Polymeric Materials PDF

Unit 3 Conducting polymers: Conducting mechanisms - Electron transport and bipolar polymers. Photoconductive polymers: Charge carriers, charge injectors, charge transport, charge trapping. Liquid crystalline polymers: Fundamentals and process, liquid crystalline displays – applications. Polymers for...

Unit 3 Conducting polymers: Conducting mechanisms - Electron transport and bipolar polymers. Photoconductive polymers: Charge carriers, charge injectors, charge transport, charge trapping. Liquid crystalline polymers: Fundamentals and process, liquid crystalline displays – applications. Polymers for light emitting diodes – introduction, polymer structures, Organic LEDs-their functioning-advantages and disadvantages over conventional LEDs – their commercial uses. Piezoelectric materials – working principle and applications. 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 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.  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 which 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 negative 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. Absorption of radiation and formation of excitons The first step to charge generation is the absorption of radiation. For low light intensities photoconductive materials are truly photoconductive only in the range of the wavelength of absorption. Since carbazole absorbs inly in the UV range PVK and other polymers containing electronically isolated carbazolyl groups are photoconductive only in the UV range. Thus, in order to produce charge carriers by visible light sensitizing dyes or electron acceptors forming coloured charge transfer complexes have to be added. Generation of charge carriers By the absorption of light the active groups are excited and form closely bound electron–hole pairs, i.e. excitons. The excitons are captured and dissociated at the donor/acceptor sites as a result of functional groups that are polarized and appropriately to cause charge separation. The key process that determines the overall photogeneration efficiency is electric field induced separation of excitons into free charge carriers. Injection of carriers An injection of carriers occurs only if an extrinsic photogenerator is used together with a charge transporting material. Usually dye particles are dispersed in a polymer matrix or evaporated on top of a conductive substrate and then covered with the charge-transporting polymer. The carriers are generated in the visible light-absorbing material and injected into the polymer. Charge injection, as well as photogeneration and charge transport, is electric field-dependent. Carrier transport The photogenerated or injected charge carriers move within the polymer under the influence of electric field. In this process the photoconductive species, for example carbazole groups in PVK, pass electrons to the electrode in the first step and thereby become cation radicals. Cation radicals of PVK are stabilised by the charge (hole) resonance among more than two neighbouring chromophores. The transport of carriers can now be regarded as a thermally activated hopping process, in which the hole hops from one localised site to another in the general direction of the electric field. The moving cation radical can accept an electron from the neighbouring neutral carbazole group which in turn becomes a hole. Effectively the hole moves within the material while electrons only jump among neighbouring species. In a more chemical terminology hole transport can therefore be described as a series of redox reactions among equivalent groups Recombination Coulombic forces eventually cause recombination of free electrons and holes at recombination sites in the circuit. This process competes with the generation of charge carriers. Trapping During transit, the carriers do not move with uniform velocity but reside most of the time in localised states (traps) and only occasionally are released from these traps to move in field direction. The traps can be shallow or deep. These terms are only relative, and refer to the release times. Shallow traps are those from which carriers are released in the time of experiment. The trapping process is responsible for the extremely low charge carrier mobilities in most of the polymers. These mobilities are usually electric field and temperature dependent. For PVK room temperature mobilities ranging from 10-8 to 10-6 cm2 /V/s at an electric field (E) of 105 V/cm have been reported An exciton is a bound state of an electron and an electron hole which are attracted to each other by the electrostatic Coulomb force. An exciplex is an excited-state complex formed between a molecule that donates electrons and one that accepts electrons. Exciplexes are generally of interest for their favorable light-emission properties https://doi.org/10.1002/1521-3773(20010903)40:17%3C3234::AID-ANIE3234%3E3.0.CO;2-B A Good photoconductive polymer should be : A pendant group (sometimes spelled pendant) or side group is a group of atoms attached to a backbone chain of a long molecule, usually a polymer. Polycarbonate Polystyrene  Organic Light Emitting Diode (OLED) Organic Light emitting diodes (OLEDs) or organic EL (organic electroluminescent) diodes that use polymers or small organic molecules as their optically active element. The electroluminescent layer is a film of organic compound that emits light in response to an electric current. The organic layer is situated between two electrodes; typically, at least one of these electrodes is transparent. OLEDs are used to create digital displays in devices such as television screens, computer monitors, portable systems such as smartphones, handheld game consoles, and PDAs.  The plastic, organic layers of an OLED are thinner, lighter and more flexible than the crystalline layers in an LED or LCD. Because the light-emitting layers of an OLED are lighter, the substrate of an OLED can be flexible instead of rigid. OLED substrates can be plastic rather than the glass used for LEDs and LCDs. OLEDs are brighter than LEDs. The organic layers of an OLED are much thinner than the corresponding inorganic crystal layers of an LED, the conductive and emissive layers of an OLED can be multi-layered. Also, LEDs and LCDs require glass for support, and glass absorbs some light. OLEDs do not require glass. OLEDs are easier to produce and can be made to larger sizes. Because OLEDs are essentially plastics, they can be made into large, thin sheets. It is much more difficult to grow and lay down so many liquid crystals. OLEDs have large fields of view, about 170 degrees. Because LCDs work by blocking light, they have an inherent viewing obstacle from certain angles. OLEDs produce their own light, so they have a much wider viewing range. Liquid crystal is a state of matter whose properties are between conventional liquids and solid crystals. Liquid crystalline polymers (LCPs) are a special type of thermoplastics that exhibit properties between highly ordered solid crystalline materials and amorphous disordered liquids over a well defined temperature range. To date, thousands of LC polymers have been synthesized. However, only a small number have become commercially important LC materials. The three most common LCPs are semi-aromatic copolyesters, copolyamides, and polyester-co-amides. These polymers contain rigid rod-like or plate-like repeat units with a high length-to-width ratio, the so called mesogenic groups, that are able to self-assemble into anisotropic liquid crystals (mesophases) upon cooling or under the action of an external field. Three very common LC mesophases are nematic, smectic A, and smectic C. Nematic mesophases show only unidimensional orientation order in the direction of the long (in rod-shaped) or short (in disc-shaped) molecular axes. This is typically the flow direction during processing of the LC resin. Smectic mesophases, on the other hand, show two-dimensional orientation order. These LC’s have a lot in common with crystalline polymers where molecules are arranged in layers (lamellae). The long axes of the (rod-like) molecules are either perpendicular to the plane of the layers (smectic A) or it is inclined at an angle (smectic C). Liquid crystallinity in polymers may occur either by dissolving a polymer in a solvent (lyotropic liquid-crystal polymers) or by heating a polymer above its glass or melting transition point (thermotropic liquid-crystal polymers). Liquid-crystal polymers are present in melted/liquid or solid form. In solid form the main example of lyotropic LCPs is the commercial aramid known as Kevlar. Chemical structure of this aramid consists of linearly substituted aromatic rings linked by amide groups. In a similar way, several series of thermotropic LCPs have been commercially produced by several companies. Liquid (left), liquid crystalline (middle) and solid (right) phase of cholesteryl benzoate. Because of their various properties, LCPs are useful for electrical and mechanical parts, food containers, and any other applications requiring chemical inertness and high strength. LCP is particularly good for microwave frequency electronics due to low relative dielectric constants, low dissipation factors, and commercial availability of laminates. Packaging, microelectromechanical systems (MEMS) is another area that LCP has recently gained more attention. The superior properties of LCPs make them especially suitable for automotive ignition system components, heater plug connectors, lamp sockets, transmission system components, pump components, coil forms and sunlight sensors and sensors for car safety belts. LCPs are also well-suited for computer fans, where their high tensile strength and rigidity enable tighter design tolerances, higher performance, and less noise, albeit at a significantly higher cost. Liquid Crystal Display (LCD)  The first filter will naturally be polarized as the light strikes it at the beginning. Thus the light passes through each layer and is guided on to the next with the help of molecules. When this happens, the molecules tend to change the plane of vibration of the light to match their own angle. When the light reaches the far side of the liquid crystal substance, it vibrates at the same angle as the final layer of molecules. The light is only allowed an entrance if the second polarized glass filter is same as the final layer. Polarisation is a process in which the vibration of light waves is restricted to a single plane, resulting in the formation of light waves known as polarised light. Liquid crystal display screen works on the principle of blocking light rather than emitting light. LCDs require a backlight as they do not emit light by themselves Types of LCD Reflective This type of LCD has a mirror layer. When a light ray within an LCD is reflected by the mirror layer, then visible patterns are produced on the LCD. Transmissive Here the LCD has a backlight, which passes through the LCD polarised glass to produce visible pattern. But because it uses backlight for working, the images displayed in such LCD types appear very dim when used under bright sunlight. Transflective This LCD type has a reflective mirror layer and a backlight. It uses both outside light and backlight, making it suitable for indoor and outdoor conditions. A backlight is a form of illumination used in liquid-crystal displays (LCDs) that provides illumination from the back or side of a display panel. 1. For making an LCD screen, a reflective mirror has to be set up in the back. 2. Followed by glass plate with a polarizing film is also added on the bottom side. 3. An electrode plane made of indium-tin oxide is kept on top. 4. The entire area of the LCD has to be covered by a common electrode and above it should be the liquid crystal substance. 5. Next comes another piece of glass with an electrode in the shape of the rectangle on the bottom and, on top, another polarizing film. 6. It must be noted that both polarizers are kept at right angles to one another. 7. When there is no current, the light passes through the front of the LCD it will be reflected by the mirror and bounced back. As the electrode is connected to a temporary battery the current from it will cause the liquid crystals between the common-plane electrode and the electrode shaped like a rectangle to untwist. Thus the light is blocked from passing through. Thus that particular rectangular area appears blank. 8. In the absence of any voltage, the perpendicular alignment layers cause the liquid crystal to adopt a twisted configuration from one plate to the other. 9. With no liquid crystal present, light passing in either direction through the cell would be absorbed because of the crossed polarizers, and the cell would appear to be dark. 10. In the presence of a liquid crystal layer, however, the cell appears to be transparent because the optics of the twisted liquid crystal match the crossed arrangement of the polarizers. Application of three to five volts across the liquid crystal destroys the twisted state and causes the molecules to orient perpendicular to the substrate plates, giving a dark appearance to the cell, as shown in the diagram. For simple displays, the liquid crystal cell is operated in a reflective mode  Piezoelectric materials A property of certain materials which expand or contract in an electrical field or a property of certain materials which generates an electrical charge when pressure is applied Direct Piezoelectric Effect Piezoelectric Material will generate electric potential when subjected to some kind of mechanical stress. Inverse Piezoelectric Effect If the piezoelectric material is exposed to an electric field (voltage) it consequently lengthens or shortens proportional to the voltage.’  Types of Piezoelectric Materials Naturally occurring crystals: Berlinite (AlPO4), cane sugar, Quartz, Rochelle salt, Topaz, Tourmaline Group Minerals, and dry bone (apatite crystals) Man-made crystals: Gallium orthophosphate (GaPO4), Langasite (La3Ga5SiO14) Man-made ceramics: Barium titanate (BaTiO3), Lead titanate (PbTiO3) Polymers: Polyvinylidene fluoride (PVDF)  Ex: PVDF CH2CF2 (Polyvinylidine fluoride) The hydrogen atoms (H’s, above), which have a net positive charge the fluorine atoms (F’s, above), which have a net negative charge end up on opposite sides of the sheet. This creates a pole direction (indicated by the small p, above) and is either directed to the top or bottom of the sheet.  When an electric field (E) is applied across the sheets Either contract in thickness and expand along the stretch direction or expand in thickness and contract along the stretch direction depending on which way the field is applied. This is due to the physical nature of the positive hydrogen atoms attracting to the negative side of the electric field and repelling from the positive side of the electric field. The negative fluorine atoms attract to the positive side of the electric field and repelling from the negative side of the electric field. Effects of the two electric field directions on a sheet of PVDF can be seen below. The electric field is in the opposite direction of The electric field is in the same direction of the the poled direction and the sheet is stretched poled direction and the sheet is contracted in length. length.  Applications Transducers (Ultrasonic, Audio, medical etc ) Switches, key boards Tissue engineering supports Energy harvesting systems Etc.