A flexible and transparent polyimide film and its preparation method

Abstract

The present invention provides a flexible and transparent polyimide film and a preparation method thereof, relating to the technical field of polymer materials. The polyimide film is prepared by the polymerization reaction of diamine and dianhydride; the diamine includes a first diamine and a second diamine; the first diamine is an aromatic diamine containing a rigid group; the second diamine is a diamine containing imidazole; the dianhydride includes a first dianhydride and a second dianhydride; the first dianhydride is selected from at least one of 3,3,4,4-diphenylsulfone tetracarboxylic dianhydride, 4,4'-biphenyltetracarboxylic dianhydride and benzophenone tetracarboxylic dianhydride; the second dianhydride is selected from 2,2'-(1,4-piperazinediyl)-disuccinic anhydride or 2R,5R,7S,10S-naphthalenetetracarboxylic dianhydride. The polyimide film prepared by the present invention significantly improves the optical properties while obtaining better heat resistance and mechanical properties.

Description

Technical Field

[0001] The present invention relates to the technical field of polymer materials, and particularly relates to a flexible transparent polyimide film and a preparation method thereof. Background Art

[0002] Polyimide (PI) refers to a class of polymers containing imide rings in the main chain, which has excellent mechanical properties, high and low temperature resistance properties, and radiation resistance properties. Moreover, it has a high resistivity and a low dielectric constant. PI has an extremely wide temperature application range and is still not brittle in liquid helium at -269°C. The thermal decomposition temperature generally exceeds 500°C, and some systems can reach above 600°C. It is one of the special engineering plastics with the highest thermal stability so far. In addition, PI also has the advantages of excellent mechanical properties, resistance to organic solvents, radiation resistance, aging resistance, flame retardancy and self-extinguishing. According to the product form, PI materials can be divided into various product forms such as films, fibers, thermosetting resins, photosensitive adhesives, slurries, separation membranes or diaphragms, aerogels, foams, etc., and have important applications in the fields of electronic microelectronics, aerospace, mechanical and electrical engineering, etc.

[0003] In recent years, with the development of high-tech industries, the flexibility and transparency of optoelectronic devices have become an inevitable development trend. Colorless transparent polyimide has received more and more attention in the fields of patterned display devices, liquid crystal alignment layers, optical films, organic photovoltaic solar panels, flexible printed circuit boards, and touch panels due to its characteristics such as transparency, light weight, impact resistance, and excellent heat resistance. To achieve the true transparency and flexibility of organic optoelectronic devices represented by OLED, in addition to transparency and heat resistance, excellent comprehensive mechanical properties of the polyimide film are also important influencing factors for its application as a flexible optoelectronic material layer.

[0004] Traditional polyimide films are brown or yellow. In order to minimize the intermolecular or intramolecular charge transfer interaction to produce colorless transparent polyimide, researchers generally introduce special groups into the main chain structure of the molecule, such as large substituent side groups that can endow a large free volume, or an asymmetric structure. However, in most cases, the introduction of these functional groups will sacrifice the flexibility of the film. Therefore, conventional transparent polyimide materials have good optical properties, but their processes are complex, the product flexibility is insufficient, resulting in a low yield.

[0005] Chinese Patent Application No. 201810688078.5 discloses a wholly aromatic colorless and transparent polyimide film and its preparation method. Using 1,4-bis(2-trifluoromethyl-4-aminophenoxy)benzene, a diamine monomer with trifluoromethyl side groups and an intermediate ether bond connection, as a raw material, it is polymerized with 2,3,3',4'-diphenylether tetracarboxylic dianhydride containing an ether bond in the middle and isomers, and then a thermoplastic polyimide material is synthesized by chemical imidization method. The polyimide film can be prepared by dissolving, coating, and volatilizing the solvent. The wholly aromatic colorless and transparent polyimide film prepared by this invention has high optical transparency and thermal stability. However, generally, wholly aromatic CPI mainly hinders the formation of intermolecular charge transfer complexes by introducing fluorine atoms or fluorine-containing groups. But wholly aromatic CPI has high cost, complex processing technology, and poor surface adhesion of the transparent film, which limits the industrial promotion of wholly aromatic CPI.

[0006] Chinese Patent Application No. 201810841797.6 provides a low birefringence and high Tg transparent polyimide film and its preparation method. The polyimide film is a polyimide polymer containing a non-linear structure obtained by polycondensation of a mixture of dianhydride and diamine. Among them, the dianhydride used includes an asymmetric dianhydride with a molar ratio of 20% to 80%, and other dianhydrides with a molar ratio of 80% to 20%; the diamine used includes a meta-substituted diamine with a molar ratio not greater than 50% and other diamines with a molar ratio not less than 50%. The transmittance of the said transparent polyimide film at 550 nm is greater than 85%, the birefringence is less than 0.005, and Tg is greater than 300 °C. However, its yellowness index is generally between 5 and 6.

[0007] Based on this, developing a new polyimide film and its preparation method, making the prepared polyimide film have high optical transparency, excellent heat resistance, optical properties and comprehensive mechanical properties at the same time. The development of such materials can meet the increasingly urgent technical needs in the fields of advanced electronics and flexible displays, which is the research focus of researchers in this field and has important significance. Summary of the Invention

[0008] In view of the above problems, the present invention provides a flexible and transparent polyimide film and its preparation method. By specific components and ratios, and optimizing the preparation method, while significantly improving the optical properties of the polyimide film, the effects of better heat resistance and mechanical strength are obtained.

[0009] To achieve the above object, the technical solution adopted by the present invention is as follows:

[0010] On the one hand, the present invention provides a flexible and transparent polyimide film, which is prepared by the polymerization reaction of diamine and dianhydride;

[0011] The diamine described above includes a first diamine and a second diamine;

[0012] The first diamine is an aromatic diamine containing a rigid group;

[0013] The second diamine is a diamine containing imidazole;

[0014] The dianhydride described above includes a first dianhydride and a second dianhydride;

[0015] The first dianhydride is selected from at least one of 3,3,4,4-diphenylsulfone tetracarboxylic dianhydride (DSDA), 4,4'-biphenyltetracarboxylic dianhydride (BPDA), and benzophenone tetracarboxylic dianhydride (BTDA);

[0016] The second dianhydride is selected from 2,2'-(1,4-piperazinediyl)-disuccinic anhydride (PDA) or 2R,5R,7S,10S-naphthalenetetracarboxylic dianhydride (HNTDA).

[0017] Preferably, the first diamine is selected from at least one of 9,9-bis[4-(4-aminophenoxy)phenyl]xanthene (BAPX), 2-(3,5-diaminophenyl)-9,9'-spirobifluorene (35DABSBF), 2-(2,4-diaminophenyl)-9,9'-spirobifluorene (24DABSBF), and 2-bis(4-aminophenyl)hexafluoropropane (BIS-A-AF);

[0018] Preferably, the second diamine is selected from at least one of 2-(4-aminophenyl)-5-aminobenzimidazole (4APBI), 2-(3-aminophenyl)-5-aminobenzimidazole (3APBI), diaminodiphenyl ether (ODA), and 2,2'-dimethyl-4,4'-diaminobiphenyl (DMBZ);

[0019] Preferably, the molar ratio of the total amount of the diamine to the total amount of the dianhydride is 1 - 1.2:1 - 1.2; more preferably, the molar ratio of the total amount of the diamine to the total amount of the dianhydride is 1:1.05.

[0020] Preferably, the molar ratio of the first diamine to the second diamine is 3 - 7:2 - 6; more preferably, the molar ratio of the first diamine to the second diamine is 4 - 6:3 - 4;

[0021] Preferably, the molar ratio of the first dianhydride to the second dianhydride is 6 - 9:1 - 4; more preferably, the molar ratio of the first dianhydride to the second dianhydride is 7 - 8:3 - 4.

[0022] Preferably, the polymerization reaction is carried out in the presence of a solvent;

[0023] Preferably, the solvent is selected from at least one of N-methyl-2-pyrrolidone (NMP), dimethylacetamide (DMAc), γ-butyrolactone (GBL), dimethyl sulfoxide (DMSO), dimethylformamide (DMF), tetramethylurea (TMU), tetrahydrofuran (THF), m-cresol and ethyl acetate.

[0024] Preferably, the polymerization reaction further comprises an auxiliary agent;

[0025] Preferably, the auxiliary agent comprises a composite anti-yellowing agent, a catalyst and polytetrafluoroethylene nanofibrillated cellulose.

[0026] Preferably, the composite anti-yellowing agent comprises an antioxidant, an ultraviolet light absorber and hydrophobic fumed silica;

[0027] Preferably, the antioxidant is selected from at least one of hindered phenol antioxidants and natural antioxidants;

[0028] Further preferably, the antioxidant is selected from at least one of antioxidant 2010 and tocopherol; more preferably, the antioxidant is antioxidant 2010 and tocopherol.

[0029] Preferably, the molar ratio of antioxidant 2010 to tocopherol is 6-10:1-2; further preferably, the molar ratio of antioxidant 2010 to tocopherol is 8:1.

[0030] Preferably, the ultraviolet light absorber is 2,4-dihydroxybenzophenone.

[0031] Preferably, the mass ratio of the antioxidant, the ultraviolet light absorber and the hydrophobic fumed silica is 10-15:3-6:2-4; further preferably, the mass ratio of the antioxidant, the ultraviolet light absorber and the hydrophobic fumed silica is 12:5:2.

[0032] Preferably, the addition amount of the composite anti-yellowing agent is 0.1-8% of the total molar amount of diamine and dianhydride; further preferably, the addition amount of the composite anti-yellowing agent is 0.1-5% of the total molar amount of diamine and dianhydride. More preferably, the addition amount of the composite anti-yellowing agent is 2% of the total molar amount of diamine and dianhydride.

[0033] Preferably, the preparation method of the composite anti-yellowing agent is as follows: Dissolve the antioxidant in 10-15 wt% ethanol, then add the ultraviolet light absorber and hydrophobic fumed silica and ultrasonicate for 10-30 min, and then dry.

[0034] Preferably, the catalyst is selected from at least one of platinum bis(acetoacetate), triethyl[(trimethoxysilyl)methylcyclopentadienyl]platinum(IV), and stannous octoate;

[0035] More preferably, the catalyst is platinum bis(acetoacetate), triethyl[(trimethoxysilyl)methylcyclopentadienyl]platinum(IV), and stannous octoate.

[0036] Preferably, the molar ratio of platinum bis(acetoacetate), triethyl[(trimethoxysilyl)methylcyclopentadienyl]platinum(IV), and stannous octoate is 8 - 12:0.1 - 1:0.1 - 1; more preferably, the molar ratio of platinum bis(acetoacetate), triethyl[(trimethoxysilyl)methylcyclopentadienyl]platinum(IV), and stannous octoate is 10:0.1:0.5.

[0037] Preferably, the addition amount of the catalyst is 0.5 - 1% of the total molar amount of diamine and dianhydride; more preferably, the addition amount of the catalyst is 1% of the total molar amount of diamine and dianhydride.

[0038] Preferably, the ratio of polytetrafluoroethylene nanocellulose to the total molar amount of diamine and dianhydride is 0.01 - 0.05:1; more preferably, the ratio of polytetrafluoroethylene nanocellulose to the total molar amount of diamine and dianhydride is 0.02:1.

[0039] On the other hand, the present invention provides a method for preparing the above-mentioned flexible transparent polyimide film, comprising the following steps:

[0040] S1: Mix diamine and dianhydride with a solvent, and add polytetrafluoroethylene nanocellulose to obtain a mixed solution;

[0041] S2: Add a composite anti-yellowing agent to the mixed solution, stir for 10 - 30 min, and then add a catalyst to obtain a precursor solution;

[0042] S3: Pour the precursor solution onto a substrate, dry for 10 - 40 min to remove part of the solvent, and obtain a semi-dry film;

[0043] S4: Place the semi-dry film in an inert gas oven for high-temperature imidization to obtain the product.

[0044] Preferably, in S3, the drying temperature is 60 - 160 °C;

[0045] Preferably, in S4, the high-temperature imidization temperature is 200 - 280 °C, and the time is 10 - 40 min.

[0046] Compared with the prior art, the present invention has the following beneficial effects:

[0047] 1. The present invention prepares a polyimide film by mixing specific polyanhydrides and specific polyamines in a specific ratio, achieving the effects of significantly improving the optical properties of the polyimide film while obtaining better heat resistance and mechanical properties.

[0048] 2. The present invention uses a composite anti - yellowing agent prepared from antioxidant 2010, tocopherol, 2,4 - dihydroxybenzophenone, and hydrophobic fumed silica, which is combined with the diamine and dianhydride of the present invention to achieve the effects of significantly improving the optical properties of the polyimide film and reducing the yellowing index.

[0049] 3. The present invention uses specific catalysts, polytetrafluoroethylene nanofibrils in combination with the diamine and dianhydride of the present invention to achieve the effects of significantly improving the optical properties of the polyimide film while obtaining better heat resistance and mechanical properties.

[0050] 4. In summary, the present invention provides a flexible transparent polyimide film. By specific components and ratios and optimizing the preparation method, it achieves the effects of significantly improving the optical properties of the polyimide film while obtaining better heat resistance and mechanical properties. Detailed Embodiments

[0051] In order to make the technical means, creative features, achieved purposes and effects of the present invention easy to understand, the following specific embodiments are used to further clarify the present invention. However, the following embodiments are only the preferred embodiments of the present invention, not all of them. Based on the embodiments in the implementation manner, other embodiments obtained by those skilled in the art without creative efforts all belong to the protection scope of the present invention. It should be noted that the raw materials used in the present invention are all ordinary commercially available products, and no specific limitation is made on their sources. The technical and scientific terms used in the embodiments have the same meanings as those commonly understood by ordinary skilled persons in the technical field to which the present invention belongs.

[0052] Examples 1 - 6

[0053] A flexible transparent polyimide film, which is prepared by the polymerization reaction of diamine and dianhydride:

[0054] Raw material composition:

[0055] 1. The diamine, dianhydride and their molar ratios are shown in Table 1:

[0056] Table 1. Diamine and dianhydride of Examples 1 - 6

[0057] 2.

[0059] Solvent (DMSO);

[0060] Compound anti-yellowing agent (0.041 mol): antioxidant 2010 (0.0231 mol), tocopherol (0.0029 mol), 2,4-dihydroxybenzophenone (0.011 mol), hydrophobic fumed silica (0.004 mol);

[0061] Catalyst (0.0205 mol): platinum bis(acetoacetate) (0.01940 mol), triethyl[(trimethoxysilyl)methylcyclopentadienyl]platinum(IV) (0.00019 mol), and stannous octoate (0.00096 mol);

[0062] Polytetrafluoroethylene nanocellulose (0.041 mol).

[0063] The preparation method of the compound anti-yellowing agent is as follows: dissolve antioxidant 2010 and tocopherol in 10 - 15% ethanol, then add 2,4-dihydroxybenzophenone and hydrophobic fumed silica and ultrasonicate for 10 - 30 min, and then dry.

[0064] The preparation method comprises the following steps:

[0065] S1: Mix diamine and dianhydride with a solvent, and add polytetrafluoroethylene nanocellulose to obtain a mixed solution;

[0066] S2: Add the compound anti-yellowing agent to the mixed solution, stir for 20 min, and then add the catalyst to obtain a precursor solution;

[0067] S3: Pour the precursor solution onto a substrate, dry at 120 °C for 35 min to remove part of the solvent, and obtain a semi-dry film;

[0068] S4: Place the semi-dry film in an inert gas oven, and perform high-temperature imidization at 260 °C for 30 min to obtain the product.

[0069] Comparative Example 1 - 11

[0070] A flexible and transparent polyimide film, which is prepared by the polymerization reaction of diamine and dianhydride:

[0071] Compared with Example 1, the difference is only that the diamine and dianhydride in the raw material composition are different, as shown in Table 2 specifically:

[0072] Table 2. Diamine and dianhydride of Comparative Examples 1 - 10

[0073]

[0074]

[0075] The rest is the same as in Example 1.

[0076] Comparative Example 12

[0077] A flexible and transparent polyimide film, which is prepared by the polymerization reaction of diamine and dianhydride:

[0078] Compared with Example 1, the only difference is that the compound anti-yellowing agent in the raw materials is different:

[0079] Specifically:

[0080] Compound anti-yellowing agent (0.041 mol): antioxidant 2010 (0.0257 mol), tocopherol (0.0032 mol), 2,4-dihydroxybenzophenone (0.0121 mol);

[0081] The preparation method of the compound anti-yellowing agent is as follows: Take antioxidant 2010 and tocopherol, dissolve them with 10 - 15% ethanol, then add 2,4-dihydroxybenzophenone for 10 - 30 min, and dry.

[0082] The rest is the same as in Example 1.

[0083] Comparative Example 13

[0084] A flexible and transparent polyimide film, which is prepared by the polymerization reaction of diamine and dianhydride:

[0085] Compared with Example 1, the only difference is that the compound anti-yellowing agent in the raw materials is different:

[0086] Specifically:

[0087] Compound anti-yellowing agent (0.041 mol): antioxidant 2010 (0.0312 mol), tocopherol (0.0039 mol), hydrophobic fumed silica (0.0059 mol);

[0088] The preparation method of the compound anti-yellowing agent is as follows: Take antioxidant 2010 and tocopherol, dissolve them with 10 - 15% ethanol, then add hydrophobic fumed silica and sonicate for 10 - 30 min, and dry.

[0089] The rest is the same as in Example 1.

[0090] Comparative Example 14

[0091] A flexible and transparent polyimide film, which is prepared by the polymerization reaction of diamine and dianhydride:

[0092] Compared with Example 1, the only difference is that the compound anti-yellowing agent in the raw materials is different:

[0093] Specifically:

[0094] Compound anti-yellowing agent (0.041 mol): antioxidant 2010 (0.013 mol), tocopherol (0.013 mol), 2,4-dihydroxybenzophenone (0.011 mol), hydrophobic fumed silica (0.004 mol);

[0095] The preparation method of the compound anti-yellowing agent is as follows: Take antioxidant 2010 and tocopherol, dissolve them in 10 - 15% ethanol, then add 2,4-dihydroxybenzophenone and hydrophobic fumed silica, ultrasonic for 10 - 30 min, and dry.

[0096] The rest is the same as in Example 1.

[0097] Comparative Example 15

[0098] A flexible and transparent polyimide film, which is prepared by the polymerization reaction of diamine and dianhydride:

[0099] Compared with Example 1, the difference lies only in the catalyst in the raw materials:

[0100] Specifically:

[0101] Catalyst (0.0205 mol): platinum bis(acetylacetonate);

[0102] The rest is the same as in Example 1.

[0103] Comparative Example 16

[0104] A flexible and transparent polyimide film, which is prepared by the polymerization reaction of diamine and dianhydride:

[0105] Compared with Example 1, the difference lies only in the catalyst in the raw materials:

[0106] Specifically:

[0107] Catalyst (0.0205 mol): triethyl[(trimethoxysilyl)methylcyclopentadienyl]platinum(IV);

[0108] The rest is the same as in Example 1.

[0109] Comparative Example 17

[0110] A flexible and transparent polyimide film, which is prepared by the polymerization reaction of diamine and dianhydride:

[0111] Compared with Example 1, the difference lies only in the catalyst in the raw materials:

[0112] Specifically:

[0113] Catalyst (0.0205 mol): stannous octanoate;

[0114] The rest is the same as in Example 1.

[0115] Comparative Example 18

[0116] A flexible and transparent polyimide film, which is prepared by the polymerization reaction of diamine and dianhydride:

[0117] Compared with Example 1, the difference is only in the raw material ratio:

[0118] Diamine: 0.3 mol of BAPX and 0.7 mol of DMBZ;

[0119] Dianhydride: 0.76 mol of DSDA and 0.29 mol of PDA;

[0120] Compound anti-yellowing agent (0.205 mol): 0.1155 mol of antioxidant 2010, 0.0145 mol of tocopherol, 0.055 mol of 2,4-dihydroxybenzophenone, and 0.02 mol of hydrophobic fumed silica;

[0121] Catalyst (0.0410 mol): 0.03880 mol of platinum bis(acetoacetate), 0.00038 mol of triethyl[(trimethoxysilyl)methylcyclopentadienyl]platinum(IV), and 0.00192 mol of stannous octanoate;

[0122] Polytetrafluoroethylene nanocellulose (0.205 mol).

[0123] The rest is the same as in Example 1.

[0124] Comparative Example 19

[0125] A flexible and transparent polyimide film, which is prepared by the polymerization reaction of diamine and dianhydride:

[0126] Compared with Example 1, the difference is only that polytetrafluoroethylene nanocellulose is not added to the raw materials.

[0127] The rest is the same as in Example 1.

[0128] Test Example

[0129] The film performance test method is as follows:

[0130] (1) The light transmittance (Tr 550 nm), yellowness index, haze, etc. of the polyimide film are tested using an X-rite Ci7800 spectrophotometer;

[0131] (2) The tensile strength, elongation at break, and elastic modulus of the polyimide film were tested using a Shimadzu AG-X plus with a 1 KN load at a test speed of 5 mm / min. The sample size was 10 mm wide * 15 mm long, the test gauge length was 50 mm, and the extensometer gauge length was 20 mm.

[0132] (3) The glass transition temperature (Tg) was measured using a dynamic mechanical analyzer (DMA850) under the following test conditions:

[0133] The load was 0.05 N, the heating rate was 3 °C / min, and the measurement was carried out in a nitrogen atmosphere in the temperature range of 200 - 400 °C. The inflection point of the curve with the maximum value was recorded as the glass transition temperature.

[0134] (4) The coefficient of thermal expansion (CTE) was tested using a thermomechanical analyzer (TMA 7100C) under the following test conditions: the load was 20 mN, the heating rate was 5 °C / min, and the temperature range was 50 - 200 °C.

[0135] The results are shown in Table 3.

[0136] Table 3. Test Results of Film Properties

[0137]

[0138]

[0139] From the data in Table 3, it can be seen that the light transmittance of the polyimide films prepared in Examples 1 - 6 of the present invention is greater than 89%, the yellowness index is less than 1.7, the Tg is above 300 °C, combining good optical transmittance, heat resistance, and good mechanical properties. Compared with Example 1, in Comparative Examples 1 - 11, the diamine and dianhydride were mainly changed, resulting in a significant decrease in their optical properties, and also a decrease in heat resistance and mechanical properties; in Comparative Examples 12 - 14, the anti-yellowing agent was mainly changed, and it can be seen that their optical properties decreased significantly, especially the yellowness index; in Comparative Examples 15 - 19, the catalyst, raw materials, and ratios were changed, leading to a significant decrease in their optical properties, and also a decrease in heat resistance and mechanical properties. The specific raw materials and ratios of the present invention have achieved a significant improvement in the optical properties of the imide film and ensured its good heat resistance and mechanical properties.

[0140] Finally, it should be noted that the above content is only used to illustrate the technical solution of the present invention, rather than limiting the protection scope of the present invention. Any simple modification or equivalent replacement of the technical solution of the present invention by those of ordinary skill in the art does not depart from the essence and scope of the technical solution of the present invention.

Claims

1. A flexible and transparent polyimide film, characterized in that, The polyimide film is prepared by the polymerization reaction of diamine and dianhydride; The diamine described contains a first diamine and a second diamine; The first diamine is an aromatic diamine containing a rigid group; the first diamine is selected from at least one of 9,9-bis[4-(4-aminophenoxy)phenyl]xanthene, 2-(3,5-diaminophenyl)-9,9'-spirobifluorene, 2-(2,4-diaminophenyl)-9,9'-spirobifluorene, and 2-bis(4-aminophenyl)hexafluoropropane; The second diamine is a diamine containing imidazole; the second diamine is selected from at least one of 2-(4-aminophenyl)-5-aminobenzimidazole and 2-(3-aminophenyl)-5-aminobenzimidazole; The molar ratio of the first diamine to the second diamine is 3-7:2-6; The dianhydride described contains a first dianhydride and a second dianhydride; The first dianhydride is selected from at least one of 3,3,4,4-diphenylsulfone tetracarboxylic dianhydride, 4,4'-biphenyltetracarboxylic dianhydride, and benzophenone tetracarboxylic dianhydride; The second dianhydride is selected from 2,2'-(1,4-piperazinediyl)-disuccinic anhydride or 2R,5R,7S,10S-naphthalenetetracarboxylic dianhydride; The molar ratio of the first dianhydride to the second dianhydride is 6-9:1-4; The polymerization reaction also contains additives; the additives include a composite anti-yellowing agent, a catalyst, and polytetrafluoroethylene nanocellulose; The addition amount of the composite anti-yellowing agent is 0.1-8% of the total molar amount of diamine and dianhydride; The addition amount of the catalyst is 0.5-1% of the total molar amount of diamine and dianhydride; the catalyst is platinum bis(acetoacetate), triethyl[(trimethoxysilyl)methylcyclopentadienyl)]platinum(IV), and stannous octoate; the molar ratio of platinum bis(acetoacetate), triethyl[(trimethoxysilyl)methylcyclopentadienyl)]platinum(IV), and stannous octoate is 8-12: 0.1-1：0.1-1； The ratio of polytetrafluoroethylene nanocellulose to the total molar amount of diamine and dianhydride is 0.01-0.05:1; The composite anti-yellowing agent contains an antioxidant, an ultraviolet light absorber, and hydrophobic fumed silica; The antioxidant is selected from at least one of hindered phenol antioxidants and natural antioxidants; The mass ratio of the antioxidant, the ultraviolet light absorber, and the hydrophobic fumed silica is 10-15:3-6:2-4.

2. The polyimide film according to claim 1, characterized in that, The molar ratio of the total amount of diamine to the total amount of dianhydride is 1-1.2:1-1.

2.

3. The polyimide film according to claim 1, wherein The polymerization reaction is carried out in the presence of a solvent; the solvent is selected from at least one of N-methyl-2-pyrrolidone, dimethylacetamide, γ-butyrolactone, dimethyl sulfoxide, dimethylformamide, tetramethylurea, tetrahydrofuran, m-cresol, and ethyl acetate.

4. The polyimide film according to claim 1, characterized in that, The antioxidant is tocopherol.

5. The polyimide film according to claim 1, wherein The ultraviolet light absorber is 2,4-dihydroxybenzophenone.

6. The polyimide film according to claim 1, wherein The preparation method of the composite anti-yellowing agent is: take the antioxidant, dissolve it with 10-15 wt% of ethanol, then add the ultraviolet light absorber and hydrophobic fumed silica, ultrasonic for 10-30 min, and dry.

7. A method for preparing the flexible transparent polyimide film according to any one of claims 1-6, characterized in that, Including the following steps: S1: Mix diamine and dianhydride with a solvent, and add polytetrafluoroethylene nanocellulose to obtain a mixed solution; S2: Add a composite anti-yellowing agent to the mixed solution, stir for 10 - 30 min, and then add a catalyst to obtain a precursor solution; S3: Pour the precursor solution onto a substrate, dry for 10 - 40 min to remove part of the solvent, and obtain a semi-dry film; S4: Place the semi-dry film in an inert gas oven for high-temperature imidization to obtain the product.

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