Applications of conductive polymers
The commercialisation exemplified by the following list of materials illustrates the effects of Heeger’s,
McDiarmid’s and Shirakawa’s work on the later development of conductive polymers. The principal interest
in the use of polymers is in low-cost manufacturing using solution-processing of film-forming polymers. Light
displays and integrated circuits, for example, could theoretically be manufactured using simple inkjet printer
techniques. 6-10
Doped polyaniline is used as a conductor and for electromagnetic shielding of electronic circuits. Polyaniline
is also manufactured as a corrosion inhibitor.
Poly(ethylenedioxythiophene) (PEDOT) doped with polystyrenesulfonic acid is manufactured as an antistatic
coating material to prevent electrical discharge exposure on photographic emulsions and also serves as
a hole injecting electrode material in polymer light-emitting devices.
Poly(phenylene vinylidene) derivatives have been major candidates for the active layer in pilot production of
electroluminescent displays (mobile telephone displays).
Poly(dialkylfluorene) derivatives are used as the emissive layer in full-colour video matrix displays.
Poly(thiophene) derivatives are promising for field-effect transistors: They may possibly find a use in
supermarket checkouts.
Poly(pyrrole) has been tested as microwave-absorbing “stealth” (radar-invisible) screen coatings and also as
the active thin layer of various sensing devices.
Other possible applications of conductive polymers include supercapacitors and electrolytic-type capacitors.
Some conductive polymers such as polyaniline show a whole range of colours as a result of their many
protonation and oxidation forms. Their electrochromic properties can be used to produce, e.g. “smart
windows” that absorb sunlight in summer. An advantage over liquid crystals is that polymers can be
fabricated in large sheets and unlimited visual angles. They do not generally respond as fast as in electron-gun
displays, because the dopant needs time to migrate into or out from the polymer - but still fast enough for
many applications. We shall return to electroluminescent polymers below.
Synthesis and processing
There is often a big step between the first chemical synthesis of a molecular substance and the development of
processing methods for practical applications. The first polyacetylenes were obtained from acetylene which
polymerized in the presence of a catalyst.
Of the two polyacetylene conformations, cis and trans, the trans form is thermodynamically more stable.
Shirakawa’s polyacetylene had mainly the cis form and was a copper-coloured flexible film which could be
converted to the silvery trans form by heating above 150° C. X-ray diffraction and scanning electron
microscopy showed that such films were polycrystalline matted fibrils. These materials were semiconductors,
the trans isomer with higher conductivity (4.4 x 10–3 S m–1) than the cis (1.7 x 10–7 S m–1). Shirakawa and
Ikeda had noticed that when (CH)x films were exposed to bromine or chlorine at room temperature for a few
minutes, there was a dramatic decrease in the infrared spectrum (decrease in transmission between 4000 and
400 cm–1). By contrast, complete halogenation, resulting in (CHBr)x, gave high IR transmission and a white
film. However, they did not investigate the corresponding conductivity, so it remained for Heeger and
McDiarmid, in collaboration, to discover the effect of doping.
The halogen doping that transforms polyacetylene to a good conductor of electricity is oxidation (or pdoping).
Reductive doping (called n-doping) is also possible using, e.g., an alkali metal.
[CH]n + 3x/2 I 2 [CH]n x+ +xI3
– oxidative doping
[CH] n + xNa [CH]n
x– + xNa + reductive doping
The doped polymer is thus a salt. However, it is not the counter ions, I3
– or Na+, but the charges on the
polymer that are the mobile charge carriers (see Mechanism of polymer conductivity, below).
By applying an electric field perpendicular to the film, the counter ions can be made to diffuse from or into
the structure, causing the doping reaction to proceed backwards or forwards. In this way the conductivity can
be switched off or on.
Processing polyacetylene and many other polymers such as polypyrrole and polythiophene was for a time
ruled out because of their failure to melt or to dissolve in any solvent. Ingenious methods developed over the
years have, however, made processing possible. In 1980, James W. Feast and co-workers at the University of
Durham synthesised polyacetylene from a soluble precursor polymer, poly(7,8-bis(trifluoromethyl)-
tricyclo[4.2.2.0]deca(3,7,9-triene). Upon heating, the dissociation product bis-trifluoromethylbenzene
evaporates to leave a polyacetylene film which is much denser than Shirakawa’s material. Another important
invention was Caltech researchers Robert H. Grubbs’ and co-workers’ production of polyacetylene by
metathesis polymerisation of cyclooctatetraene in the presence of a titanium alkylidene complex as catalyst.
Grubbs’ polyacetylene reportedly had a conductivity of about 35,000 S m–1, but was as intractable and
unstable as other polyacetylenes. However, by attaching alkyl substituents to the cyclooctatetraene molecule,
Grubbs and his group managed to prepare a soluble substituted polyacetylene that could be cast in any form
desired, although the alkyl substituents seemed to lower the conductivity considerably.
Another advance in electrical properties, but unfortunately not in processing, came in 1987 when BASF
(Badishe Anilinen und Soda Fabrik) scientists Herbert Naarman and Nicholas Theophilou in West Germany
developed a polymerisation method based on Shirakawa’s method, at 150°C. When doped, their material was
claimed to have a conductivity of more than 107 Sm–1, i.e., of the same order as that of copper’s. This
polyacetylene may have a higher conductivity because of its greater order and fewer defects than previous
preparations.
Other polymers with interesting properties have been developed: added to those already listed are
polyparaphenylene, polyparaphenylenevinylene, polypyrrole, polythiophene and polyaniline and their
derivatives. These materials generally show much lower conductivity than polyacetylene, ca 102–104 Sm–1,
which is more than enough for many purposes. These polymers have the advantage of relatively high stability
and processibility, e.g. poly(3-dodecylthiophene), can be prepared as a melt-spun, strong film in the undoped
state and then doped to a conductivity of 105 Sm–1.
Mechanism of polymer conductivity – role of doping
In a metal there is a high density of electronic states with electrons with relatively low binding energy, and
”free electrons” move easily from atom to atom under an applied electric field. The conductivity of the
material can be measured with standard procedures, a value for metallic copper around 108 S m–1 having been
measured.
