Electroactive polymer with oligoanilines in the main chain and azo chromophores in the side chain: Synthesis, characterization and dielectric properties

Article abstract of DOI:10.1039/c0jm02960b

By an oxidative coupling polymerization approach, we have synthesized a novel electroactive polymer with good solubility containing alternating phenyl-capped aniline tetramer in the main chain and azo chromophores in the side chain. Dielectric properties of the as-synthesized polymer were investigated in detail and gratifying results have been observed. Firstly, a large enhancement in the dielectric constant was achieved utilizing the method of doping the conjugated oligoaniline segments with hydrochloric acid. Secondly, the adjustment of dielectric constant was implemented primarily by means of exposing the samples to UV and visible irradiation, mainly owing to the photoisomerization derived from azo chromophores in the side chain. The detailed characteristics of the as-synthesized polymer were systematically studied by Fourier-transform infrared (FTIR) spectra, nuclear magnetic resonance ( ¹H NMR) and gel permeation chromatography (GPC). UV-vis spectra were used to monitor the photoisomerization and doping process of the polymer. The thermal characteristics of the polyamide were evaluated by thermogravimetric analysis (TGA). Moreover, the electrochemical activity of the polymer was explored by cyclic voltammogram (CV) measurement, showing that the intrinsic electroactivity of the oligoaniline was maintained in the polymer.

Full text of DOI:10.1039/c0jm02960b

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Electroactive polymer with oligoanilines in the main chain and azo
chromophores in the side chain: synthesis, characterization and dielectric
properties
*
Libing He, Danming Chao, Xiaoteng Jia, Hongtao Liu, Lei Yao, Xincai Liu and Ce Wang
*
Received 6th September 2010, Accepted 1st November 2010
DOI: 10.1039/c0jm02960b
By an oxidative coupling polymerization approach, we have synthesized a novel electroactive polymer
with good solubility containing alternating phenyl-capped aniline tetramer in the main chain and azo
chromophores in the side chain. Dielectric properties of the as-synthesized polymer were investigated in
detail and gratifying results have been observed. Firstly, a large enhancement in the dielectric constant
was achieved utilizing the method of doping the conjugated oligoaniline segments with hydrochloric
acid. Secondly, the adjustment of dielectric constant was implemented primarily by means of exposing
the samples to UV and visible irradiation, mainly owing to the photoisomerization derived from azo
chromophores in the side chain. The detailed characteristics of the as-synthesized polymer were
systematically studied by Fourier-transform infrared (FTIR) spectra, nuclear magnetic resonance
(1H NMR) and gel permeation chromatography (GPC). UV-vis spectra were used to monitor the
photoisomerization and doping process of the polymer. The thermal characteristics of the polyamide
were evaluated by thermogravimetric analysis (TGA). Moreover, the electrochemical activity of the
polymer was explored by cyclic voltammogram (CV) measurement, showing that the intrinsic
electroactivity of the oligoaniline was maintained in the polymer.
reported a double polyester capped aniline trimer ABA block
polymer synthesized via ring-opening polymerization in anhy-
drous toluene solution.26 Chen and co-workers synthesized
Introduction
Among conducting polymers, polyaniline (PANI) has been
widely studied for its low cost, good environmental stability, easy
synthesis, and its oxidation- or protonation-adjustable electrical
properties as well as its potential for a variety of applications.1–7
However, the major drawback of PANI is the issue of defect
sites, ill-defined structure, poor solubility and processability,
which greatly impeded its practical applications, and seriously
hindered a better understanding of the structure-property
correlations and the conducting mechanism.8,9
a
biodegradable and electroactive triblock copolymer by
coupling an carboxy-capped aniline pentamer with two bi-
hydroxyl-capped polylactide via a condensation reaction, which
would be potentially used as scaffold materials for neuronal or
cardiovascular tissue engineering.24
The above mentioned examples reflect some exciting
developments in functional electroactive polymers containing
oligoanilines. However, to the best of our knowledge, only a little
attention has been paid to the dielectric properties of such kinds
of polymers.27 Since PANI has been utilized as a conductive filler
to improve the dielectric constant of a polymer matrix success-
fully,28–30 we had an optimistic view of obtaining high dielectric
constant polymers by introducing oligoanilines into polymer
chains. Furthermore, for the sake of achieving a multifunctional
polymer, incorporation of other functional groups into the
polymer would be a feasible way. As we have known, azo
chromophores have specific property of photoisomerization. In
order to meet the needs of changing polymer materials from
structural materials to functional materials, the current trend is
to employ the azobenzene moiety as a ‘‘trigger’’ to induce
morphological changes which can be light driven.31 Accordingly,
if azo chromophores are introduced into a copolymer containing
oligoanilines, the functional groups would endow the polymer
with not only electroactivity but also photoactivity, resulting in
the possibility of adjusting its dielectric properties.
Monodispersed oligoanilines, for their remarkably similar
electronic properties to those of PANI,10–13 have been considered
as the model compounds of PANI. Furthermore, oligoanilines
have a wide range of applications, such as anti-static and anti-
corrosion coatings, lightweight batteries, gas sensors, and tissue
engineering, etc.14–17 However, it is a pity that oligoanilines lack
good stability and mechanical properties compared with that of
polymers. Therefore, the attempts of incorporating a well-
defined oligoaniline into the positive mechanical and physical
properties of processable polymers might open new avenues for
applications. By modifying conventional processable polymers
with oligoanilines via covalent bonds, a series of novel functional
polymers have been created, which could avoid the drawbacks of
agglomeration and phase separation in PANI/insulating polymer
blends prepared by physical mixing methods. Up to now, some
graft,18,19 alternating20–23 and block-like polymers24,25 containing
oligoaniline groups have been synthesized. Recently, Liu et al.
Bearing all this in mind, we successfully synthesized a novel
electroactive polymer with alternating phenyl-capped aniline
tetramers in main chain and azo chromophores in side chain
by an oxidative coupling polymerization approach. The
Alan G. MacDiarmid Institute, Jilin University, Changchun, 130012,
People’s Republic of China. E-mail: cwang@jlu.edu.cn; jfylxc@jlu.edu.
cn; Fax: +86-431-85168924
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as-synthesized polymer exhibited very good solubility in
comparison to the similar polymer with azo chromophores in
main chain in our previous work.32 Its electroactive properties
were similar to oligoaniline, as characterized by electrochemical
method. Besides, UV-vis spectra have showed that the intrinsic
photoisomerization of the azo chromophores was maintained in
the polymer. Moreover, the investigation of its dielectric
properties in HCl-doped form demonstrated a large enhance-
ment in the dielectric constant and a strong frequency depen-
dence of the dielectric properties associated with the interfacial
polarization, all of which derived from the existence of the
conjugated oligoaniline segments in the main chain. We also
found that the photoisomerization of azo groups could affect the
dielectric constant obviously, and the reversible change of the
photoisomerization led to a reversible change of the dielectric
constants.
films cast from DMF solution onto a glassy carbon electrode.
The film was cycled in 1.0 M H2SO4 aqueous solution in the
range from ꢁ200 mV to 1000 mV. A Perkin-Elmer PYRIS 1
TGA was used to investigate the thermal stability of the PANI in
the temperature range from 100 ꢀC to 800 ꢀC under nitrogen
protection. Dielectric spectroscopy was performed using
a QuadTech1920 LCR meter. Measurements of the capacitance
(C) over the range of 100 Hz to 1 MHz were carried out with the
polymer powder-pressed pellet, approximately 1.3 cm in dia-
meter with a thickness of 0.5 mm. Two identical aluminium foil
strips were used to clamp the polymer pellet on its two surfaces,
which were used as both electrode plates and lead wires. In order
to make the aluminium foil well contact with the polymer pellet,
two glass plates with two clips were used to clip the aluminium
foil strips. The dielectric constant (30) was calculated from
capacitance using the following equation:
cd
30 ¼
Experimental
l2 E0
Materials
where d is the thickness and E0 is the permittivity of the free
space, is 8.854 ꢂ 10ꢁ12 F$mꢁ1. The contact part between
aluminium foil strip and pellet is an approximate square, l is the
side length of the square.
p-Phenylenediamine (99%), 2,6-difluorobenzoyl chloride (99%),
p-hydroxy-diphenylamine (99%), and aniline (99%) were
purchased from Aldrich. Ammonium persulfate (APS, 98%) was
purchased from Beijing Chemical Factory. N,N-Dime-
thylformamide (DMF, 99.5%), N,N-dimethylacetamide (DMAc,
99.5%), acetone (99.5%), toluene (99.5%), dichloromethane
(DCM, 99%), Petroleum ether (bp. 60–90 ꢀC), hydrochloric acid
(37%), hydrazine hydrate (50%) and ammonium hydroxide
(30%) were purchased from Tianjin Chemical Factory. Trie-
thylamine (99.7%), sodium hydroxide (99%) and sodium nitrite
(99%) were purchased from Shanghai Guoyao Chemical
Factory. All the above chemicals were used as received without
further purification. Distilled and deionized water was used.
Anhydrous potassium carbonate was dried at 120 ꢀC for 24 h
before use.
Synthesis of monomer 1
Concentrated hydrochloric acid (20 mL) was added dropwise to
aniline (40 g, 0.43 mol) with stirring at room temperature.
Subsequently, the solution was added dropwise to a mixture of
aniline (80 g, 0.86 mol) sodium nitrite (14 g, 0.ꢀ20 mol) and
distilled water (20 mL) over a period of 4 h at 35 C. Then the
ꢀ
mixture was heated up to 50 C for translocation, and adjusted
pH to 8 with sodium hydroxide solution. Finally it was trans-
ferred into the separating funnel. The product was obtained from
organic phase by vacuum distillation and purified by column
chromatography. (yield 76%).
MALDI-TOF-MS: The calculated mass-charge ratio for
monomer 1 was 197.2. And the ion peak at the mass-charge ratio
197.5 was observed in the experiment. 1H NMR (500 MHz,
25 ꢀC, DMSO-d6, TMS, ppm): d ¼ 7.74(d, 2H, Hc), d ¼ 7.66 (d,
2H, Hd), d ¼ 7.51 (t, 2H, He), d ¼ 7.41 (t, 1H, Hf), d ¼ 6.67 (d,
2H, Hb),d ¼ 7.29 (s, 2H, Ha). FTIR (KBr pellet, cmꢁ1): 3475
(m, yNH), 3382 (m, yNH), 1619 (s, yC]C of benzenoid rings), 1597
(vs, yC]C of benzenoid rings), 1504 (m, yC]C of benzenoid
rings), 1462 (w, yC]C of benzenoid rings), 1415 (m, yN]N), 1307
(m, yC–N), 833 (m, dCH), 769 (m, dCH), 690 (m, dCH).
Instruments
The 1H NMR spectra were run on a BRUKER-500 spectrometer
to determine the chemical structure and tetramethylsilane was
used as the internal standard. Mass spectra (MS) was performed
on an AXIMA-CFR laser desorption ionization flying time
spectrometer (COMPACT). FTIR measurements were recorded
on a BRUKER VECTOR 22 Spectrometer. The samples were
mixed with KBr and pressed into a plate for measurement at
25 ꢀC. The number-average molecular weight (Mn), weight-
average molecular weight (Mw), and molecular weight distribu-
tion 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 eluent at a flow rate of 1 mL minꢁ1.The sample was
prepared in DMF (high-performance, liquid-chromatogrphy-
grade). Before analysis, the sample was filtered through a 0.2-mm
poly(tetrafluoroethylene) syringe filter. Calibration was accom-
plished with monodispersed polystyrene standards. UV-vis
spectra were performed on UV-2501 PC Spectrometer (SHI-
MADZU) in DMF. The cyclic voltammetry (CV) was performed
with a CHI 660A Electrochemical Workstation (CH Instru-
ments, USA) in a conventional three-electrode cell by using thin
Synthesis of monomer 2
A solution of 2,6-difluorobenzoyl chloride (1.77 g, 1 mmol) in
50 mL of dichloromethane was added dropwise to a stirring
mixture of monomer 1 (1.97 g, 1 mmol) and triethylamine (3 mL)
in dichloromethane (100 mL) over a period of 20 min. The
reaction proceeded readily at room temperature with the
formation of a yellow solution. The reaction mixture was stirred
for 6 h, then it was poured into 200 mL distilled water to
precipitate the product, which was suction filtered and subse-
quently washed with pure water and diethyl ether. Then it was
purified by recrystallization with dichloromethane. After drying
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in vacuum at 50 ꢀC for 24 h, analytically product was obtained.
(yield 82%).
GPC: The number-average molecular weight (Mn) is 14 kDa,
weight-average molecular weight (Mw) is 29 kDa, and molecular
1
MALDI-TOF-MS: The calculated mass-charge ratio for
monomer 2 was 337.3. And the ion peak at the mass-charge ratio
336.8 was observed in the experiment. 1H NMR (500 MHz,
25 ꢀC, DMSO-d6, TMS, ppm): d ¼ 11.17(s, 1H, Hc), d ¼ 7.96 (d,
2H, He), d ¼ 7.93 (d, 2H, Hf), d ¼ 7.89 (d, 2H, Hd), d ¼ 7.97 (m,
4H, Ha+g+h), d ¼ 7.29 (t, 2H, Hb). FTIR (KBr pellet, cmꢁ1): 3444
(m, yNH), 3288 (m, yNH), 1664 (vs, yC]O), 1626 (s, yC]C of
benzenoid rings), 1597 (s, yC]C of benzenoid rings), 1527 (s,
yC]C of benzenoid rings), 1468 (m, yC]C of benzenoid rings),
1409 (m, yN]N), 1317 (m, yC–N), 840 (m, dCH), 790 (m, dCH), 684
(s, dCH).
weight distribution of the polymer is 2.07. H NMR (500 MHz,
25 ꢀC, DMSO-d6, TMS, ppm): d ¼ 10.97(s, –CO–NH–), d ¼ 7.95
(s, –NH), d ¼ 7.85–7.92 (m, Ar–H), d ¼ 7.53–7.57 (m, Ar–H), d ¼
7.02–7.21 (m, Ar–H), d ¼ 6.49–6.51 (m, Ar–H). FTIR (KBr
pellet, cmꢁ1): 3450 (s, yNH), 1654 (m, yC]O), 1594 (s, yC]C of
quinoid rings), 1498 (vs, yC]C of benzenoid rings), 1450 (s, yC]C
of benzenoid rings), 1404 (m, yN¼N), 1300 (m, yC–N), 1228 (m,
yC–O–C), 821 (m, dCH), 748 (w, dCH), 688 (m, dCH).
Reduction of the polymer
The as-synthesized polymer (0.5 g) was dispersed into a stirring
mixture solution (3 mL hydrazine hydrate in 40 mL of 1.0 M
ammonium hydroxide) and stirred for 15 h. The de-doped
polymer was reduced to the leucoemeraldine state. Then it was
filtered using a Buchner funnel and water aspirator, and the filter
was washed with distilled water several times, followed by drying
under dynamic vacuum at 40 ꢀC for 24 h.
Synthesis of monomer 3
A mixture of DMAc (20 mL), toluene (6 mL), anhydrous
potassium carbonate (0.7255 g, 5.25 mmol), p-hydroxyl-diphe-
nylamine (1.8522 g, 10 mmol), and monomer 2 (1.3493 g,
4 mmol) were added into 50 mL three-necked round-bottom
flask. Then it was heated to 130 ꢀC under nitrogen with stirring
for 2 h to remove the water by azeotropic distillation with
toluene. After removing the toluene, the mixture was heated up
to 150 ꢀC for 8 h to ensure completion of the reaction. Then the
solution was cooled to room temperature and poured into
distilled water to precipitation of the product. The crude product
filtered from the solution was washed with distilled water several
times, and was purified by recrystallization with dried dichloro-
methane. The purified product was dried under dynamic vacuum
Results and discussion
Synthesis and characterization
The polymer with phenyl-capped aniline tetramer in the main
chain and azo chromophores in the side chain was synthesized
through six steps as shown in Scheme 1. Monomer 1 was
obtained via aiazonium salt coupled reaction and translocation,
and monomer 2 was synthesized from the reaction between
monomer 1 and 2, 6-difluorobenzoyl chloride at room temper-
ature. Monomer 3 was obtained from conventional aromatic
nucleophilic reaction between p-hydroxy-diphenylamine and
monomer 2 in the presence of K2CO3, with DMAc as the solvent,
ꢀ
at 45 C for 24 h. (yield 83%).
MALDI-TOF-MS: The calculated mass-charge ratio for
monomer 2 was 667.7. And the ion peak at the mass-charge ratio
1
667.1 was observed in the experiment. H NMR (500 MHz, 25
ꢀ
under the protection of nitrogen, at 150 C, after removing the
ꢀC, DMSO-d6, TMS, ppm): d ¼ 10.96(s, 1H, Hi), d ¼ 8.12
(d, 2H, He), d ¼ 7.91 (m, 6H, Hj+k+l), d ¼ 7.57 (m, 3H, Ha+m), d ¼
7.31 (t, 1H, Hn), d ¼ 7.22 (t, 4H, Hg), d ¼ 7.08 (m, 12H, Hc+d+f),
d ¼ 6.80 (t, 2H, Hh), d ¼ 6.55 (d, 2H, Hb). FTIR (KBr pellet,
cmꢁ1): 3451 (s, yNH), 3301 (m, yNH), 1668 (m, yC]O), 1592 (s,
yC]C of benzenoid rings), 1504 (vs, yC]C of benzenoid rings),
1455 (m, yC]C of benzenoid rings), 1403 (w, yN]N), 1322 (m, yC–
N), 1234 (m, yC–O–C), 846 (w, dCH), 746 (m, dCH), 692 (m, dCH).
water from the reaction system by the azeotropic distillation of
Synthesis of the polymer
Monomer 3 (1.0016g, 1.5 mmol) and p-phenylenediamine
(0.1622 g, 1.5 mmol) were dissolved in a 20 mL mixture solution
(16 mL DMF, 2 mL distilled water, and 2 mL concentrated
hydrochloric acid). A solution of (NH4)2S2O8 (0.6848 g in 4 mL
1.0 M HCl aqueous solution) was added dropwise to the above
solution with stirring at room temperature over a period of 30
min. After the addition, the resulting solution reacted for another
12 h. Then it was poured into 50 mL distilled water to precipitate
the product. The product was washed with distilled water three
ꢀ
times, followed by drying under dynamic vacuum at 40 C for
24 h. Finally, the crude product was extracted with acetone using
a Soxhlet extractor for 20 h until the extracted solution became
colorless, and the purified powder was dried under vacuum at
40 ꢀC for 24 h. (yield 78%).
Scheme 1 Synthetic route of the monomers and polymer.
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toluene. The chemical structures of the monomers were
confirmed by FTIR, 1H NMR and MS. An characteristic
absorption bond of N]N at around 1415 cmꢁ1 was observed in
the FT-IR spectrum of monomer 1, and the characteristic
absorption bond of C]O and C–O–C were found in the FT-IR
spectrum of monomer 2 and monomer 3 respectively, which all
supported the proposed molecular structure of the monomers. In
the polymer chain.33–35 The subsequent weight loss is associated
with gradual decomposition of the backbone. From the TGA
curve, it can be seen that 5% weight loss temperature and 10%
weight loss temperature of the polymer were 320 ꢀC and 393 ꢀC,
respectively, which indicated that the as-synthesized polymer had
good thermal stability.
1
the H NMR spectrum of monomer 3, the signals at 10.96 and
Electrochemical activity
8.12 ppm could be assigned to amino protons (–CO–NH– and
–NH– respectively), signals around d ¼ 6.55–7.91 ppm allow
estimation of the other expected splitting of the aromatic proton
signals. In addition, the data from MS also verified the structure
of the monomers.
The electrochemical behavior of the polymer was examined by
cyclic voltammetry (CV) measurements which performed in
1.0 M H2SO4 aqueous solution using Ag/Ag+ as the reference
electrode with a scan rate of 50 mV sꢁ1 (Fig. 2). The DMF
solution of the polymer was cast on the glassy carbon working
electrode and was evaporated to form thin solid film. Under these
conditions, the CV of the polymer showed two pairs of redox
current peaks at 208 mV/362 mV and 538 mV/644 mV respec-
tively, revealing that the polymer exhibited an intrinsic electro-
activity similar to that of PANI20 and oligoaniline.36 We
proposed that the first redox peak at 208 mV/362 mV corre-
sponds to the transition from the leucoemeraldine state to the
emeraldine oxidation state, and the second redox peak at 538
mV/644 mV was attributed to the transition from the emeraldine
oxidation state to the pernigraniline oxidation state (Scheme 2).
The polymer was synthesized through the reaction of mono-
mer 3 and p-phenylenediamine by an oxidative coupling poly-
merization approach developed in our previous work,21,22 which
has been proved to be a convenient method to prepare a series of
novel electroactive polymers with oligoaniline groups in the
backbone under mild conditions. In the FT-IR spectrum, the
characteristic absorption bond around 3450 cmꢁ1 was attributed
to stretching vibrations of N–H. The signals at 1654 cmꢁ1 and
1594 cmꢁ1 corresponded to the stretching vibration of C]O and
quinone rings respectively. Besides, the stretching vibration of
N]N at 1404 cmꢁ1, aryl ether linkages at 1228 cmꢁ1 were also
observed. In the 1H NMR spectrum of the polymer, the signals at
10.97 ppm could be assigned to amino protons (–CO–NH–). The
protons of –NH– appear at d ¼ 7.95. The aromatic protons
appear during d ¼ 6.49 and 7.92 (multiplet). All of the 1H NMR
signals supported the proposed molecular structure of the poly-
mer. The polymer featured excellent solubility. It could be
completely soluble in DMF, DMSO and NMP solvents and even
almost completely soluble in THF. However, the film prepared
from the DMF solution of the polymer did not exhibited very
good ductility and was brittle, probably because of the rigidity of
oligoaniline segments. And the further research on the process-
ability of the polymer is ongoing.
The UV-visible spectra pre- and post doping with hydrochloric
acid
The UV-visible spectra are shown in Fig. 3. At the beginning, the
polymer in the emeraldine oxidation state without doping was
dissolved in DMF solution (Fig. 3 a), then one drop of 37 wt%
HCl was added to the above solutions to obtain the doped
polymer (Fig. 3 b). From the spectrum a, four major peaks were
observed, and among them, the peaks at about 300 and 583 nm
could be attributed to the transitions in the oligoaniline
segments, namely to the p–p* transition in the benzoid ring and
to the exciton-type transition between the HOMO orbital of the
benzoid ring and the LUMO orbital of the quinoid ring. From
the spectrum b, there are four absorption peaks were observed.
Thereinto, absorption peaks at about 295 nm, 435 nm and
600 nm, were ascribed to p–p* transition in the benzoid ring,
polaron-p* transition and p–polaron transition to reflect the
formation of conducting phase of the polymer. The spectra in
Thermal properties
The thermal properties of the polymer were investigated by TGA
under nitrogen atmosphere (Fig. 1). The polymer did not show
an obvious loss step. The initial weight loss of the polymer
started at around 155 ꢀC, which could be attributed to expulsion
of loosely bounded water, solvent and the bonded dopant from
Fig. 2 Cyclic voltammogram of the polymer modified carbon-glassy
electrode in 1.0 M H2SO4 supporting electrolyte at 50 mV sꢁ1 versus Ag/
AgCl reference electrode.
Fig. 1 TGA thermogram of the polymer under nitrogen atmosphere.
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Scheme 2 Molecular structures of the polymer at various oxidation
states: leucoemeraldine, emeraldine, and pernigraniline.
Fig. 4 UV-vis spectra changes during ultraviolet irradiation (a) and
visible light irradiation (b).
Dielectric properties
Measurements of dielectric response were carried out with the
polymer powder-pressed pellet at room temperature. As shown
in Fig. 5, dielectric constants and dielectric loss of the polymer in
leucoemeraldine form, emeraldine base form and HCl-doped
form were presented. And the polymer in HCl-doped form gave
an 30 value of 161.9 detected at the frequency of 100 Hz, then
decreased to 31.3, 19.5 and 14.3, as the frequency was increased
to 10 kHz, 100 kHz and 1 MHz respectively, possessing a much
higher dielectric constant than that of leucoemeraldine form and
emeraldine base form over the range of 100 Hz to 1 MHz. A
possible explanation to that was given below: By doping the
polymer with HCl, a great quantity of protons were introduced
to the oligoaniline segments to form polarons and bipolarons,
leading to a charge delocalization and an great enhancement of
the polarity of the main chains.39 Then, high interfacial polari-
zation was easy to initiate, resulting in large dielectric responses.
In contrast with doped polymers, carriers inside the pristine
electroactive polymer (in leucoemeraldine form or emeraldine
base form) occupied localized states and their movements in
response to the electric field were suppressed. Hence they were
incapable of providing enough space charge accumulated at the
interface of the polymer, and then the pristine electroactive
polymer possessed dielectric constant corresponding to that of
traditional polymers.40 It was noted that the dielectric constant
showed a frequency dependence and the dielectric constant
decreased significantly on the increase of frequency, resulting
from the AC-conductivity increasing with the frequency at
a higher frequency41 and the dipoles and charge carriers within
the polymer failing to keep up with increasing AC frequency.42
Fig. 5 (b) showed the frequency dependence of dielectric loss in
the experimental frequency range. The dielectric loss of the
Fig. 3 UV-vis spectra of (a) emeraldine base of the polymer and
(b) polymer doped with HCl.
turn showed the efficient doping of the polymer chains with HCl,
and the polymer exhibited the similar spectral properties to that
of PANI.37,38
Photoisomerization of the polymer
In order to study its trans-cis isomerization, the polymer in the
emeraldine oxidation state without doping was dissolved in
DMF solution and UV-vis spectra changes were monitored
during the ultraviolet irradiation. As shown in Fig. 4 (a), the
intensity of the absorption at about 335 nm, which was asso-
ciated with p–p* transition of the trans-azobenzene group,
gradually decreased as the time of irradiation increased. And
correspondingly, the absorption at 451 nm associated with n–p*
transition of the cis-azobenzene group increased gradually. The
trans-cis isomerization reached steady state 6 min later, then the
above solution was transferred to visible radiation, and UV-Vis
spectra changes were obtained. As shown in Fig. 4 (b), the
intensity of the absorption associated with p–p* transition of the
trans-azobenzene group, gradually increased and the absorption
associated with n–p* transition of the cis-azobenzene group
decreased. However, the intensity of the absorption at about 335
nm didn’t reach the original values, probably because of mild
ultraviolet photodegradation. From the result described above,
the obtained polymer retained the photoisomerization charac-
teristics of azobenzene.
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Fig. 5 Frequency dependence of (a) dielectric constant and (b) dielectric
loss of the polymer in leucoemeraldine form, emeraldine base form and
HCl-doped form, respectively.
Fig. 6 Frequency dependence of (a) dielectric constant and (b) dielectric
loss of the polymer under ultraviolet irradiation and visible light irradi-
ation in turn.
polymer in doped form was higher than that of pristine polymers,
which was attributed to the enhancement of the polarity of the
polymer chains. In a word, from the discussion, it could be
concluded that it was a feasible way to obtain high dielectric
constant polymers by synthesizing such kind of electroactive
polymers. Futhermore, influencing factors of dielectric property,
such as the structure of the polymer, the content of the oligoa-
niline segments, dopant types and doping levels, would be major
research issues in our future work.
excluded, as the temperature of the sample during the whole
process changed scarcely.
A preliminary explanation of those changes was given below:
first, the azo chromophores in the side chains changed from the
trans isomer to cis isomer with their dipole moments increased
under the UV irradiation, leading to a increase of the dielectric
constants of the polymer. Second, the trans-cis isomerization of
azo chromophores could change the side chains from rodlike to
globularity, which might influence the shapes of polymer chains,
accompanied by the changes of the dipole moments of the
polymer chains. Then the dielectric constants of the material
might vary to some extent. And an inverse process occurred
under the visible irradiation. What puzzled us was that the
dielectric loss in the whole process changed unregularly. Alto-
gether, the adjustment of dielectric constant was implemented
primarily utilizing the reversible photoisomerization of azo
chromophores.
The influence of photoisomerization derived from azo chro-
mophores on dielectric properties of the polymer was also
studied. As shown in Fig. 6, before exposure to UV irradiation,
the polymer powder-pressed pellet in HCl-doped form gave an
initial value of 161.9 detected at the frequency of 100 Hz, then
decreased to 31.3, 19.5 and 14.3, as the frequency was increased
to 10 kHz, 100 kHz and 1 MHz, respectively. Afterward, the
pellet was exposed to UV irradiation, and the dielectric constants
increased gradually as the time of irradiation increased. When
the irradiation time reached 10 min, the sample reached steady
state and came to a value of 354.7 detected at the frequency of
100 Hz, then decreased to 16.8, as the frequency was increased to
1 MHz, respectively. Subsequently, the pellet was exposed to
visible irradiation, and the dielectric constants decreased grad-
ually as the time of irradiation increased. The dielectric constants
approached but didn’t reach its initial value as the time of irra-
diation increased to about 25 min, probably because of the
occurrence of secondary reaction. Then, it was exposed to UV
and visible irradiation in turn again and a similar process was
observed, which indicated that the reversible change of the
photoisomerization resulted in a reversible change of the
dielectric constants. The influence of temperature could be
Conclusion
In summary, a novel functional alternating polymer with well-
defined oligoaniline in the main chain and azo group in the side
chain has been prepared by an oxidative coupling polymerization
approach. The polymer exhibited similar electroactive properties
to oligoaniline and possessed excellent solubility and good
thermal resistance. Dielectric constants and dielectric loss of the
polymer in leucoemeraldine form, emeraldine base form and
HCl-doped form were investigated and the results showed that
This journal is ª The Royal Society of Chemistry 2011
J. Mater. Chem., 2011, 21, 1852–1858 | 1857

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17 S. P. Surwade, S. R. Agnihotra, V. Dua, N. Manohar, S. Jain,
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a large enhancement in the dielectric constant was achieved in the
HCl-doped form. Furthermore, we obtained the polymer with
adjustable dielectic constants by introducing the azo groups into
the side chains, and a preliminary explanation was given.
21 D. M. Chao, X. F. Lu, J. Y. Chen, X. C. Liu, W. J. Zhang and Y. Wei,
Polymer, 2006, 47, 2643.
Acknowledgements
22 D. M. Chao, X. F. Lu, J. Y. Chen, X. G. Zhao, L. F. Wang,
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The work has been supported by National 973 Project (No.
2007CB936203 and S2009061009), NSF China (No. 50973038
and 50873045), and National 863 Project (No. 2007AA03Z324)
24 L. H. Huanga, J. Hua, L. Lang, X. Wang, P. B. Zhang, X. B. Jing,
X. H. Wang, X. S. Chen, P. I. Lelkesc, A. G. MacDiarmid and
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This journal is ª The Royal Society of Chemistry 2011