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Characterization of porous structure through the
dynamical properties of ions confined in
sulfonated polyimide ionomers films Anne-Laure
ROLLET4, Jean-François BLACHOT1, Alfred
DELVILLE3, Olivier DIAT1, Armel GUILLERMO2,
Patrice PORION3, Laurent RUBATAT1 and Gérard
GEBEL1 1 Groupe des Polymères Conducteurs
Ioniques SI3M/DRMFC/ CEA-Grenoble / 17 rue de
Martyrs, 38054 Grenoble Cedex 9, France (
ggebel_at_cea.fr odiat_at_cea.fr rubatat_at_drfmc.ce,g.ce
a.fr ) 2 Laboratoire de Spectrométrie Physique
-CNRS-LSP/UMR C5588-Université J.Fourier, B.P.87,
38402 St Martin d'Hères (armel.GUILLERMO_at_grenoble-
ujf.fr ) 3 Centre de Recherche sur la Matière
Divisée- CNRS Université dOrléans, UMR 6619 /
1B rue de la Férollerie, 45071 Orléans cedex 02,
France ( porion_at_cnrs-orleans.fr
delville_at_cnrs-orleans.fr ) 4 Centre de Recherche
les Matériaux à Haute Température-CNRS UPR 4212 /
1D avenue de la Recherche Scientifique, 45071
Orléans cedex 02, France ( rollet_at_crns-orleans.fr
)
The ionomer membranes Ionomers S. Schlick,
Ionomers characterization, theory and
applications (CRC Press, Boca Raton, 1996) M. R.
Tant, K. A. Mauritz, and G. L. Wilkes, Ionomers
synthesis, structure, properties and
applications (Chapman and Hall, London, 1997).
are weakly charged polymers their equivalent
weight noted EW (weight of polymer per mole of
charges) ranges from 500g/equiv to 1500g/equiv.
The presence of the charges in moderate amount
leads to a complex organization of the polymer
with a phase separation between hydrophilic and
hydrophobic domains. The films are prepared by
evaporation of ionomer solution and they can be
swollen when immersed in electrolyte solution.
The latter is located in the hydrophilic domains
that are highly charged because of the
aggregation of the ionic groups carried by the
polymer the ionic concentration is several
mol/L. Moreover the counterions can be easily
exchanged and their concentration is set by the
Donnan equilibrium. Due to their properties
(ionic selectivity, conductivity, interesting gas
permeation properties...) they are widely used in
electrochemical devices such as electrodialysis
and fuel cell... In this study, we were
interested in sulfonated PolyImide (sPI) ionomer
which chemical formula is given in below.
sPI chemical formula
Radiotracers Time gt 1min
NMR PFGSE Time from 10-3s to 1s
The radiotracer method used here is based on the
rotating electrode principle Rollet, A.-L. 
Simonin, J.-P.  Turq, P. Phys. Chem. Chem.
Phys., 2000, 2(5), 1029.. A circular piece of
membrane was glued on a Plexiglas cylinder with a
mastic. The cell was first immersed into an
electrolyte solution of a given molality m. After
a sufficient time, it was introduced into a
radiolabelled electrolyte solution of the same
composition. When equilibrium was reached, the
cell was mounted on a rotating electrode spindle
and immersed into a non-radioactive solution of
molality m. Then successive aliquots of this
solution were taken as a function of time.
This sequence is repeated with 15 rising gradient
strength and D is obtained by a fit of
LnS(q)/S(0) versus q2.
The self-diffusion coefficient of N(CH3)4 ions
have been measured by Pulsed Field Gradient Spin
Echo (PFGSE) NMR. At time t0 the spins are
labeled with respect to their position by
applying a pulse of magnetic field gradient g
after a evolution time D, a second pulse of
magnetic field gradient reverse to the first one
is applied. The second pulse cancels the effect
of the first one, unless the spins have moved
during the time D, in that case the second
gradient pulse does not refocus all the spins and
the echo signal is less intense. This decrease is
a function the self-diffusion coefficient D.
B0
The NMR experiments reveals an anisotropic and
confined diffusion of N(CH3)4 in sPI ionomer
membrane the dimension of the characteristic
volume is about 7 7 4 µm3
The self-diffusion coefficient of both cations,
Na and N(CH3)4 are very low compare with the
ones in non confined solution (1.310-9m2/s and
1.210-9m2/s respectively)
Existence of a foliated organization
Micron scale
Nanometer scale anisotropic orientation of the
polymer chains with a poor phase segregation
between hydrophilic and hydrophobic domains.
The conductivity experiments have been done with
a mercury cell. A piece of swollen membrane
(0.383cm2) separates 2 compartments filled with
Hg. The conductivity is thus measured in the
transverse direction. The spectra were recorded
over a frequency range from few Hertz to several
hundred of kiloHertz. The value of the membrane
resistance was determined when the imaginary part
of the impedance is equal to zero (frequency
about 50000Hz).
The nematic order of the polymer chains is not
cancelled by powder average.
Two dynamical regimes can be observed a power
law at high frequency and a plateau at low
frequency. The cross-over is here about t
1/w310-7s and it can be related to a
characteristic distance l (2Dt)1/2 . Assuming
that D ranges from 10-11 to 10-9m2/s, l ranges
from 310-9 to 310-8m. The distance determined
by SAXS experiments is 1.610-8m at this scale,
the self-diffusion at 310-7s can thus be
estimated at D410-10m2/s appreciately. This
value is largely smaller to the one in non
confined solution.
The conductivity is low compare with Nafion
ionomer membrane (cNa 1.110-2S/cm) whereas the
pores are smaller in this latter membrane, the
hypothesis of the existence of polymer chains in
the hydrophilic pores in sPI is thus supported.
Time from 10-4 to 10-9s NMR relaxation
Time10-5s Conductivity
Conclusion The transport properties of cations
confined in ionomer membranes have been
determined using several techniques with
different window time a method using
radiotracers, conductivity, pulsed field gradient
NMR and NMR quadrupolar relaxation rates
determination. These properties are related to
the structure of the membrane. They indicate the
existence of a multiscale and anisotropic
structure organization of the polymer. The
N(CH3)4 ions do not interact in a strong way
with the polymer chain except by sterical effect.
The measurement at small timescale confirms the
presence of polymer chains in the hydrophilic
pores that greatly slow down the transport of the
ionic species.