New soluble polyimides with high optical transparency and light color containing pendant trifluoromethyl and methyl groups

Article abstract of DOI:10.1002/cjoc.201100754

A new aromatic diamine containing trifluoromethyl and methyl groups, namely α,α-bis(4-amino-3-methylphenyl)-4-(trifluoromethyl)phenylmethane (1), was synthesized from 2-methylaniline and 4-(trifluoromethyl)benzaldehyde. A series of fluorinated polyimides (PIs) were prepared from the diamine with four commercially available aromatic tetracarboxylic dianhydrides using a one-step high-temperature polycondensation procedure. These obtained PIs showed excellent solubility, with the dissolvability at a concentration of 10 wt% in most solvents, and they could afford flexible and strong films. Thin films of these PIs exhibited high optical transparency and light color, with the cutoff wavelength at 324-357 nm and transmittance higher than 74% at 450 nm. Moreover, these PIs possessed eminent thermal stability and good mechanical properties. Copyright

Full text of DOI:10.1002/cjoc.201100754

FULL PAPER
DOI: 10.1002/cjoc.201100754
New Soluble Polyimides with High Optical Transparency and
Light Color Containing Pendant Trifluoromethyl and Methyl
Groups
Wang, Chenyi*,a(汪称意)
Zhao, Xiaoyana(赵晓燕)
Li, Guang*,b(李光)
a School of Material Science and Engineering, School of Petrochemical Engineering, Changzhou University,
Changzhou, Jiangsu 213164, China
b State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, College of Material Science
and Engineering, Donghua University, Shanghai 201620, China
A new aromatic diamine containing trifluoromethyl and methyl groups, namely α,α-bis(4-amino-3-methyl-
phenyl)-4-(trifluoromethyl)phenylmethane (1), was synthesized from 2-methylaniline and 4-(trifluoromethyl)-
benzaldehyde. A series of fluorinated polyimides (PIs) were prepared from the diamine with four commercially
available aromatic tetracarboxylic dianhydrides using a one-step high-temperature polycondensation procedure.
These obtained PIs showed excellent solubility, with the dissolvability at a concentration of 10 wt% in most sol-
vents, and they could afford flexible and strong films. Thin films of these PIs exhibited high optical transparency
and light color, with the cutoff wavelength at 324—357 nm and transmittance higher than 74% at 450 nm. More-
over, these PIs possessed eminent thermal stability and good mechanical properties.
Keywords fluorinated polyimides, solubility, optical transparency
Introduction
Recently, considerable attention has been devoted to
the fluorinated PIs, especially the trifluoromethyl-
containing poly(ether imide)s (PEIs).[14,18,20-26] As
well-known, the bulky free volume and high electro-
negativity of the fluorinated groups could endow aro-
matic PIs with many attractive features, such as good
organo-solubility, high optical transparency, light color
as well as low water uptake and dielectric constants.[27]
Thus, the develepment of novel trifluoromethyl substi-
tuted monomers for functional PIs has become a subject
of intense research interest. In this study, a new
CF3-containing aromatic diamine was synthesized in
high yield by a facile condensation reaction. Subse-
quently, a series of fluorinated PIs with pendant
trifluoromethyl and methyl groups were prepared from
the diamine and different aromatic dianhydrides. The
fluorinated PIs should have diminished aromatic conju-
gation and intermolecular CTC formation because of
incorporation of bulky free volume and noncoplanar
structure into the polymer backbones. The effects of the
structures on the properties of the PIs were detailedly
investigated and discussed.
Functional polyimide films with high optical trans-
parency and light color are a class of important ad-
vanced materials and play crucial roles for some appli-
cations in liquid crystal display devices, flexible solar
cell substrates, optical waveguides and so on.[1-6] How-
ever, the wholly aromatic polyimides (PIs) are often
pale yellow or deep reddish yellow and have strong ab-
sorption in the visible region observed from their
UV-visible spectra because of their high conjugated aro-
matic structures and/or the intermolecular charge-trans-
fer complex (CTC) formation.[7] Moreover, most aro-
matic PIs are difficult for both solution-casting and melt
processing due to their poor solubility in organic sol-
vents and high melting temperatures, which highly re-
stricts their application in many fields.[8] Purely com-
mercial PIs derived from pyromellitic dianhydride
(PMDA) or 3,3',4,4'-biphenyltetracarboxylic dianhydride
(BPDA), for instance, are soluble only in a few strongly
protic acids. To overcome these problems, much re-
search efforts have been focused on the synthesis of
soluble and optically transparent PIs without much de-
terioration of their own excellent properties. Several
successful approaches including insertion of flexible
linkage and bulky substituents on the main chain or
utilization of noncoplanar or unsymmetrical monomers
have been developed.[9-19]
Experimental
Materials
2-Methylaniline, 4-(trifluoromethyl)-benzaldehyde
*
E-mail: wangcy1981@gmail.com; lig@dhu.edu.cn
Received January 4, 2012; accepted February 14, 2012; published online XXXX, 2012.
Chin. J. Chem. 2012, XX, 1—6
© 2012 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
1

Wang, Zhao & Li
FULL PAPER
and trifluoromethanesulfonic acid purchased from
Shanghai Darui Finechemical Co. Ltd were used as re-
ceived. Commercially available aromatic tetracarbox-
ylic dianhydrides such as pyromellitic dianhydride
(PMDA or 2a), 3,3',4,4'-biphenyltetracarboxylic dian-
hydride (BPDA or 2b), 3,3',4,4'-benzophenone-tetra-
carboxylic dianhydride (BTDA or 2c), and 4,4'-oxy-
diphthalic dianhydride (ODPA or 2d) were all purified
by recrystallization from acetic anhydride and then
dried in vacuo at 180 ℃ for 10 h. Commercially avail-
able N-methyl-2-pyrrolidinone (NMP), m-cresol, N,N-
dimethylformamide (DMF), N,N-dimethyl-acetamide
(DMAc), dimethyl sulfoxide (DMSO), tetrahydrofuran
(THF) and other solvents were purified by distillation
prior to use.
uct was further purified by recrystallization from tolu-
ene to give white crystals (31.48 g, 85%); m.p. 188—
189 ℃ by DSC (10 ℃/min).
FT-IR (KBr) ν: 3441, 3354, 2924, 1638, 1598, 1163,
－
1
1
1104, 1058 cm . H NMR (400 MHz, DMSO-d6) δ:
7.57 (d, J＝8.0 Hz, 2H), 7.23 (d, J＝8.0 Hz, 2H), 6.63
(s, 2H), 6.56 (d, J＝8.0 Hz, 2H), 6.49 (d, J＝8.0 Hz,
2H), 5.22 (s, 1H), 4.65 (s, 4H), 1.94 (s, 6H). 13C NMR
(100 MHz, DMSO-d6) δ: 151.2, 145.2, 131.8, 131.0,
130.0, 127.4, 126.8 (JC-F＝30.8 Hz), 125.3, 124.9 (JC-F
＝270.2 Hz), 121.4, 114.3, 54.9, 18.0. Anal. calcd for
C22H21F3N2 (370.41): C 71.34, H 5.71, N 7.56; found C
71.25, H 5.80, N 7.49.
Polymer synthesis
The general procedure for the preparation of the
fluorinated PIs 3a—3d was illustrated as follows. Dia-
mine 1 (2.0 mmol) and dianhydride monomer 2a—2d
(2.0 mmol) were first dissolved in 15.0 mL of m-cresol
in a 50 mL three-necked round-bottomed flask. After
the mixture was stirred at room temperature for 30 min,
isoquinoline (ca. 5 drops) was added, and further stirred
for 3 h at 120 ℃. Then the mixture was heated at 185
℃ for 8 h. Water formed during the imidization was
continuously removed with a stream of nitrogen. The
whole polymerization proceeded smoothly, and homo-
geneous, transparent, viscous polymer solutions were
produced. White, fibrous polymer resins were obtained
when the resultant polymer solution was poured into an
excess of ethanol. The polymer was separated by filtra-
tion and washed with ethanol for several times and dried
in a vacuum oven at 120 ℃ for 10 h.
Measurements
NMR spectra were measured on a Bruker AV400 in-
strument with dimethyl sulfoxide-d6 (DMSO-d6) or
CDCl3 as solvent and tetramethylsilane (TMS) as an
internal standard. FT-IR spectra were recorded on a
Nicolet NEXUS 670 spectrometer. Elemental analysis
was carried out on an Elmentar Vario EL system. Gel
permeation chromatography-Light scattering (GPC-LS)
on soluble polymers was performed on a BI-MwA sys-
tem using THF as the eluent. Differential scanning calo-
rimetric (DSC) analysis was performed on a PE Dia-
mond DSC instrument at a heating rate of 20 ℃/min in
nitrogen and air atmosphere. Glass transition tempera-
tures were read at the middle of the transition in the heat
capacity from the second heating scan after cooling
from 400 ℃ at a cooling rate of 20 ℃/min. Thermo-
gravimetric analysis (TGA) of the polymer samples was
measured on a Netzsch TG 209F1 instrument at a heat-
ing rate of 20 ℃/min in nitrogen and air atmosphere.
Ultraviolet-visible spectra of the polymer films were
recorded on a PerkinElmer Lambda 35 UV-Visible
spectrophotometer at room temperature. The tensile
properties were performed on an Instron 5565 Tensile
Apparatus with a 5 kg load cell at a crosshead speed of
5 mm/min on strips approximately 40—60 um thick and
0.5 cm wide with a 2 cm gauge length. An average of at
least five individual determinations was used.
Results and Discussion
Monomer synthesis
New CF3-containing aromatic diamine, α,α-bis(4-
amino-3-methylphenyl)-4-(trifluoromethyl)-phenylmeth-
ane (1), was synthesized via a condensation reaction of
4-(trifluoromethyl)benzaldehyde and 2-methylaniline
catalyzed by hydrochloric acid at 150 ℃, as shown in
Scheme 1. The results of FT-IR, and NMR spectra, and
elemental analysis all confirmed the chemical structure
of the diamine monomer. The FT-IR spectrum of dia-
mine 1 showed the characteristic absorptions of the N—
H stretching band of the amino groups at 3300—3500
Monomer synthesis
In a 100 mL, four-necked, round-bottomed flask
equipped with a mechanical stirrer, a condenser and a
nitrogen inlet, 2-methylaniline (25.72 g, 0.24 mol) was
added and heated to reflux under nitrogen atmosphere.
4-(Trifluoromethyl)benzaldehyde (17.41 g, 0.10 mol)
dissolved in 4 mL (12 mol/L) of hydrochloric acid was
added dropwise over a period of 1 h. After the comple-
tion of addition, the reaction mixture was maintained at
reflux (about 150 ℃) for another 10 h. The reaction
mixture was then cooled to 60 ℃, and 100 mL of 15%
aqueous solution of sodium hydroxide was added to the
resulting suspension. The mixture was poured into
methanol to precipitate a pale-white solid which was
filtered, washed repeatedly with water. The crude prod-
cm－ and the C—F stretching at about 1050—1200
1
－
1
1
cm . The H and 13C NMR spectra of diamine 1 were
shown in Figure 1a and 1b, respectively. In Figure 1a,
the signal at δ 4.65 was peculiar to the protons of amino
groups and the signal at δ 1.94 corresponded to the pro-
tons of methyl groups. In the 13C NMR spectrum, all the
carbon-13 atoms resonated in the region of δ 15—155,
and there appeared clear quatets because the C—F cou-
pling. The coupling effect decreased with the increase
of the distance between C and F atoms. For instance,
C-12 (JC-F＝270.2 Hz) showed much stronger coupling
effect with F than C-1 (JC-F＝30.8 Hz). Moreover, the
2
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Chin. J. Chem. 2012, XX, 1—6

New Soluble Polyimides with High Optical Transparency and Light Color
Scheme 1 Synthesis of aromatic diamine 1
H3C
H2N
CH3
NH2
CHO
H3C
H
HCl
H2N
+
150 ℃, 10 h
CF3
CF3
1
Figure 1 1H and 13C NMR spectra of diamine 1 in DMSO-d6.
all PIs 3a—3d can be prepared conveniently from aro-
matic diamine 1 and various aromatic dianhydrides
2a—2d (PMDA, BPDA, BTDA and ODPA) via a
one-step, high-temperature solution polycondensation
procedure, as shown in Scheme 2. The polymerization
of diamine 1 with aromatic dianhydrides 2a—2d pro-
ceeded smoothly at 185 ℃, and homogeneous, trans-
parent, viscous polymer solutions were produced. White
or pale yellow, and fibrous polymer resins were ob-
tained when the resultant polymer solution was poured
into an excess of ethanol. The PIs were obtained in al-
most quantitative yields and the inherent viscosities
elemental analysis values were in good agreement with
the calculated values. All the above characterizations
indicated the successful preparation of the fluorinated
diamine monomer 1.
Polymer synthesis
Generally, there are two main synthetic methods to
PIs, namely two-step and one-step polymerizations.
Commercial PIs are mainly prepared by two-step po-
lymerizations. In this method, the poly(amic acid) (PAA)
was first prepared from dianhydride and diamine in a
polar aprotic solvent, then the PAA was cyclodehy-
drated at elevated temperatures or by adding a cyclizing
agent such as acetic anhydride. Compared to two-step
polymerizations, one-step polymerization is more con-
venient and fully cyclized PIs were obtained directly
from their corresponding teteracarboxylic acid dianhy-
dride and diamine using high boiling solvents. This
method was usually used when working with soluble
PIs and is considered more practical for polymerizing
less reactive dianhydrides and diamines. In this study,
were between 0.54 and 0.87 dL•g－ in DMAc solution
1
(Table 1). The weight-average molecular weights (Mw)
and number average molecular weights (Mn) were
measured based on GPC (Table 1), the Mw and Mn val-
ues of the PIs dissolved in THF were 41400 to 105200
and 20900 to 57500, respectively. The polydispersity
index (PDI) of the 3 series was in the range of 1.77—
1.98. These data indicate that the obtained PIs have
moderate molecular weights and relatively narrow PDI.
Chin. J. Chem. 2012, XX, 1—6
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3

Wang, Zhao & Li
FULL PAPER
Scheme 2 Synthesis of fluorinated PIs 3a—3d
H3C
CH3
O
N
O
O
O
O
O
O
O
O
N
O
m-cresol
isoquinoline
H
+
Ar
Ar
1
8 h
185 ℃,
n
2a—
2d
CF3
— 3d
3a
O
C
O
:
Ar
c
d
a
b
Table 1 Inherent viscositiesa and GPC-LS molecular weights of
fluorinated PIs
disappears completely here, and new peaks appear at δ
8.26 and δ 8.08—8.13 which correspond to the protons
in the dianhydride units. All the results confirm the
complete imidization between diamine and dianhy-
drides.
GPC-LS data
1
Polymer
ŋinh/(dL•g－
)
Mn×10－
Mw×10－
PDI
1.98
1.83
1.90
1.77
4
4
3a
3b
3c
3d
0.54
0.87
0.70
0.62
2.09
4.14
Polymer solubility
5.75
10.52
8.27
The solubility of these fluorinated PIs was tested
qualitatively in various organic solvents, and the results
are reported in Table 2. The obtained PIs 3a—3d were
soluble and had high dissolvability at a concentration of
4.35
3.86
6.83
a Measured at a concentration of 0.5 g/dL in DMAc at 30 ℃.
The structures of the PIs 3a—3d were characterized
1
by FT-IR and H NMR spectra. All the PIs showed
characteristic imide absorption bands around 1782 and
1730 cm－ (imide carbonyl asymmetrical and symmet-
1
rical stretchings), 1374 cm－ (C—N stretching), and
1
1040 and 735 cm－ (imide ring deformation), together
1
with strong C—F stretching absorption peak at about
－
1
1100 cm , while the amino groups at the region of
3300—3500 cm－ disappeared. A representative of
1
1
FT-IR and H NMR spectra of PI 3b are illustrated in
Figure 2 and 3 respectively. The assignments of each
proton designated in the 1H NMR spectrum are in com-
plete agreement with the proposed polymer structures.
The sharp peak at δ 4.65 corresponding to the amine
1
Figure 2 FT-IR spectrum of PI 3b.
protons in H NMR spectrum of diamine 1 (Figure 1a)
Figure 3 1H NMR spectrum of PI 3b in CDCl3.
4
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Chin. J. Chem. 2012, XX, 1—6

New Soluble Polyimides with High Optical Transparency and Light Color
10 wt% in most tested solvents. It is surprising that PI
3a and 3b, which were prepared from the very rigid di-
anhydrides PMDA and BPDA, respectively, were
quickly soluble in NMP, DMAc, CHCl3, CH2Cl2 and
THF at room temperature. It should be noted that good
solubility in low-boiling-point solvents is critical for
preparing PI films or coatings at a relatively low proc-
essing temperature, which is desirable for advanced
manufacturing applications. The good solubility of these
PIs was governed by the structural modification through
incorporation of the bulky 4-(trifluoromethyl)phenyl
groups in the polymer backbone, which could decrease
the packing density and intermolecular interactions of
macromolecular chain, thereby enhancing solubility.
Moreover, the methyl group introduced at the ortho-
position of nitrogen in the polymer chains appeared to
hinder the rotation of C—N imide bond and force the
two aromatic rings to adopt a noncoplanar structure,
which was also effective to improve solubility.
Table 3 Thermal properties of PIs 3a—3d
T10c/℃
a
b
Polymer Tg /℃
Td /℃
Char yieldd/%
In N2 In air
538 450
541 465
534 453
529 447
3a
3b
3c
3d
no
518
517
516
519
58
56
58
55
348
324
309
a From the second trace of DSC measurements conducted at a
b
heating rate of 20 ℃/min. Onset decomposition temperature at
20 ℃/min heating rate. c 10% weight loss temperature in TGA at
20 ℃/min heating rate. d Residual weight retention at 800 ℃.
Table 2 Solubility behavior of fluorinated PIsa
Polymer NMP DMAc DMF DMSO CHCl3 CH2Cl2 THF Acetone
3a
3b
3c
3d
○
○
○
○
＋＋
○
＋
＋＋
○
＋
＋
○
○
○
○
＋＋
○
○
○
○
○
－
－
－
S
○
＋＋
○
○
○
○
○
Figure 4 TGA curves of PI 3b at a heating rate of 20 ℃/min.
a ○, 100 mg sample dissolved in 1 mL solvent (10 wt%); ＋＋,
soluble at 5 wt%; ＋, soluble at 1 wt%; S, swelling; －, insolu-
ble.
bility even with the introduction of methyl and
trifluoromethyl pendent groups.
Optical and mechanical properties
Thermal properties
As depicted in introduction, most commercially
aromatic PIs display rather deep yellow and strong ab-
sorption at UV-visible spectra because of their high
conjugated aromatic structures and/or the intermolecular
CTC formation. Unlike those commercial PIs, PI 3a—
3d films exhibit high optical transparency at UV-visible
spectra and nearly colorless. Thin films were measured
for optical transparency with UV-visible spectroscopy.
Figure 5 shows the UV-visible transmittance spectra of
these PIs, and the cutoff wavelength (λcutoff) values and
the percentages transmittance at 450 nm from these
spectra are listed in Table 4. All the PI films had shorter
λcutoff than 360 nm, and exhibited high optical transpar-
ency of 74.4%—81.5%. For comparison, a photograph
of one piece of commercial Kapton and one piece of PI
4d film is shown in Figure 6. The films were photo-
graphed against a special background to highlight the
transparency. It is easy to find that PI 3d film exhibited
much lighter color than Kapton film. The good optical
transparency and light color of these PIs were mainly
attributed to the reducing of the conjugation and inter-
molecular CTC formation along the PI backbone. It may
be concluded that bulky pendant groups together with
the noncoplanar structure and methane moieties in the
polymer chains were effective to reduce the conjugation
and intermolecular CTC formation, so to improve the
optical properties.
DSC and TGA were used to evaluate the thermal
properties of the PIs 3a—3d. The results are summa-
rized in Table 3. It was inspiring that these PIs showed
extra high Tg, and the values were all beyond 300 ℃. PI
3a based on rigid dianhydride PMDA even shows no
distinct Tg at determined temperature region. Apparently,
the 3 series exhibited 30—60 ℃ of Tg increment com-
pared with those of structurally fluorinated PEIs derived
from the trifluoromethyl-substituted bis(ether amine)s
and corresponding dianhydrides.[20-26] High Tg values
mainly originated from the inherent rigidity of these
macromolecular backbones. Meanwhile, the introduc-
tion of methyl substituent into the ortho-position of the
imide nitrogen hinders the rotation of the two aromatic
rings around C—N bonds, so further increases the Tg
values.
The thermal and thermooxidative stabilities of these
PIs were evaluated in both nitrogen and air atmospheres.
Typical TGA curves for PI 3b are reproduced in Figure
4. These PIs showed good thermal stability and had no
notable weight loss below 510 ℃ in nitrogen atmos-
phere. The 10% weight loss temperatures (T10) in nitro-
gen and in air atmospheres stayed in the range of 529—
541 ℃ and 447—465 ℃, respectively. They left more
than 55% char yield at 800 ℃ in nitrogen. The TGA
data indicated that these PIs had fairly high thermal sta-
Chin. J. Chem. 2012, XX, 1—6
© 2012 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.cjc.wiley-vch.de
5

Wang, Zhao & Li
FULL PAPER
capability, high thermal stability and optical transpar-
ency, light color, and good mechanical properties. The
eminent combination of several properties guaranteed
the obtained PIs as potential high-temperature resistant
materials for optical and photoelectric applications.
Acknowledgement
This word was supported by the National Natural
Science Foundation of China (No. 51103016), Natural
Science Foundation of Jiangsu Province (Nos.
BK2011232 and BK2011241), the Grant-in-aid for
Youth Innovation Fundation of Changzhou University
(Nos. ZMF11020017 and ZMF1102037), and the Open-
ing Project of State Key Laboratory for Modification of
Chemical Fibers and Polymer Materials (No. LK1001).
Figure 5 UV-visible spectra of the PI films (about 30—40 µm
thickness).
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5.9
11.7
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[26] Yang, F. C.; Zhao, J. J.; Li, Y. F.; Zhang, S. J.; Shao, Y.; Shao, H.;
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(Pan, B.; Lu, Z.)
Conclusions
A new CF3-containing aromatic diamine has been
successfully synthesized in high yield by a facile
2-methylaniline condensation reaction. A series of
fluorinated PIs with bulky free volume and noncoplanar
structure, were prepared by one-step high-temperature
polycondensation procedure. The obtained PIs were
characterized by excellent solubility, good film-forming
6
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Chin. J. Chem. 2012, XX, 1—6