Two-step thermal imidization process in the synthesis of PIs using ODPA dianhydride monomer and BAPF diamine monomer as an example: (a) poly(amic acid) and (b) polyimide. 

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... this rapidly changing world of technology, various technologies are booming, and one of them - the flexible device is being intensely pursued [1-6]. Its applications have emerged in many new territories such as solar cell, flexible display, e-book, and RFID based on polymer or metal substrate. Recently, polymers have received much attention to be the substrate in the flexible device because of its better properties on lightweight, flexibility, thermal ability, dielectric properties, chemical resistance and lower cost. However, there are several drawbacks and limitations of using a polymer substrate, for example, low glass transition temperature (T g ) to survive high-temperature process, high coefficient of thermal expansion (CTE), and high water vapor permeation rate (WVTR). In particular, low T g is one of the critical issues because the requirement of deposition temperature for transparent conductive oxides (TCOs) films such as indium-tin-oxides (ITO) and aluminum-zinc-oxides (AZO) is o quite high, >200 C. At present, commercial plastic substrates such as poly (ethylene terephthalate) (PET) and poly (arylene ether nitrile) (PEN) can only undergo a thin film deposition at o o temperature lower than 80 C and 150 C, respectively. Polyimides possess high T g , excellent thermal stability and mechanical strength for packaging, passivation, and dielectric applications in microelectronics. However, polyimides have limited applications as flexible substrate in optoelectronics due to their certain color and difficult solubility. In this study, our objective is to develop polyimide films with high T g o (250-300 C) and high transmittance (> 80%) in visible range [7], as the flexible substrate for optoelectric applications. To achieve this goal, aromatic polyimides are first selected due to their excellent thermal and mechanical properties by means of their backbone with aromatic rings and high strength imide bond. For improving the transparency of polyimide films, one can introduce flexible linkage, bulky group, or the trifluoromethyl (CF 3 ) substituent into the main chain of polyimide [8, 9]. The effects of such approaches on the transmittance and glass transition temperature are of significant interests for the design of polyimide films in flexible substrate application. In this study, we introduced ether linkage, bulky group (CH 3 ), or trifluoromethyl (CF 3 ) substituent into diamine and/or dianhydride monomers to examine their structure-property relationship in terms of transmittance (400 nm to 1000 nm range), T g , thermal stability, and film quality. The structural effect of ether, bulky groups, and their combination will be also addressed. Four different starting diamine monomers: 2,2-bis[4-(4-amino-phenoxy)] hexafluoropropane (BAF; a1), 2,2-bis[4-(4-aminophenoxy)pheny] hexafluoropropane (BAPF; a2), 4,4 ’ -diaminodiphenyl ether (ODA; a3), and (2,2-bis[4-(4-amino phenoxy)pheny]propane (BAPP; a4) were chosen in this study. Meanwhile, two starting dianhydride monomers: [2,2 ’ -bis(3,4-dicarboxyphenyl) hexafluoropropane dianhydride] (6FDA; b1) and [4 , 4 ’ -oxydiphthalic anhydride] (ODPA; b2) were used. The chemical structures of the starting diamine and dianhydride monomers are schematically illustrated in Fig. 1. N,N-Dimethyl-acetamide (DMAc) (99.5% purity) used as a solvent was procured from TCI. DMAc was purified and isolated from UV and water prior to use. 4 , 4 ’ -oxydiphthalic anhydride (ODPA) was recrystalized by acetic anhydride and then washed by n-hexane before o drying in the vacuum oven at 60 C for 3 hrs. In addition, all o diamine and dianhydride monomers were heated to 60 C for 30 minutes to remove water before the synthesis of polyamic acid. The polyamic solutions were synthesized by adding diamine monomer and dianhydride monomer at 1:1 molar ratio, which was further stirred at room temperature for 24 hr in a flask isolated from air and water. An uniform poly (amic acid) solution with high viscosity was obtained as schematically shown by the case of ODPA-BAPF poly(amic acid) in Fig. 2(a). The poly(amic acid) solution was then coated onto a glass substrate using doctor blade technique to form a smooth and uniform thick film. The PAA film was then cured on a hot o o plate at 150 C for 1 hr and then at 300 C for 2 hrs to form a polyimide film as illustrated in Fig. 2 (b). The nominal thickness of polyimide film in this study is 70-80 μ m unless specified otherwise. Glass transition temperature of polyimide films was measured by a differential scanning calorimetry (DSC) (Q500, o o TA Instruments). The heating rate was 5 C /mins from 90 C o to 350 C with 2 thermal cycles. The T g was determined based nd on the 2 heating curve. The decomposition temperature of polyimide film was determined by a thermo-gravimetric analysis (TGA) using Diamond DSC4000 (Perkin Elmer). The o sample was heated at 90 C for 5 minutes, then the o o temperature was raised from 100 C to 750 C at a heating rate o of 5 C/min under air ambient. A UV/VIS spectrophotometry, Evolution 300 (Thermo Scientific) was used to determine the transmission spectrum of a polyimide film in the UV/VIS region (50 to 1000 nm). The thermal properties such as T g and decomposition temperature (T d ) of polyimide films are summarized in Tables 1 and 2 for OPDA- and 6FDA-based polyimides, respectively. The thermal decomposition temperatures (T d ) of all polyimide films in this study are in a range of 478 o C – 507 o C. This indicates these polyimide films exhibit excellent thermal stability for intended optoelectronic applications. Next, we examined the effect of introducing flexible ether linkage (a2, a3, a4, b2), bulky CH 3 side-group (a4), or CF 3 substituent (a1, a2, b1) into diamine and/or dianhydride monomers on the T g ’ s of OPDA- and 6FDA-based polyimides. The polyimides films with 6FDA as the anhydride are found to possess higher T g than those with ODPA as the anhydride. In addition, under the same anhydride (either ODPA or 6FDA), the T g of polyimide films shows the decreasing trend: BAF > BAPF > ODA > BAPP. Furthermore, 6FDA-BAF polyimide o has the highest T g of 297 C, while ODPA-BAPP polyimide o posseses the lowest T g of 224 C, among 8 different polyimides. BAF-based polyimides such as 6FDA-BAF and o ODPA-BAF polyimides show relatively high T g , > 290 C. However, ODPA-BAF polyimide is brittle and not usable as a flexible substrate. The brittleness may be due to semicrystalline and/or low molecular weigh [10]. The other polyimides have excellent film quality in flexibility as a flexible substrate. Fig. 3 shows the transmission spectra of these polyimide films in the range between 0 and 600 nm. Their transmittance at 400 nm and 550 nm are summarized in Tables 1 and 2. Overall, all polyimide films with 70-80  m thickness exhibit excellent transparency, 84-92% at wavelength between 500 nm and 1000 nm. Specifically, the polyimides with 6FDA as the anhydride show better transparency (89-92%) than those with ODPA as the anhydride (84-89%). In addition, under the same anhydride (either ODPA or 6FDA), the effect of diamine monomer on the transmittance of polyimide films shows the following trend: BAF > BAPF > ODA ~ BAPP. When the full UV-Vis spectrum is considered, however, ODPA-BAPF and 6FDA-ODA polyimides extend their transparency below 500 nm and into UV region. Specifically, their transmittance at 400 nm are relatively high at 78%. Excluding 6FDA-BAPP and OPDA-BAF, the polyimides using 6FDA as the anhydride have higher transmittance in 400-500 nm range than the those with ODPA as the dianhydride. Based on T g , transmittance, and film quality, 6FDA-BAF polyimide film offers the best combination in o materials properties with relative high T g (297 C) and 92% transparency between 500 and 1000 nm or 41% at 400 nm. In this paper, we incorporated ether linkage in both diamine monomer such as ODA (a3) and in dianhydride monomer such as ODPA (b2). On the other hand, trifluormethyl bulky group was introduced in diamine monomer such as BAF (a1) and in dianhydride monomer such as 6FDA (b1). In addition, a combined ether linkage and bulky groups either CH 3 or CF 3 substituent were built into diamine monomers, namely: BAPF (a2) and BAPP (a4). The molecular structure of an ether linkage, -C-O- is sp hybrid orbitals. Although the central oxygen atom in the ether linkage has four bonding orbitals, two of them are empty orbitals. Therefore, two lone pairs of electrons can warp the C-O linkage more easily and make C-O linkage more flexible. 3 In contrast, CH 3 and CF 3 bulky groups are full bonded sp structures. Thus, the introduction of these bulky groups into the main chains of polyimide makes the polymer chains rigid and hard to rotate [11]. The impact of flexible ether linkage can be illustrated by the use of 6FDA and ODPA dianhydride monomers. The flexible ether linkage in ODPA makes the T g ’ s o f OPDA-based polyimides lower than 6FDA-based polyimides as summarized in Tables 1 and 2. Flexibility in polyimide c h a i n s r e d u c e s t h e f i l m s ’ s T g . Under the same dianhydride monomer (either 6FDA or ODPA), ODA-based polyimide has higher T g than that of BAPP-based polyimide. This indicates the polyimide chains with combined – CH 3 side-group and ether linkage (for example, ODPA-BAPP) is more ...