Novel photoactive hyperbranched poly(aryl ether)s containing azobenzene chromophores for optical storage

Article abstract of DOI:10.1039/b807956k

Two novel hyperbranched poly (aryl ether)s with azobenzene moieties in both the main chain and side chain of the branched arms were prepared by using B₃ + A₂ methodology via a nucleophilic aromatic substitution polycondensation. Stru

Full text of DOI:10.1039/b807956k

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Novel photoactive hyperbranched poly(aryl ether)s containing azobenzene
chromophores for optical storage
Xingbo Chen,a Yunhe Zhang,a Baijun Liu,a Jingjing Zhang,a Hui Wang,a Wenyi Zhang,b Qidai Chen,b
c
Songhao Pei and Zhenhua Jiang
a
*
Received 9th May 2008, Accepted 6th August 2008
First published as an Advance Article on the web 23rd September 2008
DOI: 10.1039/b807956k
Two novel hyperbranched poly (aryl ether)s with azobenzene moieties in both the main chain and side
chain of the branched arms were prepared by using B3 + A2 methodology via a nucleophilic aromatic
substitution polycondensation. Structures of the hyperbranched poly (aryl ether)s with azobenzene
1
moieties (azo-HPAEs) were characterized by means of IR, UV-visible, H NMR spectroscopy, and
XRD. It is shown that both the azo-HPAEs exhibit high glass transition temperatures (Tg), excellent
thermal stability and homogeneous photochromic behavior. By exposing their spin-coating films to an
interference pattern of laser beam, both the azo-HPAEs could be used for rapid (within 30 s)
fabrication of surface-relief gratings (SRGs). The obtained SRGs present no shape changes even at
temperatures up to 200 ꢀC. Especially, the azo-HPAE with azobenzene moieties in the side chain shows
larger photoinduced birefringence intensity and better reversible optical storage than the one with
azobenzene moieties in the main chain upon irradiation with 532 nm Nd:YAG laser. The novel
azo-PAEs are photoactive to UV light (355 nm), making them potentially useful for high density
information storage.
The incorporation of azo moieties into hyperbranched polymers
Introduction
could significantly enlarge their potential applications due to
their ease of preparation.
Polymers bearing azobenzene moieties (azo-polymers) have
been considered as promising materials for optical data storage,
optical switches, electro-optical (EO) modulators, and in other
electro-optic areas1–5 because of the unique reversible
photoisomerization between the trans and cis isomers and an
orientational distribution perpendicular to the direction of the
polarization of the incident laser beam of the azobenzene
chromophore. The photoinduced isomerization can cause
significant bulk, surface property variation and polarity of
the polymers, such as photoinduced phase transition, photoin-
duced surface-relief gratings (SRGs), and photoinduced
birefringence.6–11 For decades, research efforts to explore new
azo-polymers have predominantly focused on the linear
azo-polymers with side-chain or main-chain architectures.12–15
More recently, dendritic polymers containing azobenzene units
have attracted considerable attention and have been intensively
investigated.16–18 The dendrimers are designed to bear azo-
benzene chromophores in the exterior and interior or throughout
the dendritic architecture. Those azo-dendrimers have exhibited
some fascinating properties, such as harvesting low-energy
photons and improved nonlinear optical properties. In contrast
to the intensive study on the azo-dendrimers, only a few hyper-
branched azo-polymers have been synthesized and studied.19,20
There is a continuous demand for high-performance polymers
as optical and electronic devices for use in a harsh environment.
However, most polymeric materials studied for optical storage
were based on aliphatic backbones, which usually had poor
thermal stability and low Tg values.17,20,21 Poly(aryl ether)s
(PAEs) are a family of high-performance engineering thermo-
plastics with excellent thermal, mechanical and electrical
properties. Functionalized poly(aryl ether)s have been widely
studied as proton-exchange membranes (PEMs), light-emitting
materials and optical materials, and some of them exhibited
many attractive properties.22–24 In comparison with the linear
poly(aryl ether)s, hyperbranched poly(aryl ether)s (HPAEs)
possess a highly branched structure and good thermal properties.
To the best of our knowledge, no azobenzene-functionalized
HPAEs as optical storage materials have been reported so far.
The presence of highly branched structure may facilitate trans–
cis isomerization of azo groups. Thus the combination of the
HPAEs backbone with photoactive azo groups could provide
a new approach to develop novel high-performance materials
with optical properties. Following this guidance, two kinds of
HPAEs with different azobenzene chromophore substitutes
located in both the main chain and side chain of the branched
arms (Scheme 1) were synthesized by using B3 + A2 methodology
via a nucleophilic aromatic substitution polycondensation, and
their thermal property, photoisomerization, formation of SRGs
and photoinduced birefringence were fully investigated. The
novel azo-PAEs are also photoactive to short wavelength light,
making it possible to fabricate their SRGs by using a UV laser
beam of 355 nm.
aAlan G. MacDiarmid Institute, Jilin University, Changchun, 130012,
China. E-mail: jiangzhenhua@jlu.edu.cn; Fax: +86 431-85168886; Tel:
+86 431-85168886
bState Key Lab on Integrated Optoelectronics, College of Electronic
Science and Engineering, Jilin University, Changchun, 130012, China
cCollege of Physics, Jilin University, Changchun, 130012, China
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Scheme 1 Schematic of azo-HPAEs 4a and 4b.
Experimental
Materials
Phloroglucinol and 2,6-difluorobenzoyl chloride were purchased
from Aldrich. 4,40-Difluorbenzophenone and 4,40-dichlor-
odiphenyl sulfone were purchased from Changzhou Huashan
Chemical Co., Ltd (China) and were purified before use. Phenol
was purchased from Beijing Chemical Factory. 4,40-Dia-
minodiphenylsulfone (DDS) and 4,40-isopropylidenediphenol
(BPA) were purchased from Shanghai Chemical Factory.
Anhydrous potassium carbonate (K2CO3) was dried at 120 ꢀC for
24 h before being used for polymerization. All other chemicals
were obtained from commercial sources and used as received.
Scheme 2 Synthesis routes to a) monomer 1, b) monomer 2 and c)
monomer 3.
N-methyl-2-pyrrolidone (NMP) (500 mL) and toluene (130 mL)
were placed in a 1000 mL three-necked flask fitted with a nitrogen
inlet, a thermometer, a Dean–Stark trap and a mechanical stirrer,
and the apparatus was purged ꢀwith nitrogen. The reaction
mixture was refluxed at 140–160 C for 6 h to ensure complete
dehydration. After removing the toluene, the reaction mixture
Measurements
Gel permeation chromatograph (GPC) was carried out using
a Waters 410 instrument with N,N-dimethylformamide (DMF) as
an eluent and polystyrene as calibration standard. Inherent
viscosity was determined on an Ubbelohde viscometer in a ther-
mostatic container with a polymer concentration of 0.5 g dLꢁ1 in
DMF at 25 ꢀC. IR spectra (KBr pellet) were recorded on a Nicolet
ꢀ
was heated to 170–190 C for 4 h under a nitrogen atmosphere.
The final solution was poured into a mixture of deionized
water (2 L) with hydrochloric acid (50 mL) under vigorous
stirring, and then the precipitate was collected by filtration. The
crude product was washed with deionized water and ethanol to
give a white powder (30% yield). m.p. 131 ꢀC; m/z (MALDI-
TOF): 721 (C45H27F3O6 requires 720.18); nmax/cmꢁ1 1656
(–C]O), 1238 (–O–); dH (500 MHz; CDCl3, Me4Si): 6.61 (1H, s,
ArH), 7.13 (2H, d, J ¼ 8.7 Hz, ArH), 7.17 (2H, t, J ¼ 8.6 Hz,
ArH), 7.81 (2H, m, ArH), 7.83 (2H, m, ArH).
Impact 410FTIR spectrophotometer. H and 13C NMR spectra
1
were recorded on a Bruker 510 (1H, 500 MHz; 13C, 125 MHz)
instrument using dimethysulfoxide-d6 (DMSO-d6) or CDCl3
as solvent. Thermogravimetric analyses were performed on
a PerkinElmer Pyris 1 TGA under a nitrogen flow (100 mL minꢁ1
)
at a heating rate of 10 C minꢁ1. Glass transition temperatures
(Tg) were determined using a DSC (Mettler Toledo DSC821e)
instrument at a heating rate of 20 ꢀC minꢁ1 and under a nitrogen
flow of 200 mL minꢁ1. The reported Tg value was recorded from
the second scan after first heating and quenching. Wide-angle
X-ray diffraction (WAXD) measurements were made using
a Siemens D5005 diffractometer equipped with a CuKa radiation
source at room temperature. The powder sample was multiplied
to increase the intensity in the range of 4–40ꢀ (2q). Polarized
optical microscopy (POM) observation was performed on a Leica
DLMP with a Linkam THMSE 600 hot stage. UV-visible
absorption spectra were recorded on a UV2501-PC spectropho-
tometer in DMF solution at room temperature.
ꢀ
4,40-Dihydroxyphenylazodiphenylsulfone (monomer 2, Scheme
2b). Monomer 2 was synthesized by a diazotization reaction
followed by coupling with phenol. Into a 1000 mL beaker
equipped with a mechanical stirrer, a dropping funnel, and
a thermometer were placed water (50 mL), ice (50 mL) and DDS
(24.8 g, 0.1 mol). Hydrochloric acid (0.8 mol, 67.2 mL) was
added drop-wise into the stirred mixture through the dropping
funnel. The solution was cooled to 0–5 ꢀC in an ice-water
bath and then a concentrated water solution of sodium nitrite
(13.8 g, 0.2 mol) was added drop-wise. The mixture was stirred
for 30 min at 0–5 ꢀC, and yielded a clear solution. The resulting
solution was filtered and added drop-wise into a solution of
NaOH (8.0 g, 0.2 mol), phenol (18.8 g, 0.2 mol), and sodium
bicarbonate (25.2 g, 0.3 mꢀol) in 100 mL water. The reaction
mixture was stirred at 0–5 C for about 2 h and then at room
Synthesis
1,3,5-Tris(4-(4-fluorobenzoyl)phenoxy)benzene (monomer 1,
Scheme 2a). Phloroglucinol (10.08 g, 0.08 mol), 4,40-difluor-
obenzophenone (210 g, 1.12 mol), K2CO3 (20 g, 0.15 mol),
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temperature for another 2 h. The final solution was added slowly
to 700 mL acid water (HCl) and a brown–orange precipitate of
the azo-compound was formed. The precipitate was collected by
filtration, and washed thoroughly with water containing a little
amount of sodium hydrogen carbonate (pH ¼ 8). The solid was
ꢀ
dried at 60 C in a vacuum oven and finally recrystallized from
a mixture of THF and ethanol to obtain a pure orange–yellow
powder (60% yield). m.p ꢂ288 ꢀC (decomp.); m/z (MALDI-
TOF): 459 (M+ + H, C24H18N4SO4 requires 458.10). nmax/cmꢁ1
3393 (–OH), 3048 (Ar–H), 1294 (–SO2–), 1263, 1140 (–SO2–);
dH (500 MHz; CDCl3, Me4Si): 10.54 (2H, s, –OH), 8.17 (4H, d,
J ¼ 8.6 Hz, ArH), 7.98 (4H, d, J ¼ 8.6 Hz, ArH), 7.86 (4H, d,
J ¼ 8.9 Hz, ArH), 6.96 (d, 4H, J ¼ 8.9 Hz, ArH); dc (125 MHz;
CDCl3, Me4Si): 162.57, 155.41, 145.80, 141.81, 129.33, 126.06,
123.50, 116.75; Calculated C, 62.87; H, 3.96; N, 12.12; O, 13.96;
Found C, 63.05; H, 3.77; N, 12.35; O, 13.77.
4-((4-Cyanophenyl)diazenyl)phenyl-2,6-difluorobenzoate (mono-
mer 3, Scheme 2c). Monomer 3, an azo-bisfluoro monomer, was
synthesized according ꢀto our previously reported procedure.24b
(85% yield); m.p. 194 C; m/z (MALDI-TOF): 364.5 (M+ + H,
C20H11F2N3O2 requires 363.08). nmax/cmꢁ1 3030 (Ar-H), 2234
(–CN), 1754 (–CO–O–), 1012 (–C–F). dH (500 MHz; CDCl3,
Me4Si): 8.05 (2H, d, J ¼ 8.8 Hz, ArH), 8.00 (2H, d, J ¼ 8.4 Hz,
ArH), 7.83 (2H, d, J ¼ 8.4 Hz, ArH), 7.54 (1H, s, ArH), 7.46
(2H, d, J ¼ 8.8 Hz, ArH), 7.06 (2H, t, J ¼ 8.2 Hz, ArH). dc
(125 MHz; CDCl3, Me4Si): 162.1, 160.0, 154.4, 153.2, 150.3,
133.8, 133.4, 124.7, 123.4, 122.5, 120.6, 118.4, 114.2, 112.4.
Azo-HPAE 4a (Scheme 3). A 75 mL three-necked flask
equipped with a mechanical stirrer, a Dean–Stark trap, a cold
water condenser, an N2 inlet/outlet and a thermometer was
charged with monomer 1 (1.80 g, 2.5 mmol), 4,40-dichlor-
odiphenyl sulfone ( 0.72 g, 2.5 mmol ), monomer 2 (0.85 g,
1.85 mmol), BPA (0.42 g, 1.85 mmol), K2CO3 (0.54 g, 3.86 mmol),
tetramethylene sulfone (TMS, 20 mL) and toluene (15 mL).
Scheme 3 Synthesis routes to azo-HPAEs 4a and 4b.
(–Ar–O–Ar–). dH (500 MHz; CDCl3, Me4Si): 8.05–7.90
(m, ArH), 7.89–7.70 (m, ArH), 7.24–6.80 (m, ArH), 6.65–6.55
(s, ArH), 1.68 (s, –CH3); GPC: number-average molecular weight
(Mn) 1.3 ꢃ 104; polydispersity (Mw/Mn) 2.03.
ꢀ
The reaction mixture was refluxed at 120 C for 2 h under N2
atmosphere to dehydrate the system. After dehydration and
removal of toluene, the reaction temperature was then increased
to 140 ꢀC and maintained at this temperature for 12 h until
a viscous solution was obtained. Then the viscous solution was
slowly poured into 500 mL water and the threadlike polymer
was obtained. After pulverized into powders, the polymer was
washed with hot deionized water and extracted in a Soxhlet
extractor with alcohol. The resulting product was dried at 100 ꢀC
under vacuum for 24 h and 4a was obtained as red solid (85%
yield). nmax/cmꢁ1 3056 (Ar-H), 2969 (–CH3), 1659 (C]O), 1235
(Ar–O–Ar). dH (500 MHz; CDCl3, Me4Si): 8.20–8.05 (m, ArH),
7.91–8.05 (m, ArH), 7.67–7.91 (m, ArH), 7.35–6.85 (m, ArH), 6.59
(s, ArH), 1.68 (s, –CH3); GPC: number-average molecular
weight (Mn) 5.6 ꢃ 103; polydispersity (Mw/Mn) 1.70.
Preparation of polymer films
Polymers were dissolved in cyclohexanone (10 wt%) and
then filtered through 0.8 mm syringe filters membranes. Polymer
films were obtained by spin coating (for SRG) or casting (for
birefringence) the polymer solution onto glass substrates, which
were cleaned in the ultrasonic bath with DMF, THF, ethanol
and distilled water subsequently. The thickness of the films was
controlled to be about 0.5–1.0 mm by adjusting the spinning rate
for fabricating SRG and the thickness of the casting films were
about 8–10 mm for the photoinduced birefringence experiment.
The residual solvent was removed by heating the films in
a vacuum oven at 100 ꢀC for 2 days. The films were stored in
a desiccator for further studies.
Azo-HPAE 4b (Scheme 3). The synthesis was carried out
following the same procedure described for 4a, but using
monomer 1 (1.8025 g, 2.5 mmol), monomer 3 (0.91 g, 2.5 mmol),
BPA (0.84 g, 3.7 mmol), K2CO3 (0.54 g, 3.86 mmol), TMS
(20 mL) and toluene (15 mL). After purification, an orange
solid of 4b was obtained (90% yield). nmax/cmꢁ1 3030 (Ar-H),
2969 (–CH3), 2223 (–CN), 1750 (–CO–O–), 1659 (–C]O), 1234
Results and discussion
Monomer synthesis and characterization
The ‘core’ molecule B3 (monomer 1) was synthesized by nucle-
ophilic substitution of excess 4,40-difluorobenzophenone with
phloroglucinol in the presence of K2CO3. The structure of
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monomer 1 is shown in Scheme 2a and confirmed by MS, IR and
1H NMR. The IR spectrum shows the characteristic bands of
linear analogues). Although the number-average molecular
weight (Mn) of 4a and 4b were only 5.6 ꢃ 103 (Mw/Mn ¼ 1.70)
and 1.3 ꢃ 104 (Mw/Mn ¼ 2.03), the real molecular weight of the
polymers may be even larger than the values referred to poly-
styrene standards because dendritic macromolecules generally
have a smaller size than linear polymers with the same molecular
weight and can hardly be expanded in solution. The broad
molecular weight distribution should be ascribed to the specific
characteristics of the nucleophilic substitution polycondensation
reaction of B3 + A2. The polymers of azo-HPAEs are well soluble
in various solvents such as chloroform, THF, DMF, DMSO,
NMP and cyclohexanone, but insoluble in acetone and ethanol.
The chemical structures of azo-HPAEs were confirmed by IR,
1H NMR and UV-vis spectra. The IR spectra of 4a and 4b are
shown in Fig. 1. In the IR spectrum of 4a, the absorption bands
at 2969, 1659 and 1234 cmꢁ1 are attributed to the stretching
vibrations of the methyl (–CH3), diphenylcarbonyl (–C]O) and
the aromatic ether group (Ar–O–Ar), respectively. In the IR
spectrum of 4b, two additional characteristic absorption bands at
2223 and 1750 cmꢁ1 attributed to the nitrile (–CN) stretching
vibration and the carbonyl stretching vibration of the conjugated
carboxylic esters (–CO–O–) could be identified. Fig. 2 shows
the signal assignments of the 1H NMR spectra of the azo-HPAEs
4a and 4b in CDCl3. Although the signals in the 1H NMR spectra
of azo-HPAEs are complicated and greatly overlapped, which
make the spectra difficult to analyze, the signals corresponding
to the protons of –CH3 (d, 1.68), the protons located at ortho-
position of –phloroglucinol–O–Ph– group (d, 6.59), azo (–Ph–N ¼
N–Ph–) group (d, 7.97) and –N]N–Ph–SO2–Ph–N]N– (d, 8.11)
are distinguished from other protons. UV-visible spectra of 4a
–C]O and –O– stretching vibrations at 1656 and 1238 cmꢁ1
,
respectively. We prepared a new bisphenol (monomer 2)
containing azo groups by a diazotization reaction, as shown
in Scheme 2b. The structure of 2 is confirmed by MS, IR,
UV-visible and 1H NMR spectroscopy. The IR spectrum exhibits
a characteristic band of –OH at 3393 cmꢁ1. UV-vis absorptions
of monomer 2 are at 376 and 470 nm, and they are assigned to
p/p* and n/p* electronic transitions of azo-aromatic chro-
mophores, respectively. In the 1H NMR spectrum of monomer 2,
all of the signals are well within agreement with the expected
structure. The signal at 10.54 ppm in the 1H NMR is assigned to
the hydroxyl proton.
Polymer synthesis and structure characterization
Azo-HPAEs were synthesized by using B3 + A2 methodology via
a typical nucleophilic substitution polycondensation reaction
with 1,3,5-tris(4-(4-fluorobenzoyl)phenoxy)benzene as
a B3
‘core’ molecule (Scheme 3). Prepared hyperbranched polymers
with B3 + A2 monomers is a novel method developed by
Jikei et al., Frechet et al., and Long et al.25–27 One advantage of
´
this approach is that the performance of polymer can be medi-
ated by adjusting the structure and chain length of B3 or A2.
Furthermore, performing direct polycondensation close to the
gel point makes the structure of the polymer’s aggregation state
also controllable. However, the solubility of the resulting
polymer of B3 and A2 is very poor, and not suitable for fabri-
cation of the casting films. By introduction of the additional
monomers, the solubility of the copolymers can be improved
greatly and qualified for fabrication of the casting films. Two
branched copolymers, azo-HPAEs 4a and 4b, were designed as
synthesized according to Scheme 3. Since the reactive groups of
the monomers have no selectivity and both the bis-phenol
monomers could react with monomer 1 and 4,40-dichlor-
odiphenyl sulfone (for 4a) or monomer 3 (for 4b) optionally.
Thus, the real structure of azo-HPAEs 4a and 4b may be more
complicated than that shown in Scheme 3. However, it is difficult
to give out the real structure of the resulting random copolymers
due to complexity of the reactions. Therefore, we represent the
three branches of azo-HPAEs 4a and 4b with different lengths, as
shown in Scheme 1.
As shown in Table 1, both the resulting azo-HPAEs, 4a and
4b, have a inherent viscosity of 0.29 dL gꢁ1 in DMF at 25 C,
ꢀ
Fig. 1 IR (KBr) spectra of azo-HPAEs 4a and 4b.
indicating their low inherent viscosity (in comparison with their
Table 1 Properties of azo-HPAEs 4a and 4b
DT5 /ꢀCd
DT10 /ꢀCe
c
c
a
b
Polymer
hiv /dL gꢁ1
Tg /ꢀC
Air
N2
Air
N2
Mn
Mw/Mn
4a
4b
0.29
0.29
150
143
422
429
405
423
461
469
442
466
5.6 ꢃ 103
1.3 ꢃ 104
1.70
2.03
a
Inherent viscosity was determined on an Ubbelohde viscometer in a thermostatic container with the polymer concentration of 0.5 g dLꢁ1 in DMF at
25 ꢀC. b Glass transition temperature from DSC. c DT ¼ decomposition temperature. d 5% weight-loss temperatures were detected at a heating rate of
e
10ꢀC minꢁ1 in nitrogen or air with a gas flow of 100 mL minꢁ1
.
10% weight-loss temperatures were detected at a heating rate of 10ꢀC minꢁ1 in nitrogen
or air with a gas flow of 100 mL minꢁ1
.
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Fig. 4 TGA curves of azo-HPAEs 4a and 4b in nitrogen and in air.
chains changed from solutions to solid films. All of the above
results obtained from IR, 1H NMR and UV-vis spectra indicate
that the designed hyperbranched azo-polymers are successful
synthesized.
Thermal properties and morphologies of azo-HPAEs
TGA curves of the azo-HPAEs are given in Fig. 4 and the
detailed experimental data are illustrated in Table 1. The TGA
thermograms of 4a and 4b indicate that their 5% weight loss
ꢀ
temperatures are all above 400 C in both air and nitrogen gas,
indicating the excellent thermal stability of azo-HPAEs 4a and
4b. Tg values of azo-HPAEs were determined by differential
scanning calorimetry (DSC). In the curves, the hyperbranched
polymers 4a and 4b show the second-order phase transition and
no other thermo changes except that glass transitions are
observed below the decomposition temperatures, indicating the
amorphous nature of azo-HPAEs 4a and 4b. Due to the aromatic
structure and the large dipole moment of the side groups, 4a and
4b show high glass transition temperatures, which ensured their
excellent thermal property. The glass transition temperatures
(Tg) of the polymers obtained by DSC are also listed in Table 1.
The wide-angle X-ray diffractions of 4a and 4b are shown in
Fig. 5. The curves of azo-HPAEs are broad and without obvious
peak features, which also indicate their amorphous character.
Fig. 2 1H NMR spectra of azo-HPAEs 4a and 4b in CDCl3.
and 4b in DMF solutions and as spin-coating thin films are given
in Fig. 3. From the spectra, we could see the characteristic
absorption bands of p/p* and n/p* electronic transitions of
azo-aromatic chromophores in the range of 300–500 nm.
Compared with the maximum absorption (lmax) of 4a and 4b in
the DMF solutions appeared at 359 and 351 nm, the lmax of the
corresponding films spectra experience a red-shift to 362 and
355 nm, contributed to the solvatochromism effect of the internal
azo-chromophores when the environments of the polymeric
Fig. 3 UV-vis absorption spectra of azo-HPAEs 4a and 4b in DMF
solutions and as spin-coated films at room temperature.
Fig. 5 Powder WAXD patters of 4a and 4b at room temperature.
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Photochromic behavior of azo-HPAEs
also shows the same reversibility of the photoisomerization
process. This result clearly indicates the homogeneous photo-
chromic behavior for trans- and cis-azobenzene chromophores in
the polymer samples. From the experiments, the proportions of
isomerization at the photostationary state of 4a and 4b are
determined to be 21.1 and 50.6%, respectively. Such a difference
might come from the fact that it is more difficult for the
azobenzene groups to change from trans- to cis- when they are
attached to the polymer main chain.
The UV-vis absorption spectrum of 4b in DMF solution exhibits
two absorption bands centered around 351 and 447 nm in the
range of 300–650 nm, corresponding to p/p* and n/p*
electronic transitions of azo-aromatic chromophores, respec-
tively. The high energy band is mainly attributed to the trans-
isomer, whereas the low energy one is mainly derived from the
metastable cis-isomer. When the solution was irradiated by light
of 360 nm, azobenzene chromophores undergo a trans-to-cis
photoisomerization process (Fig. 6a). The absorption band at
351 nm decreases gradually and the absorption band at 447 nm
increases gradually with the prolonged irradiation time. The
photostationary state of the photoisomerization is reached
after irradiation for 300 s. On the other hand, when the irradiated
sample was annealed at a certain temperature or irradiated
with visible light, the absorption peak at 351 nm will gradually
recover to the starting value before irradiation (Fig. 6b),
confirming the reversibility of the photoisomerization process.
The proportion of isomerization at the photostationary state
can be estimated from eqn 1:
Preparation and characterization of surface-relief gratings
Formation of photoinduced surface-relief gratings (SRGs) is one
of the most interesting properties of azo-polymers in recent
years.2,10,11,20,21 By exposure of the polymer films to the inter-
fering laser beams, SRGs will be formed on the films at
a temperature well below the Tg of the samples. This function
could be potentially applied to holographic memories, reversible
optical data storage and waveguide couplers.
The experimental setup for the surface-relief gratings forma-
tion is shown in Fig. 7. A polarized Nd:YAG nanosecond pulsed
UV laser beam (Spectra-Physics, Quanta-Ray-150, 355 nm), with
pulse duration of 10 ns and repetition rate of 10 Hz, was used as
the light source. The spot of the laser beam is 10 mm in diameter,
and the intensity of the recording laser conforms to a Gauss
distribution. Surface-relief gratings were optically inscribed on
the spin-coating films with polarized interfering laser beams,
where the laser beam was split by beam splitting (BS) and the
reflected half-beam coincided with the other half on the film
surface. The surface profiles of the resulting gratings were
observed using an atomic force microscope (AFM) in the tapping
mode.
F ¼ 100(1 ꢁ At/A0) %
(1)
where A0 and At are the absorptions at the maximum before and
after irradiation, respectively. The hyperbranched polymer 4a
It is interesting to have found that SRGs of 4a and 4b could be
rapidly fabricated within 30 s at room temperature with a modest
intensity (about 60 mW cmꢁ2). Fig. 8 shows the typical AFM
plane and three-dimensional images of the surface structures.
The gratings profile of 4a and 4b exhibit the same regularly
sinusoidal shapes with grating spacings of ¢1.6 mm, as set by the
interference pattern. From the AFM measurement, the surface
modulation depths of 4a and 4b are about 145 and 100 nm,
respectively. In addition, the modulation depth could be well
adjusted by the irradiation time and the irradiation energy.
The spatial period depend on both the angle (q) between the
two interfering beams and the wavelength (l) of the writing
beams.
Fig. 6 Changes in the UV-vis absorption spectra of 4b in DMF solution
at 25 ꢀC with different irradiation time under (a) 360 nm light irradiation
and (b) visible light irradiation.
Fig. 7 Experimental setup for SRGs inscription. (BS ¼ beam splitting;
RF ¼ flector).
5024 | J. Mater. Chem., 2008, 18, 5019–5026
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of the remaining reliefs is about 120 nm. But for 4b, most of the
SRGs are removed and the amplitude of the reliefs remained is
only about 10 nm. It is concluded that both the SRGs of the
hyperbranched polymer 4a and 4b films have good thermal shape
stability and the main chain type azo-polymer 4a shows even
better persistent fixation than the side chain type azo-polymer 4b.
Photoinduced birefringence
Birefringence measurements were produced by a pump beam
(532 nm of Nd:YAG laser) polarized at 45ꢀ 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 photo detector and connected through a lock-
in amplifier to a computer. The probe light was modulated at
980 Hz by a mechanical chopper. A birefringence On induced by
532 nm pump laser resulted in transmission of the 632.8 nm
probe beam through polarizer A. The intensity of this trans-
mitted beam could be described by the well-known eqn 2:30,31
Fig. 8 AFM images of the SRGs formed on azo-HPAEs 4a and 4b films.
(a) The plane view of the SRGs (the image size is 10 ꢃ 10 mm). (b) Typical
3-D view of the SRGs.
It ¼ Iosin2(pOnd/l)
(2)
Another important requirement of a SRGs is the shape
stability in terms of long-term storage and durability at higher
temperatures.21,28,29 To test whether the SRGs based on such
hyperbranched polymers could withstand high temperature, we
observed the changes of the surface profiles of the films from
25 to 300 ꢀC at a heating rate of 10 ꢀC minꢁ1 by polarized optical
microscope (POM) with a Linkam THMS 600 hot stage. The
azo-HPAEs 4a and 4b films could keep good stability at 200 ꢀC.
Surface-relief gratings of the films of 4b became blurry when they
were heated to 220 ꢀC and then evidently removed at 260 ꢀC. In
contrast to SRGs of 4b, the induced inscriptions on 4a films are
only partially erasable by thermal treatment and kept good
stability at the temperature of 260 ꢀC. The gratings of 4a could not
where d is the sample’s thickness, l 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. 10 shows the measured photoinduced birefringence of 4a
and 4b as a function of time. The time at which the pump laser
was switched on or off was marked with a letter. From the
multiple ‘‘writing–erasing’’ photoinduced birefringence cycles on
the films we could see that at the beginning of the experiments, no
light is transmitted through the analyzer (0–20 s) due to the
random orientation of the azo compounds. When the polarized
Nd:YAG laser beam radiation was introduced (marked A in
Fig. 10), an anisotropic orientation distribution is expected as
a consequence of the accumulation of the azo chromophores that
are oriented perpendicular to the polarization vector of the
writing beam. Light is thus transmitted through the analyzer due
ꢀ
be totally erased by heating, even at 300 C. The morphologies
of the film surfaces after heating were also observed by AFM.
Fig. 9 shows the AFM images of 4a and 4b films after heating to
300 ꢀC. Clearly, in the 4a film, the SRGs structure could be
still kept pretty well after the thermal treatment and the amplitude
Fig. 10 Multiple ‘writing–erasing’ photoinduced birefringence on the
films of azo-HPAEs 4a and 4b at room temperature. At point A the
linearly polarized laser (‘writing’ beam) was on; at point B the linearly
polarized laser was switched off; at point C the circularly polarized laser
(‘erasing’ beam) was on.
Fig. 9 AFM images of the SRCs on 4a and 4b films after heating
treatment. (a) Plane (10 ꢃ 10 mm) and (b) typical 3-D view of the SRGs.
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The polymer with azobenzene moieties in the side chain presents
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