Is chemical crosslinking necessary for the photoinduced bending of polymer films?

Article abstract of DOI:10.1039/b715855f

Freestanding crosslinked liquid-crystalline polymer films obtained by self-assembly through intermolecular hydrogen bonding showed photoinduced bending and unbending. The structural change at the microscopic level, caused by trans-cis photoisomerization of the azobenzene moieties at the hydrogen-bonded crosslinks, is successfully converted into a macroscopic deformation in the liquid-crystalline polymer films. The Royal Society of Chemistry.

Full text of DOI:10.1039/b715855f

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www.rsc.org/materials | Journal of Materials Chemistry
Is chemical crosslinking necessary for the photoinduced bending of
polymer films?{
Jun-ichi Mamiya,a Akira Yoshitake,a Mizuho Kondo,a Yanlei Yub and Tomiki Ikeda*a
Received 15th October 2007, Accepted 29th October 2007
First published as an Advance Article on the web 1st November 2007
DOI: 10.1039/b715855f
isomerization of the azobenzenes may influence the crosslinking of
the three-dimensional polymer networks. In this communication,
we investigated the photoresponsive behavior of azobenzene
CLCP films formed by self-assembly. We succeeded in fabricating
recyclable CLCP films that could crosslink/de-crosslink reversibly
using the hydrogen bonding at the crosslinks, and we showed
photoinduced bending and unbending as well.
Freestanding crosslinked liquid-crystalline polymer films
obtained by self-assembly through intermolecular hydrogen
bonding showed photoinduced bending and unbending.
The structural change at the microscopic level, caused by
trans–cis photoisomerization of the azobenzene moieties at the
hydrogen-bonded crosslinks, is successfully converted into a
macroscopic deformation in the liquid-crystalline polymer films.
The structure of the LC copolymer (1) used in this study is
shown in Fig. 1, which has both a carboxyl group and an
azobenzene moiety. The mesomorphic properties of the copolymer
1 were studied by differential scanning calorimetry (DSC). The
copolymer 1 exhibited a glass transition (Tg) at 38 uC, and an LC–I
phase transition at 106 uC upon heating (Fig. 2). Crosslinkers 2
and 3 that are capable of recognizing hydrogen-bond donor
molecules at the pyridyl ends were selected as hydrogen-bond
acceptors (Fig. 1). A complex of the copolymer and the crosslinker
was obtained from a THF solution containing equimolar amounts
of the carboxyl group and the pyridine moiety. In the infrared (IR)
spectra of the complexes of 1 + 2 and 1 + 3, the absorption bands
corresponding to the hydrogen-bonded O–H stretch of the
Soft materials have attracted much attention because of their
dynamic properties.1 For effective muscle-like actuation, soft
materials with stratified structures and high molecular orders are
necessary. Crosslinked liquid-crystalline polymers (CLCPs) are
superior soft materials that possess both the order of LCs and the
elasticity of polymer networks.2,3 CLCPs exhibit a spontaneous
contraction along the director axis when heated above nematic–
isotropic (I) phase transition temperatures.4 When azobenzene
chromophores are incorporated into CLCPs, they can undergo the
contraction isothermally because of the change in LC alignment
caused by light.5 Furthermore, a three-dimensional deformation,
bending, of CLCP films containing azobenzenes has been observed
upon exposure to light.6 Due to the limitation of absorption of
photons, a surface contraction caused by the photoinduced
change in alignment of LCs contributes to the bending. By using
photodeformation of these materials, one can convert light energy
into mechanical work directly (photomechanical effects).3,5–7
Hydrogen bonds have been used for the preparation of a
wide variety of self-assembled soft materials, among which LC
phases with well-defined structures have been formed by exploiting
intermolecular hydrogen bonding.8 Various synthetic approaches
for the construction of dynamically functional materials using
hydrogen bonds have been demonstrated in such self-assembly
systems as supramolecular LCs and polymers.8,9 However, there
are few examples of supramolecular crosslinking of polymers by
low-molecular-weight (LMW) crosslinkers.10 Crosslinked poly-
mers by supramolecular interactions are expected to function as
recyclable materials responsive to external stimuli, because LMW
crosslinkers can be connected and disconnected reversibly.
carboxyl groups were observed at 1930 and 2500 cm21 11
On the
.
other hand, in the single copolymer 1, no absorption due to
the hydrogen bonding appeared. It was also found that when each
Since the hydrogen bonds crosslink polymer chains to form
three-dimensional polymer networks, if an azobenzene moiety as a
photoresponsive group is introduced into the crosslinker, trans–cis
aChemical Resources Laboratory, Tokyo Institute of Technology, R1-11,
4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan.
E-mail: tikeda@res.titech.ac.jp; http://www.res.titech.ac.jp./polymer/;
Fax: +81-45-924-5275; Tel: +81-45-924-5240
bDepartment of Materials Science, Fudan University, 220 Handan Road,
Shanghai, China 200433
{ Electronic supplementary information (ESI) available: Experimental
details. See DOI: 10.1039/b715855f
Fig. 1 Chemical structures of the hydrogen-bond donor (1) and
acceptors (2 and 3) used in this study.
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photoisomerization behavior. The photoinduced bending and
unbending of the hydrogen-bonded films of 1 + 3 is similar to that
of the chemically-bonded films reported previously.6 These results
indicate that a structural change caused by photoisomerization of
the azobenzene moieties at the crosslinks plays an important role
in the photoinduced bending of the hydrogen-bonded CLCP films.
In other words, it means that the crosslinks formed by non-
covalent bonds can convert the motion of the mesogens into a
macroscopic change of the CLCP films.
Irradiation with UV light gives rise to trans–cis isomerization of
the azobenzene mesogens in CLCPs. However, the CLCP films of
1 + 2 showed no bending upon UV-light irradiation. This is
presumably because the photoisomerization of the azobenzene
mesogens in the copolymer 1 induces no decrease in alignment
order of the hydrogen-bonded LC structure. On the other hand,
trans-azobenzenes in both the copolymer and the crosslinker
isomerize to cis-azobenzenes in the CLCP films of 1 + 3. By
photoisomerization of the azobenzene moieties at the crosslinks,
hydrogen-bonds are partly destroyed, leading to a decrease in
alignment order of the hydrogen-bonded LC structure only in the
film surface (Fig. 4). As the azobenzene mesogens are aligned
parallel to the rubbing direction, an anisotropic contraction is
generated along the alignment direction of the azobenzene moieties
upon irradiation with UV light.
Fig. 2 DSC thermograms of the copolymer and the complexes on
heating (scan rate = 10 uC min21).
crosslinker was mixed with the copolymer 1, the temperature range
of the LC phase in the complexes became broader than that in the
copolymer 1 (Fig. 2). It is observed clearly that the mesophases of
the complexes are stabilized by the formation of the hydrogen
bonding between 1 and the crosslinkers
Next, the recyclability of the hydrogen-bonded CLCP films was
investigated (ESI{). The chemically-bonded CLCP films were
undissolved in various solvents. However, the hydrogen-bonded
CLCP films were dissolved in THF. Reprecipitation from the
THF solution in diethyl ether led to dissociation of the hydrogen
bonding. We could reconstruct the CLCP films by self-assembly
through hydrogen bonding and repeatedly induce the photo-
induced bending of the reformed CLCP films.
Hydrogen-bonded azobenzene CLCPs were sandwiched
between two sodium chloride (NaCl) plates with rubbing
treatment. The cell thickness was controlled with silica spacers
with a diameter of 20 mm. Freestanding films were obtained after
the NaCl substrates were dissolved in water. The optical
anisotropy of the CLCP films was evaluated by polarizing optical
microscopy. The regular maximum and minimum values with 90u
separations showed that the azobenzene mesogens are preferen-
tially aligned along the rubbing direction.
In summary, freestanding hydrogen-bonded CLCP films
containing azobenzenes have been obtained. The supramolecular
crosslinking of the linear copolymers with LMW crosslinkers was
We observed the photoresponsive behavior of the hydrogen-
bonded CLCP films above their Tgs of 1 + 2 and 1 + 3. In 1 + 2,
no bending was observed upon exposure to UV light at 366 nm
and the film showed no deformation upon further visible-light
irradiation at 60 uC (Fig. 3a). On the other hand, when the CLCP
film of 1+3 was exposed to 366 nm light at 50 uC, the film bent
toward the actinic light source along the alignment direction of the
mesogens (Fig. 3b). The bent films reverted to the initial flat states
when irradiated with visible light at .540 nm. Here, crosslinker 2
has no photoresponsive property, while crosslinker 3 shows
Fig. 4 Plausible mechanism of bending in the hydrogen-bonded CLCP
films of 1 + 3. (a) Network structures of the hydrogen-bonded CLCP films
consisting of the copolymers and the crosslinkers. Schematic illustration of
molecular alignment in the hydrogen-bonded CLCP film before (b) and
after (c) irradiation with UV light.
Fig. 3 Photoresponsive behavior of the hydrogen-bonded CLCP films of
1 + 2 (a) and 1 + 3 (b). Size of the films: 2 mm 6 3 mm 6 20 mm. UV
light intensity, 18 mW cm22; visible light intensity, 21 mW cm22
.
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