Preparation and characterization of highly transparent and colorless semi-aromatic polyimide films derived from alicyclic dianhydride and aromatic diamines

Article abstract of DOI:10.1016/j.polymer.2012.05.047

A novel meta-substituted aromatic diamine containing trifluoromethyl and sulfonyl groups in the backbone, i.e., 2,2′-bis[4-(3-amino-5- trifluoromethylphenoxy)phenyl]sulfone (m-6FBAPS), was synthesized and characterized. A series of semi-aromatic polyimides were prepared from 1,2,4,5-cyclohexanetetracarboxylic dianhydride (CHDA) and various aromatic diamines. These polymers exhibited excellent solubility, high thermal stabilities and good mechanical properties. All semi-aromatic polyimide films showed considerably improved optical transparency as comparing with traditional aromatic ones due to the incorporation of alicyclic moieties. They gave the UV cutoff wavelengths of 292-314 nm, transmittance at 450 nm > 91% and yellowness indices <3.8. The extremely transparent and entirely colorless films were obtained from the polymers incorporated with bulky electron-withdrawing trifluoromethyl and sulfonyl groups in diamine moieties, such as PI-2, PI-5 and PI-6. Their excellent optical properties are attributed to the distorted molecular conformation combined with the weakened electron-accepting and electron-donating properties of dianydride and diamines, which significantly restrained the formation of inter-/intra-molecular charge transfer interactions.

Full text of DOI:10.1016/j.polymer.2012.05.047

Polymer 53 (2012) 3529e3539
Contents lists available at SciVerse ScienceDirect
Polymer
journal homepage: www.elsevier.com/locate/polymer
Preparation and characterization of highly transparent and colorless
semi-aromatic polyimide films derived from alicyclic dianhydride and
aromatic diamines
*
Lei Zhai, Shiyong Yang, Lin Fan
Laboratory of Advanced Polymer Materials, Institute of Chemistry, Chinese Academy of Sciences, Zhongguancun, Beijing 100190, China
a r t i c l e i n f o
a b s t r a c t
Article history:
A novel meta-substituted aromatic diamine containing trifluoromethyl and sulfonyl groups in the back-
bone, i.e., 2,20-bis[4-(3-amino-5-trifluoromethylphenoxy)phenyl]sulfone (m-6FBAPS), was synthesized
Received 26 April 2012
Received in revised form
22 May 2012
and characterized.
A
series of semi-aromatic polyimides were prepared from 1,2,4,5-
cyclohexanetetracarboxylic dianhydride (CHDA) and various aromatic diamines. These polymers exhibi-
ted excellent solubility, high thermal stabilities and good mechanical properties. All semi-aromatic poly-
imide films showed considerably improved optical transparency as comparing with traditional aromatic
ones due to the incorporation of alicyclic moieties. They gave the UV cutoff wavelengths of 292e314 nm,
transmittance at 450 nm > 91% and yellowness indices <3.8. The extremely transparent and entirely
colorless films were obtained from the polymers incorporated with bulky electron-withdrawing tri-
fluoromethyl and sulfonyl groups in diamine moieties, such as PI-2, PI-5 and PI-6. Their excellent optical
properties are attributed to the distorted molecular conformation combined with the weakened electron-
accepting and electron-donating properties of dianydride and diamines, which significantly restrained the
formation of inter-/intra-molecular charge transfer interactions.
Accepted 24 May 2012
Available online 2 June 2012
Keywords:
Semi-aromatic polyimides
High transparency
Colorless
Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction
devices in the fields of displays, memory, lighting, solar cells,
sensors and waveguides [7e13].
Aromatic polyimides (PIs) are well recognized as high perfor-
mance polymers owing to their excellent combination properties,
such as high thermal stability, good mechanical property, low
dielectric constant and excellent chemical resistance, which make
them good candidates for microelectronics and optoelectronic
applications [1e3]. However, one of the great disadvantages of the
aromatic polyimide films for optoelectronic application is their
yellowish nature. It is well known that the commercial aromatic
polyimide films, such as Kapton and Upilex, give strong coloration
from deep yellow to dark brown. Deep coloration greatly limits the
widespread application of the polyimide films in areas where
transparency and colorlessness are the basic needs [4e6]. In recent
years, many efforts have been made to develop highly optical
transparent and colorless polyimide films along with high thermal
stability and excellent mechanical properties for applications in
prospective flexible substrates of electronic or micro-optical
It has been demonstrated that the origin of coloration in
aromatic PIs is caused by the intra- and/or inter-molecular charge
transfer complex (CTC) formation between the alternating
electron-acceptor (dianhydride) and electron-donor (diamine)
moieties. In this regard, the effective strategy for obtaining trans-
parent and less-colored or colorless PIs is to use weak electron-
accepting dianhydrides and/or electron-donating diamines to
suppress the charge transfer (CT) interactions [14e16]. In the past
decades, many attempts have been made to improve the optical
properties of PIs by incorporation of fluorine, chlorine, or sulfone
groups, introducing of unsymmetrical and bulky pendent units, as
well as adopting the alicyclic moieties in the polymer structure
[17e19].
Fluorinated aromatic PIs, in which fluorine atoms or tri-
fluoromethyl groups are introduced into the polymer main chain or
side chain, have been extensively investigated due to their unique
characteristics. Our previous research on fluorinated aromatic PIs
demonstrated that the incorporation of bulky fluorine atoms or
trifluoromethyl groups into the polymer structures resulted in an
enhanced solubility and a lowered dielectric constant, as well as an
improved optical transparency [20e22]. This is attributed to their
* Corresponding author. Tel.: þ86 10 6256 4819; fax: þ86 10 6256 9562.
E-mail address: fanlin@iccas.ac.cn (L. Fan).
0032-3861/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.polymer.2012.05.047

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L. Zhai et al. / Polymer 53 (2012) 3529e3539
large free volume, low polarization of C-F bond and the high elec-
tron negativity. Various other investigations on fluorinated
aromatic PIs also obtained similar results [23e26]. Ando and Ha
developed the chlorine-containing aromatic PIs based on chlori-
nated diamines [27,28]. They found that these PIs exhibited good
transparency, high refractive index and low birefringence due to
the high atomatic polarizability of chlorine and the bulky molecular
structure of chlorinated PIs. St. Clair reported the aromatic PIs
incorporated with bulky electron-withdrawing sulfonyl groups and
indicated that the highly transparent PI films could be obtained
because of the reduction of the inter-chain and intra-chain CTC
formation [29]. The aromatic PIs with non-coplanar biphenyl
structures and meta-substituted phenyl groups in the backbone, as
well as the bulky pendant phenyl groups in the side chain have
been studied by some researchers [30e32]. It is suggested that
these polyimide films have good solubility combine with high
transparency, which related to the expanded free volumes and the
weakened CTC formation. However, these aromatic polyimide films
still have weak coloration due to the presence of CT interactions
caused by the electron-donors and electron-acceptors from
aromatic backbones.
Non-aromatic PIs derived from dianhydrides and/or diamines
with alicyclic structures have been developed by Matsumoto
[33e35] and other researchers [36e38]. It is suggested that the
non-aromatic PIs displayed good solubility, low dielectric constant
and high transparency, which is related with their relatively lower
molecular density and polarity, and especially the absence or inhi-
bition of intra- and/or inter-molecular CT interactions [39,40].
Therefore, the incorporation of alicyclic units in PIs is considered as
one of the effective ways to enhance the transparency in the
UV-visible region and other desired properties. Unfortunately, the
fully alicyclic PIs prepared from alicyclic dianhydrides and diamines
possess much lower thermal stability than the corresponding fully
aromatic PIs, which is a major drawback of fully aliphatic PIs for
application. In order to improve thermal stability of alicyclic PIs, the
semi-aromatic PIs derived from aromatic dianhydrides/alicyclic
diamines or alicyclic dianhydrides/aromatic diamines have been
developed [41e43]. However, it is difficult to obtain the semi-
aromatic PIs with high molecular weights as alicyclic diamines
applied, owing to the salt formation of carboxyl group of the
poly(amic acid)s (PAA) with highly basic amino group of the alicyclic
diamines [44,45]. In recent years, the semi-acromatic PIs prepared
from alicyclic dianhydrides and aromatic diamines have attracted
more attention due to their highly transparency without sacrificing
thermal stabilityand mechanical toughness, which make them good
candidate for applied as colorless and transparent substrates.
As a part of systematic approach to develop the fluorinated
PIs, in this study, a series of semi-aromatic PIs were prepared
from 1,2,4,5-cyclohexanetetracarboxylic dianhydride (CHDA) and
various non-fluorinated and fluorinated aromatic diamines with
different molecular structures. In order to get highly transparent
and colorless polyimide films with good mechanical and thermal
properties, the alicyclic dianhydride CHDA with high reactivity was
applied, meanwhile, aromatic diamines incorporated with strong
electron-withdrawing trifluoromethyl and sulfonyl substituents in
the structure were used. The incorporation of trifluoromethyl
substituents is also expected to endow the PIs with good solubility
and low moisture absorption due to the expanded free volume and
hydrophobic nature. A novel meta-substituted aromatic diamine
containing trifluoromethyl and sulfonyl groups, i.e., 2,20-bis[4-
(3-amino-5-trifluoromethylphenoxy)phenyl]sulfone (m-6FBAPS),
was synthesized and characterized. The solubility, thermal stability
and mechanical properties of the semi-aromatic PIs were evalu-
ated. The optical transparency and coloration of these polyimide
films were investigated by their physical appearance, UV-visible
spectra and color intensities. The relationship between the poly-
mer structure and their optical properties was also systematically
discussed in detail.
2. Experimental
2.1. Materials
3,5-Dinitrobenzotrifluoride and 4,40-sulfonyl diphenol (Acros)
were used as received. 1,2,4,5-Cyclohexanetetracarboxylic dianhy-
dride (CHDA, TCI) was dried in a vacuum oven at 160 ꢀC for 12 h
prior to use. 4,40-Diaminodiphenyl ether (4,40-ODA, Shandong
Wanda Chemical Co., China), 2,20-bis(trifluoromethyl)-4,40-dia-
minobiphenyl (TFDB, Changzhou Sunlight Fine Chemicals Co.,
China),
1,4-bis(4-amino-2-trifluoromethylphenoxy)benzene
(6FAPB) and 4,40-bis(4-amino-2-trifluoromethylphenoxy)biphenyl
(6FBAB) (Beijing POME Sci-tech Co., China) were purified by
recrystallization from ethanol or 2-methoxyethanol. 2,20-Bis[4-(4-
amino-2-trifluoromethylphenoxy)phenyl]sulfone (p-6FBAPS) was
synthesized in our laboratory according to the literature [46].
Commercially available solvents, i.e., dimethyl sulfoxide (DMSO),
N,N-dimethylacetamide (DMAc), N,N-dimethylformamide (DMF)
and N-methyl-2-pyrrolidinone (NMP), (Beihua Fine Chemicals Co.,
China) were purified by vacuum distillation and dehydrated with
4Å molecular sieves prior to use. Other solvents and regents were
used as received.
2.2. Monomer synthesis
2.2.1. 2,20-Bis[4-(3-nitro-5-trifluoromethylphenoxy)phenyl]sulfone
(m-6FBNPS)
A mixture of 4,40-sulfonyl diphenol (25.1 g, 0.1 mol), anhydrous
potassium carbonate (69.1 g, 0.5 mol), and anhydrous DMSO
(300 mL) was placed into a 500 mL three-necked round-bottom
flask equipped with a mechanical stirrer, a nitrogen inlet, and
a condenser. The mixture was stirred at 60 ꢀC and for 4 h in nitrogen
and then 3,5-dinitrobenzotrifluoride (52.0 g, 0.22 mol) was added
in three portions. The reaction solution was heated to 80 ꢀC and
kept for 6 h, followed by reacted at 120 ꢀC for another 12 h. After
that, the solution was cooled down to room temperature, and then
concentrated hydrochloric acid (37 wt.%, 85 mL) was slowly added
with agitation. The obtained mixture was vacuum distillation to
remove most of solvent, and then poured into 2 L of ethanol/water
(volume ratio 1/3). The resulting precipitate was collected by
filtration, washed with water, and dried under vacuum. The crude
product was purified by recrystallization from ethyl acetate to give
yellow crystals (51.5 g, 85%). mp: 155e157 ꢀC. 1H NMR (CDCl3,
d,
ppm): 7.17e7.20 (d, 4H), 7.64 (s, 2H), 8.03 (s, 4H), 8.05 (s, 2H), 8.30
(s, 2H). FTIR (KBr, cmꢁ1): 3093, 1626, 1585, 1531, 1485, 1354, 1325,
1286, 1267, 1153, 1111, 1049. Mass spectrum (EI, m/z, % relative
intensity): 628 [Mþ, 80], 330 [(M ꢁ 298)þ, 100]. Elemental analysis:
calculated for C26H14F6N2O8S (628.45): C, 49.69%; H, 2.25%; N,
4.46%. Found: C, 49.46%; H, 2.29%; N, 4.43%.
2.2.2. 2,20-Bis[4-(3-amino-5-trifluoromethylphenoxy)phenyl]
sulfone (m-6FBAPS)
A suspension solution of the m-6FBNPS (37.7 g, 0.06 mol) and 5%
Pd/C (1.5 g) in ethanol (200 mL) was heated at 70e80 ꢀC with
stirring. Then hydrazine monohydrate (40 mL) was added dropwise
within 1 h, The mixture was reacted at reflux temperature for
refluxed for 8 h and then filtered to remove Pd/C. The filtrate was
concentrated with a vacuum evaporator, and then poured into
800 mL of water to give white precipitate. The resulting precipitate
was collected, washed with water, and dried in vacuum at 120 ꢀC.
The product was recrystallized from ethanol to give white crystals

L. Zhai et al. / Polymer 53 (2012) 3529e3539
3531
(31.5 g, 80%). mp: 160e162 ꢀC. 1H NMR (400 MHz, DMSO-d6,
d
,
measurement of polyimide films were carried out on a TA Q400
instrument in nitrogen at a heating rate of 10 ꢀC/min. The CTE of the
samples were determined by calculating the results in a tempera-
ture range of 50e200 ꢀC. Mechanical properties were measured on
an Instron-3365 tensile apparatus with 120 ꢂ 10 ꢂ 0.06 mm
specimens at 25 ꢀC in accordance with GB/T 1040.3-2006 at
a drawing rate of 5.0 mm/min. Ultraviolet-visible (UV-vis) spectra
of the polymer films were recorded on a Hitachi U3900 spectro-
photometer at room temperature. The color intensity of polymer
films was evaluated by X-rite Color i7 spectrophotometer, using an
observational angle of 8ꢀ and a CIE (Commission International de
I’Eclairage) standard D65 illuminant. A CIE LAB color difference
equation was used. Solubility was determined by dissolving poly-
mer resin in different solvents with 15 wt.% of solid content at room
temperature. The absolute viscosity was measured using a Brook-
field DV-IIþ viscometer at 25 ꢀC. Water uptake was determined by
weighing the changes of a specimen before and after immersing in
water at 25 ꢀC for 24 h.
ppm): 5.84 (s, 4H), 6.45 (s, 2H), 6.50 (s, 2H), 6.72 (s, 2H), 7.17-7.19 (d,
4H), 7.94-7.96 (d, 4H). FTIR (KBr, cmꢁ1): 3300e3500, 1632, 1583,
1487, 1471, 1370, 1282, 1236, 1151, 1107, 1006. Mass spectrum (EI, m/
z, % relative intensity): 568 [Mþ, 100]. Elemental analysis: calcu-
lated for C26H18F6N2O4S (568.49): C, 54.93%; H, 3.19%; N, 4.93%.
Found: C, 54.78%; H, 3.13%; N, 4.83%.
2.3. Polymer synthesis
A series of semi-aromatic PIs were prepared from alicyclic dia-
nhydride CHDA and aromatic diamines, i.e., 4,40-ODA, TFDB, 6FAPB,
6FBAB, p-6FBAPS and m-6FBAPS, respectively. In a typical experi-
ment, PI-6, which derived from CHDA and m-6FBAPS, was prepared
in the following procedure.
To a completely dried 500 mL three-necked flask equipped with
a mechanical stirrer, a nitrogen inlet, a thermometer, and a Dean-
Stark trap, m-6FBAPS (28.42 g, 0.05 mol) and anhydrous DMAc
(160 mL) were fitted. The mixture was stirred at ambient temper-
ature under nitrogen flow until m-6FBAPS was completely dis-
solved to give a homogeneous solution. Then, CHDA (11.21 g,
0.05 mol), isoquinoline (0.3 g) and toluene (30 mL) were added and
the solution was stirred in nitrogen at room temperature for 6 h.
The reaction solution was gradually heated to 160 ꢀC and main-
tained at the temperature for 12 h, in which the water evolved in
the procedure of imidization was removed simultaneously by
azeotropic distillation. After the solution was cooled down to
80e100 ꢀC, the viscous solution was poured slowly into excess
ethanol to give white fiber-like precipitate, which was then
collected by filtration, washed thoroughly with ethanol, and dried
in vacuum at 120 ꢀC for 8 h.
3. Results and discussion
3.1. Monomer synthesis
The meta-substituted aromatic diamine m-6FBAPS, which con-
taining trifluoromethyl and sulfonyl groups in the structure, was
prepared by a two-step reaction as shown in Scheme 1. At first, the
intermediate dinitro-compound m-6FBNPS was synthesized by the
nitro nucleophilic displacement reaction of 3,5-dinitrobenzotri-
fluoride with 4,40-sulfonyl diphenol in the presence of potassium
carbonate in DMSO, which was then reduced with hydrazine
monohydrate and Pd/C in ethanol to afford m-6FBAPS in high yield
of 80%.
The other semi-aromatic PIs, i.e., PI-1 (CHDA/4,40-ODA), PI-2
(CHDA/TFDB), PI-3 (CHDA/6FAPB), PI-4 (CHDA/6FBAB) and PI-5
(CHDA/p-6FBAPS) were synthesized by the similar procedure,
except m-6FBAPS was replaced by the other aromatic diamines.
The chemical structures of the synthesized dinitro-compound
m-6FBNPS and the corresponding diamine m-6FBAPS were
confirmed by 1H NMR, FTIR, MS and elemental analysis. Fig. 1
2.4. Film preparation
Polyimide films were obtained via the casting of the polyimide
solutions onto glass plates followed by baking to remove the
residual solvent. The polyimide resin was dissolved in DMAc with
stirring to give a homogeneous polyimide solution with a solid
content of 25 wt.%. The polyimide solution was filtered by a 0.2-mm
Teflon syringe filter and then spread onto a glass plate, which was
then thermally baked in an oven at 80 ꢀC for 2 h, 120 ꢀC for 1 h, at
150 ꢀC for 1 h and 180 ꢀC for 1 h, successively. The free-standing
polyimide film was obtained by immersion the glass plate in
water followed by drying in an oven at 100 ꢀC.
2.5. Measurements
1H NMR spectra were performed on a Bruker Avance 400
Spectrometer operating at 400 MHz in CDCl3 and DMSO-d6. FTIR
spectra were recorded on a Perkin-Elmer 782 Fourier transform
spectrophotometer. The number average molecular weights (Mn)
and polydispersities (Mw/Mn) were determined by tandem gel
permeation chromatography-multiangle laser light scattering
(GPC-MLLS) system using DMF containing 0.02 M LiBr as eluent at
flow rate of 1.0 mL/min at 50 ꢀC. Wide-angle X-ray diffraction
(WAXRD) measurements were conducted on a Rigaku D/max-2500
X-ray diffractometer with Cu/Ka radiation, operated at 40 kV and
200 mA. Differential scanning calorimetry (DSC) and thermal
gravimetric analysis (TGA) were carried out using a TA Q100 and
a TA Q50 instruments, respectively, at a heating rate of 10 ꢀC/min in
nitrogen. The in-plane coefficients of thermal expansion (CTE)
Scheme 1. Synthesis of diamine m-6FBAPS.

3532
L. Zhai et al. / Polymer 53 (2012) 3529e3539
Fig. 1. 1H NMR spectra of m-6FBNPS in CDCl3 (a) and m-6FBAPS in DMSO-d6 (b).
shows the 1H NMR spectra of compounds m-6FBNPS and m-
6FBAPS, in which all the protons in the structures could be assigned
clearly according to the integral values of the intensity. The signals
assigned to the aromatic protons of m-6FBNPS appeared in the
range of 7.17e8.30 ppm, and those of m-6FBAPS shifted to higher
field between 6.45 and 7.96 ppm. In 1H NMR spectrum of m-
6FBNPS, the aromatic proton H1 resonated at the furthest down-
field region of 8.30 ppm owing to the strong electron-withdrawing
effect of -NO2 and -CF3 groups in ortho-position. The aromatic
protons H2, H3 and H5, which were adjacent to the -NO2, -CF3 and
-SO2 groups, respectively, were observed in the region of 8.05e7.64.
The aromatic proton H4 ortho-oriented to ether linkage showed the
doublet peaks in the upfield region of 7.17e7.20 ppm because of the
electron-donation effect of the ether group. After reduction, the
aromatic protons H1-H3 of m-6FBAPS were observed at
6.45e6.72 ppm, which shifted to the upfield region due to the
electron donating property of amine groups. Moreover, the signal
assigned to the protons in amino groups was detected at 5.84 ppm.
The structures of m-6FBNPS and m-6FBAPS were further
confirmed by means of FTIR spectra. As shown in Fig. 2, the char-
acteristic absorptions at 1531 and 1325 cmꢁ1 were assigned to the
N-O asymmetric and symmetric stretching vibrations of nitro
groups in m-6FBNPS, which disappeared in the spectrum of m-
6FBAPS. Whereas the characteristic absorptions in the region of
3300e3500 cmꢁ1 due to the N-H stretching vibration of amino
groups were detected for m-6FBAPS, demonstrating that the nitro
groups in m-6FBNPS were converted into the amine groups in
aromatic diamine. Meanwhile, the characteristic absorptions
around 1580 cmꢁ1 (aromatic C]C), 1350e1370 and 1150 cmꢁ1
(sulfonyl S]O), as well as at about 1110 cmꢁ1 (trifluoromethyl C-F)
were also detected in the FTIR spectra for both of dinitro and
diamine compounds. In addition, the elementary analysis values of
Fig. 2. FTIR spectra of m-6FBNPS and m-6FBAPS.

L. Zhai et al. / Polymer 53 (2012) 3529e3539
3533
m-6FBNPS and m-6FBAPS were in good agreement with the
calculated ones. The characterization results demonstrated that the
aromatic diamine m-6FBAPS was successfully synthesized and the
obtained monomer was pure enough to be employed for prepara-
tion of PIs.
about 1780 cmꢁ1 (C]O, asymmetric),1720 cmꢁ1 (C]O, symmetric)
and 1380 cmꢁ1 (C-N, asymmetric), respectively. The absorptions
assigned to the stretching vibrations of C-F were detected at 1250
and 1140 cmꢁ1 in the spectra of PI-2wPI-6. Moreover, for the PIs
prepared from aromatic diamines p-6FBAPS and m-6FBAPS, i.e., PI-
5 and PI-6, the absorptions at attributed to the sulfonyl groups were
also detected at 1300 cmꢁ1 (S]O, asymmetric) and 1110 cmꢁ1 (S]
O, symmetric). The characteristic N-H stretching vibrations of
amino groups in poly(amic acid) at around 3300e3500 cmꢁ1 were
not observed in the FTIR spectra for all the PIs, implying that the
polymers were fully imidized.
3.2. Polymer synthesis and characterization
3.2.1. Polymer synthesis
A series of semi-aromatic PIs were synthesized via one-pot
solution polycondensation at high temperature from alicyclic dia-
nhydride CHDA and various aromatic diamines in DMAc as shown
in Scheme 2. It is considered that the one-pot method is suitable for
low reactive monomers to synthesize soluble PIs, however, the
polycondensation should be implemented at high temperature
[47]. Initially, CHDA and aromatic diamine were reacted at ambient
condition in the presence of isoquinoline, which acted as a catalyst,
to yield poly(amic acid). Then, the intermediate without isolation
was thermally cyclodehydrated at elevated temperature to give the
corresponding polymer. The water by-product was removed by
azeotropic distillation with toluene to ensure the thermal cycliza-
tion being processed completely. The homogeneous and trans-
parent viscous polyimide solutions were produced, implying that
the polycondensation reactivity of the alicyclic dianhydride CHDA
was appropriate to the selected aromatic diamines. The white fiber-
like precipitates were obtained by pouring the polymer solutions
slowly into excess of ethanol, indicating that the polymers have
relatively high molecular weight.
Table 1 summarized the physical properties, GPC data and
elemental analysis of the semi-aromatic PIs. It can be seen that all
the semi-aromatic PIs were prepared in high yields of 96e98%. The
fluorinated polyimides PI-2wPI-6 gave the fluorine content of
15.1e22.4%. These polymers exhibited reasonable high molecular
weight and narrow molecular weight distribution with Mn and Mw
in the range of 4.5e5.3 ꢂ 104 and 6.1e7.4 ꢂ 104, as well as Mw/Mn of
1.3e1.4, respectively. The elemental analysis results of PIs were in
good accordance with the calculated ones, indicating that the
expected polymer structures were obtained by the complete
polycondensation.
3.2.2. Solubility
The solubility of the semi-aromatic PIs was measured by dis-
solving the polyimide resins in different organic solvents with
15 wt.% of solid content. As shown in Table 2, all of the semi-
aromatic PIs could be easily dissolved in the strong polar
solvents, such as NMP, m-cresol, DMAc and DMF, at room temper-
ature to afford homogenous polymer solutions, which is related to
their alicyclic feature. It was noted that the solubility of semi-
aromatic PIs depended on their backbone structure. The semi-
aromatic PIs with relatively higher fluorine content displayed
better solubility. For instance, fluorinated polyimides PI-2, PI-3 and
PI-4, which have fluorine content over 16.5%, could even be
completely dissolved in less polar solvents, such as THF, acetone
and dioxane. PI-5 and PI-6, which containing fluorinated and
sulfonyl groups in the backbones, showed moderate solubility in
different organic solvents. In contrast, the non-fluorinated poly-
imide PI-1 derived from CHDA/4,40-ODA had relatively poor solu-
bility and the polyimide resin was hardly dissolved in any one of the
low boiling point solvents even after heating.
The solubility of semi-aromatic PIs was quantitatively evaluated.
Fig. 4 depicts the dependence of the absolute viscosities of PI-2, PI-5
and PI-6 solutions in DMAc on the solid contents. The absolute
viscosities of these PIs in DMAc with 25 wt.% of solid content were
no more than 3500 mPa s at 25 ꢀC, which exhibited a gradually
increasing for PI-2 and an abruptly enhancing for PI-5 and PI-6 as
the solid content in excess of 30 wt.%. Moreover, the absolute
viscosity of these polyimide solutions with same concentration
increased in the order of PI-2 < PI-6 < PI-5. The absolute viscosity
for PI-2 at the solid content of 40 wt.% was 9.2 ꢂ 104 mPa s, which is
Fig. 3 shows the FTIR spectra of the semi-aromatic PIs, in which
the characteristic absorptions of imide groups were observed at
Scheme 2. Synthesis of semi-aromatic polyimides.

3534
L. Zhai et al. / Polymer 53 (2012) 3529e3539
Table 1
Physical properties, GPC data and elemental analysis of semi-aromatic polyimides.
PIs
Yield (%)
Fa (%)
GPC data
Formula
Elemental analysis (%)
C
Mn ꢂ 10ꢁ4
Mw ꢂ 10ꢁ4
Mw/Mn
H
N
PI-1
PI-2
PI-3
PI-4
PI-5
PI-6
98
96
97
96
98
97
0
4.5
6.1
1.36
(C22H16N2O5)n
Calcd
Found
Calcd
Found
Calcd
Found
Calcd
Found
Calcd
Found
Calcd
Found
68.04
68.75
56.70
55.20
58.45
58.69
62.43
62.67
57.15
56.75
57.15
57.41
4.15
4.04
2.78
2.88
2.94
3.15
3.20
3.59
2.93
3.05
2.93
2.97
7.21
7.02
5.51
5.40
4.54
4.50
4.04
3.93
3.70
3.93
3.70
3.49
22.4
18.5
16.5
15.1
15.1
4.6
5.3
5.0
5.3
4.8
6.8
7.4
7.1
7.1
7.0
1.47
1.38
1.41
1.44
1.45
(C24H14F6N2O4)n
(C30H18F6N2O6)n
(C36H22F6N2O6)n
(C36H22F6N2O8S)n
(C36H22F6N2O8S)n
a
F: fluorine content.
only half of that for PI-5 and PI-6. The excellent solubility of PI-2 is
attributed to the presence of the bulky trifluoromethyl groups in
the polymer backbone with high loading, which inhibited close
chain packing, consequently reduced the interaction of polymer
chains [48]. As comparing the solubility of PI-5 and PI-6, it is found
that the latter showed better solubility than the former, although
they have the same fluorine content. This can be explained by the
synergetic effects of the meta-linkage backbone and pendent bulky
trifluormethyl substituents in the polymer structure, which dis-
rupted the regularity of the molecular chains and hindered the
dense chain stacking, thus increased the solubility [49]. The poly-
mer solutions were very stable and no phase separation or
precipitation was observed after storage for several weeks.
owing to the presence of flexible ether linkage and their spatially
extended molecular structures.
Fig. 5 compares the TGA curves of the semi-aromatic PIs. All of
the polymers did not show significant weight loss before the
scanning temperature reached upon to 350 ꢀC in nitrogen, indi-
cating that no thermal decomposition occurred. From the TGA
results listed in Table 3, it is revealed that these polymers have
reasonably good thermal stability with the decomposition
temperatures at 5 wt.% and 10 wt.% of weight loss in the range of
436e474 ꢀC and 454e486 ꢀC in nitrogen, respectively. However, as
the temperature was scanned up over 450 ꢀC, all the PIs except PI-2
showed rapid thermal decomposition. The residual weight reten-
tions at 700 ꢀC for these polymers were higher than 37%. As
comparing the TGA data of these polymers, it is also found that PI-2
showed apparently better thermal stability with 21e38 ꢀC higher T5
than the other PIs due to its rigid biphenyl structure.
It has been reported that the decomposition of dianhydride
moieties of the polyimide chains generally appeared at lower
temperature than that of the diamine moieties [28]. Therefore, it
could be deduced that the semi-aromatic PIs derived from alicyclic
dianhydride exhibit inferior thermal stability compared to the
aromatic PIs. As comparing the thermal stabilities of semi-aromatic
polyimide PI-3 (CHDA/6FAPB) with that of fully aromatic polyimide
derived from pyromellitic dianhydride (PMDA) and 6FAPB (ref-PIa),
which had been reported in our previous research [50], it is
confirmed that the Tg and Ts values of the former were 24 ꢀC and
94 ꢀC lower than those of latter, respectively.
3.2.3. Thermal properties
The thermal properties of theses semi-aromatic PIs were eval-
uated by DSC and TGA, which are summarized in Table 3. The glass
transition temperatures (Tg) of these PIs determined by DSC were in
the range of 268e370 ꢀC, which depended on the chemical struc-
ture of aromatic diamine segment. PI-1 and PI-2 exhibited much
higher Tg values (>330 ꢀC) because of the effect of the rigid polymer
backbone. It is worth to be mentioned that PI-2 derived from TFDB
gave the highest Tg of 370 ꢀC, which is related to its highly rigid
biphenyl linkage in the polymer backbone. On the contrary,
PI-3wPI-6 showed the Tg values almost 100 ꢀC lower than PI-2,
3.3. Properties of semi-aromatic polyimide films
The semi-aromatic polyimide films were prepared by casting
the corresponding polymer solutions with a solid content of
25 wt.% in DMAc on a glass plate, followed by thermal baking to
remove the solvent completely at relatively low temperature in
a schedule of 80 ꢀC/2 h, 120 ꢀC/1 h, 150 ꢀC/1 h and 180 ꢀC/1 h. All of
the semi-aromatic PIs displayed pretty good film forming abilities.
Tough, flexible and transparent polyimide films were obtained.
3.3.1. Mechanical and physical properties
The unoriented semi-aromatic polyimide films were employed
to investigate the mechanical and physical properties and the
results are illustrated in Table 4. All the polyimide films exhibited
good mechanical properties, and the tensile strength, Young’s
modulus and elongation at breakage of these polyimide films were
in the range of 93e117 MPa, 2.2e2.9 GPa and 5.5e11.6%, respec-
tively. As comparing the mechanical properties of these films, it is
found that PI-3 and PI-4 displayed better flexibility with the
Fig. 3. FTIR spectra of semi-aromatic polyimides.

L. Zhai et al. / Polymer 53 (2012) 3529e3539
3535
Table 2
Solubility of semi-aromatic polyimidesa.
PIs
NMP
m-cresol
DMAc
DMF
THF
Acetone
Dioxane
Ethyl acetate
Chloroform
PI-1
PI-2
PI-3
PI-4
PI-5
PI-6
þþ
þþ
þþ
þþ
þþ
þþ
þþ
þþ
þþ
þþ
þþ
þþ
þþ
þþ
þþ
þþ
þþ
þþ
þþ
þþ
þþ
þþ
þþ
þþ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
þ
þ
ꢁ
ꢁ
þþ
þþ
þþ
þþ
þþ
þþ
þþ
þþ
þ
þþ
þþ
þþ
ꢁ
þþ
þ
þ
ꢁ
þ
þ
þ
a
þþ: Soluble at room temperature; þ: partially soluble; ꢁ: insoluble even on heating.
elongation at breakage >9%, whereas, PI-2, PI-5 and PI-6 showed
more stiffen with the Young’s modulus over 2.7 GPa. This result can
be easily explained by the difference in flexibility/rigidity of poly-
mer chain. These semi-aromatic polyimide films have good
pliability and sufficient toughness for application as flexible
substrates.
3.3.2. Optical transparency
Optical transparency of the semi-aromatic polyimide films were
evaluated by their physical appearance, UV-vis spectra and color
intensities. Fig. 7 shows the optical photographs of polyimide films.
It is noted that these films were transparent and essentially color-
less even as the film with thickness of 70 mm, which is significantly
The in-plane coefficient of thermal expansion (CTE) for the
semi-aromatic polyimide films were evaluated by TMA analysis and
the results are provided in Fig. 6 and Table 4.These polyimide films
different from the deep yellow or brown color of traditional
aromatic polyimide films. The improved optical transparency of
these films is associated with the incorporation of alicyclic moieties
in the polymer structure. As comparing the coloration of these
films, it is interestingly observed that PI-2, PI-5 and PI-6 produced
entirely colorless films, whereas, PI-1, PI-3 and PI-4 provided
slightly yellow films. The difference of optical transparency
between these polyimide films is also confirmed by their UV-vis
spectra and color intensities.
except PI-2 displayed
a similar dimension change with the
temperature increasing and their in-plane CTEs between 50 and
200 ꢀC were around 56e65 ppm/ꢀC. It is noteworthy that PI-2
derived from TFDB exhibited extremely low CTE of 35 ppm/ꢀC,
which is attributed to the presence of rigid biphenyl unit in the
main chain. It is well known that a low CTE is quite benefit to apply
the optical transparent and thermal stable polyimides as substrates
for flexible displays and flexible solar cells, in which the polymer
substrates need to be exposed to high temperature >200 ꢀC during
the vacuum deposition processes of inorganic thin films [51].
Water uptake of polyimide films is one of key factors affecting
the film performance as flexible substrates. The polymer films with
a high water uptake tend to be deteriorated in dimensional stability
upon moisture absorption [52]. From the water uptakes of the
semi-aromatic polyimide films summarized in Table 4, it is noted
that fluorinated polyimides exhibited relatively lower water
uptakes. The water uptake for non-fluorinated polyimide PI-1 was
1.18%, whereas that for fluorinated polyimides PI-2wPI-6 was no
more than 0.95%. Partially, the PI-2 derived from CHDA/TFDB,
which has the highest fluorine content, gave the lowest water
uptake of 0.59%. The low moisture absorption of the fluorinated PIs
were mainly attributed to the existence of hydrophobic tri-
fluoromethyl groups in the polymer structure, which inhibited
adsorption of moisture molecule on the polymer surface [53].
Fig. 8 shows the UV-vis spectra of the semi-aromatic polyimide
films with thickness of 25 mm. The optical transparency gained from
the UV-vis spectra, including cutoff wavelength (absorption edge,
l0) and transmittance at different wavelength, are summarized in
Table 5. As expected from the alicyclic nature, all of the semi-
aromatic polyimide films exhibited excellent optical transparency,
which gave the UV cutoff wavelengths shorter than 314 nm and
transmittance at 450 nm higher than 91%. It is contrasted sharply
with the typical aromatic polyimide film derived from PMDA and
4,40-ODA (ref-PIb), which showed deep yellow color and exhibited
l0 of 444 nm and transmittance at 450 nm of 2%. The excellent
optical transparency of semi-aromatic polyimide films is attributed
to the weak electron withdrawing properties of the alicyclic imide
moieties, which effectively suppressed the inter- and intra-
molecular charge transfer interactions [10].
As comparing the optical transparency of the fluorinated poly-
imide films, i.e., PI-2wPI-6, it is found that PI-2, PI-5 and PI-6
revealed relatively better transparency with l0 < 298 nm and
T500 > 95%. Their excellent optical property is thought to be due to
the steric hindrance effect of bulky trifluomethyl (-CF3) groups
combined with the inductive effect of electron-withdrawing -CF3
and sulfonyl (SO2) groups, which obstructed the dense chain
stacking and broke the electron-conjugation, and consequently,
significantly restrained the formation of inter- and intra-molecular
charge transfer complexes (CTC). It is also detected that UV
absorption edge of PI-3 and PI-4 shifted to the longer wavelength
Table 3
Thermal properties of semi-aromatic polyimidesa.
PIs
Tg (ꢀC)
T5 (ꢀC)
T10 (ꢀC)
Rw (%)
PI-1
PI-2
PI-3
PI-4
PI-5
PI-6
ref-PIa
333
370
268
276
280
275
292
442
474
447
453
445
436
541
458
486
462
466
460
454
598
37
40
43
51
44
43
e
a
Tg: determined by DSC; T5, T10: the decomposition temperatures at 5% and 10%
weight loss, respectively; Rw: residual weight retention at 700 ꢀC in nitrogen; ref-
PIa: aromatic polyimide derived from PMDA and 6FAPB.
Fig. 4. Dependence of the absolute viscosity of polyimide solution in DMAc on solid
content.

3536
L. Zhai et al. / Polymer 53 (2012) 3529e3539
Fig. 6. TMA curves of the semi-aromatic polyimides (in nitrogen).
Fig. 5. TGA curves of semi-aromatic polyimides (in nitrogen).
colorimeter. For comparison, the color intensity of fully aromatic
polyimide ref-PIb was also detected. As illustrated in Table 6, all of
with l0 of 302 and 314 nm, respectively, although they have the
moderate fluorine loading. It is considered that the flexible main
chain and the conjugation of biphenyl rings lead to the relatively
close chain packing. However, there is no significant difference in
UV transmittance between non-fluorinated and fluorinated semi-
aromatic polyimides. For example, non-fluorinated PI-1 exhibited
similar UV cutoff wavelength and transmittance with fluorinated
PI-2. This result seems to disagree with the observation from
physical appearance of these films. We suspect that this may be
related to the relatively high proportion of alicyclic moieties of PI-1,
which strongly affected the optical properties of polyimide films.
From the wide-angle X-ray diffraction (WAXRD) patterns of the
semi-aromatic polyimide films shown in Fig. 9, it can be seen that
the semi-aromatic polyimide films with thickness of 50e53
mm
showed high lightness with L* values over 90 and exhibited
nearly zero a* values combined with extremely low b* values of
3.3e3.8, indicating the extremely transparent and essentially
colorless of these films. As a contrast, the ref-PIb film revealed
a lower L* value of 79.6, a slightly higher a* value of 6.3 and
a significantly higher b* value of 107.7, which suggested the deep
yellow to brown coloration. On the other hand, as comparing the
color intensity of semi-aromatic polyimide PI-5 (CHDA/p-6FBAPS)
with that of fluorinated aromatic polyimide derived from
4,40-(hexafluoroisopropylidene)diphthalic dianhydride (6FDA) and
p-6FBAPS (ref-PIc), which reported by Yang’s research group [46], it
is noted that the former gave the b* value only half of that for the
latter. Therefore, it can be concluded that adoption of weak electron
accepting alicyclic dianhydride is quite effective in eliminating the
characteristic coloration of fully aromatic polyimides. Furthermore,
as comparing the color intensities of these films, it is found that the
highly transparent and entirely colorless PI-2, PI-5 and PI-6 films
gave the relatively lower b* values than the others. The relationship
between the color intensity and polymer structure has been
investigated systematically by Ando and his coworkers [55]. They
suggested that the coloration of polyimides is dominated by the
electron-donation properties of diamines as the dianhydride
structure is fixed. Therefore, the excellent optical transparency of
PI-2, PI-5 and PI-6 is ascribable not only to the adoption of weak
electron accepting CHDA but also to the incorporation of weak
electron donating diamines. The bulky electron-withdrawing -CF3
and SO2 groups as well as the “separator” groups of ether linkages
in the diamines could reduce the electron donating ability and
overall conjugation in the polymer chain [29].
all of the polymers displayed broad peaks centered 2
q
¼ 15e18ꢀ
due to the diffraction of intermolecular packing having some
regularity combined with amorphous halo [54]. This indicated that
these polymers are non-crystalline in morphologic structures. The
amorphous nature of these semi-aromatic PIs could be attributed to
the introduction of alicyclic structure in dianhydride units as well
as bulky trifluoromethyl and sulfonyl groups in diamine moieties,
which resulted in loose chain packaging and aggregation. The inter-
chain distances (d-spacing) of these PIs can be calculated from the
maxima of the broad peaks by Bragg’s equation. PI-2, PI-5 and PI-6
exhibited the diffraction peak at 2
of 5.3e5.8 Å, whereas, PI-1, PI-3 and PI-4 gave the diffraction peak
q
¼ 15.2e16.6ꢀ with the d-spacing
at 2
q
¼ 16.9e17.6ꢀ with the smaller d-spacing of 4.9e5.1 Å. It is
suggested the PI-2, PI-5 and PI-6 have relatively looser polymer
chain packing. This result is consistent with that finding from UV-
vis spectra.
Optical transparency of these the semi-aromatic polyimide films
was also evaluated by the color intensities from the lightness (L*),
redness (a*), and yellowness (b*) indices detected by a color-eye
3.3.3. Molecular structures and molecular orbital energies
Table 4
It is well known that the optical transparency and coloration of
polyimide films are strongly depended on their molecular struc-
ture. In order to get a comprehensive understanding of the rela-
tionship between optical properties and polymer structure, we
tried to investigate the three-dimensional molecular structures of
the model compounds for semi-aromatic PIs. The structure geom-
etries were optimized by the density functional theory (DFT)
calculations using a large basis-set function [56e58]. The molecular
orbital energies of these model compounds were determined by
molecular orbital (MO) calculation.
Mechanical and physical properties of semi-aromatic polyimidesa.
PIs
TS (MPa)
TM (GPa)
Eb (%)
CTE (ppm/ꢀC)
Wu (wt.%)
PI-1
PI-2
PI-3
PI-4
PI-5
PI-6
117
98
106
108
98
2.4
2.7
2.2
2.3
2.7
2.9
7.8
6.6
9.1
11.6
6.6
5.5
56
35
62
61
62
65
1.18
0.59
0.81
0.95
0.92
0.88
93
a
TS: tensile strength; TM: Young’s modulus; Eb: elongation at break; CTE: the in-
plane coefficient of thermal expansion at 50e200 ꢀC; Wu: water uptake.

L. Zhai et al. / Polymer 53 (2012) 3529e3539
3537
Fig. 7. Photographs of semi-aromatic polyimide films (film thickness: 70 mm).
Fig. 8. UV-vis spectra of semi-aromatic polyimide films (film thickness: 25
insert is the photograph of ref-PIb film.
mm). The
Fig. 9. XRD patterns of semi-aromatic polyimides.
Fig. 10 depicts the optimized molecular structures of the model
compounds for semi-aromatic PIs. It can be seen that the model
compound for non-fluorinated PI-1 gave the dihedral angle
between phenyl ring and imide ring of 43ꢀ owing to the slightly
twisted structure. The model compounds for fluorinated
Table 6
Color coordinates of semi-aromatic polyimide filmsa.
PIs
Film thickness (
m
m)
L*
a*
b*
Table 5
PI-1
PI-2
PI-3
PI-4
PI-5
PI-6
ref-PIb
ref-PIc
51
53
52
53
50
52
50
53
91.9
92.0
92.6
90.1
92.4
92.3
79.6
95.0
ꢁ0.2
ꢁ0.2
0.2
ꢁ0.2
ꢁ0.1
ꢁ0.2
6.3
3.8
3.6
3.7
3.8
3.3
3.4
Optical transparency of semi-aromatic polyimide filmsa.
PIs
l0 (nm)
T400 (%)
T450 (%)
T500 (%)
PI-1
PI-2
PI-3
PI-4
PI-5
PI-6
ref-PIb
293
292
302
314
298
297
444
84
85
88
87
88
90
0
91
93
92
93
93
94
2
94
95
95
95
96
97
58
107.7
6.8
ꢁ2.1
a
The color parameters were calculated according to a CIE LAB equation. L* refers
to lightness; 100 means white, while 0 indicates black. A positive a* means red color,
a negative a* indicates green color. A positive b* means yellow color, a negative b*
indicates blue color. ref-PIc: fluorinated aromatic polyimide film derived from 6FDA
and p-6FBAPS.
l0: UV cutoff wavelength; T400, T450, T500: transmittance at 400, 450 and 500 nm,
respectively; ref-PIb: aromatic polyimide film derived from PMDA and 4,40-ODA.

3538
L. Zhai et al. / Polymer 53 (2012) 3529e3539
Fig. 10. Molecular structures of the model compounds for semi-aromatic polyimides. The geometries were optimized by the DFT [B3LYP/6-311G(d,p)] calculations. Gray, white, red,
blue, yellow and brown of each molecular structure display C, H, O, N, F and S, respectively.
polyimides exhibited the similar dihedral angles of 38e43ꢀ, which
suggested that the structures are less affected by the meta-
substituted -CF3 groups. From the calculated molecular orbital
energies listed in Table 7, it is found that all of the semi-aromatic PIs
gave the much higher εLUMO values and consequently much larger
conformation with bond angles of 106-107ꢀ due to the introduction
of SO2 into the backbone, which no longer allows dense chain
stacking, and in turn, interchain CTC formation. Meanwhile, the
strong electron-withdrawing effect of sulfonyl groups along with
the CF3 groups broke the phenyl-phenyl and imide-phenyl elec-
tron-conjugation and resulted in the suppression of the intra-
molecular CT interaction. As a consequence, the excellent optical
properties of semi-aromatic polyimides PI-2, PI-5 and PI-6 are
attributed to their distorted molecular conformation combined
with the weakened electron-accepting and electron-donating
property of dianydride and diamines, which significantly
restrained the formation of inter- and intra-molecular CT
interactions.
Dε values than fully aromatic ref-PIb, which is due to the incorpo-
ration of weak electron accepting alicyclic dianhydride CHDA. In
general, the large energy gap is favorable for preventing the CTC
formation. Therefore, the absence of phenyl rings in dianhydride
moiety could suppress the inter- and intra-molecular CT interac-
tions, and further improve the optical transparency and eliminating
the coloration of PIs.
The model compounds for PI-1, PI-3 and PI-4, which have
phenyl rings separated by ether groups, exhibited the similar bond
angles at ca. 120ꢀ. These compounds have flexible backbones and
gave the relatively linear structures, which are favorable for the
close chain packing. This can be verified by their relatively nar-
rower interchain distances as discussed for WAXRD. Therefore, the
slightly yellow coloration of the films prepared from these poly-
mers can be interpreted by the formation of inter-molecular CT
interactions to some extent. It is also noted that the PI-4 gave the
4. Conclusions
A novel meta-substituted aromatic diamine m-6FBAPS con-
taining trifluoromethyl and sulfonyl groups in the backbone was
synthesized and characterized. A series of semi-aromatic PIs were
prepared based on alicyclic dianhydride CHDA and various
aromatic diamines. The semi-aromatic PIs displayed good solubility
due to the synergetic effects of alicyclic moieties and bulky tri-
fluoromethyl substituents in the polymer structure. They also
exhibited high thermal stability with Tg of 268e370 ꢀC and T5 of
436e474 ꢀC, respectively. The flexible and toughness films could be
obtained via the casting of the polyimide solutions directly. In
particular, PI-2 derived from CHDA/TFDB demonstrated relatively
higher Tg of 370 ꢀC and extremely lower CTE of 35 ppm/ꢀC, which is
attributed to the presence of rigid biphenyl unit in the main chain.
The semi-aromatic polyimide films were transparent and essen-
lowest
Dε value due to the conjugation of biphenyl rings with
dihedral angles of 140ꢀ. This is the reason why the absorption edge
for PI-4 shifted to the longest wavelength of 314 nm. On the
contrary, the model compound for PI-2 showed the highest
Dε
values because of phenyl-phenyl electron-conjugation obstructed
by the extremely distorted biphenyl conformation with dihedral
angle of 75ꢀ, which related with the steric hindrance effect of the
bulky -CF3 substituents. Moreover, the model compounds for PI-5
and PI-6 also revealed the highly distorted molecular
tially colorless even as the film with thickness of 70 mm, which is
Table 7
considerably different from the deep yellow or brown color of
traditional aromatic polyimide films. They exhibited optical trans-
parency with UV cutoff wavelengths of 292e314 nm, transmittance
at 450 nm higher than 91% and yellowness indices (b*) no more
than 3.8. The improved optical transparency of these films is
associated with the incorporation of alicyclic moieties in the
polymer structure. The extremely transparent and entirely color-
less films were obtained from the semi-aromatic polyimides PI-2,
Molecular orbital energies of HOMO and LUMO as well as energy gaps of semi-
aromatic polyimidesa.
PIs
PI-1
PI-2
PI-3
PI-4
PI-5
PI-6
ref-PIb
εLUMO (eV)
εHOMO (eV)
ꢁ1.15
ꢁ6.23
5.08
ꢁ1.89
ꢁ7.20
5.31
ꢁ1.40
ꢁ6.62
5.22
ꢁ1.38
ꢁ6.33
4.95
ꢁ1.72
ꢁ6.93
5.21
ꢁ1.84
ꢁ6.90
5.06
ꢁ3.43
ꢁ6.30
2.87
D
ε (eV)
a
εHOMO
:
the highest occupied molecular orbital energy; εLUMO
ε: energy gap,
:
the lowest
unoccupied molecular orbital energy;
D
D
ε ¼ εLUMOꢁεHOMO.

L. Zhai et al. / Polymer 53 (2012) 3529e3539
3539
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