Most of the organic polymers are considered to be insulators in their nature. However there is existence of the conducting polymers which are formed from the organic polymers and they can conduct electricity. The ICPs (Intrinsic conducting polymers) exists which contains an alternating double and single bond found along the backbone of the polymer (conjugate bonds) or they are composed of the aromatic ring like phenylene, anthracene, thiophene, pyrrole and also naphthalene that are connected to each other by a single bond of carbon to carbon. The conductive polymers allow an excellent control on the electrical stimuli (Richard, Nigel and Sarah 2341). They have very good optical and electrical characteristics, higher conductivity ratio and they can form biodegradable, biocompatible and also porous products. In addition the greater advantage of the conductive polymer is the physical, electrical and chemical characteristics which can be transformed to the vast applications.
The idea of the conductive polymers started several years ago, and at present, there are more than twenty-five conductive organic polymers. There was the merging of the positive characteristics of the metals and the conventional polymers, i.e., the ability for the charge conduction, electrical conductivity, and optical characteristics and also the ease in the synthesis and processing. The first work of the conductive polymers was prompted by the discovery of the and the observation that was done on the polyacetylene. This is a polymer that is only known for being a semiconductor and it increases to the ten million folds when there is an oxidation of the polyacetylene with iodine vapor. The underlying phenomenon of oxidation was named doping and therefore are essential in the conduction of the polymers since it’s the only process which they can gain their ability to conduct electricity. Since the polyacetylene became very difficult to undergo synthesis and it was also unstable while in the air, it led to the intense research and study of the new and better conductive organic polymer. Therefore in this paper, we are going to discuss the conductive polymer materials, their use, and application in various fields and how they are made.
How the conductive organic polymers are made
The conductive polymers are made by both doping and doping method. The electrical conduction of the conjugate and nondoped polymers like the polyacetylene is because of the existence of the conducting band that is similar to the metal. For the conjugated polymers, “the three of four electrons of the valence from strong σ bonds through sp2 hybridization where electrons are strongly localized” (polymerdatabase.com). The other remaining electrons that are not paired with every carbon atom are retained in the “Pz orbital.” They overlap with the neighboring “Pz orbital” so that they can form the “π bond.” The electrons of π in the “conjugate Pz orbital” continue to overlap so that they can form the extended “Pz orbital system” which allows the movement of the electrons freely (Ghasemi). These imply that the π electrons are delocalized. However, the non-doped polymers are considered to have low conductivity.
It’s only when the electron is detached from the valence band through oxidation which is p doping, or an electron is added to a conducting band through reduction which is n doping in case the polymer turns out to be highly conductive. There are four different methods of doping. The first one is Redox p-doping. In this method, some π bonds are being oxidized through giving treatment to the polymer by use of the oxidizing agent like the chlorine, iodine, arsenic pentafluoride and many more. The second method of doping is by redox n-doping where some π bonds are being reduced through treatment of the polymer by the use of the reducing agent like sodium naphthalene and lithium. The third method of doping is by use of the electrochemical n- and p-doping. In this method, the doping process is achieved through anodic reduction (N) or even cathode reduction (P). The last method of doping is by photo-induced doping in which the polymer should be exposed to higher radiation of the energy which allows electrons to move to the electrical conducting band. For this case, both the negative charge and the positive charge are being localized to the few bonds.
Doping of the polymers raises the electrical conductivity by many different orders of the magnitude. It has been reported that the values of conduction are very high as 102 S/m to 104 S/m. The other method that is essential in increasing the conductivity is through mechanical alignment of the polymers chains. For instance, the conductivity of the polyacetylene is very high at the rate of 105 S/m and is several magnitudes below the conductivity of copper and silver which have 108 S/m, but it has sufficient electronic applications like the “polymer-based transistors, lasers and the light emitting diodes.
Conductive polymeric materials and their uses
In the present, intense researchers have been subjected to the intrinsic conducting polymers. These materials are considered to be polymeric or oligomeric materials that are composed of the phenylene rings and the related units like those of the anthracene, naphthalene and also heteroaromatic rings like the pyrrole and the thiophene (Wallace and Smyth). These are connected with one another by a single bond of carbon to carbon or by the vinylene group which is denoted by –C=C-. This kind of polymeric materials has unique thermophysical and electrical characteristics. Because they have low halogen content and their aromatic structure, they illustrate an excellent thermal, chemical and also oxidation stability. They are practically not soluble in the common solvents. The materials have the potential of conducting electricity especially when they are doped.
They have a very high restriction on the mobility of repeat unit which results to a very high softening point and melting point due to their aromatically ring structure and the absence of the free rotating groups of electrons. This causes the processing and the synthesis of these kinds of resins expensive and difficult (Guiseppi 2712). Thus the viscosities of the melting point are often very high in which the injection molding and the processing methods are not practical or even feasible. Therefore, some of the conductive polymers materials are illustrated below.
The first one is polyphenylenes. They are one of the important classes of the conductive polymers. The phenylene unit from these types of the polymers is connected with the other by the single bond of carbon-carbon atoms. This results in the linear polymers that have the backbone that comprises only the aromatic ring. More so the largest attention has been received in the illustration of the PPP (“Poly Para Phenylene”). This type of polymer has been considered to be so stable up to the temperature range of 500 to 600 degrees Celsius with the minimal of slow oxidation. It is not soluble in many solvents and has a high melting point (Zhou, Cui, and Hines). This polymer possess unusual optical and electronic properties, and it can be processed to the crystalline skinny film, for instance through vacuum deposition, it is electrically conducting when it is doped. PPP can use photoconductive and also the potential of making electroluminescence applications like those of the light emitting diodes.
The second conducting polymer material is polyphenylene vinylenes. The “Poly (para-phenylene vinylenes)” denoted by PPVs and the derivatives are the other class of the electrically conducting polymers that are being studied thoroughly due to a lot of interesting and possibility of the useful photoelectrical and optical properties. The unit of the phenylene that is found in the polyphenylene vinylenes is connected to the other phenylene by the double bond of carbon to carbon leading to the outcome of the rigid and rods like linear polymer that comprises of the only aromatic ring and the double bond (Logan 112). This polymer can be processed to form a highly ordered thin crystalline film that can be able to conduct electricity when it is doped. Similar to PPP, PPVs are capable being used in the electroluminescence applications and can also be used in the form of an emissive layer to the light emitting diodes of the organic-based polymers like the electroluminescent displays. Also, PPVs were the first conductive polymer materials to be used to serve this purpose. Polyphenylene vinylenes and the other copolymers are also being used as the efficient acceptors in the PSCs (polymeric solar cells).
The other essential electrical conductive organic polymer is the doped polyaniline. PAN denotes it. This type of the polymer is not considered as part of the family of the polyphenylene since it possesses amine groups as one of the components of its backbone. Polyaniline is considered to be an attractive conductive organic polymer since its somehow not expensive, its synthesis is easy and its chemically easy to modify. With no surprise, it is the most conductive polymer that has been studied, and it finds many applications as the conductor and also as an electromagnetic shield for the electronic circuit. Also, polyaniline is also being used as the corrosion inhibitor and also in the manufacture of the conducting nanofiber.
The fourth conductive polymeric material is polypyrrole. This is one of the promising conductive polymeric materials and is denoted by PPy. PPy can be processed easily, and it possesses a lot of interesting characteristics of electrical conduction. The polymer is very stable both regarding electrical and thermal conduction. In comparison to the other full aromatic organic polymers, PPy has been experimentally illustrated as an electrical insulator (Shirakawa, Louis, and MacDiarmid). When PPy is oxidized with an oxidizing agent, it turns and become an electrical conductor. The electrical conductivity of the polypyrrole highly depends on the techniques that are being used in its preparation, and the polymer additives that are being used are essential in increasing the conductivity by two orders of the magnitude. The uses of this polymer include coating of the anti-electrostatic used to act as the gas sensors, used as a solid electrolytic capacitor and as one of the components in the electronic devices.
The fifth conducting polymeric material is the polythiophenes. This type of polymer and its derivatives are very promising since the when polythiophenes have not doped the conductivity of the electricity is considered to be very low. However, when the polythiophenes are doped with low level or less than one percent of the doping materials the conductivity of electricity increases many times. Especially for the regioregular PATs (poly 3-alkylthiophenes) are of greater interest due to their respective structural orders that lead to the greater charge mobility carriers. These types of the polymers are very soluble and also fusible, and therefore they are used to demonstrate some novel properties like “thermochromism” and “solvatochromism.” The absorption or even emission can be adjusted from ultraviolet to the IR by just changing the polythiophenes substituents.
The last type of the conductive polymeric material is the polyacetylene or sometimes called polyethylene. It is composed of the repeating units of C2H2 which are very rigid and rod-like. They contain a long chain of carbon that has an alternating double bond and single bonds within the atoms of carbon. It is all known that the electrically conductive polymers began by doping of the organic polymers. The polyacetylene was the first polymer that had conductivity synthesis. The electrical conductivity of this polymer was discovered by Alan Heeger, Shirakawa Hideki and also MacDiarmid Alan who both received the Nobel Prize in the field of Chemistry in 2000 due to this discovery (Skotheim and Elsenbaumer). They first synthesized the polymer in 1974. In that year they prepared polyacetylene from acetylene in the form of thin silvery film including the Ziegler-Natta catalyst. The result was a metallic appearance of the polymer, but the attempt didn’t yield the conductive polymer. After three years of research, they discovered that oxidation with the halogen vapor would lead to the production of a very conductive film of polyacetylene. The conductivity of this polymer became significantly higher as compared to the initial conductive polymer that was known. Hence, this discovery initiated the development of many other electric conductive organic polymers. Despite the fact that the discovery of the polyacetylene began the development of the conductive polymeric materials, these type of polymer does not have any commercial use and application.
Application of conductive polymeric materials
There are many applications of the conductive polymeric materials in different fields. Some of them have been illustrated below. The first one is supercapacitors. The supercapacitor consists of the electrode whose materials are classified in three different categories including the transition metal oxide, the conducting polymer, and the higher surface carbon. This supercapacitor which is sometimes called the electrochemical capacitor is essential in the development of the hybrid electric vehicle and is also used in the electronic devices which are portable for their urgency and increased demand (Murate and Sarac 232). The Supercapacitor have been used in this appliance due to the increased power supply, simplicity, long life cycle and a higher dynamic of the charge propagation.
The second one is the light emitting diodes (LED). The polymer light emitting diode is based on the PPV and at the moment is coming out as the commercial products. In a comparison of the organic and inorganic materials used to develop LEDs, the benefit of the polymer electroluminescence is that the device has the fast response time in processing ability, a low voltage of operation, the possibility of covering a larger area and many more. The polymers found in the electronic company have established the passive role of being the insulators and are now being involved in the active roles like designing of the microlithography applications.
The third application is in the solar cell. The polymeric materials are involved in the designing of the low-cost electronic devices like the photovoltaic devices and the other organic electronics have been received with greater attention. In a comparison of the organic technology and the silicon-based photovoltaic materials used in the solar panels, the organic photovoltaic provide low thermal budget, the very high speed of processing and low-cost processing solution. The fourth application is on the field effect transistors where the conducting polymers are found in different applications like smart pixels and also the sensors. The last application is on the biosensors which are a device used in biological sensing of the elements that are connected to the transducer with the aim of producing an electronic signal that is proportional to the set chemicals. The conducting polymers have been applied to the biosensors to enhance speed, stability and also sensitivity. Therefore it has been recommended for the medical diagnosis.
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