Novel soluble fluorinated poly(ether imide)s with different pendant groups: Synthesis, thermal, dielectric, and optical properties

Article abstract of DOI:10.1002/pola.24111

Three types of new bis(ether dianhydride) monomers, [4,4'-(2-(3'- methylphenyl)-1,4-phenylenedioxy)-diphthalic anhydride (4a)], [4,4'-(2-(3'-trifluoromethylphenyl)-1,4-phenylenedioxy)-dlphthalic anhydride (4b)], and [4,4'-(2-(3',5'-ditrifluoromethylphenyl)-1,4-phenylenedioxy)- diphthalic anhydride (4c)] were prepared via a multistep reaction sequence. Three series of soluble poly(ether imide)s (PEIs) were prepared from the obtained dianhydrides by a two-step chemical imidization method. Experimental results indicated that all the PEIs had glass transition temperature in the range of 200-230 °C and the temperature of 5% weight loss in the range of 520-590 °C under nitrogen. The PEIs showed excellent solubility in a variety of organic solvents due to introduction of the bulky pendant groups and were capable of forming tough films. The casting films of PEIs (80-91 μm in thickness) had tensile strengths in the range from 88 to 117 MPa, tensile modulus from 2.14 to 2.47 GPa, and elongation at break from 15 to 27%. The casting films showed UV-Vis absorption edges at 357-377 nm, low dielectric constants of 2.73-2.82, and water uptakes lower than 0.66 wt %. The spin-coated films of PEIs presented a minimum birefringence value as low as 0.0122 at 650 nm and low optical absorption at the optical communication wavelengths of 1310 and 1550 nm.

Full text of DOI:10.1002/pola.24111

Novel Soluble Fluorinated Poly(ether imide)s with Different Pendant
Groups: Synthesis, Thermal, Dielectric, and Optical Properties
YU LIU, YAN XING, YUNHE ZHANG, SHAOWEI GUAN, HAIBO ZHANG, YU WANG, YONGPENG WANG, ZHENHUA JIANG
Alan G. MacDiarmid Laboratory, College of Chemistry, Jilin University, Qianjin Street 2699, Changchun 130023, People’s
Republic of China
Received 5 January 2010; accepted 29 April 2010
DOI: 10.1002/pola.24111
Published online in Wiley InterScience (www.interscience.wiley.com).
ABSTRACT: Three types of new bis(ether dianhydride) mono-
mers,
[4,40-(2-(30-methylphenyl)-1,4-phenylenedioxy)-diphthalic
casting films of PEIs (80–91 lm in thickness) had tensile
strengths in the range from 88 to 117 MPa, tensile modulus
from 2.14 to 2.47 GPa, and elongation at break from 15 to
27%. The casting films showed UV-Vis absorption edges at
357–377 nm, low dielectric constants of 2.73–2.82, and
water uptakes lower than 0.66 wt %. The spin-coated films of
PEIs presented a minimum birefringence value as low as
anhydride (4a)], [4,40-(2-(30-trifluoromethylphenyl)-1,4-phenylene-
dioxy)-diphthalic anhydride (4b)], and [4,40-(2-(30,50-ditrifluoro-
methylphenyl)-1,4-phenylenedioxy)-diphthalic anhydride (4c)]
were prepared via a multistep reaction sequence. Three series
of soluble poly(ether imide)s (PEIs) were prepared from the
obtained dianhydrides by
a
two-step chemical imidization
0.0122 at 650 nm and low optical absorption at the optical
C
V
method. Experimental results indicated that all the PEIs had
glass transition temperature in the range of 200–230 ꢀC and the
temperature of 5% weight loss in the range of 520–590 ꢀC
under nitrogen. The PEIs showed excellent solubility in a vari-
ety of organic solvents due to introduction of the bulky pend-
ant groups and were capable of forming tough films. The
communication wavelengths of 1310 and 1550 nm.
2010
Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48:
3281–3289, 2010
KEYWORDS: dielectric properties; fluoropolymers; optics; poly-
imides; synthesis
INTRODUCTION Aromatic polyimides (PI) are well known
high performance materials because of their excellent ther-
mal stability, mechanical, electric, and optical properties.1–3
High optical transparency is one of the most attractive prop-
erties of polyimides materials for a variety of applications
such as optical waveguides.4,5 However, most of the conven-
tional aromatic polyimide films show considerable coloration
from light yellow to dark brown due to their highly conju-
gated aromatic structures, the formation of intermolecular
charge-transfer complexes and electronic polarization inter-
actions. In addition, the aromatic polyimides are difficult to
be processed because of their poor solubility and high pro-
cessing temperature, which are caused by their rigid poly-
mer backbones and strong interchain interaction.6,7
their excellent properties.8–12 Moreover, it is an effective
method to improve high optical transparency through intro-
ducing trifluoromethyl into polyimides. It has been demon-
strated that the introduction of bulky trifluoromethyl group
into polyimide backbones resulted in an enhanced solubility
and optical transparency together with a lower dielectric
constant, attributed to low polarizability of the CAF bond
and the increase in free volume. The fluorinated polyimides
also provided other merits such as good thermal and
thermo-oxidative stability, and low moisture absorption. In
addition, the introduction of bulky trifluoromethyl moieties
into the polymers could decrease the refractive index and
the birefringence effectively.13–17
At present, most dissoluble aromatic polyimides containing
fluorine were prepared by using different kinds of aromatic
diamine containing fluorine rather than fluorinated aromatic
dianhydrides, except for 6FDA, which is currently commer-
cial available.18–21 However, the widespread applications
and commercialization for 6FDA are also hampered by the
difficulty in handling of poisonous gaseous chemicals in the
synthetic procedure. As part of our continuing efforts to
gain colorless and soluble polyimides with high thermal
stability and wide application potentials in optoelectronics,
three novel bis(ether dianhydride)s, bis(ether dianhydride)
The development of soluble polyimides with high optical
transparency has been to attract major interesting in the
research of aromatic polymers. Fortunately, considerable
efforts have got good progresses on the processing ability
and solubility of polyimides on the basis of structure design
and modification of aromatic dianhydride and diamine
monomers. Introducing noncoplanar moiety into the back-
bone of the polyimides, contributed either by the dianhy-
drides or from the diamines, has been proven to be a suc-
cessful approach to attain the solubility without sacrificing
Additional Supporting Information may be found in the online version of this article. Correspondence to: Z. Jiang (E-mail: jiangzhenhua@jlu.edu.cn)
C
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Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 48, 3281–3289 (2010) 2010 Wiley Periodicals, Inc.
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
SCHEME 1 Synthetic route of bis(ether anhydride) monomers.
[4,40-(2-(30-methylphenyl)-1,4-phenylenedioxy)-diphthalic an-
hydride (4a)], [4,40-(2-(30-trifluoromethylphenyl)-1,4-phenyl-
enedioxy)-diphthalic anhydride (4b)], and [4,40-(2-(30,50-di-
trifluoromethylphenyl)-1,4-phenylenedioxy)-diphthalic anhy-
dride (4c)] were designed and synthesized. Three series of
soluble poly(ether imide)s (PEIs) (4aI–4aIII, 4bI–4bIII, and
4cI–4cIII) were prepared by polycondensation of the mono-
mers with various aromatic diamines. This study deals with
the synthesis and basic characterization of the PEIs on the ba-
sis of bis(ether anhydride)s derived from hydroquinone and
its methylphenyl, trifluoromethylphenyl, and ditrifluoromethyl-
phenyl pendant derivatives. The effect of methylphenyl, tri-
fluoromethyl, and ditrifluoromethyl groups on solubility, ther-
mal properties, mechanical properties, dielectric constant, and
optical properties of the PEIs were investigated. It was charac-
terized that the prepared PEIs exhibited good solubility, excel-
lent thermal, and mechanical properties, low water uptake,
low dielectric constant, high optical transparency, and low op-
tical absorption in the optical communication wavelengths.
Gel permeation chromatograms (GPC) were obtained on a
Waters 410 instrument with tetrahydrofuran (DMF) as an
eluent at a flow rate of 1 mL/min using polystyrene as a
standard. Inherent viscosity was determined on an Ubblohde
viscometer in thermostatic container with the polymer con-
centration of 0.5 g/dL in DMAc at 25 ꢀC. Differential scan-
ning calorimetry (DSC) measurements were performed on a
e
ꢀ
Mettler Toledo DSC 821 instrument at a heating rate of 20 C/
min under nitrogen. Thermal gravimetric analyses (TGA)
were determined in nitrogen atmosphere using a heating
ꢀ
rate of 10 C/min and polymers were contained within open
aluminum pans on a PerkinElmer TGA-7. The birefringences
of the polymer films at 650 nm were determined from cou-
pling angles of TE (transverse electric) of TM (transverse
magnetic) optical guided modes with a gadolinium gallium
garnet (GGG) prism. UV-visible transparency was measured by
a Shimadzu UV 2501-PC spectrophotometer. Wide-angle X-ray
diffraction (WAXD) measurements were made at room tem-
perature using a Rigaku/max-rA diffractometer equipped with
a Cu Ka radiation source. The mechanical tests in tension
were carried out using a Shimadzu AG-I at a constant cross-
head speed of 10 mm/min. The e at 1.0 MHz frequency was
calculated from the equation as follows: e ¼ 1.10nAV2, where
nAV is average refractive index (i.e. nAV ¼ (2nTE þ nTM)/3).
The equilibrium moisture absorption was determined by
weighing the changes in vacuum-dried film specimens before
EXPERIMENTAL
Materials
All the reagents were purchased from commercial sources
and were used as received. The (3-methylphenyl)-1,4-hy-
droquinone, (3-fluoromethylphenyl)-1,4-hydroquinone, and
(3,5-difluoromethylphenyl)-1,4-hydroquinone were synthe-
sized according to the literature.22 1,4-bis(4-amino-2-trifluoro-
methylphenoxy) benzene (6FAPB) was synthesized in our
laboratory according to the literature.10 4,40-oxydianiline
(ODA) (TCI) and 1,4-bis(4-aminophenoxy) benzene (APB)
(TCI) were used without further purification.
ꢀ
and after immersion in deionized water at 25 C for 3 days.
Monomer Synthesis
The monomers, 4a–4c were synthesized by the nitrodisplace-
ment reaction of 4-nitrophthalodinitrile with 1a–1c in N,N-
dimethylformamide (DMF) in presence of potassium carbon-
ate as the base, followed by the alkaline hydrolysis of the in-
termediate bis(ether dinitrile)s and cyclodehydration of the
resulting bis(ether diacid)s.23 The synthetic procedure is
shown in Scheme 1 and the elemental analysis, IR, and 1H
NMR data of the target bis(ether anhydride)s are given in
Supporting Information.
Measurements
IR spectra (KBr) were measured on a Nicolet Impact 410
Fourier transform infrared spectrometer. 1H NMR spectra
were recorded on a Bruker 510 NMR spectrometer (500
MHz) with tetramethyl silane as a reference. Elemental anal-
ysis was performed on an Elemental Analyses MOD-1106.
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FIGURE 1 1H NMR spectrum of bis(ether anhydride) 4a in
DMSO-d6.
thoroughly with methanol, and dried at 80 ꢀC in vacuum.
Other PEIs were synthesized in an analogous procedure.
Characterization of the Polymers
PEI-4cI: IR (KBr, cmꢁ1): 1780 and 1721 (asymmetric, sym-
metric imide C¼¼O stretch), 1619–1480 (aromatic C¼¼C
stretch), 1382 (CAN stretch), 1100–1300 (CAO and CAF
1
stretching). H NMR (500 MHz, DMSO-d6, d, ppm): 8.26 (2H),
8.06 (1H), 8.00 (1H), 7.90 (2H), 7.87 (1H), 7.69 (3H), 7.56
(2H), 7.47 (1H), 7.43 (1H), 7.37 (2H), 7.23 (4H), 7.19 (2H).
The elemental analysis (Table S1), IR and 1H NMR data of
4aI and 4bI are listed in Supporting Information.
RESULTS AND DISCUSSION
Monomer Synthesis
SCHEME 2 Synthetic procedures of PEIs.
The bis(ether dinitrile)s 4a, 4b, and 4c were prepared by a
three-step reaction sequence, shown in Scheme 1. First, the
intermediate bis(ether dinitrile)s were obtained from the
Polymer Synthesis
PEIs were synthesized by polycondensation of dianhydride
monomers 4a, 4b, and 4c with diamine monomers ODA (I),
APB (II), and 6FAPB (III). The resulting PEIs were abbrevi-
ated as 4aI–4aIII, 4bI–4bIII, and 4cI–4cIII, respectively
(Scheme 2).
In a typical experiment, PEI-4aI, which was derived from
dianhydride 4a and ODA (I), was prepared as follows: dia-
nhydride 4a (0.9848 g, 2 mmol) was dissolved in 10 mL of
DMAc. To this diamine solution, ODA (0.4005 g, 2 mmol) and
an additional 4 mL of DMAc were added. The mixture was
stirred for 24 h at room temperature under nitrogen to yield
a viscous poly(amic acid) (PAA) solution with 10 wt % solid
content. Chemical imidization was carried out via the addi-
tion of 2 mL of an acetic anhydride/pyridine (5/4 v/v) mix-
ture into the PAA solution (with mechanical stirrꢀing) at ambi-
ent temperature. The mixture was stirred at 60 C for 4 h to
yield a polyimide solution, which was poured into methanol
to give a precipitate, 4aI was collected by filtration, washed
FIGURE 2 1H NMR spectrum of PEI-4bI in DMSO-d6.
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
TABLE 1 Inherent Viscosity, GPC Data, and Thermal Properties of the PEIs
GPC Data
DSC
TGA
Inherent Viscosity
inh(dL/g)a
b
b
c
d
d
Polymer
g
Mw ꢂ 104
Mw/Mn
Tg (ꢀC)
T5% (ꢀC)
T10% (ꢀC)
Char Yielde (%)
4aI
0.84
13.5
16.2
13.5
7.1
1.7
1.6
1.9
1.4
1.4
1.5
1.4
1.6
1.5
230
211
208
216
203
200
226
220
218
531
520
536
590
575
585
578
578
576
551
544
574
604
597
603
602
602
598
57
58
61
62
60
67
47
57
50
4aII
4aIII
4bI
4bII
4bIII
4cI
0.86
0.82
0.65
0.71
0.67
0.73
0.85
0.75
7.8
8.1
13.2
17.2
9.3
4cII
4cIII
a
d
Determined with 0.5% solutions in a solvent (DMAc) at 25 ꢀC.
Relative to polystyrene standard, using DMF as the eluent.
Temperature at 5 and 10% weight loss were recorded by TGA at a
b
c
heating at 10 ꢀC/min in nitrogen.
e
Baseline shift in the second heating DSC traces, with a heating rate of
Residual weight (%) when heated to 800 ꢀC.
20 ꢀC/min in nitrogen.
nucleophilic nitrodisplacement of 4-nitrophthalonitrile with
the phenoxide ions of hydroquinones in DMF. The purified
bis(ether diacid)s by ethanolic potassium hydroxide, and
then the generated bis(ether diacid)s were cyclodehydrated
to yield the final bis(ether dianhydride)s. The structures of
the bis(ether anhydride)s products were confirmed by ele-
mental analysis, IR, and NMR spectra.
spectra of bis(ether anhydride) 4a, in which the proton
ortho-positioned at C¼¼O groups (H8) appeared in the down-
field of the spectra (8.14 ppm) owing to the strong electron-
withdrawing effect of the C¼¼O group, the methyl group
appeared in 2.27 ppm (Fig. 1). In 13C NMR spectra, the car-
bonyl carbon atoms (C1, C2, C28, C29) appeared in the down-
field of the spectrum and the methyl carbon atom (C21
appeared in the upfield. (Supporting Information, Figure S2)
)
The cyano group (ACN) of bis(ether dinitrile) 2a was evi-
dent from the peak at 2233 cmꢁ1 in the IR spectra (Support-
ing Information, Figure S1). In the IR spectra of bis(ether
diacid) 3a, the cyano stretching vibration was absent, but a
broad OAH absorption appeared in the region between 2500
and 3500 cmꢁ1 and carbonyl stretching absorption around
1700 cmꢁ1. The IR spectra of the bis(ether anhydride) 4a
shows characteristic cyclic anhydride absorption at 1854
and 1773 cmꢁ1, suggesting the asymmetric and symmetric
stretching vibrations of C¼¼O. Figure 1 shows the 1H NMR
Synthesis and Characterization of Polymers
The three types bis(ether anhydride)s 4a, 4b, and 4c were
reacted with different diamine I–III with a conventional two-
step procedure of ring-opening polyaddition at room temper-
ature to form PAA and sequential chemical imidization by
adding mixture of acetic anhydride and pyridine (volume ra-
tio 5:4) into PAA solution at ambient temperature for 30
min followed by heating at 60 ꢀC for 4 h to give poly(ether
TABLE 2 Solubility of the PEIs in Organic Solvents
Solventsa
Polymer
NMP
DMAc
DMF
DMSO
THF
Py
CHCl3
Actone
Ethanol
4aI
þþ
þþ
þþ
þþ
þþ
þþ
þþ
þþ
þþ
þþ
þþ
þþ
þþ
þþ
þþ
þþ
þþ
þþ
þ
þþ
þþ
þþ
þþ
þþ
þþ
þþ
þþ
þþ
þ
þþ
þþ
þþ
þþ
þþ
þþ
þþ
þþ
þþ
þþ
þþ
þþ
þþ
þþ
þþ
þþ
þþ
þþ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
4aII
4aIII
4bI
þ
þ
ꢁ
þþ
þ
þþ
þþ
þþ
þþ
þþ
þþ
þ
þ
þþ
þ
þþ
þþ
þ
ꢁ
4bII
4bIII
4cI
ꢁ
þ
þ
4cII
4cIII
þ
þþ
ꢁ
a
Qualitative solubility was determined with as 10 mg of polymer in 1 mL
of solvent. þþ: soluble at room temperature; þ: soluble on heating at
100 ꢀC.
NMP, N-methyl-2-pyrrolidone; DMAc, N,N-dimethylacetamide; DMF,
N,N-dimethylformamide; DMSO, dimethyl sulfoxide; Py, pyridine; THF,
tetrahydrofuran.
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TABLE 3 Mechanical Properties of the PEIs
PEIs were in the range of 71,000–162,000 with the Mw/Mn
values of 1.4–1.9 and the intrinsic viscosities of the PEIs were
in the range of 0.65–0.86 g/dL (Table 1).
Film
Tensile
Strength
(MPa)
Elongation
at Break
(%)
Youngs’
Modulus
(GPa)
Thickness
(lm)
Polymers
Solubility of the Polyimides
The solubility of synthesized PEIs was tested in a variety of
organic solvents at 1.0% (w/v) and the results are summar-
ized in Table 2. All the PEIs showed excellent solubility in
aprotic polar solvents such as DMSO, DMAc, DMF, and NMP
and were also soluble in less polar solvents such as pyridine
and THF at room temperature. Some of them were even
soluble in acetone. The improved solubility is attributed to
the introduction of bulky pendant methylphenyl, trifluorome-
thylphenyl, and ditrifluoromethylphenyl groups which could
inhibit close packing and reduce the interchain interactions
(Supporting Information, Fig. S3).
4aI
91
85
80
90
85
87
87
85
89
113
93
20
25
27
18
22
22
15
27
20
2.40
2.21
2.19
2.47
2.18
2.16
2.34
2.15
2.14
4aII
4aIII
4bI
89
117
92
4bII
4bIII
4cI
90
107
88
4cII
4cIII
92
The fluorinated PEIs 4aIII, 4bIII, and 4cIII exhibited better
solubility than the PEIs derived from unfluorinated aromatic
diamines. PEI-4cIII even exhibited excellent solubility in ace-
tone. The good solubility of PEI-4cIII was attributed to the
high fluorine content in the polymer backbone, in which
four CF3 groups were attached to each polymer unit. It
should be noted that the good solubility of the PEIs in low
boiling point solvents is beneficial to prepare the polymer
films at relative low processing temperatures, which is desir-
able feature for advanced microelectronics manufacturing
applications.
imide)s 4aI–4aIII, 4bI–4bIII, and 4cI–4cIII as displayed in
Scheme 2.
The structures of the PEIs were characterized by FTIR and
1H NMR spectra. The FTIR spectrum of PEI-4aI exhibited
characteristic imide carbonyl asymmetrical and symmetrical
stretches at 1788 and 1727 cmꢁ1 and the peak of amide car-
bonyl at 1650 cmꢁ1 disappeared, indicating the complete
1
imidization during polycondensation. In H NMR spectrum of
soluble PEI-4bI, all protons resonated in the region of 7.1–
8.0 ppm (Fig. 2). The signal of H8 in the ortho-positioned at
ACF3 group appeared at the farthest downfield region of the
spectrum because of the strong electron–withdrawing effect
of the ACF3 group. The resonance of H15 shifted to a higher
field because of the electron-donating effect of aromatic
ether. The above results demonstrated that the bis(ether an-
hydride) held a good polymerization activity to form PEIs.
Meanwhile, complete chemical imidization could be achieved
at lower temperatures than those for thermal imidization,
which should be profitable to get soluble polyimide. It is char-
acterized that the molecular weights (Mw) of the resulting
Thermal Properties of the Polyimides
The thermal behavior data of all the PEIs are also listed in
Table 1. Generally, the decreasing order of Tg correlated with
both molecular packing and chain conformation (chain rigid-
ity and linearity) of the polymers. The Tg values of series 4a,
4b, and 4c were, respectively, in the ranges of 208–230,
200–216, and 218–226 ꢀC, which increased in the order of
ODA > APB > 6FAPB. As we expected, the Tg values of these
PEIs depended on the structures of the diamine components
employed and decreased with increasing flexibility of the PEI
TABLE 4 Optical Properties and Dielectric Constants of the PEIs
Refractive Index
Dnd
k0 (nm)
UV-Transmittancee (%)
eopt
Water Uptake (%)
a
b
c
f
Polymer
nTE
nTM
nAV
4aI
1.6028
1.6053
1.5995
1.5925
1.5992
1.5898
1.5838
1.5908
1.5796
1.5853
1.5900
1.5848
1.5763
1.5821
1.5759
1.5674
1.5758
1.5674
1.5970
1.6002
1.5946
1.5871
1.5935
1.5852
1.5792
1.5851
1.5755
0.0175
0.0153
0.0147
0.0162
0.0171
0.0139
0.0164
0.0150
0.0122
377
373
369
372
369
362
366
365
357
e
88
87
89
89
87
88
87
87
88
2.81
2.82
2.80
2.77
2.79
2.76
2.74
2.76
2.73
0.66
0.55
0.38
0.64
0.53
0.35
0.51
0.42
0.23
4aII
4aIII
4bI
4bII
4bIII
4cI
4cII
4cIII
a
In-plane refractive index at 650 nm: nTE.
UV-transmittance at 800 nm (%).
b
c
f
Out-of-plane refractive index at 650 nm: nTM.
Average refractive index: nAV ¼ (2nTE þ nTM)/3.
Birefringence Dn ¼ nTE ꢁ nTM.
Dielectric constant estimated from the refractive index at 650 nm:
eopt ¼ 1.10 n2
.
AV
d
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
backbones. 4aIII, 4bIII, and 4cIII, derived from 6FAPB exhib-
ited low Tg because of the presence of two flexible ether
units and two CF3 groups. In comparison with the analogous
nonfluorinated PEIs, fluorinated PEI exhibited a lower Tg as
a result of reduced interchain interactions and poor chain
packing due to the bulky pendant CF3 groups.24 Here, the
PEIs series 4b exhibited lower Tg values than the PEIs series
4a, 4c. The phenomenon is possibly attributed to the differ-
ent molecular packing density of the PEIs with different phe-
nyl pendants. A possible explanation for the lower Tg values
of 4b PEIs is that the substitution of methyl with trifluoro-
methyl or ditrifluoromethyl group would lead to an internal
plasticization in addition to the geometry and free volume
factors.22,25
Thermal stabilities of the PEIs were evaluated by TGA (Table
1). All the PEIs had excellent thermal stability, and no
obvious decomposition was observed below 500 ꢀC. Film
samples of polyimides underwent 5% weight losses at 520–
ꢀ
590 C under nitrogen when subjected to TGA with a heating
ꢀ
rate of 10 C/min. Series 4b, 4c possessed higher decomposi-
tion temperatures than that of Series 4a. This result was
attributed to the presence of stronger CAF bonds with
higher stability than CAH under nitrogen. Their char yield at
800 ꢀC in nitrogen was in the range of 47–67 wt %. The
TGA data indicated that these PEIs had fairly high thermal
stability regardless of the introduction of the pendant
groups.
Mechanical Properties of the Polyimides
The mechanical properties of PEI films are summarized in
Table 3. The PEI films have tensile strength of 88–117 MPa,
tensile modulus of 2.14–2.47 GPa, and elongation at break-
age of 15–27%. The tensile modulus of the PEIs enhanced
with increasing in the imide concentration accompanied poly-
mer backbones. PEI–4aI, 4bI, and 4cI, which had the highest
imide concentration and the lowest ether concentration,
exhibited the highest tensile modulus compared with PEIs
from diamine II, III.
Dielectric Constants of the Polyimides
The dielectric constants for the PEI thin films were evaluated
by optical method (2.73–2.82), as listed in Table 4. The
dielectric constant (eopt) of the materials at optical frequen-
cies could be estimated from the refractive index (nAV
)
according to Maxwell’s equation, eopt ꢃ n2. The eopt at
around 1 MHz is evaluated to be eopt ꢃ 1.1n2, including an
additional contribution of approximately 10% from the IR
absorption.26–28 The estimated values are lower than Kapton
films (ncap ¼ 3.5 at 1 kHz) and Ultem (ncap¼ 3.15 at 1
kHz).29,30 The low dielectric constants were attributed to the
bulky pendant groups, which has low polarizability and large
free volume of the polymer.
Considering the type of dianhydride backbone, the dielectric
constant form lists in the following order: 4a > 4b > 4c.
The decreased dielectric constants could be explained by the
attribution of the bulky ditrifluoromethyl phenyl pendant
groups, which has lower polarizability and larger free vol-
ume of the polymer chain than trifluoromethyl phenyl
FIGURE 3 UV-Visible spectra of the PEI films.
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FIGURE 4 Photographs of a piece of the PEI-4cIII film (89 lm thick) and a piece of the Kapton film (90 lm thick).
pendant and methyl phenyl pendant groups. PEI having dia-
mine III exhibited lowest dielectric constants because of pres-
ence of bulky CF3 groups. As expected, the fluorinated PEI
series 4c exhibited lower water absorptions (0.23–0.51%)
than series 4b (0.35–0.64%) and 4a (0.38–0.66%) due to
content of two hydrophobic CF3 groups. The low water
absorptions ensure that these polymers have stable dielectric
performance.
tions.31 Moreover, CF3 group might weaken chain-to-chain
cohesive force due to lower polarizability of the CAF bond.
The decrease in intermolecular CTC formation was under-
standable also from the significant solubility of the PEIs pre-
pared from dianhydrides. All the series 4c revealed shorter
k0 than their respective series 4b and 4a. A photograph of
one piece of the PEI 4cIII is shown in Figure 4(A). For com-
parison, another photograph of a commercially Kapton film
is shown in Figure 4(B) either. The films were photographed
against a special background to highlight the transparency. It
is easy to find that PEI 4cIII film exhibited much lighter
color than Kapton film.
Optical Properties of the Polyimides
Thin films (pale-yellow, ꢄ90 lm thick) were measured for
optical transparency with UV-visible spectroscopy. Figure 3
shows the UV-visible transmittance spectra of the PEI films,
the cut-off wavelength (absorption edge, k0) values and the
percentages transmittance at 800 nm from these spectra are
listed in Table 4. All the PEIs had shorter k0 than 380 nm,
and exhibited high optical transparency of 87–89%. Because
of the highly conjugated aromatic structures and intermolec-
ular charge-transfer complex (CTC) formation of PI, most
polymers between UV and the visible area had strong
absorption. However, these PEIs which had flexible group
like ether linkage and CF3 group with bulky side groups in
the center of dianhydride reduced the intermolecular CTC
between altermating electron-donor (diamine) and electron-
acceptor (dianhydride) moieties. The ODA, APB, and 6FAPB
produced nearly colorless polyimide films and these could
be explained from the decreased intermolecular interac-
It was reported that the replacement of CAH bonds with
CAF bonds gives high optical transparency of the polymeric
material in the near-infrared (NIR) telecommunication
region.32 The propagation losses of the fluorinated PEIs se-
ries 4b and 4c were measured by the near-infrared (NIR)
absorption spectra, and the spectra are shown in the Figure
5. There were CAH bond vibrational absorptiong peaks
(2mCAH 1650 nm and 3dCAH, 1100 nm) and related peak
(2mCAH þ dCAH, 1400 nm). However, it had small light
absorptions at telecommunication wavelengths of 1310 and
1550 nm due to high fluorine content. Furthermore, the
near-infrared absorption of series 4c was weaker than that
of series 4b at the key telecommunication wavelengths of
1310 and 1550 nm because of the presence of two ACF3
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
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Three series of soluble poly(ether imide)s (PEIs) were pre-
pared from the dianhydrides (4a, 4b, and 4c) and diamines
by two-step chemical imidization methods, respectively.
Because of the presence of pendant groups and aryl ether
groups along the polymer backbone of the repeat unit, the
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