Modern Polymer Spect..

208 4 T/ihmtional Spectroscopy of Ivitact arid Doped Coiijirgnted Polymers
4.2 Materials
The conductivity of iodine-doped polyacetylene first reported by Shirakawa et al.
[I] in 19'7'7 was 30 S cm-'. Since then, the conductivity reported for doped polyacetylene
has kept increasing, the highest conductivity obtained so far for an iodinedoped
stretched polyacetylene film [1?] being > 10' S cm-', a value comparable
with that of copper (6 x 10' S cni-I).
A film of intact polyacetylene usually shows a conductivity lower than lo-'
S cm-' . However, the conductivity increases dramatically when the film is exposed
to oxidizing agents (electron acceptors) such as iodine, AsF5, H'S04, etc. or reducing
agents (electron donors) such as alkali metals. This process is referred to as duping,
by analogy with the doping of inorganic semiconductors. The polymers that are not
doped are referred to as ititact polymers in this review. The main process of doping
is a redox reaction between the polymer chains and acceptors (or donors). Upon
doping, an ionic complex consisting of positively (or negatively) charged polymer
chains and counter ions such as 13-, AsF6-, etc. (or Naf, K+, etc.) is formed.
Counter anions or cations are generated by reduction of acceptors or oxidation of
donors, respectively. The use of an acceptor causes the p-type doping, and that of
a donor the n-type doping. The electrical conductivity can be controlled by the
content of a dopant. A sharp increase in conductivity is observed when the dopant
content is < 1 mole% per C2H2 unit. After this sharp increase, the conductivity becomes
gradually higher with further increase in the dopant content. At low doping
levels, polyacetylene does not exhibit metallic properties, whereas its conductivity is
high. When the dopant content is more than about 13 mole% per C2Hz unit, polyacetylene
shows metallic properties such as a Pauli susceptibility [24, 251, a linear
temperature dependence of the thermoelectric power [26], and a high reflectivity in
the infrared region [27], though the temperature dependence of conductivity is not
like that of a metal. The origin of such properties of heavily doped polyacetylene is
not yet fully understood.
Following polyacetylene, a large number of conducting polymers have been
reported. The chemical structures of typical conducting polymers are depicted in
Figure 4-1. These conducting polymers have conjugated n-electrons in common,
(a) (b) (C) (d)
pq..J-@--q)n
s \ i n
(e)
(f)
p, //
aniline
Figure 4-1. Chemical structures of conducting
polymers. ia) Trurrs-polyacetylene;
(b) cis-polyacetylene; (c) polyj p-
phenylene); (d) polypyrrole; (e) polythiophene:
if 1 poly( p-phenylenevinylene);
(g) poly(2,5-thienylenevinylenei; (h) poly-
(leucoemeraldine base form); (i j
(h) (0 polyisothianaphthene.