Nano-segregated polymeric film exhibiting high ionic conductivities

Article abstract of DOI:10.1021/ja0549594

Nanostructures can be used for the fabrication of highly functional materials transporting ions and charges. We demonstrate a new design strategy for polymeric higher ion-conductors. Phase-segregated layers of alternating mobile tetra(ethylene oxide)s (TEOs) and rigid aromatic cores where the TEO moieties are grafted from aromatic layers have been shown to be efficient to transport lithium triflate. Such segregated structures at the nanometer scale (nano-segregated structures) were prepared by in-situ photopolymerization of an aligned methacrylate liquid crystalline monomer comprising a terphenyl rigid rod mesogen having a TEO terminal chain. The ion-conductive TEO moiety remains in the highly mobile state even after polymerization, which is indicated by its low glass transition temperature (-45 °C). This nanostructured film exhibits an ionic conductivity parallel to the layer of 10^-3 S cm^-1 at room temperature. The highest ionic conductivity is in the level of 10 ^-2 S cm^-1 observed at 150 °C. The anisotropic ionic conductivities have been observed for the nano-segregated film.

Full text of DOI:10.1021/ja0549594

Published on Web 10/14/2005
Nano-Segregated Polymeric Film Exhibiting High Ionic
Conductivities
Kenji Kishimoto,† Tomoyuki Suzawa,† Tomoki Yokota,† Tomohiro Mukai,‡
Hiroyuki Ohno,‡ and Takashi Kato*,†
Department of Chemistry and Biotechnology, School of Engineering, The UniVersity of Tokyo,
Hongo, Bunkyo-ku, Tokyo 113-8656, Japan, and Department of Biotechnology, Tokyo UniVersity
of Agriculture and Technology, Nakacho, Koganei, Tokyo 184-8588, Japan
Received July 23, 2005; E-mail: kato@chiral.t.u-tokyo.ac.jp
Abstract: Nanostructures can be used for the fabrication of highly functional materials transporting ions
and charges. We demonstrate a new design strategy for polymeric higher ion-conductors. Phase-segregated
layers of alternating mobile tetra(ethylene oxide)s (TEOs) and rigid aromatic cores where the TEO moieties
are grafted from aromatic layers have been shown to be efficient to transport lithium triflate. Such segregated
structures at the nanometer scale (nano-segregated structures) were prepared by in-situ photopolymerization
of an aligned methacrylate liquid crystalline monomer comprising a terphenyl rigid rod mesogen having a
TEO terminal chain. The ion-conductive TEO moiety remains in the highly mobile state even after
polymerization, which is indicated by its low glass transition temperature (-45 °C). This nanostructured
film exhibits an ionic conductivity parallel to the layer of 10-3 S cm-1 at room temperature. The highest
ionic conductivity is in the level of 10-2 S cm-1 observed at 150 °C. The anisotropic ionic conductivities
have been observed for the nano-segregated film.
Introduction
Self-organized nanostructures of liquid crystals can be applied
to anisotropically functional materials.1-29 Supramolecular as-
sembly of molecules and nano-segregation can be used for the
† The University of Tokyo.
‡ Tokyo University of Agriculture and Technology.
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Figure 1. Design strategy for higher ion-conductive polymeric films. (a)
Liquid crystalline ion-conductive nanostructured polymers having oligo-
(ethylene oxide) chains (indicated in red) as spacer units of the side chains.
(b) Comb-shaped polymers having flexible ion-conductive oligo(ethylene
oxide) side chains (in red). (c) Combined design of the advantages of (a)
and (b): a new type of liquid crystalline nanostructured ion-conductor. The
nanolayers consist of liquidlike ion-conductive (in red) and rigid aromatic
mesogenic (in blue) domains.
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preparation of such anisotropic materials.1-10 These materials
have great potential as low-dimensional conductors of elec-
trons2,11-15 and ions2,16-29 due to their phase-segregated struc-
tures. Macroscopic alignment of phase-segregated ordered
nanostructures consisting of conductive and insulating parts is
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Nano-Segregated Polymeric Film
A R T I C L E S
Figure 2. Molecular structures of ion-conductive polymers.
to control macroscopically oriented structures of liquid crystals
to achieve desired functions. Recently, we have reported that
the self-assembly of block molecules consisting of ion-conduc-
tive oligo(ethylene oxide)s and ion-insulating mesogenic rod
parts leads to the formation of two-dimensional (2-D) ion-
conductors.25-29 They exhibit anisotropic ion conduction due
to the homeotropic alignment in the liquid crystalline (LC)
smectic phases on the substrates. Such nanostructures were fixed
by in-situ photopolymerization of a block liquid crystalline
monomer having a tetra(ethylene oxide) (TEO) moiety, which
results in the fabrication of self-standing 2-D nanostructured
films (Figure 1a).25 However, the ionic conductivities still
remained at values of less than 10-5 S cm-1 at room temper-
ature. We predicted that the ionic conductivity should increase
for polymeric self-standing films if a faster molecular motion
of the ion-conductive parts is achieved. For non-LC poly-
(ethylene oxide)-based (PEO-based) ion-conductors, several
approaches to improving the motion of ion-conductive moieties
have been reported.30-37 For example, polymers with side chains
of oligo(ethylene oxide)s were developed to improve ionic
conductivities (Figure 1b). In these polymers, the depression
of the glass transition temperatures (Tg) of the oligo(ethylene
oxide)s or the suppression of the crystallization of the molecules
was intended for solid-state materials.34-37 Although this design
led to the enhancement of the motion of ethylene oxide chains,
their ionic conductivities were also limited below 10-5 S cm-1
at room temperature.37
We here demonstrate the new design of an anisotropic
polymeric ion-conductive film exhibiting high ionic conductivi-
ties of a value of 10-3 S cm-1 at room temperature. This film
has formed a layered nano-segregated structure consisting of
alternating a highly mobile ion-conductive part and a rigid
mesogenic part.
Results and Discussion
The design strategy for anisotropic higher ion-conductive
polymeric films in the present study is shown in Figure 1. The
key for such organization presented in Figure 1c is to use the
molecular structure of P1 shown in Figure 2. P1 consists of
three molecular blocks: a polymer backbone, a mesogenic
aromatic core, and a TEO part (Figure 2). In particular, the TEO
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Polym. Sci., Part A: Polym. Chem. 2003, 41, 3486-3492.
(21) Mukai, T.; Yoshio, M.; Kato, T.; Yoshizawa, M.; Ohno, H. Chem. Commun.
2005, 1333-1335.
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M.; Tu¨rk, S.; Ra¨der, H. J. Macromolecules 2004, 37, 4222-4234.
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2000, 12, 513-516.
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(31) Meyer, W. H. AdV. Mater. 1998, 10, 439-448.
(32) Takeoka, S.; Ohno, H.; Tsuchida, E. Polym. AdV. Technol. 1993, 4, 53-
73.
(25) Kishimoto, K.; Yoshio, M.; Mukai, T.; Yoshizawa, M.; Ohno, H.; Kato,
T. J. Am. Chem. Soc. 2003, 125, 3196-3197.
(26) Ohtake, T.; Ogasawara, M.; Ito-Akita, K.; Nishina, N.; Ujiie, S.; Ohno,
H.; Kato, T. Chem. Mater. 2000, 12, 782-789.
(33) Hawker, C. J.; Chu, F.; Pomery, P. J.; Hill, D. J. T. Macromolecules 1996,
29, 3831-3838.
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T.; Ohno, H.; Kato, T. Macromolecules 2000, 33, 8109-8111.
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1999, 31, 1155-1158.
(34) Matsumi, N.; Mizumo, T.; Ohno, H. Chem. Lett. 2004, 372-373.
(35) Nishimoto, A.; Agehara, K.; Furuya, N.; Watanabe, T.; Watanabe, M.
Macromolecules 1999, 32, 1541-1548.
(36) Cowie, J. M. G.; Martin, A. C. S. Polym. Commun. 1985, 26, 298-300.
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A R T I C L E S
Kishimoto et al.
Figure 3. Preparation of macroscopically oriented and nano-segregated ion-conductive polymeric films. (a) Molecular structure of M1/LiOSO2CF3 (left),
homeotropically oriented M1/LiOSO2CF3 in the smectic A phase (60 °C) on the substrate (center), and oriented nano-segregated structure of P1/LiOSO2CF3
with a highly mobile TEO moiety (right); (b) molecular structure of M2/LiOSO2CF3 (left), oriented M2/LiOSO2CF3 in the smectic A phase (80 °C) (center),
and nano-segregated structure of P2/LiOSO2CF3 where the motion of the TEO moiety is restricted by the rigid insulating layer and polymer backbones
(right).
Scheme 1. Synthesis of Mesogenic Cores
chain is introduced on to the extremity of the rigid aromatic
core to achieve high segmental motion of the chains. The
nanolayered structure shown in Figure 1a for P2 and the side-
chain polymer of low Tg shown in Figure 1b have been
combined for the design of P1 to achieve the formation of ion-
conductive paths consisting of highly mobile liquidlike orga-
nized layers (Figure 1c).
Synthesis. Synthetic routes of liquid crystalline monomers
are shown in Schemes 1-3. Compounds M1 and M2 were
prepared by etherification of terphenyl mesogenic compounds38
(Scheme 1) and tetra(ethylene oxide) derivatives, followed by
esterification with methacryloyl chloride under light-resistant
condition (Schemes 2 and 3). The terphenyl mesogenic cores
The target nanostructure of the film shown in Figure 1c was
fixed by the in-situ photopolymerization of a homeotropic
monodomain formed by an LC monomer (M1) (Figure 3a).
(38) (a) Gray, G. W.; Hird, M.; Lacey, D.; Toyne, K. J. J. Chem. Soc., Perkin
Trans. 2 1989, 2041-2053. (b) Dong, C. C.; Hird, M.; Goodby, J. W.;
Styring, P.; Toyne, K. J. Ferroelectrics 1996, 180, 245-257. (c) Mehl, G.
H.; Kouwer, P. H. J. Angew. Chem., Int. Ed. 2003, 42, 6015-6018.
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Scheme 2. Synthesis of Liquid Crystalline Monomer M1
Scheme 3. Synthesis of Liquid Crystalline Monomer M2
Table 1. Phase Transition Temperatures of M1 and M2, and the
Lithium Salt Complexes of M1, M2, and P1-P3
center). This oriented liquid crystal in the smectic A phase at
60 °C was photopolymerized with UV irradiation (around 365
nm, 30 mW cm-2 for 20 min) in the presence of a photoinitiator,
2,2-dimethoxy-2-phenylacetophenone (0.5 wt %) (Figure 3a,
right). A transparent film (P1/LiOSO2CF3) was obtained by this
polymerization of the lithium salt complexes of M1. A cross-
shaped pattern was also observed for the conoscopic image of
the polymer complex, which indicates the preservation of
macroscopic orientation after the polymerization (Figure 3a,
right). The 1H NMR spectrum for the polymer complex revealed
that over 98% of the monomer were consumed through the
reaction. For comparison, the oriented film of P2 having the
same aromatic core as P1 was also obtained by the same method
of alignment and polymerization for the lithium complex of M2
(Figure 3b, and see Supporting Information). The film of P3/
LiOSO2CF3 was prepared by casting the complex of lithium
salt and the sample obtained by a solution polymerization in
tetrahydrofuran (THF) using 2,2′-azobis(isobutyronitrile) as a
radical initiator.
d
compound
Tg
Tsolid
Tsolid
Tsolid
TSmC
TSmA
-Iso
-solid
-SmC
-SmA
-SmA
M1a
M2a
M1/LiOSO2CF3
M2/LiOSO2CF3
P1/LiOSO2CF3
P2/LiOSO2CF3
P3/LiOSO2CF3
-79
-71
-61
-65
-45
46
26
72
17
67
72
80
54
75
78
105
80
112
202
210
50
8
49
a,c
a,c
b,c
161
b,c
b,c
94
101
-44
a Phase transition temperatures detected on the first cooling of DSC
thermogram. b Phase transition temperatures detected on the second heating
of DSC thermogram. c Lithium salt concentration: [Li+]/[CH2CH2O] )
0.05. d Glass transition temperatures of the TEO moieties; abbreviations
are as follows: SmC: smectic C phase, SmA: smectic A phase, Iso:
isotropic phase.
were obtained by a palladium-catalyzed cross coupling reaction
from 4-bromo-4′-alkoxybiphenyl derivatives and difluorophe-
nylboronic acid compounds (Scheme 1). The methacryl group
was introduced on the extremity of the alkyl chain for compound
M1, while compound M2 has the polymerizable group on the
extremity of the TEO moiety.
Formation of Macroscopically Oriented Polymer Com-
plexes. M1 complexed with lithium triflate (LiOSO2CF3: 0.05
mol % to the ethylene oxide unit) showed smectic A (80-54
°C) and C phases (54-17 °C) on cooling, while M1 alone
exhibited a smectic A phase in a narrow temperature range
(Table 1). The lithium salt complexes of M1 aligned spontane-
ously on the glass or ITO substrates (Figure 3a, center). This
monodomain of the oriented molecules was achieved by cooling
from the isotropic to the smectic A phase. The uniform
orientation was confirmed by the orthoscopic and conoscopic
images taken with a polarized optical microscope (Figure 3a,
The phase behavior of the lithium salt complexes of P1-3
is given in Table 1. The complex of P1 and LiOSO2CF3 (0.05
mol % to the ethylene oxide unit) shows the LC smectic A phase
up to 202 °C, which is 122 °C higher than that of the
corresponding monomer complex. However, at room temper-
ature, the polymer complex does not show any liquid crystalline
phases and exhibits a solid phase below 161 °C. For the complex
based on P2, the solid phase is observed below 94 °C.
The field emission scanning electron micrograph (FE-SEM)
images of the cross-section for the film of P1/LiOSO2CF3
formed on the glass substrates show the formation of layered
nanostructures (Figure 4). The average distance of the lines is
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A R T I C L E S
Kishimoto et al.
Chart 1
Figure 4. FE-SEM images of a macroscopically oriented P1/LiOSO2CF3
film. (a) Cross-section of the edge of the polymer complex film formed on
the glass substrates. (b) Magnified view of the image in (a). The line textures
aligned parallel to the substrates.
the ions can be transported faster in the well-oriented layer
structures of the smectic A and ordered solid phases than in
the isotropic liquids. It should be noted that the ionic conduc-
tivities of P1/LiOSO2CF3 are much higher than those of PEO-
based LC or non-LC polymers (10-5-10-4 S cm-1).16,17,22-37,39-42
To understand such high conductivities, the ionic conductivi-
ties of the lithium salt complexes of P2 and P3 were also
measured (Figure 5).
The ionic conductivities perpendicular to the layer (σ ) for
the P1/LiOSO2CF3 complex are higher than those of the ionic
conductivities parallel to the layer (σ|) for the P2/LiOSO2CF3
complex. The relatively higher values of σ for the P1 complex
may be due to faster movement of ions within the layers.
The ionic conductivities parallel to the layer (σ|) of the P2/
LiOSO2CF3 complex are lower than those of the complex of
the analogous LC polymer P4 having a cyclohexylbiphenyl
mesogen previously reported (Chart 1).25 For example, the σ|
value of P2/LiOSO2CF3 was 1.7 × 10-7 S cm-1 observed at
65 °C, which is one order lower than that of P4/LiOSO2CF3
observed at the same temperature (3.0 × 10-6 S cm-1). The
terphenyl mesogenic cores of P2 may be packed more tightly
with stronger intermolecular interactions than those of the
cyclohexylbiphenyl cores of P4.
The highest ionic conductivities of the complex of P1 at room
temperature can be ascribed to the formation of nanoscale
liquidlike layers in the solid film. The glass transition of the
TEO moiety of the P1/LiOSO2CF3 complex is observed at -45
°C (Table 1). This observation shows that the TEO moiety is
highly mobile even at around room temperature. The Tg of the
complex of P2 is seen at 46 °C, which is above room
temperature. This shows that the segmental motion of the TEO
chain of the P2 complex is limited at room temperature.
Figure 5. Anisotropic ionic conductivities of the polymer complexes of
P1/ LiOSO2CF3 and P2/LiOSO2CF3 and ionic conductivities of the complex
of P3.
estimated to be 10-50 nm. The layer spacing of the film
measured using small-angle X-ray scattering of the polymer is
6.3 nm (see Supporting Information), suggesting that one line
consists of several layers. These results show that the layered
LC nanostructures of the monomeric complex were successfully
locked in the solid-state polymer film by photopolymerization.
Anisotropic Measurements of Ionic Conductivity. The
anisotropic ionic conductivities of the polymeric complexes were
measured by the complex impedance method as previously
reported.18-21,25-29 Figure 5 presents ionic conductivities parallel
(σ|) and perpendicular (σ ) to the layer. The oriented complex
of P1 and LiOSO2CF3 shows high ionic conductivities in-plane
direction of the smectic layers at room temperature. The σ| value
at 35 °C is 1.3 × 10-3 S cm-1. At the same temperature, the
σ value is 1.8 × 10-6 S cm-1, and the anisotropy in the ionic
conductivities (σ|/σ ) is about 7.5 × 102. The highest σ| value
of P1/LiOSO2CF3 is 1.4 × 10-2 S cm-1 observed at 150 °C in
(39) Kosonen, H.; Valkama, S.; Hartikainen, J.; Eerika¨inen, H.; Torkkeli, M.;
Jokela, K.; Serimaa, R.; Sundholm, F.; ten Brinke, G.; Ikkala, O.
Macromolecules 2002, 35, 10149-10154.
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the oriented solid phase, while the σ value is 1.1 × 10-4
S
cm-1. The σ| values start to fall above this temperature, and
the anisotropy in the ionic conductivities disappears in the
isotropic phase. The σ| values do not show sudden changes at
the solid-smectic A phase transition. These results suggest that
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temperature controller (heating rate: 2.0 °C min-1). Detection limits
of resistance for the impedance analyzer were 102-108 Ω. The ionic
conductivity was calculated to be the product of 1/resistance (Ω-1) times
cell constant (cm-1). For anisotropic ionic conductivity measurements,
The complex of non-LC polymer (P3) exhibits the same level
of the glass transition temperature as the complex of P1 (Table
1). However, the conductivities for P3/LiOSO2CF3 are lower
than the σ| values measured for the P1 complex (Figure 5).
This difference suggests that, in the self-organized conducting
paths, ions can be transported more smoothly due to less
entanglement and more cooperative motion of the grafted TEO
chains from the mesogens (Figure 1c).
two types of cells with comb-shaped gold (cell constant: 100 cm-1
)
and ITO electrodes (cell constant: 0.1 cm-1) were used (see Supporting
Information). The ionic conductivities parallel (σ|) and perpendicular
(σ ) to the smectic layer direction were measured with gold and ITO
cells, respectively.
Conclusions
Acknowledgment. This study was partially supported by
Grant-in-Aid for Creative Scientific Research of “Invention of
Conjugated Electronic Structures and Novel Functions” (No.
16GS0209) (T.K.) and Grant-in-Aid for Scientific Research (B)
(No. 17350065) (T.K.) from the Japan Society for the Promotion
of Science (JSPS) and Grant-in-Aid for The 21st Century COE
Program for Frontiers in Fundamental Chemistry (T.K. and
K.K.) from the Ministry of Education, Culture, Sports, Science,
and Technology. K.K. is thankful for financial support from
JSPS Research Fellowships for Young Scientists.
We have proposed a new strategy for high ion-conductive
materials. The combination of the formation of nanolayers and
grafting the chain from the aggregated mesogenic layers has
been shown to be a promising approach for new highly ion-
conductive organic soft materials. This nanostructured film
exhibits not only high ionic conductivities but also two-
dimensional ion conduction. The key to the formation of such
oriented nanostructured film is to use the liquid crystalline
alignment of the smectic phase followed by photopolymeriza-
tion. The concept of nano-segregation of rigid and liquidlike
layers may bring a new design of solid-state polymeric materials
exhibiting transporting properties.
Supporting Information Available: Synthesis of liquid
crystalline monomers and non-LC polymer P3, sample prepara-
tion of polymerizable complexes, photopolymerization, FE-SEM
images of P2/LiOSO2CF3, X-ray charts, calculated molecular
conformation, ionic conductivity measurement cells. This
material is available free of charge via the Internet at
http://pubs.acs.org.
Experimental Section
Anisotropic Ionic Conductivity Measurements (complex imped-
ance method). Dynamic ionic conductivities were measured using a
Schlumberger Solartron 1260 impedance analyzer (frequency range:
10 Hz-10 MHz, applied voltage: 0.3 V) and a custom setup
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