Polymers 1 20-09-24.pdf

Polymers  Polymers Polymers are organic macromolecules, a long carbon chain, composed by structural repeating entities, called mer. These smallest units, for instance, are bonded by covalent bonds, repeating successively along a chain. A monomer, molecule co...

Polymers  Polymers Polymers are organic macromolecules, a long carbon chain, composed by structural repeating entities, called mer. These smallest units, for instance, are bonded by covalent bonds, repeating successively along a chain. A monomer, molecule composed by one mer, is the raw material to produce a polymer. The majority of polymers are insulators, due to an unavailability of free electrons to create the conductivity. Therefore, these type of materials do not show a high conductivity.  Conducting Polymers The polymers that are used in daily basis are insulators. However, some polymers can conduct electricity under certain conditions. Hence, there are some mechanisms through which electrons can be made available in organic molecules. The Nobel Prize in Chemistry 2000 was awarded jointly to Alan J. Heeger, Alan G. MacDiarmid and Hideki Shirakawa “for the discovery and development of conductive polymers.” These materials, based on doped polyacetylene and other conjugated polymers, are sometimes called synthetic metals.  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 would serve both current-carrying and ion- conduction functions by replacing traditional electrode and electrolyte substances Chemical structure of some types of conjugated polymers: (a) Trans-polyacetylene-trans-Pac, (b) Poly(p-phenylene)- PPP, (c) Poly(pphenylene-vinylene)-PPV, (d) Poly(p-phenylene-sulphide)-PPS, (e) Polypyrrole-PPy, (f) Polythiophene-PTh, (g) Poly(3,4-ethylenedioxythiophene)PEDOT, (h) Polyaniline-PAni and (i) Polyfluorene-PFO  Applications Biosensors Light weight Batteries Supercapacitors Transparent antistatic coatings for metals and electronic devices Electromagnetic shielding light-emitting diodes (LEDs) electrodes, biosensors, transistors, and ultrathin, flexible screens for computer and TV monitors.  Types of Conducting Polymers Conducting Polymers according to their Composition The main chain No hetero atoms Heteroatoms present contains Nitrogen containing Sulphur containing Aromatic cycles Poly(p-phenylenes) The N is in the The S is in the aromatic Poly(naphthalenes) aromatic cycle: cycle: Poly(fluorenes) Poly(pyrroles) Poly(thiophenes) Poly(indoles) The S is outside the aromatic The N is outside the cycle: aromatic cycle: Poly(p-phenylene sulphide) Polyanilines Double bonds Poly(acetylenes)  Types of Conducting Polymers Linear-backbone “polymer blacks” (polyacetylene, polypyrrole, polyaniline, etc.) and their copolymers are the main class of conductive polymers. Intrinsically conducting polymers These polymers have a solid backbone with conjugated double bonds that allow for electron delocalization. They can conduct electricity due to thermal or light activation of electrons. Polyacetylene is an example of an ICP Extrinsically conducting polymers These polymers are composites that contain added conductive elements, such as carbon black, embedded in a non- conducting polymer.  Conductive Polymers or Intrinsically Conducting Polymers Conductive polymers or more precisely, intrinsically conducting polymers (ICPs) are organic polymers that conduct electricity. Such compounds may have metallic conductivity or can be semiconductors. The biggest advantage of conducting polymers is their processability, mainly by dispersion. Conductive polymers are organic materials, but they are generally 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. Conducting polymers have backbones of continuous sp2 hybridized carbon centres. One valence electron on each centre resides in a pz orbital, which is orthogonal to the other three sigma bonds. The electrons in these delocalized orbitals have high mobilities.  Types of Conducting Polymers Intrinsically conducting polymers are substances which have a π-bond backbone. There are certain electrons that are extra in this type of polymers. These extra electrons flow from one point to another in the polymer, as a result they have the ability to conduct electricity. Conduction of electricity in this type of polymers is due to conjugation in the backbone of polymer. The conjugation can be due to either π electrons or due to doped ingredients. Conduction due to conjugated π electrons: In these types of polymers, due to the presence of double bonds and lone pair of electrons conduction of electricity takes place. Actually due to overlapping of conjugated π electrons, valence and conduction bands are developed throughout the backbone of the polymer. Electrical conduction can occur only after attainment of required energy of activation either thermally or photochemically because there is some gap between the valence and conduction bands. So the electrons need to be excited by some means. Polyacetylene, polyaniline, etc., are these types of conducting polymers.  Doped Conducting Polymers The conduction power of semiconductor can be enhanced by adding some foreign material or desired impurities. These impurities are called doping agent or dopant. Appropriate doping agent increase the conductivity of semiconductors up to 104 times. The increase in conduction is due to participation of impurity elements in between the valence band and conduction band and thus making a bridge through which electrons can jump easily from the valence band to the conduction band. Actually the conjugated π electrons have very low ionization potential and high electron affinities. The foreign materials develop positive or negative charge through oxidation or reduction of the semiconductor. Doping are mainly two types. 1. p-type doping through oxidation of materials: In this type of doping some electrons from the conjugated π bonds are removed through oxidation creating a positive hole called polaron inside the polymer. The positive hole or polaron can move throughout the polymeric chain and make it conducting polymer.  Doped Conducting Polymers The polymers which have conjugation in the backbone when treated with electron-deficient species (Lewis acid) like FeCl3 or I2 vapour or I2/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.  Doped Conducting Polymers 2. n-type doping through reduction of materials: 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 hole or 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. When Lewis bases (electron rich species) are treated with polymer having conjugation, due to reduction of the polymers, negative charge develops. Actually by the addition of one electron, polaron and by the addition of the second electron, bipolaron are formed. In bipolaron, due to the delocalization of charge, conduction takes place.  Doped Conducting Polymers 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.  Extrinsically Conducting Polymers (ECPs) Those conducting polymers which owe their conductivity due to the presence of externally added ingredients in them are called extrinsically conducting polymers. Extrinsically conducting polymers (ECP’s) are of two types. These are: (1) conducting elements filled polymers (CEFP) i.e., the polymers filled with conducting element, and (2) blended conducting polymers (BCP). 1. Conducting Elements Filled Polymers (CEFP): In this type, 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. The conduction power of polymer can be enhanced by adding some foreign conducting material or good conductor in powder (carbon dust) form or granule from (metallic fibers). The role of polymer is to bind the conducting materials.  Extrinsically Conducting Polymers (ECPs) 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. The filler (ingredients) that percolate have more surface area, more porosity and filamentous nature due to which they can enhance conducting properties. Important characteristics of these polymers are : (a) They possess good bulk conductivity; (b) They are cheaper; (c) They are light in weight; (d) They are mechanically durable and strong; (e) They are easily processable in different forms, shapes, and sizes. 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.  Polyaniline The conductivity of polyaniline is dependent upon the dopant concentration, and it gives metal- like conductivity only when the pH is less than 3. Polyaniline exists in different forms which are shown in Figure. Leucoemeraldine exists in a sufficiently reduced state, and pernigraniline exists in a fully oxidized state. Polyaniline becomes conductive only when it is in a moderately oxidized state and acts as an insulator in a fully oxidized state.