Relation between the spontaneous polarization and alkyl chain length in ferroelectric switching of columnar liquid-crystalline ureas

Article abstract of DOI:10.1080/15421400903400795

Relation between the spontaneous polarization and alkyl chain length was investigated in ferroelectric switching of N,N'-bis(3,4,5-trialkoxyphenyl)urea (1-7) possessing n-C₄H₉, n-C₈H₁₇, n-C₉H₁₉, n-C₁₀H₂₁, n-C ₁₁H₂₃, n-C₁₂H₂₅, and n-C ₁₆H₃₃, respectively. The phase behaviors were investigated by differential scanning calorimetry, polarized optical microscopy, and the column diameters were obtained by X-ray diffraction. Compound 2-7 exhibited rectangular and hexagonal columnar phases. All of the hexagonal columnar phases showed ferroelectric switching under a triangular wave voltage. From the plot of the spontaneous polarization against the carbon atom number of the alkyl chains, correlation between them were clarified and it was strongly suggested that the switching current peaks originated in switching of the polar columns, not in the movement of ionic impurities.

Full text of DOI:10.1080/15421400903400795

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Relation between the Spontaneous
Polarization and Alkyl Chain Length in
Ferroelectric Switching of Columnar
Liquid-Crystalline Ureas
Keiki Kishikawa a , Shoichiro Nakahara a , Masanori Natsukawa a ,
Keiichi Suzuki a & Shigeo Kohmoto a
a Department of Applied Chemistry and Biotechnology, Faculty of
Engineering , Chiba University , Inage-ku, Chiba, Japan
Published online: 22 Feb 2010.
To cite this article: Keiki Kishikawa , Shoichiro Nakahara , Masanori Natsukawa , Keiichi Suzuki &
Shigeo Kohmoto (2010) Relation between the Spontaneous Polarization and Alkyl Chain Length in
Ferroelectric Switching of Columnar Liquid-Crystalline Ureas, Molecular Crystals and Liquid Crystals,
516:1, 107-113, DOI: 10.1080/15421400903400795
To link to this article: http://dx.doi.org/10.1080/15421400903400795
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Mol. Cryst. Liq. Cryst., Vol. 516: pp. 107–113, 2010
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400903400795
Relation between the Spontaneous Polarization
and Alkyl Chain Length in Ferroelectric Switching
of Columnar Liquid-Crystalline Ureas
KEIKI KISHIKAWA, SHOICHIRO NAKAHARA,
MASANORI NATSUKAWA, KEIICHI SUZUKI,
AND SHIGEO KOHMOTO
Department of Applied Chemistry and Biotechnology, Faculty
of Engineering, Chiba University, Inage-ku, Chiba, Japan
Relation between the spontaneous polarization and alkyl chain length was investigated
in ferroelectric switching of N,N0-bis(3,4,5-trialkoxyphenyl)urea (1-7) possessing
n-C4H9, n-C8H17, n-C9H19, n-C10H21, n-C11H23, n-C12H25, and n-C16H33, respec-
tively. The phase behaviors were investigated by differential scanning calorimetry,
polarized optical microscopy, and the column diameters were obtained by X-ray
diffraction. Compound 2-7 exhibited rectangular and hexagonal columnar phases.
All of the hexagonal columnar phases showed ferroelectric switching under a tri-
angular wave voltage. From the plot of the spontaneous polarization against the
carbon atom number of the alkyl chains, correlation between them were clarified
and it was strongly suggested that the switching current peaks originated in switching
of the polar columns, not in the movement of ionic impurities.
Keywords Ferroelectric columnar phase; liquid crystal; polarity; switching; urea
Introduction
Realization of ferroelectric columnar liquid crystal phases with achiral molecules is
an important scientific challenge and no one has achieved the columnar phases yet.
To realize the columnar phases, molecules have to pack one-directionally for
generation of polarity in the columns. So far, many shape-assisted approaches were
performed. Bowlic and conical molecular shapes were effective for one-dimensional
packing of molecules [1–11]. However, the polar columns of those molecules were
organized in anti-parallel to cancel out their macroscopic polarities, and any switch-
ing behaviors were not observed on applying an electric field. In 2005, our group
developed a hydrogen bond-assisted approach, and realized ferroelectrically switch-
able hexagonal columnar (Colh) liquid crystal phases which indicated switching
We are grateful to Mukai Science and Technology Foundation, the Asahi Glass Foun-
dation and Grant-in-Aid for Scientific Research (B) 19350090.
Address correspondence to Keiki Kishikawa, Department of Applied Chemistry and
Biotechnology, Faculty of Engineering, Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba
263-8522, Japan. E-mail: kishikawa@faculty.chiba-u.jp
107

108
K. Kishikawa et al.
Figure 1. Molecular packing structure in the column and its switching behavior under apply-
ing an electric field.
behaviors under applying a triangular wave voltage (Fig. 1) [12,13]. The switching of
the macroscopic polarities was confirmed by observation of their second harmonic
generation [14–16]. The response time in the switching was very short (1 ms). The
response time is much faster than that of ion movement in liquid crystal phases.
However, some scientists claimed that the switching might originate in movement
of ionic impurities. In order to clarify that the switching peaks came from the polar
switching of the columns, in this paper, we investigated the relation between the
spontaneous polarization and the alkyl chain length of the liquid crystalline ureas.
If the peaks originated in the movement of some ionic impurities, the amount of
the impurities of one sample was different from that of other sample and there would
be no relation between the switching current peak area and the column diameters.
On the other hand, if the switching current peaks originate in the polar switching
of the columns, there would be some relation between the peak area and the column
diameter. In this study, we prepared N,N0-bis(3,4,5-trialkoxyphenyl)ureas 1–7
(Fig. 2), respectively, and their column diameters and spontaneous polarizations
(Ps) were investigated.
Figure 2. Molecular structures of 1–7.

Spontaneous Polarization of Ferroelectrically Switchable Columnar Phases 109
Experimental
Synthesis of 1–7
The liquid-crystalline ureas were synthesized by the following prodcedure [12].
3,4,5-Trialkoxyaniline (4.91 mmol), N,N0-carbonyldiimidazole (4.00 mmol), and
N,N-dimetylformamide (50 ml) were added into a 200 ml-three-necked round
bottom flask. The solution was stirred under Ar atmosphere for 6 h at room
temperature. Water (50 ml) was added and the solution was extracted with
chloroform (50 ml ꢀ 3). The organic phase was dried over anhydrous MgSO4
Table 1. Phase behaviors of 1–7
Compound
Phase transiltion (ꢁC (kcal=mol))a
1 (R ¼ n-C4H9)
2 (R ¼ n-C8H17)b
3 (R ¼ n-C9H19)
4 (R ¼ n-C10H25)
5 (R ¼ n-C11H23)
6 (R ¼ n-C12H25)b
7 (R ¼ n-C16H33)b
aThe transition temperatures (ꢁC) and enthalpies (in parentheses, kcal= mol) were determ-
ined by DSC (5ꢁC=min) and are given above and below the arrows. Cr1, Cr2, and Cr indicate
crystal phases, and Colr, Colh, and I indicate rectangular columnar, hexagonal columnar, and
isotropic phases, respectively.
bPhase behaviors reported in Reference 12.
cThe peaks could not be separated. h1 þ h2 ¼ 15.9 kcal=mol.

110
K. Kishikawa et al.
and concentrated in vacuum. The products were separated by silica gel column
chromatography eluting with chloroform to give the corresponding urea as white
crystalline powder.
Phase Identification and Measurement of Column Diameters
The phase identification and phase behavior were performed by polarized optical
microscopy (POM) and differential scanning calorimetry (DSC). The intercolumnar
distances in their Colh phases were measured by X-ray diffractometer (XRD). Their
column diameters were calculated from the d(100) peaks, respectively.
Switching Experiment
The compounds were sandwiched by two glass plates which were coated
with indium tin oxide and polyimide films (made by E.H.C, cell gap: 5 mm).
The cell was set on a hot stage (INSTEC) provided with a polarized light
optical microscope (Nikon, Eclipse POL400). The switching experiment was
performed in the Colh phase with a triangular wave voltage (6 Hz, Vpp 200 V).
The current was monitored by measuring the voltage of the two terminal of a
1 kX resistance.
Results and Discussion
Investigation of the Phase Behavior and Measurement of Column Diameters
The phase behaviors of compounds 1–7 were shown in Table 1. Compound 1
exhibited no liquid crystal phase. Compounds 2–7 showed two kinds of columnar
phases, rectangular columnar (Colr) and Colh phases, which showed typical textures
of Colr and Colh phases in POM, respectively. Their superstructures were analyzed
by XRD, and the column diameters were shown in Table 2. The column diameter
increased with increase in the alkyl chain length. However, in the case of 6 (R ¼
n-C12H25), the diameter was smaller than that of 5 (R ¼ n-C11H23). Though the
reason was not clear, it was assumed that the averaged repeat distance in the direc-
tion of the column axis might be longer than those of 2–5, and 7.
Table 2. Column diameters of the Colh phases of 2–7
a
˚
Column diameter (A)
Compound
2 (R ¼ n-C8H17)
3 (R ¼ n-C9H19)
4 (R ¼ n-C10H21)
5 (R ¼ n-C11H23)
6 (R ¼ n-C12H25)
7 (R ¼ n-C16H33)
21.9
23.8
24.4
25.4
24.3
28.8
aThe column diameters were calculated from d(100) of the
hexagonal columnar phases in XRD.

Spontaneous Polarization of Ferroelectrically Switchable Columnar Phases 111
Figure 3. Plot of Ps value against the carbon atom number of alkyl chain (n). (The Ps values
are plotted with filled squares. The solid line is an approximation curve in second order).
Explanation for Correlation between the Ps and the Alkyl Chain Length
In Figure 3, the Ps value was plotted against the carbon atom number of the alkyl
chain. A decrease in the Ps value was observed with increase of the carbon atom
number of the alkyl chains. This was explained by the schematic representation
shown in Figure 4. In each switching experiment, the electrode area and the cell
gap were fixed (1.0 cm2 and 5 mm). With increase in the column diameter, the number
of the columns on the electrode area decrease. Accordingly, the Ps value decreased
with increase of the carbon atom number of the alkyl chains. This strongly suggested
that the switching current peaks under applying a triangular wave voltage originated
in switching of the polar columns, not in movement of ionic impurities.
Figure 4. Schematic representation of small and large columns on the electrodes. With
increase in carbon atom number of the alkyl chain number, the column diameter increases
and the number of the columns on the electrode decreases.

112
K. Kishikawa et al.
Figure 5. Structure of N,N0-diphenylurea and the net atomic charges of its carbonyl-oxygen
and carbonyl-carbon atoms.
Estimation of Ps Value of One Column
Then, the Ps value of one column was estimated as follows. The column length was
assumed to be 5 mm. The cross section (S) of the unit cell in the Colh phase of 16 is
p
S ¼ (28.8)2 ꢀ ( 3=2) ꢀ 10ꢂ16 (cm2). The number (N) of the columns on the electrode
is N ¼ 1=S on the 1 cm2 electrode. Accordingly, the Ps value of one column is
p
Ps0 ¼ (650 ꢀ 10ꢂ9)=N ¼ (650 ꢀ 10ꢂ9) ꢀ S ¼ (650 ꢀ 10ꢂ9) ꢀ (28.8)2 ꢀ ( 3=2) ꢀ 10ꢂ16
¼
4.67 ꢀ 10ꢂ20 (C). This value was 29.2% of the elementary electric charge
(¼1.60 ꢀ 10ꢂ19 C). Net atomic charges of N,N0-diphenylurea molecule (Fig. 5) were
calculated by AM1. The atomic charges of the carbonyl-oxygen and carbony-carbon
atoms were ꢂ0.367 and þ0.394. In the urea column, negative and positive charges
of molecules are canceled out by formation of the one-dimensional aggregates.
However, atomic charges of the two terminal molecules of the column are not can-
celed out. Accordingly, the negative and positive atomic charges ꢂ0.367 and þ0.394
remain at the two terminal molecules, respectively. The order of Ps0 (¼ (elementaly
electric charge) ꢀ 0.292) obtained by the switching experiment was reasonable as
the Ps value of the polar column.
Conclusions
We investigated relation between the spontaneous polarization and the alkyl chain
length in ferroelectrically switchable columnar phases, and it showed that they were
correlated. The correlation indicated that each polar column had a certain amount of
polarity, which did not depend on the column diameter. This also suggested strongly
that switching behavior originated in the switching of the polar columns not in
movement of ionic impurities. Further, the Ps value of one polar column (Ps0) esti-
mated from this experiment was a reasonable value, which supported our switching
mechanism.
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