Design and synthesis of a novel quinoxaline diamine and its polyimides with high-Tg and red color

Article abstract of DOI:10.1016/j.polymer.2019.121612

To produce high-temperature polyimide (PI) films meeting the demand of flexible-display, heteroaromatic monomers were extensively investigated. Quinoxaline is one kind of the most important N-heterocycles, which possesses pharmacological activities and thermotolerance. In this work, newly designed diamine monomer 3/2-(4-aminophenyl)-2/3-methylquinoxalin-6-amine (QMDA) was successfully synthesized. Their corresponding quinoxaline-containing PI films were prepared by cooperating QMDA with several aromatic dianhydrides, and divided into two series according to different thermal imidization procedures. The firm scaffold of quinoxaline was proved to enhance the thermal stability of the obtained PIs. The effect of final annealing temperature on molecular packing and thermal properties of the PI films was also investigated. Under higher annealing temperature, PI films were found to pack more tightly due to the formation of more cross-linkings in polymer matrix. The restriction of chian movements brought these PIs higher T_g more than 500 °C or even prevented the occurace of glass transition.

Full text of DOI:10.1016/j.polymer.2019.121612

Accepted Manuscript
Design and synthesis of a novel quinoxaline diamine and its polyimides with high-Tg
and red color
Tuanqing Liu, Feng Zheng, Tongmei Ding, Shuyu Zhang, Qinghua Lu
PII:
S0032-3861(19)30596-8
DOI:
https://doi.org/10.1016/j.polymer.2019.121612
Article Number: 121612
Reference:
JPOL 121612
To appear in: Polymer
Received Date: 2 May 2019
Revised Date: 6 June 2019
Accepted Date: 23 June 2019
Please cite this article as: Liu T, Zheng F, Ding T, Zhang S, Lu Q, Design and synthesis of a novel
quinoxaline diamine and its polyimides with high-Tg and red color, Polymer (2019), doi: https://
doi.org/10.1016/j.polymer.2019.121612.
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to
our customers we are providing this early version of the manuscript. The manuscript will undergo
copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please
note that during the production process errors may be discovered which could affect the content, and all
legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT
Design and synthesis of a novel quinoxaline diamine
and its polyimides with high-Tg and red color
a
b
a
a*
a,b*
Tuanqing Liu , Feng Zheng , Tongmei Ding , Shuyu Zhang , Qinghua Lu
a
School of Chemistry and Chemical Engineering, Shanghai Key Laboratory of Electrical
Insulation and Thermal Aging, Shanghai Jiao Tong University, Shanghai 200240, China
School of Chemical Science and Engineering, Tongji University, 1239 Siping Road., Shanghai,
b
200092, China
(for TOC only)
1

ACCEPTED MANUSCRIPT
ABSTRACT: To produce high-temperature polyimide (PI) films meeting the demand of
flexible-display, heteroaromatic monomers were extensively investigated. Quinoxaline is one
kind of the most important N-heterocycles, which possesses pharmacological activities and
thermotolerance.
In
this
work,
newly
designed
diamine
monomer
3
/2-(4-aminophenyl)-2/3-methylquinoxalin-6-amine (QMDA) was successfully synthesized.
Their corresponding quinoxaline-containing PI films were prepared by cooperating QMDA with
several aromatic dianhydrides, and divided into two series according to different thermal
imidization procedures. The firm scaffold of quinoxaline was proved to enhance the thermal
stability of the obtained PIs. The effect of final annealing temperature on molecular packing and
thermal properties of the PI films was also investigated. Under higher annealing temperature, PI
films were found to pack more tightly due to the formation of more cross-linkings in polymer
matrix. The restriction of chian movements brought these PIs higher T more than 500 °C or
g
even prevented the occurace of glass transition.
Keywords: Quinoxaline; Polyimide; High thermal stability
1
.Introduction
In recent years, considerable attention has been paid to flexible active-matrix organic
1
-2,3
light-emitting-diode (AMOLED).
One of the most critical technologies in producing
4
-6
AMOLED is the fabrication of low-temperature polysilicon thin-film transistors (LTPS TFTs).
Thus, many high-temperature flexible substrates have arisen to fulfill such fabrication
requirements. Among various organic polymer materials, aromatic polyimides (PIs) have
attracted extensive interest because of their excellent thermal performance, chemical resistance
2

ACCEPTED MANUSCRIPT
7
-9
g
and mechanical properties. However, the glass transition temperature (T ) of conventional PIs
1
1
are usually lower than 400 °C, which is still not sufficient to stand the LTPS fabrication
temperature. Many researches have focused on developing PIs with higher T by introducing
g
heterocycle rings into the backbone of polyimides, which breaks through the limited
1
2
performance of benzene ring. Heterocyclic skeleton can significantly improve the bond energy
due to the resonance between the heterocyclic and benzene, leading to the improvement of
1
3
thermal stability.
In a common two-step approach to prepare PI, the first step is to prepare a precursor poly
amic acid) PAA by the polycondensation of diamine and dianhydride monomers in an aprotic
(
1
0
polar solvent at room temperature. Therefore, the root to obtain high-T PIs ultimately is to
g
design the novel diamine and dianhydride monomers. In previous reports, extensive aromatic
1
4
15
heterocyclic monomers have been deeply investigated, such as benzimidazole , benzioxazole ,
1
6
17
18
19
benzothiazole , quinazoline , pyridine and pyrimidine . Quinoxaline is a typical class of
nitrogen-containing heterocyclic compounds, commonly found in various natural products,
2
0
bioactive molecules and drugs. Derivatives of quinoxaline are widely applied in medicine ,
2
1
22
23
insecticide , biochemistry , and organic functional materials for their antibacterial activity,
bioactivity, chemical stability and heat resistance. Nevertheless, high-T polyimides containing
g
quinoxaline diamine are rarely reported. Bulky pendent groups having quinoxaline were used to
2
g
4
prepare organosoluble polyimides with the increase of T , but the role of quinoxaline in
improving heat resistance has not been well studied. The skeleton of quinoxaline exhibits highest
activation barrier to rupture on account of C=N possessing the shorter bond length and higher
bond energy in comparison with C-N, C-C, C-O, C-S, and C=S. So, it is significant to design and
3

ACCEPTED MANUSCRIPT
g
synthesize novel monomers equipped with quinoxaline to acquire high-T polyimide, which
could fill the gap in the field of aromatic heterocyclic polyimide.
In this work, we report for the first time on the successful preparation of novel quinoxaline
containing polyimide films and the characterization of their thermal properties. The newly
designed quinoxaline diamine 3/2-(4-aminophenyl)-2/3-methylquinoxalin-6-amine (QMDA) was
synthesized using an efficient and high yield approach, which was then subjected to prepare
corresponding PIs with different dianhydride monomers. It is known that in the duration of
converting PAA into PI, the thermal imidization process, including solvent evaporation,
imidization and annealing, would also have impact on the final PI product. Taking this step into
account, different annealing temperatures (380 °C and 420 °C) were applied. Two series of PIs
(
PI-380 and PI-420) were synthesized accordingly to achieve higher T , and to investigate the
g
impact of cross-linkings on their thermal behavior as well.
2
. Experimental
2
.1 Materials
All commercially available chemicals were used as received. 1,2,4,5-benzenetetracarboxylic
dianhydride
,3’,4,4’-benzophenonetetracarboxylic dianhydride (BTDA), 3,3’,4,4’-oxydiphthalic dianhydride
ODPA) were obtained from ChinaTech (Tianjin) Chemical Co., Ltd, without further
purification.
(PMDA),
3,3’,4,4’-biphenyltetracarboxylic
dianhydride
(BPDA),
3
(
2
.2 Monomer synthesis
2
5
2.2.1 Synthesis of (E)-1-nitro-4-(2-nitroprop-1-en-1-yl)benzene(a)
4

ACCEPTED MANUSCRIPT
Scheme 1 shows the synthetic process of the OMDA momoner. A solution of
-nitrobenzaldehyde (19.3 g, 128 mmol), nitroethane (25.2 mL, 352 mmol) and ammonium
4
acetate (11.1 g, 144 mmol) in HOAc (60 mL) was heated to 120 °C for 2 h. The reaction was
monitored by thin layer chromatography (TLC). The reaction was quenched by an addition of
water (60 mL). The precipitate was filtered, recrystallized in EtOH. The resultant crystal was
1
filtered and dried to afford a pale yellow powder (23.98 g, 90%). H NMR (CDCl , 400 MHz): δ
3
1
3
8
.33-8.30 (m, 2H), 8.08 (s, 1H), 7.60-7.58 (m, 2H), 2.45 (d, J = 1 Hz, 3H) (Fig. S1). C NMR
CDCl , 125 MHz): δ 150.2, 148.1, 138.9, 130.6, 130.3, 124.0, 14.1 (Fig. S2). FTIR (KBr,
cm ):3116, 2941, 1521, 1344, 1325.
(
3
-1
2.2.2 Synthesis of b
A solution of (E)-1-nitro-4-(2-nitroprop-1-en-1-yl)benzene (a) (7.6 g, 37 mmol),
4
-nitrobenzene-1,2-diamine (5.6 g, 37 mmol) and Copper (II) bromide (816 mg, 3.7 mmol) in
EtOH (100 mL) was heated to 110 °C for 12 h. The reaction was monitored by TLC. The
reaction was quenched by an addition of water (100 mL). The aquous layer was extracted with
EtOAc (100 mL*2), washed with brine (100 mL*2). The combined organic layers were dried
over anhydrous MgSO
4
, concentrated under reduced pressure, and purified by flash column
1
chromatograph on silica gel to afford a yellow powder (6.01 g, 53%). H NMR (CDCl , 500
3
MHz): δ 8.97-8.91 (1H), 8.52-8.47 (1H), 8.42-8.40 (2H), 8.24-8.19 (1H), 7.92-7.90 (2H),
1
3
2
1
1
.86-2.85 (3H) (Fig. 2). C NMR (CDCl , 125 MHz): δ 155.6-155.4, 154.8-154.6, 148.5,
3
48.2-147.7, 144.1-144.0, 143.9-143.2, 140.4-140.0, 131.0-130.3, 130.2, 125.7-125.0, 123.9,
-1
23.9-123.1, 24.7-24.5 (Fig. S3). FTIR (KBr, cm ):3072, 2931, 1519, 1342.
2.2.3 Synthesis of QMDA
5

ACCEPTED MANUSCRIPT
To a solution of nitro-mixture (b) (7.0 g, 22 mmol) in MeOH (200 mL) was added 5% Pd/C
(
0.7 g). The reaction atmosphere was replaced with H for three times. The reaction mixture was
2
stirred at room temperature for 12 h. The reaction was monitored by TLC. The reaction was
filtered through celite pad. The filtrate was concentrated under reduced pressure, and
1
recrystallized in MeOH to afford a yellow powder (4.44g, 79%). H NMR (DMSO-d , 500 MHz):
6
δ 7.69-7.67 (1H), 7.46-7.40 (2H), 7.17-7.14 (1H), 6.91-6.88 (1H), 6.70-6.68 (2H), 5.90-5.84
1
3
(
2H), 5.47-5.40 (2H), 2.65-2.64 (3H) (Fig. 3). C NMR (DMSO-d
6
, 125 MHz): δ 154.3-151.9,
1
1
2
50.1-150.0, 150.0-146.5, 149.8-149.4, 143.3-142.8, 134.9-134.4, 130.7-130.5, 129.5-128.8,
-1
26.9-126.7, 121.6-121.5, 113.6, 105.7-105.1, 24.8-24.4 (Fig. S4). FTIR (KBr, cm ):3336, 3235,
+
916, 1606.HRMS (ESI) m/z calculated for C H N [M+H ] 251.1298, found 251.1297.
1
5
14
4
Scheme 1. Synthetic route of QMDA.
2
.3 Synthesis of polyimide films
Polyimides PI-380 and PI-420 were synthesized from QMDA with four aromatic dianhydrides:
PMDA (m), BPDA (b), BTDA (t) and ODPA (o) using different final curing temperatures: 380
°
C and 420 °C. Synthesis of PI-m-380 was used as an example to illustrate the general synthetic
6

ACCEPTED MANUSCRIPT
route. 5.0062 g (20 mmol) of QMDA was charged into a 250 mL three-neck round-bottom flask,
equipped with a stirrer and a nitrogen inlet, containing 50 mL of DMAc. After the diamine was
completely dissolved by stirring under nitrogen, 4.3624 g (20 mmol) of PMDA was gradually
added. A viscous solution of PAA-m with a solid content of 16.7 wt% was obtained after stirring
at room temperature for about 12 h. Bubbles were removed by planetary mixer. The above
poly(amic acid) PAA-m-380 solution was cast onto a clean, dry glass plate at room temperature
with a 250-µm-depth blade. The PAA-m-380 films were vacuumed in a vacuum oven for 1h and
then cured in a muffle with a curing program of 115 °C for 15 min, 140 °C for 15 min, 200 °C
for 30 min, 250 °C for 5 min, and 380 °C for 90 min. PI-420 represents that the final curing
condition of polyimide film was 420 °C for 90 min. Scheme 2 shows the synthetic process and
chemical structures of the polyimides.
7

ACCEPTED MANUSCRIPT
Scheme 2. Synthesis of polyimides containing quinoxaline.
2
.4 Characterization
The chemical characterizations of the diamine QMDA were carried out as follows. Thin layer
chromatography (TLC) was performed on silica gel F254 pre-coated glass plates (HSGF ,
2
54
1
13
Huanghai) and visualized by irradiation under a 254 nm UV lamp. H NMR and C NMR
spectra were recorded on a Bruker AVANCE III HD 400 or 500 spectrometer. The residual
solvent peak was used as an internal reference (CDCl : δ = 7.26 ppm, δ = 77.16 ppm;
3
H
C
DMSO-d
6
: δ
H
= 2.50 ppm, δ
C
= 39.52 ppm). Fourier transform infrared (FTIR) spectra were
8

ACCEPTED MANUSCRIPT
-1
obtained on a Nicolet 6700 FTIR spectrometer (Perkin Elmer, Inc., USA) for 4000–650 cm in
3
2 scans. High-resolution mass spectra (HRMS) were obtained on a Waters ACQUITYTM
UPLC & Q-TOF MS.
The chemical characterization of PI films was monitored in attenuated total-reflectance (ATR)
-1
mode on a Nicolet 6700 FTIR spectrometer (Perkin Elmer, Inc., USA) for 4000–650 cm in 32
scans. Dynamic mechanical analyses (DMA) of the PI films were carried out on a Q800 DMA
-1
(
TA Instruments, USA) at a heating rate of 5 °C min with a load frequency of 1 Hz under
nitrogen. The decomposition behavior of the PI films was investigated by using Discovery 550
-1
TGA (TA Instruments, USA) with a heating rate of 10 °C min under nitrogen. The CTEs of the
films were collected using a Q400 TMA (TA Instruments, USA) at a temperature increase of 5
-1
-1
°
C min in a nitrogen flow of 50 mL min under a static load set at 0.05 N. The average CTE
values were obtained from the second heating run after the first run, which was carried out to
eliminate the residual stress. Film densities were obtained from a Furbs Multifunctional
Densimeter FK-120S (Furbs Cop., China), using the solid model and 95% alcohol as a medium.
The UV-Vis transmission spectra of the films were recorded by UV-1800 (Shimadzu, Japan)
spectrophotometer at room temperature with a spectral width of 200-800 nm. The mechanical
properties were measured on an CMT1104 universal electromechanical tester (SUST., China)
with a strain rate of 10 mm/min. The electronic properties were tested by the parallel-plate
capacitor method with Concept 40 Dielectric/impedance spectrometer (Novocontrol Company,
Germany) in a dry chamber. Copper electrodes were vacuum-deposited on both surfaces of dried
films.
3. Results and discussion
9

ACCEPTED MANUSCRIPT
3
.1 Synthesis and characterization
/2-(4-aminophenyl)-2/3-methylquinoxalin-6-amine (QMDA) containing the quinoxaline
3
moiety was designed and prepared from 4-nitrobenzaldehyde, nitroethane with excess
ammonium acetate in the first step. Then the nucleophilic addition of
4
-nitrobenzene-1,2-diamine proceeded as shown in Scheme 1. The reduction of dinitro
compounds into corresponding diamines was accomplished using hydrogen with 5% Pd/C as a
1
13
catalyst. Chemical structure of the targeted monomer was characterized by FTIR, H NMR, C
NMR and HRMS spectroscopy. The FTIR spectra of a, b and QMDA are shown in Fig. 1. The
-1
-1
stretching vibration peaks of –NO shifted from 1521, 1344, 1325 cm to 1519, 1342 cm ,
2
indicating that the nitro group attached to the aliphatic chain had transferred to benzene, that is
(E)-1-nitro-4-(2-nitroprop-1-en-1-yl)benzene (a) had transformed to dinitro compounds (b). The
-1
absorption peaks at 3336 and 1606 cm originated from –NH group demonstrated that diamine
2
1
(
QMDA) was successfully synthesized. The H NMR (Fig. 2) spectra of dinitro compounds(b)
shows two sets of singlets due to the formation of isomers including
2
3
-methyl-6-nitro-3-(4-nitrophenyl)quinoxaline
and
-methyl-6-nitro-2-(4-nitrophenyl)quinoxaline. After the reduction, the quinoxaline diamine
isomers (QMDA) maintained the same ratio with b (Fig. 3).
10

ACCEPTED MANUSCRIPT
Figure 1. FTIR spectra of a, b and QMDA.
1
Figure 2. H NMR spectrum of b in CDCl3
.
11

ACCEPTED MANUSCRIPT
1
Figure 3. H NMR spectrum of QMDA in DMSO-d .
6
Two series of PIs were then synthesized from QMDA and four aromatic dianhydrides under
different final annealing temperatures according to Scheme 2. The imidization of polyimide
films involved three processes, the solvent evaporation, the imidization and the annealing stage
with increased temperature. Since the annealing temperature would be expected to affect the
properties of the obtained PI films, two final anneling temperatures, 380 °C and 420 °C, were
perfomed to investigate such effects.
The successful preparation of PAAs as well as PIs were confirmed by ATR as shown in Fig.
-1
S5 and Fig. S6. The characteristic absorption bands at 1780 cm (asymmetric stretching of imide
-1
-1
C=O), 1716 cm (symmetric stretching of imide C=O), 1360 cm (C-N-C stretching of imide
-1
ring) and 725 cm (bending vibration of imide C=O) suggested the formation of an imide ring
1
9, 26
-1
-1
structure.
Peaks at ~1660 cm (amide-I of PAAs) and 1550 cm (amide-II of PAAs) were
hardly observed, as shown in Fig. 4, which indicated the high imidization degree of the obtained
12

ACCEPTED MANUSCRIPT
1
8
-1
PI films. In addition, the characteristic stretching vibration of C=N at 1604 cm of the PI films
suggested the incorporation of quinoxaline moiety into the PI main chains.
Figure 4. ATR spectra of PAA-m, PI-m-380 and PI-m-420.
3.2 The chain packing and crystallization
QMDA exhibits linear and rigid molecular structure, which is conducive to the orientation of
alignment and compactly packing of polymer chains. The chain packing and crystallization of
the polyimide films were studied by the film density and wide-angle X-ray diffraction (WAXD).
2
7
The packing coefficient (K) was estimated from eq (1):
Vint
NA
∑
M
ρ
∆Vi
K = V
=
(1)
true
K is the ratio of the intrinsic volume (Vint) to the calculated molar volume (Vtrue) of the PI films,
whereas V is molar volume of the atomic groups that make up the repeating unit and V is
int
true
obtained from the experimental measurement of the density (ρ) of the polyimide film. ∆Vi
13

ACCEPTED MANUSCRIPT
represents the volume increments of the atoms, M is the molecular mass of the repeating unit and
N is Avogadro’s number.
A
As listed in Table 1, the film density values and the packing coefficients of PI-380 and PI-420
-3
were found to be higher than the most reported polyimides, ranging from 1.350-1.484 g×cm
and 0.665-0.723, respectively. This is probably due to the strong intermolecular π-π stacking
interactions among quinoxaline moiety and aromatic rings. The density values and packing
coefficients of the PI-420 series were higher than those of the PI-380 regardless of the
dianhydrides employed. This should be attributed to the dehydrogenation of aromatic rings when
the temperature exceeded 400 °C, as a result, large quantities of free radicals were generated and
recombined, leading to the formation of cross-linking. With the same diamine monomer QMDA,
the packing coefficient of different PI films could be regulated by the rigidity of dianhydrides. In
current study, the linearity and rigidity of the dianhydrides is in the order of
PMDA>BPDA>BTDA>ODPA. The packing coefficients of PI-o-380 and PI-o-420, containing
ODPA, were found to be comparatively higher than other PIs. This may be due to that PI-o were
much easier to form cross-linking because of the flexible ether linkage of ODPA moiety. As
shown in Fig. 5, the entire chain segments of PI-o were packed tightly under 380 °C. Increasing
the annealing temperature up to 420 °C had no significant effect on further improving the
packing coefficient. It suggests from Fig. 5 that with the increasing flexibility of dianhydride in
PI, the impact of high annealing temperature on packing coefficient become more and more
negligible. On the contrary, a higher anneling temperature would be necessary to promote the
complete formation of cross-linkings in the PIs containing rigid moieties.
14

ACCEPTED MANUSCRIPT
Table 1
Density and packing coefficient of polyimides.
Curing
Temperature( C)
-3
Dianhydride PIs
o
ρ
(g×cm )
K
PI-m-380
380
420
380
420
380
420
380
420
1.416
1.469
1.350
1.388
1.362
1.378
1.480
1.484
0.672
0.698
0.665
0.683
0.665
0.672
0.721
0.723
PMDA
BPDA
BTDA
ODPA
PI-m-420
PI-b-380
PI-b-420
PI-t-380
PI-t-420
PI-o-380
PI-o-420
Figure 5. Packing coefficients (K) of PIs.
Fig. 6 shows the wide-angle X-ray diffraction (WXRD) pattern of PIs. The two characteristic
diffraction peaks at 2θ of 15 ° suggests the face to face π-π stacking interactions, and the single
15

ACCEPTED MANUSCRIPT
peak at 2θ of 25 ° could be attributed to the side to side π-π stacking interactions. It was
worthnoting that the crystallinity of PI-m-420 was sharply enhanced comparison to PI-m-380
because PMDA was too rigidity to be packed completely under 380 °C. This phenomenon was
not observed for the PIs with other dianhydrides, and further confirms that the effect of anneal
temperature on pack coefficient of PIs are dependent on the flexibility of monomers.
Figure 6. WXRD diffraction of PIs.
3.3 Film color and charge transfer complexes
Conjugated charge transfer between electron-donating diamines and electron-withdrowing
dianhydride results in the formation of charge transfer complexes (CTC) within and between
molecular chains, so polyimides are usually golden yellow. For quinoxaline-type polyimides, the
lone pair electrons on the nitrogen atoms in the quinoxaline structure also participate in the
formation of the conjugated system, further reducing the energy of absorption of visible light, so
the polyimide film showed red color (Fig.7a). It has been reported that the cutoff wavelength is
16

ACCEPTED MANUSCRIPT
2
8
positively coreelated with the conjugated property of the used monomer.
For the
quinoxaline-type PIs, the cutoff wavelengths were measured at more than 510 nm as shown in
Fig. 7b and 7c. As CTC become more intense with the increase of annealing temperature, the
films cured under 420 °C exhibit deeper color than those cured under 380 °C. In addition, the
deepest color and the largest cutoff wavelength was observed in PI-o-420, indicating that PI
prepared from ODPA (Table S1) was more susceptible to oxidative crosslinking at elevated
temperatures.
Figure 7. (a) Picture of PI films and UV-Vis transmission spectra of (b) PI-380 and (c) PI-420.
3.4 Thermal properties
17

ACCEPTED MANUSCRIPT
Thermal stability and decomposition behavior of PI films were investigated through thermo
gravimetric analysis (TGA). The TGA curves of PIs are illustrated in Fig. S7. The temperature of
% weight loss (Tꢀ5%), the temperature of 10% weight loss (T
ꢀ10%
d
) and the residual weight
5
d
percentage at 800 °C (R
loss temperatures of PI-380 and PI-420 were observed in the ranges of 453-494 °C and 499-536
C, respectively. After heating to 800 °C the char yields were in the range of 54-68% for PI-380
w
) under nitrogen atmosphere are summarized in Table 3. The 5% weight
°
and 64-70% for PI-420. Therefore, the quinoxaline structure maintains the high thermal stability
of aromatic polyimides. Higher final annealing temperature forced the molecular chains packed
more tightly, therefore the heat resistance of PIs was further improved.
Table 3
Thermal properties of polyimide films.
(a) Thermal properties of PI-380.
ꢀ
5%
o
ꢀ10%
o
o
PIs
T
( C)
T
( C)
T
g
( C)
R
w
(%)
CTE (ppm/K)
19.65
d
d
PI-m-380
PI-b-380
PI-t-380
PI-o-380
453
494
486
488
515
553
529
530
376
355
377
376
54
68
63
63
25.70
41.07
44.66
(b) Thermal properties of PI-420.
ꢀ
5%
o
ꢀ10%
o
o
PIs
T
( C)
T
( C)
T
g
( C)
R
w
(%)
CTE (ppm/K)
12.30
d
d
PI-m-420
PI-b-420
PI-t-420
PI-o-420
499
536
509
502
537
573
543
540
>500
>500
474
64
70
66
66
24.32
40.41
>500
43.96
18

ACCEPTED MANUSCRIPT
g
The glass-transition temperature (T ) values were obtained from the peak temperature of tan
(
delta) in dynamic thermomechanical analysis (DMA). The T values of PI-380 ranged from
g
3
55-377 °C as shown in Fig. 8a. For PI-420 films, the T value of PI-t-420 reached up to 474 °C
g
and the others may even exceeded 500 °C which is beyond the highest detection limit of the
DMA instrument employed (Fig. 8b). There is a consensus about T indicates the highest usage
g
temperature of polymer materials. When the temperature exceeds T , the movement of polymer
g
segments occurs and the polymer mechanical properties degrade. As curing at 420 °C, more
cross-linking points would be formed in the polymer chains of PI-420 and rudeced their free
volume as confirmed by their higher packing coefficients. This could lead to a restriction of the
movenmet of the PI segements, and thus enhanced the thermal performace.
Figure 8. DMA curves of (a) PI-380 and (b) PI-420 films.
The coefficient of thermal expansion (CTE) was measured by thermomechanical analysis
(TMA). The values of the PI-380 ranged from 19.65 ppm/K to 44.66 ppm/K, whereas the CTE
for PI-420 extended from 12.30 ppm/K to 43.96 ppm/K (Fig. S8). For both PI series, the CTE
values increased in the order of PMDA<BPDA<BTDA<ODPA, which basically consists with
the reduced rigidity of the employed dianhydrides. In addition, PI-420 films were determined to
19

ACCEPTED MANUSCRIPT
have lower CTE comparing with PI-380, as the in-plane chain orientation in PI-420 films
increased.
3.5 Mechanical properties
The mechanical properties of PIs including tensile strength, elongation at break and initial
modulus are deeply impacted by the rigidity of dianhydride and thermal imidization procedure,
as summarized in Table 4. For the PIs cured under 380 °C, the tensile strength and elongation at
break increased with the increase of dianhydride flexibility. The reason should be attributed to
the synergistic effect of rigid-rod QMDA and flexible dianhydride, which realized the
2
9
transformation of fracture mechanism from brittle to ductile fracture . Meanwhile, the most
rigid combination PI-m-380 had the highest initial modulus. As annealing temperature reached
up to 420 °C , the molecular chains were tightly packed which impeded the rotation of molecular
chains, resulting in the weakened mechanical properties due to brittle fracture. PI-m-420 and
PI-t-420 were too fragile to run tensile tests. The mechanical properties of PI-o also decreased
more sharply than that of PI-b which is consistent with the highest packing coefficient of PI-o.
Table 4
Mechanical and electronic properties of the polyimides.
Tensile strength Elongation
Initial modulus Dielectric
Dielectric loss
(10 )
a
-3
(MPa)
(%)
(GPa)
5.2±0.3
3.5±0.1
3.2±0.4
3.5±0.3
3.1±0.3
-
Constant
PI-m-380 95±4
PI-b-380 141±4
2.4±0.6
5.7±0.1
8.8±0.4
9.7±0.3
5.2±0.5
-
3.84
9.5
6.1
4.7
4.5
5.2
3.8
4.00
PI-t-380
157±9
3.88
PI-o-380 174±11
PI-b-420 122±10
3.81
3.72
PI-t-420
-
3.96
20

ACCEPTED MANUSCRIPT
PI-o-420 75±8
3.0±0.8
2.6±0.4
3.97
3.7
a
Dielectric constant at 1kHz.
3.6 Electronic properties
The variation curves of dielectric constant and dielectric loss of PIs over whole frequency
measuring range are shown in Fig. 9. In general, the dielectric constants of the resulting PIs were
not higher than 4 at 1 kHz and the dielectric loss ranged from 0.0037 to 0.0095 (Table 4). With
the increase in test frequency, the dielectric constants reduced slightly while the dielectric loss
increased. In lower frequency, higher dipole orientation degree was achieved by the prompt
response of dipoles to the alternation of applied electric field and less electrical energy was
dissipated with heat loss, therefore the PIs demonstrated higher dielectric constant and lower
3
0
dielectric loss .
Figure 9. Dielectric properties of the PIs.
4
. Conclusions
21

ACCEPTED MANUSCRIPT
A novel diamine QMDA containing quinoxaline was designed and synthesized, and then
employed to prepare new polyimide films with several aromatic dianhydrides for the first time.
Quinoxaline group can provide higher bond energy of C=N and stronger intermolecular π-π
stacking interaction, and thus brought PIs with high-T and enhanced their thermal properties.
g
Polyimide films were divided into two series: PI-380 and PI-420 on the basis of the final
annealing temperature. At a higher anneal temperature of 420 °C PI films were found to have
g
strengthened thermostability with extremely high T values exceeding 500 °C, 10% weight-loss
temperatures (Tꢀ10%) over 537 °C, and CTEs from 12.30 to 43.96 ppm/K. This could be achieved
d
by generating more cross-linkings in polyimide chains, leading to the chain segments tend to
cross-linking and stack closely under 420 °C. A new approach to improve thermal properties of
PIs by raising the final annealing temperature has been investigated. Thus,
quinoxaline-containing polyimide films exhibit a good combination of properties required for
fabricating LTPS TFTs and demonstrate a promising application prospect for flexible AMOLED.
Acknowledgments
This work was supported financially by Shanghai Key Projects of Basic Research
(16JC1403900) and National Natural Science Foundation of China (Key Program 51733007).
Appendix A. supplementary data
The supporting information is available from the website: https://doi.org/10.1016/j.polymer.××
1
13
H NMR and C NMR mass spectra of a, b and QMDA; ATR spectra of PAAs and PIs;
UV-Vis cut-off wavelengths of PI films ; TGA, TMA curves of PI-380 and PI-420 films.
22

ACCEPTED MANUSCRIPT
Notes
The authors declare no competing financial interest.
References
1) Huang, X.; Hu, K.; Lin, L.; Shan, Q.; Yang, X.;Gao, X. Development of AMOLED
display: from rigid to flexible. SID Symposium Digest of Technical Papers 2016, 47 (1),
12-414.
2) Xu, C. X.; Shu, S.; Lu, J. N.; Yuan, G. C.; Yao, Q.; Wang, L.; Xu, Z. Q.;Sun, Z. Y.
(
4
(
Foldable AMOLED display utilizing novel COE structure. SID Symposium Digest of Technical
Papers 2018, 49 (1), 310-313.
(3)
Ma, R.; Pang, H.; Mandlik, P.; Levermore, P. A.; Rajan, K.; Silvernail, J.; Krall, E.;
Paynter, J.; Hack, M.;Brown, J. J. Flexible OLEDs for lighting applications. SID Symposium
Digest of Technical Papers 2012, 43 (1), 772-775.
(4)
Chen, W. H.; Hsieh, M. C.; Wang, T. T. J.; Chang, T. C.; Yang, M. J.; Su, B. Y.; Yeh, Y.
H.; Ho, J. C.; Chen, G.; Tsai, C. C.;Lee, C. C. Stress absorbing LTPS-TFT for highly flexible
AMOLED. SID Symposium Digest of Technical Papers 2017, 48 (1), 1742-1745.
(5)
Chiang, M.; Cheng, C.; Tu, C.; Liu, C.; Huang, T.; Lu, J.; Sugiura, N.;Lin, Y. Handling
technology of plastic substrates in flexible display manufacturing. SID Symposium Digest of
Technical Papers 2014, 45 (1), 46-49.
(6)
Gao, X.; Lin, L.; Liu, Y.;Huang, X. LTPS TFT process on polyimide substrate for
flexible AMOLED. J. Disp. Technol. 2015, 11 (8), 666-669.
23

ACCEPTED MANUSCRIPT
(7)
Liu, J.; Lee, T. M.; Wen, C.;Leu, C. High performance organic-inorganic hybrid plastic
substrate for flexible display and electronics. SID Symposium Digest of Technical Papers 2010,
1 (1), 913-916.
8) Kim, M. K.; Kim, D. W.; Moon, S. H.; Shin, D. W.; Oh, T. S.;Yoo, J. B. One step
4
(
process of decomposition and polymerization to fabricate SiO hollow spheres/polyimide
2
composite for foldable OLEDs. Mater.Sci.Eng.,B 2017, 217, 7-11.
(
9)
coefficients of thermal expansion. Polymer 2017, 111, 91-102.
10) Chen, W.; Chen, W.; Zhang, B.; Yang, S.;Liu, C. Thermal imidization process of
polyimide film: interplay between solvent evaporation and imidization. Polymer 2017, 109,
05-215.
Hasegawa, M.; Hoshino, Y.; Katsura, N.;Ishii, J. Superheat-resistant polymers with low
(
2
(11) Sroog, C. E. Polyimides. Prog. Polym. Sci. 1991, 16 (4), 561-694.
(12) Varma, I. K.; Goel, R. N.;Varma, D. S. Effect of structure on the thermal stability of
polyimides. J. Polym. Sci., Polym. Chem. Ed. 1979, 17 (3), 703-713.
13) Luo, L.; Zhang, J.; Huang, J.; Feng, Y.; Peng, C.; Wang, X.;Liu, X. The dominant factor
(
for mechanical property of polyimide films containing heterocyclic moieties: in-plane
orientation, crystallization, or hydrogen bonding. J. Appl. Polym. Sci. 2016, 133 (39).
(
14) urRehman, S.; Song, G.; Jia, H.; Zhou, H.; Zhao, X.; Dang, G.;Chen, C. Synthesis and
characterization of benzimidazole-based low CTE block copolyimides. J. Appl. Polym. Sci.
013, 129 (5), 2561-2570.
15) Cheng, Y.; Dong, J.; Yang, C.; Wu, T.; Zhao, X.; Zhang, Q. Synthesis of
poly(benzobisoxazole-co-imide) and fabrication of high-performance fibers. Polymer 2017, 133,
0-59.
2
(
5
24

ACCEPTED MANUSCRIPT
(16) Dalton, M. J.; Kannan, R.; Jakubiak, R.; Haley, J. E.; Tan, L. S. Synthesis and
characterization of novel aromatic imide polymer and co-polymers containing
diphenylaminofluorene-benzothiazole as two-photon chromophoric units. J. Polym. Sci. Pol.
Chem. 2009, 50 (1), 0032-3934.
(17) Niu, H.; Huang, M.; Qi, S.; Han, E.; Tian, G.; Wang, X.;Wu, D. High-performance
copolyimide fibers containing quinazolinone moiety: preparation, structure and properties.
Polymer 2013, 54 (6), 1700-1708.
(
18) Xia, A.; Guo, H.; Qiu, X.; Ding, M.;Gao, L. Syntheses and properties of polyimides
derived from diamines containing 2,5-disubstituted pyridine group. J. Appl. Polym. Sci. 2006,
02 (2), 1844-1851.
19) Xia, A.; Lü, G.; Qiu, X.; Guo, H.; Zhao, J.; Ding, M.;Gao, L. Syntheses and properties of
novel polyimides derived from 2-(4-aminophenyl)-5-aminopyrimidine. J. Appl. Polym. Sci.
006, 102 (6), 5871-5876.
20) Sheardown, M. J.; Nielsen, E. O.; Hansen, A. J.; Jacobsen, P; Honore, T.
,3-Dihydroxy-6-nitro-7-sulfamoyl-benzo (F) quinoxaline: a neuroprotectant for cerebral
ischemia. Science. 1990, 247 (4942), 571-574.
21) Behrends, A.; Hardeland, R.; Ness, H. Photocatalytic actions of the pesticide metabolite
-hydroxyquinoxaline: destruction of antioxidant vitamins and biogenic amines-implications of
organic redox cycling. Redox. Report. 2004, 9 (5), 279-288.
22) Kim, Y. B.; Kim, Y. H.; Park, J. Y. Synthesis and biological activity of new quinoxaline
antibiotics of echinomycin analogues. Bioorg. Med. Chem. Lett. 2004, 14 (2), 541-544.
1
(
2
(
2
(
2
(
25

ACCEPTED MANUSCRIPT
(23) Zhang, H.; Wu, Y. Z.; Zhang, W. W.; Li, E. P.; Shen, C.; Jiang, H. Y.; Tian, H.; Zhu, W.
H. Low cost and stable quinoxaline-based holetransporting materials with a D–A–D molecular
configuration for efficient perovskite solar cells. Chem. Sci. 2018, 9 (27), 5919-5928.
(24) Fumihiko, A.; Sigeki, k.; Koichi, M.; Kiyoshi, N.; Masatoshi, M. Preparation of
polyimides from 2,3-bis(4-aminophenyl)quinoxalines and aromatic tetracarboxylic dianhydrides.
Macromol. Rapid. Commun. 1990, 11 (12), 673-677.
(
25) Yazdanbakhsh, L. N.; Azim, Z. H.; Florenci, V. G. Regioselective opening of
nitroepoxides with unsymmetrical diamines. J. Org. Chem. 2018, 83 (3), 1252-1258.
26) Chung, I. S.; Park, C. E.; Ree, M.; Kim, S. Y. Soluble polyimides containing
benzimidazole rings for interlevel dielectrics. Chem. Mater. 2001, 13 (9), 2801-2806.
27) Slonimskii, G. L.; Askadskii, A. A.; Kitaigorodskii, A. I. The packing of polymer
molecules. Polym. Sci. U.S.S.R. 1970, 12 (3), 556-577.
28) Luo, L. B.; Yao, J.; Wang, X.; Li, K.; Huang, J. Y.; Li, B. Y.; Wang, H. N.; Feng, Y.;
(
(
(
Liu, X. Y. The evolution of macromolecular packing and sudden crystallization in rigid-rod
polyimide via effect of multiple H-bonding on charge transfer (CT) interactions. Polymer. 2014,
5
5, 4258-4269.
(29) Feng, Y.; Luo, L. B.; Huang, J.; Li, K.; Li, B.;Wang, H.; Liu, X. Y. Effect of molecular
rigidity and hydrogen bond interaction on mechanical properties of polyimide fibers. J. Appl.
Polym. Sci. 2016, 133 (28).
(30) Zhou, H.; Lei, H. Y.; Wang, J. H.; Qi, S. L.; Tian, G. F.; Wu, D. Z. Breaking the mutual
restraint between low permittivity and low thermal expansion in polyimide films via a branched
crosslink structure. Polymer. 2018, 162, 116-120.
26

ACCEPTED MANUSCRIPT
Design and synthesis of a novel quinoxaline diamine
and its polyimides with high-T and red color
g
a
b
a
a*
a,b*
Tuanqing Liu , Feng Zheng , Tongmei Ding , Shuyu Zhang , Qinghua Lu
Highlights:
•
•
•
A novel diamine derived from quinoxaline was designed and synthesized
Quinoxaline polyimides exhibited high thermal stability and red color
Improving final annealing temperature enhanced the thermostability of polyimides