Transparent, flexible and highly conductive ion gels from ionic liquid compatible cyclic carbonate network

Article abstract of DOI:10.1039/b921517d

Transparent, flexible, self-standing and highly ion conductive ion gels have been synthesised from novel ionic liquid compatible cyclic carbonate (CC) network polymer. The use of dual functional cyclic carbonate methacrylate (CCMA) monomer for the synthes

Full text of DOI:10.1039/b921517d

View Article Online / Journal Homepage / Table of Contents for this issue
COMMUNICATION
www.rsc.org/chemcomm | ChemComm
Transparent, flexible and highly conductive ion gels from ionic liquid
compatible cyclic carbonate networkw
Satyasankar Jana,* Anbanandam Parthiban and Christina L. L. Chai*
Received (in Cambridge, UK) 14th October 2009, Accepted 4th December 2009
First published as an Advance Article on the web 6th January 2010
DOI: 10.1039/b921517d
Transparent, flexible, self-standing and highly ion conductive ion
gels have been synthesised from novel ionic liquid compatible
cyclic carbonate (CC) network polymer. The use of dual
functional cyclic carbonate methacrylate (CCMA) monomer
for the synthesis has enabled versatile and operationally simple
routes to such type of ion gels.
Combination of macromolecules with ionic liquids (ILs) offers
new progress, challenges and opportunities in polymer
science.1 ILs are of much interest as green solvents for different
Fig.
1 Structure of ionic liquids used (a) [BMIM][PF6], (b)
kinds of polymerizations1,2 mainly due to their non-flammable
and non-volatile nature and are also used for solubilization of
poorly soluble biopolymers.1 The other unique properties of
ILs such as high ionic conductivity, thermal and (electro)-
chemical stability and wide electrochemical window makes the
polymer-IL combination useful as polymer electrolyte for
various electrochemical devices.1,3 One such material that is
of interest for lithium batteries,3,4 membranes,5 chemical
sensors,6 thin film transistors7 and solar cells8 is the IL swelled
polymer network, termed as ion gel.9 In general, ion gels are
prepared either by physical assembly of (co)polymers3b,7,10 or
chemical crosslinking of monomers in an IL. Ion gels by the
latter method is usually obtained either by the polymerization
of vinyl monomers in the presence of a crosslinker9b,11 or
by the polyaddition reaction of (macro)monomers having
multifunctional reactive groups.9a,12 However (co)polymer
networks used to synthesize all these types of ion gels reported
to date are based on either poly(ethylene oxide) (PEO) or
poly(methyl methacrylate) (PMMA) as the primary ionic
liquid-solubilizing units.
[EMIM][TFSI] and (c) dual functional cyclic carbonate methacrylate
(CCMA) monomer.
Fig. 2 Photograph of transparent, flexible and self-standing disc
shaped ion gels (a) IG5 and (b) IG6 after eight months of storage.
The CCMA monomer was synthesized using a N,N0-dicyclo-
hexylcarbodiimide (DCC) mediated coupling reaction of glycerol
1,2-carbonate and methacrylic acid. Poly(cyclic carbonate
methacrylate) (PCCMA), synthesised by free radical poly-
merisation, was only soluble in highly polar solvents like
DMF and DMSO and insoluble in most common organic
solvents presumably because of the presence of the rigid and
highly polar CC pendant groups. Noteworthy is the observation
that the linear PCCMA homopolymer is soluble in a wide
range of ILs such as 1-butyl-3-methylimidazolium hexafluoro-
phosphate ([BMIM][PF6]), 1-ethyl-3-methylimidazolium tetra-
fluoroborate ([EMIM][BF4]), 1-butyl-4-methylpyridinium
tetrafluoroborate ([BMP][BF4]) and 1-ethyl-3-methylimidazolium
bis(trifluoromethylsulfonyl)imide ([EMIM][TFSI]) etc. The
ionic liquid [EMIM][TFSI] was synthesized using a modified
method of Watanabe et al.9b
Here we report the synthesis of ion gels based on a novel
ionic liquid compatible side chain cyclic carbonate (CC)
network polymer. Due to the dual functional nature of
precursor cyclic carbonate methacrylate (CCMA) (Fig. 1),
these ion gels were synthesized by both free radical poly-
merization (route A) and polyaddition reaction (route B)
(Fig. 2). Additionally, the cyclic carbonate (CC) group present
in these ion gels could potentially act as a coordinating unit for
metal ions (e.g. Li+) as in the case of ethylene and propylene
carbonates13 and hence these ion gels are potential alternatives
to these flammable solvents in lithium ion batteries.3a
The compatibility of PCCMA in [EMIM][TFSI] was
confirmed by polymerising CCMA monomer in [EMIM][TFSI]
at 80 1C using benzoyl peroxide (BPO) initiator under nitrogen
for 16 h. This gave a completely homogeneous and transparent
viscous polymerization mixture. Upon addition to DCM, a
fine white powder with 498% yield was collected and the
resultant polymer was readily soluble in DMF and DMSO.
In contrast, polymerization of CCMA in ILs (e.g.
[BMIM][PF6], [EMIM][TFSI]) in the presence of only 5 mol%
of poly(ethylene glycol) dimethacrylate 550 (PEGDMA 550)
Institute of Chemical and Engineering Sciences, Agency for Science,
Technology and Research (A*STAR), 1 Pesek Road, Jurong Island,
Singapore 627833. E-mail: satyasankar_jana@ices.a-star.edu.sg,
christina_chai@ ices.a-star.edu.sg; Fax: (+65) 6316 6184;
Tel: (+65) 67963918 (SJ), 67963902 (CLLC)
w Electronic supplementary information (ESI) available: Details of
monomer, IL, polymer, ion gel synthesis, impedance measurement/
spectra and other characterizations. See DOI: 10.1039/b921517d
ꢀc
This journal is The Royal Society of Chemistry 2010
1488 | Chem. Commun., 2010, 46, 1488–1490

View Article Online
Table 1 Details of ion gels synthesized by route A using [EMIM][TFSI]
The first two ion gels prepared using [BMIM][PF6] (IG1 and
IG2 in Table 1) showed high ionic conductivity in the
impedance measurement and the conductivity is directly
proportional to the IL content, since conductivity is the result
of the mobility of the cations and anions of the IL. Ion gels
were also prepared using [EMIM][TFSI] (IG3 to IG10 in
Table 1). It should be noted that this IL possess lower
viscosity, higher ionic conductivity, higher thermal stability,
better hydrophobicity as well as the potential for less toxic gas
evolution as compared to the use of [BMIM][PF6] as the IL.14
In this class of ion gels, ionic conductivity also increases with
an increase in the ionic liquid composition from IG3 to IG6
(Table 1). The ion conductivity values are comparable to the
values for ion gels obtained using other polymeric networks as
reported in the literature.9b,12c Noteworthy is the observation
that ion gel IG6 contains only 19% polymer and has the
highest ion conductivity.
Composition
Electrolyted
content
(Wt%)
Ionic
conductivitye
Ion gel
code
CCMA : ILb LiTFSI
(S cmꢁ1
)
IG1a
IG2a
IG3
IG4
IG5
IG6
IG7
IG8
IG9
IG10
5 : 5
3 : 7
7 : 3
5 : 5
4 : 6
3 : 7
7 : 3
5 : 5
4 : 6
3 : 7
No
No
No
No
No
No
57.1
75.6
43.9
64.7
73.3
81.0
52.8
68.4
75.5
82.2
7.988 ꢂ 10ꢁ6
1.717 ꢂ 10ꢁ4
6.967 ꢂ 10ꢁ6
1.833 ꢂ 10ꢁ4
4.780 ꢂ 10ꢁ4
1.131 ꢂ 10ꢁ3
1.009 ꢂ 10ꢁ5
3.489 ꢂ 10ꢁ4
8.996 ꢂ 10ꢁ4
3.241 ꢂ 10ꢁ3
Yesc
Yesc
Yesc
Yesc
a
b
c
Prepared using [BMIM][PF6]; Molar ratio, 25 mol% of CCMA
d
e
used; IL + LiTFSI; Measured at 23 1C.
using BPO initiator under a N2 atmosphere produced an
insoluble, transparent, flexible and self-standing ion gels.
A number of ion gels were prepared with different ratios of
CCMA to IL as shown in Table 1. All ion gels were highly
transparent, flexible and did not leach IL upon storage
(Fig. 2). The presence of absorption bands at 1354 cmꢁ1 (from
[EMIM][TFSI]) and 1800 cmꢁ1 (CC carbonyl) in the FT-IR
spectra of these gels confirms their composition (Fig. 3). The
thermal stability of these ion gels increased with the increase in
the IL content as is indicated by thermogravimetric analysis
(Fig. 4). Unlike the linear homopolymer PCCMA, these ion
gels were insoluble in DMF or DMSO due to the network
structure prevailing in the latter. IL from these ion gels can be
extracted quantitatively by solvent extraction using suitable
solvents like acetone and DCM.
Similarly transparent, flexible and self-standing ion gels
were prepared by mixing LiTFSI in a ratio of 1 : 4 to the
respective CCMA monomer used (IG7 to IG10 in Table 1)
before polymerisation. The ionic conductivity of these ion gels
were higher than the respective ion gels without the lithium
salt (e.g. IG10 vs. IG6) because of the contribution from the
ions of LiTFSI.
In general, the incorporation of IL to the CCMA network
not only dramatically improves the ionic conductivity but also
introduces flexibility to the CCMA network. Films derived
from the homopolymer PCCMA and samples produced after
solvent extraction of ion gels based on CCMA network are
highly brittle. Incorporation of IL also improves the thermal
stability of the polymer as is observed from the TGA curves
shown in Fig. 4.
The versatility of the use of cyclic carbonate methacrylate
(CCMA) monomer in ion gel design is further demonstrated
with the synthesis of ion gels using Route B (Scheme 1) where
the reactivity of the CC group towards nucleophiles is
exploited15 (by route B in Scheme 1). For this purpose two
PCCMA-b-PEG-b-PCCMA triblock copolymers with controlled
structures were synthesized by ATRP using two low molecular
weight difunctional poly(ethylene glycol) (PEG) macroinitiators
(M.Wt. 598 and 898 Da, respectively) as reported by us.16
Gelation of these block copolymers in [EMIM][TFSI] with
Fig. 3 FT-IR spectra of ion gels (a) IG3, (b) IG6 and (c) [EMIM][TFSI].
Fig. 4 Thermogravimetric analysis of (a) [EMIM][TFSI], ion gels
Scheme 1 Synthesis of ion gels in two different routes by exploiting
(b) IG6, (c) IG4, (d) IG3 and (e) sample after solvent extraction of IG4.
the dual functionality of CCMA.
ꢀc
This journal is The Royal Society of Chemistry 2010
Chem. Commun., 2010, 46, 1488–1490 | 1489

View Article Online
Table 2 Details of ion gels synthesized by route B
Ion gel codea
Copolymer usedb
Diamine usedc
Temp. (1C)
Ionic conductivityd (S cmꢁ1
)
IG11
IG12
IG13
IG14
P1
P2
P2
P2
XDA
HMD
DETA
XDA
70
6.249 ꢂ 10ꢁ4
7.734 ꢂ 10ꢁ4
9.288 ꢂ 10ꢁ4
5.011 ꢂ 10ꢁ4
RT
RT
70
a
b
[EMIM][TFSI] content B67.5%; Copolymer P1: (CCMA)43-(EG)7-(CCMA)43; Mn 16350 (calculated); Mn 20221 (GPC in DMF), PDI 1.18.
copolymer P2: (CCMA)47-(EG)13-(CCMA)47; Mn 18470 (calculated); Mn 22460 (GPC in DMF), PDI 1.19; 10 mol% relative to CC units of
c
d
polymer is used; HMD = hexamethylenediamine, DETA = diethylenetriamine and XDA = p-xylylenediamine; Measured at 23 1C.
various diamines produced transparent, flexible and self-standing
ion gels with high ionic conductivity (Table 2 and see ESI).w
Hexamethylenediamine (HMD) and diethylenetriamine
references therein.
Notes and references
1 T. Ueki and M. Watanabe, Macromolecules, 2008, 41, 3739, and
(DETA) produced ion gels at room temperature whereas
p-xylylenediamine (XDA) gave ion gels at 70 1C without the
need for reactions under nitrogen. It was also observed that
the homopolymer PCCMA can also yield ion gels using this
method. This is one of the simplest ways to prepare stable
ion gels.
2 (a) N. Winterton, J. Mater. Chem., 2006, 16, 4281 and references
therein; (b) B. Ochiai, Y. Satoh and T. Endo, J. Polym. Sci.,
Part A: Polym. Chem., 2009, 47, 4629.
3 (a) A. M. Christie, S. J. Lilley, E. Staunton, Y. G. Andreev and
P. G. Bruce, Nature, 2005, 433, 50; (b) Y. He and T. P. Lodge,
Macromolecules, 2008, 41, 167.
4 (a) J. H. Shin, W. A. Henderson and S. Passerini, Electrochem.
Commun., 2003, 5, 1016; (b) H. Shobukawa, H. Tokuda, S. Tabata
and M. Watanabe, Electrochim. Acta, 2004, 50, 305; (c) J. H. Shin,
W. A. Henderson and S. Passerini, J. Electrochem. Soc., 2005, 152,
A978.
5 (a) J. Huang, A. Riisager, P. Wasserscheid and R. Fehrmann,
Chem. Commun., 2006, 4027; (b) J. B. Tang, H. D. Tang,
W. L. Sun, H. Plancher, M. Radosz and Y. Q. Shen, Chem.
Commun., 2005, 3325.
The ionic liquid content of the ion gels synthesized by route
B was about 67.5% (Table 2). In general, these ion gels possess
higher ionic conductivity than the ion gels synthesized using
route A with similar IL content (see IG4 and IG5 in Table 1).
The higher ionic conductivity of ion gels synthesized by route
B may be due to the higher mobility of ionic species caused by
the presence of larger amounts of loosely bound IL in
the crosslinked gel. Various other factors which may also
contribute to this process i.e.: the more flexible nature of ion
gels obtained by route B as a result of the ring opening of rigid
cyclic carbonate units and the copolymeric nature of the ion
gel, the presence of less uniform crosslinking sites, the ordering
of IL induced by the physico-chemical interactions between
the block copolymer and the IL before crosslinking and so on.
Thermal stability of these gels is similar to the ion gels
synthesized by route A as is demonstrated by thermo-
gravimetric analysis (see ESI).w
In conclusion, we have synthesized a new class of flexible,
self-standing and highly ion conductive ion gels using novel
ionic liquid compatible cyclic carbonate network. Since cyclic
carbonate network and ionic liquids are highly compatible,
the gels formed were completely transparent and stable.
Most importantly, the use of the dual functional cyclic
carbonate methacrylate monomer for this purpose has
enabled the synthesis of ion gels in two different routes.
Incorporation of lithium salt to these ion gels dramatically
improved the ionic conductivity. Due to this highly compatible
nature, ionic liquids may be useful as plasticizer for
CCMA based polymers which are otherwise very brittle and
CCMA based polymers can be used as support materials for
small molecules or catalysts, if required, for other reactions
performed in IL.
6 P. G. Boswell, E. C. Lugert, J. Rabai, E. A. Amin and
P. Buhlmann, J. Am. Chem. Soc., 2005, 127, 16976.
7 (a) J. Lee, M. J. Panzer, Y. He, T. P. Lodge and C. D. Frisbie,
J. Am. Chem. Soc., 2007, 129, 4532; (b) J. H. Cho, J. Lee, Y. He,
B. Kim, T. P. Lodge and C. D. Frisbie, Adv. Mater., 2008, 20, 686;
(c) J. H. Cho, J. Lee, Y. Xia, B. Kim, Y. He, J. M. Renn,
T. P. Lodge and C. D. Frisbie, Nat. Mater., 2008, 11, 900.
8 (a) M. Watanabe and S. Yanagida, Chem. Commun., 2005, 740;
(b) R. Kawano and M. Watanabe, Chem. Commun., 2005, 2107;
(c) R. Kawano, M. K. Nazeeruddin, A. Sato, M. Gratzel and
M. Watanabe, Chem. Commun., 2005, 2107.
9 (a) M. A. Lligshirm, S. K. Spear, R. Subramanian, J. D. Holbrey,
J. G. Huddleston and R. D. Rogers, Chem. Mater., 2004, 16, 3091;
(b) M. A. B. H. Susan, T. Kaneko, A. Noda and M. Watanabe,
J. Am. Chem. Soc., 2005, 127, 4976; (c) K. Hanabusa, H. Fukui,
M. Suzuki and H. Shirai, Langmuir, 2005, 21, 10383.
10 (a) Y. He and T. P. Lodge, Chem. Commun., 2007, 2732; (b) Y. He,
P. G. Boswell, P. Buhlmann and T. P. Lodge, J. Phys. Chem. B,
2007, 111, 4645; (c) A. Noro, Y. Matsushita and T. P. Lodge,
Macromolecules, 2008, 41, 5839.
11 S. Seki, M. A. B. H. Susan, T. Kaneko, A. Noda and
M. Watanabe, J. Phys. Chem., 2005, 109, 3886.
12 (a) M. A. Neouze, J. L. Bideau, P. Guveau, S. Bellayer and
A. Vioux, Chem. Mater., 2006, 18, 3931; (b) K. Matsumoto
and T. Endo, Macromolecules, 2008, 41, 6981; (c) K. Matsumoto and
T. Endo, Macromolecules, 2008, 42, 4580.
13 (a) J. M. Tarascon and M. Armand, Nature, 2001, 414, 359;
(b) F. B. Dias, L. Plomp and J. B. J. Veldhuis, J. Power Sources,
2000, 88, 169; (c) J. H. Golden, B. G. M. Chew, D. B. Zax,
F. J. DiSalvo, J. M. J. Frechet and J. M. Tarascon, Macromole-
cules, 1995, 28, 3468; (d) J. Britz, W. H. Meyer and G. Wegner,
Macromolecules, 2007, 40, 7558.
14 Z. Bai, Y. He and T. P. Lodge, Langmuir, 2008, 24, 5284.
15 (a) A. Steblynko, W. Choi, F. Sanda and T. Endo, J. Polym. Sci.,
Part A: Polym. Chem., 2000, 38, 2375; (b) D. C. Webster, Prog.
Org. Coat., 2003, 47, 77.
This work was supported by the Science and Engineering
Research council of A*STAR (Agency for Science, Technology
and Research), Singapore. We thank Mr H. Yu and Dr T.
Zhiqun for their assistance during ion gel characterization.
16 S. Jana, H. Yu, A. Parthiban and C. L. L. Chai, manuscript
submitted to J. Polym. Sci.: Part A: Polym. Chem.
ꢀc
This journal is The Royal Society of Chemistry 2010
1490 | Chem. Commun., 2010, 46, 1488–1490