Biomedical Engineering Reference
In-Depth Information
H
+
CI
-
NH
NH
NH
Leuco-emeraldine salt (LS)
+ H
+
+ e
-
- H
+
- e
-
CI
-
NH
NH
NH
Emeraldine salt (ES)
+ H
+
+ e
-
- H
+
- e
-
CI
-
NH
N
N
Pernigraniline salt (PS)
H
+
FIGURE 8.4
REDOX cycle of PANi in HCl aqueous solution. The emeraldine salt is oxidized
into pernigraniline (PS) salt or reduced into leuco-emeraldine (LS) salt. (Tourillon, G. and F.
Garnier. 1984.
J. Electroanal. Chem
. 161:51.)
Polythiophene (PT) and its derivatives are not as well studied as PPy and PANi,
but they are also a subject of research as actuators at the present time. Most actuators
based on PTs have been fabricated from monomers synthesized for particular actu-
ation purposes. Tourillon and Garnier (1984) studied the behavior of substituted PTs
in acetonitrile. The swelling of the polymer was observed when this was oxidized.
They have demonstrated, for example, that the thickness varies from 160 nm in the
undoped state to 200 nm in the oxidized doped state. The doping level was deter-
mined by elemental microanalysis to about 25% (one positive charge developed in
every four monomeric units).
The dopants do not modify the electrochemical properties of the PTs, contrary
to the case of polypyrrole. This difference has been attributed to different types of
morphology; polythiophene derivatives show a fibrillar structure, whereas polypyr-
role is much more compact. Besides, the doping process becomes more difficult by
the steric hindrance increase leading to a decreasing of conductivity. Unlike PPy
and PANi actuators, in PT actuators the actuation mechanism is not simply due to
ion intercalation in the polymer chains. In this class of materials, the actuation results
from stacking of thiophene oligomers upon oxidation, producing a reversible molec-
ular displacement, which promotes large strains. One possible molecular rearrange-
ment is the formation of dimers by the tendency of orbitals to align due to Pauli's
exclusion principle during oxidation of the material.
Figure 8.5 shows the actuation mechanism of a material composed by hinge
molecules (as a calyxarene) interconnected with seven rigid chains of thiophene.
