JAEA-Review-2010-065.pdf:15.99MB - 日本原子力研究開発機構

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Conductometric Analysis of Track Etching in
Poly(vinylidene fluoride)
T. Yamaki, H. Koshikawa, S. Sawada, S. Hasegawa, M. Asano and Y. Maekawa
Environment and Industrial Materials Research Division, QuBS, JAEA
We have recently been working on ion-track membranes
of poly(vinylidene fluoride) (PVDF) because they have
attracted a renewed interest for their applications to fuel
cells. Researchers previously employed a concentrated
alkaline solution with a KMnO4 additive at high
temperatures to produce visible tracks in the PVDF films.
These severe conditions provided irreversible chemical
damage all over the film including the non-irradiated part.
On the other hand, we have attempted to prepare
PVDF-based ion-track membranes efficiently without any
oxidant additives 1-3) . The aim of this study is to investigate
the formation of the PVDF track membranes in detail by
conductometric analysis. Interestingly, instead of the
oxidant agent-induced activation, high-voltage application to
the conductometry cell was found to increase the etch rate,
governing the pore sizes and shapes, which is indicative of
4)
pore development affected by the etching products .
A 25 m-thick PVDF film was irradiated at room
temperature with 450 MeV 129 Xe from the TIARA cyclotron.
The fluence was fixed at 3 × 107 ions/cm 2 . Track etching
was performed by mounting the irradiated film as a dividing
wall in the conductometry cell made of Teflon containing a
9 mol/dm 3 aqueous KOH solution at 80 ºC. The electrical
conductance of the solution through the film, g(t), was
monitored between two platinum electrodes as a function of
the etching time under a sine AC voltage with a frequency
of 1 kHz using an LCR meter. Assuming cylindrical pores,
we calculated the effective pore diameter, deff, at any given
time by
deff = (4Lg(t)/KNS) 1/2 ,
where L is the membrane thickness, K is the specific
conductivity of the KOH etchant, N is the pore density
generally corresponding to the ion fluence (corrected for the
contribution of the microscopically-nonhomogeneous
irradiation), and S is the area of the measured sample. The
amplitude of the voltage between the electrodes was
constant at either 0.3, 1.0 or 2.0 V.
Figure 1 compares the conductometry results obtained at
the different applied voltages. Basically, there were
several characteristic regions during the course of the
etching. In the beginning, the plateau at a nearly zero
diameter represented the process of etchant penetration into
tracks and approach of the etched cones from both sides of
the film. As soon as these two approaching cones made
contact with each other, the deff suddenly increased. The
time associated with this contact is called the breakthrough
time, TB. After the pore breakthrough, the deff increased, in
other words, the pores grew in the transverse direction.
Lastly, the rate of the deff increase became negligibly low,
JAEA-Review 2010-065
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and eventually the curve reached the second plateau.
Chemical etching in the unmodified area, that is, bulk
etching occurred beyond the width of the latent track.
As also seen in (a), a T B value is estimated to be 4.2 h at
an applied voltage of 0.3 V. When the applied voltage was
increased from 0.3 to 1.0 V, we did not see any significant
change in the curve. When we further increased the
voltage to 2.0 V, the curve exhibited a shorter T B and time
taken until the d eff value started to level off, while it still
showed an invariant growth rate and final d eff. In other
words, the etching seemed to be accelerated at the higher
voltages. Cornelius et al. 5) similarly found that the etch
rate for the polycarbonate-based track membranes varied
with the applied voltage. As a probable cause, they took
into account the creation of dissolution products during the
etching process which should attach to the pore walls and
affect further etching. If the dissolved species are ionic or
polar, they are possibly more efficiently pulled out of the
pore due to electrophoretic migration, reducing the effect of
their adsorption on the inner pore surface. That is, pore
development will finally be activated by a concurrent
decrease in the susceptibility of the materials to chemical
attack, and this activating effect due to voltage application
worked well for only the etching before the breakthrough.
References
1) T. Yamaki et al., Kobunshi Ronbunshu 65 (2008) 273.
2) R. Rohani et al., Nucl. Instrum. Meth. Phys. Res. 267
(2009) 554.
3) T. Yamaki et al., GSI Sci. Rep. 2008 (2009) 350.
4) N. Nuryanthi et al., Electrochemistry 78 (2010) 146.
5) T.W. Cornelius et al., Nucl. Instrum. Meth. Phys. Res.
265 (2007) 553.
d eff (nm)
200
150
100
50
0
(b)
(c)
(a)
0 5 10 15 20 25 30
Etching time (h)
Fig. 1 Plots of the d eff value as a function of the etching
time. The voltage applied to the conductometry cell
was (a) 0.3, (b) 1.0, or (c) 2.0 V.