High heat-resistant transparent soluble polyimide film and its preparation method and application

Abstract

The present invention discloses a highly heat-resistant transparent soluble polyimide film, its preparation method and application. By polycondensing triarylmethane diamines with large bulky side groups and 3,3',4,4'-biphenyltetracarboxylic dianhydride or 4,4'-oxydiphthalic anhydride, a highly heat-resistant transparent soluble polyimide film is prepared, solving the problem that the existing polyimide cannot balance the thermal properties, optical transparency and solubility.

Description

Technical Field

[0001] The present invention belongs to the technical field of polyimide films, and particularly relates to a high heat-resistant transparent soluble polyimide film, a preparation method thereof, and an application thereof. Background Art

[0002] In the fields of optoelectronic devices such as image display, optical films, organic photovoltaics, flexible printed circuit boards, and touch panels, colorless transparent polymers have attracted much attention as plastic substrates. Replacing fragile inorganic glass substrates (with a thickness of about 400 - 700 μm) with plastic substrates (with a thickness of about 50 μm) to improve portability and flexibility is a major trend in the field of optoelectronic devices currently.

[0003] In addition to certain requirements for optical transparency, solubility, dimensional stability, and coefficient of thermal expansion (CTE), etc., the extremely high processing temperature of such substrates also poses high requirements for the heat resistance of the materials. For example, when preparing flexible active matrix organic light-emitting display devices (AMOLEDs), the processing temperature on the flexible polymer film substrate can be higher than 300 °C; the temperature of the lead-free solder reflow process is about 270 °C; the temperature of the silicon-based thin-film transistor (TFT) manufacturing process will remain at 400 °C for a short time. Currently, the glass transition temperatures (T g ) of commonly used commercial colorless engineering plastics are not sufficient to support such high-temperature manufacturing processes, such as polyethylene terephthalate (78 °C), polyethylene naphthalate (120 °C), polycarbonate (145 °C), and polyethersulfone (225 °C). Currently, polyimide (PI) (T g can reach above 400 °C) is one of the few materials that can be used as a flexible substrate for optoelectronic devices.

[0004] Traditional PI is generally a brown transparent material. This is because there are strong electron donors and acceptors in the PI molecular structure, forming strong charge transfer complexes (CTCs) within or between molecular chains, causing strong absorption of PI in the visible light range, which severely limits the application of PI in the optoelectronic engineering field. Therefore, in recent years, high-temperature resistant colorless transparent PI (CPI) has gradually attracted attention. However, the molecular structure design that is beneficial to the optical transparency of the material often reduces the thermal performance of the material to a certain extent; and structural factors that increase the thermal performance, such as rigid aromatic structures and highly conjugated structures, will bring about the CTC effect and often damage the optical transparency of the material. On the other hand, the high rigidity and intermolecular interactions of traditional PI reduce its solubility and result in poor molding and processing performance. Methods such as introducing flexible chain segments and bulky groups can improve solubility, but often damage the thermal performance of the material. Therefore, through molecular design, the CTC effect needs to be inhibited to a certain extent to achieve a good balance of various properties of polyimide.

[0005] In summary, the existing PI has the problem that its thermal properties, optical transparency, and solubility cannot be balanced simultaneously. SUMMARY OF THE INVENTION

[0006] Aiming at the problem that the existing PI cannot balance its thermal properties, optical transparency, and solubility simultaneously, the present invention provides a highly heat-resistant, transparent, and soluble polyimide film and its preparation method by polycondensing triarylmethane diamines with large bulky side groups and 3,3',4,4'-biphenyltetracarboxylic dianhydride (BPDA) or 4,4'-oxydiphthalic anhydride (OPDA).

[0007] Another object of the present invention is to provide the application of the above-mentioned highly heat-resistant, transparent, and soluble polyimide film.

[0008] The present invention uses 3,3'-diisopropyl-4,4'-diamino-3”-phenoxytriphenylmethane to polycondense with rigid dianhydride (BPDA) or flexible dianhydride (OPDA). The resulting PI, due to the sp 3 hybridization of the central carbon of the diamine monomer, forms a helical structure with three aromatic rings, making the polymer present a twisted state, increasing the free volume and chain spacing, and endowing the resulting polymer with good solubility; such polymers contain both aromatic rings and aliphatic chains, which can provide high-temperature stability and good molding and processing properties for PI at the same time, and have a large bulky twisted structure themselves, and large bulky groups phenoxy are introduced at the benzene ring substituents, which is beneficial to the inhibition of the CTC effect; the introduction of isopropyl groups forms a large torsional angle between the imide ring and the benzene ring, increasing the single bond rotation barrier and keeping the main chain with a certain rigidity, which is beneficial to maintaining the excellent heat resistance of aromatic PI.

[0009] The object of the present invention is achieved through the following solutions:

[0010] A highly heat-resistant, transparent, and soluble polyimide film has the following general structural formula:

[0011]

[0012] Where Ar is: The dotted line is the position of the connecting bond; n represents the average number of repeating structural units, and the value range of n is 25-35.

[0013] The preparation method of the above-mentioned highly heat-resistant, transparent, and soluble polyimide includes the following steps:

[0014] (1) Under a nitrogen atmosphere, mix a triarylmethane diamine monomer with large bulky side groups, 3,3',4,4'-biphenyltetracarboxylic dianhydride or 4,4'-oxydiphthalic anhydride monomer, a polar aprotic solvent, and a catalyst isoquinoline, and stir at room temperature until clear.

[0015] (2) The clarified solution obtained in step (1) is placed at room temperature and continuously stirred for 10 hours. Then, o-dichlorobenzene is added and a water separator is installed, and the temperature is raised to 185 °C and stirred for 10 hours to obtain a polyimide solution.

[0016] (3) The polyimide solution obtained in step (2) is dropped into ethanol with a mass 30 - 50 times that of the polyimide solution. Fibrous precipitates are separated out. The precipitates are filtered, washed with ethanol and then dried to obtain fibrous polyimide.

[0017] (4) The fibrous polyimide obtained in step (3) is dissolved in a polar aprotic solvent, and the solid content is controlled to be 10 - 20 wt%. After complete dissolution, it is cast on a dry and clean silica glass plate, left standing at room temperature for 1 hour under vacuum conditions, then heated for drying and cooled to obtain a highly heat-resistant transparent soluble polyimide film.

[0018] To further achieve the object of the present invention, preferably, the triarylmethane diamine monomer with a bulky side group in step (1) is 3,3'-diisopropyl-4,4'-diamino-3”-phenoxytriphenylmethane, and its structural formula is The structural formula of 3,3',4,4'-biphenyltetracarboxylic dianhydride is The structural formula of 4,4'-oxybisphthalic anhydride is

[0019] Preferably, the molar ratio of the triarylmethane diamine monomer with a bulky side group to 3,3',4,4'-biphenyltetracarboxylic dianhydride or 4,4'-oxybisphthalic anhydride monomer in step (1) is 1:1.

[0020] Preferably, the polar aprotic solvent is one of N,N-dimethylformamide, N,N-dimethylacetamide, N-methylpyrrolidone, and m-cresol.

[0021] Preferably, the amount of the polar aprotic solvent used in step (1) is 6 - 9 times the total mass of the triarylmethane diamine monomer with a bulky side group and 3,3',4,4'-biphenyltetracarboxylic dianhydride or 4,4'-oxybisphthalic anhydride monomer.

[0022] Preferably, the amount of the catalyst isoquinoline used in step (1) is 1 - 1.5 times the molar amount of the triarylmethane diamine monomer with a bulky side group.

[0023] Preferably, the stirring time at room temperature in step (1) is 30 - 60 minutes.

[0024] Preferably, the amount of o-dichlorobenzene used in step (2) is 15 - 20% of the volume of the polar aprotic solvent.

[0025] Preferably, the drying in step (3) is to place the precipitate in a vacuum or at normal pressure at 100 °C for 6-8 hours; the drying in step (4) is to dry at 40 °C for 2-4 hours, then raise the temperature to 50-80 °C and dry for 2-4 hours, continue to raise the temperature to 120-150 °C and dry for 2-4 hours, and finally raise the temperature to 180-200 °C and dry for 2-4 hours.

[0026] The application of the high heat-resistant transparent soluble polyimide film described in the present invention on flexible display substrates and flexible solar cell substrates.

[0027] Compared with the prior art, the advantages of the present invention are as follows:

[0028] In the present invention, a triarylmethane diamine with a large bulky side group is polycondensed with BPDA or OPDA to obtain a high heat-resistant transparent soluble polyimide film, which has a light color and excellent properties. Its glass transition temperature exceeds 320 °C, and the light transmittance at 450 nm exceeds 80%, and it is easily soluble in most conventional organic solvents. Description of the Drawings

[0029] Figure 1 It is the infrared spectrogram of the polyimides obtained in Example 1, Example 2 and Comparative Example 1, Comparative Example 2, Comparative Example 3, wherein: a is the polyimide film product obtained in Example 1, b is the polyimide film product obtained in Example 2, c is the polyimide film product obtained in Comparative Example 1, d is the polyimide film product obtained in Comparative Example 2, and e is the polyimide film product obtained in Comparative Example 3.

[0030] Figure 2 It is the nuclear magnetic resonance hydrogen spectrum of the polyimide obtained in Example 1 (using deuterated dimethyl sulfoxide as the solvent).

[0031] Figure 3 It is the nuclear magnetic resonance hydrogen spectrum of the polyimide obtained in Example 2 (using deuterated dimethyl sulfoxide as the solvent).

[0032] Figure 4 It is the nuclear magnetic resonance hydrogen spectrum of the polyimide obtained in Comparative Example 1 (using deuterated dimethyl sulfoxide as the solvent).

[0033] Figure 5 It is the nuclear magnetic resonance hydrogen spectrum of the polyimide obtained in Comparative Example 2 (using deuterated dimethyl sulfoxide as the solvent).

[0034] Figure 6 It is the nuclear magnetic resonance hydrogen spectrum of the polyimide obtained in Comparative Example 3 (using deuterated dimethyl sulfoxide as the solvent).

[0035] Figure 7 In it, (a) is the nuclear magnetic resonance hydrogen spectrum of the raw material PAPOT; (b) is the nuclear magnetic resonance carbon spectrum of the raw material PAPOT.

[0036] Figure 8 In (a) is the 1H NMR spectrum of the raw material PAPNT; (b) is the 13C NMR spectrum of the raw material PAPNT.

[0037] Figure 9 Are the physical pictures of the polyimides obtained in Example 1, Example 2 and Comparative Example 1, Comparative Example 2, Comparative Example 3, where: a is the polyimide film product obtained in Example 1, b is the polyimide film product obtained in Example 2, c is the polyimide film product obtained in Comparative Example 1, d is the polyimide film product obtained in Comparative Example 2, and e is the polyimide film product obtained in Comparative Example 3. Detailed implementation mode

[0038] To better understand the present invention, the following further illustrates the specific implementation of the present invention in conjunction with examples, but the implementation and protection scope of the present invention are not limited thereto.

[0039] Raw material preparation

[0040] 3,3'-Diisopropyl-4,4'-diamino-3”-phenoxytriphenylmethane (PAPOT)

[0041] Under the protection of an argon atmosphere, 13.50 g (100 mmol) of 2-isopropylaniline was added to a 50 mL two-neck reaction flask, and the temperature was raised to 120 °C. 4.95 g (25 mmol) of m-phenoxybenzaldehyde and 2.0 mL (25 mmol) of concentrated hydrochloric acid were mixed and added dropwise within 1 h. After stirring for 0.5 h, a reflux tube was installed, and the mixed solution was stirred at 150 °C for 10 h. After the reaction was completed, NaOH(aq) was added until the pH of the system was about 9, and then extracted with dichloromethane and washed repeatedly with water until the pH of the organic phase was 7. The organic phase was dried over anhydrous sodium sulfate, filtered, and purified by a silica gel column (V 乙酸乙酯 :V 石油醚 =1:4), and the obtained powder was dried under low-temperature vacuum overnight to obtain 5.78 g of a white solid powder with a yield of 51.4%. 11H NMR (500 MHz, DMSO-d6) δ 7.32–7.25 (m, 2H), 7.22 (d, J = 7.8 Hz, 1H), 7.08–7.00 (m, 1H), 6.91–6.86 (m, 2H), 6.83 (s, 1H), 6.76 (td, J = 7.0, 6.1, 2.4 Hz, 1H), 6.70 (dt, J = 4.5, 2.0 Hz, 2H), 6.65 (dq, J = 6.2, 2.0 Hz, 1H), 6.54 (td, J = 7.3, 6.3, 2.1 Hz, 2H), 6.49–6.42 (m, 2H), 5.17–5.12 (m, 1H), 4.67–4.62 (m, 4H), 2.91–2.82 (m, 2H), 0.98 (ddd, J = 8.5, 6.6, 4.2 Hz, 12H). 13 13C NMR (101 MHz, DMSO-d6) δ 157.41, 156.51, 149.05, 143.64, 132.52, 130.37, 129.93, 127.02, 125.76, 124.84, 123.55, 119.78, 118.51, 116.59, 115.07, 55.39, 26.80, 22.84.

[0042] 3,3'-Diisopropyl-4,4'-diaminodiphenyl-4”-naphthylmethane (PAPNT)

[0043] Under the protection of an argon atmosphere, 13.50 g (100 mmol) of 2-isopropylaniline was added to a 50 mL two-necked reaction flask, and the temperature was raised to 120 °C. 6.24 g (40 mmol) of 1-naphthaldehyde and 3.5 mL (40 mmol) of concentrated hydrochloric acid were mixed and placed in a dropping funnel. The mixture was added dropwise within 1 h. After stirring for 0.5 h, a reflux tube was installed, and the mixed solution was stirred at 150 °C for 10 h. After the reaction was completed, NaOH(aq) was added until the pH of the system was about 9, and then extracted with dichloromethane and washed repeatedly with water until the pH of the organic phase was 7. The organic phase was dried over anhydrous sodium sulfate, filtered, and purified by silica gel column (V 乙酸乙酯 : V 石油醚 = 1:4), and the obtained powder was dried under low-temperature vacuum overnight to obtain 9.47 g of a white solid powder with a yield of 58.0%. 11H NMR (500 MHz, DMSO-d6) δ 8.08–8.03 (m, 1H), 7.86–7.81 (m, 1H), 7.69 (d, J = 8.2 Hz, 1H), 7.43–7.31 (m, 3H), 6.90 (dt, J = 7.3, 0.8 Hz, 1H), 6.77 (d, J = 2.1 Hz, 2H), 6.52 (dd, J = 8.2, 2.1 Hz, 2H), 6.46 (d, J = 8.1 Hz, 2H), 5.94 (s, 1H), 4.64 (s, 4H), 2.87 (hept, J = 6.8 Hz, 2H), 0.99 (dd, J = 15.1, 6.7 Hz, 12H). 13 13C NMR (101 MHz, DMSO-d6) δ 143.61, 142.58, 133.95, 132.54, 131.97, 131.39, 128.86, 127.24, 126.82 (d, J = 2.1 Hz), 126.17 (d, J = 4.7 Hz), 125.66 (d, J = 8.9 Hz), 124.94, 115.08, 51.68, 26.86, 22.89.

[0044] Example 1

[0045] Under nitrogen protection, 0.225 g of 3,3'-diisopropyl-4,4'-diamino-3”-phenoxytriphenylmethane (PAPOT), 3.114 g of m-cresol and 0.09 g of isoquinoline were added to a 10 mL single-necked flask, stirred at room temperature until completely dissolved, and then 0.147 g of 3,3',4,4'-biphenyltetracarboxylic dianhydride (BPDA) was slowly added. The reaction was continued at room temperature for 10 h, then 0.5 mL of o-dichlorobenzene was added, a water separator was installed, and the temperature was raised to 185 °C and the reaction was continued for 10 hours to obtain a red-black viscous polyimide solution;

[0046] The polyimide solution was dropped into 157.8 g of ethanol. While stirring, fibrous precipitates were precipitated. It was filtered with a Buchner funnel and washed with a small amount of ethanol; the precipitate was vacuum dried at 100 °C for 6 hours to obtain white fibrous polyimide;

[0047] The dried fibrous polyimide was dissolved in N,N-dimethylacetamide, and the solid content was controlled to be 20 wt%. After being fully dissolved, it was cast on a dry and clean silica glass plate, allowed to stand at room temperature for 1 hour under vacuum conditions, dried at 40 °C for 2 hours, then the temperature was raised to 80 °C and dried for 4 hours, and then the temperature was raised to 120 °C and dried for 4 hours, and finally the temperature was raised to 200 °C and dried for 4 hours. After cooling, a PAPOT-BPDA type polyimide film was obtained.

[0048] The PAPOT-BPDA polyimide film can be dissolved in common polar solvents, as shown in Table 1. When it is dissolved in tetrahydrofuran, the number-average molecular weight measured by the GPC method is 1.98×10 4 g / mol, PDI = 1.34, and the average number of repeating units is 28; the glass transition temperature measured by a synchronous thermal analyzer (DSC) is 332 °C; the transmittance of the film at 450 nm measured by an ultraviolet-visible spectrophotometer is 71%.

[0049] The infrared (IR) spectrum of the film product is as shown in the appendix Figure 1 . The peaks at 3020 - 2830 cm -1 are the C-H stretching vibration peaks of methyl groups, and the peaks at 1778 cm -1 and 1716 cm -1 correspond to the characteristic peaks of the asymmetric and symmetric stretching vibrations of C=O in the imide ring, respectively. The peak at 1620 cm -1 (aromatic ring skeleton vibration), the peak at 1366 cm -1 (C-N stretching vibration), and the peak at 741 cm -1 (C=O bending vibration); the nuclear magnetic resonance hydrogen spectrum (500 MHz, DMSO-d6) of the film product is as shown in the appendix Figure 2 . The chemical shifts (ppm) are assigned as δ8.40(s,2H), 8.34(s,2H), 8.06(d,J = 8.2Hz,2H), 7.38(s,1H), 7.32(d,J = 8.1Hz,4H), 7.26(s,2H), 7.14(d,J = 8.6Hz,2H), 7.09–7.01(m,2H), 6.99–6.94(m,2H), 6.89(s,2H), 5.81(s,1H), 2.78–2.75(m,2H), 1.02–0.98(m,12H). It can be seen from the above that the molecular structural formula of the obtained polyimide is:

[0050] where n = 28.

[0051] Example 2

[0052] Under nitrogen protection, 0.225 g of 3,3'-diisopropyl-4,4'-diamino-3”-phenoxytriphenylmethane (PAPOT), 3.114 g of m-cresol, and 0.09 g of isoquinoline were added to a 10 mL single-necked flask, stirred at room temperature until completely dissolved, and then 0.155 g of 4,4'-oxybisphthalic anhydride (OPDA) was slowly added. The reaction was continued at room temperature for 10 h, then 0.5 mL of o-dichlorobenzene was added, a water separator was installed, and the temperature was raised to 185 °C and the reaction was continued for 10 hours to obtain a red-black viscous polyimide solution;

[0053] The polyimide solution was dropped into 157.8 g of ethanol. While stirring, fibrous precipitates were formed. The precipitates were filtered by suction using a Buchner funnel and washed with a small amount of ethanol; the precipitates were dried in vacuo at 100 °C for 6 hours to obtain white fibrous polyimide;

[0054] The dried fibrous polyimide was dissolved in N,N-dimethylacetamide, with the solid content controlled at 20 wt%. After complete dissolution, it was cast on a dry and clean silica glass plate, allowed to stand at room temperature for 1 hour under vacuum conditions, dried at 40 °C for 2 hours, then heated to 80 °C and dried for 4 hours, then further heated to 120 °C and dried for 4 hours, and finally heated to 200 °C and dried for 4 hours. After cooling, a PAPOT-OPDA type polyimide film was obtained.

[0055] The PAPOT-OPDA type polyimide film can be dissolved in common polar solvents, as shown in Table 1. It was dissolved in tetrahydrofuran, and the number-average molecular weight measured by GPC method was 2.15×10 4 g / mol, PDI = 1.96, and the average number of repeating units was 30; the glass transition temperature measured by a synchronous thermal analyzer (DSC) was 321 °C; the transmittance of the film at 450 nm measured by an ultraviolet-visible spectrophotometer was 81%.

[0056] The infrared (IR) spectrum of the film product is as shown in the appendix Figure 1 The C-H stretching vibration peak of methyl was at 3020 - 2830 cm -1 The characteristic peaks corresponding to the asymmetric and symmetric stretching vibrations of C=O in the imide ring were at 1779 cm -1 and 1720 cm -1 respectively, 1609 cm -1 (aromatic ring skeletal vibration), 1368 cm -1 (C-N stretching vibration), 746 cm -1 (C=O bending vibration); the nuclear magnetic resonance hydrogen spectrum (500 MHz, DMSO-d6) of the film product is as shown in the appendix Figure 3 , and the chemical shifts (ppm) were assigned as δ8.02 (d, J = 7.9 Hz, 2H), 7.61 (d, J = 10.1 Hz, 4H), 7.39–7.19 (m, 7H), 7.16–6.98 (m, 4H), 6.95 (d, J = 7.8 Hz, 2H), 6.87 (d, J = 6.8 Hz, 2H), 5.78 (s, 1H), 2.74 (p, J = 6.9 Hz, 2H), 0.98 (q, J = 6.1, 4.9 Hz, 12H). It can be seen from the above that the molecular structural formula of the obtained polyimide is:

[0057] where n = 30.

[0058] Comparative Example 1

[0059] Under nitrogen protection, 0.225 g of 3,3'-diisopropyl-4,4'-diamino-3”-phenoxytriphenylmethane (PAPOT), 3.114 g of m-cresol and 0.09 g of isoquinoline were added to a 10 mL single-necked flask, stirred at room temperature until completely dissolved, and then 0.109 g of pyromellitic dianhydride (PMDA) was slowly added. The reaction was continued at room temperature for 10 hours, then 0.5 mL of o-dichlorobenzene was added, a water separator was installed, and the temperature was raised to 185 °C and the reaction was continued for 10 hours to obtain a red-black viscous polyimide solution;

[0060] The polyimide solution was dropped into 157.8 g of ethanol, and fibrous precipitates were formed while stirring. The precipitates were filtered with a Buchner funnel and washed with a small amount of ethanol; the precipitates were vacuum dried at 100 °C for 6 hours to obtain white fibrous polyimide;

[0061] The dried fibrous polyimide was dissolved in N,N-dimethylacetamide, and the solid content was controlled to be 20 wt%. After complete dissolution, it was cast on a dry and clean silica glass plate, allowed to stand at room temperature for 1 hour under vacuum conditions, dried at 40 °C for 2 hours, then heated to 80 °C and dried for 4 hours, then heated to 120 °C and dried for 4 hours, and finally heated to 200 °C and dried for 4 hours. After cooling, a PAPOT-PMDA type polyimide film was obtained.

[0062] The PAPOT-PMDA type polyimide film was soluble in tetrahydrofuran. The number-average molecular weight was measured by GPC method to be 3.02×10 4 g / mol, PDI = 1.13, and the average number of repeating units was 48; the glass transition temperature was measured by a synchronous thermal analyzer (DSC) to be 212 °C; the transmittance of the film at 450 nm was measured by an ultraviolet-visible spectrophotometer to be 56%.

[0063] The infrared (IR) spectrum of the film product is as attached Figure 1 shown. The C-H stretching vibration peak of methyl is at 3020 - 2830 cm -1 The characteristic peaks corresponding to the asymmetric and symmetric stretching vibrations of C=O in the imide ring are at 1777 cm -1 and 1721 cm -1 respectively. 1582 cm -1 (aromatic ring skeletal vibration), 1363 cm -1 (C-N stretching vibration), 727 cm -1 (C=O bending vibration); the nuclear magnetic resonance hydrogen spectrum (400 MHz, DMSO-d6) of the film product is as attached Figure 4, the chemical shifts (ppm) are attributed to δ 8.44 (d, J = 4.9 Hz, 2H), 7.40 (dd, J = 21.7, 8.2 Hz, 5H), 7.31 (s, 2H), 7.23 (d, J = 8.5 Hz, 2H), 7.17–7.06 (m, 2H), 7.02 (d, J = 7.9 Hz, 2H), 6.95 (d, J = 8.0 Hz, 2H), 5.88 (s, 1H), 2.84–2.78 (m, 2H), 1.07–1.03 (m, 12H). As can be seen above, the molecular structural formula of the obtained polyimide is:

[0064] where n = 48.

[0065] By comparison, it is found that the glass transition temperature of the PAPOT-PMDA type polyimide film prepared in this comparative example is 212 °C, and the light transmittance at 450 nm is only 56%, which is significantly lower than that of the polyimide films obtained in Examples 1-2.

[0066] Comparative Example 2

[0067] In this comparative example, it was originally planned to synthesize the substance with the following chemical formula:

[0068]

[0069] However, the polymerization activity of the monomer was low, and the resulting polymer had a small molecular weight and was difficult to cast into a film. Therefore, it was changed to synthesize the substance with the following chemical formula:

[0070]

[0071] Under nitrogen protection, 0.204 g of 3,3'-diisopropyl-4,4'-diaminodiphenyl-4”-naphthylmethane (PAPNT), 3.114 g of m-cresol, and 0.09 g of isoquinoline were added to a 10 mL single-necked flask, stirred at room temperature until completely dissolved, and then 0.147 g of 3,3',4,4'-biphenyltetracarboxylic dianhydride (BPDA) was slowly added. The reaction was continued at room temperature for 10 hours, then 0.5 mL of o-dichlorobenzene was added, a water separator was installed, and the temperature was raised to 185 °C and the reaction was continued for 10 hours to obtain a red-black viscous polyimide solution;

[0072] The polyimide solution was dropped into 157.8 g of ethanol, and fibrous precipitates were precipitated while stirring. It was filtered with a Buchner funnel and washed with a small amount of ethanol; the precipitate was vacuum dried at 100 °C for 6 hours to obtain white fibrous polyimide;

[0073] Take the dry fibrous polyimide and dissolve it in N,N-dimethylacetamide. Control the solid content to be 20 wt%, and after complete dissolution, cast it on a dry and clean silica glass plate. Let it stand at room temperature for 1 hour under vacuum conditions, dry at 40 °C for 2 hours, then heat up to 80 °C and dry for 4 hours, then further heat up to 120 °C and dry for 4 hours, and finally heat up to 200 °C and dry for 4 hours. After cooling, a PAPNT-BPDA type polyimide film is obtained.

[0074] This PAPNT-BPDA type polyimide film can be dissolved in tetrahydrofuran. The number-average molecular weight measured by the GPC method is 4.10×10 4 g / mol, PDI = 1.29, and the average number of repeating units is 62; the glass transition temperature measured by a synchronous thermal analyzer (DSC) is 342 °C; the light transmittance of the film at 450 nm measured by an ultraviolet-visible spectrophotometer is 67%.

[0075] The infrared (IR) spectrum of this film product is as shown in Figure 1 The peak at 3020 - 2830 cm -1 is the C-H stretching vibration peak of methyl, and the peaks at 1776 cm -1 and 1715 cm -1 correspond to the characteristic peaks of the asymmetric and symmetric stretching vibrations of C=O in the imide ring respectively, 1620 cm -1 (aromatic ring skeleton vibration), 1364 cm -1 (C-N stretching vibration), 740 cm -1 (C=O bending vibration); the nuclear magnetic resonance hydrogen spectrum (500 MHz, DMSO-d6) of this film product is as shown in Figure 5 , and the chemical shift (ppm) assignments are δ8.47–8.17(m,5H),8.05(s,2H),7.93(s,1H),7.84(s,1H),7.50(s,3H),7.30(s,4H),7.08(d,J=22.7Hz,3H),6.63(s,1H),2.75(d,J=8.7Hz,2H),1.00(dt,J=17.0,7.0Hz,12H). It can be seen from the above that the molecular structural formula of the obtained polyimide is:

[0076] where n = 62.

[0077] By comparison, it is found that the glass transition temperature of the PAPNT-BPDA type polyimide film prepared in this comparative example is 342 °C, which is slightly higher than that of the polyimide film obtained in Example 1, but the light transmittance at 450 nm is only 67%, which is significantly lower than that of Example 1.

[0078] Comparative Example 3

[0079] In this comparative example, it was originally planned to synthesize a substance with the following chemical formula:

[0080]

[0081] However, the polymerization activity of the monomer was relatively low, and the molecular weight of the resulting polymer was small, making it difficult to form a film. Therefore, the synthesis was changed to a substance with the following chemical formula:

[0082]

[0083] Under nitrogen protection, 0.204 g of 3,3'-diisopropyl-4,4'-diaminodiphenyl-4”-naphthylmethane (PAPNT), 3.114 g of m-cresol, and 0.09 g of isoquinoline were added to a 10 mL single-necked flask, stirred at room temperature until completely dissolved, and then 0.155 g of 4,4'-oxybisphthalic anhydride (OPDA) was slowly added. The reaction was continued at room temperature for 10 hours, then 0.5 mL of ortho-dichlorobenzene was added, a water separator was installed, and the temperature was raised to 185 °C and the reaction was continued for 10 hours to obtain a red-black viscous polyimide solution;

[0084] The polyimide solution was dropped into 157.8 g of ethanol. While stirring, fibrous precipitates were precipitated, filtered with a Buchner funnel and washed with a small amount of ethanol; the precipitate was dried in vacuo at 100 °C for 6 hours to obtain white fibrous polyimide;

[0085] The dried fibrous polyimide was dissolved in N,N-dimethylacetamide, and the solid content was controlled to be 20 wt%. After complete dissolution, it was cast on a dry and clean silica glass plate, allowed to stand at room temperature for 1 hour under vacuum conditions, dried at 40 °C for 2 hours, then heated to 80 °C and dried for 4 hours, then heated to 120 °C and dried for 4 hours, and finally heated to 200 °C and dried for 4 hours. After cooling, a PAPNT-OPDA type polyimide film was obtained.

[0086] This PAPNT-OPDA type polyimide film can be dissolved in tetrahydrofuran. The number-average molecular weight measured by GPC method was 3.54×10 4 g / mol, PDI = 1.50, and the average number of repeating units was 52; the glass transition temperature measured by a synchronous thermal analyzer (DSC) was 330 °C; the transmittance of the film at 450 nm measured by an ultraviolet-visible spectrophotometer was 74%.

[0087] The infrared (IR) spectrum of this film product is as shown in the appendix Figure 1 The peaks at 3020 - 2830 cm -1 are the C-H stretching vibration peaks of methyl groups, and the peaks at 1778 cm -1 and 1716 cm -1correspond to the characteristic peaks of the asymmetric and symmetric stretching vibrations of C=O in the imide ring at 1607 cm -1 (aromatic ring skeleton vibration) at 1367 cm -1 (C-N stretching vibration) at 746 cm -1 (C=O bending vibration); The nuclear magnetic resonance hydrogen spectrum (500 MHz, DMSO-d6) of the thin film product is as attached Figure 6 , and the chemical shifts (ppm) are attributed to δ8.22 (d, J = 8.1 Hz, 1H), 8.01 (d, J = 8.0 Hz, 2H), 7.92 (d, J = 7.4 Hz, 1H), 7.85–7.80 (m, 1H), 7.61 (s, 4H), 7.50–7.46 (m, 3H), 7.30–7.22 (m, 4H), 7.06 (dd, J = 19.1, 7.6 Hz, 3H), 6.61 (s, 1H), 2.72 (q, J = 7.1 Hz, 2H), 0.96 (dt, J = 15.0, 5.6 Hz, 12H). It can be seen from the above that the molecular structural formula of the obtained polyimide is:

[0088] where n = 52.

[0089] By comparison, it is found that the glass transition temperature of the PAPNT-OPDA type polyimide film prepared in this comparative example is 330 °C, which is slightly higher than that of the polyimide film obtained in Example 2, but the light transmittance at 450 nm is 74%, which is significantly lower than that of Example 2.

[0090] The experimental results of the dissolution properties of the polyimide films prepared in Examples 1-2 and Comparative Examples 1-3 in different solvents are shown in Table 1. Test conditions: 10 mg of the sample was dissolved in 1 mL of the solvent at room temperature or by heating, and its dissolution state was observed after standing for 24 hours. In Table 1, "++" means soluble at room temperature; "+" means soluble by heating to the boiling point; "+-" means partially soluble even after heating to the boiling point.

[0091] As can be seen from Table 1, the polyimides of Examples 1-2 have better dissolution properties than Comparative Examples 1-3. They are not only easily soluble in high-boiling-point solvents such as N,N-dimethylacetamide, N,N-dimethylacetamide, dimethyl sulfoxide, N-methylpyrrolidone, and m-cresol, but also have excellent dissolution properties in common low-boiling-point solvents such as chloroform, tetrahydrofuran, and acetone.

[0092] Table 1 Dissolution properties of the polyimide films prepared in Examples 1-2 and Comparative Examples 1-3

[0093]

[0094] Aiming at the problem that the existing PI cannot balance thermal performance, optical transparency and solubility, the present invention uses 3,3'-diisopropyl-4,4'-diamino-3”-phenoxytriphenylmethane to polycondense with rigid dianhydride (BPDA) or flexible dianhydride (OPDA) to obtain a highly heat-resistant, transparent and soluble polyimide film. Due to the sp 3 hybridization of the central carbon of the diamine monomer, three aromatic rings form a helical structure, making the polymer present a twisted state, increasing the free volume and chain spacing, and endowing the obtained polymer with good solubility; such polymers contain both aromatic rings and aliphatic chains, which can provide high-temperature stability and good molding and processing performance for PI at the same time, and have a large-volume twisted structure itself, and a large-volume group phenoxy is introduced at the benzene ring substituent, which is beneficial to the inhibition of the CTC effect; the introduction of isopropyl groups forms a large torsional angle between the imide ring and the benzene ring, increasing the single-bond rotation barrier and keeping the main chain rigid to a certain extent, which is beneficial to maintaining the excellent heat resistance of aromatic PI. Therefore, the polyimide provided by the present invention has both high heat resistance, high light transmittance and excellent solubility, its glass transition temperature is above 320 °C, it can adapt to the high-temperature environment required for processing, the light transmittance of the film at 450 nm exceeds 80%, and it can be dissolved in most common organic solvents and can be reprocessed at a lower temperature; it has great potential in the fields of flexible display substrates and flexible solar cell substrates.

[0095] The implementation modes of the present invention are not limited by the above-mentioned embodiments. Any other changes, modifications, substitutions, combinations, and simplifications made without departing from the spirit and principle of the present invention shall be equivalent replacement methods and are all included in the protection scope of the present invention.

Claims

1. A high heat-resistant transparent soluble polyimide film, characterized in that, It has the following structural general formula: wherein Ar is: or ; n represents the average number of repeating structural units, and the value range of n is 25 - 35.

2. The preparation method of the high heat-resistant transparent soluble polyimide film according to claim 1 includes the following steps: (1) Under a nitrogen atmosphere, a triarylmethane diamine monomer with a large bulky side group is mixed with 3,3',4,4'-biphenyltetracarboxylic dianhydride or 4,4'-oxydiphthalic anhydride monomer, a polar aprotic solvent, and a catalyst isoquinoline, and stirred at room temperature until clear; (2) The clear solution obtained in step (1) is placed at room temperature and continuously stirred for 10 hours, o-dichlorobenzene is added and a water separator is installed, and then the temperature is raised to 185 °C and stirred for 10 hours to obtain a polyimide solution; (3) The polyimide solution obtained in step (2) is dropped into ethanol with a mass 30 - 50 times that of it, fibrous precipitates are precipitated, the precipitate is filtered, washed with ethanol and dried to obtain fibrous polyimide; (4) The fibrous polyimide obtained in step (3) is dissolved in a polar aprotic solvent, the solid content is controlled to be 10 - 20 wt%, and after being fully dissolved, it is cast on a dry and clean silica glass plate, left standing at room temperature for 1 hour under vacuum conditions, dried by heating, and cooled to obtain a high heat-resistant transparent soluble polyimide film.

3. The preparation method of the highly heat-resistant transparent soluble polyimide film according to claim 2, characterized in that, The triarylmethane diamine monomer with large bulky side groups described in step (1) is 3,3'-diisopropyl-4,4'-diamino-3''-phenoxytriphenylmethane, and its structural formula is .

4. The preparation method of the high heat-resistant transparent soluble polyimide film according to claim 2, characterized in that in step (1), the molar ratio of the triarylmethane diamine monomer with a large bulky side group to 3,3',4,4'-biphenyltetracarboxylic dianhydride or 4,4'-oxydiphthalic anhydride monomer is 1:

1.

5. The preparation method of the high heat-resistant transparent soluble polyimide film according to claim 2, characterized in that, The polar aprotic solvent is one of N,N-dimethylformamide, N,N-dimethylacetamide, N-methylpyrrolidone, and m-cresol.

6. The preparation method of the high heat-resistant transparent soluble polyimide film according to claim 2, characterized in that In step (1), the amount of the polar aprotic solvent used is 6 - 9 times the total mass of the triarylmethane diamine monomer with a large bulky side group and 3,3',4,4'-biphenyltetracarboxylic dianhydride or 4,4'-oxydiphthalic anhydride monomer.

7. The preparation method of the high heat-resistant transparent soluble polyimide film according to claim 2, characterized in that, In step (1), the amount of the catalyst isoquinoline used is 1 - 1.5 times the molar number of the triarylmethane diamine monomer with a large bulky side group.

8. The preparation method of the high heat-resistant transparent soluble polyimide film according to claim 2, characterized in that, In step (2), the amount of o-dichlorobenzene used is 15 - 20% of the volume of the polar aprotic solvent in step (1).

9. The preparation method of the high heat-resistant transparent soluble polyimide film according to claim 2, characterized in that, In step (3), the drying is to dry the precipitate under vacuum or normal pressure at 100 °C for 6 - 8 hours; in step (4), the drying is to dry at 40 °C for 2 - 4 hours, then raise the temperature to 50 - 80 °C and dry for 2 - 4 hours, continue to raise the temperature to 120 - 150 °C and dry for 2 - 4 hours, and finally raise the temperature to 180 - 200 °C and dry for 2 - 4 hours.

10. The application of the high heat-resistant transparent soluble polyimide film according to claim 1 in flexible display substrates and flexible solar cell substrates.

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