Conductive polymer

Source: Wikipedia, the free encyclopedia.
This article is about bulk applications of conductive polymers. For single-molecule electronics, see Molecular scale electronics.
polyphenylene sulfide
(X = S).

Conductive polymers or, more precisely, intrinsically conducting polymers (ICPs) are

conduct electricity.[1][2] Such compounds may have metallic conductivity or can be semiconductors. The main advantage of conductive polymers is that they are easy to process, mainly by dispersion. Conductive polymers are generally not thermoplastics, i.e., they are not thermoformable. But, like insulating polymers, they are organic materials. 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[3] and by advanced dispersion techniques.[4]

History

Polyaniline was first described in the mid-19th century by Henry Letheby, who investigated the electrochemical and chemical oxidation products of aniline in acidic media. He noted that reduced form was colourless but the oxidized forms were deep blue.[5]

The first highly-conductive organic compounds were the

charge transfer complexes.[6] In the 1950s, researchers reported that polycyclic aromatic compounds formed semi-conducting charge-transfer complex salts with halogens.[3] In 1954, researchers at Bell Labs and elsewhere reported organic charge transfer complexes with resistivities as low as 8 ohms-cm.[7][8] In the early 1970s, researchers demonstrated salts of tetrathiafulvalene show[9] almost metallic conductivity, while superconductivity was demonstrated in 1980. Broad research on charge transfer salts continues today. While these compounds were technically not polymers, this indicated that organic compounds can carry current. While organic conductors were previously intermittently discussed, the field was particularly energized by the prediction of superconductivity[10] following the discovery of BCS theory
.

In 1963 Australians B.A. Bolto, D.E. Weiss, and coworkers reported derivatives of

semiconductors. Subsequently, DeSurville and coworkers reported high conductivity in a polyaniline.[12] Likewise, in 1980, Diaz and Logan reported films of polyaniline that can serve as electrodes.[13]

While mostly operating in the

quantum tunneling, negative resistance, phonon-assisted hopping and polarons. In 1977, Alan J. Heeger, Alan MacDiarmid and Hideki Shirakawa reported similar high conductivity in oxidized iodine-doped polyacetylene.[14] For this research, they were awarded the 2000 Nobel Prize in Chemistry "for the discovery and development of conductive polymers."[15] Polyacetylene itself did not find practical applications, but drew the attention of scientists and encouraged the rapid growth of the field.[5] Since the late 1980s, organic light-emitting diodes (OLEDs) have emerged as an important application of conducting polymers.[16][17]

Types

Linear-backbone "polymer blacks" (

electroluminescent semiconducting polymers. Today, poly(3-alkylthiophenes) are the archetypical materials for solar cells and transistors.[3]

The following table presents some organic conductive polymers according to their composition. The well-studied classes are written in bold and the less well studied ones are in italic.

The main chain contains No heteroatom
Heteroatoms
present
Nitrogen-containing Sulfur-containing
Aromatic cycles The N is in the aromatic cycle:

The N is outside the aromatic cycle:

The S is in the aromatic cycle:

The S is outside the aromatic cycle:

  • poly(p-phenylene sulfide)
    (PPS)
Double bonds
Aromatic cycles and double bonds

Synthesis

Conductive polymers are prepared by many methods. Most conductive polymers are prepared by oxidative coupling of monocyclic precursors. Such reactions entail dehydrogenation:

n H–[X]–H → H–[X]n–H + 2(n–1) H+ + 2(n–1) e−

The low

molecular weight
need not be high to achieve the desired properties.

There are two main methods used to synthesize conductive polymers,

Cyclic Voltammetry and Potentiostatic method by applying cyclic voltage[18]
and constant voltage. The advantage of Electro (co)polymerization are the high purity of products. But the method can only synthesize a few products at a time.

Molecular basis of electrical conductivity

The conductivity of such polymers is the result of several processes. For example, in traditional polymers such as

p-type semiconductors
, respectively.

Although typically "doping" conductive polymers involves oxidizing or reducing the material, conductive organic polymers associated with a protic solvent may also be "self-doped."

Undoped conjugated polymers are semiconductors or insulators. In such compounds, the energy gap can be > 2 eV, which is too great for thermally activated conduction. Therefore, undoped conjugated polymers, such as polythiophenes,

negative differential resistance
and voltage-controlled "switching" analogous to that seen in inorganic amorphous semiconductors.

Despite intensive research, the relationship between morphology, chain structure and conductivity is still poorly understood.[22] Generally, it is assumed that conductivity should be higher for the higher degree of crystallinity and better alignment of the chains, however this could not be confirmed for polyaniline and was only recently confirmed for PEDOT,[26][27] which are largely amorphous.

Properties and applications

Conductive polymers show promise in antistatic materials

biosensors,[28] flexible transparent displays, electromagnetic shielding and possibly replacement for the popular transparent conductor indium tin oxide. Another use is for microwave-absorbent coatings, particularly radar-absorptive coatings on stealth aircraft. Conducting polymers are rapidly gaining attraction in new applications with increasingly processable materials with better electrical and physical properties and lower costs. The new nano-structured forms of conducting polymers particularly, augment this field with their higher surface area and better dispersability. Research reports showed that nanostructured conducting polymers in the form of nanofibers and nanosponges, showed significantly improved capacitance values as compared to their non-nanostructured counterparts.[29][30]

With the availability of stable and reproducible dispersions, PEDOT and

polystyrene sulfonic acid), polyaniline is widely used for printed circuit board manufacturing – in the final finish, for protecting copper from corrosion and preventing its solderability.[4] Moreover, Polyindole is also starting to gain attention for various applications due to its high redox activity,[31] thermal stability,[30] and slow degradation properties than competitors polyaniline and polypyrrole.[32]

Electroluminescence

solar panels, and optical amplifiers
.

Barriers to applications

Since most conductive polymers require oxidative doping, the properties of the resulting state are crucial. Such materials are salt-like (polymer salt), which makes them less soluble in organic solvents and water and hence harder to process. Furthermore, the charged organic backbone is often unstable towards atmospheric moisture. Improving processability for many polymers requires the introduction of solubilizing substituents, which can further complicate the synthesis.

Experimental and theoretical thermodynamical evidence suggests that conductive polymers may even be completely and principally insoluble so that they can only be processed by dispersion.[4]

Trends

Most recent emphasis is on

polymer solar cells.[33] The Organic Electronics Association is an international platform to promote applications of organic semiconductors. Conductive polymer products with embedded and improved electromagnetic interference (EMI) and electrostatic discharge (ESD) protection have led to both prototypes and products. For example, Polymer Electronics Research Center at University of Auckland is developing a range of novel DNA sensor technologies based on conducting polymers, photoluminescent polymers and inorganic nanocrystals (quantum dots) for simple, rapid and sensitive gene detection. Typical conductive polymers must be "doped" to produce high conductivity. As of 2001, there remains to be discovered an organic polymer that is intrinsically electrically conducting.[34] Recently (as of 2020), researchers from IMDEA Nanoscience Institute reported experimental demonstration of the rational engineering of 1D polymers that are located near the quantum phase transition from the topologically trivial to non-trivial class, thus featuring a narrow bandgap.[35]

See also

References

  1. ISBN 978-3-540-75929-4
    .
  2. ^ Conducting Polymers, Editor: Toribio Fernandez Otero, Royal Society of Chemistry, Cambridge 2016, https://pubs.rsc.org/en/content/ebook/978-1-78262-374-8
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  7. ^ a b Okamoto, Yoshikuko and Brenner, Walter (1964) "Polymers", Ch. 7, pp. 125–158 in Organic Semiconductors. Reinhold
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  9. doi:10.1021/ja00784a066
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  10. doi:10.1103/PhysRev.134.A1416
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  11. doi:10.1071/ch9631090
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  12. doi:10.1016/0013-4686(68)80071-4
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  13. doi:10.1016/S0022-0728(80)80081-7
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  14. doi:10.1039/C39770000578. Archived
    from the original on 2017-09-25. Retrieved 2018-04-29.
  15. ^ "The Nobel Prize in Chemistry 2000". Retrieved 2009-06-02.
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    S2CID 43158308
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  17. S2CID 4328634
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  18. doi:10.1039/c4py00850b
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  19. doi:10.1103/RevModPhys.60.781
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  20. ISBN 9789814518215
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  21. ^ Handbook of Organic Conductive Molecules and Polymers; Vol. 1–4, edited by H.S. Nalwa (John Wiley & Sons Ltd., Chichester, 1997).
  22. ^ a b Skotheim, T.A.; Elsenbaumer, R.L.; Reynolds, J.R., eds. (1998). Handbook of Conducting Polymers. Vol. 1, 2. New York: Marcel Dekker.
  23. S2CID 44646344
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  25. doi:10.1103/PhysRevLett.51.1191
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  26. doi:10.1063/1.3580514
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  27. doi:10.1021/cm203216m
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  28. PMID 18405677
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  29. doi:10.1039/C7TA06242G
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  30. ^
    doi:10.1016/j.electacta.2017.07.038
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  31. doi:10.1021/acsaem.7b00057
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  32. S2CID 99946069
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  33. ^ Overview on Organic Electronics Archived 2017-03-02 at the Wayback Machine. Mrs.org. Retrieved on 2017-02-16.
  34. ^ Conjugated Polymers: Electronic Conductors Archived 2015-02-11 at the Wayback Machine (April 2001)
  35. S2CID 207930507
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Further reading

External links
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