Biomedical Engineering Reference
In-Depth Information
shrinks and, when they get in, it expands. According to the properties of volume
change observed in the CCP, Baughman et al. (1991) reported on the possibility of
using this volume change due to charge insertion for mechanical actuation.
The behavior of polypyrrole is dramatically altered with chemical doping. The
p-doped polypyrrole conductive conjugated polymers have a number of applications
in electrochromic devices, rechargeable batteries, capacitors, ionic membranes,
charge dissipation, and electromagnetic shielding. Generally, polypyrrole (PPy) is
partially oxidized to produce p-doped materials as shown next.
Otero et al. (1992b) and Otero (1998) reported how a bilayer of PPy(ClO
) with
another neutral flexible polymer substrate could bend in a cantilever fashion in
LiClO
4
-
electrolyte solution. Otero and colleagues further studied the behavior of this
bilayer in solutions of acetonitrile/LiClO
4
, propylene carbonate/LiClO
, and
4
4
water/LiClO
. As one of the pioneers in this field, Otero and coworkers (1992a,
1992b, 1993, 1994, 1995, 1996a, 1996b, 1997, 1998) have developed a model
explaining the volume change in the PC, taking into account the electrostatic repul-
sions between charged polymeric chains (Otero et al., 1992a, 1992b, 1993, 1994).
According to this model, when a CCP like polypyrrole (PPy) is subjected to an
oxidation reaction, positive charges are generated along the polymeric chains. These
positive charges produce electrostatic repulsions between them. Due to these repul-
sions, some conformational changes are generated in the polymeric structure.
Tourillon and Garnier (1984) also demonstrated that these polymer network
reconfigurations are accompanied by the expulsion of the anions into the electrolytic
medium by XPS measurements. Pei and Inganäs (1993) have studied the behavior
of PPy(DBS
4
) films. In this case, they observed an increase of mass when hydrated
cations were inserted into the CCP network and a decrease in mass when they were
taken out. Thus, the CCP became swollen during the reduction and shrunk during
the oxidation. Smela and Gadegaard (2001) have studied the
-
volume change
in these PPy films by atomic force microscopy (AFM). They found that the film
thickness increased between 30 and 40% in the reduced state compared to the
oxidized state (Smela and Gadegaard, 2001).
Polyaniline (PANi) is another commonly used CCP for robotic actuator appli-
cations. Okabayashi et al. (1987) determined that a volume variation in PANi accord-
ing to its oxidation state can occur. They observed the weight change of PANi in
propylene carbonate/LiClO
in-situ
during a REDOX reaction by an electrogravimetric
method. The weight of the polymer increased during the oxidized doped (with ClO
4
4
-
anions solvated) up to eight times. The emeraldine form of PANi also can be
electrochemically oxidized or reduced in aqueous or acidic environment, resulting
in pernigraniline (PS) and leuco-emeraldine (LS) salts, respectively, as is shown in
figure 8.4. The REDOX reaction occurs with motion of protons and electrons in
strong acid (pH < 3).
The addition of protons and electrons in nitrogen is observed during the reduc-
tion; this leads to the phenyl ring changing to quinonoid structures and vice versa
during oxidation and reduction, respectively (fig. 8.5). In this case, the structural
changes (phenyl to quinonoid to phenyl) lead to deformation and strain in PANi.
Kaneto et al. (1995) have shown that PANi is more compact in the reduced state
than in the oxidized state.
