**2. Experiments**

#### **2.1. Materials**

high curing temperature of PAAs often causes serious damage for the temperature-sensitive components in TFT-LCDs, such as the color filters which would be destroyed when the tem‐

Besides the curing temperature consideration, the high VHR and low RDC values of the de‐ vices are also highly desired for advanced TFT-LCD fabrication in order to achieve a highresolution display (high contrast, low image sticking, etc) [8]. VHR and RDC values of the TFT-LCD devices are influenced by many factors, including the characteristics of LC materi‐ als, the features of the PI alignment layers and the display modes of the devices. Among the factors, the effects of the chemical structures of the PI alignment layers are often critical. For instance, it has been well established that the highly conjugated molecular skeletons in wholly-aromatic PIs often lead to low VHR and high RDC values for the devices [9]. Thus, PI alignment layers with low conjugated structures have been paid increasing attentions.

Considering the above-mentioned structure-property relationship for PI alignment layers used for advanced TFT-LCDs, alicyclic or semi-alicyclic PIs have been confirmed to be the best candidates as AL materials up to now. Especially, semi-alicyclic PIs derived from alicy‐ clic dianhydrides and aromatic diamines possess the best combined properties, including good thermal stability, good solubility in organic solvents, acceptable mechanical properties, good optical transparency, high VHR and low RDC values. Thus, semi-alicyclic PIs have been widely investigated as AL materials for advanced TFT-LCDs [10-12]. Figure 1 illus‐ trates the developing trajectory of PI alignment layers with different kinds of display modes.

C) have been developed in the past decades [5-7].

C [4]. Hence, PI alignment layers with low curing tempera‐

peratures are higher than 230 o

270 Optoelectronics - Advanced Materials and Devices

**Figure 1.** Developing trends of PI alignment layers for LCDs.

tures (<220 o

Styrene, *p*-methylstyrene, *p*-*tert*-butylstyrene and *p*-fluorostyrene were purchased from To‐ kyo Chemical Industry Co., Ltd., Japan (TCI) and used as received. It is unnecessary to re‐ move the inhibitors in the chemicals. Maleic anhydride was obtained from Beijing Yili Fine Chemicals, China and used as received. 4,4'-Methylenedianiline (MDA, TCI, Japan) was re‐ crystallized from ethanol and dried in vacuum at 80 o C overnight prior to use. 2,4-Diamino- (*n*-hexadecanoxy)benzene (16PDA) was synthesized in our laboratory and purified by continuous recrystallization from ethanol. Commercially available *N*-methyl-2-pyrrolidi‐ none (NMP), *N,N-*dimethylacetamide (DMAc), cyclopentanone (CPA), γ-butyrolactone (GBL) and ethylene glycol monobutyl ether (butyl cellosolve, BC) were purified by distillation pri‐ or to use. The other commercially available reagents were used without further purification.

#### **2.2. Measurements**

Inherent viscosity was measured using an Ubbelohde viscometer with a 0.5 g/dL NMP solu‐ tion at 25 o C. Absolute viscosity was measured using a Brookfield DV-II+ Pro viscometer at 25 o C. Fourier transform infrared (FT IR) spectra were obtained with a Tensor 27 Fourier transform spectrometer. Ultraviolet-visible (UV-vis) spectra were recorded on a Hitachi U-3210 spectrophotometer at room temperature. The cutoff wavelength was defined as the point where the transmittance drops below 1% in the spectrum. Prior to test, PI samples were dried at 100 o C for 1 h to remove the absorbed moisture. Yellow index (YI) and haze values of the PI films were measured using an X-rite color i7 spectrophotometer with PI film samples at a thickness of 30-40 μm in accordance with the procedure described in ASTM D1925 "Test method for yellowness index of plastics" and in ASTM D1003 "Standard test method for haze and luminous transmittance of transparent plastics", respectively. The col‐ or parameters were calculated according to a CIE Lab equation. *L*\* is the lightness, where 100 means white and 0 implies black. A positive *a*\* means a red color, and a negative one indi‐ cates a green color. A positive *b*\* means a yellow color, and a negative one indicates a blue color. Nuclear magnetic resonances (1 H NMR and 13C NMR) were performed on a AV 400 spectrometer operating at 400 MHz in DMSO-*d6* or CDCl3. Differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) were recorded on a TA-Q series thermal anal‐ ysis system at a heating rate of 10 o C/min and 20 <sup>o</sup> C/min in nitrogen or air, respectively. Gel permeation chromatography (GPC) measurements were performed using a Waters 1515 HPLC pump equipped with a Waters 2414 refractive index detector. Two Waters Styragel HR 4 columns kept at 35o C±0.1o C were used with HPLC grade NMP as the mobile phase at a flow rate of 1.0 mL/min. Number average weight (*M*n), weight average molecular weight (*M*w) and polydispersity (*M*w/*M*n) were then determined with polystyrene as a standard.

being dried in vacuum at 80 o

Yield: 51.44 g (73.4%).

Melting point: 229 o

Melting point: 199 o

Melting point: 218 o

1410, 1222, 1055, and 925. 1

and 26.8. MS (EI): 239 (M+

Melting point: 201 o

H, 5.66%; Found: C, 67.23%, H, 5.70%.

1405, 1229, 1058, and 928. 1

1412, 1223, 1076, and 916. 1

C for 24h, the pure MTDA was obtained as white crystals.

C (DSC peak temperature). FT IR (KBr, cm-1): 2941, 1855, 1778, 1506,

C (DSC peak temperature). FT IR (KBr, cm-1): 2966, 1861, 1780, 1493,

C (DSC peak temperature). FT IR (KBr, cm-1): 2962, 1859, 1789, 1504,

H NMR (DMSO-*d*6, ppm): 7.70 (*s*, 1H), 7.34-7.32 (*d*, 2H), 7.13-7.11


C (DSC peak temperature). FT IR (KBr, cm-1): 2912, 1864, 1782, 1664,

H NMR (DMSO-*d*6, ppm): 7.69-7.67 (*d*, 1H), 7.38-7.27 (*m*, 2H),

(*m*, 1H), 4.56-4.54 (*d*, 1H), 4.12-4.10 (*m*, 1H), 3.83-3.81 (*m*, 1H), 3.18-3.13 (*m*, 1H), 3.09 (*m*, 1H), 2.82-2.78 (*m*, 1H), 2.32 (*s*, 3H), 1.97-1.95 (*m*, 1H), and 1.86-1.85 (*m*, 1H). 13C NMR (DMSO-*d*6, ppm): 175.8, 175.3, 173.8, 172.4, 138.1, 135.0, 131.8, 130.5, 129.9, 127.7, 45.1, 44.2, 36.4, 32.3,

*3,4-Dicarboxy-1,2,3,4-tetrahydro-1-naphthalene succinic dianhydride (TDA).* The dianhydride was synthesized from styrene and maleic anhydride through a similar route to MTDA. The

7.20-7.19 (*d*, 1H), 4.67-4.64 (*d*, 1H), 3.73-3.58 (*m*, 2H), 3.40-3.33 (*m*, 1H), 2.85-2.80 (*m*, 2H), and 2.12-2.07 (*m*, 2H). 13C NMR (DMSO-*d*6, ppm): 173.8, 173.5, 172.1, 170.8, 136.4, 129.5, 128.4,

*3,4-Dicarboxy-1,2,3,4-tetrahydro-6-tert-butyl-1-naphthalene succinic dianhydride (TTDA).* The di‐ anhydride was synthesized from *p*-*tert*-butylstyrene and maleic anhydride through a similar

(*d*, 1H), 4.65-4.62 (*d*, 1H), 3.74-3.67 (*m*, 1H), 3.62-3.58 (*m*, 1H), 3.31-3.30 (*m*, 1H), 2.93-2.76(*m*, 2H), 2.44-2.39(*m*, 1H), 2.13-2.07 (*m*, 1H) and 1.29(*s*, 9H). 13C NMR (DMSO-*d*6, ppm): 174.4, 174.0, 172.7, 171.3, 150.2, 134.0, 128.4, 128.1, 126.6, 125.0, 43.3, 42.8, 38.0, 36.8, 34.7, 33.3, 31.5,

*3,4-Dicarboxy-1,2,3,4-tetrahydro-6-fluoro-1-naphthalene succinic dianhydride (FTDA).* The dia‐ nhydride was synthesized from *p*-fluorostyrene and maleic anhydride through a similar

1441, 1376, 1304, 1262, 1211, 1151, 1080, 1027, 967, 914, 870, 819, 754, 633, 594, 558 and 498. 1

mental analysis: calculated for C16H11FO6: C, 60.38%, H, 3.48%; Found: C, 59.90%, H, 3.53%.

NMR (DMSO-*d*6, ppm): 7.51-7.49 (*d*, 1H), 7.29-7.27 (*m*, 2H), 7.17-7.14 (*m*, 1H), 4.69-4.68 (*d*, 1H), 3.75-3.71 (*m*, 1H), 3.60-3.57(*m*, 1H), 3.44-3.43(*m*, 1H), 2.85-2.83 (*m*, 2H), 2.57-2.53(*m*, 1H) and 2.10-2.06 (*m*, 1H). 13C NMR (DMSO-*d*6, ppm): 173.9, 172.3, 171.2, 162.3, 160.7, 133.1,

route to MTDA. The product was obtained as white crystals. Yield: 55.60 g (77.3%).

131.3, 130.6, 116.2, 114.9, 43.3, 42.4, 37.3, 36.7, 33.2 and 26.5. MS (EI): 146 (M+

26.4, and 22.0. MS (EI, m/e, percentage of relative intensity): 142 (M+

128.1, 127.5, 127.4, 42.8, 42.1, 37.3, 36.7, 32.8, and 26.2. MS (EI): 128 (M+

analysis: calculated for C16H12O6: C, 64.00%, H, 4.03%; Found: C, 64.10%, H, 4.03%.

route to MTDA. The product was obtained as white crystals. Yield: 58.49 g (73.6%).

product was obtained as white crystals. Yield: 50.35 g (75.2%).

analysis: calculated for C17H14O6: C, 64.97%, H, 4.49%; Found: C, 64.32%, H, 4.44%.

H NMR (DMSO-*d*6, ppm): 7.35 (*s*, 1H), 7.15-7.13 (*m*, 1H), 7.10-7.09


Organo-soluble Semi-alicyclic Polyimides http://dx.doi.org/10.5772/51182 273


H


Solubility was determined as follows: 1.5 g of the PI resin was mixed with 8.5 g of the tested solvent at room temperature (15 wt% solid content), which was then mechanically stirred in nitrogen for 24 h. The solubility was determined visually as three grades: completely soluble (++), partially soluble (+), and insoluble (-). The complete solubility is defined as a homoge‐ nous and clean solution is obtained, in which no phase separation, precipitation or gel for‐ mation is detected.

The electrical characteristics, including VHR, RDC and pretilt angle values of the LC test cells were measured on a Toyo Model 6254 measurement system. VHR measurements were performed at LC test cells with a gap of 5-6 μm. The peak value of the square wave voltage and pulse duration was +5V and 60 μs, respectively. RDC measurements were performed using the "flick free" method. The test cells were first addressed with +5V direct circuit (DC) offset voltage for 3600 s. After the time, the +5V DC offset was switched off. The resulting flicker was monitored and the DC offset voltage was increased until the flickering was no longer visible. The compensating voltage was the residual DC voltage (RDC). Pretilt angles measurements were performed using the crystal rotation method with LC cells with a gap of 50 μm fabricated by anti-parallel rubbing process. The values of all measurements of VHR, RDC and pretilt angles are averages of at least 10 independent LC cells.

#### **2.3. Monomer synthesis**

*3,4-Dicarboxy-1,2,3,4-tetrahydro-6-methyl-1-naphthalene succinic dianhydride (MTDA).* Into a 500-mL three-necked flask equipped with a mechanical stirrer, a gas inlet and a condenser, 43.75 g (0.446 mol) of maleic anhydride, 80.60 g (0.682 mol) of *p*-methylstyrene, and 0.1138 g (0.5 mmol) of 2,5-di-*tert*-butyl hydroquinone were added. Nitrogen was introduced to re‐ move the air in the system. Then, nitric oxide (NO) gas was introduced from a gas inlet plac‐ ing under the surface of the reaction solution. The reaction mixture were heated to 120o C and maintained for 5 h under an atmosphere of nitric oxide. The produced red-brown nitro‐ gen oxide gas was trapped by passing through an aqueous solution of 20 wt% sodium hy‐ droxide. An orange precipitate formed during the reaction. After the reaction, the mixture was cooled to room temperature and 60 ml of acetonitrile was then added. The reaction mix‐ ture was heated to reflux for another 0.5 h. Then, 60 ml of toluene was added and the reac‐ tion mixture was cooled to temperature. The produced white needle crystals were collected by filtration and the solid was washed with toluene and petroleum ether in succession. After

being dried in vacuum at 80 o C for 24h, the pure MTDA was obtained as white crystals. Yield: 51.44 g (73.4%).

color. Nuclear magnetic resonances (1

272 Optoelectronics - Advanced Materials and Devices

ysis system at a heating rate of 10 o

C±0.1o

HR 4 columns kept at 35o

mation is detected.

**2.3. Monomer synthesis**

H NMR and 13C NMR) were performed on a AV 400

C were used with HPLC grade NMP as the mobile phase at a

C/min in nitrogen or air, respectively. Gel

C

spectrometer operating at 400 MHz in DMSO-*d6* or CDCl3. Differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) were recorded on a TA-Q series thermal anal‐

permeation chromatography (GPC) measurements were performed using a Waters 1515 HPLC pump equipped with a Waters 2414 refractive index detector. Two Waters Styragel

flow rate of 1.0 mL/min. Number average weight (*M*n), weight average molecular weight (*M*w) and polydispersity (*M*w/*M*n) were then determined with polystyrene as a standard.

Solubility was determined as follows: 1.5 g of the PI resin was mixed with 8.5 g of the tested solvent at room temperature (15 wt% solid content), which was then mechanically stirred in nitrogen for 24 h. The solubility was determined visually as three grades: completely soluble (++), partially soluble (+), and insoluble (-). The complete solubility is defined as a homoge‐ nous and clean solution is obtained, in which no phase separation, precipitation or gel for‐

The electrical characteristics, including VHR, RDC and pretilt angle values of the LC test cells were measured on a Toyo Model 6254 measurement system. VHR measurements were performed at LC test cells with a gap of 5-6 μm. The peak value of the square wave voltage and pulse duration was +5V and 60 μs, respectively. RDC measurements were performed using the "flick free" method. The test cells were first addressed with +5V direct circuit (DC) offset voltage for 3600 s. After the time, the +5V DC offset was switched off. The resulting flicker was monitored and the DC offset voltage was increased until the flickering was no longer visible. The compensating voltage was the residual DC voltage (RDC). Pretilt angles measurements were performed using the crystal rotation method with LC cells with a gap of 50 μm fabricated by anti-parallel rubbing process. The values of all measurements of VHR,

*3,4-Dicarboxy-1,2,3,4-tetrahydro-6-methyl-1-naphthalene succinic dianhydride (MTDA).* Into a 500-mL three-necked flask equipped with a mechanical stirrer, a gas inlet and a condenser, 43.75 g (0.446 mol) of maleic anhydride, 80.60 g (0.682 mol) of *p*-methylstyrene, and 0.1138 g (0.5 mmol) of 2,5-di-*tert*-butyl hydroquinone were added. Nitrogen was introduced to re‐ move the air in the system. Then, nitric oxide (NO) gas was introduced from a gas inlet plac‐ ing under the surface of the reaction solution. The reaction mixture were heated to 120o

and maintained for 5 h under an atmosphere of nitric oxide. The produced red-brown nitro‐ gen oxide gas was trapped by passing through an aqueous solution of 20 wt% sodium hy‐ droxide. An orange precipitate formed during the reaction. After the reaction, the mixture was cooled to room temperature and 60 ml of acetonitrile was then added. The reaction mix‐ ture was heated to reflux for another 0.5 h. Then, 60 ml of toluene was added and the reac‐ tion mixture was cooled to temperature. The produced white needle crystals were collected by filtration and the solid was washed with toluene and petroleum ether in succession. After

RDC and pretilt angles are averages of at least 10 independent LC cells.

C/min and 20 <sup>o</sup>

Melting point: 229 o C (DSC peak temperature). FT IR (KBr, cm-1): 2941, 1855, 1778, 1506, 1412, 1223, 1076, and 916. 1 H NMR (DMSO-*d*6, ppm): 7.35 (*s*, 1H), 7.15-7.13 (*m*, 1H), 7.10-7.09 (*m*, 1H), 4.56-4.54 (*d*, 1H), 4.12-4.10 (*m*, 1H), 3.83-3.81 (*m*, 1H), 3.18-3.13 (*m*, 1H), 3.09 (*m*, 1H), 2.82-2.78 (*m*, 1H), 2.32 (*s*, 3H), 1.97-1.95 (*m*, 1H), and 1.86-1.85 (*m*, 1H). 13C NMR (DMSO-*d*6, ppm): 175.8, 175.3, 173.8, 172.4, 138.1, 135.0, 131.8, 130.5, 129.9, 127.7, 45.1, 44.2, 36.4, 32.3, 26.4, and 22.0. MS (EI, m/e, percentage of relative intensity): 142 (M+ -172, 100). Elemental analysis: calculated for C17H14O6: C, 64.97%, H, 4.49%; Found: C, 64.32%, H, 4.44%.

*3,4-Dicarboxy-1,2,3,4-tetrahydro-1-naphthalene succinic dianhydride (TDA).* The dianhydride was synthesized from styrene and maleic anhydride through a similar route to MTDA. The product was obtained as white crystals. Yield: 50.35 g (75.2%).

Melting point: 199 o C (DSC peak temperature). FT IR (KBr, cm-1): 2966, 1861, 1780, 1493, 1405, 1229, 1058, and 928. 1 H NMR (DMSO-*d*6, ppm): 7.69-7.67 (*d*, 1H), 7.38-7.27 (*m*, 2H), 7.20-7.19 (*d*, 1H), 4.67-4.64 (*d*, 1H), 3.73-3.58 (*m*, 2H), 3.40-3.33 (*m*, 1H), 2.85-2.80 (*m*, 2H), and 2.12-2.07 (*m*, 2H). 13C NMR (DMSO-*d*6, ppm): 173.8, 173.5, 172.1, 170.8, 136.4, 129.5, 128.4, 128.1, 127.5, 127.4, 42.8, 42.1, 37.3, 36.7, 32.8, and 26.2. MS (EI): 128 (M+ -172, 100). Elemental analysis: calculated for C16H12O6: C, 64.00%, H, 4.03%; Found: C, 64.10%, H, 4.03%.

*3,4-Dicarboxy-1,2,3,4-tetrahydro-6-tert-butyl-1-naphthalene succinic dianhydride (TTDA).* The di‐ anhydride was synthesized from *p*-*tert*-butylstyrene and maleic anhydride through a similar route to MTDA. The product was obtained as white crystals. Yield: 58.49 g (73.6%).

Melting point: 218 o C (DSC peak temperature). FT IR (KBr, cm-1): 2962, 1859, 1789, 1504, 1410, 1222, 1055, and 925. 1 H NMR (DMSO-*d*6, ppm): 7.70 (*s*, 1H), 7.34-7.32 (*d*, 2H), 7.13-7.11 (*d*, 1H), 4.65-4.62 (*d*, 1H), 3.74-3.67 (*m*, 1H), 3.62-3.58 (*m*, 1H), 3.31-3.30 (*m*, 1H), 2.93-2.76(*m*, 2H), 2.44-2.39(*m*, 1H), 2.13-2.07 (*m*, 1H) and 1.29(*s*, 9H). 13C NMR (DMSO-*d*6, ppm): 174.4, 174.0, 172.7, 171.3, 150.2, 134.0, 128.4, 128.1, 126.6, 125.0, 43.3, 42.8, 38.0, 36.8, 34.7, 33.3, 31.5, and 26.8. MS (EI): 239 (M+ -118, 100). Elemental analysis: calculated for C20H20O6: C, 67.41%, H, 5.66%; Found: C, 67.23%, H, 5.70%.

*3,4-Dicarboxy-1,2,3,4-tetrahydro-6-fluoro-1-naphthalene succinic dianhydride (FTDA).* The dia‐ nhydride was synthesized from *p*-fluorostyrene and maleic anhydride through a similar route to MTDA. The product was obtained as white crystals. Yield: 55.60 g (77.3%).

Melting point: 201 o C (DSC peak temperature). FT IR (KBr, cm-1): 2912, 1864, 1782, 1664, 1441, 1376, 1304, 1262, 1211, 1151, 1080, 1027, 967, 914, 870, 819, 754, 633, 594, 558 and 498. 1 H NMR (DMSO-*d*6, ppm): 7.51-7.49 (*d*, 1H), 7.29-7.27 (*m*, 2H), 7.17-7.14 (*m*, 1H), 4.69-4.68 (*d*, 1H), 3.75-3.71 (*m*, 1H), 3.60-3.57(*m*, 1H), 3.44-3.43(*m*, 1H), 2.85-2.83 (*m*, 2H), 2.57-2.53(*m*, 1H) and 2.10-2.06 (*m*, 1H). 13C NMR (DMSO-*d*6, ppm): 173.9, 172.3, 171.2, 162.3, 160.7, 133.1, 131.3, 130.6, 116.2, 114.9, 43.3, 42.4, 37.3, 36.7, 33.2 and 26.5. MS (EI): 146 (M+ -172, 100). Ele‐ mental analysis: calculated for C16H11FO6: C, 60.38%, H, 3.48%; Found: C, 59.90%, H, 3.53%.

### **2.4. Polyimide synthesis**

The general procedure for the synthesis of PIs can be illustrated by the preparation of PI-8 (Table 1). Into a 250 mL three-necked, round-bottomed flask equipped with a mechanical stirrer, a Dean-Stark trap and a nitrogen inlet, MDA (17.8434 g, 0.09 mol) and 16PDA (3.457 g, 0.01 mol) was dissolved in *m*-cresol (100 g) to give a clear diamine solution. Then, FTDA (31.825 g, 0.1 mol) was added in one batch and an additional volume of *m*-cresol (112 g) was added to wash the residual dianhydride, and at the same time to adjust the solid content of the reaction system to be 20 wt%. After stirring in nitrogen for 1 h, a mixture of toluene (230 ml) and isoquinoline (catalytic amount) was then added. The reaction mixture was heated to 180 o C and maintained for 6 h. During the reaction, the toluene-water azeotrope was distil‐ led out of the system and collected in the Dean-Stark trap. After cooling to room tempera‐ ture, the viscous solution was slowly poured into an excess of ethanol to yield a silky resin. The resin was collected and dried at 80 o C in vacuo for 24 h. Yield: 47.54 g (96%).

ter). In addition, this reaction is easily to expand to a large scale. A scale of kilograms per

**O O**

**O**

**C**

**O O**

**O**

**R**

**O O**

**O**

**O O**

**O**

**O O**

**O**

**F**

**O O**

Organo-soluble Semi-alicyclic Polyimides http://dx.doi.org/10.5772/51182 275

**O**

H-13C

**NO (g) 120 <sup>o</sup> C, 5 h**

> **O O**

**O**

**R=-H, TDA R=-CH3, MTDA R=-C(CH3)3, TTDA R=-F, FTDA**

Figure 2 shows the FT IR spectra of the dianhydrides, in which the characteristic bands of carbonyl groups in the anhydride moiety were clearly observed at around 1863 and 1782 cm-1 for all of the compounds. In addition, the characteristic absorption of methyl group ap‐ peared at 2941 cm-1 for MTDA and TTDA. The 13C NMR and the two-dimensional 1

heteronuclear single-quantum coherence (HSQC) spectra of TTDA and FTDA are illustrated in Figure 3, together with the assignments of the observed resonances. As depicted in Figure 3, 18 carbon signals are clearly revealed for TTDA and 15 signals for FTDA. The absorptions of the protons cohered well with those of the corresponding carbon signals. This result is consistent with their proposed structures. Interestingly, the two pairs of protons in methyl‐ ene groups (H3,3' and H6,6') for both of the dianhydrides exhibited individual absorptions in

batch has been successfully achieved in our lab.

**O O**

> **O O**

**O**

**Scheme 1** Synthesis of new alicyclic dianhydrides via a low-cost route

**+**

**R**

**O O**

**O**

**CH3**

**O**

**O O**

**O**

**Figure 2.** FT IR spectra of tetralin dianhydrides

PI-8 resin (15 g) was dissolved in NMP (85 g) at room temperature to afford a 15 wt% solu‐ tion. The solution was filtered through a 0.45 μm Teflon syringe filter to remove any undis‐ solved impurities. Then, the solution was spin-coated on a clean silicon wafer or quartz substrate. The thickness of the PI film was controlled by regulating the spinning rate. PI-8 films with thicknesses ranged from 10~100 μm were obtained by thermally baking the solu‐ tion in a flowing nitrogen according to the following heating procedure: 80 o C/2 h, 150 o C/1 h, 200 o C/1 h, and 220 o C/1 h. The other PI resin and films were prepared according to a simi‐ lar procedure as mentioned above.
