Thermal and photochemical properties of polyethylene imines having azobenzene side chain groups

Article abstract of DOI:10.1080/15421400600932637

Polyethylene imines (PEIs) having 4-butylazobenezene group through methylene spacer groups were prepared, and their thermal as well as photochemical properties was investigated. PEI104 having longer methylene spacer was found to form an out-of-plane struc

Full text of DOI:10.1080/15421400600932637

Mol. Cryst. Liq. Cryst., Vol. 458, pp. 173–179, 2006
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400600932637
Thermal and Photochemical Properties of Polyethylene
Imines Having Azobenzene Side Chain Groups
Yoshinari Matsumura
Masaki Moritsugu
Graduate School of Science and Technology, Kumamoto University,
Kurokami, Kumamoto
Tomonari Ogata
Takamasa Nonaka
Seiji Kurihara
Department of Applied Chemistry & Biochemistry, Faculty of
Engineering, Kumamoto University, Kurokami, Kumamoto
Seiji Ujiie
Department of Applied Chemistry, Faculty of Engineering,
Oita University, Dannoharu, Oita
Polyethylene imines (PEIs) having 4-butylazobenezene group through methylene
spacer groups were prepared, and their thermal as well as photochemical proper-
ties was investigated. PEI104 having longer methylene spacer was found to form
an out-of-plane structure by heating into an isotropic phase and following cooling.
The photochemical control of the molecular orientation of PEI104 between in-plane
and the out-of-plane structures could be achieved by combination of thermal and
photochemical processes.
Keywords: azobenzene; homeotropic; molecular orientation; out-of-plane; polyethylene
imine
1. INTRODUCTION
There have been many studies on the photo-control of the mole-
cular orientation of polymer liquid crystals (PLCs) containing azo-
benzene compounds. The azobenzene compounds show a reversible
Address correspondence to Seiji Kurihara, Department of Applied Chemistry &
Biochemistry, Faculty of Engineering, Kumamoto University, Kurokami Kumamoto,
860-8555, Japan. E-mail: kurihara@gpo.kumamoto-u.ac.jp
173

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Y. Matsumura et al.
photoisomerization between trans-form and cis-form. There are two
typical liquid crystalline systems in relating to the photo-cotnrol of
the molecular orientation by means of photoisomerization of azoben-
zene compounds: one is ‘‘photochemical phase transition.’’ between a
liquid crystalline phase and a random state [1–3], the other is photo-
alignment between in-plane and out-of-plane structures, which is
called as a ‘‘command surface’’ [4] It is expected that larger change
in the refractive index can be induced in the command surface system
compared to the photochemical phase transition, because Dðn ꢀ n Þ is
?
k
larger than Dðn ꢀ nÞ: Dðn ꢀ n Þ and Dðn ꢀ nÞ express differences in
?
k
k
k
refractive indices between n , n , and n, where n , n are extra-
?
?
k
k
ordinary and ordinary refractive indices of a liquid crystal in a liquid
crystalline phase, and n is a refractive index in an isotropic phase,
respectively. The command surface system consists of the command
surface and low molecular weight liquid crystals. Therefore, the sys-
tem requires a cell, and its memory stability is not so high.
On the other hand, it was reported that some polyethylene imines
(PEIs) having azobenzene groups as side groups show liquid crystal-
line phases, and align perpendicular direction to the substrate sponta-
neously by heating into an isotropic phase and following cooling
without any alignment process [5,6]. If we control photochemically
the molecular orientation of the PEIs between the in-plane and the
out-of-plane structures, we can fabricate simply the optical switching
system showing excellent memory stability as well as larger change in
the refractive index by coating PEIs on a substrate without any align-
ment process. In this study, we investigated the thermal and photo-
chemical properties of PEIs having azobenzene side groups with
respect to the change in the molecular orientation between the
in-plane and the out-of-plane structures.
2. EXPERIMENTAL
2.1 Materials
The azobenzene derivatives, (4-(10-bromo-n-decyloxy)-40-butylazobene-
zene, and 4-(6-bromo-n-hexyloxy)-40-butylazobenezene), were synthe-
sized by diazo-coupling reaction of 4-n-buthylaniline (4.8 g, 33 mmol)
with phenol (4.5 g, 48 mmol) in water, and following etherification with
1,6-dibromohexane or 1,10-dibromodecane in acetone at 70ꢁC for 24 h.
The resulting azo-compounds were recrystallized from methanol. Two
PEIs, PEI104 (m ¼ 10) and PEI64 (m ¼ 6), were synthesized by react-
ing polyethylene imine (DP ¼ ca.20) with 4-(10-bromo-n-decyloxy)-40-
butylazo-benezene, and 4-(6-bromo-n-hexyloxy)-40-butylazo-benzene,

Thermal and Photochemical Properties of Polyethylene Imines
175
SCHEME 1 Scheme synthetic route for PEI-104.
respectively, in dimethyl formamide (DMF) at 100ꢁC (Scheme). After
evaporation of half amount of DMF, PEIs were obtained by repricipita-
tion of the solution inꢁto methanol. Both PEIs showed LC phases:
ꢁ
ꢁ
ꢁ
ꢁ
.
.
.
.
.
.
.
.
.
.
K 30 C SmA1 39 C N 94 C I for PEI64, and SmF 68 C SmA1 114 C I
for PEI104.
2.2 Control of Molecular Alignment
Uniaxially oriented PEI films were prepared by spin coating PEI tolu-
ene solutions (3 wt%, 600 rpm) on a glass substrate coated with poly-
imide and rubbed. In order to explore the molecular orientation of
the films prepared, the order parameter was evaluated by measuring
polarized absorption spectra (Shimadzu UV-1600). In addition, the
transmitted light intensity (365nm) through the PEI films was mea-
sured as a function of angle between light polarization and rubbing
direction (Fig. 1). Photo-irradiation was carried out using a 500 W
high pressure Hg lump (Ushio SX-UI 500H) equipped with a filter
(Sigma koki Co. UTVAF-35). The photoisomerization behavior was
investigated by measuring absorption spectra with Ocean (USB2000
Ocean optics) under irradiation of UV light. Xenon lump was used
as a monitor light source. To improve the molecular orientation, PEI
films was heated into the isotropic phase and cooled rapidly to the
room temperature.
3. RESULTS AND DISCUSSION
Figure 2(A) shows changes in absorbance at absorption maximum
(345nm) of PEI104 film by varying temperature. The absorbance of
the film was about 0.35 at 25ꢁC immediately after spin coating. The

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Y. Matsumura et al.
FIGURE 1
absorbance was increased by heating, and decreased in a temperature
range from 50 to 70ꢁC. The absorbance was increased again by heating
higher than 70ꢁC up to 120ꢁC. On the other hand, the absorbance
decreased linearly with decreasing temperature, and the absorbance
became 0.2 at 25ꢁC. After heating to an isotropic phase once, the
FIGURE 2 Changes in absorption spectra of EI LC films by heating and
cooling.

Thermal and Photochemical Properties of Polyethylene Imines
177
FIGURE 3 Reorientation of PEI liquid crystals after isotropic state.
PEI film was found not to recover the initial absorbance (0.4) by
further heating and cooling cycle. This result means that the meso-
genic azobenzene groups transform from in-plane structure to out-
of-plane structure by the heating and cooling cycle (Fig. 3). Contrary
to PEI104, PEI64 didn’t show such hysteresis of the change in the
absorbance by the heating-cooling cycle as shown in Figure 2(B).
These results can be explained in terms of the hydrophobicity of the
side chains of the PEIs. In the membrane consisting of amphiphilic
compounds on the cleaned glass substrate, the polar groups are in con-
tact with the surface of the glass substrate, contrary the hydrophobic
groups are spatially directed to air. Namely, the azo-side groups in
PEI104 is more hydrophobic, and consequently tend to align perpen-
dicular to the glass substrate.
On the other hand, it is known that the trans-cis photoisomeriza-
tion disorganizes the molecular orientation of the liquid crystals.
Therefore, it is expected that the transformation from the in-plane
structure to out-of-plane will be induced by photoisomerization of
azo-dyes attached in the PEIs, because the photochemical pertur-
bation on the molecular ordering can be considered to be compatible
with thermal effect. Actually the decrease in the absorbance was
observed for the PEI104 film, while no change was observed for the
PEI64 film.
In order to improve the photo-switching properties, uniaxially
oriented films were prepared by spin coating PEI solutions on a glass
substrate which is coated with polyimide and rubbed. After the spin
coating, the films were heated into the isotropic phase and cooled rap-
idly to the room temperature. Figure 4 shows angular dependence of
absorbance of linerly polarized light (365 nm) between rubbing direc-
tion and light polarization, where 0ꢁ is corresponding to perpendicular
to the rubbing direction, and 90ꢁ is parallel to the rubbing direction,
respectively. As can be seen in Figure 4, the absorbances at 0 and
270ꢁ are larger than others, indicating the uniaxially molecular orien-
tation of azo groups in the films before UV irradiation. The UV
irradiation on both films caused change in the angular dependence

178
Y. Matsumura et al.
FIGURE 4 Angler dependence of orientation of PEI LCs on the incidence
angle of xenon light.

Thermal and Photochemical Properties of Polyethylene Imines
179
of the absorbance. No angular dependence of the absorbance was
observed for both films immediately after the UV irradiation. This is
due to the photochemical phase transition from LC phase to the iso-
tropic phase. By keeping the PEI64 film in the dark, no further change
in the angular dependence of the absorbance was observed. On the
other hand, the absorbance of the PEI104 film was decreased
extremely with time, indicating the formation of the out-of-plane
structure.
CONCLUSIONS
Transformation of PEI104 film from the in-plane structure to the
out-of-plane structure was achieved by thermally and trans-cis photo-
isomerization of the azo-side groups. On the other hand, PEI6 did not
show such transformation thermally as well as photochemically. The
transformaton was attributed to more hydrophobicity of the azo-side
groups having long spacer. However, the time more than 24 h was
required for the photochemical transformation, because the slow ther-
mal back reaction limited the photo-response properties.
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