A reactive azobenzene liquid-crystalline block copolymer as a promising material for practical application of light-driven soft actuators

Article abstract of DOI:10.1039/c5tc00595g

Cross-linked liquid-crystalline polymers containing azobenzene have been drawing great research interest of scientists due to their significant values in the design of light-driven soft actuators. However, poor processabilities due to the chemical cross-l

Full text of DOI:10.1039/c5tc00595g

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Reactive Azobenzene Liquid-Crystalline Block
Copolymer as a Promising Material for Practical
Application of Light-driven Soft Actuators
Cite this: DOI: 10.1039/x0xx00000x
Received 00th January 2012,
Accepted 00th January 2012
Jiuꢀan Lv, Wei Wang, Wei Wu, Yanlei Yu*
DOI: 10.1039/x0xx00000x
www.rsc.org/
rapid deformation of materials,1a,b and polymers are one of the most
Cross-linked
liquid-crystalline
polymers
containing
promising materials for the fabrication and the engineering of the
actuators due to their advantageous properties, such as their high
processability, softness, easy fabrication characteristics, excellent
corrosion resistance, and low manufacturing costs.2 Several types of
photosensitive polymers have been developed as soft actuator
materials, including gels,3 shapeꢀmemory polymers,4 organic
crystals,5 and crossꢀlinked liquidꢀcrystalline polymers (CLCPs).1a In
particular, the CLCPs are superior soft materials that possess both
the order of liquid crystals (LCs) and the elasticity of polymer
networks. With the aid of the CLCPs, it is possible to convert small
amounts of external energy into macroscopic amounts of mechanical
energy, which is essential for applications in actuators. By
incorporating photochromic molecules such as azobenzene (azo)
moieties into the CLCPs, deformations such as reversible contraction
and expansion,6 and even bending7,8 have been induced by
photochemical reactions of the azobenzene chromophores.
Subsequently, numerous lightꢀdriven soft actuators such as
swimmer,9 motor,10 oscillator,11 microrobot,12 artificial cilia,13
micropump,14 microvalve,15 inchworm and robotic arm16 have
already been fabricated from the photoresponsive CLCPs.
Generally, there are two major methods to prepare the azoꢀ
CLCPs: the one step method and the two step method. In the former,
the CLCPs are usually synthesized in conventional LC cells through
common free radical copolymerization of acrylic monomers and
crosslinkers; thus the size and the shape of the acquired CLCPs are
limited and wellꢀdefined CLCPs are not easily obtained.17 In the
latter, wellꢀdefined weak networks are synthesized in the first step,
then these networks are stretched unidirectionally to align mesogens
and establish order. In the second step, crossꢀlinking reactions fix
the network anisotropy.18 The advantage of the two step method is
that the induced network anisotropy in the first step is reproducible;
thus well aligned elastomers are achieved. However, the two step
method is only suitable for producing the CLCPs with polysiloxane
as the main chain. The both methods mentioned above are not
azobenzene have been drawing the great research interests of
the scientists due to their significant values in design of light-
driven soft actuators. However, poor processabilities due to
the chemical cross-linking severely prevent their practical
applications. Here, we designed and synthesized a novel
reactive block copolymer PEO-b-PAZO bearing N-
hydroxysuccinimide carboxylate substituted azobenzene
groups. This copolymer is compatible with traditional
polymer processing methodology including melt and solution
processing methods, providing a facile way to prepare the
photoresponsive films and fibers, and enabling post-
crosslinking after processing through a mild reaction between
N-hydroxysuccinimide carboxylate substituents and
a
difunctional amine. Most importantly, the prepared fibers
and films showed reversible photodeformation regulated by
alternating irradiation of UV and visible light (365 nm/470
nm). We envision that this novel liquid crystalline polymer
will open up new possibilities for low-cost fabrication of high
performance photo-driven actuators on a large scale.
Actuators that reversibly change their shape and/or size in response
to external stimuli have been extensively studied for novel
applications in wideꢀranging industrial and medical fields.1 Among
them, the lightꢀdriven soft actuators are most attractive, because light
as an external stimulus enables the remote control and gives rise to
Department of Materials Science & State Key Laboratory of
Molecular Engineering of Polymers, Fudan University, 220 Handan
Road, Shanghai 200433, China. E-mail: ylyu@fudan.edu.cn
Electronic supplementary information (ESI) available. See DOI:
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applicable for widespread applications of the CLCPs as acutators
due to their limitation of processability. A great challenge still
remains about how to ameliorate the processability of the azoꢀ
CLCPs, which requires the precise coordination of rational
molecular design strategy and processing methodology.
Lee et al. firstly reported an uncrossꢀlinked liquidꢀcrystalline
polymer (UCLCP) which exhibited reversible photoinduced bending
behavior in various directions controlled by polarized UV light.19
Due to lack of chemical crossꢀlinking, such properties of the
UCLCP as strength, heat resistance, and reversibility are not good.
Followingly, electron beam (EB) irradiation polymerization was
employed to prepare the azoꢀCLCPs, which can be carried out
without initiator at any temperature and any state of the monomers.20
Although this approach shows excellent processing freedom
compared with the previous methodology, it still remains some
drawbacks for practical applications due to the requirement of
unique and expensive processing equipment.
Fig. 2 SAXS spectrum of the block copolymer PEOꢀbꢀPAZO. The
sample was cooled from 200 ºC and then annealed at 140 ºC for 1 h.
Herein, we designed a reactive block copolymer PEOꢀbꢀPAZO
containing polymethacrylate, which bears Nꢀhydroxysuccinimide
carboxylate substituted azo groups, and poly(ethylene oxide) PEO
(Fig. 1a). The molecular design strategy of the copolymer is based
on two principal points: (1) Nꢀhydroxysuccinimide substituents in
the azo mesogens offer a superior approach after processing to postꢀ
crosslink polymer chains by using a difunctional primary amine
under mild reaction conditions; (2) the introduction of a soft PEO
block can supply enough free volume for the photoisomerization of
the azo groups and the photochemical phase transition, enabling us
to obtain more excellent photoresponsive CLCPs.21
The LC monomer was synthesized according to the similar
procedures reported previously (Scheme S1).22,23 Macroꢀchain
transfer agent was synthesized according to the previously reported
method (Scheme S2).24 A glass tube was charged with macroꢀchainꢀ
transfer agent (102 mg, 0.05 mmol), 2,2’ꢀazobisisobutyronitrile (1.5
mg, 0.01 mmol) and the LC monomer (500 mg, 0.61 mmol) in
anhydrous tetrahydrofuran (THF, 2 mL); it was then degassed by
three freezeꢀevacuateꢀthaw cycles and sealed in vacuo. The
polymerization was carried out at 70 °C for 48 h. The block polymer
was precipitated through the addition of the mixture into absolute
diethyl ether, collected by filtration, and dried in a vacuum oven at
40 °C. Molecular weight as well as polydispersity index was
measured by Gel Permeation Chromatography (GPC, Shimazu, LCꢀ
10ADvp) with THF as the eluent at a flow rate of 0.8 mL/min.
Thermodynamic properties of the LC monomer and the block
Experimental
Synthesis and characterization of block copolymer PEO-b-
PAZO: The block copolymer was synthesized by RAFT polymer were determined by differential scanning calorimetry (DSC;
polymerization of a LC monomer, Nꢀhydroxysuccinimide 4ꢀ[(ꢀωꢀ TA, Q2000) at heating and cooling rates of 3 °C/min for the LC
methacryloxyundecyloxy))phenylazo]
benzoate,
and
a
monomer and 10 °C/min for the block polymer, respectively. Three
dithiobenzoylꢀterminated PEOꢀbased macroꢀchain transfer agent. scans were applied to check the reproducibility. The optical
anisotropy of the block polymer fiber was studied using a polarizing
optical microscope (POM; Leika, DM2500p). 1DꢀSAXS
experiments were performed with a small angle Xꢀray scattering
(SAXS) instrument (Bruker NanoSTAR). Fourier transform infrared
(FTꢀIR) spectra were recorded on a Nicolet Nexus 470 spectrometer.
Preparation of CLCP fibers: As shown in Fig 1b, the block
copolymer fibers were drawn from a PEOꢀbꢀPAZO melt in its
isotropic state. In a typical experiment, a small amount of the
copolymer (20 mg) was heated to 180 °C on a glass substrate placed
on a hot stage (Mettler, FPꢀ90 and FPꢀ82), then the fibers were
prepared by quickly drawing out the melt with a toothpick. The
fibers were left at room temperature for about 1 h to be stabilized.
The average diameter of the fibers was about 100 ꢀm. Then the
drawn fibers were immersed into a solution of 1,6ꢀhexanediamine in
methanol (1 × 10ꢀ3 M) to undergo the chemical crosslinking reaction
for about 2 h. After being washed with methanol several times and
then dried at ambient temperature for 24 h, freeꢀstanding crosslinked
fibers were finally obtained.
Preparation of CLCP films: A THF solution (8 mg/ml, 0.5 ml) of
PEOꢀbꢀPAZO was cast onto a glass substrate using a spin coater at a
low speed of 1200 r/min for 10 s. After the solvent was evaporated at
ambient temperature for 1 h, a yellow polymer film was formed on
the substrate. The obtained film was then immersed into a solution of
1,6ꢀhexanediamine in methanol (1 × 10ꢀ3 M) and the chemical
crosslinking reaction took place at room temperature for 4 h. After
being washed with methanol several times and then dried at ambient
temperature for 24 h, a freeꢀstanding CLCP film with the thickness
Fig. 1 (a) Chemical structure of the block copolymer PEOꢀbꢀPAZO
used in this study. Mn: numberꢀaverage molecular weight, Mw:
weightꢀaverage molecular weight, G: glassy phase, SmA: smetic A
phase, I: isotropic phase. (b) Experimental schematic to show the
preparation procedures of the photoresponsive CLCP fibers and
films.
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of about 20 ꢀm was obtained by peeling off from the glass substrate
(Fig. 1b).
Photoirradiation: For photoinduced bending experiments, a 365ꢀ
nm UV LED (Omron, ZUVꢀC30H, 100 mW cmꢀ2) was used.
Linearly polarized UV light (LPL) was obtained from the same
instrument through
a polarized UV filter. For unbending
experiments, a 470ꢀnm visible LED (CCS, PJꢀ1505ꢀ2CA, HLVꢀ
24GRꢀ3W, 120 mW cmꢀ2) was used to irradiate the samples.
Photographs of bending and unbending behavior were recorded with
a digital camera (Olympus, VHXꢀ500F).
Results and discussion
Mesomorphic property of PEO-
b-PAZO
The copolymer PEOꢀ
b
ꢀPAZO showed a numberꢀaverage
molecular weight of Mn= 1 × 104 g/mol with a molecular
weight distribution of Mw/Mn = 1.4 (Figure S5). The
Fig. 4 IR spectra of the copolymer PEOꢀbꢀPAZO (a) and the
crosslinked copolymer film (b).
mesomorphic property of PEOꢀ
the combined techniques of DSC, POM, and SAXS. According
to the DSC curves in Fig. S6, PEOꢀ ꢀPAZO exhibits a phase
bꢀPAZO was investigated by
phase.
Furthermore, the SAXS spectrum of PEOꢀbꢀPAZO was
b
measured when the sample was cooled from 200 ºC and
annealed at 140 ºC for 1 h (Fig. 2). The first and second
scattering peak is q1 = 1.96 nmꢀ1 and q2 = 3.93 nmꢀ1,
respectively, with the ratio of q1 to q2 being 1/2. This result
demonstrates the presence of a longꢀrange ordered lamellar
structure with a layer spacing d = 3.20 nm calculated from the
formula d = 2π/q1. The d value agrees with the calculated sideꢀ
transition at 154 ºC (clearing point) in the heating process and
at 148 ºC in the cooling process. In addition, a glass transition
at 70 ºC and 69 ºC was observed in the heating and cooling
scan, respectively. The POM observation revealed the presence
of a broken focalꢀconic fanꢀshaped texture when the melt of
PEOꢀ
b
ꢀPAZO was heated to 140ºC and annealed for 60 min
ꢀPAZO exhibits a smectic
(Inset of Fig. S6), suggesting PEOꢀ
b
chain length of PEOꢀ
bꢀPAZO (3.26 nm) perfectly, which
implies that PEOꢀ ꢀPAZO features a SmA structure.
b
Photochemical isomerization of PEO-
b-PAZO
The photochemical isomerization of the crosslinked PEOꢀ
b
ꢀ
PAZO film prepared by the method demonstrated in the Fig. 1b
was investigated. When irradiated with UV light at 365 nm, the
PEOꢀ
until a photostationary state was eventually reached (Fig. 3a).
With the increase of irradiation time, the intensity of the π π*
transition band around 350 nm decreased, whereas the intensity
of the n π* transition band around 450 nm slightly increased.
Fig. 3b shows the change on the UVꢀvis spectrum of the PEOꢀ
ꢀPAZO film under irradiation of visible light at 470 nm. It can
be seen clearly that the PEOꢀ ꢀPAZO film undergoes cisꢀtrans
bꢀPAZO film underwent transꢀcis photoisomerization
→
→
b
b
back isomerization upon the visible light irradiation, but the
finally recovered absorbance of transꢀazobenzene is lower than
that before the UV light irradiation, with the recovery of the
transꢀisomer being 98%. These results indicate that the CLCP
film composed of PEOꢀbꢀPAZO is able to generate the
photoisomerization in response to UV and visible light.
Photoinduced bending and unbending behavior
As shown in Fig. 1b, a freestanding CLCP fiber was prepared
from the PEOꢀbꢀPAZO melt by dipꢀdrawing method and then
immersed into a solution of 1,6ꢀhexanediamine in methanol to
undergo the chemicalꢀcrosslinking reaction. Fig. 4 shows the IR
spectra of PEOꢀbꢀPAZO and the crosslinked counterpart. The
absorption band corresponding to the C=O stretch was observed
1644 cmꢀ1 for the amide group, whereas the absorption band
was observed at 1771 cmꢀ1 and 1742 cmꢀ1 for the Nꢀ
hydroxysuccinimide ester in the uncrosslinked copolymer.
Fig. 3 Changes in UVꢀvis absorption spectra of the crossꢀlinked
PEOꢀbꢀPAZO film (the film thickness is 2 ꢀm) at 25 ºC in
dependence of time: (a) upon irradiation with 365 nm light; (b)
upon irradiation with 470 nm light at the photostationary state.
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POM observation of the CLCP fiber was performed when
the fiber was placed between crossed polarizers. Typical
polarized optical micrographs are shown in Fig. 5a. The
highest transmittance occurred at an angle of 45° between the
polarization direction of either polarizer and the drawing
direction, whereas the lowest transmittance appeared when the
polarization direction was parallel to the drawing direction.
Therefore, a periodic change of dark and bright images was
observed by rotating the CLCP fiber with an interval of 45°.
The results indicate that the mesogens are preferentially
oriented to the drawing direction; therefore, the fiber can be
called the monodomain CLCP fiber. The photoinduced bending
and unbending behavior of the fiber was shown in Fig. 5b,
when the fiber was heated to 80 ºC by a hot stage. As irradiated
with 365 nm UV light, the fiber began to bend toward the light
source along the fiber axis, and reached the maximum bending
angle in 30 s. Then the bent fiber reverted to the initial flat state
upon irradiation with 470 nm visible light in 3 min. Because of
the alignment of the azobenzene mesogens along the fiber axis,
irradiation with UV light leads to a reduction of the alignment
order along the fiber axis in the surface of the CLCP fiber,
which contributes to the bending deformation. In addition, the
effect of heat on unbending behaviour was also studied,
because the experiment was conducted at 80 ºC. Due to the
thermalꢀinduced disordered alignment, the CLCP fiber
underwent back reaction indeed. However, the thermalꢀinduced
unbending behaviour was too slow and negligible compared
with photoꢀinduced recover process. This phenomenon testified
that irradiation of visible light is necessary to trigger the back
reaction within 3 minutes.
Fig. 6 Images of the precise control of bending direction of the
CLCP film using linearly polarized UV light. White arrows in the
images indicate the direction of polarized light. The size of the
film is 4 mm × 3 mm × 20 ꢁm.
Furthermore, a freestanding CLCP film was fabricated by
casting a THF solution of PEOꢀbꢀPAZO on a glass substrate,
followed by the postꢀcrosslinking reaction as well (Scheme S3).
Since the obtained CLCP film was a polydomain film without
preꢀorientation, the CLCP film could undergo directional
bending upon irradiation with linearly polarized light (LPL) by
varying the polarization direction.3b Fig. 6 shows a sequence of
frames of bending behavior of the CLCP film heated to 80 ºC
by a hot stage. The first frame in Fig. 6 shows the film before
light irradiation. On irradiation with 365ꢀnm LPL with a
polarization direction at zero degree, the film bent towards the
light source with the direction of bending parallel to the LPL
direction, as shown in second frame in Fig. 6, counting
clockwise. When the bent film exposed to visible light at 470
nm, it reverted completely to the initial flat state. Other bending
behaviors of the film upon photoꢀirradiation with LPL
directions at 45°, 90° and 135° are shown in the fourth, sixth
and eighth frames of Fig. 6, respectively. Furthermore, the bent
film could be restored from each bent state to its original flat
state upon irradiation with 470ꢀnm visible light. Moreover, the
bendingꢀunbending cycle was repeated without apparent fatigue.
Similar to the case of the reported CLCP films,6 the bending of
the film was ascribed to the contraction in the surface of film
induced by the transꢀcis isomerization of azobenzene and the
alignment change of the mesogens with irradiation of UV light.
Conclusions
We successfully synthesized a novel reactive block copolymer
PEOꢀ
the polydomain CLCP film through the mild chemical
crosslinking reaction between PEOꢀ ꢀPAZO and 1,6ꢀ
hexanediamine at room temperature. Both of the CLCP fiber
and film showed reversible photoinduced bending and
unbending behaviors in response to UV and visible light,
respectively. The monodomain CLCP fiber bent toward the
light source along the fiber axis as well as the alignment
direction of mesogens; therefore the fiber is able to bend toward
bꢀPAZO, and prepared the monodomain CLCP fiber and
Fig. 5 (a) Polarizing optical micrographs of the texture of the
CLCP fiber observed at room temperature. The black arrows
indicate the directions of optical axes of the two polarizers, while
the dashed line shows the drawing direction. (b) Images of
photoinduced bending and unbending of the CLCP fiber upon
irradiation with UV light at 365 nm (100 mW cm ꢀ2) and visible
light at 470 nm (120 mW cm ꢀ2). The size of the fiber is 20 mm ×
100 ꢁm.
b
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any direction when we change the irradiation direction of the
incident UV light. On the other hand, the polydomain CLCP
film underwent directional bending behaviors upon exposure to
linearly polarized UV light. This reactive block copolymer with
excellent processability allows fabrication of the freestanding
CLCPs with good photomechanical properties and unlimited
shape. Either melt or solution processing method also benefits
the realization of widely practical applications of the CLCPs as
lightꢀdriven actuators on a large scale.
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Journal of Materials Chemistry C
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DOI: 10.1039/C5TC00595G
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Journal of Materials Chemistry C
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DOI: 10.1039/C5TC00595G
Reactive Azobenzene Liquid-Crystalline Block Copolymer as a Promising Material for Practical Application of
Light-driven Soft Actuators
An liquid-crystalline block copolymer composed of poly (ethylene oxide) methyl ester (PEO) and poly (methacrylate)
bearing an N-hydroxysuccinimide carboxylate-substituted azobenzene mesogen was synthesized by RAFT polymerization.
The freestanding fiber and film were prepared by facile techniques of dip-drawing or casting and thereafter immersing to
undergo the post-crosslinking. Both of the CLCP fiber and film showed reversible bending and unbending behaviors in
response to UV and visible light, respectively.