3D interconnected ionic nano-channels formed in polymer films: Self-organization and polymerization of thermotropic bicontinuous cubic liquid crystals

Article abstract of DOI:10.1021/ja106707z

Thermotropic bicontinuous cubic (Cub_bi) liquid-crystalline (LC) compounds based on a polymerizable ammonium moiety complexed with a lithium salt have been designed to obtain lithium ion-conductive all solid polymeric films having 3D interconnec

Full text of DOI:10.1021/ja106707z

ARTICLE
pubs.acs.org/JACS
3D Interconnected Ionic Nano-Channels Formed in Polymer Films:
Self-Organization and Polymerization of Thermotropic Bicontinuous
Cubic Liquid Crystals
Takahiro Ichikawa,† Masafumi Yoshio,† Atsushi Hamasaki,‡ Junko Kagimoto,‡ Hiroyuki Ohno,‡ and
Takashi Kato*,†
†Department of Chemistry and Biotechnology, School of Engineering, The University of Tokyo, Hongo, Bunkyo-ku,
Tokyo 113-8656, Japan
‡Department of Biotechnology, Tokyo University of Agriculture and Technology, Nakacho, Koganei, Tokyo 184-8588, Japan
S Supporting Information
b
ABSTRACT: Thermotropic bicontinuous cubic (Cubbi) liquid-crystalline
(LC) compounds based on a polymerizable ammonium moiety complexed
with a lithium salt have been designed to obtain lithium ion-conductive all
solid polymeric films having 3D interconnected ionic channels. The mono-
mer shows a Cubbi phase from -5 to 19 °C on heating. The complexes
retain the ability to form the Cubbi LC phase. They also form hexagonal
columnar (Colh) LC phases at temperatures higher than those of the Cubbi
phases. The complex of the monomer and LiBF4 at the molar ratio of 4: 1 exhibits the Cubbi and Colh phases between -6 to 19 °C
and 19 to 56 °C, respectively, on heating. The Cubbi LC structure formed by the complex has been successfully preserved by in situ
photopolymerization through UV irradiation in the presence of a photoinitiator. The resultant nanostructured film is optically
transparent and free-standing. The X-ray analysis of the film confirms the preservation of the self-assembled nanostructure. The
polymer film with the Cubbi LC nanostructure exhibits higher ionic conductivities than the polymer films obtained by
photopolymerization of the complex in the Colh and isotropic phases. It is found that the 3D interconnected ionic channels
derived from the Cubbi phase function as efficient ion-conductive pathways.
’ INTRODUCTION
interconnected ionic channels.11,12 Very recently, lyotropic
Cubbi liquid crystals were used for the preparation of ion-
conductive polymeric materials with 3D ion channels through
the polymerization of materials composed of a polymerizable
lithium sulfonate with a propylenecarbonate as a solvent.13 The
preservation of thermotropic functional nanostructures in solid
polymer films will make a great progress for the development of
all-solid-type transport nanomaterials. The in situ photopolym-
erization of LC molecules incorporating polymerizable moieties
is a useful approach.7,17 But in the case of thermotropic liquid
crystals with nanosegregated stuctures, the introduction of con-
ventional polymerizable moieties such as acrylates into LC
molecules thermally destabilizes the original LC phases due to
their steric hindrance and the change in polarity of molecules.7a
The balance of size and shape of nanosegregated parts is delicate
for the induction of desired thermotropic LC phases.
Nanostructured liquid crystals1 have attracted increasing
attention as functional materials because they can be used in a
variety of fields.2 Layered, cylindrical, and globular nanostruc-
tures have been applied for the development of low-dimensional
transport materials for charge,3-5 ions,3,4,6-8 and molecules.9
Nanostructured LC assemblies exhibiting Cubbi phases10 formed
through self-organization are emerging as a new generation of
nanostructured soft materials because of their three-dimensional
nanonetwork structures.11-15 In particular, these nanochannels
formed by LC ionic molecules have been applied for the trans-
port pathways of substances, such as ions11-13 and gases.14 The
construction of 3D interconnected ionic networks based on
liquid crystals leads to new development of ion-conductive
membrane materials because ordered 3D structures with well-
defined size and shape of channels should be useful for efficient
transportation of ions.11-13 Although intensive research has fo-
cused on the preparation of thermotropic ionic liquid crystals,15
very limited numbers of examples exhibiting thermotropic Cubbi
phases have been reported.11,12,16 Moreover, no research on the
ion-conductive properties for these thermotropic Cubbi LC ma-
terials has been achieved, with one exception.11 It was prelimi-
narily reported that thermotropic Cubbi LC ammonium salts
functioned as alignment-free ion conductors by forming 3D
It is not easy to design a polymerizable molecule forming
thermotropic Cubbi phases. Until now, no study has been
reported on the preparation of ion-conductive polymer films
preserving thermotropic LC Cubbi nanostructures, although Gin
and co-workers have achieved the 3D nanostructures using
Received: July 28, 2010
Published: January 27, 2011
r
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Figure 2. (a) SAXS pattern of compound 1 in the Cubbi phase at 15 °C;
(b) polarizing optical microscopic image of compound 1 in the Cubbi
phase at 15 °C.
Figure 1. Molecular structures of taper-shaped ammonium salts 1-3.
lyotropic LC Cubbi systems.13 There are no reports on the
preservation of preformed Cubbi structures formed by thermo-
tropic ionic LC molecules except for nonionic LC molecules
having polymerizable groups.18 In contrast, in the case of lyo-
tropic liquid crystals, there are several reports on the polymer-
ization of Cubbi structures.13,19 The achievement of ion-con-
ductive polymer films formed by thermotropic LC systems
including no organic solvent is important in the development
of all solid polymer electrolytes for safety and easier fabrication.
Herein, we report on the development of an ion-conductive
polymer film having 3D ion-channel networks derived from a
thermotropic Cubbi phase. The flexible and mechanically robust
films have been prepared by photopolymerization of an LC
ammonium salt complexed with LiBF4. We have found that
efficient ion transportation occurs in the solid film preserving the
Cubbi structures.
phase has been confirmed by small-angle X-ray scattering (SAXS)
and polarizing optical microscope. The SAXS pattern of compound
1 at 15 °C shows two peaks at 33.2 and 28.7 Å (Figure 2a). The
√ √
reciprocal d-spacing ratio of these peaks is 6: 8, which can be
indexed to the (211), (220) reflections of a gyroid Cubbi phase
with the Ia3d symmetry with a lattice constant a = 81.2 Å. No
birefringence has been observed for compound 1 under crossed
Nicols condition on cooling (Figure2b) though increased viscosity
has been observed.
For compound 2, no mesomorphic behavior has been ob-
served. The comparison of the thermal properties of these two
compounds suggests that 1,3-diene group is much more suitable
than the acrylate group for the polymerizable group of thermo-
tropic LC materials.
Complexation of the LC Monomer with a Lithium Salt.
Compound 1 was complexed with LiBF4 to obtain lithium ion-
conductive materials. The mixtures were obtained by slow
evaporation of the tetrahydrofuran solution of compound 1
and the requisite amount of LiBF4. The phase transition behavior
of the complexes of compound 1 as a function of lithium salt
concentration is shown in Figure 3. The addition of LiBF4
induces the formation of hexagonal columnar (Colh) phases in
the higher temperature region than those of the Cubbi phase. The
LC state is thermally stabilized in the presence of the lithium salt.
For example, upon cooling the isotropic liquid of the mixture of 1
and LiBF4 at the molar ratio of 4:1, a fan-texture appears at 56 °C
under polarizing optical microscopic observation, which indi-
cates the formation of a Colh phase (Figure 4a). On further
cooling of the Colh phase, the birefringence disappears entirely at
around 20 °C (Figure 4b). The differential scanning calorimetry
(DSC) curves for this complex (Figure 4c) shows the endother-
mic peaks at 19 and 56 °C upon heating, which correspond to the
Cubbi-Colh and Colh-Iso phase transitions, respectively. The
values of transition enthalpies are 0.67 and 0.29 J g-1. These
enthalpy values increase with the increase of mole fraction of
the lithium salt in the complexes (See Supporting Information).
The complex of 1 and LiBF4 at the molar ratio of 2:1 gives the
enthalpy value of 0.73 J g-1 at the Cubbi-Colh transition and
0.49 J g-1 at the Colh-Iso transition. The value of enthalpy at the
Cubbi-Iso transition also increases by the complexation of 1
with LiBF4. The complex of 1 and LiBF4 at the molar ratio of 10:1
’ RESULTS AND DISCUSSION
Molecular Design. We have designed and synthesized poly-
merizable ionic compounds 1 and 2 shown in Figure 1. We
previously reported that ammonium-based compound 3 having
three alkyl chains showed the Cubbi phase between 32 and 49 °C
on heating.11 In the present work, for the design of polymerizable
LC monomers, two kinds of photopolymerizable groups, acrylate
and 1,3-diene, have been incorporated into the alkyl chains of
compound 3. Acrylate groups were widely used for the design
of LC monomers.1a,17a,17b However, the acrylate groups at
the extremity of the alkyl chains tend to reduce the temperature
range of LC phases of the compounds due to their bulkiness and
polarity.7a Alternatively, 1,3-diene tails were employed for ther-
motropic LC systems by Gin and co-workers.20 They pointed out
that 1,3-diene substituted monomers were more likely to lead to
the exhibition of desired LC properties than acrylate substituted
monomers because the physical properties of 1,3-diene tails
resemble normal alkyl tails.20a In the present work, these poly-
merizable groups have been introduced into only two of three
alkyl chains of 1 and 2 because we found that a fan-shaped
imidazolium salt with only two acrylate groups showed a columnar
LC phase.7a
Liquid-Crystalline Properties. Compound 1 exhibits a Cubbi
phase from -5 to 19 °C on heating. The formation of the Cubbi
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√
estimated to be 5.0 from Z = 3NAa2hF/2M, where NA is
Avogadro’s number (6.02 ꢀ 1023 mol-1), h is the layer thickness
(4.5 Å), and M is molecular weight. The density (F) of the
material is 1.22 g cm-3, which has been determined by a flotation
method in D-(þ)-sucrose/H2O at 20 °C. The value of h has been
taken from the halo of X-ray due to disordered aliphatic chains
and benzene ring in the direction of the column axis.
These results suggest that the lithium salt is incorporated in
the ionic parts of the nanosegregated structures and enhances
the ionic interactions.6b These interactions stabilize the self-
assembled nanostructures and widen the temperature range of
the LC phases.6b The induction of the columnar phase can be
attributed to the change of the volume balance between hydro-
phobic and ionic parts of the molecule.
Fixation of the LC Structures by Photopolymerization.
Ion-conductive polymeric solid materials have been prepared as
shown in Figure 5. The complexation of monomer 1 with LiBF4
(1/LiBF4, molar ratio = 4:1) and subsequent photopolymeriza-
tions of the 1/LiBF4 in the Cubbi, Colh, and isotropic phases has
given three types of polymeric films of Film-B, Film-C, and Film-
I, as shown in the bottom of Figure 5. In particular, the Cubbi LC
structure for the complex has been successfully preserved into a
solid polymer film.
Figure 3. Phase transition behavior for a mixture of LC compound 1
and LiBF4 on the heating process. Iso: isotropic; Colh: hexagonal
columnar; Cubbi: bicontinuous cubic; Cr: crystalline.
The complex 1/LiBF4 exhibits the Cubbi phase between -6
and 19 °C on heating. The UV irradiation with a super high-
pressure mercury lamp (around 365 nm, 30 mW cm-2 for 1 h)
was performed for the sample in the presence of a photoinitiator,
2,2-dimethoxy-2-phenylacetophenone (1.0 wt %) at 10 °C. A
transparent free-standing film that shows no birefringence under
the crossed Nicols condition, has been obtained (Film-B)
(Figure 5). In the infrared (IR) spectrum of the photopolymer-
ized sample (Film-B), the intensity of the IR band at 1650 cm-1
attributable to the stretching of 1,3-diene group decreases, which
indicates that the cross-linking reaction occurs. The resultant film
is thermally stable, and no peaks of phase transitions are detected
up to 120 °C in DSC measurements. Furthermore, the film is
flexible, mechanically robust, and insoluble in water and organic
solvents. These results suggest that the covalently bonded net-
work structures are formed by the reaction of the 1,3-diene
groups.
The preservation of the nanostructure has been confirmed by
the comparison of SAXS patterns obtained before and after
photoirradiation (Figure 6). The SAXS pattern of complex 1/
LiBF4 at 10 °C shows two peaks at 36.1 and 31.2 Å, which can be
assigned to the (211) and (220) reflections, while those peaks of
Film-B are observed at 34.7 and 30.2 Å. These results show that
the Film-B retains the Cubbi structure although a contraction of
unit cell from 88 to 85 Å (Figure 7) occurs due to the formation
of networks upon polymerization. The SAXS pattern of Film-B at
100 °C, which is about 40 °C higher than the clearing point of
the nonpolymerized complex 1/LiBF4, shows a Cubbi ordering of
the sample. These results show that the nanostructure fixed by
the photopolymerization is retained even at a higher temperature
region (Figure 7).
Figure 4. Polarizing optical microscopic images of the mixture of 1 with
LiBF4 (the molar ratio of 4:1): (a) for the Colh phase; (b) for the Cubbi
phase and DSC thermograms (c) of the mixture at the scanning
rate of 5 °C min-1. The insets show magnified views of the DSC
thermograms.
shows the enthalpy value of 0.56 J g-1 at the Cubbi-Iso
transition at 22 °C, while compound 1 alone exhibits the
transition enthalpy of 0.07 J g-1 at 19 °C. These results suggest
that the bicontinuous cubic and columnar structures are ther-
mally stabilized by the complexation of compound 1 with LiBF4.
The molecular assembled structures of the complexes of 1 with
LiBF4 have been further examined by X-ray diffraction measure-
ments. The wide-angle X-ray diffraction pattern at 40 °C for the
complex of 1 with LiBF4 at the molar ratio of 4:1 gives two peaks
with d-spacing of 32.7 and 16.4 Å, which correspond to the (100)
and (200) reflections, respectively. A definite assignment of the
molecular packing cannot be made by the X-ray diffraction in the
absence of unequivocal higher order reflections. Despite the lack
of (110) reflection, a hexagonal columnar packing is suggested
from the polarizing optical microscopic image shown in
Figure 4a. The calculated lattice parameter a is 37.8 Å. The
number (Z) of molecules per unit cell in the hexagonal lattice is
Our present approach to the use of a thermotropic ionic liquid
crystal with polymerizable groups is the first example for the
development of ion-active polymer films having the Cubbi
structure with 3D ion-channels, although Gin and co-workers
have recently reported a lithium ion-conductive polymer material
having a Cubbi structure based on a lyotropic LC system.13
For Film-C and Film-I, the monomer complexes of 1 and
LiBF4 were polymerized in the columnar LC state at 40 °C and in
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Figure 5. Schematic illustration of the preparation of ion-conductive polymeric films. The complex of 1 with LiBF4 forms the Cubbi and Colh phases.
Self-assembly and subsequent photopolymerization of the materials in each state yields various polymeric materials having three different structures: (a)
Film-B forming the Cubbi nanostructure; (b) Film-C forming the randomly oriented columnar nanostructure; and (c) Film-I forming the disordered
amorphous (isotropic) state.
Figure 7. The comparison of self-assembled nanostructures before and
after UV-irradiation.
Information). The lattice parameter is slightly decreased after
polymerization. A fan-texture indicating the random orientation
of the columns is observed for the Film-C at room temperature
under a polarizing microscope. These results suggest that the
columnar structure is also fixed into a solid polymer film. In
addition, we have tried to align the columnar structures for the
lithium salt complexes by using either mechanical shearing or
amine-functionalized glass substrate.7a However, for these am-
monium salts, no uniform alignments of the columns parallel and
vertical to the surface of substrates were obtained, although we
previously achieved macroscopic orientation of the columnar LC
phase formed for an acrylate monomer having an imidazolium
moiety.7a The structure-property relationships of these materi-
als will be discussed in the next section.
Figure 6. Small-angle X-ray scattering patterns of (a) the monomer
complex 1/LiBF4 at 10 °C and (b) Film-B obtained by photopolymer-
ization of the complex 1/LiBF4 in the Cubbi phase. The SAXS
measurement for the Film-B was performed at room temperature.
Ionic Conductivities of the Films with Nano-Ion Channels.
The ionic conductivities of monomer complex 1/LiBF4 have been
measured using comb-shaped gold electrodes (Figure 8, O). The
ionic conductivities for complex 1/LiBF4 increase on heating until
the isotropic phase at 70 °C, respectively. The wide-angle X-ray
diffraction pattern of the Film-C shows two peaks at 31.3 and
16.1 Å with the reciprocal d-spacing ratio of 1:2 (See Supporting
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enable the efficient ion transportation at the interface of LC
domains, which results in the efficient ion transportation even in
the polydomain states.
’ CONCLUSIONS
We have demonstrated that the 3D interconnected ionic
channels formed in the nanosegregated solid film are effective
for ion conduction. This nanostructured film has been prepared
by photopolymerization of the Li salt-doped polymerizable LC
ammonium salt 1 in the thermotropic Cubbi phase. The selection
of the 1,3-diene group as a polymerizable moiety is a key for the
design of thermotropic Cubbi liquid crystal in the present work.
The ionic conductivities of the polymer film with the Cubbi LC
structure are higher than those of the polymer films of the Colh
and disordered amorphous states. Moreover, the solid polymer
film with the 3D ionic channels shows conductivities comparable
to those observed for the isotropic liquid state of monomer 1.
The present work shows great potential for preservation of the
thermotropic Cubbi phase into the solid state films for transport-
ing membrane materials.
Figure 8. Ionic conductivities of the complex 1/LiBF4 as a function of
temperature: (O) before photopolymerization and (b) after photopo-
lymerization in the Cubbi phase.
’ EXPERIMENTAL SECTION
General Procedures. 1H NMR and 13C NMR spectra were
obtained on a JEOL JNM-LA400 at 400 and 100 MHz in CDCl3,
respectively. Chemical shifts of 1H and 13C NMR signals were quoted to
(CH3)4Si (δ = 0.00) and CDCl3 (δ = 77.0) as internal standards,
respectively. Matrix-associated laser desorption ionization-time-of-flight
mass spectra (MALDI-TOF MS) were taken on a PerSeptive Biosys-
tems Voyager-DE STR spectrometer using dithranol as the matrix.
Elemental analyses were carried out on a Yanaco MT-6 CHN autocorder
and an Exeter Analytical CE440 instrument. IR spectra were obtained
using a JASCO FT/IR-660 Plus spectrometer. The thermal properties of
the materials were examined by a DSC using a Netzsch DSC204 Phoenix.
The heating and cooling rates were 5 °C min-1. Transition temperatures
were taken at the onset of the transition peaks. A polarizing optical
microscope Olympus BX51 equipped with a Mettler FP82HT hot stage
was used for visual observation. Wide-angle X-ray diffraction (WAXD)
patterns were obtained using a Rigaku RINT-2500 diffractometer with
Cu KR radiation. Two-dimensional small-angle X-ray scattering (2D
SAXS) patterns of the materials were also recorded using an image plate
detector (R-AXIS DS3C). UV irradiation was carried out using a high-
pressure mercury lamp (Ushio, 500 W) with appropriate glass filters
(Asahi Technoglass UV-35 and UVD-36C).
Figure 9. Ionic conductivities of polymeric films as a function of
temperature: (b) Film-B forming the Cubbi nanostructure; (2) Film-
I forming the nonordered isotropic state; and (9) Film-C forming the
randomly oriented columnar structure.
the Cubbi-Colh transition occurs at 19 °C. Then the conductivities
suddenly decrease at the region of the phase transition from the
Cubbi to Colh phase. These results show that the interconnected
ion channels formed in the Cubbi structures of complex 1/LiBF4
are efficient for ion conduction. The polymerized material of 1/
LiBF4 shows a simple increasing trend for the ionic conductivities
on heating until over 120 °C (Figure 8, b). It should be noted that
between 60 and 120 °C, the ionic conductivities of the solid
polymerized sample (Film-B) are slightly higher than those of the
mobile isotropic liquid state of the complex based on 1. Generally,
the cross-linking of molecules suppresses the diffusion of molecules
themselves,7a,21 which leads to the decrease of ionic conductivity.
The present results reveal that the continuous ion-channel struc-
tures preserved in the solid state are effective for ion transportation.
The ionic conductivities of the Film-C and Film-I have been
measured. The comparison of these results with those for Film-B
has revealed that the use of the Cubbi structure is advantageous
for the ion transportation. The ionic conductivities for these films
are shown in Figure 9. Film-B (b) exhibits higher conductivities
than other films. For example, the conductivity of 3.1 ꢀ 10-4 S
cm-1 at 90 °C is observed for Film-B. The value is 4 times higher
than that of Film-I (2). In contrast, the value of the conductivity
Materials. All chemical reagents and solvents were obtained from
commercial sources and used without purification. All reactions were
carried out under an argon atmosphere in anhydrous solvents.
Measurement of Ionic Conductivities. Temperature depen-
dence of the ionic conductivities was measured in the heating process by
the alternating current impedance method using a Schlumberger
Solartron 1260 impedance analyzer (frequency range: 10 Hz-10 MHz,
applied voltage: 0.3 V) equipped with a temperature controller. The
cooling rate was fixed to 2 K min-1. Ionic conductivities were practically
calculated to be the product of 1/Rb (Ω-1) times cell constants (cm-1
)
of the comb-shaped gold electrodes, which were calibrated with KCl
aqueous solution (1.00 mmol L-1) as a standard conductive solution.
The impedance data (Z) were modeled as a connection of two RC
circuits in series.
’ ASSOCIATED CONTENT
of Film-C (9) at the same temperature is 8.8ꢀ10-6 S cm-1
,
S
which is 36 times lower than that of Film-B. The 3D inter-
Supporting Information. Synthesis of compounds 1 and
b
connected ionic channels derived from the Cubbi phases may
2, and DSC thermograms of compounds 1 and 2, and Film-B,
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WAXD patterns of 1 in the Cubbi LC phase, complex of 1/LiBF4
in the Cubbi and Colh phases, and Film-C, IR spectra of complex
1/LiBF4 before and after photopolymerization, and thermal
properties of 1 and its complexes with LiBF4. This material is
available free of charge via the Internet at http://pubs.acs.org.
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’ AUTHOR INFORMATION
Corresponding Author
kato@chiral.t.u-tokyo.ac.jp
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’ ACKNOWLEDGMENT
This study was partially supported by the Funding Program
for World-Leading Innovative R&D on Science and Technology
(FIRST) from the Cabinet Office, Government of Japan, the
Global COE Program for Chemistry Innovation from the
Ministry of Education, Culture, Sports, Science, and Technology,
and Grant-in-Aid for Scientific Research (A) (No. 19205017)
from the Japan Society for the Promotion of Science (JSPS). T.I.
is grateful for financial support from the JSPS Research Fellow-
ship for Young Scientists.
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