Multifunctional hyperbranched polyamide: Synthesis and properties

Article abstract of DOI:10.1016/j.polymer.2013.04.021

A novel multifunctional hyperbranched polyamide bearing oligoaniline, azobenzene and triphenylbenzene (PAOAF) has been prepared through a pyridine/LiCl-mediated acylation reaction. The structure of PAOAF was confirmed via nuclear magnetic resonance (NMR),

Full text of DOI:10.1016/j.polymer.2013.04.021

Polymer 54 (2013) 3223e3229
Contents lists available at SciVerse ScienceDirect
Polymer
journal homepage: www.elsevier.com/locate/polymer
Multifunctional hyperbranched polyamide: Synthesis and properties
,
,
,
,
,
Danming Chao a 1, Libing He a 1, Erik B. Berda b, Shutao Wang a 1, Xiaoteng Jia a 1, Ce Wang a
*
a Alan G. MacDiarmid Institute, College of Chemistry, Jilin University, Changchun 130012, PR China
b Department of Chemistry and Materials Science Program, University of New Hampshire, Durham, NH 03824, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
A
novel multifunctional hyperbranched polyamide bearing oligoaniline, azobenzene and triphe-
Received 15 January 2013
Received in revised form
3 April 2013
Accepted 11 April 2013
Available online 17 April 2013
nylbenzene (PAOAF) has been prepared through a pyridine/LiCl-mediated acylation reaction. The
structure of PAOAF was confirmed via nuclear magnetic resonance (NMR), Fourier-transform infrared
spectra (FTIR), X-ray diffraction (XRD) and gel permeation chromatography (GPC). The thermal stability
was probed via thermogravimetric analysis (TGA). The electrochemical activity of PAOAF was explored by
cyclic voltammetry in 1.0 M H2SO4 confirming a surface controlled process. Tunable dielectric property of
PAOAF has been accomplished by controlling the isomerization of azobenzene groups using UV/Vis
Keywords:
irradiation. The electrochromic performance of
a PAOAF/ITO electrode was studied by spec-
Multifunctional
Electrochromic performance
Oligoaniline
trochronoamperometry in detail, exhibiting good electrochromic properties with high contrast value and
satisfactory coloration efficiency. Moreover, the fluorescence of PAOAF was modulated by controlling the
oxidation degree of oligoaniline segments because of the energy migration occurring between oligoa-
niline and triphenylbenzene groups.
Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction
conjugated polymers prepared by in situ electrochemical deposi-
tion, which were used as a highly-efficient blue-emitting and
The novel properties of multifunctional polymers make them
useful for an extensive range of application, including organic field-
effect transistors, photovoltaic cells, bioengineering materials and
sensors [1,2]. Exploring architectures, applications, and funda-
mental principles of various attributes of multifunctional polymers
has recently attracted a significant academic and commercial in-
terest. Among several functions reported for multifunctional
polymers, electroactivity, shape-memory, magnetic, biomedical
and bioinformatics, are particularly attractive [3e9]. In the Wang
research group [3], biodegradable and biocompatible poly(p-diox-
anone)epoly(tetramethyleneoxide)glycol multiblock copolymers
with excellent shape memory effect and mechanical properties
were developed by coupling PPDO-diol and PTMEG-diol with 1,6-
hexamethylene diisocyanate (HDI). Lendlein and coworkers [6]
studied the shape-memory capability as well as the hydrolytic
and enzymatic in vitro degradation behavior of electro-spun scaf-
folds prepared from a multiblock copolymer, containing hydrolyt-
ically degradable poly(p-dioxanone) and poly(e-caprolactone)
segments. Ma et al. [7] presented cross-linked multifunctional
electron-transport layer.
Polyaniline (PANI) is one of the most frequently studied func-
tional polymers potential for advanced applications such as elec-
troluminescence,
biosensors,
anticorrosion
coatings,
electrochemical capacitors, gas separation membranes, and elec-
tromagnetic interference shielding materials [10e17] because of its
good environmental stability, ease of synthesis and high conduc-
tivity [18e21]. Recently, ample work has been conducted on the
preparation of multifunctional polymers bearing polyaniline olig-
omer segments, which could combine the good solubility and
processability inherent to the oligoanilines with the functional
benefits of other macromolecules. Chen and coworkers prepared
and characterized a biodegradable and electroactive polymer blend
materials based on PEG/tetraaniline and PLLA, the 20 wt% (PEG/
tetraaniline) blend shows the best biocompatibility and an elec-
troactivity that accelerates the differentiation of rat C6 glioma cells
[22]. Albertsson et al. synthesized degradable and electroactive
hydrogels based on polylactide, polycaprolactone, and aniline
pentamer. Tunable electrical conductivity and swelling behavior
were reported [23,24]. Yeh et al. prepared an intrinsically dopable
polyimide membrane containing aniline trimer that exhibits
excellent gas separation capabilities as well as mechanically and
thermally enhanced properties [25]. In our group, we have recently
reported a novel poly(aryl ether) containing pendant oligoaniline
* Corresponding author. Tel./fax: þ86 431 85168292.
E-mail addresses: cwang@jlu.edu.cn, chaodanming@gmail.com (C. Wang).
Tel./fax: þ86 431 85168292.
1
0032-3861/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.polymer.2013.04.021

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D. Chao et al. / Polymer 54 (2013) 3223e3229
and anthracene moieties. This material exhibits an interesting
fluorescent response to redox active species [26]. We also designed
and synthesized a multifunctional polyamide containing oligoani-
line and azobenzene groups. Taking advantage of surface relief
gratings based on azo-chromophores, we have fabricated materials
with electrochemically responsive diffraction gratings [27]. In this
paper, we describe a versatile post-polymerization modification
strategy to prepare multifunctional polyamide bearing oligoaniline,
azobenzene and triphenylbenzene groups (PAOAF). The properties
of PAOAF, such as thermal stability, electroactivity, electrochromic
performance and fluorescence modulating are described in detail.
mixture was allowed to warm to reflux and stirred for 10 h. The
excess thionyl chloride was removed at reduced pressure to give
the red product. (Yield: 96%).
FTIR (KBr pellet, cmꢂ1): 3081 (m, yCeH of benzenoid rings), 2972
(m, yCeH of methyl groups), 1763 (vs,
yC of benzenoid rings), 1421 (m,
dCH), 686 (m, dCH). 1H NMR (500 MHz, 25 ꢁC, CDCl3, TMS, ppm):
¼ 8.13 (s, 2H, due to H4 (the protons have been defined in Scheme
1)), ¼ 7.66 (d, 2H, due to H2), ¼ 2.81
¼ 8.02 (d, 2H, due to H1),
(s, 6H, due to H3).
y
C]O), 1747 (s,
yC]O), 1593 (m,
]
C
yN]N), 822 (m, dCH), 760 (w,
d
d
d
d
2.3. Synthesis of 1,3,5-tri(4-aminophenyl) benzene (TAPB)
2. Experimental
SiCl4 (360 mmol) was added dropwise over a period of 1 h to a
stirring solution of 4-nitroacetophenone (12 mmol) in 120 mL
ethanol. Following the addition of SiCl4, the resulting solution was
heated to reflux for 10 h. After cooling to room temperature, the
mixture was poured into 200 mL saturated NH4Cl aqueous solution.
The solvent was removed at reduced pressure to give the yellow
crude product, followed by drying under dynamic vacuum at 40 ꢁC
for 24 h. Then the crude product, Pd/C catalyst and 300 mL ethanol
was added into a 500 mL three-necked round-bottom flask
equipped magnetic stirring. Hydrazine hydrate (40 mL) was added
dropwise to the above mixture, and then was heated to reflux for
10 h. The resulting solution was concentrated and recrystallized
twice from ethanol, followed by drying under dynamic vacuum at
room temperature for 24 h. (Yield: 81%).
2.1. Materials
3-Methyl-4-nitrobenzoic acid and N-phenyl-p-phenylenedi-
amine were purchased from Aldrich. a-D-Glucose, acetic acid and
thionyl chloride were purchased from Shanghai Chemical Factory.
4-Nitroacetophenone, SiCl4, hydrazine hydrate and Pd/C catalyst
were purchased from Beijing Chemical Factory. All other reagents
were obtained from commercial sources and used as received
without further purification. Distilled and deionized water was
used. Optically transparent ITO glass substrates (Reintech elec-
tronic technologies CO., 10
U/square) with dimensions of
6.0 ꢀ 0.6 cm2 were used as thin film electrodes.
MALDI-TOF-MS: m/z calculated for C24H21N3 ¼ 351.4. Found
2.2. Synthesis of azo(3-methylbenzoyl)-4,40-dicarbonyl dichloride
(DDBMBC)
351.3. FTIR (KBr pellet, cmꢂ1): 3434 (s, yNH), 3355 (s, yNH), 1621 (s,
yC
(vs, yC
1425 (m,
1H NMR (d6-DMSO):
been defined in Scheme 1)),
6H, due to H3),
] of benzenoid rings), 1606 (s, yC] of benzenoid rings), 1513
C
C
]
C
of benzenoid rings), 1473 (m, yC] of benzenoid rings),
C
The synthesis route was described in Scheme 1. The reaction was
conducted in a 500 mL three-necked round-bottom flask equipped
with gas inlet and heated in an oil bath. a-D-Glucose (1250 mmol),
y
N]N), 871 (w, dCH), 829 (s, dCH), 707 (m, dCH), 649 (m, dCH).
¼ 7.48 (s, 3H, due to H1 (the protons have
¼ 7.47 (d, 6H, due to H2), ¼ 6.66 (d,
d
d
d
dissolved in 500 mL H2O, was added dropwise to stirring mixture of
3-methyl-4-nitrobenzoic acid (78 mmol), sodium hydrate
(1250 mmol) and H2O (250 mL) at 50 ꢁC. Then the reaction carried
out readily in an atmosphere with the formation of a brown solu-
tion. The pink precipitate obtained by adding acetic acid, was dis-
solved in 1.0 M K2CO3 aqueous solution, filtered and reprecipitated
with hydrochloric acid, followed by drying under dynamic vacuum
at room temperature for 24 h. (Yield: 52%).
d
¼ 5.23 (s, 6H, due to H4). A typical elemental
analysis for C24H21N3: Calcd. C 82.02, H 6.02, N 11.96; Found C 82.11,
H 5.98, N 11.91.
2.4. Synthesis of parent aniline tetramer (PAT)
The synthetic route for the preparation of parent aniline
tetramer is depicted in Scheme 1. Firstly, the parent aniline
tetramer in emeraldine state was synthesized by oxidative coupling
of N-phenyl-p-phenylenediamine in the presence of ferric chloride
according to the literature [28]. The obtained product (5.0 g) was
then dispersed into a stirring mixture solution (30 mL of hydrazine
hydrate in 400 mL of 1.0 M ammonium hydroxide) and stirred for
12 h. Then it was filtered and washed with distilled water for
several times, followed by drying in vacuum. Finally, the PAT was
reduced to the leucoemeraldine oxidation state. (Yield: 94%).
MALDI-TOF-MS: m/z calculated for C24H22N4 ¼ 366.5. Found
366.6. FTIR (KBr, cmꢂ1): 3390 (s, yNH), 3020 (m, yCH), 1600 and 1527
The obtained acid product was dispersed in 150 mL thionyl
chloride. A drop of DMF was added as a catalyst. The reaction
(s, yC
dCH), 692 (m, dCH). 1H NMR (d6-DMSO):
protons have been defined in Scheme 1)),
¼ 7.43 (s, 1H, due to H2), ¼ 7.12 (t, 2H, due to H6),
12H, due to H8), ¼ 6.51 (d, 2H, due to
¼ 6.65 (t, 1H, due to H5),
H7),
] of benzenoid rings), 1305 (s, yCeN), 815 (m, dCH), 746 (m,
C
d
¼ 7.78 (s,1H, due to H4 (the
¼ 7.68 (s, 1H, due to H3),
¼ 6.87 (m,
d
d
d
d
d
d
d
¼ 4.62 (s, 2H, due to H1). A typical elemental analysis for
C24H22N4: Calcd. C 78.66, H 6.05, N 15.29; Found C 78.49, H 6.01, N
15.50.
2.5. Synthesis of multifunctional hyperbranched polyamide
Scheme 1. Synthesis of azo(3-methylbenzoyl)-4,40-dicarbonyl dichloride (DDBMBC),
1,3,5-tri(4-aminophenyl) benzene (TAPB) and parent aniline tetramer (PAT).
A typical polymer synthesis procedure (shown in Scheme 2) is
as follows. A mixture of DDBMBC (1.3407 g, 4 mmol), DMAc

D. Chao et al. / Polymer 54 (2013) 3223e3229
3225
Scheme 2. Synthesis of multifunctional hyperbranched polyamide.
(40 mL), toluene (10 mL), anhydrous lithium chloride (7 mmol),
2.6. Fabrication of electrochromic device
pyridine (0.7910 g, 10 mmol) were added to a 100 mL three-necked
round-bottom flask and cooled by ice bath under nitrogen with
mechanical stirring for 0.5 h. TAPB (0.7029 g, 2 mmol), dissolved in
5 mL DMAc, was added dropwise to the above solution. The reac-
tion proceeded in an ice bath with stirring for 2 h. Then PAT
(0.7329 g, 2 mmol) was dissolved in 5 mL DMAc and added to the
reaction to cap the hyperbranched polymer. The solution was stir-
red for another 6 h to ensure the completion of the reaction. The
solution was poured into 200 mL water, which yielded a brown
precipitate. The precipitate was washed with water, ethanol and
dichloromethane several times, filtered and dried under dynamic
vacuum at 80 ꢁC for 48 h (90% yield).
The ITO substrates were washed ultrasonically in acetone for
3 min and then in the deionized water for another 3 min, followed
by drying in the air before use. Polymer (0.03 g) was dissolved in
1 mL DMAc to form a solution, and filtered through 0.2-mm poly(-
tetrafluoroethylene) syringe filter. Then, polymer films were
spin-coated onto the ITO substrates using the DMAc solution of
polymer. The spin-coating process started at 500 rpm for 5 s and
then 1000 rpm for 30 s. A copper tape (1.0 cm ꢀ 0.5 cm) was applied
to the top edge of ITO substrates, before electrochromic measure-
ments, as the bus bar.
FTIR (KBr pellet, cmꢂ1): 3386 (s, yNH), 2926 (m, yCeH of methyl
2.7. Characterization techniques
groups),1693 (vs,
yC C of benzenoid rings), 1429 (m,
dCH), 776 (w, dCH). 1H NMR (500 MHz, 25 ꢁC, DMSO-d6, TMS, ppm):
¼ 10.51 (m, due to eNHeCOe), ¼ 8.17e7.49 (m, due to HeAr
and eNHe), ¼ 6.97 (m, due to HeAr),
¼ 7.16 (m, due to HeAr),
¼ 6.71 (m, due to HeAr), ¼ 2.74 (m, due to eCH3). GPC results:
Mn: 5000, Mw: 11,000, PDI: 2.2.
y
C]O),1602 (s,
y
C]C of benzenoid rings),1512 (vs,
]
y
N]N), 1299 (m, yCeN), 823 (m,
Mass spectroscopy (MS) was performed on an AXIMA-CFR laser
desorption ionization flying time spectrometer (COMPACT).
Fourier-transform infrared spectra (FTIR) measurements were
recorded on a BRUKER VECTOR 22 spectrometer. The nuclear
magnetic resonance spectra (NMR) were run on a BRUKER-500
spectrometer. The weight percentages of carbon, hydrogen,
d
d
d
d
d
d
Fig. 1. X-ray diffraction pattern (a) and TGA thermogram (b) (under nitrogen atmosphere) of PAOAF.

3226
D. Chao et al. / Polymer 54 (2013) 3223e3229
The conductivity was measured three times at different current
values, and the average value was taken as the conductivity.
Dielectric spectroscopy was performed using a QuadTech1920 LCR
meter. Measurements of the capacitance (C) over the range of
100 Hze1 MHz were carried out with the polymer powder-pressed
pellet. Two identical aluminum foil strips were used to clamp the
polymer pellet on its two surfaces, which were used as both elec-
trode plates and lead wires. In order to make the aluminum foil well
contact with the polymer pellet, two glass plates with two clips
were used to clip the aluminum foil strips. The dielectric constant
(ε0) was calculated from capacitance using the following equation:
Cd
ε0 ¼
l2E0
where d is the thickness and E0 is the permittivity of the free space,
is 8.854 ꢀ 10ꢂ12 F mꢂ1. The contact part between aluminum foil
strip and pellet is an approximate square, l is the side length of the
square.
Fig. 2. CV of the PAOAF electrode in 1.0 M H2SO4 at different potential scan rates: 10e
100 mV/s. Inset shows the relationships between the oxidation peaks and reduction
current vs. potential scan rate.
3. Results and discussion
nitrogen and oxygen in the samples were measured by a Flash Ea
1112 elemental analysis instrument. The number-average molecu-
lar weight (Mn), weight-average molecular weight (Mw), and mo-
lecular weight distribution of the polymer were measured with a
gel permeation chromatography (GPC) instrument equipped with a
Shimadzu GPC-802D gel column and SPD-M10AVP detector with
DMF as an eluant at a flow rate of 1 mL/min. Wide-angle X-ray
diffraction (WAXD) measurements were made using a Siemens
D5005 diffractometer equipped with a CuKa radiation source at
room temperature. A PerkineElmer PYRIS 1 TGA was used to
investigate the thermal stability of the polymer. Transecis isom-
erization behavior of PAOAF was investigated by exposing to the UV
lamp (365 nm after filter, PHILIPS HPA400S 400 W) and visible
lamp (420 nm after filter, CHF-XM 500 W). The CV was investigated
on a CHI 660A Electrochemical Workstation (CH Instruments, USA)
with a conventional three-electrode cell, using an Ag/AgCl cell as
the reference electrode, a platinum wire electrode as the counter
3.1. Synthesis, structure and thermal properties
The multifunctional hyperbranched polyamide was synthesized
by a two-step procedure. The lithium chloride was used as a cata-
lyst and pyridine was used to absorb the hydrochloride acid, which
comes from the polycondensation reaction. The reaction proceeded
in the ice bath. The structure of PAOAF was confirmed by FTIR, 1H
NMR, GPC and XRD. FTIR spectra of PAOAF displayed the charac-
teristic absorption bonds around 3386 cmꢂ1 corresponding to NeH
stretching vibration, around 1299 cmꢂ1 based on CeN stretching
vibration of amide groups. The vibration at 1602 cmꢂ1 and
1512 cmꢂ1 is attributed to the stretching vibration of C]C in the
benzene rings. In the 1H NMR spectra of PAOAF, the signal at
d
¼ 10.51 ppm is attributed to the amino protons of the amide
groups, and the signal at
¼ 2.74 is ascribed to the methyl protons.
Furthermore, aromatic protons appeared at
¼ 8.17e6.17. Accord-
d
d
ing to the integral area ratio of amino protons (amino and amide
groups) and methyl protons from the 1H NMR spectra, the triphe-
nylbenzene, azobenzene and oligoaniline were about 1:1.96:0.92,
which almost the same with the feeding ratio (1:2:1) of the reac-
tion. The number average molecular weight and polydispersity
index of PAOAF, obtained by GPC, are 5000 and 2.2 respectively.
The wide-angle XRD pattern of PAOAF (Fig. 1a) shows a broad
peak from 10ꢁ to 35ꢁ, which indicates that the hyperbranched
polymer existed in a form of noncrystalline state. This is mainly due
to the fact that the branched structure decreases the interchain
electrode, and a glassy carbon electrode (F3.0 mm) as the working
electrode. Spectroelectrochemical measurements were carried out
in a cell built from a 1 cm commercial cuvette using a UV-2501 PC
Spectrometer. The ITO-coated glass was used as the working elec-
trode, a Pt wire as the counter electrode, an Ag/AgCl cell as the
reference electrode and 0.5 mol/L H2SO4 as the electrolyte. Fluo-
rescence spectra were investigated on a fluorometer of SPEXF212
model (SPEX) in dilute DMAc solution.
The electrical conductivity of the 1 mol/L HCl-doped PAOAF
was determined by the standard Van Der Pauw four-probe method.
hydrogen bonding and
pep interactions and disturbs the close
Fig. 3. UVeVis spectra changes of PAOAF during ultraviolet irradiation (a) and visible light irradiation (b).

D. Chao et al. / Polymer 54 (2013) 3223e3229
3227
Fig. 4. Frequency dependence of dielectric constant and dielectric loss of PAOAF in LEB, EB and HCl-doped EB form (a), and under UV/Vis irradiation (b), respectively.
packing and regularity of the polymer chains. The amorphous
structure is a basic characteristic for hyperbranched polymers [29].
Moreover, PAOAF exhibited outstanding solubility in polar solvents
such as N,N-dimethylacetamide (DMAc), N,N-dimethylformamide
(DMF), dimethyl sulfoxide (DMSO), N-methyl-2-pyrrolidone
(NMP), tetrahydrofuran (THF) due to the hyperbranched structure.
The thermal properties of pristine PAOAF were evaluated by TGA
(Fig. 1b). An obvious mass loss beginning at 280 ꢁC is observed
corresponding to the decomposition of main chain. 5% Weight loss
temperatures of the obtained polymer are 300 ꢁC, which indicates
that the PAOAF has moderate thermal stability and general degra-
dation temperature the same as typical polyaniline materials [30].
But compared with aramid and copolyaniline material [31,32], the
thermal stability of PAOAF is much lower, which due to the weak
interchain interaction originated from its hyperbranched structure.
[33,34]. UVeVis spectra changes were obtained with time intervals
about 2 min and the transecis isomerization reached to a steady
state 8 min later, and then the above solution was transferred to
visible radiation. As shown in Fig. 3(b), the intensity of the absorp-
tion, associated with
pep* transition of the trans-azobenzene
group, gradually increased with the anticipated red shift. The ab-
sorption at about 340 nm nearly reached the intensity of its original
value after 16 min. These results confirm that PAOAF retained the
photoisomerization characteristics of azobenzene.
3.4. Electrical and dielectric properties
The electrical conductivity value of PAOAF was determined by
the standard four probe method. Firstly, PAOAF in EB was prepared
by oxidizing the obtained PAOAF with ammonium persulfate. Then
HCl-doped PAOAF in EB was made by dispersing the polymer
powder in 1 M HCl solution at room temperature for 8 h. Finally
three samples of PAOAF were measured four times at different
current values. The average of 12 measurements was about
7.6 ꢀ 10ꢂ6 S/cm. The electrical conductivity value of PAOAF is much
lower than that of HCl-doped PANI, which was attributed to the low
conductive percent in the polymer architecture.
Recently, some high dielectric constant materials have been re-
ported by incorporating oligoaniline segment into the polymer ar-
chitecture, which avoids the drawbacks of agglomeration and phase
separation in PANI/insulating polymer blends [35]. Consequently,
dielectric response properties of PAOAF were investigated in here.
3.2. Electrochemical activity
The cyclic voltammetry (CV) was used to study the electro-
chemical behaviorof PAOAFand the results were shown in Fig. 2. The
PAOAF DMAc solution was cast on the g-c working electrode and
then evaporated to form a thin solid film. The CV of the decorated
electrode was performed in 1.0 M H2SO4 at different potential scan
rates from 10 to 100 mV/s. The CV of the PAOAF-decorated electrode
exhibited two redox processes with the reduction peaks at 200 and
460 mV. The first reduction peak corresponded to the emeraldine
base (EB)/leucoemeraldine base (LEB) transition, and the second
reduction peak corresponded to the pernigraniline base (PNB)/
emeraldine base (EB) transition. A linear dependence of the peak
currents as a function of scan rates in the region of 10e100 mV/s
(inset of Fig. 2) confirmed both a surface controlled process and a
well-adhered electroactive polymer film. The PAOAF film was found
to be very stable with almost no change in its CV diagram after 60
repeated cyclic scans between 0 and 800 mV.
3.3. Photoisomerization of PAOAF
Transecis isomerization of azobenzene groups was investigated
in details by irradiating the PAOAF DMAc solution with UV/visible
light. The UVeVis spectra changes were recorded during the ultra-
violet and visible irradiation. As shown in Fig. 3(a), the intensity of
the absorption of PAOAF DMAc solution at about 340 nm, associated
with the
pep* transition of the trans-azobenzene groups and
benzenoid rings, gradually decreased as irradiation time increased.
This occurred concurrently with a blue shift, due to the trans-to-cis
photoisomerization process of the azobenzene chromophore. The
Fig. 5. Optical transmittance of PAOAF electrodes (0.6 ꢀ 3 cm2) in 0.5 M H2SO4 at
different potentials. Inset shows photographs of PAOAF/ITO electrode at different
potentials.
blue shift of the absorptionwas attributed tothe overlap of the
pep*
transition of the trans-azobenzene groups and benzenoid rings

3228
D. Chao et al. / Polymer 54 (2013) 3223e3229
in conjunction with the acquisition of UVeVis spectral data (Fig. 5).
The optical contrast value (%
T) was obtained from the trans-
D
mittance difference at 700 nm between its coloring (oxidization)
and bleaching (reduction) states. This PAOAF/ITO electrode
exhibited good contrast with a high optical transmittance change
(%DT) of 25% (69.24% (at ꢂ0.2 V) 4 44.69% (at 0.8 V)) at 700 nm.
The color of the PAOAF/ITO changed drastically from gray (at 0.0 V),
to green (at ca. 0.4 V), and finally to dark blue (at ca. 0.8 V).
Electrochromic switching studies were performed to monitor
changes in the optical contrast at 700 nm during repeated potential
stepping between reductive state (ꢂ0.2 V) and oxidized state at
0.8 V with a residence time of 60 s. Fig. 6 shows the first 10 cycles
results. Switching time in this case is the time required to bring the
PAOAF to its most reduced state from its most oxidized state or vice
versa. It was defined as the time required for reaching 90% of the
full change in the coloring/bleaching process. The PAOAF revealed a
switching time of 7.8 s at 0.8 V for the coloring process at 700 nm
and 3.0 s for bleaching (ꢂ0.2 V). The coloration efficiency (CE,
h
¼
DOD/Q) is generally used to measure the power requirements of
an electrochromic material. It was calculated by monitoring the
Fig. 6. Current consumption (a) and absorbance changes (b) monitored at 700 nm of
PAOAF/ITO electrode in 0.5 mol/L H2SO4 for the first 10 cycles.
amount of ejected charge (Q) as a function of the change in optical
density (DOD) of the polymer film. The new multifunctional PAOAF
exhibited a high CE up to 97.14 cm2/C (at 700 nm).
Dielectric constants and dielectric loss of PAOAF in LEB, EB and HCl-
doped EB (emeraldine salt) were presented in Fig. 4a. The emer-
aldine salt of PAOAF possessed a higher dielectric constant than that
of EB state from 100 Hz to 1 MHz, and gave a value of 20 detected at
the frequency of 100 Hz, then decreased to 5.0, as the frequency was
increased to 1 MHz. The frequency dependence of dielectric loss was
shown in the inset of Fig. 4a. Due to the enhancement of the polarity
of the polymer chains, the dielectric loss of emeraldine salt was
higher than that of LEB and EB. The influence of photoisomerization
derived from azobenzene groups on the dielectric properties of
PAOAF was also investigated. The dielectric properties of emeraldine
salt PAOAF exposure to UV/Vis irradiation were shown in Fig. 4b.
After exposing to the UV irradiation, the dielectric constant and
dielectric loss increased gradually as the time of irradiation
increased. When the polymer reached the steady state after 10 min
UV irradiation, the dielectric constant increased to 26.5 at 100 Hz,
and 5.7 at 1 MHz. After 20 min visible light irradiation, the values of
its dielectric constant and dielectric loss went back to its original
value, which indicated that the dielectric properties of as-
synthesized PAOAF could be tuned by controlling the photo-
isomerization of azobenzene groups using UV/Vis irradiation.
3.6. Photophysical properties and fluorescence tuning
Photophysical properties of PAT, DDBMBC, TAPB and PAOAF
were characterized by UVeVis spectra and fluorescence emission
spectra (Fig. 7). In the UVeVis spectra, PAT, TAPB and PAOAF in the
DMAc are peaked at 322, 300 and 326 nm, respectively, associated
with
shows azobenzene’s characteristic absorption bands at 341 nm,
assigned to the * transition of the trans-azobenzene groups. In
pep* transition in the conjugated structure. DDBMBC also
pep
the fluorescence spectra, parent aniline tetramer shows blue fluo-
rescence at 440 nm derived from benzenoid units. DDBMBC also
exhibits a weak fluorescence at 382 and 405 nm derived from
benzenoid units. Moreover, TAPB presents strong emission with
maximum at 414 nm, and TAPB units should be the main chro-
mophore in the polymer structure because of the low fluorescent
essence of the PAT and DDBMBC [36,37]. As a result, PAOAF displays
fluorescence at 420 nm.
Furthermore, to investigate the fluorescence of PAOAF with
diverse oxidation degree, the titrations were carried out by adding
1.0 ꢀ 10ꢂ3 M (NH4)2S2O8 DMAc solution (containing 3 vol% H2O)
into the reduced PAOAF DMAc solution. Firstly, the as-prepared
luminescence solution at LB (0.00 eq S2O82ꢂ) exhibits the ex-
pected fluorescence signal (Fig. 8). With the increase of (NH4)2S2O8
concentration, the fluorescence intensity of the emission maximum
3.5. Spectroelectrochemistry and electrochromic performances
Spectroelectrochemical measurements were performed on
PAOAF/ITO electrode by increasing the potential from ꢂ0.2 to 1.0 V
Fig. 7. UVeVis spectra (a) and fluorescence spectra (b) of PAT, DDBMBC, TAPB and PAOAF in DMAc solution.

D. Chao et al. / Polymer 54 (2013) 3223e3229
3229
Acknowledgments
This work has been supported in part by the National Natural
Science Foundation of China (No. 21104024 and 21274052), and the
National 973 Project (No. S2009061009). Erik B. Berda would like to
thank the NSF for support through grant NSF EEC 0832785.
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A
multifunctional hyperbranched polyamide containing
pendant aniline tetramer, azobenzene and triphenylbenzene
groups was synthesized and characterized. Based on the synergistic
interplay of these three distinct functional groups, the obtained
hyperbranched polyamide shows reversible electroactivity, tunable
dielectric property and the expected spectroscopic properties.
PAOAF displayed good electrochromic performance with high
contrast value, outstanding coloration efficiency, and acceptable
switching times. Moreover, the fluorescence intensity of PAOAF
could be tuned by modulating the oxidation degree of oligoaniline
group. Taking into account the molecular diversity and tailoring as
well as reversible redox properties, this multifunctional hyper-
branched polymer could be competitive material for electro-
chromic devices, optic devices, sensors, and anticorrosion coatings.
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