Biomedical Engineering Reference
In-Depth Information
N
H
N
H
n(CIO
4
+
)
cq
+
N
H
N
H
N
H
Solid
Oxidation
Reduction
(CIO
4
+
)
C
+
C
+
N
H
N
H
ne
+
+
N
H
N
H
N
H
Solid
FIGURE 8.3
Sketch representing a REDOX reaction of PPy due to the presence of ionic
ClO
anions.
4
-
under polarization at different potentials. The volume changes in the CCP are due
to ionic movement produced during an electrochemical reaction. For example, if a
p-doped polymer such as PPy is oxidized, there are two possibilities to maintain the
electroneutrality. If the polymer is doped with a mobile and small anion in an
electrolytic solution with both mobile cations and anions, the insertion and de-
insertion of the anions during the REDOX reaction will maintain the electroneutrality
(equations 8.1 and 8.2 and fig. 8.3) (see Otero, 1998; Burgmayer et al., 1982;
Matencio et al., 1995; and Baughman et al., 1991):
reduction
oxidation
←
→
+
−
−
0
−
()
PPy
(
CLO
)
+
e
P
Py
+
CLO
aq
(8.1)
4
4
If the polymer is doped with a nonmobile and big anion as dodecyl benzene
sulfonate anion (DBS
,
cations will be inserted and de-inserted during the REDOX reaction to maintain the
electro neutrality:
-
) into an electrolyte containing mobile and small cations X
+
reduction
oxidation
←
→
+
−
+
0
(
−+
PPy
(
DBS
)
+
X
aq
PPy
DBS X
)
(8.2)
Note that chemical equation (8.1) shows that in the first case the CCP is expanded
during the oxidation phase due to the insertion of ClO
anions. In the second case
(chemical reaction 8.2), the polymer is expanded during the reduction by the insertion
of X
4
-
cations.
These investigations showed that the ionic flow in the CCP depends on cation
and anion size, thickness of the CP film, applied voltage, and the time scale.
Upon application of a voltage or imposition of an electric field, a volume change
is induced because the oxidation state of CCP is modified. During the volume
increase of CCP, the inserted ions and their hydrated shells occupy any available
free space. When the inserted anions or cations get out of the polymer, the network
+
