It has been a convention and a natural selection for every science student to define and distinguish materials on the basis of their properties, one such property being electrical conductivity. Through the ages we see metals as our optimum choice for conducting electric current. An addition to the Power Set of conductors was Semiconductor, that allow electric charge to pass under induced doping mechanism. Above all this Polymer has been unanimously accepted and known to be an insulator or in some cases dielectric, and so has been used to coat metal wires for a safety purpose for the property of insulation.
The name conducting polymer is kind of an oxymoron. But this was proved wrong, after it was predicted back in 1962 by Walmslay. Walmslay along with a colleague presented the idea of in poly-acetylene. According to that solitons are form of mobile defects that can be charged to give conductive properties. Similarly in 1963, Weiss and his colleagues worked on conducting polymers as they reported high conductivity in polypyrrole oxided with iodine. The journey continued when in 1973 a key discovery was made by Walatka and his co-workers as they found an inorganic polymer that is a metal, the polymer was polysulphur nitride. They predicted that the characteristic of metal, found in this polymer, is intrinsic. It was due to an unpaired electron present between each S-N unit, which showed that the valance band is half occupied. Hence forbidden gap is absent, which allows for the movement of these unpaired electrons whenever an electric field is applied.
So in 1977, the hard-work over the years was paid off, after Heeger, Mcdiarmad and Shirakawa discovered the property of conductance in polyacetylene (PA) by the oxidation of halogens vapours as chlorine, bromine and iodine. These vapours made the conducting property of PA films to increase by a billion times. Such form of halogen treatment is called doping just as so the term used in semi-conductor study. The doped form of PA showed the conductance of tenth of a million S/m compared to other polymers, it’s the highest, as for example Teflon ten quadrillionth S/mand if compared to metals silver and copper show 100 million S/m polymer is that along the backbone, conjugated double bonds are present. By conjugation it is meant that between the carbon atoms there are single and double bonds in alternating positions. Sigma bond that gives a strong chemical bond is present in every bond. In addition to it localised weak pi bond is also present in double bonds. To make the material conductive, charge carriers in the form of extra electron or holes are injected that allows for the current to pass.
Conductivity:
Conductivity of a material can be defined from ohms law that states that “current through a conductor between two points is directly proportional to the potential difference across the two points” Where R is the resistance, the reciprocal of resistance is called conductance and is measure in Siemens and of Conductivity is Siemens/meter. Conductivity of a material depends on the number of charge carriers and the mobility of the charge, as in how fast they can move. The conductivity of a material also depends on temperature as in it shows a decreasing character, when the material is metal and increasing when it’s a semi conductor or insulator.
What makes materials conductive?
In most of the materials be it polymer or any crystalline structure metal, the property of conductance is directional, called anistropic property. For example, carbon allotropic forms diamond and graphite show different characteristics adding to that we compare polyacetylene character as well. Diamond, graphite and polyacetylene has three, two and one dimensional structured carbon atoms respectively. Unlike graphite and diamond, Polyacetylene has hydrogen atoms between the carbon atoms. Due to the presence of strong sigma bond and symmetrical structure diamond is isotropic and insulator while in graphite and polyacetylene the presence of mobile pi electrons gives space for the material to conduct electrical charge in certain directions.
Synthesis & Processing:
With temperature change Shirakawa was able to form copper coloured Cis and silvery trans polyacetylene. With trans showing modest conductivity compared to Cis. Also the trans form is more stable thermodynamically. Shirakawa, during experimentation, found that the transmission was reduced when the film was exposed for a few minutes to bromine and chlorine, but when exposed for higher rates the film gave high IR transmission. The doping was studied by Mcdiarmid and Heeger. The halogen doping that induces conductivity in polymers is oxidation (p type). The doped polymer forms a salt. And the charge on the polymer is what constitute the charge carrier and by applying electric field in the direction perpendicular to the film, the counter ions can be made to diffuse both from and into the structure. Thus, giving the character of turning conductivity on and off. The later years synthesis include one as formation of a more denser film of polyacetylene, left after the evaporation of bis-triflouromethylebenzene. An advancement in electrical properties was achieved in the year 1987 when the conductivity of PA was claimed to be equal to copper’s. The experiment was same as Shirakawa.
Apart from polyacetylene other polymers have also been developed that are more stable to air and oxygen effects and also are more easily process able, compared to PA, but have lower conductivities. These include polyparaphenylene, polyparaphenylenevinylene, polypyrrole, polythiophene and polyaniline and their derivatives.
Mechanism:
The lower binding energy of electrons in metals gives them highly dense electronic state as well as provides the free electrons to move easily, under the application of electric field, from atom to atom. The structure also has a defining role In determining the electrical properties of materials. As in metals, we see there is an overlap of orbitals across each atom giving a molecular orbital. Hence, greater atomic orbitals intersection more will be molecular orbitals formation. These molecular orbitals combine to form an energy gap or band. In metals the valance band is unfilled. So we can say that molecular orbital above some energy will also be unfilled or will be empty. The gap or the energy spacing between two bands, highest energy and other lowest energy is called band gap. The conduction band is one that has the lowest energy and is the unoccupied band while the opposite is valance band. The electric field response is shown by a material when the energy gap is zero or the conduction band is partially filled. For chained structure the same model goes but it can be added through using quantum mechanical model and so making use of Pauli Exclusion Principle we find that increasing the polymer length decreases the band gap.
Another model is Ab-inito model that considers ladders of filled and empty orbitals. The combined wave function of the orbitals is called state. Cases are, one when there are even number of electrons in the lower part of the energy ladder and occupying same orbital, these electrons will have opposite spins and so the spin angular momentum be zero. Another case is when the electron is excited to move to a higher energy state. The energy difference is called band gap. If the spin of these different state electrons is same than such is called triplet and acts as a paramagnetic and non magnetic if the spin is opposite.
We mentioned that the band gap will be zero if the chain lengthens but such is not predicted and its experimentally found that it depends on the wavelength of the first absorption band. it has also been seen beyond a certain limit no change is expected in further conjugation. So the result shows that with energy levels and band gap of significant size the PA acts as a semi conductor.
Question arises, as if why does PA behave as metal when doped? Some models suggest an answer to that as the conjugation of the double bond. That is different from ordinary in a way that the next bond is known to other and another reason presented is the delocalised pi electrons between these double bonds that are evenly spaced but it has been found that they are not. Such idea was presented back in 1930s when a student of Heisenberg put forward the hypothetical presentation that the chain of sodium if distorted as in making the equal chains into alternating long and short ones we see that the metal becomes an insulator or semiconductor and the conductivity decreases with temperature.
Dopant role is found to either add electron or remove it. For example if iodine atom undergoes oxidation it is predicted to produce a hole that is delocalised but it does not do that completely. As the electron is removed a radical cation is formed the radical is called polaron and is localised because of the coulomb attraction to its counterion. The counterion is found to have a lower mobility and is made to surround the polaron so to allow the charge carrying to be made. Another reason for localisation of polaron is the equilibrium change of the radical cation.
If another electron is removed from the already oxidised section the electron then moves as a pair. A defect in conducting polymer called solitons formed after thermal isomerisation of trans PA. The stable free radical formed is neutral although it does not itself carry as it propagates but can help in transfer from chain to chain.
Another mechanism of charge transfer intersolitons hopping. In this the solitons move all around and exchange electrons with adjacent localised soliton that are charged.
Special Features:
- Molecular Disorder:
Unlike inorganic semiconductors, polymers exhibit molecular character with lacking of long range order. The motion involving electrons is one dimensional in individual molecules. The dimensional reduction gives polymer the nature to generate electronic properties by certain states called Fermi surface instabilities.
- Nature of Doping:
Doping occurring in inorganic semiconductor involves the dopant to occupy a position within the lattice of the material being doped. Thus, resulting in electron- rich or deficient sites, there is no charge transfer between the two sites. In polymers we see the doping of a material involves charge transfer reaction, the polymer undergoes oxidation and reduction.
- Solitons, bipolarons and polarons
Conductivity increase with doping was thought to be due to formation of unfilled band of electron. In PA and PPP it has been assumed that the conductivity was dependant on spinless charge carriers. Increase in conductivity is due to formation of polaron,bipolaron and soliton. These particles form due to electron phonon interaction and these are the charge carrying species. In degenerate polymers as trans, solitons are the charge carriers whereas in non generate the polarons and when combined bipolarons are the charge carriers.
Applications:
- Polyaniline is used as inhibitor (reducing rate of corrosion) and as in electromagnetic shielding as a conductor.
- Poly(ethylenedioxythiophene) is used in coating to protect a material from electrical discharge and also used as an injector of holes in light emitting diodes.
- The derivatives of polythiopene are used in field effective transistors.
- Polypyrrole can be used in stealth devices as it has been found that it can absorb microwaves and so can be used in sensing devices.
- Poly(phenylene vinylidene) can be used in protecting electroluminescent displays.
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