Polyimide material, film and film preparation method

Abstract

The invention provides a polyimide material, a film and a film preparation method. The polyimide material is prepared from polyamide acid, a surface-modified silicon oxide nanocage and a modifier according to the following proportion through a copolymerization-curing reaction, the polyamic acid modified silicon oxide nanocage comprises the following components in parts by weight: 100 parts of polyamic acid, 5-20 parts of surface modified silicon oxide nanocage and 2-8 parts of modifier. The invention relates to a polyimide substrate material modified by introducing a surface modified silicon oxide nanocage (silsesquioxane) and synergistically optimizing the proportion of a diamine monomer, and compounding a special modifier, and a preparation method thereof. The material can effectively reduce OLED display ghost shadows, is suitable for a substrate of a flexible OLED display panel, is especially suitable for OLED devices with high-frequency driving and long-time display, and can be widely applied to the OLED display fields of smart phones, tablet personal computers, flexible wearable devices, vehicle-mounted displays and the like.

Description

Technical Field

[0001] This invention relates to the field of organic electroluminescent (OLED) display technology, and in particular to a polyimide material, a thin film, and a method for preparing the thin film. Background Technology

[0002] OLED display technology, with its superior characteristics such as self-emissiveness, high contrast, wide viewing angle, fast response, and flexibility, has become the mainstream direction of next-generation display technology. As the core supporting component of the device, the OLED substrate material directly determines the display performance, mechanical stability, and lifespan of the OLED panel. Its core performance requirements include: excellent heat resistance (suitable for high-temperature processes such as OLED device evaporation and encapsulation, glass transition temperature Tg ≥ 250℃), good optical transparency (visible light transmittance ≥ 85%, reducing light loss), low coefficient of thermal expansion (CTE ≤ 30ppm / K, matching the thermal expansion characteristics of adjacent layers such as metal electrodes and light-emitting layers, avoiding warping and cracking during the manufacturing process), excellent mechanical properties (tensile strength ≥ 100MPa, elongation at break ≥ 5%, meeting the requirements for flexible bending), and good dielectric properties and adhesion to the substrate.

[0003] Currently, polyimide (PI) is the primary substrate material for OLEDs. Polyimide is characterized by its high temperature resistance, chemical corrosion resistance, and excellent mechanical properties, making it the preferred material for flexible OLED substrates. However, a key technical challenge exists when using existing polyimide materials in OLED substrates—it easily leads to image retention in OLED displays.

[0004] In the current technology, the industry's improvements to the OLED image retention problem mainly focus on the light-emitting layer material and driving circuit optimization, with fewer improvements to the polyimide substrate material, and these improvements have obvious limitations. At the same time, although there have been attempts to introduce siloxane cage-like materials into the polyimide system to improve transparency, these have not been specifically designed to address the core pain point of OLED substrate image retention, nor have they solved problems such as "single modification effect, poor synergy of various performances, and insufficient adaptation to OLED process requirements". Summary of the Invention

[0005] To overcome the above-mentioned technical defects, the present invention provides a polyimide material, a film, and a method for preparing the film, so as to solve the problems involved in the background art.

[0006] According to the present invention, a polyimide material is provided, which is prepared by copolymerization-curing reaction of polyamic acid, surface-modified silica nanocages and modifier in the following proportions; the components by weight are: 100 parts of polyamic acid, 5-20 parts of surface-modified silica nanocages and 2-8 parts of modifier.

[0007] Preferably, the polyamic acid is prepared by polymerizing dianhydride monomer and diamine monomer in a polar solvent, wherein the molar ratio of dianhydride monomer to diamine monomer is 1:0.95-1.05.

[0008] Preferably, the dianhydride monomer is selected from one or more mixtures of 3,3',4,4'-diphenyl ether tetracarboxylic dianhydride, 4,4'-hexafluoroisopropylphthalic anhydride, 3,3',4,4'-biphenyl tetracarboxylic dianhydride, and benzophenone tetracarboxylic dianhydride; Preferably, the dianhydride monomer is a mixture of 3,3',4,4'-diphenyl ether tetracarboxylic dianhydride and 4,4'-hexafluoroisopropylphthalic anhydride in a molar ratio of 1:0.3-0.7.

[0009] Preferably, the diamine monomer is selected from one or more of p-phenylenediamine (PDA), 4,4'-diaminodiphenyl ether (ODA), and siloxane-containing diamines; Preferably, the diamine monomer is a complex system of a rigid aromatic diamine and a flexible siloxane-containing diamine; Preferably, the diamine monomer is a mixture of p-phenylenediamine and siloxane-containing diamine in a molar ratio of 3-7:1; Preferably, the flexible siloxane diamine is an amino-terminated polydimethylsiloxane diamine.

[0010] Preferably, the surface-modified silica nanocage is a cage-like silsesquioxane with the molecular formula (RSiO). 1.5 ) n n can be 8, 10, or 12, with n preferably being 8, and the particle size being 2-8 nm. The surface-modifying groups of the surface-modified silica nanocage are one or more of amino, epoxy, or acyl chloride groups; The surface-modifying groups are mixed modifying groups in which amino and epoxy groups are present in a molar ratio of 1:0.5-1.5.

[0011] Preferably, the modifier is a compound system of water-soluble acrylic resin and imidazole compound in a weight ratio of 2:1.

[0012] Preferably, the copolymerization-curing reaction is carried out in a solvent and accelerated by a catalyst; Preferably, the solvent is selected from N,N-dimethylacetamide (DMAC) and N-methyl-2-pyrrolidone (NMP); Preferably, the catalyst is a compound system of isoquinoline and pyridine in a weight ratio of 1:1.

[0013] Secondly, this embodiment provides a polyimide film, which is a film obtained using the aforementioned polyimide material.

[0014] Thirdly, a method for preparing a polyimide film includes: Step 1: Prepare polyamic acid; Step 2: Prepare a surface-modified silica nanocage dispersion; Step 3: Slowly add the prepared silica nanocage dispersion and modifier to the polyamic acid solution prepared in Step 1, and stir at room temperature for 2-4 hours to ensure uniform dispersion of each component and obtain a mixed slurry. Step 4: The prepared mixed slurry is uniformly coated on a clean substrate using a doctor blade coating method, with a coating thickness of 5-50 μm. After coating, it is placed in an oven for gradient curing.

[0015] Preferably, the gradient curing process includes: Low temperature stage: Curing at 80℃ for 1 hour to remove low-boiling-point solvents and moisture from the slurry; Medium temperature stage: Curing at 100℃ for 2 hours, curing at 150℃ for 3 hours, and curing at 200℃ for 2 hours to promote the initial imidization and crosslinking reaction of polyamic acid; High temperature stage: Curing at 250℃ for 1 hour, curing at 300℃ for 2 hours, curing at 350℃ for 1 hour, curing at 400℃ for 1 hour, and curing at 450℃ for 1 hour. After curing, the film is allowed to cool naturally to room temperature and then peeled off from the substrate to obtain a polyimide film containing silica nanocages.

[0016] This invention relates to a polyimide material, a thin film, and a method for preparing the thin film, which has the following advantages compared with the prior art: (1) Significantly reduced image retention, specifically optimized for OLED substrates. This invention introduces amino-epoxy mixed-modified silica nanocages. The inorganic cage structure has no charge trapping sites, which can effectively dilute the polar group density in the polyimide molecular chain and reduce carrier accumulation. At the same time, the specific ratio of PDA to siloxane diamine in a ratio of 3-7:1 improves the mobility of the molecular chain and facilitates carrier desorption. The two work together to shorten the image retention time of OLED devices by more than 80%, with the preferred solution shortening it by 90%, solving the core pain point of existing polyimide substrates. Moreover, no additional charge trapping sites are introduced, avoiding the performance imbalance of existing modification solutions.

[0017] (2) The modifier compound system achieves multiple performance gains. The present invention uses a water-soluble acrylic resin and imidazole compound compound modifier in a weight ratio of 2:1 to improve the crosslinking efficiency of silica nanocages and polyamic acid, avoid agglomeration, and ensure uniform material performance; improve the surface flatness and film-forming properties of polyimide film, and reduce the problem of uneven light emission in OLED devices; optimize the adhesion of the material to glass substrate, a-Si, and OLED light-emitting layer / electrode layer, with adhesion to glass substrate ≥0.8N / cm and adhesion to a-Si ≥0.3N / cm, which is superior to the prior art; reduce the dielectric constant of the material, with dielectric constant Dk≤3.0 and dielectric loss factor Df≤0.005, which is suitable for high-frequency driven OLED devices and reduces signal attenuation; does not introduce additional polar groups, has no charge trapping risk, and takes into account both modification effect and anti-residual requirements.

[0018] (3) The synergistic ratio of diamine monomers achieves synergistic improvement of multiple properties, solving the problem of performance imbalance in existing technologies. This invention focuses on optimizing the compound ratio of PDA and siloxane diamine (3-7:1), synergistically with the modification effect of silicon oxide nanocages, to solve the dilemma in existing technologies where "increased flexibility leads to decreased heat resistance, and increased transparency leads to intensified charge trapping." It eliminates the CTC effect, improves transparency (visible light transmittance ≥90%, yellowness index ≤2), and eliminates the yellowing problem of traditional polyimides. While improving molecular chain flexibility, it maintains heat resistance (glass transition temperature Tg ≥350℃, 5% thermogravimetric temperature ≥600℃), adapting to the 450℃ high-temperature baking requirements of OLED devices. It improves the dimensional stability of the material (coefficient of thermal expansion CTE ≤25ppm / K), matching the thermal expansion characteristics of metal electrodes and the light-emitting layer, avoiding warping and cracking during manufacturing. It optimizes mechanical properties (tensile strength ≥120MPa, elongation at break ≥8%), meeting the bending requirements of flexible OLEDs (bending cycles ≥100,000, bending radius ≤1mm).

[0019] (4) Excellent compatibility and dispersibility, resulting in uniform substrate performance. This invention achieves dual-site covalent bonding between silica nanocages and polyamic acid molecular chains by amino-epoxy mixed modification (molar ratio 1:0.5-1.5). Combined with high-speed dispersion and ultrasonic dispersion processes, the silica nanocages are uniformly dispersed in polyimide, with a particle size distribution controlled at 2-8 nm and no obvious agglomerates. At the same time, a special modifier further improves the compatibility of the system, avoids phase separation, ensures the performance uniformity of the OLED substrate, and improves device yield.

[0020] (5) The preparation process has significant advantages and is easy to industrialize. The process is simple and highly controllable, requiring no special equipment. It adopts gradient curing and high-speed dispersion processes to reduce production costs. The polymerization and modification processes are carried out at room temperature or medium and low temperature, which is energy-saving and environmentally friendly. By optimizing the selection of solvents, curing process and the distribution ratio of each component, the product qualification rate is improved (≥98%), which is suitable for large-scale industrial production.

[0021] In summary, compared with existing technologies, especially those that only introduce cage-like siloxanes into the polyimide system, this invention has the aforementioned significant and non-obvious beneficial effects. The core innovation lies in the combination of surface-modified silica nanocages, synergistic diamine monomer ratio, and the formulation of a dedicated modifier, achieving a synergistic gain effect of 1+1+1>3. It has a wide range of applications and broad market prospects. This material is not only suitable for flexible OLED substrates but also for rigid OLED substrates, adaptable to various OLED display products such as smartphones, tablets, flexible wearable devices, and automotive displays. Furthermore, its applications can be extended to optical waveguide materials, liquid crystal display alignment films, and other fields. Its modification approach can be extended to other polyimide materials requiring reduced charge trapping and improved overall performance, providing a new direction for polyimide modification technology. Detailed Implementation

[0022] In the following description, numerous specific details are set forth in order to provide a more thorough understanding of the invention. However, it will be apparent to those skilled in the art that the invention can be practiced without one or more of these details. In other instances, certain technical features well-known in the art have not been described in order to avoid obscuring the invention.

[0023] Industry improvements to address OLED image retention primarily focus on emissive layer materials and driving circuit optimization, with limited improvements to the polyimide substrate material, which also have significant limitations. Through in-depth research, the applicant discovered that the essence of OLED image retention is that after an OLED device displays a fixed image for an extended period, when the display is turned off or the image is switched, a trace of the original image remains temporarily, affecting the display effect. The core cause is closely related to the charge trapping effect, molecular chain rigidity, and thermal stability of the polyimide substrate. Specifically, this includes the following aspects: Charge trapping effect: There are a large number of polar groups (such as imide bonds and aromatic rings) in the existing polyimide molecular chain. These groups easily trap the charge carriers (electrons and holes) generated during the operation of OLED devices. After the charge carriers accumulate for a long time, they are difficult to release quickly, which causes the luminous intensity of the light-emitting layer to fail to recover in time, forming a ghost image. Excessive rigidity of molecular chains: Traditional polyimide molecular chains are mostly rigid aromatic ring structures with poor molecular chain mobility. When the heat generated by the device is transferred to the substrate, the molecular chains cannot release stress in time through movement, which can easily lead to micro-defects on the substrate surface, further aggravating charge trapping, and affecting the uniformity of charge carrier transport, thus aggravating image retention. Insufficient thermal stability: In order to improve flexibility and solubility, some flexible polyimides introduce flexible groups, which leads to a decrease in thermal stability. In the medium and low temperature (80-150℃) environment where OLEDs operate for a long time, the molecular chains are prone to slight degradation or orientation changes, which leads to a decrease in the dielectric properties and surface smoothness of the substrate, indirectly causing image retention. The contradiction between transparency and charge trapping: Traditional polyimides are often yellow or dark brown due to electronic polarization. To improve transparency, existing technologies often introduce fluorine-containing groups or aliphatic structures. However, these structures reduce the regularity of the molecular chain and further aggravate charge trapping, creating a dilemma of "increased transparency - increased image retention".

[0024] Among them, silica nanocages (silsesquioxane, POSS) are nanomaterials with a cage-like structure and a molecular formula of (RSiO). 1.5 ) n (n is typically 8, 10, or 12), its core characteristics are: a central inorganic silica cage structure (high temperature resistant, chemically inert, and without charge trapping sites), and the ability to graft organic functional groups onto the surface (which can react with polyimide molecular chains to improve compatibility), uniform particle size (1-10 nm), and excellent dispersibility. Introducing silica nanocages (silsesquioxanes) into polyimide materials has the potential to solve the problem of image retention in existing polyimide substrates, but existing technologies have not addressed this issue specifically. Based on this, this invention proposes a triple-core improved design: (1) A customized surface modification scheme for silica nanocages was adopted, using amino-epoxy mixed modification to achieve covalent bond connection with polyamic acid molecular chains, solving the problems of uneven dispersion and poor compatibility, ensuring that the inorganic cage structure without charge trapping sites is uniformly dispersed in the polyimide system, and diluting polar groups from the source. (2) Synergistically optimize the ratio of diamine monomers, and combine rigid aromatic diamines (PDA / ODA) with flexible siloxane diamines in a specific ratio to eliminate the CTC (charge transfer complex) effect. While improving the mobility of molecular chains, heat resistance and flexibility are also taken into account, avoiding the performance imbalance caused by the single monomer ratio. (3) A special modifier for water-soluble acrylic resin and imidazole compounds is compounded. This compounding system is not a conventional setting in this field. It can further improve the crosslinking stability of silicon dioxide nanocages and polyimide, reduce the dielectric constant, and at the same time improve the adhesion between the substrate and the functional layers of OLED, so as to achieve multiple performance synergistic optimization.

[0025] Currently, there is no publicly available technical solution that combines surface-modified silica nanocages, synergistic diamine monomer ratios, and compounded special modifiers to specifically address the image retention problem on OLED substrates. Based on this, the present invention proposes a novel modified polyimide material, filling a gap in the industry.

[0026] Specifically, this invention designs a polyimide material, characterized in that it is prepared by copolymerization-curing reaction of polyamic acid, surface-modified silica nanocages and modifier in a solvent in the following proportions under the action of a catalyst. The copolymerization-curing reaction is carried out in a solvent and accelerated by a catalyst. The components by weight are: 100 parts polyamic acid, 5-20 parts surface-modified silica nanocages, 2-8 parts modifier; solvent: 150-300 parts; catalyst: 0.5-2 parts.

[0027] As a preferred embodiment, the polyamic acid is prepared by polymerizing dianhydride monomers and diamine monomers in a polar solvent, wherein the molar ratio of dianhydride monomers to diamine monomers is 1:0.95-1.05; the dianhydride monomers are selected from one or more mixtures of 3,3',4,4'-diphenyl ether tetracarboxylic dianhydride (ODPA), 4,4'-hexafluoroisopropylphthalic anhydride (6FDA), 3,3',4,4'-biphenyl tetracarboxylic dianhydride (BPDA), and benzophenone tetracarboxylic dianhydride (BTDA); preferably, a mixture of ODPA and 6FDA in a molar ratio of 1:0.3-0.7 is preferred, which balances transparency and heat resistance while reducing yellowing caused by electronic polarization; The diamine monomer is a complex system of rigid aromatic diamine and flexible siloxane-containing diamine. The diamine monomer is selected from one or more mixtures of p-phenylenediamine (PDA), 4,4'-diaminodiphenyl ether (ODA), and siloxane-containing diamines. Preferably, it is a mixture of PDA and siloxane-containing diamine in a molar ratio of 3-7:1, wherein the siloxane-containing diamine is amino-terminated polydimethylsiloxane diamine. This specific ratio is the core optimization point, which can improve the flexibility and solubility of polyimide while eliminating the CTC (charge transfer complex) effect, further improving transparency and reducing charge trapping, while avoiding the decrease in heat resistance caused by a single flexible siloxane-containing diamine, thus achieving a synergistic balance of "flexibility-heat resistance-anti-charge trapping". The polyamic acid has a solid content of 10-30 wt%, a viscosity of 1000-7000 Pa·s, and a molecular weight distribution of 1.05-1.25, ensuring subsequent film formation and processability.

[0028] As a preferred option, surface-modified silica nanocages (silsesquioxanes): cage-like silsesquioxanes (POSS) with the molecular formula (RSiO) are selected. 1.5)8, with a particle size of 2-8 nm; the surface modification group is one or more of amino, epoxy or acyl chloride, preferably a mixed modification group of amino and epoxy in a molar ratio of 1:0.5-1.5. This mixed modification is directionally designed and can react with the carboxyl and amino groups of polyamic acid respectively to form a stable cross-linked structure and avoid the aggregation of silica nanocages. The surface modification rate of the silica nanocage is ≥80% to ensure its reactivity with polyamic acid; the addition amount is limited to 5-20 parts: if the addition amount is too small, it cannot effectively reduce charge trapping and the effect of improving image retention is not obvious; if the addition amount is too large, it will lead to excessive cross-linking of polyimide molecular chains, increase rigidity, affect flexibility and solubility, and may also reduce transparency.

[0029] As a preferred embodiment, the modifier is a compound system of water-soluble acrylic resin and imidazole compound in a weight ratio of 2:1. This compounding scheme is not a conventional setting in the art and is one of the core innovations of this invention. Among them, the water-soluble acrylic resin can improve the film-forming properties and surface smoothness of polyimide, and the imidazole compound can act as a crosslinking promoter to improve the crosslinking efficiency of silica nanocages and polyamic acid molecular chains, while optimizing the dielectric properties of the material and its adhesion to the OLED light-emitting layer / electrode layer. The compounding of the two can achieve multiple gains in "film-forming properties-crosslinking stability-adhesion-dielectric properties", and will not introduce additional charge trapping sites, thus avoiding the aggravation of image retention. The amount of modifier added is limited to 2-8 parts. If the amount added is too small, the modification effect will be insufficient, and if the amount added is too large, the heat resistance of the material will be reduced.

[0030] As a preferred embodiment, the solvent is selected from one of N,N-dimethylacetamide (DMAC) and N-methyl-2-pyrrolidone (NMP), i.e., pure DMAC or pure NMP, without using any mixed solvents; The solvent must have good solubility to fully dissolve polyamic acid, surface-modified silica nanocages, and special modifiers, and have a moderate boiling point (150-220℃) to facilitate slow evaporation during subsequent curing and avoid the formation of bubbles and defects in the substrate. The amount of solvent added is limited to 150-300 parts to ensure that the system viscosity is moderate, which is convenient for stirring, dispersion, and coating. Pure NMP is preferred (it has better solubility and is suitable for high-temperature curing requirements).

[0031] As a preferred embodiment, the catalyst is preferably a compound system of isoquinoline and pyridine in a weight ratio of 1:1, which can effectively promote the imidization reaction of polyamic acid and the crosslinking reaction between silica nanocages and polyamic acid, ensuring an imidization rate of ≥98% and improving the structural stability of the material.

[0032] This invention also provides a method for preparing a polyimide film, characterized by comprising the following steps: Step 1: Prepare polyamic acid; Under inert gas (nitrogen or argon) protection, a diamine monomer (PDA to siloxane-containing diamine molar ratio of 3-7:1) is added to a solvent in a specific ratio and stirred at room temperature (25-30℃) for 1-2 hours until the diamine monomer is completely dissolved to obtain a diamine solution. Then, dianhydride monomer is added to the diamine solution in batches, controlling the addition time to 0.5-5 hours to avoid excessive reaction and local overheating. After the addition is complete, the reaction is stirred at room temperature for 10-20 hours to obtain a polyamic acid solution. The molar ratio of dianhydride monomer to diamine monomer is 1:0.95-1.05, and the mass ratio of the total mass of diamine and dianhydride to the mass of solvent is 1-3:10. High-speed dispersion equipment (3000-5000 rpm) can be used to assist dispersion during polymerization to avoid dianhydride monomer agglomeration.

[0033] Step 2: Prepare a surface-modified silica nanocage dispersion; Unmodified cage-like silsesquioxane (POSS) was added to anhydrous ethanol and ultrasonically dispersed for 30-60 min to obtain a POSS dispersion. Then, a mixed modification reagent of aminosilane coupling agent (KH550) and epoxysilane coupling agent (KH560) in a molar ratio of 1:0.5-1.5 was added, the temperature was raised to 60-80℃, and the reaction was stirred for 4-6 h to complete the surface modification. After the reaction was completed, the mixture was centrifuged (8000-10000 rpm, 10-15 min) to remove unreacted modification reagents and impurities, and then vacuum dried (80-100℃, 2-4 h) to obtain surface-modified silica nanocages. The surface-modified silica nanocages were added to a portion of the solvent (the same solvent used in step 1), ultrasonically dispersed for 1-2 hours, and then dispersed in a high-speed dispersion device (5000-8000 rpm) for 30-60 minutes to obtain a silica nanocage dispersion. The dispersion was made uniform without agglomeration and with a uniform particle size distribution.

[0034] Step 3: Slowly add the silica nanocage dispersion prepared in Step 2, the special modifier compounded with water-soluble acrylic resin and imidazole compounds at a weight ratio of 2:1 to the polyamic acid solution prepared in Step 1, and stir at room temperature for 2-4 hours to ensure that each component is uniformly dispersed to obtain a mixed slurry.

[0035] Step 4: The mixed slurry prepared in step 3 is uniformly coated on a clean substrate (glass substrate or stainless steel substrate) using a blade coating method, with a coating thickness of 5-50 μm. After coating, it is placed in an oven for gradient curing to meet the baking requirements of OLED substrate polyimide at 450℃. Specifically: (1) Low temperature stage: Curing at 80℃ for 1h to remove low boiling point solvents and water in the slurry; (2) Medium temperature stage: Curing at 100℃ for 2h, 150℃ for 3h, and 200℃ for 2h to promote the initial imidization and crosslinking reaction of polyamic acid; (3) High temperature stage: Curing at 250℃ for 1h, 300℃ for 2h, 350℃ for 1h, 400℃ for 1h, and 450℃ for 1h to ensure complete imidization reaction (imidization rate ≥98%) and sufficient crosslinking between silicon oxide nanocages and polyimide molecular chains to meet the baking requirements of OLED substrate. After curing, the film is naturally cooled to room temperature and then peeled off from the substrate to obtain an OLED substrate polyimide film containing silicon oxide nanocages (silsesquioxane). The film thickness is 20-50 μm, which meets the thickness requirements of OLED substrates.

[0036] Step 5: Post-processing The peeled polyimide film is annealed at 200-250℃ for 1-2 hours to eliminate internal stress generated during curing and further improve the dimensional stability and surface smoothness of the material. Then, plasma cleaning (100-200W power, 5-10min) is used to remove impurities and residual solvents from the film surface and improve the adhesion between the film and the OLED light-emitting layer and electrode layer.

[0037] The components, by weight, are as follows: 100 parts polyamic acid; 10-15 parts surface-modified silica nanocages (sesquioxane); 4-6 parts modifier (water-soluble acrylic resin: imidazole compound = 2:1); 200-250 parts solvent (pure NMP); and 1-1.5 parts catalyst (isoquinoline: pyridine = 1:1).

[0038] In the corresponding preparation method, the diamine monomer is a compound system of PDA and siloxane-containing diamine in a molar ratio of 5:1, and the dianhydride monomer is a compound system of ODPA and 6FDA in a molar ratio of 1:0.5. The polymerization reaction time in step 1 is 15 h, the surface modification agent in step 2 is a mixture of KH550 and KH560 in a molar ratio of 1:1, the surface modification reaction temperature is 70 °C, the stirring time in step 3 is 3 h, and the gradient curing in step 4 is carried out according to the above standard process. The polyimide material prepared by this preferred scheme has the best image retention improvement effect, while taking into account heat resistance, transparency and mechanical properties, making it suitable for high-end flexible OLED displays.

[0039] The present invention will be further described below with reference to the embodiments. These embodiments are intended to explain the invention and should not be construed as limiting it. Where specific techniques and reaction conditions are not specified in the embodiments, they can be performed according to the techniques or conditions described in the literature or product instructions in the art. All reagents, instruments, or equipment without a specified manufacturer are commercially available.

[0040] Experimental materials: Dihydride monomers: 3,3',4,4'-diphenyl ether tetracarboxylic dianhydride (ODPA), 4,4'-hexafluoroisopropylphthalic anhydride (6FDA), commercially available with a purity ≥99%; Diamine monomers: p-phenylenediamine (PDA), 4,4'-diaminodiphenyl ether (ODA), amino-terminated polydimethylsiloxane diamine, commercially available with a purity ≥99%; Silica nanocages: cage-like silsesquioxane (POSS, molecular formula (RSiO1.5)8), particle size 5nm, commercially available; Surface modification agents: aminosilane coupling agent (KH550), epoxysilane coupling agent (KH560), commercially available; Modifiers: water-soluble acrylic resin (30% solid content), imidazole compounds (2-methylimidazole), commercially available; Solvents: N-methyl-2-pyrrolidone (NMP), N,N- Dimethylacetamide (DMAC), commercially available anhydrous grade; catalysts: isoquinoline and pyridine, commercially available analytical grade; other materials: inert gas (nitrogen), anhydrous ethanol, and glass substrate, all of which are commercially available conventional products.

[0041] Example 1: This embodiment provides an OLED substrate polyimide material containing silica nanocages (silsesquioxanes). The components, by weight, are as follows: polyamic acid: 100 parts; surface-modified silica nanocages: 5 parts; special modifier (water-soluble acrylic resin: 2-methylimidazole = 2:1): 2 parts; solvent (pure NMP): 150 parts; catalyst (isoquinoline: pyridine = 1:1): 0.5 parts.

[0042] The preparation method of this embodiment includes the following steps: Step 1: Under nitrogen protection, PDA and siloxane-containing diamine were added to pure NMP at a molar ratio of 3:1 and stirred at room temperature for 1 hour until completely dissolved. Then, a mixture of ODPA and 6FDA at a molar ratio of 1:0.3 (dianhydride to diamine molar ratio 1:0.95) was added in batches over a period of 0.5 hours. The mixture was stirred at room temperature for 10 hours to obtain a polyamic acid solution. Step 2: Add POSS to anhydrous ethanol, sonicate for 30 min, add a mixture of KH550 and KH560 in a molar ratio of 1:0.5, heat to 60℃, stir for 4 h, centrifuge, and vacuum dry (80℃, 2 h) to obtain surface-modified silica nanocages; add them to a portion of pure NMP, sonicate for 1 h, and disperse at high speed (5000 rpm) for 30 min to obtain a dispersion; Step 3: Slowly add the dispersion and special modifier to the polyamic acid solution, stir at room temperature for 2 hours to obtain a mixed slurry; Step 4: Apply the mixed slurry onto the glass substrate with a doctor blade to a thickness of 50 μm, and perform gradient curing: 80℃ / 1h → 100℃ / 2h → 150℃ / 3h → 200℃ / 2h → 250℃ / 1h → 300℃ / 2h → 350℃ / 1h → 400℃ / 1h → 450℃ / 1h. Cool and peel off to obtain a polyimide film (20 μm thick). Step 5: Anneal at 200℃ for 1 hour, then plasma clean (100W, 5 minutes) to obtain the finished product.

[0043] Example 2: This embodiment discloses an OLED substrate polyimide material containing silica nanocages (silsesquioxanes). The components, by weight, are as follows: polyamic acid: 100 parts; surface-modified silica nanocages: 12 parts; special modifier (water-soluble acrylic resin: 2-methylimidazole = 2:1): 5 parts; solvent (pure NMP): 220 parts; catalyst (isoquinoline: pyridine = 1:1): 1.2 parts.

[0044] The preparation method of this embodiment includes the following steps: Step 1: Under nitrogen protection, PDA and siloxane-containing diamine were added to pure NMP at a molar ratio of 5:1 and stirred at room temperature for 1.5 hours until completely dissolved. Then, a mixture of ODPA and 6FDA at a molar ratio of 1:0.5 (dianhydride to diamine molar ratio of 1:1.0) was added in batches over a period of 3 hours. The mixture was stirred at room temperature for 15 hours to obtain a polyamic acid solution. Step 2: Add POSS to anhydrous ethanol, sonicate for 45 min, add a mixture of KH550 and KH560 in a molar ratio of 1:1 as a modification agent, heat to 70℃, stir for 5 h, centrifuge, and vacuum dry (90℃, 3 h) to obtain surface-modified silica nanocages; add it to a portion of pure NMP, sonicate for 1.5 h, and disperse at high speed (6500 rpm) for 45 min to obtain a dispersion. Step 3: Slowly add the dispersion and special modifier to the polyamic acid solution, stir at room temperature for 3 hours to obtain a mixed slurry; Step 4: Apply the mixed slurry onto the glass substrate with a doctor blade to a thickness of 100 μm, and perform gradient curing: 80℃ / 1h → 100℃ / 2h → 150℃ / 3h → 200℃ / 2h → 250℃ / 1h → 300℃ / 2h → 350℃ / 1h → 400℃ / 1h → 450℃ / 1h. Cool and peel off to obtain a polyimide film (thickness 35 μm). Step 5: Anneal at 220℃ for 1.5 hours, then plasma clean (150W, 8 minutes) to obtain the finished product.

[0045] Example 3: This embodiment provides an OLED substrate polyimide material containing silica nanocages (silsesquioxanes). The components, by weight, are as follows: polyamic acid: 100 parts; surface-modified silica nanocages: 20 parts; special modifier (water-soluble acrylic resin: 2-methylimidazole = 2:1): 8 parts; solvent (pure NMP): 300 parts; catalyst (isoquinoline: pyridine = 1:1): 2 parts.

[0046] The preparation method of this embodiment includes the following steps: Step 1: Under nitrogen protection, PDA and siloxane-containing diamine were added to pure NMP at a molar ratio of 7:1 and stirred at room temperature for 2 hours until completely dissolved. Then, a mixture of ODPA and 6FDA at a molar ratio of 1:0.7 (dianhydride to diamine molar ratio 1:1.05) was added in batches over a period of 5 hours. The mixture was stirred at room temperature for 20 hours to obtain a polyamic acid solution. Step 2: Add POSS to anhydrous ethanol and sonicate for 60 min. Add a mixture of KH550 and KH560 in a molar ratio of 1:1.5 as a modification agent. Heat to 80℃ and stir for 6 h. Centrifuge and vacuum dry (100℃, 4 h) to obtain surface-modified silica nanocages. Add them to a portion of pure NMP and sonicate for 2 h. Then disperse at high speed (8000 rpm) for 60 min to obtain a dispersion. Step 3: Slowly add the dispersion and special modifier to the polyamic acid solution, stir at room temperature for 4 hours to obtain a mixed slurry; Step 4: Apply the mixed slurry onto the glass substrate with a doctor blade to a thickness of 150 μm, and perform gradient curing: 80℃ / 1h → 100℃ / 2h → 150℃ / 3h → 200℃ / 2h → 250℃ / 1h → 300℃ / 2h → 350℃ / 1h → 400℃ / 1h → 450℃ / 1h. Cool and peel off to obtain a polyimide film (50 μm thick). Step 5: Anneal at 250℃ for 2 hours, then plasma clean (200W, 10 minutes) to obtain the finished product.

[0047] Comparative Example 1: The difference between this comparative example and Example 2 is that: no silica nanocages or modifiers were introduced, and the diamine ratio was adjusted; the molar ratio of PDA to siloxane-containing diamine was changed from 5:1 to 1:1. This comparative example provides an OLED substrate polyimide material, with the following components by weight: polyamic acid: 100 parts; solvent (pure NMP): 220 parts; catalyst (isoquinoline: pyridine = 1:1): 1.2 parts; the polyamic acid is prepared by polymerizing PDA and siloxane-containing diamine in a molar ratio of 1:1 (not the optimized ratio of this invention), and ODPA and 6FDA in a molar ratio of 1:0.5, without silica nanocages or special modifiers.

[0048] The preparation method of this comparative example is completely consistent with that of Example 2, except that the silica nanocage and special modifier are not added and the diamine monomer ratio is adjusted (the diamine ratio is adjusted so that the molar ratio of PDA to siloxane diamine is 1:1).

[0049] Comparative Example 2: The difference between this comparative example and Example 2 is that the silica nanocages are not surface-modified and no modifier is introduced. This comparative example provides an OLED substrate polyimide material, and the components by weight are: polyamic acid: 100 parts; surface-modified silica nanocages: 12 parts; solvent (pure NMP): 220 parts; catalyst (isoquinoline: pyridine = 2:1): 1.2 parts.

[0050] The preparation method of this comparative example is completely consistent with that of Example 2, except that no surface modification or special modifier is added in step 2.

[0051] Comparative Example 3:The difference between this comparative example and Example 2 is that no modifier was introduced. This comparative example provides an OLED substrate polyimide material containing silica nanocages (silsesquioxanes). The components, by weight, are: polyamic acid: 100 parts; surface-modified silica nanocages: 12 parts; solvent (pure NMP): 220 parts; catalyst (isoquinoline: pyridine = 1:1): 1.2 parts.

[0052] The preparation method of this comparative example is completely the same as that of Example 2, except that no special modifier is added.

[0053] Comparative Example 4: The difference between this comparative example and Example 2 is that the diamine ratio has been adjusted; the molar ratio of PDA to siloxane-containing diamine has been changed from 5:1 to 1:1. This comparative example provides an OLED substrate polyimide material, with the following components by weight: polyamic acid: 100 parts; surface-modified silica nanocages: 12 parts; modifier (water-soluble acrylic resin: 2-methylimidazole = 2:1): 5 parts; solvent (pure NMP): 220 parts; catalyst (isoquinoline: pyridine = 1:1): 1.2 parts; the polyamic acid is prepared by polymerizing PDA and siloxane-containing diamine in a molar ratio of 1:1 and ODPA and 6FDA in a molar ratio of 1:0.5.

[0054] The preparation method of this comparative example is completely consistent with that of Example 2, except that the diamine monomer ratio is adjusted (the diamine ratio is adjusted so that the molar ratio of PDA to siloxane-containing diamine is 1:1).

[0055] Performance testing: Performance tests were conducted on Examples 1 to 3 and Comparative Examples 1 to 4, respectively. The performance testing methods included the following: (1) Image retention test: OLED devices were prepared using polyimide film as substrate. A fixed black and white image was displayed for a long time (24h) in an environment of 25℃ and 50% humidity. After the display was turned off, the image retention time was recorded by a high-speed camera and compared with the image retention time of existing unmodified polyimide substrates. (2) Heat resistance test: The glass transition temperature (Tg) was determined by differential scanning calorimetry (DSC) at a heating rate of 20℃ / min; the 5% thermal weight loss temperature (Td5%) was determined by thermogravimetric analysis (TGA) at a heating rate of 10℃ / min under nitrogen atmosphere. (3) Optical transparency test: The transmittance of visible light (400-760nm) and the yellowness index were measured using a UV-Vis spectrophotometer; (4) Thermal expansion coefficient test: The thermal expansion coefficient (CTE) in the range of 50-400℃ was determined using a thermomechanical analyzer (TMA). (5) Mechanical property testing: Tensile strength and elongation at break were determined using a universal testing machine at a tensile rate of 5 mm / min and a sample size of 100 mm × 10 mm × 0.03 mm. (6) Adhesion test: The adhesion between the film and the glass substrate and a-Si was determined by cross-cut test; (7) Dispersion test: The dispersion of silica nanocages in polyimide was observed using transmission electron microscopy (TEM) to determine whether agglomeration exists; (8) Dielectric performance test: The dielectric constant (Dk) and dielectric loss factor (Df) were tested at a frequency of 10 GHz using the coaxial resonant cavity method.

[0056] The performance test results of Examples 1 to 3 and Comparative Examples 1 to 4, based on the above performance tests, are shown in the table below: .

[0057] discuss: Analysis of the test results of the examples and comparative examples shows that: (1) The polyimide materials prepared in Examples 1-3 have a significant effect on improving image retention (shortening by more than 80%), which is far superior to the comparative examples. This proves that the surface-modified silica nanocage, the synergistically optimized diamine monomer ratio, and the compounded special modifier are the core elements for improving image retention, and none of them can be omitted. (2) Example 2 (preferred scheme, molar ratio of PDA to siloxane diamine 5:1, 5 parts of special modifier, and 12 parts of silica nanocage) has the best performance in all aspects, and the image retention improvement effect reaches 90%. This proves that the optimal ratio of each component in the present invention achieves synergistic gains in multiple performances. (3) Comparative example 3 shows that without the special modifier, even with silica nanocage and optimized diamine ratio, the image retention improvement effect, adhesion, and dispersibility will all decrease. This proves that the compounded modifier of water-soluble acrylic resin and imidazole compound is a non-conventional setting in the field and can achieve significant performance gains. (4) Comparative Example 4 shows that even with silica nanocages and a special modifier, the image retention improvement effect and heat resistance will significantly decrease when the diamine monomer ratio is not optimized. This proves that the synergistic ratio of PDA to siloxane diamine of 3-7:1 and the modification effect of silica nanocages form a key synergistic effect, solving the performance imbalance problem of the prior art. (5) Comparative Examples 1-2 show that the polyimide system alone or the unmodified silica nanocage system cannot solve the image retention problem and has poor overall performance, further highlighting the necessity of the three core innovations of this invention.

[0058] It should also be noted that the various specific technical features described in the above embodiments can be combined in any suitable manner without contradiction. To avoid unnecessary repetition, the present invention will not describe the various possible combinations separately.

Claims

1. A polyimide material, characterized in that, The polyimide material is prepared by copolymerization-curing reaction of polyamic acid, surface-modified silica nanocages and modifiers in the following proportions: 100 parts polyamic acid, 5-20 parts surface-modified silica nanocages and 2-8 parts modifiers.

2. The polyimide material according to claim 1, characterized in that, The polyamic acid is prepared by polymerizing dianhydride monomer and diamine monomer in a polar solvent, wherein the molar ratio of dianhydride monomer to diamine monomer is 1:0.95-1.

05.

3. The polyimide material according to claim 2, characterized in that, The dianhydride monomer is selected from one or more mixtures of 3,3',4,4'-diphenyl ether tetracarboxylic dianhydride, 4,4'-hexafluoroisopropylphthalic anhydride, 3,3',4,4'-biphenyl tetracarboxylic dianhydride, and benzophenone tetracarboxylic dianhydride; Preferably, the dianhydride monomer is a mixture of 3,3',4,4'-diphenyl ether tetracarboxylic dianhydride and 4,4'-hexafluoroisopropylphthalic anhydride in a molar ratio of 1:0.3-0.

7.

4. The polyimide material according to claim 2, characterized in that, The diamine monomers are p-phenylenediamine (PDA), 4,4'-diaminodiphenyl ether (ODA), and one or more mixtures of siloxane-containing diamines; Preferably, the diamine monomer is a complex system of a rigid aromatic diamine and a flexible siloxane-containing diamine; Preferably, the diamine monomer is a mixture of p-phenylenediamine and siloxane-containing diamine in a molar ratio of 3-7:1; Preferably, the flexible siloxane diamine is an amino-terminated polydimethylsiloxane diamine.

5. The polyimide material according to claim 1, characterized in that, The surface-modified silica nanocages are cage-like silsesquioxanes with the molecular formula (RSiO). 1.5 ) n n can be 8, 10, or 12, with n preferably being 8, and the particle size being 2-8 nm. The surface-modifying groups of the surface-modified silica nanocage are one or more of amino, epoxy, or acyl chloride groups; The surface-modifying groups are mixed modifying groups in which amino and epoxy groups are present in a molar ratio of 1:0.5-1.

5.

6. The polyimide material according to claim 1, characterized in that, The modifier is a compound system of water-soluble acrylic resin and imidazole compound in a weight ratio of 2:

1.

7. The polyimide material according to claim 1, characterized in that, The copolymerization-curing reaction is carried out in a solvent and accelerated by a catalyst; The solvent is selected from N,N-dimethylacetamide (DMAC) and N-methyl-2-pyrrolidone (NMP); The catalyst is a compound system of isoquinoline and pyridine in a weight ratio of 1:

1.

8. A polyimide film, characterized in that, A film obtained using the polyimide material according to any one of claims 1 to 8.

9. A method for preparing a polyimide film, characterized in that, include: Step 1: Prepare polyamic acid; Step 2: Prepare a surface-modified silica nanocage dispersion; Step 3: Slowly add the prepared silica nanocage dispersion and modifier to the polyamic acid solution prepared in Step 1, and stir at room temperature for 2-4 hours to ensure uniform dispersion of each component and obtain a mixed slurry. Step 4: The prepared mixed slurry is uniformly coated on a clean substrate using a doctor blade coating method, with a coating thickness of 5-50 μm. After coating, the product is placed in an oven for gradient curing.

10. The polyimide film according to claim 9, characterized in that, The gradient solidification process includes: Low temperature stage: Curing at 80℃ for 1 hour to remove low-boiling-point solvents and moisture from the slurry; Medium temperature stage: Curing at 100℃ for 2 hours, curing at 150℃ for 3 hours, and curing at 200℃ for 2 hours to promote the initial imidization and crosslinking reaction of polyamic acid; High temperature stage: Curing at 250℃ for 1 hour, curing at 300℃ for 2 hours, curing at 350℃ for 1 hour, curing at 400℃ for 1 hour, and curing at 450℃ for 1 hour. After curing, the film is allowed to cool naturally to room temperature and then peeled off from the substrate to obtain a polyimide film containing silica nanocages.