Ion gels prepared by in situ radical polymerization of vinyl monomers in an ionic liquid and their characterization as polymer electrolytes

Article abstract of DOI:10.1021/ja045155b

To realize polymer electrolytes with high ionic conductivity, we exploited the high ionic conductivity of an ionic liquid. In situ free radical polymerization of compatible vinyl monomers in a room temperature ionic liquid, 1-ethyl-3-methyl imidazolium bis(trifluoromethane sulfonyl)imide (EMITFSI), afforded a novel series of polymer electrolytes. Polymer gels obtained by the polymerization of methyl methacrylate (MMA) in EMITFSI in the presence of a small amount of a cross-linker gave self-standing, flexible, and transparent films. The glass transition temperatures of the gels, which we named ion gels , decreased with increasing mole fraction of EMITFSI and behaved as a completely compatible binary system of poly(methyl methacrylate) (PMMA) and EMITFSI. The temperature dependence of the ionic conductivity of the ion gels followed the Vogel-Tamman-Fulcher (VTF) equation, and the ionic conductivity at ambient temperature reached a value close to 10^-2 S cm^-1. Similarly to the behavior of the ionic liquid, the cation in the ion gels diffused faster than the anion. The number of carrier ions, calculated from the Nernst-Einstein equation, was found to increase for an ion gel from the corresponding value for the ionic liquid itself. The cation transference number increased with decreasing EMITFSI concentration due to interaction between the PMMA matrix and the TFSI^- anion, which prohibited the formation of ion clusters or associates, as was the case for the ionic liquid itself.

Full text of DOI:10.1021/ja045155b

Published on Web 03/11/2005
Ion Gels Prepared by in Situ Radical Polymerization of Vinyl
Monomers in an Ionic Liquid and Their Characterization as
Polymer Electrolytes
Md. Abu Bin Hasan Susan,† Taketo Kaneko, Akihiro Noda, and
Masayoshi Watanabe*
Contribution from the Department of Chemistry and Biotechnology, Yokohama National
UniVersity, and Core Research for EVolutional Science and Technology (CREST), Japan Science
and Technology Agency, 79-5 Tokiwadai, Hodogaya-ku, Yokohama 240-8501, Japan
Received August 11, 2004; E-mail: mwatanab@ynu.ac.jp
Abstract: To realize polymer electrolytes with high ionic conductivity, we exploited the high ionic conductivity
of an ionic liquid. In situ free radical polymerization of compatible vinyl monomers in a room temperature
ionic liquid, 1-ethyl-3-methyl imidazolium bis(trifluoromethane sulfonyl)imide (EMITFSI), afforded a novel
series of polymer electrolytes. Polymer gels obtained by the polymerization of methyl methacrylate (MMA)
in EMITFSI in the presence of a small amount of a cross-linker gave self-standing, flexible, and transparent
films. The glass transition temperatures of the gels, which we named “ion gels”, decreased with increasing
mole fraction of EMITFSI and behaved as a completely compatible binary system of poly(methyl
methacrylate) (PMMA) and EMITFSI. The temperature dependence of the ionic conductivity of the ion
gels followed the Vogel-Tamman-Fulcher (VTF) equation, and the ionic conductivity at ambient temperature
reached a value close to 10-2 S cm-1. Similarly to the behavior of the ionic liquid, the cation in the ion gels
diffused faster than the anion. The number of carrier ions, calculated from the Nernst-Einstein equation,
was found to increase for an ion gel from the corresponding value for the ionic liquid itself. The cation
transference number increased with decreasing EMITFSI concentration due to interaction between the
PMMA matrix and the TFSI- anion, which prohibited the formation of ion clusters or associates, as was
the case for the ionic liquid itself.
Introduction
good electrochemical stability, has therefore been an intriguing
domain of recent research.
The development of novel highly ion-conducting polymer
electrolytes for potential application in a wide variety of solid-
state electrochemical devices has received an upsurge of interest,
driven by the intrinsic properties of the polymer electrolytes,
such as thin-film forming ability, flexibility, and transparency
as well as the relatively high ionic conductivity and wide
electrochemical window.1 Conventional electrolytes of this
variety, typically polyether-based polymer electrolytes, are solid
solutions of electrolyte salts in polymers. The ionic motion in
these polymer electrolytes is coupled with the local segmental
motion, where the relaxation time for the segmental motion is
coupled with the conduction relaxation time, and a decrease in
the ionic mobility caused by the rapid increase in the glass
transition temperature (Tg) offsets the effect of the increasing
carrier-ion density with increasing ion concentration. These facts
are mirrored in the appearance of a maximum conductivity in
polyethers at a relatively low salt concentration with increasing
salt concentration. Innovation of systems, where the polymer
electrolytes have a glass transition low enough to remain rubbery
at room temperature while preserving high conductivity and
The notion that a small amount of a high-molecular weight
polymer would suffice to give rubbery compliance with high
ionic conductivity is evident for certain salt-polymer systems,
which show high ionic conductivity with significantly low Tg
in the range of high salt concentrations.2 In these salt-polymer
systems, known as polymer-in-salt electrolytes, the number of
carrier ions and their mobility increase with increasing the
electrolyte concentration. Consequently, high ionic conductivity
that is not coupled with the segmental motion of the polymers
is likely to be achieved. However, the electrolyte salts for such
a polymer-in-electrolyte system need to possess some essential
characteristics, such as low Tg and melting point, Tm, and
capability of forming (supercooled) liquids at ambient temper-
ature while retaining high ionic conductivity and compatibility
with the host polymers.2
Ionic liquids, also known as room-temperature molten salts,
due to, inter alia, their immeasurably low vapor pressure, high
ionic conductivity, and greater thermal and electrochemical
(2) (a) Watanabe, M. In Soild State Ionics: Materials and Applications;
Chowdari, B. V. R., Chandra, S., Singh, S., Srivastava, P. C., Eds.; World
Scientific: Singapore 1992; pp 149-157. (b) Angell, C. A.; Liu, C.;
Sanchez, E. Nature 1993, 362, 137. (c) Watanabe, M.; Yamada, S.; Sanui,
K.; Ogata, N. J. Chem. Soc., Chem. Commun. 1993, 929.
† Present address: Department of Chemistry, University of Dhaka, Dhaka
1000, Bangladesh.
(1) MacCallum, J. R., Vincent, C. A., Eds. Polymer Electrolyte ReViews 1 and
2; Elsevier: London, 1987 and 1989.
9
4976
J. AM. CHEM. SOC. 2005, 127, 4976-4983
10.1021/ja045155b CCC: $30.25 © 2005 American Chemical Society

Ion Gels as Polymer Electrolytes
A R T I C L E S
stability3-5 are considered to be suitable electrolyte salts for
polymer-in-salt systems. Ionic liquids meet all of the require-
ments of plasticizing salts and offer the potential for improved
thermal and mechanical properties and may expand the tem-
perature range where flexible polymers can be used. We have
preliminarily reported polymer-in-salt electrolytes, prepared by
dissolving compatible polymers in chloroaluminate2a,c,6a and
non-chloroaluminate molten salts,6b which exhibit high ionic
conductivity as well as a rubbery electrolyte property. Scott et
al. have reported ionic liquids based on imidazolium salts as
excellent plasticizers for poly(methyl methacrylate) with im-
proved thermal stability and ability to significantly reduce the
glass transition temperatures.7 Carlin and co-workers also
reported highly ion-conducting rubbery gel electrolytes from
non-chloroaluminate ionic liquids and poly(vinylidene fluoride-
co-hexafluoropropylene).8
More recent studies provide insight into the kinetics and the
types of polymers formed by the free radical polymerization
reactions of vinyl monomers in ionic liquids as a solvent.14 Ionic
liquids have also been proved to be suitable for controlled atom
transfer radical polymerization (ATRP),15 reverse ATRP,16 and
radical addition fragmentation transfer (RAFT) polymerization17
of acrylates and styrene in ionic liquids. In a recent study,
Snedden et al.18 have investigated the polymer-ionic liquid
interactions to the design of composite materials and discussed
the solubility of a range of polymers in the selected ionic liquids,
polymerization and copolymerization in the ionic liquids, the
characterization of targeted polymer-ionic liquid composites,
and the role of cross-linking and gelation.
Through numerous attempts so far made, the prospect of ionic
liquids, as electrolyte salts for polymer electrolytes, has been
well understood; however, the art of realizing highly conducting
polymer electrolytes and characterizing their ion transport
behavior remains in a rudimentary stage. To bring perspective
in this burgeoning field of polymer electrolytes, we aim at
understanding the ion-transport property of the highly conductive
polymer electrolytes based on ionic liquids. This paper deals
with in situ polymerization of vinyl monomers in 1-ethyl-3-
methyl imidazolium bis(trifluoromethane sulfonyl)imide (EMITF-
SI) to present a new methodology affording highly conductive
polymer electrolytes. The compatibility of the ionic liquid with
the polymer matrix has been analyzed to develop the criteria
for such a binary system to belong to this novel category of
polymer electrolytes. Finally, the characterization and ion
transport behavior of ion gels have been targeted to achieve
the goal of molecular design of highly conductive polymer
electrolytes.
While most of the studies are concerned with the doping of
polymers with electrolyte salts, the solubility of a large number
of vinyl monomers in ionic liquids has prompted us to carry
out polymerization of the vinyl monomers in ionic liquids to
afford polymer electrolytes.9a In situ free radical polymerization
of the compatible vinyl monomers in ionic liquids yields
transparent, mechanically strong, and highly conductive polymer
electrolyte films. Like conventional polymer gels, the obtained
polymer electrolytes are comprised of polymer networks and
liquids,9 ionic liquid in our case. If the incorporation of ionic
liquid into polymer networks affords a completely compatible
combination, we name the obtained polymer gels “ion gels”,10
which are distinctly discriminated from conventional polymer
gels in terms of nonvolatility and high thermal stability of the
liquids incorporated in the network polymers. Since task-oriented
properties, such as proton conduction, lithium-ion conduction,
and electronic charge transport, can be molecularly designed
into the ionic liquids, the scope and the utility of the ion gels
as new polymer electrolytes will immediately expand to fuel
cells,11 lithium batteries,12 and solar cells.13
Experimental Section
Preparation and Characterization of EMITFSI. EMITFSI was
prepared following the procedure reported earlier with a slight
modification.19 1-Ethyl-3-methyl imidazolium bromide (EMIBr) was
first prepared by the quaternization reaction of 1-methyl imidazole with
ethyl bromide at 80 °C for 24 h in cyclohexane under refluxing
condition. The crude product was purified by repeated recrystallization
using a mixture of ethyl acetate and isopropyl alcohol (volume ratio,
1:1) as the solvent. This yielded white crystalline hygroscopic EMIBr
(Tm ) 78 °C). The structure of the prepared EMIBr was confirmed by
1H NMR spectra (DMSO-d6, chemical shift, δ/ppm relative to TMS):
9.34 (s,1H), 7.88 (t, 1H, and J ) 1.65 Hz) 7.79 (t, 1H, and J ) 1.65
(3) Wilkes, J. S.; Zaworotko, M. J. J. Chem. Soc., Chem. Commun. 1992, 965.
(4) (a) Noda, A.; Hayamizu, K.; Watanabe, M. J. Phys. Chem. B 2001, 105,
4603. (b) Tokuda, H.; Hayamizu, K.; Ishii, K.; Susan, M. A. B. H.;
Watanabe, M. J. Phys. Chem. B 2004, 108, 16593. (c) Tokuda, H.;
Hayamizu, K.; Ishii, K.; Susan, M. A. B. H.; Watanabe, M. J. Phys. Chem.
B 2005, in press.
(5) Wasserscheid, P., Welton, T., Eds. Ionic Liquids in Synthesis; Wiley-VCH
Verlag: Weinheim, Germany, 2003.
(6) (a) Watanabe, M.; Yamada, S.; Ogata, N. Electrochim. Acta 1995, 40, 2285.
(b) Watanabe, M.; Mizumura, T. Solid State Ionics 1996, 86-88, 353.
(7) (a) Scott, M. P.; Brazel, C. S.; Benton, M. G.; Mays, J. W.; Holbrey, J. D.;
Rogers, R. D. Chem. Commun. 2002, 1370. (b) Scott, M. P.; Rahman, M.;
Brazel, C. S. Eur. Polym. J. 2003, 39, 1947. (c) Benton, M. G.; Brazel, C.
S. In Ionic Liquids: Industrial Applications for Green Chemistry; Rogers,
R. D., Seddon, K. R., Eds.; ACS Symposium Series 818; American
Chemical Society: Washington, DC, 2002; pp 125-133.
(8) (a) Carlin, R. T.; Delong, H. C.; Fuller, J.; Trulove, P. C. J. Electrochem.
Soc. 1994, 141, L73. (b) Carlin, R. T.; Fuller, J. Chem. Commun. 1997,
1345. (c) Fuller, J.; Breda, A. C.; Carlin, R. T. J. Electrochem. Soc. 1997,
144, L67. (d) Fuller, J.; Breda, A. C.; Carlin, R. T. J. Electroanal. Chem.
1998, 45, 29.
(13) (a) Kawano, R.; Watanabe, M. Chem. Commun. 2003, 330. (b) Kawano,
R.; Matsui, H.; Matsuyama, C.; Sato, A.; Susan, M. A. B. H.; Tanabe, N.;
Watanabe, M. J. Photochem. Photobiol., A 2004, 164, 87.
(14) (a) Harrison, S.; MacKenzie, S. R.; Haddleton, D. M. Chem. Commun.
2002, 2850. (b) Hong, K.; Zhang, H.; Mays, J. W.; Visser, A. E.; Brazel,
C. S.; Holbrey, J. D.; Reichert, W. M.; Rogers, R. D. Chem. Commun.
2002, 1368. (c) Zhang, H.; Hong, K.; Mays, J. W. Macromolecules 2002,
35, 5738. (d) Harrison, S.; MacKenzie, S. R.; Haddleton, D. M. Macro-
molecules 2003, 36, 5072.
(15) (a) Wang, J.-C.; Matyjaszewski K. J. Am. Chem. Soc. 1995, 117, 5614.
(b) Carmichael, A. C.; Haddleton, D. M.; Bon S. A. F.; Seddon, K. R.
Chem. Commun. 2000, 1237. (c) Biedron T.; Kubisa, P. Macromol. Rapid
Commun. 2001, 22, 1237. (d) Sorbu, T.; Matyjaszewski, K. Macromol.
Chem. Phys. 2001, 202, 3379. (e) Biedron T.; Kubisa, P. J. Polym. Sci.,
Part A: Polym. Chem. 2002, 40, 2799. (f) Zhao, Y.; Zhang, J.-M.; Jiang,
J.; Chen, C. F.; Xi, F. J. Polym. Sci., Part A: Polym. Chem. 2002, 40,
3360.
(9) (a) Noda, A.; Watanabe, M. Electrochim. Acta 2000, 45, 1265. (b)
Tiyapiboonchaiya, C.; MacFarlane, D. C.; Sun, J.; Forsyth, M. Macromol.
Chem. Phys. 2002, 203, 1906. (c) Washiro, S.; Yoshizawa, M.; Nakajima,
H.; Ohno, H. Polymer 2004, 45, 1577.
(10) Freemantle, M. C&E News May 3, 2004, 82 (18), 26.
(11) (a) Susan, M. A. B. H.; Noda, A.; Mitsushima, S.; Watanabe, M. Chem.
Commun. 2003, 938. (b) Noda, A.; Susan, M. A. B. H.; Kudo, K.;
Mitsushima, S.; Hayamizu, K.; Watanabe, M. J. Phys. Chem. B 2003, 107,
4024. (c) Susan, M. A. B. H.; Yoo, M.; Nakamoto, H.; Watanabe, M. Chem.
Lett. 2003, 32, 836. (d) Susan, M. A. B. H.; Nakamoto, H.; Yoo, M.;
Watanabe, M. Trans. Mater. Res. Soc. Jpn. 2004, 29, 1043.
(12) Shoubukawa, H.; Tokuda, H.; Tabata, S.; Watanabe, M. Electrochim. Acta
2004, 50, 304.
(16) Ma, H. Y.; Wan, X. H.; Chen, X. F.; Zhou, Q. F. J. Polym. Sci., Part A:
Polym. Chem. 2003, 41, 143.
(17) Perrier, S.; Davies, T. P.; Carmichael, A. J.; Haddleton, D. M. Chem.
Commun. 2002, 2226.
(18) Snedden, P.; Cooper, A. I.; Scott, K.; Winterton, N. Macromolecules 2003,
36, 4549.
(19) Bonhoˆte, P.; Dias, A.-P.; Papageorgiou, N.; Kalyanasundaram, K.; Gra¨tzel,
M. Inorg. Chem. 1996, 35, 1168.
9
J. AM. CHEM. SOC. VOL. 127, NO. 13, 2005 4977

A R T I C L E S
Susan et al.
Table 1. Compatibility of EMITFSI with Monomersa and Their
Because of the favorable compatibility of PMMA with EMITFSI, a
network polymer of PMMA was chosen as a matrix of the EMITFSI.
Deoxygenated MMA, ethylene glycol dimethacrylate (EGDMA, 2 mol
% based on MMA), and BPO (2 mol % based on MMA) as an initiator
were dissolved in EMITFSI. The mixtures were injected with a syringe
between two glass plates separated by a poly(tetrafluoroethylene)
(Teflon) spacer (0.5 mm in thickness), and in situ radical polymerization
was conducted at 80 °C for 24 h. As for the samples for conductivity
and NMR measurements, a further heat treatment at 140 °C for 30
min was carried out in order to complete the polymerization (vide infra).
The composition of [MMA]/[EMITFSI] was varied from 9/1 to 3/7,
and in all cases the obtained films of ion gels were flexible, transparent,
and self-standing. In addition, PMMA (bulk) films, which did not
contain any EMITFSI, were also prepared as a reference material
following the polymerization technique identical to that of an ion gel.
Thermal Properties. Differential scanning calorimetry (DSC) was
carried out on a Seiko Instruments DSC 220C under a nitrogen
atmosphere. The samples were tightly sealed in Al pans in the glovebox.
The measurements were conducted with the samples heated to 200 °C
followed by cooling to -150 °C, and then heating again up to 200 °C
at a cooling and heating rate of 10 °C min-1. The glass transition
temperature (the midpoint temperature of the heat capacity change, Tg)
and melting point (onset temperature of the endothermic peak, Tm) were
determined from the DSC thermograms during the programmed
reheating steps. Thermogravimetric (TG) measurements of the samples
were conducted using open Al pans on a Seiko Instruments thermo-
gravimetry/differential thermal analyzer (TG-DTA 6200) from room
temperature to 550 °C at a heating rate of 10 °C min-1 under a nitrogen
atmosphere.
Polymersb
monomer
polymer
methyl methacrylate
acrylonitrile
vinyl acetate
compatible
compatible
compatible
compatible
compatible
compatible
compatible
compatible
phase-separated
no polymerization
phase-separated
phase-separated
compatible
styrene
2-hydroxyethyl methacrylate
methyl acrylate
acrylamide
phase-separated
a Ionic liquids and monomers are mixed at 1:1 molar ratio. b Polymer-
ization was carried out in the presence of 2 mol% BPO for monomers at
80 °C for 12 h.
Hz), 4.24 (q, 2H, and J ) 7.26 Hz) 3.89 (s and 3H), 1.42 (t, 3H, and
J ) 7.26 Hz). It was further characterized by fast atom bombardment
mass spectroscopic (FAB-MS, glycerol matrix) results. Positive FAB:
molecular weight of EMI cation ) 111.17 and the mass-to-charge ratio,
m/z, for the molecular ion peak ) 111. The quasi-molecular ion peaks
with an m/z of 301 and 303 in the positive FAB corresponded to the
species with two EMI cations attached to a bromide (Br-) with the
indication of the formation of ion clusters in the EMIBr system.20
Finally, the anion exchange reaction from Br- to bis(trifluoromethane
sulfonyl)imide (TFSI-) was carried out by heating an equimolar mixture
of EMIBr and LiTFSI (kindly supplied by Dai-ichi Kogyo Seiyaku
Co.) in water at 70 °C for 24 h to be phase-separated into hydrophobic
ionic liquid and aqueous phases. The obtained EMITFSI was repeatedly
washed with pure water and then dehydrated under vacuum at 120 °C
for 72 h. This yielded a clear, colorless liquid (Tm ) -16 °C, vide
1
infra). The structure of EMITFSI was identified by H NMR spectra
Dynamic mechanical analysis (DMA) was performed under a
nitrogen atmosphere by using a Seiko Instruments DMS 210. The
samples of the films of the ion gels were cut into strips (5 mm × 30
mm) in the argon atmosphere glovebox and were rapidly cooled from
ambient temperature to -150 °C, and the measurements were performed
at the frequency of 1 Hz with the heating rate of 2 °C min-1 up to 150
°C.
(DMSO-d6, δ/ppm relative to TMS): 9.10 (s and 1H), 7.77 (t, 1H, and
J ) 1.65 Hz), 7.68 (t, 1H, and J ) 1.65 Hz), 4.19 (q, 2H, and J ) 7.26
Hz), 3.85 (s and 3H), 1.42 (t, 3H, and J ) 7.26 Hz). The FAB-MS
(glycerol matrix) results (Positive FAB: molecular weight of EMI
cation )111.17 and m/z the molecular ion peak ) 111. Negative
FAB: molecular weight of TFSI anion ) 280.17 and m/z of the
molecular ion peak ) 280) further characterized EMITFSI. Similarly
to EMIBr, the quasi-molecular ion peak with an m/z of 502 in the
positive FAB corresponded to the species with two EMI cations attached
to a TFSI anion and suggested ion cluster formation in the ionic liquid
system.20 The samples were stored in an argon atmosphere glovebox
(VAC, [O2] < 1 ppm, [H2O] < 1 ppm).
Radical Polymerization of Vinyl Monomers in Ionic Liquid and
Preparation of Ion Gels. Using EMITFSI as a polymerization solvent,
we tried to polymerize seven common vinyl monomers. Prior to
polymerization, methyl methacrylate (MMA), acrylonitrile, vinyl
acetate, styrene, 2-hydroxyethyl methacrylate, methyl acrylate, and
acrylamide were purified by distillation or recrystallization and bubbled
with nitrogen gas to ensure an oxygen-free atmosphere in the case of
liquid monomers. The equimolar mixtures of EMITFSI and the
monomers were compatible, and therefore, radical polymerization was
carried out by heating the solutions at 80 °C for 12 h in the presence
of 2 mol % of benzoyl peroxide (BPO) as an initiator. The obtained
polymers, poly(methyl methacrylate) (PMMA), and poly(methyl acryl-
ate) showed good compatibility with EMITFSI in the resulting vinyl
polymers, while polyacrylonitrile, polystyrene, poly(2-hydroxyethyl
methacrylate), and polyacrylamide were incompatible with the ionic
liquid. Under the experimental conditions, free radical polymerization
reaction did not occur for vinyl acetate, possibly due to the inhibition
caused by a trace amount of dissolved oxygen in the system. The
compatibility of the vinyl monomers and polymers with EMITFSI is
summarized in Table 1.
Ionic Conductivity Measurements. The ionic conductivity (σ) was
determined in a hermetic cell by means of the complex impedance
measurements with stainless steel blocking electrodes, using a computer-
controlled Hewlett-Packard 4192A LF impedance analyzer over the
frequency range from 5 Hz to 13 MHz at an AC amplitude of 10 mV.
An ionic liquid sample was filled between two mirror-finished stainless
steel electrodes using a Teflon ring spacer in a constant volume
cylindrical cell (13 mm outer diameter, 7 mm inner diameter, 2 mm
thickness) and was sealed in a Teflon container in the glovebox. For
the polymer electrolytes, a film (12 mm diameter, 1.13 cm2 area, ca.
0.5 mm thickness) was sandwiched between mirror-finished stainless
steel electrodes, sealed in a Teflon container, and subjected to the
impedance measurements. The measurements were conducted at
controlled temperatures while cooling from 100 to -20 °C in a Tabai
Espec SU-220 temperature chamber. The samples were thermally
equilibrated at each temperature for at least 1 h prior to the measure-
ments. The accuracy of all of the results was checked by replicate
measurements under identical experimental conditions.
Self-Diffusion Coefficient Measurements. The self-diffusion coef-
ficients of the ions and the polymer matrix in the ion gels were
determined by pulsed-gradient spin-echo NMR (PGSE-NMR) mea-
surements, which were made by using a JEOL GSH-200 spectrometer
with a 4.7 T wide bore superconducting magnet controlled by a
TecMAG Galaxy and a Mac NMR systems equipped with JEOL pulse
1
field gradient probes and a current amplifier. The 19F and H spectra
were measured with a 19F/1H probe. The measurements were carried
out with cooling from 100 to 30 °C, and the samples were thermally
equilibrated at each temperature for 30 min before the measurements.
After the measurements of 90° pulses, the longitudinal relaxation time
(T1), and the transversal relaxation time (T2), the self-diffusion coef-
(20) (a) Abdul-Sada, A. K.; Greenway, A. M.; Hitchcock, P. B.; Mohammed,
T. J.; Seddon, K. R.; Zora, J. A. J. Chem. Soc., Chem. Commun. 1996,
1753. (b) Abdul-Sada, A. K.; Elaiwi, A. E.; Greenway, A. M.; Seddon, K.
R. Eur. J. Mass Spectrom. 1997, 3, 245.
9
4978 J. AM. CHEM. SOC. VOL. 127, NO. 13, 2005

Ion Gels as Polymer Electrolytes
A R T I C L E S
Figure 1. TG curves for MMA network polymers with different mole
fractions of dissolved EMITFSI and EMITFSI bulk at a heating rate of 10
°C min-1
.
ficients were measured using the simple Hahn spin-echo (i.e., 90° -
τ - 180° - τ - Acquisition) sequence incorporating a gradient
pulse in each τ period, unless otherwise noted. The echo signal
attenuation, E, is related to the experimental parameters by21
E ) exp(-γ2g2Dδ2(∆-δ/3))
(1)
where γ is the gyromagnetic ratio, g is the amplitude of the gradient
pulses of the duration δ, and ∆ is the interval between the leading
edges of the gradient pulses. In the present experiments, the g value
used was constant (3.34 T/m), the δ was in the range of 0 to 3 ms, and
the value of ∆ was 50 ms. The self-diffusion coefficient of the EMI+
cation in the ion gel was obtained from the signal attenuation of the
1H on the 3-methyl group, and that of the TFSI- anion was determined
from the signal attenuation of the 19F nucleus. The accuracy of the
experimental results was checked by replicate measurements under
identical experimental conditions. The PGSE measurement involved
the collection of spin-echo amplitudes as the gradient pulse strengths
were varied, and the value of D could, therefore, be determined from
the slope of a plot of ln E versus -γ2g2Dδ2(∆ - δ /3)(R2 > 0.996 for
EMI+ and TFSI-). The self-diffusion coefficient values of the ions in
EMITFSI (bulk) were used from our previous report.4
Figure 2. DSC thermograms for MMA network polymers with different
mole fractions of dissolved EMITFSI and EMITFSI bulk at a heating rate
of 10 °C min-1
.
loss process for PMMA in the ion gels is higher than that of
the bulk PMMA, which is indicative of the higher thermal
stability of the ion gels compared with that of PMMA itself,
despite a decrease in the Tg with the incorporation of EMITFSI.
Similar behavior has been observed in other polymer/ionic liquid
systems.18
Figure 2 shows the DSC thermograms during the heating
scans from -150 to 200 °C (reheating step) for the ion gels
along with those for bulk EMITFSI and PMMA. EMITFSI
shows a sharp endothermic peak in the DSC thermogram
corresponding to Tm (-16 °C), together with a heat capacity
change corresponding to Tg (-86 °C). The coexistence of the
Tm and Tg indicates relatively slow crystallization kinetics of
EMITFSI, which has been widely observed in ionic liquids.4
On the other hand, the PMMA bulk exhibits Tg at 96 °C. It is
readily seen that the ion gels have only the heat capacity
changes, corresponding to glass transition, in the DSC thermo-
grams, which vary with composition.
Results and Discussion
Characterization of EMITFSI and Ion Gels. Figure 1
shows the TG curves for PMMA network polymers with a
different mole fraction of dissolved EMITFSI. The TG curve
for EMITFSI (bulk) shows a one-step weight loss process with
the detection of noticeable change only at above 400 °C
(temperature corresponding to 10% weight loss, Td ) 444 °C)
indicating high thermal stability as an ionic liquid over a wide
range of temperature. The TG curves for the network polymers
with dissolved EMITFSI, on the other hand, show two-step
weight loss processes: the initial losses corresponding to the
network polymer and the final step corresponding to the weight
loss corresponding to EMITFSI in the system. The reference
polymer, PMMA, undergoes a one-step weight loss process with
a Td of 257 °C. It should be noted that the onset of the weight
The DSC results for the heating scans from room temperature
to 200 °C (first step) are also interesting (Supporting Information
1). The PMMA bulk shows an exothermic peak at 140 °C, which
can be ascribed to the heat of polymerization of unreacted MMA
monomers in the system. With increasing EMITFSI content in
the ion gel, the intensity of the exothermic peak gradually
decreases, and for [MMA]/[EMITFSI] ) 7/3 or higher com-
positions, the peak disappears, indicating the absence of
unreacted MMA monomer in the ion gels. This may indicate
enhancement of the free radical polymerization of MMA in the
(21) (a) Stejskal, E. O. J. Chem. Phys. 1965, 43, 3597. (b) Price, W. S. Concepts
Magn. Reson. 1997, 9, 299. (c) Price, W. S. Concepts Magn. Reson. 1998,
10, 199.
9
J. AM. CHEM. SOC. VOL. 127, NO. 13, 2005 4979

A R T I C L E S
Susan et al.
Figure 4. Storage modulus (E′), loss of modulus (E′′), and loss of tangent
(tan δ) for EMITFSI dissolved in MMA network polymers ([MMA]/
[EMITFSI] ) 8/2) as a function of temperature at 1 Hz.
Figure 3. Glass transition temperature (Tg) for MMA network polymers
with dissolved EMITFSI as a function of EMITFSI concentration. The points
are the experimental Tg’s, and the lines are the fitted profiles. Dotted line:
Gordon-Taylor equation. Solid line: Fox equation.
and EMITFSI, respectively, while wPMMA and wPMMA are the
weight fractions, respectively, in the systems. The experimental
results fit well both of the equations (Figure 3), and therefore,
the binary system of PMMA and EMITFSI behaves as a
compatible system. This is further revealed from the DMA
results, which give a single peak for the temperature dependence
of loss tangent (tan δ) obtained from the storage modulus (E′)
divided by the loss of modulus (E′′) for EMITFSI dissolved in
the network polymer (Figure 4).
Ionic Conductivity and Self-Diffusion Behavior in Ion
Gels. The temperature dependence of ionic conductivity (σ) for
the network polymer electrolytes based on PMMA and EMITF-
SI along with that for bulk EMITFSI is depicted in Figure 5.
The σ has been found to increase with an increase in the content
of EMITFSI in the ion gels. At high EMITFSI concentrations,
the σ reaches the value of ca. 10-2 S cm-1 at 30 °C, which is
much higher than the values ever reported for conventional
polymer electrolytes at ambient temperature. The high ionic
conductivity of the ion gels9 is due to the high self-dissociating
and ion-transporting abilities of the constituent ionic liquid and
the decoupling of the ion transport from polymer segmental
motion (vide infra).
presence of EMITFSI. This is in good agreement with the
corroborative study by Harrison et al.,14a wherein by means of
pulse laser polymerization technique, they have shown that the
rate constant of MMA propagation increases considerably when
polymerization is carried out in an ionic liquid.
Compatibility of EMITFSI with PMMA in Ion Gels.
Despite two distinct components (PMMA and EMITFSI) in the
binary systems, the polymer electrolytes exhibit a single Tg in
the range of the compositions, and the endothermic peak
corresponding to the melting point of EMITFSI vanishes (vide
supra). Therefore, at all compositions of the ion gels, the system
at temperatures higher than the Tg is in the rubbery state. Figure
3 shows the composition dependence of the Tg with varying
[MMA]/[EMITFSI] compositions, which shows a decrease in
the Tg with an increase in the mole fraction of EMITFSI and
approaches the value of EMITFSI at high salt concentrations.
These suggest that the network polymer of PMMA is compatible
with dissolved EMITFSI. The relationship for the dependence
of the Tg on composition can be understood, as was rationalized
by the Gordon and Taylor equation22a for a copolymer, based
on free volume considerations, and by the Fox equation22b for
a plasticized polymer and a copolymer, based on additivity of
the weight fraction. If the Tg of the polymer and the solvent,
PMMA and EMITFSI in our case, are represented by Tg PMMA
and Tg EMITFSI, respectively, the Gordon-Taylor equation and
Fox equation are given by:23
The Arrhenius plots of the temperature dependency of the σ
exhibits convex upward curved profiles; therefore, experimental
data have been fitted with the Vogel-Tamman-Fulcher (VTF)
equation24 for conductivity for such electrolytic materials (Figure
5). The VTF equation is
σ ) σ0 exp[-B/(T - T0)]
(4)
Tg ) VEMITFSI × TgEMITFSI + VPMMA
TgPMMA
wPMMA
×
(VEMITFSI + VPMMA ) 1) (2)
where the constants, σ0 (S cm-1), B (K), and T0 (K) are
adjustable parameters. The best-fit parameters are shown in
wEMITFSI
1
)
+
(wEMITFSI + wPMMA ) 1) (3)
(23) Usually, the Fox equation is used to predict the glass transition temperatures
for random copolymers. However, the equation is expected to be applicable
to polymer-diluent systems, which are compatible and not too strongly
polar. Data on copolymers and on polymer-diluent systems illustrated this
to be substantially true.9b,22b,c Free volume considerations, as in the Gordon-
Taylor equation, can also predict new Tg’s when polymers are plasticized
by molecular solvents (diluents) and are compatible with them at the
molecular level.22a,c
Tg TgEMITFSI TgPMMA
where VPMMA and VEMITFSI are the volume fractions of PMMA
(22) (a) Gordon, M.; Taylor, J. S. J. Appl. Chem. 1952, 2, 493. (b) Fox, T. G.
Bull. Am. Phys. Soc. 1956, 1, 123. (c) Nielsen, l. E. Mechanical Properties
of Polymers and Composites; Marcel Dekker: New York, 1975; Chapter
1.
(24) (a) Vogel, H. Phys. Z. 1921, 22, 645. (b) Fulcher, G. S. J. Am. Ceram.
Soc. 1923, 8, 339.
9
4980 J. AM. CHEM. SOC. VOL. 127, NO. 13, 2005

Ion Gels as Polymer Electrolytes
A R T I C L E S
Figure 5. Temperature dependence of ionic conductivity for MMA network
polymers with different mole fractions of dissolved EMITFSI and EMITFSI
bulk.
Figure 6. Temperature dependence of self-diffusion coefficient for MMA
network polymers with dissolved EMITFSI ([MMA]/[EMITFSI] ) 3/7)
and EMITFSI bulk.
Table 2. VTF Equation Parameters for Ionic Conductivity
Data and Glass Transition Temperatures for MMA Network
Polymer with Dissolved EMITFSI and EMITFSI Bulk
gels. Figure 6 shows the temperature dependency of the self-
diffusion coefficients of different diffusive species in the ion
gel with [PMMA]/[EMITFSI] ) 3/7 along with those in
EMITFSI (bulk). The experimental data have been fitted with
the VTF equation24 for diffusivity:
σ ) σ0 exp[-B/(T - T0)]
1
[MMA]/[EMITFSI]
σ
0/S cm-
B/K
T0/K
R2
Tga/K
0/10
3/7
4/6
5/5
6/4
7/3
8/2
9/1
5.51 × 10-1
3.08 × 10-1
2.72 × 10-1
2.78 × 10-1
2.89 × 10-1
2.51 × 10-1
1.58 × 10-1
4.13 × 10-3
5.34 × 102
4.79 × 102
5.67 × 102
6.66 × 102
8.21 × 102
1.06 × 103
1.30 × 103
2.21 × 103
167
178
176
175
177
177
194
272
0.999
0.999
0.999
0.999
0.999
0.999
0.999
0.999
187
192
196
202
214
235
273
324
D ) D0 exp[- B/(T - T0)]
(5)
where D0 (cm2 s-1), B (K), and T0 (K) are constants. The best-
fit parameters of the self-diffusion coefficients are summarized
in Table 3. The EMI cation diffuses faster than the TFSI anion
in the ion gel, similar to that in the ionic liquids. Interestingly,
the decrease in the ionic diffusivity in the ion gel is rather small,
compared with that of bulk EMITFSI, although the ion gel
behaves as a soft solid.
a The midpoint temperature of the heat capacity change (Tg) during
heating scans from -150 °C by using differential scanning calorimetry.
Table 3. VTF Equation Parameters for Self-Diffusion Coefficient
Data for EMITFSI Bulk and MMA Network Polymer with
Dissolved EMITFSI
Ion Transport Mechanism in Ion Gels. A comparison of
the temperature dependency of the σ and self-diffusion coef-
ficient (DEMI + DTFSI) for EMITFSI (bulk) and an ion gel
([PMMA]/[EMITFSI] ) 3/7) shows that the gelation results in
a decrease in both the ionic conductivity and self-diffusion
coefficient, as is the case for conventional polymer-in-salt
electrolytes. However, the ratio of the ionic conductivity (σion
gel/σionic liquid) for the ion gel ([PMMA]/[EMITFSI] ) 3/7) and
ionic liquid (EMITFSI bulk) at a particular temperature is
significantly higher than that of the self-diffusion coefficient
D ) D0 exp[-B/(T - T0)]
1
D0/cm2 s-
B/K
T0/K
R2
Ionic Liquid (EMITFSI Bulk)
cation
anion
cation + anion
1.56 × 10-4
2.55 × 10-4
3.45 × 10-4
9.14 × 102
1.30 × 103
1.03 × 103
134
100
123
0.999
0.999
0.999
Ion Gel ([MMA]/[EMITFSI] ) 3/7)
cation
anion
cation + anion
4.27 × 10-5
1.30 × 10-4
1.10 × 10-4
6.87 × 102
1.33 × 103
8.58 × 102
165
105
147
0.999
0.999
0.999
(Dion /Dionic liquid) for a range of temperatures (Figure 7).
gel
The Nernst-Einstein equation, which correlates ionic con-
ductivity with diffusivity, is given by
Table 2. A close value of the Tg to T0 for EMITFSI (bulk) is
normal and well rationalized. However, with a decrease in the
EMITFSI content in the PMMA/EMITFSI electrolytes, the
difference between the T0 and Tg has been found to increase,
and the ion conduction, therefore, showed a tendency to be
decoupled from the structure glass transition. We shall discuss
the phenomenological detail of the physical processes in the
ion gel system elsewhere.
σ ) nq2(Dcation + Danion)/kT
(6)
where n is the number of ion carriers, q is the electric charge
on each ionic carrier, k is the Boltzmann constant, and T is the
absolute temperature. The larger value of the σion gel/σionic liquid
compared to the Dion gel/Dionic liquid at a particular temperature in
the ion gel, therefore, indicates an increase in the number of
ion carriers when EMITFSI has been incorporated into network
polymers by radical polymerization. The carrier number (n),
The diffusion behavior, studied by PGSE-NMR measure-
ments, in association with the ionic conductivity provides
valuable information on the ion transport behavior in the ion
9
J. AM. CHEM. SOC. VOL. 127, NO. 13, 2005 4981

A R T I C L E S
Susan et al.
Figure 9. Self-diffusion coefficients for EMITFSI dissolved in MMA
network polymers as a function of the mole fraction of EMITFSI at 30 °C.
Figure 7. Ratio of ionic conductivity and self-diffusion coefficient for
MMA network polymers with dissolved EMITFSI ([MMA]/[EMITFSI] )
3/7) to those for bulk EMITFSI at different temperatures.
molecular ion peaks for ion clusters for a number of imidazolium
salts. Because of such ionic aggregation, not all of the diffusive
species in ionic liquids contribute to the ionic conduction, since
some associated species are neutral and do not carry electric
charge. In fact, ion conduction in EMITSFI has been found to
be contributed by 30-50% of the total ions in the bulk EMITFSI
system.4 If the polymer matrix can have some specific interac-
tion with either the cation or anion, the tendency to form ion
clusters or aggregates should decrease, resulting in the increase
in the number of ion carriers in the ion gels. To clarify, we
have plotted the self-diffusion coefficients for different [PMMA]/
[EMITFSI] compositions as a function of EMITFSI concentra-
tion at 30 °C in Figure 9. It is apparent here that a decrease in
the self-diffusion coefficient for the TFSI- anion is larger than
that for the EMI+ cation with increasing PMMA content in the
ion gels. This is indicative of an interaction between the polymer
matrix and the TFSI- anion in the ion gels. The cation
transference number (Dcation/(Dcation + Dcanion)) of the EMI+
cation in the ion gels at 30 °C changed from 0.619 to 0.703 for
the change in the mole fraction of EMITFSI from 1.0 to 0.5.
The increase in the cation transference number with decreasing
EMITFSI concentration corresponds to the relative increase in
the cationic mobility, resulting from the interaction between the
PMMA matrix and the TFSI- anion, which prohibits the
formation of ion clusters or associates. This interaction leads
to an increase in the n in the ion gels (Figure 8). It is a tradeoff
between the generation of new ionic carriers by the specific
interaction of the anion with PMMA and the decrease in total
ionic species in the solid polymer matrix with increasing mole
fraction of PMMA, which determines the overall carrier ion
density for the ionic conduction in the ion gels.
Figure 8. Carrier numbers for EMITFSI dissolved in MMA network
polymers as a function of the mole fraction of EMITFSI at 30 °C.
calculated from eq 6 for some ion gels with different composi-
tions and bulk EMITFSI, has been plotted against the mole
fraction of EMITFSI at 30 °C in Figure 8. It is worth mentioning
that, due to the very small T2 values for some of the ion gels,
the self-diffusion coefficients used for the calculation of n for
all of the ion gels in Figure 8 were determined using a stimulated
echo sequence (i.e., 90° - τ1 - 90° - τ2- 90° - τ1
-
Acquisition) to ensure precise results. Surprisingly, the n for
the ion gel with [PMMA]/[HTFSI] ) 3/7 is higher than that of
bulk EMITFSI, though the volume fraction of EMITFSI
decreases. The nonconductive species in the ionic liquids seem
to interact with the polymer matrix, which might cause the
generation of new carrier ions in the ion gels.
Since ionic liquids are highly concentrated electrolytes, an
ion is surrounded by the oppositely charged ions and, therefore,
may form ion clusters or aggregates. This is supported by FAB-
MS results (vide supra), which is in agreement with the
observation of Abdul-Sada et al.,20 wherein they also evidenced
Conclusion
An ionic liquid has been successfully used with compatible
polymers for the preparation of a novel series of polymer
electrolytes. In situ radical polymerization of suitable vinyl
monomers in an ionic liquid generates ion gels that exhibit high
ionic conductivities at room temperature and possess sufficient
mechanical strength, transparency, and flexibility, which are
considered to be the basic requirements for polymer electrolytes.
9
4982 J. AM. CHEM. SOC. VOL. 127, NO. 13, 2005

Ion Gels as Polymer Electrolytes
A R T I C L E S
The ion gels are completely compatible binary systems of a
network polymer and an ionic liquid, but the liquid is nonvolatile
and thermally stable, giving them distinct characteristics over
conventional polymer gels. The specific interaction of the
polymer matrix with the ionic component of the ionic liquid
(the TFSI- anion in our case) fascinatingly aids to increase the
total carrier number in the ion gel from the corresponding value
in the ionic liquid. The elucidation of the mechanism of the
compatibility of the polymer matrix with EMITFSI as well as
other ionic liquids and investigation of the type of interaction
between the polymer matrix and an ionic liquid by spectroscopic
methods (e.g., IR, Raman) need to be further explored. We
would, therefore, explore a systematic understanding of the
physical processes involved and determination of the factors
controlling ion transport behavior in these novel polymer
electrolytes aiming at tuning the molecular engineering aspects
of ion gels to have desirable properties.
Acknowledgment. This research was supported in part by
Grant-in-Aid for Scientific Research (No. 14350452 and No.
16205024) from the Japanese Ministry of Education, Science,
Sports, and Culture and CREST-JST. We thank Dr. Kikuko
Hayamizu, National Institute of Advanced Industrial Science
and Technology, for discussions and help in PGSE-NMR
measurements. M.A.B.H.S. also acknowledges a postdoctoral
fellowship from the JSPS.
Supporting Information Available: DSC thermograms for
MMA network polymers with dissolved EMITFSI in the first
heating step from room temperature to 200 °C at a heating rate
of 10 °C min-1. This material is available free of charge via
the Internet at http://pubs.acs.org.
JA045155B
9
J. AM. CHEM. SOC. VOL. 127, NO. 13, 2005 4983