Biomedical Engineering Reference
In-Depth Information
H
H
H
H
H
H
FIGURE 13.1
Polypyrrole chemical structure.
SO
2
Na
FIGURE 13.2
Chemical structure of sodium dodecylbenzenesulfonate (NaDBS).
move from one pyrrole unit to the other on the chain. If there are defects in the synthesized PPy, or
if there are intentionally introduced dopant ions, such as the DBS
-
ion in this work, the double bonds
can become ionized (electrons removed or added) to form structures known as solitons, polarons, and
bipolarons. In PPy, the conduction mechanism is “bipolaron hopping” along the polymer chain. In
a heavily doped polymer, the presence of many bipolarons generates bipolaron conduction bands in
the material band-gap, and the polymer becomes conductive. In the case of PPy doped with DBS, the
conductivity is on the order of 10 Ω/cm, depending on the oxidation state and temperature.
6
13.2.2.3
Doping of Polypyrrole with DBS Ions
The presence of sodium dodecylbenzenesulfonate (NaDBS) in the synthesis solution serves
several functions: fi rst, NaDBS functions as a conducting electrolyte to allow oxidation of pyr-
role monomers; second, NaDBS is an amphiphilic surfactant that allows a high concentration of
hydrophobic pyrrole monomer to be dissolved in an aqueous solution; and third and most important,
the DBS
−
ion becomes incorporated into the polymer matrix during synthesis and serves as a dop-
ant ion. DBS is a large molecule with a long hydrophobic tail (Figure 13.2).
Due to its size and the favorable hydrophobic-hydrophobic interactions with the PPy, DBS ion
becomes permanently embedded in the polymer matrix, acting as a permanent dopant and also
altering the polymer nanostructure, as discussed in Section 13.4. The degree of the doping is far
higher in PPy than the typical doping in inorganic semiconductors; in PPy, there is roughly one
DBS ion per four pyrrole units.
13.2.2.4
Electrochemical Cycling of PPy(DBS)
For electrochemical testing of PPy doped with DBS ions, the experimental setup shown in
Figure 13.3 can be used with an NaDBS aqueous electrolyte.
The voltage range versus the Ag/AgCl reference is 0 to
−
1 V, corresponding to approximately
0 to
2.5 V measured between WE and CE. This range ensures complete oxidation (at 0 V versus
Ag/AgCl) and reduction (at
−
1 V versus Ag/AgCl) of the PPy without undesirable effects such as
irreversible overoxidation or electrolysis that begin to occur at more positive or negative voltages.
The reversible reaction is described in Equation 13.1, where the left side, PPy
+
(DBS
−
) represents the
polymer in its oxidized state, while the right side, PPy
0
(DBS
−
Na
+
) represents the reduced polymer.
As the application of negative voltage adds electrons to the polymer backbone (reduction), charge
−
