Chemistry Reference
In-Depth Information
Polyaniline
was first prepared at the beginning of the last century. Several oxidation states are
known. The conductivity and the color of the material vary progressively with oxidation. Only one
form, however, known as the emeraldine salt, is truly conducting. The material can be prepared
readily by electrochemical or chemical oxidation of aniline in aqueous acid media. Common
oxidants, such as ammonium peroxydisulfate, can be used. Flexible emeraldine films can be cast
from solutions of
-methylpyrrolidone and made conductive by protonic doping. This is done by
dipping the films in acid or by exposing them to acid vapors. The process results in protonation of the
imine nitrogen atoms:
N
N
N
N
N
n
H
H
H
H
insulator
N
N
N
N
n
H
H
emerald bas, insulator
H
N
N
N
N
n
H
H
H
H
The conductivity of the emeraldine salt increases with decrease in pH of the acid used to dope it.
In this respect, polyaniline, in its emeraldine form, differs from other conductive polymers because
it does not require partial oxidation or reduction for doping. Protonation of the imine nitrogens is
sufficient to make it a very conductive material.
A different method of synthesizing polyaniline was reported [
77
]. It uses an enzyme, horseradish
peroxidase, in the presence of hydrogen peroxide to polymerize aniline. To prevent reactions at the
ortho
positions of the phenyl rings that yield insoluble branched materials, a polyelectrolyte template,
like sulfonated polystyrene, was used. The polyelectrolyte aligns the monomers, dopes the
polyaniline to the conducting form, and forms an irreversible complex with the polyaniline to keep
it water-soluble [
77
]. The conductivity of the complex increases with increasing polyaniline to
sulfonated polystyrene molar ratios. Conductivities of 0.005 S/cm are obtained with the pure complex
and increase to 0.15 S/cm after additional doping by exposure to HCl vapor [
77
].
Lee reported [
78
] a new synthesis of polyaniline, where aniline hydrochloride monomer acts as a
surfactant. The organic phase serves to diffuse water away from water-insoluble oligomers. As a
result, the chains grow, because the monomer radicals can meet the active polymer chain ends at the
organic aqueous interfaces. This results in formation of a polymer with high structural integrity. This
enables the electrons or holes to travel in a perpendicular direction.
