Photoalignment and holographic properties of azobenzene-containing block copolymers with oxyethylene and alkyl spacers

Article abstract of DOI:10.1080/15421400903196120

Two azobenzene-containing block copolymers with an oxyethylene (P1) or an alkyl (P2) spacer in the side chain were prepared by atom transfer radical polymerization (ATRP). Neither of the block copolymers with a low azobenzene content showed a liquid-cryst

Full text of DOI:10.1080/15421400903196120

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Photoalignment and
Holographic Properties of
Azobenzene-Containing Block
Copolymers with Oxyethylene
and Alkyl Spacers
Yumiko Naka a , Haifeng Yu a , Atsushi Shishido a
Tomiki Ikeda a
&
a Chemical Resources Laboratory, Tokyo Institute of
Technology, Midori-ku, Yokohama, Japan
Published online: 10 Nov 2009.
To cite this article: Yumiko Naka , Haifeng Yu , Atsushi Shishido & Tomiki Ikeda
(2009): Photoalignment and Holographic Properties of Azobenzene-Containing Block
Copolymers with Oxyethylene and Alkyl Spacers, Molecular Crystals and Liquid
Crystals, 513:1, 131-141
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Mol. Cryst. Liq. Cryst., Vol. 513, pp. 131–141, 2009
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400903196120
Photoalignment and Holographic Properties of
Azobenzene-Containing Block Copolymers with
Oxyethylene and Alkyl Spacers
Yumiko Naka, Haifeng Yu, Atsushi Shishido, and
Tomiki Ikeda
Chemical Resources Laboratory, Tokyo Institute of Technology,
Midori-ku, Yokohama, Japan
Two azobenzene-containing block copolymers with an oxyethylene (P1) or an alkyl
(P2) spacer in the side chain were prepared by atom transfer radical polymeriza-
tion (ATRP). Neither of the block copolymers with a low azobenzene content
showed a liquid-crystalline phase. However, their glass transition temperatures
(Tgs) were lowered by introducing flexible spacers. In particular, Tg was lowered
by about 30ꢀC in P1. Their photoalignment and holographic behavior was inves-
tigated at various temperatures in thin films. Rapid changes in refractive index
were induced in the P1 films upon irradiation at a temperature below Tg. But
the photoinduced birefringence in the P1 film was lower than that in the P2 films.
Similar results were obtained in holographic recording. The azobenzene-
containing block copolymer with the oxyethylene spacer (P1) used in this study
has potential application for dynamic hologram.
Keywords: atom transfer radical polymerization (ATRP); azobenzene; block copolymer;
holography; oxyethylene
INTRODUCTION
Generally, holographic materials require various properties such as
high diffraction efficiency, high transparency, quick response, heat-,
moisture- and light-resistance. However, it is difficult to attain all of
these requirements. Polymeric materials containing azobenzene
moieties have been studied for holograms, because of their preferable
properties to holograms such as reversible storage and high diffraction
Address correspondence to Tomiki Ikeda, Chemical Resources Laboratory, Tokyo
Institute of Technology, R1-11, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan.
E-mail: tikeda@res.titech.ac.jp
131

132
Y. Naka et al.
efficiency [1–11]. Holographic gratings can be formed by photoisomeri-
zation and photoinduced alignment of azobenzene molecules.
In our previous studies, azobenzene-containing polymers with alkyl
chains as spacers were selected for holographic materials [2–8].
Recently, it was found that the photoinduced birefringence (Dn) of block
copolymer films was higher than that of random copolymer films with a
similar azobenzene content, thanks to the cooperative motion of liquid
crystals in the block copolymer films [5,6]. However, photoisomerization
and alignment of azobenzene molecules take place slowly in the block
copolymer films than those in the random copolymer films by micro-
phase separation upon irradiation. Schmidt et al. reported that the
higher refractive-index anisotropy was obtained by an increase of free
volume and a decrease of the interface layer in block copolymer films
[9,10]. Zhao et al. indicated that the nanoscale phase separation might
restrict the motion of azobenzene moieties in block copolymer films [11].
Recently, it has been demonstrated that oxyethylene chains as
flexible spacers not only decouple mesogens from the main chains in
liquid-crystalline polymers, but also decrease interaction among meso-
gens [12,13]. The oxyethylene moiety is a very flexible and non-bulky
chain. Altomare et al. synthesized azobenzene-containing liquid-
crystalline polymers with oxyethylene segments and investigated
their isomerization behavior [14]. Azobenzene moieties in polymers
with the oxyethylene chain may be aligned more easily than those
with an alkyl spacer thanks to weak interaction among them in block
copolymer films. However, photoinduced changes in alignments of
azobenzene moieties have not been reported in block polymers with
the oxyethylene spacer.
In this study, photoresponsive and holographic properties were
investigated in the block copolymer films with oxyethylene spacers
as a flexible unit. For comparison, an azobenzene-containing block
copolymer with alkyl spacers was also prepared, and its photorespon-
sive behavior was explored. The block copolymers with the narrow
polydispersity and a similar azobenzene content were prepared by
atom transfer radical polymerization (ATRP).
EXPERIMENTAL
Material
As shown in Scheme 1(b), two block copolymers with an oxyethylene
or an alkyl spacer in the azobenzene side chain were synthesized by
using a bromo-terminated poly(methyl methacrylate) (PMMA-Br) as
a macroinitiator by ATRP [5,6], which are named as P1 and P2,

Photoalignment and Holographic Properties
133
SCHEME 1 Synthetic routes of two azobenzene monomers with an oxyethy-
lene and an alkyl spacer (a) and their corresponding block copolymers (b),
P1 and P2, used in this study.
respectively. The two monomers, 4-(8-methacryloyloxy-3,6-dioxaoctyl-
1-oxy)-40-ethoxyazobenzene (1) and 4-[6-(methacryloyloxy)hexyloxy]-
40-ethoxyazobenzene (2), were prepared according to the reported

134
Y. Naka et al.
procedure shown in Scheme 1(a) [7]. Molecular weight of the poly-
mers was measured by GPC (Japan Spectroscopy Co., model
DG-980-50; column, Shodex GPC K802 þ K804 þ K805; eluent,
chloroform) using standard polystyrenes for calibration. The thermo-
dynamic properties of polymers were analyzed with a differential
scanning calorimeter (DSC, Seiko I&E SSC-5200 and DSC220C) at
heating and cooling rates of 10ꢀC=min. At least three scans were per-
formed to check the reproducibility. The mole fraction of the azoben-
zene moiety in the copolymers was estimated by the absorbance of
the copolymers and the monomers in toluene measured by UV-vis
spectroscopy (JASCO, V-550).
Film Preparation
The block copolymer films with a thickness of about 1.9 mm were pre-
pared by casting their toluene solution onto glass substrates. The film
thickness was measured with a surface profiler (Veeco Instruments
Inc., Dektak 3ST). Before measurement, the films were annealed at
140ꢀC in a vacuum oven for 40 h. The heating and cooling rates were
controlled at 0.5ꢀC=min and 0.1ꢀC=min, respectively. Atomic force
microscopy (AFM) images of surface modulation were detected with
a scanning probe microscope (Veeco Instruments Inc., Nanoscope IV)
in a tapping mode at room temperature.
Optical Setups for Photoinduced Alignment and Formation
of Gratings
The sample film was first fixed on a rubber heater to control the
temperature of the film (Fig. 1(a)). Then an s-polarized beam at
488 nm from an Arþ laser (Spectra Physics, Inc., BeamLok2065-7S)
(100 mW=cm2) was used to induce alignment of azobenzene moieties
in the sample film. Figure 1(b) shows an optical setup for photoinduced
alignment. The intensity of a probe beam at 633 nm from a He-Ne
laser (NEC, Co., GLS5370) transmitted through a pair of crossed
polarizers, with the sample film fixed on the heater between them,
was measured with a photodiode.
Gratings with a fringe spacing of 2 mm were formed upon irradia-
tion with two interference beams on a film at an incident angle of 7ꢀ
(Fig. 1(c)). The total intensity of the beams was 100 mW=cm2. The
grating formation was evaluated by monitoring the first-order dif-
fraction beam (I1) at 633 nm from a He-Ne laser with a photodiode
in real time. The first-order diffraction efficiency (g) was defined

Photoalignment and Holographic Properties
135
FIGURE 1 Optical setup around the sample (a), for the evaluation of the
photoinduced birefringence (Dn) (b) and for the formation of gratings (c). A,
analyzer; BS, beam splitter; GP, Glan-Thomson prisms; L, lens; M, mirror;
PH, pinhole; S, sample; P, polarizer; PD, photodiode; H, rubber heater.
as the ratio of I1 to the intensity of the transmitted beam before
recording (I0).
I1
g ¼
ð1Þ
I0
RESULTS AND DISCUSSION
Properties of Polymers
Results of characterization of the polymers are summarized in Table 1.
Although both block copolymers had almost a similar azobenzene

136
Y. Naka et al.
TABLE 1 Summary of Properties of the Block Copolymers
Used in this Study
Composition
ratio (x:y)
Phase transition
temperature (ꢀC)
Mn
Mw=Mn
PNMA-Br
P1
P2
12,000
18,000
19,000
1.12
1.04
1.04
–
6.94
9.91
G 118 I
G 87 I
G 105 I
Mn, number-average molecular weight; Mw, Weight-average mole-
cular weight; G, glassy; I, isotropic.
content as evaluated by UV-vis absorption spectra, their Tg values
were different: Tg of P1 was 87ꢀC, while that of P2 was 105ꢀC. Tg of
a homopolymer with an oxyethylene spacer is lower than that with
an alkyl spacer [6,8,15]. It has been reported that a flexible spacer
leads to a decrease of Tg and a separation of the motion of a main chain
from those of mesogens [12–14]. The oxyethylene spacer in the azoben-
zene block may decouple azobenzene moieties from the main chain.
The P1 film with an oxyethylene spacer showed microphase separa-
tion with spherical microdomains after annealing (Fig. 2(a)). This
might be due to the introduction of hydrophilic segments of oxyethy-
lene. Although P2 has a similar azobenzene content, its film did not
show any microphase separation (Fig. 2(b)).
Effect of Temperature on Photoinduced Birefringence
To explore the change in alignment of azobenzene moieties, transmit-
tance was monitored through a pair of crossed polarizers upon irradia-
tion with a linearly polarized beam. The photoinduced birefringence
(Dn) was estimated from the change in transmittance by Eq. (2),
FIGURE 2 AFM phase images of P1 (a) and P2 (b) after annealing at 140ꢀC.

Photoalignment and Holographic Properties
137
ꢀ
ꢁ
pdDn
T ¼ sin2
ð2Þ
k
where d is the film thickness and k is the wavelength of the probe
beam, respectively. The experiments were carried out at room tem-
perature, at 55ꢀC and at a temperature just below Tg (T ¼ Tg ꢁ 15ꢀC),
respectively. In both films, birefringence increased and finally reached
to a constant value upon irradiation as shown in Figure 3. It was found
that the photoinduced birefringence in both block copolymer films was
reduced with an increase in temperature (Fig. 3). Moreover, birefrin-
gence could not be induced above Tg. This is presumably because the
degree of alignment of the azobenzene moieties is suppressed by
enhancing the mobility of the main chain at high temperature. After
the laser beam was switched off, Dn decreased in the P1 film, while
Dn was kept constant at a maximum value in the P2 film. These
results suggest that the mobility of azobenzene moieties in P1 with
the oxyethylene spacer is higher than that in P2 with the alkyl spacer
at the same temperature. To investigate the alignment of azobenzene
moieties, polarized absorption spectra were measured after irradiation
with a linearly polarized beam. Figure 4 shows polarized absorption
spectra of P1 and P2 films at room temperature after photoalignment.
A and A are absorbance parallel and perpendicular to the polariza-
k
?
tion direction of the actinic beam, respectively. Before irradiation,
the same spectra were obtained, while A was higher than A after
?
k
irradiation with a linearly polarized beam. These results indicate
that trans-azobenzene molecules are aligned perpendicular to the
FIGURE 3 Photoinduced birefringence of P1 (a) and P2 (b) in film at room
temperature, at 55ꢀC, and at a temperature slightly below Tg.

138
Y. Naka et al.
FIGURE 4 Polarized absorption spectra of P1 (a) and P2 (b) in film after
irradiation with an s-polarized beam at 488 nm. A and A are absorbance
k
?
parallel and perpendicular to the polarization direction of the actinic beam,
respectively.
polarization direction of the actinic beam in film upon irradiation. The
order parameter (S) was estimated from A and A by Eq. (3).
?
k
A ꢁ A
?
k
S ¼
ð3Þ
A þ 2A
?
k
A higher alignment order of azobenzene moieties was induced in the
P2 film, similar to the photoinduced birefringence. The values of S
were 0.14 (P1) and 0.23 (P2), respectively.
Modulation of Refractive Index
To investigate the grating formation, the copolymer film was irra-
diated with two interference beams. All the experiments were per-
formed in the Raman-Nath regime. Modulation of refractive index
(Dn0) between the bright and dark fringes of the interference pattern
was estimated by Eq. (4), assuming that the diffraction is due to the
change in the refractive index alone,
ꢀ
ꢁ
2
pDn0d
g ¼
ð4Þ
k
where d is the film thickness and k is the wavelength of the probe
beam, respectively. When the interference beams were turned on,
the Dn0 increased in both films as shown in Figure 5. Continuous irra-
diation with the interference beams led to a decay in Dn0. Since trans-
azobenzene moieties are also aligned perpendicular to the polarization
direction of the actinic beam in the dark area as well as in the bright

Photoalignment and Holographic Properties
139
FIGURE 5 Modulation of refractive index in P1 (a) and P2 (b) in film at room
temperature, at 55ꢀC, and at a temperature slightly below Tg.
areas, the difference in the refractive index between the dark and
bright fringes becomes smaller. After the actinic light was switched
off, the films of P1 and P2 behaved differently. In the P2 film, the
Dn0 increased rather monotonically and remained unchanged, whereas
the Dn0 decreased after the actinic beam was turned off in the P1 film.
Considering the results of Dn described in the preceding section, it is
reasonable to infer that the improvement of motion of azobenzene
moieties in the P1 film is really due to the incorporation of oxyethy-
lene moieties. To explore the effect of temperature on the grating
formation, the response time is plotted as a function of temperature
of the films in Figure 6. Here the response time is defined as the time
FIGURE 6 Response times of P1 and P2 films at various temperatures.
(Squares) P1; (circles) P2.

140
Y. Naka et al.
necessary to reach the maximum value of Dn0. It was observed that the
response time decreased with an increase in temperature in both
films. This could be interpreted in terms of the increased mobility of
azobenzene moieties as well as their enhanced cis-trans thermal iso-
merization with temperature. The response time of P1 was shorter
than that of P2 at each temperature. The oxyethylene spacer of P1
may effectively decouple the azobenzene moieties from the main chain.
CONCLUSIONS
Two block copolymers with oxyethylene (P1) and alkyl (P2) spacers in
the azobenzene block were successfully prepared by ATRP with a low
polydispersity of 1.04. The P1 film showed spherical microphase
separation, whereas no nanostructure was observed in the P2 film.
Glass transition temperature (Tg) was lowered by about 20ꢀC in P1
compared with P2. Their photoalignment and holographic properties
were investigated in thin films. Although photoinduced birefringence
(Dn) in P2 was kept constant, Dn in P1 was reduced after the laser
beam was switched off. Moreover, the modulation of refractive index
(Dn0) between the bright and dark fringes with the interference pat-
tern increased in the P2 film, while in the P1 film Dn0 decreased
immediately after the actinic light was switched off. The changes in
Dn and Dn0 strongly suggest that the oxyethylene spacers effectively
decouple the azobenzene moieties from the main chain. It was found
that the Dn and Dn0 in both block copolymer films were reduced with
an increase in temperature. Rapid change in molecular alignment can
be induced in P1 during photoirradiation at high temperatures, which
suggests its potential applications as dynamic holographic materials.
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