Photo-responsive polymers and their applications to optical memory

Article abstract of DOI:10.1080/15421400500377602

We synthesized photo-responsive azo polymers from novel malonic ester and silicate ester monomers having two symmetrical azobenzene groups in the side chain. The possibility of their applications as reversible data storage media was investigated utilizing optical data recording techniques. As a result, the azo polymers had very excellent stability and resolution of the stored data as well as the sensitivity to pump beam. The polymeric thin films also could be used as erasable digital data materials for a practical use by an Ar ion pulse laser (pulse width: 10 s). In addition, the polymeric thick films (500-1000 m) from new monomer and methyl methacrylate showed good angular selectivity for holographic memory by using optical two wave mixing technique.

Full text of DOI:10.1080/15421400500377602

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Photo-responsive Polymers and
their Applications to Optical
Memory
Min-Jeong Lee a , Doo-Hee Jung a & Yang-Kyoo Han a
a Center for Functionalized Organics and Organic-
Inorganics, Department of Chemistry , Hanyang
University , Sungdong-Gu, Seoul, Korea
Published online: 20 Aug 2006.
To cite this article: Min-Jeong Lee , Doo-Hee Jung & Yang-Kyoo Han (2006) Photo-
responsive Polymers and their Applications to Optical Memory, Molecular Crystals and
Liquid Crystals, 444:1, 41-50, DOI: 10.1080/15421400500377602
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Mol. Cryst. Liq. Cryst., Vol. 444, pp. 41–50, 2006
Copyright # Taylor & Francis LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400500377602
Photo-responsive Polymers and their Applications to
Optical Memory
Min-Jeong Lee
Doo-Hee Jung
Yang-Kyoo Han
Center for Functionalized Organics and Organic-Inorganics,
Department of Chemistry, Hanyang University, Sungdong-Gu,
Seoul, Korea
We synthesized photo-responsive azo polymers from novel malonic ester and
silicate ester monomers having two symmetrical azobenzene groups in the side
chain. The possibility of their applications as reversible data storage media was
investigated utilizing optical data recording techniques. As a result, the azo poly-
mers had very excellent stability and resolution of the stored data as well as the
sensitivity to pump beam. The polymeric thin films also could be used as erasable
digital data materials for a practical use by an Ar ion pulse laser (pulse width:
10 ls). In addition, the polymeric thick films (500–1000 lm) from new monomer
and methyl methacrylate showed good angular selectivity for holographic memory
by using optical two wave mixing technique.
Keywords: angular selectivity; data resolution; data storage stability; digital data bit;
holographic memory; photo-responsive azo polymers
INTRODUCTION
Recently photo-responsive polymers with chromophore groups have
attracted much attention in optical data storage, optical holography,
and integrated optics [1,2]. Furthermore, since they show a great
potential as recording media for data storage and retrieval, they
have been the subjects of a number of papers. So far azobenzene,
This work was supported by the Korea Research Foundation Grant (KRF-2002-041-
C00193). Min-Jeong Lee appreciates financial support from the 2001 scholarship
program of Hanyang University. Doo-Hee Jung also thanks financial support from the
BK 21 Program for the Center for Functionalized Organics and Organic-Inorganics,
Department of Chemistry, Hanyang University.
Address correspondence to Yang-Kyoo Han, Center for Functionalized Organics and
Organic-Inorganics, Department of Chemistry, Hanyang University, Sungdong-Gu,
Seoul 133-791, Korea. E-mail: ykhan@hanyang.ac.kr
41

42
M.-J. Lee et al.
spiropyran, fulguid and their derivatives as a photo-responsive group,
which is sensitive to light, have been introduced into the side chain of
the polymers. They induce the change of optical birefringence and
absorbance under irradiation of a linearly polarized light. Especially,
azo dye molecules are much more sensitive to the light than any other
photo-responsive groups [3,4], That is why azo containing polymers
are mainly used as electronic materials. We, therefore, have exten-
sively studied on the synthesis and characterization of novel liquid
crystalline polymers with two symmetrical azobenzene groups in the
side chain. Moreover, we investigated their applications to reversible
optical information storage media through a photoisomerization of
the azobenzene group, a photochromophore [5,6].
As a series of our works, the present work reports the synthesis of
photo-responsive polymers from new malonic ester and silicate ester
liquid crystalline monomers and their applications to data storage
media for optical memory.
EXPERIMENTAL
As optical recording media, novel poly(malonic esters) [7] and poly-
(silicate ester) [8] were prepared by the procedure already reported
(Fig. 1). New thermotropic liquid crystalline (LC) malonic ester mono-
mer (MDR1) was synthesized through reacting in THF at 0ꢀC for 24 h
malonyl dichloride and disperse red 1(DR1). It was then condensed
with 1,6-dibromohexane or a, a0-dibromo-p-xylene in THF in the pres-
ence of sodium hydride at 65ꢀC for 24 h to give poly(malonic esters)
with two symmetrical DR1, a photoresponsive group. On the other
hand, silicate ester monomer (MSi-DR1) was synthesized by reacting
3-methacryloxypropylmethyldichlorosilane and DR1 in methylene
chloride at room temperature for 48 h. The monomer MSi-DR1 was
polymerized with a radical initiator at 60ꢀC for 72 h to produce
homopolymer (PSi-DR1) and copolymers with different composition
ratio. The results for polymerization are listed in Table 1.
The polymeric thin films were cast from the polymer solution
(5 wt%) in CHCl3 onto a glass plate for 30 seconds using a spin caster.
The thickness of the films was controlled by varying the spin speed
and the concentration of the polymer solution. We also prepared the
polymeric thick films from the LC monomer (MDR1 or MSi-DR1)
and methyl methacrylate (MMA) to carry out a fundamental experi-
ment for holographic memory: the novel LC monomer was dissolved
in the MMA solution including radical initiator AIBN and then the
mixed monomer solution was charged into the reaction plate, which
is made from two glass plates, through a syringe. The plate was placed

Photo-responsive Azo Polymers for Optical Memory
43
FIGURE 1 Synthesis of novel photo-responsive polymers with DR1 group in
the side chain: poly(malonic esters), PDR1; poly(silicate esters), PSi-DR1.
in an oven and polymerized for 72 h at 60ꢀC to obtain a thick film. The
film thickness (500–1000 mm) is determined by the thickness of rubber
band used as a spacer between the glass plates.
TABLE 1 Polymerization Results for the Homopolymer (PSi-DR1) and the
Copolymers
Copolymer compositiona Conversion
Tg Initial decomposꢀition
temperatureb ( C)
Polymer
MSi:MMA
(%)
Mw (ꢀC)
PSi-DR1
100:0
57:43
19:81
3:97
65
73
81
79
6500 47
11600 68
18700 76
25800 93
195
208
221
245
PSi-PMMA-1
PSi-PMMA-2
PSi-PMMA-3
acalculated from the characteristic peaks of 1H-NMR spectra.
bTemperature of 1.0% weight loss in TGA thermogram.

44
M.-J. Lee et al.
The prepared polymers showed maximum absorbances (kmax
)
around 465 nm in UV-VIS absorption spectra due to the stable trans
conformation of azobenzene. On the basis of the UV-VIS results, line-
arly polarized argon laser with 488 nm was used as a pump beam to
record optical data in the polymeric thin films. The optical anisotropy
of the polymer derived from the pump beam in a write-in step was
measured by using low power Ga=As laser of 847 nm, a probe beam.
For holographic memory, Nd-YAG laser with 532 nm was used as
reference and object beams.
RESULTS AND DISCUSSION
Phase Transition Behavior
The new malonic ester monomer MDR1 showed enantiotropic beha-
vior in which LC phase appeared at 117 and 86ꢀC on both heating
and cooling cycles by DSC. It, however, was observed to show a typical
smectic batonnet texture only on a cooling cycle (96ꢀC) by means of
optical polarizing microscope (OPM). On the other hand, the poly-
(malonic esters) (PDR1) obtained from the MDR1 and dibromide com-
pounds had no mesophase. The PDR1-6 having six methylene units
(X: ꢁ(CH2)6ꢁ) in the polymer backbone exhibited only isotropization
temperature (76ꢀC) on a heating cycle without a mesophase. Its glass
transition temperature (Tg) was about 50ꢀC. In addition, introducing a
rigid p-xylene unit instead of flexible methylene group into the poly-
mer backbone enhanced Tg (96ꢀC) of the corresponding polymer
(PDR1-p-Xylene). The weight average molecular weights of the
PDR1 polymers were in the range of 5,000 to 15,000, which were con-
trolled by polymerization time.
In addition, MSi-DR1 monomer also showed monotropic behavior in
which LC phase appeared at 80ꢀC on cooling cycle of DSC. It, however,
had a typical smectic batonnet texture on both heating (76–87ꢀC) and
cooling cycles (87–65ꢀC) by OPM. The homopolymer (PSi-DR1) and
copolymers with MMA exhibited only the melting temperature on a
heating cycle without any mesophase.
Application to Optical Data Storage Media
The application of the prepared photo-responsive polymers as optical
data recording media was performed according to our previous litera-
ture [6,7]. As a result, Figure 2 shows writing, relaxation, and erasing
profiles for the PDR1 thin films with irradiation time: When the polar-
ized argon laser was turned on, optical birefringence rapidly increased

Photo-responsive Azo Polymers for Optical Memory
45
FIGURE 2 Writing, relaxation, erasing processes with temperature: (a)
PDR1-6 and (b) PDR1-p-xylene thin films.
in a write-in process. This means that polymer chains were aligned
perpendicular to the plane of laser polarization through a trans-cis iso-
merization of azobenzene group incorporated into the polymer. Such
an orientation gives rise to high transmission of the probe beam
during a read-out process. However, the value of birefringence caused

46
M.-J. Lee et al.
by the high transmission rapidly decreased about 20% within the first
few minutes and then maintained a certain value, when the writing
laser was turned off. The stored data also can be easily erased by a
circularly polarized Ar laser. In the experiments, the optical birefrin-
gence (Dn) derived from the film was calculated from the transmitted
intensity of the probe beam measured in a read-out process and the
transmitted intensity I can be expressed as
ꢀ
ꢁ
pDnd
I ¼ I0 sin2
sin2ð2hÞ
k
where, I0 and k is the incident power and the wavelength of the probe
laser, d is the thickness of the film, and h is the angle between the
polarization angle of the pump beam and the optic axis of a polarizer
used in the read-out process [5]. The measurement was carried out at
h ¼ 45ꢀ to obtain the highest transmitted intensity from the equation.
On the other hand, the stored information (high birefringence) was
drastically decreased as the ambient temperature was increased. Such
decay was quite rapid in the PDR1-6 with flexible methylene group in
the polymer main chain. As shown in Figure 2(a), especially, the
recorded data was erased completely when the temperature went up
to the Tg (50ꢀC) of the film. Thus, we introduced a rigid group like
p-xylene unit instead of the methylene group in the main chain to
enhance the storage stability of the data with temperature. In the case
of the PDR1-p-xylene film, the recorded data was very stable up to
75ꢀC, as shown in Figure 2(b). It indicates that the stability of the
optical data is closely related to the Tg of the film using as recording
material. To investigate the capability of the film as a rewritable
medium, we also performed the reversibility of the data for a write-
erase-rewrite process. As a result, the PDR1 film exhibited excellent
property as an erasable material as shown in Figure 3.
For a practical application, the focused linearly and circularly polar-
ized laser beams (pulse width: 10 ms) were irradiated to write and
erase bits, digital data, respectively. For this experiment, we used
the schematic optic-setup reported in our previous work [7]. Before
preparing a polymeric thin film, Al reflection layer was deposited onto
a substrate material using a sputtering method in order to obtain
higher contrast. Figure 4 shows the digital bits of a 0.5 mm size stored
in the PDR1-p-Xylene thin film. The resolution of the bits was getting
better with increase in the power intensity of the pulsed laser beam
from 1.0 to 5.0 mW=Cm2. On the other hand, irradiating the circularly
polarized Ar laser can erase the digital data and repeating the write-in
process also can rewrite them.

Photo-responsive Azo Polymers for Optical Memory
47
FIGURE 3 Reversibility of the data storage for the PDR1-p-Xylene.
On the basis of the above results, we found that the poly(malonic
esters) with DR1 as a chromophore were excellent as reversible media
for data storage and retrieval. However, their high crystallinity and
poor solubility make it difficult to prepare the corresponding thick
FIGURE 4 Digital bits (0.5 mm) stored in the PDR1-xylene film with the
power intensity of Ar ion pulsed laser (5.0, 4.0, 3.0, 2.0, and 1.0 mW): pulse
width, 10 ms.

48
M.-J. Lee et al.
FIGURE 5 Holographic image (1.0 ꢂ 2.0 mm) inscribed in the polymeric
MDR1 thick film.
film. Thus we made the thick film (530 mm) from the LC monomer
MDR1 (5 wt% vs. MMA) and the MMA to examine the possibility of
application as a holographic device. Figure 5 shows the hologram
image (1.0 ꢂ 2.0 mm) stored in the MDR1=PMMA blend typed thick
film using a two wave mixing optic-setup for holographic memory
[7]. High concentration of the MDR1 in the blend thick film is needed
FIGURE 6 Influence of temperature on the data storage for the copolymer
PSi-PMMA-3.

Photo-responsive Azo Polymers for Optical Memory
49
to improve both resolution and diffraction efficiency of the image
inscribed in the thick film. However, it was impossible to prepare
the thick film including the MDR1 of more than 5 wt% because of
its poor solubility against the MMA.
Therefore, we synthesized new acrylate typed monomer MSi with
two symmetrical azobenzene groups in the side chain to enhance both
content and solubility of the photo-responsive monomer. The monomer
was polymerized in the presence of radical initiator to produce
corresponding homopolymer and copolymers with different MSi
composition, which are summarized in Table 1. Figure 6 is a writing-
relaxation-erasing profile of the digital data with temperature for the
copolymer thin film (PSi-PMMA-3). The result was similar to the
trend of the PDR1-p-Xylene film. The recorded data also was very
stable up to the Tg of the copolymer-3. Based on these results, we
prepared the copolymer thick film (860 mm) with the MSi of 10 wt%
in terms of the MMA for holographic memory device utilizing the
special reaction plate. The resolution of the holographic image was
better than the one inscribed in the MDR1-PMMA blend thick film.
FIGURE 7 Angular selectivity of the copolymer thick film for the holographic
memory.

50
M.-J. Lee et al.
Figure 7 displays the angular selectivity of the copolymer thick film,
indicating how many images we can store per unit volume. The thick
film had good angular selectivity of 0.035ꢀ angle [9]. It shows a strong
potential of the film as a volume hologram material.
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