Synthesis, characterization and thermal and photo alignment behavior of polyethylene imines having butyl substituent azobenzene side chain group

Article abstract of DOI:10.1080/15421406.2011.600610

New type of polyethylene imine having butyl (-C₄H₉) substituent azobenzene side chain group through methylene spacer group (PEI64) was successfully synthesized by solution polycondensation reaction. Synthesized polymer was characteri

Full text of DOI:10.1080/15421406.2011.600610

Mol. Cryst. Liq. Cryst., Vol. 550: pp. 134–148, 2011
Copyright © Taylor & Francis Group, LLC
ISSN: 1542-1406 print/1563-5287 online
DOI: 10.1080/15421406.2011.600610
Synthesis, Characterization and Thermal and Photo
Alignment Behavior of Polyethylene Imines Having
Butyl Substituent Azobenzene Side Chain Group
MOHAMMAD KAMRUZZAMAN,1 YUTAKA KUWAHARA,1
TOMONARI OGATA,1 SEIJI UJIIE,2 AND SEIJI KURIHARA1,∗
1Department of Applied Chemistry and Biochemistry, Graduate School
of Science and Technology, Kumamoto University, Kurokami 2-39-1,
Kumamoto 860-8555, Japan
2Department of Applied Chemistry, Faculty of Engineering, Oita University,
Dannoharu 700, Oita 870-1192, Japan
New type of polyethylene imine having butyl (–C4H9) substituent azobenzene side chain
group through methylene spacer group (PEI64) was successfully synthesized by solution
polycondensation reaction. Synthesized polymer was characterized by DSC, Polarized
Optical Microscopic and X-ray diffraction analysis. The effect of reaction parameters
on degree of substitution of PEI64 also studied and optimized the reaction conditions.
Synthesized polymer possessed liquid crystalline (LC) properties. Photochemical, ther-
mooptical as well as photoorientational behavior of the polymer were investigated
elaborately. Spin coated PEI64 film showed out-of-plane molecular orientation on an-
nealing. PEI64 both in solution and in solid film exhibited photoresponsive properties
upon irradiation of UV and visible light. PEI64 film also exhibited reversible molecular
orientation from random state to out-of-plane and from out-of-plane to random state on
annealing, non-polarized UV and visible light irradiation.
Keywords azobenzene polymer; polyethylene imines; thermal alignment behavior;
liquid crystalline properties; out-of-plane ordering
Introduction
In recent years, azobenzene containing polymers have been the object of intensive investi-
gations because of their potential applications in the field of photonics and optoelectronics.
It was shown that under irradiation with an appropriate wavelength of light the azoben-
zene moieties change their spatial orientation [1–4], through photochemical trans-cis-trans
isomerization (Weigert effect). In this way anisotropy—dichroism and birefringence—is
generated in optically isotropic polymers with azobenzene moieties in the side chain. The
mechanism for the alignment involves the photochemical trans-cis isomerization and pho-
tochemical and/or thermal cis-trans isomerization of the azobenzene groups. By repetition
of these trans-cis-trans isomerization cycles and the resultant molecular motion, the op-
tical axis of the azobenzene groups becomes aligned perpendicular to the electric vector
of the polarized actinic light. This process was used for optical switching [5], digital and
∗Corresponding author. E-mail: kurihara@gpo.kumamoto-u.ac.jp
134

Alignment Behavior of PEI Having Azobenzene Side Group
135
holographic data storage [6] and optical modulators [7]. In addition the azobenzene deriva-
tives are known to undergo trans/cis photo-isomerization upon irradiation. This is not a
simple switching process between the two photochromic isomers, because a steady state is
strongly dependent on the substitution and the wavelength of the excitation.
“Photochemical phase transition” between a liquid crystalline phase and a random state
[8–10] and photoalignment between in-plane and out-of-plane structures, which is called
“command surface” [11, 12] are the two typical liquid crystalline systems in relating to the
photo-cotnrol of the molecular orientation by means of photoisomerization of azobenzene
compounds. 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 ꢀ(n
ꢀ
− n ) is larger than ꢀ(n − n), where n is extraordinary index and n is ordinary index.
⊥
ꢀ
ꢀ
⊥
The command surface system consists of azobenzene monolayer on a glass substrate and
low molecular weight liquid crystals. Therefore, the system requires a cell, and its memory
stability is not so high due to thermal back reaction of azobenzene, and fluidity of low
molecular weight liquid crystals. In contrast, in photochemical phase transition system,
polymeric materials could be used to improve memory stability. However, to obtain ꢀ(n
ꢀ
− n ) relating to the photochemical phase transition, alignment layer is required.
⊥
On the contrary, in literature so far very few papers are published on spontaneous out-
of-plane molecular orientation of polymeric liquid crystals (PLCs) [4, 13–16]. In addition,
Ujiie et al. [17, 18] reported that some polyethylene imines (PEIs) having azobenzene
groups as side groups show liquid crystalline phases and align perpendicular direction to
the substrate spontaneously by heating into an isotropic phase and following cooling without
any alignment process. So by photochemically controlling the molecular orientation of PEIs
between random state and out-of-plane structures, one can fabricate the optical switching
system showing excellent memory stability by coating PEIs on a substrate without any
alignment process.
So in this article, we synthesized polyethylene imine having butyl substituent azoben-
zene side chain group through methylene spacer group (PEI64) and investigated its photo-
chemical as well as thermal alignment behavior on annealing, UV and non-polarized visible
light irradiation. And, we also demonstrated the multiple reorientation of the azobenzene
chromophores in PEI64.
Experimental
Materials
4-Butyl aniline was purchased from Wako Pure Chemical Industries Ltd and used as re-
ceived. 1,6-Dibromohexane was purchased from TCI and polyethylene imine of low molec-
ular weight (Mn = ca.1800) and high molecular weight (Mn = ca. 10,000) were purchased
from Polysciences Inc. Reagents were used without further purification, unless stated. N,N-
dimethylformamide (DMF) was purchased from Wako Pure Chemical Industries Ltd and
was dried up with molecular sieves prior to use.
Synthesis of Azobenzene Monomers
Synthesis of 4-Butyl-4ꢀ-hydroxyazobenzene (4Az-OH)
To a solution of 4.9 g (33 mmol) of 4-butylaniline dissolved in 240 ml of water and
14 cm3 (170 mmol) of HCl was added dropwise a solution of 3.3 g (48 mmol) of NaNO2

136
M. Kamruzzaman et al.
Scheme 1. Synthetic route for monomer and polymer.
dissolved in 140 ml water at such a rate that the temperature did not exceed 5◦C. At the
same temperature, a solution of 1.8 g (44 mmol) sodium hydroxide, 15.9 g (150 mmol) of
sodium carbonate and 4.5 g (48 mmol) of phenol in 130 ml water was added slowly to the
resulting diazonium salt solution. After stirring at room temperature for 1 h, the resulting
mixture was acidified with concentrated HCl to adjust pH 4 to 5. The precipitated product
was filtered, washed with excess amount of water and dried under vacuum.
1
Yield: 59% as solid. H-NMR (CDCl3, δ): 7.95 (2H, dd, aromatic), 7.86 (2H, dd,
aromatic), 7.06 (2H, d, aromatic), 6.95 (2H, d, aromatic), 5.29 (1H, s, -OH), 1.03 (3H,
s, -CH3). Anal. Calcd for C16H18N2O (254.33): C- 75.56; H- 7.13; N- 11.01. Found: C-
74.98; H- 7.14; N- 10.88.
Synthesis of 4-(6-Bromo-n-hexyloxy)-4ꢀ-butyl azobenzene [4AzO6Br]
4-(6-bromo-n-hexyloxy)-4ꢁ-butyl azobenezene was synthesized by the following procedure:
To a mixture of 1.12 g (4.4 mmol) of 4-butyl-4ꢁ-hydroxyazobenzene and 0.91 g (6.6 mmol)
potassium carbonate dissolved in 100 ml of dried acetone, 5.39 g (22.3 mmol) of 1,6-
dibromohexane was added. After refluxing for 48 h at 70◦C the reaction mixture was filtered
and evaporated the solvent from filtrate and the product 4AzO6Br was re-crystallized from
methanol twice. The reaction scheme is shown in Scheme 1.
Yield: 76.1% as solid. Anal. Calcd for C22H29BrN2O (417.38): C- 63.31; H- 7.00; N-
6.71. Found: C- 63.89; H- 6.76; N- 6.91.
Synthesis of Polyethylene Imines Having Azobenzene Side Chain Group (PEI64)
Polyethylene imines having butyl substituent azobenzene side chain group with six methy-
lene spacer groups was synthesized using different reaction conditions. The typical pro-
cedure is as follows: 4-(6-Bromo-n-hexyloxy)-4ꢁ-butyl azobenzene 1.75 g (4.2 mmol),
polyethylene imine 0.18 g (4.0 mmol), potassium carbonate 1.8 g (6.0 mmol) and 50 ml
dry DMF were added into a round bottom flask equipped with a condenser. With con-
tinuous stirring the reaction mixture was refluxed at 100◦C for 72 h. After the reaction,

Alignment Behavior of PEI Having Azobenzene Side Group
137
the mixture was filtered and evaporated the half amount of DMF from filtrate and then
poured into methanol. The resulting polymer (PEI64) was purified by precipitation from
chloroform/THF into methanol two to three times. The removal of the monomer was mon-
itored using thin-layer chromatography. Finally, the product was dried in vacuum for 24 h
(Scheme 1).
Characterization
To identify the structure and composition of the synthesized monomers and polymer,
elemental analyses were performed with a YANAKO CHN CORDER MT-6. The structures
of the compounds were also determined using 1H NMR spectroscopy with CDCl3 or acetone
as solvent and tetramethylsilane as internal standard. The spectra were recorded at ambient
temperature with a JEOL JNM-EX400, 400 MHz NMR-spectrometer.
The phase transition behavior of the polymer was studied by differential scanning
calorimetry (DSC; Seiko SSC-5020) with a heating rate of 10 K/min and polarizing optical
microscopy (Olympus BHSP polarizing microscope; Mettler FP-80 and FP 82 hot stage
and controller).
X-ray diffraction was used to confirm the nature of the LC phases and to determine the
spacing of the smectic layers. Rigaku, RINT 2100/PC XRD machine (X-Ray, 40 kV/200
mA) equipped with a θ-θ wide angle goniometer and scintillation detector was used for
X-ray diffraction (XRD) measurement. Cu K alpha (λ 1.5406 Å) was used as target. XRD
sample was made by casting solution (40 mg polymer in 1 ml of THF) onto a 1.5 × 1.5
cm2 glass substrate. Thick film of PEI64 was heated to 100 for 10 min, cooled to room
temperature and measured the XRD.
Thin films for thermo-optical experiments were obtained by spin-coating technique
using solutions in THF on quartz glass substrates. For drying, the spin-coated films were
kept at room temperature during 1 h. Typical coating was performed as follows: speed of
rotation 3000 rpm and time of rotation 30 sec with 4 wt% of polymer in THF.
Photoirradiation was performed by using a 500 W high-pressure Hg lamp with adequate
cut filter for UV and Vis light at room temperature. The orientational order was studied
using polarized UV-Vis spectroscopy (Perkin Elmer Lambda 650 UV/Vis Spectroscopy),
because the transition moment of the trans isomer of azobenzene moiety is directed along
the long axis of this group. For this purpose the angular dependence of the absorbance
was measured. The values of order parameter determined by spectroscopic method were
calculated by Eq. (1) [4]
A − A
ꢀ
⊥
S =
(1)
A − 2A
ꢀ
⊥
where A is the absorbance at the preferred direction; A is the absorbance perpendicular to
ꢀ
⊥
this direction. To study the out-of-plane thermal/photoorientation phenomena we measured
the angular distribution of the polarized absorption spectra at an angle of 45◦ to film normal
and calculated the order parameter using Eq. (1).
Results and Discussion
Synthesis and Characterization of PEI64
In this study, liquid crystalline polyethylene imine having butyl substituent azobenzene
side chain group through methylene spacer group (PEI64) was successfully synthesized

138
M. Kamruzzaman et al.
Table 1. Reaction conditions and properties of synthesized polyethylene imine PEI64
Reaction conditions
Temp. Reaction Azo
PEI Yield
Degree of
Phase transition
Polymer ◦C time, h (mmol) (mmol) (%) substitution (%)a temperature, ◦C
PEI64 100 72 4.2 4.0 78.8 66.3 G12 S 102 N 148 I
aDetermined from UV absorption spectra. Molecular weight of PEI = ca 1800, Reaction solvent =
DMF (50 ml).
by solution polycondensation reaction. For good thermal and photo alignment behavior,
polyethylene imines should have high degree of substitution. It was assumed that as the
introduction of azobenzene group into polyethylene imine main chain increased the out-of-
plane alignment behavior of the polymers also increased. So in synthesis step, to achieve
higher degree of substitution in polyethylene imines, reaction conditions were optimized
by changing the reaction parameters such as temperature, reaction time, molar ratio of
azobenzene monomer and PEI, and reaction solvent. The degree of substitution of PEI64
has considerable effect on all of these parameters. With the increase in reaction temperature
and reaction time, the degree of substitution increased, and best results were obtained at
a temperature of 100◦C and reaction time of 72 h. At high temperature more than 100◦C
and longer reaction time (above 72 h), the degree of substitution did increase little but the
solubility of PEI64 decreased probably due to the formation of gel. So finally we synthesized
PEI64 by optimizing the reaction conditions as temperature 100◦C, reaction time 72 h, molar
ratio of azobenzene monomer to PEI = 4.2:4.0, and solvent (DMF) = 50 ml. Synthesized
polymer showed good solubility in DMF, chloroform, THF and cyclohexanone. Table 1
represents the reaction conditions and properties of synthesized polyethylene imine.
Thermal characteristics of synthesized polyethylene imine were studied using DSC
with a heating rate of 10 K/min and polarized optical microscopy (POM) analysis. Figure 1
shows the DSC thermogram of PEI64 for the second heating scan. PEI64 showed two
endothermic peaks with low glass transition temperatures. On the basis of the DSC and
POM measurements, these two peaks were assigned to smectic-nematic, and nematic-
isotropic thermal phase transitions. And, the polymer is stable up to 190◦C.
120 140 160
80
100
00
20
40
60
Temperature, °C
Figure 1. DSC thermograms of synthesized polyethylene imine (PEI64) obtained at 10◦C min−1 for
the second heating run.

Alignment Behavior of PEI Having Azobenzene Side Group
139
1.5
Before annealing
After annealing
1.0
0.5
0.0
200
300
400
500
600
Wavelength, nm
Figure 2. UV-vis absorption spectra of PEI64 single layer film before and after annealing at 100◦C
during 5 min. Longer time of annealing does not lead to additional changes.
Thermooptical Alignment Behavior of PEI64 in Film
To investigate the thermal induced optical anisotropy in PEI64 solid film, the changes in
absorbance of spin coated polymeric films on annealing were demonstrated. In this study,
all experiments were performed for polymeric films with a thickness ranging from 200 to
300 nm. The films were prepared by spin coating technique. Upon annealing the test films
at room temperature, no marked changes took place. However, when the films were kept at
the temperature, which is above its glass transition temperature corresponding to LC state,
one may observe the significant spectral changes (Figure 2). On annealing, the absorption
in the region of the π–π∗ transition is related to out-of-plane ordering was decreased by
about 60%. The position of the absorption maximum remained almost unchanged [Figure
2]. It should be noted that, the profiles of the corresponding spectra and absorption in the
region of the ϕ–ϕ∗ transitions were preserved.
On the other hand, the absorption maxima of the PEI64 solid film was blue-shifted
compared to that in THF [Figure 3], indicating the formation of H-aggregate in the solid
film. In addition, the relative intensity of π–π∗ absorption band to absorption band around
250 nm which is assigned to the ϕ–ϕ∗ transition of the aromatic ring [15, 19], was varied as
shown in Figure 4. It has been reported that the ϕ–ϕ∗ transition is insensitive to the molecular
orientation, consequently, the change in the relative intensity of π–π∗ absorption band to
ϕ–ϕ∗ absorption band (Aπ–π /Aϕ–ϕ ) is related to the out-of-plane molecular orientation
[15, 16]. Therefore, the change in π–π∗ absorption intensity and blue shift of the solid
film may be attributed to the out-of-plane molecular orientation and consequent formation
of H-aggregate [15, 20]. From Figure 4, it is also clear that the out-of-plane molecular
orientation proceeded in a few minutes.
∗
∗
Therefore, to quantify the thermal induced anisotropy in PEI64 solid film, polarized
absorption spectra were measured and explored the angular dependency of absorbance
before and after annealing of the film. Annealing of the PEI64 films brought about not only
decrease in absorbance corresponding to the π–π∗ transition without polarizer, but also
change in the polarized absorption spectra as shown in Figure 5. The order parameter, the

140
M. Kamruzzaman et al.
1.5
1.2
0.9
0.6
0.3
0.0
In THF Solution
In Film
200
300
400
Wavelength, nm
500
600
Figure 3. Normalized UV-vis absorption spectra of polyethylene imine, PEI64 in THF solution and
in solid film.
degree of out-of-plane ordering, was calculated from polarized absorption spectra by using
equation (1).
The polarized absorption spectra were recorded at an angle of 45◦ to the normal of
the films, and by following rotation of the film for polar plot to evaluate the quantity of
out-of-plane molecular orientation. From polarized absorption spectra, polar plot for poly-
meric film was also plotted and is shown in Figure 6. After annealing at a temperature
corresponding to the LC state of the polymer, the absorbance parallel to the polarization
direction of actinic light was higher than that perpendicular to the polarization direction of
actinic light. From polar plot, it is clear that, before annealing, little angular dependency
was observed, indicating that the azobenzene chromophores were slightly orientated per-
pendicularly on glass substrate in solid film. But after annealing, PEI64 solid film showed
∗
∗
Figure 4. Changes in Aπ-π /Aϕ-ϕ values of PEI64 single layer film on annealing time.

Alignment Behavior of PEI Having Azobenzene Side Group
141
Figure 5. Polarized UV-vis. absorption spectra of PEI64 single layer film before and after annealing
at 100◦C during 5 min. A and A donate absorption parallel and perpendicular to the polarization
ꢀ
⊥
direction of actinic light respectively. Spectral measurements were made at 45◦ to film normal.
big angular dependency and the shape of the corresponding polar plot is purely concave,
allowing to assess the level of order parameter [4]. The out-of-plane order parameter value
before and after annealing were 0.107 and 0.381 respectively.
Influence of Annealing Temperature on Out-of-Plane Molecular Orientation
PEI64 solid film showed different spectral changes at different annealing temperatures. To
investigate the effect of annealing temperature on molecular orientation, absorption spectra
Figure 6. Polar plot for PEI64 (λ = 313 nm) single layer film before and after annealing. Spectral
measurements were made at 45◦ to film normal.

142
M. Kamruzzaman et al.
Figure 7. (a) Changes in absorbance of PEI64 single layer film at absorption maxima (λmax. = 330
nm) on annealing temperature and (b) Changes in out-of-plane order parameter of PEI64 single layer
film on annealing temperature. Annealing time = 5 min.
of PEI64 solid film was measured after annealing for 3 min at different temperatures
ranging from 40 to 120◦C. As shown in Figure 7(a), absorbance of PEI64 film at 330 nm
was decreased by heating up to 100◦C and again increased as the temperatures increased.
Figure 7(b) plots the thermally enhanced order parameters of the polyethylene imine film
as functions of the annealing temperatures. As the temperature increased the values of order
parameter increased showing big optical anisotropy at elevated temperatures. But at high
temperature (above their LC state) again the optical anisotropy decreased and that seems
to be related to the change in the mesophase from smectic to nematic phases [21–23].
The orientation of the azobenzene side chain groups was furthermore investigated
by XRD measurements. Figure 8 shows the X-ray diffraction patterns for synthesized
polyethylene imine film on glass substrate cast from THF, and confirms that the polymer
is smectic layer structures [24–26]. PEI64 exhibited strong few diffraction peaks in small
angle region. On the basis of X-ray diffraction patterns, the smectic layer spacings were
calculated from the XRD peaks using Bragg’s law and the results are listed in Table 2.
Moreover, molecular length of azobenzene side chain group (Scheme 2) in PEI64 was
calculated by MOPAC (PM3 level) method and compared the value with smectic layer
spacings. The value of smectic layer spacings is slightly higher than the calculated value.
We also calculated the value of L/Lꢁ (where L is the smectic layer spacing and Lꢁ is the
Figure 8. X-ray diffraction pattern of synthesized polymer PEI64 in it’s LC state.

Alignment Behavior of PEI Having Azobenzene Side Group
143
Scheme 2. Molecular model of polyethylene imine PEI64. Black color for carbon atom, gray for
hydrogen atom, red for oxygen atom and blue for nitrogen atom.
calculated molecular length of azobenzene side chain group) and listed in Table 2. For
PEI64, the value of L/Lꢁ is comparatively large, indicating loose packing of molecules in
PEI64.
Photoinduced Optical Anisotropy in PEImM Solid Films
To investigate the photo alignment behavior of PEI64, the photochemical behavior of
PEI64 in solution and solid state was examined. The changes in the UV-vis. absorption
spectra of PEI64 in THF by irradiation of UV and visible light is shown in Figure 9.
Before irradiation, PEI64 in THF showed a strong absorption at 352 nm and a weak
absorption around 460 nm, corresponding to π–π∗ transition and n–π∗ transition of the
azo-chromophores, respectively. UV irradiation caused decrease and increase in absorption
at 352 and 460 nm, respectively, indicating photoisomerization from trans-form to cis-
form. It became photostationary in 10 sec in case of trans-cis photoisomerization by UV
light irradiation and 30 sec in case of cis-trans isomerization by visible light irradiation.
Photoisomerization of the polymer in solution shows isosbestic points, indicating uniform
photoreactions. The subsequent cis-trans isomerization by visible light irradiation rebuilt
the initial state completely, which demonstrates the reversibility of the reaction. In addition,
it is obvious that both trans-form and cis-form of PEI64 have weak absorption around
488 nm. Therefore, the irradiation of light in this wavelength region should cause the
photoisomerization from the trans-form to the cis-form for the trans-form, and vice verse
for the cis-form. Consequently, trans–cis–trans photoisomerization cycle of PEI64 will be
brought about by irradiation of the light in this region [19, 26, 27].
In order to understand photochemical and photoinduced optical anisotropy in poly-
meric films, the spectral changes were investigated in polymeric film under irradiation with
Table 2. Smectic layer spacings and molecular length of PEI64
Polymer 2θ(1) 2θ(2) dist. 1 (Å), L dist. 2 (Å) Molecular length (Å)a, Lꢁ L L/Lꢁ
PEI64
2.32 4.75
37.70
8.70
27.6
1.37
aCalculated by MOPAC (PM3) method.

144
M. Kamruzzaman et al.
Figure 9. Changes in UV-vis. absorption spectra of PEI64 in THF by irradiation of (a) UV (365 nm)
and (b) visible light (435 nm). The intensity of UV and visible light was 7.0 mWcm−2 and 80 mWcm−2
respectively.
UV and non-polarized visible light of different wavelengths. Depending on the irradia-
tion wavelength, different steady states of cis-trans photo-isomerization were established
allowing or preventing the photo-orientation of azobenzene chromophores in PEI64.
Upon irradiation of non polarizedUVlight onspincoatedsolidfilms of PEIs, significant
spectral changes were observed in polymers which are typical for trans-cis photoisomer-
ization [1, 27, 28]. In PEI64 film, photostationary state with a little high content of the
cis isomeric form was achieved within 2 or 3 min as shown in Figure 10(a). In film, due
to molecular aggregation, trans-cis photoisomerization rate is not as high as in solution.
On the subsequent irradiation with visible light of 435 nm, the reverse changes occurred
establishing a steady state with a high concentration of trans isomers [Figures 10(b)]. The
recovery of the initial spectrum was complete, even the value of absorption maxima cor-
responding to π–π∗ transition exceeded the initial value. This phenomenon was related to
the partial out-of-plane molecular ordering of azobenzene moieties on glass substrate.
Upon a prolonged irradiation (>5 s), one may observe a decrease in the absorption in
both regions corresponding to the π–π∗ and n–π∗ transitions as shown in Figures 10(c).
Within the first 5 seconds of irradiation with visible light, the back cis–trans photoisomer-
ization occurred, while upon continued irradiation, the azobenzene groups were oriented
along the normal of film due to photo-orientation [4]. In this case, the transition moment of
these groups become oriented perpendicular to the plane of films; as a result, the probability
of light absorption and, thus, optical density markedly decreased [4]. So irradiation with
non-polarized visible light (435 nm) caused the induction of anisotropy in the films by
photoorientation of azobenzene side groups. It was established that the photoorientation
process took place in the steady state of the photoisomerization via numerous angular-
selective photo-isomerization cycles leading to a preferred orientation of the azobenzene
groups perpendicularly to the electric field vector of the excitation light.
Multiple Thermal and Photo Reorientation of PEImM Films
Since the synthesized polyethylene imine showed thermal alignment behavior and cis- trans
photo isomerization on UV and visible light irradiation as described in the above section, it
is essential to investigate the multiple reorientation behavior of azobenzene chromophores

Alignment Behavior of PEI Having Azobenzene Side Group
145
Figure 10. Changes of absorption spectra of PEI64 polymeric film during (a) UV irradiation
(365 nm), (b) first 05 sec visible light irradiation and (c) further visible light irradiation (435 nm).
The intensity of UV and visible light was 7.0 mWcm−2 and 80 mWcm−2 respectively.
in PEI64. In order to show the possibility of reversible reorientation of azobenzene chro-
mophores in PEI64, molecular ordering behaviors were observed at different conditions.
And synthesized polymer showed reversible molecular alignment behavior from random
state to out-of-plane order and from out-of-plane order to random state. In all cases we ob-
served the angular dependency on absorbance at different conditions and plotted the polar
plots. Figure 11 illustrates the polar plots for PEI64, showing the changes in absorbance on
annealing, UV and non polarized visible light irradiation. The shape of the corresponding
polar plots at various conditions has demonstrated the nature of the alignment behaviors.
Figure 12 also demonstrates the reversibility of the alignment behavior of PEI64 at
different conditions in terms of order parameter, estimated by measuring polarized ab-
sorption spectra. Polymeric film showed very little out-of-plane ordering before annealing.
Upon annealing, polymeric film showed high order parameter value showing out-of-plane
molecular alignment. Irradiation with the 365 nm light brings the film always into the
isotropic state and erases any previously induced anisotropy. So after the irradiation of
UV light on annealed films, the value of order parameter for the polymeric film decreased
via cis-trans photoisomerization and reached to almost initial level. Namely the azoben-
zene molecules aligned randomly on the glass substrate. Following non polarized visible

146
M. Kamruzzaman et al.
Figure 11. Polar plots for PEI64 (λ = 250 nm and λ = 313 nm) single layer film (a) before annealing,
(b) after annealing, (c) after non-polarized UV irradiation and (d) after non-polarized visible light
irradiation. Spectral measurements were made at 45◦ to film normal.
Figure 12. Reversibility of alignment behavior of polyethylene imine, PEI64 on different experi-
mental conditions (1-fresh film, 2-annealing, 3-UV irradiation, 4-visible irradiation, 5-UV irradiation,
6-annealing, 7-UV irradiation and 8-visible irradiation).

Alignment Behavior of PEI Having Azobenzene Side Group
147
Scheme 3. Schematic representation of the alignment behavior of azobenzene chromophores in
PEI64 on glass substrate at different conditions.
light irradiation again increased the order parameter value, indicating the transformation
of molecular orientation of azobenzene molecules from random state to out-of-plane order
due to photo-orientation. The efficiency of the induction of anisotropy in the polymeric
film was higher for the 435 nm non-polarized visible light compared with that of annealing
because the pre-irradiation with the 365 nm light strongly increased the effectiveness of
photoorientation process [29]. Again upon UV irradiation, the azobenzene group aligned
randomly on the glass substrate. It should be noted that after each step of irradiation with
the 365 nm UV light the angular-dependent spectra were exactly the same. This indicates
a complete erasure of the previously written orientation was achieved. In this way, the
reversibility of the alignment behavior of azobenzene groups in polyethylene imine could
be achieved by changing the conditions such as annealing, UV and non polarized visible
light irradiation (Scheme 3). All these results indicated that the optical information in the
polymer can be rewritten in the system without any memory effects of previous irradiation
or annealing. Several steps of annealing (writing) and the irradiation by light of 365 nm
(erasing) and 435 nm non polarized visible light (writing) showed that optical anisotropy
in the polymeric film could be completely erased and rewritten again without any losses;
the orientation direction is always perpendicular to the electric field vector.
Conclusion
Liquid crystalline polyethylene imine (PEI64) having butyl (–C4H9) substituent azobenzene
side chain group through methylene spacer group was successfully synthesized by solution
polycondensation reaction and characterized by DSC, Polarized Optical Microscopic and
XRD analysis. The effect of reaction parameters on degree of substitution of PEI64 also
studied and optimized the reaction conditions. The synthesized polymer exhibited nematic
and smectic phases with low glass transition temperature. Photochemical, thermooptical
as well as photoorientational behavior of the polymer were investigated elaborately. Spin
coated PEI64 film showed out-of-plane molecular ordering on annealing. PEI64, both in
solution and in solid film exhibited photoresponsive properties upon irradiation of UV and
visible light. PEI64 film also exhibited reversible molecular orientation from random state
to out-of-plane and from out-of-plane to random state on annealing, non-polarized UV
and visible light irradiation. This reversibility of molecular ordering has been achieved
by the combination of thermal and photochemical processes. The reversible molecular
ordering of azobenzene chromophores showed that optical anisotropy in the film can be

148
M. Kamruzzaman et al.
completely erased and rewritten again without any memory effect of the previous irradiation
or annealing.
References
[1] Natansohn, A. and Rochon, P. Chem. Rev., 102, 4139, (2002).
[2] Yaroshchuk, O. V., Kiselev, A. D., Zakrevskyy, Yu., Bidna, T., Kelly, J., Chien, L-C., and Lindau
J. Phys. Rev. E 68, DOI 011803, (2003).
[3] Schab-Balcerak, E., Sapich, B., and Stumpe, J. Polymer, 46, 49, (2005).
[4] Bobrovsky, A., Boiko, N., Shibaev, V., and Stumpe, J. J. Photochem. and Photobio A: Chem.,
163, 347, (2004).
[5] Tang, T., Zeng, F., Wu, S., Tong, Z., Luo, D., and She, W. Optical Materials, 27, 585, (2004).
[6] Han, Y-K. and Ko, B-S. Optical Materials, 21, 621, (2002).
[7] Djurado, D., Delabouglise, D., Caix-Cecillon, C., Cecchetto, L., Decker, I., and Petit, J. P. Solid
State Ionics, 154–155, 29, (2002).
[8] Ikeda, T., Horiuchi, D. B., Karanjit, D. B., Kurihara, S., and Tazuke, S. Macromolecules, 23,
36, (1990).
[9] Ikeda, T. and Tsutsumi, O. Science, 268, 1873, (1995).
[10] Kurihara, S., Yoneyama, D., and Nonaka, T. Chem. Mater., 13, 2807, (2001).
[11] Ichimura, K. Chem. Rev., 100, 1847, (2000).
[12] Ichimura, K., Suzuki, Y., Seki, and T. Lungmuir, 4, 1214, (1988).
[13] Ujiie, S. and Iimura, K. Macromolecules, 25, 3174, (1992).
[14] Ujiie, S. and Iimura, K. Poly. J., 25, 347, (1993).
[15] Sapich, B., Vix, A. B. E., Rabe, J. P., Stumpe, J., Wilbert, G., and Zentel, R. Thin Solid Films,
514, 165, (2006).
[16] Uekusa, T., Nagano, S., and Seki, T. Macromolecules, 42, 312, (2009).
[17] Ujiie, S. and Iimura, K. Chem. Lett., 19, 995, (1990).
[18] Ujiie, S. and Yano, Y. Chem. Commun., 1, 79, (2000).
[19] Fabian, J. and Hartmann, H. Light Absorption of Organic Colarants; Springer-Verlag: Berlin,
pp. 32–79, (1980).
[20] Kasha, M., Rawls, H. R., and Bayoumi, M. A. El. Pure Appl. Chem., 11, 371, (1965).
[21] Wu, Y., Demachi, Y., Tsutsumi, O., Kanazawa, A., Shiono, T., and Ikeda, T. Macromolecules,
31, 1104, (1998).
[22] Hvilsted, S., Andruzzi, F., Kulinna, C., Siesler, H. W., and Ramanujam, P. S. Macromolecules,
28, 2172, (1995).
[23] Ivanov, S., Yakovlev, I., Kostromin, S., Shibaev, V., La¨sker, L., Stumpe, J., and Kreysig, D.
Makromol. Chem., Rapid Commun., 12, 709, (1991).
[24] Liu, J. H., Yang, P. C., Chiu, Y. H., and Suda, Y. J Polym Sci Part A: Polym Chem., 45, 2026,
(2007).
[25] Uekusa, T., Nagano, S., and Seki, T. Langmuir, 23, 4642, (2007).
[26] Freiberg, S., Labarthet, F. L., Rochon, P., and Natansohn, A. Macromolecules, 36, 2680, (2003).
[27] Alam, M. Z., Ohmachi, T., Ogata, T., Nonaka, T., and Kurihara, S. Optical Materials, 29, 365,
(2006).
[28] Ishiguro, M., Sato, D., Shishido, A., and Ikeda T. Langmuir, 23, 332, (2007).
[29] Rais, D., Zakrevskyy, Y., Stumpe, J., Nesˇpu˚rek, S., and Sedla´kova´, Z. Optical Materials, 30,
1335 (2008).

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