Synthesis and electrochromic properties of aromatic polyimides bearing pendent triphenylamine units

Article abstract of DOI:10.1016/j.polymer.2014.03.031

A series of aromatic polyimides with pendent triphenylamine group were synthesized from equimolar mixtures of 4,4′-oxydianiline (ODA) and 4-(3,5-diaminobenzamido)triphenylamine (4), 4-(3,5-diaminobenzamido)-4′, 4″-di-tert-butyltriphenylamine (t-Bu-4) or 4-(3,5-diaminobenzamido)- 4′,4″-dimethoxytriphenylamine (MeO-4) with two aromatic tetracarboxylic dianhydrides (DSDA or 6FDA) via a conventional two-step procedure that included a ring-opening polyaddition to give poly(amic acid)s, followed by chemical imidization. These polyimides exhibited good solubility in polar organic solvents and could be solution-cast into flexible and strong films. They showed excellent thermal stability, with T_g values in the range of 284-309°C. The polyimides derived from diamines t-Bu-4 and MeO-4 exhibited reversible electrochemical oxidation, accompanied by strong color changes with high contrast ratio and electrochromic stability. For the polyimides derived from diamine 4, the coupling reaction between the triphenylamine radical cations occurred during the oxidative process forming a tetraphenylbenzidine structure, which resulted in an additional oxidation state and color change together with enhanced near-IR absorption at fully oxidized state.

Full text of DOI:10.1016/j.polymer.2014.03.031

Polymer 55 (2014) 2411e2421
Contents lists available at ScienceDirect
Polymer
journal homepage: www.elsevier.com/locate/polymer
Synthesis and electrochromic properties of aromatic polyimides
bearing pendent triphenylamine units
*
Sheng-Huei Hsiao , Yu-Tan Chou
Department of Chemical Engineering and Biotechnology, National Taipei University of Technology, Taipei 10608, Taiwan, ROC
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of aromatic polyimides with pendent triphenylamine group were synthesized from equimolar
mixtures of 4,40-oxydianiline (ODA) and 4-(3,5-diaminobenzamido)triphenylamine (4), 4-(3,5-
diaminobenzamido)-40,400-di-tert-butyltriphenylamine (t-Bu-4) or 4-(3,5-diaminobenzamido)-40,400-
dimethoxytriphenylamine (MeO-4) with two aromatic tetracarboxylic dianhydrides (DSDA or 6FDA) via
a conventional two-step procedure that included a ring-opening polyaddition to give poly(amic acid)s,
followed by chemical imidization. These polyimides exhibited good solubility in polar organic solvents
and could be solution-cast into flexible and strong films. They showed excellent thermal stability, with Tg
values in the range of 284e309 ꢀC. The polyimides derived from diamines t-Bu-4 and MeO-4 exhibited
reversible electrochemical oxidation, accompanied by strong color changes with high contrast ratio and
electrochromic stability. For the polyimides derived from diamine 4, the coupling reaction between the
triphenylamine radical cations occurred during the oxidative process forming a tetraphenylbenzidine
structure, which resulted in an additional oxidation state and color change together with enhanced near-
IR absorption at fully oxidized state.
Received 9 January 2014
Received in revised form
12 March 2014
Accepted 19 March 2014
Available online 25 March 2014
Keywords:
Polyimides
Triphenylamine
Electrochromism
Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction
backbone [9e20]. Incorporation of three-dimensional, propeller-
shaped triphenylamine (TPA) unit into the polyimide backbone not
Aromatic polyimides are commercially important materials
used extensively in a wide range of optoelectronic applications due
to their excellent chemical, thermal, and dielectric properties [1e4].
However, the technological applications of conventional poly-
imides are limited by processing difficulties because of high
melting or glass-transition temperatures (Tg) and limited solubility
in most organic solvents due to their rigid backbones and strong
interchain interactions. Thus, polyimide processing is generally
carried out via the poly(amic acid) precursor, which is then con-
verted to polyimide by vigorous thermal cyclodehydration. This
process has inherent problems such as the emission of volatile by-
products and storage instability of poly(amic acid) solution. To
overcome these problems, many attempts have been made to
synthesize soluble and processable polyimides in fully imidized
form while maintaining their excellent properties [5e8]. One of the
common methods used for increasing solubility and processability
of polyimides without much sacrificing high thermal stability is the
introduction of bulky, packing-disruptive groups into the polymer
only resulted in enhanced solubility [21e24] but also led to new
electronic functionality of polyimides, such as electrochromic [25e
30] and memory [31e36] characteristics, due to the redox-activity
of the triarylamino core.
The anodic oxidation pathways of TPA have been extensively
studied by Adams and co-workers [37]. The electrogenerated cation
radical of TPA is not stable and could dimerize to form tetraphe-
nylbenzidine by tail to tail coupling with the loss of two protons per
dimer. When the phenyl groups were incorporated by bulky or
electron-donating substituents such as tert-butyl and methoxy
groups at the para-coupling sites of the TPA, the coupling reaction
were greatly prevented [38e40]. It has been demonstrated that
TPA-based polyimides generally exhibited poor electrochemical
and electrochromic stability as compared to their polyamide ana-
logs because of the strong electron-withdrawing imide group,
which increases the oxidation potential of the TPA unit and de-
stabilizes the resultant amino radical cation upon oxidation [41,42].
Attaching the TPA units as pendent groups on the polyimide
backbone may improve the electrochemical and elcetrochromic
stability of this kind of electroactive polymers. In this work, aro-
matic polyimides containing pendent TPA units were synthesized
by incorporating the diamine components of 4, t-Bu-4 or MeO-4
* Corresponding author. Tel.: þ886 2 27712171x2548; fax: þ886 2 27317117.
E-mail address: shhsiao@ntut.edu.tw (S.-H. Hsiao).
http://dx.doi.org/10.1016/j.polymer.2014.03.031
0032-3861/Ó 2014 Elsevier Ltd. All rights reserved.

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S.-H. Hsiao, Y.-T. Chou / Polymer 55 (2014) 2411e2421
Scheme 1. Synthesis of amino compounds 2, t-Bu-2, and MeO-2.
into the polymer chain. The polyimides are expected to exhibit high
thermal stability due to their aryl imide backbones, together with
good solubility and redox-activity because of the laterally attached
TPA groups. Furthermore, the incorporation of methoxy or tert-
butyl substituents is expected to give extra electrochemical and
electrochromic stability of the resulting polyimides.
butyltriphenylamine (t-Bu-4) and 4-(3,5-diaminobenzamido)-
40,400-dimethoxytriphenylamine (MeO-4) were prepared by the
Pd/C-catalyzed hydrazine reduction of the corresponding dinitro
compounds 3, t-Bu-3 and MeO-3 obtained from the condensation
of 3,5-dinitrobenzoyl chloride with amino compounds 2, t-Bu-2
and MeO-2, respectively, in N-methyl-2-pyrrolidone (NMP) in the
presence of triethylamine (Scheme 2). Details of the synthetic
procedures and characterization data of these diamine monomers
are included in the Supporting Information (SI). N,N-Dimethyla-
cetamide (DMAc, Fluka) was dried over calcium hydride for 24 h,
2. Experimental
2.1. Materials
ꢀ
distilled under reduced pressure, and stored over 4 A molecular
sieves in a sealed bottle. 3,30,4,40-Diphenylsulfonetetracarboxylic
dianhydride (DSDA; New Japan Chemical Co.) and 2,2-bis(3,4-
4-Aminotriphenylamine (2) (mp ¼ 148e149 ꢀC) [25] and 4-
amino-40,400-di-tert-butyltriphenylamine (t-Bu-2) (mp ¼ 139e
141 ꢀC) [43] were synthesized by hydrazine Pd/C-catalyzed
reduction of 4-nitrotriphenylamine (1) and 4,40-di-tert-butyl-400-
nitrotriphenylamine (t-Bu-1) resulting from the fluoro-
displacement reaction of p-fluoronitrobenzene with the sodium
amide of aniline and bis(4-tert-butylphenyl)amine formed in situ
by treatment with sodium hydride (Scheme 1). According to a
reported method [44], 4-amino-40,400-dimethoxytriphenylamine
(MeO-2) (mp ¼ 133e134 ꢀC) was synthesized by hydrazine Pd/C-
catalyzed reduction of 4-nitro-40,400-dimethoxytriphenylamine
(MeO-1) resulting from the Ullmann reaction of 4-nitroaniline
with two equivalent amount of iodoanisole by using copper
powder and potassium carbonate (K2CO3) in 1,2-dichlorobenzene.
According to a known means [45e47], 4-(3,5-diaminobenzamido)
triphenylamine (4), 4-(3,5-diaminobenzamido)-40,400-di-tert-
dicarboxyphenyl)hexafluoropropane
dianhydride
(6FDA;
Hoechst Celanese) were heated at 200 ꢀC in vacuo for 3 h before
use. Tetra-n-butylammonium perchlorate (Bu4NClO4) was ob-
tained from Acros and recrystallized twice from ethyl acetate and
then dried in vacuo before use. All other reagents were used as
received from commercial sources.
2.2. Synthesis of polyimides
The polyimides were prepared from DSDA or 6FDA with an
equimolar mixture of 4,40-oxydianiline (ODA) and diamine 4, t-Bu-4,
or MeO-4 by the conventional two-step method via chemical
imidization reaction. A typical example for the preparation of
polymer 5b is given. A mixture of diamine 4 (0.1330 g, 0.25 mmol)
Scheme 2. Synthesis of diamine monomers 4, t-Bu-4, and MeO-4.

S.-H. Hsiao, Y.-T. Chou / Polymer 55 (2014) 2411e2421
2413
and ODA (0.0675 g, 0.25 mmol) was dissolved in 4.8 mL of DMAc in a
50-mL round-bottom flask. Then dianhydride 6FDA (0.2995 g,
0.50 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 3 h to yield a
viscous poly(amic acid) solution. The inherent viscosity of the
resulting poly(amic acid) was 1.10 dL/g, measured in DMAc at a
concentration of 0.5 g/dL at 30 ꢀC. Then, 2 mL of acetic anhydride and
1 mL of pyridine were added to the obtained poly(amic acid) solu-
tion, 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 an excess of methanol giving rise to a pale yellow pre-
cipitate that was collected by filtration, washed thoroughly with hot
water and methanol, and dried. The inherent viscosity of the
resulting polyimide 5b was 0.81 dL/g, measured in DMAc at a con-
centration of 0.5 g/dL at 30 ꢀC. A polymer solution was made by the
dissolution of about 0.4 g of the polyimide sample in 5 mL of hot
DMAc. The homogeneous solution was poured into a 7-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.
The IR spectrum of 5b (film) exhibited characteristic imide ab-
sorption bands at 1783 cmꢁ1 (asymmetrical C]O stretch),
1729 cmꢁ1 (symmetrical C]O stretch). 1H NMR (500 MHz, DMSO-
CHNS elemental analyzer. 1H and 13C NMR spectra were measured
on a Bruker Avance 500 FT-NMR system. The chemical shifts in the
NMR spectra were reported in parts per million (ppm) using tet-
ramethylsilane as an internal standard. Splitting patterns were
designed as s (singlet), d (doublet), t (triplet), or m (multiplet). The
inherent viscosities were determined with
a Cannon-Fenske
viscometer at 30 ꢀC. Wide-angle X-ray diffraction (WAXD) mea-
surements were performed at room temperature (ca. 25 ꢀC) on a
Shimadzu XRD-6000 X-ray diffractometer with a graphite mono-
chromator (operating at 40 kV and 30 mA), using nickeꢀl-filtered Cu-
ꢀ
K
a
radiation (
l
¼ 1.5418 A). The scanning rate was 2 /min over a
range of 2
q
¼ 10e40ꢀ. Thermogravimetric analysis (TGA) was per-
formed with a PerkineElmer Pyris 1 TGA. Experiments were carried
out on approximately 4e6 mg of samples heated in flowing nitro-
gen or air (flow rate ¼ 40 cm3/min) at a heating rate of 20 ꢀC/min.
DSC analyses were performed on a PerkineElmer Pyris 1 DSC at a
scan rate of 20 ꢀC/min in flowing nitrogen. Ultravioletevisible (UVe
Vis) spectra of the polymer films were recorded on an Agilent 8453
UVeVisible spectrometer. Cyclic voltammetry (CV) was performed
with a CH Instruments 611C electrochemical analyzer using ITO as a
working electrode (the coating area of the polymer film is
approximately 1 cm2, 0.8 cm ꢂ 1.25 cm) and a platinum wire as an
auxiliary electrode at a scan rate of 50 mV/s against a Ag/AgCl
reference electrode in acetonitrile (CH3CN) or N,N-dime-
thylformamide (DMF) solution of 0.1 M Bu4NClO4 under a nitrogen
atmosphere. Spectroelectrochemistry analyses were carried out
with an electrolytic cell, which was composed of a 1 cm cuvette, ITO
d6,
d
, ppm): 6.97 (6H, He þ Hg), 7.01 (2H, Hd), 7.23 (8H, Hi þ Hf), 7.48
(4H, Hh), 7.67 (2H, Hc), 7.75 (4H, Hj þ Hj’), 7.80 (1H, Ha), 7.96 (4H,
Hl þ Hl’), 8.13 (1H, Hb), 8.12 (4H, Hk þ Hk’), 10.41 (1H, amide proton).
2.3. Fabrication of electrochromic device
as a working electrode, a platinum wire as an auxiliary electrode,
and a Ag/AgCl reference electrode. Absorption spectra in the
spectroelectrochemical experiments were measured with an Agi-
lent 8453 UVeVis diode array spectrophotometer. Colorimetric
measurements were obtained using a Minolta CS-100A Chroma
Meter and the results are expressed in term of lightness (L*) and
color coordinates (a*, b*). Coloration efficiency is derived from the
Electrochromic polymer film was prepared by dropping solution
of the polyimide (4 mg/mL in DMAc) onto an ITO-coated glass
substrate (20 ꢂ 30 ꢂ 0.7 mm, 50e100
U/square). The polymers
were drop-coated onto an active area (about 20 mm ꢂ 20 mm) then
dried in vacuum. 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 dissolved in
dry acetonitrile (5 g), 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 slowly
heated until gelation. The gel electrolyte was spread on the
polymer-coated side of the electrode, and the electrodes were
sandwiched. Finally, a transparent epoxy resin was used to seal the
device.
equation:
h
¼
DOD/Qd, DOD is optical density change at specific
absorption wavelength and Qd is ejected charge determined from
the in situ experiments.
3. Results and discussion
3.1. Polymer synthesis
Homopolymerization of any one of the diamine monomers 4, t-
Bu-4, and MeO-4 with dianhydride DSDA or 6FDA just produced
low-molecular-weight oligomers, which could not afford flexible
films. Less favorable results from homopolymerization may be
attributed to the formation of macrocyclic oligomers which limits
ultimate molecular weights achievable during the polyaddition
2.4. Instrumentation and measurements
Infrared (IR) spectra were recorded on a Horiba FT-720 FT-IR
spectrometer. Elemental analyses were run in a Heraeus VarioEL Z

2414
S.-H. Hsiao, Y.-T. Chou / Polymer 55 (2014) 2411e2421
Scheme 3. Synthesis of polyimides and images of their cast films.
Table 1
Inherent viscosity and solubility behavior of polyimides.
reactions of these meta-oriented phenylenediamines with the
a
Polymer code hinh (dL/g)
Solubility in various solventsb
dianhydrides. In order to obtain polymers with sufficient molecular
weights to permit the casting of tough films, we carried out the
copolymerization of an equimolar mixture of ODA and each of the
newly synthesized diamines with dianhydrides DSDA and 6FDA,
respectively (Scheme 3). As shown in Table 1, the resulting pol-
y(amic acid)s exhibited inherent viscosities of 0.89e1.24 dL/g.
These poly(amic acid)s were chemically cyclodehydrated into the
corresponding polyimides by treatment with a mixture of acetic
anhydride and pyridine. The obtained polyimides were soluble in
polar solvents such as DMAc. Therefore, the characterization of
solution viscosity was carried out without any difficulty, and the
inherent viscosities of these polyimides were recorded in the range
of 0.60e0.88 dL/g, as measured in DMAc at 30 ꢀC. All the polyimides
could be solution cast to flexible and strong films (see the photos
PAA
PI
NMP DMAc DMF DMSO m-cresol THF
5a
5b
t-Bu-5a
t-Bu-5b
MeO-5a
MeO-5b
0.89
1.10
1.24
1.19
1.17
1.21
0.66
0.81
0.60
0.88
0.62
0.86
þþ
þþ
þþ
þþ
þþ
þþ
þþ
þþ
þþ
þþ
þþ
þþ
þþ
þþ
þþ
þþ
þþ
þþ
þþ
þþ
þþ
þþ
þþ
þþ
þh
þh
þh
þh
þꢁ
þh
e
þþ
þþ
þþ
þþ
þþ
Notation: þþ, soluble at room temperature; þh, soluble on heating at 100 ꢀC; þꢁ,
partially soluble; ꢁ, insoluble even on heating. NMP: N-methyl-2-pyrrolidone;
DMAc: N,N-dimethylacetamide; DMF: N,N-dimethylformamide; DMSO: dimethyl
sulfoxide; THF: tetrahydrofuran.
Inherent viscosity measured at a concentration of 0.5 g/dL in DMAc at 30 ꢀC.
PAA: poly(amic acid).
a
b
Qualitative solubility was determined by using 10 mg sample in 1 mL of stirred
solvent.

S.-H. Hsiao, Y.-T. Chou / Polymer 55 (2014) 2411e2421
2415
shown in Scheme 1), indicating that they are high-molecular-
weight polymers.
assigned to the pendent amide linkages. The signal appearing at
1.22 ppm in the 1H NMR spectrum of polyimide t-Bu-5b (Fig. S9) is
peculiar to the tert-butyl groups, and that appearing at 3.70 ppm in
the 1H NMR spectrum of polyimide MeO-5b (Fig. 1) is assigned to
the methoxy substituents. Assignments of each proton are in good
agreement with the structures of their repeating units.
Structural features of these polyimides were characterized by IR
and NMR analysis. A typical set of IR spectra of polyimide 5b and its
poly(amic acid) precursor are illustrated in SI Fig. S7. All polyimides
exhibited characteristic imide group absorptions around 1780 and
1720 cmꢁ1 (typical of imide carbonyl asymmetrical and symmet-
rical stretch), 1380 cmꢁ1 (CeN stretch), and 1100 and 720 cmꢁ1
(imide ring deformation). Representative 1H NMR and HeH COSY
NMR spectra of polyimide MeO-5b in DMSO-d6 are presented in
Fig. 1. The NMR spectra of 5b and t-Bu-5b are included in SI Figs. S8
3.2. Solubility of polyimides
The solubility of the polyimides in several organic solvents was
investigated qualitatively, and the results are summarized in
Table 1. All samples have been found soluble in aprotic polar sol-
vents (NMP, DMAc, DMF, and DMSO) at room temperature. Except
and S9. All the aromatic protons resonated in the region of
d 6.8e
8.2 ppm. The signals appearing around 10.3e10.4 ppm were
Fig. 1. (a)1H NMR spectrum and (b) HeH COSY spectrum of polyimide MeO-5b.

2416
S.-H. Hsiao, Y.-T. Chou / Polymer 55 (2014) 2411e2421
for polyimide 5a, all of them are also readily soluble in THF. The
high solubility can be attributed in part to the introduction of the
bulky pendent TPA moiety into the repeat unit. The excellent sol-
ubility makes these polymers potential candidates for practical
applications by common solution processes to afford high perfor-
mance thin films for optoelectronic devices. As shown in Fig. S10
(SI), all the polyimides showed amorphous WAXD patterns. This
result is reasonable because the bulky pendent groups and random
repeat units do not favor their close chain packing.
3.4. Electrochemical properties
The anodic and cathodic electrochemical properties of the cast
polyimide films on the ITO electrodes were assessed by cyclic vol-
tammetry (CV) using a conventional three-electrode cell assembly.
The oxidation and reduction cycles of the film samples were
measured in acetonitrile and DMF, respectively, using Bu4NClO4 as
the electrolyte. All the polyimides showed a reversible oxidation
process in their first CV scans at around 0.66e0.97 V (Eo1x/2) that
corresponds to the TPA oxidation. Typical CV curves of polyimides
5a, t-Bu-5a, and MeO-5a are shown in Fig. 2. As expected, poly-
3.3. Thermal properties
amide MeO-5a exhibited
a
lower oxidation potential
0.69 V) and 6b
(Eonset 0.58 V) than t-Bu-5a (Eonset
¼
¼
The thermal stability and phase-transition temperatures of
these polyimides were investigated by TGA, DSC, and TMA tech-
niques. The thermal behavior data are summarized in Table 2. DSC
measurements were conducted with a heating rate of 20 ꢀC/min in
nitrogen. Quenching from an elevated temperature of about 400 ꢀC
to 50 ꢀC gave predominantly amorphous samples so that the glass
transition temperatures (Tg) of all the polyimides could be easily
determined in the second heating traces of DSC (Fig. S11, SI). Tg
values of these polyimides were recorded in the range 284e309 ꢀC.
Higher Tg values for the polyimides t-Bu-5a and t-Bu-5b as
compared with the other counterparts can be attributed to the
bulky tert-butyl groups thus restricting the segment mobility. The
softening temperatures (Ts) (may be referred to as apparent Tg) of
the polymer film samples were determined by TMA method with a
loaded penetration probe. These polyimides exhibited Ts values of
284e295 ꢀC.
Fig. S12 compares the TGA thermograms of 5a, t-Bu-5a and
MeO-5a in both air and nitrogen atmospheres. All the prepared
polyimides exhibited good thermal stability with no significant
weight loss up to 450 ꢀC in nitrogen or air atmosphere. Decom-
position temperatures (Td) at a 10% weight loss of these polymers
in nitrogen and air were recorded in the range of 469e552 and
486e552 ꢀC, respectively. The amount of carbonized residue
(char yield) of these polymers in nitrogen atmosphere was more
than 45% at 800 ꢀC. Polyimides t-Bu-5 and MeO-5 series
exhibited a lower Td value as compared with the corresponding 5
ones without substituents on the pendent phenyl rings. This is
reasonable when considering the less stable aliphatic segments.
The DSDA polyimides showed a relatively lower Td and char yield
as compared to the 6FDA polyimides due to the less stable sul-
fonyl group.
(Eonset ¼ 0.88 V) because of the electron-donating methoxy sub-
stituents on the pendent phenyl rings of the TPA unit.
The repetitive CV scanning results for polyimide 5a are shown
in Fig. S13. Upon repetitive scanning for polyimide 5a over the
voltage range from 0 to 1.20 V, a new oxidation wave appeared as
a shoulder at 0.86 V since the second scan. This is a typical
oxidation wave of the benzidine group, indicating the occurrence
of the oxidative coupling between the TPA units. As suggested in
Fig. S14, the TPA radical cations may dimerize to a tetraphe-
nylbenzidine (TPB) segment that is easier to oxidize than the
parent TPA unit. The electrochemical properties of the analogous
polyimide MeO-5a are completely different. Repetitive scans
between 0 and 0.90 V at a scan rate of 50 mV/s provided the same
patterns as that observed in the first scan. The curves were
affected by only the diffusion contribution, and no new waves
were detected under these experimental conditions. This
reversible oxidation process suggests that the radical cations of
MeO-5a are more stable than those produced from the oxidation
of 5a. The tert-butyl-substituted analogs showed
a similar
Table 2
Thermal properties of the polyimides.
Polymera code Tg (ꢀC)b Ts (ꢀC)c Td at 10% weight loss (ꢀC)d
Char yield
(wt %)e
In N2
In air
5a
5b
t-Bu-5a
t-Bu-5b
MeO-5a
MeO-5b
302
296
309
298
284
287
293
284
300
291
289
295
510
552
490
542
469
531
543
552
486
530
493
531
48
54
45
54
48
53
a
All the polymer films were heated at 300 ꢀC for 1 h prior to DSC, TMA and TGA
experiments.
b
Midpoint temperature of the baseline shift on the second DSC heating trace
(rate ¼ 20 ꢀC/min) of the sample after quenching from 400 to 50 ꢀC (cooling
rate ¼ 200 ꢀC/min) in nitrogen.
c
Softening temperature measured by TMA with a constant applied load of 10 mN
at a heating rate of 10 ꢀC/min.
d
Decomposition temperature at which a 10% weight loss was recorded by TGA at
Fig. 2. Anodic cyclic voltammograms of the cast films of polyimides 5a, t-Bu-5a, and
MeO-5a on an ITO-coated glass substrate in 0.1 M Bu4NClO4/CH3CN solution at a scan
rate of 50 mV/s.
a heating rate of 20 ꢀC/min.
e
Residual weight % at 800 ꢀC at a scan rate 20 ꢀC/min in nitrogen.

S.-H. Hsiao, Y.-T. Chou / Polymer 55 (2014) 2411e2421
2417
Fig. 3. Cyclic voltammograms of the cast films of polyimides (a) 5a, (b) 5b, (c) t-Bu-5a, (d) t-Bu-5b, (e) MeO-5a, and (f) MeO-5b on an ITO-coated glass substrate in 0.1 M Bu4NClO4/
CH3CN (for anodic process) and DMF (for cathodic process) solution at a scan rate of 50 and 100 mV/s, respectively.
electrochemical behavior as that of MeO-5a. Thus, incorporation
of bulky tert-butyl or electron-donating methoxy groups on the
active sites of the TPA unit greatly prevents the coupling reaction
of these electroactive polyimides. As illustrated in Fig. 3, all the
polyimides exhibited additional two quasi-reversible reduction
peaks at Epc ¼ ꢁ0.87wꢁ1.12 V and ꢁ1.06wꢁ1.24 V due to the
electron-charging on the phthalimide units. Due to the strong
electron-withdrawing property of the sulfonyl group, the imide
segment derived from DSDA can be reduced at a less negative
potential.
Table 3
Redox potentials and energy levels of polyimides.
Polymer code
Thin film (nm)
Oxidation potential (V)a
Reduction potential (V)b
Eogpt (eV)c
HOMO (eV)d
LUMO (eV)d
abs
abs
ox
ox
red
red1
red2
l
l
E
E
E
E
E
1/2
max
onset
onset
1/2
onset
1/2
5a
5b
t-Bu-5a
t-Bu-5b
MeO-5a
MeO-5b
301
297
304
300
297
295
397
397
402
407
408
409
0.88
0.79
0.69
0.72
0.58
0.57
0.92
0.97
0.81
0.87
0.66
0.70
ꢁ0.68
ꢁ0.91
ꢁ0.70
ꢁ0.90
ꢁ0.71
ꢁ0.94
ꢁ0.82
ꢁ1.05
ꢁ0.83
ꢁ1.06
ꢁ0.84
ꢁ1.03
ꢁ1.01
ꢁ1.17
ꢁ1.03
ꢁ1.19
ꢁ1.01
ꢁ1.19
3.18
3.12
3.05
3.05
2.94
2.90
5.28
5.33
5.17
5.23
5.02
5.06
2.10
2.21
2.12
2.18
2.08
2.16
a
vs. Ag/AgCl in CH3CN.
b
c
vs. Ag/AgCl in DMF. E1/2: the half-wave potential (Average potential of the redox couple peaks).
The data were calculated from polymer films by the equation: Eogpt ¼ 1240/lonset
.
d
ox
The HOMO and LUMO energy levels were calculated from E
values of CV curves and were referenced to ferrocene (4.8 eV relative to the vacuum energy level).
onset

2418
S.-H. Hsiao, Y.-T. Chou / Polymer 55 (2014) 2411e2421
The energy levels of the highest occupied molecular orbital
(HOMO) and lowest unoccupied molecular orbital (LUMO) of the
TPA moiety. Further increase of the applied voltage to 1.1 V led to an
additional strong absorption in the near-IR region (around 875 nm)
with the color changed to blue (L*: 55; a*: ꢁ6; b*: ꢁ24). This result
may be caused by the oxidation of the TPB moiety formed in situ
during the potential scan. After the first electrochemical series of
polyimide 5b was recorded from 0 to 1.1 V and then back to 0 V, we
re-applied the electrode voltage. As shown in Fig. 4b, when the
applied voltage was stepped from 0 to 0.8 V, in addition to a small
band at 750 nm, a new small absorption at 490 nm emerged.
Meanwhile, the film changed from colorless to orange (L*: 70; a*: 3;
b*: 15). The new orange oxidized state may be attributed to the first
oxidation of the TPB unit resulted from the coupling reaction
occurred on the active sites of the side-chain TPA unit (Fig. S14).
When the applied voltage was stepped to 1.1 V, the spectral and
color changes of the film are similar to those observed in the first
spectroelectrochemical series. These results indicated that the
occurrence of the second step oxidation of the TPB moieties asso-
ciated with the oxidation of the residual TPA units. A similar
spectral change was also observed for polyimide 5a.
Spectral changes of polyimide MeO-5b at various applied po-
tentials are illustrated in Fig. 4c. When the applied potential was
gradually increased from 0 to 0.8 V, the electrochromic film of
MeO-5b showed new absorptions at around 380 and 750 nm with
increasing intensity. As shown in Fig. 4d, the spectral changes of the
second spectroelectrochemical series are very similar to those in
the first series. For polyimide t-Bu-5b, its film revealed a medium
absorption at 388 nm and a strong absorption band centered at
750 nm at an applied potential of 1.0 V (Fig. 5a). Both of these two
polymer films were associated with significant color change from
colorless (L*: 96; a*: 1; b*: 2) to cyan (L*: 61; a*: ꢁ2; b*: ꢁ8) or blue
(L*: 30; a*: ꢁ3; b*: ꢁ29) upon electro-oxidation. No remarkable
absorptions at 490 nm and in the near-IR region were observed
ox
corresponding polymers were estimated from the E
values.
1/2
Assuming that the HOMO energy level for the ferrocene/ferroce-
nium (Fe/Feþ) standard is 4.8 eV with respect to the zero vacuum
level, the HOMO and LUMO values for these polyimides were
calculated to be in the range of 5.02e5.33 eV and 2.08e2.21 eV,
respectively. The redox potentials and energy levels of all the pol-
yimides are summarized in Table 3.
3.5. Spectroelectrochemical and electrochromic properties
The electro-optical properties of the polymer films were
investigated using the changes in electronic absorption spectra at
various applied voltages. For these investigations, the polyimide
film was cast on an ITO-coated glass slide (a piece that fit in the
UVevis cuvette), and a homemade electrochemical cell was built
from a commercial UVevis cuvette. The cell was place in the optical
path of the sample light beam in a commercial diode array spec-
trophotometer. This procedure allowed us to obtain electronic ab-
sorption spectra under potential control in a 0.1 M Bu4NClO4/
acetonitrile or DMF solution. The result of the 5b film upon electro-
oxidation is presented in Fig. 4a as a series of UVeviseNIR ab-
sorption curves correlated to electrode potentials. In the neutral
form, at 0.0 V, polyimide 5b exhibited strong absorption at wave-
length around 301 nm, characteristic for TPA, but it is almost
transparent in the visible region (L*: 91; a*: 2; b*: 1). As the applied
voltage was stepped from 0 to 0.8 V, the intensity of the absorption
peak at 301 nm decreased slightly and new peaks at 400, 500 and
750 nm gradually increased in intensity. In the same time, the film
turned into a greenish-blue color. We attribute these spectral
changes to the formation of a stable cation radical of the side-chain
Fig. 4. Spectral changes of the cast film of polyimides on an ITO-coated glass in 0.1 M Bu4NClO4/CH3CN at various applied potentials (vs. Ag/AgCl): (a) the first series for polyimide
5b, (b) the second series for polyimide 5b, (c) the first series for polyimide MeO-5b, and (d) the second series for polyimide MeO-5b.

S.-H. Hsiao, Y.-T. Chou / Polymer 55 (2014) 2411e2421
2419
Fig. 5. Spectroelectrochemistry of polyimide t-Bu-5b thin film on the ITO-coated glass substrate in 0.1 M Bu4NClO4/CH3CN (for the anodic oxidation) or DMF (for the cathodic
reduction) at various applied potentials.
during the oxidation process of these two polyimides. This result
illustrated that almost no benzidine moieties was produced during
the oxidation process. Fig. 5b illustrates the spectral and coloration
changes of t-Bu-5b on electro-reduction. However, the color
change from colorless to light yellow (L*: 81; a*: 1; b*: 5) is not
strong as that observed in the anodic scanning.
The coloration efficiencies of these three polyimides were calcu-
lated to be in the range of 211e320 cm2/C.
The switching stability of the polyimides was investigated by
monitoring the electrochromic contrast (D%T) of the thin films on
repeated square-wave potential steps between their neutral and
oxidized states. Fig. 7aec shows the results of first ten cycles of
polyimides 5b, t-Bu-5b, and MeO-5b, respectively. Polyimides t-
Bu-5b and MeO-5b exhibited better reversibility of electrochromic
characteristics than 5b in the first ten cycles. Therefore, the intro-
duction of the para-substituted methoxy or tert-butyl groups on the
pendent diphenylamino unit effectively prevent the tail to tail
coupling reaction and could increase the electrochromic stability of
these polyimides. After 50 cycles polyimide MeO-5b still preserved
good electrochromic reversibility and moderate optical contrast
(Fig. 7d).
3.6. Electrochromic switching and stability
Electrochemical switching studies for the polyimides were
performed to monitor the percent transmittance change (D%T) as a
function of time at their absorption maximum (lmax) and to
determine the response time by stepping potential repeatedly be-
tween the neutral and oxidized states. The active area of the
polymer film on ITO glass is about 1 cm2. Fig. 6 depicts the optical
transmittance change of polyimides 5b, t-Bu-5b, and MeO-5b as a
function of time at 750 nm by applying square-wave potential steps
of 10 s (complete cycle time is 20 s) between 0 and 1.1,1.0, and 0.8 V,
respectively. The switching time was defined as the time that was
required to reach 90% of the full change in absorbance after
switching potential because it is difficult to perceive any further
color change with naked eye beyond this point. The optical contrast
3.7. Electrochromic device
Based on the foregoing results, it can be concluded that these
polyimides can be used in the construction of electrochromic de-
vices and optical display due to the fast response time and the
robustness of the polymers. Therefore, we fabricated as preliminary
investigations single layer electrochromic cells (Fig. 8). The poly-
imide films were cast onto the ITO-coated glass and then dried.
Afterward, the gel electrolyte was spread on the polymer-coated
side of the electrode and the electrodes were sandwiched. To pre-
vent leakage, an epoxy resin was applied to seal the device. As a
typical example, an electrochromic cell based on polyimide MeO-
5b was fabricated. The polymer film is colorless or very pale yellow
in the neutral form. When the applied voltage was increased to
1.8 V, the film changed to cyan in color due to electro-oxidation.
When the potential was subsequently set back at 0 V, the
measured as D%T of these polymers between neutral and oxidized
blue states was found to be 72% for 5b, 85% for t-Bu-5b, and 83% for
MeO-5b, respectively. The electrochromic properties of the poly-
imide films during n-doping processes are summarized in Table 4.
Table 4
Electrochromic properties of polyimides.
%T Response timea
D
ODb Qd
CEd
c
Polymer code lmax Voltage
D
(nm) (V)
(mC/cm2) (cm2/C)
tc (s)
tb (s)
5b
t-Bu-5b
MeO-5b
750 1.1
750 1.0
750 0.8
72 3.2
85 3.2
83 2.2
2.6
5.4
3.1
0.41 1.94
0.55 1.84
0.54 1.73
211
298
320
a
Time for 90% of the full-transmittance change.
b
Optical density (
D
ODb) ¼ log[Tbleached/Tcolored], where Tcolored and Tbleached are
Fig. 6. Optical transmittance change monitored at 750 nm for the polyimide thin films
(w150 nm in thickness) on ITO-glass slide in 0.1 M Bu4NClO4/CH3CN. (The dotted line
represents the applied voltage.)
the maximum transmittance in the oxidized and neutral states, respectively.
c
Qd is ejected charge, determined from the in situ experiments.
d
Coloration efficiency (CE) ¼
DOD/Qd.

2420
S.-H. Hsiao, Y.-T. Chou / Polymer 55 (2014) 2411e2421
Fig. 8. (a) Photos of single-layer ITO-coated glass electrochromic device, using poly-
imide MeO-5b as active layer. (b) Schematic diagram of polyimide ECD sandwich cell.
dimerize to a benzidine structure which led to additional oxidation
states and color change together with a strong near-IR absorption
at their fully oxidized states. Thus, these characteristics suggest that
this new class of polyimides has great potential for use in opto-
electronics applications such as electrochromic devices.
Acknowledgment
We thank the financial support from the National Science
Council in Taiwan, ROC.
Appendix A. Supplementary data
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.polymer.2014.03.031.
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