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Enhanced Reactivity of Ion-track Grafting for Fuel-cell
Electrolyte Membranes
T. Sekine a, b) , S. Sawada a) , T. Yamaki a) , H. Koshikawa a) , M. Asano a) , Y. Maekawa a) ,
A. Suzuki b) and T. Terai b)
a) Environment and Industrial Materials Research Division, QuBS, JAEA,
b) Department of Nuclear Engineering and Management, The University of Tokyo
Ion-track grafting involves irradiation of a base polymer
film with swift heavy ions and the subsequent graft
polymerization directly into the latent tracks. We have
recently applied this technique to the development of
proton exchange membranes (PEMs) 1) . A key for success
here is to achieve high graft levels in as small a number of
tracks as possible because it is very important to keep a
balance between proton conductivity (prefers higher graft
levels) and mechanical strength (prefers lower fluences) of
the resulting membrane. We found that a mixture of water
and isopropanol (H2O/iPrOH) is an effective medium for
enhancing the reactivity of the ion-track grafting of styrene.
A 25 m-thick poly(ethylene-co-tetrafluoro- ethylene)
(ETFE) film was bombarded by 3.5 MeV/n 129 Xe from the
TIARA cyclotron. The fluence was 3.0 × 10 7 or 3.0 ×
10 8 ions/cm 2 , at which almost no ion tracks overlapped.
Just after ion-beam irradiation, the film was exposed to air
to produce peroxide groups, and then immersed in a
grafting solution (a volume ratio of styrene to the grafting
medium was 1:4) at 60 °C for 1 - 48 hours. The percentage
composition of the H2O/iPrOH mixture, RH2O/iPrOH, was
varied from 0 (pure iPrOH) to 100% (pure H2O). The
degree of grafting (DOG) was calculated by: DOG(%) =
(Wg – W0)/W0 × 100, where Wg and W0 denote the weights
of the styrene-grafted and as-irradiated films, respectively.
Figure 1 plots the DOG of the films irradiated at 3.0 ×
10 8 ions/cm 2 and styrene-grafted for 6 hours, as a function
of RH2O/iPrOH. All of the obtained DOGs were higher than
that for the medium-free grafting (i.e., in neat styrene)
(8.4%, a result not shown) and, interestingly, the DOG
peaked at RH2O/iPrOH = 36%. Rager et al. 2) and Gürsel et
3)
al. similarly used the H2O/iPrOH mixture for -ray
pre-irradiation grafting of styrene into a fluorinated base
film. Our results agree with their previous ones, and can
be discussed by considering the well-known Trommsdorff
effect 3) , in which poor solubility of the grafted polystyrene
in polar media leads to an increased polymerization rate
due to a slower termination rate. The lower RH2O/iPrOH yielded a homogeneous and compatible grafting solution,
though, in contrast, the higher RH2O/iPrOH gave an
immiscible grafting mixture, which composed a lower
water-rich phase with the ETFE base film and a upper
styrene-rich phase. The creation of these two phases would
reduce the monomer accessibility within the base film. This
is probably why the DOG decreased at RH2O/iPrOH > 36%.
Figure 2 shows the DOG vs. time curves for the films
irradiated at fluences of 3.0 × 10 7 and 3.0 × 10 8 ions/cm 2
and styrene-grafted at RH2O/iPrOH = 36%. The DOG
increased during the first 24 hours and then leveled off at a
JAEA-Review 2010-065
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saturated value. At the same grafting time, the DOG was
naturally smaller at the lower fluence because the number of
peroxides acting as a starting point of graft polymerization is
proportional to the fluence. It should be mentioned here that,
for the first time, we obtained a DOG of > 10% even at the
lower fluence of 3.0 × 10 7 ions/cm 2 . This DOG corresponds
to an ion exchange capacity of > 0.9 meq/g, which is
comparable to that of a benchmark fuel-cell PEM called
Nafion. We thus expect that the obtained result could be the
technical breakthrough for PEMs endowed with both of high
proton conductivity and mechanical strength.
Characterization of the sulfonated membranes is now in
progress.
DOG (%)
Fig. 1 The DOG as a function of R H2O/iPrOH. The styrene
grafting was performed for 24 hours after the 129 Xe
irradiation at 3.0 × 10 8 ions/cm 2 .
DOG DOG (%)
40
30
20
10
0
0 20 40 60 80 100
R RH2O/iPrOH H2O/iPrOH (%)
70
60
50
40
30
20
10
3×108 ions/cm2 3×108 ions/cm2 3×107 ions/cm2 3×107 ions/cm2 0
0 10 20 30 40 50
Time (h)
Fig. 2 The DOG as a function of grafting time at fluences
of 3.0 × 10 7 and 3.0 × 10 8 ions/cm 2 . The 129 Xe-irradiated
films underwent the styrene grafting at RH2O/iPrOH = 36%.
References
1) T. Yamaki, J. Power Sources, in press.
2) T. Rager et al., Helv. Chim. Acta, 86 (2003) 1966.
3) S.A. Gürsel et al., Nucl. Instrum. Meth. Phys. Res. 265
(2007) 198.