Photomechanical effects of ferroelectric liquid-crystalline elastomers containing azobenzene chromophores

Article abstract of DOI:10.1002/anie.200603053

(Figure Presented) Bending over backwards: Ferroelectric liquid-crystalline (LC) elastomer films with a high LC order and low glass transition temperature bend when irradiated with light (see picture). At 366 nm the films bend toward the actinic light source along the alignment direction of the mesogens, and they recover their initial flat state after exposure to visible light. The mechanical force generated by photoirradiation reaches about 220 kPa.

Full text of DOI:10.1002/anie.200603053

Angewandte
Chemie
DOI: 10.1002/anie.200603053
Liquid-Crystalline Elastomers
Photomechanical Effects of Ferroelectric Liquid-Crystalline
Elastomers Containing Azobenzene Chromophores
Yanlei Yu, Taketo Maeda, Jun-ichi Mamiya, and Tomiki Ikeda*
Photodeformable smart materials that can undergo a shape or
volume change in response to light are attracting increasing
attention. On the one hand, light is a clean energy and can be
controlled rapidly and remotely; on the other hand, by using
the deformation of these materials, one can convert light
energy into mechanical energy directly (photomechanical
effects). Most photodeformable smart materials contain
photochromic compounds, such as azobenzene, stilbene, and
spiropyran. The chromophores change their molecular struc-
ture upon exposure to light; thus, their incorporation into
polymer systems gives rise to conformation changes of the
polymer chains and concomitant changes in the physical and
chemical properties of the polymer solutions and solids
through photoisomerization, including photoinduced contrac-
tions/expansions of rubbery networks and swollen gels.[1–12]
However, the low elastic modulus and low yield strength of
gels provide important limitations in the performance of
actuation, while for the solid polymer networks, deformations
of less than 10% limit their practical applications.
Large photoinduced contractions/expansions have been
acquired by Finkelmann et al. and other research groups by
incorporating azobenzene derivatives into liquid-crystalline
elastomers (LCEs) as a trigger.[13–15] LCEs with the rubber
elasticity of polymer networks exhibit a simultaneous aniso-
tropic orientational symmetry of liquid-crystalline (LC)
phases. The driving force for their large changes in shape is
suggested to arise from a variation of LC alignment order:
upon irradiation with UV light, LC systems containing
azobenzene chromophores experience a reduction in align-
ment order and even an LC-I phase transition as a result of
the trans–cis photoisomerization of the azobenzene moieties,
because the rodlike trans-azobenzene moieties stabilize the
LC alignment, whereas the bent cis forms lower the LC order
parameter.
heated above their glass transition temperatures in air.[16] As a
consequence of the limitation of absorption of photons, a
surface contraction caused by the photoinduced change in
alignment of liquid crystals contributes to the bending.
Several examples of the three-dimensional deformations
have been reported, such as twisting of the azobenzene-
containing LCEs.[17–19] In comparison with the contraction/
expansion that is a two-dimensional action, the bending could
be advantageous for artificial hands and medical microrobots
that are capable of completing particular manipulations.
However, the following three factors limit their actual
applications: 1) a slow response in the order of minutes[16a]
or seconds;[16d] 2) difficulty in inducing effective bending at
room temperature in air; and 3) the low force generated upon
photoirradiation.
In our previous work, we found that the bending speed
was different in the nematic LCE films with various cross-
linking densities.[16c] The larger the order parameter and the
mobility of polymer segments, the faster the bending.
According to these results, if we use films with a higher
order of liquid crystals and a lower glass transition temper-
ature (Tg), it should be easy to acquire high-speed bending at
room temperature. It is well-known that ferroelectric liquid
crystals have two advantages: their high degree of order of
mesogens, and the fact that their molecular alignment can be
controlled quickly by applying an electric field, because of the
presence of spontaneous polarization.
Zentel et al. synthesized ferroelectric LCEs from electric-
field-induced oriented LC polysiloxanes by a radical photo-
cross-linking process.[20] Later, Finkelmann et al. prepared
chiral smectic C (SmC*) elastomers by two successive defor-
mation processes,[21a] and more perfectly by mechanical shear
deformation;[21b] they successfully obtained a monodomain
SmC* elastomer by a mechanical shear field.[21c] Here, we
used a photopolymerization method to prepare new ferro-
electric LCE films from a mixture of molecules 1 and 2
(Figure 1; 80:20) in the SmC* phase in a rubbing-treated
indium tin oxide (ITO) glass cell under an electric field, as it
has been reported that highly ordered polymers can be
obtained by in situ photopolymerization of oriented LC
monomers.[22] In the obtained ferroelectric LCE films, the
long axis of the molecules (director n) in each layer is aligned
in the same direction in the smectic layers oriented roughly
perpendicular to the surface of the glass cell (Figure 2a).
Moreover, n is not parallel to the rubbing direction of the
alignment layer but has a tilt with it.[23]
Furthermore, a three-dimensional deformation, photo-
induced bending, has been achieved by using nematic LCE
films containing azobenzenes swollen in suitable solvents or
[*] T. Maeda, Dr. J. Mamiya, Prof. Dr. T. Ikeda
Chemical Resources Laboratory
Tokyo Institute of Technology, R1-11
4259 Nagatsuta, Midori-ku, Yokohama 226-8503 (Japan)
Fax : (+81)45-924-5275
E-mail: tikeda@res.titech.ac.jp
Homepage: http://www.res.titech.ac.jp/polymer/
Prof. Dr. Y. Yu
Department of Materials Science
Fudan University
The photoresponsive behavior of the ferroelectric LCE
films was investigated by irradiation with non-polarized light.
The first frame in Figure 2b shows the ferroelectric LCE film
before photoirradiation; counting clockwise, the second and
220 Handan Road, Shanghai 200433 (China)
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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at wavelengths above 540 nm for 5 s. Then, after further
irradiation for 25 s, it is clear that the bent film recovers its
initial flat state completely (the first frame in Figure 2b). This
bending and unbending behavior was reversible just by
changing the wavelength of the incident light.
As mentioned above, the ferroelectric LCE films hold a
smectic layer structure with n in the layers aligned in the same
direction; therefore, irradiation by UV light reduces the
alignment order of the azobenzene mesogens along n in each
layer.[23] Moreover, the extinction coefficient of the azoben-
zene moieties around 360 nm is large, which leads to a
contraction in the film surface and finally bending of a whole
film along n.
Figure 1. Chemical structures and phase transition temperatures of the
In our previous work on nematic LCE films, we found that
the bending time decreased significantly with an increase in
light intensity, because actinic light with a higher intensity
produces a higher concentration of cis-azobenzene moieties,
and thus a larger surface contraction.[16b] This effect of the
light intensity on the bending behavior was also observed in
the ferroelectric LCE films. Notably, when exposed to an
LCmonomer ( 1), the cross-linker (2), and the monomer mixture used
for preparation of the ferroelectric LCE film. SmA, smectic A; SmC*,
chiral smectic C; SmCA*, chiral smectic CA.
argon-ion laser at 364 nm with
a light intensity of
280 mWcmÀ2, the film underwent fast bending within
500 ms (for a movie, see the Supporting Information), which
is one order of magnitude faster than the bending of the
nematic LCE films.[16]
The mechanical force generated in the ferroelectric LCE
films upon photoirradiation was measured with a thermome-
chanical analyzer (Simadzu, TMA-60) as follows: both ends
of a film were clamped and 20 mN was initially loaded on the
film; then the film was heated to 508C and exposed to UV
light. Figure 3 shows the change of the load on the film upon
photoirradiation at different light intensities. As the length of
Figure 2. Bending and unbending of the ferroelectric LCE film upon
alternate irradiation with UV and visible light at room temperature.
a) Azobenzene mesogens are aligned parallel to each other to form
layers with a tilt between the director (n) and the normal to the
smectic layer that is parallel to the rubbing direction. b) Photographic
frames of the film bending toward the actinic light source along the
alignment direction of mesogens in response to irradiation at 366 nm
(17 mWcmÀ2), and being flattened again by visible light at wavelengths
above 540 nm (110 mWcmÀ2 at 547 nm). The size of the film is
10 mm”4 mm”10 mm.
Figure 3. Change of the load on the ferroelectric LCE film when
exposed to UV light at 366 nm with different intensities at 508C. The
cross-sectional area of the film was 5 mm”20 mm. An external force of
20 mN was loaded initially on the film to keep its length unchanged.
third show how the film curls up after irradiation at 366 nm at
a light intensity of 17 mWcmÀ2 for 5 and 10 s, respectively.
Notably, the film bends toward the actinic light source with a
tilt to the rubbing direction of the alignment layer: along n. As
the third frame in Figure 2b shows, the film even curls up to a
corkscrew spiral shape. More importantly, the bending was
induced at room temperature, because the ferroelectric LCE
films has a low Tg of 208C. The fourth frame in Figure 2b
shows the state of the bent film after exposure to visible light
the film was kept unchanged, the increase of the load
indicates the generation of the mechanical force upon photo-
irradiation. The maximum loads reached 38 and 42 mN when
the light intensity was 3.5 and 5.5 mWcmÀ2, respectively,
which indicates that a mechanical force of 18 and 22 mN is
generated. The cross-sectional area of the film was 5 mm ”
20 mm, and the force was calculated to be 180–220 kPa, similar
to the contraction force of human muscles (around 300 kPa).
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zenes with a high LC order and a low glass transition
temperature were prepared by photopolymerization under an
electric field. Irradiation with 366-nm light caused the films to
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along the alignment direction of mesogens, and the bent films
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visible light. The bending took place within 500 ms upon
irradiation by a laser beam. In addition, the mechanical force
generated by photoirradiation reached about 220 kPa, similar
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The preparation of 1 and 2 is shown in the Supporting Information.
The ferroelectric LCE films were prepared by photopolymerization
of a monomer mixture of 1 and 2 (80:20) containing 1 mol% of a
photoinitiator (Irgacure 784) in the SmC* phase. First, the molten
mixture was injected into an ITO glass cell at 1178C (in an isotropic
phase), which was coated with rubbing-treated polyimide alignment
layers. Then, after the sample was cooled slowly (2 KminÀ1) to a
polymerization temperature of 908C (in the SmC* phase), an electric
field of 1 VmmÀ1 (applied voltage/cell thickness) was applied and
photoirradiation was performed for 2 h at > 540 nm (3 mWcmÀ2 at
547 nm) with a 500-W high-pressure mercury lamp through glass
filters (Toshiba, Y-52, and IRA-25S). After polymerization, the ITO
cells were opened and the LCE films were removed with a cutter. The
detailed measurement methods and the properties of 1 and 2, the
monomer mixture, and the ferroelectric LCE films are described in
the Supporting Information.
Received: July 28, 2006
Revised: October 31, 2006
Published online: December 20, 2006
Keywords: chromophores · elastomers · liquid crystals ·
.
mechanical properties · thin films
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