Biomedical Engineering Reference
In-Depth Information
polymer chains. The low-angle peak is attributed to the presence of parallel planes that are normal
to
z
-direction, or in-plane of the polymer fi lm, and cause a Bragg refl ection at an angle that is related
to the spacing between the planes. It was found that the “lattice spacings”
d
are related to the size of
the dopant ions according to the linear relationship:
d
(
n
)
=
(0.19
n
+
1.2) nm
where
n
is the number of carbon atoms in the hydrophobic tail of the
n
-alkyl sulfonate dopant. This
feature led authors to the conclusion that the morphology of the PPy doped with large amphiphilic
ions such as
n
-alkyl sulfonates is that of alternating planes of polymer chains and dopant ions
stacked tail to tail, and the interplane distance is related to the size of the dopant ion.
25
XRD results
for chemically synthesized PPy doped with dodecylbenzenesulfonic acid included a sharp peak at
2
θ
3.34°, corresponding to
d
-spacing of 26.4 Å,
26
while XRD data for electropolymerized PPy
doped with different isomers of DBS were as follows:
d
=
=
39.2, 31.7, 24.2 Å for PPy-(1D)BS, PPy-
(2D)BS, and PPy-(6D)BS, respectively.
27
The XRD data from the literature give weight to the hypothesis that PPy(DBS) fi lm morphol-
ogy is strongly anisotropic, made up of alternating planes of polymer chains, and dopant ions that
lie in the plane of the fi lm. The XRD refl ection data from large area thick PPy(DBS) fi lms on Au/Ti
conducting seed layer electropolymerized in our laboratory are shown in Figure 13.22. A peak at
2
θ
1.54 Å). This peak
is somewhat broad, most likely due to the presence of different isomers in the DBS that is avail-
able commercially and is in agreement with the published data. It is likely that the anisotropy in
the volume change of PPy fi lm undergoing electrochemical redox is related to the morphological
=
3.7° is clearly visible and corresponds to
d
=
23.9 Å (
λ
=
2
d
sin
θ
,
λ
=
(A)
PPy(DBS) nanowires
(B)
PPy(DBS) nanowires + alumina
23.9 Å
Au
Film peak 2
= 3.7
°
,
d
= 23.9 Å
DBS double layer
Polypyrrole
PPy(DBS) film
0
5
10
15
20
25
30
35
40
2
(
°
)
FIGURE 13.22
(A) XRD data for large area PPy(DBS) fi lm (50 µm thick, bottom spectrum), PPy(DBS)
nanowires (50 µm long) embedded in alumina (middle) and with alumina removed (top). (B) Schematic rep-
resentation graphically shows the layered structure of PPy(DBS) fi lms, corresponding to the bottom XRD
spectrum on the left.
