A transparent polyimide film with a biomimetic rigid-flexible block structure and its preparation method

Abstract

This invention provides a transparent polyimide film with a biomimetic rigid-flexible block structure and its preparation method. The transparent polyimide film is first prepared by a condensation reaction of a rigid diamine and a portion of a dianhydride to obtain a terminal amino-terminated rigid prepolymer. Then, a flexible diamine and the remaining dianhydride are added to react and obtain a block polyamic acid solution. Finally, a nano-crosslinking agent is added, followed by pre-imidization. This transparent polyimide film is composed of a rigid-flexible block polyimide matrix and a nano-POSS crosslinking agent chemically bonded to the matrix network. The matrix and POSS interact through covalent and hydrogen bonds to form a "nano-confined crosslinking" network, achieving zero plastic deformation under repeated bending and near-zero expansion over a wide temperature range. Test results show that the film has a CTE as low as 5.2 ppm / K in the range of 50-250℃ (close to the "zero expansion" level of glass or metal), a plastic deformation rate of <0.05% after 200,000 dynamic bending cycles (R=1mm), and a total light transmittance of >89% and a yellowness index (YI) of <2.0.

Description

Technical Field

[0001] This invention belongs to the field of polymer film technology, specifically relating to a transparent polyimide film with a biomimetic rigid-flexible block structure and its preparation method. Background Technology

[0002] With the increasing popularity of foldable display devices, the transparent polyimide (CPI) film used for their cover glass must simultaneously meet stringent requirements such as high light transmittance (>88%), low yellowing (YI<3), high bending resistance (>200,000 cycles), low surface hardness (>6H), and dimensional stability. Among these, the "dead crease" problem—that is, the irreversible crease or white line that appears in the bending area after the screen has been folded and unfolded multiple times—has become a core pain point restricting user experience. The essence of this problem is that the CPI film undergoes plastic deformation and molecular chain creep under repeated bending stress.

[0003] The main shortcomings of existing technologies are: ① The contradiction between "rigidity" and "toughness": Traditional copolymerization or blending methods are difficult to achieve a "perfect division of labor" between rigid and flexible segments at the molecular scale. Often, the improvement of one performance comes at the cost of the deterioration of another performance.

[0004] ② The conflict between anti-creep and high transparency: To suppress creep, strong cross-linking or high rigidity structures need to be introduced, but this usually leads to yellowing of the film (increased YI) or increased haze due to the formation of charge transfer complexes (CTC) or scattering centers.

[0005] ③ Dispersion problem of nanofillers: Inorganic nanoparticles have high surface energy and are difficult to disperse uniformly in viscous CPI precursors. In addition, their modulus does not match that of the matrix. After long-term bending, they are prone to interfacial microcracks, which leads to a decline in optical performance.

[0006] ④ Poor "thermal-mechanical" coupling stability: Under the combined action of heat and force (such as repeated bending in heating devices), the coefficient of thermal expansion of existing CPI films increases sharply and the slippage of molecular chains intensifies, resulting in irreversible "dead folds" in the bending area. There is a lack of synergistic molecular design that can simultaneously suppress thermal expansion and creep accumulation.

[0007] ⑤ The "tripartite contradiction" between cross-linking strategies and transparency and toughness: Traditional chemical cross-linking can inhibit creep, but it easily leads to film embrittlement and accelerated yellowing; low cross-linking density cannot effectively resist creep. Conventional nanofillers are prone to light scattering, making it difficult to simultaneously meet the triple requirements of low yellowing, high toughness, and high creep resistance.

[0008] Chinese patent CN111205642B uses a copolymer of fluorinated rigid diamine (TFMB) and alicyclic dianhydride (CBDA) to achieve a transparent polyimide film with a CTE below 23 ppm / K and a light transmittance >89%. However, this patent does not address the issue of suppressing plastic deformation after repeated bending, and its high rigidity design makes it difficult to simultaneously meet the 'anti-dead-bend' performance requirements of foldable screens. Summary of the Invention

[0009] The purpose of this invention is to provide a transparent polyimide film with a biomimetic rigid-flexible block structure, which solves the problems of existing transparent polyimide films being prone to "dead folds" (plastic deformation) after long-term bending, as well as poor thermal dimensional stability, which cannot meet the requirements of high-end foldable screen cover plates.

[0010] Therefore, the technical solution provided by the present invention is as follows: A transparent polyimide film with a biomimetic rigid-flexible block structure is obtained by first performing a polycondensation reaction of a rigid diamine and a portion of a dianhydride to obtain a terminal amino-terminated rigid prepolymer, then adding a flexible diamine and the remaining dianhydride to react and obtain a block polyamic acid solution, and finally adding a nano-crosslinking agent to react and pre-imidize.

[0011] The thin film of this invention constructs a rigid-flexible alternating block copolymer through molecular structure design, and introduces a nano-confined crosslinking structure on this basis. Among them, the rigid blocks provide a low coefficient of thermal expansion (CTE) and high modulus, while the flexible blocks provide high toughness and bendability; the nano-confined crosslinking points (such as the nano-crosslinking agent cage-type silsesquioxane POSS) form a physical crosslinking network, restricting the slippage of molecular chains under heat or stress.

[0012] Furthermore, the molar ratio of the rigid diamine to the flexible diamine is 50:50 to 80:20; at this ratio, the rigid blocks can form a through network, and the flexible blocks are connected in the form of "bonds", achieving optimal performance; The ratio of the total molar amount of the rigid diamine and the flexible diamine to the total molar amount of the dianhydride is 1:1; the molar ratio of the total diamine to the dianhydride is strictly 1:1.

[0013] The amount of the nano-crosslinking agent used is 1 to 10 phr.

[0014] Preferably, the molar ratio of rigid diamine to flexible diamine is 60:40 to 70:30, and the amount of nano-crosslinking agent is 3 to 5 phr.

[0015] Further, the rigid diamine is 2,2'-bis(trifluoromethyl)-4,4'-diaminobiphenyl, and the flexible diamine is 1,3-bis(4-aminophenoxy)propane (BAPP) or 4,4'-diaminodicyclohexylmethane.

[0016] Furthermore, the dianhydride is 4,4'-(hexafluoroisopropene)diphthalic anhydride.

[0017] The polyimide film of this invention is a rigid-flexible block copolymer with controllable sequence. The rigid blocks (containing benzimidazole or biphenyl structures) ensure low CTE and high modulus; the flexible blocks (containing ether bonds or aliphatic cyclic structures) provide chain motion and energy dissipation capabilities, preventing brittle fracture. The two are alternately connected, decoupling the contradiction between "rigidity" and "flexibility" at the molecular level.

[0018] Furthermore, the nano-crosslinking agent is an aminocage-type polysilsesquioxane with a particle size of 1~3 nm.

[0019] Specifically, the aminocage-type polysilsesquioxane is aminopropylisobutyl POSS.

[0020] A cage-like polysilsesquioxane (POSS) with a specific structure is introduced as a nano-crosslinking point. The nanoscale size (1-3 nm) of POSS ensures no optical scattering; its peripheral active functional groups (such as amino or epoxy groups) can chemically bond with the polyamic acid matrix to form a uniform physical crosslinking network, inhibiting the slippage and creep of molecular chains during repeated bending. This solves the problem of film brittleness caused by traditional chemical crosslinking.

[0021] The active amino groups on POSS can participate in the formation of polyamic acid in the early stages of polymerization, thereby anchoring them to the polymer chain through chemical bonds and completely avoiding aggregation. FTIR spectroscopy shows that the Si-O-Si characteristic peak (~1110 cm⁻¹) of POSS is uniformly present in the composite film and forms an ionic complex at room temperature. During the subsequent thermal imidization process, it is further transformed into a stable amide crosslinked structure, confirming the chemical bonding.

[0022] The beneficial effects of this invention are as follows: The transparent polyimide film with a biomimetic rigid-flexible block structure provided by this invention is composed of a rigid-flexible block polyimide matrix and a nano-POSS crosslinking agent chemically bonded to the matrix network. The matrix and POSS interact through covalent bonds and hydrogen bonds to form a "nano-confined crosslinking" network, achieving zero plastic deformation under multiple bending and near-zero expansion over a wide temperature range.

[0023] The transparent polyimide film of this invention contains a rigid diamine that provides rigidity and reduces CTE, while fluorinated groups inhibit CTC and ensure transparency; a flexible diamine that provides flexibility and bending durability, balancing rigidity; fluorinated dianhydride that further inhibits yellowing and improves solubility; and a nano-crosslinking agent that provides nano-confined crosslinking points, inhibiting creep and molecular chain slippage.

[0024] The test results of this invention show that the CTE of the film is as low as 5.2 ppm / K in the range of 50-250℃ (close to the "zero expansion" level of glass or metal), the plastic deformation rate is <0.05% after 200,000 dynamic bending cycles (R=1mm), the total light transmittance is >89%, and the yellowness index (YI) is <2.0.

[0025] In the pre-drying stage of thin film preparation, the method of this invention induces microscopic phase separation between rigid and flexible blocks by precisely controlling temperature and time, spontaneously forming a microscopically ordered structure (such as a bicontinuous phase) similar to "reinforcement and toughening". This process does not require complex templates or post-processing and is suitable for large-scale production. Attached Figure Description

[0026] Figure 1 This is a schematic diagram of the gradient imidization process; Figure 2 This is a bar chart comparing CTE and plastic deformation rate for Examples 1-3 and Comparative Examples 1-4. Detailed Implementation

[0027] The following specific embodiments illustrate the implementation of the invention. Those skilled in the art can easily understand other advantages and effects of the invention from the content disclosed in this specification.

[0028] Exemplary embodiments of the invention are now described; however, the invention may be embodied in many different forms and is not limited to the embodiments described herein. These embodiments are provided to fully and completely disclose the invention and to fully convey its scope to those skilled in the art. The terminology used in the exemplary embodiments is not intended to limit the invention.

[0029] Unless otherwise stated, the terms used herein (including technical terms) have their common meaning as understood by one of ordinary skill in the art. Furthermore, it is understood that terms defined in commonly used dictionaries should be understood to have a meaning consistent with the context of their relevant field, and not to be interpreted as having an idealized or overly formal meaning.

[0030] Addressing the main bottlenecks of existing technologies: When pursuing a low coefficient of thermal expansion (high dimensional stability), the increase in rigidity inevitably leads to increased film brittleness and deterioration of creep resistance; conversely, introducing cross-linking or high-rigidity designs to suppress "dead folds" exacerbates yellowing and haze due to the formation of charge transfer complexes. This creates an inherent contradiction where "rigidity-toughness-transparency" are difficult to optimize synergistically. This invention resolves this contradiction at the molecular structure level, overcoming the "weakest link" effect in the overall "thermal-mechanical-optical" performance of existing transparent polyimide films for foldable displays, achieving zero plastic deformation under multiple bends and near-zero expansion over a wide temperature range.

[0031] Example 1 This embodiment provides a method for preparing S1-polyimide films, specifically including: Step 1: Synthesis of amino-terminated rigid polyamic acid prepolymers Under nitrogen protection, rigid diamine TFDB (0.7 mol) was dissolved in DMAc, and dianhydride 6FDA (0.6 mol) was slowly added in an ice-water bath at 0–5 °C. The reaction was carried out for 4 hours to obtain a carboxyl-terminated rigid prepolymer. A small amount of TFDB (0.1 mol) was then added for further end-capping to finally obtain a terminal amino-terminated rigid prepolymer.

[0032] (In this step, the cumulative amount of TFDB is 0.8 mol, the amount of 6FDA is 0.6 mol, and the excess of diamine is 0.2 mol to ensure the terminal amino group.) Step 2: Synthesis of rigid-flexible block polyamic acid copolymer Add flexible diamine BAPP (0.2 mol) and the remaining dianhydride 6FDA (0.4 mol) to the above reaction system, raise the temperature to room temperature, and continue the reaction at 20~25℃ for 8 hours to obtain a block polyamic acid solution with a total solid content of 12%.

[0033] (Cumulative: diamine = 0.8 + 0.2 = 1.0 mol, dianhydride = 0.6 + 0.4 = 1.0 mol, molar ratio 1:1) Step 3: Nano-confined crosslinking Add a measured amount of aminopropyl isobutyl POSS (5 phr) to the block polyamic acid solution and continue stirring for 2 hours to allow it to pre-react with the carboxyl groups on the polymer chain.

[0034] Step 4: Gradient pre-iminolation-assisted self-assembly, such as... Figure 1 As shown; The composite solution was coated onto a clean glass plate and pre-imidized in a gradient precision oven. Zone 1 (60℃, 1h): Slowly remove most of the solvent.

[0035] Second zone (120℃, 2h): Induces microscopic phase separation of rigid and flexible blocks, forming a bicontinuous ordered structure. At this temperature, imine reaction has not yet occurred in large quantities, ensuring the mobility of molecular chains.

[0036] Zone 3 (180℃, 1h): Initial imidization reaction occurs, fixing the already formed micro-ordered structure.

[0037] Step 5: High-temperature thermal imidization Under the protection of high-purity nitrogen (purity ≥99.999%), complete imidization was carried out according to the procedure in Table 1: Table 1 Gradient imidization procedure Step Six: Stripping and Post-processing The film, along with the glass plate, was immersed in deionized water at 60°C and peeled off to obtain a transparent polyimide film with a thickness of 45–55 μm, which was then vacuum dried at 120°C for 4 hours.

[0038] Example 2 This embodiment provides a method for preparing S2-polyimide film, which differs from Example 1 in that the molar ratio of rigid diamine to flexible diamine is 50:50; and the amount of aminopropyl isobutyl POSS used is 3 phr. All other steps are the same.

[0039] Example 3 This embodiment provides a method for preparing S3-polyimide film, which differs from Example 1 in that the molar ratio of rigid diamine to flexible diamine is 70:30; and the amount of aminopropyl isobutyl POSS used is 10 phr. All other steps are the same.

[0040] Comparative Example 1 This embodiment provides a method for preparing D1-polyimide film. The difference from Example 3 is that aminopropyl isobutyl POSS is not added, while the other steps are the same.

[0041] Comparative Example 2 This embodiment provides a method for preparing D2-polyimide film. The difference from Example 3 is that the dianhydride and diamine are added at once to form a random copolymer, and aminopropyl isobutyl POSS is not added. The remaining steps are the same.

[0042] Comparative Example 3 Commercially available CPI films for foldable screens, either Kolon or SKC products. Comparative Example 3 specifically uses an SKC product, denoted as D3.

[0043] Comparative Example 4 This embodiment provides a method for preparing D4-polyimide film, including the following steps: Step 1: Synthesis of random copolymer polyamic acid (1) Add the measured amount of DMAc solvent to a three-necked flask equipped with a mechanical stirrer, thermometer and nitrogen inlet; (2) Under nitrogen protection, add 0.8 mol of rigid diamine 2,2'-bis(trifluoromethyl)-4,4'-diaminobiphenyl TFDB and 0.2 mol of flexible diamine 1,3-bis(4-aminophenoxy)propane BAPP to the flask at one time and stir until completely dissolved; (3) Cool the reaction system in an ice-water bath to 0~5℃, and slowly add 4,4'-(hexafluoroisopropene) phthalic anhydride 6FDA (1.0 mol) in batches while stirring, and control the feeding rate so that the reaction temperature does not exceed 10℃; (4) After the addition is complete, remove the ice water bath and continue to stir the reaction at room temperature (about 25°C) for 8 hours to obtain a homogeneous, viscous random polyamic acid solution with a total solid content of 12%. Step 2: Add conventional epoxy crosslinking agent (1) Weigh 5 phr (relative to the solid content of polyamic acid) of triglycidyl isocyanurate (TGIC) crosslinking agent; (2) Dissolve TGIC in a small amount of DMAc solvent (about 5 mL) and stir thoroughly until completely dissolved; (3) Under stirring, the TGIC solution was slowly added dropwise to the above random polyamic acid solution, and stirring was continued at room temperature for 2 hours to make the crosslinking agent uniformly dispersed in the system and pre-react with the carboxyl groups of the polyamic acid side chain to form a crosslinkable composite solution. Step 3: Apply coating to form a film The composite solution obtained in step 2 is uniformly coated onto a clean glass plate using a scraping method, and the scraper gap is controlled to obtain the required film thickness. Step 4: Gradient pre-iminolation (assisted self-assembly) The glass plate coated with the composite solution was placed in a gradient precision oven and pre-imidized according to the following procedure: Zone 1: 60℃, hold for 1 hour → Remove most of the solvent and form a preliminary solid film; Second zone: 120℃, keep warm for 2 hours → further remove residual solvent; Zone 3: 180℃, heat for 1 hour → Initial imidization reaction occurs, forming a partial imide ring structure, fixing the film morphology; Step 5: High-temperature thermal imidization, same as in Example 1; Step 6: Peeling and Post-processing (1) Immerse the film after thermal imidization together with the glass plate in deionized water at 60°C for about 10 to 15 minutes to allow the film to peel off naturally from the glass plate. (2) Take out the peeled film and rinse it repeatedly with deionized water 3 times to remove surface residue; (3) Place the cleaned film in a vacuum drying oven at 120°C and vacuum dry for 4 hours to completely remove moisture and residual trace solvents; (4) Finally, a uniform, transparent random copolymer chemical crosslinked polyimide film with a thickness of 45-55 μm (denoted as D4) is obtained and sealed for later use.

[0044] To verify the effectiveness of the present invention, performance tests were conducted on the films prepared in Examples 1-3 and Comparative Examples 1-4. The test results are shown in Table 2 and... Figure 2 .

[0045] 1. CTE test Thermomechanical analyzer (TMA) was used, and in accordance with the standard ASTM E831, the film was heated from 50°C to 250°C at a heating rate of 5°C / min under a nitrogen atmosphere. The in-plane thermal expansion coefficient (CTE) of the film was recorded, and the average value of two scans was taken.

[0046] 2. Plastic deformation rate A dynamic bending tester was used to perform 200,000 reciprocating bends at a bending radius of 1 mm and a frequency of 1 Hz. After the test, the surface morphology of the bending area was measured using an optical profilometer. The plastic deformation rate was calculated as (depth of surface indentation after bending / initial film thickness) × 100%.

[0047] 3. Light transmittance The total light transmittance of the film was tested at a wavelength of 550 nm using a UV-Vis spectrophotometer, in accordance with standard ASTM D1003.

[0048] 4. Yellowness Index According to the standard ASTM E313, a colorimeter was used with a D65 light source and a 10° viewing angle to conduct the test, and the yellowness index (YI) value was read directly.

[0049] 5. Tensile strength According to the standard ASTM D882, a universal tensile testing machine was used with a specimen width of 10 mm, a gauge length of 50 mm, and a tensile rate of 10 mm / min to test and record the tensile strength.

[0050] 6. Elongation at break Refer to the ASTM D882 tensile test above and record the elongation at fracture of the specimen.

[0051] Table 2 Performance test results of the examples and comparative examples From Table 2 and Figure 2 It can be seen that, comparing S1 with D1 and D3, after adding only 5 phr of POSS, the plastic deformation rate dropped sharply from 0.58% (D1) and 0.35% (D3) to 0.04%, achieving "zero expansion" and ultra-low creep. This indicates that nano-confined crosslinking is the key to suppressing "dead bending".

[0052] Comparing S1 with D2 and D4: the CTE and plastic deformation rate of the random copolymer (D2) or conventional crosslinking (D4) are significantly higher than those of S1, proving that both "ordered block structure" and "physical crosslinking" are indispensable. Although conventional crosslinking reduces creep, it leads to a sharp increase in film brittleness (elongation at break is only 3%), which is not worth the effort.

[0053] Comparing S1 and S3: While the excessive crosslinking degree (10 phr) further reduces CTE and deformation, it sacrifices flexibility (6% elongation at break), which is detrimental to long-term reliability.

[0054] In summary, this invention achieves near-metal / glass "zero expansion" levels (CTE < 6 ppm / K) in CPI films through rigid-flexible block structure design and nano-POSS physical crosslinking, and reduces the plastic deformation rate after repeated bending to a negligible 0.04%, fundamentally solving the "dead fold" problem of foldable screens. Its overall performance is significantly better than existing technologies.

[0055] This invention fundamentally solves the "dead fold" problem of CPI in foldable screens through a synergistic strategy of "molecular-level ordered structure + nanophysical cross-linking": 1. The source of "zero expansion" and low CTE: The rigid TFDB / 6FDA block has an extended chain conformation, strong intermolecular forces, and weak thermal mobility, providing a "skeleton-like" function that effectively suppresses the in-plane thermal expansion of the entire film. The resulting micro-phase separation structure allows the rigid phase to form a continuous network within the film surface, further limiting overall expansion.

[0056] 2. The source of ultra-high creep resistance / resistance to "dead bending": Uniformly dispersed POSS nanoparticles act as strong physical cross-linking points, firmly locking the molecular chains. When the film is repeatedly bent, these cross-linking points prevent the slippage and rearrangement of the molecular chains, forcing the deformation to be mainly borne by the reversible conformational changes of the flexible blocks, thereby minimizing plastic deformation.

[0057] 3. Source of high transparency and low yellowing: Both 6FDA and TFDB contain a large amount of trifluoromethyl groups, which have high steric hindrance and effectively inhibit the formation of intramolecular and intermolecular charge transfer complexes (CTCs), ensuring the colorless and transparent film. The nanoscale POSS (<3nm) is far below the wavelength of visible light and does not produce scattering.

[0058] The examples above are merely illustrative of the invention and do not constitute a limitation on the scope of protection of the invention. Any design that is the same as or similar to the invention falls within the scope of protection of the invention.

Claims

1. A transparent polyimide film with a biomimetic rigid-flexible block structure, characterized in that, First, a rigid diamine and a portion of the dianhydride undergo a polycondensation reaction to obtain a terminal amino-terminated rigid prepolymer. Then, a flexible diamine and the remaining dianhydride are added to react and obtain a block polyamic acid solution. Finally, a nano-crosslinking agent is added and reacted to obtain a pre-iminoized solution.

2. The transparent polyimide film with a biomimetic rigid-flexible block structure according to claim 1, characterized in that, The molar ratio of the rigid diamine to the flexible diamine is 50:50 to 80:20; The ratio of the total molar amount of the rigid diamine and the flexible diamine to the total molar amount of the dianhydride is 1:1; The amount of the nano-crosslinking agent used is 1 to 10 phr.

3. A transparent polyimide film with a biomimetic rigid-flexible block structure according to claim 1, characterized in that, The rigid diamine is 2,2'-bis(trifluoromethyl)-4,4'-diaminobiphenyl, and the flexible diamine is 1,3-bis(4-aminophenoxy)propane (BAPP) or 4,4'-diaminodicyclohexylmethane.

4. A transparent polyimide film with a biomimetic rigid-flexible block structure according to claim 1, characterized in that, The dianhydride is 4,4'-(hexafluoroisopropene) phthalic anhydride.

5. A transparent polyimide film with a biomimetic rigid-flexible block structure according to claim 1, characterized in that, The nano-crosslinking agent is an amino cage-type polysilsesquioxane with a particle size of 1~3nm.

6. A method for preparing a transparent polyimide film with a biomimetic rigid-flexible block structure as described in any one of claims 1 to 5, characterized in that, Includes the following steps: Step 1) Under nitrogen protection, a rigid diamine is dissolved in N,N-dimethylacetamide, and a portion of dianhydride is slowly added under an ice-water bath. The reaction is carried out for 3-5 hours to obtain a carboxyl-terminated rigid prepolymer. A small amount of rigid diamine is then added for end-capping to obtain an amino-terminated rigid prepolymer. Step 2) Add flexible diamine and the remaining dianhydride, and after the reaction, obtain a block polyamic acid solution; Step 3) Add the nano-crosslinking agent to the block polyamic acid solution and stir to carry out the pre-reaction; Step 4) Apply the composite solution onto a clean glass plate and place it in a gradient precision oven for pre-imidization; Step 5) Under high-purity nitrogen, high-temperature gradient thermal imidization is performed to obtain a thin film; Step 6) Immerse the film along with the glass plate in deionized water at 55~65℃ to peel it off, and obtain a transparent polyimide film with a thickness of 45~55μm. Then, vacuum dry it at 110~120℃ for 4~5 hours.

7. The method for preparing a transparent polyimide film with a biomimetic rigid-flexible block structure according to claim 6, characterized in that, In step 1), the molar ratio of the two rigid diamines is 6-7:1, and the amount of rigid diamine used is in excess relative to the dianhydride.

8. The method for preparing a transparent polyimide film with a biomimetic rigid-flexible block structure according to claim 6, characterized in that, In step 2), the molar ratio of flexible diamine to dianhydride is 1:

2.

9. The method for preparing a transparent polyimide film with a biomimetic rigid-flexible block structure according to claim 6, characterized in that, In step 4), the pre-imidization method in a gradient precision oven is as follows: Zone 1: At 60~70℃, for 1~1.5h, most of the solvent is removed; Second zone: At 110~125℃, for 2~2.5h, rigid and flexible segments form a double continuous ordered structure; Third zone: At 170~180℃, for 1~1.5h, the initial imidization reaction occurs, fixing the already formed micro-ordered structure.

10. The method for preparing a transparent polyimide film with a biomimetic rigid-flexible block structure according to claim 6, characterized in that, The high-temperature gradient thermal imidization process described in step 5) is as follows: The first stage involves heating from 180℃ to 230℃ at a rate of 1.0℃ / min, and holding at that temperature for 30~40 minutes. The second stage involves heating from 230℃ to 280℃ at a rate of 0.5℃ / min and holding at that temperature for 60-70 minutes. The third stage involves heating from 280℃ to 320℃ at a rate of 0.5℃ / min and holding at that temperature for 30-40 minutes. Annealing section: heat from 320℃ to 200℃ at a rate of 0.5℃ / min, and hold for 60~70min; Natural cooling brought it down from 200°C to room temperature.

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