Solution-processable colorless polyimides derived from hydrogenated pyromellitic dianhydride with controlled steric structure

Article abstract of DOI:10.1002/pola.26407

This work presents novel colorless polyimides (PIs) derived from 1R,2S,4S,5R-cyclohexanetetracarboxylic dianhydride (H″-PMDA). Isomer effects were also discussed by comparing with PI systems derived from conventional hydrogenated pyromellitic dianhydride, that is, 1S,2R,4S,5R-cyclohexanetetracarboxylic dianhydride (H-PMDA). H″-PMDA was much more reactive with various diamines than H-PMDA, and the former led to PI precursors with much higher molecular weights. The results can be explained from the quite different steric structures of these isomers. The thermally imidized H″-PMDA-based films were colorless regardless of diamines because of inhibited charge-transfer interaction. In particular, the H″-PMDA/4, 4′-oxydianiline system simultaneously achieved a very high T_g exceeding 300 °C, high toughness (elongation at break > 70%), and good solution processability. In contrast, the H-PMDA-based counterparts were essentially insoluble. The outstanding solubility of the former probably results from disturbed chain stacking by its nonplanar steric structure. An advantage of chemical imidization process is also proposed. In some cases, a copolymerization approach with an aromatic tetracarboxylic dianhydride was effective to improve the thermal expansion property. The results suggest that the H″-PMDA-based PI systems can be promising candidates for novel high-temperature plastic substrate materials in electronic paper displays. A potential application as optical compensation film materials in liquid crystal displays (LCD) is also proposed in this work.

Full text of DOI:10.1002/pola.26407

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Solution-Processable Colorless Polyimides Derived from Hydrogenated
Pyromellitic Dianhydride with Controlled Steric Structure
Masatoshi Hasegawa,1 Daiki Hirano,1 Mari Fujii,1 Misako Haga,1 Eiichiro Takezawa,2
Shinya Yamaguchi,2 Atsushi Ishikawa,2 Takashi Kagayama2
1Department of Chemistry, Faculty of Science, Toho University, 2-2-1 Miyama, Funabashi, Chiba 274-8510, Japan
2Iwatani Industrial Gases, Corp., 10 Otakasu-cho, Amagasaki, Hyogo 660-0842, Japan
Correspondence to: M. Hasegawa (E-mail: mhasegaw@chem.sci.toho-u.ac.jp)
Received 24 August 2012; accepted 2 October 2012; published online 31 October 2012
DOI: 10.1002/pola.26407
ABSTRACT: This work presents novel colorless polyimides (PIs)
derived from 1R,2S,4S,5R-cyclohexanetetracarboxylic dianhy-
dride (H00-PMDA). Isomer effects were also discussed by com-
paring with PI systems derived from conventional
hydrogenated pyromellitic dianhydride, that is, 1S,2R,4S,5R-
cyclohexanetetracarboxylic dianhydride (H-PMDA). H00-PMDA
was much more reactive with various diamines than H-PMDA,
and the former led to PI precursors with much higher molecu-
lar weights. The results can be explained from the quite differ-
ent steric structures of these isomers. The thermally imidized
H00-PMDA-based films were colorless regardless of diamines
because of inhibited charge-transfer interaction. In particular,
the H00-PMDA/4,40-oxydianiline system simultaneously achieved
a very high Tg exceeding 300 ꢀC, high toughness (elongation at
break > 70%), and good solution processability. In contrast,
the H-PMDA-based counterparts were essentially insoluble.
The outstanding solubility of the former probably results from
disturbed chain stacking by its nonplanar steric structure. An
advantage of chemical imidization process is also proposed. In
some cases, a copolymerization approach with an aromatic tet-
racarboxylic dianhydride was effective to improve the thermal
expansion property. The results suggest that the H00-PMDA-
based PI systems can be promising candidates for novel high-
temperature plastic substrate materials in electronic paper dis-
plays. A potential application as optical compensation film
materials in liquid crystal displays (LCD) is also proposed in
C
V
this work. 2012 Wiley Periodicals, Inc. J. Polym. Sci., Part A:
Polym. Chem. 2013, 51, 575–592
KEYWORDS: coefficient of thermal expansion (CTE); cycloali-
phatic tetracarboxylic dianhydride; optical compensation films;
plastic substrates; polyimides; solution processability; tough-
ness; transparency
INTRODUCTION Recently, colorless high-temperature poly-
meric materials are greatly required as various optical com-
ponent materials, for example, plastic substrates in various
display devices, liquid crystal alignment layers, color filters,
optical compensation films, antireflection films, core/clad
materials in light waveguides, and optical lenses. If current
fragile inorganic glass substrates were replaced by plastic
substrates, flat panel displays can be lightened. However, it
is not easy to develop practically useful plastic substrates
simultaneously possessing excellent optical transparency
are widely used in electric and microelectronic devices for
their simple two-step manufacturing processes, excellent elec-
tric insulation ability, an extremely high purity (the absence of
volatile organic compounds and metallic/ionic contamina-
tions), nonflammability, thermo-oxidative stability (long-term
heat resistance), and considerably high Tg’s overcoming the
reflow soldering processes (short-term heat resistance).1–12
However, intensive coloration intrinsic to conventional aro-
matic PI films often disturbs their optical applications.
No attention has been paid to the development of colorless
polybenzazole films in industrial standpoints, probably
owing to their complicated manufacturing processes, poor
processability, and limited commercial sources of the mono-
mers. We have previously reported that novel PBO systems
polycondensed in hot polyphosphoric acid from aromatic
bis(o-aminophenol)s and cycloaliphatic (alicyclic) dicarbox-
ylic acids are highly soluble in various common organic sol-
vents (solution processable) and provides colorless cast films
(noncoloration) and sufficiently high heat resistance (Tg
>
ꢀ
ꢀ
250 C, more desirably > 300 C) overcoming some thermal
processes such as thin-film transistor formation; even poly
(ether sulfone) (PES), which possesses the highest heat
resistance among current colorless engineering plastics
(Tg ¼ 225 ꢀC), is not applicable for this purpose. Polymeric
ꢀ
materials with very high Tg exceeding 300 C are practically
limited to polyimides (PI) and polybenzazoles [polybenzoxa-
zoles (PBO), polybenzimidazoles, and polybenzthiazoles]. PIs
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2012 Wiley Periodicals, Inc.
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with good toughness.13 However, the results also revealed
that most of commercially available aliphatic dicarboxylic
acids show no sufficient polymerizability with bis(o-amino-
phenol)s except for 1,4- or 1,3-cyclohexanedicarboxylic acids.
This means a narrow width of structural modification in col-
orless PBO systems.
substrate materials. Steric structure effects on the polymeriz-
ability with diamines and the PI film properties are also dis-
cussed by comparing with isomeric systems using conven-
tional
hydrogenated
PMDA,
that
is,
1S,2R,4S,5R-
cyclohexanetetracarboxylic dianhydride (H-PMDA).28
A
potential application for the H00-PMDA-based PIs is also pro-
posed in this work.
On the other hand, much effort has been devoted to erase PI
film coloration from academic and industrial interests. The
most effective strategy is to use nonaromatic (cycloaliphatic)
monomers either in diamines or tetracarboxylic dianhydrides
or both,14–17 thereby charge-transfer (CT) interactions are
inhibited.18 However, a serious problem occurs when ali-
phatic diamines were used, that is, the formation of insoluble
salt at the initial stage in the polymerization process of PI
precursors [poly(amic acid)s (PAAs)].14,15 In particular,
trans-1,4-cyclohexanediamine (t-CHDA) tends to form the
‘‘tightest’’ salt, for example, the polyaddition with pyromel-
litic dianhydride (PMDA) is completely inhibited,19,20
although even reaction systems using other flexible cycloali-
phatic diamines need very long reaction times until the salt
is completely dissolved. This situation causes poor productiv-
ity/reproducibility for the PAA polymerization process,
which must be a great obstacle for commercialization of col-
orless PI systems using aliphatic diamines. In contrast, the
combinations of cycloaliphatic tetracarboxylic dianhydrides and
aromatic diamines can provide colorless PIs without the salt
formation in the PAA polymerization processes. However, cyclo-
aliphatic tetracarboxylic dianhydrides [e.g., commercially avail-
able bicyclo[2.2.2]oct-7-ene-2,3,5,6-tetracarboxylic dianhydride
(BTA)] are often not highly reactive with aromatic dia-
mines,21,22 as a result, tend to give PAAs with lower molecular
weights, which reduce chain entanglement closely related to
film flexibility (formability). Exceptionally, 1,2,3,4-cyclobutane-
tetracarboxylic dianhydride (CBDA)23 shows very high poly-
merizability with diamines like common aromatic tetracarbox-
ylic dianhydrides,19,20,24 probably owing to a strain in the acid
anhydride ring of CBDA. Therefore, it leads to colorless PIs with
very high molecular weights.14,19,20,23 However, CBDA-based
PIs essentially possess no solution processability.14
EXPERIMENTAL
Materials
Monomers, Polyaddition, Imidization, and Film
Preparation
The abbreviations, sources, purification methods, and melt-
ing points of the monomers used in this work are shown in
Table 1 and Figure 1. H00-PMDA and H-PMDA were supplied
by Iwatani Industrial Gases and New Japan Chemical, respec-
tively. The oak ridge thermal ellipsoid plot program (ORTEP)
diagrams obtained by the single-crystal X-ray diffraction
measurements for H00-PMDA25 and H-PMDA28 are also
depicted in Figure 1. The central cyclohexane unit of H00-
PMDA in crystal takes an intermediate conformation
between a boat and a twist-boat form. H00-PMDA was synthe-
sized as previously described;25 PMDA was hydrolyzed with
an NaOH aqueous solution. The pyromellitic acid tetraso-
dium salt formed was hydrogenated in a high-pressure
hydrogen atmosphere at 160 ꢀC in the presence of a ruthe-
nium/carbon catalyst. The reaction mixture was heated at a
precisely controlled temperature and neutralized with conc.
HCl. The precipitated tetracarboxylic acid was collected by
filtration and washed with toluene. The white precipitate
was dissolved in acetic anhydride (Ac2O) and refluxed at an
established temperature, then recrystallized by cooling to
room temperature.
PAAs were prepared by equimolar polyaddition of tetracar-
boxylic dianhydrides and diamines as shown in Figure 2. A
typical procedure is as follows: tetracarboxylic dianhydride
powder (10 mmol) was added in a dry N,N-dimethylaceta-
mide (DMAc) solution of diamine (10 mmol) at an initial
total solid content of 30 wt % with continuous stirring. The
reaction mixture was stirred with a magnetic spinner at
room temperature until it becomes homogeneous with a
maximum solution viscosity (typically for 72 h). In this case,
if necessary, the reaction mixture was gradually diluted with
a minimum quantity of the same solvent to ensure effective
stirring. The polymerization conditions and the results are
listed in Tableꢀ2. The PAA solutions were stored in a sealed
bottle at ꢁ20 C without dilution to avoid a gradual molecu-
lar weight decrease.
To solve such crucial problems in acquiring useful colorless
PI materials, we have previously developed cycloaliphatic
tetracarboxylic dianhydrides derived from hydrogenated tri-
mellitic anhydride (HTA) and various diols.22 The HTA-
derived tetracarboxylic dianhydrides showed high polymeriz-
ability with diamines and led to colorless poly(ester imide)s
(PEsIs), which also possess outstanding solution processabil-
ity, relatively high Tg, and good toughness. The incorporation
of rigid biphenyl units into the colorless PEsI main chains
PI films were prepared upon different methods (thermal and
chemical imidization). The reaction schemes are shown in
Figure 2. PI films were prepared by the conventional two-
step process; PAA solution was bar-coated on a glass sub-
strate and dried at 60 C/2 h for DMAc solutions or 80 C/2
h for N-methyl-2-pyrrolidone (NMP) solutions in an air-con-
vection oven. We first attempted thermal imidization under a
relatively mild condition with a maximum temperature of
250 ꢀC from the viewpoints of the film transparency.
22
ꢀ
was capable of enhancing the Tg up to 295 C. However, it
was difficult to obtain Tg’s exceeding 300 ꢀC as far as the
HTA-derived tetracarboxylic dianhydrides were used.
ꢀ
ꢀ
In the present article, we proposes novel colorless PI materi-
als derived from a cycloaliphatic tetracarboxylic dianhydride
with a controlled steric structure, that is, 1R,2S,4S,5R-cyclo-
hexanetetracarboxylic dianhydride (H00-PMDA),25–27 which
show excellent combined properties compatible to plastic
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TABLE 1 The Conditions of Pretreatment and Purification for Monomers
Solvent for
Vacuum-Drying
Condition
Melting
Point (ꢀC)
Monomers
Source
Recrystallization
1R,2S,4S,5R-Cyclohexanetetracarboxylic
dianhydride (H00-PMDA)
Iwatani Industrial
Gases
–
150 ꢀC/24 h
150 ꢀC/24 h
150 ꢀC/24 h
150 ꢀC/12 h
267 (273a)
1S,2R,4S,5R-Cyclohexanetetracarboxylic
dianhydride (H-PMDA)
New Japan Chemical
–
303
–
Bicyclo[2.2.2]oct-7-ene-2,3,5,6-tetracarboxylic
dianhydride (BTA)
Tokyo Chemical
Industry (TCI)
Ac2O/DMF (3/1, v/v)
1,2,3,4-Cyclobutanetetracarboxylic
dianhydride (CBDA)
Nissan Chemical
Industries
–
241
p-Phenylenediamine (p-PDA)
4,40-Oxydianiline (4,40-ODA)
TCI
Ethyl acetateb
30 ꢀC/24 h
50 ꢀC/24 h
142
191
Wako Chemical
Toluene/DMF
(10/1, v/v)b
3,40-Oxydianiline (3,40-ODA)
JFE Chemical
–
50 ꢀC/24 h
74
Bis(4-amino-2-trifluoromethylphenyl)
ether (TFODA)
Ihara Chemical
EtOH/H2O (11/5, v/v)
145 ꢀC/12 h
129
m-Tolidine (m-TOL)
Wakayama Seika
Toluene/CHX
(7/3, v/v)
70 ꢀC/24 h
86
o-Tolidine (o-TOL)
2,20-Bis(trifluoromethyl)benzidine (TFMB)
Wakayama Seika
Wakayama Seika
TCI
Toluene
80 ꢀC/24 h
50 ꢀC/24 h
50 ꢀC/24 h
129
184
198
–
–
2,2-Bis(4-aminophenyl)hexafluoropropane
(BA6F)
1,4-Bis(4-aminophenoxy)benzene (TPEQ)
Wakayama Seika
Wakayama Seika
Toluene/DMF
(15/1, v/v)
100 ꢀC/24 h
50 ꢀC/24 h
172
115
1,3-Bis(4-aminophenoxy)benzene (TPER)
Ethanol/H2O
(11/9, v/v)
4-Aminophenyl-40-aminobenzoate (APAB)
4,40-Diaminobenzanilide (DABA)
4,40-Bis(4-aminophenoxy)biphenyl (BAPB)
Wakayama Seika
TCI
–
50 ꢀC/24 h
50 ꢀC/24 h
50 ꢀC/24 h
185
205
197
c
–
Wakayama Seika
Toluene/DMF
(15/1, v/v)
Bis[4-(4-aminophenoxy)phenyl] sulfone
(BAPS)
Wakayama Seika
Wakayama Seika
Wakayama Seika
Toluene
50 ꢀC/24 h
50 ꢀC/24 h
50 ꢀC/24 h
194
129
159
2,2-Bis[4-(4-aminophenoxy)phenyl] propane
(BAPP)
–
–
2,2-Bis[4-(4-aminophenoxy)phenyl]
hexafluoropropane (HFBAPP)
trans-1,4-Cyclohexanediamine (t-CHDA)
Iwatani Industrial Gases
New Japan Chemical
–
–
30 ꢀC/24 h
–
70
4,40-Methylenebis(cyclohexylamine) (MBCHA)
(mixture of isomers)
none
4,40-Methylenebis(2-methylcyclohexylamine)
(M-MBCHA)
Aldrich
–
–
–
(mixture of isomers)
a
Repeatedly recrystallized from acetic anhydride (Ac2O).
Vacuum sublimated for complete decoloration after recrystallization.
by recrystallization in the presence of activated carbon. DMF, N,N-dime-
thylformamide; CHX, cyclohexane.
b
c
It contains a colored contamination which is difficult to remove even
However, the PI films obtained were often somewhat brittle.
This is probably due to partial retro-polyaddition although
the generated terminal functional groups can recombine by
additional heating at higher temperatures.29 Then, to obtain
flexible PI films while avoiding film coloꢀration, thermal iꢀmid-
ization was carried out typically at 200 C/0.5 h þ 320 C/1
h in vacuum as adhered on the substrate, and successively
annealed in vacuum at 10–20 ꢀC lower temperatures than
the Tg’s after peeling them off from the substrate to elimi-
nate residual stress without undesirable significant film de-
formation. The imidization and annealing conditions were
properly adjusted to form better quality of PI films. The
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FIGURE 1 Molecular structures of monomers and ORTEP diagrams of H00-PMDA and H-PMDA.
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/pyr-
idine (7/3, v/v) to adequately diluted PAA solutions at a
feed molar ratio of [Ac2O]/[COOH]PAA ¼ 5 with continuous
vigorous stirring at room temperature. The reaction mixture
was additionally stirred at room temperature for 12 h in a
sealed flask while monitoring the solution homogeneity. This
procedure is effective to suppress an undesirable significant
ginh decrease, for example, ginh ¼ 1.25 dL gꢁ1 (PAA) and ginh
¼ 1.32 dL gꢁ1 (PI) for H00-PMDA/2,20-bis(trifluoromethyl)-
benzidine (TFMB) system in contrast to simple thermal imid-
ization in solution. In some systems using aromatic diamines
with rigid structures, chemical imidization was not applica-
ble because of inhomogenization such as gelation due to
insufficient solubility of the imidized forms, by which
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FIGURE 2 Reaction schemes of polyaddition and imidization of PAAs.
imidization is not completed. 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 as a poor solvent. The fibrous
white precipitate obtained was repeatedly washed with
uum. After peeling them off from the substrate, the PI films
(typically 20 lm thick) were additionally annealed in a simi-
lar manner. The samples prepared via chemical imidization
(powder or film) are denoted as ‘‘(C)’’.
In this work, the chemical compositions of PAA and PI systems
are represented with the abbreviations of the monomer com-
ponents used [tetracarboxylic dianhydrides (X) and diamines
(Y)] as X/Y for homopolymers and X1;X2/Y1 for copolymers.
ꢀ
methanol/water, collected by filtration, and dried at 80 C in
vacuum for 12 h. The excess of the chemical imidization rea-
gent added should be eliminated to avoid undesirable film
coloration as observed in some cases. The dried precipitate
was redissolved in pure DMAc at a solid content of 10–15
wt %. The homogeneous solutiꢀon formed was coated on a
Model Compounds
A model compound was prepared as follows; aniline (22
mmol) was dissolved in dry DMAc in a three-necked flask
ꢀ
glass substrate and dried at 60 C/2 h þ 250 C/1 h in vac-
TABLE 2 Results of polyaddition of H00-PMDA with various diamines
Solid Content
(wt %)
Reaction
Time (h)
ginh (PAA)
Category of Diamine
Aromatic-flexible (I)
Diamine
Solvent
(dL gꢁ1
)
4,40-ODA
3,40-ODA
TPEQ
30!17
30!21
30!11
30!11
30!12
30!12
30
DMAc
DMAc
DMAc
DMAc
DMAc
DMAc
DMAc
DMAc
DMAc
DMAc
DMAc
NMP
72
72
72
72
72
72
72
72
72
72
72
120
72
72
72
72
72
96
96
96
2.47
2.44
3.76
2.21
3.63
3.89
0.60
1.21
1.22
4.02
TPER
BAPP
BAPB
BAPS
TFODA
BA6F
30!22
30!25
30!17
30!8
HFBAPP
p-PDA
p-PDA
o-TOL
a
Aromatic-rigid (II)
–
10!8
1.21
1.62
3.01
1.55
1.35
1.25
1.25
1.83
0.61
30!15
30!12
30!17
30!14
30!15
20!16
20!18
20!15
DMAc
DMAc
DMAc
DMAc
DMAc
DMAc
DMAc
DMAc
m-TOL
APAB
DABA
TFMB
Cycloaliphatic-flexible (III)
Cycloaliphatic-rigid (IV)
MBCHA
M-MBCHA
t-CHDA
a
Gelation.
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with a nitrogen inlet and a condenser. H00-PMDA powder (10
mmol) was added in the solution and stirred at room tem-
perature for 24 h, then refluxed in an oil bath regulated at
ꢀ
190 C for 3 h while keeping nitrogen bubbling. The reaction
mixture was cooled to room temperature and slowly poured
into a large amount of water. The white precipitate obtained
was repeatedly washed with fresh water, collected by filtra-
tion, and dried at 100 ꢀC in vacuum for 12 h. The product
purified by recrystallization from toluene provided white
plate crystal with a very sharp endothermic peak (m.p. 266.6
ꢀC) by differential scanning calorimetry (DSC, Bruker-AXS,
DSC3100) at a heating rate of 5 K minꢁ1
.
IR (Jasco FTIR 5300, KBr): 3065 cmꢁ1 (CaromAH stretching),
2936 cmꢁ1 (CaliphAH), 1779/1709 cmꢁ1 (imide C¼¼O), 1497
1
cmꢁ1 (phenyl group). H NMR [JEOL, JNM-ECP400, 400 MHz,
dimethyl sulfoxide (DMSO)-d6, d, ppm]: 7.5–7.2 (10H, N-phe-
nyl group, relative integrated intensity: 10.00), 3.2–2.3 (m,
8H, central cyclohexane unit, relative integrated intensity:
8.00). Anal. Calcd (%) for C22H18N2O4 (374.40): C, 70.58; H,
4.85; N, 7.48. Found: C, 70.86; H, 4.90; N, 7.29.
FIGURE 3 FTIR spectrum of thin PI film for H00-PMDA/BAPP.
nals for the NHCO (d ¼ 10.0 ppm) and the COOH groups
(12.3 ppm) in the 1H NMR spectra. Imidization was also
monitored using separately prepared thin films (4–5 lm
thick) to avoid spectral over-saturation by the transmission-
mode IR spectroscopy (Jasco FTIR 5300 infrared spectrome-
ter). In this case, it is preferable to use thin film samples
with nonuniform thickness to erase significant interference
fringes, which deform the IR spectra. A typical FTIR spec-
trum of a thin PI film is shown in Figure 3. The spectrum
indicate that imidization was completed on the basis of the
perfect disappearance of the 1660/1540 cmꢁ1 bands (amide
C¼¼O stretching) and the ꢂ2600/ꢂ1700 cmꢁ1 broad bands
(hydrogen-bonding carboxylic acid, OAH/C¼¼O) inherent to
PAAs, in addition to the appearance of some specific bands
originating from the imide ring formation at 1779 cmꢁ1
(C¼¼O, imide-I) and 1715 cmꢁ1 (C¼¼O, imide-II), and 1387
cmꢁ1 (NACarom, imide-III). Other IR bands are also observed
at 3054 cmꢁ1 (CaromAH), 2969 cmꢁ1 (CaliphAH), 1503 cmꢁ1
(1,4-phenylene group), and 1242 cmꢁ1 (CaromAOACarom).
The H-PMDA-derived model compound was also synthesized
in a similar manner. Recrystallization from ethanol led to a
white needꢀle crystal with a very sharp endothermic peak at
m.p. 240.3 C by DSC.
IR (KBr): 3065 cmꢁ1 (CaromAH), 2984/2915 cmꢁ1
(CaliphAH), 1769/1709 cmꢁ1 (imide C¼¼O), 1497 cmꢁ1 (phe-
nyl). 1H NMR (400 MHz, DMSO-d6, d, ppm): 7.5–7.25 (10H,
N-phenyl group, relative integrated intensity: 10.00), 3.2–2.0
(m, 8H, central cyclohexane unit, relative integrated inten-
sity: 7.87). Anal. Calcd (%) for C22H18N2O4 (374.40): C,
70.58; H, 4.85; N, 7.48. Found: C, 70.24; H, 4.86; N, 7.40. The
chemical imidization method led to essentially the same
product as that obtained by thermal imidization.
Measurements
Inherent Viscosities and Gel Permeation Chromatography
The reduced viscosities (gred) of PAAs were measured at a solid
ꢀ
content of 0.5 wt % at 30 C on an Ostwald viscometer. The
Thermomechanical Analysis
measurements were conducted as prompt as possible to avoid
the depolymerization of PAAs, which immediately proceeds by
dilution. An extrapolation to zero concentration for determina-
tion of the inherent viscosities (ginh) is difficult due to an abnor-
mal increase in the gred values in the lower concentration range,
which arises from a polyelectrolyte effect of PAAs. Instead, the
The linear coefficients of thermal expansion (CTE) along the
film plane direction for PI specimens (15 mm long, 5 mm
wide, and typically 20 lm thick) in the glassy region were
measured as an average in the range of 100–200 ꢀC at a
heating rate of 5 K minꢁ1 on a thermomechanical analyzer
[Bruker-AXS, thermomechanical analysis (TMA) 4000] with a
fixed load (0.5 g per a unit film thickness in lm, typically, 10
g load for 20 lm-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 elimination of adsorbed water.
g
g
red values measured at 0.5 wt % can be practically regarded as
inh as often similarly dealt with in the literature.
The molecular weights for some soluble PIs were deter-
mined by gel permeation chromatography (GPC) using tetra-
hydrofuran (THF) as an eluent at room temperature on a
Jasco, LC-2000 Plus HPLC system with a GPC column (Sho-
dex, KF-806L) and an UV detector (Jasco, UV-2075) at a flow
rate of 1 mL minꢁ1. The calibration was carried out with
standard polystyrenes (Shodex, SM-105).
Dynamic Mechanical Analysis
The storage modulus (E0) and the loss energy (E00) of PI films
were measured at a heating rate of 5 K minꢁ1 on the same
TMA instrument; the measurements were conducted at a si-
nusoidal load frequency of 0.1 Hz with an amplitude of 15 gf
Monitoring of Imidization
Full imidization for the chemically imidized samples was
confirmed from complete disappearance of the proton sig-
in
a nitrogen atmosphere. The Tg’s of PI films were
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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 lm by softening above the Tg’s.
10 nm bandpass of a monochromator through an optical
fiber cable.
Thermogravimetric Analysis
Thermal stability of PI films was evaluated from the 5%
weight loss temperatures (T5d) on a thermo-balance (Bruker-
AXS, TG-DTA2000). Thermogravimetric analysis (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 tem-
perature while mounting in the sample chamber under a con-
tinuous dry N2 flow. After the data resetting to 0% weight
loss, TGA data were collected from the second heating run.
Mechanical Properties
Tensile modulus (E), tensile strength (rb), and elongation at
break (eb) of PI specimens (film dimension: 30 mm long, 3
mm wide, typically 20 lm thick, specimen numbers > 15)
were measured on a mechanical testing machine (A & D,
Tensilon UTM-II) at a cross head speed of 8 mm minꢁ1 at
room temperature. In this case, the specimens were taken
from high-quality large film samples (10 ꢃ 10 cm2) without
any defects such as fine bubbles. For some systems, it was
difficult to carry out the mechanical stretching test owing to
considerable film brittleness or the difficulty of high-quality
large film preparation.
Solubility
The solubility of chemically imidized powder samples or
thermally imidized film samples was examined by immersing
these samples (20 mg) in various solvents (1 mL) in a test
tube at room temperature. When the samples were insoluble
at room temperature, they were heated at 60 ꢀC for THF or
Extent of Water Absorption
The water absorption % (WA) of PI films was determined
from the relation:
ꢀ
at 100 C for other solvents.
WA ¼ ½ðW ꢁ W0Þ=W0ꢄ ꢃ 100
Film Transparency
where W0 is the weight of film samples just after vacuum
drying at 50 ꢀC for 24 h, W denotes the weight of the films that
were immersed in water at 23 ꢀC for 24 h and carefully blotted
dry with a tissue paper. In this case, large film specimens (> 0.1
g) were used to minimize the experimental errors.
The optical transparency of PI films (20 lm thick) was eval-
uated from the light transmittance (%) at a wavelength (k)
of 400 nm (T400) and the cutoff wavelength (kcut, k at T ꢂ
0%) on a Jasco V-530 UV/vis spectrophotometer. The total
light transmittance (Ttotal, JIS K 7361-1) and the diffuse
transmittance (Tdiff, JIS K 7136) of PI films were measured
on a double-beam haze meter with an integrating sphere
(Nippon Denshoku Industries, NDH 4000). The haze (turbid-
ity) of PI films was calculated from the relation:
Refractive Indices, Birefringence, and Optically Estimated
Dielectric Constants
The in-plane (nin or nx ¼ ny) and the out-of-plane (nout or
nz) refractive indices of PI films were measured with a Na
lamp at 589.3 nm on an Abbe refractometer (Atago, 4T, nD
range: 1.47–1.87) equipped with a polarizer by using a con-
tact liquid (sulfur-saturated methylene iodide nD ¼ 1.78–
1.80) and a test piece (nD ¼ 1.92). The birefringence of PI
films, which represents a relative extent of in-plane chain
orientation (chain alignment parallel to the film plane), was
calculated from the relation:
Haze ¼ (Tdiff/Ttotal) ꢃ 100
RESULTS AND DISCUSSION
Polymerizability of H00-PMDA with Diamines and Effect of
Steric Structure
The reactivity of tetracarboxylic dianhydrides with diamines
can be represented strictly by the second-order rate con-
stants. However, in actual PAA polymerization processes, it is
difficult to determine the rate constants since the reaction
mixtures are inhomogeneous in the initial stage; the amic
acid formation proceeds with gradual dissolution of tetracar-
boxylic dianhydride powder fed into diamine solutions.
Therefore, in the present work, the polymerizability was dis-
cussed on the basis of the ginh values of the finally obtained
polymers. This parameter (ginh-based polymerizability) is
more important in the practical viewpoints because the ginh
of PAAs is closely related to the PI film flexibility as repre-
sented by eb;31 empirically speaking, PI films usually become
very brittle in systems with very low ginh values (<0.3 dL
Dnth ¼ nin ꢁ nout
The dielectric constants were estimated from the optical
data on the basis of an empirical relation:
eopt ¼ 1:1n2av
where nav represents the average refractive indices
expressed as
nav ¼ ð2nin þ noutÞ=3
The eopt values are known to be approximate to the dielec-
tric constants determined with a precision LCR meter at a
frequency of 1 or 10 MHz for PI systems.20,30
g
ꢁ1) owing to poor chain entanglement. Insufficient polymer-
Dnth was also measured at some wavelengths in the visible
range on an Abbe refractometer (Atago, 1T, nD range: 1.30–
1.70); monochromatic light was introduced from a Xe lamp
in a fluorescence spectrometer (Hitachi, F-2000) with a
izability, which increases unreacted functional groups at the
chain ends, may also cause another negative result, that is,
undesirable film coloration arising from partial decomposi-
tion of the terminal amino groups as mentioned later.
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ylic dianhydride and 1,3-bis(4-aminophenoxy)benzene
(TPER) led to a PAA with ginh ¼ 0.18 dL gꢁ1, on the other
hand, that with ginh ¼ 0.20 dL gꢁ1 when the all-exo isomer
was used. The prominent isomer effect observed in the pres-
ent work can be explained on the basis of the significantly
different steric structures as schematically depicted in Figure
5; in the H-PMDA system with the all-exo configuration, after
one functional group reacted, the another surviving func-
tional group may be partially ‘‘covered/shielded’’ by the adja-
cent growing chain, thereby the approaching/attack of other
reactants to the surviving functional group is prevented. In
contrast, such steric hindrance is probably less effective in
the H00-PMDA system since the functional groups orient
almost opposite directions each other. An alternative possible
interpretation for the present isomer effect is based on the
difference of cyclic oligomer formability. A previous article
reported that the cyclic oligomers can be formed in the
reaction of mellophanic dianhydride (MLPDA) and nonsubsti-
tuted flexible diamines with a symmetric structure (e.g., 4,40-
methylenedianiline and 4,40-ODA),33 although the reactions
using asymmetric or rigid diamines at considerably high
monomer concentrations most likely lead preferably to the
corresponding linear polymers.34 In this context, one may
imagine that the chain structure of H-PMDA/4,40-ODA (ginh
¼ 0.57 dL gꢁ1) may be more suitable for the cyclic oligomer
FIGURE 4 Comparisons of polymerization reactivity for H00-
PMDA systems with other systems: (l) H-PMDA, (ꢃ) BTA, and
(~) CBDA systems. The dotted line denotes the relation: Y ¼ X.
Table 2 shows the results of polyaddition of H00-PMDA and
various diamines. When aromatic diamines were used, the
polyaddition smoothly proceeded and led to homogeneous/
viscous PAA solutions with high ginh values (1.2–4.0 dL gꢁ1
)
formation than the asymmetric H00-PMDA counterpart (ginh
¼
except for the systems using 1,4-phenylenediamine (p-PDA,
in DMAc) and bis[4-(4-aminophenoxy)phenyl] sulfone
(BAPS) with an electron-withdrawing sulfone group. The
reaction with TFMB also proceeded without the polymeriz-
ability problems predicted from the presence of the electron-
withdrawing CF3 substituents and led to a sufficiently high
ginh value of 1.25 dL gꢁ1 (Mw ¼ 1.8 ꢃ 105, Mn ¼ 6.3 ꢃ 104
for the chemically imidized powder sample). On the other
hand, the use of cycloaliphatic diamines caused salt forma-
tion in the initial stage. However, the salt formed in the H00-
PMDA systems was not so tight; the salt started to dissolve
after several hours, consequently, the reaction mixtures were
almost homogenized by stirring at room temperature for 12–
24 h. In contrast, the conventional hydrogenated PMDA, that
is, H-PMDA shows much lower polymerizability; for example,
ginh ¼ 0.57 dL gꢁ1 even when highly reactive 4,40-ODA was
selected as a diamine. In particular, the systems using less
reactive or rigid diamines give rise to a crucial problem, that
is, poor film-formability, corresponding to very low ginh val-
ues (e.g., 0.1–0.3 dL gꢁ1 for the H-PMDA/TFMB system). The
significant difference of the polymerizability between H00-
PMDA and H-PMDA is observed independent of the catego-
ries of the diamines used (aromatic/cycloaliphatic and flexi-
ble/rigid) as illustrated in Figure 4. One also notices that
BTA is less reactive than H-PMDA. In contrast, CBDA shows
comparable or higher polymerizability than H00-PMDA.
2.47 dL gꢁ1). However, as far as H-PMDA was used, no dras-
tic ginh enhancement was observed even under an estab-
lished reaction condition unfavorable for the cyclic oligomer
formation; for example, the ginh remained rather low (0.22
dL gꢁ1) even in the reaction of H-PMDA and rigid/highly re-
active 4,40-diaminobenzanilide (DABA) at a high solid con-
tent. This result suggests that the cyclic oligomer formation
is probably not the primary reason why H-PMDA possesses
much lower polymerizability than H00-PMDA.
Properties of H00-PMDA-based PI films Prepared on
Thermal Imidization
Optical Properties
As shown in Table 3, H00-PMDA was very effective to com-
pletely erase PI film coloration for all systems examined as
suggested from very high light transmittance (T400 > 80%),
unlike that the degree of PI film coloration usually strongly
depends on the diamines when common aromatic tetracar-
boxylic dianhydrides were used. The H00-PMDA-based PI films
(T) tend to be somewhat superior in the film transparency
to the H-PMDA-based counterparts, for example, T400
¼
83.4% (H00-PMDA/TPEQ) versus 79.4% (H-PMDA/TPEQ),
and 83.9% (H00-PMDA/BAPP) versus 74.7% (H-PMDA/BAPP).
The slightly decreased transparency of the H-PMDA-based
systems may be due to partial deterioration of a trace
amount of the terminal amino groups, which probably
becomes practically negligible when the molecular weights
are considerably high as in the H00-PMDA-based systems.
Thus, we observed a prominent steric structure effect on the
PAA polymerizability between H00-PMDA and H-PMDA. How-
ever, there are no clear descriptions on this effect for other
cycloaliphatic dianhydride systems in the literature. For
example, Matsumoto et al.32 reported that the polyaddition
of bicyclo[2.2.2]octane-2-endo,3-endo,5-exo,6-exo-tetracarbox-
The cutoff wavelength (kcut) is an index representing the
transparency in the ultraviolet range. The kcut values (244–
319 nm) for the H00-PMDA/aromatic diamine systems (T)
were much lower (i.e., higher uv transmittance) than those
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FIGURE 5 Predicted steric hindrance for polyaddition reaction by the growing chains adjacent to the functional groups for H00-
PMDA and H-PMDA systems. A model reaction with aniline is depicted here for simplicity.
of another type of semiaromatic PI systems derived from ar-
omatic tetracarboxylic dianhydrides and cycloaliphatic dia-
mines (e.g., kcut ¼ 330 nm for PMDA/4,40-methylenebis(cy-
clohexylamine) (MBCHA) system and 366 nm for 3,30,4,40-
biphenyltetracarboxylic dianhydride (s-BPDA)/MBCHA sys-
tem, where there exists electronic conjugation between the
imide C¼¼O groups and the adjacent aromatic ring. The pres-
ent results originate from a combined effect of an overall
decrease in the p electrons and the nonextended conjugation
in the H00-PMDA-based diimide unit.
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TABLE 3 Film Properties of H00-PMDA-Based Homo-PIs
T400
Curea (%)
kcut
Tg
CTE
WA
E
eb
rb
Td5
Td5
No. Diamine
(nm) Dnth
eopt
(ꢀC) (ppm Kꢁ1
)
(%)
(GPa) (%) (MPa) (N2) (ꢀC) (air) (ꢀC)
1
2
4,40-ODA
T
C
T
C
T
T
T
T
T
C
T
C
T
C
T
C
T
T
C
T
C
T
T
T
C
T
T
T
86.0
92.9
86.0
87.8
83.4
81.9
83.9
84.5
81.0
83.2
83.3
83.8
88.5
88.6
82.5
87.0
80.5
83.8
87.6
86.4
86.6
83.5
78.6
83.5
89.5
86.6
84.7
81.4
292
306
288
306
305
290
294
315
301
315
296
297
277
277
290
296
278
319
322
294
307
313
339
295
291
244
248
258
0.0012 2.89 328
0.0099 2.82 302
0.0007 2.88 277
0.0090 2.86 253
0.0018 2.92 285
0.0008 2.82 239
0.0007 2.89 256
0.0000 2.99 289
0.0002 2.93 283
0.0050 2.87 283
0.0010 2.65 292
0.0204 2.64 290
0.0004 2.66 327
0.0135 2.64 319
0.0005 2.73 264
0.0202 2.75 277
0.0020 2.82 407
0.0257 2.92 360
0.0306 2.80 330
0.0096 2.87 369
0.0149 2.83 368
0.0293 2.91 290
0.0277 2.97 381
0.0016 2.68 330
0.0253 2.65 339
0.0002 2.62 285
0.0005 2.58 270
0.0005 2.66 346
54.9
57.1
54.5
52.1
60.8
61.9
63.1
58.9
56.2
54.0
54.8
38.8
56.1
55.7
63.8
59.4
42.7
46.1
45.6
47.5
46.8
46.1
40.5
47.2
43.5
65.8
62.8
56.6
2.32 2.87
2.91 2.31
2.83 2.10
2.86 1.69
1.89 2.04
1.37 1.54
1.43 1.68
1.62 2.11
73
65
74
53
31
29
70
51
9
114
145
86
484
–
455
–
3,40-ODA
449
–
451
–
86
3
4
5
6
7
TPEQ
TPER
BAPP
BAPB
BAPS
105
64
480
–
470
–
80
477
469
467
–
457
420
447
–
116
85
–
–
2.03
1.73
6
50
8
TFODA
BA6F
1.12 2.43
1.93
1.75 2.39
1.95
1.41 2.07
1.59
56
59
8
122
85
463
476
463
464
451
475
460
486
486
462
453
431
462
463
477
443
430
445
421
409
443
414
412
402
424
421
404
427
420
428
441
422
422
373
360
394
–
9
81
–
9
95
10
HFBAPP
61
58
20
9
115
42
–
11
12
p-PDA
o-TOL
3.02 2.53
2.69 3.14
2.92 2.95
3.31 3.17
3.42 3.03
92
173
116
119
106
97
6
13
m-TOL
11
7
14
15
16
APAB
DABA
TFMB
–
3.22
7
5.74 2.61
1.55 2.22
27
18
28
14
8
133
108
117
109
112
132
–
2.23
17
18
19
a
MBCHA
M-MBCHA
t-CHDA
1.52 1.88
2.72 2.16
–
2.78
10
Imidization (cure) processes; C, chemical imidization; T, thermal
imidization.
The H00-PMDA-based PI films (T) also essentially showed
very low birefringence (Dnth < 0.002) except for some rigid
diamine systems, reflecting to the three-dimensionally ran-
dom chain orientation. Lower Dnth values correspond to
lower optical retardation (Rth ¼ Dnth d, where d ¼ film
thickness), which is a desirable property as plastic substrates
in LCDs. On the other hand, the systems using rigid diamines
(o- and m-tolidine, APAB, and DABA) showed one-order
higher Dnth values (ꢂ0.01–0.03), although they are still in a
low level. This is probably due to a certain degree of in-
plane chain orientation behavior, which occurs upon the
thermal imidization process with a close relation to the back-
bone stiffness/linearity.35,36
plete removal of p electrons was also effective for further
reducing eopt, for example, 2.58 for H00-PMDA/M-MBCHA.
Thus, the results are attributed to the same reason as that
for reducing kcut as mentioned above.
Thermal Properties
The H00-PMDA-based PIs derived from ether-containing dia-
mines were essentially thermoplastic as suggested from an
abrupt E0 decrease above the Tg in the dynamic mechanical
analysis (DMA) curves. When 4,40-ODA was used as a typical
flexible diamine, the PI film (T) exhibited a very high Tg of
328 ꢀC as listed in Table 3, although the Tg inevitably
decreased by incorporation of meta-linkages (3,40-ODA) and
bulky substituents (TFODA) and an increase in flexible ether
linkages (TPEQ, TPER, BAPP, etc.). On the other hand, the use
of rigid diamines (typically, p-PDA, ꢀo-TOL, m-TOL, and DABA)
drastically enhanced the Tg (>360 C). The Tg’s of H00-PMDA-
based systems were compared with those of the correspond-
ing H-PMDA-based counterparts as shown in Figure 6; the
The H00-PMDA-based PI systems (T) also display relatively
low eopt values ranging 2.82–2.99 even when nonfluorinated
common aromatic diamines were used. The incorporation of
less polar CF3 group gave rise to a significant decrease in
e
opt, for example, 2.65 for the H00-PMDA/TFODA system. Com-
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for example, 40.5 ppm Kꢁ1 for H00-PMDA/DABA, which is
somewhat lower than for the isomeric H-PMDA/DABA sys-
tem (46.7 ppm Kꢁ1). However, the CTE level of the former is
still far from a criterion for so-called ‘‘low-CTE systems’’
(< ꢂ20 ppm Kꢁ1) in contrast to a very low CTE (0.2 ppm
K
ꢁ1) of the aromatic counterpart, that is, PMDA/DABA.39 It
is generally accepted that there is
a good correlation
between CTE and the degree of in-plane chain orientation in
a fixed system.35,36 Therefore, the common level of the CTE
values for the H00-PMDA-based systems reflect that the imid-
ization-induced in-plane orientation phenomenon became
ineffective by their highly bent/nonplanar chain structure.
Water Absorption Behavior
The H00-PMDA/4,40-ODA film (T) showed a WA value of
2.32% (Table 3), which is a common level to conventional PI
systems. In the systems using nonfluorinated flexible dia-
mines, WA tends to decrease as the diamine molecular
weight is increased: 2.32% (4,40-ODA) > 1.89% (TPEQ) >
1.62% (BAPB) > 1.43% (BAPP). This trend is probably related
to a decrease in the content of highly polarizing imide groups.
As expected, the incorporation of CF3 group with the lowest
polarizability per a unit volume led to a further decrease in
WA (1.12% for the TFODA system). A secondary factor influ-
encing WA is the degree of chain packing,22 as illustrated from
a higher WA (1.55%) for a highly bent/nonplanar structure of
H00-PMDA/TFMB than that for the aromatic counterpart, a rod-
like structure of PMDA/TFMB (WA ¼ 0.47%40).
FIGURE 6 Comparison of Tg of thermally imidized PI films for
H00-PMDA-based and H-PMDA-based systems (see Table 3 for
the sample numbers inserted). The dotted line denotes the
relation: Y ¼ X.
ꢀ
former always shows somewhat (7–24 C) lower Tg’s than the
latter. The results can be explained in terms of the difference
of the degree of the dipole–dipole intermolecular interaction
between the imide C¼¼O groups, which is believed to be an
origin of strong intermolecular forces acting as physical cross-
links in aromatic PIs;37,38 assuming the presence of a similar
intermolecular interaction in the present systems, the asym-
metric nonplanar steric structure of the H00-PMDA-diimide
moiety is probably disadvantageous for denser chain stacking.
However, it should be noted that the present trend, that is, Tg
(H00-PMDA system) < Tg (H-PMDA system) is limited to the
cases using flexible diamines; the combinations of H-PMDA
and rigid diamines (e.g., p-PDA, o- and m-TOL) caused a cru-
cial problem, that is, poor film formability. In this viewpoint,
H00-PMDA is much superior to H-PMDA in aiming at a higher
level of heat resistance by using such rigid diamines. The Tg-
influencing overall structural rigidity for the H00-PMDA-based
systems can be relatively speculated from the order of the Tg
in analogous systems using a fixed diamine, 2,2-bis[4-(4-ami-
Mechanical Properties
The H00-PMDA-based PI films (T) provide common tensile
modulus (E) ranging 1.54–3.22 GPa without significant dif-
ference with those of the H-PMDA-based counterparts. On
the other hand, a prominent isomer effect was observed in
the comparison of film toughness as shown in Figure 7; the
use of ether-containing diamines led to highly tough films,
for example, eb ¼ 73% for H00-PMDA/4,40-ODA and 70% for
H00-PMDA/BAPP, which are comparable to eb of a typical
tough PI film (eb ¼ 85% for PMDA/4,40-ODA).40 On the one
hand, the corresponding H-PMDA systems displayed further
enhanced eb values in spite of their lower ginh as shown in
nophenoxy)phenyl]propane (BAPP): Tg ¼ 287 ꢀC (PMDA/
34
ꢀ
ꢀ
BAPP) > 280 C (MLPDA/BAPP) > 265 C (H-PMDA/BAPP)
> 256 C (H00-PMDA/BAPP).
ꢀ
As shown in Table 3, the H00-PMDA-based PIs derived from
aromatic diamines also possess relatively good thermal sta-
bility (chemical heat resistance) as suggested from the Td5
ꢀ
values (in N2) exceeding 450 C, although a decrease in T5d is
inevitable when cycloaliphatic diamines were used. The H00-
PMDA-based PIs still maintain high T5d values even in a
thermo-oxidative atmosphere (in air), for example, 455 ꢀC
for H00-PMDA/4,40-ODA.
The H00-PMDA-based PI systems (T) derived from flexible
diamines (category I and III) provided CTE values ranging
54–66 ppm Kꢁ1 (Table 3), which are a common level to flex-
ible polymer systems. The present CTE values were also
comparable to those for the H-PMDA-based systems. On the
other hand, the combinations of H00-PMDA and rigid diamines
(category II) brought about an appreciable decrease in CTE;
FIGURE 7 Comparison of eb of thermally imidized films for H00-
PMDA-based (blue/left bar) and H-PMDA-based PIs (red/right
bar). The systems without red/right bars practically represent
eb ¼ 0% or difficulty of mechanical stretching test for the ab-
sence of film-forming ability.
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TABLE 4 Solubility of H00-PMDA-Based Homo-PIs
Solvent
Diamine
4,40-ODA
Sample Form (Cure)
NMP
DMAc
DMF
DMSO
m-Cresol
CPN
GBL
THF
Film (T)
þ
þþ
þ
þþ
6
þ
6
6
þ
þ
þ
þ
6
ꢁ
þ
6
þ
þ
þ
þ
þ
þ
þ
þ
ꢁ
þ
þ
þ
þ
6
ꢁ
þ
þ
ꢁ
ꢁ
ꢁ
ꢁ
6
ꢁ
Powder (C)
Film (T)
þþ
þþ
þþ
6
þþ
þþ
þþ
6
þþ
þ
þþ
6
ꢁ
þþ
þ
þþ
6
ꢁ
3,40-ODA
þ
ꢁ
Powder (C)
Film (T)
þ
ꢁ
TPEQ
TPER
BAPP
BAPB
BAPS
ꢁ
ꢁ
Film (T)
ꢁ
6
ꢁ
6
ꢁ
6
ꢁ
6
ꢁ
ꢁ
6
ꢁ
Film (T)
6
6
ꢁ
ꢁ
Film (T)
6
6
6
6
6
ꢁ
Film (T)
þ
þþ
þ
þþ
þ
þþ
þþ
þþ
ꢁ
þ
þþ
þ
þ
6
þ
þþ
þ
þþ
þþ
þþ
þþ
þþ
ꢁ
þ
þþ
þ
þ
6
þ
þ
þþ
þ
þþ
þ
þþ
þþ
þþ
ꢁ
þ
þþ
þ
þ
6
ꢁ
þþ
þ
þþ
þ
þþ
þ
ꢁ
Powder (C)
Film (T)
þþ
þþ
þþ
þþ
þþ
þþ
þþ
ꢁ
þþ
þ
þþ
þ
þ
þ
þ
TFODA
BA6F
ꢁ
Powder (C)
Film (T)
ꢁ
þ
Powder (C)
Film (T)
þþ
þ
HFBAPP
Powder (C)
Film (T)
þþ
ꢁ
þþ
ꢁ
þþ
ꢁ
p-PDA
o-TOL
Film (T)
þ
þþ
þ
þ
6
ꢁ
þ
ꢁ
Powder (C)
Film (T)
þ
þ
ꢁ
m-TOL
6
ꢁ
ꢁ
Powder (C)
Film (T)
þ
6
ꢁ
þ
þþ
ꢁ
ꢁ
ꢁ
þ
ꢁ
APAB
DABA
TFMB
ꢁ
ꢁ
Film (T)
þ
þ
þþ
ꢁ
ꢁ
ꢁ
þ
þ
þþ
ꢁ
ꢁ
ꢁ
þ
þ
þþ
ꢁ
ꢁ
ꢁ
þ
þ
þþ
ꢁ
ꢁ
ꢁ
þ
ꢁ
Film (T)
þ
ꢁ
Powder (C)
Film (T)
þþ
ꢁ
þþ
ꢁ
MBCHA
M-MBCHA
t-CHDA
Film (T)
ꢁ
ꢁ
ꢁ
ꢁ
Film (T)
(þþ) Soluble at room temperature, (þ) soluble upon heating, (6) deformed
or swelled, and (ꢁ) insoluble. CPN, cyclopentanone; GBL, c-butyrolactone.
Figure 7. It is also obvious how the toughening effect gener-
ated by the combination of H-PMDA and flexible diamines is
prominent from a comparison: eb ¼ 236% for H-PMDA/
BAPP (ginh ¼ 0.73 dL gꢁ1) versus eb ¼ 169% for PMDA/
BAPP (ginh ¼ 2.30 dL gꢁ1). Thus, H-PMDA monomer is
undoubtedly effective for film toughening. This may stand
for the importance of its molecular shape (see Fig. 1) which
contributes to chain entanglement just like ether linkages,
although the detailed mechanism is not clear. However, it
should be noted that the present situation, that is, eb (H00-
PMDA systems) ꢅ eb (H-PMDA systems) is limited to the
systems using flexible diamines; when rigid diamines were
used, the H-PMDA systems completely lost film-forming abil-
ity probably owing to their too low molecular weights (e.g.,
ginh ¼ 0.10 dL gꢁ1 for H-PMDA/TFMB). Thus, the excellent
film-forming ability irrespective of diamines is a prominent
feature of H00-PMDA.
Solution Processability
It is empirically known that thermally imidized film samples
tend to decrease solubility compared to the corresponding
chemically imidized powder samples as discussed later.
Nonetheless, the H00-PMDA/4,40-ODA system (T) possesses
good solubility in hot NMP while keeping homogeneity even
after cooling to room temperature. The H00-PMDA/3,40-ODA
system (T) displayed further improved solubility as shown
in Table 4. Even the rigid diamine systems (DABA, o-, and m-
TOL) display good solubility in hot amide solvents. In gen-
eral, highly organo-soluble PIs include some solubility-pro-
moting groups (e.g., CF3, –SO2–, and bulky side groups) and
significantly distorted units in the structures.41–43 It should
be noted that the H00-PMDA-based systems (T) show good
solubility even without any solubility-promoting groups. The
introduction of such solubility-promoting groups caused fur-
ther solubility improvement by a combined effect. However,
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TABLE 5 Solubility of H-PMDA-Based PIs
Solvent
DMSO
Diamine
Sample Form (Cure)
NMP
DMAc
DMF
m-Cresol
CPN
GBL
THF
4,40-ODA
3,40-ODA
TPEQ
Film (T)
Film (T)
Film (T)
Film (T)
Film (T)
Film (T)
Powder (C)
Film (T)
Film (T)
6
6
6
6
6
6
6
6
6
þ
þþ
–
6
6
6
6
6
þ
þþ
–
þ
6
þ
6
þ
þ
þþ
–
–
–
–
6
–
6
–
–
6
6
–
TPER
6
6
–
–
–
BAPP
6
6
–
–
–
BAPS
þþ
þþ
–
þþ
þþ
–
–
–
–
HFBAPP
MBCHA
M-MBCHA
þþ
–
þþ
–
þþ
–
–
–
–
–
–
–
–
–
(þþ) Soluble at room temperature, (þ) soluble upon heating, (6)
deformed or swelled, and (ꢁ) insoluble.
H00-PMDA was less effective for the nonaromatic systems
derived from cycloaliphatic diamines.
and CBDA/TFMB) are surely categorical in colorless high-Tg
systems as plotted in the target area in this figure. However,
in fact, these are beyond the present candidates because of
their poor solution processability. The TA-BPFL/t-CHDA sys-
tem (see the bottom of Figure 8 for its monomer structure)
shows a high Tg exceeding 300 ꢀC, good transparency, and
NMP-solubility in contrast to the fact that the t-CHDA-based
PIs are usually quite insoluble. However, this PI film is some-
what brittle,30 which may be a negative feature of fluorene-
containing cardo-type PIs. On the other hand, 6FDA/TFMB
system, which contains solubility-promoting CF3 groups with
a high content, is an only limited case possessing the present
target properties. It should be noted that some of the present
H00-PMDA-based PI systems achieved the target properties
without any solubility-promoting substituents. As an represen-
tative example, the data for the H00-PMDA/4,40-ODA system
(*), which is also highly tough and solution processable, is
plotted within the target area in Figure 8.
Comparing with the solubility of the H-PMDA-based counter-
parts (Table 5), a prominent isomer effect is obvious; the H-
PMDA-based samples (T) are essentially insoluble. The sig-
nificant difference of the solubility between these isomeric
systems originates from the quite different steric structures
of H00-PMDA and H-PMDA, although no distinct isomer effect
on the solubility was recognized in other analogous PI sys-
tems derived from bicyclo[2.2.2]octane-2-endo,3-endo,5-
exo,6-exo-tetracarboxylic dianhydride and the all-exo iso-
mer.32 The order of the solubility for the H00- and H-PMDA-
based model compounds also corresponds to the present
results. The much better solubility for the H00-PMDA/aro-
matic diamine systems than for the H-PMDA-based counter-
parts can be also explained by reduced intermolecular
dipole-dipole interaction based on the highly bent/nonplanar
steric structure at the H00-PMDA-based diimide unit. Compar-
ing with analogous systems that we have so far investigated,
the dependence of the relative solubility on the tetracarbox-
ylic dianhydrides used can be represented by the following
order: H00-PMDA ꢆ MLPDA34 > H-PMDA ꢇ PMDA, reflecting
their structural planarity/linearity at the diimide fragments
rather than their aromaticity. We have previously investigated
the property difference between MLPDA- and PMDA-based PI
systems and found that the former possesses always slightly
lower Tg’s than the latter, but the solubility difference between
them was remarkable.34 One notices that a very similar iso-
mer effect are also observed between H00-PMDA and H-PMDA
systems (small Tg difference but large solubility difference).
Effects of Chemical Imidization
If the selected PIs were originally soluble, chemical imidiza-
tion process (Fig. 2) can be applied since imidization is com-
pleted without gelation/precipitation. A great merit of this
process in a practical viewpoint is that PI films can be
formed by simple casting (coating and drying) of homogene-
ous PI solutions without subsequent thermal imidization.
Another advantage is that a significant molecular weight
decrease is suppressed, in contrast to thermal imidization in
solution. The H00-PMDA-based systems were essentially com-
patible to the chemical imidization process. On the other
hand, this process was often not applicable to the H-PMDA-
based systems owing to a trend of inhomogenization except
for H-PMDA/HFBAPP and H-PMDA/BAPS. When cycloali-
phatic diamines were used, a curious phenomenon is often
observed, that is, chemical imidization is not completed even
if the reaction mixtures remained homogeneous. No discus-
sion has been done for the mechanism in the literature. A
possible explanation is as follows; the intramolecular nucleo-
philic attack of the amide group to the adjacent carbonyl
Figure 8 shows a Tg–transparency–solubility diagram. Many
of conventional engineering plastics are originally colorless,
however, their Tg’s do not exceed 230 ꢀC. PIs tend to lose
the film transparency and the solubility as soon as Tg
ꢀ
exceeds 300 C in the X-axis, suggesting how it is difficult to
achieve our target properties (T400 > 80%, Tg > 300 ꢀC,
good solution processability, and high toughness). Some
semicycloaliphatic PIs (PMDA/MBCHA, H-PMDA/4,40-ODA,
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FIGURE 8 Tg–transparency–solubility diagram: (
^
, h, *) soluble and (~) insoluble systems, 1: bisphenol A-type polycarbonate, 2:
R
PES, 3: poly(ether imide) (UltemV1000, Saudi Basic Industries), 4: HTA-44BP/o-TOL, 5: 6FDA/TFMB, 6: TA-BPFL/TFMB, 7: TA-BPFL/t-
CHDA, 8: ODPA/MBCHA, 9: H-PMDA/BAPP, 10: PMDA/MBCHA, 11: H-PMDA/4,40-ODA, 12: CBDA/TFMB, 13: s-BPDA/TFMB, 14:
HQDEA/o-TOL, 15: s-BPDA/4,40-ODA, 16: DSDA/4,40-ODA, 17: TA-HQ/4,40-ODA, 18: s-BPDA/p-PDA, 19: TA-HQ/m-TOL 20: PMDA/
TFMB, 21: H00-PMDA/4,40-ODA system.
group for cyclodehydration (imidization) is promoted by the
electron-withdrawing AOAc group in the expected intermediate
as depicted in Figure 2. If the amide groups were partially pro-
tonated by free acetic acid as a by-product arising from Ac2O,
its nucleophilic reactivity would be significantly reduced. Recall
that, at the stage of aliphatic diamine monomers, they possess
much higher basicity than aromatic ones. Therefore, such proto-
nation may occur in case of the aliphatic amide groups if suffi-
cient proton-accepting ability would remain.
chemical imidization process as illustrated from slightly
higher T400 values than those of the corresponding thermally
imidized films. The results probably originate from an end-
capping effect of the thermally less stable terminal amino
groups with Ac2O during chemical imidization as shown in
Figure 2. The lower maximum temperature in the film for-
mation process also probably contributes to the increased
transparency of the films (C).
Another merit of the chemical imidization process is the
drastically improved solution processability as shown in Ta-
ble 4. This is probably attributed to looser chain packing in
the chemically imidized samples. For example, the H00-PMDA/
4,40-ODA sample (C) became soluble in c-butyrolactone
Table 3 also lists the properties of the H00-PMDA-based PI
films (C). The properties do not essentially depend on the
imidization methods. However, one notices that the transpar-
ency of the PI films appreciably increased by adopting the
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(GBL) without heating. In addition, H00-PMDA/TFMB (C) is
highly soluble at room temperature even in GBL and cyclo-
pentanone (CPN), which behave as poor solvents for many
aromatic PI systems. A large number of previous studies on
organo-soluble PIs primarily discussed the solubility in polar
amide solvents,41–43 however, it seems that no great attention
has been paid for the solubility in less hygroscopic solvents such
as GBL. These solvents are very useful to avoid serious troubles
by moisture absorption in some applications, for example,
screen-printing using PI solutions; in this case, the use of amide
solvents often gives rise to jamming of screen-printing plates.
One also notices another feature of the H00-PMDA-based films
(C); they tend to provide slightly lower CTE than the ther-
mally imidized counterparts, in accordance with higher Dnth
values as compiled in Table 3. The results indicate that the
casting process induced a certain degree of in-plane chain ori-
entation.44 The H00-PMDA/TFODA film (C), as well as H00-
PMDA/TFMB (C), displayed the lowest CTE (ꢂ40 ppm Kꢁ1
)
as shown in Table 3. However, it may be a lower limit of CTE
achievable in the homo PI systems derived from H00-PMDA.
Attempts to Decrease CTE
Recently, plastic substrate materials possessing excellent dimen-
sional stability against thermal cycles in device fabrication proc-
esses are required in some cases. Electronic components (e.g.,
thin-film transistor) formed on the plastic substrates inevitably
undergo a positional shift accompanied with reversible expan-
sion/contraction of the substrates in the glassy temperature
region (< Tg) during multiple thermal cycles. However, if such
dimensional change was appreciably irreversible or hysterical,
it would become a great obstacle for the device fabrication. The
direct strategy is to decrease CTE (ultimately, to aim at zero
CTE), although it is not easy to realize low CTE characteristics
without sacrificing the target properties mentioned above. The
conventional copolymerization approach with rigid comono-
mers for reducing CTE often brings about serious deterioration
of film transparency and solution processability. In this work,
we used some rigid comonomers to improve the dimensional
stability. Table 6 shows the properties of the H00-PMDA/TFMB-
based copolymers and the comonomer structures. In spite of a
high rigid comonomer content (50 mol %), chemical imidiza-
tion smoothly proceeded without gelation/precipitation. This is
probably due to a still surviving influence of the originally out-
standing solubility of the pristine H00-PMDA/TFMB system. The
modification with PMDA caused a drastic CTE decrease (26.7
ppm Kꢁ1), but accompanied with an undesirable decrease in
the film transparency. On the other hand, the use of s-BPDA and
TA-HQ as comonomers enabled us to reduce CTE while main-
taining high transparency as shown in Table 6. These copoly-
mers (C) retain good solubility in various solvents.
H00-PMDA was also used as a property-modifier at a minor
content (30 mol %) to improve the film flexibility of origi-
nally colorless and low-CTE PI systems, that is, s-BPDA/t-
CHDA (eb ¼ 9%, CTE ¼ 10 ppm Kꢁ1 19) and TA-HQ/t-CHDA
(eb ¼ 4%, CTE ¼ 13 ppm Kꢁ1 39). The use of H00-PMDA as a
comonomer somewhat improved the film toughness (eb
20%) but with an increase in CTE, although the chemical
¼
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ventional aromatic polymers other than PIs usually do not
generate such high Dnth values by simple solution casting as
illustrated from the fact a NMP-cast PES film showed a con-
siderably low Dnth approximate to zero. Heat resistance of the
optical compensation film itself is beyond the present target
properties. The higher Dnth values mean that even the thinner
films can accomplish an aiming Rth value on the basis of the
relation: Rth ¼ Dnth d. In this work, we also established
another target, that is, considerably low wavelength depend-
ence (dispersion) of Rth. The magnitude of the Rth-wavelength
dispersion can be simply represented by an optical retarda-
tion (birefringence) ratio: R450/R550 ¼ Dn450/Dn550 where the
subscripts denote wavelength (k), although actual normal
wavelength dispersion curve (Dn450/Dn550 > 1) is asymptotic
as represented by an empirical equation: Dnth ¼ a(1 þ b/k2
þ c/k4) or Dnth ¼ A þ B/(k2 – C2)46 where a, b, c, A, B, and C
are constants depending on the materials.
FIGURE 9 Relationship between CTE and Dnth for H00-PMDA -
based systems: (~) thermally imidized homo PIs, (n) chemi-
We measured the Dn450/Dn550 values for a variety of solution
processable colorless PIs because it is expected to provide
higher Dnth by the casting-induced in-plane orientation behav-
ior. The results revealed that the dispersion parameter distrib-
uted within 1.07–1.16 in most cases. In this case, a good cor-
relation was observed; Dn450/Dn550 decreases with
decreasing kcut. This relation is convincing from an asymptotic
shape of the Dnth–k curves where the slope (in other words,
Dn450/Dn550) decreases with increasing k. A blue-shift of light
transmission curves (i.e., a decrease in kcut) probably causes a
blue-shift of the Dnth–k curves, consequently, the slope further
reduces in the longer wavelength range. Indeed, a thermally
imidized wholly cycloaliphatic PI film showed an ultimately
low wavelength dispersion (Dn450/Dn550 ¼ 1.00) although it
was not compatible to the present film formation process.
cally imidized homo PIs, (*) thermally imidized co-PIs, and (^)
chemically imidized co-PIs.
imidization process remained not applicable. On the one
hand, the incorporation of H00-PMDA to TA-HQ/t-CHDA main-
tained low CTE (14.2 ppm Kꢁ1), which results from a high
extent of in-plane chain orientation as illustrated from a very
high Dnth exceeding 0.1. The data of Dnth and CTE were plot-
ted for all the H00-PMDA-based systems investigated in this
work as shown in Figure 9. The results suggest that the pres-
ent approach is undoubtedly effective to decrease CTE, how-
ever, a significant increase in Dnth is inevitable. Even nonaro-
matic PI systems with a lower polarization anisotropy were
difficult to erase Dnth while keeping low CTE characteristics,
for example, CTE ¼ 26 ppm Kꢁ1 and Dnth ¼ 0.02 for CBDA/t-
CHDA PI film.20 In this context, the present materials are not
perfect as the plastic substrates for LCDs where backlight
passes through the substrates, since the birefringence of the
substrates themselves causes optical retardation as an origin
of a contrast decrease at higher view angles. Instead, the pres-
ent materials can be applied as the plastic substrates for elec-
tronic paper displays with an external light reflection mode.
In this context, the H00-PMDA-based PI systems are advanta-
geous owing to their low kcut values (Table 3) based on the
less extended electronic conjugation. For the present pur-
pose, the H00-PMDA/TFMB system was chosen. The PI film
(12 lm thick) was prepared by coating/drying a CPN solu-
ꢀ
tion (10 wt %) of the chemꢀically imidized sample at 60 C/1
ꢀ
h þ 100 C/10 min þ 150 C/15 min on a glass substrate in
Potential Application of H00-PMDA -based PI systems
On the basis of the aforementioned features of the H00-PMDA-
based PIs, we also propose a potential application as a novel
coating-type optical compensation film material (negative-C
plate, nx ¼ ny > nz, i.e., Dnth > 0) with low wavelength de-
pendence of Rth for vertical alignment-mode LCDs.45 The tar-
get materials need to be formed by solution casting on tria-
cetylcellulose (TAC) films as a standard protective film for
iodine-doped poly(vinyl alcohol) polarizers. For this purpose,
the casting solvents should be sufficiently volatile and non-
an air-convention oven and additional heat treatment at 150
ꢀ
corrosive to the TAC substrates [typically, CPN (b.p. 131 C)].
The H00-PMDA/TFMB system (C) has excellent CPN-process-
ability, that is, good solution stability even at high solid con-
tents (ꢆ10 wt % in CPN at room temperature). Other
desired characteristics are noncoloration, nonturbidity, and a
sufficient magnitude of Dnth (desirably, > 0.02) which needs
to generate by simple coating/drying at temperatures lower
FIGURE 10 Birefringence (Dnth) as a function of wavelength for
H00-PMDA/TFMB PI film cast from cyclopentanone solution.
ꢀ
than 150 C to avoid thermal deformation of TAC films. Con-
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ꢀC/30 min in vacuum after peeling the film off from the sub-
strate. The PI film was sufficiently flexible and colorless
without appreciable turbidity (Ttotal ¼ 91%, Haze ¼ 0.36%,
3 Polyimides: Materials, Chemistry and Characterization; Feger,
C.; Khojasteh, M. M.; McGrath, J. E., Eds.; Elsevier: Amster-
dam, 1989.
4 Advances in Polyimide Science and Technology; Feger, C.;
Khojasteh, M. M.; Htoo, M. S., Eds.; Technomic: Lancaster,
1993.
T400 ¼ 87.4%, kcut ¼ 291 nm). Figure 10 shows Dn450
/
Dn550 as a function of k. The H00-PMDA/TFMB film displayed
low wavelength dispersion (Dn450/Dn550 ¼ 1.04), which
slightly fluctuated within 1.04–1.06 depending upon the drying
temperature program, the solvent used, and the PI molecular
weight. This system also provided a sufficient magnitude of
Dnth approximate to 0.03. We also tried to further improve the
low wavelength dispersion property by copolymerizing a fluo-
rene-containing monomer while maintaining the desired prop-
erties mentioned above. The results will be reported elsewhere.
5 Polyamic Acid and Polyimides: Synthesis, Transformation
and Structure; Bessonov, M. I.; Zubkov, V. A., Eds.; CRC Press:
Boca Raton, FL, 1993.
6 Polyimides: Fundamentals and Applications; Ghosh, M. K.;
Mittal, K. L., Eds.; Marcel Dekker: New York, 1996.
7 Polyimides: Trends in Materials and Applications; Feger, C.;
Khojasteh, M. M.; Molis, S. E., Eds.; Society of Plastic Engi-
neers: New York, 1996.
8 Advances in Polyimides and Low Dielectric Polymers; Sach-
dev, H. S.; Khojasteh, M. M.; Feger, C., Eds.; Society of Plastic
Engineers: New York, 1999.
Recently, the present investigation was extended to another
isomer (i.e., H0-PMDA). Studies of the basic properties and
some potential applications for the H0-PMDA-based PI sys-
tems are in progress.26,45
9 Polyimides and Other High Temperature Polymers: Synthe-
sis, Characterization and Application, Vol. 1; Mittal, K. L., Ed.;
VSP: Zeist, The Netherland, 2001.
CONCLUSIONS
10 The Latest Polyimides: Fundamentals and Applications;
Imai, Y.; Yokota, R., Eds.; NTS: Tokyo, 2002.
H00-PMDA showed much higher polymerizability with various
diamines than H-PMDA. The prominent isomer effect can be
explained on the basis of these steric structures.
11 The Latest Polyimides: Fundamentals and Applications,
2nd ed.; Ando, S.; Ueda, M.; Kakimoto, M.; Kochi, M.; Takeichi,
T.; Hasegawa, M.; Yokota, R., Eds.; NTS: Tokyo, 2010.
The thermally imidized H00-PMDA-based films were essentially
colorless regardless of diamines, owing to inhibited CT interac-
tion. In addition, some of the H00-PMDA-based PI films displayed
12 Sroog, C. E. Prog. Polym. Sci. 1991, 16, 561–694.
13 Miyazaki, T.; Hasegawa, M. High Perform. Polym. 2009, 21,
219–244.
ꢀ
excellent combined properties, for example, Tg ¼ 328 C, eb ¼
14 Volksen, W.; Cha, H. J.; Sanchez, M. I.; Yoon, D. Y. React.
Funct. Polym. 1996, 30, 61–69.
73%, and good solution processability for H00-PMDA/4,40-ODA.
On the other hand, the H-PMDA-based counterparts are essen-
tially insoluble in common organic solvents. The outstanding
solubility of the former probably results from disturbed chain
stacking by a highly bent/nonplanar steric structure at the H00-
PMDA-based diimide fragments. Chemical imidization brought
about some prominent advantages compared to thermal imid-
ization: improved film transparency, slightly decreased CTE,
and drastically enhanced solution processability. In some cases,
copolymerization with rigid aromatic tetracarboxylic dianhy-
drides was effective to reduce CTE while maintaining the origi-
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