An Examination Of The Applications Of Conducting Polymers Biology Essay

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Conducting polymers are the plastic that conducts electricity. Conductive polymers found applications in electronic industry by shielding against electromagnetic interference.

FIGURE:-SOME POLYMERS

Conducting polymers are also organic polymers. Therefore they can combine the mechanical properties like toughness, flexibility, malleability, elasticity etc. of plastics with high electrical conductivity.

POLYACETYLENE (polyethene) was the first conducting polymer discovered.

POLYPYROLE was the first aromatic polymer to be produced.

Some uses of conducting polymers in microelectronics is depicted by the below given figure.

DISCOVERY OF CONDUCTING POLYMERS:-

In 1958, polyacetylene was first synthesized by Natta et al. as a black powder. This was found to be a semi-conductor with conductivity between 7 x 10-11 to 7 x 10-3 Sm-1, depending upon how the polymer was processed and manipulated.

In 1967, a Japanese postgraduate of Hideki Shirakawa by mistake produced a metallic looking film, when he was trying to produce polyacetylene. It was found that he has added a 1000 times more ZIEGLA-NATTA catalyst.

This film was having same conductivity as that of black powder (polyacetylene).On investigation, it was found that, he has produced trans-polyacetylene. On repeating the experiment at different temperature, he was able to produce cis-polyacetylene. [2]

FIGURE 2:- cis and Trans poly (acetylene)

HISTORY OF CONDUCTING POLYMERS:-

CHANCE CONVERSATIONS and experimental accidents, as well as informed in sight, often play a pivotal role in orchestrating the direction of scientific endeavour. The CHEM COMM paper in 1977, in which a polymer chemist Hideki Shirakawa, an organic chemist Alan MacDiarmid, and physicist Alan Heeger presented their discovery of the simplest conducting polymer, polyacetylene (doped with halogens ), was the result of one of the most famous examples. This work reinforced the then emerging concept of organic materials behaving not as traditional insulators but as metals or semiconductors. It has driven many advances in materials science, electronic theory and technological applications. The discovery of conducting polyacetylene was recognized by the award of the CHEMISTRY NOBEL PRIZE in 2000.[3]

FIGURE3:- Hideki Shirakawa, Alan MacDiarmid and Alan Heeger.

FIGURE4:- The original 1977 CHEMCOMM paper.

FIGURE5:-Scanning electron micrograph of highly oriented polyacetylene fibrils synthesised by the liquid-crystal polymerisation method under a magnetic field.

WHY SOME POLYMERS CONDUCT?

It is well known that graphite is a good conductor, previously it was thought that polymers which substitute a carbon (e.g. adding hydrogen's to make hydrocarbons) for another atom could not conduct, however our greater knowledge of conjugated systems has enabled the discovery of conducting polymers.

As in conjugated system, electrons are loosely bound, electron flow may be possible. As the polymers are covalently bounded, the material needs to be doped for the flow of electron to occur.

Doping is either the addition of electrons (reduction reaction) or the removal of electrons (oxidation reaction) from the polymer.  Once doping has occurred, the electrons in the pi-bonds are able to "jump" around the polymer chain.  As the electrons are moving along the molecule a electric current occurs.

However the conductivity of material is limited as the electrons have to "jump" across molecules so for better conductivity the molecules must be well ordered and closely packed to limit the distance "jumped" by the electrons.  This occurs better in trans-undoped poly-actelyene.  By doping, the conductivity of the polymer increases from 10-3 S m-1 to 3000 S m-1 . [4]

TYPES OF CONDUCTING POLYMERS:-

There are three types of conducting polymers,

1. Proton conducting polymers.

2. Ion conducting polymers.

3. Electron conducting polymers.

1. PROTON CONDUCTING POLYMERS:-

FIGURE 6:- Synthesis of polybenzimidazoles.

The development of efficient proton-conducting membranes is of the greatest importance for the design and improvement of low-temperature fuel cells. Poly-benzimidazoles have been considered as promising alternative materials for the fabrication of proton-exchange membranes, and a good number of very recent publications have appeared on this topic. These polymers have very good thermal stability and proton conductivity especially, when doped with acids. Among many possible polybenzimidazole derivatives, the polymer most extensively examined is poly [2, 29-(m-phenylene)-5, 59-bibenzimidazole] (PBI). In addition to the commercial polymer, several laboratories have also centered on the synthesis and development of new materials; as a result, the polymer known as ABPBI [poly (2, 5-benzimidazole)] has been reported as a proton-conducting membrane.

ABPBI was obtained by condensation of 3, 4-diaminobenzoic acid monomers in polyphosphoric acid (PPA). MPPBBI and SMPPBBI were prepared by condensation of tetraaminobenzene and sulfonated or non-sulfonated isophthalic acid. About 250 mg of polybenzimidazole (ABPBI, MPPBBI, or SMPPBBI) were dissolved in 10 mL of the acid. Each polybenzimidazole sample was precipitated from solution by addition of different amounts of water. The resulting doped polymers were filtered off under vacuum and dried at 100 °C. The amount of phosphoric acid present in each sample was calculated by elemental analysis.

Thermal stability of proton conducting polymers:-

FIGURE 7:- TGA of the non-doped polymers in nitrogen; heating rate 10 °C/min: (a) PBI, (b) ABPBI, (c) MPPBBI, and (d) SMPPBBI.

An initial weight loss is observed in all cases between room temperature and about 150 °C (Fig. 4). This is assigned to the loss of variable amounts of loosely bound water. Although the figure shows obvious differences in thermal stability, it must be noted that all polymers are stable at least up to temperatures of about 400 °C.

FIGURE 8:- TGA of acid-doped polybenzimidazoles and 85% H3PO4 in nitrogen; heating rate 10 °C/min: (a) SMPPBBI z 3.6 H3PO4, (b) MPPBBI z 5.4 H3PO4, (c) ABPBI z 5.0 H3PO4, and (d) 85% H3PO4.

TGA of acid-doped polymers can be compared and related in part to that of phosphoric acid (Fig. 5) and the corresponding un-doped polymers (Fig. 4). First, all doped derivatives exhibit an initial weight loss below 200 °C most likely corresponding to water loss. The acid-doped sulfonated polymer shows (as its un-doped counterpart) a significant loss at 450 °C corresponding to the sulfonic acid group. On the other hand, and contrary to the stable weight featured by the un-doped polymers, the TGA curves of all acid-doped derivatives indicate a slow and continuous weight loss that parallels that of phosphoric acid. Then, the most significant losses for acid-doped polymers take place at 690 and 920 °C. [5]

ION CONDUCTING POLYMERS:-

Polymers such as PEO complexed with salts form good ionic conductors which are used as electrolyte in many devices. These materials are biphasic with crystalline as well as amorphous phases present in a single sample. In addition there is continuous rearrangement of the polymer chains, which aids in the ion transport process. In a recent study of PEO-NH4ClO4 systems it was shown that there are two distinct morphological regimes for varying salt concentration and the ion-conductivity can be related to the morphology.

Ion conducting polymers have a biphasic character with crystalline as well as amorphous phases. There is moreover, a dynamic disorder due to motion of polymer chain segments. The PEO-NH4ClO4 system undergoes a crossover from a DLA-type morphology for low salt fraction (X) to a structure with polygonal spherulites. In the present communication we show that the low X regime exhibits a variation of diffusivity with crystallinity typical of a quenched system, whereas the high X regime has dynamic disorder with rapid rearrangement. [6]

THERMAL STABILITY OF IONIC CONDUCTING POLYMERS:-

EXLAINING WITH THE HELP OF PROTON EXCHANGE MEMBRANE FUELL CELLS (PEMFC).

One of the main component of the PEMFC is the ion conducting polymer membrane. Two types of ion conducting membranes could be used: anionic or cationic membranes. Anionic membranes are used for alkaline fuel cells. We will consider here only the cationic membranes. In these membranes the polymer generally contains carboxylic or sulfonic acid functional groups. For the time being the reference. membranes for PEMFC are probably still the per fluorinated membranes, specially the NAFION® from Du Pont. These membranes exist in different thickness.

One of the problem concerning the perfluorinated membranes is their lack of mechanical stability above 100°C. So one of the objective in FC for different laboratories is to increase the working temperature of the FC in order to increase the performances. For that it is necessary to increase the temperature stability of the polymer membranes.

For that purpose, different attempts have been made for the synthesis of thermostable proton conducting polymers. Introduction of the protogenic group is made by co-polymerisation or polycondensation of the appropriate monomers or by chemical reactions (e.g. sulfonation or grafting) on the polymer.

Several thermo-stable proton conducting polymers have been prepared generally based on the existing thermo-stable polymer: polysulfone, polyimides and so on. [7]

FIGURE9:- Formula of NAFION

ELECTRON CONDUCTING POLYMERS:-

The prospect of powerful electronic circuits made from printable plastics has moved a step closer with the discovery of a cheap, stable organic polymer semiconductor that carries charge through the movement of electrons. Recent studies shows that a polymer based on naphthalene-bis (dicarboximide) can be readily processed by a range of printing technologies, including ink-jet and gravure printing, and combined with other components to create 'complementary' circuit components that use electron-transporting polymer material. The potential of naphthalene-based polymers to act as electronic conductors was first reported in 2008 by Xugang Guo and Mark Watson of the University of Kentucky.

FIGURE 10:-The n- and p-type semiconductors can be combined in a printed transistor.

The new class of compound has two key features: it can accept low-energy electrons readily, and is highly 'regioregular' - it forms a highly ordered polymer backbone with no distortions, enabling charge to be transported down the polymer chain extremely efficiently. The reason, it is able to accept electrons easily is that four C=O bonds surround the central core of each building block of the polymer. These pull electrons away from the centre, resulting in an electron-depleted region. Low-energy electrons can therefore drift with little impediment into the core of the molecule and be passed along the chain. [8]

THERMAL STABILITY OF ELECTRON CONDUCTING POLYMERS:-

Basically confining the study to only POLY-PYRROLE.

Poly-pyrrole is synthesis by chemical or electrochemical polymerization from pyrrole. Oxidative coupling was used to synthesis poly-pyrrole.

FIGURE11:- synthesis of poly pyrrole

Thermo-gravimetric analysis (TGA) and differential thermal analysis (DTA) can be used to assess the thermal stability of the polymer. There have been very few reports on the thermogravimetry of poly-pyrrole. Mohammad et al. showed that poly-pyrrole doped with BF4− ion lost weight in three indistinct steps in N2 as well as in air. Steps 1 and 2 were identical in N2 as well as in air. Step 1 had a weight loss of 3% in the temperature range of 318-398K in N2 and similar in air. Step 2 ranged from 398 to 558K with an additional weight loss of 19.5% in N2 and same in air. Step 3 was monitored up to 973Kandwas found to be incomplete with further weight loss of 29% in N2and 55% in air.

Fig. 9a and b shows the data obtained on the thermal stability of poly-pyrrole prepared from the ternary eutectic melt, in the doped and de-doped forms, respectively, in air. The TG of de-doped sample shows only one major weight loss step. There is a small weight loss of 10% below 383 K. A gradual weight loss of another 15% occurs between 383 and 673 K. Thereafter there is a rapid change in mass above 673 K, completing the degradation at 933 K. . The TG data obtained in atmosphere (Fig. 9c and d) show the polymer decomposition to occur between 752 and 1025 K. The dopant elimination at 463 K (Fig. 9c) is more distinct in N2 atmosphere than in air.

FIGURE12:-

The results show that the polymer is stable in air up to 673K in the de-doped form and up to 553K in the doped form. [9]

BIPOLAR CONDUCTING POLYMERS:-

Bipolar conducting polymers, in which both hole transport and electron transport contribute to electronic conductivity, have been explored by chemical template synthesis of p-type polypyrrole (PPy) in the matrix of an n-type conjugated ladder polymer, poly(benzimidazole−benzophenanthroline) (BBL). Transmission electron microscopy images of the conducting polymer blends show that 5−20 nm diameter Ã- 100−180 nm long rodlike PPy particles are randomly and homogeneously distributed in the BBL matrix, with connectivity of the PPy phase occurring at a volume fraction of about 0.17. The volume fraction dependence of conductivity of the BBL/PPy blends did not exhibit a percolation threshold at volume fractions as low as 0.007 nor can it be described by percolation-type effective medium theory. Room temperature conductivities as high as 60−70 S/cm were observed in the blends compared to 2 S/cm in pure PPy. The enhanced conductivity and the non-percolation nature of these blends originate from bipolar charge transport involving both conjugated polymer components of the blends. Existence of the oxidized (p-type) poly-pyrrole and reduced (n-type) BBL that facilitate bipolar charge transport in these blends was established by cyclic voltammetry. [10]

DOPING OF CONDUCTING POLYMERS:-

The conductivity of certain organic polymers can be raised to metallic levels by chemical or electrochemical `p-doping' (oxidation), or `n-doping' (reduction). Polyacetylene, (CH)x, the prototype conducting polymer, has been studied more extensively than any other conducting polymer and the doping concepts involved appear to be applicable to other polymer systems. The doping of an organic polymer to achieve certain metallic properties is phenomenologically similar to the doping of a classical inorganic semiconductor in that very large increases in conductivity are observed when the material takes up very small amounts of certain chemical species. However, mechanistically it is different in that the doping of an organic polymer involves simply the partial oxidation or reduction of the polymer, each oxidation state exhibiting its own characteristic reduction potential. The dopant ion incorporated may be derived from the chemical dopant species or it may be completely unrelated to it. The reduction potentials of neutral trans-(CH)x and its various oxidized or reduced states, and also the band gap of cis- and trnas-(CH)x have been determined electrochemically. The reduction potentials have been used, together with known standard reduction potentials of a variety of redox couples, to rationalize the doping of (CH)x to achieve metallic conductivity by using a number of dopant species, including I2, Li, AgClO4, gaseous O2, H2O2 or benzoquinone (the last three species in aqueous HBF4 and aqueous HClO4, etc. The stability of p-doped polyacetylene in aqueous acidic media is ascribed to the fact that a positive charge on a CH unit in trans-(CH)x is delocalized over approximately fifteen carbon atoms in what is termed a `positive soliton'. This reduces the ease of nucleophilic attack of the partly oxidized polymer chain. The O2-doping of (CH)x permits the use of (CH)x as an electrocatalytic electrode for the spontaneous reduction of oxygen at one atmosphere pressure and at room temperature in strong aqueous HBF4 solutions. It is concluded that reduction potentials can be used to rationalize the ability of certain dopants to increase the conductivity of selected organic polymers by many orders of magnitude and that they may also be used to predict new chemical species that are thermodynamically capable of acting as p- or n-dopants. [11]

ENHANCING THE CONDUCTIVITY OF POLYMERS WITH THE HELP OF CARBON & GRAPHITE:-

Plastics and polymers are inherently low in thermal and electrical conductivity. For this reason applications that require conductive properties, which could also benefit from the use of polymer components because of their light weight, high strength/weight ratio, easy moldability, etc, cannot take advantage of this desirable material. Research is in progress on inherently conductive polymers, and some polymers with reasonable conductivity values are commercially available. However, at the present time admixing inert, conductive fillers into non-conductive polymers remains a very effective and economical way to produce an electrically or thermally conductive polymer component.

Graphite and carbon offer the benefit of low density and cost when compared to metallic substances used for the same function. Carbon materials provide electrical conduction through the pi bonding system that exists between adjacent carbon atoms in the carbon/graphite structure. Thermal conduction is affected by overlapping sigma bonds which are part of the same molecular bonding system. Regardless of whether or not the conduction is thermal or electrical, electrons provide the pathway for energy transfer.

Many carbons, and especially graphite, have thermal and electrical conductivities many orders of magnitude higher than most polymers or plastics. [12]

USES OF CONDUCTING POLYMERS:-

1. Uses in electronics:-

Conductive polymers have found two main kind of application in electronics so far:-

As material for construction of various devices.

As a selective layers in chemical sensors.

In either case, interaction with ambient gases is critical. It may compromise the performance of a device based on conducting polymers, whereas it is beneficial in sensors. Conductivity has been the primary property of interest. Work function - related to conductivity, but in principle a different property-has received only scant attention. Our aim here is to discuss the usability of conducting polymers in both types of electronic applications in light of these two parameters. [13]

OTHER USES:-

Conducting polymers finds its use in a wide range of applications, such as light emitting diodes and solar cells.

Conducting polymers as corrosion inhibitors: Polyaniline and polypyrrole are the polymers used for prevention of corrosion of metals like steel, copper, silver.

Charge dissipates in electron beam writing and scanning electron microscope metrology: In electron beam writing, a beam of electron is directly focused on a coating of resist on the substrate and a pattern to be generated is controlled by computer.

Metallization : it means deposition of a patterned film of conducting material on a substrate to form interconnections between electronic components.

Separation application: polyaniline polymer in the form of free standing films or supported films can be used for gas separation applications.

Electrically conducting textiles: polymers coated fabric show instant dissipation of static electricity with the excellent tunneling characteristic and this property finds numerous industrial applications like coated fabric, high speed composite rollers, carpets, uniforms, gloves and computer keyboards.

Chemical and biological sensors: chemical sensors are analytical devices that converts the chemical energy of a targeted molecules into a proportionate measurable single, usually electrical or optical. Conductive polymer polyaniline is used as a sensor because of its inherent ability to undergo molecular interactions and produce measurable signals. [14]

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