated conducting polymers contain alternate double bonds.
Doped conducting polymers are formed due to the interaction with charge transfer agents, that is, dopants. Polymers with conducting elements contain conducting fibers. In the polymers with conducting blends, conventional non-conducting polymers are blended with conducting polymers. Polyacetylene is the first example of synthetic conducting polymer. Acetylene on passing over Ziegler Nauticatalyst at room temperature forms polyacetylene.
Ziegler Nauticatalyst. Vapors of Acetylene at room temperature. Polyacetylene film containing a mixture of both cis and trans structures is formed. Cis form is in copper color while trans structure silvery.
At lower temperature, that is, around minus 80 degrees centigrade, cis form exists. Sysform, on heating above 150°C, converts to, trans-structure. In a conducting polymer, sp2 hybridized carbon chain, constitute, the backbone for, conduction.
On each sp2 hybridized carbon, there will be a single electron in the unhybridized pz orbital. The unhybridized pz orbital lies orthogonal to sp2 hybridized orbitals. The delocalization of electrons present in the pz orbitals results in the conduction sp2 hybridized carbon containing 3 sp2 hybrid orbitals. These 3 sp2 hybrid orbitals lie in the same plane.
Unhybridized p orbital lying orthogonal to sp2 hybrid orbitals contains an electron. According to band theory, in the case of insulator, between the valence band, and, the conduction band, the forbidden gap is more. While in the case of semiconductor, the forbidden gap is, moderate.
In the case of conductors, the forbidden gap is negligible. In conjugated polymers, the forbidden gap can be decreased by introducing charge carriers through doping. This process facilitates conduction in conjugated polymers. The requirements for a conducting polymer for electrical conduction are The molecule should have a linear backbone. The molecule should have extended conjugation.
Charge carriers, that is, either positive holes or electrons, should be introduced by appropriate dopants. The conducting properties of a conjugated polymer can be enhanced by doping with suitable dopants. P-doping, the polymer, which has conjugation in the backbone, when treated with, electron-deficient species, that is, Lewis acids, like, FeCl3, or, iodine vapor, or, iodine vapor in ClCl4, there takes place, oxidation, and, a positive charge is created, in the molecule.
Removal of, one electron, from the pi backbone of, a conjugated polymer, forms, a radical cation, that is, polarin, which, on losing another electron, forms, bipolarin. The delocalization of positive charges, causes electrical conduction. Polyacetylene, is a neutral molecule, with extended conjugation. Its conductivity is, very low.
On doping, With suitable dopants, conductivity increases. Polyacetylene, on treatment with, iodine, in carbon tetra chloride, forms, a radical cation, that is, a polarin. Polarin, can be a.
radical cation, or radical anion. In this case, polarin is a radical cation. Polarin, on treatment with iodine in carbon tetrachloride, converts to bipolarin.
bipolarin of polyacetylene in this mechanism contains two positive charges that is dication in soliton charge delocalization takes place further delocalization of charges can be observed. And doping, when, Lewis bases, that is, electron-rich species, are treated with polymer, having conjugation, due to reduction of the polymer, negative charge develops. For example, When polyacetylene is treated with sodium naphthalide, negative charge is developed in the polymer.
In this mechanism also, the first step is the formation of polarin, and in the second step, bipolarin is formed. This can be accomplished by dipping the polyacetylene film in the solution of an alkali metal naphthalide dissolved in tetrahydrofuran, or bion. Electrochemical Method Actually, by the addition of one electron, polarion, and by the addition of second electron, bipolarion, are formed. In bipolarion, due to the delocalization of the charge, conduction takes place.
Polyacetylene is a neutral molecule with extended conjugation. Its conductivity is very low. On doping, with suitable dopants, conductivity increases. Polyacetylene, on treatment with, sodium naphthalide, forms, a radical anion. Polarin, can be, a radical cation, or, radical anion, in this case.
Polarin is a radical anion. On treatment with sodium naphthalide, Polarin converts to bipolarin. Bipolarin of polyacetylene in this mechanism contains two negative charges, that is, a dianion charge delocalization the mechanism of conduction in doped polyacetylene can be visualized as mechanism of conduction in polyacetylene In the conventional organic polymers, all the carbon atoms are sp3 hybridized, and do not conduct electricity, owing to the tightly bound sigma electrons. But another class of polymers, called as conducting polymers, have all sp2 hybridized carbon atoms, in the polymeric chain.
Each sp2 hybridized carbon atom contains, one unhybridized p orbital, at orthogonal position to the other sp2 hybridized orbitals. Each unhybridized p orbital on the sp2 carbon atom contains one electron which can be delocalized and responsible for electrical conduction. These unhybridized p orbitals combine to form pi bonds in an alternate manner. Thus, so formed pi bonds are conjugated. Movement of electrons in these conjugated or orbitals results in electrical conductivity.
The conductivity of doped polyacetylene is more than the undoped molecule. Because, doping of polyacetylene creates charge carriers in the molecule. The charge carriers can be either holes, or electrons. Polyacetylene can be either oxidized, or reduced, to introduce charge carriers. The movement of charge carriers towards opposite electrodes is responsible for electrical conduction.
P-doping is an oxidative process in which holes are created in the polyacetylene by removing the electrons from the valence band on treating with Lewis acids like iodine or bromine. mechanism. In the first step, polyacetylene is treated with vapors of iodine at room temperature. As a result, an electron is removed and a radical cation, that is a positive polarion is formed. Due to the movement of charge carriers, electrical conduction takes place.
Electrical conduction takes place in a positive bipolarion also. Positive bipolarion is a dicanion. When polyacetylene is treated with sodium naphthalide, that is when polyacetylene is endoped, reduction takes place. In the process, electrons are added to the conduction band of polyacetylene. As a result, negative polarin and a dianion are formed.
Mechanism Other conducting polymers benzene, polyparaphenylene, aniline, polyaniline, pyrrole, polypyrrole, thiophene, polytheophene. Few important applications of conducting polymers. Conducting polymers are used in the preparation of biosensors, artificial nerves, artificial intelligent materials, flat panel displays, flexible panel displays, ion exchange membranes, rechargeable batteries, printed circuit boards, corrosion inhibitors, antistatics.
Further, conducting polymers are used to detect the gases and to absorb harmful radiation from electronic devices thanks for watching please like subscribe and share