Synthesis of reactive azobenzene main-chain liquid crystalline polymers via michael addition polymerization and photomechanical effects of their supramolecular hydrogen-bonded fibers

Article abstract of DOI:10.1021/ma401655k

A new and efficient strategy for obtaining a series of reactive azobenzene (azo)-containing main-chain liquid crystalline polymers (LCPs) is described, which involves the first design and synthesis of acrylate-type azo monomers with different length of flexible spacers and an amino end-group (in its trifluoroacetate salt form) and their subsequent Michael addition polymerization under mild reaction conditions. The resulting polymers showed rather high thermal stability, relatively low glass transition temperatures, a broad temperature range of smectic C liquid crystalline phase, and reversible photoresponsive behavior. The presence of secondary amino groups in the backbones of these azo main-chain LCPs not only made them highly reactive precursors for various new functional linear and cross-linked azo LCPs but also led to the formation of hydrogen-bonding interactions among their polymer chains. Supramolecular hydrogen-bonding cross-linked LCP fibers were directly fabricated by using the simple melt spinning method, which proved to have a high order of mesogen along the fiber axis and exhibit good mechanical properties, fast and reversible photoinduced bending and unbending behaviors, and large photoinduced stress (240 kPa) at close to ambient temperature as well as excellent photodeformation fatigue resistance.

Full text of DOI:10.1021/ma401655k

Article
pubs.acs.org/Macromolecules
Synthesis of Reactive Azobenzene Main-Chain Liquid Crystalline
Polymers via Michael Addition Polymerization and Photomechanical
Effects of Their Supramolecular Hydrogen-Bonded Fibers
Liangjing Fang, Hongtao Zhang, Zidong Li, Ying Zhang, Yuying Zhang, and Huiqi Zhang*
Key Laboratory of Functional Polymer Materials, Ministry of Education, Synergetic Innovation Center of Chemical Science and
Engineering (Tianjin), and Department of Chemistry, Nankai University, Tianjin 300071, People’s Republic of China
*
S Supporting Information
ABSTRACT: A new and efficient strategy for obtaining a
series of reactive azobenzene (azo)-containing main-chain
liquid crystalline polymers (LCPs) is described, which involves
the first design and synthesis of acrylate-type azo monomers
with different length of flexible spacers and an amino end-
group (in its trifluoroacetate salt form) and their subsequent
Michael addition polymerization under mild reaction con-
ditions. The resulting polymers showed rather high thermal
stability, relatively low glass transition temperatures, a broad
temperature range of smectic C liquid crystalline phase, and
reversible photoresponsive behavior. The presence of secondary amino groups in the backbones of these azo main-chain LCPs
not only made them highly reactive precursors for various new functional linear and cross-linked azo LCPs but also led to the
formation of hydrogen-bonding interactions among their polymer chains. Supramolecular hydrogen-bonding cross-linked LCP
fibers were directly fabricated by using the simple melt spinning method, which proved to have a high order of mesogen along
the fiber axis and exhibit good mechanical properties, fast and reversible photoinduced bending and unbending behaviors, and
large photoinduced stress (240 kPa) at close to ambient temperature as well as excellent photodeformation fatigue resistance.
INTRODUCTION
modification with other functional species, which makes them
highly promising functional materials for such applications as
the improvement of the thermal stability of the photoinduced
■
(
Azobenzene (azo)-containing liquid crystalline polymers
LCPs) have drawn considerable attention in recent years
because of their combined intriguing properties of anisotropic
18−20
surface relief grating,
preparation of photoresponsive
liquid crystalline elastomers (LCEs) for photomechanical
liquid crystals, photoresponsive materials, and flexible poly-
8,9,12,21−23
1
−17
devices,
and so forth.
mers.
The liquid crystalline properties of the azo polymers
In general, the reactive azo LCPs can be divided into side-
chain and main-chain types according to the location of the azo
groups in the polymers. Since the reactive azo side-chain LCPs
can be easily prepared either by the copolymerization of a
monovinyl azo monomer without a reactive group and another
are related to the rod-like trans-azo mesogens, and their
photoresponsive properties stem from the photoinduced fast
and reversible isomerization between the trans and cis isomers
of the azo groups upon exposure to UV or visible light, which
triggers significant changes in their physicochemical properties
and can lead to photoinduced reorientation of azo groups in the
polymers. Many potential applications have been proposed for
these polymers, such as optical data storage, liquid crystal
displays, molecular switches, nonlinear optical devices, surface
relief gratings, and photomechanical systems.
1
8,22,24
monovinyl monomer with a reactive group
or by the
direct polymerization of a monovinyl azo monomer with a
2
3,25,26
reactive group,
such functional side-chain azo LCPs have
been prepared and used in different areas in most cases. In
sharp contrast, the reactive azo main-chain LCPs have been
27
rarely reported up to now, although they have some distinct
advantages over the side-chain ones such as their better thermal
So far, a great number of azo LCPs with various structures
have been developed for different purposes, such as comb-
shaped LCPs with azo mesogens in the side chains,
2
8
12
1
−4,6,13
stability and much stronger chain anisotropy, which is
probably due to the availability of rather limited synthetic
1
,12
LCPs with azo units in the main chains, and cross-linked azo
2
8
4
,5,7−12,14−17
methods. As far as we know, only Acierno et al. reported the
synthesis of a series of reactive main-chain segmented azo LCPs
with pendent vinyl groups via solution polycondensation
LCPs.
In addition, azo LCPs can also be classified
into nonreactive and reactive ones, depending on whether they
contain reactive groups or not. Among them, the reactive azo
LCPs are particularly interesting because they could offer many
new possibilities for generating advanced photoresponsive
materials by their transformation into cross-linked LCP
networks through the cross-linking or by their further
Received: August 7, 2013
Revised: August 31, 2013
©
XXXX American Chemical Society
A
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Article
Scheme 1. Synthetic Route and Chemical Structures of the Acrylate-Type Azo Monomers with an Amino End-Group in Its
Trifluoroacetate Salt Form (M-m (m = 2, 6, 10)) and Their Main-Chain LCPs (MP-m (m = 2, 6, 10)) Prepared via Michael
Addition Polymerization as Well as the Chemical Reactivity and Supramolecular Hydrogen Bonding Interaction of These Main-
Chain LCPs (TEA: Triethylamine; HDI: 1,6-Hexamethylene Diisocyanate)
approach and studies on their photoresponsive properties both
reaction conditions, high functional group tolerance, and high
conversions, the preparation of LCPs with azo units in their
27
34
in the solutions and in thin films. In addition, several main-
chain azo polymers bearing reactive groups have also been
prepared for different purposes, but they did not show liquid
main chains by Michael addition reaction has never been reported.
The chemical structures, thermal and phase transition proper-
ties, and photoresponsivity of the resulting main-chain azo
polymers were characterized in detail. In addition, the high
chemical reactivity of these main-chain azo polymers and the
photoinduced bending behaviors of the supramolecular hydro-
gen-bonded LCP fibers made from them by the simple melt
spinning method were also demonstrated. To our knowledge, the
findings we report here represent the first successful example of the
supramolecular photomechanical system based on the main-chain
LCPs with azo mesogens in their backbones that can show good
mechanical properties, fast and reversible photoinduced bending
and unbending behaviors and large photoinduced stress at close to
ambient temperature, and excellent photodeformation fatigue
resistance.
2
9−33
crystalline properties.
On the basis of the above
statements, it can be concluded that despite the promising
future of reactive azo main-chain LCPs in many applications,
their synthesis still remains a challenge. Therefore, the
development of new azo-containing main-chain LCPs bearing
reactive substituents (particularly those with high reactivity
under mild conditions) is of significant importance.
Herein, we report for the first time an efficient strategy for
preparing a series of azo main-chain LCPs with reactive
secondary amino groups in the polymer backbones (i.e., MP-m
(m = 2, 6, 10), Scheme 1) by the rational design and synthesis
of acrylate-type azo monomers with different length of flexible
spacers and an amino end-group in its trifluoroacetate salt form
(
M-m (m = 2, 6, 10)) and their subsequent Michael addition
polymerization in the presence of triethylamine (TEA) under
mild reaction conditions. Note that although Michael addition
between acrylates and amines has found widespread
applications in the macromolecular design due to its mild
EXPERIMENTAL SECTION
■
Materials. Tetrahydrofuran (THF, Tianjin Jiangtian Chemicals,
China, 99%) was refluxed over sodium and then distilled. Dichloro-
methane (CH Cl , Tianjin Jiangtian Chemicals, 99%) and chloroform
2
2
B
dx.doi.org/10.1021/ma401655k | Macromolecules XXXX, XXX, XXX−XXX

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Article
(
CHCl , Tianjin Jiangtian Chemicals, 99%) were refluxed over calcium
(q, 2H, −CH N−), 1.94−1.19 (m, 18H, −(CH ) − and −CH CN−),
3
2
2
8
2
hydride and then distilled. N,N-Dimethylformamide (DMF, Tianjin
Jiangtian Chemicals, 99.5%) was dried with anhydrous magnesium
sulfate and then distilled under vacuum. Triethylamine (TEA, Tianjin
Jiangtian Chemicals, 99%) was dried with anhydrous sodium sulfate
and then distilled. Thionyl chloride (Tianjin Jiangtian Chemicals,
1.39 (s, 9H, −CH ).
3
Trifluoroacetate Salt of 3-Amino-1-Propyl 4-((4-(ω-
Acryloyloxyethyloxy))phenylazo)benzoate (M-2). Trifluoroacetic
acid (27 mL, 0.36 mol) was added dropwise into a solution of Boc-M-
2 (1.78 g, 3.6 mmol) in dried CH Cl (27 mL) in an ice bath. The
2 2
9
9.5%) was purified by distillation just prior to use. Methacryloyl
chloride was prepared by the reaction between methacrylic acid
Tianjin Jiangtian Chemicals, 99%) and thionyl chloride. Methacrylic
reaction mixture was then stirred at ambient temperature for 5 h,
which was subsequently poured into a large amount of ether (250
mL). The precipitate was filtered, washed with ether several times, and
then dried under vacuum at 40 °C for 24 h to obtain the orange-yellow
product (yield: 93%); mp 132−134 °C (determined by POM at a
heating rate of 10 °C/min). UV−vis (DMF): λmax/nm (ε/L mol−
(
anhydride was prepared by reacting methacrylic acid with methacryloyl
chloride in the presence of an aqueous solution of sodium hydroxide.
Acrylic chloride was prepared by the reaction between acrylic acid
1
−1
(
Tianjin Damao Chemical Reagent Factory, China, 99.5%) and
cm ) = 360 (23 680), around 450 (not available due to the overlap of
1
benzoyl chloride (Tianjin Jiangtian Chemicals, 98%). 4-((4-Hydroxy)-
phenylazo)benzoic acid, 4-((4-ω-hydroxyalkyloxy)phenylazo)benzoic
acid (HAzoA-m (m = 2, 6, 10)), and 4-((4-(ω-acryloyloxyalkyloxy))-
phenylazo)benzoic acid (AAzoA-m (m = 2, 6, 10)) were synthesized
the absorption bands). H NMR (400 MHz, DMSO-d
8.24−8.14 (d, 2H, Ar−H), 8.02−7.94 (m, 4H, Ar−H), 7.88−7.72 (br,
3H, −NH
), 7.29−7.17 (d, 2H, Ar−H), 6.43−6.34 (dd, 1H, CHC−
COO−), 6.30−6.19 (dd, 1H, CCH−COO−), 6.02−5.95 (dd, 1H,
CHC−COO−), 4.54−4.47 (t, 2H, Ar−COOCH −), 4.43−4.34
(m, 4H, CC−COOCH − and Ar−OCH −), 3.06−2.97 (t, 2H,
−CH N−), 2.11−1.98 (m, 2H, −CH CN−).
6
): δ (ppm) =
3
3
5,36
according to our previously reported procedure (Scheme 1).
3-
2
(
tert-Butoxylcarbonylamino)-1-propanol was prepared following a
2
2
37
literature method. 4-(Dimethylamino)pyridine (DMAP, Merck,
analytical grade (AR)), N,N′-dicyclohexylcarbodiimide (DCC, Tianjin
Jiangtian Chemicals, AR), and all the other chemicals were
commercially available and used without further purification.
2
2
Trifluoroacetate Salt of 3-Amino-1-Propyl 4-((4-(ω-
Acryloyloxyhexyloxy))phenylazo)benzoate (M-6). Prepared as
for M-2 (yield: 95%); mp 131−133 °C (determined by POM at a
heating rate of 10 °C/min). UV−vis (DMF): λ /nm (ε/L mol−
1
3
-(tert-Butoxylcarbonylamino)-1-Propyl 4-((4-(ω-
Acryloyloxyethyloxy))phenylazo)benzoate (Boc-M-2). AAzoA-2
3.40 g, 10 mmol), 3-(tert-butoxylcarbonylamino)-1-propanol (3.50 g,
0 mmol), and DMAP (0.244 g, 2 mmol) were dissolved in dried THF
150 mL), and the solution was cooled to 0 °C. To this mixture was
added dropwise a solution of DCC (1.94 g, 10 mmol) in dried THF
50 mL) with stirring. After being stirred at ambient temperature for
4 h, the reaction mixture was filtered and the filtrate was poured into
max
−
1
cm ) = 361 (23 900), around 450 (not available due to the overlap of
1
(
2
(
the absorption bands). H NMR (400 MHz, DMSO-d
8.29−8.15 (d, 2H, Ar−H), 8.02−7.91 (m, 4H, Ar−H), 7.89−7.78 (br,
3H, −NH
), 7.21−7.13 (d, 2H, Ar−H), 6.37−6.29 (dd, 1H, CHC−
COO−), 6.21−6.13 (dd, 1H, CCH−COO−), 5.95−5.91 (dd, 1H,
CHC−COO−), 4.43−4.37 (t, 2H, Ar−COOCH −), 4.17−4.07
(m, 4H, CC−COOCH − and Ar−OCH −), 3.07−2.98 (t, 2H,
−CH N−), 2.10−1.35 (m, 10H, −(CH − and −CH CN−).
6
): δ (ppm) =
3
(
2
2
2
2
a large amount of water (400 mL). The resulting precipitate was
collected and then purified with silica gel column chromatography by
using a mixture of ethyl acetate and CH Cl (1/12 v/v) as the eluent
to provide an orange product (yield: 70%). Melting point (mp) 104−
1
heating rate of 10 °C/min). UV−vis (DMF): λmax/nm (ε/L mol
cm ) = 362 (26 640), around 450 (not available due to the overlap of
the absorption bands). H NMR (400 MHz, DMSO-d ): δ (ppm) =
2
2
)
4
2
Trifluoroacetate Salt of 3-Amino-1-Propyl 4-((4-(ω-
Acryloyloxydecyloxy))phenylazo)benzoate (M-10). Prepared as
for M-2 (yield: 96%); mp 129−131 °C (determined by POM at a
2
2
heating rate of 10 °C/min). UV−vis (DMF): λ /nm (ε/L mol−
1
06 °C (determined by polarizing optical microscope (POM) at a
max
−1
−1
cm ) = 361 (24 120), around 450 (not available due to the overlap of
−1
1
the absorption bands). H NMR (400 MHz, DMSO-d
): δ (ppm) =
6
1
8.25−8.15 (d, 2H, Ar−H), 8.04−7.91 (m, 4H, Ar−H), 7.89−7.72 (br,
3H, −NH
), 7.24−7.13 (d, 2H, Ar−H), 6.36−6.28 (dd, 1H, CHC−
COO−), 6.22−6.13 (dd, 1H, CCH−COO−), 5.97−5.91 (dd, 1H,
CHC−COO−), 4.43−4.35 (t, 2H, Ar−COOCH −), 4.15−4.05
(m, 4H, CC−COOCH − and Ar−OCH −), 3.07−2.97 (t, 2H,
−CH N−), 2.10−1.22 (m, 18H, −(CH − and −CH CN−).
6
8
2
.24−8.13 (d, 2H, Ar−H), 8.02−7.90 (m, 4H, Ar−H), 7.27−7.17 (d,
3
H, Ar−H), 6.98−6.87 (br, 1H, −NH), 6.42−6.33 (dd, 1H, CHC−
COO−), 6.29−6.18 (dd, 1H, CCH−COO−), 6.05−5.95 (dd, 1H,
2
CHC−COO−), 4.57−4.47 (t, 2H, Ar−COOCH −), 4.44−4.38 (t,
2
2
2
2
H, CC−COOCH −), 4.35−4.27 (t, 2H, Ar−OCH −), 3.18−3.04
2
2
)
8
2
2
2
(
q, 2H, −CH N−), 1.92−1.81 (m, 2H, −CH CN−), 1.37 (s, 9H,
Synthesis of the Main-Chain Azo Polymers (MP-m (m = 2, 6,
0)) via Michael Addition Polymerization. The typical Michael
2
2
1
−
CH ).
3
addition polymerization of M-6 is presented as follows: M-6 (1.134 g,
2 mmol), triethylamine (TEA, 0.58 mL, 4 mmol), and a mixture of
freshly distilled methanol and DMF (1/1 v/v, 80 mL) were added into
a one-neck round-bottom flask (250 mL) successively, and a clear
solution was obtained after the ultrasonic treatment. The reaction
mixture was bubbled with argon for 20 min, sealed, and then immersed
into a thermostated oil bath at 40 °C in the dark. After a certain time
of polymerization with stirring (note that the precipitation of the
resulting polymers was observed during the polymerization process),
the reaction mixture was cooled to room temperature and then poured
into methanol (200 mL). The precipitate was filtered, and the
obtained solid was washed thoroughly with warm methanol until no
monomer was detected with thin layer chromatography. Finally, the
product was dried at 40 °C under vacuum for 24 h to provide the
orange-yellow main-chain azo polymer MP-6. The yield of MP-6 was
34, 40, 54, 56, and 58% for a polymerization time of 3, 6, 12, 24, and
48 h, respectively (Figure S1 in the Supporting Information).
Michael addition polymerizations of M-2 and M-10 were performed
similarly as for M-6 except that the polymerization time was fixed at 24
h, leading to their corresponding orange-yellow main-chain azo
polymers MP-2 and MP-10 in a yield of 46% and 64%, respectively.
Cross-Linking of the Main-Chain Azo Polymer MP-6. The
synthesis of the cross-linked main-chain azo polymers CL-MP-6-f (f =
2, 4) (CL-MP-6 refers to the cross-linked MP-6 and f represents the
3
-(tert-Butoxylcarbonylamino)-1-Propyl 4-((4-(ω-
Acryloyloxyhexyloxy))phenylazo)benzoate (Boc-M-6). Prepared
as for Boc-M-2 (yield: 65%); mp 96−98 °C (determined by POM at a
−1
heating rate of 10 °C/min). UV−vis (DMF): λ /nm (ε/L mol
max
−1
cm ) = 363 (27 200), around 450 (not available due to the overlap of
1
the absorption bands). H NMR (400 MHz, DMSO-d ): δ (ppm) =
6
8
2
.22−8.12 (d, 2H, Ar−H), 8.03−7.91 (m, 4H, Ar−H), 7.20−7.12 (d,
H, Ar−H), 7.02−6.93 (br, 1H, −NH), 6.38−6.29 (dd, 1H, CHC−
COO−), 6.24−6.13 (dd, 1H, CCH−COO−), 5.98−5.91 (dd, 1H,
CHC−COO−), 4.37−4.27 (t, 2H, Ar−COOCH −), 4.16−4.06
2
(
−
9
m, 4H, CC−COOCH − and Ar−OCH −), 3.16−3.06 (q, 2H,
2
2
CH N−), 1.93−1.29 (m, 10H, −(CH ) − and −CH CN−), 1.38 (s,
2
2
4
2
H, −CH ).
3
3
-(tert-Butoxylcarbonylamino)-1-Propyl 4-((4-(ω-
Acryloyloxydecyloxy))phenylazo)benzoate (Boc-M-10). Pre-
pared as for Boc-M-2 (yield: 67%); mp 92−94 °C (determined by
POM at a heating rate of 10 °C/min). UV−vis (DMF): λ /nm (ε/L
max
−1
−1
mol cm ) = 363 (26 840), around 450 (not available due to the
1
overlap of the absorption bands). H NMR (400 MHz, DMSO-d ): δ
6
(
7
ppm) = 8.28−8.14 (d, 2H, Ar−H), 8.08−7.92 (m, 4H, Ar−H), 7.26−
.14 (d, 2H, Ar−H), 7.04−6.93 (br, 1H, -NH), 6.42−6.31 (dd, 1H,
CHC−COO−), 6.25−6.13 (dd, 1H, CCH−COO−), 6.01−5.91
(
4
dd, 1H, CHC−COO−), 4.39−4.28 (t, 2H, Ar−COOCH −),
2
.19−4.06 (m, 4H, CC−COOCH − and Ar−OCH −), 3.19−3.08
2
2
C
dx.doi.org/10.1021/ma401655k | Macromolecules XXXX, XXX, XXX−XXX

Macromolecules
Article
molar ratio of the repeat unit (or reactive secondary amino group) in
the main-chain azo polymer to 1,6-hexamethylene diisocyanate
The melting points of the azo monomers, the liquid crystalline textures
of the azo polymers, and the alignment states of the polymer fibers
were observed by using an Olympus BX51 polarizing optical
microscope (POM, with crossed polarizer and analyzer) equipped
with a Linksys 32 THMSE600 hot stage and a digital camera
(Micropublisher 5.0 RTV). Small-angle X-ray scattering (SAXS)
measurements were carried out on a Bruker NanoSTAR SAXS system
using Cu Kα (λ = 1.542 Å) as the radiation source. The working
voltage and current are 40 kV and 35 mA, respectively. The distance
between the sample and the detector is 27.15 cm. A temperature
control unit in conjunction with the instrument was utilized to study
the structure evaluations of the liquid crystalline mesophases by
heating the samples to the desired temperatures. A Bio-Rad FTS6000
FT-IR spectrometer equipped with an UMA-500 microscope, an MCT
detector (cooled with liquid nitrogen), and a Linkam FTIR600 hot
stage and temperature control system was utilized to carry out the
variable temperature FT-IR measurements. The studied polymer
sample was sandwiched between two KBr slides, and its temperature
(
HDI)) by cross-linking MP-6 with HDI was performed as follows:
MP-6 (22.7 mg, 0.05 mmol of repeat unit in MP-6) was dissolved in
dried chloroform (1 mL). After the solution was bubbled with argon in
an ice bath for 5 min, HDI (0.025 mmol) was added. The reaction
mixture was further bubbled with argon for 10 min in an ice bath, and
the flask was then sealed and immersed into a thermostated oil bath at
6
0 °C. Finally, the cross-linked product was washed thoroughly with
warm methanol and then dried at 40 °C under vacuum for 24 h to
provide orange-yellow CL-MP-6−2 (yield: 87%).
CL-MP-6−4 was prepared similarly as for CL-MP-6−2, but a lower
amount of HDI (0.0125 mmol) was added into the reaction system.
The yield of CL-MP-6−4 was 85%.
Polymer Analogous Reactions of MP-m (m = 2, 6, 10) with
Anhydrides. A typical procedure for the polymer analogous reaction
of MP-6 with an anhydride is presented as follows (Scheme S1 in the
Supporting Information): MP-6 (22.7 mg, 0.05 mmol of repeat unit in
MP-6) was dissolved in dried chloroform (2 mL). To this solution was
added acetic anhydride (0.2 mL, 2 mmol). After being stirred at
ambient temperature for 5 h, the reaction mixture was poured into
ether (20 mL). The precipitate was collected through centrifugation,
washed with ether, and then dried at 40 °C under vacuum for 24 h to
provide the orange-yellow acetic anhydride-modified main-chain azo
polymer A-MP-6 (yield: 96%).
−1
was changed at a rate of 10 °C min . Sixteen scans were conducted
for each spectrum at the selected temperatures in the wavenumber
−1
−1
range of 700−4000 cm with a resolution of 8 cm . An UV−vis
scanning spectrophotometer (TU1900, Beijing Purkinje General
Instrument Co., Ltd.) was utilized to obtain the UV−vis spectra of
the studied azo monomer and polymer solutions at 25 °C. The
photochemical isomerization of the polymer solution was investigated
by irradiating it first with a 365 nm UV lamp (12 W) until the
photostationary state was reached and then with a 450 nm visible light
The polymer analogous reactions of MP-6 with Boc anhydride (or
namely di-tert-butyl dicarbonate) and methacrylic anhydride were
performed similarly following the above procedure, leading to the
corresponding orange-yellow anhydride-modified main-chain polymers
B-MP-6 and M-MP-6 in a yield of 91% and 93%, respectively.
Acetic anhydride-modified main-chain azo polymers MP-2 and MP-
lamp (18 W, wavelength range 400−550 nm, λ = 450 nm; a filter
max
was put between the samples and the lamp during the study in order
to block the light with wavelength λ < 430 nm). The photoinduced
bending and unbending behaviors of the MP-m (m = 2, 6, 10) fibers
were investigated as follows: Part of a polymer fiber was first pasted
onto an aluminum block, which was heated to a certain temperature
with a hot stage under the control of a thermocouple (IKA C-MAG
HP 7). Irradiation of the polymer fiber with 365 nm UV light and
visible light (λ > 430 nm) was performed using a high-pressure
mercury lamp (USHIO SP-7) through glass filters. The photographs of
the bending and unbending behaviors of the polymer fiber were taken
by using a CCD camera (KBier-1202). The UV-induced bending and
thermally induced unbending behaviors of the MP-6 fiber were
investigated similarly. Thermomechanical analysis (TMA) was
performed in a stretch mode under an external tension at a constant
temperature by using a dynamic mechanical analyzer (DMA) (TA
Instruments, DMA-Q800) to study the mechanical properties (at 28
°C) and photoinduced stress (at 35 °C) of the azo main-chain LCP
fibers.
1
0 (i.e., A-MP-2 and A-MP-10) were prepared similarly as for A-MP-6,
and the yields of A-MP-2 and A-MP-10 were 96% and 95%,
respectively.
Fabrication of MP-m (m = 2, 6, 10) Fibers. The main-chain azo
polymer fibers were prepared by using the simple melt spinning
2
3,38
method as reported previously,
but in the absence of any chemical
cross-linker. A typical procedure for the fabrication of MP-6 fiber is
presented as follows: 5 mg of MP-6 was heated to 200 °C on a glass
slide placed on a hot stage (IKA, C-MAG HP 7), which was kept at
that temperature for 2 min (the sample was in an isotropic state). The
temperature was then gradually lowered to 130 °C where the sample
was in a liquid crystalline state. The MP-6 fiber was then drawn by
dipping the tip of a metallic tweezer into the melt sample and pulling it
quickly.
The MP-2 and MP-10 fibers were fabricated similarly as for the MP-
6
fiber, except that the spinning temperature was changed from 130 °C
for MP-6 to 115 and 120 °C for MP-2 and MP-10, respectively.
1
RESULTS AND DISCUSSION
Characterization. H NMR spectra were recorded on a Varian
■
Unity plus-400 spectrometer (400 MHz). The molecular weights and
molar mass dispersities (Đ) of the acetic anhydride-modified main-
chain azo polymers (i.e., A-MP-m (m = 2, 6, 10)) were determined
with a gel permeation chromatograph (GPC) equipped with an
Agilent 1200 series manual injector, an Agilent 1200 HPLC pump, an
Agilent 1200 refractive index detector, and two Waters Styragel HT
columns (HT 4 and HT 3) with 5K-600 K and 500−30K molecular
ranges. Chloroform was used as the eluent at a flow rate of 1.0 mL
Synthesis and Characterization of the Reactive Azo
Main-Chain LCPs. It has been well demonstrated that the
Michael addition between acrylates and amines is highly
versatile in the macromolecular design due to its mild reaction
conditions, high functional group tolerance, and high
34
conversions. Therefore, it can be expected that Michael
addition polymerization of the acrylate-type azo monomers
with an amino end-group should take place easily under
suitable reaction conditions, leading to the main-chain azo
polymers (Scheme 1). To confirm our hypothesis, a series of
acrylate-type azo monomers with different length of flexible
spacers and an amino end-group in its trifluoroacetate salt form
−
1
min . The calibration curve was obtained by using polystyrene (PS)
standards. Thermogravimetric analyses (TGA) were performed on a
Netzsch TG 209 instrument in a nitrogen atmosphere at a heating rate
−1
of 10 °C min . Differential scanning calorimetry (DSC, Netzsch 200
F3) was utilized to study the phase transitions of the azo polymers at a
−1
heating/cooling rate of 10 °C min under nitrogen. The temperature
and heat flow scale were calibrated with standard materials including
indium (70−190 °C), tin (150−270 °C), zinc (350−450 °C), bismuth
(
i.e., trifluoroacetate salt of 3-amino-1-propyl 4-((4-(ω-
acryloyloxyalkyloxy))phenylazo)benzoate, M-m (m = 2, 6,
0)) were first designed and synthesized according to the
1
(
190−310 °C), and mercury (−100−0 °C) in different temperature
synthetic route illustrated in Scheme 1. 4-((4-Hydroxy)-
phenylazo)benzoic acid, 4-((4-ω-hydroxyalkyloxy)phenylazo)-
benzoic acid (HAzoA-m, m = 2, 6, 10), and 4-((4-ω-
acryloyloxyalkyloxy)phenylazo)benzoic acid (AAzoA-m, m =
ranges. The glass transition temperatures (T ) of the azo polymers
g
were determined as the midpoints of the step changes of the heat
capacities, while the phase transition temperatures were measured
from the maximum/minimum of the endothermic/exothermic peaks.
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2
, 6, 10) were prepared according to our previously reported
3
5,36
procedure.
3-(tert-Butoxylcarbonylamino)-1-propyl 4-((4-
(ω-acryloyloxyalkyloxy))phenylazo)benzoate (Boc-M-m, m =
2, 6, 10) was readily prepared in quite good yields (65−70%)
through the coupling reaction between AAzoA-m (m = 2, 6, 10)
and 3-(tert-butoxylcarbonylamino)-1-propanol in the presence
of DMAP and DCC. Boc-M-m (m = 2, 6, 10) were then reacted
with trifluoroacetic acid to provide the deprotected acryalte azo
monomers with an amino group in its trifluoroacetate salt form
(M-m (m = 2, 6, 10)) in almost quantitative yields. The purity
of Boc-M-m and M-m (m = 2, 6, 10) was satisfactory, as verified
1
with both thin layer chromatography and the H NMR
technique (Figure 1a,b).
The Michael addition polymerization of M-6 was first carried
out by stirring it with TEA in a mixture of freshly distilled
methanol and DMF (1/1 v/v) under argon at 40 °C for
different times. A series of orange-yellow azo polymers (MP-6)
were readily obtained by pouring the reaction mixtures into
methanol and thoroughly washing the resulting precipitates
with warm methanol until no monomer could be detected by
1
thin layer chromatography and H NMR technique. The yields
of the azo polymer MP-6 proved to be dependent on the
polymerization time, and they increased largely in the first 12 h
and then almost leveled off at a polymerization time of 24 h
(yield = 56%) (Figure S1). Therefore, the Michael addition
polymerizations of M-2 and M-10 were also performed
similarly as for M-6 for a reaction time of 24 h, leading to
their corresponding orange-yellow azo polymers MP-2 and
MP-10 in a yield of 46% and 64%, respectively. All the obtained
azo polymers proved to be soluble in chloroform (instead of
THF). In this context, it is worth mentioning that although
methanol is a nonsolvent for our azo polymers, its combined
use with DMF as the solvent for the Michael addition
polymerization proved to lead to considerably enhanced
polymerization rates and azo polymers with relatively high
molecular weights.
1
The H NMR spectra of one representative azo polymer MP-
6
1
and its acetamide A-MP-6 (Scheme S1) are shown in Figure
c,d. A detailed comparison of the proton signals from the
methylene groups connected with the nitrogen atom in MP-6
and A-MP-6 revealed the presence of a secondary amino group
in the repeat unit of MP-6 instead of a tertiary one because the
acetamidation of MP-6 led to the significant shift of these
methylene proton signals (i.e., peaks a and m). This result
demonstrates that the azo polymers prepared by Michael
addition polymerization are indeed main-chain linear polymers
instead of hyperbranched ones. In addition, the chemical shifts
and peak integrations of all the protons in the polymer are in
excellent agreement with its expected structure.
1
Figure 1. H NMR spectra of Boc-M-6 (a) and M-6 (b) in DMSO-d
6
and those of MP-6 (c), A-MP-6 (d), B-MP-6 (e), and M-MP-6 (f) in
CDCl3.
The molecular weights and molar mass dispersities (Đ) of
MP-m (m = 2, 6, 10) were characterized by using GPC with
chloroform as the eluent. However, no GPC signal was
observed for these polymers, probably due to the existence of
strong interaction between the secondary amino groups of the
polymers and GPC columns. This hypothesis was verified by
the fact that the molecular weights and Đ values of the azo
polymers were readily obtained by GPC after they were allowed
to react with acetic anhydride to protect their secondary amino
groups. The number-average molecular weights of the resulting
polymers (i.e., A-MP-m (m = 2, 6, 10)) determined by GPC
The thermal and phase transition behaviors of the main-
chain azo polymers MP-m (m = 2, 6, 10) were then studied by
using TGA, DSC, POM, and SAXS. The temperatures at 5%
weight loss of the polymers determined by TGA under nitrogen
were found to be ≥271 °C, demonstrating their rather good
thermal stability (Table 1). The DSC study showed that all the
main-chain azo polymers exhibited one glass transition step and
one phase transition peak during both the second heating and
the first cooling scans (Figure 2). A glass transition temperature
(T ) of 50, 30, and 43 °C was observed in the DSC second
g
heating scan for MP-2, MP-6, and MP-10, respectively (Table
1). The POM observation revealed that there existed
enantiotropic schlieren liquid crystalline textures for all the
(i.e., Mn,GPC) proved to range from 6800 to 7230, and their Đ
values were around 1.5 (Table 1).
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Table 1. Characterization and Thermal Transition Data of
the Azo Main-Chain LCPs
thermal
a
c
f
g
yield
transition
ΔH
Td
si
b
b
sample
MP-2
(%)
Mn,GPC
Đ
T (°C)
(kJ/mol) (°C)
d
e
46
6800
1.46
1.52
1.51
G 50 SC 139 I
I 133 SC 46 G
G 30 SC 159 I
I 148 SC 27 G
G 43 SC 142 I
I 129 SC 44 G
3.0
−3.3
11.2
271
277
284
d
e
MP-6
56
64
6960
7230
−10.1
12.5
d
e
MP-10
−11.4
a
All the polymers were obtained at a polymerization time of 24 h.
The number-average molecular weights (Mn,GPC) and molar mass
b
dispersities (Đ) of the polymers were determined by GPC with
chloroform as the eluent after their modification with acetic anhydride
c
(
polystyrene standards). G = glassy, S = smectic C, I = isotropic.
C
d
−1
e
DSC second heating scan under nitrogen (10 °C min ). DSC first
cooling scan under nitrogen (−10 °C min ). Enthalpy of the
transition between the smectic and isotropic phases. The temper-
−1
f
g
Figure 3. POM images upon cooling: MP-2 at 118 °C (annealed for
3
0 min) (a), MP-6 at 144 °C (annealed for 30 min) (b), and MP-10 at
atures at 5% weight loss of the polymers under nitrogen determined by
TGA heating experiments (10 °C min ).
−1
124 °C (annealed for 30 min) (c).
3
9−41
length (with an evident even−odd effect).
However, no
regular changes with spacer length were observed for our main-
chain azo polymers, just as reported by Xu and co-workers.
42
The real reason is not very clear yet, and further investigation is
ongoing to provide a reasonable explanation for this
phenomenon.
It is well-known that SAXS can provide useful information
concerning molecular arrangement, packing mode of the
mesogen, and type of order in the mesophase of a liquid
crystalline polymer, and it has thus been widely utilized for
accurately assigning the exact nature of the liquid crystalline
2
6,43,44
phases.
Therefore, the liquid crystalline mesophases of
the main-chain azo polymers MP-m (m = 2, 6, 10) were further
studied with in situ variable temperature SAXS. Figure 4a shows
the SAXS patterns of MP-2, MP-6, and MP-10, which were
Figure 2. DSC curves of the main-chain azo polymers from the first
cooling scan (a) and from the second heating scan (b) (±10 °C
−
1
min ).
main-chain azo polymers (Figure 3) and the phase transition
peaks in the DSC curves of these azo polymers represented the
transition between liquid crystalline and isotropic phases. The
clearing temperatures (T ) determined from the DSC second
cl
heating scan were 139, 159, and 142 °C for MP-2, MP-6, and
MP-10, respectively (Table 1). The above results demonstrate
that all the obtained main-chain azo polymers have a relatively
broad range of liquid crystalline mesophases and low T values.
g
Figure 4. (a) SAXS patterns of the main-chain azo polymers upon
cooling from the isotropic states: MP-2 at 118 °C (scan time: 20 min)
a1), MP-6 at 144 °C (scan time: 20 min) (a2), and MP-10 at 124 °C
(scan time: 20 min) (a3). (b) Proposed liquid crystalline structure for
It is worth mentioning here that the influence of the flexible
spacer length has been reported for main-chain LCPs with rod-
like mesogens and the transition temperatures (T or T )
(
g
cl
usually have a tendency to decrease with increasing the spacer
MP-m (m = 2, 6, 10).
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obtained at their liquid crystalline temperatures (with a certain
time of annealing) upon cooling from the isotropic states. It can
be seen clearly that each polymer exhibited one strong
scattering peak at the small angle area, suggesting the existence
of an ordered smectic lamellar structure with a layer spacing d
and methacrylic anhydride at ambient temperature (Scheme
S1). A series of new functionalized main-chain azo polymers
(i.e., A-MP-6, B-MP-6, and M-MP-6) were readily synthesized
in high yields, with all of them showing obvious liquid
crystalline phases (Figures S2 and S3b−d). Figure 1d−f
44
1
(d = 2π/q, Table 2). This d value is smaller than the
presents the H NMR spectra of these anhydride-modified
MP-6.
Table 2. SAXS Characterization Data of the Azo Main-Chain
LCPs
Photoresponsive Behaviors of the Azo Main-Chain
LCPs and Their Supramolecular Hydrogen-Bonded
Fibers. The photoresponsive properties of the representative
azo main-chain LCP MP-6 were first investigated in chloro-
form. By irradiation at 365 nm, the studied polymer solution
underwent trans to cis photoisomerization until a photosta-
tionary state was eventually reached (Figure 5a). The intensity
a
b
c
d
q1
d
l
θ
liquid crystalline
e
(nm−1) (nm) (nm)
sample
d/l
(deg)
mesophase
MP-2
MP-6
MP-10
2.59
2.30
2.00
2.43
2.73
3.14
2.79
3.19
3.59
0.87
0.86
0.87
60
60
60
SC
SC
SC
a
q refers to the scattering vector of the first-order diffraction peak of
1
b
the azo polymers, as shown in Figure 4a. d denotes the layer spacing
of the ordered smectic lamellar structures in the studied azo polymers
determined by SAXS, which can be evaluated by using the equation d
c
=
2π/q . l refers to the calculated molecular length for the fully
1
d
extended liquid crystalline unit in the azo polymers. θ represents the
angle between the director of the azo mesogen and the smectic
lamellar layer surface in the liquid crystalline states of the azo
e
polymers, as shown in Figure 4b and sin θ = d/l. S refers to smectic
C
C liquid crystalline mesophase.
calculated molecular length (l) for the fully extended liquid
crystalline unit, suggesting a smectic C structure with a tilted
angle of 60° between the director of the azo mesogen and the
smectic lamellar layer surface, as schematically shown in Figure
4
(
b. This result agrees well with the schlieren textures observed
with POM) for the main-chain azo polymers and the general
rule “schlieren texture is frequently found in the smectic C
45
phase instead of smectic A phase”.
Chemical Reactivity of the Azo Main-Chain LCPs. It is
anticipated that the presence of the secondary amino groups in
MP-m (m = 2, 6, 10) should make these azo polymers reactive
precursors for further cross-linking and functionalization.
Herein, we started to study the chemical cross-linking of the
main-chain azo LCPs by using 1,6-hexamethylene diisocyanate
(
HDI) as the model cross-linker. The representative reactive
azo main-chain LCP MP-6 was found to be able to directly
react with HDI to provide the cross-linked main-chain azo
polymers. Obvious gelating phenomenon was observed after
the addition of HDI into the polymer solutions, suggesting the
occurrence of the cross-linking reaction. Cross-linked polymers
Figure 5. UV−vis spectral changes in dependence of time for the
solution of MP-6 in chloroform (C = 50 μM repeat unit of the studied
polymer) at 25 °C upon irradiation with 365 nm UV light (a) and
upon irradiating the polymer solution at the photostationary state with
visible light (λ > 430 nm) (b).
(namely CL-MP-6-f (f = 2, 4)) were obtained by reacting MP-6
with varying amounts of HDI, where CL-MP-6 denotes the
cross-linked polymer of MP-6 and f represents the molar ratio
of the repeat unit (or reactive secondary amino group) in the
polymer to HDI, respectively. The DSC study showed that no
phase transition peak was discernible for the obtained cross-
linked polymers in the studied temperature range (Figure S2).
This, together with their presence of obvious optical
birefringence in both the heating and cooling processes (as
observed by POM) (Figure S3e,f), revealed that all these cross-
linked main-chain azo polymers exhibited rather stable liquid
crystalline phases. However, such cross-linked liquid crystalline
phases proved to be rather difficult to develop, even after a long
annealing time, probably due to the possible confinement effect
of the π → π* transition band around 360 nm decreased,
whereas that of the n → π* transition band around 450 nm
slightly increased. The existence of isobestic points at 304 and
410 nm is characteristic for the presence of two distinct
absorbing species in equilibrium with each other and at the
same time proves that no side reaction took place during the
46
photoisomerization process in the range studied. Irradiation
of the above polymer solution at the photostationary state with
visible light (λ > 430 nm) led to the cis to trans back-
isomerization process (Figure 5b). However, the finally
recovered absorbance of trans-isomer was lower than that
before UV irradiation with the recovery of the trans-isomer
being around 87%, just as previously reported by our
26
of the cross-linking on the movement of the mesogens.
2
6,36,43
47,48
The high chemical reactivity of the above-obtained azo main-
chain LCPs was further demonstrated by reacting MP-6 with
some anhydrides including acetic anhydride, Boc anhydride,
group
and by others.
Its real cause is not totally
clear yet but might be that, at the same wavelength where cis to
trans photochemical back-isomerization was performed, there
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was also a weak absorption from the trans-isomer, which
eventually led to an equilibrium with cis to trans and trans to cis
isomerizations taking place under the same visible light after
26,49
most of the cis-isomer returned to the trans-isomer.
Nevertheless, the photoisomerization became completely
reversible upon the subsequent cycles of UV and visible light
irradiation (Figure S4).
The photoresponsive behaviors of the polymer fibers of MP-
m (m = 2, 6, 10) were then studied. The fibers of these azo
main-chain LCPs were fabricated by using the simple melt
23,38
spinning method as reported previously,
but in the absence
of any chemical cross-linker. POM (with crossed polarizer and
analyzer) observation demonstrated that the obtained main-
chain LCP fibers had a rather high order of mesogen along the
fiber axis, as revealed by the sharp contrast inversion every 45°
upon rotating the sample with respect to the analyzer (Figure
2
3,38
6a and Figure S5).
In addition, it can be envisioned that
Figure 7. FT-IR spectra of MP-6 at different temperatures during the
second heating (a) and first cooling (b) processes.
spectra of MP-6 obtained during both the second heating and
first cooling processes. It can be seen clearly that MP-6 exhibits
−1
two infrared bands around 3550 cm (very weak shoulder)
and 3360 cm− (medium peak) for νN−H at ambient
temperature, which can be assigned to the “free” (higher than
1
−
1
3
400 cm ) and hydrogen-bonded N−H stretching modes,
respectively. Moreover, the N−H stretching vibration also
proved to be dependent on the sample’s temperature. The
hydrogen-bonded N−H stretching peak (with a frequency
−1
lower than 3400 cm ) became weaker and shifted toward
higher frequencies upon increasing the sample temperature,
while the free N−H stretching peak with a frequency higher
than 3400 cm− became stronger in the meantime (Figure 7a),
indicating that the hydrogen bonding in the polymer got
weaker and some of the hydrogen-bonded N−H groups were
transformed to free ones with increasing temperature (note that
the hydrogen bonding still existed in the polymer even at 200
°C). On the other hand, the intensity and frequency of the N−
H stretching vibration were fully recovered upon decreasing the
sample temperature, suggesting that the hydrogen bonding
interaction was reversible, just as expected (Figure 7b). It is
noteworthy that the N−H bending vibration band of the
studied polymer also showed an obvious reversible change in its
1
Figure 6. (a) POM images of the textures of the MP-6 fiber taken at
room temperature. Sample angle to the analyzer: θ = 0° (1); θ = 45°
(
2). (b) Photographs of the MP-6 fiber that exhibit photoinduced
bending and unbending behaviors upon irradiation with 365 nm UV
−2
−2
light (150 mW cm ) and visible light (λ > 430 nm, 120 mW cm ) at
0 °C. (c) The reversible deformation of the MP-6 fiber characterized
6
by tracing the bent distance from its straight state at 60 °C. The size of
the fiber is 11 mm × 21 μm.
−1 50
frequency (from 1641 to 1665 cm ) following the change of
the sample temperatures, whereas negligible frequency shift
−1
hydrogen bonding interactions should be present among the
main-chain azo polymer chains because of their presence of the
secondary amino groups, thus leading to supramolecular
hydrogen bonding-cross-linked fibers.
To confirm the presence of hydrogen bonding interactions
among the azo main-chain LCP chains, the variable temper-
ature FT-IR experiments were performed for the representative
polymer MP-6. Figure 7 shows the variable temperature FT-IR
(from 1719 to 1723 cm ) was observed for the CO
stretching band from the ester groups in the polymer, which
clearly demonstrated that the hydrogen bonding mainly formed
between N−H and N−H (both from the secondary amino
groups) instead of between the N−H and CO (from the
ester groups in the polymer).
With the above-obtained supramolecular hydrogen-bonded
polymer fibers in hand, we started to study their photo-
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mechanical properties. Part of MP-6 fiber was first pasted onto
an aluminum block and heated to 60 °C by a hot stage (Figure
S6a). Figure 6b shows the photoinduced bending and
unbending of the MP-6 fiber upon irradiation with UV light
and visible light, respectively. It can be seen clearly that the MP-
trans thermal back-isomerization of the azo groups in the fiber.
This hypothesis was indeed verified by the experimental results
(Figure S7a). The UV light-induced bending and thermally
induced unbending proved to be reversible although the
thermally induced unbending took a relatively longer time (140
s) for the fiber to revert to its initial straight state in comparison
with the visible light-induced unbending process (60 s).
Moreover, such bending and unbending could also be repeated
over 100 cycles, and both their bending/unbending times and
bending angles remained almost constant, again demonstrating
the excellent fatigue resistance of the fiber. Figure S7b shows
the initial 10 cycles of the reversible deformation of the MP-6
fiber.
6
fiber bent to the left side toward the actinic light source along
the fiber axis when it was exposed to 365 nm UV light (150
−2
mW cm ) from the left side of the sample, and it took a
bending time of 15 s for the fiber to reach its maximum
bending. The bent fiber could revert to its initial straight state
−2
upon irradiation with visible light (λ > 430 nm, 120 mW cm )
for 60 s. In addition, upon subsequent irradiation with the same
UV and visible light from the right side, the MP-6 fiber also
bent to the right side toward the actinic light source and then
reverted to its initial straight state, respectively. The above
results unambiguously demonstrated that the bending and
unbending of the fiber were fast and reversible and could be
easily controlled by just changing the wavelength of the
incident light. In addition, the bending direction of the fiber
could be directly controlled by changing the irradiation
direction of the light, thus allowing the well-controlled three-
dimensional photomobility of the fiber on demand. Similar with
the MP-6 fiber, MP-2 and MP-10 fibers also showed fast and
reversible photoinduced bending and unbending under the
similar conditions (Figure S6b,c). These reversible photo-
induced bending and unbending of the physically cross-linked
azo main-chain LCP fibers are quite similar to those of the
previously reported chemically cross-linked azo LCP fibers and
The dependence of the bending behaviors of the supra-
molecular hydrogen-bonded fibers on the light intensity and
temperature was studied by measuring the bending times for
the fibers to bend to their largest extent (which proved to be
almost independent of the light intensity and temperature (T >
T )) upon exposure to 365 nm UV light (Figure 8). The
g
23
can also be explained by the same mechanism. However, in
comparison with the chemically cross-linked azo LCPs
normally used for the photomechanical studies, this physically
cross-linked photomechanical system has some distinct
advantages such as greater flexibility in fabrication and more
51−53
freedom for further processing.
Although a few photo-
deformable LCPs with physically cross-linked networks have
been disclosed, they are all based on the LCPs with azo
2
2,52,53
moieties in the side chains.
To our knowledge, our findings
presented here is the first successful example of the physically cross-
linked photomechanical system based on the main-chain LCPs with
azo units in the polymer backbones.
The reversible deformation of the azo main-chain LCP fibers
upon irradiation with UV light and visible light was also
studied, which was found to be capable of being repeated over
100 cycles, and both their bending/unbending times and
bending angles remained almost constant, thus demonstrating
the excellent fatigue resistance of these photodeformable
54
Figure 8. (a) Effect of light intensity on the bending time (i.e., the
time required to reach the maximum bending) of the MP-6 fiber upon
its exposure to 365 nm UV light at 60 °C. (b) Effect of temperature on
the bending time of the MP-6 fiber upon its exposure to 365 nm UV
supramolecular hydrogen-bonded LCP fibers. Figure 6c
shows the initial 10 cycles of the reversible deformation of
the MP-6 fiber. Note that the oriented fibers of the main-chain
azo polymers obtained by the reaction of the secondary amino
groups of MP-6 with anhydrides were also prepared via the
melt spinning method, but they could not show any
photoinduced bending behavior, thus revealing that the
presence of hydrogen bonding interactions between the
polymer chains is necessary for achieving the photomechanical
properties of such fibers.
In addition to the above reversible photoinduced bending
and unbending of the polymer fibers upon irradiation with UV
light and visible light, we also studied the UV-induced bending
and thermally induced unbending behaviors of the MP-6 fiber.
It is expected that an unbending process should take place for
the UV light-deformed polymer fiber when the UV light
irradiation was turned off because of the occurrence of the cis to
−
2
light (150 mW cm ). The size of the fiber is 11 mm × 21 μm.
experimental results revealed that the photoinduced bending
could take place only when the fibers were heated to a
temperature above the T of MP-m (m = 2, 6, 10), which is
g
easily understandable because the relaxation of polymer
segments is necessary to transfer the changes in structure and
alignment of the azo moieties at the fiber surface to the
transformation of the conformation in the entire physically
cross-linked network, just as in the chemically cross-linked azo
55
LCP systems. Figure 8 shows that a rapid decrease in the
bending time was observed for the MP-6 fiber both with an
increase in the 365 nm UV light intensity from 25 to 100 mW
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cm− at 60 °C and with an increase in the temperature from 35
2
was loaded onto the fiber to keep its length constant. The
stretching direction was controlled to be parallel to the fiber
axis. Figure 9b shows the change in the stress of the fiber upon
its exposure to 365 nm UV light with different intensities at 35
°C. A clear increase in the stress was observed for the fiber,
indicating the generation of the mechanical force in the fiber
under photoirradiation. The generated maximum stress was
determined to be 130 and 240 kPa when the light intensity was
to 50 °C under the 365 nm UV light irradiation with a fixed
−
2
intensity (150 mW cm ). However, the bending time
remained almost unchanged with a further increase in the
light intensity and temperature. Similar results were also
55
reported and explained previously by Yu et al. Note that the
photoinduced bending of the MP-6 fiber could be realized even
at close to ambient temperature (i.e., 35 °C) within 1 min
−2
(
Figure 8b), mainly because of the low T of MP-6. The MP-2
50 and 150 mW cm , respectively, which are close to the stress
56
g
and MP-10 fibers also showed similar dependence of the
bending behaviors on the light intensity and temperature as the
MP-6 fiber (Figures S8 and S9).
generated by the chemically cross-linked azo LCP films and
23
fibers2 as well as that of the human muscles (around 300
3,56
kPa).
The higher intensity of UV light generated larger
The mechanical properties and photoinduced mechanical
force (or stress) of the photodeformable azo main-chain LCP
fibers were also investigated because they are important
parameters for the potential applications of such fibers in
artificial muscles and light-driven actuators. Figure 9a presents
stress, which could be ascribed to the higher concentration of
cis-azo moieties produced by the actinic light with a higher
intensity and thus the larger surface contraction.
CONCLUSIONS
■
We have demonstrated for the first time the efficient synthesis
of a series of azo main-chain LCPs with reactive secondary
amino groups in the polymer backbones via the facile Michael
addition polymerization under mild reaction conditions. These
main-chain azo polymers showed rather high thermal stability,
relatively low glass transition temperatures, a broad temper-
ature range of smectic C liquid crystalline mesophases, and
reversible photochemical behaviors. In addition, they proved to
be highly reactive precursors for various advanced functional
linear and cross-linked azo main-chain LCPs. Furthermore,
these azo main-chain LCPs could be directly used to fabricate
supramolecular hydrogen-bonded LCP fibers by the simple
melt spinning method, which showed a high order of mesogen
along the fiber axis and exhibited fast and reversible
photoinduced bending and unbending behaviors with control-
lable three-dimensional photomobile directions upon their
exposure to UV and visible light even at close to ambient
temperature. In particular, the as-prepared supramolecular
photomechanical system showed very good photodeformation
fatigue resistance. The stress generated by one fiber under the
UV light irradiation at 35 °C reached 240 kPa, which is similar
to the contraction forces of both the chemically cross-linked
photomechanical system and human muscles (around 300
kPa). Considering its highly appealing properties such as
flexible preparation, high reconstruction and recycle ability, and
good mechanical and photomechanical properties, we believe
that such supramolecular hydrogen-bonded photomechanical
system hold great promise in many potential applications such
as artificial muscles and actuators.
Figure 9. (a) Stress−strain curve of the MP-6 fiber (size: 10 mm × 25
μm) measured at 28 °C. (b) Change in the stress of the MP-6 fiber
size: 9 mm × 24 μm) upon its exposure to 365 nm UV light with
(
different light intensities at 35 °C. An external stress of 1100 kPa was
loaded initially on the fiber to keep its length constant.
ASSOCIATED CONTENT
■
*
S
Supporting Information
the stress−strain curve of the MP-6 fiber determined by
thermomechanical analysis (TMA) at 28 °C. The polymer fiber
achieves a tensile strength of 44 MPa and an elastic moduli of
Scheme for the polymer analogous reactions of MP-6 with a
series of anhydrides, dependence of the yields of MP-6 on the
polymerization time, DSC curves and POM images of the
anhydride-modified linear polymers and HDI-cross-linked
polymers of MP-6, UV and visible light-induced photo-
isomerization cycles of the MP-6 solution, POM images of
the textures of the MP-2 and MP-10 fibers, photographs of the
photoinduced bending and unbending of the MP-2 and MP-10
fibers as well as those of the UV-induced bending and thermally
induced unbending of the MP-6 fiber, and the effects of light
intensity and temperature on the bending times of the MP-2
and MP-10 fibers. This material is available free of charge via
the Internet at http://pubs.acs.org.
5
.8 GPa (i.e., elastic moduli = stress/strain), which is about 2.8
and 14.5 times as large as the tensile strength (i.e., 16 MPa) and
elastic moduli (i.e., 400 MPa) of a chemically cross-linked azo
5
4
LCP film, respectively, thus demonstrating the good
mechanical properties of our supramolecular hydrogen-bonded
main-chain LCP fibers. The mechanical stress generated in the
MP-6 fiber upon photoirradiation was also measured by TMA.
One fiber with a length of 9 mm and a diameter of 24 μm was
fixed by clamping its two ends and then heated to 35 °C, which
is 5 °C higher than the T of MP-6. An initial stress of 1100 kPa
g
J
dx.doi.org/10.1021/ma401655k | Macromolecules XXXX, XXX, XXX−XXX

Macromolecules
Article
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31) Xu, C.; Wu, B.; Becker, M. W.; Dalton, L. R.; Ranon, P. M.; Shi,
AUTHOR INFORMATION
Corresponding Author
E-mail zhanghuiqi@nankai.edu.cn; Tel +86-22-23507193
H.Z.).
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Notes
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The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
The authors gratefully acknowledge the financial support from
National Natural Science Foundation of China (20974048,
1103075), Natural Science Foundation of Tianjin
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dx.doi.org/10.1021/ma401655k | Macromolecules XXXX, XXX, XXX−XXX