Hyperbranched azo-polymers synthesized by azo-coupling reaction of an AB2 monomer and postpolymerization modification

Article abstract of DOI:10.1021/ma0511393

A series of hyperbranched azo-polymers have been synthesized by two-stage azo-coupling reactions, in which a hyperbranched precursor azo-polymer (HPAP) was prepared by step-growth polymerization of an AB₂ monomer through azo-coupling reaction and then was further modified by postpolymerization azo-coupling reaction at the peripheral groups. The AB₂ monomer, di[2-(N-ethylanilino)ethyl]-5-aminoisophthalate, was synthesized from the nucleophilic substitution reaction between N-ethyl-N-(2-chloroethyl)aniline and 3,5-bis(carboxyl)aniline. On the basis of the monomer, HPAP was prepared by step-growth polycondensation of the diazonium salt of the AB₂ monomer. The hyperbranched precursor polymer was then reacted with diazonium salts of 4-nitroaniline, 4-aminobenzoic acid, and 4-aminobenzonitrile to introduce different types of donor-acceptor azo-chromophores at the peripheral positions. The structure and properties of the azo-polymers were characterized by the spectroscopic methods and thermal analysis. The hyperbranched azo-polymers were used to fabricate surface relief gratings (SRGs) by exposing spin-coated thin films of the polymers to an interference pattern of Ar ⁺ laser beams at modest intensity (150 mW/cm²). The type of electron-withdrawing groups in the parapositions of the terminal azobenzene units shows a significant effect on the SRG inscription rate. The synthetic scheme demonstrated in this work is a feasible way to prepare a variety of hyperbranched azo-polymers under extremely mild conditions. The hyperbranched azo-polymers can potentially be used for applications such as reversible optical data storage, photoswitching, sensors, and other photodriven devices.

Full text of DOI:10.1021/ma0511393

Macromolecules 2005, 38, 8657-8663
8657
Hyperbranched Azo-Polymers Synthesized by Azo-Coupling Reaction of
an AB2 Monomer and Postpolymerization Modification
Pengchao Che, Yaning He, and Xiaogong Wang*
Department of Chemical Engineering and School of Materials Science and Engineering, Tsinghua
University, Beijing, P. R. China 100084
Received June 1, 2005; Revised Manuscript Received August 15, 2005
ABSTRACT: A series of hyperbranched azo-polymers have been synthesized by two-stage azo-coupling
reactions, in which a hyperbranched precursor azo-polymer (HPAP) was prepared by step-growth
polymerization of an AB2 monomer through azo-coupling reaction and then was further modified by
postpolymerization azo-coupling reaction at the peripheral groups. The AB2 monomer, di[2-(N-ethyl-
anilino)ethyl]-5-aminoisophthalate, was synthesized from the nucleophilic substitution reaction between
N-ethyl-N-(2-chloroethyl)aniline and 3,5-bis(carboxyl)aniline. On the basis of the monomer, HPAP was
prepared by step-growth polycondensation of the diazonium salt of the AB2 monomer. The hyperbranched
precursor polymer was then reacted with diazonium salts of 4-nitroaniline, 4-aminobenzoic acid, and
4-aminobenzonitrile to introduce different types of donor-acceptor azo-chromophores at the peripheral
positions. The structure and properties of the azo-polymers were characterized by the spectroscopic
methods and thermal analysis. The hyperbranched azo-polymers were used to fabricate surface relief
gratings (SRGs) by exposing spin-coated thin films of the polymers to an interference pattern of Ar+
laser beams at modest intensity (150 mW/cm2). The type of electron-withdrawing groups in the para-
positions of the terminal azobenzene units shows a significant effect on the SRG inscription rate. The
synthetic scheme demonstrated in this work is a feasible way to prepare a variety of hyperbranched
azo-polymers under extremely mild conditions. The hyperbranched azo-polymers can potentially be used
for applications such as reversible optical data storage, photoswitching, sensors, and other photodriven
devices.
Introduction
trans-cis isomerization of azo-chromophores, azo-
polymers have been wildly investigated for diversified
potential applications. The photoinduced isomerization
can cause significant bulk and surface property varia-
tion of the polymers, such as photoinduced phase
transition, photoinduced anisotropy, and photoinduced
surface relief gratings (SRGs), among others.26-31 For
azo-polymers containing donor-acceptor-type azo-chro-
mophores, the nonlinear optical property is another
important function that has been well studied.21 Poly-
mers with such properties are promising for applications
in holographic gratings, reversible optical storage sys-
tems, electrooptical (EO) modulators, and sensors. For
decades, research efforts to explore new azo-polymers
have predominantly focused on the linear azo-polymers
with side-chain or main-chain architectures.21-25 More
recently, dendrimers containing azobenzene units have
attracted considerable attention and have been inten-
sively investigated. The dendrimers are designed to bear
azo-chromophores in the exterior and interior or through-
out the dendritic architecture.32-34 Those azo-dendrim-
ers have exhibited some fascinating properties, such as
harvesting low-energy photons35 and improving both the
nonlinear optical properties and thermal stability of the
azo-polymers.34b,36 In contrast to the intensive study on
the azo-dendrimers, only few hyperbranched azo-
polymers have been synthesized and studied.37,38
Hyperbranched polymers, as one type of dendritic
polymers, have received great attention during the past
decade.1-4 Similar to other dendritic polymers, hyper-
branched polymers are characterized by a highly
branched backbone and a large number of terminal
functional groups. Because of the structural character-
istics, hyperbranched polymers show many interesting
properties of the dendritic polymers, such as good
solubility, low melt and solution viscosity, and the
feasibility to be further modified through various chemi-
cal reactions of the peripheral groups.5-9 In comparison
with dendrimers, which have a well-controlled size and
shape and are prepared by multistep reactions through
a divergent or convergent scheme, hyperbranched poly-
mers have less regular structure and a broader molec-
ular weight distribution. On the other hand, hyper-
branched polymers can be prepared through some
relatively simple methods, such as one-step polymeri-
zation of ABx-type monomers (where x is generally 2 or
3). The preparation does not need the tedious isolation
and purification procedures required for preparing
dendrimers through the generation-by-generation
scheme.10-14 Therefore, hyperbranched polymers are
considered as a unique type of dendritic polymer, which
possesses interesting properties of the dendritic struc-
tures and feasibility for large-scale manufactures at the
same time.15-20
Formation of photoinduced surface relief gratings
(SRGs) is one of the most interesting properties of azo-
polymers found in recent years.24,30,31,39-41 By exposure
of the polymer films to the interfering laser beams,
SRGs are formed on the films at a temperature well
below the glass transition temperatures (Tgs) of the
samples. In contrast to irreversible surface patterns
prepared by conventional methods such as chemical
Polymers containing azobenzene-type chromophores
(azo-polymers for short) have been extensively studied
in recent years.21-25 On the basis of the optically induced
* Corresponding author:
wxg-dce@mail.tsinghua.edu.cn.
Fax 86-10-62770304; e-mail
10.1021/ma0511393 CCC: $30.25 © 2005 American Chemical Society
Published on Web 09/22/2005

8658 Che et al.
Macromolecules, Vol. 38, No. 21, 2005
min under N2 protection. The molecular weights and their
distributions of the polymers were determined by gel perme-
ation chromatography (GPC) utilizing a Waters model 515
pump and a model 2410 differential refractometer with three
Styragel columns HT2, HT3, and HT4 connected in a serial
fashion. THF was used as the eluent at a flow rate of 1.0 mL/
min. Polystyrene standards with dispersity of 1.08-1.12
obtained from Waters were employed to calibrate the instru-
ment. Infrared spectra were measured using a Nicolet 560-IR
spectrometer by incorporating the sample in a KBr disk.
Elemental analysis of C, H, and N was completed through the
Heratus CHN-Rapid method. UV-vis spectra of the azo-
polymers in solutions and as spin-coated films were recorded
on a Perking-Elmer Lambda Bio-40 spectrometer. The surface
images of the surface relief gratings were monitored using
atomic force microscopy (AFM) (Nanoscope IIIa, tapping
mode).
etching and laser ablation, SRGs can be removed by
heating samples to a temperature above their Tgs or
erased optically even below the Tgs.42,43 This function
can be potentially applied to the fabrication of diffractive
optical elements and waveguide couplers.44-46 Since the
phenomenon was reported, the factors controlling the
SRG formation have been actively investigated. The
SRG formation at low or middle light intensity is due
to neither a thermally driven process nor the photoab-
lation occurring in the irradiation regions.24,47 The
photoinduced dynamic process depends on the intensity
and polarization state of the interfering beams, the
thickness of the films, and the molecular weight of
polymers.40,43,47 The rate of the SRG formation and the
modulation depth are also closely related to the polymer
structure and the irradiating energy of the writing
beam.24,40,47,48 Although several models have been pro-
posed to describe the SRG formation, no general agree-
ment about the forming mechanism has been reached
until the present time.24,47 Hyperbranched azo-polymers
are expected to show both SRG-forming ability and the
interesting properties of dendritic polymers. Meanwhile,
because of the significantly different molecular archi-
tecture compared to their linear analogues, study on the
structure-property relationship of hyperbranched azo-
polymers can shed new light on the molecular origin of
the SRG formation. However, to our knowledge, no
systematic study on the SRG properties of hyper-
branched azo-polymers (or even other dendritic azo-
polymers) has been reported, which is partly due to the
lack of a straightforward way to prepare hyperbranched
azo-polymers under mild conditions.
Recently, we found that hyperbranched azo-polymers
can be obtained through azo-coupling reaction of an AB2
monomer under extremely mild conditions.49 For years,
the azo-coupling reaction, which can be carried out in
different ways, has been well developed to prepare
various types of dyestuff.22 By exploiting the knowledge
base, a variety of hyperbranched azo-polymers can be
designed and prepared. Through this scheme, the strict
reaction conditions such as high temperature and
vacuum can be avoided. Moreover, the hyperbranched
azo-polymer contains a large amount of peripheral
diazonium salt groups, which can be further modified
to introduce other types of terminal groups.
Materials. N-Ethyl-N-hydroxyethylaniline and 3,5-bis(car-
boxyl)aniline were purchased as commercial products from
Acros. All other reagents and solvents such as dimethyl
sulfoxide (DMSO) and N,N-dimethylformamide (DMF) were
used as received without further purification.
N-Ethyl-N-(2-chloroethyl)aniline (ECA). N-Ethyl-N-hy-
droxyethylaniline (18.2 g, 0.11 mol) was slowly added into a
round-bottom flask containing phosphorus oxychloride (10.2
mL 0.11 mol) with ice bath cooling. After the addition was
completed, the mixture was heated to 110 °C and reaction was
carried out at this temperature for 1 h under a nitrogen
atmosphere protection. Then the mixture was poured into
benzene (50 mL). A proper amount of water (about 50 mL)
was mixed with the benzene solution to extract the unreacted
N-ethyl-N-hydroxyethylaniline and other water-soluble impu-
rities. The aqueous layer was washed with benzene (5 mL)
three times to extract the remaining product. The benzene
solutions were mixed and dried with anhydrous magnesium
sulfate. Oily product was obtained by removal of the benzene
through vacuum evaporation. Yield 83%; bp 102-104 °C/399
Pa. IR (KBr): 2970 (s; C-H), 1600 1500 (s; benzene ring), 1340
1
(s; C-N) and 720 (s; C-Cl) cm-1. H NMR (DMSO-d6): δ )
1.16 (t, -CH3, 3H), 3.40 (m, N-CH2CH3, 2H), 3.60 (s,
N-CH2CH2-Cl, 4H), 6.68 (m, Ar-H, 3H, ortho and para to
N), 7.22 (t, Ar-H, 2H, meso to N). Elemental analysis: C10H14-
NCl (183.68) Calcd: C, 65.40; H, 7.87; N, 7.63; Cl, 19.30.
Found: C, 64.10; H, 7.70; N, 7.81; Cl, 18.74.
Di[2-(N-ethylanilino)ethyl]-5-aminoisophthalate (DEE-
AP). DEEAP was prepared by the nucleophilic substitution
reaction between ECA and 3,5-bis(carboxyl)aniline. ECA (1.835
g, 10 mmol) and 3,5-bis(carboxyl)aniline (0.724 g, 4 mmol) were
dissolved in DMSO (50 mL) in a 250 mL round-bottom flask.
Potassium carbonate (4 g) and potassium iodide (1 g) were
added into the DMSO solution. The reaction was carried out
at 110 °C for 7 h with stirring, and then the mixture was
poured into an excessive amount of water. The precipitated
DEEAP solid was collected by filtration and washed with
plenty of water. The product was dried in a vacuum oven at
70 °C for 24 h. Yield 80%; mp 110-112 °C. IR (KBr): 3460
3373 (s; N-H), 2970 (s; C-H), 1700 (s; COOR), 1597 1506 (s;
In this work, we develop a new synthetic scheme
through two-stage azo-coupling reactions based on a
newly designed AB2 monomer. A hyperbranched precur-
sor azo-polymer (HPAP), which contains a large number
of peripheral anilino groups, was prepared by step-
growth polycondensation of the monomer through azo-
coupling reaction. HPAP was further modified through
postpolymerization azo-coupling reactions to introduce
different peripheral azo-chromophores. The scheme can
be used to prepare a variety of azo-polymers that have
a highly branched azo-backbone and bear different types
of azo-chromophores at the peripheral positions. In this
paper, the synthesis and characterization of the mono-
mer, the precursor polymer, and the hyperbranched azo-
polymers are reported. Their SRG formation properties
and their relationship with the molecular structure are
discussed in detail.
1
benzene ring), and 1348 (s; N-C). H NMR (DMSO-d6): δ )
1.11 (t, -CH3, 6H), 3.40 (m, N-CH2CH3, 4H), 3.65 (t,
-COOCH2CH2N-, 4H), 4.39 (t, -COOCH2CH2N-, 4H), 5.74
(s, Ar-NH2, 2H), 6.58 (t, Ar-H, 2H, para to N), 6.74 (d, Ar-
H, 4H, ortho to N), 7.15 (t, Ar-H, 4H, meso to N), 7.40 (s,
Ar-H, 2H, ortho to NH2 and ortho to COO-), 7.68 (s, Ar-H,
1H, para to NH2 and ortho to COO-).
Hyperbranched Precursor Azo-Polymer (HPAP). DEE-
AP (1.900 g, 4 mmol) was dissolved in a homogeneous mixture
of glacial acetic acid (20.0 mL) and sulfuric acid (1.4 mL).
Aqueous solution of sodium nitride (0.345 g, 5 mmol) in 0.9
mL water was added dropwise into the DEEAP solution at 0
°C, and the solution was kept at this temperature for 5 min.
Then the clear solution of the diazonium salt was added
dropwise into DMF (60 mL) at 0 °C. The reaction was carried
out with stirring at the temperature for 12 h. The solution
was then poured into an excess amount of water. The precipi-
tate was collected by filtration and washed with water repeat-
Experimental Section
Characterization. 1H NMR and 13C NMR spectra were
obtained on a JEOL JNM-ECA600 NMR spectrometer. The
glass transition temperatures (Tgs) of the polymers were tested
with a TA Instruments DSC 2910 at a heating rate of 10 °C/

Macromolecules, Vol. 38, No. 21, 2005
Hyperbranched Azo-Polymers 8659
edly. After being dried, the product was dissolved in DMF (30
mL) again and then dropped into an excess amount of ethanol
with stirring. The precipitate was collected by filtration,
washed with ethanol, and then dried in a vacuum oven at 70
°C for 48 h. Yield: 78%. GPC: Mn 142 000, MWD 1.44. DSC:
Tg 91 °C. 1H NMR (DMSO-d6): δ ) 1.08, 3.38, 3.62, 4.43, 6.54,
6.73, 6.92, 7.11, 7.72, 8.33. 13C NMR (DMSO-d6): δ ) 165.0,
153.2, 151.3, 148.0, 143.1, 131.6, 129.6, 126.7, 126.1, 116.2,
112.4, 112.1, 63.6, 48.6, 44.9, 12.5. UV-vis (DMF): 425 nm;
UV-vis (film): 436 nm.
Scheme 1. Synthetic Route of the Precursor
Hyperbranched Azo-Polymer
HPAP-NT. The diazonium salt of 4-nitroaniline was pre-
pared by adding an aqueous solution of sodium nitrite (0.200
g, 2.9 mmol) dropwise into a solution of 4-nitroaniline (0.345
g, 2.5 mmol) in a homogeneous mixture of sulfuric acid (1 mL)
and glacial acetic acid (10 mL). The mixture was stirred at 0
°C for 5 min. The diazonium salt solution was added dropwise
into a solution of HPAP (0.972 g, 2 mmol) in DMF (50 mL) at
0 °C. After the reaction was carried out at 0 °C for 10 h, the
mixture was poured into an excess amount of water. The
precipitate was collected by filtration and washed with water
repeatedly. After being dried, the product was dissolved in
DMF (10 mL) again and then dropped into an excess amount
of ethanol with stirring. The precipitate was collected by
filtration, washed with ethanol, and then dried in a vacuum
oven at 70 °C for 48 h. Yield: 76%. DSC: Tg 139 °C. 1H NMR
(DMSO-d6): δ ) 1.06, 3.45, 3.74, 4.45, 6.80, 7.73, 8.21. 13C
NMR (DMSO-d6): δ ) 165.0, 156.0, 153.2, 152.0, 146.7, 143.5,
131.4, 129.0, 126.4, 126.0, 125.0, 122.8, 112.2, 63.5, 48.2, 45.1,
12.5. UV-vis (DMF): 450 nm; UV-vis (film): 443 nm.
Scheme 2. Synthetic Route of HPAP-NT, HPAP-CA,
and HPAP-CN
HPAP-CA. HPAP-CA was synthesized and purified via a
procedure similar to that described above for the HPAP-NT
preparation. In the reaction, the diazonium salt of 4-ami-
nobenzenic acid (2.5 mmol) was reacted with HPAP (0.972 g,
2 mmol) to obtain HPAP-CA. Yield: 79%. DSC: Tg 167 °C. 1H
NMR (DMSO-d6): δ ) 1.08, 3.45, 3.76, 4.46, 6.86, 7.69, 7.99,
8.26. 13C NMR (DMSO-d6): δ ) 166.6, 164.3, 154.9, 152.0,
151.2, 142.7, 130.7, 130.1, 129.0, 125.5, 125.2, 121.4, 111.4,
47.8, 44.4, 11.8. UV-vis (DMF): 432 nm; UV-vis (film): 424
nm.
HPAP-CN. HPAP-CN was synthesized and purified via a
procedure similar to that described above for the HPAP-NT
preparation. In the reaction, the diazonium salt of 4-ami-
nobenzonitrile (2.5 mmol) was reacted with HPAP (0.972 g, 2
1
mmol) to obtain HPAP-CN. Yield: 80%. DSC: Tg 133 °C. H
NMR (DMSO-d6): δ ) 1.08, 3.47, 3.77, 4.42, 6.83, 7.77, 8.24.
13C NMR (DMSO-d6): δ ) 165.0, 155.3, 153.2, 151.5, 143.3,
133.8, 131.4, 129.6, 126.7, 126.2, 122.8, 116.2, 112.5, 63.6, 48.7,
44.6, 12.5. UV-vis (DMF): 439 nm; UV-vis (film): 438 nm.
Preparation of Polymer Films. Suitable amounts of the
polymer samples were dissolved in DMF to obtain solutions
with concentrations about 0.1 g/mL. The solutions were filtered
through 0.45 µm membranes. The films were prepared by spin-
coating the solutions onto clear glass slides. By adjusting the
spining rate, the thickness of the films was controlled to be
about 1.0 µm. After dried at 70 °C under vacuum for 48 h, the
films were stored in a desiccator for further studies.
Results and Discussion
Preparation of Hyperbranched Azo-Polymers.
The way to prepare the hyperbranched azo-polymers
through the two-stage azo-coupling reactions is shown
in Scheme 1 and Scheme 2. N-Ethyl-N-(2-chloroethyl)-
aniline (ECA) was prepared by chloridizing N-ethyl-N-
hydroxyethylaniline with phosphorus oxychloride (POCl3).
The AB2 monomer, di[2-(N-ethylanilino)ethyl]-5-ami-
noisophthalate (DEEAP), was synthesized by nucleo-
philic substitution reaction between ECA and 3,5-
bis(carboxyl)aniline in DMSO, using K2CO3 and KI as
HCl absorbent and catalyst. The hyperbranched precur-
sor azo-polymer (HPAP) was prepared by step-growth
polymerization of DEEAP through azo-coupling reaction
(Scheme 1). In the reaction, the diazonium salt of
DEEAP was prepared and then added into a suitable
amount of DMF with ice bath cooling. HPAP was
obtained after reaction in DMF at 0 °C for 12 h. HPAP
Surface Relief Grating (SRG) Fabrication. The experi-
mental setup for SRG fabrication was similar to those reported
before.30,31 A linearly polarized Ar+ laser beam (488 nm, 150
mW/cm2) was used as the light source. SRGs were optically
inscribed on the polymer films with p-polarized interfering
laser beams, where the Ar+ laser beam was split by a mirror
and the reflected half-beam coincided with the other half on
the film surface. The surface profiles of the resulting gratings
were recorded by using a Nanoscope atomic force microscope
(AFM) in the tapping mode. The diffraction efficiency of the
gratings was probed by measuring the first-order diffracted
beam intensity of an unpolarized low power He-Ne laser beam
(633 nm) in transmission mode. Even only weak absorption
at the probed wavelength, the absorption effect of the polymer
films was corrected in the calculation of the diffraction
efficiency.

8660 Che et al.
Macromolecules, Vol. 38, No. 21, 2005
Figure 1. 1H NMR spectra of (a) DEEAP and (b) HPAP in
DMSO-d6.
was further functionalized to incorporate three types of
peripheral azo-chromophores by postpolymerization azo-
coupling reactions (Scheme 2). In these reactions, HPAP
was reacted respectively with the diazonium salts of
4-nitroaniline, 4-aminobenzoic acid, and 4-aminoben-
zonitrile in DMF to obtain HPAP-NT, HPAP-CA, and
HPAP-CN.
1H NMR spectra of the AB2 monomer DEEAP and
precursor polymer HPAP are given in Figure 1. In the
spectrum of DEEAP (Figure 1a), the peak at 5.74 ppm
corresponds to the resonance of the protons of the amino
groups (protons Hj). After polymerization, the resonance
disappears completely in the spectrum of HPAP (Figure
1b). Moreover, because of the increase of conjugated
length and the introduction of strong electron-with-
drawing groups, the chemical shifts of the protons of
both N-ethylanilino and 5-aminoisophthalate moieties
shift to lower magnetic field after forming the azo-
linkages. In the 1H NMR spectrum of HPAP, new peaks
appearing at 8.33, 7.72, and 6.92 ppm are assigned to
the resonances of the protons in azobenzene units. These
spectroscopic characters clearly indicate that the amino
groups of DEEAP reacted almost completely in the azo-
coupling reaction, and an equal amount of azobenzene
units was produced by the reaction. The number-
average molecular weight (Mn) and the polydispersity
index of HPAP, obtained by gel permeation chromatog-
raphy (GPC), are 142 000 and 1.44. According to the
GPC result, the average number of the monomer units
in the polymer is estimated to be 290. The absorption
band corresponding to the azobenzene units can be
observed from the UV-vis spectrum of HPAP. Results
obtained form1H NMR, 13C NMR, UV-vis, and GPC all
suggest that the designed hyperbranched azo-polymer
with a reasonably high molecular weight has been
synthesized.
Figure 2. 1H NMR spectra of (a) HPAP-NT, (b) HPAP-CA,
and (c) HPAP-CN in DMSO-d6.
1H NMR spectra of HPAP-NT, HPAP-CA, and HPAP-
CN are shown in Figure 2. The 6.54 ppm resonance
(Figure 1b), corresponding to the protons in the para
positions of the amino groups of the peripheral anilino
moieties, disappears completely after the azo-coupling
modifications (Figure 2a-c). Because of the presence of
the electron-withdrawing groups introduced by the azo-
coupling reactions, the resonances of the protons ortho
and meta to the amino groups of the peripheral anilino
moieties shift to lower magnetic field. These indicate
that the peripheral anilino moieties are almost com-
pletely reacted, and the postpolymerization azo-coupling
reactions occur at the para positions of peripheral
aniline moieties of HPAP. According to those results,
the reaction yields of the terminal anilino groups (the
degree of functionalization) are about 100%. In our
previous work, the postpolymerization azo-coupling
reactions have been used to modify linear precursor
polymers to incorporate azo-chromophores.48,50 The cur-

Macromolecules, Vol. 38, No. 21, 2005
Hyperbranched Azo-Polymers 8661
Figure 3. DSC curves of the hyperbranched azo-polymers.
Table 1. Tg and Td of the Hyperbranched Azo-Polymers
Figure 4. TGA curves of the hyperbranched azo-polymers.
polymer
Tg (°C)
Td (°C)
HPAP
91
139
167
133
274
238
241
246
HPAP-NT
HPAP-CA
HPAP-CN
rent research shows that the same scheme can be
successfully used to introduce various peripheral azo-
chromophores into a hyperbranched precursor polymer.
Thermal Properties. The phase transition behav-
iors of the precursor polymer (HPAP) and the function-
alized polymers (HPAP-NT, HPAP-CA, and HPAP-CN)
were studied by using the differential scanning calo-
rimetry (DSC). DSC curves of the hyperbranched poly-
mers are given in Figure 3. All the hyperbranched
polymers show the second-order phase transition of a
typical amorphous substance. The glass transition tem-
peratures (Tgs) of the polymers obtained by DSC are
listed in Table 1. Tg of HPAP is about 91 °C. After
introducing peripheral azo-chromophores, Tgs of the
functionalized polymers become much higher than Tg
of the precursor polymer. This is because of the signifi-
cant increase both in the size and the dipole moment of
the numerous terminal groups. The Tgs of HPAP-NT
and HPAP-CN are 139 and 133 °C, which are about 48
and 42 °C higher than the precursor polymer. HPAP-
CA exhibits the highest Tg of 167 °C among this series.
The reason for such high Tg is attributed to intermo-
lecular hydrogen bonding between carboxyl units of the
terminal chromophores. The significant increase of Tg
after postpolymerization azo-coupling reaction has been
observed for linear epoxy-based polymers.48,50 Introduc-
ing the azo-chromophores shows quite similar effect to
increase the Tgs whether the azo-chromophores are in
the side-chain positions or in the peripheral positions.
The thermal stability of the polymers was character-
ized by the thermogravimetric analysis (TGA). TGA
curves of the precursor and functionalized polymers are
shown in Figure 4, and the decomposition temperatures
(Tds) of the polymers are given in Table 1. The precursor
polymer is thermally stable up to 274 °C under a
nitrogen atmosphere. HPAP-NT, HPAP-CA, and HPAP-
CN start to lose the weight at 238, 241, and 246 °C.
After introducing the peripheral azo-chromophores, the
thermal stability of the hyperbranched azo-polymers
decreased.
Figure 5. UV-vis spectra of the hyperbranched azo-poly-
mers: (a) in DMF solutions and (b) as spin-coated films.
Table 2. λmax of the Hyperbranched Azo-Polymers in DMF
Solutions and as Spin-Coated Films
λmax (nm)
(in DMF solutions)
λmax (nm)
(as spin-coated films)
polymer
HPAP
425
450
432
439
436
443
424
438
HPAP-NT
HPAP-CA
HPAP-CN
corresponding to the π-π* transition, are listed in Table
2. As containing the donor-acceptor-type azo-chro-
mophores, all the polymers show typical spectral char-
acteristics of the pseudo-stilbene-type azo-chromo-
phores.21,23 For those azo-chromophores, the (n-π*)
state has a larger transition energy gap than that of
(π-π*) state. Therefore, the low-intensity n-π* band
appears at a shorter wavelength than π-π* band and
cannot be observed from the spectra. The spectrum of
HPAP in the DMF solution shows the absorption
maximum at 425 nm, and the λmax red-shifts to 436 nm
for the spin-coated film of the polymer. After incorpora-
tion of the peripheral azo-chromophores, UV-vis spec-
Spectral Characteristics. Figure 5 shows the UV-
vis spectra of the precursor polymer HPAP and the
hyperbranched azo-polymers HPAP-NT, HPAP-CA, and
HPAP-CN. Parts a and b of Figure 5 show spectra of
the samples in the DMF solutions and as spin-coated
thin films, respectively. The λmax values of the polymers,

8662 Che et al.
Macromolecules, Vol. 38, No. 21, 2005
Figure 6. AFM image of the SRGs formed on HPAP-CA film
(10.0 µm × 10.0 µm).
Figure 7. Diffraction efficiency of the surface relief gratings
inscribed on the films of HPAP (1), HPAP-NT (2), HPAP-CN
(b), and HPAP-CA (9) as a function of the irradiation time.
The intensity of irradiation beam is about 150 mW/cm2.
tra of HPAP-NT, HPAP-CA, and HPAP-CN show some
significant differences. In addition to the azo-chro-
mophores throughout the hyperbranched architecture,
HPAP-NT, HPAP-CA, and HPAP-CN have another type
of azo-chromophore at the terminal positions. The
absorption bands of both types of azo-chromophores
overlap, and only one broader peak can be observed in
the spectra. The λmax of the overlapped bands shifts to
a longer wavelength or shorter wavelength, depending
on the relative positions of the absorption bands of the
internal azo-chromophores and the terminal azo-chro-
mophores. The λmax of HPAP-NT, HPAP-CA, and HPAP-
CN in DMF solutions appears at 450, 432, and 439 nm.
In contrast to HPAP, the λmax of HPAP-NT, HPAP-CA,
and HPAP-CN films shows a blue shift compared with
the λmax of the corresponding solution spectra, which
appears at 443, 424, and 438 nm for HPAP-NT, HPAP-
CA, and HPAP-CN solid films, respectively. The exact
nature of this observation is not clear at the current
stage. One possible explanation is that the absorption
band shifts observed for the solutions and films are due
to different mechanisms. When the environments of the
polymeric chains change from solutions to solid films,
the red shift of HPAP might be caused by a solvato-
chromic effect of the internal azo-chromophores, whereas
the blue shift of HPAP-NT, HPAP-CA, and HPAP-CN
could predominately result from the H-aggregation of
the terminal azo-chromophores. The name H-aggrega-
tion originated in the observation of hypsochromic or
H-bands in UV-vis spectra of ionic dyes, which mean
those transitions that are blue-shifted and thus appear
at shorter wavelengths than molecular absorption band
(M-band).51 The appearance of H-bands has been at-
tributed to face-to-face aggregation through the forma-
tion of dimers, trimers, or even n-mers.52,53 In the
current case, the terminal azo-chromophores could form
H-aggregates more easily in the solid states than in
DMF solutions.
Surface Relief Grating (SRG) Behavior. Spin-
coated thin films of the hyperbranched azo-polymers
were used to carry out SGR inscription experiments.
The experimental setup and conditions were similar to
those reported before.30,31 Two p-polarized Ar+ laser
beams (488 nm) were used to produce the interference
pattern on the polymer films. Although using two
circularly polarized interfering beams can produce SRGs
more efficiently, the p/p-polarization condition has also
been frequently used to fabricate good quality SRGs,
and the result is relatively easier to be explained.40,41
AFM observation indicated that sinusoidal surface relief
structures with regular spaces were fabricated on the
film surfaces of the azo-polymers. Figure 6 shows a
typical AFM image of the surface structure formed on
the HPAP-CA film after irradiated for 1000 s at room
temperature. From the AFM measurement, the surface
modulation depth is about 195 nm, and the grating
spacing is about 890 nm. The modulation depth depends
on the irradiation time, and the spatial period depends
on both the angle θ between the two interfering beams
and the wavelength λ of the writing beams. With the
same irradiation time (1000 s), the modulation depth
measured by AMF is significantly different for the films
of HPAP, HPAP-NT, HPAP-CA, and HPAP-CN due to
the different grating formation rates, which will be
discussed in the following part.
The rates of grating formation can be probed by
measuring the first-order diffraction efficiency of the
SRGs during the grating inscription process.30,31,40 In
current study, the diffraction efficiency was recorded in-
situ and used to characterize the rate of the surface
modification. The rate of grating formation depends on
both the structures of the polymers and the conditions
such as film thickness, the light intensity, and the angle
between the two interfering beams.40 To attribute the
difference of the SRG-forming rates only to the polymer
structures, thick films (about 1 µm) were used and the
other conditions were controlled to be the same in the
experiments. Relatively low light intensity (150 mW/
cm2) was used to write the gratings in order to avoid
the sample damage and the other possible side effects
caused by high-intensity laser irradiation.
A plot of the diffraction efficiency as a function of the
irradiation time for the hyperbranched azo-polymers is
given in Figure 7. The precursor polymer HPAP exhibits
the slowest inscription rate for the SRG formation
among the four polymers. The rates of SGR formation
are much faster for HPAP-NT, HPAP-CA, and HPAP-
CN, which are functionalized by the peripheral azo-
chromophores. It is obvious that the introduction of the
terminal azobenzene chromophores increases the amount
of azo-chromophores in the polymeric systems. It might
be even more important that these peripheral azo-
chromophores have less steric hindrance and are easier
to undergo the photoinduced trans-cis isomerization.
HPAP-CA, HPAP-NT, and HPAP-CN also show a
considerable difference in the grating formation rates,
in which the only structural difference distinguished
with each other is the electron-withdrawing groups in
the terminal azo-chromophores. With the same back-
bone structure, HPAP-CA shows the fastest inscription
rate and HPAP-NT shows the slowest inscription rate
among the series. HPAP-CN shows an intermediate
inscription rate between those of HPAP-CA and HPAP-
NT.

Macromolecules, Vol. 38, No. 21, 2005
Hyperbranched Azo-Polymers 8663
(19) Tuner, S.; Walter, F.; Voit, B.; Mourey, T. Macromolecules
1994, 27, 1611-1616.
(20) Hadjichristidis, N.; Pispas, S.; Pitsikalis, M.; Iatrou, H.;
Vlahos, C. Adv. Polym. Sci. 1999, 142, 71-127.
(21) Xie, S.; Natansohn, A.; Rochon, P. Chem. Mater. 1993, 5,
403-411.
(22) Kumar, G. S. Azo Functional Polymers: Functional Group
Approach in Macromolecular Design; Technomic Publishing
Co. Inc.: Lancaster, 1993.
(23) Rau, H. Photochemistry and Photophysics; Rabek, J. F., Ed.;
CRC Press: Boca Raton, FL, 1990; Vol. II, Chapter 4.
(24) Natansohn, A.; Rochon, P. Chem. Rev. 2002, 102, 4139-4175.
(25) Delaire, J. A.; Nakatani, K. Chem. Rev. 2000, 100, 1817-
1845.
There is no directly comparable dendritic polymer
system that has been reported before for the SRG study.
One of the most closely related systems is a series of
epoxy-based linear azo-polymers bearing the similar
type azo-chromophores.48 For the linear polymers, the
rate of SRG formation is closely related with the
electron-withdrawing groups in the azobenzene units.
The linear polymers containing 4-carboxyazobenzene
and 4-cyanoazobenzene groups show an obviously faster
SRG formation rate than that of polymer containing
4-nitroazobenzene groups. In current study, a similar
tendency was observed for the hyperbranched polymers
containing those types of azo-chromophores in the
terminal positions. Although hyperbranched azo-poly-
mers are obviously different with their linear analogues
in backbone architecture and located positions of the
azo-chromophores, the influence of the electron-with-
drawing groups appears to be the same.
(26) Todorov, T.; Markovski, P.; Tomova, N.; Dragostinova, V.;
Stoyanova, K. Opt. Quantum Electron. 1984, 16, 471-476.
(27) Ikeda, T.; Tsutsumi, O. Science 1995, 268, 1873-1875.
(28) Ichimura, K.; Oh, S. K.; Nakagawa, M. Science 2000, 288,
1624-1626.
(29) Hugel, T.; Holland, N. B.; Cattani, A.; Moroder, L.; Seitz, M.;
Gaub, H. E. Science 2002, 296, 1103-1106.
(30) Rochon, P.; Batalla, E.; Natansohn, A. Appl. Phys. Lett. 1995,
66, 136-138.
Conclusions
(31) Kim, D. Y.; Tripathy, S. K.; Li, L.; Kumar, J. Appl. Phys. Lett.
1995, 66, 1166-1168.
A series of hyperbranched azo-polymers were pre-
pared by step-growth polycondensation of an AB2 mono-
mer through the azo-coupling reaction and postpoly-
merization azo-coupling modification. This synthetic
scheme can be used to prepare a variety of hyper-
branched azo-polymers with different peripheral azo-
chromophores under extremely mild conditions. Intro-
ducing the terminal azo-chromophores can significantly
modify the thermal properties, spectral characteristics,
and SRG formation behavior. The type of terminal azo-
chromophores plays an important role to influence the
rate of SRG formation. The hyperbranched azo-polymers
can potentially be used for applications such as revers-
ible optical data storage, photoswitching, sensors, and
other photodriven devices.
(32) (a) Archut, A.; Vogtle, F.; Cola, L.; Azzellini, G.; Balzani, V.;
Ramanujam, P.; Berg, R. Chem.sEur. J. 1998, 4, 699-706.
(b) Archut, A.; Azzellini, G.; Balzani, V.; Cola, L.; Vogtle, F.
J. Am. Chem. Soc. 1998, 120, 12187-12191. (c) Schenning,
A.; Elissen-Roman, C.; Weener, J.; Baars, M.; Gaast, S.;
Meijer, E. J. Am. Chem. Soc. 1998, 120, 8199-8208. (d)
Weener, J.; Meijer, E. Adv. Mater. 2000, 12, 741-746. (e)
Nithyanandhan, J.; Jayaraman, N.; Davis, R.; Das, S. Chem.s
Eur. J. 2004, 10, 689-698.
(33) (a) Junge, D.; McGrath, D. Chem. Commun. 1997, 857-858.
(b) Junge, D.; McGrath, D. J. Am. Chem. Soc. 1999, 121,
4912-4913. (c) Sidorenko, A.; Houphouet-Boigny, C.; Vilavi-
cencio, O.; Hashemzadeh, M.; McGrath, D.; Tsukruk, V.
Langmuir 2000, 16, 10569-10572. (d) Ghosh, S.; Banthia,
A. Tetrahedron Lett. 2001, 42, 501-503.
(34) (a) Nagasaki, T.; Tamagaki, S.; Ogino, K. Chem. Lett. 1997,
717-718. (b) Yokoyama, S.; Nakahama, T.; Otomo, A.;
Mashiko, S. Chem. Lett. 1997, 1137-1138. (c) Yokoyama, S.;
Nakahama, T.; Otomo, A.; Mashiko, S. J. Am. Chem. Soc.
2000, 122, 3174-3181.
(35) Aida, T.; Jiang, D. Nature (London) 1997, 388, 454-456.
(36) Yokoyama, S.; Nakahama, T.; Otoma, A.; Mashiko, S. Thin
Solid Films 1998, 331, 248-253.
Acknowledgment. The financial support from the
NSFC under Project 50273018 and the MOST under
Project 2002AA313130 is gratefully acknowledged.
(37) Wang, G. J.; Wang, X. G. Chem. Lett. 2002, 1, 78-79.
(38) He, Y. N.; Wang, X. G.; Zhou, Q. X. Synth. Met. 2003, 132,
245-248.
References and Notes
(1) Kim, Y.; Webster, O. Macromolecules 1992, 25, 5561-5572.
(2) Inoue, K. Prog. Polym. Sci. 2000, 25, 453-571.
(3) Jikei, M.; Kakimoto, M. Prog. Polym. Sci. 2001, 26, 1233-
1285.
(4) Gao, C.; Yan, D. Prog. Polym. Sci. 2004, 29, 183-275.
(5) (a) Mori, H.; Seng, D.; Zhang, M. Langmuir 2002, 18, 3682-
3693. (b) Mori, H.; Seng, D.; Lechner, H.; Zhang, M. Macro-
molecules 2002, 35, 9270-9281.
(6) Kim, Y. J. Polym. Sci., Part A: Polym. Chem. 1998, 36, 1685-
1734.
(7) Borkovec, M.; Koper, G. Colloid Polym. Sci. 1998, 109, 142-
152.
(39) Ramanujam, P. S.; Holme, N. C. R.; Hvilsted, S. Appl. Phys.
Lett. 1996, 68, 1329-1331.
(40) Tripathy, S. K.; Kim, D. Y.; Li, L.; Kumar, J. CHEMTECH
1998, 28, 34-40.
(41) Viswanathan, N. K.; Kim, D. Y.; Bian, S.; Williams, J.; Liu,
W.; Li, L.; Samuelson, L.; Kumar, J.; Tripathy, S. K. J. Mater.
Chem. 1999, 9, 1941-1955.
(42) Ho, M. S.; Barrett, C.; Paterson, J.; Esteghamatian, M.;
Natansohn, A.; Rochon, P. Macromolecules 1996, 29, 4613-
4618.
(43) Jiang, X. L.; Li, L.; Kumar, J.; Kim, D. Y.; Tripathy, S. K.
Appl. Phys. Lett. 1998, 72, 2502-2504.
(44) Paterson, J.; Natansohn, A.; Rochon, P.; Callender, C.;
Robitaile, L. Appl. Phys. Lett. 1996, 69, 3318-3320.
(45) Rochon, P.; Natansohn, A.; Callender, C.; Robitaile, L. Appl.
Phys. Lett. 1997, 71, 1008-1010.
(46) Stockermans, R.; Rochon, P. Appl. Opt. 1999, 38, 3714-3719.
(47) Barrett, C. J.; Yager, K. G. Curr. Opin. Solid State Mater.
Sci. 2001, 5, 487-494.
(48) He, Y. N.; Wang, X. G.; Zhou, Q. X. Polymer 2002, 43, 7325-
7333.
(8) Hong, C.; Pan, C. Polymer 2001, 42, 9385-9391.
(9) Smits, R.; Koper, G.; Mandel, M. Phys. Chem. 1993, 194,
1953-1963.
(10) Yang, G.; Jikei, M.; Kakimoto, M. Macromolecules 1999, 32,
2215-2220.
(11) Miller, T.; Neenan, T.; Kwock, E.; Stein, S. J. Am. Chem. Soc.
1993, 115, 356-357.
(12) Kumar, A.; Ramakrishnan, S. Macromolecules 1996, 29,
2524-2530.
(13) Bolton, D.; Wooley, K. Macromolecules 1997, 30, 1890-1896.
(14) Turner, S.; Voit, B.; Mourey, T. Macromolecules 1993, 26,
4617-4623.
(49) Che, P. C.; He, Y. N.; Zhang, Y.; Wang, X. G. Chem. Lett.
2004, 33, 22-23.
(50) Wang, X. G.; Kumar, J.; Tripathy, S. K.; Li, L.; Chen, J.;
Marturunkakul, S. Macromolecules 1997, 30, 219-225.
(51) Herz, A. H. Photogr. Sci. Eng. 1974, 18, 323-335.
(52) McRae, E. G.; Kasha, M. J. Chem. Phys. 1958, 28, 721-722.
(53) DeVoe, H. J. Chem. Phys. 1964, 41, 393-400.
(15) Ding, J.; Chuy, C.; Holdcroft, S. Chem. Mater. 2001, 13,
2231-2233.
(16) Percec, V.; Kawasumi, M. Macromolecules 1992, 25, 3843-
3850.
(17) Mathias, L.; Carothers, T. J. Am. Chem. Soc. 1991, 113,
4043-4044.
(18) Hawker, C.; Chu, F. Macromolecules 1996, 29, 4370-4380.
MA0511393