Advances in inherently conducting polymers Page: 4 of 21
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The polymer first discovered as a highly conductive material
when doped is polyacetylene. The polymer is obtained in the cis
and/or trans configuration as silvery cohesive films, of a
fibrillar morphology, by liegler-Matta catalysis (1,2).
Polyacetylene is known as the prototype material in this area of
research. One attractive feature is that the polymer films can be
stretched to form an anisotropic material (2).
The oxidative-coupling method (4) is one of the best methods
to obtain a poly(p-phenylene), PPP, with a high molecular weight.
The product consists of mostly uncrooslinked paralinkages. The
powdery material can be annealed in an inert atmosphere up to
400'C for 24 hours, thus increasing its crystallinity by
crosslinking toward a graphite-like structure. The annealiLg
process reuulta in an increase of the conductivity.
The first processable polymer known to be rendered
conductive was poly(phenylenesulfide), PPS. The synthesis
employed was generally step-growth condensation reactions (5).
Films are prepared by melting the polymer powder ( a 270*C) or
by casting it from a 10% solution in diphenylether 220*C.
3lectrochemical polymerisation (6) is a novel method of
synthesis for some conducting polymers. The most-studied ones are
polypyrroles and polythiophenes. Until recently, most highly
conductive polymers obtained by using this technique were
insoluble. The electrosynthesis of soluble conducting polymers of
this type will be discussed in a subsequent section.
another class of material of interest is that oO ladder-
chain polymers. They consist of mixed aromatic hetercayclic
backbones having rigid, planar, ladder-chain structures that are
melt-processable. These materials axe usually obtained by
condensation polymerizations. Polyquinolines asi polyquinoxalines
are known examples of high-temperature polymers that could be
rendered conductive (7).
Polyanilines (6-10) are examples of intrinsically conducting
polymers that do not involve doping in the same way as do the
other polymers. The polymer can be obtained in various forms and
structures, the conductivity of which varies from the insulating
regime to the conducting regime, depending upon the pa of the
synthesis medium (11). Polyamiline can also be synthesized
electrochemically in an aqueous nediua that consists of 4S% nF4.
I. SLUCTnOMIC STOCTURE
A. Pristine (Undaped) Polymers
In order to understand transport in conducting -lymera it
is essential to examine their electronic structure m terms of
band structure and excitations. Trans-polyacetylent is unique
among conducting polymers in having a dejsuerate ground state
(12), i.e., two geometric structures having the same energy. The
two structures differ by the alternation of tho siagle and double
bonds. This degeneracy results in the formation of the neutral
soliton recitation (radical) having a spin 1/8 that separates the
two ground states (Fig. la) and can propagate freely on a one-
dimensional basis along the ahafn (13,14). The ground state in
polyparomatics is nondegenerate (15). The ground state
corresponds to an aromatic structure with f single-like bond
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Aldissi, M. Advances in inherently conducting polymers, article, September 1, 1987; New Mexico. (digital.library.unt.edu/ark:/67531/metadc1108118/m1/4/: accessed February 19, 2019), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.