Biomedical Engineering Reference
In-Depth Information
take place: the polymer starts out in the oxidized state, and as the voltage is lowered, the potential
barrier for adding electrons onto the polymer backbone is overcome, and the current due to poly-
mer reduction begins to fl ow. The reduction current reaches its peak at
0.6 V and begins to taper
off as the polymer matrix becomes fully reduced. This reduction is accompanied by simultaneous
movement of Na + ions from the electrolyte into the polymer to compensate the negative charge on
DBS dopant ions. At
1 V, the current has reached its minimum, signifying that the reduction
of the polymer is complete. If the voltage is lowered further, the current would begin to increase
again due to water electrolysis, which would damage the polymer. Therefore, the applied voltage
is limited to
1 V, as this voltage is low enough to reduce the polymer without negative effects of
electrolysis. As the voltage is then increased on the return scan, oxidation current can be observed
due to the electrons being removed from the polymer and the corresponding movement of Na + ions
out of the PPy matrix. The peak occurs at
0.35 V, and the current reaches a minimum at 0 V,
corresponding to a complete oxidation of the PPy. At higher voltages, an irreversible oxidation of
the polymer occurs, accompanied by chemical changes to the PPy structure. If allowed, this higher
voltage would render PPy unusable; therefore, the applied oxidation voltage is limited to 0 V.
13.2.2.5 Volume Change Due to Reversible Electrochemical
Redox Reaction of PPy(DBS)
1 V, movement of Na + in and out of
the PPy matrix occurs as described in Section 13.2.2.4. Each Na + ion is accompanied by a solvation
shell of several water molecules, and, in addition, osmotic pressure causes more water molecules
to enter the polymer alongside the sodium ions. 7 The additional volume of Na + ions and water that
have entered the PPy fi lm causes conformational changes of the PPy molecules and in the overall
dimensions of the deposited PPy (Figure 13.5).
As seen in the fi gure, the applied voltage and the entering sodium ions and water molecules
cause a conformational change of the PPy chains (dotted lines), with the resulting volume change of
the fi lm. If the fi lm has length L and thickness H , the expansion of the fi lm under reducing voltage
can be as high as
As the polymer is electrochemically cycled between 0 and
30%. The increase in length is used in so-called bilayer
actuators in which a PPy fi lm is laminated to a thin fi lm of another material that does not undergo
volume changes with applied voltage; this modifi cation allows the magnifi cation of the relatively
small changes in the length of PPy fi lm and results in large-scale movement. The reversible increase
in thickness, on the other hand, is relatively large compared with the size of the synthesized fi lm and
can be used directly in micro- and nanoactuators discussed in Sections 13.3 and 13.4. The strong
anisotropy of the volume change in PPy(DBS) fi lms is likely due to the layered orientation of the
polymer chains and the dopant anions. It heavily depends on the synthesis conditions, with impli-
cations for the fabrication of micro- and nanostructures. The evidence for the layered structure of
Δ
L / L
=
2-3% and
Δ
H / H
=
Na +
H 2 O
Na +
(A)
(B)
H 2 O
H
H
H
L
L
L
FIGURE 13.5 Schematic cross-section of a polypyrrole fi lm. (A) Oxidized fi lm ( V versus Ag/AgCl = 0 V)
with sodium ions and water expelled from the polymer matrix, polymer backbone charged positive ( + signs),
compensating the charge on immobile DBS ions (encircled ( ) signs). (B) Reduced fi lm with the neutral
polymer backbone and Na + ions entering the fi lm (encircled ( + ) signs) to compensate the charge on DBS ions.
 
 
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