A comparative study on the properties of aromatic polyamides with methyl- or trifluoromethyl-substituted triphenylamine groups

Article abstract of DOI:10.1016/j.jfluchem.2016.06.001

Two triphenylamine-based diamine monomers, 4,4′-diamino-4″-methyltriphenylamine and 4,4′-diamino-4″-(trifluoromethyl)triphenylamine, were synthesized via the cesium fluoride-mediated double N-arylation reactions of p-toluidine and p-(trifluoromethyl)anili

Full text of DOI:10.1016/j.jfluchem.2016.06.001

Accepted Manuscript
Title: A comparative study on the properties of aromatic
polyamides with methyl- or trifluoromethyl-substituted
triphenylamine groups
Author: Sheng-Huei Hsiao Kai-Han Lin
PII:
S0022-1139(16)30145-2
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.jfluchem.2016.06.001
FLUOR 8786
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FLUOR
Received date:
Revised date:
Accepted date:
28-4-2016
2-6-2016
6-6-2016
Please cite this article as: Sheng-Huei Hsiao, Kai-Han Lin, A comparative
study on the properties of aromatic polyamides with methyl- or
trifluoromethyl-substituted triphenylamine groups, Journal of Fluorine Chemistry
http://dx.doi.org/10.1016/j.jfluchem.2016.06.001
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A comparative study on the properties of aromatic
polyamides with methyl- or
trifluoromethyl-substituted triphenylamine groups
1
2
Sheng-Huei Hsiao* , Kai-Han Lin
1
Department of Chemical Engineering and Biotechnology, National Taipei University
of Technology, Taipei, Taiwan
2
Department of Chemical Engineering, Tatung University, Taipei, Taiwan
*
Corresponding author.
E-mail: shhsiao@ntut.edu.tw (S.-H. Hsiao)
1

Graphical Abstract-Pictogram
Redox-Active and Electrochromic Aramids
A
new fluorine-containing, triphenylamine-based diamine monomer,
4
,4’-diamino-4”-(trifluoromethyl)triphenylamine, was synthesized and led to a
series of electroactive aromatic polyamides. The electrochemical and
electrochromic properties of the polyamides were investigated and compared with
the
those
of
analogous
polyamides
based
on
4
,4’-diamino-4”-methyltriphenylamine.
2

Highlights

Two triphenylamine-based diamine monomers with methyl or trifluoromethyl
substituent have been synthesized.

New redox-active and electrochromic polyamides with either 4-methyltriphenylamine or
4
-(trifluoromethyl)triphenylamine units were prepared.


The polyamides showed green or dark golden coloring upon oxidation.
Effects of the –CF substituent on the properties of the polyamides have been
3
investigated.
ABSTRACT
Two
triphenylamine-based
diamine
monomers,
and
4
,4’-diamino-4”-methyltriphenylamine
4
,4’-diamino-4”-(trifluoromethyl)triphenylamine, were synthesized via the cesium
fluoride-mediated double N-arylation reactions of p-toluidine and
p-(trifluoromethyl)aniline, respectively, with p-fluoronitrobenzene, followed by
palladium-catalyzed hydrazine reduction of the dinitro intermediates. New
redox-active aromatic polyamides containing main-chain triphenylamine unit with
methyl or trifluoromethyl (–CF ) group on the pendent phenyl ring were prepared
3
by the phosphorylation polycondensation reactions of the synthesized diamine
monomers with commercially available aromatic dicarboxylic acids. The polyamides
were readily soluble in polar organic solvents and could afford flexible and strong
films via solution casting. Cyclic voltammograms of the polyamide films cast onto
the indium-tin oxide (ITO)-coated glass substrate revealed reversible
electrochemical oxidation processes accompanied with color change from pale
yellow to dark golden or green. Comparative studies of the methyl and –CF
3
3

substituents on the properties of the polyamides, such as solubility, thermal,
electrochemical and electrochromic properties, are investigated.
Keywords:
Fluorinated
polyamides,
Triphenylamine,
Trifluoromethyl,
Electrochemistry, Electrochromism, Redox polymers
1
. Introduction
Wholly aromatic polyamides (aramids) are a kind of high performance polymers
owing to their high mechanical properties, good chemical resistance, and excellent
thermal stability [1−3]. However, the main problem of aramids is associated with
their insolubility due to their rigid backbones and strong interchain interactions,
leading to processing difficulties. Therefore, many efforts have been made to
increase the solubility and processability of aramids through structural modification
of their monomers [4−11]. One of the common approaches to increasing solubility
without much compromising their thermal and mechanical stability is the use of
monomers with bulky packing-disruptive moieties [12−16]. Introduction of
trifluoromethyl (–CF
strategies for structural modification leading to substantial solubility enhancement
17−21]. Moreover, the incorporation of bulky groups increases the interchain
3
) groups into aramids has been one of the most widely used
[
spacing and reduces the packing efficiency thereby increasing the intrinsic
microporosity. This ultimate behavior finds application in gas separation membrane
technology, where much effort has been focused to enhance gas permeability
through an increment in intrinsic microporosity which could be achieved by chemical
modification of the polymer chain [22−24].
Triphenylamine (TPA) derivatives and polymers are well-known for their
4

electroactive and photoactive properties that may find optoelectronic applications
in electrophotography, electroluminescent diodes, field-effect transistors, solar
cells, memory devices, and electrochromic or electrofluorochromic devices [25−33].
TPAs can be easily oxidized to form stable radical cations as long as the
para-position of the phenyl rings is protected, and the oxidation process is always
associated with a strong change of coloration. During the past decade, a huge
number of high-performance polymers (typically, aromatic polyamides and
polyimides) carrying the redox-active TPA unit have been prepared and evaluated
for electrochromic applications [34−44]. It has been demonstrated that aromatic
polyamides bearing the propeller-shaped TPA unit in the backbone were
amorphous and easily soluble in polar organic solvents and could be solution-cast
into flexible and strong films with high thermal stability. Thus, incorporation of
three-dimensional, packing-disruptive TPA units into the aramid backbone not only
resulted in enhanced solubility but also led to new electronic functions of aramids
such as electrochromic characteristics.
As reported in the pioneering works by Nelson and Adams et al. [45,46],
unsubstituted TPA undergoes dimerization to tetraphenylbenzidine (TPB) after the
formation of an unstable monocation radical. This is accompanied by the loss of
two protons per dimer and the dimer is more easily oxidized than TPA and also can
+•
undergo further oxidations in two discrete one-electron steps to give TPB and
+2
finally the quinoidal TPB . Quantitative data have been obtained for several
4
-substituted TPAs in the form of second-order coupling rate constants, and it was
generally found that electron-donating substituents such as methoxy group tended
to stabilize the cation radicals while electron-withdrawing groups such as nitro
group had the opposite effect [47]. As a continuation of our efforts in developing
TPA-functionalized high performance polymers, herein we synthesize two
5

TPA-based diamine monomers, 4,4’-diamino-4”-methyltriphenylamine and
,4’-diamino-4”-(trifluoromethyl)triphenylamine, and their derived aromatic
polyamides containing the electroactive TPA unit with electron-donating –CH
group or electron-withdrawing –CF group para substituted on the pendent phenyl
ring. The effect of incorporating the –CH and –CF substituents on the thermal,
4
3
3
3
3
electrochemical and electrochromic properties of the polyamides will be
investigated.
2
. Experimental section
2
.1 Materials
p-Toluidine (TCI), p-(trifluoromethyl)aniline (TCI), p-fluoronitrobenzene (Acros),
1
0 % palladium on charcoal (Pd/C, Fluka), cesium fluoride (CsF, Acros), triphenyl
phosphite (TPP, TCI), and hydrazine monohydrate (Acros) were used without
further purification. Pyridine (Py; Wako) and N-methyl-2-pyrrolidone (NMP; Fluka)
were dried over calcium hydride for 24 h, distilled under reduced pressure, and
stored over 4 Å molecular sieves in sealed bottles. The commercially available
aromatic dicarboxylic acids including terephthalic acid (5a, Wako), isophthalic acid
(
5b, Wako), 4,4’-biphenydicarboxylic acid (5c, TCI), 4,4’-dicarboxydiphenyl ether (5d,
TCI), bis(4-carboxyphenyl) sulfone (5e, New Japan Chemicals Co.),
,2-bis(4-carboxyphenyl)hexafluoropropane (5f, TCI), 1,4-naphthalenedicarboxylic
2
acid (5g, Wako), and 2,6-naphthalenedicarboxylic acid (5h, TCI) were used as
received.
6

2
4
.2 Synthesis of TPA-diamine monomers
,4’-Dinitro-4’’-(trifluoromethyl)triphenylamine (1). In a 250 mL round-bottom flask
equipped with a stirring bar, a mixture of 11.3 g (0.07 mol) of
p-(trifluoromethyl)aniline, 19.8 g (0.14 mol) of p-fluoronitrobenzene, and 24.3 g of
CsF in 120 mL of dimethyl sulfoxide (DMSO) was heated at 120 C for about 48 h.
After cooling, the mixture was poured into 500 mL of water, and the precipitate was
collected by filtration and washed thoroughly with methanol and water. The crude
product was washed several times with methanol and water to afford 25.0 g (yield
8
8 %) of dinitro compound 1 as yellow solid, which could be further recrystallized
from N,N-dimethylformamide (DMF)/methanol to afford yellow crystals with a
o
o
melting point of 294 C (by DSC, 2 C/min). ANAL. Calcd for C19
H
12
F
3
N
3
O
4
(403.31): C,
5
1
6.58%; H, 3.00%; N, 10.42%. Found: C, 56.42%; H, 2.81%; N, 10.46%. IR (KBr): 1586,
−
1
1
323 (−NO
2
), 1117 cm (C–F stretch). H NMR (500 MHz, DMSO-d , δ, ppm): 8.34
6
(d, J = 9.1 Hz, 4H, H
a b c
) , 7.37 (d, J = 9.1 Hz, 4H, H ), 7.29 (d, J = 9.1 Hz 2H, H ), 8.23 (d,
1
3
5
1
J = 9.1 Hz, 2H, H
d
). C NMR (125 MHz, DMSO-d
6
, δ, ppm): 160.75 (C ), 151.50 (C ),
4
7
8
2
2
9
1
44.02 (C ), 143.13 (C ), 126.85 (C , quartet, JC−F = 30 Hz), 126.76 (C ), 124.68 (C ,
quartet, 1JC−F = 254 Hz), 120.03 (C3).
O N
NO2
2
1
2
4
a
N
3
5
b
6
7
c
d
8
9
CF3
1
4
,4’-Diamino-4”-(trifluoromethyl)triphenylamine (2). In a 500-mL round-bottom
flask equipped with a stirring bar, a mixture of 10.0 g (0.025 mol) of dinitro
7

compound 1, 0.2 g of 10% Pd/C, 7.5 mL of hydrazine monohydrate, and 150 mL of
ethanol was heated at reflux for 10 h. The solution was filtered to remove Pd/C
catalyst, and the filtrate was distilled to remove the solvent. The residue was
o
washed with water and dried in vacuo at 100 C to give 8.6 g (yield: 95%) of grey
product; the crude product was further recrystallized from ethanol to afford grey
o
o
crystals with a melting point of 114 C (by DSC, 2 C/min). ANAL. Calcd for
C
19
H
16
F
3
3
N (343.35): C, 66.47%; H, 4.70%; N, 12.24%. Found: C, 66.35%; H, 4.78%; N,
−
1
-1
1
1
2.35%. IR (KBr): 3474, 3379 cm (−NH
2
stretch), 1114 cm (C−F stretch). H NMR
(
500 MHz, DMSO-d
6
, δ, ppm): 7.35 (d, J = 8.9 Hz, 2H, H
d
), 6.93 (d, J = 8.6 Hz, 4H, H ),
b
1
3
6
c
.67 (d, J = 8.9 Hz, 2H, H ), 6.65 (d, J = 8.6 Hz, 2H, H
a
), 5.11 (s, 4H, −NH
2
)。 C NMR
5
1
4
3
(
125 MHz, DMSO-d
6
, δ, ppm)：152.88 (C ), 146.77 (C ), 134.81 (C ), 128.28 (C ),
7
9
1
8
2
1
1
26.15 (C ), 125.34 (C , quartet, JC−F = 270 Hz), 116.93 (C , quartet, JC−F = 31 Hz),
2
6
15.32 (C ), 114.59 (C ).
H N
NH2
2
1
2
4
a
N
3
b
5
6
7
c
d
8
9
CF3
2
4
,4’-Dinitro-4’’-methyltriphenylamine (3). The compound 3 was prepared by a
similar procedure described for the synthesis of compound 1, except that
p-toluidine was used instead of p-(trifluoromethyl)aniline; mp = 249 C (by DSC, 2
o
−1
1
C/min). IR (KBr): 1579, 1339 cm (−NO
2
). H NMR (500 MHz, DMSO-d
) , 7.22 (d, J = 9 Hz, 2H, H ), 7.18 (d, J = 9 Hz, 4H, H
). C NMR (125 MHz, DMSO-d
6
, δ, ppm):
8
.18 (d, J = 9 Hz, 4H, H
a
d
b
), 7.08 (d,
1
3
5
1
J = 9 Hz, 4H, H
c
6
, δ, ppm): 158.51 (C ), 152.03 (C ),
4
7
8
2
3
6
9
1
41.98 (C ), 137.13 (C ), 129.53 (C ), 125.82(C ), 122.04 (C ), 116.16 (C ), 51.71 (C ).
8

O N
NO2
2
1
2
4
a
N
3
b
5
6
7
c
d
8
9
CH3
3
4
,4’-Diamino-4”-methyltriphenylamine (4). The diamine monomer 4 was prepared
by the Pd/C-catalyzed reduction of dinitro compound 3 using the similar procedure
o
described for the synthesis of 2; mp = 146 C (by DSC, 2 C/min). IR (KBr): 3454,
−
1
1
3
2
338 cm (−NH
H, H ), 6.76 (d, J = 8.5 Hz, 4H, H
) 4.88 (s, 4H, −NH
2
stretch). H NMR (500 MHz, DMSO-d
), 6.59 (d, J = 8.5 Hz, 2H, H
). C NMR (125 MHz, DMSO-d
6
, δ, ppm): 6.90 (d, J = 8.5 Hz,
d
b
c
), 6.55 (d, J = 8.5 Hz, 4H,
1
3
5
H
a
2
6
, δ, ppm): 147.00 (C ), 145.02
1
4
7
8
3
6
2
(
C ), 137.21 (C ), 129.49 (C ), 127.48 (C ), 127.48 (C ), 118.44 (C ), 115.28 (C ),
2
0.34 (C9).
H N
NH2
2
1
2
4
a
N
3
b
5
6
7
c
d
8
9
CH3
4
2
.3 Polymer synthesis
The Yamazaki-Higashi phosphorylation polycondensation technique [48] was
used to prepare the polyamides. A typical example for the preparation of polyamide
6
a is given. A flask was charged with 0.5150 g (1.5 mmol) of diamine monomer 2,
9

0
.2492 g (1.5 mmol) of terephthalic acid (5a), 0.2 g of CaCl
2
, 1.5 mL of TPP, 0.5 mL
o
pyridine, and 3 mL of NMP. This mixture was heated with stirring at 120 C for 3 h.
The resulting viscous polymer solution was poured slowly into 300 mL of methanol
with stirring. The fiber-like precipitate was filtered off, washed thoroughly with
methanol and hot water, and dried. The inherent viscosity of polyamide 6a was
0
.98 dL/g, as measured at a concentration of 0.5 dL/g in N,N-dimethylacetamide
o
(
DMAc) at 30 C.
2
.4 Instrumentation and measurements
Infrared spectra were recorded on a PerkinElmer FT-IR System spectrum GX. 1H
1
3
and C NMR spectra were obtained on a Bruker AV-500 FT NMR spectrometer. The
inherent viscosities of the polyamides were determined with a Cannon-Fenske
o
viscometer at 30 C. Weight-average molecular weights and number-average
molecular weights were obtained via size exclusion chromatography (SEC) on the
basis of polystyrene calibration using Water 2410 apparatus and tetrahydrofuran
(THF) as the eluent. A universal tester LLOYD LRX with a load cell of 5 kg was used
to study the stress-strain behavior of the samples. A gauge length of 2 cm and a
crosshead speed of 5 mm/min were used for this study. Measurements were
performed at room temperature with film specimens (0.5 cm wide, 6 cm long, and
about 0.09 mm thick), and an average of at least three replicates was used.
Wide-angle X-ray diffraction (WAXD) measurements were performed at room
o
temperature (ca. 25 C) on a Shimadzu XRD-6000 X-ray diffractometer (operating at
4
0 kV and 30 mA) with graphite-monochromatized Cu Kα radiation (λ= 1.5418 Å).
o
o
The scanning rate was 2 /min over a range of 2θ= 10−40 . Thermogravimetric
analysis (TGA) was conducted with a PerkinElmer Pyris 1 TGA. Experiments were
10

carried out on approximately 4－8 mg of samples in flowing nitrogen (flow rate 30
3
o
cm /min) at a heating rate of 20 C/min. Thermomechanical analysis (TMA) was
conducted with a PerkinElmer TMA 7 instrument. The TMA experiments were
o
o
o
conducted from 50 C to 350 C at a scanning rate of 10 C/min by using a
penetration probe 1.0 mm in diameter under an applied constant load of 10 mN.
Softening temperatures (T ) were taken as the onset temperature of probe
s
displacement on the TMA traces. Electrochemistry was performed with a
Bioanalytical System model CV-27 potentiostat and a BAS X–Y recorder.
Voltammograms are presented with the positive potential pointing to the left and
with increasing anodic currents pointing downward. Cyclic voltammetry was
conducted with the use of a three-electrode cell in which ITO (the polymer film
area was ca. 0.7 cm x 0.5 cm) 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
homemade
Ag/AgCl,
KCl
(saturated)
reference
electrode.
The
spectroelectrochemical cell was composed of a 1-cm cuvette, ITO as a working
electrode, a platinum wire as an auxiliary electrode, and an Ag/AgCl reference
electrode. Absorption spectra were measured with an Agilent 8453 UV–vis
spectrophotometer.
3
. Results and discussion
3
.1. Monomer synthesis
The TPA-based diamine monomers 2 and 4 were prepared by the synthetic
routes outlined in Scheme 1. The first step involved a nucleophilic aromatic
11

fluoro-displacement of p-fluoronitrobenzene with p-(trifluoromethyl)aniline and
p-toluidine, respectively, in the presence of CsF in DMSO. Diamines 2 and 4 were
readily obtained in high yields by the Pd/C-catalyzed reduction of the intermediate
dinitro compounds 1 and 3 with hydrazine monohydrate in refluxing ethanol. The
structures of dinitro precursors and the target diamine monomers were confirmed
by elemental analyses as well as FTIR and NMR spectroscopy.
FTIR spectra of all the synthesized compounds are included in the
supplementary data (Fig. S1). The nitro group of 1 and 3 gave two characteristic
−
1
bands around 1585 and 1325 cm , which disappeared after reduction. The diamine
monomers 2 and 4 showed a typical –NH stretching absorption pair in the region
2
−1
1
13
of 3300-3500 cm . The H NMR and C NMR spectra of diamines 2 and 4 are
1
illustrated in Fig. 1 and Fig. 2, respectively. The H NMR spectra of diamines 2 and 4
confirmed that the signals at 5.11 and 4.88 ppm corresponding to the primary
1
3
aromatic amine protons. All the C atoms in diamine 2 resonated in the region
00−150 ppm, and the quartets centered at about 125.34 ppm due to the −CF
carbons. The one-bond C−F coupling constant was about 270 Hz. The methyl
1
3
1
3
carbons in diamine 4 showed a singlet at 20.34 ppm in its C NMR spectrum. The
assignments of each carbon and proton are also given in the figures, which agree
well with the expected structures of diamine monomers 2 and 4.
12

3
.2. Polymer synthesis
According to the phosphorylation polyamidation technique developed by Yamazaki and
his co-workers [48], two series of aromatic polyamides 6a−h and 7a−h were synthesized
from the diamine monomers 2 and 4 with various aromatic dicarboxylic acids 5a−h via
solution polycondensation using TPP and pyridine as condensing agents in the NMP solution
containing dissolved CaCl
2
(Scheme 2). All the polymerization reactions proceeded
homogeneously throughout the reaction and afforded clear and highly viscous polymer
solutions. The polymer products precipitated in a tough, fiber-like form when the resulting
polymer solutions were slowly pouring into stirring methanol. As shown in Table 1, the
obtained polyamides exhibited inherent viscosities in the range of 0.42−1.31 dL/g and could
be solution-cast into creasable, strong films from DMAc solutions, indicating
high-molecular-weight polymers. The THF-soluble polyamide 6f showed a weight-average
molecular weight (Mw) and number-average molecular weight (Mn) of 41000 and 31000,
respectively, on the basis of SEC. Fig. S2 (Supplementary data) shows a typical IR spectrum
for polyamide 6a. The characteristic IR absorption bands of amide group can be found at
−
1
−1
around 3288 cm (N-H stretching) and 1654 cm (amide carbonyl).
3
.3. Solubility and film properties
The solubility behaviors of all polyamides are reported in Table 1. All the polyamides
6
a−h and 7a−h were highly soluble in the polar organic solvents such as NMP, DMAc, DMF,
and DMSO. The high solubility of these polyamides is apparently due to the presence of TPA
unit. The substituents on the TPA unit seem does not play a dominant role since these two
series polyamides display a similar solubility behavior. Among these polyamides, polymer 6f
was soluble in all the test solvents including less polar m-cresol and THF. This can be
attributable to the additional contribution of the hexafluoroisopropylidene [−C(CF
fragment in the polymer backbone together with the bulky –CF substituent which interfere
the interchain interactions and close chain packing. Therefore, the good solubility makes
3
)
2
−]
3
13

these polyamides candidates for practical applications via solution processing techniques to
afford high performance thin films for optoelectronic devices.
The WAXD patterns of polyamides 6a-h and 7a-h are illustrated in Fig. S3. The results
indicate that all the 6 and 7 series polyamides are amorphous in nature. Thus, the
amorphous nature of most 6 and 7 series polyamides also reflected in their excellent
solubility and good film-forming ability. Fig. 3 shows the appearance of the DMAc-cast films
of typical polyamides 6h and 7h. The 6 series polymers exhibited lighter color than the
corresponding 7 series ones due to the bulky –CF
the polyamide films are summarized in Table 2. The polymer films had a tensile strength of
6-117 MPa, an elongation at break of 7-49%, and a tensile modulus of 1.6-2.9 GPa. Most of
3
substituent. The mechanical properties of
7
the polymer films exhibited high tensile strengths; thus, they could be considered as strong
materials.
3
.4. Thermal properties
TMA and TGA were used to investigate the thermal properties of the polyamide films,
and some of the thermal behavior data are reported in Table 3. To avoid the effects of the
absorbed moisture and residual solvent in the film samples, we heated the samples at 300
oC for 1 h prior to all thermal analyses. The softening temperatures (T
) of the thoroughly
s
dried polymer films were measured with TMA by the penetration method. They were
obtained from the onset temperatures of the probe displacement on the TMA traces. As a
typical example, the TMA trace of polyamide 7b is illustrated in Fig. S4. The T values of
s
o
these polymers were observed in the range of 259−317 C. The lowest T
s
value of 6d and 7d
in each series polyamides can be explained in terms of the flexible ether linkage in their
diacid component. In most cases, the 6 series polyamides revealed slightly lower T when
compared to their corresponding 7 series analogs. This result implies that the bulky –CF
s
3
substituent in the 6 series polymers leads to an increase in steric hindrance for close chain
packing and thus an enhanced fractional free volume.
The thermal and thermo-oxidative stability of these polyamides were measured by TGA.
The temperatures of 10 % weight loss in nitrogen and air atmospheres determined from the
original TGA thermograms are included in Table 3. All polymers exhibited good thermal
o
stabilities with decomposition temperature (T
d
) at 10 % weight loss above 450 C in both
14

o
nitrogen and air atmospheres. The anaerobic char yield at 800 C for all polymers was in the
range of 65−74 wt %. All the polyamides exhibited a lower T as compared with their
d
corresponding counterparts without the substituted group [49], implying that the earlier
weight loss is associated with the decomposition of the substituent on the TPA unit.
3
.5. Optical and electrochemical properties
The optical and electrochemical properties of the polyamides were investigated with
UV–Vis absorption spectroscopy and cyclic voltammetry (CV). The relevant data are
summarized Table 4. These polymers exhibited strong UV–Vis absorption bands at 300–346
nm in solid films, which were assignable to the π–π* transitions in the backbones. The cutoff
wavelengths (absorption edge) read from the UV–Vis absorption spectra are also indicated
in Table 4 and were recorded in the range of 397–455 nm, dependent on the diacid
component and the substituent on the TPA unit. The cutoff wavelengths (λ onset) of the
polyamides generally increased with increasing electron affinity of the corresponding diacid
residue, and the 6 series polyamides revealed a slightly lowered λonset as compared to the
corresponding 7 series polymers. This result suggests that the coloration of the polymer film
was caused by charge-transfer (CT) interactions.
The redox behavior of the 6 and 7 series polyamides was investigated by CV conducted
for the cast films on an ITO-coated glass substrate as the working electrode in dry
acetonitrile (CH CN) containing 0.1 M TBAP as an electrolyte under a nitrogen atmosphere.
3
The typical CV diagrams for polyamides 6a and 7d are shown in Fig. 4. There is one reversible
oxidation redox couple at E1/2 value of 1.00 V for polyamide 6a and at E1/2 = 0.84 V for
polyamide 7d in the oxidative scan. Upon oxidation, the polymer films of 6a and 7d changed
color from original colorless or pale yellow to dark golden and pale green, respectively. As
can be seen from Table 4, the 6 series polyamides displayed a higher E1/2 value (0.97−1.03 V)
in comparison with the 7 series polyamides (0.83−0.88 V), attributable to the
electron-withdrawing trifluoromethyl substituent on the TPA unit. Methyl group, on the
other hand, inductively donates electrons. This is the same hyperconjugative donating effect
that causes alkyl substituents to stabilize alkenes and carbocations. When compared with
15

the structurally related polyamides (the 8 series) based on 4-fluorotriphenylamine [50], the
trifluoromethyl-substituted 6 series polymers also showed a higher oxidation potential. The
withdrawal or donation of electrons by a substituent group is controlled by an interplay of
inductive effects and resonance effects. The trifluoromethyl groups inductively withdraw
electrons through the σ bond linked to the benzene ring and are strongly deactivating. The
direct fluorine substituent is weakly deactivating owing to the interplay of inductive effects
and resonance effects. The energy levels of the highest occupied molecular orbital (HOMO)
and the lowest unoccupied molecular orbital (LUMO) of the corresponding polyamides were
estimated from the E1/2 values. Assuming that the HOMO energy level for the
+
ferrocene/ferrocenium (Fc/Fc ) standard is 4.8 eV with respect to the zero vacuum level, the
HOMO levels for these polyamides were calculated to be 5.19−5.39 eV. Their LUMO energy
levels deduced from the band gap calculated from the absorption edge were 2.06−2.59 eV.
3
.6. Spectroelectrochemical and electrochromic properties
The electrochromism of thin films from the polyamides was examined via the casting of
polymer solutions onto an ITO-coated glass substrate, and their absorption profiles were
monitored with a UV–Vis spectrometer at different applied potentials. The electrode
preparation and solution conditions were identical to those used in cyclic voltammetry. The
typical spectral changes of polyamide 6a and 7d are shown in Figs. 5 and 6, respectively.
When the applied potentials increased positively from 0.71 to 1.36 V, the absorption peak of
polyamide 6a at 342 nm decreased gradually, whereas three new bands grew up at 404, 540
and 855 nm because of the electron oxidation. Meanwhile, the film color changed from pale
yellow to chrome yellow or dark golden at electrode potential of 1.2 V. Upon
electro-oxidation, the polymer film of 7d showed new absorption bands at 397, 641, and
8
09 nm, which appeared the complementary color of pale green after oxidation.
To explore the potential application of the polyamide films as electrochromic materials,
a square-wave potential step method coupled with UV-Vis spectroscopy were used to
evaluate their electrochromic properties. In Fig. 7 are depicted the chronoabsorptograms
obtained for polyamide 6a film in TBAP/CH
3
CN for the color change pale yellow  dark
golden. The switching times estimated from the absorbance-time curves and indicated in Fig.
16

8
were determined by considering 90% of the full optical change. Thin films from polyamide
6
a would require 5 s for coloring after applying a voltage of 1.23 V and 2 s for bleaching after
switching off the potential. Because the electron-withdrawing −CF group destabilizes the
3
TPA radical cation formed upon oxidation, a considerable loss of optical contrast was
observed after ten redox switches for polyamide 6a. In general, the 6 series polyamides
displayed an inferior electrochromic performance as compared to the 8 series based on
4
-fluorotriphenylamine [50] because of less electrochemical stability of the CF TPA unit. Thin
3
films from polyamide 7d would require 5 s for coloring (at 1.26 V) and 2 s for bleaching.
After ten continuous cyclic scans between 0.0 and 1.26 V, the polyamide 7d film still did not
show significant loss in optical contrast. It is reasonable that the CH
series polyamides exhibited much better electrochromic stability than the CF
series because the former ones formed more stable TPA radical cations in the oxidative
processes. Although the CF TPA-based 6 series polyamides showed a less electrochromic
performance, the bulkiness of the −CF group and their good film quality make them
3
TPA-functionalized 7
3
TPA-based 6
3
3
potential application as gas separation membranes. Their other optoelectronic applications
such as memory devices are still deserved to be developed.
4
. Conclusions
New TPA-functionalized aromatic polyamides with −CH
3
or −CF substituents were
3
synthesized from the phosphorylation polycondensation reactions of TPA-based diamines 2
and 4 with various aromatic dicarboxylic acids. Because of the presence of the bulky,
packing-disruptive TPA unit, all the polymers were amorphous, had good solubility in many
polar aprotic solvents, and could afford flexible and strong films with good mechanical
o
properties. All the polyamides had softening temperatures higher than 275 C and high
o
thermal stability, with 10% weight loss temperature being recorded above 450 C in both
nitrogen and air atmospheres. Introduction of electron-withdrawing −CF
the oxidation potential of the polyamides and decreased the electrochemical and
electrochromic stability of the polyamides. The CH TPA-containing polyamides exhibited
3
group increased
3
good redox stability and might be good candidates for electrochromic applications.
Acknowledgement
17

The authors thank the Ministry of Science and Technology, Taiwan for the financial support
Grant No. MOST 104-2221-E-027-106).
(
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in the online version, at
http://dx.doi.org/10.1016/j.jfluchem.2016.xx.xxx.
References
1
2
3
4
5
6
7
8
. P.E. Cassidy PE, Thermally Stable Polymers. Marcel Dekker, New York, 1980.
. H.H. Yang, Aromatic High-Strength Fibers. Wiley, New York, 1989, pp. 66-289.
. H.H. Yang, Kevlar Aramid Fiber. Wiley, New York, 1993.
. Y. Imai, High Perform. Polym. 7 (1995) 337−345.
. Y. Imai, React. Funct. Polym. 30 (1996) 3−15.
. S.H. Hsiao, P.C. Huang PC, J. Polym. Sci. Part A: Polym. Chem. 35 (1997) 2421−2429.
. G.C. Eastmond, J. Paprotny, R.S. Irwin RS, Polymer 40 (1999) 469−486.
. J.F. Espeso, E. Ferrero, J.C. de la Campa, A.E. Lozano, J. de Abajo, J. Polym. Sci. Part A:
Polym. Chem. 39 (2001) 475−485.
9
1
1
1
1
. S.-H. Hsiao, Y.-H. Chang, Eur. Polym. J. 40 (2004) 1749−1757.
0. M. Ghaemy, S.M. Amini Nasab, React. Funct. Polym. 70 (2010) 306−313.
1. J.M. Garcia, F.C. Garcia, F. Serna, J.L. de la Pena, Prog. Polym. Sci. 35 (2010) 623−686.
2. G.-S. Liou, S.-H. Hsiao, J. Polym. Sci. Part A: Polym. Chem. 40 (2002) 1781−1789.
3. G.-S. Liou GS, S.-H. Hsiao, M. Ishida, M. Kakimoto, Y. Imai, J. Polym. Sci. Part A: Polym.
Chem. 40 (2002) 2810−2818.
1
1
1
1
1
1
4. S.-H. Hsiao, W.-T. Chen, J. Polym. Sci. Part A: Polym. Chem. 41 (2003) 420−431.
5. S.-H. Hsiao, Y.-M. Chang, J. Polym. Sci. Part A: Polym. Chem. 42 (2004) 4056−4062.
6. H. Behniafar, S. Khosravi-borna, Polym. Int. 58 (2009) 1299−1307.
7. S.-R. Sheng, X.-L. Pei, Z.-Z. Huang, X.-L. Liu, C.-S. Song, Eur. Polym. J. 45 (2009) 230−236.
8. S.-H. Hsiao, W.-J. Guo, C.-L. Chung, W.-T. Chen, Eur. Polym. J. 46 (2010) 1878−1890.
9. H. Behniafar, M. Sedaghatdoost, J. Fluorine Chem. 132 (2011) 276−284.
18

2
2
0. M. Ghaemy, F.R. Berenjestanaki, J. Fluorine Chem. 144 (2014) 86−93.
1. X.-L. Liu, D. Wu, R. Sun, L.-M. Yu, J.-W. Jiang, S.-R. Sheng, J. Fluorine Chem. 154 (2013)
1
6−22.
2
2
2
2. P. Bandyopadhyay, D. Bera, S. Ghosh, S. Banerjee, RSC Adv. 4 (2014) 28078−28092.
3. D. Bera, V. Padmanabhan, S. Banerjee, Macromolecules 48 (2015) 4541−4554.
4. D. Bera, P. Bandyopadhyay, S. Ghosh, S. Banerjee, V. Padmanabhan, J. Membr. Sci. 474
(2015) 20−31.
2
2
2
2
2
3
3
3
3
3
3
5. M. Thelakkat, Macromol. Mater. Eng. 287 (2002) 442−461.
6. Y. Shirota, H. Kageyama, Chem. Rev. 107 (2007) 953−1010.
7. Z. Ning, H. Tian, Chem. Commun. (2009) 5483−5495.
8. A. Iwan, D. Sek, Prog. Polym. Sci. 36 (2011) 1277−1325.
9. H.-J. Yen, G.-S. Liou, Polym. Chem. 3 (2012) 255−264.
0. M. Liang, J. Chen, Chem. Soc. Rev. 42 (2013) 3453−3488.
1. W.-P. Lin, S.-J. Liu, T. Gong, Q. Zhao, W. Huang, Adv. Mater. 26 (2014) 570−606.
2. J.-H. Wu, G.-S. Liou, Adv. Funct. Mater. 24 (2014) 6422−6429.
3. H.-J. Yen, G.-S. Liou, Polym. J. 48 (2016) 117−138.
4. S.-H. Cheng, S.-H. Hsiao, T.-H. Su, G.-S. Liou, Macromolecules 38 (2005) 307−316.
5. S.-H. Hsiao, Y.-M. Chang, H.-W. Chen, G.-S. Liou, J. Polym. Sci. Part A: Polym. Chem. 44
(2016) 4579−4592.
3
3
3
3
4
4
4
4
4
4
6. C.-W. Chang, G.-S. Liou, S.-H. Hsiao, J. Mater. Chem. 17 (2007) 1007−1015.
7. S.-H. Hsiao, G.-S. Liou, Y.-C. Kung, H.-J. Yen, Macromolecules 41 (2008) 2800−2808.
8. H.-J. Yen, G.-S. Liou, Chem. Mater. 21 (2009) 4062−4070.
9. Y.-C. Kung, S.-H. Hsiao, J. Mater. Chem. 20 (2010) 5841−5849.
0. L.-T. Huang, H.-J. Yen, G.-S. Liou, Macromolecules 44 (2011) 9595−9610.
1. H.-J. Yen, K.-Y. Lin, G.-S. Liou, J Polym Sci Part A: Polym Chem 50 (2012) 61−69.
2. S.-H. Hsiao, H.-M. Wang, P.-C. Chang, Y.-R. Kung, T.-M. Lee, J. Polym. Res. 20 (2013) 154.
3. H.-M. Wang, S.-H. Hsiao, J. Polym. Sci. Part A: Polym. Chem. 52 (2014) 272−286.
4. S.-H. Hsiao, S.-L. Cheng, J. Polym. Sci. Part A: Polym. Chem. 53 (2015) 496−510.
5. E.T. Seo, R.F. Nelson, J.M. Fritsch, L.S. Marcoux, D.W. Leedy, R.N. Adams, J. Am. Chem. Soc.
8
8 (1966) 3498−3503.
4
4
6. R.F. Nelson, R.N. Adams, J. Am. Chem. Soc. 90 (1968) 3925−3930.
7. S.C. Creason, J. Wheeler, R.F. Nelson, J. Org. Chem. 37 (1972) 4440−4446.
19

4
4
5
8. N. Yamazaki, M. Matsumoto, F. Higashi, J. Polym. Sci. Polym. Chem. Ed. 13 (1975)
373−1380.
9. Y.-C. Kung, G.-S. Liou, S.-H. Hsiao, J. Polym. Sci. Part A: Polym. Chem. 47 (2009)
740−1755.
0. S.-H. Hsiao, Y.-R. Kung, J. Fluorine Chem. 186 (2016) 79−90.
1
1
20

H N
NH2
2
1
2
4
a
N
3
b
5
6
7
c
d
8
9
CF3
-
NH
2
H N
NH2
2
1
2
-
NH
2
4
a
N
3
b
5
-
CH
3
6
7
c
d
8
9
CH3
1
Fig. 1. H NMR spectra of (a) diamine 2 and (b) diamine 4 in DMSO-d .
6
21

H N
NH2
2
1
2
4
a
N
3
b
5
6
7
c
d
8
9
CF3
H N
NH2
2
1
2
4
a
N
3
b
5
6
7
c
d
8
9
CH3
1
3
Fig. 2. C NMR spectra of (a) diamine 2 and (b) diamine 4 in DMSO-d .
6
22

Fig. 3. Typical DMAc-cast films of polyamides 6h (right) and 7h (left).
23

Fig. 4. Cyclic voltammograms of the cast films of polyamides (a) 6a and (b) 7d on an
indium-tin oxide (ITO)-coated glass substrate in 0.1 M TBAP/CH
mV/s.
3
CN at a scan rate of 100
24

1
.2 V
Fig. 5. Spectral changes of polyamide 6a film on the ITO-glass substrate in 0.1 M
TBAP/CH CN at different electrode potentials: (a) 0.0, (b) 0.71, (c) 0.78, (d) 0.85, (e) 0.92, (f)
.00, (g) 1.08, (h) 1.15, and (i) 1.23 V.
3
1
25

1
.2 V
Fig. 6. Spectral changes of polyamide 7d film on the ITO-glass substrate in 0.1 M
TBAP/CH CN at different electrode potentials: (a) 0.0, (b) 0.60, (c) 0.69, (d) 0.78, (e) 0.88, (f)
.97, (g) 1.07, (h) 1.17, and (i) 1.26 V.
3
0
26

Fig. 7. Potential step absorptometry of polyamide 6a in 0.1 M TBAP/CH
3
CN by applying a
potential step (0 V  1.23 V).
27

Fig. 8. Potential step absorptometry of polyamide 7d in 0.1 M TBAP/CH
3
CN by applying a
potential step (0 V  1.26 V).
28

O N
2
NO2
H2N
NH2
NH2
N
N
CF3
CsF
NH NH H O
2 2 2
2
F
NO2
CF3
CF3
NH2
DMSO
EtOH, Pd/C
1
2
O N
NO2
H N
NH2
2
2
N
N
CH3
CH3
CH3
4
3
Scheme 1. Synthesis routes to diamine monomers 2 and 4.
29