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Nanoscale Structures of Radiation-Grafted
Polymer Electrolyte Membranes Investigated
by Dissipative Particle Dynamics Simulation
S. Sawada a) , T. Yamaki a) , A. Suzuki b) , T. Terai b) and Y. Maekawa a)
a) Environment and Industrial Materials Research Division, QuBS, JAEA,
b) Graduate School of Engineering, The University of Tokyo
A radiation-grafting method is one of the promising
techniques for preparing polymer electrolyte membranes
(PEMs) for fuel cell applications. In the research of the
radiation-grafted PEMs, much effort has been devoted to
improve their properties (proton conductivity, chemical
stability, mechanical strength, etc.) by the best combination
of base polymer films and graft monomers. As part of their
basic science studies, we investigated the nanoscale
structures of the PEMs by a dissipative particle dynamics
(DPD) simulation. The simulation target was our original
PEM synthesized by the radiation grafting of styrene into
crosslinked-polytetrafluoroethylene (PTFE) and subsequent
1-3)
sulfonation .
4)
According to our recent report , we modeled the grafted
electrolyte polymers with ion exchange capacities (IECs) of
0.89 - 2.2 meq/g. Based on the molecular structures, the
atom groups of (CF2) 6 –CH(C6H4SO3H)CH2, and 7
water molecules were represented as particles A, B and W,
respectively. The interaction parameters between the two
particles of the A-B, A-W, or B-W pair were calculated by a
full-atomistic simulation. The molecular architecture (e.g.,
the length of PTFE main chains, the length of graft chains,
and the number of graft chains per molecule) was
determined based on the experimental data. As the initial
state, the electrolyte polymers and a fixed number of
particles W were randomly located in a cubic cell with the
three-dimensional periodic boundaries. After 50,000 time
steps, all the systems reached the equilibrated states. For
comparison, a DPD simulation was also performed on
5)
Nafion .
Figure 1 shows the snapshots of the simulation results in
the equilibrated states. As in both (a) and (b), particles W
appeared to be mixed with particles B while, in contrast,
particles W in Nafion aggregated alone to grow larger.
This finding indicates that, unlike Nafion, in which water
clusters were excluded from any polymer parts, the
water-filled regions were not clearly separated from the
poly(styrene sulfonic acid) (PSSA) graft chains.
In order to obtain quantitative information on nanoscale
structures, we calculated the radial distribution function
between particles W, GWW(r). The GWW(r) was defined as:
n W
GWW
r
, (1)
2
4π
r r
φW
where nW is the average number of particles W existing
in the distance between r and r +Δr from any particle W.
Figure 2 shows the GWW(r) for the hydrated electrolyte
JAEA-Review 2010-065
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polymers. In the case of Nafion, there were sharp peaks at
r = 0.60, 1.0 and 1.5 nm. These three peaks are considered
to originate from the first, second and third nearest W
particles, respectively, in the same cluster. At r = 3.4 nm,
the GWW(r) dropped to under 1, which suggests the edge of a
5)
water cluster . Accordingly, the diameter of the water
cluster, dC, was twice as much as this value, i.e., 6.8 nm.
For the grafted electrolyte polymers, the GWW(r) dropped to
under 1 at r = 0.88 nm, corresponding to dC of 1.8 nm. This
result is in good agreement with the diameter obtained
previously by the small-angle X-ray scattering analysis
6)
(1.7 nm ). It was found that a low of these small clusters
would exist surrounded by PSSA graft chains. Such unique
nanoscale structures were possibly related to the high proton
conductivity and low water diffusivity of the grafted PEMs.
-(CF 2) 6-
-CH-CH 2-
SO 3 H
A
B
7H2O W
(a) (b)
Fig. 1 Snapshots of the simulated polymer electrolyte in
the IEC of (a) 0.89 and (b) 2.2 meq/g.
G WW(r)
3
2
1
Grafted PEM (0.89 meq/g)
Grafted PEM (2.2 meq/g)
Nafion
0
0 1 2 3 4 5
r (nm)
Fig. 2 The GWW(r) as a function of r.
References
1) T. Yamaki et al., Polymer 45 (2004) 6569.
2) S. Sawada et al., Trans. Mater. Res. Soc. Jpn. 30 (2005)
943.
3) S. Sawada et al., Solid State Inoics 179 (2008) 1611.
4) S. Sawada et al., Kobunshi Ronbunshu 67 (2010) 224.
5) S. Yamamoto et al., Polym. J. 35 (2003) 519.
6) M. Elomaa et al., J. Mater. Chem. 10 (2000) 2678.