Photo-crosslinkable liquid-crystalline azo-polymer for surface relief gratings and persistent fixation

Article abstract of DOI:10.1039/b900146h

Highly efficient migration has been observed for soft liquid-crystalline azobenzene-containing polymer films to form surface relief gratings (SRGs). The efficient migration is probably driven by the interfacial instability at boundaries between the smecti

Full text of DOI:10.1039/b900146h

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New Journal of Chemistry
An international journal of the chemical sciences
Volume 33 | Number 6 | June 2009 | Pages 1157–1440
www.rsc.org/njc
Themed issue: MOLMAT and New Horizons of Photochromism
ISSN 1144-0546
PAPER
Takahiro Seki et al.
Photo-crosslinkable liquid-crystalline
azo-polymer for surface relief gratings
and persistent fixation
1144-0546(2009)33:6;1-Y

PAPER
www.rsc.org/njc | New Journal of Chemistry
Photo-crosslinkable liquid-crystalline azo-polymer for surface relief
gratings and persistent fixation
Wenhan Li, Shusaku Nagano and Takahiro Seki*
Received (in Montpellier, France) 5th January 2009, Accepted 3rd March 2009
First published as an Advance Article on the web 6th April 2009
DOI: 10.1039/b900146h
Highly efficient migration has been observed for soft liquid-crystalline azobenzene-containing
polymer films to form surface relief gratings (SRGs). The efficient migration is probably driven by
the interfacial instability at boundaries between the smectic liquid-crystalline phase and photo-
induced isotropic one. However, this migration system suffers from long-term and heat stability.
This work proposes a new post-fixation method by utilizing the [2 + 2] photo-dimerization
reaction of a cinnamoyl unit introduced into the polymer. The polymer synthesis, the properties
of SRG formation, and drastic improvement of the stability towards heating and solvent exposure
are described.
a high Ti (liquid to isotropic transition (isotropization)
temperature). Still, these strategies may lead to great
Introduction
The research groups of Rochon and Natansohn,1 and
difficulties in material design. Furthermore, polymers with a
Tripathy,2 followed by Ramanujam and Hvilsted3 first showed
that interference irradiation to an azobenzene (Az)-containing
high Tg or Ti lack the fluidity necessary for the mass migration.
The inscribing rate may be enhanced with a highly fluid film,
polymer film produces surface relief gratings (SRGs) which
but this choice consequently leads to poor stability for SRG
is attributed to a mass migration of polymer chains over
micrometer distances.4,5 Since then, a number of researchers
To overcome this problem, we had earlier proposed a post-
structures.
have been exploring such phenomena for basic interests
crosslinking strategy after the SRG formation via
a
and applications.6–11 From the standpoint of potential
applications, holographic optical recording, waveguide forma-
tion, and liquid-crystal alignment etc. can be proposed.5
However, conventional amorphous systems require a high
light energy dose for the relief formation.
formalization reaction (acetal formation between two hydroxyl
groups by formaldehyde) in the vapor phase.15,17 This
strategy, however, sometimes suffers from damage of the relief
structure and lacks reproducibility. Also, in view of health
safety, use of hazardous formaldehyde should be avoided.
Furthermore, procedures totally consisting of photo-processes
will be more favorable for the actual fabrication process.
We have recently developed a family of soft liquid-
crystalline Az polymers applicable for SRG formation.12–18
The use of soft liquid-crystalline Az polymer allowed marked
enhancement in the SRG formation upon patterned
irradiation. The photon dose for the completion of migration
is approximately 10ꢀ3 fold lower than that of widely reported
amorphous and liquid-crystalline polymers. From the
knowledge obtained by our previous investigations, the
mechanism of the SRG formation in this system could
be assumed as follows. The patterned visible irradiation
gives rise to spatial distributions of trans-rich and cis-
rich regions. The film material starts to move from the trans-
rich smectic liquid-crystalline regions to cis-rich isotropic ones,
which is possibly initiated by the disparities of the viscosity
and sharp gradient of surface tension at the boundary
regions.7,18
In the above contexts, we propose herein
a newly
designed polymer material, a cinnamate-containing photo-
crosslinkable Az polymer. With this polymer, the [2 + 2]
photo-dimerization exerted by exposure to suitable UV light is
expected to immobilize the SRG inscription. The process
involves two steps: rapid SRG inscription via light exposure,
followed by on-demand post-fixation for persistent storage.
The latter step plays a role in enhancing both thermal stability
and insolubility of the polymer. Without fixation, the SRG
structure may be erased and regenerated many times. The
chemical structure of the polymer is indicated in Fig. 1, and
this polymer is abbreviated as p5Az10Me-PE4.5-Ci. Kimura
et al.19 formerly proposed a series of amorphous UV-curable
Az polymers containing
a cinnamate moiety, however,
An important requirement for SRGs is the shape stability in
terms of long-term storage and durability at higher
temperatures. The stability can be improved when one
the stabilization after UV irradiation was not effective.
They reported that heating above 200 1C, corresponding to
Tg, destroyed the SRG structure even for the UV-cured
film. Furthermore, their process required a high light dose
(4100 J cmꢀ2) to inscribe SRG structures. We report herein
a new system in which efficient mass migration occurs at
moderate irradiation conditions, and the photo-crosslinking
significantly improves stability towards heat and solvent
exposure.
employs anamorphous polymer with a high Tg (glass
transition temperature) or a liquid-crystalline polymer with
Department of Molecular Design and Engineering, Graduate School of
Engineering, Nagoya University, Chikusa, 464-8603 Nagoya, Japan.
E-mail: tseki@apchem.nagoya-u.ac.jp
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A mixture of 5Az10-OH (5 mmol) and triethylamine
(10 mmol) in dried tetrahydrofuran (THF) was added to a
solution of acryloyl chloride (10 mmol) in THF under
a nitrogen atmosphere, and was stirred at room temperature
for 5 h. The resulting raw product was washed in water several
times and dried over magnesium sulfate. After removing
the solvent, the solid residue was recrystallized twice from
methanol to give pure 4-(10-methacryloyloxydecyloxy)-40-
pentylazobenzene (5Az10Me) as a yellow powder in 87%
yield; mp 75–76 1C. 1H NMR (d[ppm], 270 MHz, CDCl3):
0.89 (3H, t, CH3–), 1.32–1.94 (25H, m, –CH2–, –CH3), 2.67
(2H, t, J = 8 Hz, –CH2Ar), 4.03 (2H, t, J = 7 Hz, –OCH2–),
4.14 (2H, t, J = 7 Hz, –CH2O–), 5.54, 6.09 (2H, s, CH2Q),
7.00 (2H, d, J = 9 Hz, Ar-H), 7.28 (2H, d, J = 9 Hz, Ar-H),
7.80 (2H, d, J = 8 Hz, Ar-H), 7.90 (2H, d, J = 9 Hz, Ar-H).
The azo-containing polymer p5Az10Me-PE4.5 was syn-
thesized by free radical polymerization in dry THF (5 ml) solution
under nitrogen, using AIBN (0.0392 mmol) as an initiator via
free radical polymerization of the 5Az10Me monomer
(0.245 mmol) and PE200 (0.49 mmol), another methacrylate
monomer containing an oligo (ethylene oxide unit) chain.
The reaction medium was heated at 70 1C for 6 h, cooled to
room temperature, and then poured in to a vigorously stirred
hexane for reprecipitation. The resulting polymer was
collected by centrifugation. The orange solid product was
dried in vacuum. The reaction yield was 70%.
Fig. 1 Chemical structure of photo-crosslinkable Az-containing
polymer, p5Az10Me-PE4.5-Ci, in this study.
Experimental
Materials
The methacrylate monomer possessing an oligo (EO) unit
(PE4.5) was kindly supplied by NOF Corp. All other reagents
were purchased commercially and used without further
purification. The detailed information on synthetic procedures
is described below.
Finally, using p5Az10Me-PE4.5 (0.223 mmol) and
cinnamoyl chloride (0.116 mmol) as reactants in dry THF
(4 ml) as solvent a polymer containing both azobenzene and
cinnamate as side chains was synthesized by stirring the
mixture at room temperature for 5 h. Reprecipitation was
carried out in a mixture solvent of 98.5% hexane and 1.5%
chloroform. After being dried in vacuum, the product was
observed as a sticky orange solid in a yield of 80%. This
product was abbreviated as p5Az10Me-PE4.5-Ci, and the
copolymerization ratio (m : n : l in Fig. 1) was determined
Synthesis
4-[(40-Pentyl)azo]phenol (5Az-OH) was synthesized by diazo-
coupling of 4-pentylaniline with phenol according to
a
conventional procedure. 4-Pentylaniline (30 mmol) was dissolved
in a mixture of concentrated hydrochloric acid and water. After
cooling, the aniline was diazotized by adding dropwise a solution
of NaNO2 (60 mmol) in water at 0–5 1C. Addition of the
diazotized solution to a solution of phenol (60 mmol), NaOH,
Na2CO3 in water led to the diazo-coupling reaction. After
neutralization and subsequent filtration, the precipitate was
recrystallized from hexane. Recrystallization gave pure 6AzOH
as a yellow powder in 87% yield; mp 72–73 1C. 1H NMR
(d[ppm], 270 MHz, CDCl3): 0.89 (3H, t, J = 6 Hz, –CH3),
1.32–1.68 (6H, m, –CH2–), 2.67 (2H, d, J = 8 Hz ArCH2–),
5.31 (1H, m, Ar–OH), 6.93 (2H, d, J = 9 Hz, Ar-H), 7.30 (2H, d,
J = 9 Hz, Ar-H), 7.79 (2H, d, J = 8 Hz, Ar-H), 7.86 (2H, d,
J = 9 Hz, Ar-H).
4-[(40-Pentyl)azo]phenol (5Az-OH) was converted to 4-(10-
hydroxydecyl)-40-pentylazobenzene (5Az10-OH) by a Williamson
esterification reaction. To a stirred dried acetone solution (20 ml)
of 5AzOH (30 mmol) were added powdered potassium carbonate
(8 mmol), potassium iodide (as a catalyst) and a dried acetone
solution of 10-bromodecanol (6 mmol). The mixture was stirred
at refluxing temperature for 20 h. After removing the precipitate,
the filtrate was concentrated to give a solid residue, which
was purified by recrystallization from hexane at least twice
1
by H NMR as described in the Results and discussion.
Measurements
1H NMR data were measured on a 400 MHz NMR A-400
(JEOL). Molecular mass data for the polymer was obtained by
gel permeation chromatography (GPC, Shodex Technology).
UV-vis absorption spectra were taken on an Agilent 8453
spectrophotometer (Agilent Technology). Polarized optical
microscopic (POM) observations were made using a BX51
(Olympus Technology). Film thickness was determined by
atomic force microscopy (Nanoscope 2100, Seiko Instru-
ments), after scratching the film with a micro-spatula. An
Hg–Xe lamp (Sanei-200S) was used for both SRG inscription
and photo-crosslinking. UV light at 350–380 nm was filtered
out for the pre-irradiation of films. To inscribe the SRG
structure, visible light at 430–500 nm was irradiated to the
film through a photo-mask, which was placed with the support
of a cover glass with a thickness of 0.15 mm as the spacer. For
photo-crosslinking, a combination of a D33S optical filter
(Toshiba Technology) and one glass slide were used to screen
out the light of wavelength 300–400 nm. The micro-
photographs of SRGs were taken by a microscope (Keyence,
1
to give a yellow powder in a yield of 91%; mp 81–82 1C. H
NMR (d[ppm], 270 MHz, CDCl3): 0.89 (3H, t, J = 7 Hz, CH3–),
1.21–1.83 (22H, m, –CH2–), 2.67 (2H, t, J = 8 Hz, CH2), 3.65
(2H, t, J = 6 Hz, –OCH2–), 4.04 (2H, t, J = 7 Hz, –OCH2–) 7.00
(2H, d, J = 9 Hz, Ar-H), 7.29 (2H, d, J = 9 Hz, Ar-H), 7.80
(2H, d, J = 9 Hz, Ar-H), 7.90 (2H, d, J = 9 Hz, Ar-H).
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1344 | New J. Chem., 2009, 33, 1343–1348

VHX-500) at different temperatures, 25, 55 and 62 1C. Differen-
tial scanning calorimetry (DSC) was undertaken with a DSC
6200 (Seiko Instruments Inc.). Unless stated otherwise, all
measurements were conducted at room temperature.
According to our former data of a similar series of co-
polymers, the liquid-crystalline phase below Ti corresponds
to a smectic A phase.17,18
Quartz substrates were formerly cleaned by sequential
ultrasonic treatments for 15 min immersing in the following
fluids, THF, saturated KOH–methanol, deionized water.
SRG inscription
The SRGs were formed on the surface of polymer thin films
prepared by spin-coating on a cleaned quartz substrate from a
2% (by weight) THF solution at 2000 rpm for 30 s. The films
were kept in the air for 30 min before use. The initial film
thickness was ca. 70 nm as evaluated by AFM. To inscribe the
SRG pattern, the films are first exposed to UV at 365 nm, and
then irradiated at 436 nm through a photo-mask having a
10 mm line-and-space pattern at 50 1C. This temperature was
selected just below Tg, which ensures the polymer film soft
enough to inscribe relief with a low energy dose, but yet rigid
enough to maintain the gratings once inscribed. In our earlier
work, the irradiation was performed at room temperature
since the former polymer had a lower Tg.15,17 Energy data
needed for the SRG formation, including the irradiation
power and irradiation time, were optimized by repeating the
irradiating processes at different conditions and comparing the
SRG structures. It was confirmed that optimized irradiation
conditions for the SRG formation were as follows; pre-
irradiation with 365 nm light: 10 mW cmꢀ2 exposure for 200 s
(2 J cmꢀ2) at room temperature, and subsequent patterned
irradiation at 436 nm: 5 mW cmꢀ2 for 200 s (1 J cmꢀ2) at
50 1C. Both energy doses are much lower than those required
for the UV-curable polymer previously reported.19 This
inscribed SRG pattern after these procedures observed by
AFM is shown in Fig. 3. The surface profile exhibited a
regular undulated shape with a grating depth of 130 nm and
a grating period of 20 mm. The top-to-top distance exactly
corresponded to the mask patterns. Thus, the highest regions
reached approximately twice the initial film thickness, indicat-
ing that full mass transfer occurred during the optimized
procedures.
Results and discussion
Characterizations of the ternary copolymer
We observed 1H NMR spectra for the determination of
the copolymerization ratio for both p5Az10Me-PE4.5 (before
introduction of cinnamoyl groups) and p5Az10Me-PE4.5-Ci
(after introduction of cinnamoyl groups). The integral
intensities of the two peaks at 6.50–6.55 ppm corresponding
to the proton of the cinnamate double bond were compared
with those at 7.8–7.9 ppm assignable to the benzene ring of the
azobenzene side chain. The side-chain content ratios was
found to be m = 0.38, n = 0.48 and l = 0.14 (see Fig. 1).
According to the GPC measurements, the final
product showed an weight-average molecular weight (Mw) of
1.51 ꢂ 104 and a number average molecular weight (Mn) of
1.18 ꢂ 104 (Mw/Mn = 1.45).
The thermophysical properties of this ternary copolymer
was evaluated by DSC at a heating rate of 10 1C minꢀ1 and
a polarized optical microscope (POM). The results are shown
in Fig. 2. A small dip in the curve at 59 1C and an endothermic
peak at 66 1C are observed in the DSC profile in the heating
process. A POM image at 63 1C clearly exhibited a bi-
refringence, indicating the liquid-crystalline phase. On the
other hand, the birefringence disappeared at the endothermal
transition, and the POM image became fully dark at 73 1C.
The above data indicate that the two characteristic transitions
at 59 and 66 1C correspond to Tg and Ti, respectively.
Changes in the UV absorption spectrum during the above
process are indicated in Fig. 4. The initial film in the trans-Az
state (solid line) showed the p–p* absorption maximum at
347 nm. The absorption band around 280 nm is ascribed to the
p–p* absorption of the cinnamoyl unit. By irradiation with
Fig. 2 Thermal analysis of polymer p5Az10Me-PE4.5-Ci as measured
by DSC and POM (500 ꢂ 900 mm).
Fig. 3 AFM image of inscribed relief gratings on a p5Az10Me-
PE4.5-Ci film surface.
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[2 + 2] Photo-dimerization (photo-crosslinking)
Next, the exposure to UV light to induce the [2 + 2] photo-
dimerization of cinnamoyl groups in the p5Az10Me-PE4.5-Ci
film was performed. At room temperature, an SRG inscribed
film was irradiated with UV light at a wavelength in the range
300–400 nm changing the time from 1 to 40 min at room
temperature (Fig. 6). As indicated, during the irradiation,
the absorption band of the cinnamoyl group at 280 nm
was gradually reduced; this should be due to the [2 + 2]
cycloaddition photo-dimerization reaction leading to the
formation of a cyclobutane ring.20–22 A sudden upward
shift in the spectrum particularly in the UV region
was observed at an early stage (within 1 min) of the photo-
crosslinking which can be ascribed to light scattering due to an
evolution of inhomogeneous domain formation in the
polymer film.
Fig. 4 UV-Vis absorption of a sample film during the SRG inscrip-
tion process. The solid line is for the original film, the dashed line is for
the film irradiated with 365 nm UV and the dotted line is for the film
subsequently exposed to 436 nm light.
The efficiency of crosslinking was confirmed by dissolving
behavior in THF,
a good dissolving solvent for the
original p5Az10Me-PE4.5-Ci. Fig. 7 shows the absorption
spectra obtained after subsequently rinsing in THF
films which were exposed to UV light at various periods.
These data indicate that UV exposure for 15 min leads to
insolubility for THF. Essentially the same results were
obtained when chloroform was employed as the rinsing
solvent.
365 nm light the Az unit was efficiently isomerized to the cis
form (dashed line). After subsequent irradiation with 436 nm
light, the Az unit was mostly reverted to the trans form (dotted
line), but in this case the band peak of the p–p* absorption was
shifted to 361 nm, indicating the disruption of partial H
aggregation in the initial film.
The microstructures of SRGs under different irradiation
energies for the crosslinking were further evaluated by
AFM. By comparing them, it was found that for the photo-
crosslinking, the best irradiation intensity and time were found
to be 50 mW cmꢀ2 at 313 nm and 15 min, respectively.
The grating pattern picture taken after photo-crosslinking at
the above conditions showed almost no change at any scale.
The grating depth was also unchanged at 130 nm, which
remained the same level as observed before the photo-
crosslinking. This irradiation also leads to the trans-to-cis
isomerization of the Az unit, however, this did not influence
the SRG structure since the irradiation was performed at room
temperature, namely below Tg.
Thermal stability of SRG before crosslinking
During heating the SRG inscribed film from 25 to 70 1C at
2 1C minꢀ1, optical microscope observations were made, as
shown in Fig. 5. Upon gradual heating, the SRG pattern
started to collapse from 55 1C, and almost disappeared
at 62 1C. This result was in accord with the data of the thermal
analysis taken by DSC and POM (Fig. 2). Thus, the SRG
inscription was readily erased by heating above Ti
of p5Az10Me-PE4.5-Ci. This observation also coincides
with our previous results obtained without the cinnamoyl
group.5
Fig. 5 Optical microscope images taken while heating an SRG
inscribed film from 25 to 70 1C at a rate of 2 1C minꢀ1. The top-right
image displays a magnified region for the box in the top-left image.
Fig. 6 UV-Vis absorption spectra of the copolymer film irradiated
with UV light at wavelength 300–400 nm at room temperature while
changing the irradiation time from 1 to 40 min.
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1346 | New J. Chem., 2009, 33, 1343–1348

system even though the content of the crosslinkable unit is less
(14 mol%). This fact suggests more efficient photo-
dimerization in the present p5Az10Me-PE4.5-Ci film. It seems
likely that the hydrophobic cinnamoyl groups are efficiently
aggregated in the nano-segregated layer of hydrophilic
oligo(ethylene oxide) side-chain layer regions.
Conclusions
A photo-crosslinkable ternary Az copolymer was newly
developed, in which a cinnamoyl group was introduced in
the liquid-crystalline Az side-chain polymer. The polymer
showed a favorable property of liquid-crystalline nature to
exert the mass transport. After pre-irradiation of 365 nm UV
light followed by patterned visible light at 436 nm at 50 1C, the
SRG structure was readily formed with a grating depth of
130 nm, which corresponds to nearly twice the level of the
initial thickness. This SRG pattern was able to be erased when
heated above the isotropization temperature. Without
crosslinking, the film was readily dissolved in chloroform
or THF. After subsequent photo-crosslinking at room
temperature, the SRG patterns remained essentially un-
changed after rinsing with the above solvents. The thermal
stability was improved drastically, i.e., the SRG structure was
not influenced after heating to 300 1C for 5 min. We expect
that this new type of photo-crosslinkable polymer will provide
wider opportunities for the application of SRG structures.
Fig. 7 UV-Vis absorption spectra of five series of films subjected to
photo-crosslinking for 1, 5, 10, 13 and 15 min and subsequently rinsed
in THF.
Thermal stability of the SRG after crosslinking
Fig. 8 displays the AFM image of the photo-crosslinked
SRG structure after the treatment in THF and chloroform,
and subsequently heated at 300 1C for 5 min. This SRG
pattern showed no significant change compared with that of
the as-prepared SRG film as shown in Fig. 3. In sharp
contrast, the SRG inscribed film without photo-crosslinking
could retain its pattern only up to 55 1C which is just below the
Ti. After photo-crosslinking, on the other hand, the SRG
pattern was still maintained at 300 1C without any reduction in
its grating depth. Thus the high temperature stability
was substantially improved as well as the resistance to solvent
exposure.
Acknowledgements
We thank Shun Mitsui of our laboratory for his technical
assistance. This work was supported by Grant-In-Aid in
Priority Areas (New Frontiers in Photochromism, No. 471)
of MEXT, Japan.
Kimura et al.19 reported that a poly(phenylmaleimide)-
based random copolymer containing Az and cinnamate
side-chain groups (cinnamate content of 18 mol%) showed
only little stability improvement. The SRG structure scarcely
endured above Tg around 200 1C. It is worth noting here that
significant stability improvement was found in the present
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