A method for preparing a high-transparency low-yellowing polyimide film based on two-step end-capping treatment

Abstract

The application first prepares polyamide acid by using the method of condensation polymerization of dianhydride and diamine (and / or polyamine) monomers, and then end-caps the terminal amino group and terminal anhydride group of the polyamide acid by using dianhydride and hydroxyl substances, so as to eliminate the generation of amino oxidation coloration and colored anthraquinone structure, and a high-performance polyamide acid resin and a polyimide film with high transparency and low yellowing are prepared. Meanwhile, the anhydride end-capping agent can further increase the molecular weight of the film-forming polymer, destroy the Rayleigh scattering generated by the nanoscale ordered structure of the polyimide, and improve the light transmittance, processability and mechanical properties of the polyimide film in the UV / vis region, while the mechanical properties and thermal properties of the polyimide itself are basically not affected. The polyimide film preparation method provided by the application has the characteristics of universality and low cost compared with other methods, and has good industrialization prospect.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of polyimide material preparation, and particularly relates to a preparation method of high-transparency low-yellowing polyimide film based on two-step end-capping treatment. BACKGROUND

[0002] Polyimide (PI) is a kind of special polymer material containing imide ring repeating units. Its temperature range is -269-500℃, long-term use temperature range is -200-300℃, part of which has no obvious melting point, high insulation performance, 10 3 The dielectric constant under Hertz is 4.0, and the dielectric loss is only 0.004-0.007, belonging to F to H grade insulation, which is one of the best organic polymer materials in comprehensive performance. However, their color is usually light yellow to brown, which limits the application of the material in the fields of microelectronics, optoelectronics and new energy, etc. Therefore, it is of great significance to research and develop colorless transparent polyimide (CPI) film, which can be used as transparent electrode, flexible display, organic light emitting device, wearable device and transparent substrate and cover plate material of flexible solar cell. Compared with the ultra-thin glass material commonly used in the field at present, CPI film has the advantages of flexibility, light weight, bending resistance, impact resistance and long service life; compared with traditional transparent polymer materials such as polystyrene, polyethylene terephthalate, polycarbonate and polyether sulfone, CPI film has wider use temperature and high and low temperature resistance, which can withstand the requirements of high temperature processing in device manufacturing and the temperature changes of use scene. In summary, colorless transparent polyimide is increasingly critical and important as a new generation of flexible high-performance material.

[0003] Optical properties of optical films, from a chemical point of view, polyimide films generally belong to the full aromatic, and generally by aromatic diamine and aromatic dianhydride by condensation reaction to prepare a prepolymer, and then by imidization. Most researchers believe that the characteristic yellow or brown yellow polyimide film shows due to its aromatic diamine electron-donating and aromatic dianhydride electron-withdrawing caused by intramolecular or intermolecular charge transfer, electronic effect (Electromeric effect) and charge transfer complex CTC effect (Charge transfer complex effect) interaction formed, PI main chain of aliphatic units and side chain groups of partial decomposition will also affect the film color. Thus causing its low light transmittance, greatly limiting its application in the field of optics. For example, the most famous PI Kapton is from pyromellitic dianhydride (PMDA) and 4,4'-diamino diphenyl ether (ODA), due to the internal and intermolecular charge transfer (CTC) interaction between polyimide skeleton leads to the characteristic absorption in the visible region, its color shows yellow brown or even to black brown, in addition, due to Rayleigh scattering, nanoscale ordered structure also reduces the UV / Vis region of the transparency.

[0004] At present, domestic researchers introduce fluorine-containing groups, alicyclic structures, non-coplanar structures, meta-substituted structures, sulfone groups, etc. into the polyimide main chain to inhibit the formation of CTC, thereby improving the light transmittance of the polyimide film and reducing the yellow index of the film. The PI films prepared accordingly can be divided into fluorine-containing, alicyclic, non-coplanar, etc. In order to reduce the color of the polyimide film and thereby improve the light transmittance, generally, the CTC is reduced, such as using an alicyclic dianhydride or diamine to destroy the conjugation effect of the polymer main chain, or introducing fluorine-containing dianhydride or diamine monomers to prevent electron transfer by the electronegativity of fluorine atoms, or using non-planar structures and meta-dianhydride or diamine monomers to hinder the formation of planar conjugated structures in the molecular chain to reduce the formation of CTC. However, such polyimides still cannot completely eliminate the possibility of yellowing, and these methods will more or less reduce the aromaticity of the polyimide, sacrifice the thermal stability of the polyimide, and affect the coefficient of thermal expansion. Moreover, the synthesis of monomers or PAA polymers generally requires relatively complex operation steps or strict water-free and clean conditions. For example, in the process of polymerizing aromatic dianhydride monomers and alicyclic diamine monomers to obtain polyamide acid (PAA), the alicyclic monomer can well control the molecular weight, adjust the required viscosity, has good transparency and solubility, relatively low dielectric constant and relatively high Tg, as well as excellent thermal performance and mechanical durability. However, the PAA generated by the reaction will form a highly alkaline salt with free diamine, and this salt seriously prevents the formation of high molecular weight PAA, making it difficult to mass-produce alicyclic PI. In order to solve the new problems, scientists propose to introduce functional groups or substituents containing fluorine groups into the molecular chain of PI to solve these problems, and there are still complex physical and chemical problems such as complex monomer preparation process, high monomer price, and yellowing during processing of high molecular materials. SUMMARY

[0005] The present application aims to provide a high-transparency low-yellowing polyimide film preparation method based on two-step end-capping treatment, which is a universal method for improving the transparency of polyimide film and reducing the yellow index. The present application believes that the characteristic yellow or brownish yellow of the polyimide film is not only due to the CTC effect, but also due to the oxidation of the terminal amino group in the polyamide acid (PAA) polymerization, storage and processing process; and another important coloration reason that is often overlooked is that the electrophilic substitution coloration reaction of the terminal anhydride group or the terminal carboxyl group in the PAA molecular chain with the diamine monomer segment generates an anthraquinone structure and exhibits color (principle as shown in Figure 1 In addition, Rayleigh scattering caused by the ordered structure of PI at the nanoscale also reduces the transparency of PI film in the UV / vis region, thereby causing low light transmittance and greatly limiting its application in the optical field.

[0006] Therefore, the application adopts a method of gradually capping the terminal amino group and the terminal anhydride group of polyamide acid to prepare high-performance polyimide film with high transparency and low yellowing. The invention principle is that first, capping the terminal amino group of the diamine monomer of polyamide acid with dianhydride monomers can eliminate the color development of amino oxidation, reduce the electron cloud density of the terminal diamine monomer, or low electrophilic substitution reactivity of the diamine monomer is used for polycondensation to prepare polyamide acid, so as to prevent the generation of an anthraquinone structure by electrophilic substitution color reaction of the anhydride group of the dianhydride monomer to the terminal diamine monomer. Then, the terminal anhydride group or the terminal carboxyl group of PAA is further capped with a hydroxyl substance to prevent the electrophilic substitution color reaction of the terminal anhydride group or the terminal carboxyl group to the diamine monomer segment of PAA, prevent the generation of a colored anthraquinone structure, and combine the steric diamine and the low electrophilic substitution activity monomer to further reduce the anthraquinone color reaction, which can further improve the colorless transparency of the polyimide resin or film. In addition, the use of a multi-hydroxyl substance as an anhydride capping agent can further increase the molecular weight of the film-forming polymer and improve the mechanical properties of the polymer. In addition, the amino and anhydride capping agent can also be used to destroy the Rayleigh scattering generated by the ordered structure of the polyimide at the nanoscale, improve the light transmittance of the PI film in the UV / vis region, and improve the processability and mechanical properties. The specific steps of the application are as follows: first, low-temperature polymerization is performed with equimolar amounts of dianhydride and diamine monomers to generate high-molecular-weight polyamide acid; then, the same or different dianhydride monomers are added for amino capping to eliminate the occurrence of amino oxidation color development and reduce the electron cloud density of the diamine monomer; then, a single-hydroxyl or multi-hydroxyl substance is added as an anhydride or carboxyl capping agent to cap the anhydride group of the dianhydride monomer, prevent the generation of a colored anthraquinone structure by the terminal anhydride group or the terminal carboxyl group to the diamine segment, and eliminate the problem of color development of the anthraquinone structure during the storage, use or thermal imidization of polyamide acid. The principle of the two-step capping method is shown in Figure 2 and the specific invention is as follows.

[0007] A high-transparency and low-yellowing polyimide film preparation method based on two-step capping treatment, characterized in that the polyimide precursor solution is a polyamide acid solution (PAA) prepared by polymerizing equimolar amounts of dianhydride and diamine monomers, and then an amino capping agent and an anhydride capping agent are sequentially added to prepare a two-step capping polyamide acid resin solution (BTPAA). The BTPAA resin solution is then subjected to a thermal imidization reaction to prepare a high-transparency and low-yellowing high-performance polyimide film.

[0008] The specific steps for preparing the BTPAA precursor solution are as follows: under the protection of N2, a certain amount of diamine monomer and organic solvent are added to a reaction bottle, and mechanical stirring is performed until the diamine monomer is completely dissolved, then an equimolar amount of dianhydride monomer and the same organic solvent as the diamine monomer are added to the solution in batches, and stirring is continuously performed at 0-60°C for 8-24h to obtain a uniform viscous polyamic acid (PAA) solution; then 0.5-5% of the molar amount of the diamine monomer is added as an amino capping agent, and stirring is continuously performed at 0-60°C for 2-8h to obtain an anhydride group capped polyamic acid (TPAA) solution; then 0.5-5% of the molar amount of the dianhydride monomer is added as an anhydride capping agent, and stirring is continuously performed at 0-60°C for 1-8h, and the product is filtered and degassed to obtain a two-step capped polyamic acid (BTPAA) solution;

[0009] The method for preparing the polyimide film by thermal imidization is as follows: the obtained two-step capped polyamic acid solution (BTPAA) is uniformly coated on a substrate to obtain a wet film, which is placed in a heating furnace, and heated at a temperature in the range of 60-350°C under inert gas protection or vacuum conditions for 2-24h at a gradient, the solvent is removed, thermal imidization is performed, after the end, natural cooling to room temperature, then it is placed in a poor solvent and boiled for 0.5-2h, the film is peeled off, and finally dried at 70-150°C for 0.2-5h to obtain a high-performance polyimide film with high transparency and low yellowing;

[0010] The structure general formula of the polyamic acid (PAA) prepared by polymerization reaction of equimolar amount of diamine monomer R1 and dianhydride monomer R2 is shown in chemical formula (1);

[0011] The full anhydride group capped polyamic acid (TPAA) obtained by adding an amino capping agent R3 to the PAA has a structure general formula shown in chemical formula (2);

[0012] The structure general formula of the two-step capped polyamic acid resin solution (BTPAA) obtained by adding an anhydride capping agent R4 or R5 to the TPAA is shown in chemical formula (3) or (4);

[0013] The structure general formula of the prepared high-transparency and low-yellowing polyimide film is shown in chemical formula (5) or (6);

[0014] m, n are positive integers from 10 to 1000 in the chemical formulas (1) - (6); X = -H, -F, -CH3, -CH2CH3, -CF3, -NH2 or -CH2Cl in the chemical formulas (7) and (8), which can be the same or different atoms or groups in the same monomer molecule, and can be 1 to 10 substitutions; Y can be a C or N atom; M can be -O-, -CONH-, -COO-, -SO2-, -CX2-, -SiX2-, -SO-, -CO- or 1 or 2 to 5 structural units in the chemical formula (7) in the same monomer molecule;

[0015] The diamine monomer is one or a combination of 2-5 monomers of aromatic diamine, alicyclic diamine, fluorine-containing diamine, non-coplanar diamine, or substituted diamine monomer, and the monomer structure or monomer unit structure R1 is one or 2-5 of formula (7); wherein the diamine monomer is 2,2'-bis(trifluoromethyl)-4,4'-diaminophenyl ether (6FODA), 2,2-bis[4-(4-aminophenoxy)phenyl]hexafluoropropane (HFBAPP), 1,3-bis(4-aminophenoxy)benzene (TPE-R), 2,2'-bis(trifluoromethyl)-(1,1'-biphenyl)-4,4'-diamine (TFMB), 1,4-bis(4-aminophenoxy)benzene (TPE-Q), 4,4'-diamino-2,2'-dimethylbiphenyl (M-Tolidine), 4,4'-diaminobiphenyl-2,2'-dicarboxylic acid (biphenyl carboxylic acid 2,2'-DCB), 4,4'-bis(4-aminophenoxy)biphenyl (BAPB), 1,3-bis(3-aminophenoxy)benzene (APB), 4,4'-bis(3-aminophenoxy)diphenyl sulfone (BAPS-M), 4-aminobenzoic acid 4-aminophenyl ester (APAB), [4-(4-aminobenzoyl)oxyphenyl] 4-aminobenzoate (ABHQ), 6,6'-bisamino-3,3'-methylene diphenic acid (MBAA), 3,3',5,5'-tetramethylbenzidine (TMB), N,N'-(2,2'-bis(trifluoromethyl)-[1,1'-biphenyl]-4,4'-diyl)bis(4-aminobenzamide) (AB-TFMB), 4,4'-bis(3-aminophenoxy)biphenyl (BAPB-M), 1,4-bis(4-amino-2-trifluoromethylphenoxy)benzene (6FAPB), 4,4'-diaminodiphenylmethane (MDA), N,N'-[[2,2,2-trifluoro-1-(trifluoromethyl)ethylidene]bis(6-hydroxy-3,1-phenylene)]bis[3-aminobenzamide] (M-6FDAP), N,N'-[[2,2,2-trifluoro-1-(trifluoromethyl)ethylidene]bis(6-hydroxy-3,1-phenylene)]bis[4-aminobenzamide] (P-6FDAP), 2,2-bis(4-aminophenyl)hexafluoropropane (FA), 4,4'-diaminobenzanilide (DABA), 2,2-bis(4-aminophenyl)propane, 2,2-bis[4-(4-aminophenoxy)phenyl]hexafluoropropane, diamino diphenyl ether, para or per-metaposition triphenyl diether diamine, 3,3'-diaminodiphenyl sulfone, 4,4'-diaminodiphenyl sulfone, 1,3-bis(3-aminopropanol)tetramethyldisiloxane, 1,3-bis(4-aminophenoxy methane)-1,1,3,3-tetramethyldisiloxane, 2,2-bis[(4-aminophenoxy)phenyl]propane (4-BAPP), bis[4-(4-phenoxy)phenyl]sulfone (BAPS), 3,3'-dimethyl-4,4'-diaminodiphenylmethane (DMMDA), 1.4-bis(4-aminophenoxy)-2-tert-butylbenzene (BATB), 1.4-bis(4-aminophenoxy)-2.5-di-tert-butylbenzene (BADTB), 4,4'-bis(3- aminophenoxy)benzophenone (3-BABP), or one or two to four of melamine;

[0016] The dianhydride monomer and the amino capping agent are one or two to five monomers in combination of aromatic tetracarboxylic dianhydride, alicyclic tetracarboxylic dianhydride, fluorine-containing tetracarboxylic dianhydride, non-coplanar tetracarboxylic dianhydride or substituted tetracarboxylic dianhydride monomers, and the monomer structure or monomer unit structure R2 or R3 is one or two to five of the structural units in formula (8); wherein the dianhydride monomer is pyromellitic dianhydride (PMDA), diphenyl ether dianhydride, 3,3',4,4'-benzophenone tetracarboxylic dianhydride (BTDA), diphenyl ether tetracarboxylic dianhydride (ODPA), biphenyl tetracarboxylic dianhydride (BPDA), bisphenol A type diether dianhydride (BPADA), 2,2'-bis[4-(3,4-dicarboxyphenoxy)phenyl]propane tetracarboxylic dianhydride (BPADA), 2,2'-bis(3,4-dicarboxyphenyl)hexafluoropropane tetracarboxylic dianhydride (6FDA), bisphenol A type diether dianhydride, bisphenol F diether dianhydride (BPFDA), bisphenol S dianhydride, 1,2,4,5-cyclohexane tetracarboxylic dianhydride (HPMDA), 2,2'-difluoromethyl-4,4',5,5'-biphenyl tetracarboxylic dianhydride, 2,2'-bis(4-biphenyl)-biphenyl tetracarboxylic dianhydride, p-arylene-bisphenyl trimellitate dianhydride, 3,3,4,4-diphenyl sulfone tetracarboxylic dianhydride, 4,4'-p-phenylenedioxybisphthalic anhydride (HQDA), bicyclohexyl-3,4,3',4'-tetracarboxylic dianhydride, cyclobutane tetracarboxylic dianhydride, cyclopentane tetracarboxylic dianhydride, bicyclo[2.2.2]oct-7-ene-2,3,5,6-tetracarboxylic dianhydride, hydrogenated pyromellitic dianhydride (HPMDA), 1,2,3,4-butanetetracarboxylic dianhydride (BDA), (4-phthalic anhydride) formyloxy-4-phthalic ester, bis[(3,4-dianhydride)phenyl]terephthalic ester (PHAP), 3,3,4,4-diphenyl sulfone tetracarboxylic dianhydride (DSDA), 4,4'-p-phenylenedioxyphthalic anhydride (HQDPA), N-[4-[4-[(1,3-dioxo-2-benzofuran-5-carbonyl)amino]-2-(trifluoromethyl)phenyl]-3-(trifluoromethyl)phenyl]-1,3-dioxo-2-benzofuran-5-formamide (TA-TFMB), diethylene glycol (4-trimellitic anhydride) (TMEG), 9,9-bis(trifluoromethyl)-2,3,6,7-oxanthrene tetracarboxylic dianhydride (6FCDA), 5-isobenzofuran carboxylic acid 1,3-dihydro-1,3-dioxo-5,5'-[(1-methylethylene)di4,1-benzene] ester (BPEDA), 1,4,5,8-naphthalene tetracarboxylic anhydride (NTDA), and the addition amount of the above dianhydride is 1 mole relative to 1 mole of the diamine; the addition amount of the above amino capping agent is 0.5-5% of the mole number of the diamine monomer used;

[0017] The anhydride end-capping agent R4 or R5 is one or two to five of 1-4 membered alcohol, alicyclic alcohol, branched alcohol, straight-chain alcohol, polyether polyol, polyethylene glycol, or a fluorinated alcohol thereof, is one or two to five of methanol, ethanol, propanol, butanol, pentanol, hexanol, ethylene glycol, propylene glycol, butylene glycol, pentylene glycol, hexylene glycol, glycerol, butanetriol, pentanetriol, hexanetriol, diethylene glycol, triethylene glycol, tetraethylene glycol, pentaethylene glycol, trimethylolpropane, pentaerythritol, hydrogenated bisphenol A, and fluorinated products of the above-mentioned alcohols, and the amount of the anhydride end-capping agent is 0.5-5% of the moles of the dianhydride monomer used.

[0018] The organic solvent is one or two to three of tetrahydrofuran, chloroform, butanone, N-methylpyrrolidone, N-ethyl-2-pyrrolidone, N,N-dimethylformamide, N,N-dimethylacetamide, dimethyl sulfoxide, dichloromethane, γ-butyrolactone, γ-valerolactone, butyl acetate, ethyl acetate, m-cresol, acetonitrile, 1,3-dimethyl-2-imidazolidinone, hexamethylphosphoric triamide, N-methyl-2-dimethylpropanamide, and N-ethyl-2-methylpropanamide; wherein the mass ratio of the monomer to the organic solvent ranges from 10-50%.

[0019] The polyamic acid PAA, TPAA, and BTPAA condensation reaction temperature is 0-60℃.

[0020] The substrate is one or two to four of glass, quartz, indium tin oxide, copper, aluminum, gallium arsenide, ceramic, stainless steel, or silicon wafer.

[0021] The poor solvent is one or two to three of water, methanol, ethanol, acetone, acetic acid, or isopropyl alcohol.

[0022] The degassing method is reduced pressure degassing or ultrasonic degassing.

[0023] The application prepares high-performance polyamide acid resin and polyimide film with high transparency and low yellowing by adopting two-step end-capping method of amino and anhydride group in sequence, reduces anthraquinone color reaction by combining steric hindrance diamine and low electrophilic substitution activity monomer, reduces its absorption in visible light region, and further improves the transparency of polyimide resin or film material. The polyimide film preparation method provided by the application has a completely different coloration theory compared with other methods for preparing colorless and transparent polyimide based on reducing CTC effect, is a new design and preparation method for preparing CPI based on controlling amino oxidation to generate nitro coloration and anthraquinone structure coloration principle. The two-step end-capped polyimide film has better comprehensive performance than the uncapped polyimide film, including higher visible light transmittance and lower yellowing index, higher heat resistance, tensile strength, elastic modulus and elongation at break, and can prevent yellowing problem of polyimide during storage, transportation and processing, thereby preparing high-performance polyimide resin and polyimide film material with high transparency and low yellowing. The polyimide film preparation method provided by the application has the characteristics of universality, easy processability and low cost compared with other methods, and has good industrialization prospect as a flexible functional material applied in high-tech fields such as battery, printed circuit board, microelectronic and wearable device.

[0024] Transmittance, ultraviolet absorption cutoff wavelength

[0025] The transmittance and ultraviolet-visible absorption spectrum of the film were measured using an ultraviolet spectrophotometer (Shimadzu, UV-3600). The size of the polyimide film used was 2 cm*1 cm, and the thickness was about 10-15 μm, which was cleaned with a lens paper. The scanning wavelength range was 200-800 nm with an interval of 1 nm.

[0026] Mechanical properties (elongation at break, tensile strength, Young's modulus)

[0027] The mechanical properties (tensile strength, elongation at break, Young's modulus) of the film were measured using an electronic universal testing machine (INSTRON-1121 type electronic universal tensile testing machine). Each sample needs to be tested five times to obtain the average value, and the tensile rate is 5 mm / min.

[0028] Infrared spectroscopy characterization

[0029] The infrared spectrum of the film was tested using a Nicolet 5700 Fourier transform infrared spectrometer to measure whether the film was imidized. The polyimide film used for testing had a size of more than 1 cm*1 cm and a thickness of less than 5 μm. The measurement conditions were: scanning times 20 times, wave number range 4000-500 cm -1 .

[0030] Thermal stability analysis

[0031] A thermal gravimetric analysis (TG) model SDT650 was used to measure the relationship between the percentage of mass loss and temperature of the PI film. The measurement conditions were: nitrogen (N2) protection, flow rate of 30 mL / min, temperature rising rate of 10°C / min, and test temperature range of 25°C-800°C.

[0032] Contact angle test

[0033] A contact angle tester model SL200B was used to test the contact angle of the PI film. The test conditions were: test temperature of 25°C, and the liquid used was distilled water. Each sample was tested three times, and the average value was taken. BRIEF DESCRIPTION OF DRAWINGS

[0034] Figure 1 Principle of color development reaction that can occur during thermal imidization of polyamic acid

[0035] Figure 2 Principle of preparing high-transparency low-yellowing polyimide film by two-step end-capping polyamic acid method

[0036] Figure 3 Chemical formulas (1)-(6)

[0037] Figure 4 Chemical formula (7) of diamine monomer or unit structure thereof

[0038] Figure 5 Chemical formula (8) of dianhydride monomer or unit structure thereof

[0039] Figure 6 PIOB R Infrared spectra of PIOB t-B (A), PIOB t-B / E (B) and PIOB t-B / E (C) films

[0040] Figure 7 UV absorption spectra of PIOB films (A) at different film formation temperatures and their end-capped films (B)

[0041] Figure 8 DSC spectrum (A) and TGA spectrum (B) of PIOB film

[0042] Figure 9 Infrared spectra of PIBB series polyimide films

[0043] Figure 10 UV absorption spectra of PIBB end-capped film

[0044] Figure 11 DSC (A) and TGA (B) curves of PIBB film

[0045] Figure 12 UV absorption spectra (A) and optical pictures (B) of PISF series films

[0046] Figure 13 DSC (A) and TGA (B) curves of PISF series films

[0047] Figure 14 Contact angle test pictures of PIOB, PIBB, PISF series films.

[0048] Example 1 two-step end-capped PIOB R Preparation of (ODA / BPDA) polyimide series films

[0049] (1) Reference PAOB R Preparation of polyamic acid solution

[0050] Under room temperature and N2 protection, 2.0024 g of diaminodiphenyl ether (ODA) and 13.35 g of DMAc solvent were added into a 250 mL three-necked flask, and mechanically stirred until ODA was completely dissolved. Then 2.9422 g of biphenyl tetracarboxylic dianhydride (BPDA) and 16.67 g of DMAc solvent were added into the solution in batches, and continuously stirred for 24 h to prepare a uniform viscous PAOB R Polyamic acid solution.

[0051] (2) Preparation of BPDA end-capped amino PAOBt-B polyamic acid solution

[0052] Under room temperature and N2 protection, BPDA (0.05884 g) was added into the PAOB polyamic acid solution obtained in step (1), and continuously stirred for 4 h to prepare a uniform viscous anhydride end-capped PAOBt-B polyamic acid solution.

[0053] (3) Preparation of ethylene glycol end-capped anhydride PAOBt-B / E polyamic acid solution

[0054] Under room temperature and N2 protection, ethylene glycol (EG, 0.01231 g) was added into the anhydride end-capped PAOBt-B polyamic acid solution obtained in step (2), and continuously stirred for 4 h to form a uniform viscous two-step end-capped PAOBt-B / E polyamic acid solution.

[0055] (4) Preparation of PIOB series polyimide films

[0056] The PAOB, PAOBt-B and PAOBt-B / E polyamic acid solutions were uniformly coated on quartz substrates by doctor blade, and placed in a tube furnace for thermal imidization according to the gradient temperature of 70℃ / 7h; 100℃ / 1h; 150℃ / 1h; 200℃ / 1h; 250℃ / 1h. Finally, the quartz substrate with the film was placed in a water bath to release the film, and PIOB was prepared. RPIOBt-B and PIOBt-B / E films.

[0057] (5) Infrared spectral characterization of PIOB series polyimide films

[0058] PIOB R Infrared spectra of PIOBt-B and PIOBt-B / E thin films, such as Figure 6 As shown, 1600cm -1 and 1500cm -1 The absorption peaks at 1290 cm⁻¹ correspond to the C=C stretching and bending vibrations in the PIOB benzene ring, respectively. -1 and 1170cm -1 The absorption at that point corresponds to C ph -N symmetric and asymmetric tensile vibration, 940cm -1 The bending vibration of the -OH group in -COOH, 3083 cm⁻¹ -1 879cm -1 831cm -1 765cm -1 and 737cm -1 It is the –C in the benzene ring ph –H's tensile and bending vibrations, 1240cm -1 and 1115cm -1 The peak value appearing at is C in ODA ph Symmetric and asymmetric tensile vibrations of -O were observed. In all PIOB films, the 3270 cm⁻¹ tensile vibrations of NH₃ and C=O in the -CONH₃ group were not observed. -1 and 1680cm -1 Characteristic peak; however, a peak at 1712 cm⁻¹ can be observed. -1 and 1773cm -1 Symmetrical and asymmetric stretching at C=O, 1374cm -1 The corresponding CO-N-CO imine stretching characteristic absorption peak indicates that the imine cyclization reaction is complete. PIOB R Spectral line at 3083 cm -1 The appearance of a small, broad peak representing the hydrogen bond association of the NH stretching vibration indicates that PIOB R The peak is terminally amino-containing; the decrease in size of this peak in PIOBt-B and PIOBt-B / E is due to the reduction of amino-terminated peaks; in PIOBt-B... R And 1860cm in PIOBt-B -1 The absorption peak is due to the stretching vibration of the anhydride C=O group, but it is 1860 cm⁻¹ in PIOBt-B / E. -1 The absorption peaks have all weakened, indicating that the anhydride group in PIOBt-B / E is capped by EG.

[0059] (6) Optical property detection of PIOB series polyimide film

[0060] The UV spectrum of PIOB precursor solution after coating on the surface of quartz plate and heat treatment at different temperature gradient is shown in Figure 7 A. The optical property data of the film are listed in Table 1. From Figure 7 A, it can be seen that after the polyamide acid precursor solution is coated at 70℃, the UV cutoff wavelength is 367nm, and the obtained film is colorless and transparent, indicating that the CTC effect of the polyamide acid resin bulk molecular structure is not enough to make the polyamide acid resin film yellow; but after heating to 100℃, the UV cutoff wavelength of the film significantly red shifts to 392-400nm, and the film has already begun to turn yellow before the imidization ring formation temperature is reached, indicating that amino oxide or anthraquinone structure is generated, making the film yellow; but after heating to 250℃ to complete the imidization reaction, the UV cutoff wavelength of the obtained polyimide film does not continue to red shift, and even slightly left shifts, indicating that the CTC effect of the polyimide film obtained after the imidization ring formation reaction does not increase, and thus it can be seen that the CTC effect of the polyamide acid bulk main chain structure is not enough to make the polyamide acid resin or polyimide film develop color. Therefore, it can be indicated that the side reaction that occurs after the polyamide acid resin is heated to 100℃ is the cause of the characteristic yellow color of the polyimide film, and the polyimide film with non-conjugated molecular structure prepared based on the CTC effect is not completely colorless, and thus how to prevent the occurrence of side reactions in the preparation of polyimide resin is an important method for the design and preparation of CPI film molecular structure.

[0061] PIOB R The UV-vis spectrum of PIOBt-B and PIOBt-B / E film is shown in Figure 7 B. From it, it can be seen that compared with PIOB R , the UV cutoff wavelength of PIOBt-B film gradually left shifts from 417nm to 412nm, indicating that after the amino group is capped, the generation of colored nitro compound or anthraquinone structure is effectively reduced, and at the same time, the visible light transmittance is also correspondingly improved, the transmittance at 450nm is increased from 17% to 27.9%, and the maximum transmittance is increased from 94.5% to 96.3%; compared with PIOBt-B film, the transmittance of PIOBt-B / E film is significantly improved, and the maximum transmittance is increased from 96.3% to 99.5%. The UV cutoff wavelength of PIOBt-B / E is left shifted from 417nm to 410nm, and the transmittance at 450nm is increased from 27.9% to 39.0%, which is mainly due to the fact that the flexible ethylene glycol capping agent destroys the nanoscale ordered structure of the rigid aromatic ring of the polyimide, avoids Rayleigh scattering, and improves the transparency of the polyimide film in the UV / vis region. The specific data are listed in Table 1.

[0062] Table 1 Optical performance data of PIOB series thin films

[0063]

[0064] (7) Thermal and mechanical properties of PIOB series polyimide films

[0065] The thermal properties of PIOB thin films were investigated using DSC and TGA tests. The test results are as follows: Figure 8 A and Figure 8 As shown in Figure B, its main thermal performance evaluation parameters include the glass transition temperature (T). g ), temperature at which thermal weight loss is 5% (T) d5％ ), 10% thermal weight loss at temperature (T) d10％ ) and R w780℃ The data is listed in Table 2, from Figure 8 The DSC curve shown in Figure A indicates that PIOB R, T of PIOBt-B and PIOBt-B / E films g The glass transition temperatures of the three films were 265℃, 268℃, and 269℃, respectively. The temperature increase was likely due to the coupling effect of the end-capping agent, which increased the molecular weight of the polyimide resin and the intermolecular forces. However, the glass transition temperatures of the three films were relatively low. g The difference is not significant, indicating that adding a small amount of end-capping agent will not affect the thermal properties of the film.

[0066] Table 2 Mechanical and thermal properties of PIOB series films

[0067]

[0068] from Figure 8 As shown in TGA curve B, the three films exhibit minimal mass loss between 100-500℃, primarily due to the loss of moisture and small molecular chains within the PIOB film. Significant mass loss occurs between 500-800℃ because the CN bonds on the polyimide backbone and the CC bonds on the amide ring break, creating free radicals that generate CO gas, leading to chain transfer and mass decrease. However, the TGA curves of the three films show… d5％ With T d10％ The temperatures all exceeded 500℃, indicating that PIOBs films possess excellent thermal stability and heat resistance. Among them, the thermal weight loss of PIOBt-B and PIOBt-B / E end-capped films and PIOBt... R Compared to the previous model, there was little change, indicating that end-capping does not significantly affect the heat resistance and thermal stability of polyimide films. However, the stability of PIOBt-B with dianhydride-terminated amino groups is slightly lower than that of unterminated polyimide films. R, possibly because of its unstable end anhydride group decomposition, or the end anhydride group affected the further condensation of the polyamic acid resin, so the molecular weight or intermolecular force of the PIOBt-B was less than that of the unblocked film PIOB R ; but the thermal stability of the ethylene glycol blocked film PIOBt-B / E was slightly higher than that of the unblocked film PIOB R , the thermal weight loss temperature Td 5％ of PIOBt-B / E was 550°C, the thermal weight loss temperature Td 10％ of 10% was 575°C, and there was still 62% residual amount when the temperature rose to 790°C, which should be due to the coupling effect of the blocking agent ethylene glycol, which increased the molecular weight and thermal stability of the polyimide resin.

[0069] As can be seen from Table 2, the tensile strength of the two blocked polyimide films PIOBt-B and PIOBt-B / E was slightly increased compared with the unblocked PIOB R film, and the PIOBt-B / E film blocked by BPDA and EG had a more obvious increase in tensile strength, from 92.8Mpa to 103Mpa. This is because the addition of the blocking agents BPDA and EG has the effect of a coupling agent, which increases the molecular weight and intermolecular force of the polyimide resin, so that the tensile strength is increased; but due to the addition of the flexible ethylene glycol blocking agent, the crystal close-packed structure of the rigid aromatic ring is destroyed to some extent, resulting in a decrease in tensile modulus and elongation at break, but due to the small amount of the blocking agent, the mechanical properties of the obtained blocked polyimide film change little, and the characteristic structure and properties of the monomer polyimide material of the dianhydride and diamine remain unchanged.

[0070] Preparation of the two-step blocked PIBB (BAPP / BPDA) polyimide series film of Example 2 R Preparation of the polyamic acid solution

[0071] Under the conditions of room temperature and nitrogen protection, 4.1052g of 2,2-bis[(4-aminophenoxy)phenyl]propane (BAPP) was dissolved in 23.27g of DMAc, and mechanically stirred until completely dissolved. 2.9422g of biphenyl tetracarboxylic dianhydride (BPDA) and 16.67g of DMAc were added in two portions, and mechanically stirred for 24h to obtain a uniform viscous PABB R polyamic acid solution.

[0072] (2) Preparation of the BPDA blocked amino PABBt-B polyamic acid solution

[0073] Under the conditions of room temperature and nitrogen protection, the PABB polyamic acid solution obtained in step (1) was further added with BPDA (0.05884g), and mechanically stirred for 5h to obtain a uniform viscous anhydride blocked PABBt-B polyamic acid solution.

[0074] (3) Preparation of BPDA / EG end-capped PABBt-B / E poly(amic acid) solution

[0075] Under the condition of room temperature and nitrogen protection, 0.01231 g of ethylene glycol was added into the anhydride end-capped PABBt-B poly(amic acid) solution obtained in step (2), and the stirring was continued for 3 h to form a uniform viscous two-step end-capped PABBt-B / E poly(amic acid) solution.

[0076] (4) Preparation of 6FDA end-capped amino PABBt-F poly(amic acid) solution

[0077] Under the condition of room temperature and nitrogen protection, 0.09 g of hexafluorodiphthalic anhydride (6FDA) was added into the PABB poly(amic acid) solution obtained in step (1), and the stirring was continued for 6 h to form a uniform viscous 6FDA end-capped amino PABBt-F poly(amic acid) solution.

[0078] (5) Preparation of 6FDA / EG end-capped PABBt-F / E poly(amic acid) solution

[0079] Under the condition of room temperature and nitrogen protection, 0.013 g of ethylene glycol was added into the PABBt-F poly(amic acid) solution obtained in step (4), and the stirring was continued for 6 h to form a uniform viscous two-step end-capped PABBt-F / E poly(amic acid) solution.

[0080] (6) Preparation of PIBB series polyimide films

[0081] The viscous PABB series solutions prepared in steps (1)-(5) above were uniformly coated on quartz substrates. In a tube furnace, vacuum was first applied, and then heating was performed, and the thermal imidization was completed by gradient heating in the following sequence: 70 °C / 8 h, 100 °C, 150 °C, 200 °C and 250 °C for 2 h each. After the imidization was completed, the quartz substrates were immersed in hot water at 60 °C for 10 min to make the films peeled off from the quartz substrates, and PIBB R , PIBBt-B, PIBBt-B / E, PIBBt-F and EPIBBt-F / E films were prepared, respectively.

[0082] (7) Infrared spectrum characterization of PIBB series polyimide films

[0083] The infrared spectrum characterization of PIBB series films is shown in Figure 9 . It can be clearly seen that the molecular characteristic vibration absorption peaks of amide ring, 1730 cm -1 and 1780 cm -1 correspond to the symmetric contraction and asymmetric contraction of C=O; the peak at 1370 cm -1 corresponds to the characteristic contraction vibration of C-N; and the peak at 1500 cm -1and 1600 cm -1 The characteristic absorption peak of the benzene ring skeleton stretching vibration is at 1600 cm -1 The characteristic absorption peak of the benzene ring skeleton stretching vibration is at 1600 cm -1 The characteristic absorption peak of the benzene ring skeleton stretching vibration is at 1600 cm

[0084] (8) Optical performance test of PIBB series polyimide films

[0085] The optical performance test of PIBB series films is shown in Table 2. Figure 10 It can be seen that the optical performance of the polyimide films prepared by two-step end-capping method is greatly improved compared with the un-capped polyimide films. This indicates that the two-step end-capping method can not only reduce the yellow index of polyimide and shift the UV cut-off wavelength to the left, but also improve the transmittance of polyimide films in the visible light region. Among them, the polyimide film capped with fluorine-containing non-conjugated monomer has a lower UV cut-off wavelength.

[0086] (9) Thermal and mechanical performance test of PIBB series polyimide films, as shown in Table 3.

[0087] Table 3 Thermal and mechanical performance of PIBB series end-capped films

[0088]

[0089] The thermal performance of PIBB R， PIBBt-B and PIBBt-B / E; PIBBt-F and PIBBt-F / E films was studied by DSC and TGA, and the test results are shown in Figure 11 and Table 3. From the DSC spectrum, it can be seen that the glass transition temperature Tg slightly increases with the addition of end-capping agent, but the overall effect is not large. From the TGA curve, it can be seen that the end-capped PIBB film has good heat resistance, among which the amino end-capped film slightly decreases due to the instability of the anhydride group, and the EG secondary end-capped film slightly increases due to the coupling effect of EG. The overall change of thermal performance is not large due to the small amount of end-capping agent.

[0090] Example 3 Preparation of two-step end-capped PISF (APS / 6FDA) polyimide film

[0091] (1) Reference PASF R Preparation of polyamic acid solution

[0092] At 0°C and under nitrogen protection, 2.4830 g of diaminodiphenyl sulfone (APS) was dissolved in 8.00 g of DMAc, mechanically stirred until completely dissolved, 4.4424 g of 6FDA and 14.00 g of DMAc were added in two portions, and after mechanical stirring for 24 h, a uniform viscous PASF was prepared R Polyamic acid solution.

[0093] (2) Preparation of 6FDA-terminated amino PISFt-F polyamic acid solution

[0094] At 0°C and under nitrogen protection, the PASF obtained in step (1) R was added with 0.10 g of 6FDA, and after continuous mechanical stirring for 3 h, an anhydride-terminated PASFt-F polyamic acid solution was formed.

[0095] (3) Preparation of ethylene glycol-terminated anhydride PISFt-F / E polyamic acid solution

[0096] At 0°C and under nitrogen protection, 0.01231 g of ethylene glycol was added to the polyamic acid solution obtained in step (2), and after continuous stirring for 2 h, a two-step-terminated PASFt-F / E polyamic acid solution was formed.

[0097] (4) Preparation of PISF series polyimide films

[0098] The PASF series viscous solutions prepared in steps (1)-(3) above were uniformly coated on quartz substrates. In a tube furnace, vacuum was first applied, and then heating was performed, and heat imidization was completed by gradient heating in the following sequence: 60°C / 8 h, 120°C, 140°C, 180°C, and 250°C for 1 h each; after imidization was completed, the quartz substrate was immersed in hot water at 50°C for 15 min, so that the film was peeled off from the quartz substrate, and PISF R , PISFt-F, and EPISFt-F / E films were prepared, respectively.

[0099] (5) Optical performance detection of PISF series films

[0100] The optical performance detection of PISF series films is shown as Figure 12 follows. It can be seen that the polyimide film prepared by the two-step termination method has greater improvement in optical performance compared with the un-terminated polyimide film. It shows that the two-step termination method can not only reduce the yellow index of polyimide, but also shift the ultraviolet cutoff wavelength to the left and improve the transmittance of polyimide film in the visible light region. It can be seen that the polyimide film prepared by using APS monomer has better colorless transparency, and the polyimide film prepared after two-step termination has lower ultraviolet cutoff wavelength and better transparency in the visible light region.

[0101] (6) Thermal and mechanical properties of PISF series polyimide films

[0102] The thermal properties of PISF were studied by DSC and TGA R， The thermal properties of PISFt-F and PISFt-F / E films were studied by DSC and TGA as shown in Figure 6. From the DSC curves, it can be seen that the glass transition temperature Tg of the films increased slightly with the addition of the end-capping agent, but the overall effect was not significant. From the TGA curves, it can be seen that the end-capped PISF films had good thermal resistance, with the amino end-capped film decreasing slightly due to the instability of the anhydride group, and the EG secondarily end-capped film increasing slightly due to the coupling effect of EG. The thermal properties did not change significantly overall due to the small amount of end-capping agent used. Figure 13

[0103] Surface energy detection of PIBB, PISF series films

[0104] Surface energy detection of PIOB, PIBB, PISF series films as shown in Figure 5 and Table 3, where the picture numbers correspond to the following polyimide films: (1) PIOBR, (2) PIOBt-B, (3) PIOBt-B / E; (4) PIBBR, (5) PIBBt-B, (6) PIBBt-B / E; (7) PIBBR, (8) PIBBt-F, (9) PIBBt-F / E; (10) PISFR, (11) PISFt-F, and (12) PISFt-F / E polyimide films. Figure 14 The contact angle detection data of the polyimide films showed that the polarity of the polyimide films and their end-capped films gradually increased, and the contact angles of the first end-capped films and the second end-capped films gradually decreased, and the surface energy increased, which was due to the introduction of the polar end-capping agent anhydride group and hydroxyl group. Among them, the contact angles of PIBBt-F and PIBBt-F / E had a larger increase than PIBBt-B, which should be due to the hydrophobic structure of the 6FDA end-capping agent causing the surface energy to decrease. R

[0105]

[0106] Preparation of two-step end-capped PIFF (TFMB / 6FDA) series polyimide films in Example 4

[0107] (1) Reference PIFF R Preparation of polyamic acid solution

[0108] Under room temperature and nitrogen protection conditions, 3.2023 g of 2,2'-bis(trifluoromethyl)-(1,1'-biphenyl)-4,4'-diamine (TFMB) was dissolved in 7.5 g of DMAc, and 4.4424 g of 6FDA and 15 g of DMAc were added in two portions. After mechanical stirring for 24 h, a uniform polyamic acid (PAFF​​​R ) solution.

[0109] (2) Preparation of 6FDA end-capped amino-PIFFt-F polyamic acid solution

[0110] Under room temperature and nitrogen protection, 0.08884 g of 6FDA was added into the anhydride end-capped polyamic acid (PAFFt-F) solution obtained in step (1), and mechanical stirring was continued for 7 h. The obtained viscous amino end-capped PAFFt-F polyamic acid solution was used as such.

[0111] (3) Preparation of ethylene glycol end-capped anhydride-PIFFt-F / E polyamic acid solution

[0112] (4) Preparation of glycerol end-capped anhydride-PIFFt-F / G polyamic acid solution

[0113] Under room temperature and nitrogen protection, 0.019 g of glycerol was added into the amino end-capped PAFFt-F polyamic acid solution obtained in step (2), and stirring was continued for 5 h. The obtained viscous two-step end-capped PAFFt-F / G polyamic acid solution was used as such.

[0114] (5) Preparation of PIFF series polyimide films

[0115] The polyamic acid solutions obtained in steps (1) to (4) above were coated on glass substrates, respectively. Vacuum was first applied, and then heating was performed in a tube furnace. The imidization was completed by heating at the following sequence and gradient: 70 °C / 5 h, 130 °C, 150 °C, 220 °C, each for 1 h. After the imidization was completed, the quartz substrates were immersed in hot water at 50 °C for 12 min, and the films were peeled off from the glass substrates. PIFFt-F, PIFFt-F / E, PIFFt-F / G films were obtained, respectively, and the UV cut-off wavelength range was 382-395 nm, and the highest transmittance was 98.5%. R

[0116] Example 5 Preparation of two-step end-capped PIFSF(TFMB / APS / 6FDA) polyimide film R Preparation of polyamic acid solution

[0117] ​Under the condition of 10°C and nitrogen protection, 3.2023 g of 2,2'-bis(trifluoromethyl)-(1,1'-biphenyl)-4,4'-diamine (TFMB) and 2.4830 g of APS were dissolved in 15 g of DMAc, and 8.8848 g of 6FDA and 30 g of DMAc were added in 4 times. After mechanical stirring for 24 h, a uniform viscous PAFSF polyamic acid solution was formed. R of polyamide acid.

[0118] (2) Preparation of 6FDA-terminated amino PAFSFt-F polyamic acid solution

[0119] Under the condition of 10°C and nitrogen protection, 0.230 g of 6FDA was added to the PAFSF R polyamic acid solution obtained in step (1), and after mechanical stirring for 8 h, a uniform viscous PAFSFt-F polyamic acid solution was prepared.

[0120] (3) Preparation of ethylene glycol-terminated anhydride PAFSFt-F / E polyamic acid solution

[0121] Under the condition of 10°C and nitrogen protection, 0.031 g of ethylene glycol was added to the PAFSFt-F polyamic acid solution obtained in step (2), and after continuous stirring for 5 h, a uniform viscous two-step-terminated PAFSFt-F / E polyamic acid solution was formed.

[0122] (4) Preparation of glycerol-terminated anhydride PIFSFT-F / G polyamic acid solution

[0123] Under the condition of 10°C and nitrogen protection, 0.0461 g of glycerol was added to the PAFSFt-F polyamic acid solution obtained in step (2), and after continuous stirring for 3 h, a uniform viscous two-step-terminated PAFSFt-F / G polyamic acid solution was formed.

[0124] (5) Preparation of PIFSFT series polyimide films

[0125] The polyamic acid solutions PAFSF R， PAFSFt-F, PAFSFt-F / E, and PAFSFt-F / G obtained in steps (1)-(4) above were respectively coated on a quartz substrate. Heat imidization was completed in a tube furnace by gradient heating in the following sequence: 70°C / 5 h, 110°C, 160°C, 210°C for 1 h each. After imidization was completed, the quartz substrate was immersed in hot water at 55°C for 15 min to peel the film from the quartz substrate, and PIFSFT R， PIFSFT-F, PIFSFT-F / E, and PIFSFT-F / G films were respectively prepared, and the ultraviolet cutoff wavelength range was 385-395 nm, and the highest transmittance was 99.0%.

[0126] Example 6 Preparation of two-step end-capped PIOBMt-B / E (ODA / BPDA / MA) polyimide film by thermal imidization method

[0127] At 20°C and under nitrogen protection, 3.1860 g of 4,4'-ODA and 44.0 mL of DMAc were added to a 250 mL three-necked flask and dissolved by mechanical stirring. Then, 4.4952 g of 4,4'-BPDA was added to the solution in the three-necked flask and stirring was continued for 12 hours. Separately, 0.0451 g of MA and 0.3130 g of 4,4'-BPDA were added to a dry and clean conical flask, 12.0 mL of DMAc was added and the solution was dissolved by stirring at 50°C. After the solution became clear, the heating was stopped and stirring was continued for 12 hours. Then, the MA / BPDA solution was slowly added to the ODA / BPDA system and stirring was continued for 6 hours. Then, 0.0971 g of BPDA was added and stirring was continued for 8 hours to form a uniform and viscous solution of anhydride end-capped amino group-containing PABMt-B polyamide acid. Then, 0.020 g of ethylene glycol was added and stirring was continued for 4 hours to form a uniform and viscous solution of two-step end-capped PABMt-B / E polyamide acid. The solution was uniformly coated on a dry and clean quartz substrate and left to stand for 4 hours. The obtained film was placed in a tube furnace and heated at 150°C for 2 hours and at 250°C for 1 hour under vacuum. A two-step end-capped polyimide film PIOBMt-B / E was obtained by thermal imidization, which had an ultraviolet cutoff wavelength of 394 nm and a maximum transmittance of 98.0%.

[0128] As the method of forming the coating layer by applying the composition for forming the coating layer on the polyimide-based film, any one or more of spin coating, dip coating, spray coating, die coating, bar coating, roll coater, meniscus coating, flexographic printing, screen printing, bead coating, air knife coating, reverse roll coating, blade coating, flow coating, and gravure coating, etc. can be used, but the present application is not limited thereto.

Claims

1. A method for preparing a high-transparency low-yellowing polyimide film based on a two-step end-capping process, characterized in that, The polyimide precursor solution is prepared by polymerizing equimolar amounts of dianhydride monomers and diamine monomers to obtain a polyamide acid (PAA) solution, then adding an amino capping agent and an anhydride capping agent in sequence to obtain a two-step capped polyamide acid resin (BTPAA) solution, and then performing thermal imidization on the BTPAA solution to obtain a polyimide film. The BTPAA solution is prepared by the following steps: under N2 protection, a certain amount of diamine monomers and an organic solvent are added to a reaction bottle, and mechanical stirring is performed until the diamine monomers are completely dissolved, then equimolar amounts of dianhydride monomers and the same organic solvent are added to the solution in batches, and stirring is continuously performed at 0-60℃ for 8-24h to obtain a uniform viscous PAA solution, then 0.5-5% of the molar amount of dianhydride monomers is added as an amino capping agent, and stirring is continuously performed at 0-60℃ for 2-8h to obtain an anhydride-capped polyamide acid (TPAA) solution, then 0.5-5% of the molar amount of hydroxyl substances is added as an anhydride capping agent, and stirring is continuously performed at 0-60℃ for 1-8h, and the product is filtered and degassed to obtain the BTPAA solution. The polyimide film is prepared by the following steps: the obtained BTPAA solution is uniformly coated on a substrate to obtain a wet film, the wet film is placed in a heating furnace, and the film is continuously heated at a temperature range of 60-350℃ under inert gas protection or vacuum conditions at a gradient of 2-24h to remove the solvent and perform thermal imidization, then the film is naturally cooled to room temperature, and the film is placed in a poor solvent for 0.5-2h to peel off the film, and finally the film is dried at 70-150℃ for 0.2-5h to obtain the polyimide film. The diamine monomers are one or two to four of diamino diphenyl ether, 2,2-bis[(4-aminophenoxy)phenyl]propane, diamino diphenyl sulfone, and 2,2'-bis(trifluoromethyl)-(1,1'-biphenyl)-4,4'-diamine; the dianhydride monomers and the amino capping agent are biphenyl tetracarboxylic dianhydride or / and hexafluoro dianhydride; and the anhydride capping agent is ethylene glycol or / and glycerol.

2. The method for preparing high-transparency low-yellowing polyimide film based on two-step end-capping treatment according to claim 1, characterized in that The organic solvent is one or two to three of tetrahydrofuran, chloroform, butanone, N-methyl pyrrolidone, N-ethyl-2-pyrrolidone, N,N-dimethylformamide, N,N-dimethylacetamide, dimethyl sulfoxide, dichloromethane, γ-butyrolactone, γ-valerolactone, butyl acetate, ethyl acetate, m-methyl phenol, acetonitrile, 1,3-dimethyl-2-imidazolidinone, hexamethylphosphoric triamide, N-methyl-2-dimethylpropionamide, and N-ethyl-2-methylpropionamide; and the total mass of the diamine and dianhydride monomers accounts for 10-50% of the mass of the organic solvent.

3. The method of claim 1, wherein the two-step end-capping process is performed by adding the first end-capping agent to the polyimide precursor solution, and then adding the second end-capping agent to the polyimide precursor solution. The substrate is one or two to four of glass, quartz, copper, aluminum, gallium arsenide, ceramic, stainless steel, or a silicon wafer.

4. The method for preparing high-transparency low-yellowing polyimide film based on two-step end-capping treatment according to claim 1, characterized in that The poor solvent is one or two to three of water, methanol, ethanol, acetone, acetic acid, and isopropanol.

5. The method of claim 1, wherein the two-step endcapping process is performed by adding the first endcapping agent to the polyimide solution and then adding the second endcapping agent to the polyimide solution. The degassing method is reduced pressure degassing or ultrasonic degassing.

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