Colorless polyimides derived from 1S,2S,4R,5R-cyclohexanetetracarboxylic dianhydride, self-orientation behavior during solution casting, and their optoelectronic applications

Article abstract of DOI:10.1016/j.polymer.2014.07.032

A novel cycloaliphatic monomer for polyimides (PI), 1S,2S,4R,5R- cyclohexanetetracarboxylic dianhydride (H′-PMDA) is proposed in this work. H′-PMDA shows high polymerizability with various diamines in contrast to its isomer, i.e., conventional hydrogenated pyromellitic dianhydride (H-PMDA) and leads to highly flexible and colorless PI films with very high T _g's. In particular, the combinations with rigid structures of diamines give rise to PIs with significantly decreased coefficients of thermal expansion (CTE) owing to high extents of in-plane chain orientation induced by thermal imidization, whereas the H-PMDA-based counterparts do not. The decreased CTE reflects structural rigidity/linearity of the H′-PMDA-based diimide units as supported by liquid crystallinity observed in the corresponding model compound. Solution casting of a chemically imidized PI derived from H′-PMDA and 2,2′-bis(trifluoromethyl)benzidine (TFMB) results in a lower CTE than that of the thermally imidized counterpart, suggesting the presence of a self-orientation phenomenon during solvent evaporation. The mechanism is proposed in this work. H′-PMDA/TFMB and its copolymer systems can be useful as plastic substrates in image display devices and/or novel coating-type optical compensation films.

Full text of DOI:10.1016/j.polymer.2014.07.032

Polymer 55 (2014) 4693e4708
Contents lists available at ScienceDirect
Polymer
journal homepage: www.elsevier.com/locate/polymer
Colorless polyimides derived from 1S,2S,4R,5R-cyclohexanetetra
carboxylic dianhydride, self-orientation behavior during solution
casting, and their optoelectronic applications
,
Masatoshi Hasegawa a *, Mari Fujii a, Junichi Ishii a, Shinya Yamaguchi b,
Eiichiro Takezawa b, Takashi Kagayama b, Atsushi Ishikawa b
a Department of Chemistry, Faculty of Science, Toho University, 2-2-1 Miyama, Funabashi, Chiba 274-8510, Japan
b Iwatani Industrial Gases, Corp., 10 Otakasu-cho, Amagasaki, Hyogo 660-0842, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
A novel cycloaliphatic monomer for polyimides (PI), 1S,2S,4R,5R-cyclohexanetetracarboxylic dianhydride
(H0-PMDA) is proposed in this work. H0-PMDA shows high polymerizability with various diamines in
contrast to its isomer, i.e., conventional hydrogenated pyromellitic dianhydride (H-PMDA) and leads to
highly flexible and colorless PI films with very high Tg's. In particular, the combinations with rigid
structures of diamines give rise to PIs with significantly decreased coefficients of thermal expansion
(CTE) owing to high extents of in-plane chain orientation induced by thermal imidization, whereas the
H-PMDA-based counterparts do not. The decreased CTE reflects structural rigidity/linearity of the H0-
PMDA-based diimide units as supported by liquid crystallinity observed in the corresponding model
compound. Solution casting of a chemically imidized PI derived from H0-PMDA and 2,20-bis(tri-
fluoromethyl)benzidine (TFMB) results in a lower CTE than that of the thermally imidized counterpart,
suggesting the presence of a self-orientation phenomenon during solvent evaporation. The mechanism is
proposed in this work. H0-PMDA/TFMB and its copolymer systems can be useful as plastic substrates in
image display devices and/or novel coating-type optical compensation films.
Received 23 June 2014
Received in revised form
19 July 2014
Accepted 21 July 2014
Available online 28 July 2014
Keywords:
Colorless polyimide
1S,2S,4R,5R-cyclohexanetetracarboxylic
dianhydride
Low coefficient of thermal expansion (CTE)
© 2014 Elsevier Ltd. All rights reserved.
1. Introduction
sulfone) (PES) is known to possess the highest level of physical heat
resistance (Tg ¼ 225 ꢀC) among current colorless engineering
The importance of optically transparent high-temperature
polymeric materials has been increasing for their extensive appli-
cations as optical components, e.g., plastic substrates of image
display devices, liquid crystal alignment layers, color filters, optical
compensation films, optical fibers, light-guiding plates, and optical
lenses. One of the recent key projects is to replace current fragile
plastics, however, it is not enough for applications to the plastic
substrate materials. Polymeric materials with very high Tg
exceeding 300 ꢀC are practically limited to polyimide (PI) systems,
which are widely used in a variety of electronic devices for their
simple two-step manufacturing processes, excellent electric insu-
lation ability based on an extremely high purity (the absence of
volatile organic compounds and metallic/ionic contaminations),
non-flammability, and thermo-oxidative stability (long-term heat
resistance) in addition to considerably high Tg's overcoming the
solder-reflowing processes (short-term heat resistance) [1e13].
However, conventional aromatic PI films are in general not suitable
for optoelectronic applications because of their intensive coloration
arising from charge-transfer (CT) interactions [14].
inorganic glass substrates (300e700
by plastic substrates (<50 m thick), thereby the display devices
mm thick) in flat panel displays
m
become drastically light and flexible. However, it is very difficult to
obtain the substrate materials simultaneously possessing the
merits of inorganic glass (i.e., excellent optical transparency, ultra-
high heat resistance, and high dimensional stability against thermal
cycles undergoing in the device fabrication processes) and organic
polymeric systems (flexibility and processability). Poly(ether
Much effort has been devoted to erase the PI film coloration. The
most effective strategy is to use aliphatic monomers (usually,
cycloaliphatic ones for avoiding significant Tg decrease) either in
diamines or tetracarboxylic dianhydrides or both [15e23]. How-
ever, most of cycloaliphatic monomers do not lead to PI films with
* Corresponding author. Tel./fax: þ81 47 472 1869.
E-mail address: mhasegaw@chem.sci.toho-u.ac.jp (M. Hasegawa).
http://dx.doi.org/10.1016/j.polymer.2014.07.032
0032-3861/© 2014 Elsevier Ltd. All rights reserved.

4694
M. Hasegawa et al. / Polymer 55 (2014) 4693e4708
low linear coefficients of thermal expansion (CTE) indispensable for
good dimensional stability because their non-linear/non-planar
steric structures disturb the PI chain alignment along the film
plane (XeY) direction (called in-plane orientation) during the
thermal imidization process. Cycloaliphatic monomers with rela-
tively high structural linearity effective for low CTE generation are
limited to 1,2,3,4-cyclobutanetetracarboxylic dianhydride (CBDA)
and trans-1,4-cyclohexanediamine (t-CHDA) at the present time.
However, the use of aliphatic diamines such as t-CHDA causes the
formation of insoluble salt at the initial stage in the polymerization
process of PI precursors [poly(amic acid)s (PAAs)], by which the
reaction is completely inhibited in some cases or needs prolong
periods until the reaction mixtures are completely homogenized
[24,25]. In contrast, the combinations between cycloaliphatic tet-
racarboxylic dianhydrides and aromatic diamines can easily pro-
duce colorless PIs without such salt formation problem. CBDA is
also advantageous from the viewpoints of the polymerizability
with aromatic diamines [23e26], whereas conventional
cycloaliphatic tetracarboxylic dianhydrides [e.g., bicyclo[2.2.2]oct-
7-ene-2,3,5,6-tetracarboxylic dianhydride (BTA)] often lead to
insufficient molecular weights of PAAs without film-forming ability
[21,22,27]. However, CBDA-based PIs basically possess no solution-
processability [16]. Thus, cycloaliphatic tetracarboxylic dianhy-
drides suitable for obtaining practically useful plastic substrate
materials are very limited.
The present work proposes a novel cycloaliphatic tetracarbox-
ylic dianhydride with a relatively linear/planar structure, i.e.,
1S,2S,4R,5R-cyclohexanetetracarboxylic dianhydride (H0-PMDA)
[28] and illustrates some distinctive properties of H0-PMDA-based
PIs [29]. As the hydrogenated forms of pyromellitic dianhydride
(PMDA), two isomers have been so far reported: 1S,2R,4S,5R-
cyclohexanetetracarboxylic dianhydride (H-PMDA) [30] and
1R,2S,4S,5R-cyclohexanetetracarboxylic dianhydride (H00-PMDA)]
[31]. H0-PMDA is an additional stereoisomer. In the present work,
isomer effects on the polymerizability and the PI film properties are
also discussed. To the best of our knowledge, this is the earliest
Fig. 1. Molecular structures of monomers used in this work.

M. Hasegawa et al. / Polymer 55 (2014) 4693e4708
4695
report described in detail on the properties of H0-PMDA-based PIs
and their optoelectronic applications.
2. Experimental
2.1. Materials
The structures, abbreviations, sources, and pre-treatment con-
ditions of the monomers used in this work are summarized in Fig. 1
and Table 1. H0-PMDA was synthesized as follows [28]; PMDA was
first hydrolyzed with a NaOH aqueous solution. The pyromellitic
acid tetrasodium salt formed was hydrogenated in a high-pressure
hydrogen atmosphere at 160 ꢀC in the presence of a ruthenium
catalyst. After hydrogenation was completed, the solution was
additionally heated at a precisely controlled temperature for
several hours, and cooled to room temperature. The solution was
then neutralized by slowly adding conc. HCl. The white precipita-
tion yielded (tetracarboxylic acid, 1) was collected by filtration,
recrystallized from water, and dried in vacuum at 80 ꢀC for 5 h. A
small portion of 1 was quantitatively converted into tetramethyl
ester to analyze the purity of 1 by gas chromatography (GC). The GC
chart for the tetramethyl ester of 1 showed a single peak at a
different elution time from that of the tetramethyl ester counter-
part derived from conventional H-PMDA, suggesting the difference
of the steric structures. The tetracarboxylic acid (1) was finally
dehydrated with acetic anhydride (Ac2O) to form tetracarboxylic
dianhydride (2). Product 2 was difficult to form a sufficient size of
single crystal by recrystallization from Ac2O for the X-ray structural
analysis. Instead, the corresponding tetracarboxylic acid (1) was
used for this purpose. The results revealed that product 1 is
1S,2S,4R,5R-cyclohexanetetracarboxylic acid with a chair form of
Fig. 2. Steric structure of 1,2,4,5-cyclohexanetetracarboxylic acid (H0-PMDA precursor)
determined by the single-crystal X-ray diffraction measurements.
the central cyclohexane unit as depicted in Fig. 2 [32]. Fig. 3 shows
the powder X-ray diffraction patterns for 1 and a compound yielded
by hydrolyzing 2 with water. A good agreement between them
suggests that the steric structure of 1 is maintained even after
dehydration with Ac2O. Therefore, product 2 can be identified as
1S,2S,4R,5R-cyclohexanetetracarboxylic dianhydride (H0-PMDA).
The most probable steric structure of H0-PMDA is shown in Fig. 4 (c)
together with those of H-PMDA and H00-PMDA which were directly
determined by the single-crystal X-ray diffraction measurements
[30,31].
PAAs were prepared by equimolar polyaddition of tetracarbox-
ylic dianhydrides and diamines as shown in Fig. 5. A typical pro-
cedure is as follows; tetracarboxylic dianhydride powder
(10 mmol) was added in a dry N,N-dimethylacetamide (DMAc) or
Table 1
The conditions of pre-treatment and purification for monomers.
Monomer
Source
Solvent for
recrystallization
Vacuum-drying
condition
Melting
point (ꢀC)
1S,2S,4R,5R-Cyclohexanetetracarboxylic
dianhydride (H0-PMDA)
1R,2S,4S,5R-Cyclohexanetetracarboxylic
dianhydride (H00-PMDA)
1S,2R,4S,5R-Cyclohexanetetracarboxylic
dianhydride (H-PMDA)
Bicyclo[2.2.2]oct-7-ene-2,3,5,6- tetracarboxylic
dianhydride (BTA)
1,2,3,4-Cyclobutanetetracarboxylic
dianhydride (CBDA)
Iwatani Industrial Gases
Iwatani Industrial Gases
New Japan Chemical
e
150 ꢀC/24 h
150 ꢀC/24 h
150 ꢀC/24 h
150 ꢀC/24 h
150 ꢀC/12 h
274 (284a)
267 (273a)
303
e
e
Tokyo Chemical Industry (TCI)
Nissan Chemical Industries
Ac2O/DMF (3/1, v/v)
e
e
241
p-Phenylenediamine (p-PDA)
TCI
Ethyl acetate
Toluene/DMF (10/1, v/v)
e
30 ꢀC/24 h
50 ꢀC/24 h
50 ꢀC/24 h
145 ꢀC/12 h
142
191
74
4,40-Oxydianiline (4,40-ODA)
Wako Chemical
JFE Chemical
Ihara Chemical
3,40-Oxydianiline (3,40-ODA)
Bis(4-amino-2-trifluoromethylphenyl)
ether (TFODA)
EtOH/H2O (11/5, v/v)
129
m-Tolidine (m-TOL)
o-Tolidine (o-TOL)
Wakayama Seika
Wakayama Seika
Wakayama Seika
Wakayama Seika
Wakayama Seika
TCI
Toluene/CHX (7/3, v/v)
Toluene
70 ꢀC/24 h
80 ꢀC/24 h
50 ꢀC/24 h
100 ꢀC/24 h
50 ꢀC/24 h
50 ꢀC/24 h
50 ꢀC/24 h
50 ꢀC/24 h
86
129
184
172
185
205
197
129
2,20-Bis(trifluoromethyl)benzidine (TFMB)
1,4-Bis(4-aminophenoxy)benzene (TPEQ)
4-Aminophenyl-40-aminobenzoate (APAB)
4,40-Diaminobenzanilide (DABA)
4,40-Bis(4-aminophenoxy)biphenyl (BAPB)
2,2-Bis[4-(4-aminophenoxy)phenyl]
propane (BAPP)
e
Toluene/DMF (15/1, v/v)
e
e
Wakayama Seika
Wakayama Seika
Toluene/DMF (15/1, v/v)
e
2,2-Bis[4-(4-aminophenoxy)phenyl]
hexafluoropropane (HFBAPP)
Wakayama Seika
e
50 ꢀC/24 h
159
Trans-1,4-cyclohexanediamine (t-CHDA)
4,40-Methylenebis(cyclohexylamine)
(MBCHA) (mixture of cis/trans isomers)
4,40-Methylenebis(2-methylcyclohexylamine)
(M-MBCHA) (mixture of cis/trans isomers)
Iwatani Industrial Gases
New Japan Chemical
e
e
30 ꢀC/24 h
e
70
none
Aldrich
e
e
e
DMF ¼ N,N-dimethylformamide. CHX ¼ cyclohexane.
a
Compound repeatedly recrystallized from acetic anhydride (Ac2O).

4696
M. Hasegawa et al. / Polymer 55 (2014) 4693e4708
for 72 h). In this case, if necessary, the reaction mixture was grad-
ually diluted with a minimum quantity of the same solvent to
ensure effective magnetic stirring. The polymerization conditions
are listed in Table 2. The formation of PAAs was confirmed by the
transmission-mode FT-IR spectroscopy (Jasco, FT/IR 4100 infrared
spectrometer) using separately prepared thin cast films (4e5 mm
thick) with non-uniform thickness to erase interference fringes. A
typical FT-IR spectrum is shown in Fig. 6 (a). The spectrum includes
some specific bands (cmꢁ1): 3318 (amide, NeH), 3119 (CaromeH),
2948 (CalipheH), ~2600/1715 (hydrogen-bonded carboxylic acid,
OeH/C]O), 1675/1535 (amide, C]O), 1489 (1,4-phenylene), and
1323 (CeF).
PI films were prepared by different methods (thermal and
chemical imidization). The reaction schemes are shown in Fig. 5. PI
films were prepared upon thermal imidization as follows; PAA
solution was bar-coated on a glass substrate and dried at 60 ꢀC/2 h
for DMAc solutions (80 ꢀC/2 h for NMP solutions) in an air-
convection oven. We first attempted thermal imidization under a
relatively mild condition (e.g., at 250 ꢀC in vacuum) for suppressing
PI film coloration. However, the PI films obtained were somewhat
brittle in many cases. This is probably due to partial retro-
polyaddition although the generated terminal functional groups
can recombine by additional heating at higher temperatures [33].
Then, to obtain flexible PI films while avoiding film coloration,
thermal imidization was carried out typically at 200 ꢀC/
20min þ 250 ꢀC/30min þ 320 (or 300) ꢀC/1 h in vacuum as adhered
on the substrate, and successively annealed in vacuum at 10e20 ꢀC
lower temperatures than the Tg's after peeling them off from the
substrate to eliminate residual stress without undesirable signifi-
cant film deformation. The imidization and annealing conditions
were properly adjusted to obtain better quality of PI films. The
thermally imidized film samples are represented as “(T)” from now
on. On the other hand, chemical imidization was conducted by
gradually adding a mixed solution of Ac2O/pyridine (7/3, v/v) to
adequately diluted PAA solutions at a fixed molar ratio of [Ac2O]/
[COOH]PAA ¼ 5 with continuous vigorous stirring at room temper-
ature. The reaction mixture was additionally stirred at room tem-
perature for 12 h in a sealed bottle while carefully monitoring the
solution homogeneity. After the chemical imidization process, the
homogeneous reaction mixture was adequately diluted with the
same solvent, and very slowly poured into a large quantity of
methanol or water as a poor solvent. The fibrous white precipitate
obtained was repeatedly washed with methanol/water, collected
by filtration, and dried at 80 ꢀC in vacuum for 12 h. The excess of the
chemical imidization reagent is completely eliminated on this
procedure, which is important for avoiding undesirable film
coloration as observed in some cases. The dried precipitate was re-
dissolved in a fresh solvent [typically, cyclopentanone (CPN)] at a
solid content of 10 e 15 wt%. The homogeneous PI solution formed
was coated on a glass substrate and dried typically at 60 ꢀC/
2 h þ 150 ꢀC/2 h on the substrate, and at 200 ꢀC/1 h in vacuum
without the substrate. The PI films were additionally annealed at
Fig. 3. Powder X-ray diffraction patterns of 1,2,4,5-cyclohexanetetracarboxylic acid: (a)
compound before dehydration (H0-PMDA precursor) and (b) compound after hydro-
lysis of H0-PMDA.
N-methyl-2-pyrrolidone (NMP) solution of diamine (10 mmol) at
an initial total solid content of 30 wt% with continuous stirring at
room temperature. The reaction mixture was stirred with a mag-
netic spinner at room temperature in a sealed bottle until it be-
comes homogeneous with a maximum solution viscosity (typically
(a)
(b)
(c)
Fig. 4. Steric structures of hydrogenated PMDA isomers: (a) H-PMDA, (b) H00-PMDA, and (c) H0-PMDA.

M. Hasegawa et al. / Polymer 55 (2014) 4693e4708
4697
Fig. 5. Reaction schemes of polyaddition and imidization of PAAs and predicted end-capping reaction of terminal amino groups during chemical imidization process.
established temperatures. The film or powder samples prepared via
the homogeneous chemical imidization process are denoted as
“(C)”. This process was not applicable to some systems, where
inhomogenization (gelation/precipitation) occurs by adding the
chemical imidization reagent owing to insufficient solubility of the
imidized forms, because imidization is not completed. For the H0-
PMDA/p-phenylenediamine (p-PDA) system, the PAA film formed
on substrates was dipped in an Ac2O/pyridine (7/3, v/v) solution at
room temperature for 24 h, rinsed with methanol, then annealed at
350 ꢀC for 1 h in vacuum.
appearance of some specific bands (cmꢁ1): 3087 (CaromeH), 2939/
2874 (CalipheH), 1788/1719 (imide, C]O), 1491 (1,4-phenylene),
1375 (imide, NeCarom), and 1312 (CeF).
In this work, the chemical compositions of PAA and PI systems
are represented with the abbreviations of the monomer compo-
nents used [tetracarboxylic dianhydrides (A) and diamines (B)] as
A/B for homopolymers and A1;A2/B1;B2 for copolymers.
2.2. Measurements
For soluble PI systems, the completion of imidization was
confirmed from perfect disappearance of the proton signals for the
2.2.1. Inherent viscosities and molecular weights
The reduced viscosities (hred) of PAAs were measured at a solid
content of 0.5 wt% at 30 ꢀC on an Ostwald viscometer. The mea-
surements were conducted as prompt as possible to avoid the
depolymerization of PAAs, which immediately proceeds by dilu-
tion. An extrapolation to zero concentration for determination of
the inherent viscosities (hinh) is difficult for PAAs owing to an
abnormal increase in the hred values in a low concentration range,
which arises from a polyelectrolyte effect of PAAs. Instead, the hred
values were measured at 0.5 wt%, which can be practically regarded
as hinh as often similarly dealt with in the literature. The number-
(Mn) and weight-average molecular weights (Mw) for highly soluble
PIs were determined by the gel permeation chromatography (GPC)
using tetrahydrofuran (THF) as an eluent at room temperature on a
Jasco, LC-2000 Plus HPLC system with a GPC column (Shodex, KF-
806L) and an ultraviolet-visible detector (Jasco, UV-2075, detec-
tion wavelength: 300 nm) at a flow rate of 1 mL minꢁ1. The cali-
bration was carried out with standard polystyrenes (Shodex, SM-
105).
NHCO [
COOH groups (
d ~ 10.0 ppm in dimethyl sulfoxide (DMSO)-d6] and the
d
¼ 12e13 ppm) in the 1H NMR spectra. Full imid-
ization was also confirmed from the FT-IR spectra of separately
prepared thin films as typically shown in Fig. 6 (b) on the basis of
the disappearance of the PAA-inherent IR bands and the
Table 2
Results of polyaddition of H0-PMDA with various diamines.
Category of
diamine
System Diamine
no.
Solid content
(initial/final)
(wt%)
Solvent hinh
(PAA)
(dL gꢁ1
)
Aromatic-
flexible (I)
1
2
3
4
5
6
7
4,40-ODA
TPEQ
BAPP
30 / 16.9
30 / 15.4
30 / 14.4
30 / 18.8
30 / 15.3
30 / 19
30 / 8.8
30 / 12.2
30 / 10
DMAc
DMAc
DMAc
DMAc
DMAc
DMAc
DMAc
NMP
1.52
1.84
1.84
1.44
1.49
1.10
ea
BAPS
TFODA
HFBAPP
p-PDA
Aromatic-rigid (II)
1.60
eb
2.2.2. Optical properties
8
o-TOL
DMAc
and
The light transmission spectra of PI films (typically 20
were measured on an ultraviolet-visible spectrophotometer (Jasco,
V-530) in the wavelength ( ) range of 200e800 nm. The light
transmittance at 400 nm (T400) and the cut-off wavelength (lcut at
mm thick)
NMP
9
10
11
12
13
14
15
m-TOL
APAB
DABA
TFMB
MBCHA
M-MBCHA 30 / 12.8
t-CHDA
t-CHDAd
t-CHDAe
t-CHDAf
30 / 10.2
30 / 12.6
30 / 15
30 / 13
30 / 17.6
DMAc
DMAc
DMAc
DMAc
DMAc
DMAc
NMP
NMP
DMAc
DMAc
3.24
0.94
0.88
2.43
0.67
0.95
ec
l
,
l
T ~ 0%) were determined from the spectra. The yellowness indices
(YI, ASTM E 313) for the PI films were also calculated on the basis of
the spectra under a standard illuminant of D65 and a standard
observer function of 2ꢀ on a Jasco color calculation software from
the relation:
Cycloaliphatic-
flexible (III)
Cycloaliphatic-
rigid (IV)
20 / 18
15 / 6.8
15 / 9.7
15
ea
ec
ec
YI ¼ 100ð1:2985x ꢁ 1:1335zÞ=y
a
Gelation.
Inhomogeneous mixture.
Formation of insoluble salt.
100% Silylated t-CHDA with N,O-bis(trimethylsilyl)trifluoroacetamide (BSTFA).
50% Silylated t-CHDA with BSTFA.
Mixture of t-CHDA and acetic acid.
b
where x, y, and z are the CIE tristimulus values (CIE: International
Commission on Illumination). The YI value takes null for an ideal
white/transparent sample. The total light transmittance (Ttot, JIS K
7361e1) and the diffuse transmittance (Tdiff, JIS K 7136) of PI films
c
d
e
f

4698
M. Hasegawa et al. / Polymer 55 (2014) 4693e4708
The εopt values are known to be approximate to the dielectric
constants directly determined with a precision LCR meter at a
frequency of 1 or 10 MHz for PI systems [25,34].
D
nth was also measured as a function of wavelength in the
visible range on an Abbe refractometer (Atago, 1T, nD range:
1.30e1.70). Monochromatic light was introduced to the refrac-
tometer from a Xe lamp source in a fluorescence spectrometer
(Hitachi, F-2000) with a 10 nm bandpass of a monochromator
through an optical fiber cable. In this case, the color compensation
dial of the refractometer was set at 30 for measuring at different
wavelengths from the D-line. The raw data were corrected using a
formula supplied by Atago Co., Ltd.
2.2.3. Linear coefficient of thermal expansion
The CTE along the XeY direction for PI specimens (15 mm long,
5 mm wide, and typically 20 mm thick) in the glassy region was
measured by thermomechanical analysis (TMA) as aꢁn1average in
the range of 100e200 ꢀC at a heating rate of 5 K min on a ther-
momechanical analyzer (Bruker-AXS, TMA 4000) with a fixed load
(0.5 g per a unit film thickness in mm, typically 10 g load for 20 mm-
thick films) in a dry nitrogen atmosphere. In this case, after the
preliminary first heating run up to 120 ꢀC and successive cooling to
room temperature in the TMA chamber, the data were collected
from the second heating run for removing an influence of adsorbed
water.
2.2.4. Glass transition temperature
The storage modulus (E0) and the loss energy (E00) of PI films
were measured by the dynamic mechanical analysis (DMA) at a
heating rate of 5 K minꢁ1 on the same TMA instrument. The mea-
surements were conducted at a sinusoidal load frequency of 0.1 Hz
with an amplitude of 15 gf in a nitrogen atmosphere. The Tg's of PI
films were determined from a peak temperature of the E00 curve. In
this instrument, the heating runs undergo an automatic stop when
the irreversible elongation
(DL) of the specimens exceeded
3000 m by softening above the Tg's.
m
Fig. 6. FT-IR spectra of thin films for H0-PMDA/TFMB: (a) PAA and (b) PI.
2.2.5. Thermal stability at elevated temperatures
Thermal stability of PI films was evaluated from the 5% weight
loss temperatures (T5d) by thermogravimetric analysis (TGA) on a
thermo-balance (Bruker-AXS, TG-DTA2000). TGA was performed at
a heating rate of 10 K minꢁ1 in nitrogen or air atmosphere. For the
measurements in N2, the preliminary first heating run up to 150 ꢀC
was carried out to eliminate adsorbed water, and the samples were
cooled to room temperature while mounting in the sample
chamber under a continuous dry N2 flow. After the data resetting to
0% weight loss, TGA data were collected from the second heating
run.
were measured on a double-beam haze meter equipped with an
integrating sphere (Nippon Denshoku Industries, NDH 4000). Haze
(turbidity) of PI films was calculated from the relation:
ꢀ
.
ꢁ
Haze ¼ Tdiff Ttot ꢂ 100
The in-plane (nin or nx ¼ ny) and out-of-plane (nout or nz)
refractive indices of PI films were measured with a sodium lamp at
589.3 nm (D-line) on an Abbe refractometer (Atago, 4T, nD range:
1.47e1.87) equipped with a polarizer by using a contact liquid
(sulfur-saturated methylene iodide with nD ¼ 1.78e1.80 or 1-
bromonaphthalene with nD ¼ 1.658 when the samples were
swollen by CH2I2) and a test piece (nD ¼ 1.92). The birefringence of
PI films, which represents a relative extent of chain alignment along
the film plane (XeY) direction, was calculated from the relation:
2.2.6. Mechanical properties
The tensile modulus (E), tensile strength (sb), and elongation at
break (εb) of PI specimens (30 mm long, 3 mm wide, typically 20 mm
thick) were measured on a mechanical testing machine (A & D,
Tensilon UTM-II) at a cross head speed of 8 mm minꢁ1 at room
temperature. The data were averaged for 10e15 samples. In this
case, the specimens were taken from high-quality large film sam-
ples (10 cm ꢂ 10 cm) without any defects such as fine bubbles. For
some systems, it was difficult to carry out the mechanical stretching
test because of film brittleness or the difficulty of high-quality large
film preparation.
Dnth ¼ nin ꢁ nout
The dielectric constants were estimated from the optical data on
the basis of an empirical relation:
εopt ¼ 1:1n2av
2.2.7. Solubility
where nav denotes the average refractive indices expressed as
nav ¼ ð2nin þ noutÞ=3
The solubility of thermally imidized film or chemically imidized
powder samples was evaluated by observing the immersed sam-
ples (10 mg) in various solvents (1 mL) in a test tube.

M. Hasegawa et al. / Polymer 55 (2014) 4693e4708
4699
2.2.8. Heat of combustion
tetracarboxylic dianhydrides tends to increase as follows: BTA < H-
PMDA << H0-PMDA < CBDA.
The heats of combustion Qc (gross calorific value) for three hy-
drogenated PMDA isomers were measured by a custom analysis
center (Ibiden Engineering Co.) on an advanced adiabatic bomb
calorimeter (Yoshida Seisakusho Co., JIS M8814) calibrated with
benzoic acid as a standard sample. In this case, the Qc values
A strained structure of the five-membered acid anhydride rings
in H0-PMDA may also contribute to its enhanced polymerizability.
The bomb calorimetry determined:
D
Uc ¼ ꢁ1.87 ꢂ 104 J gꢁ1 for H0-
PMDA, e1.82 ꢂ 104 J gꢁ1 for H-PMDA, and ꢁ1.80 ꢂ 104 J gꢁ1 for H00-
correspond to the internal energy change of combustion (
enthalpy change of combustion ( Hc) was calculated from the
relation: Hc V z Uc ng$RT (P is pressure,
Uc þ P
volume change, R the gas constant, and T absolute temperature),
ng (in mol) is a change in the amount of substance for the gaseous
phases in the following combustion reaction: C10H8O6 (s) þ 9 O2
(g) ¼ 10 CO2 (g) þ 4H2O (l) ( ng ¼ 1 in the present case). To compare
thermodynamic stability of three hydrogenated PMDA isomers,
each standard enthalpy of formation at 25 ꢀC ( Hfꢀ298) for the
stereoisomers was estimated from the thermochemical equations
with the Hc values of the stereoisomers, graphite, and H2 (g).
D
Uc). The
PMDA. Using these experimental data, the heats of formation were
D
estimated:
D
Hfꢀ
¼
ꢁ8.88
ꢂ
102 kJ molꢁ1 for H0-PMDA,
298
D
¼
D
D
D
þ
D
DV
e1.00 ꢂ 103 kJ molꢁ1 for H-PMDA, and ꢁ1.05 ꢂ 103 kJ molꢁ1 for H00-
PMDA. The results suggest that H0-PMDA is somewhat thermody-
namically less stable than the other isomers, probably relating to a
strained structure of the former.
D
D
Table 2 lists the results of the polyaddition between H0-PMDA
and different categories of diamines. In the aromatic diamine sys-
tems, the reactions smoothly proceeded and led to homogeneous/
viscous PAA solutions with high hinh values (0.9e3.2 dL gꢁ1) except
for the cases using p-PDA and o-tolidine (o-TOL). The reactions
between H0-PMDA and these rigid diamines in DMAc disturbed the
formation of homogeneous PAA solutions. This is probably attrib-
uted to partial precipitation of the initially formed rigid amic acid
oligomers with poor solubility. The PAA chains for H0-PMDA/p-PDA
most likely possess a mixed sequence of cis-1,3- and trans-1,4-
cychohexanedicarboxamide units, in which the central cyclo-
hexane moieties take a chair form (Fig. 7), as suggested from the
fact that PMDA/diamine systems provide PAAs with a composition
D
D
2.2.9. Liquid crystallinity
Liquid crystallinity of low molecular weight model compounds
was examined on an Olympus BX51 polarizing optical microscope
(POM) equipped with a digital camera (Nikon Coolpix 950) and a
temperature-controllable hot stage (Mettler Toledo, FP82HT hot
stage and FP 90 central processor).
of
meta(60)/para(40)
[36].
Therefore,
the
trans-1,4-
3. Results and discussion
cychohexanedicarboxamide units greatly contribute to enhancing
the main chain rigidity, as a result, decreasing the PAA solubility. On
the other hand, neither the PAA counterpart from H-PMDA nor H00-
PMDA contains such rigid structural unit, corresponding to the fact
that these isomers cause no precipitation problem during the
polymerization process even when very rigid diamines such as p-
PDA were used. A similar precipitation problem as in the H0-PMDA/
p-PDA system was observed in a comparative reaction system, i.e.,
3.1. Polymerizability of H0-PMDA with diamines and isomer effect
The polyaddition reactivity of tetracarboxylic dianhydrides with
diamines can be represented by the second-order rate constants in
a homogeneous state. However, the PAA polymerization actually
proceeds in an inhomogeneous mixture composed of tetracarbox-
ylic dianhydride powder fed into diamine solutions. Thus, instead
of the rate constants, we simply compared the polymerizability of
three hydrogenated PMDA isomers on the basis of the hinh values of
the finally obtained PAAs in the present work. This parameter is
practically useful because it is closely related to the PI film flexi-
bility as often represented by the elongation at break (εb) [35]; in
many cases, the film specimens become very brittle at both stages
of PAA and PI when the hinh values are rather low (empirically
speaking, < ~ 0.3 dL gꢁ1) owing to poor chain entanglement. For
example, the BTA/4,40-ODA system leads to a PAA without film-
polymerization of
a
polyamide from pure trans-1,4-
cyclohexanedicarbonyl chloride (t-1,4-CHDC) and TFMB in DMAc
in the presence of pyridine as an acid acceptor. In contrast, the use
of trans(24)/cis(76) mixture of 1,4-CHDC, instead of pure t-CHDC,
allowed the formation of a homogeneous polyamide solution. This
also supports that the trans-1,4-cychohexanedicarboxamide moi-
ety [Fig. 7 (a)] behaves indeed as a very rigid structural unit. In the
present work, a pronounced solvent effect was observed in the H0-
PMDA/p-PDA system; the reaction mixture was homogenized in
NMP in contrast to DMAc and provided a PAA with a sufficiently
high hinh value of 1.6 dL gꢁ1. This is probably attributed to better
affinity between the PAA and NMP (namely, higher solubility) than
forming ability as
a result of a rather low hinh value
(0.19e0.38 dL gꢁ1) [21,22,27], which is attributed to insufficient
polymerizability of BTA. The use of H-PMDA instead of BTA slightly
increases the hinh value (0.57 dL gꢁ1). The insufficient polymer-
izability of BTA and H-PMDA can be explained in terms of self-steric
hindrance during the polymerization, which is related to an all-exo
configuration for the four C]O groups; once one functional group
reacted with a terminal amino group, the another surviving func-
tional group may undergo steric hindrance arising from “covering/
shielding” by the adjacent growing chain, consequently, the
approaching/attack of other terminal amino groups to the
remaining functional is disturbed [22]. On the other hand, the re-
action of H0-PMDA and 4,40-ODA gave rise to a sufficiently high hinh
of 1.52 dL gꢁ1, which is somewhat lower than hinh ¼ 2.26 dL gꢁ1 for
CBDA/4,40-ODA. The strikingly improved polymerizability of H0-
PMDA compared to H-PMDA probably results from a unique
configuration of H0-PMDA as depicted in Fig. 4 (c) where the
functional groups orient almost opposite directions each other as in
PMDA and CBDA, thereby the self-steric hindrance mentioned
above becomes less effective. Thus, when 4,40-ODA was used as a
fixed diamine, the hinh-based polymerizability of the cycloaliphatic
Fig. 7. Structures of cyclohexanediamide units in the PAA main chains for H0-PMDA/p-
PDA
system:
(a)
trans-1,4-cyclohexanedicarboxamide
and
(b)
cis-1,3-
cyclohexanedicarboxamide.

4700
M. Hasegawa et al. / Polymer 55 (2014) 4693e4708
PAA/DMAc. However, even the use of NMP was less effective to
obtain a homogeneous PAA solution for the H0-PMDA/o-TOL
system.
silylation method [37] nor the addition of acetic acid [38] was
successful. The success/failure of the reactions between t-CHDA
and various tetracarboxylic dianhydrides can be roughly classified
into three categories as shown in Fig. 8; in Group 1, the initially
formed salt gradually dissolves just by stirring at room tempera-
ture, and homogeneous PAA solutions are formed after stirring for
several days. The tetracarboxylic dianhydrides of this group (e.g., H-
PMDA and H00-PMDA) commonly consist of non-planar/non-linear
structures. In Group 2, the reaction mixtures including the salt
become homogeneous by heating at 80e120 ꢀC for a short period.
The tetracarboxylic dianhydrides of this group possess relatively
rigid crank-shaft-like structures. In Group 3, once the salt was
initially formed, it does not dissolve any longer even though the
reaction mixtures were diluted and heated. The tetracarboxylic
dianhydrides of this group consist of “rod-like” structures. The
classification of Fig. 8 suggests that the insolubility of the initially
formed salt is closely related to the structural rigidity of the tetra-
carboxylic dianhydride monomers used, consequently, overall ri-
gidity of the low molecular weight amic acids formed in the initial
stage. The present work revealed that H0-PMDA belongs to Group 3
as shown in Table 2, suggesting that it behaves in the reaction with
t-CHDA as an extremely rigid monomer like as PMDA and 2,3,6,7-
naphthalenetetracarboxylic dianhydride (NTDA) [39].
TFMB is somewhat inferior in the reactivity to common aromatic
diamines such as 4,40-ODA owing to the presence of the electron-
withdrawing CF3 substituents in TFMB. Nonetheless, the reaction
between H0-PMDA and TFMB led to a PAA with a high molecular
weight enough for flexible film formation [Mw ¼ 1.2 ꢂ 105,
Mn ¼ 5.2 ꢂ 104 for the chemically imidized powder sample with hinh
(PI) ¼ 1.23 dL gꢁ1] in contrast to the absence of film-forming ability
for the isomeric H-PMDA/TFMB system. The use of specially purified
H0-PMDA by repeated recrystallization produced a PAA with an
extremely high hinh of 9.04 dL gꢁ1 as a maximum value.
The reactions between H0-PMDA and flexible cycloaliphatic di-
amines (category III) caused salt formation in the initial stage as
generally observed when aliphatic diamines were used. However,
the reaction mixtures were finally homogenized by gradual disso-
lution of the salt during stirring at room temperature for several
days. The system derived from H0-PMDA and t-CHDA (category IV)
is greatly expected to display valuable combined properties (low
CTE, transparency, and low birefringence as discussed later).
However in fact, the polymerization was completely inhibited by
the formation of quite insoluble salt. In this system, neither the
Fig. 8. Classification based on polymerizability between t-CHDA and various tetracarboxylic dianhydrides in DMAc.

M. Hasegawa et al. / Polymer 55 (2014) 4693e4708
4701
Even when ether-linked flexible aromatic diamines (category I)
were chosen, the resultant PI films maintained still high Tg ranging
280e310 ꢀC. The results are probably related to the structural ri-
gidity/planarity of the H0-PMDA-based diimide (H0-PMDI) moieties
in the PI chains, which probably contributes to intermolecular face-
to-face stacking between the diimide units for activating the
dipoleedipole interactions between the imide C]O groups [40,41].
As suggested from the steric structure of H0-PMDA monomer in
Fig. 4 (c), the molecular planarity of the H0-PMDI unit must be
rather high, although it is somewhat inferior to that of aromatic
PMDI. The impact of the diimide molecular planarity on the Tg is
illustrated in the following comparisons: case (1) Tg ¼ 256 ꢀC for H0-
PMDA/BAPP vs. 287 ꢀC for PMDA/BAPP [42] (
D
Tg ¼ 31 ꢀC), and
Fig. 9. Comparisons of hinh-based polymerizability for hydrogenated PMDA isomers
with various diamines: (left/black bar) H-PMDA, (center/red bar) H0-PMDA, and (right/
blue bar) H00-PMDA systems. The absence of center/red bars represents the systems in
which homogeneous PAA solutions were not obtained (#8 and #15). (For interpreta-
tion of the references to color in this figure legend, the reader is referred to the web
version of this article.)
case (2) Tg ¼ 287 ꢀC for H0-PMDA/HFBAPP vs. 327 ꢀC for PMDA/
HFBAPP [42] (
D
Tg ¼ 40 ꢀC).
Fig.10 shows an isomer effect on the Tg's for the PIs derived from
three hydrogenated PMDA isomers. The H0-PMDA-based PIs tend to
possess comparable to (or slightly higher Tg's than) those of the H00-
PMDA-based counterparts except for some diamine systems. The
results also suggest that H-PMDA is the most effective for
enhancing the Tg among these isomers. This predominance may be
related to the molecular shape of the H-PMDI unit just like a “dish”,
which is more suitable for stacking. Thus, the order of Tg can be
represented in the following: H00-PMDA < (z) H0-PMDA < H-PMDA.
As shown in Table 3, the H0-PMDA-based PIs derived from aro-
matic diamines also possess relatively good thermal stability
(chemical heat resistance) as suggested from the T5d values (in N2)
exceeding 450 ꢀC, although a decrease in T5d is inevitable when
cycloaliphatic diamines were used. The H0-PMDA-based PIs still
maintain high T5d values even in a thermo-oxidative atmosphere (in
air), e.g., 451 ꢀC for H0-PMDA/4,40-ODA.
We compared the polymerizability for three hydrogenated
PMDA isomers as shown in Fig. 9. When ether-linked flexible aro-
matic diamines (category I) were used, the hinh-based polymer-
izability tends to increase as follows: H-PMDA << H0-PMDA < H00-
PMDA except for some diamines (#4 and #5). When a less reactive
sulfone-containing diamine (BAPS, #4) was used, H0-PMDA was
specifically more effective for enhancing hinh than H00-PMDA. On
the other hand, in the reactions with rigid aromatic diamines
(category II), the polymerizability of H0-PMDA was roughly com-
parable to that of H00-PMDA except for the o-TOL system (#8). H0-
PMDA was somewhat superior to H00-PMDA in the polymerizability
with very rigid diamines (p-PDA and m-TOL). This feature is
desirable for our current aim addressed to lowering CTE as dis-
cussed later.
3.2.2. Thermal expansion properties
One of our greatest interests concerns the dimensional stability
of PI films as represented by CTE. The CTE values of the thermally
imidized H0-PMDA-based PI films (T) are summarized in Table 3.
The PI systems derived from flexible ether-containing diamines
(category I), as well as methylene-linked cycloaliphatic diamines
(mixture of cis/trans isomers, category III), show a common level of
CTE values ranging 50e62 ppm Kꢁ1. The results are attributed to a
3.2. Properties of H0-PMDA-based PIs and isomer effects
3.2.1. Heat resistance
The Tg values of the H0-PMDA-based PI films are listed in Table 3.
The systems obtained from H0-PMDA and rigid aromatic diamines
(category II) showed considerably high Tg ranging 340e420 ꢀC.
Table 3
Film properties of H0-PMDA -based PIs.
No.
Diamine
Curea
T400 (%)
lcut (nm)
Dnth
εopt
Tg (ꢀC)
CTE (ppm/K)
E (GPa)
εb ave/max (%)
sb (MPa)
T5d (N2) (ꢀC)
T5d (air) (ꢀC)
1
2
3
4
5
4,40-ODA
TPEQ
BAPP
BAPS
TFODA
T
T
T
T
T
C
T
C
T
D
T
T
T
C
76.0
79.4
79.0
80.5
74.8
75.8
78.9
87.7
64.6
73.6
78.6
77.7
71.8
90.3
294
298
294
303
298
300
295
286
292
291
297
341
293
292
0.0003
0.0000
0.0001
0.0000
0.0003
0.0116
0.0005
0.0209
0.0056
0.0516
0.0095
0.0467
0.0026
0.0201
0.0384b
0.0468c
0.0001
0.0000
2.88
2.91
2.91
2.93
2.65
2.65
2.74
2.72
2.82
2.88
2.86
2.96
2.65
2.64
294
278
256
284
310
311
287
287
407
418
340
350
344
357
50.2
57.2
59.3
59.6
59.2
49.5
61.7
51.7
39.1
27.4
49.3
30.6
47.2
46.0
35.2b
29.8c
62.8
61.1
2.42
2.60
1.99
1.89
9.4/13.5
11.2/14.4
41.4/115
8.9/25.4
95
93
71
90
453
459
456
444
482
453
445
502
471
451
434
427
422
408
411
412
463
452
2.38
93.9/119
110
6
7
HFBAPP
1.56
2.53
2.83
114/173
19.0/30.8
5.3/6.5
62
130
94
p-PDA
9
11
12
m-TOL
DABA
TFMB
404
440
455
491
401
435
425
451
e
e
e
2.73
56.8/83.5
110
13
14
MBCHA
M-MBCHA
T
T
78.3
72.1
258
266
2.62
2.59
303
290
1.83
2.07
11.6/14.6
5.5/8.1
76
84
450
439
400
366
a
Imidization (cure) processes, T: thermal imidization, C: chemical imidization in homogeneous solutions, and D: imidization by dipping PAA film into Ac2O/pyridine at
room temperature for 24 h and successive annealing at 350 ꢀC/1 h in vacuum.
b
CPN-cast PI film formed under a slow drying condition at 30 ꢀC/1 h þ 60 ꢀC/10min þ 100 ꢀC/10min þ 150 ꢀC/15 min in air þ300 ꢀC/1 h in vacuum. PAA with
hinh ¼ 5.75 dL gꢁ1 was used for chemical imidization.
c
CPN-cast PI film formed under the slow drying condition. PAA with hinh ¼ 9.04 dL gꢁ1 was used for chemical imidization.

4702
M. Hasegawa et al. / Polymer 55 (2014) 4693e4708
Fig. 10. Comparisons of Tg for thermally imidized PI films from hydrogenated PMDA
isomers: (left/black bar) H-PMDA, (center/red bar) H0-PMDA, and (right/blue bar) H00-
PMDA systems. (For interpretation of the references to color in this figure legend, the
reader is referred to the web version of this article.)
Fig. 12. Comparisons of CTE for thermally imidized PI films from H0-PMDA-based and
other isomeric systems: ( ) H0-PMDA vs. H00-PMDA and ( ) H0-PMDA vs. H-PMDA. The
dotted line denotes the relation: Y ¼ X.
low degree of in-plane orientation as suggested from their very low
birefringence (Dnth < 0.001). On the other hand, the use of rigid
diamines (category II) caused suppressed thermal expansion
behavior, e.g., CTE ¼ 30.6 ppm Kꢁ1 for H0-PMDA/DABA and
39.1 ppm Kꢁ1 for H0-PMDA/p-PDA. A good correlation is observed in
distorted steric structures. Thus, H0-PMDA is an outstandingly
effective monomer for reducing CTE. The magnitude of in-plane
chain orientation as an origin of the suppressed thermal expan-
sion behavior is closely related to the PI backbone structures; Fig.13
depicts schematic illustrations of the extended chain forms for the
isomeric PI systems derived from three hydrogenated PMDA iso-
mers and p-PDA as a typical rigid diamine for simplicity. The H0-
PMDA-based system possesses a backbone structure with high
linearity as depicted in Fig. 13 (b), which arises from the above-
mentioned specific steric structure of H0-PMDA [Fig. 4 (c)]. A
similar correlation between the low CTE property and the main
chain linearity is also observed in the crank-shaft-shaped CBDA-
based PI systems depicted in Fig. 13 (d) [23]. On the other hand, the
non-planar steric structures of H-PMDA and H00-PMDA are
responsible for a significant decrease in the overall PI chain line-
arity as shown in Fig. 13 (a) and (c), which corresponds to the
increased CTE values for their systems. Thus, the overall chain
linearity in the extended forms can be a useful indicator for pre-
dicting the possibility of low CTE generation. This criterion con-
forms well to not only the present semi-cycloaliphatic PI systems
but also conventional wholly aromatic PIs [43] and poly(ester
imide) systems [45].
Fig. 11; CTE decreases with increasing
D
nth for H0-PMDA/aromatic
diamine systems. Thus, the distinctly decreased CTE for the H0-
PMDA/rigid diamine systems (T) results from an enhanced in-plane
chain orientation, which was induced upon thermal imidization
[43,44]. Fig. 12 shows the comparisons of CTE between H0-PMDA-
based PIs and other isomeric systems (H-PMDA- and H00-PMDA-
based PIs). When flexible ether-containing diamines were used, the
data points were plotted near the Y ¼ X line or slightly below, thus
indicating no prominent effect of H0-PMDA on reducing CTE. The
results mean that CTE is governed by not “local” structural rigidity/
linearity of the diimide units but “overall” main chain linearity; a
positive contribution of H0-PMDA to an increase in the structural
rigidity/linearity was almost canceled by incorporating the highly
flexible units from the diamines. On the other hand, when H0-PMDA
was combined with rigid diamines (typically DABA, #11), the plots
obviously deviate downward from the Y ¼ X line, indicating that the
H0-PMDA/DABA system shows a much lower CTE than the corre-
sponding isomeric systems. In this case, there exists distinct dif-
ferences of the overall chain linearity; in contrast to H0-PMDA, the
use of H-PMDA and H00-PMDA spoils a great effect of DABA on
enhancing the structural rigidity/linearity because of their
The high structural rigidity/linearity of the H0-PMDI unit is also
supported by a model compound approach; a low molecular
weight diimide compound was prepared from H0-PMDA and 4-n-
octylaniline in a similar manner to a previously reported procedure
[22]. This compound exhibited a liquid crystalline morphology in
the temperature range of 121.1e128.0 ꢀC in the heating process and
127.7e103.2 ꢀC in the cooling process on POM as shown in Fig. 14,
whereas the H-PMDA-based counterpart did not. Thus, the N,N0-
diphenyl-substituted H0-PMDI unit can behave as a mesogenic
structure.
For the H0-PMDA-based systems derived from non-fluorinated
common diamines, the PI film specimens (T) were essentially
insoluble in various solvents [NMP, DMAc, DMF, DMSO, m-cresol,
CPN, g-butyrolactone (GBL), and THF]. Therefore, it was difficult to
complete chemical imidization while keeping a homogeneous
state. In contrast, the use of CF3-containing diamines (TFODA,
HFBAPP, and even rigid TFMB) made it possible, and their PI powder
samples (C) were highly soluble in various solvents at room tem-
perature. The properties of the CF3-containing H0-PMDA-based PI
films (C) are also summarized in Table 3. A positive effect of the
chemical imidization process was observed; the PI films (C) showed
lower CTE values by approximately 10 ppm Kꢁ1 than those of the
Fig. 11. Relationship between CTE and
films prepared via (C) thermal and ( ) chemical imidization processes.
D
nth for H0-PMDA/aromatic diamine systems: PI

M. Hasegawa et al. / Polymer 55 (2014) 4693e4708
4703
Fig. 13. Schematic illustrations for simplified extended PI chains: (a) H-PMDA, (b) H0-PMDA, (c) H00-PMDA, and (d) CBDA systems. For simplicity, p-phenylene unit is depicted as a
typical rigid diamine fragment. The diimide units are represented as edge-view.
thermally imidized counterparts for the systems using TFODA and
HFBAPP. The decreased CTE for the former results from an increased
degree of in-plane chain orientation (Fig. 11) as suggested from the
increased Dnth values (Table 3).
For the rigid H0-PMDA/p-PDA system which was not compatible
to the homogeneous chemical imidization process, we attempted to
prepare the PI film by dipping the PAA film formed on a substrate
into a Ac2O/pyridine solution, followed by rinsing with water and
heat treatment in vacuum. This procedure provided a decreased
CTE of 27.4 ppm Kꢁ1 compared to the thermally imidized
counterpart.
3.2.3. Self-chain orientation behavior during solution casing
process
The relatively high
D
nth values for the H0-PMDA-based PI films
(C) also mean the presence of a self-orientation phenomenon
during the simple solution casting process without structural
transformation from PAAs into PIs [46]. The present results look
curious because solution casting of common soluble polymers
usually causes no significant in-plane orientation; for example, we
Fig. 14. POM photograph for H0-PMDA-based model compound at 123.3 ꢀC in the
cooling process.
observed almost zero
Dnth and a common level of CTE
(65.0 ppm Kꢁ1) for an NMP-cast PES film. We propose a possible

4704
M. Hasegawa et al. / Polymer 55 (2014) 4693e4708
mechanism for the self-orientation behavior in the H0-PMDA-based
systems (C); as schematically depicted in Fig. 15 (a), no orientation
occurs yet just after the PI solutions were coated onto a substrate.
The coating viscosity increases with solvent evaporation and must
be abruptly enhanced near the rubbery state at a solid content C*.
Above C*, the coating probably begins to undergo shrinkage stress
[47]. For the present PI systems, we hypothesize that such
shrinkage stress (in other words, apparent stretching effect) may
act as a trigger for the “permanent” in-plane chain orientation
occurring at the drying temperatures (Tdry). When the coatings on
the substrate were cooled from Tdry to room temperature, the
thermal stress arising from the film/substrate CTE difference also
contributes “temporally” to the total in-plane orientation. However,
in our PI film specimens, such contribution from the thermal stress
is canceled by removal of the substrate. Besides, it is most likely
that there exists a good correlation between the shrinkage stress
and the tensile modulus of the PI systems chosen. This corresponds
well to our empirical rule that the stiff/linear chain structures,
which generally increase the modulus, are indispensable for dras-
tically reducing CTE of the PI films. Therefore, we challenged to
further decrease the CTE for the rigid H0-PMDA/TFMB system. The
PI molecular weight can be also an important factor for this purpose
[48]; as schematically depicted in Fig. 15 (b, left), an increase in the
molecular weight (M1 > M2) raises the coating viscosity, as a result,
should contribute to an increase in the shrinkage stress owing to a
greater thickness decrement between CPI ¼ C* and 100 wt%. Flex-
ible PI systems are probably disadvantageous to induce higher
shrinkage stress because of their originally lower modulus. Another
possible factor is the drying temperature. A slow drying process by
elevating Tdry little by little and holding at each step for a period,
which corresponds to a drying program with a lower temperature
ramp, provides a higher coating viscosity (i.e., a higher shrinkage
stress) than a rapid drying process as schematically depicted in
Fig. 15 (b, right). In addition, an increase in Tdry (or increase in the
heating rate) may also contribute to orientational relaxation
harmful for low CTE generation. Indeed, the combination of a
molecular weight increase and very slow drying caused to a further
decrease in CTE (to 29.8 ppm Kꢁ1) for H0-PMDA/TFMB as shown in
Table 3. To apply such slow drying process, adequately volatile and
less hygroscopic solvents (e.g., CPN) are more desirable than
frequently used amide solvents with higher boiling points (e.g.,
NMP). The H0-PMDA/TFMB powder sample (C) allowed to form a
Solvent evaporation
PI solution
Local ordering ?
Half-dried
PI film
Fully dried
PI film
CPI = 10−20 wt%
CPI = 100 wt%
CPI = C*
Substrate
(a)
250
M1
M2
Rapid
150
50
Slow
0
100
0
1.5
3
C*M1 C*M2
PI solid content (wt%)
Time (h)
M1
M2
Slow
Rapid
Flexible
system
0
100
0
100
C*M1 C*M2
C*Slow C*Rapid
PI solid content (wt%)
PI solid content (wt%)
(b)
Fig. 15. (a) Schematic illustration for film formation processes via coating/drying of PI solution and (b) influences of PI molecular weight (left, M1 > M2) and heating rate in the
drying processes (right) on the viscosity of the coatings and predicted internal stress at the drying temperatures. h* denotes a viscosity at a rubbery state where the solid content
(CPI) is represented as C*.

M. Hasegawa et al. / Polymer 55 (2014) 4693e4708
4705
highly stable CPN solution at room temperature with a high solid
content (10 wt%) enough for applying common casting processes.
However, the shrinkage stress effect independently seems to be
less activeas the driving forcefor inducing the actual high level of in-
plane chain orientation (recall the fact that PES and other flexible PI
systems never show prominent in-plane orientation by solution
casting in a similar manner). Therefore, an additional factor relating
to attractive intermolecular interactions and chain rigidity/linearity
should be considered; i.e., the formation of a local ordered structure
consisting of highly oriented chains (probably with a very small
domain size undetectable by POM) [see Fig. 15 (a)]. This idea comes
from our previous experience; a partially imidized PAA of s-BPDA/p-
PDA (s-BPDA: 3,304,40-biphenyltetracarboxylic dianhydride) forms a
liquid crystal-like ordered structure in NMP, and thermal imidiza-
tion of the cast PAA film including the ordered structure results in an
appreciably lower CTE than that of the counterpart from the ordi-
nary PAA [49]. The fact that liquid crystallinity was observed in the
H0-PMDA-based model compound (Fig. 14) suggests that a similar
local ordered structure may exist even in the actual H0-PMDA/rigid
diamine PI films. Such local ordering should be much more advan-
tageous in rigid/linear PI systems such as H0-PMDA/TFMB than in
non-rigid systems. On this assumption, we propose the self-
orientation mechanism; each domain can be forcibly aligned along
the XeY direction by a flow field from the apparent stretching effect
as schematically depicted in Fig. 15 (a).
(PMDA/TFMB, εb < 20% [39]). Thus, the film toughness was prom-
inently improved by exchanging aromatic PMDA for non-aromatic
H0-PMDA and exchanging p-PDA for CF3-containing TFMB. The re-
sults may reflect an effect of chain slippage in the plastic defor-
mation region during the stretching test, which can be allowed
when intermolecular forces were significantly weakened. In
contrast to the good film toughness of H0-PMDA/TFMB, the H-
PMDA/TFMB system possessed no film-forming ability because of
its insufficient molecular weight from the polymerizability prob-
lem, namely, insufficient chain entanglement.
3.3. Performance balance and applicability as plastic substrate
materials for H0-PMDA-based systems
Fig. 16 shows spider charts for the present target properties,
which represent performance balance for the hydrogenated PMDA
isomers. The chars for CBDA and aromatic PMDA are also depicted
for a comparison. The each property was evaluated with five levels
on the basis of criteria listed in Table 4. Two fixed diamines, 4,40-
ODA and TFMB were chosen as a typically flexible and a rigid
diamine, respectively. The 4,40-ODA systems are highly transparent
(except the combination with PMDA) but commonly not suitable
for low CTE generation owing to the absence of the overall chain
linearity. In addition, these are essentially less soluble in common
solvents except H00-PMDA/4,40-ODA. As evaluated from a relatively
spread shape of the spider chart for the H00-PMDA/4,40-ODA system
[Fig. 16 (c)], H00-PMDA possesses comparatively good performance
balance although it is difficult to achieve low CTE. On the other
hand, the TFMB systems display quite different performance bal-
ance. The combination of CBDA and TFMB is suitable for low CTE
generation but not for enhancing the solubility and the film
toughness as shown in Fig. 16 (h). It should be noted that H0-PMDA/
TFMB [Fig. 16 (f)] possesses an outstanding performance balance
among the TFMB systems, although there is room for further
improvement of low CTE property.
3.2.4. Other properties
The H0-PMDA-based systems provides very clear (non-turbid) PI
films (e.g., Ttot ¼ 88.7% and Haze ¼ 1.7% for the H0-PMDA/4,40-ODA
film imidized at 300 ꢀC/0.5 h in vacuum). The H0-PMDA-based PI
films (T) show slightly lower T400 values than the H00-PMDA-based
counterparts [22], however, the formers are essentially colorless as
illustrated from a low YI value less than 5 (e.g., 4.3 for H0-PMDA/
4,40-ODA). As listed in Table 3, a positive effect of the film prepa-
ration process via homogeneous chemical imidization on the op-
tical transparency was also observed; the H0-PMDA-based PI films
(C) tend to show appreciably higher light transmittance than the
corresponding PI films (T) as shown particularly in the HFBAPP
system. The improved transparency for the PI films (C) may be
attributed to an end-capping effect of the unstable terminal amino
groups by Ac2O during the chemical imidization process (Fig. 5). In
particular, the transparency of the H0-PMDA/TFMB film (C) is so
high as illustrated from its optical data (Ttot ¼ 90.5%, YI ¼ 1.8, and
Haze ¼ 2.0%) that this system is suitable for some optical applica-
tions as proposed later.
Our first attempt for further decreasing the CTE of H0-PMDA/
TFMB by copolymerization using DABA was less effective because of
a gelation problem at a high DABA content during the chemical
imidization process. This motivated us to develop a novel CF3-
containing rigid diamine [52]. The combination of this diamine
with H0-PMDA led to a solution-processable colorless PI with a
further decreased CTE (16 ppm Kꢁ1). The results will be reported
elsewhere in detail. Thus, the present work revealed how H0-PMDA
is a promising tetracarboxylic dianhydride monomer for obtaining
novel plastic substrate materials. However, there seems to be a
problem in its application as the plastic substrates in liquid crystal
displays (LCD) with a backlight transmission mode, that is, rela-
The H0-PMDA-based PI films are expected to show low dielectric
constants as suggested from the optically estimated ones (εopt
)
ranging 2.8e2.9 even when common aromatic diamines were used.
The incorporation of CF3 groups and complete removal of the ar-
omatic units were both very effective to significantly reduce εopt
(2.59e2.65) in accordance with previous reports [8,50].
tively high birefringence (
in general desirable to possess zero
substrates because nth is responsible for optical retardation
(Rth nth$d, where d is film thickness), which may cause a
decrease in the image contrast at a high viewing angle. However, as
shown in Fig. 11, there seems to be in principle a sort of “trade-off
D
nth). The plastic substrates for LCDs are
Dnth like current inorganic glass
D
¼
D
Table 3 also summarizes the mechanical properties of the H0-
PMDA-based systems. Essentially flexible PI films were formed
upon thermal imidization. In particular, the use of BAPP and
HFBAPP was very effective to enhance the film toughness as sug-
gested from their very high εb values. Even if the molecular weights
were sufficiently high, the combinations of rigid tetracarboxylic
dianhydrides and rigid diamines generally lead to brittle PI films
owing to insufficient chain entanglement as typically observed in
PMDA/p-PDA system (εb < 3% [51]). Against this general criterion,
the H0-PMDA/p-PDA system with a stiff/linear backbone structure
exhibited sufficient film flexibility as shown in Table 3. It should be
noted that the H0-PMDA/TFMB system with less polarized and
bulky CF3 groups displayed a further enhanced εb value (>50%),
which is much higher than that of the aromatic counterpart
relationship” between low CTE and low
Dnth, suggesting great
difficultness of obtaining ideal plastic substrate materials for LCDs.
Instead, the present H0-PMDA-based low-CTE colorless PI systems
can be applied as the substrates for bottom-emission-type flexible
organic emitting diode (OLED) displays without the optical retar-
dation problem mentioned above and/or electronic paper displays
with an external light reflection mode.
3.4. Potential application as optical compensation layer
We investigated the potentiality of H0-PMDA/TFMB as a novel
coating-type optical compensation film material (negative-C plate,

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M. Hasegawa et al. / Polymer 55 (2014) 4693e4708
Fig. 16. Performance balance represented by 5-level evaluation for the systems derived from 4,40-ODA (upper) and TFMB (lower) with several tetracarboxylic dianhydrides: (a) H-
PMDA, (b, f) H0-PMDA, (c, g) H00-PMDA, (d, h) CBDA, and (e, i) PMDA. The criteria and abbreviations of the properties are shown in Table 4. The spider chart for H-PMDA/TFMB system
is not shown because of the absence of film-forming ability.
nx ¼ ny > nz, i.e.,
D
n
th > 0) with low wavelength dependence of Rth
nth is
representing a slope in the
ultimately low wavelength dispersion property with
values close to 1.00. However, this subject is not easy because
n450 n550 ranges 1.07e1.16 for most of colorless PI systems (e.g.,
1.11e1.12 for 6FDA/TFMB). This parameter is sensitive to not only
the PI chemical structure but also the drying temperature program,
solvent, and PI molecular weight. We observed that the parameter
D
n
the
l
curves. Our goal is to achieve an
for vertical alignment-mode LCDs. In this case, higher
D
D
n450 n550
/D
preferable because thinner compensation films can be used to
obtain an aimed Rth value. Another merit is that the H0-PMDA/TFMB
layer can be directly formed on a triacetylcellulose (TAC) film as a
standard protective layer for iodine-doped poly(vinyl alcohol)
polarizers from a CPN solution of this PI without corrosion of TAC.
In this case, thermal deformation of TAC can be also avoided by
simply drying at lower temperatures than ~ 150 ꢀC without sub-
sequent thermal imidization process at elevated temperatures
D
/D
Dn450/Dn550 tends to reduce with decreasing lcut, which is probably
closely related to the extents of electronic conjugation through the
main chains. In this context, the H0-PMDA/TFMB system with a low
lcut may be suitable for achieving a low wavelength dispersion
property. For the present purpose, H0-PMDA/TFMB was modified
with a minor content of a non-conjugated cycloaliphatic diamine,
(>250 ꢀC). The wavelength dependence of
normal dispersion type, that is, nth decreases with increasing
Dnth (or Rth) is usually a
D
wavelength (l) as expressed by an asymptotic empirical equation:
D
nth ¼ A þ B/(l2 e C) where A, B, and C are constants depending on
isophoronediamine (IPDA). Fig. 17 shows a
cast H0-PMDA/TFMB(80);IPDA(20) copolyimide film. Indeed, this
system exhibited a low wavelength dispersion property ( n450/
Dnthel curve for a CPN-
the materials [53]. The birefringence ratio at
l
¼ 450 and 550 nm,
i.e.,
Dn450
/Dn550 (¼ R450/R550
)
can be
a
good indicator for
D

M. Hasegawa et al. / Polymer 55 (2014) 4693e4708
4707
Table 4
Criteria for 5-step evaluation of the target properties: polymerizability (Po), heat resistance (HR), film toughness (To), low thermal expansion property (LT), film transparency
(Tr), and solution-processability (SP).
Abbreviation
Po
Property
Relative level
1
2
3
4
5
hinh (PAA)
<0.3
0.4e0.6
0.7e0.9
1.0e1.5
>2.0
(dL gꢁ1
Tg (ꢀC)
εb (%)
)
HR
To
<200
220e240
5e10
250e270
20e30
280e300
40e60
>350
>100
No film-forming
ability or < 2
>70
LT
Tr
SP
CTE (ppm Kꢁ1
T400 (%)
Qualitative
solubility
)
60e50
20e30
Partially
soluble or
swelled
45e35
40e60
Soluble in
hot amide
solvents
30e20
70e75
Soluble in
amide
solvents at r.t.
<10
>80
Soluble in
non-amide
solvents at r.t.
<5
Insoluble upon
heating
D
n550 ¼ 1.03). The partial incorporation of 1,3-dimethyl-substituted
CBDA (DM-CBDA) as a comonomer into H0-PMDA/TFMB desirably
enhanced nth while maintaining a low wavelength dispersion
294 ꢀC for H0-PMDA/4,40-ODA). The use of some flexible diamines
(e.g., BAPP) was effective to drastically enhance the film toughness.
The combinations of H0-PMDA and rigid diamines (typically, DABA)
caused a prominent decrease in CTE owing to an activated
imidization-induced in-plane chain orientation phenomenon,
whereas the counterparts from H-PMDA and H00-PMDA did not. The
results suggest how the H0-PMDA-based diimide moiety behaves as
a rigid/linear structural unit as supported by the fact that the H0-
PMDA-based model compound showed liquid crystallinity. Some of
the H0-PMDA-based PI films were also prepared via the chemical
imidization process in a homogeneous state. For example, the PI
powder sample for H0-PMDA/TFMB was highly soluble in common
solvents (even in some hygroscopic solvents such as CPN). This
procedure was more advantageous for increasing transparency and
decreasing CTE of the PI films than the conventional thermal
imidization process. The CTE for H0-PMDA/TFMB further decreased
(to 29.8 ppm Kꢁ1) by increasing the molecular weight and drying
the coatings more slowly. The results suggest the presence of a self-
orientation phenomenon during solution casting. The mechanism
was discussed in this work. A potential application was also pro-
posed as novel coating-type optical compensation film materials
with low wavelength dispersion of Rth for vertical alignment-mode
LCDs. The CPN-cast H0-PMDA/TFMB(80); IPDA(20) copolyimide film
D
property, excellent transparency (T400 ¼ 89.2%), a relatively low CTE
(30.3 ppm Kꢁ1), and a very high Tg (335 ꢀC). The results give us an
idea: although high
Dnth is generally an undesirable property for
the plastic substrates in LCDs as mentioned above, we propose that
H0-PMDA/TFMB and the modified systems with the relatively high
Dnth values can be applied to a new type of LCD substrates pos-
sessing both of good dimensional stability based on their low CTE
property and the optical compensation functionality.
4. Conclusion
This work proposed a cycloaliphatic tetracarboxylic dianhydride
monomer, H0-PMDA, useful for obtaining novel optoelectronic
materials. H0-PMDA showed much higher polymerizability with
various diamines than conventional H-PMDA and led to PAAs with
high molecular weights. Consequently, even the combination with
TFMB including electron-withdrawing CF3 groups provided a
highly flexible PI film in contrast to the absence of film-forming
ability for H-PMDA/TFMB. The enhanced polymerizability of H0-
PMDA is attributed to its unique steric structure. Exceptionally, the
polyaddition with t-CHDA did not proceed by insoluble salt for-
mation, which is probably related to the structural rigidity of the
oligomers formed. The thermally imidized H0-PMDA-based PI films
were essentially colorless regardless of diamines owing to inhibited
CT interactions. The PIs also possessed relatively high Tg's (e.g.,
achieved
a
low wavelength dispersion property
(
D
n450
/
D
n550 ¼ 1.03) in addition to non-coloration and a high
Dnth value
exceeding 0.03. Thus, the H0-PMDA-based PI systems can be useful
materials for high-temperature transparent plastic substrates with
high dimensional stability and/or novel coating-type optical
compensation films.
Acknowledgments
We would like to thank Mr. R. Masaka, Mr. D. Hirano, Mr.
S. Tsukuda, and Mr. Y. Watanabe in our research group for their
partial experimental support. We are also indebted to New Japan
Chemical, Nissan Chemical Industries, Wakayama Seika, JFE
Chemical, and Ihara Chemical for their monomer supply.
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