Enhanced redox stability and electrochromic properties of aromatic polyamides based on N,N-bis(4-carboxyphenyl)-N′,N′-bis(4-tert- butylphenyl)-1,4-phenylenediamine

Article abstract of DOI:10.1002/pola.24421

A new bis(triphenylamine)-type dicarboxylic acid monomer, N,N-bis(4-carboxyphenyl)-N′,N′-bis(4-tert-butylphenyl)-1, 4-phenylenediamine, was prepared by a well-established procedure and led to a new family of redox-active aromatic polyamides with di-tert-b

Full text of DOI:10.1002/pola.24421

Enhanced Redox Stability and Electrochromic Properties of
Aromatic Polyamides Based on N,N-Bis(4-carboxyphenyl)-
N 0,N 0-bis(4-tert-butylphenyl)-1,4-phenylenediamine
HUI-MIN WANG, SHENG-HUEI HSIAO
Department of Chemical Engineering and Biotechnology, National Taipei University of Technology, 1 Chunghsiao East Road,
3rd Section,10608 Taipei, Taiwan, Republic of China
Received 25 August 2010; accepted 26 September 2010
DOI: 10.1002/pola.24421
Published online 18 November 2010 in Wiley Online Library (wileyonlinelibrary.com).
ABSTRACT: A new bis(triphenylamine)-type dicarboxylic acid
monomer, N,N-bis(4-carboxyphenyl)-N 0,N 0-bis(4-tert-butyl-
phenyl)-1,4-phenylenediamine, was prepared by a well-estab-
lished procedure and led to a new family of redox-active
aromatic polyamides with di-tert-butyl-substituted N,N,N 0,N 0-
tetraphenylphenylenediamine (TPPA) segments. The resulting
polyamides were amorphous with good solubility in many or-
ganic solvents, and most of them could be solution cast into
flexible polymer films. The polyamides exhibited high thermal
stability with glass-transition temperatures in the range of 247–
293 ꢀC and 10% weight-loss temperatures in excess of 500 ꢀC.
They showed well-defined and reversible redox couples during
oxidative scanning, with a strong color change from a colorless
or pale yellowish neutral form to green and blue oxidized
forms. They had enhanced redox stability and electrochromic
performance when compared with the corresponding analogs
without tert-butyl substituents on the TPPA unit. The polyamide
with TPPA units in both the diacid and diamine components
shows multicolored electrochromic behavior. A polyamide con-
taining both the cathodic coloring anthraquinone chromophore
and the anodic coloring TPPA chromophore has the ability to
show red, green, and blue states, toward single-component
C
V
RGB electrochromics. 2010 Wiley Periodicals, Inc. J Polym Sci
Part A: Polym Chem 49: 337–351, 2011
KEYWORDS: cyclic voltammetry; electrochemistry; electrochro-
mic polymers; polyamides; redox polymers; spectroelectro-
chemistry; triphenylamine
INTRODUCTION Electrochromic materials comprise redox-
active species that exhibit significant, lasting, and reversible
changes in color upon reduction or oxidation.1 Research in
electrochromic devices has received a great deal of attention
because of the importance of this interesting phenomenon in
applications such as electrochromic windows,2 displays,3
memory devices,4 electronic papers,5 and adaptive camou-
flages.6 There is a huge amount of chemical species that
exhibited electrochromic properties,7 including metal coordi-
nation complexes,8 metal oxide (especially tungsten oxide),9
viologens (4,40-bipyridium salts),10 and conducting polymers
(such as polyanilines, polypyrroles, and polythiophenes).11
The use of conjugated polymers as active layers in electro-
chromic devices became popular because of the outstanding
electrochromic properties such as fast switching time, ease
of synthesis, and wide range of colors, which can be
achieved by simple chemical synthesis.12 For efficient opera-
tion of an electrochromic device, it is necessary to take a
number of properties into consideration: electrochromic effi-
ciency, optical contrast, response time, stability, and durabili-
ty.11(b) The difficulty in achieving satisfactory values for all
these parameters at the same time stimulates the develop-
ment of new methods of preparation of electrochromic films,
new materials, and components for the devices.13
Aromatic polyamides are considered to be high-performance
polymeric materials because of their excellent physical and
chemical properties, thermal and oxidative stability, flame re-
sistance, and superior mechanical properties.14 A major
problem with aromatic polyamides (particularly those pre-
pared with p-phenylene units) is their low solubility, high
thermal transition temperatures, and resultant difficulty in
fabrication. One approach for solving this has been the incor-
poration of bulky groups in the main chain or as the pendent
groups.15 On the other hand, there is continuously a vast
effort directed toward expanding the application of the aro-
matic polyamides as optically active materials, luminescent
and electroactive films, gas or ion-exchange membranes, and
so forth by incorporating new chemical functionalities in the
polyamide backbone or lateral structure.16
Triarylamine derivatives are well known for photo- and elec-
troactive properties that find optoelectronic applications as
photoconductors, hole transporters, and light emitters.17
Polymers bearing triarylamine units are receiving
Additional Supporting Information may be found in the online version of this article. Correspondence to: S.-H. Hsiao (E-mail: shhsiao@ntut.edu.tw)
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Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 49, 337–351 (2011) 2010 Wiley Periodicals, Inc.
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
considerable interest as ideal hole-transporting materials in
various optoelectronic device applications such as organic
light-emitting diodes because of the strong electron-donating
and hole-transporting/injecting properties of triarylamine
units.18 Triarylamines can be easily oxidized to form stable
radical cations, and the oxidation process is always associ-
ated with a noticeable change of coloration. Thus, many tri-
arylamine-based electrochromic polymers have been
reported in literature.19 In recent years, Liou and coworkers
have developed a number of high-performance polymers
such as aromatic polyamides and polyimides carrying the tri-
arylamine unit as an electrochromic functional moiety.20 Our
strategy was to synthesize the triarylamine-containing mono-
mers such as diamines and dicarboxylic acids that were then
reacted with the corresponding comonomers through con-
ventional polycondensation techniques. The obtained poly-
mers possessed a well-defined structure, characteristically
high molecular weights, and high thermal stability. Because
of the incorporation of packing-disruptive, three-dimensional
triarylamine units along the polymer backbone, most of
these polymers exhibited good solubility in polar organic sol-
vents. They may form uniform, transparent amorphous thin
films by solution casting and spin-coating methods. This is
advantageous for their ready fabrication of large-area, thin-
film devices.
quinone. This polyamide showed interesting ambipolar and
multicolored electrochromism because it contains two types
of electroactive groups, a p-type dopable TPPA unit and an
n-type dopable anthraquinone unit.
EXPERIMENTAL
Materials
According to the reported procedure,27 N,N-bis(4-carboxy-
phenyl)-N0,N0-diphenyl-1,4-phenylenediamine (20) (mp: 316–
ꢀ
318 C) was synthesized by the fluoro-displacement reaction
of p-fluorobenzonitrile with the sodium amide of 4-aminotri-
phenylamine formed in situ by treatment with sodium
hydride in dimethyl sulfoxide (DMSO), followed by the alka-
line hydrolysis of the intermediate dinitrile compound. As
described previously,21(e) 4-amino-40,400-di-tert-butyltriphe-
nylamine (mp ¼ 139–141 ꢀC) was prepared by the fluoro-
displacement reaction of p-fluoronitrobenzene with the so-
dium amide of bis(4-tert-butylphenyl)amine formed in situ
by treatment with sodium hydride in DMSO, followed by hy-
drazine Pd/C-catalyzed reduction in refluxing ethanol. DMSO
and N-methyl-2-pyrrolidone (NMP) were dried over calcium
hydride for 24 h, distilled under reduced pressure, and
stored over 4-Å molecular sieves in a sealed bottle. Commer-
cially available aromatic diamines such as p-phenylenedi-
amine (3a) (TCI), benzidine (3b) (Wako), 4,40-diaminodo-
phenyl ether (3c) (TCI), 9,9-bis(4-aminophenyl)fluorene (3e)
(TCI), and 2,6-diaminoanthraquinone (3f) (TCI) were used
as received. 2-Triflouromethyl-4,40-diaminodiphenyl ether
(3d) (mp: 112–113 ꢀC) was synthesized by the chloro-dis-
placement of 2-chloro-5-nitrobenzotrifluoride with 4-nitro-
phenol in the presence of potassium carbonate, followed by
the hydrazine Pd/C-catalytic reduction of the intermediate
dinitro compound.28 N,N-Bis(4-aminophenyl)-N0,N0-bis(4-tert-
butylphenyl)-1,4-phenylenediamine (3g) (mp: 211–213 ꢀC)
was synthesized from readily available reagents according to
a reported synthetic procedure.21(e) Calcium chloride was
Recent studies have demonstrated that aromatic polyamides
containing N,N,N0,N0-tetraphenylphenylenediamine (TPPA)
segments show attractive electrochromic properties such as
high coloration efficiency, high optical contrast, fast response
time, long-term cycling stability, and multicolored electro-
chromism.21 The radical cations of these TPPA-bearing polya-
mides generally showed rather strong bands in the near
infrared (NIR) region because of a mixed-valence-type transi-
tion (charge transfer transition between pairs of chromo-
phores in different oxidation states22). Therefore, they can
be potentially useful as NIR electrochromic materials. In a
continuation of our interest in developing electrochromic
high-performance polymers having the TPPA moiety, herein
we synthesize a new di-tert-butyl-substituted TPPA-contain-
ing dicarboxylic acid, N,N-bis(4-carboxyphenyl)-N0,N0-bis(4-
tert-butylphenyl)-1,4-phenylenediamine (2), and its derived
polyamides. With such a configuration, the electrochemically
active sites of the pendent phenyl groups of the TPPA seg-
ments are blocked, giving the polyamides extra redox stabil-
ity upon oxidation. As a result, the resultant polyamides are
expected to exhibit an enhanced electrochemical and electro-
chromic stability, which is of great importance in practical
applications. On the other hand, anthraquinone is an n-type
electron-accepting unit that can be chemically or electro-
chemically reduced to the radical anion and dianion
states.23,24 Anthraquinone derivatives and polymers have
been used as n-type semiconductors and electron acceptors
in fundamental studies of NIR electrochromics,24 molecular
wires or redox-controlled switches,25 and cathode materials
for rechargeable lithium batteries.26 Therefore, one of the
polyamides reported in this work was prepared from the
newly obtained diacid monomer (2) and 2,6-diaminoanthra-
ꢀ
dried under vacuum at 200 C for 3 h before use. Tetrabuty-
lammonium perchlorate, TBAClO4, was dried in vacuo before
use. All other reagents were used as received from commer-
cial sources.
Monomer Synthesis
N,N-Bis(4-cyanophenyl)-N0,N0-bis(4-tert-butylphenyl)-1,4-
phenylenediamine (1)
A mixture of 18.6 g (0.05 mol) of 4-amino-40,400-di-tert-butyl-
triphenylamine, 12.1 g (0.1 mol) of p-fluorobenzonitrile, and
15.1 g (0.1 mol) of cesium fluoride (CsF) in 50 mL of DMSO
was heated with stirring at 150 ꢀC for 18 h under nitrogen.
After cooling, the mixture was poured into 300 mL of metha-
nol, and the precipitate was collected by filtration and washed
thoroughly with hot water. Recrystallization from DMF/meth-
anol yielded 11.2 g of the desired dinitrile compound 1 as yel-
lowish needles in 40% yield; mp ¼ 251–253 ꢀC.
IR (KBr): 2221 cmꢁ1 (CBN stretching). 1H NMR (500 MHz,
DMSO-d6, d, ppm): 1.27 (s, 18H, t-butyl), 6.94 (d, J ¼ 8.8 Hz,
2H, Hc), 7.01 (d, J ¼ 8.6 Hz, 4H, Hb), 7.07 (d, J ¼ 8.8 Hz, 2H,
Hd), 7.13 (d, J ¼ 8.7 Hz, 4H, He), 7.35 (d, J ¼ 8.6 Hz, 4H, Ha),
7.73 (d, J ¼ 8.7 Hz, 4H, Hf). 13C NMR (125 MHz, DMSO-d6, d,
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pH value was adjusted by dilute hydrochloric acid to near 3.
The crude precipitate was washed thoroughly with water
and recrystallized from acetic acid and dried to give 1.8 g of
yellowish dicarboxylic acid monomer 2 in 60% yield; mp ¼
ꢀ
321–323 C.
IR (KBr): 2700–3400 (OAH stretching), 1674 cmꢁ1 (C ¼ O
stretching). 1H NMR (500 MHz, DMSO-d6, d, ppm) [for the
peak assignments, see Fig. 1(a)]: 1.26 (s, 18H, t-butyl), 6.91
(d, J ¼ 8.8 Hz, 2H, Hc), 6.98 (d, J ¼ 8.6 Hz, 4H, Hb), 7.01 (d,
J ¼ 8.8 Hz, 2H, Hd), 7.06 (d, J ¼ 8.7 Hz, 4H, He), 7.30 (d, J ¼
8.6 Hz, 4H, Ha), 7.85 (d, J ¼ 8.7 Hz, 4H, Hf), 12.58 (br, 2H,
ACOOH). 13C NMR (125 MHz, DMSO-d6, d, ppm) [for the
peak assignments, see Fig. 1(b)]: 31.1 (C1), 33.9 (C2), 121.5
(C12), 123.2 (C8), 123.7 (C5), 124.2 (C14), 126.2 (C4), 127.8
(C9), 130.9 (C13), 138.8 (C7), 144.3 (C3), 145.2 (C10), 145.5
(C6), 150.3 (C11), 166.7 (ACOOH). Crystal data: yellowish
crystal grown during the slow crystallization in methanol,
0.27 mm ꢃ 0.18 mm ꢃ 0.06 mm, monoclinic C2/c with a ¼
26.0783(7), b ¼ 13.3857(4), c ¼ 9.8273(3) Å; a ¼ 90ꢀ, b ¼
98.845(3)ꢀ, c ¼ 90ꢀ, where density of crystal Dc ¼ 1.201
Mg/m3 for Z ¼ 4 and V ¼ 3389.68(17) Å3. The molecular
structure of diacid 2 solved by single-crystal X-ray analysis is
shown in Figure 2.
Synthesis of Polyamides
The synthesis of polyamide 4b is used as an example to
illustrate the general synthetic route. A mixture of 0.3677 g
(0.6 mmol) of the dicarboxylic acid monomer 2, 0.1105 g
(0.6 mmol) of benzidine (3b), 0.15 g of calcium chloride, 0.6
mL of triphenyl phosphite (TPP), 0.2 mL of pyridine, and 2.0
mL of NMP was heated with stirring at 120 ꢀC for 3 h. The
highly viscous polymer solution was poured slowly into 200
FIGURE 1 (a) 1H NMR spectrum and (b) 13C NMR spectrum of
diacid monomer 2 in DMSO-d6.
ppm): 31.1 (C1), 34.0 (C2), 104.1 (C14), 119.0 (ACN), 122.2
(C12), 122.8 (C8), 124.2 (C5), 126.4 (C4), 128.4 (C9), 133.7
(C13), 137.4 (C10), 143.2 (C3), 145.9 (C6), 146.1 (C7), 149.9
(C11).
N,N-Bis(4-carboxyphenyl)-N0,N0-bis
(4-tert-butylphenyl)-1,4-phenylenediamine (2)
A mixture of 3.2 g of potassium hydroxide and 2.8 g of the
obtained dinitrile compound 1 in 60 mL of ethanol and 20
mL of distillated water was stirred at ꢂ100 ꢀC until no fur-
ther ammonia was generated. The time taken to reach this
stage was about 4–5 days. The solution was cooled, and the
FIGURE 2 Molecular structure of dicarboxylic acid 2 by single-
crystal X-ray analysis.
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
mL of stirring methanol giving rise to a stringy, fiber-like
precipitate that was collected by filtration, washed thor-
oughly with ꢀhot water and methanol, and dried under vac-
uum at 100 C. Reprecipitations from N,N-dimethylacetamide
(DMAc) into methanol were carried out twice for further pu-
rification. The inherent viscosity of the obtained polyamide
4b was 0.80 dL/g, measured at a concentration of 0.5 g/dL
an Agilent 8453 UV–visible spectrometer. Electrochemistry
was performed with a CHI 750A electrochemical analyzer.
Voltammograms are presented with the positive potential
pointing to the left and with increasing anodic currents
pointing downward. Cyclic voltammetry (CV) was conducted
with the use of a three-electrode cell in which ITO (polymer
films area about 0.8 cm ꢃ 1.25 cm) was used as a working
electrode. A platinum wire was used as an auxiliary elec-
trode. All cell potentials were taken with the use of a home-
made Ag/AgCl, KCl (sat.) reference electrode. Ferrocene was
used as an external reference for calibration (þ0.48 V vs.
Ag/AgCl). Spectroelectrochemistry analyses were carried out
with an electrolytic cell, which was composed of a 1 cm cuv-
ette, ITO as a working electrode, a platinum wire as an auxil-
iary electrode, and a Ag/AgCl as a reference electrode.
Absorption spectra in the spectroelectrochemical experi-
ments were also measured with an Agilent 8453 UV–visible
spectrophotometer. Photoluminescence (PL) spectra were
measured with a Varian Cary Eclipse fluorescence spectro-
photometer. Fluorescence quantum yields (UF) values of the
samples in NMP were measured by using quinine sulfate in
1 N H2SO4 as a reference standard (UF ¼ 54.6%).29
ꢀ
in DMAc–5 wt % LiCl at 30 C.
The IR spectrum of 4b (film) exhibited characteristic amide
absorption bands at 3313 cmꢁ1 (NAH str.) and 1662 cmꢁ1
(amide carbonyl str.). 1H NMR (500 MHz, DMSO-d6, d, ppm)
(for the peak assignments, see Fig. 3): 1.28 (s, 18H, Hh), 7.00
(d, J ¼ 8.9 Hz, 2H, Hc), 7.01 (d, J ¼ 8.5 Hz, 4H, Hb), 7.09 (d,
J ¼ 8.9 Hz, 2H, Hd), 7.16 (d, J ¼ 8.2 Hz, 4H, He), 7.36 (d, J ¼
8.5 Hz, 4H, Ha), 7.68 (d, J ¼ 8.6 Hz, 4H, Hg), 7.87 (d, J ¼ 8.2
Hz, 4H, Hh), 7.95 (d, J ¼ 8.2 Hz, 4H, Hf), 10.21 (s, 2H,
amide).
Preparation of the Polyamide Films
A solution of polymer was made by dissolving about 0.6 g of
the polyamide sample in 8 mL of DMAc. The homogeneous
solution was poured into a 7-cm glass Petri dish, which was
ꢀ
placed in a 90 C oven overnight to remove most of the sol-
vent. The cast film was then released from ꢀthe glass sub-
strate and was further dried in vacuo at 160 C for 8 h. The
obtained films were about 50–60 lm in thickness and were
used for X-ray diffraction measurements, solubility tests, and
thermal analyses.
RESULTS AND DISCUSSION
Monomer Synthesis
The new aromatic dicarboxylic acid monomer 2 was success-
fully synthesized by the CsF-assisted N,N-diarylation of 4-
amino-40,400-di-tert-butyltriphenylamine with p-fluorobenzo-
nitrile, followed by alkaline hydrolysis of the intermediate
dicyano compound 1 (Scheme 1). IR and 1H and 13C NMR
spectroscopic techniques were used to identify the struc-
tures of the synthesized compounds 1 and 2. The transfor-
mation of cyano to carboxylic acid functionality could be
readily confirmed by the change of IR spectra. As shown in
the Supporting Information Figure S1, the cyano groups of
compound 1 gave a characteristic band at 2221 cmꢁ1. After
hydrolysis, the characteristic absorption of the cyano group
disappeared, and the carboxylic acid group showed a typical
carbonyl absorption band at 1693 cmꢁ1 (C¼¼O stretching) to-
gether with the appearance of broad bands in the range of
2700–3400 cmꢁ1 (OAH stretching). The 1H NMR and 13C
NMR spectra included in the Supporting Information Figures
S2 and S3 confirm the correct structure of the intermediate
compound 1. Figure 1 illustrates the 1H NMR and 13C NMR
spectra of the target dicarboxylic acid monomer 2. Assign-
ments of each carbon and proton are assisted by the two-
dimensional NMR spectra shown in the Supporting Informa-
tion Figure S4, and these spectra agree well with the pro-
posed molecular structure of 2. The 13C NMR spectra also
confirm that the cyano groups were completely converted
into the carboxylic acid groups by the disappearance of the
resonance peak for the cyano carbon at 119.0 ppm and the
appearance of the carbonyl peak at 166.7 ppm. Other impor-
tant evidence of this change is the shifting of the carbon res-
onance signals of C14 adjacent to the cyano or carboxyl
group. The C14 of dinitrile compound 1 resonated at a higher
field (104.1 ppm) than the other aromatic carbons because
Measurements
Infrared (IR) spectra were recorded on a Horiba FT-720
FTIR spectrometer. Elemental analyses were run in a Her-
aeus VarioEL III CHNS elemental analyzer. 1H and 13C NMR
spectra were measured on a Bruker Avance 500 FT-NMR
system with tetramethylsilane as an internal standard. X-ray
single-crystal diffraction experiment of diacid compound 2
was carried out on a Norius Kappa CCD four-circle diffrac-
tometer equipped with graphite-monochromated Mo-Ka radi-
ation. The structure was solved by direct methods using
SHELXL-97 software. The inherent viscosities were deter-
ꢀ
mined with a Cannon-Fenske viscometer at 30 C. Weight-av-
erage molecular weights (Mw) and number-average molecu-
lar weights (Mn) of the polyamides were obtained via gel
permeation chromatography on the basis of polystyrene cali-
bration using Waters 2410 as an apparatus and tetrahydro-
furan (THF) as the eluent. Wide-angle X-ray diffraction
(WAXD) measurements were performed at room tempera-
ꢀ
ture (ca. 25 C) on a Shimadzu XRD-6000 X-ray diffractome-
ter with a graphite monochromator (operating at 40 kV and
30 mA), using nickel-filtered Cu-Ka radiation (k ¼ 1.5418 Å).
The scanning rate was 2ꢀ/min over a range of 2h ¼ 10ꢀ–40ꢀ.
Thermogravimetric analysis (TGA) was performed with a
Perkin-Elmer Pyris 1 TGA. Experiments were carried out on
ꢂ4–6 mg of samples heated in flowing nitrogen or air (flow
rate ¼ 40 cm3/min) at a heating rate of 20 ꢀC/min. DSC
analyses were performed on a Perkin-Elmer Pyris 1 DSC at a
scan rate of 20 ꢀC/min in flowing nitrogen. Ultraviolet–visi-
ble (UV–vis) spectra of the polymer films were recorded on
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ARTICLE
packing of polymer chains and enhance the solubility of
formed polyamides.
Polymer Synthesis
According to the phosphorylation polyamidation technique
described by Yamazaki et al.,30 a series of novel polyamides
(4a–4g) with di-tert-butyl-substituted TPPA units were syn-
thesized from the dicarboxylic acid 2 and various diamines
3a–3g via solution polycondensation using TPP and pyridine
as condensing agents (Scheme 2). All the polymerizations
proceeded homogeneously throughout the reaction and
afforded clear and highly viscous polymer solutions, which
precipitated in a tough, fiber-like form when the resulting
polymer solutions were slowly poured into stirring methanol.
As shown in the Supporting Information Table S1, the
obtained polyamides had inherent viscosities in the range of
0.33–0.80 dL/g, and most of them could be solution cast into
flexible and tough films, indicating high-molecular-weight
polymers. The less favorable result obtained from diamine 3f
can be explained by its less basicity. The weight-average mo-
lecular weights (Mw) of the THF-soluble polyamide 4d was
36,000 with polydispersity index (Mw/Mn) of 2.06. The for-
mation of polyamides was also confirmed by IR and NMR
spectroscopy. A typical IR spectrum for polyamide 4b is given
in the Supporting Information Figure S5. The characteristic
IR absorption bands of the amide group appeared around
3300 (NAH stretching) and 1650 cmꢁ1 (amide carbonyl).
SCHEME 1 Synthesis of dicarboxylic acid monomer 2.
of the anisotropic shielding by the p electrons of CBN. After
hydrolysis, the resonance peak of C14 shifted to a lower field
(124.20 ppm) because of the lack of an anisotropic field. The
signals appeared at 1.26 ppm in the 1H NMR spectrum and
33.9 ppm (a quaternary carbon) and 31.1 ppm (a methyl
carbon) in the 13C NMR spectrum are assigned to the tert-
butyl groups. The spectroscopic data convincingly prove the
successful synthesis of the goal dicarboxylic acid monomer.
The molecular structure of 2 was also confirmed by X-ray
crystal analysis acquired from the single crystal obtained by
slow crystallization of a methanol solution. As shown in Fig-
ure 2, compound 2 displays a propeller-shaped configuration
of the triphenylamine cores, and the benzene rings are not
in the same plane. This conformation will hinder the close
1
Figure 3 shows the H NMR and H-H COSY spectra of polyam-
ide 4b in DMSO-d6. All the resonance peaks could be readily
assigned to the hydrogen atoms in the repeating unit. The
resonance signal appearing at 10.21 ppm in the 1H NMR
spectrum also supports the formation of amide linkages.
SCHEME 2 Synthesis of polyamides
4a–4g.
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
formed an opaque film via solution casting from NMP and
showed less solubility. Except for 4f, the other polymers
could afford flexible and tough films. The WAXD patterns of
the polyamides shown in the Supporting Information Figure
S6 indicate that these polymers are essentially amorphous.
Thermal Properties
The thermal properties of the polyamides were investigated
by TGA and DSC techniques. A typical set of TGA and DSC
curves of the representative polyamide 4g are shown in Fig-
ure 4, and the thermal behavior data of all polyamides are
also summarized in the Supporting Information Table S1.
These polyamides show a high thermal stability; there is no
obvious weight loss before the scanning temperature reaches
ꢀ
to 450 C in nitrogen or in air. Their decomposition tempera-
tures (Td) at a 10% weight loss in nitrogen and air were
ꢀ
recorded at 453–540 and 439–560 C, respectively. The car-
bonized residue (char yield) of the polymers was more than
57% at 800 ꢀC in nitrogen atmosphere, ascribed to their
high aromatic content. The glass-transition temperatures (Tg)
of all the aromatic polyamides were observed in the range of
ꢀ
247–293 C by DSC. The relatively lower Tg and thermal sta-
bility of polyamide 4f may be attributable to its lower molec-
ular weights. All the polyamide samples exhibit no crystalli-
zation or melting transition in DSC measurements,
supporting the amorphous nature of these polyamides. The
thermal analysis results revealed that these polyamides
exhibited excellent thermal stability, which in turn is benefi-
cial to increase the service time in device application and
enhance the morphological stability to the spin-coated film.
Absorption and Photoluminescence
The optical properties of the polyamides were investigated
by UV–vis and PL spectroscopy. The relative data are sum-
marized in the Supporting Information Table S2. These poly-
mers exhibited strong absorption bands at kmax ¼ 351–361
nm in dilute NMP solution, which are mainly assignable to
the combinations of p–p* and n–p* transitions resulting
FIGURE 3 (a) 1H NMR spectrum and (b) aromatic portion of the
H-H COSY spectrum of polyamide 4b in DMSO-d6. [Color figure
can be viewed in the online issue, which is available at
wileyonlinelibrary.com.]
Properties of the Polyamides
Solubility and Film Morphology
The solubility behavior of polyamides was tested qualita-
tively, and the results are also summarized in Supporting In-
formation Table S1. Most of these polymers exhibited good
solubility in highly polar organic solvents because of the
introduction of the three-dimensional TPPA moiety and
bulky pendent tert-butyl substituents into the repeat unit.
Thus, the excellent solubility makes these polymers potential
candidates for practical applications by spin-coating or ink-
jet-printing processes to afford high-performance thin films
for optoelectronic devices. Among these polyamides, polyam-
ide 4d with the CF3 substituents showed the best solubility.
The result can be attributed to the bulkiness of pendent CF3
groups, which increased the disorder in the chains and hin-
dered dense chain packing, thus reducing the interchain
interactions. It is worth noting that polyamides 4a and 4c
FIGURE 4 (a) TGA and (b) DSC curves of polyamide 4g with a
heating rate of 20 ꢀC/min. [Color figure can be viewed in the
online issue, which is available at wileyonlinelibrary.com.]
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ARTICLE
FIGURE 5 (a) Cyclic voltammograms of polyamide 4b film on the ITO-coated glass substrate in 0.1 M TBAClO4/CH3CN at a scan
rate of 50 mV/s. (b) Cyclic voltammograms of polyamide 4b films at various potential scan rates. (c) Plots of the current densities
versus the scan rates for polyamide 4b films. [Color figure can be viewed in the online issue, which is available at
wileyonlinelibrary.com.]
from the conjugated TPPA units. In the solid state, the polya-
mides showed similar absorption profiles as those measured
in solution. Supporting Information Figure S7 shows the so-
lution UV–vis absorption and PL spectra of polyamides 4b,
4d, 4e, and 4g in NMP (concentration: 1 ꢃ 10ꢁ5 mol/L).
Their PL spectra in dilute NMP solution showed maximum
bands around 515–521 nm in the green region with quan-
tum yields ranging from 0.15 to 1.08%. Although these poly-
amides exhibited a rather low UF, their solution and solid
film showed an obvious green PL when irradiated by a
standard laboratory UV lamp (see the photos shown on the
top of Supporting Information Fig. S7). In the solid-state PL
spectra, the polyamides exhibited fluorescence emission at
498–515 nm. The solid-state emission spectra were similar
to those recorded from the NMP solution. The polymer films
were measured for optical transparency using UV–vis spec-
troscopy, and the cutoff wavelengths (absorption edge; k0)
were recorded in the range of 409–429 nm (Supporting In-
formation Fig. S8). In general, these polyamide films revealed
a light color and high optical transparency.
Electrochemical Properties
The electrochemical behavior of the polyamides was investi-
gated by CV conducted for the cast film on the ITO-coated
ELECTROCHROMIC PROPERTIES OF AROMATIC POLYAMIDES, WANG AND HSIAO
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
potentials (E1/2) of 0.69 V (Eonset ¼ 0.58 V) and 1.07 V for
polyamide 4b in the oxidative scan. The polyamide 4b films
were found to be very stable to p-type doping, with almost
no change in its CV diagram after 50 repeated cyclic scans
between 0.0 and 1.3 V. The two oxidation waves of 4b are
separated by about 360 mV, emphasizing the strong elec-
tronic coupling between the two redox amino centers of the
TPPA unit through the p-phenylene linker. The color of the
film changed from colorless to green and then blue as a
result of electrooxidation of the TPPA moiety. Figure 5(b,c)
displays the cyclic voltammograms and variations of the cur-
rent densities of polyamide 4b films upon an increase of the
scan rate up to 300 mV/s. A linear relationship existed
between the peak current and the scan rate, indicating that
the electroactive polymer film has good conductivity and
strong adherence to the ITO electrode.
The first cyclic voltammograms for polyamides 4d (with the
tert-butyl substituents on the TPPA unit) and 40d (without
the tert-butyl substituents) are illustrated in Figure 6. We
found that that polyamide 4d is more easily being oxidized
than its analogous polyamide 40d (Eonset 0.60 vs. 0.65 V).
Polyamide 4d also displays a more stable electrochemical
stability than 40d for a long-term scanning between 0 and
1.3 V, because the active sites of the TPPA unit in the former
are blocked with tert-butyl groups. Thus, the introduction of
bulky and electron-donating tert-butyl group not only greatly
prevents the coupling reaction but also slightly lowers the
oxidation potentials of the present polyamides. Polyamides
4e and 4f show a strong resemblance in the anodic CV dia-
grams as those of 4b and 4d. The oxidation potentials of
these four polyamides are compiled in Table 1.
FIGURE 6 Cyclic voltammograms of the cast films of poly-
amides 4d and 40d on the ITO-coated glass slide in 0.1 M TBA-
ClO4/CH3CN at
a scan rate of 50 mV/s. [Color figure can
be viewed in the online issue, which is available at
wileyonlinelibrary.com.]
glass substrate as the working electrode in dry acetonitrile
(CH3CN) (for the anodic scan) or DMF (for the cathodic
scan) containing 0.1 M of TBAClO4 as an electrolyte and sat-
urated Ag/AgCl as the reference electrode under nitrogen
atmosphere. The typical cyclic voltammograms for the repre-
sentative polyamide 4b are illustrated in Figure 5(a). There
are two reversible oxidation redox couples at half-wave
TABLE 1 Redox Potentials and Energy Levels of Polyamides
Oxidation Potential (V)a
HOMOc (eV)
LUMOd (eV)
Polymer
Code
Eg
(eV)b
ox1
ox2
1=2
ox3
1=2
ox4
1=2
Eonset
E
E
E
E
Eonset
E1/2
Eonset
E1/2
1=2
4b
4d
40d
4e
0.58
0.60
0.65
0.61
0.65
0.45
0.42
0.69
0.72
0.76
0.70
0.76
0.57
0.59
1.07
1.08
1.13
1.08
1.12
0.76
0.97
–
–
–
–
–
–
–
–
–
–
2.95
2.89
2.92
2.99
2.74
2.87
2.72
4.94
4.96
5.01
4.97
5.01
4.81
4.78
5.05
5.08
5.12
5.06
5.12
4.93
4.95
1.99
2.07
2.09
1.98
2.27
1.94
1.86
2.10
2.19
2.20
2.07
2.38
2.06
1.99
4f
4g
400be
0.93
–
1.08
–
a
From cyclic voltammograms versus Ag/AgCl in CH3CN.
b
c
Energy gap ¼ 1240/Abs konset of the polymer film.
The HOMO energy levels were calculated from E1/2 or Eonset and were referenced to ferrocene (4.8 eV;
onset ¼ 0.36 V).
d
LUMO ¼ HOMO ꢁ Eg.
e
Data from ref. 21(e).
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Spectroelectrochemical and Electrochromic Properties
Spectroelectrochemical measurements were performed on
films of polymers drop coated onto ITO-coated glass slides
immerged in electrolyte solution. The electrode preparations
and solution conditions were identical to those used in the
CV experiments. Figure 8(a) presents the UV–vis–NIR absorp-
tion spectra of polyamide 4d film at various applied poten-
tials. In the neutral form, polyamide 4d exhibited strong
absorptions at 309 and 351 nm, characteristic for p–p* tran-
sitions, but it was almost transparent in the visible and NIR
regions. The band gap of polymer 4d was calculated as 2.89
eV from the onset of the p–p* transition at 429 nm. Upon
electrooxidation of the 4d film (increasing applied voltage
from 0 to 0.9 V), the absorption of p–p* transition at 351 nm
gradually decreased, whereas a new absorption peak at 421
nm and a broadband having its maximum around 929 nm in
the NIR region grew up. We attribute this spectral change to
the formation of a stable monocation radical from the TPPA
moiety. The absorption band in the NIR region is assigned to
an intervalence charge-transfer (IV-CT) between states in
which the positive charge is centered at different amino cen-
ters.22 Upon further oxidation at applied voltages to 1.25 V,
the intensity of the IV-CT band gradually decreased, with a
formation of a new strong absorption band centered at about
636 nm. The observed electronic absorption changes in the
film of 4d at various potentials are fully reversible and are
associated with strong color changes; indeed, they even can
be seen readily by the naked eye. From the photos shown in
Figure 8(b) inset, it can be seen that the film changed from a
transmissive neutral state (nearly colorless) to a highly
absorbing semioxidized state (yellowish green) and a fully
oxidized state (blue). Dynamic changes in % transmittance of
421, 636, and 929 nm at varying applied potentials are illus-
trated in Figure 8(b). The electrochromic polymer 4d shows
a high optical contrast both in the visible and NIR regions
with a transmittance change (D%T) of 45% at 421 nm and
52% at 929 nm for yellowish green coloring at electrode
potential of 0.9 V and 83% at 636 nm for blue coloring at
electrode potential of 1.25 V. Spectral changes for the refer-
enced polyamide 40d upon electrooxidation are shown in the
Supporting Information Figure S9.
FIGURE 7 Cyclic voltammograms of the cast films of polya-
mides 4g, 4b, and 400b on the ITO-coated glass substrate in
0.1 M TBAClO4/CH3CN solution. [Color figure can be viewed in
the online issue, which is available at wileyonlinelibrary.com.]
Figure 7 compares the cyclic voltammograms of polyamide
4g and structurally related polyamides 4b and 400b. Polyam-
ide 4g exhibits four redox-active amino centers in each repeat
unit and shows four corresponding quasi-reversible anodic
redox couples. The E1/2 values of these four oxidation proc-
esses are estimated to be 0.57, 0.76, 0.93, and 1.08 V, respec-
tively. Polyamides 4b and 400b, which have isomeric repeating
unit with the TPPA unit in different segments, each revealed
two well-resolved, reversible redox couples in their CV dia-
grams, corresponding to two sequential electron removals
from the electroactive TPPA unit in their polymer backbones.
The first and second oxidation potentials of polyamide 400b
are lower than the corresponding ones of polyamide 4b. This
can be rationalized because the TPPA segment is incorpo-
rated between the amide linkages in the former case. There-
fore, the amino units became more easily oxidized. According
to the order of oxidation peaks of polyamide 4b and 400b, we
propose a possible four-step reaction sequence for the elec-
trochemical oxidation of polyamide 4g as shown in the Sup-
porting Information Scheme S1.
The electrooptical properties of polyamide 4g were also
deciphered using the changes in electronic absorption spec-
tra upon oxidation process [see Fig. 9(a)]. Upon electrooxida-
tion, the polymer film exhibited a multicolored electrochro-
mic behavior, a color change from colorless to pale yellow,
yellowish green, green, and blue [Fig. 9(b)]. When the
applied potentials increased positively from 0 to 0.70, 0.90,
1.05, and 1.20 V, respectively, corresponding to the first, sec-
ond, third, and fourth electron oxidation, the characteristic
absorption bands at 309 and 350 nm for neutral-form poly-
amide 4g decreased gradually and bathochromically shifted
to 424 nm, accompanied by the concomitant formation of
new broad long-wavelength absorption bands centered
around 1022, 984, 914, and 753 nm, respectively. The spec-
tral changes are apparently arisen from the sequential oxida-
tion processes of the amino centers in the two TPPA
The HOMO (highest occupied molecular orbital) energy lev-
els of the investigated polyamides were calculated from the
oxidation onset potentials (Eonset) or half-wave potentials of
the first oxidation wave (E1o=x21) and by comparison with ferro-
cene (4.8 eV). These data together with absorption spectra
were then used to obtain the LUMO (lowest unoccupied mo-
lecular orbital) energy levels (Table 1). According to the
HOMO and LUMO energy levels obtained, the polyamides in
this study appear to be appropriate as hole injection and
transport materials.
ELECTROCHROMIC PROPERTIES OF AROMATIC POLYAMIDES, WANG AND HSIAO
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
ety and the electron-deficient anthraquinone moiety in its
repeating unit. As shown in Figure 10(a), 4f had reversible
anodic peaks at E1/2 ¼ 0.76 and 1.12 V, respectively,
ascribed to the oxidation of the TPPA units. As expected, 4f
has somewhat higher oxidation potential than other counter-
parts like 4d because of the electron-deficient nature of the
anthraquinone unit. However, 4f revealed a very similar
spectroelectrochemical and electrochromic behavior as that
of 4d: a color change from pale yellow (neutral) to yellowish
green (radical cation) and then to blue (dication). On the
other hand, irreversible reduction waves for the formation of
radical anion and dianion of the anthraquinone units in 4f
[Fig.10(b)] were observed in the negative side of the voltam-
mograms at peak potentials Epc ¼ ꢁ1.51 and ꢁ1.89 V,
respectively. It is believed that
a
little unwanted
FIGURE 8 (a) Spectroelectrochemistry of the polyamide 4d thin
film on the ITO-coated glass substrate in 0.1 M TBAClO4/CH3CN
at various applied potentials; (b) Optical change in %T as a
function of applied potential for the three absorption bands at
421, 636, and 929 nm (inset, the photo shows the color change
of the film on an ITO electrode at indicated potentials).
segments of different electronic nature in the polymer back-
bone of 4g. The possible oxidation order of the redox centers
of polyamide 4g is proposed in Figure 9(c). The E1/2 value
indicated in each amino center was taken from the CV dia-
gram of 4g. Thus, the incorporation of TPPA units into both
of the diacid and diamine components causes the polyamide
to exhibit different oxidation levels, which make it to be a
multielectrochromic material. In addition, these electrochro-
mic TPPA-polyamides can also become highly absorptive in
the NIR region upon electrochemical oxidation.
FIGURE 9 (a) Spectral changes of polyamide 4g on the ITO-
coated glass slide (in CH3CN containing 0.1 M TBAClO4 as the
supporting electrolyte) at various electrode potentials (vs. Ag/
AgCl). (b) The color changes of the polymer films at various re-
dox states. (c) The possible electrooxidation order for the
amino centers in the repeating unit of polyamide 4g.
Redox and Electrochromic Behavior of Polyamide 4f
Polyamide 4f exhibited an amphoteric redox behavior
because of the presence of both the electron-rich TPPA moi-
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ARTICLE
FIGURE 10 (a) Cyclic voltammograms of the polyamide 4f film on an ITO-coated glass substrate in 0.1 M TBAClO4 acetonitrile (for
the oxidation process) and DMF (for the reduction process) solutions at a scan rate of 50 and 100 mV/s, respectively. (b) Reaction
scheme for the electroreduction of anthraquinone in an aprotic medium. [Color figure can be viewed in the online issue, which is
available at wileyonlinelibrary.com.]
delamination or dissolution of the radical anion and dianion
products in the DMF/electrolyte solution is the main cause
of the irreversible behavior. A UV–vis spectrum was first
recorded on a pale yellow film of neutral 4f on ITO glass at
0 V (Fig. 11). Applying a potential negative to ꢁ2.0 V
resulted in a modest change in the absorption spectrum. The
absorbance at 357 nm slightly dropped, and a new absorp-
tion band at 489 nm arose with a concomitant color change
of the film to red. When the potential was subsequently set
back at 0 V, the original absorption spectrum was obtained.
The experiment proved that polymer 4f could be switched
reversibly between its neutral and dianionic states.
FIGURE 11 Three-dimensional spec-
troelectrochemical surfaces of polyam-
ide 4f thin film on the ITO-coated glass
substrate (in DMF containing 0.1
M
TBAClO4 as the supporting electrolyte)
between 0.0 and ꢁ2.0 V. The photo
shows the color change of the film on
an ITO electrode at indicated potentials.
[Color figure can be viewed in the
online issue, which is available at
wileyonlinelibrary.com.]
ELECTROCHROMIC PROPERTIES OF AROMATIC POLYAMIDES, WANG AND HSIAO
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
TABLE 2 Electrochromic Properties of Polyamides 4d and 40d
Response
Timeb
CEe
a
d
kmax
Qd
Polymer (nm) D%T tc (s) tb (s) DODc (mC/cm2) (cm2/C)
4d
40d
a
421
929
636
415
885
594
45
58
84
42
49
73
3.4
3.5
3.8
3.8
3.8
4.5
1.2
1.3
1.2
1.3
1.3
2.4
0.384 1.84
0.385 1.84
0.899 5.52
0.372 2.07
0.315 2.07
0.724 7.69
209
209
163
180
152
94
Wavelength of absorption maximum.
b
c
Time for 90% of the full-transmittance change.
Optical density (DOD) log[Tbleached/Tcolored], where Tcolored and
¼
Tbleached are the maximum transmittance in the oxidized and neutral
states, respectively.
d
Qd is ejected charge, determined from the in situ experiments.
Coloration efficiency (CE) ¼ DOD/Qd.
FIGURE 12 (a) Optical transmittance changes and (b) current
densities monitored for polyamides 4d at 929 nm and polya-
mides 40d at 885 nm, stepped between 0.0 and 0.95 V (vs. Ag/
AgCl). The dotted line represents the applied voltage. [Color
figure can be viewed in the online issue, which is available at
wileyonlinelibrary.com.]
e
maximum (at 929 nm for 4d and 885 nm for 40d) by apply-
ing squarewave potential steps of 10 s (complete cycle time
is 20 s) between 0 and 0.95 V. The response time was calcu-
lated at 90% of the full-transmittance change because it is
difficult to perceive any further color change with naked eye
beyond this point. Both polymers attained 90% of a com-
plete coloring and bleaching switch in less than 4 and 2 s,
respectively, but polymer 4d revealed a slightly faster
response time than the corresponding 40d for films of near-
identical optical density. Electrochromic contrast attainable
with 4d (e.g., 58% when m ¼ 10 s) on repeated squarewave
potential stepping was found higher than that of 40d (e.g.,
49% when m ¼ 10 s). Coloration efficiency (CE; g) is a useful
term for measuring the power efficiency of the
Electrochromic Switching and Stability
Electrochromic switching studies for the polyamides were
performed to monitor the percent transmittance changes
(D%T) as a function of time at their absorption maximum
(kmax) and to determine the response time by stepping
potential repeatedly between the neutral and oxidized states.
The active area of the polymer film on ITO glass is ꢂ1 cm2.
As a typical example, Figure 12 depicts the optical transmit-
tance and current density changes of polyamides 4d and 40d
as a function of time at their longer wavelength absorption
FIGURE 13 Squarewave potential-step absorptiometry of drop-coated films of (a) polyamide 4d at 929 nm and (b) polyamide 40d
at 885 nm on the ITO-coated glass slides in 0.1 M TBAClO4/CH3CN solution (0 to þ0.95 V vs. Ag/AgCl). The switch time was 10 s
step for 40 s (two cycles), then 5 s step for 40 s (four cycles), and 2 s step for 20 s (five cycles). [Color figure can be viewed in the
online issue, which is available at wileyonlinelibrary.com.]
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ARTICLE
FIGURE 14 Electrochromic switching between 0 and 0.95 V (vs. Ag/AgCl) of polyamides (a) 4d and (b) 40d thin films on the ITO-
coated glass substrate in 0.1M TBAClO4/CH3CN with a cycle time of 20 s.
electrochromic devices and can be calculated via optical den-
sity using the following equation:12
squarewave potential steps of 10 s in a 0.1 M TBAClO4/
CH3CN electrolyte solution between 0 and 0.95 V. In over
100 cycles under ambient conditions, polyamide 4d showed
a variation of electrochromic contrast of less than 7%,
whereas polyamide 40d exhibited a much larger loss of about
25% (Fig. 14). Therefore, the introduction of the tert-butyl
group at the active sites of the TPPA unit results in an
enhanced electrochemical stability and thus an increased
electrochromic stability of these polymers.
DODðkmax
Þ
;
g ¼
Q
where Q is the injected/extracted charge per unit electrode
area (mC/cm2) and DOD is the change in absorbance at a
specific wavelength maximum during a redox step. The CE
for the 4d film at 929 nm was calculated to be 209 cm2/C,
which is higher than that of 40d film (152 cm2/C at 885
nm). The switching times and CEs at both short and long
wavelengths for the green electrochromic switching and at
the wavelength maxima for the blue electrochromic switch-
ing (636 nm for 4d and 594 nm for 40d at an applied volt-
age of 1.25 V) are summarized in Table 2. In general, poly-
amide 4d displays a better electrochromic performance, that
is, a faster response speed and higher CE, when compared
with analogous 40d without the tert-butyl groups on the
TPPA units.
CONCLUSIONS
A series of novel electroactive TPPA-bearing aromatic polya-
mides were readily prepared from the newly synthesized
dicarboxylic acid monomer, N,N-bis(4-carboxyphenyl)-N0,N0-
bis(4-tert-butylphenyl)-1,4-phenylenediamine, with various
aromatic diamines via the phosphorylation polyamidation
reaction. By substitution of the electrochemically active sites
of the TPPA segment with sterically bulky tert-butyl groups,
the new polyamides exhibit enhanced redox stability and
electrochromic properties in comparison with previously
reported analogs without tert-butyl substituents on the TPPA
units. In addition to high Tg values and good thermal stabil-
ity, all the obtained polyamides also revealed good electro-
chemical and electrochromic stability and multicolor electro-
chromic behavior (highly transmissive-to-green and then to-
blue switching). Moreover, the anthraquinone and TPPA-con-
taining polyamide 4f exhibited a RGB electrochromism. The
new materials described here represent a novel entry to the
polymeric electrochromics that may find potential applica-
tion in the fabrication of electrochromic devices.
Moreover, the electrochromic contrasts (D%T) for polya-
mides 4d and 40d were monitored as a function of time at
their longer wavelength absorption maximum by applying
squarewave potential of 10, 5, and 2 s (Fig. 13). As polymer
4d is switched between 0 and 0.95 V with a 10 s hold at
each potential, the % transmittance contrast for 929 nm is
58%. As the time the oxidizing and reducing potentials are
held is decreased to 5 s, the optical contrast decreases only
slightly to 54% and subsequently to 44% for a 2 s hold. For
the referenced polymer 40d, the drop in optical contrast is
larger than that of 4d at a faster switching (49% when m ¼
10 s and 27% when m ¼ 2 s). The long-term stability of poly-
amides 4d and 40d was investigated by monitoring the elec-
trochromic contrast (D%T) of the thin films upon repeated
The authors thank the National Science Council in Taiwan
(ROC) for the financial support of this work.
ELECTROCHROMIC PROPERTIES OF AROMATIC POLYAMIDES, WANG AND HSIAO
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
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