Superheat-resistant polymers with low coefficients of thermal expansion

Accepted Manuscript Superheat-resistant polymers with low coefficients of thermal expansion Masatoshi Hasegawa, Yuki Hoshino, Natsumi Katsura, Junichi Ishii PII:

S0032-3861(17)30046-0

DOI:

10.1016/j.polymer.2017.01.028

Reference:

JPOL 19345

To appear in:

Polymer

Received Date: 28 November 2016 Revised Date:

10 January 2017

Accepted Date: 14 January 2017

Please cite this article as: Hasegawa M, Hoshino Y, Katsura N, Ishii J, Superheat-resistant polymers with low coefficients of thermal expansion, Polymer (2017), doi: 10.1016/j.polymer.2017.01.028. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Graphical Abstract

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Superheat-resistant polymers with low coefficients of thermal expansion

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Masatoshi Hasegawa*, Yuki Hoshino, Natsumi Katsura, Junichi Ishii

Poly(benzoxazole imide)s (PBOIs)

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(●) PBOIs (△,▲) Aromatic PIs

550

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Td5 (N2) (°C)

600

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s-BPDA/p-PDA

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450

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-10

0

Optoelectronic Application

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40

50

60

70

–1

CTE (ppm K ) R

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B ITO electrode

Light-emitting layer

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Superheat-resistant Plastic substrate Top emission-mode OLED displays

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Superheat-resistant polymers with low coefficients of thermal expansion

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Masatoshi Hasegawa*, Yuki Hoshino, Natsumi Katsura, Junichi Ishii

Department of Chemistry, Faculty of Science, Toho University, 2-2-1 Miyama, Funabashi, Chiba

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274-8510, Japan

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* Corresponding author. Tel.: +81-47-472-1869, Fax: +81-47-472-1869.

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E-mail address: [email protected] (M. Hasegawa)

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ABSTRACT Here, we report superheat-resistant polymers, poly(benzoxazole imide)s (PBOI), with very low

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coefficients of thermal expansion (CTE) and sufficient ductility for applications as the plastic substrates of organic light-emitting diode (OLED) displays. Diamines incorporating benzoxazole

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(BO) units were synthesized for this purpose. A PBOI derived from a BO-containing diamine and

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3,3′,4,4′-biphenyltetracarboxylic dianhydride (s-BPDA) has excellent properties that are superior to those of s-BPDA/p-phenylenediamine (p-PDA) such as an undetectable Tg, a very low CTE –1

(9.6 ppm K ), a very high the 5% weight loss temperature (Td5 = 592 °C) in N2, and good ductility while remaining the highest level of non-flammability (UL-94, V-0). The use of another

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BO-incorporating diamine permitted further improvement in the target properties: an extremely –1

high Td5 (609 °C) in N2 and a considerably low CTE (3.3 ppm K ) close to that of a silicon wafer.

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These results show that the PBOIs examined in this work are promising candidates for use as the

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plastic substrates in OLED displays.

Keywords: Polyimides; Polybenzoxazoles; Heat resistance; Coefficient of thermal expansion (CTE); Plastic substrates; Organic light-emitting diode (OLED) displays

1. Introduction

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Currently, fragile inorganic glass substrates are used in various image display devices. Much effort has been devoted to replacing these inorganic glass substrates (300–700 µm thick) with

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plastic substrates (< 50 µm thick), saving the weight and reducing the thickness of display devices. Plastic substrate materials must have excellent optical transparency and outstanding heat

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resistance (thermal stability at elevated temperatures). The latter property is particularly

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important in organic light-emitting diode (OLED) displays to avoid contamination of the OLED elements by volatile organic compounds (VOCs). These VOCs arise from the thermal decomposition of the plastic substrate, when exposed to a very severe high-temperature environment for a short period during OLED device fabrication. Furthermore, if the upper

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temperature limit of the plastic substrate materials could be enhanced, semiconductor elements could be prepared at higher processing temperatures, affording higher performance of the

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semiconductor devices. Therefore, it is crucial and endless to improve the heat resistance of

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plastic substrates from this point of view. Aromatic polyimides (PI), which are the most reliable heat-resistant polymeric materials at present, have been used as electrically insulating materials in a variety of electronic devices [1–6]. Since outstandingly heat resistant PI films such as Kapton H® (DuPont) and Upilex S® (Ube Industries) were developed, no significant attention has been paid to further enhance the heat resistance of PIs. This is probably attributed to the following reason; their heat resistance (glass

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transition temperatures, Tg > 350 ºC as the physical heat resistance and thermal decomposition temperatures, Td > 500 ºC in a nitrogen atmosphere as the chemical heat resistance) was rather

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“over-performance” to an actual solder-reflow process at 260 °C, which is a primary thermal process in various electronic devices. In contrast, attempts to improve the optical transparency of

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PI films have been made, because the intense coloration of conventional PI films, which arises

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from charge-transfer (CT) interactions [7], often disrupts their various optical applications. Therefore, optically transparent (colorless) PIs and other high-temperature polymers have been developed in both academia and industry [8–22]. However, optically transparent (colorless) PIs are much inferior to conventional, wholly aromatic PIs with regard to their thermal stability at

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elevated temperatures; this lack of thermal stability arises because of the presence of thermally less stable aliphatic units. Consequently, optically transparent PIs are not used as the plastic

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substrate in OLED displays. Fortunately, in the top emission-mode OLED displays, the optical

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transparency of the plastic substrate is not always required because the radiated luminescence from the emitting layer is directed away from the substrates; in contrast, in the bottom emission-mode OLED displays, the emitted light passes through the substrates, as schematically depicted in Supplementary Data 1. Accordingly, highly optically transparent plastic substrates are not necessary in the top emission-mode OLED displays, and we have focused on the development of higher-performance plastic substrates for this type of displays.

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Recently, there has been increased demand for plastic substrate materials with higher Tg values and greater dimensional stability over the course of the multiple heating-cooling cycles

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used in device fabrication. When plastic substrates have insufficient thermal dimensional stability, significant expansion and contraction, concomitant with the heating-cooling cycles, occurs even

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at temperatures below the Tg of the substrate materials. This expansion and contraction can cause

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serious problems such as misalignment and adhesion failure of various electronic components, laminate warpage, and transparent electrode breakdown. The most direct strategy for improving the thermal dimensional stability of plastic substrates is to lower the coefficients of thermal expansion (CTE) in the film plane (X-Y) direction in the glassy state (a CTE < 10 ppm K–1, or,

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ideally, zero is desirable). Concerning the thermal dimensional stability, the Tg of the substrate material must be as high as possible or practically absent (i.e., undetectable by high-sensitivity

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methods such as dynamic mechanical analysis).

from 3,3′,4,4′-biphenyltetracarboxylic

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The PI derived

dianhydride (s-BPDA)

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p-phenylenediamine (p-PDA) is known to possess not only the highest Td among the current –1

commercially available wholly aromatic PI films but also a low CTE (10–15 ppm K ), which depend on the film processing conditions such as the molecular weight of the PI precursors [poly(amic acid) (PAAs)], heating programs, film thickness, and solvent type [23,24]. However, even the excellent properties of this PI are not sufficient to meet the growing demand of further

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improving the Td and CTE values from the side of semiconductor device designing. Copolymerization with proper commercially available comonomers is often carried out to

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improve the properties of PIs. However, as mentioned later, this simple approach often fails to simultaneously improve the Td and CTE of s-BPDA/p-PDA at the same time. Here, we report

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novel superheat-resistant polymers that have improved properties compared to those of

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s-BPDA/p-PDA, i.e., PIs incorporating benzoxazole (BO) units: poly(benzoxazole imide)s (PBOI) [25].

2. Experimental

2.1.1. Monomer synthesis

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2.1. Materials

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shown in Fig. 1.

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A series of BO-incorporating diamines were synthesized according to the reaction scheme

DAR-4AB. 4,6-Diaminoresorcinol dihydrochloride [DAR, 10 mmol, Tokyo Chemical Industry (TCI)] was dissolved in anhydrous N-methyl-2-pyrrolidone (NMP, 40 mL) in the presence of pyridine (40 mmol, 3.2 mL) as an HCl acceptor in a sealed three-neck flask. In a separate sealed flask, nitrobenzoyl chloride (4-NBC, 30 mmol, TCI) was dissolved in dry NMP (10 mL). The DAR solution was cooled to 0 °C, and the 4-NBC solution was added slowly using

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a syringe with continuous magnetic stirring. The solution was stirred at 0 °C for 3 h, and additionally at room temperature for 12 h. To the reaction mixture, p-toluenesulfonic acid

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monohydrate (p-TSA, 18 mmol) was added, and refluxed at 200 °C for 6 h. The precipitate was collected by filtration, and the obtained greenish needle-like crystals were repeatedly washed

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with NMP, water, and ethanol, followed by vacuum-drying at 120 °C for 12 h. The obtained compound was insoluble in deuterated solvents such as dimethyl sulfoxide (DMSO)-d6;

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consequently, we could not measure its 1H NMR spectrum. The FT-IR spectrum of the product involved several specific bands (KBr, cm–1): 3095 (Carom-H), 1602 (oxazole C=N), 1516/1350 (NO2), suggesting that the product is the desired dinitro compound.

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The dinitro compound (1.00 g) was dissolved in NMP (50 mL), and Pd/C (0.10 g) was added as a catalyst. The reaction mixture was refluxed at 120 °C for 7 h in a hydrogen atmosphere. The

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progress of the catalytic reduction was monitored by thin-layer chromatography. Subsequently,

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the catalyst was removed by hot filtration, and the filtrate was cooled to room temperature and slowly poured into a large quantity of water. A deep-blue precipitate formed was collected by filtration, followed by repeatedly washing with water. The precipitate was then dried under vacuum at 120 °C for 12 h (total yield: 48%). The product showed a sharp endothermic peak for melting at 414 °C in the differential scanning calorimetry (DSC) thermogram. The molecular structure of the product was confirmed to be the desired BO-containing diamine (DAR-4AB)

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from the following data. FT-IR (KBr, cm–1): 3469/3316/3197 (amine N-H), 1620 (NH2 deformation + oxazole C=N), 1503 (1,4-phenylene), 1174 (oxazole C-O-C). 1H NMR (400 MHz,

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DMSO-d6, δ, ppm): 8.07 (s, 1H, 4-proton of the central BO unit), 7.88–7.86 [m, 5H, 7-proton of BO + 3,3′,5,5′-protons of the terminal aniline (AN)], 6.70 (d, 4H, J = 8.6 Hz, 2,2′,6,6′-protons of

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AN), 6.00 (s, 4H, amine). Anal. Calcd (%) for C20H14O2N4 (342.36): C, 70.17; H, 4.12; N, 16.37.

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Found: C, 69.68; H, 4.29; N, 16.15.

Other BO-incorporating diamines (Fig. 1) were also synthesized by the catalytic reduction of

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the dinitro compounds prepared from bis(o-aminophenol) (p-HAB, 5 mmol) and 4-NBC (10 mmol) and characterized in a similar manner.

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p-HAB-4AB. FT-IR (KBr, cm–1): 3454/3380/3318/3210 (amine N-H), 3042 (Carom-H), 1499 (1,4-phenylene), 1607 (NH2 deformation + oxazole C=N + biphenyl), 1499 (1,4-phenylene),

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1173 (oxazole C-O-C). 1H NMR (400 MHz, DMSO-d6, δ, ppm): 8.06 (s, 2H, 7,7′-protons of BO), 7.89 (d, 4H, J = 8.6 Hz, 3,3′,5,5′-protons of AN), 7.73 (m, 4H, 4,4′- + 5,5′-protons of BO), 6.72 (d, 4H, J = 8.6 Hz, 2,2′,6,6′-protons of AN), 6.03 (s, 4H, amine). Anal. Calcd (%) for C26H18O2N4 (418.45): C, 74.63; H, 4.34; N, 13.39. Found: C, 74.41; H, 4.47; N, 13.26.

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5ABO-4AB. FT-IR (KBr, cm–1): 3396/3324/3201 (amine N-H), 3029 (Carom-H), 1647 (NH2 deformation + oxazole C=N stretching), 1497 (1,4-phenylene), 1173 (oxazole C-O-C). 1H NMR

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(400 MHz, DMSO-d6, δ, ppm): 7.78 (d, 2H, J = 8.6 Hz, 3,5-protons of AN), 7.28 (d, 1H, J = 8.6

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Hz, 7-proton of BO), 6.77 (sd, 1H, J = 2.0 Hz, 4-proton of BO), 6.66 (d, 2H, J = 8.6 Hz, 2,6-protons of AN), 6.54 (dd, 1H, J = 8.6, 2.1 Hz, 6-proton of BO), 5.88 (s, 2H, 5-NH2 on BO), 4.98 (s, 2H, amine of AN). Anal. Calcd (%) for C13H11ON3 (225.25): C, 69.32; H, 4.92; N, 18.66.

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Found: C, 69.50; H, 4.83; N, 18.77.

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6ABO-4AB. FT-IR (KBr, cm–1): 3341/3200 (amine N-H), 1617 (NH2 deformation + oxazole

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C=N stretching), 1499 (1,4-phenylene). 1H NMR (400 MHz, DMSO-d6, δ, ppm): 7.73 (d, 2H, J = 8.6 Hz, 3,5-protons of AN), 7.27 (d, 1H, J = 8.4 Hz, 4-proton of BO), 6.75 (sd, 1H, J = 1.9 Hz, 7-proton of BO), 6.65 (d, 2H, J = 8.6 Hz, 2,6-protons of AN), 6.56 (dd, 1H, J = 8.4, 2.0 Hz, 5-proton of BO), 5.77 (s, 2H, 6-NH2 on BO), 5.24 (s, 2H, amine of AN). Anal. Calcd (%) for C13H11ON3 (225.25): C, 69.32; H, 4.92; N, 18.66. Found: C, 69.27; H, 4.92; N, 18.30.

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ABI-4AB. FT-IR (KBr, cm–1): 3464/3374/3201 (amine + imidazole N-H), 3054 (Carom-H),

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1634 (NH2 deformation), 1620 (imidazole C=N), 1478 (1,4-phenylene). 1H NMR (400 MHz, DMSO-d6, δ, ppm): 11.83 [s (broad), 1H, 1-proton of the benzimidazole (BI) unit], 7.72 (d, 2H, J

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= 8.4 Hz, 3,5-protons of BI), 7.15 [s (broad), 1H, 4-proton of BI], 6.63–6.61 (m, 3H, 7-proton of

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BI + 2,6-protons of AN), 6.44 (d, 1H, J = 8.2 Hz, 5- or 6-proton of BI), 5.46 (s, 2H, amine of AN), 4.80 [s (broad), 2H, NH2 on BI]. Anal. Calcd (%) for C13H12N4 (224.27): C, 69.62; H, 5.39; N,

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24.98. Found: C, 69.55; H, 5.44; N, 24.87.

An imide-incorporating diamine was synthesized for comparison with the corresponding

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BO-incorporating diamine as follows: 4-Nitroaniline (4-NA, 20 mmol, TCI) was dissolved in dry

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NMP (6 mL). In a separate flask, 4-nitrophthalic anhydride (4-NPA, 20 mmol, TCI) was dissolved in NMP (9 mL). To this solution, the 4-NA solution was added slowly with continuous magnetic stirring, followed by stirring for 12 h at room temperature. The reaction mixture was refluxed at 200 °C for 7 h in a nitrogen atmosphere. The precipitate formed was collected by filtration and washed with fresh NMP and methanol and then vacuum-dried at 120 °C for 12 h. The product was recrystallized from DMSO, washed with methanol, and vacuum-dried at 200 °C

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for 12 h, yielding brown needle-like crystals. Catalytic reduction of the obtained dinitro compound (14 mmol) was carried out in NMP in the presence of Pd/C in a hydrogen atmosphere,

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as mentioned above, yielding a yellow powder (total yield: 64%). The product was confirmed to be the desired imide-incorporating diamine (4API-pPDA) from the following data. FT-IR (KBr,

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cm–1): 3438/3354/3235 (amine N-H), 1759/1698 (imide C=O), 1504 (1,4-phenylene), 1391

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(N-Carom), 749 (imide ring deformation). 1H NMR (400 MHz, DMSO-d6, δ, ppm): 7.52 [d, 1H (measured integrated intensity: 1.00H), J = 8.2 Hz, 6-proton of the phthalimide (PhI) unit], 6.96– 6.92 (m, 3H (3.01H), 3,5-protons of AN + 3-proton of PhI), 6.83 (dd, 1H (1.01H), J = 8.2, 2.1 Hz, 5-proton of PhI), 6.60 (d, 2H (2.00H), J = 8.8 Hz, 2,6-protons of AN), 6.49 (s, 2H (2.01H),

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4-NH2 on PhI), 5.28 (s, 2H (2.00H), amine of AN).

2.1.2. Other monomers and raw materials The structures, abbreviations, sources, and pre-treatment conditions of the other monomers and raw materials used in this study are summarized in Table 1 and Fig. 1. In this work, we used tetracarboxylic dianhydrides with rigid/linear structures; i.e., pyromellitic dianhydride (PMDA), 3,3′,4,4′-diphenyltetracarboxylic dianhydride (s-BPDA), and 2,3,6,7-naphthalenetetracarboxylic

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dianhydride (NTDA).

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2.1.3. Polymerization and thermal imidization for preparation of PBOI films

The PBOI precursors (PBOAA) were prepared by the equimolar polyaddition of

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tetracarboxylic dianhydrides and diamines, as shown in Fig. 2. The actual PBOAA chain

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structures consist of a para/meta random sequence with respect to the bonding sites of the amide linkages, formed by the nucleophilic attack of the amino groups to the anhydride C=O groups with approximately equivalent reactivity (Fig. 2) [26]. A typical polymerization procedure is as follows: Diamines (10 mmol) were dissolved in a dry NMP. Then, tetracarboxylic dianhydride

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powder (10 mmol) was added to the diamine solutions at room temperature with continuous stirring. The initial total solid content was 30 wt%. The reaction mixture was stirred at room

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temperature in a sealed bottle until it became homogeneous with a maximum solution viscosity

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(typically after 72 h). If necessary, the reaction mixture was gradually diluted with the minimal quantity of the same solvent to ensure effective magnetic stirring. The formation of the PBOAAs was confirmed by transmission-mode FT-IR spectroscopy (Jasco, FT/IR 4100 infrared spectrometer) using separately prepared thin cast films (4–5 µm thick) with non-uniform thicknesses to prevent interference fringes. A typical FT-IR spectrum is shown in Fig. 3(a). The spectrum demonstrates specific bands (cm–1): 3393 (amide N-H), 2626/1705 (hydrogen-bonded

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carboxylic acid O-H/C=O), 1667/1531 (amide C=O), 1602 (biphenyl + oxazole C=N), and 1499 (1,4-phenylene), 1178 (oxazole C-O-C).

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The PBOI films were prepared via thermal imidization process: The PBOAA solution was bar-coated on a glass substrate, dried at 80 °C for 3 h in an air-convection oven, heated at

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established imidization temperatures (Ti = typically 250 °C + 350 °C) for each 1 h under vacuum

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on the substrate, and finally annealed without the substrate at established annealing temperatures (Ta = typically 400 °C) for 1 h in vacuum to eliminate residual stress. The imidization and the annealing conditions were finely tuned to yield better quality PBOI films. Complete imidization was also confirmed from the FT-IR spectra of separately prepared thin

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films as shown in Fig. 3(b), characterized by specific bands at 3070 (Carom-H), 1773/1719 (imide C=O), 1618 (biphenyl + oxazole C=N), 1500 (1,4-phenylene), 1362 (imide N-Carom), and 742

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cm–1 (imide, ring deformation).

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In this paper, the chemical compositions of the PBOAA and PBOI systems are denoted by the abbreviations of the monomer components used, i.e., tetracarboxylic dianhydrides (A) and diamines (B) as A/B for homopolymers and A1;A2/B1;B2 for copolymers.

2.2. Measurements 2.2.1. Reduced viscosity

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The reduced viscosities (ηred), which can be treated as the inherent viscosities (ηinh), of PBOAAs were measured in DMAc at a solid content of 0.5 wt% at 30 °C on an Ostwald

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viscometer.

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2.2.2. Linear coefficient of thermal expansion

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The CTE values in the film plane (X-Y) direction for the PBOI specimens (15 mm long, 5 mm wide, and typically 20 µm thick) in the glassy region were measured in a N2 atmosphere by thermomechanical analysis (TMA). The average value was measured in the range of 100–200 °C at a heating rate of 5 °C min–1 on a thermomechanical analyzer (Netzsch, TMA 4000) with a

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fixed load (0.5 g per unit film thickness in µm, i.e., a 10 g load for 20 µm-thick films) in a dry nitrogen atmosphere. In this case, after the preliminary heating run to 120 °C followed by

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successive cooling to room temperature in the TMA chamber, the data were collected from the

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second heating run to remove the influence of any adsorbed water.

2.2.3. Glass transition temperature The storage modulus (E′) and the loss energy (E″) of PBOI films were measured by dynamic mechanical analysis (DMA) in a N2 atmosphere at a heating rate of 5 °C min–1 on the TMA instrument (as before). The measurements were conducted at a sinusoidal load frequency of 0.1

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Hz with an amplitude of 10 gf in a nitrogen atmosphere. The Tg values of PBOI films were

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determined from the peak temperature of the E″ curve.

2.2.4. Thermal stability

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The thermal stability of the PBOI films at elevated temperatures was evaluated from the 5%

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weight loss temperatures (Td5) by thermogravimetric analysis (TGA) on a thermo-balance (Bruker-AXS, TG-DTA2000). TGA was performed using platinum pans at a heating rate of 10 °C min−1 in a nitrogen or air atmosphere. For the measurements in dry N2 gas the preliminary first heating run up to 150 °C was carried out to eliminate adsorbed water, and the samples were then

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cooled to room temperature while mounted in the sample chamber under a continuous flow of dry N2. After the data were reset to 0% weight loss, the TGA data were collected from the second

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heating run. For the measurements in air, the data were collected from the first run, and the small

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weight loss arising from the desorption of water from the samples was observed at around 100 °C was compensated by an offset at 150 °C to 0% weight loss for the data analysis.

2.2.5. Mechanical properties The tensile modulus (E), tensile strength (σb), and elongation at break (εb) of the PBOI specimens (film dimension: 30 mm long, 3 mm wide, typically 20 µm thick; specimen numbers >

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15) were measured on a mechanical testing machine (A & D, Tensilon UTM-II) at a crosshead speed of 8 mm min–1 at room temperature. The specimens were cut from high-quality film

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out using a data processing program (Softbrain, UtpsAcS Ver. 4.09).

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samples (10 cm × 10 cm) free of any defects, such as fine bubbles. The data analysis was carried

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2.2.6. Birefringence

The in-plane (nin or nx = ny) and out-of-plane (nout or nz) refractive indices of PBOI films were measured with a sodium lamp at 589.3 nm (D-line) on an Abbe refractometer (Atago, 4T, nD range: 1.47–1.87) equipped with a polarizer using a contact liquid (sulfur-saturated methylene

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iodide nD = 1.78–1.80) and a test piece (nD = 1.92). The thickness-direction birefringence (∆nth) of PBOI films, which represents the relative extent of chain alignment along the X-Y direction, was

∆nth = nin – nout

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calculated from the following relationship:

2.2.7. Water uptake

The extent of water absorption (WA, JIS K 7209) of the PBOI films was determined from the following relationship: WA = [(W – W0) / W0] × 100 16

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where W0 is the weight of film sample after vacuum-drying at 50 °C for 24 h and W denotes the weight of the films immersed in water at 23 °C for 24 h followed by carefully blotting with tissue

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paper. In this case, large film specimens (> 0.1 g) were used to minimize experimental errors.

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2.2.8. Flame retardancy

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The flame retardancy of the PBOI films was evaluated according to the UL-94 vertical burning test standard (five samples were tested using specimens with a fixed size: 125 mm long, 13 mm wide, 20 µm thick).

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3. Results and discussion

3.1. Influence of connecting groups on the thermal expansion properties and thermal

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stability

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Regarding the suppression of VOC emissions from the substrates in OLED displays, our interest has focused on short-term heat resistance, namely, the temperature of the earlier stages of thermal decomposition in an inert atmosphere. However, the 1% weight loss temperatures were not always highly reproducible. Therefore, in this study, we have used the 5% weight loss temperatures (Td5) in a nitrogen atmosphere as a more reproducible parameter, reflecting the resistance of the substrates to the generation of VOCs at higher temperatures.

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First, for para-linked PIs derived from s-BPDA and a series of diamines (H2N-Ph-Z-Ph-NH2, Ph = 1,4-phenylene group), we briefly surveyed the influence of the connecting groups (Z) in PI

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main chains on the CTE and Td5 (in N2). As shown in Fig. 4, the sulfonyl groups (Z = -SO2-), which are known to promote solubility, were less thermally stable than the methylene (-CH2-) and

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thioether (-S-) groups, as suggested by the lower Td5 value of the sulfonyl-containing PI film.

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Furthermore, as it has been believed, the ether groups (-O-) led to a PI film with the highest thermal stability among the PIs with the flexible connecting groups examined in this work. However, the incorporation of the ether groups, as well as the other flexible connecting groups, caused an unfavorable and significant increase in the CTE. Unexpectedly, the use of carbonyl

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groups (Z = -CO-) led to superior thermal stability compared to the use of ether groups, probably reflecting the higher bonding energies of the C=O double bond and the Ph-CO-Ph single bonds,

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as in the imide group (Ph-CO-N-CO-Ph), than that of the Ph-O-Ph single bonds. However, the

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use of the corresponding carbonyl-containing diamine (4,4′-diaminobenzophenone) was less effective in obtaining a low CTE. This failure can be attributed to a decrease in the overall chain linearity on the incorporation of the C=O group, as in the case of other flexible connecting groups (Z = -O-, -S-, -SO2-, and -CH2-), whereby the imidization-induced in-plane chain orientation is disturbed [27]. In contrast, the direct combination of the carbonyl and ether groups [i.e., ester groups (Z = -COO-], as well as the amide groups (-NHCO-), dramatically reduced the CTE. This

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effect is probably closely related to the fact that the ester and amide connecting groups do not disturb the formation of the extended chains with very high linearity [28–30]. Unfortunately, the

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ester and amide groups were not valuable in terms of ultimately enhancing the thermal stability of PI films as shown in Fig. 4. Therefore, the connecting groups (Z) most effective for achieving

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both an ultralow CTE and an ultimately high Td5 are almost limited to the non-substituted

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p-phenylene group [i.e., the use of 4,4″-diamino-p-terphenyl (DATP)] as shown in Fig. 4. However, DATP is not in general use as an industrial monomer for PIs, because of the complexity of its synthesis and its high cost. A combination between the diamine without the connecting groups (i.e., benzidine) and s-BPDA can be an alternative candidate, however,

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benzidine is not commonly used in industry because of its strong carcinogenicity. In contrast, although substituted benzidines (e.g., o-tolidine and m-tolidine, which have methyl substituents)

EP

are commercially available on an industrial scale, we have removed them from the candidate

AC C

diamines because of the lower heat resistance of the substituents themselves. A similar situation has been observed when these connecting groups are introduced to the tetracarboxylic anhydrides; for example, the incorporation of a flexible ether group into s-BPDA, which corresponds to 4,4′-oxydiphthalic anhydride (ODPA), causes a significant increase in the CTE even when combined with rigid diamines [31]. Therefore, it is challenging to find practically useful polymers with both very low CTE properties and very high thermal stability,

19

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excluding the s-BPDA/p-PDA polyimide system.

RI PT

3.2. Difficulty in simultaneously improving the CTE and Td5 of s-BPDA/p-PDA.

The rod-like PI derived from PMDA and p-PDA has a much lower CTE (3.3 ppm K–1) than

SC

s-BPDA/p-PDA, although the former film is very brittle (εb = 0%) because there is no chain

M AN U

entanglement [32]. This result suggests that modification of s-BPDA/p-PDA by copolymerization using PMDA results in lower CTE according to the additivity rule, which is often observed in the relationships between physical properties (e.g., Tg, CTE, and dielectric constants) of copolymers and their copolymer compositions. The copolymerization of p-PDA with s-BPDA (50 mol%) and

TE D

PMDA (50 mol%) was indeed successful in reducing the CTE (7.4 ppm K–1) compared to the homo s-BPDA/p-PDA system (CTE = 12.4 ppm K–1). However, this approach also caused an

EP

appreciable decrease in Td5 (568 °C in N2) compared to that of homo s-BPDA/p-PDA (Td5 =

AC C

588 °C in N2). Thus, these results highlight the difficulty in obtaining polymeric systems superior to s-BPDA/p-PDA in both CTE and Td5.

3.3. A molecular design strategy for a polymer superior to s-BPDA/p-PDA. Polymeric materials with outstanding heat resistance comparable to PIs are practically limited to

polybenzazoles,

including

polybenzoxazole

20

(PBO),

polybenzthiazoles

(PBT),

and

ACCEPTED MANUSCRIPT

polybenzimidazoles (PBI). Among the polybenzazoles, practically useful systems are limited to PBOs because of the poor monomer availability for PBTs and PBIs. PBOs have been applied as

RI PT

organic super-fibers [33] and photosensitive protective (buffer-coat) layer materials in semiconductor integrated circuits [34]. PBOs can be promising materials for the development of

SC

new plastic substrates for OLED displays, as suggested by the fact that the highly crystalline PBO fiber [35] [Zylon®, Toyobo, poly(p-phenylene-2,6-benzobisoxazole)], which is derived from

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terephthalic acid (TPA) and DAR, had a very high Td5 (662 °C) in N2 as determined by our TGA measurements in accordance with a previous report [36].

To reveal whether PBOs are superior to the corresponding PIs regarding their thermal stability,

TE D

we compared the Td5 values of PBOs and PI film samples prepared under almost fixed thermal conditions (Ta = 400 °C or lower). Then we first attempted to prepare high-quality PBO films

EP

consisting of rigid/linear chain structures. However, it was difficult to form a high-quality

AC C

TPA/DAR-type PBO film by coating a homogeneous polyphosphoric acid (PPA) solution of the as-polymerized PBO onto a substrate, followed by successive immersion in a water (coagulation) bath, because of the non-volatility of PPA. –1

In contrast, we obtained a high-quality PBO film with a relatively low CTE of 17 ppm K by using activated derivatives of 3,3′-diamino-4,4′-dihydroxybiphenyl (m-HAB) and TPA via the two-step method: The polymerization of a PBO precursor using a silylation technique using

21

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trimethylsilyl chloride [37], followed by solution casting, and thermal cyclization as shown in Supplementary Data 2 [38]. The use of a modified silylation agent containing bulkier alkyl

RI PT

groups (tert-butyldimethylsilyl chloride) also permitted the polymerization of the PBO precursor in NMP from the silylated DAR and terephthaloyl chloride (TPC) and the formation of a

SC

free-standing DAR/TPA-type PBO film on heating the precursor film at 500 °C for 24 h in

reported in the literature [39].

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vacuum without substrate, although the CTE and εb values of this PBO film have not been

However, even if these PBO films have low CTE values and sufficient ductility, the use of dicarboxylic acid dichlorides such as TPC, which is indispensable during the PBO precursor

TE D

polymerization via silylation, should be avoided to prevent halogen contaminations, which can act as a trigger for serious electrical troubles in OLED displays. Thus, the current rigid PBO

EP

systems are not suited to use as the plastic substrates for OLED displays.

AC C

A possible solution to this problem is to use BO-containing PI (PBOI) systems for which a simple and clean film formation process (PAA polymerization, solution casting, and thermal imidization) can be applied using BO-incorporating monomers. The PBOI precursors (PBOAAs) are expected to be highly soluble in common amide solvents like as conventional PAAs, because of the presence of a high content of the COOH side groups in PBOAAs. These groups contribute to the solvation of the PBOAA chains through the formation of hydrogen-bonding between the

22

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COOH side groups and NMP, in addition to the hydrogen-bonding of the NHCO groups in the main chains and NMP (the former is stronger than the latter) [40,41]. The para/meta random

RI PT

sequence (Fig. 2) also enhances the solubility of PBOAAs. Therefore, based on these structural features, the PBOAAs should be highly soluble PBOAAs and require no further modification,

SC

such as silyl-esterification of the COOH side groups. In addition, PBOAAs release only water on

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thermal imidization in contrast to the silylated PBO precursors [38,39].

3.4. Superiority of the BO unit over the benzimide unit concerning thermal stability. To confirm whether our idea using BO-incorporating PI monomers is a reasonable strategy to

TE D

achieve our goal, we compared the Td5 values (in N2) of the copolymer films obtained from a BO-incorporating diamine (5ABO-4AB, 50 mol%) and an imide-incorporating diamine

EP

(4API-pPDA, 50 mol%), and those of the relevant homopolymers without an imide-incorporating

AC C

diamine. When PMDA was used as the tetracarboxylic dianhydride, no differences in the Td5 values were observed between the copolymer and the homopolymer systems, as shown in Fig. 5. This result probably reflects the fact that the PMDA-based diimide units common to both systems behaved as “thermostability-determining” units, in which thermal decomposition occurs at lower temperatures than those of the other units. In contrast, when s-BPDA, as well as NTDA, was used instead of PMDA, the homo PBOIs (m = 100 mol%, in Fig. 5) displayed a distinctly higher Td5

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value than that of the copolymer (m = 50 mol%). These results suggest that the BO units are probably superior to the benzimide units concerning the high-temperature stability in a nitrogen

3.5. Properties of the PBOI films

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3.5.1. 5ABO-4AB-based PBOIs and related systems

SC

RI PT

atmosphere.

Based on the results mentioned above, we undertook a systematic examination of the simplest PBOIs using 5ABO-4AB; these PBOIs contain no flexible connecting groups such as ether group, and we were concerned that the removal of the flexible connecting groups might cause serious

TE D

embrittlement of the films. A previous paper [42] has reported basic properties of PI films prepared from commercially available 5ABO-4AB and a series of tetracarboxylic dianhydrides

EP

[PMDA, s-BPDA, and other common tetracarboxylic dianhydrides with flexible connecting

AC C

groups (e.g., ODPA)], although the previous report did not give thought to some specific target in contrast to our study. Therefore, although some basic properties of PMDA/5ABO-4AB and s-BPDA/5ABO-4AB were reported in the previous paper [42], we intentionally started our systematic investigations from these systems, because it was important to compare our target properties under fixed conditions for sample preparation and measurement. Table 2 summarizes the properties of the 5ABO-4AB-based PBOI films. The

24

ACCEPTED MANUSCRIPT

–1

PMDA/5ABO-4AB film (#1) has a very low CTE of 3.4 ppm K and a high tensile modulus (E) of 7.8 GPa, basically consistent with the previous report [42]. We observed that there was no

RI PT

distinct glass transition in the DMA curve for the PMDA/5ABO-4AB film as for the PMDA/p-PDA PI system [32], and this is probably due to the absence of molecular motion even

SC

at elevated temperatures, reflecting the linearity and stiffness of the main chains. The

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PMDA/5ABO-4AB film (#1) also showed an appreciably lower WA value (2.48%) than that of PMDA/4,4′-ODA (WA = 2.85%) when prepared under the same conditions. This can be attributed to the decreased content of the highly polarized imide C=O group present in the polymer on changing 4,4′-ODA to 5ABO-4AB, suggesting that the BO units have lower polarizability than

TE D

the corresponding benzimide units. Interestingly, PMDA/5ABO-4AB (#1) retained a certain degree of film ductility as suggested from its εb value listed in Table 2, despite its similarity to

EP

the PMDA/p-PDA PI system, i.e., its very low CTE, very high E, and the absence of a distinct Tg

AC C

in the DMA measurements. A possible explanation for the unexpectedly good ductility of the PMDA/5ABO-4AB film is that chain slippage occurs in this PBOI; this is possible because the interchain interactions should be weaker in this PBOI than in the PI because of the lower imide C=O content in the former. The imide C=O groups can be responsible for the interchain dipole-dipole interaction [43]. The use of s-BPDA (#2) as an alternative tetracarboxylic dianhydride resulted in a PI film

25

ACCEPTED MANUSCRIPT

with an increased Td5 (579 °C in N2) and a reduced WA (1.27%), again probably resulting from the reduced imide C=O content (in other words, the relative content of aromatic units is greater). –1

RI PT

However, this PBOI film (#2) has an undesirably high CTE (23.0 ppm K ), which can be attributed to the slight decrease in the main chain linearity on the incorporation of the slightly

SC

bent, “crank-shaft-like” structure of the s-BPDA-based diimide units.

On the other hand, the combination of NTDA and 5ABO-4AB (#3) was effective in

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enhancing the thermal stability (Td5 = 601 °C in N2) while maintaining low CTE property (CTE = –1

12.9 ppm K ). Thus, NTDA was superior to s-BPDA in reducing the CTE of the resultant PI films, consistent with our previous results [24]. In contrast, the fact that NTDA is more effective

TE D

in enhancing Td5 than s-BPDA was not previously known. It was difficult to activate these advantages of NTDA without using the BO-incorporating diamines; for example, a thermally

EP

imidized NTDA/p-PDA film formed from a DMAc-cast PAA film is practically worthless

AC C

because the film is not ductile (εb = 0% [24]), as was the case with the PMDA/p-PDA film. This suggests that the BO-incorporating diamines have additional functionality compared to the substituents-free common rigid diamines (e.g., p-PDA); i.e., they prevent significant film embrittlement [43].

Isomer effects were also investigated by comparing the systems using 5ABO-4AB and 6ABO-4AB. These isomeric diamines are structurally similar regarding their BO units.

26

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Nonetheless, the 5ABO-4AB-based systems (#1, 2) had clearer superior properties (lower CTE and higher Td5) compared to their 6ABO-4AB-based counterparts (#4, 5) when PMDA and

RI PT

s-BPDA were used as the tetracarboxylic dianhydrides. Although we have not proposed a detailed mechanism for the isomer effects observed here, it is likely that the intensity of the intermolecular

SC

BO-BO interactions between the 5ABO-4AB-based BO units somewhat differs from those

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between the 6ABO-4AB-based BO units, and this difference influences the in-plane orientation behavior during thermal imidization. This theory is based on our previous experience; we have previously observed that, while a BO-containing model compound derived from m-HAB and alkyl-substituted benzoic acid (R-BA) exhibited liquid crystallinity, the counterpart from p-HAB

TE D

and R-BA did not, despite their very similar structures [44].

We also investigated the effect of using a diamine incorporating a benzimidazole (BI) unit on

EP

the target properties. The film properties of PIs derived from a BI-incorporating diamine

AC C

(ABI-4AB) are shown in Table 2. An important feature of these PIs (#6, 7) is the extremely reduced CTE values, as illustrated by the negative values. The expected intermolecular hydrogen bonding (NH···N=C) between the BI units in the ABI-4AB-based PIs, which was observed in a PBI film [45], most likely promoted in-plane orientation during thermal imidization, resulting in considerably low CTE values, as suggested from our previous studies which demonstrated a great role of strong interchain attractive forces to inducing significant in-plane chain orientation [27].

27

ACCEPTED MANUSCRIPT

The ABI-4AB-based PI films undoubtedly have high Td5 values in N2, however, these were appreciably lower than those of the 5ABO-4AB-based counterparts. In addition, a serious

RI PT

problem was observed in the BI-incorporating PI films: i.e., their high water-absorbing character (e.g., WA ~ 9% for PMDA/ABI-4AB). The hygroscopic nature arises from the hydrogen-bonding

3.5.2. p-HAB-4AB-based PBOI systems

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SC

ability of the BI units. Consequently, we did not investigate the BI-incorporating PIs further.

Based on the observed positive effect of 5ABO-4AB on the target properties, p-HAB-4AB, which is a dimer form of 5ABO-4AB, was used in the next step. Here, a further improvement in

TE D

the thermal stability can be expected from the concomitant increase in BO content in the structures. However, our first concern was the possibility of gelation during polymerization;

EP

gelation is predicted from the significant decrease in the solubility of the p-HAB-4AB-based

AC C

PBOAAs. The use of p-HAB-4AB contributes to not only an increase in the PBOAA chain rigidity but also a decrease in the content of the COOH side groups, which aid solvation with amide solvents [40,41]. Nonetheless, no gelation occurred in any of the p-HAB-4AB-based systems examined in this work, and PBOAAs with sufficiently high ηinh values were obtained as listed in Table 3. This advantageous property of p-HAB-4AB (a sort of gelation-inhibiting effect) can be attributed to the relatively weak intermolecular BO-BO interactions, as suggested from

28

ACCEPTED MANUSCRIPT

our

experience

in

which

the

corresponding

imide-type

diamine

[N,N′-bis(4-aminophenyl)-3,3′4,4′-biphenyltetracarboxydiimide] tends to cause gelation when

RI PT

combined with rigid tetracarboxylic dianhydrides, owing to interchain dipole-dipole interactions between the imide C=O groups [46,47].

SC

Table 3 summarizes the film properties of p-HAB-4AB-based PBOIs. As expected, the use of p-HAB-4AB was more effective in improving the target properties than that of 5ABO-4AB; e.g.,

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the Td5 of PMDA/p-HAB-4AB (#8) is 11 °C higher (576 °C) in N2 than that of the 5ABO-4AB-derived counterpart (#1). In addition, this PBOI film has other positive effects such –1

as a reduced CTE (–0.3 ppm K ), increased film ductility (εbmax = 49%), and decreased water

TE D

uptake (WA = 1.78%).

The combination of s-BPDA and p-HAB-4AB (#9) led to further improved film properties

–1

EP

compared to s-BPDA/5ABO-4AB (#2) such as an undetectable Tg on DMA, a lower CTE (9.6

AC C

ppm K ), a higher Td5 (592 °C) in N2, and higher film ductility (εbmax = 56%) for the former than the latter (Table 3). The resulting low CTE property of the s-BPDA/p-HAB-4AB film (21 µm thick) corresponds to a very high ∆nth value of 0.11, which is indicative of a high level of in-plane chain orientation. Notably, the Td5 value in N2 of the s-BPDA/p-HAB-4AB film (#9) was appreciably higher than that of the s-BPDA/p-PDA film (Td5 = 588 °C in N2) thermally imidized under the same conditions. This PBOI (#9) was also superior to s-BPDA/p-PDA concerning the

29

ACCEPTED MANUSCRIPT

ductility (εbmax was 33% for the s-BPDA/p-PDA film). In addition, the UL-94 vertical burning test proved that this PBOI film (#9) maintained the highest level of non-flammability (V-0).

RI PT

We attempted to reduce the CTE of this PBOI film (#9) further without sacrificing the considerably high Td5 (N2) by copolymerization with PMDA. The copolymer (#11) showed a –1

SC

slightly decreased CTE (7.4 ppm K ), but, unfortunately, the Td5 (in N2) also decreased to 579 °C,

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which was a much lower value than that expected from the additivity rule.

The use of NTDA (#10) instead of s-BPDA in combination with p-HAB-4AB further –1

improved the target properties, as shown by a decreased CTE (8.4 ppm K ) and an increased Td5 in N2 (603 °C) while still maintaining sufficient ductility of the film as shown in Table 3.

TE D

We also carried out copolymerization of NTDA with p-HAB-4AB (50 mol%) and p-PDA (50 mol%) with the expectation of the “gelation-inhibiting effect” of p-HAB-4AB mentioned above.

EP

As expected, this approach using p-HAB-4AB as a comonomer was effective for obtaining a

AC C

homogeneous PBOAA solution, in contrast to the fact that the polyaddition of NTDA and p-PDA in NMP was unsuccessful because of gelation. Surprisingly, despite our initial concerns based on the additivity rule, the CTE of this thermally imidized copolymer film (#12) was significantly –1

reduced (0.8 ppm K ) without a concomitant decrease in Td5 in N2 (604 °C). These p-HAB-4AB-based PBOIs with low CTE values showed very high Tg values (Table 3) with very broad glass transition behavior on DMA (poor thermoplasticity). For the

30

ACCEPTED MANUSCRIPT

s-BPDA/p-HAB-4AB system, it was difficult to determine the Tg because of its monotonous DMA curve without distinct transitions. As well as the higher-Tg property, the absence of distinct

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glass transition is an advantageous property in terms of the dimensional stability maintained over

SC

the wider temperature ranges.

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3.5.3. DAR-4AB-based PBOI systems

As a BO-incorporating diamine with greater structural linearity than that of p-HAB-4AB, an ideally rod-like structure of DAR-4AB was used with the expectation of further decreasing the CTE. However, unlike p-HAB-4AB, DAR-4AB caused gelation on the polyaddition with PMDA

TE D

in NMP at room temperature, probably resulting from the rigid chain structure in the resultant PBOAA. In contrast, when DAR-AB was combined with s-BPDA, the polymerization proceeded

EP

in a homogeneous state. The properties of the thermally imidized s-BPDA/DAR-4AB film (#13)

AC C

are given in Table 4. Comparing the properties between s-BPDA/DAR-4AB (#13) and s-BPDA/p-HAB-4AB (#9, Table 3), there seems to be no obvious advantage in using DAR-4AB instead of p-HAB-4AB concerning the improvement of the target properties [particularly, CTE and Td5 (N2)], regardless of the tetracarboxylic dianhydrides used. Indeed, the combination of DAR-4AB and NTDA was not successful because of the difficulty in the polymerization due to gelation. Nonetheless, copolymerization of DAR-4AB (50 mol%) and 5ABO-4AB (50 mol%)

31

ACCEPTED MANUSCRIPT

with NTDA (#14) enabled us to derive a great potential of DAR-4AB; in this system, a homogeneous PBOAA solution was obtained without gelation, and the thermally imidized PBOI

RI PT

copolymer film (#14) simultaneously achieved an ultra-high Td5 in N2 (609 °C), which is the –1

highest value among the PBOIs examined in this work, and an ultra-low CTE of 3.3 ppm K ,

SC

which is close to the value of a silicon wafer. A comparison between this PBOI copolymer (#14) and the corresponding DAR-4AB-free PBOI (#3) shows that the former has a much lower CTE

improving the target properties.

TE D

3.5.4. CTE–thermal stability relationship

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and much higher Td5 (N2) than the latter. This result highlights the effectiveness of DAR-4AB in

Figure 6 shows the relationship between Td5 (N2) and CTE for the PBOIs examined in this

EP

work and conventional wholly aromatic PIs. The results illustrate that most of conventional wholly aromatic PIs with connecting groups have Td5 (N2) values ranging from 510–570 °C and

AC C

–1

common CTE values, which range from 40–60 ppm K . There is no doubt about that the s-BPDA/p-PDA film has the highest Td5 (N2) value and practically the lowest CTE value among the conventional PI systems. A loose trend was observed for the conventional PI systems; PIs with lower CTE values tend to have higher Td5 (N2) values. This trend [an increase in Td5 (N2) with decreasing CTE] is related to the increase in the chain rigidity/linearity for the conventional

32

ACCEPTED MANUSCRIPT

PIs on the gradual removal of the flexible connecting groups from the PI structures. On the other hand, in Fig. 6, the plotting points of some PBOI systems (●), e.g., s-BPDA/p-HAB-4AB (#9),

RI PT

show that they achieved both higher Td5 and lower CTE than those of s-BPDA/p-PDA (▲). In addition, these PBOI films maintain good ductility. Therefore, these PBOIs are promising

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SC

candidates for use as superheat-resistant plastic substrates in OLED display devices.

4. Conclusions

In this study, we investigated superheat-resistant polymers possessing both very low CTE values and sufficient ductility, i.e., para-linked PBOI systems without flexible connecting groups

TE D

in the main chains. BO-incorporating diamines (5ABO-4AB, p-HAB-4AB, and DAR-4AB) were synthesized for this purpose. The combinations of 5ABO-4AB and the rigid tetracarboxylic

EP

dianhydrides (PMDA. s-BPDA, and NTDA) were more effective in enhancing the Td5 in N2 than

AC C

the use of the corresponding imide-type diamine (4API-pPDA). Comparative PIs derived from a benzimidazole (BI)-incorporating diamine (ABI-4AB) and the rigid tetracarboxylic dianhydrides led to negative CTE values; however, they also resulted in an undesirable property, i.e., very high water uptake (WA > 6%) resulting from the hydrogen-bonding ability of the BI units. In contrast, the 5ABO-4AB-based PBOIs possessed relatively low WA values. The use of p-HAB-4AB as a dimer form of 5ABO-4AB led to homogeneous solutions of

33

ACCEPTED MANUSCRIPT

PBOAAs with high molecular weights, without gelation problems, and it was effective in further improving the target properties of the PBOI films; for example, the s-BPDA/p-HAB-4AB system

–1

RI PT

achieved excellent combined properties superior to those of s-BPDA/p-PDA, i.e., an undetectable Tg in DMA measurements, a very low CTE of 9.6 ppm K , a considerably high Td5 of 592 °C in

SC

N2, and sufficient ductility (εbmax = 56%) while maintaining the highest level of non-flammability

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(V-0 in the UL-94 vertical burning test). Based on weaker interchain interactions in the PBOIs, a chain-slippage mechanism was proposed for explaining the unexpectedly good film ductility observed in these rigid PBOIs without flexible connecting groups in the main chains. The combination of NTDA with DAR-4AB (50 mol%) and 5ABO-4AB (50 mol%) permitted

TE D

further improvement of the target properties, i.e., an extremely high Td5 of 609 °C (N2) and an –1

ultra-low CTE of 3.3 ppm K , close to that of a silicon wafer. These results show that some of

AC C

in OLED displays.

EP

the PBOIs examined in this work are promising candidates for use as plastic substrate materials

Acknowledgement

We are grateful to Mr. N. Okubo, Ms. N. Hamano, Ms. M. Murai, and Mr. N. Iwata in our research group for their partial support in the experimental work.

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Supplementary Data

AC C

EP

TE D

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SC

RI PT

Supplementary Data may be found in the online version of this article.

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RI PT

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TE D

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EP

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AC C

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SC

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TE D

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EP

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AC C

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31. Hasegawa M and Koyanaka M, High Perform Polym 15: 47–64 (2003).

32. Sensui N, Ishii J, Takata A, Oami Y, Hasegawa M and Yokota R, High Perform Polym 21:

SC

709–728 (2009).

M AN U

33. Kuroki T, Tanaka Y, Hokudoh T and Yabuki K, J Appl Polym Sci 65: 1031–1036 (1997). 34. Makabe H, Banba T and Hirano T, J Photopolym Sci Technol 10: 307–312 (1997). 35. Kitagawa T, Murase H and Yabuki K, J Polym Sci Part B Polym Phys 36: 39–48 (1998). 36. Wolfe JF and Arnold FE, Macromolecules 14: 909–915 (1981).

TE D

37. Maruyama Y, Oishi Y, Kakimoto M and Imai Y, Macromolecules 21: 2305–2309 (1988). 38. Hasegawa M, Kobayashi J and Vladimirov L, J Photopolym Sci Technol 17: 253–258 (2004).

EP

39. Fukumaru T, Fujigaya T, Nakashima N, Macromolecules 45: 4247–4257 (2012).

AC C

40. Brekner MJ and Feger C, J Polym Sci Part A Polym Chem 25: 2005−2020 (1987). 41. Brekner MJ and C. Feger C, J Polym Sci Part A Polym Chem 25: 2479–2491 (1987). 42. Song G, Wang D, Zhao X, Dang G, Zhou H, Chen C, High Perform Polym 25: 354–360 (2012).

43. Hasegawa M, Kaneki T, Tsukui M, Okubo N, Ishii J, Polymer 99: 292–306 (2016). 44. Hasegawa M, Suyama N, Shimoyama N, Aoki H, Nunokawa T, Kimura T, Polym Int 60:

38

ACCEPTED MANUSCRIPT

1240–1247 (2011). 45. Musto P, Karasz FE, MacKnight WJ, Polymer 30: 1012–1021 (1989).

RI PT

46. Baklagina YG, Milevskaya IS, Efanova NV, Sidorovich AV, Zubkov VA, Vysokomol Soedin A18: 1235–1242 (1976).

AC C

EP

TE D

M AN U

SC

47. Obata Y, Okuyama K, Kurihara S, Kitano Y, Jinda T, Macromolecules 28: 1547–1551 (1995).

39

ACCEPTED MANUSCRIPT

Table 1. Pre-treatment and purification conditions for the monomers and raw materials.

Commercially available monomers and raw materials Pyromellitic dianhydride (PMDA) 3,3′,4,4′-Biphenyltetracarboxylic dianhydride (s-BPDA) 2,3,6,7-Naphthalenetetracarboxylic dianhydride (NTDA) c p-Phenylenediamine (p-PDA)

Mitsubishi Gas Chemical Tokyo Chemical Industry (TCI)

-----

Vacuum-drying condition

Melting point (°C)

160 °C / 24 h

286

200 °C / 12 h

300

200 °C / 12 h

360

30 °C / 24 h

142

50 °C / 24 h

191

60 °C / 24 h

205

50 °C / 12 h

185

-----

50 °C / 12 h

91

-----

JFE Chemical

-----

TCI

Ethyl acetate Toluene/DMF (10/1, v/v) -----

Wako Chemical

4,4′-Diaminobenzanilide (DABA) 4-Aminophenyl 4-aminobenzoate (APAB) 4,4′-Methylenedianiline (MDA)

TCI Wakayama Seika TCI

4,4′-Diaminodiphenyl sulfone (DDS)

TCI

-----

50 °C / 12 h

177 b

TCI

-----

50 °C / 12 h

108 b

TCI

-----

50 °C / 12 h

245 b

TCI

-----

50 °C / 12 h

242 b

TCI

-----

-----

-----

Wakayama Seika

DMAc/water (1/1, v/v)

60 °C / 24 h

300

-----

GBL

120 °C / 12 h

354

-----

----Purified by Sublimation ----Toluene/DMF (4/1, v/v) -----

120 °C / 12 h

414

120 °C / 12 h

234

120 °C / 12 h

190

120 °C / 12 h

233

120 °C / 12 h

280

p-HAB-4AB DAR-4AB

EP

5ABO-4AB

TE D

4,4″-Diamino-p-terphenyl (DATP) 4,6-Diaminoresorcinol dihydrochloride (DAR-2HCl) 3,3′-Dihydroxy-4,4′-diaminobiphenyl (p-HAB) Diamines synthesized in this work

---------

ABI-4AB

-----

AC C

6ABO-4AB

4API-pPDA

-----

M AN U

4,4′-Diaminobenzophenone (DABP)

SC

4,4′-Oxydianiline (4,4′-ODA)

4,4′-Thiodianiline (TDA)

a

Solvent for recrystallization

Source

-----

a

RI PT

Monomer or raw material

Data determined from endothermic peak temperatures in DSC thermograms recorded at a –1 heating rate of 5 °C min . b Data from material safety data sheet. c Gift from Sony Chemical & Information Device. DMF = N,N-Dimethylformamide, DMAc = N,N-dimethylacetamide, and GBL = γ-butyrolactone.

40

ACCEPTED MANUSCRIPT

Table 2. Film properties of the 5ABO-4AB-derived PBOIs and related systems. CTE (ppm K–1)

Td5 (N2) (°C)

Td5 (air) (°C)

3.4 (–16.2) b 23.0 (4.0) b

565 (540) b 579 (579) b

547 (515) b

1.48

ND a (412) b ND a (321) b

NTDA

2.79

ND a

12.9

601

6ABO-4AB

PMDA

1.23

408

13.0

557

5

ibid

s-BPDA

2.74

26.8

572

---

6

ABI-4AB

PMDA

0.63

ND a

–2.8

556

7

ibid

NTDA

1.27

ND a

–2.7

588

ηinh (PAA) (dL g−1)

1

5ABO-4AB

PMDA

1.31

2

ibid

s-BPDA

3

ibid

4

ND: No distinct Tg was detected by DMA.

b

Data from ref. 42.

AC C

EP

TE D

a

41

E (GPa)

εb ave/max

(%)

σb (GPa)

WA (%)

7.48

16/24

0.25

2.48

7.70

28/34

0.33

1.27

575

---

---

---

2.44

---

---

---

---

---

SC

Tetracarboxylic dianhydride

---

---

---

---

525

---

---

---

8.96

550

---

---

---

6.39

M AN U

Diamine (mol%)

RI PT

Tg (ºC)

Sample No.

ACCEPTED MANUSCRIPT

Table 3. Film properties of the p-HAB-4AB-derived PBOIs. Tetracarboxylic dianhydride

ηinh (PAA) (dL g−1)

Tg (ºC)

CTE (ppm K−1)

Td5 (N2) (°C)

8

p-HAB-4AB

PMDA

0.76

384

–0.3

576

9

s-BPDA

1.83

ND a

9.6

592

10

NTDA

1.15

408

8.4

603

11

PMDA (50) s-BPDA (50)

2.19

322

7.6

NTDA

1.26

428

0.8

12

TE D EP 42

E (GPa)

ave/max

549

3.5

26/49

1.78

567

4.1

38/56

1.36

592

3.8

28/39

---

(%)

WA (%)

579

567

---

---

---

604

---

---

---

---

M AN U

ND: No distinct Tg was detected by DMA.

AC C

a

p-HAB-4AB50 p-PDA50

εb

Td5 (air) (°C)

RI PT

Diamine (mol%)

SC

Sample No.

ACCEPTED MANUSCRIPT

Table 4. Film properties of the DAR-4AB-derived PBOIs. Diamine (mol%)

Tetracarboxylic dianhydride

ηinh (PAA) (dL g−1)

Tg (ºC)

CTE (ppm K−1)

Td5 (N2) (°C)

13

DAR-4AB

s-BPDA

0.77

ND a

12.4

594

---

5.5

11/14

14

DAR-4AB50 5ABO-4AB50

NTDA

1.29

ND a

3.3

609

543

6.80

8/15

EP

TE D 43

E (GPa)

ave/max

(%)

RI PT

M AN U

SC

ND: No distinct Tg was detected by DMA.

AC C

a

Td5 (air) (°C)

εb

Sample No.

ACCEPTED MANUSCRIPT

BO-incorporating diamines (1) Pyridine

H2 Pd/C

RI PT

(2) p-TSA

6ABO-4AB (1) Pyridine (2) p-TSA

M AN U

H2

ABI-4AB

SC

5ABO-4AB

Pd/C

p-HAB-4AB

p-PDA

TE D

Common diamines

DAR-4AB

4,4′-ODA

s-BPDA

NTDA

AC C

PMDA

EP

Rigid Tetracarboxylic dianhydrides

Fig. 1. Reaction schemes for the synthesis of monomers incorporating benzoxazole (BO) units and the structures of the other monomers.

44

RI PT

ACCEPTED MANUSCRIPT

in NMP

(1) casting

at r.t. Tetracarboxylic dianhydride

M AN U

PBOAA solution

SC

(2) ∆

Diamine

PBOI film

AC C

EP

TE D

Fig. 2. Reaction schemes for the polyaddition and thermal imidization of PBOI precursors.

45

ACCEPTED MANUSCRIPT

RI PT SC

1178

2626

3393

1667 1602 1531

M AN U

1705

Transmittance (%)

(a)

TE D

(b)

742 1773

1618

EP

3070

1500

AC C

Transmittance (%)

1499

1719

1362

Wavenumber (cm–1) Fig. 3. FT-IR spectra of thin films for s-BPDA/p-HAB-4AB: (a) PBOI precursor and (b) PBOI.

46

ACCEPTED MANUSCRIPT

600

RI PT

(a) Td5 (N2) (°C)

580

SC

560

M AN U

540

520

500 60

(b)

30 20

AC C

10

TE D

40

EP

CTE (ppm K−1)

50

0

Fig. 4. Effect of connection group (Z) on the 5% weight loss temperature (Td5) in N2 (a) and CTE (b).

47

RI PT

ACCEPTED MANUSCRIPT

m

100 – m

620 610

SC

590 580

X=

570 560 550 50 1 100

TE D

540

M AN U

Td5 (N2) (°C)

600

50 2 100

50 3 100

EP

5ABO-4AB content (m) (mol%)

AC C

Fig. 5. Comparison of the thermal stability of benzimide and benzoxazole units.

48

RI PT

ACCEPTED MANUSCRIPT

650 14

10 3 13

9 8

2

11 1

550

500

450 -10

0

10

M AN U

Td5 (N2) (°C)

600

SC

12

20

30

40

50

60

70

–1

EP

TE D

CTE (ppm K )

AC C

Fig. 6 CTE–Td5 diagram for the PBOIs examined in this work (●), conventional aromatic PIs (∆,▲), and s-BPDA/p-PDA (▲). The numbers inserted in this figure denote the sample numbers of the PBOIs.

49

ACCEPTED MANUSCRIPT Highlights ►Poly(benzoxazole imide)s (PBOIs) achieved a higher 5% weight loss temperature in N2 and a lower CTE than those of a conventional aromatic polyimide (PI) (s-BPDA/p-PDA). ►The

AC C

EP

TE D

M AN U

SC

RI PT

PBOIs are promising candidates for use as the plastic substrates in OLED displays.