Synthesis of a new class of triphenylamine-containing poly(ether-imide)s for electrochromic applications

Article abstract of DOI:10.1002/pola.27064

A novel triphenylamine (TPA)-containing bis(ether anhydride) monomer, namely 4,4′-bis(3,4-dicarboxyphenoxy)triphenylamine dianhydride, was synthesized and reacted with various aromatic diamines leading to a series of new poly(ether-imide)s (PEI). Most of these PEIs were soluble in organic solvents and could be easily solution cast into flexible and strong films. The polymer films exhibited good thermal stability with glass-transition temperatures in the range 211-299°C. The polymer films exhibited reversible electrochemical processes and stable color changes (from transparent to navy blue) with high coloration efficiency and contrast ratio upon electro-oxidation. During the electrochemical oxidation process, a crosslinked polymer structure was developed due to the coupling reaction between the TPA radical cation moieties in the polymer chains. These polymers can be used to fabricate electrochromic devices with high coloration efficiency, high redox stability, and fast response time.

Full text of DOI:10.1002/pola.27064

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Synthesis of a New Class of Triphenylamine-Containing
Poly(ether-imide)s for Electrochromic Applications
Sheng-Huei Hsiao,1 Pei-Chi Chang,1 Hui-Min Wang,1 Yu-Ruei Kung,2 Tzong-Ming Lee2
1Department of Chemical Engineering and Biotechnology, National Taipei University of Technology, Taipei 10608, Taiwan
2Material and Chemical Research Laboratories, Industrial Technology Research Institute, Hsinchu 31040, Taiwan
Correspondence to: S.-H. Hsiao (E-mail: shhsiao@ntut.edu.tw)
Received 13 November 2013; accepted 7 December 2013; published online 24 December 2013
DOI: 10.1002/pola.27064
ABSTRACT: A novel triphenylamine (TPA)-containing bis(ether
anhydride) monomer, namely 4,40-bis(3,4-dicarboxyphenoxy)tri-
phenylamine dianhydride, was synthesized and reacted with
various aromatic diamines leading to a series of new poly-
(ether-imide)s (PEI). Most of these PEIs were soluble in organic
solvents and could be easily solution cast into flexible and
strong films. The polymer films exhibited good thermal stabil-
ity with glass-transition temperatures in the range 211–299 ꢀC.
The polymer films exhibited reversible electrochemical proc-
esses and stable color changes (from transparent to navy blue)
with high coloration efficiency and contrast ratio upon electro-
oxidation. During the electrochemical oxidation process,
a
crosslinked polymer structure was developed due to the cou-
pling reaction between the TPA radical cation moieties in the
polymer chains. These polymers can be used to fabricate elec-
trochromic devices with high coloration efficiency, high redox
C
V
stability, and fast response time. 2013 Wiley Periodicals, Inc.
J. Polym. Sci., Part A: Polym. Chem. 2014, 52, 825–838
KEYWORDS: bis(ether anhydride); electrochemistry; electrochro-
mic polymers; polyimides; redox polymers; triphenylamine
Triarylamine derivatives are important organic materials for
electronic and optoelectronic devices. Owing to the electron-
donating nature of the nitrogen atom in triarylamines, they act
as hole-transport materials in various applications like xerog-
raphy, light-emitting diodes, and solar cells.14 Polymers bear-
ing triarylamine units have attracted much attention 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.15 Triarylamines can be easily
oxidized to form stable radical cations, and the oxidation pro-
cess is always associated with a strong change of coloration.
Various triarylamine-based polymer systems have been
reported to be potential electrochromic materials.16
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 atten-
tion because of the importance of this interesting phenom-
enon in applications such as electrochromic windows,2
displays,3 memory devices,4 electronic papers,5 and adapt-
ive camouflages.6 There is a huge amount of chemical spe-
cies that exhibited electrochromic properties,7 including
metal coordination 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 electrochromic 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 synthe-
sis.12 For efficient operation of an electrochromic device, it
is necessary to take a number of properties into considera-
tion: electrochromic efficiency, optical contrast, response
time, stability, and durability.11(b) The difficulty in achieving
satisfactory values for all these parameters at the same
time stimulates the development of new methods of prepa-
ration of electrochromic films, new materials, and compo-
nents for the devices.13
Aromatic polyimides are known for their outstanding prop-
erties, such as unique high thermal stability, good mechanical
and electrical properties as well as excellent chemical resist-
ance.17 They are commercially important materials used
extensively in photoresists, alignment layers in liquid crystal
displays, and high-temperature coatings. However, wholly
aromatic polyimides are difficult to process because of lim-
ited solubility and extremely high softening temperatures
that stem from the rigidity of the backbone and strong inter-
chain interactions. To overcome these drawbacks, various
Additional Supporting Information may be found in the online version of this article.
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attempts have been made to develop structurally modified
polymers existing increased solubility with retention of their
good thermal stability.18 Several major approaches include
the incorporation of flexible linkages, noncoplanar moieties,
asymmetric units, or bulky pendant groups into the polyi-
mide backbones.19 In recent years, Liou and coworkers have
carried out extensive studies on the design and synthesis of
triarylamine-based high-performance polymers such as aro-
matic polyimides20 and polyamides21 for potential applica-
tion in electrochromic devices. The triarylamine-containing
monomers such as diamines and dicarboxylic acids could be
easily prepared using a well-established procedure, and they
could react with the corresponding comonomers through
conventional polycondensation techniques, producing the
desired triarylamine-based polymers. In general, these poly-
mers exhibit good solubility to organic solvents due to the
introduction of bulky, packing-disruptive triarylamine moi-
eties. Thus, they could be readily solution-cast into flexible
electrochromic films with good mechanical property and
high thermal stability. Thus, incorporation of three-
dimensional, packing-disruptive triarylamine units into the
polyimide backbone not only resulted in enhanced solubility
but also led to new electronic functions of polyimides such
as electrochromic characteristics.
ether (6c), and 9,9-bis(4-aminophenyl)fluorene (6e) were
used as received from Tokyo Chemical Industry. According to
a procedure reported previously,24 2-triflouromethyl-4,40-
diaminodiphenyl ether (6d, mp: 112–113 ꢀC) was synthe-
sized by the chloro-displacement reaction of 2-chloro-5-
nitrobenzotrifluoride with p-nitrophenol in the presence of
potassium carbonate, followed by the hydrazine palladium-
catalyzed reduction of the intermediate dinitro compound.
4,40-Diaminotriphenylamine (6f, mp: 186–188 ꢀC) and 4,40-
diamino-400-tert-butyltriphenylamine (6g, mp: 113–115 ꢀC)25
were synthesized by the cesium fluoride-mediated condensa-
tion of p-fluoronitrobenzene with aniline and 4-tert-butylani-
line, respectively, followed by
a
palladium-catalyzed
hydrazine reduction of the dinitro intermediates. Tetrabuty-
lammonium perchlorate, Bu4NClO4, were recrystallized from
ethyl acetate under nitrogen atmosphere and then dried in
vacuo before use. All other reagents and solvents were used
as received from commercial sources.
Monomer Synthesis
4,40-Bis(3,4-dicyanophenoxy)triphenylamine (3)
A mixture of 6.93 g (0.025 mol) of bisphenol 2, 8.66 g
(0.050 mol) of 4-nitrophthalonitrile, and 6.91 g (0.050 mol)
of K2CO3 in 60 mL of DMF was stirred at room temperature
for about 48 h. Then, the reaction mixture was poured into
600 mL of water, and the precipitated light brown solid was
collected and washed thoroughly with methanol and water.
The yield of bis(ether dinitrile) 3 was 12.34 g (93%),
It has also been demonstrated that triarylamine-based polyi-
mides generally exhibited poor electrochemical and electro-
chromic stability as compared to their polyamide analogs
because of the strong electron-withdrawing imide group, which
increases the oxidation potential of the triarylamine unit and
destabilizes the resultant amino radical cation upon oxida-
tion.22 Incorporating a spacer between the triarylamine core
and the imide ring may improve the electrochemical and elce-
trochromic stability of this kind of electroactive polymers.
Thus, a new kind of triphenylamine (TPA)-containing bis(ether
anhydride), namely 4,40-bis(3,4-dicarboxyphenoxy)triphenyl-
amine dianhydride, was synthesized and led to a series of pol-
y(ether-imide)s (PEIs) with main-chain TPA groups. By the
incorporation of the phenoxy spacer between the TPA and
imide units, the resulting PEIs are expected to exhibit enhanced
electrochemical and electrochromic properties. The three-
dimensional TPA kinked structure is certainly responsible for
decreasing interchain interactions, thus improving the solubil-
ity of polyimides. Physical, thermal, electrochemical, and elec-
trochromic properties of these PEIs were also investigated.
ꢀ
mp 5 223–225 C. IR (KBr) (Supporting Information Fig. S1):
2231 cm21 (CBN stretch).
1H NMR (500 MHz, DMSO-d6, d, ppm) (Supporting Informa-
tion Fig. S2): 7.08 (t, J 5 7.5 Hz, 1H, Ha), 7.12 (d, J 5 8.0 Hz,
2H, Hc), 7.15 (s, 8H, Hd 1 He), 7.35 (t, J 5 10.0 Hz, 2H, Hb),
7.45 (d, J 5 10.0 Hz, 2H, Hf), 7.80 (s, 2H, Hh), 8.55 (d, J 5 8.5
Hz, 2H, Hg). 13C NMR (125 MHz, DMSO-d6, d, ppm): 107.98
(C12), 115.39 (ACN), 115.89 (ACN), 116.66 (C13), 121.60
(C7), 121.75 (C10), 122.28 (C14), 123.34 (C1), 123.86 (C3),
125.36 (C6), 129.66 (C2), 136.24 (C11), 144.75 (C5), 146.96
(C4), 148.79 (C8), 161.34 (C9).
EXPERIMENTAL
Materials
4,40-Bis(3,4-dicarboxyphenoxy)triphenylamine (4)
N,N-Dimethylacetamide (DMAc, Fluka) was dried over cal-
cium hydride overnight, distilled under reduced pressure,
and stored over 4 Å molecular sieves in a sealed bottle.
According to a literature method,23 4,40-dihydroxytriphenyl-
amine (2) was prepared by demethylation of 4,40-dimethoxy-
triphenylamine (1) which was obtained from Ullmann
reaction between aniline and 4-iodoanisole. Commercially
available diamine monomers including p-phenylenediamine
(6a), 4,40-diaminodiphenyl ether (6b), 3,4-diaminodiphenyl
In a 250-mL flask, a suspension of bis(ether dinitrile) 3
(10.59 g, 0.020mol) in an ethanol/water mixture (100 mL/
100 mL) containing 27 g (0.48 mol) of dissolved KOH was
heated at a reflux temperature. Refluxing was continued for
48 h until the evolution of ammonia had ceased. The result-
ing clear solution was filtered hot to remove any possible
insoluble impurities. The filtrate was allowed to cool and
was then acidified by hydrochloric acid to a pH of 2–3. The
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precipitated pale green product was filtered off, washed with
water until it was neutral, and dried. The yield of bis(ether
diacid) 4 was 10.29 g (85%).
anhydrous DMAc in a 50-mL round-bottom flask. Then
bis(ether anhydride) 5 (0.6798 g, 1.19 mmol) was added to
the diamine solution in one portion. Thus, the solid content
of the solution is approximately 10 wt %. The mixture was
stirred at room temperature for about 18 h to yield a viscous
poly(amic acid) solution. The inherent viscosity of the result-
ing poly(amic acid) was 0.68 dL g21, measured in DMAc at a
concentration of 0.50 g dL21 at 30 ꢀC. The poly(amic acid)
film was obtained by casting from the reaction polymer solu-
tion onto a glass Petri-dish and drying at 90 ꢀC overnight.
The poly(amic acid) in the form of solid film was converted
IR (KBr) (Supporting Information Fig. S1): 2500–3500 cm21
(OAH stretch), 1714 cm21 (C@O stretch). 1H NMR (500
MHz, DMSO-d6, d, ppm) (Supporting Information Fig. S3):
7.05 (t, J 5 7.3 Hz, 1H, Ha), 7.07 (d, J 5 7.7 Hz, 2H, Hc), 7.11
(s, 8H, Hd and He), 7.15 (d, J 5 8.6 Hz, 2H, Hf), 7.24 (s, 2H,
Hh), 7.33 (t, J 5 7.6 Hz, 2H, Hb), 7.85 (d, J 5 8.6 Hz, 2H, Hg).
13C NMR (125 MHz, DMSO-d6, d, ppm): 117.04 (C14), 118.64
(C10), 121.16 (C7), 122.94 (C1), 123.35 (C3), 125.52 (C6),
126.14 (C12), 129.67 (C2), 132.01 (C11), 136.82 (C13), 143.96
(C5), 147.31 (C4), 150.41 (C8), 159.48 (C9), 167.49 (ACOOH),
168.44 (ACOOH).
ꢀ
to PEI 7d byꢀsuccessive heating under vacuum at 150 C for
ꢀ
30 min, 200 C for 30 min, 250 C for 30 min and then 300
ꢀC for 1 h. The inherent viscosity of the resulting PEI 7d was
0.44 dL g21,ꢀ measured at a concentration of 0.5 g dL21 in
DMAc at 30 C. The IR spectrum of 7d (Supporting Informa-
tion Fig. S4) exhibited characteristic imide absorption bands
at 1776 cm21 (asymmetric C@O stretch) and 1722 cm21
(symmetric C@O stretch).
1H NMR (500 MHz, CDCl3, d, ppm) (Supporting Information
Fig. S5): 7.03 (d, J 5 8.5 Hz, 4H, He), 7.10 (t, J 5 7.0 Hz, 1H,
Ha), 7.15 (d, 1H, Hk), 7.18 (d, J 5 7.0 Hz, 6H, Hc, and Hd),
7.21 (d, J 5 10.5 Hz, 2H, Hj), 7.33 (t, J 5 8.0 Hz, 2H, Hb), 7.39
(d, J 5 7.5 Hz, 2H. Hf), 7.46 (d, J 5 8.5 Hz, 2H, Hi), 7.48 (s,
2H, Hh), 7.57 (d, J 5 8.0 Hz, 1H, Hl), 7.79 (s, 1H, Hm), 7.91 (d,
J 5 7.5 Hz, 2H, Hg).
4,40-Bis(3,4-dicarboxyphenoxy)triphenylamine
Dianhydride (5)
In a 200-mL flask, a mixture of 9.08 g (0.015 mol) of
bis(ether diacid) 4 dissolved in 50 mL of acetic anhydride
was heated at a reflux temperature for 3 h. The resulting
clear solution was filtered to remove any insoluble impur-
ities. After cooling to room temperature, the precipitated oli-
vine product was filtered and dried at 120 ꢀC in vacuo. The
yield of bis(ether anhydride) 5 was 5.00 g (67%). IR (KBr)
(Supporting Information Fig. S1): 1849 cm21 (asymmetric
C@O stretch), 1774 cm21 (symmetric C@O stretch).
For the chemical imidization method, 4 mL of acetic anhy-
dride and 2 mL of pyridine were added to the poly(amic
acid) solution obtained by a similar process as above, and
the mixture was heated at 100 ꢀC for 1 h to effect a com-
plete imidization. The homogenous polymer solution was
poured slowly into 150 mL of stirring methanol giving rise
to yellow precipitate that was collected by filtration, washed
thoroughly with hot water and methanol, and dried. A poly-
mer solution was made by the dissolution of about 0.5 g of
the polyimide sample in 10 mL of hot DMAc. The homoge-
neous solution was poured into a 9-cm glass Petri dish,
which was placed in a 90 ꢀC oven overnight for the slow
release of the solvent, and then the film was stripped off
from the glass substrate and further dried in vacuum at 160
ꢀC for 6 h.
1H NMR (500 MHz, DMSO-d6, d, ppm) (Fig. 1): 7.09 (t,
J 5 7.3 Hz, 1H, Ha), 7.13 (t, J 5 8.1 Hz, 2H, Hc), 7.17 (s, 8H,
Hd 1 He), 7.36 (t, J 5 7.6 Hz, 2H, Hb), 7.50 (s, 2H, Hh), 7.56
(d, J 5 8.4 Hz, 2H, Hf), 8.08 (d, J 5 8.4 Hz, 2H. Hg). 13C NMR
(125 MHz, DMSO-d6, d, ppm): 112.35 (C14), 121.61 (C7),
123.39 (C1), 123.93 (C3), 124.36 (C10), 124.56 (C12), 125.36
(C6), 127.80 (C11), 129.68 (C2), 134.04 (C13), 144.70 (C5),
146.98 (C4), 149.24 (C8), 162.39 (anhydride carbonyl),
162.60 (anhydride carbonyl), 164.18 (C9). Anal. Calcd. For
C34H19NO8 (569.53): C, 71.70%; H, 3.36%; N, 2.46%. Found:
Fabrication of Electrochromic Devices
C, 71.45%; H, 3.29%; N, 2.41%.
Electrochromic polymer films were prepared by dropping
solution of the PEIs (3 mg mL21 in DMAc) onto an ITO-
coated glass substrate (20 3 30 3 0.7 mm, 50–100 X
cm22). A gel electrolyte based on PMMA (Mw: 120,000) and
LiClO4 was plasticized with propylene carbonate to form a
highly transparent and conductive gel. PMMA (1 g) was dis-
solved in dry acetonitrile (4 mL), and LiClO4 (0.1 g) was
added to the polymer solution as supporting electrolyte.
Then propylene carbonate (1.5 g) was added as plasticizer.
The mixture was then gently heated until gelation. The gel
electrolyte was spread on the polymer-coated side of the
electrode, and the electrodes were sandwiched. Finally, an
epoxy resin was used to seal the device.
Polymer Synthesis
A typical polymerization procedure is as follows. The dia-
mine 6d (0.3202 g, 1.19 mmol) was dissolved in 9.5 mL of
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Measurements
Infrared (IR) spectra were recorded on a Horiba FT-720 FTIR
spectrometer. 1H and 13C NMR spectra were measured on a
Bruker AVANCE 500 FT-NMR system with tetramethylsilane
as an internal standard. Elemental analysis was performed on
a Heraeus VarioEL III CHNS elemental analyzer. The inherent
viscosities were determined with a Cannon-Fenske viscometer
ꢀ
at 30 C. Wide-angle X-ray diffraction (WAXD) measurements
were performed at room temperature (ca. 25 ꢀC) on a Shi-
madzu XRD-6000 X-ray diffractometer with a graphite mono-
chromator (operating at 40 kV and 30 mA), using nickel-
filtered Cu-Ka radiation (k 5 1.5418 Å). The scanning rate
was 2ꢀ min21 over a range of 2h 5 10–40ꢀ. Thermogravimet-
ric analysis (TGA) was performed with a Perkin-Elmer Pyris 1
TGA. Experiments were carried out on approximately 4–6 mg
of samples heated in flowing nitrogen or air (flow rate 5 40
cm3 min21) at a heating rate of 20 ꢀC min21. DSC analyses
were performed on a Perkin-Elmer Pyris 1 DSC at a scan rate
21
ꢀ
of 20 C min in flowing nitrogen. Thermomechanical analy-
sis (TMA) was determined with a Perkin-Elmer TMA 7 instru-
ment. The TMA experiments were carried out from 50 to 400
21
ꢀ
ꢀC at a scan rate of 10 C min with a penetration probe 1.0
mm in diameter under an applied constant load of 10 mN.
Softening temperatures (Ts) were taken as the onset tempera-
tures of probe displacement on the TMA traces. Electrochem-
istry was performed with a CHI 750A electrochemical
analyzer. Cyclic voltammetry was conducted with the use of a
three-electrode cell in which ITO (polymer films area about
0.8 3 1.25 cm2) was used as a working electrode. A platinum
wire was used as an auxiliary electrode. All cell potentials
were taken with the use of a home-made Ag/AgCl, KCl (sat.)
reference electrode. Ferrocene was used as an external refer-
ence for calibration (10.48 V vs. Ag/AgCl). Voltammograms
are presented with the positive potential pointing to the left
and with increasing anodic currents pointing downwards.
Spectroelectrochemistry analyses were carried out with an
electrolytic cell, which was composed of a 1 cm cuvette, ITO
as a working electrode, a platinum wire as an auxiliary elec-
trode, and a Ag/AgCl reference electrode. Absorption spectra
in the spectroelectrochemical experiments were measured
with an Agilent 8453 UV–visible diode array spectrophotome-
ter. Color measurements were performed by using a Konica
Minolta CS-100 ChromaMeter with viewing geometry as rec-
ommended by Commision Internationale de l’Eclairage.
SCHEME 1 Synthetic route to the target bis(ether anhydride)
monomer 5.
formed in situ by treatment of K2CO3 in DMF. Alkaline hydrolysis
of the bis(ether dinitrile) 3 with aqueous potassium hydroxide
in ethanol followed by acidification with aqueous HCl led to 4,40-
bis(3,4-dicarboxyphenoxy)triphenylamine (4), which was subse-
quently cyclodehydrated with acetic anhydride to generate the
target bis(ether anhydride) monomer 5. The yield in each step
was reasonable, and the molecular structure of the synthesized
compounds could be affirmed by IR and NMR spectroscopic
techniques. The FTIR spectra of intermediate compounds 3 and
4 and the target monomer 5 are included in Supporting Informa-
tion (Fig. S1). The proton and carbon-13 NMR spectra of inter-
mediate compounds 3 and 4 are also shown in SI (Supporting
Information Figs. S2 and S3). Figure 1 depicts the proton and
carbon-13 NMR spectra of bis(ether anhydride) 5 in dimethyl
sulfoxide-d6. Assignments of each carbon and proton assisted by
the two-dimensional NMR spectroscopy are also indicated in
these spectra, and they are in good agreement with the proposed
molecular structures of these compounds.
RESULTS AND DISCUSSION
Monomer Synthesis
The new triphenylamine-based bis(ether anhydride) 5 was syn-
thesized via the synthetic route outlined in Scheme 1. According
to a reported method,23 the 4,40-dimethoxytriphenylamine (1)
was synthesized by Ullmann condensation between aniline and
4-iodoanisole by using Cu and K2CO3 in tri(ethylene glycol)
dimethyl ether (TEGDME). Demethylation of dimethoxy com-
pound 1 with boron tribromide (BBr3) gave the 4,40-dihydroxy-
triphenylamine (2). 4,40-Bis(3,4-dicyanophenoxy)triphenylamine
(3) was obtained from the nitro-displacement of 4-
nitrophthalonitrile with the potassium salt of compound 2
Polymer Synthesis
PEIs 7a–7g were prepared in a conventional two-step
method by the reactions of equal molar amounts of bis(ether
anhydride) monomer 5 with various aromatic diamines (6a–
6g) to form poly(amic acid)s, followed by thermal or chemi-
cal cyclodehydration (Scheme 2). As shown in Table 1, the
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FIGURE 1 (a) 1H NMR, (b) 13C NMR, (c) H-H COSY, and (d) C-H HMQC spectra of the target bis(ether anhydride) 5 in DMSO-d6.
[Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
SCHEME 2 Synthesis of poly(ether-imide)s 7a–7g.
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TABLE 1 Inherent Viscosity and Solubility Behavior of Poly(ether-imide)s Prepared via Thermal (-T) or Chemical (-C) Imidization
ginh (dL g21 a Solubility in Various Solventsb
Poly(amic acid) Polyimide
)
Polymer Code
NMP
DMAc
DMF
DMSO
m-Cresol
THF
CHCl3
7a-T
7a-C
7b-T
7b-C
7c-T
7c-C
7d-T
7d-C
7e-T
7e-C
7f-T
0.50
0.69
0.35
0.68
0.44
0.64
0.60
–
–
–
–
–
1–
1h
1h
1h
1h
1h
1–
1h
1–
1h
1–
1h
1–
1h
–
–
–
1–
1–
11
1h
11
1–
11
1–
11
–
1–
1–
11
1–
11
1–
11
1–
11
–
1–
1–
11
1–
11
1–
11
1–
11
–
1–
1–
1h
1–
1h
1–
1h
1–
1h
–
1–
–
–
–
1–
11
11
11
–
0.54
–
1–
1–
11
1–
11
1–
11
–
0.37
–
0.44
–
11
1–
11
1–
11
1–
1–
0.36
–
7f-C
7g-T
7g-C
0.36
–
11
1–
1h
11
1–
1h
1h
1–
1h
1h
–
11
1–
11
–
1h
a
c
Inherent viscosity measured at a concentration of 0.5 dL g21 in DMAc
Solvent: NMP: N-methyl-2-pyrrolidone; DMAc: N,N-dimethylaceta-
mide; DMF: N,N-dimethylformamide; DMSO: dimethyl sulfoxide; THF:
tetrahydrofuran.
at 30 ꢀC.
b
The qualitative solubility was tested with 10 mg of a sample in 1 mL
of stirred solvent.11, soluble at room temperature; 1h, soluble on
heating; 12, partially soluble; 2, insoluble even on heating.
poly(amic acid) precursors had inherent viscosities in the
range of 0.35–0.69 dL g21. The resulting poly(amic acid) sol-
utions were poured into a clean glass Petri-dish and dried to
form thin solid films. The thermal conversion to polyimides
was carried out by step-by-step heating of the poly(amic
acid) films to 300 ꢀC. The transformation from poly(amic
acid) to a polyimide could also be carried out via chemical
cyclodehydration by using acetic anhydride and pyridine. All
the polyimides showed the characteristic absorption bands
of the imide ring near 1776 (asymmetric C@O stretching)
and 1722 cm21 (symmetric C@O stretching). Typical IR
spectra of PEI 7d and its poly(amic acid) precursor are illus-
Table 1, most of the chemically imidized samples of 7b–7g
are soluble in NMP, DMAc, DMF, and THF. The good solubility
of these polyimides can be attributed to the presence of
three-dimensional TPA units, together with the flexible ether
linkage along the polymer backbone. In addition, the chemi-
cally imidized samples generally revealed a higher solubility
than those prepared by the thermal imidization method. The
lower solubility of the thermally cured PEIs may be attrib-
uted to the presence of partial interchain crosslinking or an
aggregation of the polymer chains during thermal curing
process.
1
Thermal Properties
trated in Supporting Information Figure S4. The H NMR and
1H-1H COSY spectra of a representative PEI 7d are illustrated
in Supporting Information Figure S5. Assignments of each
proton are in good agreement with the structure of its
repeating unit.
The thermal stability and phase-transition temperatures of
these PEIs were determined by TGA, DSC, and TMA techni-
ques. The thermal behavior data are included in Table 2.
Typical TGA and TMA curves of the representative PEI 7f
are shown in Figure 2. The TGA results showed an excellent
thermal stability of these PEIs, even though they revealed
high solubility. All of these polymers exhibited a similar TGA
pattern with no significant weight loss below 500 ꢀC in air
or nitrogen atmosphere. The decomposition temperatures
(Td) at 10% weight-loss in nitrogen and air were recorded in
the range of 539–598 and 551–596 ꢀC, respectively. The
amount of carbonized residue (char yield) of these polymers
in nitrogen atmosphere was more than 58% at 800 ꢀC. The
glass-transition temperatures (Tg) of all the polymers were
observed in the range of 211–299 ꢀC by DSC (Supporting
Information Fig. S7). The decreasing order of Tg generally
correlated with that of chain flexibility. As expected, the poly-
mer 7e showed the highest Tg due to the presence of rigid
Polymer Properties
Basic Characterization
As evidenced by their WAXD patterns (see Supporting Infor-
mation Fig. S6), all the PEI films exhibited an amorphous
nature. This can be attributed in part to the bulky, packing-
disruptive TPA unit along the polymer backbone, which does
not favor their close chain packing. The solubility behavior
of the polyimides depended on their chain packing ability
and intermolecular interactions that was affected by the
rigidity, symmetry, and regularity of the molecular backbone.
PEI 7a derived from more rigid p-phenylenediamine showed
a poor solubility in all the test solvents. The other PEIs
exhibited a better solubility to organic solvents. As shown in
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TABLE 2 Thermal Properties of the Poly(ether-imide)sa
Td at 10%
Weight
Loss (ꢀC)d
Td at 5% Weight
Loss (ꢀC)d
Polymer Code
Tg (ꢀC)b
Ts (oC)c
N2
Air
N2
Air
Char Yield (wt %)e
7a
7b
7c
7d
7e
7f
239
227
211
227
299
252
260
232
227
208
224
300
252
256
505
536
513
572
571
527
536
518
526
513
562
571
513
509
539
563
542
598
597
555
558
557
569
551
596
595
552
555
61
58
60
66
66
65
60
7g
a
c
All the polymer films were heated at 300 ꢀC for 1 h before DSC, TMA,
and TGA experiments.
Softening temperature determined by TMA at a heating rate of 10 ꢀC min21
.
d
Decomposition temperature at which a 5 or 10% weight loss was
b
Midpoint temperature of the baseline shift on the second DSC heating
recorded by TGA at a heating rate of 20 ꢀC min21 and a gas flow rate of
20 cm3 min21
e
trace (rate 5 20 ꢀC min21) of the sample after quenching from 400 to 50
ꢀC (rate 5 200 ꢀC min21) in nitrogen.
.
Residual weight % at 800 ꢀC at a scan rate 20 ꢀC min21 in nitrogen.
9,9-diphenylfluorene segments. Polymer 7c exhibited the
lowest Tg value of 211 C because of the presence of flexible
mides based on N,N-bis(3,4-dicarboxyphenyl)aniline dianhy-
dride26 due to the incorporation of more flexible phenoxy
spacer in the dianhydride component.
ꢀ
m-phenoxy linkage in its diamine residue. The softening tem-
peratures (Ts) (may be referred to as apparent Tg) of the
polymer film samples determined by the TMA method with
a loaded penetration probe are also listed in Table 2. They
were obtained from the onset temperature of the probe dis-
placement on the TMA trace. As a representative example,
the TMA trace of PEI 7f is illustrated in the inset of Figure 2.
In most cases, the Ts values of the PEI films obtained by
TMA are comparable to the Tg values measured by the DSC
experiments. The Tg values of the present PEIs are similar to
those of analogous ones in which the bis(phenoxy)triphenyl-
amine segment is in the diamine side;23(b) however, they are
lower than those (Tg 5 298–351 ꢀC) reported for the polyi-
Electrochemical Properties
The electrochemical behavior of each PEI was investigated
by CV conducted for the cast film on the ITO-coated glass
substrate as the working electrode in dry acetonitrile (for
the anodic scan) or DMF (for the cathodic scan) containing
Bu4NClO4 as an electrolyte and saturated Ag/AgCl as the ref-
erence electrode under nitrogen atmosphere. Figure
3
depicts the CV diagrams for 7d, which was representative for
the other 7a–7e analogs. PEI 7d exhibited an amphoteric
redox behavior because of the presence of both the electron-
FIGURE 3 Cyclic voltammograms of the cast film of poly(ether-
imide) 7d on an ITO-coated glass substrate in 0.1 M Bu4NClO4/
CH3CN (for the anodic process) and DMF (for the cathodic pro-
cess) solution at a scan rate of 50 and 100 mV s21, respec-
tively. [Color figure can be viewed in the online issue, which is
available at wileyonlinelibrary.com.]
FIGURE 2 TMA and TGA curves of poly(ether-imide) 7f with a
heating rate of 10 and 20 ꢀC min21, respectively. [Color figure
can be viewed in the online issue, which is available at
wileyonlinelibrary.com.]
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TABLE 3 Optical and Electrochemical Properties of the Poly(ether-imide)s
Oxidation
Potential (V)b
Reduction
Potential (V)c
Bandgap
(eV)d,e
Energy Level
(eV)e
Thin Film (nm)a
Abs
Abs
ox
ox
red
opt
ec
Polymer Code
kmax
konset
Eonset
E1/2
Eonset
E1/2
Eg
Eg
HOMO
LUMO
7a
7b
7c
7d
7e
7f
298
297
302
298
301
298
302
304
557
585
647
566
574
606
556
613
0.89
0.88
0.87
0.93
0.95
0.97
0.94
0.79
1.01
21.05
21.03
20.96
21.03
20.89
21.13
21.08
20.96
21.35
21.29
21.28
21.29
21.28
21.30
21.30
21.24
2.23
2.12
1.92
2.19
2.16
2.05
2.23
2.79
2.36
2.26
2.22
2.31
2.27
2.31
2.32
2.20
5.37
5.33
5.30
5.38
5.35
5.37
5.38
5.32
3.01
3.07
3.08
3.07
3.08
3.06
3.06
3.12
0.97
0.94
1.02
0.99
1.01
7g
7d’
1.02
0.86, 0.96
a
d
ec,
Eg electrochemical band gap is derived from the difference between
Versus Ag/AgCl in CH3CN. E1/2 5 average potential of the redox couple
peaks.
HOMO and LUMO values.
b
e
ox
Versus Ag/AgCl in DMF.
Bandgaps calculated from absorption edge of the polymer films:
The HOMO and LUMO energy levels were calculated from E1/2 and
c
red
E1/2
values of CV curves and were referenced to ferrocene (4.8 eV rel-
Egopt 5 1240/konset
.
ative to the vacuum energy level).
rich TPA moiety and the electron-deficient imide ring in its
repeating unit. The anodic peak at 1.14 V is ascribed to the
oxidation of the TPA units, and the cathodic peak at 21.34
V can be attributed to the formation of radical anion of the
imide unit. For the first positive scan, we observed an oxi-
dation peak at about 1.14 V. From the first reverse negative
potential scan, we detected two cathodic peaks one at 0.89
V and the other at 0.80 V. After the second scan, a new
oxidation potential appeared at about 0.90 V that was the
complementary anodic process for the second cathodic
peak at 0.8 V. The observation of a new oxidation couple in
the second potential scan indicates that the polymer 7d
radical cations were involved in very fast electrochemical
reactions that produced a substance that was easier to oxi-
dize than was the parent 7d. The other 7a–7e analogs
showed a similar CV behavior to that of 7d, and their
redox potential according to the first CV diagrams are sum-
marized in Table 3.
The energy levels of the highest occupied molecular
orbital (HOMO) and lowest unoccupied molecular orbital
(LUMO) of the corresponding polymers were estimated
ox
from the E1/2 values. Assuming that the HOMO energy
level for the ferrocene/ferrocenium (Fc/Fc1) standard is
4.80 eV with respect to the zero vacuum level, the HOMO
and LUMO values for these PEIs were calculated to be in
the range of 5.30–5.38 and 3.01–3.08 eV, respectively
(Table 3).
Spectroelectrochemical Studies
The electro-optical properties of the polymer films were
investigated using the changes in electronic absorption spec-
tra at various applied voltages. As a typical example, the
result of the 7d film upon electro-oxidation (p-doping) is
presented in Figure 6 as a series of UV–vis-NIR absorption
curves correlated to electrode potentials. In the neutral form,
polymer 7d exhibited strong absorption at wavelength around
310 nm, characteristic for triphenylamine, but it almost trans-
parent in the visible region. When the applied voltage was
stepped from 0 to 1.14 V, the intensity of the absorption peak
around 310 nm decreased gradually, and new peaks at 367 and
741 nm gradually increased in intensity [Fig. 6(a)]. Conse-
quently, the film turned from colorless to blue. We attribute
these spectral changes to the formation of a cation radical of
the TPA moiety. As shown in Figure 6(b), when the applied
voltage returned from 1.14 to 0.80 V, the absorption peaks at
367 and 741 nm decreased in intensity while a new absorption
band at 478 nm appeared. Meanwhile, the film changed color
from blue to yellow-orange. After the first electrochemical
series of 7d was recorded from 0.00 to 1.02 V and then back to
0 V, we reapplied the electrode voltage and recorded its
absorption profile. As shown in Figure 6(c), an additional
yellow-orange oxidized state was observed in the rescan film at
applied voltage of 0.89 V. The film showed a multicolored elec-
trochromism from colorless neutral state to yellow-orange (L:
82; a: 6; b: 40) and blue (L: 80; a: 220; b: 216) oxidized states.
The cyclic voltammograms and differential pulse voltammo-
grams (DPV) of PEIs 7f and 7g are illustrated in Figures 4
and 5, respectively. For comparison, the CV diagrams of
structurally related polyimides 7d and 8 are also included in
Figure 4. Polymers 7f and 7g with a TPA unit in both the
dianhydride and diamine components are expected to dis-
play two oxidation processes due to the different electronic
characters of these two TPA segments in their repeating
units. However, these two oxidation processes occurred
almost simultaneously by a two-electron loss event. There-
fore, these two oxidation waves emerged (around 1.1–1.2 V)
and were not well resolved even in their DPV diagrams. The
repetitive CV diagrams of polymer 7g shown in Figure 5
reveal another new oxidation couple at lower potentials
(waves at about 0.90 and 0.76 V). This behavior is similar to
that observed for PEIs 7a–7e and can be attributed to the
concomitant dimerization of the TPA units upon oxidation of
these PEIs.
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The CV and DPV diagrams of the electrochemically cross-
linked film of 7d0 on the ITO glass substrate in 0.1 M Bu4N-
ClO4/CH3CN are displayed in Figure 8. The CV diagram
reveals one emerged oxidation peak at 1.04 V and two asso-
ciated cathodic waves at 0.78 and 0.89 V. As mentioned
above, the first anodic peak (by DPV) corresponds to oxida-
tions of the TPB units to form radical cations, followed sub-
sequently by oxidation to dicationic species. In addition, the
oxidation process of the unreacted TPA units in polyimide
7d0 probably occurred concurrently with the second oxida-
tion of TPB units. Upon oxidation of the 7d0 film, an obvious
electrochromic behavior was observed. The film in the neu-
tral state is almost colorless. It changed color to yellowish
orange at an applied voltage of 0.90 V and to blue at 1.10 V.
Spectroelectrochemical and Electrochromic Properties of
the 7d’ Film
The result of the 7d0 film upon electro-oxidation (p-doping)
is presented in Figure 9 as a series of UV–vis-NIR absorption
curves correlated to electrode potentials. In the neutral form,
polymer 7d0 exhibited strong absorption at wavelength
around 303 nm, characteristic for triphenylamine, but it
almost transmissive in the visible region. When the applied
voltage was stepped from 0 to 0.91 V, the new peaks at 485
nm gradually increased intensity. Meanwhile, the film
changed color from colorless to yellowish orange (L: 82; a:
6; b: 40). This spectral and color change can be attributed to
the first oxidation of the TPB units in the 7d0 film. When the
applied potential became more anodic at 1.03 V, the absorb-
ance at 485 nm decreased and a strong broadband centered
at 839 nm appeared, which we assigned to the formation of
TPA radical cations and TPB dications. Meanwhile, the film
changed color to blue (L: 80; a: 220; b: 216).
FIGURE 4 (a) Cyclic voltammograms of the cast films of polyi-
mides 7d, 7f, and 8 on an ITO-coated glass substrate in 0.1 M
Bu4NClO4/CH3CN solution at a scan rate of 50 mV s21. (b)
Differential pulse voltammogram (DPV) of 7f. [Color figure
can be viewed in the online issue, which is available at
wileyonlinelibrary.com.]
For the explanation of this spectral change, repetitive CV dia-
grams of 7d are also included in Figure 6(d). The new yellow-
orange state should be related to the oxidation of the benzidine
group formed by the oxidative coupling between TPA units.16(c)
Electrochemical Crosslinking of the Poly(ether-imide)s
To obtain a crosslinked film of PEI 7d, the solution of 7d in
Bu4NClO4/CH2Cl2 was repeated scanned between 0 and 1.4
V. Upon repetitive scanning, we observed a progressive
growth in all peak currents and an insoluble polymer film
being deposited on the ITO glass surface (Fig. 7). This behav-
ior suggests that the oxidative coupling of the radical cations
of TPA units in the polymer chain which might produce a
tetraphenylbenzidine (TPB) structure (Scheme 3).16(a,c),27
Thus, the linear PEI 7d may formed a partially crosslinked
structure (we coded the partially crosslinked polymer as
7d0) after repeated CV scans.
FIGURE 5 (a) Repetitive cyclic voltammograms of the cast film
of poly(ether-imide) 7g on an ITO-coated glass substrate in 0.1
M Bu4NClO4/CH3CN solution at a scan rate of 50 mV s21
.
(b) Differential pulse voltammograms of 7g. [Color figure
can be viewed in the online issue, which is available at
wileyonlinelibrary.com.]
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FIGURE 6 (a) Absorption spectra of thin film of poly(ether-imide) 7d on an ITO electrode at various applied voltages from 0 to 1.14 V,
(b) spectral change from 1.14 back to 0.80 V, (c) the second spectroelectrochemical series, and (d) the color changes and repetitive
cyclic voltammograms of the 7d film. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
Electrochromic switching studies for the PEI films were per-
formed to monitor the percent transmittance changes (DT%)
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 about 1 cm2.
Figure 10 depicts the optical transmittance as a function of
time, current densities monitored and the first cycle optical
transmittance changes for the 7d0 film at 485 and 839 nm
by applying square-wave potential steps between 0 and 0.91
V for a resident time of 10 s and between 0 and 1.03 V for a
resident time of 11 s. The response time was calculated at
90% of the full-transmittance change, because it is difficult
to perceive any further color change with naked eye beyond
this point. As shown in Figure 10(c), the 7d0 film attained
90% of a complete orange coloring and bleaching in 4 and
2.3 s, respectively. The optical contrast measured as DT% of
7d0 between neutral colorless and oxidized yellow states
was found to be 22% at 485 nm. The 7d0 film attained 90%
of a complete blue coloring and bleaching in 4.6 and 3.5 s,
respectively [Fig. 10(c)]. The optical contrast measured as
FIGURE 7 Repeated cyclic voltammograms of poly(ether-imide)
7d between 0 and 1.4 V in 0.1 M Bu4NClO4/CH2Cl2 solution, with
a scan rate of 150 mV s21. The resulting partially crosslinked
polymer is coded as 7d0. [Color figure can be viewed in the online
issue, which is available at wileyonlinelibrary.com.]
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SCHEME 3 Proposed coupling reaction of the TPA radical cations in the PEIs, together with the following oxidation reactions.
DT% of polyimide 7d0 between neutral colorless and oxi-
dized blue states was found to be 81% at 839 nm. The elec-
trochromic coloring efficiency (CE) for the yellowish orange
electro-oxidation, the same as was already observed for the
solution spectroelectrochemistry experiments. When the
potential was subsequently set back at 0 V, the polymer film
turned back to original colorless. We believe that optimiza-
tion could further improve the device performance and fully
explore the potential of these electrochromic PEIs.
coloring (g 5 DOD485/Q) was estimated to be 391 cm2 C21
,
and that for the blue coloring (g 5 DOD839/Q) of the 7d0 film
was estimated to be 384 cm2 C21 (Table 4).
Finally, we fabricated as preliminary investigations single
layer electrochromic cells (Fig. 11). The polyimide films were
spray-coated onto ITO-glass and then dried. Afterward, the
gel electrolyte was spread on the polymer-coated side of the
electrode and the electrodes were sandwiched. To prevent
leakage, an epoxy resin was applied to seal the device. As a
typical example, an electrochromic cell based on PEI 7d0 was
fabricated. The polymer film is colorless in neutral form.
When the voltage was increased (to a maximum of 1.29 V),
the color changed to yellowish orange and blue due to
CONCLUSIONS
A new bis(ether anhydride) monomer containing the triphe-
nylamine group, namely 4,40-bis(3,4-dicarboxyphenoxy)tri-
phenylamine dianhydride, was successfully synthesized and
was polymerized with aromatic diamines to a series of elec-
troactive PEIs. Most of these PEIs were easily soluble in
polar organic solvents and could afford flexible and strong
films with high thermal stability. The PEIs could be further
converted into a crosslinked film via oxidative coupling of
their TPA radical cations in an electrochemical cell. The
FIGURE 8 Cyclic voltammograms of the deposited film of elec-
trochemically crosslinked PEI 7d0 on an ITO-coated glass sub-
FIGURE 9 Spectroelectrochemistry of the 7d0 thin film on the
ITO-coated glass substrate in 0.1 M Bu4NClO4/CH3CN. [Color
figure can be viewed in the online issue, which is available at
wileyonlinelibrary.com.]
strate in 0.1 M Bu4NClO4/CH3CN at a scan rate of 50 mV s21
.
[Color figure can be viewed in the online issue, which is avail-
able at wileyonlinelibrary.com.]
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FIGURE 10 (a) Optical transmittance changes, (b) current densities monitored and (c) the first cycle optical transmittance changes
for the 7d0 film at 485 and 839 nm on the ITO-glass slide (coated area: 1 cm2) (in CH3CN with 0.1 M Bu4NClO4 as the supporting
electrolyte). The dotted line represents the applied voltage. [Color figure can be viewed in the online issue, which is available at
wileyonlinelibrary.com.]
TABLE 4 Electrochromic Properties of the 7d0 Film
Response Timeb
DODc
Qd (mC cm22 d
)
CE (cm2 C21 e
)
a
Polymer
kmax (nm)
D%T
tc (s)
tb (s)
7d’
485
839
22
81
4
2.3
3.5
0.133
0.529
0.34
1.38
391
384
4.6
a
d
Wavelength of absorption maximum.
Qd is ejected charge, determined from the in situ experiments.
Coloration efficiency (CE) 5 DOD/Qd.
b
c
e
Time for 90% of the full-transmittance change.
Optical Density (DOD) 5 log[Tbleached/Tcolored], where Tcolored and
Tbleached are the maximum transmittance in the oxidized and neutral
states, respectively.
crosslinked PEI films exhibit reversible electrochemical oxi-
dation accompanied by strong color changes that can be
switched through modulation of the applied potential. Elec-
trochemical and spectral results showed that these polymers
can be employed as potential anodically coloring materials in
the development of electrochromic devices. Further develop-
ment of materials of this type would seem to be warranted
by the encouraging initial results presented here.
ACKNOWLEDGMENTS
The authors thank the financial supports from National Science
Council of Taiwan and Industrial Technology Research Institute.
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