Preparation and characterization of crosslinked azobenzene liquid-crystalline polymer fibers

Article abstract of DOI:10.1080/15421400701681083

We report the synthesis and physical properties of crosslinked liquid-crystalline polymer (CLCP) fibers consisting of a liquid-crystalline monomer and an acrylate monomer with a hydroxyl group that acts as a reactive site for crosslinking. By differential

Full text of DOI:10.1080/15421400701681083

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Preparation and
Characterization of Crosslinked
Azobenzene Liquid-Crystalline
Polymer Fibers
Taiki Yoshino a , Jun-ichi Mamiya a , Motoi Kinoshita a
, Tomiki Ikeda a & Yanlei Yu b
a Chemical Resources Laboratory , Tokyo Institute of
Technology , Yokohama, Japan
b Department of Materials Science , Fudan
University , Shanghai, China
Published online: 22 Sep 2010.
To cite this article: Taiki Yoshino , Jun-ichi Mamiya , Motoi Kinoshita , Tomiki Ikeda
& Yanlei Yu (2007) Preparation and Characterization of Crosslinked Azobenzene
Liquid-Crystalline Polymer Fibers, Molecular Crystals and Liquid Crystals, 478:1, 233/
[989]-243/[999], DOI: 10.1080/15421400701681083
To link to this article: http://dx.doi.org/10.1080/15421400701681083
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Mol. Cryst. Liq. Cryst., Vol. 478, pp. 233=[989]–243=[999], 2007
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400701681083
Preparation and Characterization of Crosslinked
Azobenzene Liquid-Crystalline Polymer Fibers
Taiki Yoshino
Jun-ichi Mamiya
Motoi Kinoshita
Tomiki Ikeda
Chemical Resources Laboratory, Tokyo Institute of Technology,
Yokohama, Japan
Yanlei Yu
Department of Materials Science, Fudan University, Shanghai, China
We report the synthesis and physical properties of crosslinked liquid-crystalline
polymer (CLCP) fibers consisting of a liquid-crystalline monomer and an acrylate
monomer with a hydroxyl group that acts as a reactive site for crosslinking. By dif-
ferential scanning calorimetry, the CLCP fibers showed a glass transition tem-
perature around 60ꢀC. Polarizing optical microscopy revealed that we obtained
fibers with high orientation of mesogens along the fiber axis. The CLCP fibers
showed deformations induced by heated the fiber to an isotropic phase.
Keywords: azobenzene; fiber; liquid crystal; photoinduced bending
INTRODUCTION
Human’s muscles are made of many bundles of fibers and their aniso-
tropic contractions are induced by electric stimuli. Recently, there has
been a considerable effort to develop artificial actuator materials that
can mimic muscle performance. To construct artificial muscles, it is
desirable to use soft materials with high mechanical flexibility. Cross-
linked liquid-crystalline polymers (CLCPs) are superior soft materials
that possess both LC orders and elasticity due to polymer networks.
CLCPs have been intensively investigated over the last two decades
Address correspondence to Tomiki Ikeda, Chemical Resources Laboratory, Tokyo
Institute of Technology, R1-11, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan.
E-mail: tikeda@res.titech.ac.jp
233=[989]

234=[990]
T. Yoshino et al.
[1–3]. One of the most characteristic properties of the CLCPs was
theoretically proposed by de Gennes et al. [4,5]. They suggested that
a change in temperature across an isotropic to nematic transition is
able to cause a strong uniaxial deformation of CLCPs at nearly con-
stant volume (thermomechanical effects). In fact, nematic CLCPs
showed anisotropic contraction along the director axis experimentally
when heated above the nematic to isotropic phase transition tempera-
ture [6]. Naciri et al. described a synthesis and physical properties of
CLCP fibers containing two side-chain mesogens. The fibers showed
an anisotropic contraction induced by the nematic-isotropic phase
transition [7]. Also self-assembled shape-memory fibers using triblock
main-chain LC polymers showed a contraction upon heating and an
elongation on cooling reversibly [8].
The phase transition of CLCPs from an LC to isotropic phase is
usually induced by heating the samples to high temperature. Some
photochromic molecules such as azobenzene could isothermally trigger
the phase transition of LCs through the photochemical reaction of the
chromophores [9]. The driving force of the photochemical phase tran-
sition is interpreted in terms of the changes in geometrical structure of
the chromophores: the trans azobenzene, for instance, has a rod-like
shape, which stabilizes the phase structure of the LC phase, while
its isomer (cis form) is bent form and acted as an impurity that dis-
rupts the orientational order. Finkelmann et al. achieved a large
photocontraction by using azobenzene-containing CLCPs, which is
ascribed to the change in the nematic order [10]. Recently, we have
realized photoinduced bending behavior of CLCP gels and films con-
taining azobenzene moieties [11–17]. The mechanism of the photo-
induced bending behavior is the difference of volume contractions
between the sample’s surface and the bulk of the sample.
The aim of this paper is the preparation of photoresponsive CLCP
fibers containing azobenzene moieties and to investigate their thermal
and optical properties.
EXPERIMENTAL
Materials
The structures of monofunctional LC monomers, 6-[4-(4-hexyloxy-
phenylazo)phenoxy]hexyl acrylate (A6AB6), and 6-{4-[4-(6-hydroxy-
hexyloxy)phenylazo]phenoxy}hexyl acrylate (A6AB6OH) and
a
crosslinker 4,4’-methylenebis(phenyl isocyanate) (MDI) used in this
study are shown in Figure 1. A6AB6 was synthesized according to
the method reported previously [18]. Scheme 1 shows the synthetic

Crosslinked Liquid-Crystalline Polymer Fibers
235=[991]
FIGURE 1 Structures and abbreviations of the LC monomers and the
crosslinker used in this study.
routes of A6AB6OH. All reagents were purchased from Tokyo Kasei
Co. or Wako Pure Chemical Industries, Ltd. The compounds synthe-
sized were identified by means of 1H NMR (JEOL, JNM-LA300),
fast atom bombardment mass spectrometry (FABMS, JEOL, JMS-
AX505W). Molecular weight of a copolymer was determined by gel per-
meation chromatography (GPC, JASCO, DG-980-50; column, Shodex
GPC K802 þ K804 þ K805; eluent, chloroform) calibrated with
standard polystyrenes.
SCHEME 1 Synthetic route for the LC monomer A6AB6OH.

236=[992]
T. Yoshino et al.
Synthesis of an LC Monomer
4-(6-Hydroxyhexyloxy)nitrobenzene (1). A mixture of 4-nitrophenol
(5.0 g, 36 mmol) and 6-chloro-1-hexanol (5.2 g, 38 mmol) was dissolved in
5 ml of DMF, and potassium carbonate (4.7 g, 34 mmol) and a trace
amount of potassium iodide were added to the solution. The resulting
solution was heated at 120ꢀC for 4 h. After the reaction mixture was
cooled to room temperature, 100 ml of water was added to the mixture.
The precipitate was collected and extracted with ethyl acetate. The ethyl
acetate solution was washed with water and dried with magnesium
sulfate. After the solvent was evaporated, 1 (8.1 g, 34 mmol) was obtained
in 94% yield.
1H NMR (300 MHz, CDCl3): d 1.37–1.86 (m, 8H, ꢁOꢁCH2ꢁ
(CH2)4ꢁCH2ꢁOH), 3.65 (t, J ¼ 6 Hz, 2H, ꢁCH2OH), 4.03 (t, J ¼ 6 Hz,
Hz, 2H, ꢁOꢁCH2ꢁ), 6.91–6.97 (m, 2H, ꢁAr), 8.17–8.22 (m, 2H, ꢁAr).
4-(6-Hydroxyhexyloxy)aniline (2). A mixture of 1 (8.1 g, 34 mmol)
and Pd=C (1.6 g) was added to 55 ml of THF, and the resulting suspen-
sion was stirred vigorously at 0ꢀC. Sodium boronhydride (5.5 g,
145 mmol) was added to the solution. The suspension was stirred at
room temperature for 2 h. About 110 ml of hydrochloric acid (1 M)
was added to the mixture at 0ꢀC. The precipitate was removed and
an aqueous solution of potassium carbonate was added to the mixture
to give pH 10. The mixture solution was extracted with ethyl acetate
and washed with water. Then the solution was dried with sodium sul-
fate. After the solvent was removed, 2 (6.5 g, 31 mmol) was obtained in
91% yield.
1H NMR (300 MHz, CDCl3): d 1.23–1.77 (m, 8H, ꢁOꢁCH2ꢁ(CH2)4
ꢁCH2ꢁOH), 3.38 (br, 2H, ꢁNH2), 3.63 (t, J ¼ 6 Hz, 2H, ꢁCH2–OH),
3.88 (t, J ¼ 6 Hz, 2H, ꢁOꢁCH2ꢁ), 6.55–6.64 (m, 2H, ꢁAr), 6.70–6.74
(m, 2H, ꢁAr).
4’-Hydroxy-4-(6-hydroxyhexyloxy)azobenzene (3). 2 (6.5 g, 31 mmol)
was dissolved in 120 ml of hydrochloric acid (1 M) and the resulting
solution was cooled at 0ꢀC. With stirring, 2.5 g (36 mmol) of sodium
nitrite in water (40 ml) was added dropwise into the solution to produce
diazonium salt. A mixture of phenol (3.5 g, 37 mmol) and sodium
hydroxide (2.4 g, 60 mmol) in water (100 ml) was added slowly at 0ꢀC.
An aqueous solution of potassium carbonate was added to the mixture
to give pH 7, and then a yellow solid precipitated. The reaction mixture
was stirred at 0ꢀC for 3 h. After hydrochloric acid (1 M) was added to
the reaction mixture to give pH 3, the precipitated solid was collected
and washed with water. The crude product was recrystallized from a mix-
ture of n-hexane and ethyl acetate to give 3 (6.8 g, 22 mmol) in 71% yield.

Crosslinked Liquid-Crystalline Polymer Fibers
237=[993]
1H NMR (300 MHz, CDCl3): d 1.38–1.58 (m, 6H, ꢁOꢁCH2ꢁ
CH2ꢁ(CH2)3ꢁCH2ꢁOH), 1.74–1.79 (m, 2H, ꢁOꢁCH2ꢁCH2ꢁ), 3.68
(t, J ¼ 6 Hz, 2H, ꢁCH2ꢁOH), 4.04 (t, J ¼ 6 Hz, 2H, ꢁOꢁCH2ꢁ),
6.92–7.00 (m, 4H, ꢁAr), 7.82–7.87 (m, 4H, ꢁAr).
6-(4-{4-[6-(2-Tetrahydropyranyloxy)hexyloxy]phenylazo}phenoxy)-
hexanol (4). A mixture of 3 (5.0 g, 16 mmol) and 2-(6-chlorohexyloxy)-
tetrahydropyran (4.0 g, 19 mmol) was dissolved in 10 ml of DMF, and
potassium carbonate (3.3 g, 24 mmol) and a trace amount of potassium
iodide were added to the solution. The resulting solution was heated at
120ꢀC for 3 h. After the reaction mixture was cooled to room tempera-
ture, 100 ml of water was added to the mixture. The precipitate was
collected and extracted with ethyl acetate. The ethyl acetate solution
was washed with water and dried with magnesium sulfate. After the
solvent was removed, the crude product was purified by recrystalliza-
tion from n-hexane and ethyl acetate to give 4 (6.0 g, 12 mmol) in 76%.
1H NMR (300 MHz, CDCl3): d 1.42–1.84 (m, 22H, ꢁArꢁOꢁCH2ꢁ
(CH2)4ꢁCH2ꢁOꢁ, ꢁOꢁCH2ꢁ (CH2)3ꢁCHꢁOꢁ), 3.37–3.41 (m, 2H,
ꢁOꢁCH2ꢁ(CH2)4ꢁCH2ꢁOꢁCHꢁ), 3.65 (t, J ¼ 6 Hz, 2H, ꢁCH2ꢁ
OH), 3.73–3.82 (m, 2H, ꢁOꢁCH2ꢁ(CH2)3ꢁCHꢁOꢁ), 4.02 (t, J ¼ 7 Hz,
Hz, 4H, ꢁArꢁOꢁCH2ꢁ), 4.57 (t, J ¼ 4 Hz, 1H, ꢁOꢁCH2ꢁ
(CH2)3ꢁCHꢁOꢁ), 6.95–6.99 (m, 4H, ꢁAr), 7.82–7.87 (m, 4H, ꢁAr).
6-(4-{4-[6-(2-Tetrahydropyranyloxy)hexyloxy]phenylazo}phenoxy)-
hexyl acrylate (5). A mixture of 4 (4.0 g, 8.0 mmol), triethylamine
(15 ml, 108 mmol) and a trace amount of hydroquinone were dissolved
in dehydrated THF (100 ml), and the resulting solution was cooled at
0ꢀC. With stirring, 2.0 ml (24 mmol) of acryloyl chloride in dehydrated
THF (40 ml) was added dropwise to the solution, and the reaction mix-
ture was stirred at room temperature for 48 h. The reaction mixture
was poured into saturated sodium hydrogen carbonate solution, and
the product was extracted with chloroform. The chloroform solution
was washed with water and dried with magnesium sulfate. After the
solvent was removed, a yellow solid obtained was purified by column
chromatography on silica gel (chloroform) and recrystallized from
ethyl acetate and methanol to give 5 (3.6 g, 6.6 mmol) in 82%.
1H NMR (300 MHz, CDCl3): d 1.40–1.79 (m, 22H, ꢁArꢁOꢁCH2ꢁ
(CH2)4ꢁCH2ꢁOꢁ, ꢁOꢁCH2ꢁ(CH2)3ꢁCHꢁOꢁ), 3.30–3.47 (m, 2H,
ꢁOꢁCH2ꢁ(CH2)4ꢁCH2ꢁOꢁCHꢁ), 3.66–3.83 (m, 2H, ꢁOꢁCH2ꢁ
(CH2)3ꢁCHꢁOꢁ), 3.99 (t, J ¼ 6 Hz, 4H, ꢁArꢁOꢁCH2ꢁ), 4.11 (t,
=
J ¼ 7 Hz, 2H, CH2 CHꢁOꢁCH2ꢁ), 5.74 (dd, J ¼ 2, 10 Hz, 1H,
=
=
cisꢁCH2 CHꢁ), 6.07 (dd, J ¼ 10, 17 Hz, 1H, CH2 CHꢁ), 6.33 (dd,
=
J ¼ 2, 17 Hz, 1H, transꢁCH2 CHꢁ), 6.90–6.93 (m, 4H, ꢁAr), 7.79–7.82
(m, 4H, ꢁAr).

238=[994]
T. Yoshino et al.
6-{4-[4-(6-Hydroxyhexyloxy)phenylazo]phenoxy}hexyl
acrylate
(A6AB6OH). 5 (3.0 g, 5.4 mmol) was dissolved in 150 ml of THF, and
the resulting solution was cooled at 0ꢀC. With stirring, 60 ml of
hydrochloric acid (1 M) was added dropwise to the solution, and the
reaction mixture was stirred at room temperature for 24 h. The mix-
ture solution was extracted with ethyl acetate and washed with water.
The ethyl acetate solution was dried with sodium sulfate. After the sol-
vent was removed, a yellow solid obtained was purified by column
chromatography on silica gel (chloroform) and recrystallized twice
from methanol to give 2.3 g (4.8 mmol) of A6AB6OH in 89% yield.
1H NMR (300 MHz, CDCl3): d 1.38–1.79 (m, 16H, ꢁOꢁCH2ꢁ
(CH2)4ꢁCH2ꢁOꢁ), 3.60 (t, J ¼ 6 Hz, 2H, ꢁCH2ꢁOH), 3.97 (t,
=
J ¼ 7 Hz, 4H, ꢁArꢁOꢁCH2ꢁ), 4.13 (t, J ¼ 7 Hz, 2H, CH2
=
CHCOOCH2ꢁ), 5.75 (dd, J ¼ 2, 10 Hz, 1H, cisꢁCH2 CHꢁ), 6.05 (dd,
=
J ¼ 10, 17 Hz, 1H, CH2 CHꢁ), 6.33 (dd, J ¼ 2, 17 Hz, 1H, trans-
=
CH2 CHꢁ), 6.90–6.93 (m, 4H, ꢁAr), 7.78–7.82 (m, 4H, ꢁAr). MS
(FABþ) m=z: 469 (Mþ ). Anal. Calcd for C27H36N2O5: C, 69.21; H,
7.74; N, 5.98. Found: C, 69.06; H, 7.62; N, 5.94.
Polymerization of Monomers
A mixture of A6AB6 (0.48 g, 1.1 mmol), A6AB6OH (0.21 g, 0.45 mmol)
and 5.0 mg (0.015 mmol, 1 mol% of monomers) of azobis(isobutyroni-
trile) (AIBN) were dissolved in 10 ml of dry toluene. The mixture
was degassed by five freeze-pump-thaw cycles and then sealed off.
The mixture was stirred and heated in an oil bath at 60ꢀC for 48 h.
The cooled solution was poured into 300 ml of methanol with stirring
to obtain a copolymer. The copolymer obtained was purified by
reprecipitation from THF into methanol three times and dried under
vacuum for 24 h to give 0.30 g of the copolymer PAB70 in 44% yield.
The number-average molecular weights (Mn) and the molecular
weight distribution (Mw=Mn) determined by GPC were 7,000 and 1.2.
Characterization Methods
The thermodynamic properties of the monomers and the copolymer
were determined by differential scanning calorimetry (DSC, Seiko
I&E, SSC-5200 and DSC220C) at heating and cooling rates of 3ꢀC=min
for the monomers and 10ꢀC=min for the copolymer. At least three
scans were performed to check the reproducibility. The mesomorphic
properties and phase transition behavior were examined with a pola-
rizing optical microscope (POM, Olympus, BH-2) equipped with a hot

Crosslinked Liquid-Crystalline Polymer Fibers
239=[995]
stage (Mettler, FP-90 and FP-82). The X-ray diffraction measurement
of the copolymer was performed by X-ray diffractctomertry (MAC
Science MXP with a thermal controller, model 5301).
RESULTS AND DISCUSSION
Mesomorphic Properties of Monomers and a Copolymer
The mesomorphic properties of the monomers and the copolymer were
studied by POM, DSC and X-ray diffraction measurement. For
A6AB6, a typical schlieren texture of a nematic phase was observed
at 86ꢀC and then isotropization occurred at 92ꢀC upon heating. When
cooled from the isotropic phase, the nematic phase appeared at 92ꢀC
and then a monotropic smectic phase at 85ꢀC. Crystallization started
at 77ꢀC. A6AB6OH also exhibited an LC phase from 93 to 106ꢀC upon
heating. In addition, it was found that PAB70 exhibited an LC phase
from 71 to 140ꢀC. X-ray diffraction measurements at 90ꢀC for
A6AB6OH or 100ꢀC for PAB70 revealed that both compounds exhi-
bited a diffuse halo in a wide-angle region, meaning that they showed
a nematic phase (Fig. 2). Thermodynamic properties of the monomers
and the copolymer are summarized in Table 1.
Preparation of CLCP Fibers
The CLCP fibers were prepared from a mixture of PAB70 and the
MDI crosslinker. A small amount of PAB70 (20 mg) was heated to
FIGURE 2 X-ray patterns of the liquid-crystalline monomer A6AB6OH and
the copolymer PAB70.

240=[996]
T. Yoshino et al.
TABLE 1 Phase Transition Temperature and Thermodynamic Parameters of
the Monomers and the Copolymer
Phase transition
temperaturea (ꢀC)
DHNI
(kJ=mol)
DSNI
(J=molꢂK)
DHSN
(kJ=mol)
DSSN
(J=molꢂK)
Compound
A6AB6
A6AB6OH
PAB70
I 92 N (85 S) 77 K
I 103 N 77 K
I 140 N 71 G
1.1
5.6
0.95
3.1
14.7
2.3
1.3
–
–
3.7
–
–
aI, isotropic; N, nematic; S, smectic; K, crystal; G, glass; DHNI, change in enthalpy of
N-I phase transition; DSNI, change in entropy of N-I phase transition; DHSN, change in
enthalpy of S-N phase transition; DSSN, change in entropy of S-N phase transition.
145ꢀC on a glass substrate placed on a hot stage. 2 mg of MDI was
added at this temperature, and then the temperature was lowered to
around 125ꢀC where PAB70 showed a nematic phase. The sample
was mixed well at this temperature for several seconds until the
mixture appeared homogeneous. At this point, the mixture
became viscous, indicating that the crosslinking had started to occur.
The CLCP fibers were formed by dipping a tip of a pick and pulling the
mixture with it as quickly as possible.
Thermal and Optical Properties of the CLCP Fiber
The DSC thermogram of the CLCP fibers is shown in Figure 3. The
glass transition temperature (Tg) of the CLCP fibers appeared around
60ꢀC. The CLCP fiber was observed by POM (Fig. 4). This figure
shows the CLCP fiber placed between crossed polarizers at room
FIGURE 3 DSC thermogram of the CLCP fibers.

Crosslinked Liquid-Crystalline Polymer Fibers
241=[997]
FIGURE 4 Polarizing optical micrographs of the texture of the CLCP fiber.
Sample angle to the analyzer: h ¼ 0ꢀ (a); h ¼ 45ꢀ (b). The textures were
observed at room temperature.
temperature. The CLCP fiber showed a contrast inversion upon
rotating the sample to every ꢃ 45ꢀ position with respect to the
analyzer. This means that the CLCP fibers have high orientation of
mesogens. When the CLCP fiber was dipped in a chloroform solution
as shown in Figure 5, the fiber was undissolved to the solution,
FIGURE 5 Photographs of the CLCP fiber dipping in chloroform solution.

242=[998]
T. Yoshino et al.
FIGURE 6 Photographs of the deformation behavior of the CLCP fibers.
Temperature: 60ꢀC (a); 180ꢀC (b). Size of the fiber: 1.3 mm ꢄ 30 mm (a);
0.75 mm ꢄ 60 mm (b).
indicating that the CLCP fiber formed network structures by
urethane bonding.
Thermomechanical Behavior of the CLCP Fiber
When the CLCP fibers were heated from an LC phase to an isotropic
phase, the length of the fiber along the fiber axis was decreased, and
when the fibers were cooled to room temperature, the fibers regained
their original length as shown in Figure 6. The contraction of the fiber
is induced by the change in the nematic order [7].
CONCLUSIONS
The CLCP fibers containing azobenzene moieties were obtained by
mixing a copolymer and a crosslinker in the nematic phase. From
DSC measurement, it was found that the CLCP fiber showed a Tg
around 60ꢀC. The CLCP fiber showed a contrast inversion upon
rotating the sample to every ꢃ45ꢀ position with respect to the
analyzer. Also the CLCP fibers showed contractions when the fibers
were heated to an isotropic phase. Furthermore, we expect that the
CLCP fibers may show photoinduced bending behavior as the CLCP
films previously reported. This is now under investigation.
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