Synthesis and characterization of transparent polyimides derived from ester-containing dianhydrides with different electron affinities

Article abstract of DOI:10.1039/c5ra16574a

Two series of poly(ester imide)s derived from bis(trimellitic acid anhydride) phenyl ester (TAHQ) and bis[(3,4-dicarboxylic anhydride) phenyl] terephthalate (PAHP), as well as poly(ether imide)s based on hydroquinone diphthalic anhydride (HQDPA), were synthesized with aromatic diamines via solution polycondensation. These polyimide films were transparent with an ultraviolet-visible absorption cut-off wavelength below 375 nm, and with tensile strengths of 42.0-83.8 MPa, tensile moduli of 2.5-4.7 GPa and elongations at break of 2.1-5.4%. Compared with the poly(ether imide)s, the poly(ester imide)s showed higher glass transition temperatures (T_g), lower water absorption (W_A) and lower temperature of 5% weight loss (T_d5%). Moreover, the poly(ester imide)s derived from PAHP with a low electron affinity of 2.04 eV by theoretical calculation achieved better transparency, lower W_A and slightly lower T_g than the corresponding TAHQ-based poly(ester imide)s.

Full text of DOI:10.1039/c5ra16574a

View Article Online
View Journal
RSC Advances
This article can be cited before page numbers have been issued, to do this please use: Y. Zhou, G. Chen,
W. Wang, L. Wei, Q. Zhang, L. Song and X. Fang, RSC Adv., 2015, DOI: 10.1039/C5RA16574A.
This is an Accepted Manuscript, which has been through the
Royal Society of Chemistry peer review process and has been
accepted for publication.
Accepted Manuscripts are published online shortly after
acceptance, before technical editing, formatting and proof reading.
Using this free service, authors can make their results available
to the community, in citable form, before we publish the edited
article. This Accepted Manuscript will be replaced by the edited,
formatted and paginated article as soon as this is available.
You can find more information about Accepted Manuscripts in the
Information for Authors.
Please note that technical editing may introduce minor changes
to the text and/or graphics, which may alter content. The journal’s
standard Terms & Conditions and the Ethical guidelines still
apply. In no event shall the Royal Society of Chemistry be held
responsible for any errors or omissions in this Accepted Manuscript
or any consequences arising from the use of any information it
contains.
www.rsc.org/advances

Page 1 of 9
RSC Advances
View Article Online
DOI: 10.1039/C5RA16574A

Please do not adjust margins
RSC Advances
Page 2 of 9
View Article Online
DOI: 10.1039/C5RA16574A
Journal Name
ARTICLE
Synthesis and characterization of transparent polyimides derived
from ester-containing dianhydrides with different electron
affinities
Received
Accepted
Yu Zhoua,b, Guofei Chena*, Wei Wanga, Lihong Weia, Qiuju Zhanga, Liping Songb, Xingzhong Fanga*
DOI:
Two series of poly(ester imide)s derived from bis(trimellitic acid anhydride) phenyl ester (TAHQ) and bis[(3,4-dicarboxylic
anhydride) phenyl] terephthalate (PAHP), as well as poly(ether imide)s based on hydroquinone diphthalic anhydride
(HQDPA), were synthesized with aromatic diamines via solution polycondensation. These polyimide films were transparent
with an ultraviolet–visible absorption cut-off wavelength below 375 nm, and with the tensile strengths of 42.0-83.8 MPa,
tensile moduli of 2.5-4.7 GPa and elongations at break of 2.1-5.4%. Compared with the poly(ether imide)s, the poly(ester
imide)s showed higher glass transition temperature (Tg), lower water absorption (WA) and lower temperature of 5% weight
loss (Td5%). Moreover, the poly(ester imide)s derived from PAHP with low electron affinity of 2.04 eV by theoretical
calculation achieved better transparency, lower WA and slightly lower Tg than the corresponding TAHQ-based poly(ester
imide)s.
www.rsc.org/
dianhydride, showed good transparency.17-25 However, poly(ether
1. Introduction
imide)s (PEIs) based on bis(ether anhydride)s decrease the Tg due to
Aromatic polyimides (PIs) have been widely utilized in a variety
of electric and microelectronic applications for their reliable and
excellent properties, i.e., considerably high glass transition
temperatures (Tg), high resistance to chemicals, good dielectric and
mechanical properties.1-5 However, the widespread applications of
polyimides in optical fields are often limited because of deep
coloration which is caused by the strong intra- and intermolecular
charge transfer (CT) interactions.6,7 So far, much effort has been
devoted to development of transparent polyimide films based on
structural design. The most effective approach for inhibiting CT
interactions is to use non-aromatic (cycloaliphatic) monomers
either in diamines or tetracarboxylic dianhydrides.8-15
Unfortunately, this approach somewhat sacrifices high-temperature
stability. For fully aromatic polyimides, the best way is
incorporation of trifluoromethyl group in the monomers, such as
4,4'-(hexafluoroisopropylidene) diphthalic anhydride (6FDA) and
2,2'-bis(trifluoromethyl)biphenyl-4,4'-diamine (TFDB), which are
commercial available but expensive.16 Another strategy for
obtaining transparent polyimides is to use dianhydrides with low
electron-acceptability and diamines with low electron-donatability
as monomers for weakening both intra- and intermolecular CT
interactions.7 For example, the polyimide films derived from
bis(ether anhydride)s with electron-donating atom “O” in phthalic
anhydride, which decreased the electron affinity (Ea) of
the introduction of flexible ether linkage.
On the other hand, the poly(ester imide)s (PEsIs) derived from
trimellitic acid anhydride achieved higher Tg and low water
absorption (WA) etc, which was widely used in electronic devices
such as integrated circuits.26-31 The introduction of polar ester
linkage in the polymer chain, increased the intermolecular
interaction, which was beneficial to improve Tg and lower water
absorption, but also increased the electron affinity of dianhydride
due to the electron-withdrawing C=O group directly substituted at
phthalic anhydride, which enhanced the electron-acceptability of
dianhydride leading to increase the coloration of resulting
polyimides at the same time.
Therefore, PEsIs derived from hydroxyl phthalic anhydride
would be a best strategy to obtain combined excellent properties.
The existence of ester group would remain the excellent properties
of poly(ester imide)s, such as high Tg and low WA, while using
electron-donating atom “O” linked with phthalic anhydride would
decrease the electron affinity of dianhydride to improve the
transparency of films (Fig. 1). To the best of our knowledge, there is
no report on the isomeric effect of such linkage on the polyimides.32
Herein, two series of PEsIs were prepared from two dianhydrides
with different electron affinities, bis(trimellitic acid anhydride)
phenyl ester (TAHQ) and bis[(3,4-dicarboxylic anhydride) phenyl]
terephthalate (PAHP), with various aromatic diamines via solution
polycondensation. The thermal stability, solubility, water
absorption, coefficient of thermal expansion (CTE), mechanical and
optical properties of these polyimides as well as the different
properties caused by the different ester linkages were well
investigated. The polyimides derived from hydroquinone diphthalic
a.Ningbo Key Laboratory of Polymer Materials, Ningbo Institute of Materials
Technology and Engineering, Chinese Academy of Sciences, Ningbo, Zhejiang,
315201, People’s Republic of China. Correspondence to: G. Chen (E-mail:
gfchen@nimte.ac.cn); X. Fang (E-mail: fxzhong@nimte.ac.cn)
b.Department of Chemistry, School of Science, Shanghai University, Shanghai,
200444, People’s Republic of China
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 1
Please do not adjust margins

Please do not adjust margins
RSC Advances
Page 3 of 9
View Article Online
DOI: 10.1039/C5RA16574A
ARTICLE
Journal Name
anhydride (HQDPA) were also prepared and investigated for performed on a Mettler Toledo-TGA/DSC I instrument to evaluate
comparison.
the thermal stability of the polyimides at a heating rate of 10
°C/min from 50 °C to 800 °C under nitrogen or air atmosphere (flow
rate of 50 mL/min). The mechanical properties of the polyimide
films such as tensile modulus, tensile strength, and elongation at
break were measured and averaged on at least five film specimens
by an Instron model 5567 tensile tester at a crosshead speed of 5
mm/min at room temperature. The specimen gauge lengths were
50 mm and the specimen widths were 10 mm. Ultraviolet-visible
(UV-vis) spectra of the polymer films were recorded on a Lambda
950 UV/Vis/NIR spectrophotometer. The wide-angle X-ray
diffraction (WAXD) measurement of the polyimide films was
undertaken on a Bruker D8 Advance with Cu Kα radiation (40 kV, 40
mA) at a scanning rate of 5°/min from 5 to 50°. The equilibrium
water absorption (WA) was determined by the weighing of the
changes in vacuum-dried film specimens before and after
immersion in deionized water at 25 °C for 24 hr. The water
absorption was calculated from the following equation:
Water absorption (%) = [(Wwet - Wdry) / Wdry] * 100%
Where Wdry is the initial weight of the dry film and Wwet is the
weight of the film after immersion in deionized water. The water
absorption of the PIs is the mean data of three parallel samples.
Thermo mechanical analysis (TMA) was performed by a METTLER
TMA/SDTA841 at a heating rate of 10 °C/min. The coefficient of
thermal expansion (CTE) was recorded at the temperature range of
100-200 °C.
Fig. 1 Chemical structure of HQDPA, PAHP and TAHQ.
2.Experimental
2.1. Materials
4-Nitrophthalonitrile (98%, Leadership Chemical Technologies Co.,
Ltd.), terephthaloyl chloride (Sinopharm Chemical Reagent Co.,
Ltd.) and hydroquinone diphthalic anhydride (HQDPA) (Hipolyking
Chemical Reagent Co., Ltd.) were used as received. Bis[4-(3-
aminodiphenoxy)phenyl] sulfone (m-BAPS, 98%, Changzhou
Sunlight Pharmaceutical Co., Ltd.) was recrystallized from ethanol
2.3. Monomer synthesis
2.3.1 4-Hydroxyphthalonitrile
To a flask containing 4-nitrophthalonitrile (103.9 g, 600 mmol) and
DMSO (800 mL) was added K2CO3 (91.6 g 660 mmol) and NaNO2
(46.0 g, 660 mmol) under nitrogen atmosphere with stirring at
room temperature. The mixture was stirred for 3 h at 160 °C, then
the resulting solution was poured into the water after cooled to
room temperature and acidified by hydrochloric acid to pH = 3. The
precipitate was filtered off, washed with water, and dried at 110 °C
under vacuum to afford 73.3 g of 4-hydroxyphthalonitrile, yield:
85%, m.p. = 219 oC. FT-IR (KBr, cm-1): 3269 cm-1 (O-H stretching),
2243 cm-1 (C≡N stretching). 1H NMR δH (400 MHz, DMSO-d6, δ,
ppm): 11.46 (s, 1H), 7.91 (d, J = 8.0 Hz, 1H), 7.39 (s, 1H) and 7.21 (d,
J = 8.0 Hz, 1H).
before
use.
Bis[4-(4-amino-2-trifluoromethylphenoxy)phenyl]
sulfone (6F-BAPS),33 and bis(trimellitic acid anhydride) phenyl ester
(TAHQ)29 were synthesized in our laboratory according to the
literature. N,N-Dimethylacetamide (DMAc) was distilled from
calcium hydride and stored over 4 Å molecular sieves. Anhydrous
tetrahydrofuran (THF) was distilled from sodium. All other reagents
for the study were commercially obtained and used as received
without further purification.
2.2. Instrumentation
Fourier transform infrared (FT-IR) spectra were recorded with a
Thermo Nicolet 6700 FT-IR spectrometer. Nuclear magnetic
resonance (NMR) spectra were performed on a Bruker 400 AVANCE
III spectrometer operating at 400 MHz for 1H NMR using dimethyl
sulfoxide-d6 (DMSO-d6) as solvent. The inherent viscosities of the
polymers were measured at 30 ± 0.1 °C with an Ubbelohde
viscometer and the concentration was 0.5 g/dL in DMAc.
Differential scanning calorimetry (DSC) measurements of the
polyimides were performed on a Mettler Toledo-DSC I at a heating
rate of 20 °C/min under nitrogen atmosphere, and the temperature
at the middle of the thermal transition from the second heating
scan was assigned as the glass transition temperature (Tg). The
melting points (m.p.) of synthesized monomers were also measured
by the DSC instrument at a heating rate of 10 °C/min and the
melting points were determined from the temperature at the
maximum point of the corresponding enthalpy peaks.
Thermogravimetric analyses (TGA) of the polyimides were
2.3.2 4-Hydroxyphthalic acid
In a 1 L flask, a suspension of 4-hydroxyphthalonitrile (69.2 g, 480
mmol) in a 800 mL water containing of dissolved KOH (230.0 g, 4.11
mol) was boiled under reflux for about 90 h. The resulting clear
solution was cooled to room temperature and filtered to remove
any possible insoluble impurities. Then filtrate was acidified by
hydrochloric acid to pH = 1, extracted with ethyl acetate three times,
washed with brine and dried over anhydrous MgSO4. After filtration
and evaporation, the crude product 82.9 g was afforded and dried
o
at room temperature under vacuum, yield: 95%, m.p. = 196 C. The
crude product was used directly for the cyclodehydration reaction
without further purification. FT-IR (KBr, cm-1): 3483, 3103 cm-1 (O-H
stretching), 1686, 1598 cm-1 (C=O stretching). 1H NMR δH (400 MHz,
2 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Please do not adjust margins
RSC Advances
Page 4 of 9
View Article Online
DOI: 10.1039/C5RA16574A
Journal Name
ARTICLE
DMSO-d6, δ, ppm): 12.78 (s, 2H), 10.37 (s, 1H), 7.62 (d, J = 8.0 Hz, scheduled heating at 150 and 200 °C for 60 min at each
1H) and 6.84-6.82 (m, 2H).
temperature.
PI-1a-C: FT-IR (film): 1782 cm-1 (asym C=O stretching), 1722
2.3.3 4-Hydroxyphthalic anhydride
cm-1 (sym C=O stretching), 1371 cm-1 (C-N stretching).
PI-1b-C: FT-IR (film): 1784 cm-1 (asym C=O stretching), 1725
cm-1 (sym C=O stretching), 1378 cm-1 (C-N stretching).
PI-2a-C: FT-IR (film): 1780 cm-1 (asym C=O stretching), 1722
cm-1 (sym C=O stretching), 1370 cm-1 (C-N stretching).
PI-2b-C: FT-IR (film): 1783 cm-1 (asym C=O stretching), 1724
cm-1 (sym C=O stretching), 1371 cm-1 (C-N stretching).
PI-3a-C: FT-IR (film): 1778 cm-1 (asym C=O stretching), 1725
cm-1 (sym C=O stretching), 1378 cm-1 (C-N stretching).
PI-3b-C: FT-IR (film): 1779 cm-1 (asym C=O stretching), 1722
cm-1 (sym C=O stretching), 1373 cm-1 (C-N stretching).
The 4-hydroxyphthalic anhydride (10.8 g) was afforded by
sublimation of 4-hydroxyphthalic acid (12.3 g, 67.5 mmol) directly,
yield: 95%, m.p. = 172 oC. FT-IR (KBr, cm-1): 3293 cm-1 (O-H
stretching), 1846, 1762 cm-1 (C=O stretching). 1H NMR (400 MHz,
DMSO-d6, δ, ppm): 11.40 (s, 1H), 7.89 (d, J = 8.0 Hz, 1H) and 7.34-
7.25 (m, 2H).
2.3.4 Bis[(3,4-dicarboxylic anhydride) phenyl] terephthalate (PAHP)
To a solution of 24.6180 g (150 mmol) of 4-hydroxyphthalic
anhydride in 400 mL of anhydrous THF was added terephthaloyl
chloride (15.2265 g, 75 mmol) in portions under stirring. Pyridine
(35.0 mL) was then added as catalyst and acid absorbent. The
reactive mixture was stirred for 24 h at room temperature and the
solid separated during reaction was filtered and washed with water,
methanol and trichloromethane. A white solid product (28.20 g)
was obtained and dried at 150 °C under vacuum, yield: 82%, m.p. =
277 oC. FT-IR (KBr, cm-1): 1846 cm-1 (asym C=O stretching), 1783 cm-
3.Results and discussion
3.1. Monomer synthesis
As shown in Scheme 1, 4-hydroxyphthalic anhydride was obtained
by substitution reaction with 4-nitrophthalonitrile, alkaline
hydrolysis in the presence of KOH, and then sublimation under the
vacuum. The bis(ester anhydride), bis[(3,4-dicarboxylic anhydride)
phenyl]terephthalate (PAHP), was afforded through esterification
reaction of terephthaloyl chloride with 4-hydroxyphthalic
anhydride. Fig. 2 shows the IR spectrum of the bis(ester anhydride)
PAHP. The strong absorption bands around 1846 cm-1 (vasym C=O)
and 1783 cm-1 (vsym C=O) were assigned to the phthalic anhydride,
while the strong absorption bands around 1737 cm-1 was assigned
to the ester group in PAHP. The observed bands in the region of
1000-1300 cm-1 were due to the C-O stretching. The 1H NMR
spectrum of PAHP is shown in Fig. 3, in which all the signals were
assigned. The aromatic protons in terephthalate (Ha) and phthalic
anhydride (Hb-Hd) resonated in the region of 8.40 and 8.23-8.01
ppm, respectively.
1
(sym C=O stretching), 1737 cm-1 (C=O stretching), 1258, 1060,
1018 cm-1 (C-O stretching). 1H NMR (400 MHz, DMSO-d6, δ, ppm):
8.40 (s, 4H), 8.23 (d, J = 8.4 Hz, 2H), 8.21 (s, 2H) and 8.01 (d, J = 8.4
Hz, 2H).
2.3.5 Polymer synthesis and film preparation
Utilizing TAHQ, PAHP and HQDPA as dianhydride monomers, six
kinds of polyimides were synthesized by polycondensation with
diamine monomer m-BAPS and 6F-BAPS via two-step procedure
with different methods (thermal imidization (H) and chemical
imidization (C)), respectively. The resulting PIs were referred to as
PI-1a to PI-3b successively.
The synthesis of the film of polyimide (PI-2a) is used below as
an example to illustrate the general synthetic route for the
preparation: dianhydride PAHP (1.8334 g, 4.0 mmol) was added in
one portion to a solution of diamine m-BAPS (1.7300 g, 4.0 mmol) in
DMAc (14.2536 g) in a 50 mL flask. The mixture was stirred in
nitrogen at room temperature for 24 h to yield a viscous poly(amic
acid) (PAA) solution with 20 wt% solid concentration. Then the
resulting viscous poly(amic acid) solution was diluted to 15 wt%
with additional DMAc.
For the thermal imidization procedure, the PAA solution was
taken out and cast onto a glass plate. The glass plate was placed in
an oven at 80 °C for 2 h to remove most of the solvent. The
semidried PAA film was further dried and imidized by heating at 150
and 260 °C for 60 min at each temperature. The resulting films PI-
2a-H were obtained by immersion the glass plate in water followed
by dry in an oven at 100 °C.
Scheme 1. Synthesis of dianhydride PAHP.
For the chemical imidization procedure, the PAA solution was
added the mixture solution of acetic anhydride (2 mL) and pyridine
(1 mL). After stirring for 24 h, the solution was diluted with DMAc
and slowly poured into a vigorously stirred ethanol. The precipitate
was collected by filtration, washed thoroughly with hot ethanol and
dried. The polyimide film PI-2a-C was prepared via the casting of 10
wt% homogenous solution of above polyimide powder in DMAc
onto a clean glass plate, which was then placed in an oven at 80 °C
overnight to evaporate most of the solvent and subjected to
Fig. 2 FT-IR spectrum of PAHP.
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 3
Please do not adjust margins

Please do not adjust margins
RSC Advances
Page 5 of 9
View Article Online
DOI: 10.1039/C5RA16574A
ARTICLE
Journal Name
Fig. 3 1H NMR spectrum of PAHP in DMSO-d6.
3.2. Synthesis of Polyimides
Scheme 2. Synthesis of polyimides.
Utilizing bis(trimellitic acid anhydride) phenyl ester (TAHQ), bis[(3,4-
dicarboxylic anhydride) phenyl] terephthalate (PAHP) and
hydroquinone diphthalic anhydride (HQDPA) as dianhydride
monomers, six kinds of polyimides were synthesized by
polycondensation with diamine monomer m-BAPS and 6F-BAPS via
two-step procedure, which is shown in Scheme 2. Whenever
aromatic diamines were used, the reaction smoothly proceeded
and led to homogeneous after 24 hours. The inherent viscosities of
prepared poly(amic acid)s (PAAs) were between 0.57-1.02 dL/g in
DMAc at 30 °C (Table 1). The polyimide films derived from TAHQ
and HQDPA were successfully afforded by thermal imidization. To
our surprise, no flexible polyimide films based on PAHP were
obtained by thermal imidization, although the PAAs had higher
inherent viscosities with 0.87-1.02 dL/g, which indicated that PAAs
were degraded quickly during the thermal imidization conditions.
Fortunately, all the polyimides were highly soluble in DMAc at room
temperature, so that polymerization were conducted by chemical
imidization (C) and the resulting polyimides could be cast to form
flexible films.
Fig. 4 1H NMR spectrum of PI-1b-C in DMSO-d6.
The chemical structures of polyimides were confirmed by 1H
NMR and FT-IR spectroscopy. The representative 1H NMR spectra of
PI-1b-C, PI-2b-C and PI-3b-C are shown in Fig. 4, Fig. 5, Fig. 6,
respectively, in which all the signals have been assigned to the
protons of the repeating unit. The aromatic protons (Ha) in the
middle benzene of dianhydride moiety in PI-1b-C, PI-2b-C and PI-
3b-C were resonated in 7.55, 8.41 and 7.33 ppm, respectively.
Furthermore, the COSY spectra of PI-1b-C, PI-2b-C and PI-3b-C (see
Figures in SI) were measured for the further assignments of
protons. The completed imidization was confirmed by the absence
of corresponding amide signals and carboxylic acid functions.
The typical IR spectra of PI-1b-C, PI-2b-C and PI-3b-C are shown
in Fig. 7. The strong absorption bands around 1783 cm-1 (vasymC=O),
1724 cm-1 (vsym C=O), and 1373 cm-1 (vC-N imide) were assigned to
the phthalimide unit, while the absorption bond around 1316 and
1147 cm-1 were assigned to the sulfone unit.
Fig. 5 1H NMR spectrum of PI-2b-C in DMSO-d6.
Fig. 6 1H NMR spectrum of PI-3b-C in DMSO-d6.
4 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Please do not adjust margins
RSC Advances
Page 6 of 9
View Article Online
DOI: 10.1039/C5RA16574A
Journal Name
ARTICLE
The CTE values of poly(ester imide)s and poly(ether imide)s are
also summarized in Table 1, which were in the range of 52.7-75.3
ppm/K and 61.0-120.0 ppm/K, respectively.
Fig. 7 FT-IR spectra of PI.
3.3. Thermal properties
The thermal behavior data of polyimides are listed in Table 1. The
Tgs of polyimides were in the range of 211-250 °C. Tg is correlated
with the structure of polymer chain. PI-1a-C, PI-2a-C and PI-3a-C
derived from m-BAPS displayed a relatively lower Tg than the other
PIs derived from 6F-BAPS due to their more flexible backbones with
meta substitution. PI-1b-C showed the highest Tg value of 250°C
because of the increase of chain rigidity with para substitution and
steric effect of the trifluoromethyl group.32 The Tg values of the
TAHQ and PAHP series of polyimides were higher than those of the
corresponding counterparts of HQDPA-based PIs. What’s more, the
PIs derived with PAHP exhibited slightly lower Tg compared with
TAHQ-based polyimides, which indicated that intermolecular
interactions were weak due to the different ester linkages.
Fig. 8 TGA curves of polyimides in nitrogen and air.
3.4 Water absorption
As summarized in Table 1, all the PI films showed low water
absorption in the range of 0.28-0.62%, which were much lower than
commercially available Kapton-H film (PMDA/4,4’-ODA) with WA of
2.9%.13 The results are probably related to a significant decrease in
the content of highly polarizing imide groups in the structures.14,29
Moreover, the PIs based on 6F-BAPS showed lower water
absorption than the PIs based on m-BAPS. This phenomenon was
probably due to the trifluoromethyl groups, which induced greater
water proofing effect.13 Furthermore, the PAHP-based PIs exbihited
lower WA than the corresponding TAHQ-based PIs. The low water
absorption ensured their applications in the microelectronic
industry.
The thermal stability of the polyimides was evaluated by dynamic
TGA conducted at a heating rate of 10 °C/min. The temperature of
5% weight loss (Td5%) in nitrogen and air atmospheres were
determined from original TGA thermograms, which are listed in the
Table 1 and the TGA curves of PI-1a-C to PI-3b-C are shown in Fig. 8.
The Td5% values of these PIs were recorded in the range of 481-
556°C in nitrogen and 451-542°C in air. It was obvious that
poly(ester imide)s showed lower Td5% than the poly(ether imide)s,
because the ester groups started to decompose at
a lower
temperature. However, there was no obvious differences on the
Td5% regardless of different ester linkage due to the similar chemical
compound.
Table 1 Thermal properties and water absorption of the polyimides.
Td5%c(oC)
PI
ηinha(dL/g)
Tgb (oC)
Char yield (%)
CTEd (ppm/K)
WAe (%)
In N2
481
494
481
484
541
556
In air
454
451
460
461
535
542
PI-1a-C
PI-1b-C
PI-2a-C
PI-2b-C
PI-3a-C
PI-3b-C
0.59
0.72
1.02
0.87
0.75
0.57
220
250
217
236
211
222
46
50
50
50
52
54
73.6
56.9
52.7
75.3
120.0
61.0
0.58
0.54
0.32
0.28
0.62
0.40
a The inherent viscosities of poly (amic acid)s were measured at a concentration of 0.5 g/dL in DMAc at 30 °C.
b Baseline shift in the second heating DSC traces, with a heating rate of 20 °C/min.
c Temperatures at which 5% weight loss were recorded by TGA at a heating rate of 10 °C /min.
d CTE is recorded from 100 to 200 oC at a heating rate of 10 °C/min by TMA.
e Water absorption of polyimide films were measured by immersing the films of these polyimides in deionized water at 25°C for 24 hr.
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 5
Please do not adjust margins

Please do not adjust margins
Page 7 of 9
RSC Advances
View Article Online
DOI: 10.1039/C5RA16574A
ARTICLE
Journal Name
Table 3 Mechanical properties of the polyimide films.
PI
Tensile strength (MPa)
Tensile modulus (GPa)
Elongation at break (%)
PI-1a-C
PI-1b-C
PI-2a-C
PI-2b-C
PI-3a-C
PI-3b-C
68.0
83.8
62.6
42.0
79.1
74.9
2.9
2.8
2.5
2.5
4.7
4.4
3.3
5.4
4.3
2.1
2.6
2.6
Table 4 Optical properties of the polyimide films.
αc (cmꢀ1)
188
151
223
97
PI
Imidization d (μm)
λ0 (nm)a
T400 (%)b
27
T450 (%)b
T500 (%)b
79
PI-1a-H
PI-1a-C
PI-1b-H
PI-1b-C
PI-2a-C
PI-2b-C
PI-3a-H
PI-3a-C
PI-3b-H
PI-3b-C
H
C
H
C
C
C
H
C
H
C
30
20
22
27
26
22
22
23
24
21
375
360
370
360
351
343
371
365
366
360
72
82
76
85
83
86
84
84
85
87
50
85
32
84
55
87
73
85
52
78
86
48
65
86
84
67
85
74
63
87
83
71
88
70
a λ0: UV cut-off wavelength.
b T400,T450,T500: transmittance at 400, 450, 500 nm, respectively.
c The absorption coefficient of the polymer at 400 nm was calculated by dividing absorbance by the film thickness.
The mechanical properties of the flexible polyimide films are listed
in Table 3. The polyimide films exhibited good mechanical
properties with tensile strengths of 42-84 MPa, tensile moduli of
2.5-4.7 GPa, and elongations at break of 2.1-5.4%. Among these
polyimide films, the tensile strength and tensile modulus of PI-1b-C
was more than 83 MPa and 2.8 GPa, respectively, which were
attributed to rigid structure in the polymers backbone. What’s more,
the TAHQ-based PEsIs showed slightly better mechanical properties
than the PAHP-based PEsIs.
3.5 Solubility
Table 2 Solubility of the polyimides.
Solventsa
PI
m-cresol DMAc DMF NMP DMSO CHCl3 THF
PI-1a-C ++
PI-1b-C ++
PI-2a-C ++
PI-2b-C ++
PI-3a-C ++
PI-3b-C ++
++
++
++
++
++
++
+
++
++
++
++
+
+
+−
+−
+−
+−
+−
++
+−
+
++
+
++
+
+−
++
+−
++
++
+−
++
++
+−
++
3.7 X-ray diffraction
++
The qualitative solubility was tested with 10 mg samples in 1 mL of
solvent.++: soluble at room temperature; +: soluble on heating;
+−: parꢀal soluble on heaꢀng.
a DMAc: N,N-dimethylacetamide; DMF: N,N-dimethylformamide;
NMP: N-methylpyrrolidone; DMSO: dimethyl sulfoxide; THF:
tetrahydrofuran.
The solubility of the polyimides was tested qualitatively in various
organic solvents, and the results are shown in Table 2. The solubility
of PI-1b-C, PI-2b-C and PI-3b-C is better than PI-1a-C, PI-2a-C and
PI-3a-C owing to CF3 group which inhibited close packing and
reduced the inter-chain interaction to enhance solubility. In almost
all cases, the PEsIs based on the m-BAPS exhibited better solubility
than the PEIs, while the 6F-BAPS-containing PEsIs showed worse
solubility than the corresponding PEIs. Furthermore, the PIs derived
from PAHP exhibited slightly better solubility compared with TAHQ-
based polyimides.
Fig. 9 Wide-angle X-ray diffractograms of the polyimide films.
The crystallinity of the polyimide films was analyzed by wide-angle
X-ray diffraction (WAXD). PI-2a-C exhibited some diffraction peaks
in the 2θ range from 15 and 30o, which suggested that polyimide
had relatively some regularity of intermolecular packing or short-
range-order structure. The WAXD patterns of the other films were
broad without obvious peak features, which indicated that they
were all amorphous (Fig. 9).
3.6 Mechanical properties
6 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Please do not adjust margins
RSC Advances
Page 8 of 9
View Article Online
DOI: 10.1039/C5RA16574A
Journal Name
ARTICLE
3.8 Optical properties
The UV-visible absorption spectra of some representative polyimide
films are illustrated in Fig. 10, while the transmittance spectra and
absorption coefficient curves are showed in Fig. S7 and S8 (see
Figures in SI). The cut-off wavelength (absorption edge, λ0),
transmittance at 400, 450, 500 nm and absorption coefficient at
400 nm from these spectra are listed in Table 4.
All polyimide films exhibited cut-off wavelengths shorter than
375 nm and were highly transparent. The lower coloration of the
polyimide derived from 6F-BAPS with the bulky and electron-
withdrawing CF3 groups in their diamine moieties could be
explained by the decreased CTC formation of polymer chains
through higher steric hindrance and lower inductive effect. A
secondary positive effect of the CF3 groups on the film transparency
was the weakened intermolecular dispersion forces due to low
polarizability of the C–F bond.18
As expected, the PAHP-based PIs exhibited better optical
properties than TAHQ-based PIs due to the lower electron affinity
(Ea) value of PAHP, which reduced the CTC effect formation. What’s
more, the PAHP-based PIs even showed lower cut-off wavelength
and higher transmittance at 400 nm than the HQDPA-based
polyimides, although HQDPA-based polyimides showed better
transparency after 450 nm. (Fig. 11) The absorption coefficients of
the polyimides at 400 nm excluding the film thickness factor were
also supported these results. On the other hand, the HQDPA based
polyimide films revealed slightly better transparent in the visible
wavelength spectrum of 400-800 nm than their respective analogue
based on the TAHQ and PAHP, among which the PI-3b-C showed
the best optical transparency, probably due to the ether moiety in
the dianhydride weakened both intra- and intermolecular charge
transfer interactions by decreasing electron-acceptability of
dianhydride.7 Furthermore, the polyimide films prepared via
chemical imidization (C) tend to show higher T400 values than the
corresponding thermally imidized (H) films. This was probably due
to an end-capping reaction of the thermally less stable terminal
amino groups with Ac2O during chemical imidization.13
Fig. 11 Photographs of the polyimide films.
To interpret the coloration of polyimide films clearly, the electron
affinities (Ea)34 of TAHQ, PAHP as well as HQDPA, were carried out
by a theoretical calculation employing the DFT method in the
Gaussian 09 package.35 The exchange correlation functional B3LYP
is carefully mixed with Hartree–Fock exchange calculated with
Kohn–Sham orbits and the B88 exchange functional plus the LYP
correlation functional.36 The all-electron basis set 6-31g (d) was
used to describe the C, O and H atoms. The calculation results for
the values of Ea of the dianhydrides were calculated from equation
1 and the results are tabulated in Table 5. As expected, the Ea
values of PAHP is 2.04 eV, which is much lower than that of TAHQ
with value of 2.17 eV, indicates that the ester group decrease the
electron acceptability of dianhydride, therefore, the corresponding
polyimides based on PAHP exhibit a relatively better transparency
than that of TAHQ. And the PIs derived from HQDPA show the best
transparency due to the lowest Ea value of 1.62 eV.
Ea = E(optimized neutral geometry) – E(optimized anion geometry)
(1)
Table 5 Electronic characteristics of TAHQ, PAHP, and HQDPA
Ea (a.u.) Calculated from
a
a
Dianhydride
Ea (eV) q1
q2
energies difference
0.0798
TAHQ
PAHP
2.17
2.04
1.62
0.814 0.816
0.814 0.816
0.813 0.816
0.0749
HQDPA
0.0597
a Refer to the values of the positive charges on the carbonyl carbon
atoms of the dianhydrides.
4.Conclusion
Two series of novel poly(ester imide)s were synthesized from
bis(trimellitic acid anhydride) phenyl ester (TAHQ), bis[(3,4-
dicarboxylic anhydride) phenyl] terephthalate (PAHP) and
hydroquinone diphthalic anhydride (HQDPA) with various aromatic
diamines by polycondensation, which could be formed transparent,
flexible, and tough films with tensile strengths of 42.0-83.8 MPa,
tensile moduli of 2.5-4.7 GPa and elongations at break of 2.1-5.4%.
The polyimide films from PAHP showed better transparency, lower
WA and slightly lower Tg compared with TAHQ-based
polyimides. The optical properties of these polyimide films are
well coincident with the electron affinities (Ea) values by
Fig. 10 UV-vis absorption spectra of polyimide films.
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 7
Please do not adjust margins

Please do not adjust margins
RSC Advances
Page 9 of 9
View Article Online
DOI: 10.1039/C5RA16574A
ARTICLE
Journal Name
theoretical calculation. This proves that poly(ester imide)s
derived from PAHP are a successful method for improving
transparency and remaining properties of poly(ester imide)s.
[24] C. P. Yang, J. M. Wang, Y. Y. Su and S. H. Hsiao, Macromol.
Chem. Phys., 2006, 207, 1049-1061.
[25] C. P. Yang and Y. Y. Su, J. Polym. Sci. Part A: Polym. Chem.,
2006, 44, 3140-3152.
Acknowledgements
[26] K. W. Lienert, Adv. Polym. Sci., 1999, 141, 45-82.
[27] M. Hasegawa and A. Tominaga, Macromol. Mater. Engng.,
2011, 296, 1002-1017.
This work was financially supported by National High Technology
Research and Development Program of China (No. 2015AA033404),
National Natural Science Foundation of China (Grant no. 51403225),
Natural Science Foundation of Ningbo City, China (No.
2015A610013 and 2012A610149) and the program for Ningbo
Municipal Science and Technology Innovative Research Team
(Grant No. 2015B11002).
[28] M. Hasegawa, Y. Sakamoto, Y. Tanaka and Y. Kobayashi, Eur.
Polym. J., 2010, 46, 1510-1524.
[29] M. Hasegawa and K. Koseki, High. Perform. Polym., 2006, 18,
697-717.
[30] Y. S. Jeng, H. H. Pei and S. W. Chun, J. Appl. Polym. Sci., 2004,
94, 730 –738.
Notes and references
[1] M. K. Ghosh and K. L. Mittal, Polyimides: fundamentals and
applications. Marcel Dekker, Marcel Dekker, New York, 1996.
[2] C. Feger, M. M. Khojasteh and S. E. Molis, Polyimides: trends in
materials and applications. New York: Society of Plastic Engineers,
1996.
[31] M. Hasegawa, Y. Tsujimura, K. Koseki and T. Miyazaki, Polym. J.,
2008, 40, 56–67.
[32] M. X. Ding, Prog. Polym. Sci., 2007, 32, 623-668.
[33] C. P. Yang, Y. Y. Su and K. L. Wu, J. Polym. Res., 2005, 12, 257-
269.
[3] K. L. Mittal, Polyimides and other high temperature polymers:
synthesis, characterization and application. The Netherland, 2001.
[4] S. Ando, M. Ueda, M. Kakimoto, M. Kochi, T. Takeichi, M.
Hasegawa and R. Yokota, The Latest Polyimides: Fundamentals and
Applications, 2nd edition, NTS, Tokyo, 2010.
[34] H. B. Wei, G. F. Chen, L. J. Cao, Q. J. Zhang, Q. Yan and X. Z.
Fang, J. Mater. Chem. A., 2013, 1, 10412-10421.
[35] M. J. Frisch, Revision A. 1, Gaussian, Inc., Wallingford, CT, 2009.
[36] C. T. Lee, W. T. Yang and R. G. Parr, Phys. Rev. B, 1988, 37, 785-
789.
[5] D. J. Liaw, K. L.Wang, Y. C. Huang, K. R. Lee, J. Y. Lai, and C. S. Ha,
Prog. Polym. Sci., 2012, 37, 907−974.
[6] M. Hasegawa and K. Horie, Prog. Polym. Sci., 2001, 26, 259-335.
[7] S. Ando, T. Matsuura and S. Sasaki, Polym. J., 1997, 29, 69-76.
[8] H. Suzuki, T. Abe, K. Takaishi, M. Narita and F. Hamada, J. Polym.
Sci. Part A: Polym. Chem., 2000, 38, 108–116.
[9] W. Volksen, H. J. Cha, M. I. Sanchez and D. Y. Yoon, React. Funct.
Polym. 1996, 30, 61–69.
[10] T. Matsumoto, Macromolecules, 1999, 32, 4933–4939.
[11] H. Seino, T. Sasaki, A. Mochizuki and M. Ueda, High. Perform.
Polym., 1999, 11, 255–262.
[12] A. S. Mathews, I. Kim and C. S. Ha, Macromol. Res., 2007, 15,
114–128.
[13] M. Hasegawa, K. Kasamatsu and K. Koseki, Eur. Polym. J., 2012,
48, 483–498.
[14] M. Hasegawa, D. Hirano, M. Fujii, M. Haga, E. Takezawa and S.
Yamaguchi, J. Polym. Sci. Part A: Polym. Chem., 2013, 51, 575–592.
[15] J. G. Liu, M. H. He, H. W. Zhou, Z. G. Qian, F. S. Wang, S. Y. Yang,
J. Polym. Sci. Part A: Polym. Chem., 2002, 40, 110-119.
[16] S. Ando, J. Potopolym. Sci. Technol. 2004, 17, 219-232.
[17] C. P. Yang, Y. Y. Su and Y. C. Chen, Eur. Polym. J., 2006, 42, 721-
732.
[18] C. P. Yang and Y. Y. Su, Polymer, 2005, 46, 5778-5788.
[19] C. P. Yang and M. Y. Hsu, J. Polym. Sci. Part A: Polym. Chem.,
2005, 43, 1270-1284.
[20] C. P. Yang, Y. Y. Su and S. H. Hsiao, J. Polym. Sci. Part A: Polym.
Chem., 2006, 44, 5909-5922.
[21] C. P. Yang, Y. Y. Su, S. J. Wen and S. H. Hsiao, Polymer, 2006, 47,
7021-7033.
[22] K. A. Watson, F. L. Palmieri and J. W. Connell, Macromolecules,
2002, 35, 4968-4974.
[23] C. L. Chung, C. P. Yang and S. H. Hsiao, J. Polym. Sci. Part A:
Polym. Chem., 2006, 44, 3092-3102.
8 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins