Phototriggered mass migration consorted with surface dewetting in thin films of a liquid crystalline azobenzene-containing dendrimer

Article abstract of DOI:10.1021/ma301170x

A dendritic molecule of poly(propylene imine) whose periphery is modified with photoresponsive mesogenic azobenzene units was synthesized. This compound exhibited highly ordered smectic B or smectic A layer structure orienting parallel to the substrate in

Full text of DOI:10.1021/ma301170x

Article
pubs.acs.org/Macromolecules
Phototriggered Mass Migration Consorted with Surface Dewetting in
Thin Films of a Liquid Crystalline Azobenzene-containing Dendrimer
Wenhan Li,† Tomoki Dohi,† Mitsuo Hara,† Shusaku Nagano,‡,§ Osamu Haba,⊥ Koichiro Yonetake,⊥
and Takahiro Seki*,†
†Department of Molecular Design and Engineering, Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa, Nagoya
464-8603, Japan
‡Venture Business Laboratory, Nagoya University, Nagoya 464-8603, Japan
§Japan Science and Technology Agency-PRESTO, 5-3, Yonbancho, Chiyoda-ku, Tokyo 102-8666 Japan
⊥Department of Organic Device Engineering, Graduate School of Science and Engineering, Yamagata University, 4-3-16 Jonan,
Yonezawa 992-8510, Japan
ABSTRACT: A dendritic molecule of poly(propylene imine) whose periphery is modified with photoresponsive mesogenic
azobenzene units was synthesized. This compound exhibited highly ordered smectic B or smectic A layer structure orienting
parallel to the substrate in the film state. When this film of 90 nm thickness on a hydrophilic substrate was exposed to UV (365
nm) light, the initial flat film morphology started to form holes followed by a drastic transformation to form separated dome
structure of micrometer levels. Upon continuous UV light irradiation, the domain height increased and reached to a level of ca. 8-
fold higher than that of the initial thickness, which can be correlated with the disordering of layer structures as revealed by
grazing angle incidence X-ray diffraction measurements. This phototriggered dewetting behavior depended on the film thickness.
The thinner film resulted in local migrations to form the smaller and more homogeneous domes. The dewetting was observed
below a threshold thickness of 100 nm. By UV light irradiation through a photomask, phototriggered migration at micrometer
distances also occurred. By the combination of phototriggered migration and concerted dewetting, characteristic hierarchical
morphologies were formed, depending on the initial film thickness. This work proposes a new possibility to control and design
the dewetting processes of thin films using the dendritic material.
introduced to interior6−11 or exterior9,12−21 of the dendritic
1. INTRODUCTION
skeleton. For LC studies, the exterior type (peripherally
Various types of molecular architectures are available in liquid
crystalline (LC) polymers.1,2 Among them LC dendrimers have
modified) dendrimers are mostly investigated.
On the other hand, since the discovery in 1995,22,23 the
been attracted great attention to elucidate effects of the highly
phenomenon of photoinduced mass migration in Az-containing
polymer films, leading to the surface relief formation, has been a
and regularly branched chain topology on the aggregation states
and LC nature.1−4 Due to their well-defined chemical
architecture, introduction of photoresponsive and photo-
reactive units have also been a subject of great interest.2,5
Dendrimers possessing photochromic azobenzene (Az) unit(s)
have already been extensively explored. The Az unit can be
Received: June 8, 2012
Revised: July 23, 2012
© XXXX American Chemical Society
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major stream of research for both chemists and physicists. At
the early stage of research, the systems are mostly limited to
amorphous Az-containing polymer films,22−26 however, recent
activities have been rapidly accumulating new knowledge,
expanding to other types of materials including amorphous low-
molecular-mass compounds27,28 and the surface of molecular
crystals.29
Scheme 1. Chemical Structure of Materials
In regard to photoinduced mass migration of dendiritic
materials, several investigations have been reported for various
types of branching architecture. Archut et al.14 first reported the
surface grating formation of thin films of dendritic molecules
containing Az units in the periphery by holographic irradiation
with laser beam. They found that the dendrimer generation and
attached position of Az influence the photoinduced migration.
Gharagozloo-Hubmann et al.30 discussed the effect of main
chain branching (linear or star-branched polymers of different
number of arms) on the surface relief grating formation. In this
case, little effect was admitted for the extent of main chain
branching. Galgano et al.31 explored a series of Az-containing
polyester possessing dendrons in the side chain. The generation
of the dendrons affected the mass migration behavior. As the
relating process, photoinduced optical anisotropy (photo-
orientation of Az) by irradiation with linearly polarized light
was also investigated for Az-containing dendrimers by
Bobrovsky et al.,20,21 and block copolymers containing a
dendritic Az block by del Barrio et al.32
Our work has been focusing onto LC Az-polymer materials
that show highly sensitive phototriggered mass migrating
systems.33−38 The mass migration is completed at uncommonly
small dose levels (<100 mJ cm−2), which is 3 orders of
magnitude smaller than those required for conventional
amorphous polymer films. Here the Az unit providing a long
lifetime of cis-isomer is employed, and thus the photo-
irradiation with UV (365 nm) light leads to the smectic LC
to isotropic phase transition.35−37 The mass transport in the LC
polymers occurs at boundary areas between the smectic LC and
isotropic phases. Probably, the disparity in the viscosity and
surface tension at the boundaries promote the mass transfer.
Basically such migrating motions are explained as a thermally
driven self-assembly process of the LC materials.
To date, such phototriggered mass migrations of LC
materials have been achieved only for linear polymers.33−37
We expected that the dendritic scaffold of constituting material
in the thin film would exhibit some characteristic effects in
phototriggered migrating behavior, because it has been pointed
out that the dendritic topology eliminates chain entanglement
and interlacing with the neighboring molecules.39,40 Yonetake
et al.41 synthesized and evaluated the LC properties of
poly(propylene imine) dendrimers peripherally modified with
cyanobiphenyl mesogens. The Az dendrimer employed in the
present study adopts the identical scaffold of this dendrimer
with the second generation (G2) (16 mesogens). The chemical
structure of this molecule is indicated in Scheme 1 (AzD6). In
thin films of this dendritic molecule, we found the UV (365
nm) light irradiation induces both dewetting42 and mass
migration on a hydrophilic substrate under proper conditions.
We report herein a new aspect of phototriggered mass
transport consorted with photoinduced surface dewetting to
form hierarchical surface morphologies by single patterned
irradiation. With regard to photoinduced surface dewetting,
Cristofolini et al.43 reported a thickness dependent photo-
induced dewetting on a hydrophobic surface using linear Az-
containing polyacrylate Langmuir−Blodgett films. This work
proposes a new possibility to engineer morphologies in thin
films through the phototriggered mass migration and surface
dewetting by employing the dendritic scaffold.
2. EXPERIMENTAL SECTION
2.1. Materials. Poly(propylene imine) dendrimer (G2) was
purchased from Aldrich and used as received. Other reagents and
solvents were obtained commercially. Tetrahydrofuran (THF) was
distilled from sodium-benzophenone ketyl just before use.
4-(4′-Hexylphenylazo)phenol. To a solution of hexylaniline (7.3
mL, 37 mmol) and concentrated hydrochloric acid (20 mdm3),
aqueous solution (25 mdm3) and NaNO2 (4.2 g, 62 mmol) was added
to the resulting a blue solution. Aqueous solution (25 mL) and NaOH
(3.0 g, 21 mmol) and phenol (8.2 g, 37 mmol) were then added. Into
this solution, concentrated hydrogen chloride was added dropwise
until the solution became acidic. After stirring for 3 h at 10 °C, the
resulting precipitate was filtered, washed with water, and dissolved in
diethyl ether (100 m dm3). The etheral solution was washed with
water, dried over anhydrous MgSO4, and concentrated under reduced
pressure. The residue was recrystallized from hexane to give a brown
powder. Yield: 4.6 g (16.4 mol, 44.2%). 1H NMR (CDCl3, 270 MHz):
δ (ppm) = 6.63−7.88 (s, 8H, ArH), 5.0 (1H, OH), 2.55 (t, 2H,
PhCH2), 1.29−1.62 (m, 8H, CH2), 0.96 (t, 3H, AlkylCH3). IR (KBr):
ν (cm−1) = 1465 (Ph), 1589 (Ph), 2854 (CH2), 2954 (CH2), 3409
(OH).
6-[4-(4-Hexylphenylazo)phenoxy]hexanol. A solution containing
4-(4′-hexylphenylazo)phenol (5.5 g, 20 mmol), 6-bromo-1-hexanol
(3.1 mdm3, 20 mmol), and KOH (1.1 g, 20 mmol) in ethanol (30
mdm3) was stirred under reflux for 12 h under nitrogen. The solvent
was evaporated, and the residue extracted with diethyl ether. The
organic layer was washed with water, dried over anhydrous MgSO4
and concentrated. The resulting solid was recrystallized from hexane to
1
give a glossy yellow crystal. Yield: 4.32 g (11 mmol, 57.9%) H NMR
(CDCl3, 270 MHz): δ (ppm) = 0.96 (t, 3H, alkyl CH3), 1.29- 1.71
(m, 16H, CH2), 2.55 (t, 2H, PhCH2), 3.65 (2H, CH2OH), 4.0 (2H,
OCH2), 6.97−7.88 (s, 8H, ArH). IR (KBr): ν (cm−1) = 1249 (Ph−
O−), 1465 (Ph), 1604 (Ph), 2854 (CH2), 2931 (CH2), 3262 (OH),
3332 (OH).
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Figure 1. Characterizations of AzD6 and its film: (a) DSC profiles, (b) POM image at room temperature, (c) X-ray diffraction patterns for a bulk
sample at various temperatures, (d) GI-XRD 2D pattern taken on a imaging plate, and (e) molecular conformation model of AzD6 in a thin film.
The picture is drawn as a fully extended state.
4-(6-Acryloxyhexyloxy)-4′-hexylazobenzene. To a solution con-
taining 6-[4-(4-hexylphenylazo)-phenoxy]-hexanol (5.4 g, 14 mmol)
and triethylamine in 2-butanone, a solution of acryloyl chloride (1.2
mdm3, 14 mmol) in 2-butanone (20 mdm3) was added dropwise at 0
°C. After stirring for 1 h at room temperature, the reaction mixture
was concentrated and extracted with diethyl ether. The organic layer
was washed with water, dried over anhydrous MgSO4 and
concentrated under reduced pressure. The residue was recrystallized
2.2. Methods. Measurements. Optical textures of the samples
were examined using the polarizing optical microscope (POM)
equipped with a hot stage (Linkam Co. TH-600RMS) under nitrogen.
Transmission X-ray diffraction (XRD) experiments were carried out by
a MiroMax007 diffractometer (Rigaku Denki Co. Ltd.) operated at 40
kV and 20 mA. Cu Kα X-ray beams monochromatized with a confocal
mirror were shone onto the specimen through a pinhole collimator of
0.2 mm in diameter. The imaging plate R-AXIS IV+2 system was
utilized as an X-ray detector at a camera length of 150 mm. Grazing
incidence angle X-ray diffraction (GI-XRD) measurements were
operated with a FR-E X-ray diffractometer (Rigaku Corp.) detected
by a R-AXIS IV two-dimensional detector using the imaging plate
(Rigaku Corp.). The incident angles of X-ray beam to the films were
set at 0.18−0.22° by using pulse controllers ATS-C310-EM and ALV-
300-HM (Chuo Precession Industrial). 0.3-mm collimated Cu Kα
radiation was used as X-ray beam and camera length was set at 300
mm. Differential scanning calorimeter (DSC) was carried out on a
DSC Q20 MO−DSC-UV under helium purging gas (0.11 MPa) at a
heating/cooling rate of 2 °C min−1. The morphology of all films was
observed by an atomic force microscope (Nanopics 2100, Seiko
instruments) using the damping mode. Original film thickness was also
obtained using the same AFM instrument by creating and measuring a
scratch on the film. UV−vis absorption spectra were taken on an
Agilent 8453 diode array spectrometer.
Film Preparation. Quartz substrates were first immersed in a
KOH−methanol mixture for several days, washed with deionized
water several times and finally cleaned by ultrasonic treatment for 15
min while immersing in deionized water. The contact angle for water
was below 10°. Films were formed on the hydrophilic quartz substrates
from chloroform solutions of AzD6 by spin-coating method. The
spinning speed was 3000 rpm. All procedures were carried out at room
temperature. The initial thickness was obtained under AFM by
scratching method for all samples. All films were prepared on
hydrophilic substrates. Freshly prepared films are subjected to light
irradiation experiments.
1
from hexane to give a yellow powder. Yield: 4.0 g (9.2 mol, 65%). H
NMR(CDCl3, 270 MHz): δ (ppm) = 0.96 (t, 3H, alkyl CH3), 1.29−
1.71 (m, 16H, CH2), 2.55 (t, 2H, PhCH2), 4.15 (t, 2H, COOCH2),
3.94 (t, 2H, PhOCH2), 5.83 (dd, 1H, CH), 6.13(dd, 1H, CH2
CH−), 6.41 (dd, 1H, CH), 6.67−7.88 (s, 8H, ArH). IR (KBr):
ν(cm−1) = 840 (CC), 1010 (C−O−C) 1211 (Ph−O−), 1465 (Ph),
1604 (Ph), 1720 (CO), 2861 (CH2), 2931 (CH2). Anal. Calcd for
C27H36N2O3: C, 74.28; H, 8.31; N, 6.42. Found: C, 74.48; H, 8.62; N,
6.35.
Az Dendrimer (AzD6). Synthesis of the Az dendrimer was
undertaken in a similar manner as described previously.41 Into a
solution of poly(propylenimine) dendrimer (G2) (0.039 g, 0.51
mmol) in THF (5 mdm3), a solution of 4-(6-acryloxyhexyloxy)-4′-
hexylazobenzene (4.9 g, 11.2 mmol) in THF (5.0 mdm3) was added.
The resulting solution was stirred at 45 °C for 2 week under nitrogen
and then poured into 500 mdm3 of hexane. The precipitate was
collected by filtration, and the obtained powder was purified by
reprecipitation by pouring THF solution into hexane. Yield 3.90 g (0.
1
50 mol, 98.8%) H NMR (CDCl3, 270 MHz): δ (ppm) = 0.96 (t,
48H, alkyl CH3), 1.29−1.71 (m, 304H, CH2), 2.35 (t, 32H, CH2C
O), 2.36 (t, 52H, CH2−N), 2.55 (t, 32H, CH2Ph), 2.75 (m, 32H,
CH2CH2CO), 3.94 (t, 32H, PhCH2O), 4.08 (t, 32H, COOCH2),
6.97−7.88 (s, 128H, ArH). IR (KBr): ν(cm−1) = 840 (CC), 1010
(C−O−C) 1249 (Ph−O−), 1465 (Ph), 1604 (Ph), 1735 (CO),
2854 (CH2), 2931 (CH2). Anal, Calcd. for C472H672N46O48: C, 73.07;
H, 8.73; N, 8.30. Found: C, 71.54; H, 9.50; N, 8.47.
Poly[4-(6-methacryloxyhexyloxy)-4′-pentylazobenzene]
(PAzMA). 4-(6-Methacryloxyhexyloxy)-4′-pentylazobenzene and its
polymerized material (Mn = 7400) was synthesized by atom transfer
radical polymerization in a similar manner as described previously.44,45
Light Irradiation. Light irradiation was performed with a Xe−Hg
UV lamp, (Sanei-200S) made by San-ei Electric through a
combination of optical filters to select 365 nm line at room
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temperature. Unless stated otherwise, nonpolarized light was used.
Incident light intensity was adjusted to 5 mW cm−2. The photomask
used for SRG formation of all the samples was a stipe patterned photo
mask with a line and space pattern of 10 μm width.
3. RESULTS AND DISCUSSION
3.1. Structural Characterizations of AzD6. Figure 1a
depicts DSC profiles of AzD6 on the heating and cooling
processes. A glass transition temperature was observed at −5
°C in the heating scan and additional two endothermal peak
was observed at 34 °C (ΔH = 0.26 J g−1) and 85 °C (ΔH =
12.0 J g−1). Tg was observed at nearly the same temperature as
for a dendrimer possessing cyanobiphenyl mesogens in the
periphery.41 Below 85 °C, a POM observation exhibited a
birefringent character of smectic liquid crystalline phase as
indicated by a focal conic fan texture (Figure 1b). Above 85 °C,
this compound became isotropic to give a dark field in the
POM observation. In the cooling process, the peaks were
observed at slightly lowered temperatures by ca. 1 °C. In this
way, the thermal transitions showed only a very small hysteresis
effect.
X-ray diffraction 2θ profiles obtained with transmission beam
along the radial direction at various temperatures were
displayed in Figure 1c. In the small angle region, a sharp
diffraction peak at 2θ = 2.60° (layer spacing, d = 3.4 nm) was
observed up to 80 °C, showing the existence of a smectic layer
structure. This intense peak corresponds to the second order
diffraction (n = 2) because a less intense peak assignable to n =
1 (d = 6.7 nm) was observed in the smaller angle region. This
layer spacing of 6.7 nm is comparable to the length when each
half of the dendrimer is stretched as shown in Figure 1e (7.8
nm). The smaller value of the actual layer spacing should show
the involvement of gauche conformations with some
interdigitation between the terminal substituent of hexyl
group of aggregated Az units. Thus, the POM observation
and X-ray analysis imply the formation of a smectic B or A
phase. The diffraction peak in the wide angle region below 30
°C became sharper, indicative of formation of a more ordered
packing structure, most likely the smectic B phase.46 XRD
measurements in the wider angle region showed that the
mesogen-to-mesogen distance is 0.43 nm, which well coincides
with that for the homologous dendrimer with cyanobiphenyl
mesogens (0.45 nm).41
GI-XRD was performed for the film (film thickness = 200
nm) to elucidate the orientation of smectic layer structure in
the trans-Az form (d). Clear diffraction spots attributed to the
half length of smectic layer (d/2 = 3.5 nm) was observed only
in the out-of-plane direction in the imaging plate, showing the
formation of highly ordered multilayered layer structure
orienting parallel to the substrate surface. The existence of
duplicated spots is due to a superposition of diffracted beams
on the film and substrate surfaces. The layer spacing was
slightly increased in the thin film state, which suggests that a
more stretched conformation is adopted in the thin film state.
Figure 2 shows UV−visible absorption spectra of AzD6 in
chloroform (dash-dotted line), and of a spincast film with 30
nm thickness before (solid line) and after (dash line) UV
exposure. The absorption peak of the π−π* long-axis transition
of Az in chloroform was positioned at 360 nm, while that of the
spincast film exhibited a large hyprochromic shift to 320 nm.
This fact indicates that the Az units form a strong side-by-side
aggregation in the spincast thin film. The π−π* long-axis
transition of Az around 340 nm was considerably small
Figure 2. UV−visible absorption spectra of AzD6 taken in chloroform
solution (dashed line), as-cast film of 30 nm thickness (dotted line)
and after UV irradiation (solid line).
compared to that of the π−π* transition of phenyl (244 nm)
band. The ratio of Aπ−π*(Az)/Aπ−π*(phenyl) in the spin-cast
film was ca. 0.4. This spectral feature indicates that the trans-Az
units are oriented mostly perpendicular to their substrate plane
as observed for linear polymer systems.35,37,47 After UV
irradiation to the film, the n−π* transition around 450 nm
band became distinct due to the photoisomerization to the cis-
form. The absorbance of π−π* band also became larger, which
is ascribed to the orientational randomization of Az. After UV
irradiation, the film became slightly opaque, which is reflected
by an uprise in the baseline particularly at shorter wavelengths.
The increased light scattering is ascribed to a surface dewetting
of the film as mentioned in more detail later.
In all aspects of structural characterizations, the AzD6 thin
film adopts highly ordered layer structure orienting parallel to
the substrate surface, Az mesogens directing normal to the
surface. Such packing and orientation feature is very similar to
that observed for relating linear polymers hitherto inves-
tigated.34−37
3.2. Photoinduced Dewetting. On the basis of the
structural feature of AzD6 film, highly sensitive mass migrations
upon patterned light irradiation were anticipated to take
place.34−38 The massive migration actually occurred, however
we noticed that unexpected surface morphologies were
obtained consequently. The resulting surface morphologies
did not reflect simply the photomask patterns, but finer sub
structures within the photopatterns were embedded. Therefore,
before performing photopatterning experiments, photoirradia-
tion with uniform 365 nm light without a photomask was
conducted.
Figure 3 shows optical microscopic observations of film
morphology of thin films (initial thickness = ca. 20 nm) of
AzD6 (molecular mass = 7753) (b) and a linear polymer of
almost identical molecular mass (PAzMA, Mn = ca. 7400) (a)
on a hydrophilic surface after irradiation with UV light at 500
mJ cm−2. The film of the linear polymer exhibited no
morphological change (a). By contrast, for the dendritic
compound of AzD6, the film showed marked morphological
change (b). Together with the observed morphology by AFM
(c), the morphological change can be recognized as the surface
dewetting providing a number of separated domains of a few
micrometers. We assume that this marked difference arise from
the chain topology. It is likely that the reduced interchain
interactions for AzD6 effectively lower the film viscosity. The
thermophysical properties should be also taken into account.
Unfortunately, we were not able to correctly determine the Tg
of PAzMA due to a lack of enough accuracy in the DSC
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cm−2) started to exhibit dewetting at 150 mJ cm−2. At this
stage, holes of 3−5 μm diameter with the film depth were
formed. At 175 mJ cm−2 irradiation, the holes grew to ca. 10
μm diameter and started to coalesce with each other. At the
same time the film became highly corrugated. Around 200 mJ
cm−2, the continuous film became separated and the
morphology was characterized by formation of separated
dome structures (see AFM image of 225 mJ cm−2). The
domes possessed round bottoms of diameters of several
micrometers and this basal bottom did not virtually changed
in shape upon further irradiation. Interestingly, the prolonged
irradiation led to significant growth of dome height. When UV
dose reached 400 mJ cm−2, the average height of domes
reached 770 nm, which corresponds, to our surprise, to ca. 8-
fold of the initial thickness. At this stage, the height growth
ceased and the morphology and the height of domes remained
unchanged even at further energy dose up to 600 mJ cm−2. The
shape change of the domes displayed a characteristic tendency.
At earlier stages, most of the domes possessed a flat top (see
AFM image at 225 mJ cm−2). Along with UV irradiation, the
flat tops were mostly lost and characterized by a smooth
droplet structure (see AFM image at 400 mJ cm−2). The flat
top should reflect that the domes are constituted with smectic
layers orienting parallel to the substrate. In Figure 5, the
average height difference in the AFM profiles is plotted against
the light energy dose. As obviously indicated, the profile
exhibited a sigmoidal shape, suggesting that the height growth
proceeds in a highly cooperative manner. The starting point of
the height growth around 170−180 mJ cm−2 corresponds to
the change in the morphological feature from the initial hole
formation to the successive dome growth. The extent of surface
dewetting is time dependent in general. Thus the data shown in
Figures 4 and 5 are expected to be changed by the time
required for the experimental procedures. However, we
obtained reproducible results as far as the irradiation time is
adjusted. This fact means that the pure motions in the dark are
negligible at room temperature in the time scale of
experimental procedures within a few hours, and that plotting
versus irradiation dose in Figure 5 instead of irradiation time
makes sense. It is further deduced that the dewetting motions
are effectively driven only by light. The molecular motion of
repeated back and forth isomerization of Az during the UV
irradiation or local molecular heating via radiationless relaxation
could activate the motions.
Figure 3. Optical microscopic images of films (initial thickness =20
nm) after irradiation with UV light of PAzMA (a) and AzD6 (b), and
topographical AFM image of AzD6 (c).
The smectic layer structure of an AzD6 film with 200 nm
thickness at each stage of UV irradiation was evaluated by GI-
XRD measurements. Figure 6a shows changes in the 2D profile
taken by the imaging plate with increase in the UV irradiation
dose. Initially a strong diffraction at 2θ = 2.6° (d/2 = 3.5 nm)
was observed in the out-of-plane direction. The intensity of this
diffraction peak reduced along with UV irradiation without
changing the diffraction spot position, indicating that disorder-
ing of the layer structure proceeded with retention of the
spacing length. The diffraction intensity at the peak was
monitored and plotted in Figure 6b. The peak intensity
immediately reduced after the initiation of UV irradiation up to
ca. 150 mJ cm−2. Beyond this stage, the peak intensity
decreased gradually in the range 200−800 mJ cm−2. At 800 mJ
cm−2, the peak intensity became considerably weakened but not
fully diminished.
measurement. However, the comparisons of the two materials
should make sense because they possess almost identical side
Az mesogenic groups with essentially the same molecular mass.
The photoinduced dewetting behavior was examined for a
AzD6 film (thickness = 50 nm) both on a clean hydrophilic
quartz surface and a hydrophobic quartz surface modified by
exposure to a vapor of 1,1,1,3,3,3-hexamethyldisilazane for 3
days (contact angle of water = 92°). With the hydrophobic
surface, this dendrimer film exhibited no dewetting behavior
upon UV light irradiation. Thus, the present work will be
focused on the explorations with the hydrophilic surfaces.
Figure 4 shows the topographical AFM images (100 μm ×
100 μm) observed in the photoinduced dewetting process of an
AzD6 films with initial thickness of 90 nm. Without UV
irradiation, the film was highly flat and did not show any
morphological change at least for a month at room temper-
ature. Upon UV irradiation, the film exhibited drastic
morphological changes. The initial highly flat state (0 mJ
These GI-XRD data can be correlated with the photo-
dinduced dewetting and dome formation processes shown in
Figures 4 and 5. At the earlier stage less than ca. 200 mJ cm−2
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Figure 4. Topographical AFM images of AzD6 film (initial thickness =90 nm) after irradiation with UV light at different periods (intensity 5 mW
cm−2). The irradiated energy doses are indicated on the top in the each figure.
irradiation, the dewetting behavior can be characterized by
formation of holes and their growth in the 2D plane directions.
After this threshold amount of irradiation, the film is separated
to form domes and successive 3D growth (height increase)
proceeds. Judging from the GI-XRD data, the dome formation
becomes predominant after considerable disordering of the
layer structure at ca. 200 mJ cm−2 (corresponding to one tenth
of the initial peak intensity in the profile of Figure 6) is
involved. Most probably, the viscosity is highly lowered around
this threshold stage. A loose layer structure is still involved at
later stages, as shown by the retention of the X-ray diffraction
spot. We assume that the tendency to form piled-up layer
structure is responsible for the height growth. As will be stated
later, the film of 200 nm subjected to GI-XRD measurements
did not exhibit dewetting. This thicker film was used for a
requirement to obtain higher quality data. Nevertheless, we
consider that the comparisons of the morphology feature with
the X-ray data is meaningful because sufficient photon doses are
provided to induce the trans-to-cis photoisomerization of Az
for both films of 90 and 200 nm thicknesses used for AFM and
X-ray measurements, respectively.
Irradiation with linearly polarized UV light was further
attempted to perform angular selective excitation in an
expectation to induce an anisotropic dewetting. In this
procedure, however, the anisotropic irradiation was not
reflected in the shape of the dome structure. During the UV-
induced dewetting, the irradiated film may become sufficiently
fluid and did not exhibit a directional photofluidization.48
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amphiphilic Az-cotaining polymers.52,53 However, precise
understanding for the origin of protrusions requires further
investigation. In any event, the difference in morphological
features observed for 90 and 100 nm films was drastic. In
occasional cases, a separated dewetted part depicted as
“occasional image” were observed, which were characterized
by a large (>10 μm diameter) dewetted dent with a film portion
left in the center. In the films thicker than 110 nm, no dewetted
areas were admitted. Thus, the thickness of 100 nm was the
threshold between the stable wetting and photoinduced
dewetting.
Such thickness dependent photoinduced dewetting is
qualitatively in good agreement with the observations by
Cristofolini et al.43 using Langmuir−Blodgett multilayers on a
hydrophobic surface with varied layer numbers. They reported
that a 4 monolayered film (8−9 nm thickness) provided fine
local dewetted structures resulting from submicrometer
migrations, whereas a 24 monolayered (ca. 50 nm thickness)
one showed a macroscopic dewettng transferring some tens of
micrometer distances. The difference has been discussed by the
dimensionality on the dewetting process. Dewetting behavior of
the thicker film is based on the three-dimensional spinodal
decomposition,49 and in contrast, the thinner film cannot
undergo such process and thus two-dimensional aggregation
proceeds to form local particles. However, there remain some
ambiguities on the wetting behavior. They observed photo-
dinduced dewetting for linear polymers with Mn = 12000
despite that we did not observed photoinduced dewetting for
our linear polymer of PAzMA (Mn = 7400). We assume that a
difference in the liquid crystal phase at room temperature leads
to this discrepancy. The linear polymer employed by
Cristofolini et al. adopt a nematic phase, while our linear
polymer shows a smectic phase. The viscosity should differ
significantly between them and this may critically influence the
migrating flow motions. Furthermore, the surface energies of
the substrate are different. They employed a hydrophobic
surface in the observation of the dewetting, whereas we used
hydrophilic surfaces. Further study is required for precise
understandings.
3.3. Mass migration with Patterned Irradiation. As the
next step, a series of AzD6 films with varied thicknesses were
prepared and exposed to patterned UV light using a photomask
instead of uniform irradiation at room temperature. The UV
irradiation was performed at 5 mW cm−2 equally for 50 s for
each film. Figure 8 shows the topographical AFM images after
the patterned irradiation (line and space pattern with 10 μm
width in typical cases) under the conditions stated above.
As shown in Figure 8, the morphologies after UV irradiation
exhibited various aspects depending on the film thickness. In
thinner films, hierarchical structures in which fine domes are
embedded in the mass transport patterns of the photomask
were observed upon a simple UV irradiation. When the film is
initially a thin film of 15 nm thickness, the irradiated regions
indicated hierarchical fine dewetted domes with the largest
number. The average diameter of the dewetting domains was
the smallest ranging from 1 to 3 μm. For a film with thickness
of 20−30 nm, the dome in the irradiated regions became larger
and typically recognized as formation of hierarchical double-
columned domes in a photoirradiated line. Here, the average
domain diameter increased to 3−5 μm. For a thicker film of
30−40 nm thickness, distorted column structure was observed.
When the film thickness reached 40−60 nm, the dome pattern
showed another typical structure constituted with an ordered
Figure 5. Average height differences in AFM images of AzD6 film
(initial thickness = 90 nm) as a function of UV irradiation dose. The
height difference was evaluated based on the images in Figure 4.
Figure 6. GI-XRD images on imaging plates at different period of UV
irradiation dose (upper), and changes in the diffraction peak intensity
corresponding to d/2 = 3.5 nm with UV irradiation dose (lower) for a
AzD6 film with 200 nm thickness.
The surface dewetting behavior of polymer thin films is
essentially dependent on the film thickness.43,49−51 Therefore,
UV irradiation was performed onto AzD6 films with various
thicknesses ranging from 20 to 100 nm. Figure 7 depicts
topographical AFM images observed after 225−250 mJ cm−2
irradiation. In thinner films of 20 or 50 nm thickness, the
dewetting migration proceeded in local regions, resulting in a
relatively dense and uniform distribution of small domes. A film
with 20 nm thickness provided most dense and uniform sized
domes with a few μm diameters. For thicker film the domain
becomes larger with the size feature more distributed, which is
well recognized for a film with 90 nm thickness. When the
thickness became more than 100 nm, the dewetting hardly
occurred any more as shown in the left image of the 100 nm
film (denoted as ‘typical image’). Instead, some protrusions of
several μm diameter were observed on the film surface. This
type of protrusions were also observed for thinner films in flat
regions at the earlier hole formation stage (see Figure 4, 150 mJ
cm−2). The formation of 3D protrusions may be related to UV-
induced expanding behaviors of the Az-containing molecular
film as has been observed for Langmuir−Blodgett films of
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Figure 7. Topographical AFM images of AzD6 films of varied initial thickness after a constant dose of uniform UV irradiation (225−250 mJ cm−2).
The initial film thickness is indicated at the top left for each figure.
single column dome lines. The average domain diameter
increased to 5−8 μm. As expected, when the film thickness
reached 100 nm, the hierarchical morphology disappeared, and
partial dent was observed in the bulged lined regions. For a film
with 120 nm thickness, no additional morphology was
admitted, which can be regarded as the “ordinary” relief
formation simply reflecting the mask pattern.
Our approach to observe dewetting behavior in photomask
line patterns is analogous with the strategy conducted on a
chemically patterned surface using self-assembled monolayers
by Sehgal et al.54 They observed the dewetting behavior of
polystyrene thin film in patterned narrow stripes. Depending
on the chemically patterned stripes, the dewetted droplets
polystyrene can be arranged in double or single line column. In
their system, the confinement is achieved by chemical
patterning by imprinting on the substrate surface, and in our
case the film material itself is spatially transformed to exert
confined dewetting by patterned irradiation.
3.4. On the Tendency To Form a 3D Structure. The
marked tendency to increase the film thickness of AzD6 in the
dewetted domes (Figure 7, 20−90 nm thickness) and also the
formation of protrusions in flat film surface (Figure 7, 100 nm)
upon UV irradiation is worth noting. We assume that the
dendritic core architecture with the defined mass will isolate
molecules with each other in the smectic layers without forming
interlacing or entanglement. This situation would apt to pull
out the isolated dendrimers from the smectic layer to form 3D
structuring. As far as the Az unit is kept in the trans-form in the
dark, the cast films did not show the surface morphology
change. Thus, UV irradiation can reduce the intermolecular
interaction considerably due to the involvement of cis-Az
isomers. Considering the data of GI-XRD measurements
(Figure 6), even at prolonged irradiation conditions, a full
photoinduced transition to the isotropic phase does not take
place but the disordered layer structure is still maintained
during the UV irradiation. We assume that the highly fluid
disordered layer structure is a requisite to form 3D
morphological growth upon UV irradiation. Systematic
investigations with other types of Az dendrimers changing
the core generation and spacer length between the Az
mesogenic unit to the core and precise evaluations of the
viscoelastic properties should be helpful to gain more insights
into these processes. Work along such directions is in progress.
The number of domes at 1000 μm2 area were counted and
plotted as a function of film thickness in Figure 9. As the film
thickness decreased, the number of dewetted domes increased
in a nonlinear manner. The figure involves data counted for
dewetted films after a uniform UV irradiation (closed circles)
and in a 10 μm line patterned UV irradiation (open circles).
The thickness dependence qualitatively agreed in both cases,
however, the number became apparently smaller for the line
patterned films. This should be the consequence of increased
thickness in the UV irradiated regions due to the mass
migration.
4. CONCLUSIONS
This work proposed UV light-triggered dewetting of thin films
of a dendritic LC molecule in which the 16 mesogenic Az units
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Figure 8. Topographical AFM images of AzD6 films of varied initial thickness after a patterned UV irradiation through a photomask at 250 mJ cm−2.
The initial film thickness is indicated at the top left for each figure.
irradiated line. We expect that after an optimization of the
experimental conditions, one can generate precisely the number
of arrayed columns simply by changing the film thickness. For
thicker films above the threshold thickness, the patterned film
shows the “ordinary” relief structure which simply reflects the
photopatterns. Controlling the dewetting process of thin
polymer films is of great technological significance. For this
purpose, “directed dewetting” has been extensively investigated
by manipulating the substrate surface such as rubbing,55,56
chemical,54,57 or topographical50,51,57,58 patterning. In such a
framework, this work proposes a new directing method of film
dewetting on an unmodified uniform substrate surface using
pattered irradiation.
Figure 9. Number of formed domes as a function of initial film
thickness after UV irradiation at 250 mJ cm−2. The domes are counted
for samples with uniform (closed) and line-and-space patterned
(open) irradiation.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: tseki@apchem.nagoya-u.ac.jp, Telephone: +81-52-
789-4668. Fax: +81-52-789-4669.
are attached in the periphery of the branched poly-
(propylamine) core (G2). With this dendritic material,
unexpected phototriggered dewetting with uniform UV
irradiation and dome-shaped hierarchical morphologies are
formed with simple patterned UV irradiation in thinner films
less than 100 nm. We assume at the moment that such
characteristic migrating motions are characteristically formed
for smectic LC films comprising of dendritic molecules. The
hierarchical structure is generated by the combination of mass
transport and surface dewetting. The dome size and density on
the surface is largely dependent on the initial film thickness. In
the patterned irradiation, thickness control will provide single
or “quasi” double column along the irradiatied line in a 10 μm
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors thank Mr. T. Hikage in conducting high intensity
X-ray analysis. The research was supported by the Grant-In-Aid
for Basic Research S (to TS, No. 23225003) from the Ministry
of Education, Culture, Sports, Science and Technology, Japan,
and PRESTO program of Japan Science and Technology
Agency (to S.N.).
I
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