Synthesis, characterization, and photoresponsive behavior of a series of azobenzene-containing side-chain poly(ether sulfone)s with various lengths of flexible spacers

Article abstract of DOI:10.1016/j.dyepig.2013.08.004

A series of azobenzene-containing side-chain poly(ether sulfone)s (azo-PES) with various spacer lengths (2, 6 or 12 methylene units) were successfully synthesized via a nucleophilic substitution grafting reaction between poly(ether sulfone)s with hydroxylphenyl side groups and azobenzene monomers. Their chemical structures and properties were characterized by means of IR, UV-vis and ¹H NMR. These azo-polymers show good thermal stability with glass transition temperatures and 5% weight-loss temperatures above 119 C and 347 C, respectively. The results of photoisomerization experiments indicated that azo-PES polymer with longer spacers correspond to higher photoisomerization rates. Upon irradiation with a 532 nm neodymium doped yttrium aluminum garnet (Nd:YAG) laser beam, they presented remnant values of birefringence larger than 82% of the saturation value of birefringence, indicating the good stability of the photoinduced orientation. No fatigue phenomena have been observed after several cycles of inscription-erasure-inscription sequences.

Full text of DOI:10.1016/j.dyepig.2013.08.004

Accepted Manuscript
Synthesis, characterization, and photoresponsive behaviour of a series of
azobenzene-containing side-chain poly(ether sulfone)s with various lengths of flexible
spacers
Yuxuan Zhang, Songhao Pei, Yongpeng Wang, Zengduo Cui, Na Li, Ye Zhu, Haibo
Zhang, Zhenhua Jiang
PII:
S0143-7208(13)00302-1
10.1016/j.dyepig.2013.08.004
DYPI 4051
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 27 April 2013
Revised Date: 12 August 2013
Accepted Date: 12 August 2013
Please cite this article as: Zhang Y, Pei S, Wang Y, Cui Z, Li N, Zhu Y, Zhang H, Jiang Z, Synthesis,
characterization, and photoresponsive behaviour of a series of azobenzene-containing side-chain
poly(ether sulfone)s with various lengths of flexible spacers, Dyes and Pigments (2013), doi: 10.1016/
j.dyepig.2013.08.004.
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ACCEPTED MANUSCRIPT
Synthesis, characterization, and photoresponsive behaviour of a series of
azobenzene-containing side-chain poly(ether sulfone)s with various lengths of flexible
spacers
a
Yuxuan Zhang , Songhao Pei b, Yongpeng Wanga, Zengduo Cui a, Na Li a, Ye Zhu a,
Haibo Zhang a,*, Zhenhua Jiang a
a Alan G. MacDiarmid Institute, Jilin University, Changchun 130012, China
b College of Physics, Jilin University, Changchun, 130012, China
* Corresponding author, Tel & Fax: (+86) 0431-85168298
E-mail address: zhanghaib@jlu.edu.cn
Abstract:
A series of azobenzene-containing side-chain poly(ether sulfone)s (azo-PES) with
various spacer lengths (2, 6 or 12 methylene units) were successfully synthesized via
a nucleophilic substitution grafting reaction between poly(ether sulfone)s with
hydroxylphenyl side groups and azobenzene monomers. Their chemical structures and
properties were characterized by means of IR, UV-vis and 1H NMR. These
azo-polymers show good thermal stability with glass transition temperatures and 5%
o
o
weight-loss temperatures above 119 C and 347 C, respectively. The results of
photoisomerization experiments indicated that azo-PES polymer with longer spacers
correspond to higher photoisomerization rates. Upon irradiation with a 532 nm
neodymium doped yttrium aluminum garnet (Nd:YAG) laser beam, they presented
remnant values of birefringence larger than 82% of the saturation value of
birefringence, indicating the good stability of the photoinduced orientation. No fatigue
phenomena have been observed after several cycles of inscription–erasure–inscription
sequences.
Keywords: Poly(ether sulfone)s; Azobenzene; Photoinduced birefringence;
Photoisomerization; Different length spacers; Grafting reaction
1. Introduction
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In recent years, polymers containing azobenzene units (azo-polymers) have
attracted considerable attention owing to their potential application in optical data
storage, optical switching and nonlinear optical materials [1-6]. Because of the unique
reversible photoisomerization and photoinduced anisotropy of the azobenzene
chromophores, azo-polymers can show a variety of photoresponsive variations, such
as photoinduced phase transition, photoinduced surface-relief-gratings (SRGs), and
photoinduced birefringence [7-9]. Photoinduced birefringence is an important feature
of azo-polymers that makes them suitable for optical storage applications. When
azo-polymer films are illuminated by linearly polarized laser light, azobenzene
chromophores undergo trans-cis isomerization accompanied by molecular
reorientation. Through the hole-burning mechanism, an excess of chromophores
occurs in the direction perpendicular to the laser polarization. This molecular
reorientation induces birefringence property changing in the materials, which could be
removed by irradiation with circularly polarized light or depolarized light [10].
Poly(arylene ether)s (PAEs) are a family of high-performance engineering
thermoplastics known for their excellent thermal, mechanical, and environmental
stabilities. Functionalized poly(arylene ether)s have received much attention due to
their potential applications in proton-exchange membranes, light-emitting materials
and optical materials [11-14]. In our previous work, we have reported some
photoresponsive azo-poly(arylene ether)s synthesized by direct copolymerization, and
investigated their corresponding optical properties [15-18]. These azo-PAEs present
photoinduced birefringence behaviour with good stability of the photoinduced
orientation and stable SRGs can be formed on them. However, most azo-PAEs show
low photoisomerization rates because of their rigid structures. It was reported that
azo-polymers with longer spacers could facilitate the alignment of azobenzene
chromophores, which was a viable method to increase their photoisomeriztion rates
[19, 20]. Herein, flexible alkyl units with different length were introduced between
the main chain of poly(ether sulfone)s and azo chromophores via a grafting reaction
in this work. The influence of spacer length on their thermal and photoresponsive
properties were also investigated.
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2. Experimental
2.1. Materials
Bis(4-fluorophenyl)sulfone was purchased from Acros Organics. 1,2-Dibromoetha-
ne and 1,6-dibromohexane were purchased from Aladdin Chemistry Co. Ltd.
1,12-Dibromododecane was purchased from TCI. 4-(4-Hydroxyphenylazo)benzon-
itrile and 2-(3-methoxyphenyl)hydroquinone was synthesized according to the
literatures, respectively[14, 21]. All the solvents were purchased from commercial
sources. Tetramethylene sulfone and N,N-dimethylformamide were dehydrated and
distilled under reduced pressure for purification. K2CO3 was dried at 120 oC for 24 h
before being used for polymerization.
2.2. Measurements
Gel permeation chromatography (GPC) was carried out using a Waters 410
instrument with N,N-dimethylformamide (DMF) as the eluent and polystyrene as the
1
calibration standard. H NMR spectra were recorded on a Bruker 510 (1H, 500 MHz)
instrument using dimethysulfoxide-d6 (DMSO-d6) and CDCl3 as solvent. FI-IR
spectra (KBr pellet) were recorded on a Nicolet Impact 410 FT-IR spectrophotometer.
Elemental analyses of C, H and N were performed on an Elementar Vario Micro Cube
elemental analyzer. Glass transition temperatures (Tgs) were determined by DSC
o
(Model Mettler DSC821e) instrument at a heating rate of 20 C/min and under a
nitrogen flow of 200 mL/min. The reported Tg value was recorded from the second
scan after the sample was first heated and then quenched. Thermo-gravimetric
analysis was performed on a Perkin Elmer Pyris 1 TGA analyzer under a nitrogen
o
atmosphere (100 mL/min) at a heating rate of 10 C/min. UV-visible absorption
spectra were recorded on a UV2501-PC spectrophotometer at room temperature.
2.3. Synthesis
2.3.1. Synthesis of 1-Bromo-2-(4-((4-cyanophenyl)diazenyl)phenoxy)ethane(AZO2,
scheme 1)
A mixture of 4-(4-hydroxyphenylazo)benzonitrile (4.46 g, 0.02 mol), 1,2-dibromo-
ethane (7.51 g, 0.04 mol), potassium carbonate (2.07 g, 0.015 mol) and acetone was
heated under reflux with stirring for 24 h. The reaction mixture was filtered hot, and
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the residue was washed with acetone. The acetone was evaporated and petroleum
o
ether (30-60 C) was added to the concentrated organic extracts. The resulting
precipitate was collected and dried. The crude product was recrystallized from ethanol
twice. Yield: 56%; m.p. 165 oC; MALDI-TOF-MS: C15H12BrN3O m/z=335(M+ + H);
IR (KBr, cm-1): 2929 2855 (-CH2-), 2231 (-CN); 1H NMR (DMSO-d6, δ, ppm): 8.05(d,
J=8.7 Hz, 2H), 7.98-7.95(m, 4H), 7.20(d, J= 9 Hz, 2H), 4.48(t, J=5.1 Hz, 2H), 3.87(t,
J=5.4 Hz, 2H). A typical element alanalysis for C15H12BrN3O: calcd. C 54.56, H 3.66,
N 12.73; found C 54.22, H 3.83, N 12.89
2.3.2. Synthesis of 1-Bromo-6-(4-((4-cyanophenyl)diazenyl)phenoxy)hexane (AZO6,
scheme1)
This monomer was synthesized by a procedure similar to that used for synthesis of
AZO2.Yield:53%; m.p. 93 oC; MALDI-TOF-MS: C19H20BrN3O m/z=386 (M+ + H);
IR (KBr, cm-1): 2929 2855 (-CH2-), 2231 (-CN); 1H NMR (DMSO-d6, δ, ppm): 8.04(d,
J=8.7 Hz, 2H), 7.97-7.93(m, 4H), 7.15(d, J=9 Hz, 2H), 4.11(t, J=6.6 Hz, 2H), 3.55(t,
J=6.6 Hz, 2H), 1.75~1.85(m, 4H), 1.40~1.48(m.4H). A typical element alanalysis for
C19H20BrN3O: calcd. C 59.08, H 5.22, N 10.88,; found C 58.87, H 5.36, N 10.54.
2.3.3. Synthesis of 1-Bromo-12-(4-((4-cyanophenyl)diazenyl)phenoxy)dodecane
(AZO12, scheme 1)
This monomer was synthesized by a procedure similar to that used for synthesis of
AZO2.Yield:50%; m.p. 110 oC; MALDI-TOF-MS: C25H32BrN3O m/z=469 (M+ + H);
IR (KBr, cm-1): 2929 2854 (-CH2-), 2230 (-CN); 1H NMR (DMSO-d6, δ, ppm): 8.04(d,
J=8.7 Hz, 2H), 7.97-7.93(m, 4H), 7.14(d, J=9 Hz, 2H), 4.11(t, J=6.6 Hz, 2H), 3.52(t,
J=6.6 Hz, 2H), 1.72~1.81(m, 4H), 1.34~1.44(m.16H). A typical element alanalysis for
C25H32BrN3O: calcd. C 63.83, H 6.86, N 8.93; found C 63.52, H 7.17, N 8.71.
2.3.4. Synthesis of poly(ether sulfone)s with methoxy side groups (MPES, scheme 2)
2-(3-Methoxyphenyl)hydroquinone (4.3246 g, 0.02 mol), bis(4-fluorophenyl)-
sulfone (5.085 g, 0.02 mol), K2CO3 (2.902 g, 0.021 mol), tetramethylene sulfone
(TMS, 24 mL) and toluene (8 mL) were put into a three-necked flask fitted with a
nitrogen inlet, a thermometer, a Dean–Stark trap, a condenser and a mechanical stirrer,
and the apparatus was purged with nitrogen. The reaction mixture was heated under
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reflux at 130oC for 3 h to ensure complete dehydration. After dehydration and
removal of toluene, the reaction mixture was heated at 170 oC for 6 h under a nitrogen
atmosphere. After being poured into deionized water (500 mL), the precipitate was
collected by filtration. The crude product was washed with hot deionized water and
o
ethanol several times in sequence. The resulting product was dried at 100 C under
vacuum for 24 h and poly(ether sulfone)s with methoxy side groups (MPES) was
obtained as a white powder. Yield: 93%; IR (KBr, cm-1): 2932䟍2849 (-OCH3), 1150
1
(-SO2-), 1231 (-Ar-O-Ar-); H NMR (DMSO-d6, δ, ppm): 7.93-7.76 (m, 4H), 7.30 (s,
H), 7.24-7.18 (m, 5H), 7.01 (t, 2H), 6.96(d, H), 6.92(s, H), 6.77(d, H), 3.55 (d, 3H)
2.3.5. Synthesis of poly(ether sulfone)s with hydroxyl side groups (HPES, scheme 2)
MPES (6.5 g) and freshly prepared pyridine hydrochloride (250 g) were put into a
three-necked flask equipped with a nitrogen inlet, a thermometer, a condenser and a
o
mechanical stirrer. The mixture was heated at 175 C until the solution became
o
homogeneous. After being cooled to 150 C, the mixture was poured into water. The
deposition was filtered and washed three times with water and dried under vacuum to
give HPES as a grey powder. Yield: 90%; IR (KBr, cm-1): 3423(-OH), 1150 (-SO2-),
1
1231 (-Ar-O-Ar-); H NMR (DMSO-d6, δ, ppm): 9.44 (s, 1H), 7.95-7.78 (m, 4H),
7.24 (s, 1H), 7.21-7.17 (m, 4H), 7.08 (m, H), 7.03-7.00 (m, 2H), 7.00 (d, H), 6.81 (d,
2H), 6.64 (d, 1H).
2.3.6. Synthesis of azo-PES polymers (scheme 2)
AZOi (6.0 mmol), HPES (6.0 mmol of repeating unit), K2CO3 (3.6 mmol) and
DMF (50 mL) were added into a 100mL three-necked flask equipped with a nitrogen
inlet, a thermometer, a condenser and a mechanical stirrer. The reaction mixture was
o
heated under reflux at 120 C for 12 h under a nitrogen atmosphere. Then the mixture
was slowly poured into aqueous HCl (500 mL, 1 mol) and azo-polymer was obtained
as a powder after washing with deionized water and ethanol several times, the orange
polymers were dried under vacuum for 24h.
azo-PES2 Yield: 84%; IR (KBr, cm-1): 2927 2855 (-CH2-), 2230 (-CN), -1150 (-SO2-),
1
1231 (-Ar-O-Ar-); H NMR (DMSO-d6, δ, ppm): 9.44 (s, -OH), 7.99-7.78 (m, ArH),
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7.23-7.01 (m, ArH), 6.87-6.81(m, ArH), 6.65-6.64 (m, ArH), 4.26 (t,-OCH2-)
azo-PES6 Yield: 82%; IR (KBr, cm-1): 2927 2855 (-CH2-), 2230 (-CN), -1150 (-SO2-),
1231 (-Ar-O-Ar-); 1H NMR (CDCl3, δ, ppm): 7.90-7.76 (m, ArH), 7.16-6.88(m, ArH),
6.77 (t, ArH), 4.03 (s,-OCH2-), 3.83 (s,-OCH2-), 1.28-1.80 (m, -CH2-)
azo-PES12 Yield: 79%; IR (KBr, cm-1): 2927 2855 (-CH2-), 2230 (-CN), 1150 (-SO2-),
1231 (-Ar-O-Ar-); 1H NMR (CDCl3, δ, ppm): 7.89-7.73 (m, ArH), 7.15-6.87(m, ArH),
6.77 (t, ArH), 4.04 (s,-OCH2-), 3.85 (s,-OCH2-), 1.28-1.81 (m, -CH2) (The azo-PES6
and azo-PES12 were soluble in CDCl3 much better than in DMSO-d6)
2.4. Preparation of polymer films
Polymer films were prepared by the following procedure. Polymers were dissolved
in cyclohexanone (10 wt%) and then filtered through 0.8 ꢀm syringe filters
membranes. Polymer films were obtained by casting the polymer solution onto glass
substrates, which were cleaned in an ultrasonic bath with DMF, THF, ethanol and
then distilled water. The thickness of the casting films were about 8 ꢀm for the
photoinduced birefringence experiment. The films were dried under vacuum for 48 h
to drive off the residual solvent and were then restored in a desiccator for further
studies.
3. Results and discussion
3.1. Monomer synthesis
A series of azobenzene monomers (AZOi) were designed and synthesized, as
shown in Scheme 1. For example, AZO2 was prepared from 4-(4-hydroxyphenylazo)
benzonitrile and 1,2-dibromoethane via a nucleophilic substitution reaction. After
recrystallized from ethanol twice, orange-red crystals of AZO2 were obtained. AZO6
and AZO12 were synthesized by a procedure similar to that used for synthesis of AZO2.
The structures of AZOi were confirmed by MS, 1H NMR and Elemental analyzer. The
yields of AZO2, AZO6 and AZO12 were 56%, 53% and 54%, respectively.
Scheme 1
3.2. Polymer synthesis and structure characterization
Azo-PES polymers were synthesized via a nucleophilic substitution grafting
reaction from AZOi and poly(ether sulfone)s with hydroxyl side groups (HPES), as
6

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shown in Scheme 2. First of all, the precursor of HPES, poly(ether sulfone)s with
methoxy side groups (MPES) was obtained by a typical nucleophilic substitution
polycondensation reaction from 2-(3-methoxyphenyl)hydroquinone and bis(4-
fluorophenyl)sulfone. Then, HPES was successfully obtained via hydrolysis of the
methoxy groups on the MPES. At last, HPES was transformed into respective
azobenzene-functionalized poly(ether sulfone)s via a nucleophilic substitution
reaction between the -CH2Br groups of the AZOi and the hydroxyl groups on the
HPES. From Table 1, it could be seen that all of azo-PES polymers had number
average molecular weights above 8.5×104 g/mol. Compared to most of the reported
azobenzene- containing poly(arylene ether)s, azo-PES polymers show higher
molecular weights, indicating that using the grafting method is viable to synthesize
azobenzene-containing poly(arylene ether)s with high molecular weight. All of
azo-PES polymers showed good solubility in common organic solvents such as
tetrahydrofuran,
dimethylacetamide,
N,N-dimethylformamide,
N-methyl-2-
pyrrolidone and cyclohexanone, but insoluble in ethanol.
Scheme 2
The chemical structures of azo-PES polymers were confirmed by IR, 1H NMR and
UV-vis spectra. The IR spectra of HPES and azo-PES polymers were shown in Fig. 1.
Compared to the IR spectrum of HPES, the IR spectrum of azo-PES polymers showed
characteristic absorption bands of -CN groups at 2230cm−1, and -CH2- groups at 2927
cm−1 and 2855 cm−1, indicating that the azobenzene chromophores were successfully
introduced into the polymer chains. Fig. 2 showed typical 1H NMR spectra of MPES,
1
HPES and azo-PES2 in DMSO-d6 with signal assignment. Comparing the H NMR
spectra of MPES and HPES䟍it was observed that the signal corresponding to the
proton of the methoxy group (δ, 3.55) disappeared with the signal of hydroxyl proton
presented at 9.44 ppm which was absent in the spectrum of MPES. Compared to the
1H NMR spectra of HPES, the 1H NMR spectra of the azo-PES2 showed new peaks at
around 4.26 ppm corresponding to the chemical shift of hydrogen in –OCH2- group.
The signal corresponding to the proton of –CH2Br group (δ, 3.87) has not been
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1
observed, indicating that there is no monomer AZO2 left in the azo-PES2. The H
NMR spectra of azo-PES6 and azo-PES12 could be characterized by a procedure
similar to that used for characterizing the 1H NMR spectra of azo-PES2. On the basis
1
of H NMR spectra, the molar percentage of the chromophore in azo-PES could be
calculated from the integral ratio of the proton of the –OCH2- group and the proton of
the Ar-H group. The grafting ratio of azo-PES2, azo-PES6 and azo-PES12 were 67%,
65% and 59%, respectively. The UV–vis spectra of HPES and azo-PES polymers in
DMF solution were shown in Fig. 3. The characteristic absorption bands at around
365 nm and 450 nm could be observed in the spectra of azo-PES polymers,
corresponding to π-π*and n–π* transitions resulted from the intramolecular charge
transfer of the azobenzene chromophores, respectively [22].
Fig. 1
Fig. 2
Fig.3
Table 1
Elemental analyzer of the polymers were performed on an Elementar Vario Micro
Cube elemental analyzer and the detailed experimental were illustrated in Table 2.
The measured values of C, H and N matched the calculated values well, and the slight
departure may come from the instrument measuring error and minor water.
Table 2
3.3. Thermal properties of azo-PES polymers
Fig. 7 showed the DSC curves of polymers, and the experimental data were listed
in Table 1. No thermal changes except for the glass transition was observed below
decomposition temperature, indicating that the polymers are amorphous. Tg values of
azo-PES polymers were much lower than that of HPES, because the introduction of
azobenzene chromophores decreased the number of hydroxyl groups and weakened
the hydrogen bonding interaction of polymer side chains. In addition the flexible
alkyls units could further decrease the rigidity of polymer chains. As the methylene
o
spacer length increased, Tg values of azo-PES polymers decreased from 171 C of
azo-PES2 to 119 oC of azo-PES12.
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The detailed experimental data from the TGA analysis were illustrated in Table 1.
From Table 1, it could be observed that the thermal stability decreased with spacer
o
o
length from 358 C to 347 C. However, the temperatures at 5% weight loss
(TGA-5%) of azo-PES polymers and HPES were all above 300 oC, indicating their
good thermal stability.
Fig. 4
3.4. Photoisomerization behaviour of azo-PES polymers
The photoisomerization behaviour of the azo-PES polymers in DMF solutions were
investigated by UV–vis spectroscopy after irradiated with 360 nm UV light. As
shown in Fig. 5A䟍UV–vis spectra of azo-PES2 were recorded over different time
intervals until the photostationary state was reached (irradiated for 70 s). When the
solution was irradiated by 360 nm light, azobenzene chromophores underwent a
trans-to-cis photoisomerization process (Fig. 5A). The intensity of the π-π* transition
band at 365 nm decreased gradually and the intensity of the n-π* transition band at
450 nm increased gradually with continued irradiation. The photostationary state of
the photoisomerization was reached after irradiation for 70 s. As shown in Fig. 5B and
Fig. 5C respectively, azo-PES6 and azo-PES12 displayed similar photoisomerization
behaviour.
Fig. 5
The trans-to-cis photoisomerization rates of azo-PES polymers in DMF solution
were analyzed using the UV absorption at 360 nm. The photoisomerization kinetic of
azo-PES polymers were further studied and are presented in Fig. 6. It could be
observed that ln((A∞-At)/ (A∞-A0)) was linearly dependent on time (where A0, At and
A∞ are the absorbances at time 0, time t and the photostationary state, respectively),
confirming that the trans-to-cis photoisomerization of azo-PES polymers obey
first-order kinetics, as reported previously. [23] The slope of the plots of ln ((A∞-At)/
(A∞-A0)) against time gave the first-order rate constants (kP) for the trans-to-cis
photoisomerization of azo-PES polymers. The rate constants of the
photoisomerization of azo-PES2, azo-PES6 and azo-PES12 were 0.0541, 0.0821 and
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1.4814 s-1, respectively. As the spacer length decreased, it could be observed that the
motion of the azobenzene chromophores became more difficult, which further
restricted the photoisomerization process.
Fig. 6
3.5. Photoinduced birefringence of photochromic film samples
Birefringence measurements were produced by a pump beam (532 nm of Nd:YAG
laser) polarized at 45o with respect to the polarization of the probe beam (632.8 nm
He:Ne laser). The samples were placed between two crossed polarizers (P and A) in
the path of the probe laser beam. The transmitted probe beam was detected by a
photodetector and connected through a lock-in amplifier to a computer. The probe
light was modulated at 980 Hz by a mechanical chopper. The birefringence, ꢁn
induced by 532 nm pump laser, resulted in transmission of the 632.8 nm probe beam
through polarizer A. The intensity of this transmitted beam could be described by the
well-known Eq. (1) [24]:
It=Iosin2(πꢁnd/λ)
(1)
where d is the sample’s thickness, λ is the probe laser wavelength and Io is the
transmitted probe light intensity when the polarizer and analyzer are parallel to each
other and the sample is not exposed to the polarized 532 nm laser light.
Fig. 7A showed the measured photoinduced birefringence of azo-PES polymers as a
function of time. The moment at which the pump laser was switched on or off was
marked with a letter. For azo-PES2, at the beginning of the experiments, it could be
seen that no light is transmitted through the analyzer (0–10 s) due to the random
orientation of the azobenzene chromophores. Under irradiation with a linearly
polarized 532 nm laser beam (approximately 60 mW/cm2, at point A), birefringence
signals were induced immediately and reached approximately 0.1139 due to the
alignment of azobenzene chromophores perpendicular to the pump laser polarization
direction through multiple trans-cis isomerization cycles of the azobenzene moieties.
The birefringence exhibited a decay after the pump laser was switched off (at point B)
because of the thermal isomerization from cis-form to trans-form and dipole
redistribution, which increase the entropy. The remaining photoinduced anisotropy
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could be erased with a circularly polarized beam (‘erasing’ beam) (at point C).
Azo-PES6 and azo-PES12 exhibited photoinduced anisotropy behaviour similar to
those described above, and the relative data are shown in Fig. 7A and Table 3. The
order of the saturation values of birefringence which depended on the azobenzene
chromophores contents was azo-PES2>azo-PES6>azo-PES12. The azobenzene
chromophores contents of azo-PES polymers corresponded to the grafting ratios of
azo-PES polymers. (The grafting ratios of azo-PES2, azo-PES6 and azo-PES12 were
67%, 65% and 59%, respectively.) With the length of flexible spacers increased, the
decay of value of birefringence became larger. For azo-PES12, it decreased to 82% of
the saturation value of birefringence. Compared to other azo-polymers [25-27],
azo-PES polymers showed much smaller birefringence decay and possess remnant
values of birefringence larger than 82% of the saturation values of birefringence,
indicating the good stability of the photoinduced orientation. This stability is mainly
attributed to the rigidity of the aromatic main chain structures of azo-PES polymers,
which suppress the relaxation process of the photoalignment.
To allow qualitative comparison of the birefringence evolution with irradiation time
for azo-PES polymers with different length spacers, the same evolution with the value
of birefringence normalized to the saturation value of birefringence was shown in Fig.
7B.[28]. From the growth curves of the three polymers, it could be seen that the
birefringence growth rate increased with increasing the methylene spacer length.
Generally, the growth process of photoinduced birefringence is attributable to the
multiple trans–cis isomerizations of the azobenzene moieties and the reorientation of
azobenzene chromophores through these isomerization cycles. Therefore, their
birefringence growth rates depended on their photoisomerization rates. As a result,
polymer azo-PES12 which had the fastest photoisomerization rate as discussed in
Section 3.4, showed the highest birefringence growth rate among these polymers.
Fig. 7
Table 2
The complete reversibility of the photoinduced birefringence of azo-PES polymers
is related to fatigue resistance properties. As shown in Fig. 8, after several irradiation
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steps with linearly polarized and circularly polarized laser beams alternatively acting
on the azo-PES12 film, the saturation values and remnant values of birefringence
were similar. This result indicated that azo-PES polymers were promising for use in
reversible optical storage applications and had excellent reproducibility, without any
degradation of the signal or material fatigue.
Fig. 8
4. Conclusions
A series of novel azobenzene-functionalized poly(ether sulfone)s with different
lengths of flexible spacers have been synthesized via a nucleophilic substitution
grafting reaction. The azo-PES polymers exhibit high molecular weights, high Tg,
good thermal stability and sensitive photoresponsive behaviour. The results of
photoisomerization experiments show that azo-PES with longer spacers correspond to
higher photoisomerization rates. Based on the results of photoinduced birefringence
experiment, all polymers possess remnant values of birefringence larger than 82% of
the saturation values of birefringence, indicating that introducing flexible spacers
between azobenzene chromophores and rigid backbone of azo-PAEs could balance
the conflict between photoisomerization rate and optic stability.
Acknowledgment
The authors gratefully acknowledge the National Natural Science Foundation of
China (Grant No. 51103056) for financial support.
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Table 1 Properties of HPES and azo-PES polymers
Tg ( oC ) a
Polymer
Mn
Mw/Mn
Td5 (oC) b
Td10 (oC) c
HPES
6.3 ×104
8.5 ×104
8.9 ×104
9.3 ×104
1.7
2.1
1.6
1.9
210
171
151
119
473
358
354
347
489
394
382
366
Azo-PES2
Azo-PES6
Azo-PES12
a Glass transition temperature by DSC.
b 5% weight-loss temperatures were detected at a heating rate of 10 oC/min in
nitrogen with a gas flow of 100 mL/min.
c 10% weight-loss temperatures were detected at a heating rate of 10 oC /min in
nitrogen with a gas flow of 100 mL/min.
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Table 2 Elemental data of MPES, HPES and azo-PES polymers.
Calculated
H
Found
H
C
N
C
N
MPES
HPES
69.75
69.22
69.98
70.73
71.46
4.21
69.53
68.78
70.53
70.32
71.96
4.42
3.51
3.69
4.67
4.73
3.87
Azo-PES2
Azo-PES6
Azo-PES12
4.02
4.23
3.78
3.07
3.91
3.43
2.86
4.53
5.06
All the values given are in percentages.
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Table 3 Photoinduced birefringence characteristics of azo-PES polymers
Saturation value of
birefringence
Remnant value of
birefringence
Remnant value of
birefringence (%) a
Polymer
Azo-PES2
Azo-PES6
Azo-PES12
0.1139
0.1129
0.1106
0.0988
0.095
87%
84%
82%
0.0904
a Remnant value of birefringence (%) = (remnant value of birefringence)/(saturation
value of birefringence)×100
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Figure Captions:
Table 1 Properties of HPES and azo-PES polymers.
Table 2 Elemental data of MPES, HPES and azo-PES polymers.
Table 3 Photoinduced birefringence characteristics of azo-PES polymers.
Fig. 1 IR (KBr) spectra of HPES and azo-PES polymers.
Fig. 2 1H NMR spectra of MPES, HPES and azo-PES2 in DMSO-d6.
Fig. 3 UV-vis absorption spectra of HPES and azo-PES polymers in DMF solutions.
Fig. 4 DSC curves of HPES and azo-PES polymers in nitrogen.
Fig. 5 Changes in UV-visible absorption spectra of azo-PES in DMF solution at 25 oC
with different irraiaton times under 360 nm irradiation. (A) azo-PES2 (B) azo-PES6 (C)
azo-PES12.
Fig. 6 First-order trans-to-cis isomerization kinetics of azo-PES polymers.
Fig. 7 (A) Typical behaviour of the photoinduced birefringence of azo-PES polymers
at room temperature: at point A, the writing laser was turned on; at point B, the
writing laser was turned off; and at point C, the circularly polarized light was
turned on. (B) Normalized photoinduced birefringence of the 10–50 s regions.
Fig. 8 Multiple writing-erasing cycles of azo-PES12 at room temperature: at point A,
the writing laser was turned on; at point B, the writing laser was turned off; and
at point C, the circularly polarized light was turned on.
Scheme 1 Synthesis of monomers.
Scheme 2 Synthesis route of azo-PES polymers.
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Highlights
· Azobenzene-containing polyethersulfone with various spacer lengths were
synthesized.
· All polymers show good thermal stability.
· The influence of spacer length on photoresponsive behaviour was investigated.
· The photoisomerization was more difficult to be induced for azobenzene-containing
poly (ether sulfone)s with shorter spacers.
· All polymers possessed remnant values of birefringence larger than 84% of the
saturation values of birefringence.

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