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Polymer 55 (2014) 4693e4708

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Polymer

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Colorless polyimides derived from 1S,2S,4R,5R-cyclohexanetetracarboxylic dianhydride, self-orientation behavior during solutioncasting, 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, Japanb Iwatani Industrial Gases, Corp., 10 Otakasu-cho, Amagasaki, Hyogo 660-0842, Japan

a r t i c l e i n f o

Article history:Received 23 June 2014Received in revised form19 July 2014Accepted 21 July 2014Available online 28 July 2014

Keywords:Colorless polyimide1S,2S,4R,5R-cyclohexanetetracarboxylicdianhydrideLow coefficient of thermal expansion (CTE)

* Corresponding author. Tel./fax: þ81 47 472 1869.E-mail address: mhasegaw@chem.sci.toho-u.ac.jp

http://dx.doi.org/10.1016/j.polymer.2014.07.0320032-3861/© 2014 Elsevier Ltd. All rights reserved.

a b s t r a c t

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 incontrast to its isomer, i.e., conventional hydrogenated pyromellitic dianhydride (H-PMDA) and leads tohighly flexible and colorless PI films with very high Tg's. In particular, the combinations with rigidstructures 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 theH-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 modelcompound. 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 isproposed in this work. H0-PMDA/TFMB and its copolymer systems can be useful as plastic substrates inimage display devices and/or novel coating-type optical compensation films.

© 2014 Elsevier Ltd. All rights reserved.

1. Introduction

The importance of optically transparent high-temperaturepolymeric materials has been increasing for their extensive appli-cations as optical components, e.g., plastic substrates of imagedisplay devices, liquid crystal alignment layers, color filters, opticalcompensation films, optical fibers, light-guiding plates, and opticallenses. One of the recent key projects is to replace current fragileinorganic glass substrates (300e700 mm thick) in flat panel displaysby plastic substrates (<50 mm thick), thereby the display devicesbecome drastically light and flexible. However, it is very difficult toobtain the substrate materials simultaneously possessing themerits of inorganic glass (i.e., excellent optical transparency, ultra-high heat resistance, and high dimensional stability against thermalcycles undergoing in the device fabrication processes) and organicpolymeric systems (flexibility and processability). Poly(ether

(M. Hasegawa).

sulfone) (PES) is known to possess the highest level of physical heatresistance (Tg ¼ 225 �C) among current colorless engineeringplastics, however, it is not enough for applications to the plasticsubstrate materials. Polymeric materials with very high Tgexceeding 300 �C are practically limited to polyimide (PI) systems,which are widely used in a variety of electronic devices for theirsimple two-step manufacturing processes, excellent electric insu-lation ability based on an extremely high purity (the absence ofvolatile organic compounds and metallic/ionic contaminations),non-flammability, and thermo-oxidative stability (long-term heatresistance) in addition to considerably high Tg's overcoming thesolder-reflowing processes (short-term heat resistance) [1e13].However, conventional aromatic PI films are in general not suitablefor optoelectronic applications because of their intensive colorationarising from charge-transfer (CT) interactions [14].

Much effort has been devoted to erase the PI film coloration. Themost effective strategy is to use aliphatic monomers (usually,cycloaliphatic ones for avoiding significant Tg decrease) either indiamines or tetracarboxylic dianhydrides or both [15e23]. How-ever, most of cycloaliphatic monomers do not lead to PI films with

M. Hasegawa et al. / Polymer 55 (2014) 4693e47084694

low linear coefficients of thermal expansion (CTE) indispensable forgood dimensional stability because their non-linear/non-planarsteric structures disturb the PI chain alignment along the filmplane (XeY) direction (called in-plane orientation) during thethermal imidization process. Cycloaliphatic monomers with rela-tively high structural linearity effective for low CTE generation arelimited 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 theformation of insoluble salt at the initial stage in the polymerizationprocess of PI precursors [poly(amic acid)s (PAAs)], by which thereaction is completely inhibited in some cases or needs prolongperiods 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 isalso advantageous from the viewpoints of the polymerizabilitywith aromatic diamines [23e26], whereas conventional

Fig. 1. Molecular structures of m

cycloaliphatic tetracarboxylic dianhydrides [e.g., bicyclo[2.2.2]oct-7-ene-2,3,5,6-tetracarboxylic dianhydride (BTA)] often lead toinsufficient 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 substratematerials 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-basedPIs [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] and1R,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 arealso discussed. To the best of our knowledge, this is the earliest

onomers used in this work.

Fig. 2. Steric structure of 1,2,4,5-cyclohexanetetracarboxylic acid (H0-PMDA precursor)determined by the single-crystal X-ray diffraction measurements.

M. Hasegawa et al. / Polymer 55 (2014) 4693e4708 4695

report described in detail on the properties of H0-PMDA-based PIsand 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. 1and Table 1. H0-PMDA was synthesized as follows [28]; PMDA wasfirst hydrolyzed with a NaOH aqueous solution. The pyromelliticacid tetrasodium salt formed was hydrogenated in a high-pressurehydrogen atmosphere at 160 �C in the presence of a rutheniumcatalyst. After hydrogenation was completed, the solution wasadditionally heated at a precisely controlled temperature forseveral hours, and cooled to room temperature. The solution wasthen 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. Asmall portion of 1 was quantitatively converted into tetramethylester to analyze the purity of 1 by gas chromatography (GC). The GCchart for the tetramethyl ester of 1 showed a single peak at adifferent elution time from that of the tetramethyl ester counter-part derived from conventional H-PMDA, suggesting the differenceof the steric structures. The tetracarboxylic acid (1) was finallydehydrated with acetic anhydride (Ac2O) to form tetracarboxylicdianhydride (2). Product 2 was difficult to form a sufficient size ofsingle crystal by recrystallization from Ac2O for the X-ray structuralanalysis. Instead, the corresponding tetracarboxylic acid (1) wasused for this purpose. The results revealed that product 1 is1S,2S,4R,5R-cyclohexanetetracarboxylic acid with a chair form of

Table 1The conditions of pre-treatment and purification for monomers.

Monomer Source

1S,2S,4R,5R-Cyclohexanetetracarboxylicdianhydride (H0-PMDA)

Iwatani Industrial Gases

1R,2S,4S,5R-Cyclohexanetetracarboxylicdianhydride (H00-PMDA)

Iwatani Industrial Gases

1S,2R,4S,5R-Cyclohexanetetracarboxylicdianhydride (H-PMDA)

New Japan Chemical

Bicyclo[2.2.2]oct-7-ene-2,3,5,6- tetracarboxylicdianhydride (BTA)

Tokyo Chemical Industry (TCI

1,2,3,4-Cyclobutanetetracarboxylicdianhydride (CBDA)

Nissan Chemical Industries

p-Phenylenediamine (p-PDA) TCI4,40-Oxydianiline (4,40-ODA) Wako Chemical3,40-Oxydianiline (3,40-ODA) JFE ChemicalBis(4-amino-2-trifluoromethylphenyl)

ether (TFODA)Ihara Chemical

m-Tolidine (m-TOL) Wakayama Seikao-Tolidine (o-TOL) Wakayama Seika2,20-Bis(trifluoromethyl)benzidine (TFMB) Wakayama Seika1,4-Bis(4-aminophenoxy)benzene (TPEQ) Wakayama Seika4-Aminophenyl-40-aminobenzoate (APAB) Wakayama Seika4,40-Diaminobenzanilide (DABA) TCI4,40-Bis(4-aminophenoxy)biphenyl (BAPB) Wakayama Seika2,2-Bis[4-(4-aminophenoxy)phenyl]

propane (BAPP)Wakayama Seika

2,2-Bis[4-(4-aminophenoxy)phenyl]hexafluoropropane (HFBAPP)

Wakayama Seika

Trans-1,4-cyclohexanediamine (t-CHDA) Iwatani Industrial Gases4,40-Methylenebis(cyclohexylamine)

(MBCHA) (mixture of cis/trans isomers)New Japan Chemical

4,40-Methylenebis(2-methylcyclohexylamine)(M-MBCHA) (mixture of cis/trans isomers)

Aldrich

DMF ¼ N,N-dimethylformamide. CHX ¼ cyclohexane.a Compound repeatedly recrystallized from acetic anhydride (Ac2O).

the central cyclohexane unit as depicted in Fig. 2 [32]. Fig. 3 showsthe powder X-ray diffraction patterns for 1 and a compound yieldedby hydrolyzing 2 with water. A good agreement between themsuggests that the steric structure of 1 is maintained even afterdehydration with Ac2O. Therefore, product 2 can be identified as1S,2S,4R,5R-cyclohexanetetracarboxylic dianhydride (H0-PMDA).Themost probable steric structure of H0-PMDA is shown in Fig. 4 (c)together with those of H-PMDA and H00-PMDAwhich were directlydetermined 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

Solvent forrecrystallization

Vacuum-dryingcondition

Meltingpoint (�C)

e 150 �C/24 h 274 (284a)

e 150 �C/24 h 267 (273a)

e 150 �C/24 h 303

) Ac2O/DMF (3/1, v/v) 150 �C/24 h e

e 150 �C/12 h 241

Ethyl acetate 30 �C/24 h 142Toluene/DMF (10/1, v/v) 50 �C/24 h 191e 50 �C/24 h 74EtOH/H2O (11/5, v/v) 145 �C/12 h 129

Toluene/CHX (7/3, v/v) 70 �C/24 h 86Toluene 80 �C/24 h 129e 50 �C/24 h 184Toluene/DMF (15/1, v/v) 100 �C/24 h 172e 50 �C/24 h 185e 50 �C/24 h 205Toluene/DMF (15/1, v/v) 50 �C/24 h 197e 50 �C/24 h 129

e 50 �C/24 h 159

e 30 �C/24 h 70e e none

e e e

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.

M. Hasegawa et al. / Polymer 55 (2014) 4693e47084696

N-methyl-2-pyrrolidone (NMP) solution of diamine (10 mmol) atan initial total solid content of 30 wt% with continuous stirring atroom 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) (bFig. 4. Steric structures of hydrogenated PMDA isome

for 72 h). In this case, if necessary, the reaction mixture was grad-ually diluted with a minimum quantity of the same solvent toensure effective magnetic stirring. The polymerization conditionsare listed in Table 2. The formation of PAAs was confirmed by thetransmission-mode FT-IR spectroscopy (Jasco, FT/IR 4100 infraredspectrometer) using separately prepared thin cast films (4e5 mmthick) with non-uniform thickness to erase interference fringes. Atypical FT-IR spectrum is shown in Fig. 6 (a). The spectrum includessome specific bands (cm�1): 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), and1323 (CeF).

PI films were prepared by different methods (thermal andchemical imidization). The reaction schemes are shown in Fig. 5. PIfilms were prepared upon thermal imidization as follows; PAAsolution was bar-coated on a glass substrate and dried at 60 �C/2 hfor DMAc solutions (80 �C/2 h for NMP solutions) in an air-convection oven. We first attempted thermal imidization under arelatively mild condition (e.g., at 250 �C in vacuum) for suppressingPI film coloration. However, the PI films obtained were somewhatbrittle in many cases. This is probably due to partial retro-polyaddition although the generated terminal functional groupscan 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/20minþ 250 �C/30minþ 320 (or 300) �C/1 h in vacuum as adheredon the substrate, and successively annealed in vacuum at 10e20 �Clower temperatures than the Tg's after peeling them off from thesubstrate to eliminate residual stress without undesirable signifi-cant film deformation. The imidization and annealing conditionswere properly adjusted to obtain better quality of PI films. Thethermally imidized film samples are represented as “(T)” from nowon. On the other hand, chemical imidization was conducted bygradually adding a mixed solution of Ac2O/pyridine (7/3, v/v) toadequately 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 thesolution homogeneity. After the chemical imidization process, thehomogeneous reaction mixture was adequately diluted with thesame solvent, and very slowly poured into a large quantity ofmethanol or water as a poor solvent. The fibrous white precipitateobtained was repeatedly washed with methanol/water, collectedby filtration, and dried at 80 �C in vacuum for 12 h. The excess of thechemical imidization reagent is completely eliminated on thisprocedure, which is important for avoiding undesirable filmcoloration as observed in some cases. The dried precipitate was re-dissolved in a fresh solvent [typically, cyclopentanone (CPN)] at asolid content of 10 e 15 wt%. The homogeneous PI solution formedwas 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 vacuumwithout the substrate. The PI films were additionally annealed at

) (c)rs: (a) H-PMDA, (b) H00-PMDA, and (c) H0-PMDA.

Fig. 5. Reaction schemes of polyaddition and imidization of PAAs and predicted end-capping reaction of terminal amino groups during chemical imidization process.

M. Hasegawa et al. / Polymer 55 (2014) 4693e4708 4697

established temperatures. The film or powder samples prepared viathe homogeneous chemical imidization process are denoted as“(C)”. This process was not applicable to some systems, whereinhomogenization (gelation/precipitation) occurs by adding thechemical imidization reagent owing to insufficient solubility of theimidized forms, because imidization is not completed. For the H0-PMDA/p-phenylenediamine (p-PDA) system, the PAA film formedon substrates was dipped in an Ac2O/pyridine (7/3, v/v) solution atroom temperature for 24 h, rinsed with methanol, then annealed at350 �C for 1 h in vacuum.

For soluble PI systems, the completion of imidization wasconfirmed from perfect disappearance of the proton signals for theNHCO [d ~ 10.0 ppm in dimethyl sulfoxide (DMSO)-d6] and theCOOH groups (d ¼ 12e13 ppm) in the 1H NMR spectra. Full imid-ization was also confirmed from the FT-IR spectra of separatelyprepared thin films as typically shown in Fig. 6 (b) on the basis ofthe disappearance of the PAA-inherent IR bands and the

Table 2Results of polyaddition of H0-PMDA with various diamines.

Category ofdiamine

Systemno.

Diamine Solid content(initial/final)(wt%)

Solvent hinh(PAA)(dL g�1)

Aromatic-flexible (I)

1 4,40-ODA 30 / 16.9 DMAc 1.522 TPEQ 30 / 15.4 DMAc 1.843 BAPP 30 / 14.4 DMAc 1.844 BAPS 30 / 18.8 DMAc 1.445 TFODA 30 / 15.3 DMAc 1.496 HFBAPP 30 / 19 DMAc 1.10

Aromatic-rigid (II) 7 p-PDA 30 / 8.8 DMAc ea

30 / 12.2 NMP 1.608 o-TOL 30 / 10 DMAc

andNMP

eb

9 m-TOL 30 / 10.2 DMAc 3.2410 APAB 30 / 12.6 DMAc 0.9411 DABA 30 / 15 DMAc 0.8812 TFMB 30 / 13 DMAc 2.43

Cycloaliphatic-flexible (III)

13 MBCHA 30 / 17.6 DMAc 0.6714 M-MBCHA 30 / 12.8 DMAc 0.95

Cycloaliphatic-rigid (IV)

15 t-CHDA 20 / 18 NMP ec

t-CHDAd 15 / 6.8 NMP ea

t-CHDAe 15 / 9.7 DMAc ec

t-CHDAf 15 DMAc ec

a Gelation.b Inhomogeneous mixture.c Formation of insoluble salt.d 100% Silylated t-CHDA with N,O-bis(trimethylsilyl)trifluoroacetamide (BSTFA).e 50% Silylated t-CHDA with BSTFA.f Mixture of t-CHDA and acetic acid.

appearance of some specific bands (cm�1): 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 systemsare represented with the abbreviations of the monomer compo-nents used [tetracarboxylic dianhydrides (A) and diamines (B)] asA/B for homopolymers and A1;A2/B1;B2 for copolymers.

2.2. Measurements

2.2.1. Inherent viscosities and molecular weightsThe 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 thedepolymerization of PAAs, which immediately proceeds by dilu-tion. An extrapolation to zero concentration for determination ofthe inherent viscosities (hinh) is difficult for PAAs owing to anabnormal increase in the hred values in a low concentration range,which arises from a polyelectrolyte effect of PAAs. Instead, the hredvalues weremeasured at 0.5 wt%, which can be practically regardedas hinh as often similarly dealt with in the literature. The number-(Mn) andweight-averagemolecular weights (Mw) for highly solublePIs were determined by the gel permeation chromatography (GPC)using tetrahydrofuran (THF) as an eluent at room temperature on aJasco, 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 min�1. The cali-bration was carried out with standard polystyrenes (Shodex, SM-105).

2.2.2. Optical propertiesThe light transmission spectra of PI films (typically 20 mm thick)

were measured on an ultraviolet-visible spectrophotometer (Jasco,V-530) in the wavelength (l) range of 200e800 nm. The lighttransmittance at 400 nm (T400) and the cut-off wavelength (lcut, l atT ~ 0%) were determined from the spectra. The yellowness indices(YI, ASTM E 313) for the PI films were also calculated on the basis ofthe spectra under a standard illuminant of D65 and a standardobserver function of 2� on a Jasco color calculation software fromthe relation:

YI ¼ 100ð1:2985x� 1:1335zÞ=y

where x, y, and z are the CIE tristimulus values (CIE: InternationalCommission on Illumination). The YI value takes null for an idealwhite/transparent sample. The total light transmittance (Ttot, JIS K7361e1) and the diffuse transmittance (Tdiff, JIS K 7136) of PI films

Fig. 6. FT-IR spectra of thin films for H0-PMDA/TFMB: (a) PAA and (b) PI.

M. Hasegawa et al. / Polymer 55 (2014) 4693e47084698

were measured on a double-beam haze meter equipped with anintegrating sphere (Nippon Denshoku Industries, NDH 4000). Haze(turbidity) of PI films was calculated from the relation:

Haze ¼�Tdiff

.Ttot

�� 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 at589.3 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 wereswollen by CH2I2) and a test piece (nD ¼ 1.92). The birefringence ofPI films, which represents a relative extent of chain alignment alongthe film plane (XeY) direction, was calculated from the relation:

Dnth ¼ nin � nout

The dielectric constants were estimated from the optical data onthe basis of an empirical relation:

εopt ¼ 1:1n2av

where nav denotes the average refractive indices expressed as

nav ¼ ð2nin þ noutÞ=3

The εopt values are known to be approximate to the dielectricconstants directly determined with a precision LCR meter at afrequency of 1 or 10 MHz for PI systems [25,34].

Dnth was also measured as a function of wavelength in thevisible 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 monochromatorthrough an optical fiber cable. In this case, the color compensationdial of the refractometer was set at 30 for measuring at differentwavelengths from the D-line. The raw data were corrected using aformula supplied by Atago Co., Ltd.

2.2.3. Linear coefficient of thermal expansionThe CTE along the XeY direction for PI specimens (15 mm long,

5 mm wide, and typically 20 mm thick) in the glassy region wasmeasured by thermomechanical analysis (TMA) as an average inthe range of 100e200 �C at a heating rate of 5 K min�1 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 thepreliminary first heating run up to 120 �C and successive cooling toroom temperature in the TMA chamber, the data were collectedfrom the second heating run for removing an influence of adsorbedwater.

2.2.4. Glass transition temperatureThe storage modulus (E0) and the loss energy (E00) of PI films

were measured by the dynamic mechanical analysis (DMA) at aheating rate of 5 K min�1 on the same TMA instrument. The mea-surements were conducted at a sinusoidal load frequency of 0.1 Hzwith an amplitude of 15 gf in a nitrogen atmosphere. The Tg's of PIfilms were determined from a peak temperature of the E00 curve. Inthis instrument, the heating runs undergo an automatic stop whenthe irreversible elongation (DL) of the specimens exceeded3000 mm by softening above the Tg's.

2.2.5. Thermal stability at elevated temperaturesThermal stability of PI films was evaluated from the 5% weight

loss temperatures (Td5) by thermogravimetric analysis (TGA) on athermo-balance (Bruker-AXS, TG-DTA2000). TGAwas performed ata heating rate of 10 K min�1 in nitrogen or air atmosphere. For themeasurements in N2, the preliminary first heating run up to 150 �Cwas carried out to eliminate adsorbed water, and the samples werecooled to room temperature while mounting in the samplechamber under a continuous dry N2 flow. After the data resetting to0% weight loss, TGA data were collected from the second heatingrun.

2.2.6. Mechanical propertiesThe tensile modulus (E), tensile strength (sb), and elongation at

break (εb) of PI specimens (30mm long, 3mmwide, typically 20 mmthick) were measured on a mechanical testing machine (A & D,Tensilon UTM-II) at a cross head speed of 8 mm min�1 at roomtemperature. The data were averaged for 10e15 samples. In thiscase, the specimens were taken from high-quality large film sam-ples (10 cm � 10 cm) without any defects such as fine bubbles. Forsome systems, it was difficult to carry out themechanical stretchingtest because of film brittleness or the difficulty of high-quality largefilm preparation.

2.2.7. SolubilityThe 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 combustionThe heats of combustion Qc (gross calorific value) for three hy-

drogenated PMDA isomers were measured by a custom analysiscenter (Ibiden Engineering Co.) on an advanced adiabatic bombcalorimeter (Yoshida Seisakusho Co., JIS M8814) calibrated withbenzoic acid as a standard sample. In this case, the Qc valuescorrespond to the internal energy change of combustion (DUc). Theenthalpy change of combustion (DHc) was calculated from therelation: DHc ¼ DUc þ PDV z DUc þ Dng$RT (P is pressure, DVvolume change, R the gas constant, and T absolute temperature),Dng (in mol) is a change in the amount of substance for the gaseousphases in the following combustion reaction: C10H8O6 (s) þ 9 O2(g)¼ 10 CO2 (g)þ 4H2O (l) (Dng¼ 1 in the present case). To comparethermodynamic stability of three hydrogenated PMDA isomers,each standard enthalpy of formation at 25 �C (DHf

�298) for the

stereoisomers was estimated from the thermochemical equationswith the DHc values of the stereoisomers, graphite, and H2 (g).

2.2.9. Liquid crystallinityLiquid 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 atemperature-controllable hot stage (Mettler Toledo, FP82HT hotstage and FP 90 central processor).

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.

3. Results and discussion

3.1. Polymerizability of H0-PMDA with diamines and isomer effect

The polyaddition reactivity of tetracarboxylic dianhydrides withdiamines can be represented by the second-order rate constants ina homogeneous state. However, the PAA polymerization actuallyproceeds in an inhomogeneous mixture composed of tetracarbox-ylic dianhydride powder fed into diamine solutions. Thus, insteadof the rate constants, we simply compared the polymerizability ofthree hydrogenated PMDA isomers on the basis of the hinh values ofthe finally obtained PAAs in the present work. This parameter ispractically useful because it is closely related to the PI film flexi-bility as often represented by the elongation at break (εb) [35]; inmany cases, the film specimens become very brittle at both stagesof PAA and PI when the hinh values are rather low (empiricallyspeaking, < ~ 0.3 dL g�1) owing to poor chain entanglement. Forexample, the BTA/4,40-ODA system leads to a PAA without film-forming ability as a result of a rather low hinh value(0.19e0.38 dL g�1) [21,22,27], which is attributed to insufficientpolymerizability of BTA. The use of H-PMDA instead of BTA slightlyincreases the hinh value (0.57 dL g�1). The insufficient polymer-izability of BTA and H-PMDA can be explained in terms of self-sterichindrance during the polymerization, which is related to an all-exoconfiguration for the four C]O groups; once one functional groupreacted 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, theapproaching/attack of other terminal amino groups to theremaining functional is disturbed [22]. On the other hand, the re-action of H0-PMDA and 4,40-ODA gave rise to a sufficiently high hinhof 1.52 dL g�1, which is somewhat lower than hinh ¼ 2.26 dL g�1 forCBDA/4,40-ODA. The strikingly improved polymerizability of H0-PMDA compared to H-PMDA probably results from a uniqueconfiguration of H0-PMDA as depicted in Fig. 4 (c) where thefunctional groups orient almost opposite directions each other as inPMDA and CBDA, thereby the self-steric hindrance mentionedabove becomes less effective. Thus, when 4,40-ODA was used as afixed diamine, the hinh-based polymerizability of the cycloaliphatic

tetracarboxylic dianhydrides tends to increase as follows: BTA < H-PMDA << H0-PMDA < CBDA.

A strained structure of the five-membered acid anhydride ringsin H0-PMDA may also contribute to its enhanced polymerizability.The bomb calorimetry determined: DUc ¼ �1.87 � 104 J g�1 for H0-PMDA, e1.82 � 104 J g�1 for H-PMDA, and �1.80 � 104 J g�1 for H00-PMDA. Using these experimental data, the heats of formation wereestimated: DHf

�298 ¼ �8.88 � 102 kJ mol�1 for H0-PMDA,

e1.00� 103 kJ mol�1 for H-PMDA, and�1.05�103 kJ mol�1 for H00-PMDA. The results suggest that H0-PMDA is somewhat thermody-namically less stable than the other isomers, probably relating to astrained structure of the former.

Table 2 lists the results of the polyaddition between H0-PMDAand 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 g�1) exceptfor the cases using p-PDA and o-tolidine (o-TOL). The reactionsbetween H0-PMDA and these rigid diamines in DMAc disturbed theformation of homogeneous PAA solutions. This is probably attrib-uted to partial precipitation of the initially formed rigid amic acidoligomers with poor solubility. The PAA chains for H0-PMDA/p-PDAmost 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 thefact that PMDA/diamine systems provide PAAs with a compositionof meta(60)/para(40) [36]. Therefore, the trans-1,4-cychohexanedicarboxamide units greatly contribute to enhancingthemain chain rigidity, as a result, decreasing the PAA solubility. Onthe other hand, neither the PAA counterpart from H-PMDA nor H00-PMDA contains such rigid structural unit, corresponding to the factthat these isomers cause no precipitation problem during thepolymerization process even when very rigid diamines such as p-PDAwere used. A similar precipitation problem as in the H0-PMDA/p-PDA system was observed in a comparative reaction system, i.e.,polymerization of a polyamide from pure trans-1,4-cyclohexanedicarbonyl chloride (t-1,4-CHDC) and TFMB in DMAcin the presence of pyridine as an acid acceptor. In contrast, the useof trans(24)/cis(76) mixture of 1,4-CHDC, instead of pure t-CHDC,allowed the formation of a homogeneous polyamide solution. Thisalso supports that the trans-1,4-cychohexanedicarboxamide moi-ety [Fig. 7 (a)] behaves indeed as a very rigid structural unit. In thepresent work, a pronounced solvent effect was observed in the H0-PMDA/p-PDA system; the reaction mixture was homogenized inNMP in contrast to DMAc and provided a PAA with a sufficientlyhigh hinh value of 1.6 dL g�1. This is probably attributed to betteraffinity between the PAA and NMP (namely, higher solubility) than

M. Hasegawa et al. / Polymer 55 (2014) 4693e47084700

PAA/DMAc. However, even the use of NMP was less effective toobtain a homogeneous PAA solution for the H0-PMDA/o-TOLsystem.

TFMB is somewhat inferior in the reactivity to common aromaticdiamines such as 4,40-ODA owing to the presence of the electron-withdrawing CF3 substituents in TFMB. Nonetheless, the reactionbetween H0-PMDA and TFMB led to a PAA with a high molecularweight enough for flexible film formation [Mw ¼ 1.2 � 105,Mn¼ 5.2�104 for the chemically imidized powder samplewith hinh(PI) ¼ 1.23 dL g�1] in contrast to the absence of film-forming abilityfor the isomeric H-PMDA/TFMB system. The use of specially purifiedH0-PMDA by repeated recrystallization produced a PAA with anextremely high hinh of 9.04 dL g�1 as a maximum value.

The reactions between H0-PMDA and flexible cycloaliphatic di-amines (category III) caused salt formation in the initial stage asgenerally 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 severaldays. The system derived from H0-PMDA and t-CHDA (category IV)is greatly expected to display valuable combined properties (lowCTE, transparency, and low birefringence as discussed later).However in fact, the polymerization was completely inhibited bythe formation of quite insoluble salt. In this system, neither the

Fig. 8. Classification based on polymerizability between t-CH

silylation method [37] nor the addition of acetic acid [38] wassuccessful. The success/failure of the reactions between t-CHDAand various tetracarboxylic dianhydrides can be roughly classifiedinto three categories as shown in Fig. 8; in Group 1, the initiallyformed salt gradually dissolves just by stirring at room tempera-ture, and homogeneous PAA solutions are formed after stirring forseveral days. The tetracarboxylic dianhydrides of this group (e.g., H-PMDA and H00-PMDA) commonly consist of non-planar/non-linearstructures. In Group 2, the reaction mixtures including the saltbecome homogeneous by heating at 80e120 �C for a short period.The tetracarboxylic dianhydrides of this group possess relativelyrigid crank-shaft-like structures. In Group 3, once the salt wasinitially formed, it does not dissolve any longer even though thereaction mixtures were diluted and heated. The tetracarboxylicdianhydrides of this group consist of “rod-like” structures. Theclassification of Fig. 8 suggests that the insolubility of the initiallyformed 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 initialstage. The present work revealed that H0-PMDA belongs to Group 3as shown in Table 2, suggesting that it behaves in the reaction witht-CHDA as an extremely rigid monomer like as PMDA and 2,3,6,7-naphthalenetetracarboxylic dianhydride (NTDA) [39].

DA and various tetracarboxylic dianhydrides in DMAc.

Fig. 9. Comparisons of hinh-based polymerizability for hydrogenated PMDA isomerswith 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 inwhich 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 webversion of this article.)

M. Hasegawa et al. / Polymer 55 (2014) 4693e4708 4701

We compared the polymerizability for three hydrogenatedPMDA 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 reactivesulfone-containing diamine (BAPS, #4) was used, H0-PMDA wasspecifically more effective for enhancing hinh than H00-PMDA. Onthe 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-PMDAwas somewhat superior to H00-PMDA in the polymerizabilitywith very rigid diamines (p-PDA and m-TOL). This feature isdesirable for our current aim addressed to lowering CTE as dis-cussed later.

3.2. Properties of H0-PMDA-based PIs and isomer effects

3.2.1. Heat resistanceThe 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 3Film properties of H0-PMDA -based PIs.

No. Diamine Curea T400 (%) lcut (nm) Dnth εopt Tg (�C) CT

1 4,40-ODA T 76.0 294 0.0003 2.88 294 502 TPEQ T 79.4 298 0.0000 2.91 278 573 BAPP T 79.0 294 0.0001 2.91 256 594 BAPS T 80.5 303 0.0000 2.93 284 595 TFODA T 74.8 298 0.0003 2.65 310 59

C 75.8 300 0.0116 2.65 311 496 HFBAPP T 78.9 295 0.0005 2.74 287 61

C 87.7 286 0.0209 2.72 287 517 p-PDA T 64.6 292 0.0056 2.82 407 39

D 73.6 291 0.0516 2.88 418 279 m-TOL T 78.6 297 0.0095 2.86 340 4911 DABA T 77.7 341 0.0467 2.96 350 3012 TFMB T 71.8 293 0.0026 2.65 344 47

C 90.3 292 0.02010.0384b

0.0468c

2.64 357 463529

13 MBCHA T 78.3 258 0.0001 2.62 303 6214 M-MBCHA T 72.1 266 0.0000 2.59 290 61

a Imidization (cure) processes, T: thermal imidization, C: chemical imidization in homroom 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/10mhinh ¼ 5.75 dL g�1 was used for chemical imidization.

c CPN-cast PI film formed under the slow drying condition. PAA with hinh ¼ 9.04 dL g

Even when ether-linked flexible aromatic diamines (category I)were chosen, the resultant PI films maintained still high Tg ranging280e310 �C. The results are probably related to the structural ri-gidity/planarity of the H0-PMDA-based diimide (H0-PMDI) moietiesin the PI chains, which probably contributes to intermolecular face-to-face stacking between the diimide units for activating thedipoleedipole interactions between the imide C]O groups [40,41].As suggested from the steric structure of H0-PMDA monomer inFig. 4 (c), the molecular planarity of the H0-PMDI unit must berather high, although it is somewhat inferior to that of aromaticPMDI. The impact of the diimide molecular planarity on the Tg isillustrated in the following comparisons: case (1) Tg¼ 256 �C for H0-PMDA/BAPP vs. 287 �C for PMDA/BAPP [42] (DTg ¼ 31 �C), andcase (2) Tg ¼ 287 �C for H0-PMDA/HFBAPP vs. 327 �C for PMDA/HFBAPP [42] (DTg ¼ 40 �C).

Fig.10 shows an isomer effect on the Tg's for the PIs derived fromthree hydrogenated PMDA isomers. The H0-PMDA-based PIs tend topossess comparable to (or slightly higher Tg's than) those of the H00-PMDA-based counterparts except for some diamine systems. Theresults also suggest that H-PMDA is the most effective forenhancing the Tg among these isomers. This predominance may berelated 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 berepresented 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 Td

5 values (in N2)exceeding 450 �C, although a decrease in Td

5 is inevitable whencycloaliphatic diamines were used. The H0-PMDA-based PIs stillmaintain high Td

5 values even in a thermo-oxidative atmosphere (inair), e.g., 451 �C for H0-PMDA/4,40-ODA.

3.2.2. Thermal expansion propertiesOne of our greatest interests concerns the dimensional stability

of PI films as represented by CTE. The CTE values of the thermallyimidized 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 ofCTE values ranging 50e62 ppm K�1. The results are attributed to a

E (ppm/K) E (GPa) εb ave/max (%) sb (MPa) Td5 (N2) (�C) Td

5 (air) (�C)

.2 2.42 9.4/13.5 95 453 451

.2 2.60 11.2/14.4 93 459 434

.3 1.99 41.4/115 71 456 427

.6 1.89 8.9/25.4 90 444 422

.2 482 408

.5 2.38 93.9/119 110 453 411

.7 445 412

.7 1.56 114/173 62 502 463

.1 2.53 19.0/30.8 130 471 452

.4 2.83 5.3/6.5 94

.3 404 401

.6 440 435

.2 e e e 455 425

.0

.2b

.8c

2.73 56.8/83.5 110 491 451

.8 1.83 11.6/14.6 76 450 400

.1 2.07 5.5/8.1 84 439 366

ogeneous solutions, and D: imidization by dipping PAA film into Ac2O/pyridine at

in þ 100 �C/10min þ 150 �C/15 min in air þ300 �C/1 h in vacuum. PAA with

�1 was used for chemical imidization.

Fig. 10. Comparisons of Tg for thermally imidized PI films from hydrogenated PMDAisomers: (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, thereader is referred to the web version of this article.)

Fig. 12. Comparisons of CTE for thermally imidized PI films from H0-PMDA-based andother isomeric systems: ( ) H0-PMDA vs. H00-PMDA and ( ) H0-PMDA vs. H-PMDA. Thedotted line denotes the relation: Y ¼ X.

M. Hasegawa et al. / Polymer 55 (2014) 4693e47084702

low degree of in-plane orientation as suggested from their very lowbirefringence (Dnth < 0.001). On the other hand, the use of rigiddiamines (category II) caused suppressed thermal expansionbehavior, e.g., CTE ¼ 30.6 ppm K�1 for H0-PMDA/DABA and39.1 ppm K�1 for H0-PMDA/p-PDA. A good correlation is observed inFig. 11; CTE decreases with increasing Dnth for H0-PMDA/aromaticdiamine systems. Thus, the distinctly decreased CTE for the H0-PMDA/rigid diamine systems (T) results from an enhanced in-planechain 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, thedata points were plotted near the Y ¼ X line or slightly below, thusindicating no prominent effect of H0-PMDA on reducing CTE. Theresults mean that CTE is governed by not “local” structural rigidity/linearity of the diimide units but “overall” main chain linearity; apositive contribution of H0-PMDA to an increase in the structuralrigidity/linearity was almost canceled by incorporating the highlyflexible units from the diamines. On the other hand, whenH0-PMDAwas combined with rigid diamines (typically DABA, #11), the plotsobviously deviate downward from the Y¼ X line, indicating that theH0-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, theuse of H-PMDA and H00-PMDA spoils a great effect of DABA onenhancing the structural rigidity/linearity because of their

Fig. 11. Relationship between CTE and Dnth for H0-PMDA/aromatic diamine systems: PIfilms prepared via (C) thermal and ( ) chemical imidization processes.

distorted steric structures. Thus, H0-PMDA is an outstandinglyeffective monomer for reducing CTE. The magnitude of in-planechain orientation as an origin of the suppressed thermal expan-sion behavior is closely related to the PI backbone structures; Fig.13depicts schematic illustrations of the extended chain forms for theisomeric 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 highlinearity as depicted in Fig. 13 (b), which arises from the above-mentioned specific steric structure of H0-PMDA [Fig. 4 (c)]. Asimilar correlation between the low CTE property and the mainchain linearity is also observed in the crank-shaft-shaped CBDA-based PI systems depicted in Fig. 13 (d) [23]. On the other hand, thenon-planar steric structures of H-PMDA and H00-PMDA areresponsible for a significant decrease in the overall PI chain line-arity as shown in Fig. 13 (a) and (c), which corresponds to theincreased CTE values for their systems. Thus, the overall chainlinearity 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 systemsbut also conventional wholly aromatic PIs [43] and poly(esterimide) systems [45].

The high structural rigidity/linearity of the H0-PMDI unit is alsosupported by a model compound approach; a low molecularweight 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 inthe temperature range of 121.1e128.0 �C in the heating process and127.7e103.2 �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 mesogenicstructure.

For the H0-PMDA-based systems derived from non-fluorinatedcommon diamines, the PI film specimens (T) were essentiallyinsoluble in various solvents [NMP, DMAc, DMF, DMSO, m-cresol,CPN, g-butyrolactone (GBL), and THF]. Therefore, it was difficult tocomplete chemical imidization while keeping a homogeneousstate. In contrast, the use of CF3-containing diamines (TFODA,HFBAPP, and even rigid TFMB)made it possible, and their PI powdersamples (C) were highly soluble in various solvents at room tem-perature. The properties of the CF3-containing H0-PMDA-based PIfilms (C) are also summarized in Table 3. A positive effect of thechemical imidization process was observed; the PI films (C) showedlower CTE values by approximately 10 ppm K�1 than those of the

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 atypical rigid diamine fragment. The diimide units are represented as edge-view.

Fig. 14. POM photograph for H0-PMDA-based model compound at 123.3 �C in thecooling process.

M. Hasegawa et al. / Polymer 55 (2014) 4693e4708 4703

thermally imidized counterparts for the systems using TFODA andHFBAPP. The decreased CTE for the former results from an increaseddegree of in-plane chain orientation (Fig. 11) as suggested from theincreased Dnth values (Table 3).

For the rigid H0-PMDA/p-PDA systemwhich was not compatibleto the homogeneous chemical imidization process, we attempted toprepare the PI film by dipping the PAA film formed on a substrateinto a Ac2O/pyridine solution, followed by rinsing with water andheat treatment in vacuum. This procedure provided a decreasedCTE of 27.4 ppm K�1 compared to the thermally imidizedcounterpart.

3.2.3. Self-chain orientation behavior during solution casingprocess

The relatively high Dnth values for the H0-PMDA-based PI films(C) also mean the presence of a self-orientation phenomenonduring the simple solution casting process without structuraltransformation from PAAs into PIs [46]. The present results lookcurious because solution casting of common soluble polymersusually causes no significant in-plane orientation; for example, weobserved almost zero Dnth and a common level of CTE(65.0 ppm K�1) for an NMP-cast PES film. We propose a possible

M. Hasegawa et al. / Polymer 55 (2014) 4693e47084704

mechanism for the self-orientation behavior in the H0-PMDA-basedsystems (C); as schematically depicted in Fig. 15 (a), no orientationoccurs yet just after the PI solutions were coated onto a substrate.The coating viscosity increases with solvent evaporation and mustbe 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 suchshrinkage stress (in other words, apparent stretching effect) mayact as a trigger for the “permanent” in-plane chain orientationoccurring at the drying temperatures (Tdry). When the coatings onthe substrate were cooled from Tdry to room temperature, thethermal stress arising from the film/substrate CTE difference alsocontributes “temporally” to the total in-plane orientation. However,in our PI film specimens, such contribution from the thermal stressis canceled by removal of the substrate. Besides, it is most likelythat there exists a good correlation between the shrinkage stressand the tensile modulus of the PI systems chosen. This correspondswell 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 tofurther decrease the CTE for the rigid H0-PMDA/TFMB system. The

(b

C*M2CC*M1

Vis

cosi

tyV

iVV

sc η*

PI solid content (wt%)0 100

M1

M2

Stre

ss

C*M2CC*M1

PI solid content (wt%)0 100

M1

M2

Flexiblesystem

Solvent evaporationPI solution

Half-PI fil

Substrate CPI = CCPI = 10−20 wt%

Local

Fig. 15. (a) Schematic illustration for film formation processes via coating/drying of PI soludrying processes (right) on the viscosity of the coatings and predicted internal stress at the(CPI) is represented as C*.

PImolecular weight can be also an important factor for this purpose[48]; as schematically depicted in Fig. 15 (b, left), an increase in themolecular weight (M1 >M2) raises the coating viscosity, as a result,should contribute to an increase in the shrinkage stress owing to agreater thickness decrement between CPI ¼ C* and 100 wt%. Flex-ible PI systems are probably disadvantageous to induce highershrinkage stress because of their originally lower modulus. Anotherpossible factor is the drying temperature. A slow drying process byelevating Tdry little by little and holding at each step for a period,which corresponds to a drying program with a lower temperatureramp, provides a higher coating viscosity (i.e., a higher shrinkagestress) than a rapid drying process as schematically depicted inFig. 15 (b, right). In addition, an increase in Tdry (or increase in theheating rate) may also contribute to orientational relaxationharmful for low CTE generation. Indeed, the combination of amolecular weight increase and very slow drying caused to a furtherdecrease in CTE (to 29.8 ppm K�1) for H0-PMDA/TFMB as shown inTable 3. To apply such slow drying process, adequately volatile andless hygroscopic solvents (e.g., CPN) are more desirable thanfrequently used amide solvents with higher boiling points (e.g.,NMP). The H0-PMDA/TFMB powder sample (C) allowed to form a

)

Time (h)

Tem

pera

ture

(ºC

)

50

atur 150

250

C*RapidCC*Slow p

PI solid content (wt%)0 100

Vis

cosi

tyV

iVV

sc η*

Rapid

Slow

Slow

Rapid

31.50

driedm Fully dried

PI film

* CPI = 100 wt%

ordering ?

(a)

tion and (b) influences of PI molecular weight (left, M1 > M2) and heating rate in thedrying temperatures. h* denotes a viscosity at a rubbery state where the solid content

M. Hasegawa et al. / Polymer 55 (2014) 4693e4708 4705

highly stable CPN solution at room temperature with a high solidcontent (10 wt%) enough for applying common casting processes.

However, the shrinkage stress effect independently seems to beless active as the driving force for inducing the actual high level of in-plane chain orientation (recall the fact that PES and other flexible PIsystems never show prominent in-plane orientation by solutioncasting in a similar manner). Therefore, an additional factor relatingto attractive intermolecular interactions and chain rigidity/linearityshould be considered; i.e., the formation of a local ordered structureconsisting of highly oriented chains (probably with a very smalldomain size undetectable by POM) [see Fig. 15 (a)]. This idea comesfromour previous experience; a partially imidized PAAof s-BPDA/p-PDA (s-BPDA: 3,304,40-biphenyltetracarboxylic dianhydride) forms aliquid crystal-like ordered structure in NMP, and thermal imidiza-tion of the cast PAAfilm including the ordered structure results in anappreciably lower CTE than that of the counterpart from the ordi-nary PAA [49]. The fact that liquid crystallinity was observed in theH0-PMDA-based model compound (Fig. 14) suggests that a similarlocal ordered structure may exist even in the actual H0-PMDA/rigiddiamine PI films. Such local ordering should be much more advan-tageous in rigid/linear PI systems such as H0-PMDA/TFMB than innon-rigid systems. On this assumption, we propose the self-orientation mechanism; each domain can be forcibly aligned alongthe XeY direction by a flow field from the apparent stretching effectas schematically depicted in Fig. 15 (a).

3.2.4. Other propertiesThe 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-ODAfilm imidized at 300 �C/0.5 h in vacuum). The H0-PMDA-based PIfilms (T) show slightly lower T400 values than the H00-PMDA-basedcounterparts [22], however, the formers are essentially colorless asillustrated 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 thecorresponding PI films (T) as shown particularly in the HFBAPPsystem. The improved transparency for the PI films (C) may beattributed to an end-capping effect of the unstable terminal aminogroups by Ac2O during the chemical imidization process (Fig. 5). Inparticular, the transparency of the H0-PMDA/TFMB film (C) is sohigh as illustrated from its optical data (Ttot ¼ 90.5%, YI ¼ 1.8, andHaze ¼ 2.0%) that this system is suitable for some optical applica-tions as proposed later.

The H0-PMDA-based PI films are expected to show low dielectricconstants as suggested from the optically estimated ones (εopt)ranging 2.8e2.9 evenwhen 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].

Table 3 also summarizes the mechanical properties of the H0-PMDA-based systems. Essentially flexible PI films were formedupon thermal imidization. In particular, the use of BAPP andHFBAPP was very effective to enhance the film toughness as sug-gested from their very high εb values. Even if the molecular weightswere sufficiently high, the combinations of rigid tetracarboxylicdianhydrides and rigid diamines generally lead to brittle PI filmsowing to insufficient chain entanglement as typically observed inPMDA/p-PDA system (εb < 3% [51]). Against this general criterion,the H0-PMDA/p-PDA system with a stiff/linear backbone structureexhibited sufficient film flexibility as shown in Table 3. It should benoted that the H0-PMDA/TFMB system with less polarized andbulky CF3 groups displayed a further enhanced εb value (>50%),which is much higher than that of the aromatic counterpart

(PMDA/TFMB, εb < 20% [39]). Thus, the film toughness was prom-inently improved by exchanging aromatic PMDA for non-aromaticH0-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 allowedwhen intermolecular forces were significantly weakened. Incontrast to the good film toughness of H0-PMDA/TFMB, the H-PMDA/TFMB system possessed no film-forming ability because ofits insufficient molecular weight from the polymerizability prob-lem, namely, insufficient chain entanglement.

3.3. Performance balance and applicability as plastic substratematerials for H0-PMDA-based systems

Fig. 16 shows spider charts for the present target properties,which represent performance balance for the hydrogenated PMDAisomers. The chars for CBDA and aromatic PMDA are also depictedfor a comparison. The each property was evaluated with five levelson the basis of criteria listed in Table 4. Two fixed diamines, 4,40-ODA and TFMB were chosen as a typically flexible and a rigiddiamine, respectively. The 4,40-ODA systems are highly transparent(except the combination with PMDA) but commonly not suitablefor low CTE generation owing to the absence of the overall chainlinearity. In addition, these are essentially less soluble in commonsolvents except H00-PMDA/4,40-ODA. As evaluated from a relativelyspread shape of the spider chart for the H00-PMDA/4,40-ODA system[Fig. 16 (c)], H00-PMDA possesses comparatively good performancebalance although it is difficult to achieve low CTE. On the otherhand, the TFMB systems display quite different performance bal-ance. The combination of CBDA and TFMB is suitable for low CTEgeneration but not for enhancing the solubility and the filmtoughness as shown in Fig. 16 (h). It should be noted that H0-PMDA/TFMB [Fig. 16 (f)] possesses an outstanding performance balanceamong the TFMB systems, although there is room for furtherimprovement of low CTE property.

Our first attempt for further decreasing the CTE of H0-PMDA/TFMB by copolymerization using DABAwas less effective because ofa gelation problem at a high DABA content during the chemicalimidization process. This motivated us to develop a novel CF3-containing rigid diamine [52]. The combination of this diaminewith H0-PMDA led to a solution-processable colorless PI with afurther decreased CTE (16 ppm K�1). The results will be reportedelsewhere in detail. Thus, the present work revealed how H0-PMDAis a promising tetracarboxylic dianhydride monomer for obtainingnovel plastic substrate materials. However, there seems to be aproblem in its application as the plastic substrates in liquid crystaldisplays (LCD) with a backlight transmission mode, that is, rela-tively high birefringence (Dnth). The plastic substrates for LCDs arein general desirable to possess zero Dnth like current inorganic glasssubstrates because Dnth is responsible for optical retardation(Rth ¼ Dnth$d, where d is film thickness), which may cause adecrease in the image contrast at a high viewing angle. However, asshown in Fig. 11, there seems to be in principle a sort of “trade-offrelationship” between low CTE and low Dnth, suggesting greatdifficultness of obtaining ideal plastic substrate materials for LCDs.Instead, the present H0-PMDA-based low-CTE colorless PI systemscan be applied as the substrates for bottom-emission-type flexibleorganic emitting diode (OLED) displays without the optical retar-dation problem mentioned above and/or electronic paper displayswith an external light reflection mode.

3.4. Potential application as optical compensation layer

We investigated the potentiality of H0-PMDA/TFMB as a novelcoating-type optical compensation film material (negative-C plate,

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 systemis not shown because of the absence of film-forming ability.

M. Hasegawa et al. / Polymer 55 (2014) 4693e47084706

nx ¼ ny > nz, i.e., Dnth > 0) with low wavelength dependence of Rthfor vertical alignment-mode LCDs. In this case, higher Dnth ispreferable because thinner compensation films can be used toobtain an aimed Rth value. Another merit is that the H0-PMDA/TFMBlayer can be directly formed on a triacetylcellulose (TAC) film as astandard 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 bysimply drying at lower temperatures than ~ 150 �C without sub-sequent thermal imidization process at elevated temperatures(>250 �C). The wavelength dependence of Dnth (or Rth) is usually anormal dispersion type, that is, Dnth decreases with increasingwavelength (l) as expressed by an asymptotic empirical equation:Dnth ¼ A þ B/(l2 e C) where A, B, and C are constants depending onthe 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

representing a slope in the Dnthel curves. Our goal is to achieve anultimately low wavelength dispersion property with Dn450/Dn550values close to 1.00. However, this subject is not easy becauseDn450/Dn550 ranges 1.07e1.16 for most of colorless PI systems (e.g.,1.11e1.12 for 6FDA/TFMB). This parameter is sensitive to not onlythe PI chemical structure but also the drying temperature program,solvent, and PI molecular weight. We observed that the parameterDn450/Dn550 tends to reduce with decreasing lcut, which is probablyclosely related to the extents of electronic conjugation through themain chains. In this context, the H0-PMDA/TFMB systemwith a lowlcut may be suitable for achieving a low wavelength dispersionproperty. For the present purpose, H0-PMDA/TFMB was modifiedwith a minor content of a non-conjugated cycloaliphatic diamine,isophoronediamine (IPDA). Fig. 17 shows a Dnthel curve for a CPN-cast H0-PMDA/TFMB(80);IPDA(20) copolyimide film. Indeed, thissystem exhibited a low wavelength dispersion property (Dn450/

Table 4Criteria 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 Property Relative level

1 2 3 4 5

Po hinh (PAA)(dL g�1)

<0.3 0.4e0.6 0.7e0.9 1.0e1.5 >2.0

HR Tg (�C) <200 220e240 250e270 280e300 >350To εb (%) No film-forming

ability or < 25e10 20e30 40e60 >100

LT CTE (ppm K�1) >70 60e50 45e35 30e20 <10Tr T400 (%) <5 20e30 40e60 70e75 >80SP Qualitative

solubilityInsoluble uponheating

Partiallysoluble orswelled

Soluble inhot amidesolvents

Soluble inamidesolvents at r.t.

Soluble innon-amidesolvents at r.t.

M. Hasegawa et al. / Polymer 55 (2014) 4693e4708 4707

Dn550¼ 1.03). The partial incorporation of 1,3-dimethyl-substitutedCBDA (DM-CBDA) as a comonomer into H0-PMDA/TFMB desirablyenhanced Dnth while maintaining a low wavelength dispersionproperty, excellent transparency (T400¼ 89.2%), a relatively low CTE(30.3 ppm K�1), and a very high Tg (335 �C). The results give us anidea: although high Dnth is generally an undesirable property forthe plastic substrates in LCDs as mentioned above, we propose thatH0-PMDA/TFMB and the modified systems with the relatively highDnth values can be applied to a new type of LCD substrates pos-sessing both of good dimensional stability based on their low CTEproperty and the optical compensation functionality.

4. Conclusion

This work proposed a cycloaliphatic tetracarboxylic dianhydridemonomer, H0-PMDA, useful for obtaining novel optoelectronicmaterials. H0-PMDA showed much higher polymerizability withvarious diamines than conventional H-PMDA and led to PAAs withhigh molecular weights. Consequently, even the combination withTFMB including electron-withdrawing CF3 groups provided ahighly flexible PI film in contrast to the absence of film-formingability for H-PMDA/TFMB. The enhanced polymerizability of H0-PMDA is attributed to its unique steric structure. Exceptionally, thepolyaddition with t-CHDA did not proceed by insoluble salt for-mation, which is probably related to the structural rigidity of theoligomers formed. The thermally imidized H0-PMDA-based PI filmswere essentially colorless regardless of diamines owing to inhibitedCT interactions. The PIs also possessed relatively high Tg's (e.g.,

Fig. 17. Birefringence (Dnth) as a function of wavelength for CPN-cast PI films: ( ) H0-PMDA/TFMB(80);IPDA(20) and ( ) H0-PMDA(50);DM-CBDA(50)/TFMB copolyimidesystems.

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 activatedimidization-induced in-plane chain orientation phenomenon,whereas the counterparts fromH-PMDA and H00-PMDA did not. Theresults suggest how the H0-PMDA-based diimide moiety behaves asa rigid/linear structural unit as supported by the fact that the H0-PMDA-basedmodel compound showed liquid crystallinity. Some ofthe H0-PMDA-based PI films were also prepared via the chemicalimidization process in a homogeneous state. For example, the PIpowder sample for H0-PMDA/TFMB was highly soluble in commonsolvents (even in some hygroscopic solvents such as CPN). Thisprocedure was more advantageous for increasing transparency anddecreasing CTE of the PI films than the conventional thermalimidization process. The CTE for H0-PMDA/TFMB further decreased(to 29.8 ppm K�1) by increasing the molecular weight and dryingthe coatings more slowly. The results suggest the presence of a self-orientation phenomenon during solution casting. The mechanismwas discussed in this work. A potential application was also pro-posed as novel coating-type optical compensation film materialswith low wavelength dispersion of Rth for vertical alignment-modeLCDs. The CPN-cast H0-PMDA/TFMB(80); IPDA(20) copolyimide filmachieved a low wavelength dispersion property (Dn450/Dn550 ¼ 1.03) in addition to non-coloration and a high Dnth valueexceeding 0.03. Thus, the H0-PMDA-based PI systems can be usefulmaterials for high-temperature transparent plastic substrates withhigh dimensional stability and/or novel coating-type opticalcompensation 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 theirpartial experimental support. We are also indebted to New JapanChemical, Nissan Chemical Industries, Wakayama Seika, JFEChemical, and Ihara Chemical for their monomer supply.

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