Chemistry Reference
In-Depth Information
created considerable interest [
61
]. Many polymeric materials were synthesized since. At present, it is
not completely understood by what mechanism the electric current passes through them. We do know,
however, that all conductive polymers, or intrinsically conducting polymers according to Inzelt [
62
],
are similar in one respect. They all consist of extended
p
-conjugated systems, namely alternating
single and double bonds along the chain.
One of the early known conductive polymers is polysulfurnitride (SN)
x
, an inorganic material that
tends to be explosive, but becomes superconducting at 0.3 K [
63
]. Since then many other conductive
polymers evolved. The most investigated ones appear to be polyacetylene, polyaniline, polypyrrole,
polythiophene, poly(phenylene sulfide), and poly(phenylene vinylene) and their derivatives. Many
derivatives of these materials and other similar ones also been reported.
The mechanism of electro polymerization that is used to form many conductive polymers is also
still not fully understood. According to Diaz et al. [
64
], the process involves a sequence of coupling
steps, with each step being activated by two species. The polymer- forming process requires two
electron per step It also includes partial oxidation of the polymer. Thus, the polymer formation and
the polymer oxidation occur simultaneously. Diaz proposed a chain propagation process for the
polymer formation. Although that is still accepted by many, his mechanism is now being questioned.
Heinze et al. [
65
] suggest that the process of polymer formation consists of oligomer formation and
oxidation followed by
-coupling of chains.
All known conducting polymers have backbones of contiguous
sp
2
-hybridized carbon centers.
In each of these centers, one valence electron resides in a
p
z
orbital. It is orthogonal to the other three
sigma bonds. When the material is oxidized or reduced, that removes some of the delocalized
electrons. The electrons then obtain high mobility. As a result, the conjugated p-orbitals form a
one-dimensional electronic band. The electrons within that band become mobile when it is partially
emptied. Depending upon the chemical structure, some polymer can also be self-oxidizing or
reducing.
Structural disorder in these polymer molecules interferes with electron mobility. Thus, for
instance, polyacetylene exhibits conductivity of 0.1-10 kS/cm. Stretch-orienting this polya-
cetylene, a process that aligns the chains and removes much of the disorder, increases conductivity
to 80 kS/cm [
66
].
Polyacetylene
can be shaped into a silvery looking film. The polymer is more thermodynamically
stable in the
s
when heated above 150
C. Partial oxidation of
the film, with iodine or other materials, transforms it and increases its conductivity 10
9
-fold. The
process of transforming a polymer to its conductive form through chemical oxidation or reduction is
called
doping
.
Two types of polyacetylene doping are possible:
trans
cis
trans
form and converts from
to
Þ
n
ð
I
3
Þ
0
:
33
ð
CH
Þ
n
þ
1
=
2I
2
!ð
CH
oxidative doping (p - type
Þ
Þ
n
Na
Þ
x
½ð
ð
CH
Þ
n
þ x
Na
!ð
CH
reductive doping (n - type
Þ
The doping process can be reversed and conductive polymers can be undoped again by applying an
electrical potential. It causes the dopant ions to diffuse in and out of the structure.
Improvements in preparations of polyacetylene came from several developments. One is the use of
metathesis polymerization of cyclooctatetraene, catalyzed by a titanium alkylidine complex. The
product has improved conductivity, though it is still intractable and unstable. By attaching
substituents, it is possible to form soluble and more stable materials that can be deposited from
solution on various substrates. Substitution, however, lowers the conductivity. This is attributed to
