Epoxy-based polymers functionalized with bisazo chromophores: Synthesis, characterization and photoresponsive behavior

Article abstract of DOI:10.1016/j.polymer.2011.05.040

In this work, two series of epoxy-based polymers bearing bisazo chromophores were synthesized from two precursor polymers (BP-AN and BP35-AN) through post-polymerization azo-coupling reactions. The polymers were prepared to contain bisazo chromophores wit

Full text of DOI:10.1016/j.polymer.2011.05.040

Polymer 52 (2011) 3344e3356
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Epoxy-based polymers functionalized with bisazo chromophores: Synthesis,
characterization and photoresponsive behavior
*
Xiaolin Wang, Jianjun Yin, Xiaogong Wang
Department of Chemical Engineering, Laboratory for Advanced Materials, Tsinghua University, Beijing 100084, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
In this work, two series of epoxy-based polymers bearing bisazo chromophores were synthesized from
two precursor polymers (BP-AN and BP35-AN) through post-polymerization azo-coupling reactions. The
polymers were prepared to contain bisazo chromophores with different electron-withdrawing groups
and different numbers of methyl substituents, which were characterized by 1H NMR, FT-IR, GPC, UVevis,
and thermal analysis. The photoinduced birefringence and surface-relief-grating (SRG) formation of the
polymers were studied by irradiating spin-coated films of the polymers with laser light at three different
wavelengths (488, 532, and 589 nm). The results show that the polymers containing bisazo chromo-
phores possess broader and red-shifted absorption bands compared with ordinary azobenzene-
containing polymers. The electron-withdrawing groups on the bisazo chromophores show significant
effect on the absorption band positions. The behavior of photoinduced birefringence and surface-relief-
grating (SRG) formation is closely correlated with the chromophore structures and excitation wavelength
of the laser light. Both electron-withdrawing groups and methyl substituents on the bisazo chromo-
phores show obvious influences on the photoinduced birefringence growth rate and SRG formation rate.
The proper methyl substitution on the bisazo chromophores shows ability to enhance the photoinduced
birefringence growth rate, birefringence saturation level, and SRG formation rate. The bisazo polymers
obtained in this study can be further explored for applications in telecommunication, data-storage,
sensors, and other optical devices.
Received 7 March 2011
Received in revised form
17 May 2011
Accepted 22 May 2011
Available online 27 May 2011
Keywords:
Epoxy-based Bisazo Polymer
Birefringence
Surface-relief-grating
Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction
interesting properties of azo polymers [1e3]. Birefringence induced
by polarized light irradiation is a result of repeated transecisetrans
In the past few decades, polymers containing aromatic azo
chromophores (azo polymers for short) have attracted considerable
attention for their unique properties and potential applications
[1e4]. Many interesting properties of azo polymers are caused by or
related with photoinduced transecis isomerization of azo chromo-
phores, where the azo group undergoes a transition from the lower
energy trans-form to less stable cis-form through the excited states.
Some well-known properties, such as photoinduced phase transi-
tion [5], photoinduced birefringence and dichroism [6], surface-
relief-grating (SRG) formation [7,8], photomechanical deformation
of thin films [9e11], and others have been intensively studied in
recent years [12e17]. The polymers with such properties can be used
in data-storage, sensors, actuators and other optical devices.
isomerization of azo chromophores, which forces the chromo-
phores and adjacent groups to align perpendicular to the polari-
zation direction of the light [3]. Photoinduced birefringence has
been widely investigated for different kinds of azo polymers, such
as side-chain azo polymers, main-chain azo polymers, and liquid
crystalline azo polymers [18e23]. SRG formation is generally
referred to the appearance of sinusoidal surface patterns on azo
polymer films induced by irradiation with interfering laser beams
[3,7,8]. The surface patterns are inscribed at a temperature well
below the glass transition temperature (Tg) of the azo polymers and
can be erased by heating the films to a temperature above the Tg.
The SRG formation has been observed for different kinds of azo
polymers such as side-chain azo polymers, main-chain azo poly-
mers, and hyperbranched azo polymers among others [2,3].
Various surface structures have been prepared by the photo-
fabrication methods based on this effect [24e26]. This unusual
response of azo polymers to light irradiation has been attributed to
transport of polymeric chains caused by the isomerization-driven
free volume expansion in bulk [27,28], the force based on the
Photoinduced birefringence and surface-relief-grating (SRG)
formation have been intensively investigated as some of the most
* Corresponding author. Tel.: þ86 10 62796171.
E-mail address: wxg-dce@mail.tsinghua.edu.cn (X. Wang).
0032-3861/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.polymer.2011.05.040

X. Wang et al. / Polymer 52 (2011) 3344e3356
3345
dipolar interaction with the optically induced electric field gradient
[29,30], a translational wormlike diffusion caused by the photo-
isomerization of the azobenzene chromophores [31], a mean-field
force caused by the molecule alignment [32], and photomechan-
ical effect occurring in thin films [33]. Although some recent
research has indicated possible correlation between the photoin-
duced chromophore orientation and the surface pattern formation
[34], a general understanding of this issue is still in a developing
process.
(BADGE,
M
¼
376) were purchased from Alfa Aesar. 4-(4-
Nitrophenylazo)aniline (Disperse Orange 3, DO3) was purchased
from ACROS. All the other materials, reagents and solvents were
commercially available products and used as received without further
purification.
2.2. Characterization
Infrared spectra were determined using a Nicolet 560-IR FT-IR
spectrophotometer by incorporating samples in KBr disks. 1H NMR
spectra were recorded using a JEOL JNM-ECA600 NMR spectrometer.
Mass spectra were determined by Agilent 6300 Ion Trap LC/MS
Systems. The molecular weights and molecular weight distributions
were determined by using a gel permeation chromatography (GPC)
apparatus at room temperature with THF as eluent (1 mL minꢀ1). The
instrument was equipped with a refractive index (RI) detector
Bisazo chromophore generally denotes the type of chromo-
phores with the conjugated aromatic rings linked by two azo
groups [35].1 A typical bisazo chromophore is composed of three
benzene rings covalently linked by two azo groups. In contrast to
the extensive study on polymers containing monoazo chromo-
phores, relatively few polymers containing bisazo chromophores
have been synthesized and investigated for the photonic properties
[36e45]. Compared with the monoazo chromophores, the conju-
gated length of the bisazo chromophore is significantly increased.
Polymers containing bisazo chromophores, which will be named as
bisazo polymers for short in the following parts, can show signifi-
cantly enhanced optical nonlinearity, birefringence and dichroism
[38,41]. Bisazo polymers have been synthesized and investigated as
novel nonlinear optical materials [36e40], all-optical switching and
anisotropic films [41e44], and thermotropic liquid crystal copoly-
mers [45]. The bisazo polymers usually possess several overlapped
absorption bands corresponding to the transitions from the ground
state to different excited states. As the excitation at different
wavelengths might cause the isomerization to occur at either or
both of the azo groups, it is possible for bisazo polymers to exhibit
distinctive responsive behavior upon the light irradiation with
different wavelengths. The absorption band positions of the bisazo
chromophores can be significantly adjusted by introducing
different electron-withdrawing groups. Consequently, the respon-
sive behavior to the different excitation wavelengths will also
depend on the electron-withdrawing groups on the chromophores.
Moreover, other substituents on the bisazo chromophores might
also affect the photoresponsive properties through different ways.
Therefore, the bisazo polymers could show great complexity in the
photoresponsive behavior correlated with both chromophore
structures and light excitation wavelength. However, to our
knowledge, a systematic study concerning the behavior of photo-
induced birefringence and SRG formation and its correlations with
chromophoric structures and excitation wavelength is still lacking
in the literature.
(Wyatt Optilab rEX) and fitted with a PLgel 5 mm mixed-D column.
The measurements were carried out at 35 ꢁC and the data were
calibrated with linear polystyrene standards. Thermal phase transi-
tions of the polymers were determined using TA Instruments DSC
2920 with a heating rate of 10 ꢁC/min in nitrogen atmosphere.
UVevis absorption spectra were recorded on a PerkineElmer Lamba
Bio-40 spectrophotometer. The surface images of the surface-relief-
gratings were monitored using an atomic force microscope (AFM,
Nanoscope IIIa) in the tapping mode.
2.3. 3,5-dimethyl-4-(40-chlorophenylazo)aniline
4-chloroaniline (1.28 g, 10 mmol) was dissolved in a homoge-
neous mixture of glacial acetic acid (12 mL), propionic acid (6 mL)
and sulfuric acid (98%, 1.5 mL). Diazonium salt was prepared by
adding an aqueous solution of sodium nitrite (0.83 g, 12 mmol in
2 mL of water) dropwise into the 4-chloroaniline solution. The
mixture was stirred at 5 ꢁC for 5 min before the excess nitrous acid
was decomposed by addition of 0.2 g of urea in a suitable amount of
water. The diazonium salt solution was then added dropwise into
a solution of 3,5-dimethylaniline (1.21 g, 10 mmol) in formic acid
(20 mL) at 0 ꢁC. The pH of the reaction mixture was adjusted to 2 by
adding a suitable amount of aqueous solution of sodium acetate.
The reaction was carried out with stirring at 0 ꢁC for 4 h and then
filtrated. The precipitate was washed with plenty of water until
a neutral stage was achieved and then dried in vacuum. Yield: 92%.
MS: m/z [M]þ: 259.14; calcd.: 259.09. 1H NMR (DMSO-d6),
d (ppm):
2.44 (s, 6H), 6.13 (br, 2H), 6.38 (s, 2H), 7.55 (d, J ¼ 8.2 Hz, 2H), 7.71 (d,
In this study, two series of epoxy-based bisazo polymers were
synthesized, which were functionalized with bisazo chromophores
bearing different electron-withdrawing groups and different
numbers of methyl substituents at different positions. The photo-
induced birefringence and SRG formation were studied by irradi-
ating the bisazo polymer films with the laser light at different
wavelengths (488, 532, and 589 nm). The influences of chromo-
phoric structure and excitation wavelength on the photoresponsive
behavior were investigated.
J ¼ 8.2 Hz, 2H).
2.4. 3,5-dimethyl-4-(40-cyanophenylazo)aniline
Via a similar procedure described above for the synthesis of 3,5-
dimethyl-4-(40-chlorophenylazo)aniline. The target compound was
synthesized through the azo-coupling reaction between 3,5-
dimethylaniline and the diazonium salt of 4-aminobenzonitrile.
Yield: 88%. MS: m/z [M]þ: 250.17; calcd.: 250.12. 1H NMR (DMSO-
d6),
d (ppm): 2.47 (s, 6H), 6.23 (br, 2H), 6.37 (s, 2H), 7.79 (d,
J ¼ 8.3 Hz, 2H), 7.93 (d, J ¼ 8.3 Hz, 2H).
2. Experimental section
2.5. 3,5-dimethyl-4-(40-nitrophenylazo)aniline
2.1. Materials
Via a similar procedure described above for the synthesis of 3,5-
dimethyl-4-(40-chlorophenylazo)aniline. The target compound was
synthesized through the azo-coupling reaction between 3,5-
dimethylaniline and the diazonium salt of 4-nitroaniline. Yield:
85%. MS: m/z [M]þ: 270.01; calcd.: 270.11. 1H NMR (DMSO-d6),
Aniline, 3,5-dimethylaniline, 4-chloroaniline, 4-aminobenzonitrile,
4-nitroaniline, 2-methyl-4-nitroaniline, bisphenol-A diglycidyl ether
1
In typical nomenclature of dyestuffs, disazo is the term generally used. In order
d
(ppm): 2.50 (s, 6H), 6.35 (br, 2H), 6.39 (s, 2H), 7.84 (d, J ¼ 8.9 Hz,
to be consistent with most polymer-related references, bisazo is used here and
after.
2H), 8.33 (d, J ¼ 8.9 Hz, 2H).

3346
X. Wang et al. / Polymer 52 (2011) 3344e3356
2.6. 3,5-dimethyl-4-(20-methyl-40-nitrophenylazo)aniline
2.10.1. BP-BAZ-35CN
Yield: 67%. DSC: Tg 143 ꢁC 1H NMR (DMSO-d6),
d (ppm): 1.55
Via a similar procedure described above for the synthesis of 3,5-
dimethyl-4-(40-chlorophenylazo)aniline. The target compound was
synthesized through the azo-coupling reaction between 3,5-
dimethylaniline and the diazonium salt of 2-methyl-4-
nitroaniline. Yield: 84%. MS: m/z [M]þ: 284.04; calcd.: 284.13. 1H
(6H), 2.43 (6H), 3.33 (2H), 3.51e3.60 (2H), 3.75e3.91 (4H), 4.11
(2H), 5.31 (1H), 5.43 (1H), 6.83e6.89 (6H), 7.08 (4H), 7.57 (2H),
7.72 (2H), 7.95 (2H), 8.02 (2H). IR (KBr, cmꢀ1): 3408, 2964, 2926,
2872, 2227, 1599, 1510, 1458, 1385, 1362, 1236, 1189, 1155, 1109,
1036, 827.
NMR (DMSO-d6),
d (ppm): 2.50 (s, 6H), 2.65 (s, 3H), 6.34 (br, 2H),
6.40 (s, 2H), 7.61 (d, J ¼ 8.9 Hz, 1H), 8.11 (m, 1H), 8.23 (d, J ¼ 2.1 Hz,
2.10.2. BP-BAZ-35NT
1H).
Yield: 66%. DSC: Tg 142 ꢁC 1H NMR (DMSO-d6),
d (ppm): 1.54
(6H), 2.43 (6H), 3.33 (2H), 3.50e3.60 (2H), 3.81e3.90 (4H), 4.11
(2H), 5.31 (1H), 5.43 (1H), 6.83 (6H), 7.07 (4H), 7.54 (2H), 7.70
(2H), 7.97 (2H), 8.35 (2H). IR (KBr, cmꢀ1): 3385, 2962, 2926,
2870, 1599, 1510, 1458, 1342, 1234, 1180, 1155, 1105, 1034, 852,
827.
2.7. BP-AN
Precursor polymer BP-AN was synthesized from the reaction
between bisphenol-A diglycidyl ether (37.6 g, 0.1 mol) and aniline
(9.3 g, 0.1 mol) according to the literature [48]. DSC: Tg 88 ꢁC. GPC:
Mn ¼ 9900, Mw ¼ 18600, MWD ¼ 1.9. 1H NMR (DMSO-d6),
d
(ppm):
2.10.3. BP-BAZ-35mNT
1.55 (6H), 3.34e3.75 (4H), 3.88 (4H), 4.03 (2H), 5.17 (1H), 5.36 (1H),
6.54 (1H), 6.73 (2H), 6.82 (4H), 7.08 (6H). IR (KBr, cmꢀ1): 3340,
2964, 2926, 2870, 1599, 1506, 1460, 1385, 1362, 1248, 1180, 1105,
1036, 827.
Yield: 68%. DSC: Tg 135 ꢁC 1H NMR (DMSO-d6),
d (ppm): 1.55
(6H), 2.48 (6H), 2.64 (3H), 3.34 (2H), 3.51e3.61 (2H), 3.78e3.91
(4H), 4.11 (2H), 5.32 (1H), 5.44 (1H), 6.83e6.87 (6H), 7.07 (4H), 7.58
(2H), 7.71 (2H), 8.09 (1H), 8.18 (1H), 8.24 (1H). IR (KBr, cmꢀ1): 3385,
2964, 2926, 2870, 1599, 1510, 1458, 1342, 1236, 1189, 1153, 1111,
1086, 1036, 928, 827.
2.8. BP35-AN
Precursor polymer BP35-AN was synthesized from the reaction
between bisphenol-A diglycidyl ether (37.6 g, 0.1 mol) and 3,5-
dimethylaniline (12.1 g, 0.1 mol) through a similar procedure for
preparing BP-AN. DSC: Tg 93 ꢁC. GPC: Mn ¼ 11400, Mw ¼ 20000,
2.10.4. BP-BAZ-NT
Yield: 61%. DSC: Tg 140 ꢁC 1H NMR (DMSO-d6),
d (ppm): 1.55
(6H), 3.34 (2H), 3.50e3.60 (2H), 3.87e3.92 (4H), 4.12 (2H),
5.17e5.45 (2H), 6.82 (6H), 7.08 (4H), 7.76 (2H), 7.95 (2H), 8.07e8.09
(4H), 8.41 (2H). IR (KBr, cmꢀ1): 3377, 2964, 2929, 2870, 1599, 1508,
1458, 1342, 1240, 1182, 1149, 1120, 1036, 829.
MWD ¼ 1.8. 1H NMR (DMSO-d6),
d (ppm): 1.55 (6H), 2.04 (6H),
3.26e3.72 (4H), 3.87 (4H), 4.04 (2H), 5.17 (1H), 5.36 (1H), 6.17 (1H),
6.32 (2H), 6.82 (4H), 7.07 (4H). IR (KBr, cmꢀ1): 3340, 2964, 2916,
2870, 1599, 1510, 1468, 1362, 1298, 1248, 1180, 1105, 1036, 825.
2.11. BP35-BAZ-35Cl, BP35-BAZ-35CN, BP35-BAZ-35NT, BP35-BAZ-
35mNT, and BP35-BAZ-NT
2.9. BP-BAZ-35Cl
BP35-BAZ-35Cl, BP35-BAZ-35CN, BP35-BAZ-35NT, BP35-BAZ-
35mNT, and BP35-BAZ-NT were synthesized through the post-
polymerization azo-coupling reactions between BP35-AN and the
diazonium salts of 3,5-dimethyl-4-(40-chlorophenylazo)aniline,
3,5-Dimethyl-4-(40-chlorophenylazo)aniline (0.65 g, 2.5 mmol)
was dissolved in THF (4 mL). Glacial acetic acid (3 mL), deionized
water (2 mL), and sulfuric acid (98%, 0.5 mL) were added into the
solution to form a homogeneous mixture. Diazonium salt was
prepared by adding an aqueous solution of sodium nitrite (0.2 g,
3 mmol in 1 mL of water) dropwise into the mixture. The mixture
was stirred at 5 ꢁC for 20 min to form the diazonium salt solution. It
was then added dropwise into a solution of BP-AN (0.94 g, 2 mmol
in term of repeat units) in DMF (120 mL) at 0 ꢁC. The solution was
stirred at 0 ꢁC for 12 h and then added into plenty of water. The
precipitate collected by filtration was washed several times and
dried in vacuum. The solid was dissolved in THF again and
precipitated with petroleum ether. The final product was collected
by filtration and dried in a vacuum oven at 60 ꢁC for 48 h. Yield:
3,5-dimethyl-4-(40-cyanophenylazo)aniline,
3,5-dimethyl-4-(40-
3,5-dimethyl-4-(20-methyl-40-nitro-
nitrophenylazo)aniline,
phenylazo)aniline, and 4-(40-nitrophenylazo)aniline, respectively.
The reactions were carried out via a similar procedure described
above for preparing BP-BAZ-35Cl. Analytical results for these
polymers are given below.
2.11.1. BP35-BAZ-35Cl
Yield: 72%. DSC: Tg 130 ꢁC 1H NMR (DMSO-d6),
d (ppm): 1.53
(6H), 2.37 (12H), 3.41 (2H), 3.53 (2H), 3.76e3.89 (4H), 4.18 (2H),
5.29 (1H), 5.43 (1H), 6.50 (2H), 6.81 (4H), 7.06 (4H), 7.46 (2H), 7.61
(2H), 7.82 (2H). IR (KBr, cmꢀ1): 3409, 2960, 2920, 2868, 1597, 1508,
1468, 1360, 1246, 1173, 1105, 1086, 1034, 1009, 829.
71%. DSC: Tg 135 ꢁC 1H NMR (DMSO-d6),
d (ppm): 1.55 (6H), 2.38
(6H), 3.34 (2H), 3.51e3.59 (2H), 3.75e3.91 (4H), 4.11 (2H), 5.32
(1H), 5.44 (1H), 6.83e6.86 (6H), 7.07 (4H), 7.57 (2H), 7.62 (2H), 7.72
(2H), 7.85 (2H). IR (KBr, cmꢀ1): 3385, 2964, 2926, 2870, 1599, 1510,
1458, 1387, 1362, 1300, 1236, 1182, 1155, 1111, 1088, 1038, 1011, 829.
2.11.2. BP35-BAZ-35CN
Yield: 69%. DSC: Tg 140 ꢁC 1H NMR (DMSO-d6),
d (ppm): 1.53
(6H), 2.41 (12H), 3.42 (2H), 3.53 (2H), 3.74e3.89 (4H), 4.10 (2H),
5.27 (1H), 5.42 (1H), 6.49(2H), 6.82 (4H), 7.07 (4H), 7.48 (2H),
7.90e7.97 (4H). IR (KBr, cmꢀ1): 3412, 2962, 2922, 2870, 2227, 1597,
1508, 1468, 1360, 1248, 1176, 1105, 1034, 886, 829.
2.10. BP-BAZ-35CN, BP-BAZ-35NT, BP-BAZ-35mNT, and BP-BAZ-NT
BP-BAZ-35CN, BP-BAZ-35NT, BP-BAZ-35mNT, and BP-BAZ-NT
were synthesized through the post-polymerization azo-coupling
reactions between BP-AN and the diazonium salts of 3,5-dimethyl-
4-(40-cyanophenylazo)aniline, 3,5-dimethyl-4-(40-nitrophenylazo)
aniline, 3,5-dimethyl-4-(20-methyl-40-nitrophenylazo)aniline, and
4-(40-nitrophenylazo)aniline, respectively. The reactions were
carried out via a similar procedure described above for preparing
BP-BAZ-35Cl. Analytical results for these polymers are given below.
2.11.3. BP35-BAZ-35NT
Yield: 67%. DSC: Tg 140 ꢁC 1H NMR (DMSO-d6),
d (ppm): 1.54
(6H), 2.38e2.41 (12H), 3.40 (2H), 3.52 (2H), 3.75e3.89 (4H), 4.09
(2H), 5.28 (1H), 5.43 (1H), 6.48 (2H), 6.81 (4H), 7.06 (4H), 7.46 (2H),
7.95 (2H), 8.33 (2H). IR (KBr, cmꢀ1): 3417, 2962, 2924, 2870, 1597,
1510, 1468, 1342, 1248, 1173, 1103, 1034, 858, 829.

X. Wang et al. / Polymer 52 (2011) 3344e3356
3347
Scheme 1. The synthetic route and chemical structure of the aminoazobenzene derivatives and bisazo polymers.
2.11.4. BP35-BAZ-35mNT
amount of the materials in N,N-dimethylformamide (DMF) with
a typical concentration of 10 wt%. The solutions were filtered
Yield: 65%. DSC: Tg 135 ꢁC 1H NMR (DMSO-d6),
d
(ppm): 1.54
(6H), 2.34e2.62 (15H), 3.41 (2H), 3.50 (2H), 3.75e3.88 (4H), 4.09
(2H), 5.28 (1H), 5.43 (1H), 6.49 (2H), 6.81 (4H), 7.07 (4H), 7.44e7.51
(3H), 8.01e8.12 (2H). IR (KBr, cmꢀ1): 3411, 2962, 2922, 2870, 1599,
1510, 1468, 1342, 1248, 1180, 1107, 1086, 1034, 829, 808.
through 0.45 mm membranes and spin-coated onto glass slides. The
film thickness was controlled to be in a range from 400 to 600 nm
by adjusting the solution concentrations and the spinning speed.
The films were dried at 60 ꢁC under vacuum for 48 h before use.
2.11.5. BP35-BAZ-NT
2.13. Photoinduced birefringence
Yield: 60%. DSC: Tg 146 ꢁC 1H NMR (DMSO-d6),
d (ppm): 1.55
(6H), 2.37e2.41 (6H), 3.42 (2H), 3.50 (2H), 3.82e3.91 (4H), 4.11
(2H), 5.36 (2H), 6.55 (2H), 6.82 (4H), 7.07 (4H), 7.87 (2H), 8.03 (4H),
8.39 (2H). IR (KBr, cmꢀ1): 3410, 2964, 2927, 2872, 1599, 1510, 1468,
1342, 1248, 1182, 1107, 1038, 829.
Photoinduced birefringence was investigated by the optical
setup similar to those reported before [41,46,47]. A linearly polar-
ized laser beam with the wavelength of 488 nm (Arþ laser), 532,
and 589 nm (diode-pumped frequency doubled solid state lasers)
was used as the light sources. A piece of the azo polymer films was
placed between two crossed linear polarizers. A low power HeeNe
laser beam at 632.8 nm was used to measure the transmitted power
through this optical setup. The writing laser beam with intensity of
200 mW/cm2 was incident perpendicular to the sample and the
polarization angle of the incident laser beam was set to be 45ꢁ with
2.12. Film preparation
Solid films of the azo polymers with smooth surfaces were
prepared by spin-coating and vacuum drying. The homogeneous
solutions of the polymers were obtained by dissolving a suitable

3348
X. Wang et al. / Polymer 52 (2011) 3344e3356
values. On the other hand, a possible drawback of the post-
functionalization scheme is that the degree of functionalization
(DF) could be less than 100%, which will be discussed in the
following parts in detail.
Two epoxy-based precursor polymers (BP-AN and BP35-AN)
were synthesized through the step polymerization between
bisphenol-A diglycidyl ether (BADGE) and aniline or 3,5-
dimethylaniline. The polymerization temperature was controlled
to be 110 ꢁC to avoid the possible side-reaction between the
secondary hydroxyl groups generated by the ring-opening reaction
and the unreacted epoxide rings. The abbreviations BP-AN and
BP35-AN denote the precursor polymer prepared by the reactions
of BADGE with aniline and 3,5-dimethylaniline, respectively. BP-AN
and BP35-AN are linear polymers with number-average molecular
weights of 9900 and 11400 and the polydispersity indexes of 1.9
and 1.8, which were determined by GPC. Both precursor polymers
are amorphous polymers with glass transition temperatures of
88 ꢁC and 93 ꢁC for BP-AN and BP35-AN. The precursor polymers
show good solubility in polar organic solvents such as tetrahydro-
furan (THF), N,N-dimethylformamide (DMF), and N,N-dimethyla-
cetamide (DMAc).
Scheme 2. The synthetic route and chemical structure of BP-BAZ-NT and BP35-BAZ-
NT.
In order to obtain the bisazo polymers, four aminoazobenzene
derivatives were synthesized by azo-coupling reactions between
3,5-dimethylaniline and the diazonium salts of 4-chloroaniline, 4-
aminobenzonitrile, 4-nitroaniline, and 2-methyl-4-nitroaniline,
respectively (Scheme 1). Because of the hyperconjugation effect of
the two methyl groups in 3,5-dimethylaniline, the azo-coupling
reactions occurred easily at the 4-position of 3,5-dimethylaniline
even under the strong acidic condition. Under the acidic condi-
tion, the amino group of 3,5-dimethylaniline was partially
protonated, which prevented the possible side-reaction between
the amino group and diazonium cation to form the diazo-
aminobenzene compounds.
respect to the optical axes of both polarizers. The angle between
incident laser beam and probing laser beam was fixed to be 8ꢁ. The
photoinduced birefringence was estimated from the change in
transmittance of the probe beam. The birefringence growth rates
and saturation levels of the bisazo polymers were obtained under
the same condition for comparison. All experiments were carried
out at room temperature under air-ambient condition.
2.14. Surface-relief-grating inscription
The bisazo polymers were then prepared by the azo-coupling
reactions between the precursor polymers and diazonium salts of
the aminoazobenzene derivatives. The post-polymerization func-
tionalization was able to introduce various bisazo chromophores at
the final stages of the synthetic routes. In the processes, BP-AN and
BP35-AN were reacted with the diazonium salts of 3,5-dimethyl-4-
(40-chlorophenylazo)aniline, 3,5-dimethyl-4-(40-cyanophenylazo)
aniline, 3,5-dimethyl-4-(40-nitrophenylazo)aniline, and 3,5-
Surface-relief-grating (SRG) inscription was carried out by the
experimental setup similar to those reported before [7,8]. A linearly
polarized laser beam with wavelength of 488 nm (Arþ laser), 532,
and 589 nm (diode-pumped frequency doubled solid state lasers)
was used as the light source. The laser beam, which was spatially
filtered, expanded, and collimated with the intensity of 200 mW/
cm2, was split by a mirror. One half of the p-polarized beam was
reflected onto the film surface and coincident with the other half
beam that was directly incident onto the surface to form an inter-
ference pattern. The formation of SRGs was probed with a low
power HeeNe laser beam at 632.8 nm by measuring the diffraction
efficiency of the first-order diffracted beam in a real time mode. The
SRG inscription was carried out at room temperature under air-
ambient condition.
dimethyl-4-(20-methyl-40-nitrophenylazo)aniline,
respectively
(Scheme 1). Through these reactions, eight epoxy-based polymers
containing the bisazo chromophores were obtained. The name
abbreviations of the polymer are given in Scheme 1. The bisazo
polymers are named as BP-BAZ-X or BP35-BAZ-X, where X is the
abbreviations to make the distinction for the different methyl
substitution and electron-withdrawing groups. For the comparison,
the other two bisazo polymers were obtained by the azo-coupling
reactions of BP-AN and BP35-AN with the diazonium salt of 4-(40-
nitrophenylazo)aniline. The synthetic route and polymer structures
are shown in Scheme 2.
3. Results and discussion
3.1. Polymer synthesis and characterization
The structures of the azo polymers were characterized by
spectroscopic analyses, thermal analysis and GPC. Fig. 1 gives 1H
NMR spectra of the precursor polymers and some representative
azo polymers. The 1H NMR spectra of the other azo polymers are
given in the Supporting Information (Fig. S1 and S2). In the 1H NMR
spectrum of BP-AN, the anilino moieties show the resonance
signals at 6.54, 7.08 and 6.73 ppm attributed to the protons at para,
meta, and ortho positions of the amino groups. In the 1H NMR
spectrum of BP35-AN, the 3,5-dimethylaniline moieties show
the resonance signals at 6.17 and 6.32 ppm corresponding to the
protons at para and ortho positions of the amino groups and the
signal at 2.04 ppm attributed to the protons of the methyl groups
at 3,5-positions. After the post-polymerization azo-coupling
The epoxy-based polymers containing the bisazo chromophores
were synthesized through the post-polymerization azo-coupling
reactions [38,48,49]. Schemes 1 and 2 give the synthetic routes of
the two series of the bisazo polymers. The post-functionalization
scheme was selected because it can yield the polymers with the
high degree of functionalization and prevent possible side-
reactions of bisazo chromophores in the step polymerization
[38,48]. One obvious advantage of this scheme for the current study
is that the degree of polymerization (DP) of the bisazo polymers
was the same as they were obtained by using the same-batch
precursor polymers. It should be more desirable to compare the
photoresponsive properties for polymers with the similar DP

X. Wang et al. / Polymer 52 (2011) 3344e3356
3349
Table 1
Tgs of the epoxy-based bisazo polymers obtained by DSC (X.L. Wang et al.).
X
Tgs (ꢁC) of azo polymers
BP-BAZ-X
BP35-BAZ-X
35Cl
135
143
142
135
140
130
140
140
135
146
35CN
35NT
35mNT
NT
reactions, where excess amounts of diazonium salts (molar ratios to
aniline moieties of the precursor polymers) were used, the 1H NMR
spectra of the precursor polymers show the following changes for
the resonance signals of aniline and 3,5-dimethylaniline moieties.
The resonances (6.54 ppm for BP-AN, and 6.17 ppm for BP35-AN)
corresponding to the protons at para positions of the amino groups
completely disappear, which indicate the substitutions occurred at
the para positions with high yield. The shifts to lower magnetic
field can be seen for signals of other protons in aniline and 3,5-
dimethylaniline residues (meta and ortho to the amino groups in
BP-AN, and ortho to the amino groups in BP35-AN). These spectral
changes clearly evidence the introduction of the electron-
withdrawing groups and increase of the conjugation length due
to the formation of the bisazo structures. Moreover, additional
resonance signals corresponding to the newly introduced benze-
noid protons appear at the even lower magnetic field range and
a new signals corresponding to the methyl groups introduced by
the reactions appear at the high magnetic field. In the 1H NMR
spectra of the bisazo polymers with the exceptions of BP-BAZ-NT
and BP35-BAZ-NT, the resonances corresponding to the protons
at para positions of the amino residues (6.54 ppm for BP-AN, and
6.17 ppm for BP35-AN) completely disappear (Fig. 1, Figs. S1 and
S2). It indicates that the degree of functionalization (DF) of the
Fig. 1. 1H NMR spectra of the precursor polymers and representative bisazo polymers.
(a) BP-AN, (b) BP35-AN, (c) BP-BAZ-35NT, (d) BP35-BAZ-35NT.
Fig. 2. The UVevis spectra of the bisazo polymer spin-coated films. (a) The bisazo
polymers obtained from BP-AN; (b) The bisazo polymers obtained from BP35-AN.

3350
X. Wang et al. / Polymer 52 (2011) 3344e3356
Table 2
The lmaxs of the bisazo polymers, spin-coated films and DMF solutions (X.L. Wang
et al.).
X
lmax (nm) (DMFesolution/film)
BP-BAZ-X
BP35-BAZ-X
35Cl
448/436
491/451
508/483
498/452
522/510
468/430
502/464
520/492
500/475
531/515
35CN
35NT
35mNT
NT
bisazo polymers is close to 100%, which can also be confirmed by
the ratios of the integration areas for the protons on the bisazo
chromophores. This high efficiency of the post-functionalization is
consistent with those reported before for post-polymerization azo-
coupling reactions used to prepare monoazo polymers [38,48,49].
For BP-BAZ-NT and BP35-BAZ-NT, the DFs are relatively low, which
are 45% and 55%, respectively. For these two polymers, the diazo-
nium salt of 4-(40-nitrophenylazo)aniline was used for the azo-
coupling reaction (Scheme 2). Above results indicate that the
3,5-dimethyl substituents on the aminoazobenzene derivatives are
critical for the azo-coupling reaction to yield the bisazo polymers
with the high DF. This substituent effect could be attributed to the
role of the 3,5-dimethyl substituents to stabilize the diazonium
cations, which increase the possibility of the diazonium reactants
to approach the reaction sites. From the 1H NMR and other
analytical results given in the Experimental Section and Supporting
Information (Figs. S1eS5), it is concluded that the target bisazo
polymers have been successfully synthesized through the post-
polymerization azo-coupling scheme.
Fig. 3. (a) Typical writing, relaxation and erasing sequence for the film of BP35-BAZ-
35CN, irradiated with the incident laser beam at 532 nm with the intensity of 200 mW/
cm2. Point A: the linearly polarized laser (writing beam) is turned on; Point B: the
writing beam is turned off; Point C: the circularly polarized laser (erasing beam) is
turned on; Point D: the writing beam is turned on again. (b) The relaxation processes
after irradiated for 5 min (curve a), 15 min (curve b), 25 min (curve c).
3.2. Thermal and spectral properties
transition can be observed from the DSC analysis before thermal
degradation (Figs. S6 and S7). The glass transition temperatures (Tgs)
of the polymers determined by DSC are given inTable 1. The Tg values
of the bisazo polymers are in range from 130 to 146 ꢁC, which are
All bisazo polymers, obtained from the two precursor polymers
with five different bisazo chromophores, showed typical thermal
transition behavior of amorphous polymers. Only the glass
Fig. 4. The birefringence as a function of time for the films of bisazo polymers with incident laser light at 488, 532, and 589 nm (a)e(c): Bisazo polymers prepared from precursor
polymer BP-AN, (a) 488 nm, (b) 532 nm, (c) 589 nm (d)e(f): Bisazo polymers prepared from precursor polymer BP35-AN, (d) 488 nm, (e) 532 nm, (f) 589 nm.

X. Wang et al. / Polymer 52 (2011) 3344e3356
3351
Table 3.1
The parameters obtained from the curve fitting of the photoinduced birefringence kinetics (X.L. Wang et al.) (BP-BAZ-X, 488 nm).
BP-BAZ-35Cl
BP-BAZ-35CN
BP-BAZ-35NT
BP-BAZ-35mNT
BP-BAZ-NT
ka(sꢀ1
kb(sꢀ1
)
)
0.013 ꢃ 0.001
0.054 ꢃ 0.002
0.52 ꢃ 0.02
0.48 ꢃ 0.02
0.9997
0.005 ꢃ 0.000
0.026 ꢃ 0.001
0.50 ꢃ 0.01
0.50 ꢃ 0.01
0.9999
0.002 ꢃ 0.000
0.013 ꢃ 0.001
0.51 ꢃ 0.02
0.49 ꢃ 0.02
0.9997
0.005 ꢃ 0.000
0.029 ꢃ 0.002
0.62 ꢃ 0.01
0.38 ꢃ 0.01
0.9999
0.007 ꢃ 0.000
0.018 ꢃ 0.002
0.55 ꢃ 0.02
0.45 ꢃ 0.02
0.9999
a
An
a
Bn
Correl. coeff.
(
D
nsꢂ10ꢀ2/ts(s))
6.2/300
7.2/900
6.3/2150
6.6/1000
6.9/600
a
An ¼ A/(A þ B), Bn ¼ B/(A þ B).
much higher than those of the corresponding precursor polymers
(88 ꢁC for BP-AN and 93 ꢁC for BP35-AN). The increases of the Tgs can
be attributed to the significant increase in the polarity of the side-
chains, which enhances the inter/intra-molecular interaction of
the polymer chains. For the bisazo polymers obtained from the same
precursor polymers, BP-BAZ-35Cl (and BP35-BAZ-35Cl) and BP-
a linearly polarized laser beam at 488, 532, and 589 nm with the
intensity of 200 mW/cm2. The experimental setup is similar to
those reported before [41,46,47]. Before the laser light irradiation,
no transmittance of the probing HeeNe laser beam (632.8 nm) was
observed because of the isotropic sample was placed between the
crossed polarizers. When the writing laser beam was switched on,
which was incident normal to the sample surfaces, the intensity of
the transmitted probing laser beam rapidly increased and then
gradually saturated after a certain time period. The photoinduced
birefringence can be determined from the detected transmittance
of probing laser by Equation (1),
BAZ-mNT (and BP35-BAZ-mNT) show
a relatively lower Tg
compared to the others in the series. These observations can be
rationalized by considering the weak electron-withdrawing ability
of chloro-substituent and the extra methylon thenitrophenylgroup,
whichcould weaken the interaction between the chromophores and
reduce the packing density of the polymer chains.
Fig. 2 gives the UVevis spectra of the bisazo polymers as spin-
coated films. The lmaxs of the bisazo polymers obtained in DMF
solutions and as spin-coated films are summarized in Table 2. The
bisazo polymers show strong absorption in visible light range. The
extinction coefficients at the maximum absorbance for the poly-
mers in DMF are given in the Supporting Information (Table S1).
The lmaxs of the bisazo polymers in DMF solutions show significant
red-shifts as compared to those of the solid films of the same
sffiffiffiffi
l
IT
I0
D
n ¼
sinꢀ1
(1)
p
d
where IT is the intensity of the transmitted probing laser beam
when the sample film is irradiated with the writing beam, I0 is the
intensity of the transmitted probing laser without the irradiation
and polarizer,
d is the film thickness (nm), and
l
is the wavelength of the probe beam (632.8 nm),
n is the birefringence [41,46].
D
Fig. 3(a) shows a typical writing-relaxation-rewriting sequence
for the film of BP35-BAZ-35CN when irradiated with the incident
laser beam at 532 nm. Before irradiation, no transmission of the
probing beam through the crossed polarizer is observed, which
indicates that the film is homogeneous and optically isotropic.
When the polymer film is irradiated (point A), the optical anisot-
ropy is induced in the polymer film, which causes growth trans-
polymers owing to the solvatochromic effect of the DMF. The lmax
s
of the bisazo polymers are strongly affected by electron-
withdrawing groups (chloro, cyano, and nitro). As the electron-
withdrawing ability of the groups increases, the lmax shows
a significant red-shift. The effect of methyl substituents on the
lmax depends on the positions of the substitution. When the
3,5-dimethyl substituents on the benzene ring bearing electron-
donating amino group (anilino moiety), the lmax shows signifi-
cant red-shift, which can be seen by comparing the corresponding
polymers in the BP-BAZ-X and BP35-BAZ-X series. On the other
hand, the 3,5-dimethyl substituents on the middle benzene ring
(between the two azo groups), a blue-shift can be observed. Simi-
larly, the 2-methyl substituent on the benzene ring bearing the
electron-withdrawing nitro group also causes the blue-shift. The
methyl-substituent effect on the lmax can be partially explained by
considering the electron-donating ability of the methyl groups,
which enhances or weakens the push-pull effect of the acceptor
and donor groups.
mittance for the probing beam. In the process,
Dn increases
gradually and reaches the saturation in about 300 s with the
saturation value of 6.9 ꢂ 10ꢀ2.When the incident laser beam was
turned off (point B),
Dn decreases to about 55% of the saturation
value in 240 s, corresponding to the partial relaxation of the
photoinduced anisotropy. When irradiating the film with a circu-
larly polarized laser beam at the same wavelength (point C), the
photoinduced anisotropy is completely erased by the irradiation for
about 100 s. When the film is exposed to the writing beam again
(point D), the photoinduced Dn increases and repeats the process of
the first round of the light irradiation. Fig. 3(b) shows the relaxation
behavior of BP35-BAZ-35CN after irradiation for 5 min (curve a),
15 min (curve b), 25 min (curve c). The relaxation occurring at room
temperature for 300 s shows the residual birefringence of 61%, 67%,
and 66%. The extension of the irradiation time after reaching the
saturation doesn’t show obvious influence on the relaxation
3.3. Photoinduced birefringence
Photoinduced birefringence behavior of the bisazo polymers
was studied by irradiating the solid thin films of the polymers with
Table 3.2
The parameters obtained from the curve-fitting of the photoinduced birefringence kinetics (X.L. Wang et al.) (BP-BAZ-X, 532 nm).
BP-BAZ-35Cl
BP-BAZ-35CN
BP-BAZ-35NT
BP-BAZ-35mNT
BP-BAZ-NT
ka(sꢀ1
kb(sꢀ1
)
)
0.015 ꢃ 0.002
0.074 ꢃ 0.003
0.22 ꢃ 0.02
0.78 ꢃ 0.02
0.9998
0.012 ꢃ 0.001
0.060 ꢃ 0.003
0.59 ꢃ 0.02
0.41 ꢃ 0.02
0.9997
0.006 ꢃ 0.000
0.044 ꢃ 0.004
0.54 ꢃ 0.03
0.46 ꢃ 0.03
0.9994
0.006 ꢃ 0.001
0.039 ꢃ 0.005
0.62 ꢃ 0.04
0.38 ꢃ 0.04
0.9992
0.009 ꢃ 0.000
0.023 ꢃ 0.004
0.81 ꢃ 0.04
0.19 ꢃ 0.04
0.9994
a
An
a
Bn
Correl. coeff.
(
D
nsꢂ10ꢀ2/ts(s))
6.0/200
6.1/400
5.7/700
6.0/800
7.5/600
a
An ¼ A/(A þ B), Bn ¼ B/(A þ B).

3352
X. Wang et al. / Polymer 52 (2011) 3344e3356
Table 3.3
The parameters obtained from the curve fitting of the photoinduced birefringence kinetics (X.L. Wang et al.) (BP-BAZ-X, 589 nm).
BP-BAZ-35Cl
BP-BAZ-35CN
BP-BAZ-35NT
BP-BAZ-35mNT
BP-BAZ-NT
ka(sꢀ1
kb(sꢀ1
)
)
0.008 ꢃ 0.000
0.057 ꢃ 0.005
0.82 ꢃ 0.04
0.18 ꢃ 0.04
0.9994
0.009 ꢃ 0.001
0.029 ꢃ 0.006
0.74 ꢃ 0.05
0.26 ꢃ 0.05
0.9996
0.007 ꢃ 0.000
0.022 ꢃ 0.001
0.60 ꢃ 0.02
0.40 ꢃ 0.02
0.9999
0.004 ꢃ 0.000
0.013 ꢃ 0.005
0.90 ꢃ 0.05
0.10 ꢃ 0.05
0.9997
0.004 ꢃ 0.001
0.010 ꢃ 0.005
0.94 ꢃ 0.04
0.06 ꢃ 0.04
0.9997
a
An
a
Bn
Correl. coeff.
(
D
nsꢂ10ꢀ2/ts(s))
5.9/550
6.6/550
7.4/750
6.2/1500
6.9/1400
a
An ¼ A/(A þ B), Bn ¼ B/(A þ B).
behavior. Other bisazo polymers show similar photoinduced bire-
fringence and relaxation behavior as discussed above for BP35-
BAZ-35CN.
The birefringence growth rates and saturation values were used
to characterize the effect of the chromophore structure and exci-
tation wavelength on the photoresponsive behavior. Fig. 4 shows
the relationship between the photoinduced birefringence and the
irradiation time for the bisazo polymers upon the irradiation at
three different wavelengths. The birefringence growths can be
fitted by a biexponential curve (Equation (2)),
two polymers (Fig. 4(c)). The saturation birefringence levels also
show a close correlation with the chromophore structure and the
excitation wavelength. When irradiated with 488, 532 and 589 nm
laser light, BP-BAZ-35CN, BP-BAZ-NT, and BP-BAZ-35NT shows the
highest saturation birefringence levels with the
0.075, and 0.074, respectively.
Dns values of 0.072,
Fig. 4(def) and Tables 3.4e3.6 show the results of the birefrin-
gence study for the series of bisazo polymers obtained from the
precursor polymer BP35-AN. Similarly, the bisazo polymer BP35-
BAZ-35Cl, containing chloro as the electron-withdrawing group,
shows the fastest birefringence growth rates when irradiated with
laser light of 488, 532, and 589 nm. When irradiated with 488, 532
and 589 nm laser light, BP35-BAZ-35CN (as well as BP35-BAZ-
35NT), BP35-BAZ-NT, and BP35-BAZ-35NT shows the highest
D
n ¼ A$ 1 ꢀ exp ðꢀk tÞg þ B$ 1 ꢀ exp ðꢀk tÞg
(2)
f
f
a
b
where
Dn is the birefringence achieved at time t, ka and kb are the
time constants for birefringence growth, and A and B are two
constants [41]. The sum of A and B represents the maximum
induced birefringence and the proportions of A and B to the sum (An
and Bn) represent the relative contribution of each term to the
induced birefringence. The fitting parameters for the birefringence
growth are listed in Tables 3.1e3.6, where the ka and kb represent
the time constants for the slow and fast growth processes. The two
processes are attributed to the orientation of chromophores and
backbone as a result of the repeated transecisetrans isomerization
cycles [41,46]. The corresponding saturation birefringence levels
saturation birefringence levels with the Dns values of 0.089, 0.081,
and 0.101, respectively. BP35-BAZ-35CN not only shows the high
saturation level, but also shows the fast birefringence growth rate
when irradiated with the 488 and 532 nm beams. BP35-BAZ-35NT
shows the highest saturation birefringence level and fast growth
rate when irradiated with the 589 nm light.
Above results indicate that the photoinduced birefringence
behavior is closely related with the chromophore structure and
excitation wavelength. Some important observations from above
study can be summarized as follows. When the bisazo polymers are
excited at the wavelengths between the lmaxs and absorption band
tails at the longer wavelength side, the bisazo polymers show the
faster birefringence growth rate and higher saturation levels (such
as BP-BAZ-35CN at 532 nm and BP35-BAZ-35Cl at 488 and 532 nm).
On the other hand, when the excitation wavelength is too close to
the absorption band tails, the birefringence growth rate and satu-
ration level decrease (such as BP-BAZ-35Cl at 589 nm). For the three
excitation wavelengths, the bisazo polymers containing chloro as
the electron-withdrawing group almost always show the fastest
birefringence growth rate. The bisazo polymers containing cyano as
the electron-withdrawing group show fast birefringence rate and
high saturation level when irradiated at 488 nm. The saturation
(Dns) and the time to reach the saturated levels (ts) are given in the
same tables. All parameters are closely related to the chromophore
structure and excitation wavelength as discussed below.
Fig. 4(aec) and Tables 3.1e3.3 show the results of the series of
bisazo polymers obtained from the precursor polymer BP-AN. The
bisazo polymer BP-BAZ-35Cl, which contains chloro as the
electron-withdrawing group, shows the fastest birefringence
growth rate when irradiated with the laser light at 488 and 532 nm
as shown in Fig. 4(a and b). For the irradiation with 589 nm laser
light, the BP-BAZ-35Cl still shows largest ka and kb in the series
(Table 3.3). However, the relative contribution of the fast process,
which is represented by Bn, becomes smaller compared with those
when irradiated with the 488 and 532 nm laser light. A slower
birefringence growth rate can be observed for BP-BAZ-35Cl for the
irradiation with the 589 nm light. Similarly, as the relative contri-
bution of the fast process becomes smaller compared with those of
BP-BAZ-35CN and BP-BAZ-35NT, a slightly slower birefringence
growth rate can be observed for BP-BAZ-35Cl compared with these
birefringence
Dns values of BP-BAZ-35CN and BP35-BAZ-35CN are
0.072 and 0.089, respectively. The methyl substituents show
important effect on the photoinduced birefringence behavior of
polymers containing bisazo chromophores with cyano and nitro as
the electron-withdrawing groups. When there are the 3,5-dimethyl
substituents on the middle benzene ring (between the two azo
Table 3.4
The parameters obtained from the curve-fitting of the photoinduced birefringence kinetics (X.L. Wang et al.) (BP35-BAZ-X, 488 nm).
BP35-BAZ-35Cl
BP35-BAZ-35CN
BP35-BAZ-35NT
BP35-BAZ-35mNT
BP35-BAZ-NT
ka(sꢀ1
)
0.054 ꢃ 0.002
0.040 ꢃ 0.002
0.37 ꢃ 0.02
0.63 ꢃ 0.02
0.9998
0.016 ꢃ 0.004
0.046 ꢃ 0.005
0.40 ꢃ 0.04
0.60 ꢃ 0.04
0.9994
0.007 ꢃ 0.001
0.034 ꢃ 0.004
0.62 ꢃ 0.03
0.38 ꢃ 0.03
0.9997
0.005 ꢃ 0.000
0.029 ꢃ 0.002
0.70 ꢃ 0.02
0.30 ꢃ 0.02
0.9998
0.078 ꢃ 0.003
0.001 ꢃ 0.000
0.43 ꢃ 0.05
0.57 ꢃ 0.05
0.9993
ki(sꢀ1
)
a
An
a
Bn
Correl. coeff.
(
D
nsꢂ10ꢀ2/ts(s))
6.4/100
8.9/250
8.9/800
6.9/900
6.3/2000
a
An ¼ A/(A þ B), Bn ¼ B/(A þ B).

X. Wang et al. / Polymer 52 (2011) 3344e3356
3353
Table 3.5
The parameters obtained from the curve-fitting of the photoinduced birefringence kinetics (X.L. Wang et al.) (BP35-BAZ-X, 532 nm).
BP35-BAZ-35Cl
BP35-BAZ-35CN
BP35-BAZ-35NT
BP35-BAZ-35mNT
BP35-BAZ-NT
ka(sꢀ1
kb(sꢀ1
)
)
0.084 ꢃ 0.005
0.290 ꢃ 0.040
0.45 ꢃ 0.03
0.55 ꢃ 0.03
0.9998
0.021 ꢃ 0.002
0.110 ꢃ 0.020
0.59 ꢃ 0.03
0.41 ꢃ 0.03
0.9994
0.009 ꢃ 0.001
0.047 ꢃ 0.005
0.56 ꢃ 0.04
0.44 ꢃ 0.04
0.9994
0.005 ꢃ 0.001
0.036 ꢃ 0.005
0.73 ꢃ 0.03
0.27 ꢃ 0.03
0.9993
0.940 ꢃ 0.040
0.001 ꢃ 0.000
0.02 ꢃ 0.02
0.98 ꢃ 0.02
0.9997
a
An
a
Bn
Correl. coeff.
(
D
nsꢂ10ꢀ2/ts(s))
5.6/100
7.0/200
7.5/600
6.6/850
8.1/3000
a
An ¼ A/(A þ B), Bn ¼ B/(A þ B).
groups), introducing the 3,5-dimethyl substituents on aniline
moieties can significantly enhance the saturation birefringence
values. This effect can obviously be seen by comparing the result of
BP-BAZ-35CN with that of BP35-BAZ-35CN when irradiated with
488 and 532 nm light and the result of BP-BAZ-35NT with that of
BP35-BAZ-35NT when irradiated with 532 and 589 nm light.
irradiation of 589 nm light. Above observations reveal that the
relative positions of the excitation wavelengths and absorption
bands show an important influence on the SRG growth rates. When
the excitation wavelength is between the lmaxs and the absorption
band tails at the longer wavelength side, the bisazo polymers show
the highest SRG growth rate. On the other hand, when the incident
laser light is too close to the absorbance band tail at the long
wavelength side, the SRG formation rate will significantly decrease,
which can be seen for BP-BAZ-35Cl irradiated with 589 nm laser
light. This result is similar to the observations for the birefringence
growth discussed above. BP-BAZ-35mNT, bearing an extra methyl
group at 4-nitrophenyl moieties of the chromophores, almost
always shows the slowest SRG growth rate upon the irradiation
with these three wavelengths.
3.4. Photoinduced surface-relief-grating
The surface-relief-grating (SRG) formation behavior was studied
by using spin-coated films of the bisazo polymers. The laser beams
with the wavelength of 488, 532, and 589 nm and the intensity of
200 mW/cm2 were used as the inscribing light. SGRs were inscribed
by exposing the films to the interference patterns generated by the
two p-polarized laser beams at these wavelengths. Under the
conditions, SRG formation was observed on films of all types of
bisazo polymers prepared in this work. Fig. 5 shows typical AFM
images of the SRG formed on the films of bisazo polymers. The
profiles of SRGs appear as regularly spaced sinusoidal surface
structures, which are similar to those well-documented for other
azo polymers [7,8,49e51]. The spatial periods of the gratings are
Fig. 6(def) shows the results obtained for the films of BP35-BAZ-
35Cl, BP35-BAZ-35CN, BP35-BAZ-35NT, and BP35-BAZ-35mNT,
which were all prepared from the precursor polymer BP35-AN. The
results also show that only when the bisazo polymers are irradiated
with the laser light at the proper excitation wavelengths, the
polymers show the fastest SRG growth rate as discussed above. The
methyl group on 4-nitrophenyl moieties of the chromophores also
shows the effect to reduce the SRG growth rate. When compared
between bisazo polymers obtained from the two different
precursor polymers, it can be seen that the 3,5-methyl substitutes
on the aniline moieties also show some influences on the SRG
growth rate. BP35-BAZ-35Cl shows a faster grating growth rate
than that of BP-BAZ-35Cl, which can be seen by comparing the
corresponding curves in Fig. 6(a and d).
about 1 mm, which can be adjusted by changing the intersection
angle between the two interfering beams. The grating growth rates,
under the irradiation of interfering laser beams, were used to
characterize the SRG formation behavior and study its correlation
with polymer structures and the irradiation wavelengths. The first
order diffraction efficiency of the SRGs, which has been used to
describe the growth rates of the SRGs [7,8,27e30], was recorded for
the purpose and is presented in Fig. 6 for discussion.
Fig. 6(aec) shows the first order diffraction efficiency as a func-
tion of the irradiation time in the course of SRG formation on the
films of BP-BAZ-35Cl, BP-BAZ-35CN, BP-BAZ-35NT, and BP-BAZ-
35mNT, which were all obtained from precursor polymer BP-AN.
When irradiated with the interfering laser beams at 488 and
532 nm, the bisazo polymers bearing chromophores with chloro
and cyano as the electron-withdrawing groups (BP-BAZ-35Cl and
BP-BAZ-35CN) show the significantly larger SRG growth rates.
However, when irradiated with the laser light at 589 nm, the SRG
growth rate of BP-BAZ-35Cl dramatically decreases and is the
slowest in the series. BP-BAZ-35NT with nitro as the electron-
withdrawing group shows the fastest SRG growth rate under the
The influences of the methyl substitution can be further
understood by comparing the SRG growth rates for BP-BAZ-35NT,
BP-BAZ-NT, BP35-BAZ-35NT, and BP35-BAZ-NT, under the irradia-
tion with the incident laser light at 488, 532, and 589 nm
(Fig. 6(gei)). These four bisazo polymers all contain nitro as the
electro-withdrawing group. BP-BAZ-35NT, BP35-BAZ-35NT, and
BP35-BAZ-NT with different numbers of the methyl substituents at
different positions on the chromophores are compared with BP-
BAZ-NT without the methyl substitution. As the bisazo polymers
show strong absorption in the three excitation wavelengths, the
change of the irradiation wavelength does not show obvious
influence on the SRG growth rate. BP-BAZ-35NT and BP35-BAZ-
35NT, which have the 3,5-dimethyl substituents on the middle
Table 3.6
The parameters obtained from the curve-fitting of the photoinduced birefringence kinetics (X.L. Wang et al.) (BP35-BAZ-X, 589 nm).
BP35-BAZ-35Cl
BP35-BAZ-35CN
BP35-BAZ-35NT
BP35-BAZ-35mNT
BP35-BAZ-NT
ka(sꢀ1
kb(sꢀ1
)
)
0.094 ꢃ 0.007
0.027 ꢃ 0.003
0.55 ꢃ 0.04
0.45 ꢃ 0.04
0.9996
0.034 ꢃ 0.002
0.014 ꢃ 0.004
0.72 ꢃ 0.04
0.28 ꢃ 0.04
0.9996
0.024 ꢃ 0.002
0.008 ꢃ 0.001
0.45 ꢃ 0.05
0.55 ꢃ 0.05
0.9997
0.010 ꢃ 0.001
0.004 ꢃ 0.000
0.03 ꢃ 0.03
0.97 ꢃ 0.03
0.9993
0.950 ꢃ 0.040
0.001 ꢃ 0.000
0.02 ꢃ 0.02
0.98 ꢃ 0.02
0.9993
a
An
a
Bn
Correl. coeff.
(
D
nsꢂ10ꢀ2/ts(s))
5.9/200
7.3/300
10.1/600
7.9/1400
9.0/5000
a
An ¼ A/(A þ B), Bn ¼ B/(A þ B).

3354
X. Wang et al. / Polymer 52 (2011) 3344e3356
Fig. 5. Typical AFM images (10
m
m ꢂ 10
m
m) of the surface-relief-grating formed on the film of BP35-BAZ-35CN with irradiation of incident laser light at 488, 532 and 589 nm: (a)
488 nm, 2D-view; (b) 488 nm, 3D-view; (c) 532 nm, 2D-view; (d) 532 nm, 3D-view; (e) 589 nm, 2D-view; (f) 589 nm, 3D-view. The thickness of the films was 500 nm. The intensity
of the incident laser light was 200 mW/cm2 and the irradiation time was 1200 s.
benzene ring (between the two azo groups), show the significant
higher SRG growth rate than BP35-BAZ-NT and BP-BAZ-NT. It could
be attributed to the lower DFs of BP35-BAZ-NT and BP-BAZ-NT as it
has been observed that the increase of DF can increase the SRG
growth rate [49]. The effect of the 3,5-dimethyl substituents on
aniline moieties depends on whether there are substituents on the
middle benzene rings. When there are the methyl substituents on
the middle benzene rings, the SRG growth rates can further
increase for the methyl substitution on the aniline moieties. This
point can be seen by comparing the SRG growth rates of BP-BAZ-
35NT and BP35-BAZ-35NT, which all have the DF close to 100%.
On the contrary, when there are no 3,5-dimethyl substituents on
the middle benzene rings, the SGR growth rate is lower for the 3,5-
dimethyl substituents on aniline moieties. It can be seen from the
lower SRG growth rate of BP35-BAZ-NT compared to that of BP-
BAZ-NT, even the former has a higher DF.
by the electron-withdrawing group on the chromophores. There-
fore, the bisazo polymers containing chromophores with different
electron-withdrawing groups show different responsive behavior
to the irradiation light with different wavelengths. Such effect has
also been observed for polymers bearing monoazo chromophores
[49e51]. On the other hand, as the bisazo polymers possess
broader absorption bands compared with monoazo polymers, the
former shows responses to the irradiation in a wider wavelength
range.
For bisazo polymers containing chromophores with the same
electron-withdrawing groups, the influences of the methyl
substituents are more complicated. The methyl substituents can
increase the mutual distance between the azo chromophores and
prevent their aggregation through dipoleedipole interaction or
pep H-aggregation. Reducing chromophore aggregation is
a favorable factor to enhance the isomerization efficiency [3,27].
On the other hand, the methyl substituents on the ortho positions
of the azo groups could have steric hindrance to the isomerization.
Above results seem to indicate that for the 3,5-methyl substitution
on the middle benzene rings, the effect to promote the isomeri-
zation efficiency is dominant. Therefore, the corresponding bisazo
polymers appear more efficient for the photoinduced birefrin-
gence growth and SRG formation when irradiated with the laser
light at the proper wavelength. For methyl substituents on the
benzene rings bearing the electron-withdrawing groups, the effect
to inhibit the isomerization of adjacent azo bonds seems to play
3.5. Discussion
Above results indicate that the photoresponsive behavior of the
bisazo polymers is closely related with the chromophore structure
and excitation wavelength. The photoinduced birefringence is
known as a result of the repeated transecisetrans isomerization of
the azo chromophores induced by the linearly polarized laser light
irradiation. The isomerization will force the chromophores and
related groups to align in the direction perpendicular to the
polarization of the incident laser beam [41e46]. The reliance of the
photoinduced birefringence behavior on the excitation wavelength
observed in this study can be attributed to the efficiency of the
transecisetrans isomerization under the irradiation [3,50]. When
the light can efficiently induce the isomerization, the birefringence
growth rate will be large. Similarly, results observed for the
SRG formation can also be attributed to the efficiency of the
transecisetrans isomerization. The excitation wavelength that can
cause the efficient responses is determined by the absorption band
positions of the bisazo chromophores, which are mainly affected
a
dominant role. Therefore, the photoinduced birefringence
growth and SRG formation become less efficiency. The influence of
the 3,5-methyl substitution on the aniline moieties is dependent
on whether there are methyl substituents on the middle benzene
rings. When both benzene rings bear the methyl substituents,
such as BP35-BAZ-35CN and BP35-BAZ-35NT, more efficient
birefringence growth and SRG formation can be observed. It can be
attributed to the effect of methyl substituents to reduce the
chromophore interaction and prevent the chromophore
aggregation.

X. Wang et al. / Polymer 52 (2011) 3344e3356
3355
Fig. 6. The first order diffraction efficiency of the SRG as a function of irradiation time in the course of SRG inscription on the films of bisazo polymers with incident laser light at
488, 532, and 589 nm (a)e(c): Bisazo polymers obtained from precursor polymer BP-AN, (a) 488 nm, (b) 532 nm, (c) 589 nm (d)e(f): Bisazo polymers obtained from precursor
polymer BP35-AN, (d) 488 nm, (e) 532 nm, (f) 589 nm (g)e(i): Bisazo polymers containing nitro as the electron-withdrawing group, (g) 488 nm, (h) 532 nm, (i) 589 nm.
4. Conclusions
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