Method, system and polyimide film for improving transparency by external electric field-induced polyimide molecular alignment

Abstract

The application discloses a method and system for improving transparency of polyimide molecular arrangement induced by an external electric field and a polyimide film. The method comprises the following steps: S1, preparing a polyimide film-forming solution, which is a polyimide precursor solution or a soluble polyimide solution; S2, coating the polyimide film-forming solution on a conductive substrate to form a wet film; and S3, applying an external electric field to the wet film during heat treatment of the wet film, so that the wet film is solidified to form a polyimide film. The system comprises a heating table, a conductive substrate, an insulating gasket, an electrode and an electric field generating device. The polyimide film is prepared by the above method. The application realizes spatial rearrangement of molecular chains by an electric field, increases the average distance between molecules and changes the stacking angle, greatly reduces the probability of electronic transition, thereby weakening the electronic transition probability of color-producing groups that absorb visible light without changing the chemical structure, and significantly improves the light transmittance.

Description

Technical Field

[0001] This invention belongs to the field of polymer material processing technology, and more specifically, relates to a method, system, and polyimide film for improving transparency by inducing the molecular arrangement of polyimide under an external electric field. Background Technology

[0002] Polyimide (PI) is a special engineering plastic that is widely used because it can maintain excellent mechanical properties, electrical insulation, chemical stability and excellent heat resistance at both extremely high and extremely low temperatures.

[0003] As the global electronics industry rapidly evolves towards flexibility, miniaturization, and high frequency, polyimide films have become a key fundamental material in fields such as flexible display panels (e.g., flexible OLED substrates), microelectronic packaging, 5G high-frequency communications, and aerospace. In the field of flexible displays, cover or substrate materials not only require extremely high mechanical reliability and thermal stability but also impose extremely stringent requirements on optical transparency. However, due to the unique structure of the PI molecular chain, traditional polyimide films often exhibit a deep yellow or even brown color, a color defect that limits their application in high-end optical devices.

[0004] The reason why polyimide films have color lies in the charge transfer complex (CTC) formed within and between the molecular chains. This is because the PI molecule is usually composed of alternating electron-rich diamine units (donors) and electron-deficient dianhydride units (acceptors). During the curing or film formation process of PI, the molecular chain segments are tightly stacked, and the orbitals between the diamine and dianhydride overlap. This makes it easier for electrons to jump from the diamine units to the dianhydride units. This transition energy level happens to fall in the visible light region, generating a strong electron absorption band, which makes the polyimide film exhibit obvious color and significantly reduces its light transmittance.

[0005] Without altering the chemical structure of PI molecules, traditional film-forming processes (such as spin coating, blade coating, and slot extrusion coating) mainly rely on mechanical forces to regulate molecular arrangement. The applied shear or tensile forces act almost entirely on the horizontal plane (XY plane) of the polyimide film. This causes PI molecular chain segments (especially rigid benzene rings) to tend to adopt a planar arrangement parallel to the substrate. This high degree of in-plane orientation not only fails to effectively suppress the formation of inter-chain CTCs, but may also deepen the color due to close packing. Summary of the Invention

[0006] To address the aforementioned deficiencies or improvement needs of existing technologies, this invention provides a method, system, and polyimide film for improving transparency by inducing the molecular alignment of polyimide under an external electric field. By applying an external electric field during the heat treatment of polyimide, the polymer molecular chains (especially benzene ring structures) are actively oriented and aligned in three-dimensional space, thereby inhibiting the formation of charge-transfer complexes from the source. This further enhances the transparency of the polyimide film based on existing technologies, thereby improving the key performance of the product.

[0007] To achieve the above objectives, according to the present invention, a method for improving transparency by inducing the molecular alignment of polyimide under an external electric field is provided, characterized by comprising the following steps: S1. Prepare a polyimide film-forming solution, wherein the polyimide film-forming solution is selected from a polyimide precursor solution or a soluble polyimide solution; S2. The polyimide film-forming solution is coated onto a conductive substrate to form a wet film on the conductive substrate. S3. During the heat treatment of the wet film, an external electric field is applied to the wet film to solidify it into a polyimide film. The external electric field is formed by connecting an electric field generating device to a conductive substrate and an electrode. The electrode is located above the wet film and there is a gap between the electrode and the wet film. The external electric field causes uneven distribution of electric field force in the wet film, which induces the polyimide molecular chains to spatially align.

[0008] The external electric field causes uneven distribution of electric force within the wet film, driving the flow of the polyimide film-forming solution and inducing the spatial orientation of molecular chains in conjunction with the electric field force.

[0009] An external electric field induces a non-uniformly distributed electric force within the wet film. This electric force gradient drives controlled flow of the polyimide film-forming solution at the microscale. The shear force generated by this flow effectively propels the polyimide molecular chains (or their precursor segments) from a state of random coiling and deep entanglement along the flow direction. The intervention of flow significantly reduces the frictional resistance of the molecular chains transitioning from a disordered to an ordered state, allowing the electric field to achieve spatial control of the molecular chains with lower energy consumption. Through the combined action of the electric field and the flow shear force, the benzene rings in the polyimide molecules are forced into a spatially stacked arrangement, which fundamentally inhibits the formation of charge-transfer complexes (CTCs).

[0010] Preferably, in step S1, the polyimide film-forming solution is a polyimide precursor solution, and the polyimide precursor solution is a polyamic acid solution; In step S3, the heat treatment is an imidization heat treatment, and the specific heat treatment process is as follows: First, the wet film is kept at 80℃~100℃ for 30 minutes to 60 minutes, then the temperature is increased to 150℃~200℃ at a rate of 5℃~10℃ / minute, and kept at 150℃~200℃ for 30 minutes to 60 minutes, then the temperature is increased to 300℃~350℃ at a rate of 5℃~10℃ / minute, and finally kept at 300℃~350℃ for 30 minutes to 90 minutes.

[0011] The initial setting stage is from 80℃ to 100℃. The main effect of this stage is to control the smooth and slow evaporation of the solvent. This slow solvent removal process ensures that the wet film remains within a low viscosity range, allowing the uneven distribution of the electric field induced by the external electric field to fully drive the solution flow and form the initial orientation of the molecular chains. Simultaneously, it avoids blistering or micropore defects on the film surface caused by drastic temperature increases, ensuring the optical uniformity of the polyimide film.

[0012] The 150℃~200℃ stage is the transitional curing stage. As the temperature rises, the imidization reaction (ring-closing reaction) begins to occur on a large scale. At this time, the molecular chain transforms from PAA to PI, and the wet film viscosity rises rapidly. The technical advantage of this stage is that the electric field force continues to act during this process, capturing the already formed orientation state and gradually fixing it as the cyclization reaction proceeds.

[0013] During the initial shaping and transition curing stages, the uneven distribution of the electric field within the wet film drives controlled flow of the polyimide film-forming solution. This micro-flow, combined with the torque generated by the electric field, creates a synergistic effect, forcing the molecular chains to align spatially along the direction of the electric field. Compared to mechanical stretching, this inside-out driving method results in a more uniform molecular arrangement of the polyimide film in the thickness direction, thus significantly improving the overall transparency of the polyimide film.

[0014] Complete imidization stage (300℃~350℃): The high temperature ensures that the degree of imidization approaches 100%, eliminating residual carboxyl groups and other chromophores, thus guaranteeing basic transparency from a chemical perspective. At this temperature, the molecular chain undergoes final rearrangement and thermal stress release. Under the synergistic effect of an external electric field, the benzene ring structure, which originally tended to be closely stacked in plane, is induced into a spatial conformation that is more conducive to light transmission.

[0015] By using a three-stage heat treatment process that limits the formation of a wet film from a polyamic acid precursor, a high degree of synchronization between the chemical reaction process (imidization) and the external electric field induction is achieved in both time and space. Utilizing the high responsiveness of the PAA stage, the viscosity change window is precisely controlled through gradient temperature control, thereby efficiently suppressing the CTC effect before the molecular chains freeze, ultimately obtaining a polyimide film with excellent optical transparency and stable quality.

[0016] Preferably, the polyamic acid solution is prepared by reacting a polyamic acid diamine monomer and a dianhydride monomer in a polar aprotic solvent; the diamine monomer is selected from 4,4'-diaminodiphenyl ether, p-phenylenediamine, or a fluorinated diamine; and the dianhydride monomer is selected from pyromellitic dianhydride, biphenyl dianhydride, or hexafluorodianhydride.

[0017] By selecting these monomers, this approach ensures that the molecular chains possess sufficient electrosensitivity during heat treatment, allowing the molecular chain segments (especially the rigid portions containing numerous benzene rings) to align orderly with the electric field lines. This arrangement directly optimizes the molecular stacking conformation in three-dimensional space, providing structural assurance for improved transparency.

[0018] Preferably, in step S1, the polyimide film-forming solution is a soluble polyimide solution; In step S3, the heat treatment is a hot annealing treatment, and the specific heat treatment process is as follows: First, hold at 80℃~120℃ for 30 minutes~90 minutes, and then raise the temperature at a rate of 5℃~10℃ / minute to (T g +10℃) ~ (T g +30℃), in (T g +10℃) ~ (T g Keep warm at a temperature of +30℃ for 30 to 60 minutes, where T g This is the glass transition temperature of the soluble polyimide.

[0019] Using imidized soluble polyimide as the film-forming material, the heat treatment process mainly involves solvent evaporation and molecular chain relaxation, rather than complex imidization reactions. The rearrangement process is accompanied by small volume shrinkage, which allows the molecular spatial orientation arrangement induced by the external electric field to be maintained more stably, thus exhibiting extremely high optical uniformity and consistent light transmittance on a macroscopic scale.

[0020] Holding the film at 80℃~120℃ for 30~90 minutes ensures that most of the solvent evaporates steadily at a controlled rate. This prevents microbubbles or skin collapse caused by the solvent boiling violently at ultra-high temperatures, maintaining the optical-grade flatness of the polyimide film.

[0021] In T g During the annealing process at 10℃~30℃, in conjunction with the induction of an external electric field, not only is molecular orientation achieved within the wet film, but the mechanical stress remaining from the coating process is also released simultaneously. This stress relaxation effect further reduces the haze of the polyimide film and improves its light transmittance.

[0022] Using (T) g +10℃) ~ (T gAt temperatures exceeding +30°C, soluble polyimide polymers transition from a glassy state to a rubbery state. The molecular chains gain sufficient energy to overcome the internal rotation barrier, resulting in a significant increase in free volume. At this temperature, the electrostatic torque generated by an external electric field can easily drive the rigid polyimide chain segments to rotate in space. Applying an external electric field within this temperature range allows the molecular chains to achieve optimal spatial rearrangement in accordance with the electric field lines, maximizing the average distance between molecules or altering the stacking conformation of the benzene rings. This conformational adjustment, completed in a softened state, more effectively suppresses the formation of charge-transfer complexes (CTCs), thereby significantly improving transparency.

[0023] The high-temperature range is strictly limited to (T) g +10℃) ~ (T g The +30℃ annealing temperature ensures sufficient chain segment kinetic energy while avoiding membrane sagging caused by excessive polymer thermal degradation or over-softening due to excessively high temperatures. A short annealing period of 30-60 minutes completes the entire process of molecular conformation induction, rearrangement, and fixation, significantly improving production efficiency while ensuring the continuous stability of the external electric field throughout the viscoelastic window.

[0024] By combining a soluble polyimide solution with two-stage annealing, an ideal external electric field control window is created. The soluble polyimide does not require further chemical transformation; instead, within the rubber-state temperature range where molecular chain activity is highest, the spatial arrangement of the molecular chains is precisely guided by the external electric field, thereby locking in a highly transparent structure with low CTC effect with extremely high process efficiency. This approach provides a stable and significantly improved technique for the fabrication of high-performance, ultra-transparent flexible electronic substrates.

[0025] Furthermore, a heating rate of 5℃~10℃ / min avoids violent solvent boiling, reduces bubbles, pinholes, or skin collapse on the wet film surface, and ensures the optical uniformity of the polyimide film. Slow heating combined with an electric field lowers the energy barrier for molecular chain de-entanglement, promoting orderly alignment of molecular chains along the electric field direction. It also reduces van der Waals forces and π-π stacking interactions between molecular chains, enhancing the regulatory effect of the external electric field on molecular conformation and suppressing CTC formation.

[0026] Preferably, in step S1, the soluble polyimide solution is formed by dissolving soluble polyimide in a solvent, wherein the soluble polyimide is a chemically modified polyimide, wherein the chemical modification includes at least one of introducing fluorine-containing groups, introducing alicyclic structures, introducing bulky side groups, or introducing distorted non-coplanar structures.

[0027] Fluorine-containing groups possess extremely high electronegativity and low electronic polarizability. During the external electric field induction process, fluorinated polyimides not only exhibit high transparency due to their inherent chemical structure, but their molecular chains also achieve more efficient ordered arrangement under the influence of the electric field. This arrangement can more thoroughly block electron transitions between diamine and dianhydride units, thereby further optimizing optical uniformity on top of already extremely high transparency.

[0028] The introduction of alicyclic structures breaks the high conjugation of fully aromatic polyimides. Under the drive of an external electric field, due to the moderate rigidity and lack of stacking kinetics of the alicyclic structure, the micro-flow induced by the external electric field can more easily drive these chain segments to rotate in space. This allows the polyimide film to maintain an extremely low absorption coefficient after curing, improving the overall transmittance in the visible light region.

[0029] The large-volume side groups significantly increase the average distance between polymer chains through strong steric hindrance. In this loosely aggregated structure, the uneven distribution of the electric field induced by the external electric field can more effectively drive the molecular chain flow and reduce chain entanglement. As a result, the orientation alignment induced by the external electric field can cover a wider range of polyimide film thicknesses, ensuring that thick films also possess excellent transparency.

[0030] The twisted non-coplanar structure fundamentally disrupts the planar regularity of the molecular chains. Under the induction of an external electric field, this twisted conformation is endowed with spatial orientation. The electric field can precisely control the deflection angle of these twisted chain segments, thereby finely controlling the birefringence and in-plane stress of the polyimide film, improving transparency while giving the polyimide film better dimensional stability.

[0031] These modified structures exhibit excellent kinetic response speed after entering the rubber state, enabling the molecular rearrangement induced by the external electric field to be completed and locked in a very short time, significantly improving production efficiency and consistency of finished product quality.

[0032] Therefore, by actively designing the chemical structure, a more responsive target is provided for the intervention of the electric field. Utilizing the interaction between the modified groups and the external electric field, the originally passive and random molecular arrangement is transformed into a controlled and ordered spatial orientation, thereby minimizing the color-forming effect at the molecular stacking level. This approach ensures that the polyimide film maintains high heat resistance and mechanical strength while achieving improved transparency.

[0033] Preferably, when the chemical structure is modified to introduce fluorinated groups, the soluble polyimide with introduced fluorinated groups is a fluorinated polyimide, which is obtained by polymerization of a monomer containing hexafluorodianhydride; the groups in hexafluorodianhydride have extremely high electronegativity and extremely low electronic polarizability. When an external electric field is applied, these strongly electron-withdrawing groups significantly change the local dipole moment orientation of the molecular chain. The introduction of hexafluorodianhydride increases the free volume between molecular chains. During heat treatment, the electric field induces these fluorinated segments to spatially align, which not only chemically inhibits the formation of charge-transfer complexes (CTCs), but also, through alignment locking, enables the polyimide film to achieve extremely high light transmittance.

[0034] Preferably, when the chemical structure is modified to introduce an alicyclic structure, the soluble polyimide with the introduced alicyclic structure is an alicyclic polyimide. The alicyclic structure lacks electronic conjugation, which inherently reduces the color of the polyimide film. Due to the lack of rigid stacking forces in aromatic structures, the alicyclic structure exhibits better molecular chain mobility at heat annealing temperatures 10°C-30°C higher than the glass transition temperature. This allows the electrostatic torque generated by the external electric field to more easily drive the alicyclic chain segments to rearrange.

[0035] Preferably, when the chemical structure is modified to introduce bulky side groups, the monomer used for the soluble polyimide with the bulky side groups is triphenylmethyldiamine. Triphenylmethyl has significant steric hindrance, which can strongly expand the polyimide molecular chains. Under the action of an external electric field, the uneven distribution of electric force generated in the wet film will drive the solution flow. Since the triphenylmethyl side groups significantly reduce inter-chain entanglement, the molecular chains can more smoothly complete the spatial orientation alignment during the flow. This structure ensures that the electric field can precisely control the orientation of the side groups, thereby blocking the CTC effect at the microscopic scale.

[0036] Preferably, when the chemical structure is modified to introduce a twisted, non-coplanar structure, the monomer used in the soluble polyimide with the introduced twisted, non-coplanar structure is a non-coplanar dianhydride. The twisted, non-coplanar structure causes the PI molecular chains to exhibit an asymmetric stereoconformity. This structure exhibits a spatial orientation effect under the induction of an external electric field. Compared to the ordinary structure, the twisted structure is more sensitive to changes in electric field strength. The external electric field induces these twisted molecular chains to align at spatial angles, thereby significantly reducing absorption in the visible light region without altering chemical stability.

[0037] By defining specific monomers such as hexafluorodianhydride, alicyclic structures, triphenylmethyldiamine, and non-coplanar dianhydrides, a chemical system highly matched to an external electric field was constructed. These structures, already possessing preliminary transparency advantages at the chemical level, significantly enhance the sensitivity of the molecular chains to electric field forces through dipole moments and spatial morphology. This deep integration of chemical pretreatment and precise external electric field control allows the polyimide molecular chains to overcome the orientation limitations of traditional film-forming processes, forming an ordered aggregated structure during curing that completely suppresses the CTC color-forming effect from its source. This not only improves the transparency of the polyimide film but also ensures optimized optical isotropy in both the thickness and planar directions.

[0038] Preferably, in step S1, the solid content of the polyimide film-forming solution is 5% to 25% by weight.

[0039] When the solid content is within 5% to 25%, the polyimide film-forming solution exhibits moderate fluidity. The low viscosity ensures that the electric field gradient generated by the external electric field can overcome the internal friction of the liquid, inducing microfluidic motion. This controlled flow can drive the molecular chains to shift and rotate within the wet film, thereby synergistically interacting with the electric field to induce efficient spatial orientation of the molecular chains (especially the benzene ring structure).

[0040] When the solid content is less than 5%, the wet film is in a semi-dilute solution or a mildly concentrated solution state. At this time, the entanglement density between molecular chains is moderate, allowing the polar groups in the imide ring or precursor to deflect with a low energy barrier when subjected to a strong electric field torque. The appropriate solid content ensures that the molecular chains have sufficient free volume for fine-tuning during heat treatment. This spatial flexibility allows the external electric field to more thoroughly disrupt the tight stacking between molecular chains, thereby effectively suppressing the formation of charge-transfer complexes (CTCs) from the source during curing. When the solid content exceeds 25%, the initial viscosity of the film-forming solution will increase significantly. Excessive viscosity will generate huge internal frictional resistance, making the electric field gradient generated by the external electric field insufficient to overcome the motion resistance of the liquid. This results in the solution failing to generate effective microflow. Due to the lack of flow synergy, the electric field force is unable to drive the highly entangled molecular chain segments to rearrange spatially, directly leading to a significant reduction in the synergistic induction effect of the external electric field.

[0041] Within a solid content range of 5% to 25%, the wet film contains sufficient solvent. During the first stage of the stepped heating process, the gradual evaporation of the solvent allows the polyimide film to remain in a viscoelastic state for a relatively long time, providing ample time for the external electric field to establish molecular orientation. Appropriate solid content ensures that the polyimide film does not shrink excessively due to too much solvent during drying, nor does it freeze the molecular orientation prematurely before imidization is complete due to too little solvent. This results in a final polyimide film with improved transparency and excellent surface smoothness.

[0042] By limiting the solid content to 5%~25%, it essentially provides a medium environment with optimal kinetic response for the external electric field-induced process. Under the premise of ensuring the morphological integrity of the polyimide film, it maximizes the use of the fluidity and low entanglement characteristics of the solution to efficiently convert the applied electric field force into the spatial orientation energy of the molecular chain, thereby constructing some aggregated structures that can suppress visible light absorption during imidization or thermal annealing.

[0043] Preferably, the solvent of the polyimide film-forming solution is selected from one or more of N-methylpyrrolidone, N,N-dimethylacetamide, N,N-dimethylformamide, γ-butyrolactone, or cyclopentanone.

[0044] These highly polar solvents serve as efficient dielectrics, ensuring that the electric field effectively penetrates the wet film surface and acts on the internal polyimide or its precursor molecular chains. The polarity of the solvent molecules synergistically interacts with the polar groups on the polymer chains (such as the carbonyl group in the imide ring), enhancing the electrostatic torque generated by the external electric field on the molecular chains. This environment makes the molecular chains more electrosensitive in the early stages of heat treatment, thus facilitating spatial orientation deflection and providing a prerequisite for suppressing the formation of charge-transfer complexes (CTCs).

[0045] The solvents listed above have excellent compatibility with the polyimide system. The good solvation effect reduces the interaction force between molecular chains, so that when the molecular chain segments are driven by the electric field, they can achieve fine-tuning of their positions in three-dimensional space at a lower energy cost.

[0046] Under the action of the electrodes, the steady evaporation of the solvent reduces the Marangoni effect caused by the surface tension gradient. Combined with the regulation of electric field convection, the final polyimide film achieves three-dimensional orientation optimization while maintaining mirror-level smoothness, which is crucial for the subsequent processing of flexible optical devices.

[0047] Preferably, in step S3, the electric field strength of the external electric field is 400V / mm to 2000V / mm.

[0048] Under an electric field strength of 400 V / mm to 2000 V / mm, the external electric field interacts with the dipoles in the molecular chain, generating sufficient electrostatic torque. This level of electric field strength is sufficient to induce the molecular chain segments in a thermally activated state to align in three-dimensional space. By driving the rearrangement of the molecular chain, the spatial distance between the electron donor and acceptor is effectively increased or their relative angle is changed, thereby suppressing the formation of charge-transfer complexes (CTCs) at the source.

[0049] An electric field strength of 400V / mm to 2000V / mm will cause uneven distribution of electric force inside the wet film. This electric force gradient drives the polyimide film-forming solution to generate controlled micro-flow. This flow can drive the entangled molecular chains to unfold. The shear force generated by the flow and the electric force produce a synergistic gain, which greatly improves the efficiency of the spatial orientation of the molecular chains along the electric field direction.

[0050] A lower limit of 400 V / mm ensures that even in the later stages of imidization where viscosity gradually increases, the electric field can overcome the resistance to molecular motion and maintain the effectiveness of molecular orientation. An upper limit of 2000 V / mm fully considers the breakdown field strength of air and the polyimide solvent system, ensuring that no arcing or film breakdown occurs under electrode conditions. This guarantees the optical integrity of the polyimide film surface and avoids surface instability (such as Taylor cones or wrinkling) that may result from excessively high electric fields. This ensures that the polyimide film maintains extremely high surface smoothness and uniformity while improving transparency.

[0051] An electric field strength of 400V / mm to 2000V / mm essentially locks in a processing window with strong induction, high safety, and wide adaptability. It utilizes an electrostatic torque sufficient to overcome the molecular rotation energy barrier, combined with the microfluidic effect induced by the external electric field, to forcibly regulate the spatial aggregation state of polyimide molecular chains without changing the chemical structure. This results in improved product transparency and ensures the stability of the production process.

[0052] Preferably, in step S3, the external electric field is a DC electrostatic field or an AC electric field with a frequency not greater than 10000Hz.

[0053] When a DC electrostatic field is used, this technical solution exhibits extremely strong directional stability: the DC electric field provides a constant and continuous electrostatic torque. Throughout the entire process of the wet film transitioning from low to high viscosity, this constant force forces the benzene ring structure to maintain a consistent tilt or alignment along the electric field lines. The continuous DC field helps overcome intermolecular van der Waals forces, achieving a larger-scale ordered arrangement. This constant induction results in a significant and stable increase in the transmittance of the final polyimide film.

[0054] The alternating current (AC) electric field mode exhibits a dynamic adjustment effect: the low-frequency alternating electric field induces minute oscillations in the molecular chains. This dynamic force field helps entangled molecular chains more easily dissociate from their original cluster state, thereby lowering the energy threshold required to achieve oriented alignment. Compared to a direct current (DC) field, the AC electric field effectively prevents space charge accumulation at the interface between the wet film and the electrode, avoiding optical defects on the film surface caused by local electric field distortion, and ensuring that the polyimide film possesses excellent optical uniformity and extremely low haze. The frequency range below 10,000 Hz ensures that the massive PI / PAA molecular chains and their polar groups (such as the carbonyl groups in the imide ring) can rotate or shift in response to changes in the electric field direction. If the frequency is too high, the molecular chain segments will not be able to respond to the electric field direction in time due to inertia and internal friction, resulting in the disappearance of the dipole orientation effect; while the setting below 10,000 Hz allows the electric field force to effectively act on the chain segments, driving them to oriented in three-dimensional space, suppressing the formation of charge-transfer complexes (CTCs) from the source. At frequencies below 10,000 Hz, the forced flow rate of the solution is ideally synchronized with the orientation response of the molecular chain segments. The combined effect of the shear force generated by the flow and the torque generated by the external electric field enables the molecular chain segments (especially structures containing large-volume side groups or fluorine-containing groups) to achieve spatial orientation with maximum efficiency, thereby improving the transparency of the product.

[0055] Both DC electrostatic fields and AC electric fields can induce uneven distribution of electric force inside the wet film.

[0056] Preferably, in step S3, the electrode is a mesh.

[0057] The mesh electrode, with its open structure, provides an unobstructed diffusion channel for solvent vapor on the wet film surface. Compared to closed plate electrodes, the mesh structure prevents solvent condensation or the formation of a saturated layer within the narrow space between the wet film and the electrode. This excellent mass transfer ensures that the solvent can smoothly and rapidly leave the film during the heat treatment stage. This effectively prevents blistering, pinholes, or skin collapse caused by solvent retention, thus guaranteeing that the polyimide film possesses an optically smooth surface and excellent optical uniformity.

[0058] The mesh electrode generates a periodically fluctuating electric field distribution at the microscale. A significant difference in electric field line density exists between the area below the electrode metal lines and the area below the mesh, thus creating a precise electric field intensity gradient on and within the wet film surface. This gradient directly leads to an uneven distribution of the electric field force within the wet film, thereby driving controlled microflow in the polyimide film-forming solution. This microflow, in synergy with the electric field force, more effectively induces the spatial orientation of entangled molecular chain segments (especially imide rings or precursor structures with dipole moments).

[0059] Under the applied electric field strength, the microscopic non-uniform electric field induced by the mesh electrode can more profoundly interfere with the aggregation state of molecules. It forcibly induces the molecular chains to deflect in three-dimensional space. This efficient rearrangement can suppress the color-forming effect of charge-transfer complexes (CTCs) from the source. Experimental results show that, with the mesh electrode-assisted process, the transmittance of the polyimide film at 550 nm is significantly improved compared with the sample without an applied external electric field, demonstrating the enhancing effect of the mesh structure on transparency.

[0060] During heat treatment (including imidization or hot annealing), the mesh electrodes allow for effective heat convection exchange between the electrodes and the substrate. This ensures that the wet film maintains a high degree of uniformity in its heating process while being induced by an external electric field. The uniform thermal field distribution, combined with stable electrostatic induction, achieves an ideal balance between the thermal stress generated inside the polyimide film and the orientation force induced by the external electric field. This results in a polyimide film with not only high transparency but also extremely low internal stress distribution, improving the dimensional stability and mechanical properties of the polyimide film.

[0061] Mesh electrodes can generate extremely high density of electric field singularities. These tiny electric field features can act specifically on the submicroscopic structure of polyimide, ensuring that the electric field force can penetrate into every entanglement node of the molecular chain, thereby improving the key performance of the product.

[0062] Therefore, the mesh electrode provides a highly efficient mass transfer and dynamically driven processing environment for the curing process of polyimide films. The permeability of the mesh structure ensures high-quality solvent removal, while the generated microscopic non-uniform electric field effectively drives molecular flow and directional rearrangement within the wet film. This design, while ensuring optical-grade surface quality, significantly improves the precision of controlling the spatial orientation of PI molecular chains, making it a key engineering guarantee for achieving high transparency in polyimide films.

[0063] Preferably, the mesh diameter of the electrode is 0.5μm to 10μm, and the distance between the electrode and the surface of the wet film is 10μm to 500μm.

[0064] The extremely small mesh size of 0.5μm to 10μm generates a high-density electric field singularity in an external electric field, causing the electric force distribution on the wet film surface to exhibit periodic variations in strength on the μm scale. Driven by this microscopic non-uniform electric field, the polyimide film-forming solution exhibits microscopic directional flow. This flow provides sufficient shear force to drive the unwinding of entangled polyimide molecular chain segments, thereby significantly reducing the kinetic energy barrier for the spatial orientation of the molecular chains. This microscopic flow, in conjunction with the electric field force, induces the non-overlapping arrangement of molecular chains (especially benzene ring structures) in three-dimensional space, thus interrupting the formation path of charge-transfer complexes (CTCs).

[0065] Within a distance range of 10μm to 500μm, when an external electric field is applied, the effective electric field force acting on the wet film is strong enough to forcefully induce molecular alignment, while effectively preventing electrical breakdown or corona discharge in the air gaps. This ensures that no electrical damage or microbubbles are generated on the surface of the polyimide film, guaranteeing that the polyimide film has optical-grade surface flatness.

[0066] When the distance is controlled within 10 μm to 500 μm and the mesh size is between 0.5 μm and 10 μm, the electric field lines can penetrate deeper into the wet film. This allows not only the film surface but also the polyimide molecular chains in the central region of the polyimide film to experience a significant directional torque, achieving a high degree of consistency in the optical properties of the polyimide film in the thickness direction (Z-axis).

[0067] By defining the distance and mesh size, a miniaturized, highly responsive control mechanism is essentially constructed above the wet film. Microscopic non-uniform electric fields precisely trigger microscopic convection and directional rearrangement of molecules, while the extremely close spatial arrangement ensures a precise balance between efficient conversion of electric field forces and solvent evaporation. This design not only achieves a significant leap in transparency but also reshapes the aggregated structure of polyimide at the microscopic level, endowing the polyimide film with excellent optical stability and surface quality.

[0068] A system for implementing the method of improving transparency by inducing polyimide molecular alignment using an external electric field includes a heating stage, a conductive substrate, an insulating pad, electrodes, and an electric field generating device, wherein: The conductive substrate is placed on the heating stage; The conductive substrate supports the insulating pad, the insulating pad supports the electrode, and there is a gap between the electrode and the conductive substrate; The electric field generating device is connected to the conductive substrate and the electrode, respectively.

[0069] A high-transparency polyimide film is prepared by the method described above for improving transparency by inducing the polyimide molecular alignment using an external electric field.

[0070] In summary, compared with the prior art, the above-described technical solutions conceived by this invention can achieve the following beneficial effects: 1) The method of improving transparency of polyimide molecules by inducing external electric field alignment according to the present invention constructs an external electric field environment by setting electrodes above the wet film and together with the conductive substrate. The polyimide precursor or soluble polyimide molecular chain contains a large number of polar groups (such as carbonyl groups in the imide ring) and highly polarizable benzene ring structures. During heat treatment, as the solvent evaporates and the thermal motion capability of the molecular chain segments increases, the external electric field generates a strong electrostatic torque on these polar structural units. This torque forces the PI molecular chains, which are in a random coil state, to rotate, attempting to align their dipole direction with the electric field vector direction.

[0071] Electrostatic torque provides additional potential energy compensation for the rotation of the chain segments. Under the action of the electric field, the rotation of the molecular chain segments no longer depends solely on random thermal motion, but on directional forced motion. This effectively reduces the activation energy required for the chain segments to break free from the bonds of adjacent chain segments (i.e., untangle). The electrostatic stress gradient generated by the external electric field (caused by the uneven distribution of the electric field force as described in this invention) generates an axial tensile force on the chain segments, accelerating the sliding speed of the molecular chains along their axial direction, making it easier for them to slip off from the tangle center.

[0072] As temperature increases, the vigorous motion of solvent molecules increases the free volume between PI molecular chains, weakening intermolecular forces. High temperatures significantly shorten the polymer's structural relaxation time, allowing sufficient time for the molecular chain segments to respond to electrostatic torque and rearrange their spatial orientation before the solvent completely evaporates and the structure freezes. The coupling of electric field force and thermal excitation enables the induction of large-scale molecular alignment even at relatively low field strengths. Through electrostatic torque-induced spatial alignment, the PI molecular chains transform from a tightly packed, parallel stack to an interlaced or rotationally oriented arrangement. Because the forced alignment by electrostatic torque disrupts the original thermodynamic equilibrium of tight stacking, it increases the average distance between molecules, thereby suppressing the formation of charge-transfer complexes (CTCs) at their source. This allows this method to significantly reduce visible light absorption and improve transparency without altering the chemical structure.

[0073] This invention achieves spatial rearrangement of molecular chains through an external electric field, significantly increasing the average distance between molecules and changing the stacking angle, thereby greatly reducing the probability of electronic transitions. Thus, without changing the chemical structure, it weakens the probability of electronic transitions of chromophores that absorb visible light from the source, achieving a significant improvement in light transmittance.

[0074] 2) The method of improving transparency of polyimide molecules induced by an external electric field according to the present invention is used during the wet film curing stage, when the polymer solution is in a sensitive period of low viscosity or viscoelastic interweaving. The external electric field formed by the conductive substrate and the electrode above it, which is not in contact with the wet film, avoids scratches, indentations, or microscopic defects introduced by uneven electrode surfaces that could result from any physical contact. The external electric field effectively avoids direct charge injection or electrochemical reactions at the electrode-wet film interface. If the electrode directly contacts the wet film, local electrical breakdown or solvent decomposition can easily occur under high voltage, generating microbubbles or impurity ions, severely damaging the optical uniformity of the polyimide film. The present invention ensures the purity of the external electric field, enabling the polyimide film to achieve optimized molecular orientation while maintaining extremely high surface smoothness and optical isotropy (at the macroscopic level), avoiding haze increases caused by interface disturbances. This is crucial for the preparation of high-performance optical films such as flexible display substrates.

[0075] 3) The method of improving transparency of polyimide molecules by inducing external electric field alignment according to the present invention involves the external electric field inducing non-uniformity of electric force distribution inside and on the surface of the wet film. This electric force gradient drives the polymer solution inside the wet film to generate minute, controlled convection or flow. This microflow and electric field have a synergistic effect. The flow process can drive the untangled molecular chain segments to unfold, reduce the entropy value of the polyimide system, effectively overcome the van der Waals forces and π-π stacking effects between molecular chains, promote the detangling of chain segments, and make the molecular chains more easily aligned with the direction of the electric field lines. This flow-electric field dual-induction mechanism greatly improves the efficiency of molecular orientation. Macroscopically, this manifests as a more uniform film formation process, eliminating local stress concentration caused by differences in solvent evaporation rates, resulting in a polyimide film with not only high transparency but also extremely low internal stress distribution, thus improving the dimensional stability of the polyimide film.

[0076] 4) The method of improving transparency of polyimide molecules by inducing external electric field in this invention can enhance transparency in both traditional rigid PI formed by wet film curing of polyimide precursor solution and chemically modified soluble polyimide. This means that, based on existing mature raw materials, products with superior performance can be obtained by adding an electric field generating device for induction, greatly reducing the research and development and production threshold of high-performance films. Furthermore, since external electric field induction is carried out simultaneously during heat treatment without adding extra process time, this induction method has extremely high energy efficiency and cost advantages compared to complex chemical synthesis and modification, aligning with the modern industrial trend of green environmental protection and energy conservation.

[0077] 5) The method of improving transparency of polyimide by inducing the molecular alignment of polyimide with an external electric field in this invention transforms the electric field force into an effective molecular conformation that suppresses CTC formation. This method not only significantly improves the transmittance of polyimide films in the visible light region, but also simultaneously optimizes the internal stress distribution, surface quality, and optical uniformity of polyimide films, opening up a completely new process route for the preparation of highly transparent and high-performance polyimide films. Attached Figure Description

[0078] Figure 1 A process flow diagram of a method for improving transparency by inducing the molecular arrangement of polyimide under an external electric field; Figure 2 This is a schematic diagram of a system for improving transparency by inducing the molecular arrangement of polyimide under an external electric field; Figure 3 The wavelength-transmittance curves of the sample in Example 2 and the sample in Comparative Example 2 were obtained by spectrophotometry. Figure 4 The wavelength-transmittance curves of the sample in Example 4 and the sample in Comparative Example 4 were obtained by spectrophotometry. Figure 5 The wavelength-transmittance curves of the sample in Example 6 and the sample in Comparative Example 8 were obtained by spectrophotometry. Figure 6 The wavelength-transmittance curves of the sample in Example 8 and the sample in Comparative Example 8 were obtained by spectrophotometry. In all the accompanying drawings, the same reference numerals denote the same technical features, specifically: 1. Heating platform; 2. Conductive substrate; 3. Insulating pad; 4. Wet film; 5. Electrode; 6. Electric field generating device. Detailed Implementation

[0079] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention. Furthermore, the technical features involved in the various embodiments of this invention described below can be combined with each other as long as they do not conflict with each other.

[0080] Example 1 A method for improving transparency by inducing the molecular alignment of polyimide using an external electric field includes the following steps: S1. Prepare a polyimide film-forming solution, wherein the polyimide film-forming solution is a polyimide precursor solution, the polyimide precursor solution is a polyamic acid solution, and the solid content of the polyimide film-forming solution is 8% by weight.

[0081] The polyamic acid solution is prepared by reacting a polyamic acid diamine monomer and a dianhydride monomer in a polar aprotic solvent; the diamine monomer is selected from 4,4'-diaminodiphenyl ether; the dianhydride monomer is selected from pyromellitic dianhydride. The solvent for the polyimide film-forming solution is selected from N-methylpyrrolidone; S2. The polyimide film-forming solution is coated onto a conductive substrate to form a wet film on the conductive substrate; wherein the coating method is spin coating, and the conductive substrate is an ITO glass substrate.

[0082] S3. During the heat treatment of the wet film, an external electric field is applied to the wet film to solidify it into a polyimide film. The external electric field is formed by connecting an electric field generating device to a conductive substrate and an electrode. The electrode is located above the wet film and there is a gap between the electrode and the wet film (i.e., the electrode does not contact the wet film). The electric field generating device is an electrostatic generator. The electric field strength of the external electric field is 900V / mm. The external electric field is a DC electrostatic field. The electrode is a mesh with a mesh diameter of 0.5μm. The distance between the electrode and the surface of the wet film is 10μm. The external electric field causes uneven distribution of electric force within the wet film, drives the flow of the polyimide film-forming solution, and induces the spatial orientation of the molecular chains in conjunction with the electric field force. The conductive substrate 2 can be placed on the heating stage 1, and the conductive substrate 2 and the wet film 4 on the conductive substrate 2 can be heated by the heating stage 1 to perform heat treatment on the wet film 4. An insulating pad 3 is placed on the heating stage 1, and the insulating pad 3 supports the electrode 5. The electrode 5 does not contact the wet film 4. An electric field generating device 6 is connected between the electrode 5 and the conductive substrate 2 to form an external electric field.

[0083] The heat treatment is an imidization heat treatment, and the specific heat treatment process is as follows: First, the wet film is kept at 80°C for 30 minutes, then heated from 80°C to 150°C at a rate of 5°C / minute, then kept at 150°C for 45 minutes, then heated from 150°C to 300°C at a rate of 6°C / minute, and finally kept at 300°C for 75 minutes.

[0084] A highly transparent polyimide film is prepared using the method described above.

[0085] The polyimide film obtained in this embodiment has a transmittance of 90.2% at 550 nm, which is 2.5 percentage points higher than that of the control sample obtained in Comparative Example 1 (the preparation method of Comparative Example 1 is to remove the application of an external electric field to the wet film in step S3 of the method in Example 1, and the other processes are the same as in Example 1. The transmittance of the control sample obtained by the preparation method of Comparative Example 1 is 87.7%).

[0086] Example 2 A method for improving transparency by inducing the molecular alignment of polyimide using an external electric field includes the following steps: S1. Prepare a polyimide film-forming solution, wherein the polyimide film-forming solution is a polyimide precursor solution; the polyimide film-forming solution is a polyimide precursor solution, the polyimide precursor solution is a polyamic acid solution, and the solid content of the polyimide film-forming solution is 5% by weight.

[0087] The polyamic acid solution is prepared by reacting polyamic acid diamine monomer and dianhydride monomer in a polar aprotic solvent; the diamine monomer is selected from p-phenylenediamine; the dianhydride monomer is selected from biphenyltetracarboxylic dianhydride. The solvent for the polyimide film-forming solution is selected from N,N-dimethylacetamide; S2. The polyimide film-forming solution is coated onto a conductive substrate to form a wet film on the conductive substrate; wherein the coating method is blade coating, and the conductive substrate is an ITO glass substrate.

[0088] S3. During the heat treatment of the wet film, an external electric field is applied to the wet film to solidify it into a polyimide film. The external electric field is formed by connecting an electric field generating device to a conductive substrate and an electrode. The electrode is located above the wet film and there is a gap between the electrode and the wet film (i.e., the electrode does not contact the wet film). The electric field generating device is an electrostatic generator. The electric field strength of the external electric field is 400V / mm. The external electric field adopts an alternating electric field with a frequency of 6500Hz. The electrode is mesh-like with a mesh diameter of 10μm. The distance between the electrode and the surface of the wet film is 350μm. The external electric field drives the polyimide film-forming solution to flow and induces the molecular chains to spatially align by causing uneven distribution of electric field force within the wet film. The conductive substrate 2 can be placed on the heating stage 1, and the conductive substrate 2 and the wet film 4 on the conductive substrate 2 can be heated by the heating stage 1 to perform heat treatment on the wet film 4. An insulating pad 3 is placed on the heating stage 1, and the insulating pad 3 supports the electrode 5. The electrode 5 does not contact the wet film 4. An electric field generating device 6 is connected between the electrode 5 and the conductive substrate 2 to form an external electric field.

[0089] The heat treatment is an imidization heat treatment, and the specific heat treatment process is as follows: First, the wet film is kept at 100°C for 45 minutes, then the temperature is increased from 100°C to 200°C at a rate of 5°C / minute, and kept at 200°C for 30 minutes, then the temperature is increased from 200°C to 320°C at a rate of 10°C / minute, and finally kept at 320°C for 30 minutes.

[0090] A highly transparent polyimide film is prepared using the method described above.

[0091] The polyimide film obtained in this embodiment has a transmittance of 89.8% at 550 nm, which is 1.7 percentage points higher than the control sample obtained in Comparative Example 2 (the preparation method of Comparative Example 2 is to remove the application of an external electric field to the wet film in step S3 of the method of Example 2, and the other processes are the same as in Example 2. The transmittance of the control sample obtained by the preparation method of Comparative Example 2 is 88.1%).

[0092] Example 3 A method for improving transparency by inducing the molecular alignment of polyimide using an external electric field includes the following steps: S1. Prepare a polyimide film-forming solution, wherein the polyimide film-forming solution is a polyimide precursor solution; the polyimide film-forming solution is a polyimide precursor solution, the polyimide precursor solution is a polyamic acid solution, and the solid content of the polyimide film-forming solution is 25% by weight.

[0093] The polyamic acid solution is prepared by reacting polyamic acid diamine monomer and dianhydride monomer in a polar aprotic solvent; the diamine monomer is selected from fluorinated diamines; the dianhydride monomer is selected from hexafluorodianhydride. The solvent for the polyimide film-forming solution is selected from γ-butyrolactone or cyclopentanone; S2. The polyimide film-forming solution is coated onto a conductive substrate to form a wet film on the conductive substrate; wherein the coating method is slot extrusion coating, and the conductive substrate is an ITO glass substrate.

[0094] S3. During the heat treatment of the wet film, an external electric field is applied to the wet film to solidify it into a polyimide film. The external electric field is formed by connecting an electric field generating device to a conductive substrate and an electrode. The electrode is located above the wet film and there is a gap between the electrode and the wet film (i.e., the electrode does not contact the wet film). The electric field generating device is a high-voltage power supply. The electric field strength of the external electric field is 2000V / mm. The external electric field uses an AC electric field with a frequency of 10000Hz. The electrode is mesh-like with a mesh diameter of 5μm. The distance between the electrode and the surface of the wet film is 500μm. The external electric field drives the polyimide film-forming solution to flow and induces the molecular chains to spatially align by causing uneven distribution of electric field force within the wet film. The conductive substrate 2 can be placed on the heating stage 1, and the conductive substrate 2 and the wet film 4 on the conductive substrate 2 can be heated by the heating stage 1 to perform heat treatment on the wet film 4. An insulating pad 3 is placed on the heating stage 1, and the insulating pad 3 supports the electrode 5. The electrode 5 does not contact the wet film 4. An electric field generating device 6 is connected between the electrode 5 and the conductive substrate 2 to form an external electric field.

[0095] The heat treatment is an imidization heat treatment, and the specific heat treatment process is as follows: First, the wet film is kept at 85°C for 60 minutes, then the temperature is increased from 85°C to 180°C at a rate of 5°C / minute, and kept at 180°C for 60 minutes, then the temperature is increased from 180°C to 350°C at a rate of 10°C / minute, and finally kept at 350°C for 90 minutes.

[0096] A highly transparent polyimide film is prepared using the method described above.

[0097] The polyimide film obtained in this embodiment has a transmittance of 92.7% at 550 nm, which is 2.6 percentage points higher than the control sample obtained in Comparative Example 3 (the preparation method of Comparative Example 3 is to remove the application of an external electric field to the wet film in step S3 of the method of Example 3, and the other processes are the same as in Example 3. The transmittance of the control sample obtained by the preparation method of Comparative Example 3 is 90.1%).

[0098] Example 4 A method for improving transparency by inducing the molecular alignment of polyimide using an external electric field includes the following steps: S1. Prepare a polyimide film-forming solution, wherein the polyimide film-forming solution is a soluble polyimide solution; the soluble polyimide solution is formed by dissolving soluble polyimide in a solvent, the soluble polyimide is a chemically modified polyimide, wherein the chemical modification is the introduction of fluorine-containing groups, the soluble polyimide with introduced fluorine-containing groups is a fluorinated polyimide, and the fluorinated polyimide is obtained by polymerization of a monomer containing hexafluorodianhydride.

[0099] The solid content of the polyimide film-forming solution is 5% by weight.

[0100] The solvent for the polyimide film-forming solution is selected from N-methylpyrrolidone.

[0101] S2. The polyimide film-forming solution is coated onto a conductive substrate to form a wet film on the conductive substrate, wherein the coating method is blade coating and the conductive substrate is an ITO glass substrate. S3. During the heat treatment of the wet film, an external electric field is applied to the wet film to solidify it into a polyimide film. The external electric field is formed by connecting an electric field generating device to a conductive substrate and an electrode. The electrode is located above the wet film and there is a gap between it and the wet film (i.e., the electrode does not contact the wet film). The electric field generating device is a high-voltage power supply. The heat treatment is a thermal annealing process, specifically as follows: first, hold at 80°C for 30 minutes, then raise the temperature from 80°C to 290°C at a rate of 10°C / minute, and hold at 290°C for 30 minutes. The glass transition temperature T of the soluble polyimide is... gThe temperature is 280℃.

[0102] The electric field strength of the external electric field is 400V / mm, and the external electric field is a DC electrostatic field.

[0103] The electrode is mesh-like with a mesh diameter of 3 μm. The distance between the electrode and the surface of the wet film is 10 μm. The external electric field causes uneven distribution of electric force within the wet film, drives the flow of the polyimide film-forming solution, and induces the spatial orientation of the molecular chains in conjunction with the electric field force.

[0104] The conductive substrate 2 can be placed on the heating stage 1, and the conductive substrate 2 and the wet film 4 on the conductive substrate 2 can be heated by the heating stage 1 to perform heat treatment on the wet film 4. An insulating pad 3 is placed on the heating stage 1, and the insulating pad 3 supports the electrode 5. The electrode 5 does not contact the wet film 4. An electric field generating device 6 is connected between the electrode 5 and the conductive substrate 2 to form an external electric field.

[0105] A highly transparent polyimide film is prepared using the method described above.

[0106] The polyimide film obtained in this embodiment has a transmittance of 92.8% at 550 nm, which is 3.2 percentage points higher than that of the control sample obtained in Comparative Example 4 (the preparation method of Comparative Example 4 is to remove the application of an external electric field to the wet film in step S3 of the method of Example 4, and the other processes are the same as those of Example 4. The transmittance of the control sample obtained by the preparation method of Comparative Example 4 is 89.6%).

[0107] Example 5 A method for improving transparency by inducing the molecular alignment of polyimide using an external electric field includes the following steps: S1. Prepare a polyimide film-forming solution, wherein the polyimide film-forming solution is a soluble polyimide solution; the polyimide film-forming solution is a soluble polyimide solution, the soluble polyimide solution is formed by dissolving soluble polyimide in a solvent, the soluble polyimide is a chemically modified polyimide, wherein the chemical structure modification is the introduction of an alicyclic structure, and the soluble polyimide with the introduced alicyclic structure is an alicyclic polyimide; The solid content of the polyimide film-forming solution is 25% by weight.

[0108] The solvent for the polyimide film-forming solution is selected from N,N-dimethylformamide.

[0109] S2. The polyimide film-forming solution is coated onto a conductive substrate to form a wet film on the conductive substrate, wherein the coating method is slot extrusion coating and the conductive substrate is an ITO glass substrate. S3. During the heat treatment of the wet film, an external electric field is applied to the wet film to solidify it into a polyimide film. The external electric field is formed by connecting an electric field generating device to a conductive substrate and electrodes connected to the electric field. The electrodes are located above the wet film and have a gap with it (i.e., the electrodes do not contact the wet film). The electric field generating device is a high-voltage power supply. The heat treatment is a thermal annealing process, specifically as follows: first, hold at 120°C for 65 minutes; then, increase the temperature from 120°C to 250°C at a rate of 5°C / minute; and then hold at 250°C for 45 minutes. The glass transition temperature T of the soluble polyimide is... g It is 220℃.

[0110] The electric field strength of the external electric field is 2000V / mm, and the external electric field adopts an alternating electric field with a frequency of 7500Hz.

[0111] The electrode is mesh-like with a mesh diameter of 0.5 μm. The distance between the electrode and the surface of the wet film is 500 μm. The external electric field causes uneven distribution of electric force within the wet film, drives the flow of the polyimide film-forming solution, and induces the spatial orientation of the molecular chains in conjunction with the electric field force.

[0112] The conductive substrate 2 can be placed on the heating stage 1, and the conductive substrate 2 and the wet film 4 on the conductive substrate 2 can be heated by the heating stage 1 to perform heat treatment on the wet film 4. An insulating pad 3 is placed on the heating stage 1, and the insulating pad 3 supports the electrode 5. The electrode 5 does not contact the wet film 4. An electric field generating device 6 is connected between the electrode 5 and the conductive substrate 2 to form an external electric field.

[0113] A highly transparent polyimide film is prepared using the method described above.

[0114] The polyimide film obtained in this embodiment has a transmittance of 91.7% at 550 nm, which is 2.2 percentage points higher than the control sample obtained in Comparative Example 5 (the preparation method of Comparative Example 5 is to omit the application of an external electric field to the wet film in step S3 of the method in Example 5, while the other processes are the same as in Example 5; the transmittance of the control sample obtained by the preparation method of Comparative Example 5 is 89.5%). Therefore, this embodiment further improves the transparency of a chemically modified soluble polyimide solution by using an external electric field.

[0115] Example 6 A method for improving transparency by inducing the molecular alignment of polyimide using an external electric field includes the following steps: S1. Prepare a polyimide film-forming solution, wherein the polyimide film-forming solution is a soluble polyimide solution; the soluble polyimide solution is formed by dissolving soluble polyimide in a solvent, and the soluble polyimide is a chemically modified polyimide, wherein the chemical modification is to introduce a twisted non-coplanar structure, and the monomer used for introducing the twisted non-coplanar structure of the soluble polyimide is a non-coplanar dianhydride.

[0116] The solid content of the polyimide film-forming solution is 20% by weight.

[0117] The solvent for the polyimide film-forming solution is selected from γ-butyrolactone or cyclopentanone.

[0118] S2. The polyimide film-forming solution is coated onto a conductive substrate to form a wet film on the conductive substrate, wherein the coating method is spin coating and the conductive substrate is an ITO glass substrate; S3. During the heat treatment of the wet film, an external electric field is applied to the wet film to solidify it into a polyimide film. The external electric field is formed by connecting an electric field generating device to a conductive substrate and an electrode. The electrode is located above the wet film and there is a gap between it and the wet film (i.e., the electrode does not contact the wet film). The electric field generating device is an electrostatic generator. The heat treatment is a thermal annealing process, specifically as follows: first, hold at 95°C for 90 minutes, then raise the temperature from 95°C to 290°C at a rate of 6°C / minute, and hold at 290°C for 60 minutes. The glass transition temperature T of the soluble polyimide is... g It is 270℃.

[0119] The electric field strength of the external electric field is 1300V / mm, and the external electric field adopts an alternating electric field with a frequency of 10000Hz.

[0120] The electrode is mesh-like with a mesh diameter of 10 μm. The distance between the electrode and the surface of the wet film is 200 μm. The external electric field causes uneven distribution of electric force within the wet film, drives the flow of the polyimide film-forming solution, and induces the spatial orientation of the molecular chains in conjunction with the electric field force.

[0121] The conductive substrate 2 can be placed on the heating stage 1, and the conductive substrate 2 and the wet film 4 on the conductive substrate 2 can be heated by the heating stage 1 to perform heat treatment on the wet film 4. An insulating pad 3 is placed on the heating stage 1, and the insulating pad 3 supports the electrode 5. The electrode 5 does not contact the wet film 4. An electric field generating device 6 is connected between the electrode 5 and the conductive substrate 2 to form an external electric field.

[0122] A highly transparent polyimide film is prepared using the method described above.

[0123] The polyimide film obtained in this embodiment has a transmittance of 91.4% at 550 nm, which is 1.9 percentage points higher than the control sample obtained in Comparative Example 6 (the preparation method of Comparative Example 6 is the same as Example 6, except that the external electric field applied to the wet film is omitted in step S3). Therefore, this embodiment further enhances the transparency of a chemically modified soluble polyimide solution by using an external electric field.

[0124] Example 7 A method for improving transparency by inducing the molecular alignment of polyimide using an external electric field includes the following steps: S1. Prepare a polyimide film-forming solution, wherein the polyimide film-forming solution is a soluble polyimide solution; the soluble polyimide solution is formed by dissolving soluble polyimide in a solvent, and the soluble polyimide is a chemically modified polyimide, wherein the chemical modification introduces bulky side groups, and the monomer used for introducing bulky side groups into the soluble polyimide is triphenylmethyldiamine; The solid content of the polyimide film-forming solution is 20% by weight.

[0125] The solvent for the polyimide film-forming solution is selected from N,N-dimethylformamide.

[0126] S2. The polyimide film-forming solution is coated onto a conductive substrate to form a wet film on the conductive substrate, wherein the coating method is spin coating and the conductive substrate is an ITO glass substrate; S3. During the heat treatment of the wet film, an external electric field is applied to the wet film to solidify it into a polyimide film. The external electric field is formed by connecting an electric field generating device to a conductive substrate and an electrode. The electrode is located above the wet film and there is a gap between it and the wet film (i.e., the electrode does not contact the wet film). The electric field generating device is a high-voltage power supply. The heat treatment is a thermal annealing process, specifically as follows: first, hold at 110°C for 40 minutes, then raise the temperature from 110°C to 325°C at a rate of 10°C / minute, and hold at 325°C for 50 minutes. The glass transition temperature T of the soluble polyimide is... g It is 310℃.

[0127] The electric field strength of the external electric field is 1800V / mm, and the external electric field adopts an alternating electric field with a frequency of 10000Hz.

[0128] The electrode is mesh-like with a mesh diameter of 8 μm. The distance between the electrode and the surface of the wet film is 400 μm. The external electric field causes uneven distribution of electric force within the wet film, drives the flow of the polyimide film-forming solution, and induces the spatial orientation of the molecular chains in conjunction with the electric field force.

[0129] The conductive substrate 2 can be placed on the heating stage 1, and the conductive substrate 2 and the wet film 4 on the conductive substrate 2 can be heated by the heating stage 1 to perform heat treatment on the wet film 4. An insulating pad 3 is placed on the heating stage 1, and the insulating pad 3 supports the electrode 5. The electrode 5 does not contact the wet film 4. An electric field generating device 6 is connected between the electrode 5 and the conductive substrate 2 to form an external electric field.

[0130] A highly transparent polyimide film is prepared using the method described above.

[0131] The polyimide film obtained in this embodiment has a transmittance of 91.5% at 550 nm, which is 2.4 percentage points higher than the control sample obtained in Comparative Example 7 (the preparation method of Comparative Example 7 is the same as that of Example 7, except that the external electric field applied to the wet film is omitted in step S3). Therefore, this embodiment further improves the transparency of the chemically modified soluble polyimide solution by using an external electric field.

[0132] Example 8 S1. Preparation of a polyimide film-forming solution, wherein the polyimide film-forming solution is a polyimide precursor solution; the polyimide film-forming solution is a polyamic acid solution, and the solid content of the polyimide film-forming solution is 15% by weight. The polyamic acid solution is prepared by reacting a polyamic acid diamine monomer and a dianhydride monomer in a polar aprotic solvent; the diamine monomer is selected from 4,4'-diaminodiphenyl ether (ODA); the dianhydride monomer is selected from pyromellitic dianhydride (PMDA). The solvent of the polyimide film-forming solution is selected from N-methylpyrrolidone (NMP).

[0133] S2. The polyimide film-forming solution is coated onto a conductive substrate to form a wet film on the conductive substrate; wherein the coating method is spin coating, and the conductive substrate is an ITO glass substrate.

[0134] S3. During the heat treatment of the wet film, an external electric field is applied to the wet film to solidify it into a polyimide film. The external electric field is formed by connecting an electric field generating device to a conductive substrate and electrodes. The electrodes are located above the wet film and have a gap between them (i.e., the electrodes do not contact the wet film). The electric field generating device is a high-voltage power supply. The electric field strength of the external electric field is 1000 V / mm, and the external electric field uses an alternating current field with a frequency of 1000 Hz. The electrodes are mesh-like with a mesh diameter of 1 μm, and the distance between the electrodes and the surface of the wet film is 200 μm. The external electric field causes uneven distribution of electric force within the wet film, driving the flow of the polyimide film-forming solution and synergistically inducing the spatial orientation of the molecular chains. The conductive substrate 2 can be placed on the heating stage 1, and the conductive substrate 2 and the wet film 4 on the conductive substrate 2 can be heated by the heating stage 1 to perform heat treatment on the wet film 4. An insulating pad 3 is placed on the heating stage 1, and the insulating pad 3 supports the electrode 5. The electrode 5 does not contact the wet film 4. An electric field generating device 6 is connected between the electrode 5 and the conductive substrate 2 to form an external electric field.

[0135] The heat treatment is an imidization heat treatment, and the specific heat treatment process is as follows: First, the wet film is kept at 80°C for 60 minutes, then the temperature is increased from 80°C to 150°C at a rate of 5°C / minute, and kept at 150°C for 60 minutes, then the temperature is increased from 150°C to 350°C at a rate of 10°C / minute, and finally kept at 350°C for 90 minutes.

[0136] A highly transparent polyimide film is prepared using the method described above.

[0137] The polyimide film obtained in this embodiment has a transmittance of 91.1% at 550 nm, which is 4.1 percentage points higher than that of the control sample obtained in Comparative Example 8 (the preparation method of Comparative Example 8 is to remove the application of an external electric field to the wet film in step S3 of the method of Example 8, and the other processes are the same as those of Example 8. The transmittance of the control sample obtained by the preparation method of Comparative Example 8 is 87%).

[0138] Example 9 S1. Prepare a polyimide film-forming solution, wherein the polyimide film-forming solution is a polyimide precursor solution; the polyimide precursor solution is a polyamic acid solution, and the solid content of the polyimide film-forming solution is 15% by weight. The polyamic acid solution is prepared by reacting a polyamic acid diamine monomer and a dianhydride monomer in a polar aprotic solvent; the diamine monomer is selected from fluorinated diamines; the dianhydride monomer is selected from hexafluorodianhydride (6FDA). The solvent of the polyimide film-forming solution is selected from N-methylpyrrolidone (NMP).

[0139] Steps S2 and S3 are the same as in Example 8.

[0140] The fluorinated polyimide film obtained in this embodiment has a transmittance of 93.5% at 550 nm, which is 3.0 percentage points higher than that of the control sample obtained in Comparative Example 9 (the preparation method of Comparative Example 9 is to remove the application of an external electric field to the wet film in step S3 of the method of Example 9, and the other processes are the same as those of Example 9. The transmittance of the control sample obtained by the preparation method of Comparative Example 9 is 90.5%).

[0141] Example 10 S1. Prepare a polyimide film-forming solution, wherein the polyimide film-forming solution is a polyimide precursor solution; the polyimide precursor solution is a polyamic acid solution, and the solid content of the polyimide film-forming solution is 25% by weight. The diamine monomer is selected from 4,4'-diaminodiphenyl ether (ODA); the dianhydride monomer is selected from pyromellitic dianhydride (PMDA). The solvent of the polyimide film-forming solution is selected from N-methylpyrrolidone (NMP).

[0142] S2. The polyimide film-forming solution is coated onto a conductive substrate to form a wet film on the conductive substrate; wherein the coating method is spin coating, and the conductive substrate is an ITO glass substrate.

[0143] S3. During the heat treatment of the wet film, an external electric field is applied to the wet film to solidify it into a polyimide film. The external electric field is formed by connecting an electric field generating device to a conductive substrate and electrodes. The electrodes are located above the wet film and have a gap between them (i.e., the electrodes do not contact the wet film). The electric field generating device is an electrostatic generator with an electric field strength of 1000 V / mm and a frequency of 1000 Hz (alternating current). The electrodes are mesh-like with a mesh diameter of 6 μm, and the distance between the electrodes and the surface of the wet film is 200 μm. The external electric field drives the solution flow by causing uneven distribution of electric force within the wet film and, in conjunction with the electric field force, induces spatial orientation of the molecular chains. The conductive substrate 2 can be placed on the heating stage 1, and the conductive substrate 2 and the wet film 4 on the conductive substrate 2 can be heated by the heating stage 1 to perform heat treatment on the wet film 4. An insulating pad 3 is placed on the heating stage 1, and the insulating pad 3 supports the electrode 5. The electrode 5 does not contact the wet film 4. An electric field generating device 6 is connected between the electrode 5 and the conductive substrate 2 to form an external electric field.

[0144] The heat treatment is an imidization heat treatment, and the specific heat treatment process is as follows: First, the wet film is kept at 80°C for 90 minutes, then the temperature is increased from 80°C to 200°C at a rate of 6°C / minute, and kept at 200°C for 60 minutes, then the temperature is increased from 200°C to 350°C at a rate of 6°C / minute, and finally kept at 350°C for 90 minutes.

[0145] A highly transparent polyimide film is prepared using the method described above.

[0146] The polyimide film obtained in this embodiment has a transmittance of 89.7% at 550 nm, which is 2.0 percentage points higher than that of the control sample obtained in Comparative Example 10 (the preparation method of Comparative Example 10 is to remove the application of an external electric field to the wet film in step S3 of the method of Example 10, and the other processes are the same as those of Example 10. The transmittance of the control sample obtained by the preparation method of Comparative Example 10 is 87.7%).

[0147] Example 11 S1. Preparation of a polyimide film-forming solution, wherein the polyimide film-forming solution is a polyimide precursor solution; the polyimide film-forming solution is a polyimide precursor solution, the polyimide precursor solution is a polyamic acid solution, and the solid content of the polyimide film-forming solution is 8% by weight. The polyamic acid solution is prepared by reacting a polyamic acid diamine monomer and a dianhydride monomer in a polar aprotic solvent; the diamine monomer is selected from 4,4'-diaminodiphenyl ether (ODA); the dianhydride monomer is selected from pyromellitic dianhydride (PMDA). The solvent of the polyimide film-forming solution is selected from N-methylpyrrolidone (NMP).

[0148] S2. The polyimide film-forming solution is coated onto a conductive substrate to form a wet film on the conductive substrate; wherein the coating method is spin coating, and the conductive substrate is an ITO glass substrate.

[0149] S3. During the heat treatment of the wet film, an external electric field is applied to the wet film to solidify it into a polyimide film. The external electric field is formed by connecting an electric field generating device to a conductive substrate and electrodes. The electrodes are located above the wet film and have a gap between them (i.e., the electrodes do not contact the wet film). The electric field generating device is a high-voltage power supply. The electric field strength is 1000 V / mm, and the external electric field uses an AC electric field with a frequency of 1000 Hz. The electrodes are mesh-like with a mesh diameter of 6 μm, and the distance between the electrodes and the surface of the wet film is 200 μm. The external electric field causes uneven distribution of electric force within the wet film, driving the flow of the polyimide film-forming solution and synergistically inducing the spatial orientation of the molecular chains. The conductive substrate 2 can be placed on the heating stage 1, and the conductive substrate 2 and the wet film 4 on the conductive substrate 2 can be heated by the heating stage 1 to perform heat treatment on the wet film 4. An insulating pad 3 is placed on the heating stage 1, and the insulating pad 3 supports the electrode 5. The electrode 5 does not contact the wet film 4. An electric field generating device 6 is connected between the electrode 5 and the conductive substrate 2 to form an external electric field.

[0150] The heat treatment is an imidization heat treatment, and the specific heat treatment process is as follows: First, the wet film is kept at 80°C for 30 minutes, then the temperature is increased from 80°C to 150°C at a rate of 5°C / minute, and kept at 150°C for 30 minutes, then the temperature is increased from 150°C to 300°C at a rate of 10°C / minute, and finally kept at 300°C for 60 minutes.

[0151] A highly transparent polyimide film is prepared using the method described above.

[0152] The polyimide film obtained in this embodiment has a transmittance of 88.8% at 550 nm, which is 1.1 percentage points higher than the control sample obtained in Comparative Example 11 (the preparation method of Comparative Example 11 is to remove the application of an external electric field to the wet film in step S3 of the method of Example 11, and the other processes are the same as in Example 11. The transmittance of the control sample obtained by the preparation method of Comparative Example 11 is 87.7%).

[0153] Example 12 A method for improving transparency by inducing the molecular alignment of polyimide using an external electric field includes the following steps: S1. Preparation of a polyimide film-forming solution, wherein the polyimide film-forming solution is a soluble polyimide solution; the soluble polyimide solution is formed by dissolving soluble polyimide in a solvent, and the soluble polyimide is a chemically modified polyimide, wherein the chemical modification involves introducing fluorine-containing groups, and the soluble polyimide with introduced fluorine-containing groups is a fluorinated polyimide, which is obtained by polymerization of a monomer containing hexafluorodianhydride. The solid content of the polyimide film-forming solution is 15% by weight. The solvent of the polyimide film-forming solution is selected from γ-butyrolactone.

[0154] S2. The polyimide film-forming solution is coated onto a conductive substrate to form a wet film on the conductive substrate; wherein the coating method is spin coating.

[0155] S3. During the heat treatment of the wet film, an external electric field is applied to the wet film to solidify it into a polyimide film. The external electric field is formed by connecting an electric field generating device to a conductive substrate and electrodes. The electrodes are located above the wet film and have a gap between them (i.e., the electrodes do not contact the wet film). The electric field generating device is an electrostatic generator. The electric field strength of the external electric field is 1000 V / mm, and the external electric field uses an alternating current field with a frequency of 1000 Hz. The electrodes are mesh-like with a mesh diameter of 5 μm. The distance between the electrodes and the surface of the wet film is 200 μm. The external electric field causes uneven distribution of electric force within the wet film, driving the flow of the polyimide film-forming solution and synergistically inducing spatial orientation of the molecular chains. The conductive substrate 2 can be placed on the heating stage 1, and the conductive substrate 2 and the wet film 4 on the conductive substrate 2 can be heated by the heating stage 1 to perform heat treatment on the wet film 4. An insulating pad 3 is placed on the heating stage 1, and the insulating pad 3 supports the electrode 5. The electrode 5 does not contact the wet film 4. An electric field generating device 6 is connected between the electrode 5 and the conductive substrate 2 to form an external electric field.

[0156] The heat treatment is a hot annealing process, and the specific heat treatment process is as follows: first, hold at 100°C for 60 minutes, then raise the temperature from 100°C to 300°C at a rate of 10°C / minute, and hold at 300°C for 60 minutes. The glass transition temperature Tg of the soluble polyimide is 280°C.

[0157] A highly transparent polyimide film is prepared using the method described above.

[0158] The thin film obtained in this embodiment has a transmittance of 92.6% at 550 nm, which is 2.6 percentage points higher than that of the control sample obtained in Comparative Example 12 (the preparation method of Comparative Example 12 is to remove the application of an external electric field to the wet film in step S3 of the method of Example 12, and the other processes are the same as those of Example 12. The transmittance of the control sample obtained by the preparation method of Comparative Example 12 is 90.0%).

[0159] Example 13 Steps S1 and S2 are the same as in Example 12.

[0160] The heat treatment process in step S3 is slightly different from that in Example 12: First, it is held at 100°C for 60 minutes, and then the temperature is increased from 100°C to 290°C at a rate of 10°C / minute, and held at 290°C for 60 minutes. The other processes in step S3 are the same as in Example 12.

[0161] A system for implementing the method of improving transparency by inducing polyimide molecular alignment using an external electric field includes a heating stage 1, a conductive substrate 2, an insulating pad 3, electrodes 5, and an electric field generating device 6, wherein: The conductive substrate 2 is placed on the heating platform 1; The conductive substrate 2 supports the insulating pad 3, the insulating pad 3 supports the electrode 5, and there is a gap between the electrode 5 and the conductive substrate 2; The electric field generating device 6 is connected to the conductive substrate 2 and the electrode 5 respectively.

[0162] A highly transparent polyimide film is prepared using the method described above.

[0163] The thin film obtained in this embodiment has a transmittance of 93.0% at 550 nm, which is 3.0 percentage points higher than that of the control sample obtained in Comparative Example 13 (the preparation method of Comparative Example 13 is to remove the application of an external electric field to the wet film in step S3 of the method of Example 13, and the other processes are the same as those of Example 13. The transmittance of the control sample obtained by the preparation method of Comparative Example 13 is 90%).

[0164] Example 14 S1. Preparation of a polyimide film-forming solution, wherein the polyimide film-forming solution is a soluble polyimide solution; the soluble polyimide solution is formed by dissolving an alicyclic soluble polyimide resin in a solvent, and the soluble polyimide is a chemically modified polyimide, wherein the chemical modification involves introducing an alicyclic structure, and the soluble polyimide with the introduced alicyclic structure is an alicyclic polyimide; the solid content of the polyimide film-forming solution is 15% by weight. The solvent of the polyimide film-forming solution is selected from cyclopentanone.

[0165] S2. The polyimide film-forming solution is coated onto a conductive substrate to form a wet film on the conductive substrate; wherein the coating method is spin coating.

[0166] S3. During the heat treatment of the wet film, an external electric field is applied to the wet film to solidify it into a polyimide film. The external electric field is formed by connecting an electric field generating device to a conductive substrate and an electrode. The electrode is located above the wet film and there is a gap between it and the wet film (i.e., the electrode does not contact the wet film). The electric field generating device is a high-voltage power supply. The heat treatment is a thermal annealing process, specifically as follows: first, hold at 100°C for 60 minutes, then raise the temperature from 100°C to 240°C at a rate of 10°C / minute, and hold at 240°C for 60 minutes. The glass transition temperature T of the soluble polyimide is... g It is 220℃.

[0167] The external electric field has an electric field strength of 1000 V / mm and is an alternating electric field with a frequency of 1000 Hz. The electrode is mesh-like with a mesh diameter of 10 μm, and the distance between the electrode and the wet film surface is 200 μm.

[0168] The external electric field causes uneven distribution of electric force within the wet film, driving the flow of the polyimide film-forming solution and synergistically inducing the spatial orientation of molecular chains.

[0169] The conductive substrate 2 can be placed on the heating stage 1, and the conductive substrate 2 and the wet film 4 on the conductive substrate 2 can be heated by the heating stage 1 to perform heat treatment on the wet film 4. An insulating pad 3 is placed on the heating stage 1, and the insulating pad 3 supports the electrode 5. The electrode 5 does not contact the wet film 4. An electric field generating device 6 is connected between the electrode 5 and the conductive substrate 2 to form an external electric field.

[0170] A highly transparent polyimide film is prepared using the method described above.

[0171] The thin film obtained in this embodiment has a transmittance of 91.8% at 550 nm, which is 2.1 percentage points higher than that of the control sample obtained in Comparative Example 14 (the preparation method of Comparative Example 14 is to remove the application of an external electric field to the wet film in step S3 of the method of Example 14, and the other processes are the same as those of Example 14. The transmittance of the control sample obtained by the preparation method of Comparative Example 14 is 89.7%).

[0172] Example 15 Steps S1 and S2 are the same as in Example 8.

[0173] S3. During the heat treatment of the wet film, an external electric field is applied to the wet film to solidify it into a polyimide film. The external electric field is formed by connecting an electric field generating device to a conductive substrate and an electrode. The electrode is located above the wet film and there is a gap between the electrode and the wet film (i.e., the electrode does not contact the wet film). The electric field generating device is a high-voltage power supply. The electric field strength of the external electric field is 500V / mm. The external electric field adopts an AC electric field with a frequency of 1000Hz. The electrode is mesh-like with a mesh diameter of 10μm. The distance between the electrode and the surface of the wet film is 200μm. The external electric field causes uneven distribution of electric force within the wet film, driving the flow of the polyimide film-forming solution and inducing the spatial orientation of molecular chains in conjunction with the electric field force. The conductive substrate 2 can be placed on the heating stage 1, and the wet film 4 on the conductive substrate 2 can be heated by the heating stage 1 to perform heat treatment on the wet film 4. An insulating pad 3 is placed on the heating stage 1, and the insulating pad 3 supports the electrode 5. The electrode 5 does not contact the wet film 4. An electric field generating device 6 is connected between the electrode 5 and the conductive substrate 2 to form an external electric field.

[0174] The heat treatment is an imidization heat treatment, and the specific heat treatment process is as follows: First, the wet film is kept at 80°C for 60 minutes, then the temperature is increased from 80°C to 150°C at a rate of 5°C / minute, and kept at 150°C for 60 minutes, then the temperature is increased from 150°C to 350°C at a rate of 8°C / minute, and finally kept at 350°C for 90 minutes.

[0175] A highly transparent polyimide film is prepared using the method described above.

[0176] The thin film obtained in this embodiment has a transmittance of 89.5% at 550 nm, which is 1.8 percentage points higher than that of the control sample obtained in Comparative Example 15 (the preparation method of Comparative Example 15 is to remove the application of an external electric field to the wet film in step S3 of the method of Example 15, and the other processes are the same as those of Example 15. The transmittance of the control sample obtained by the preparation method of Comparative Example 15 is 87.7%).

[0177] Example 16 Steps S1 and S2 are the same as in Example 8.

[0178] S3. During the heat treatment of the wet film, an external electric field is applied to the wet film to solidify it into a polyimide film. The external electric field is formed by connecting an electric field generating device to a conductive substrate and electrodes. The electrodes are located above the wet film and have a gap between them (i.e., the electrodes do not contact the wet film). The electric field generating device is a high-voltage power supply. The electric field strength of the external electric field is 1800 V / mm, and the external electric field uses an alternating current field with a frequency of 1000 Hz. The electrodes are mesh-like, with a mesh diameter of 7 μm, and the distance between the electrodes and the surface of the wet film is 200 μm. The external electric field causes uneven distribution of electric force within the wet film, driving the flow of the polyimide film-forming solution and synergistically inducing spatial orientation of the molecular chains. The conductive substrate 2 can be placed on the heating stage 1, and the conductive substrate 2 and the wet film 4 on the conductive substrate 2 can be heated by the heating stage 1 to perform heat treatment on the wet film 4. An insulating pad 3 is placed on the heating stage 1, and the insulating pad 3 supports the electrode 5. The electrode 5 does not contact the wet film 4. An electric field generating device 6 is connected between the electrode 5 and the conductive substrate 2 to form an external electric field.

[0179] The heat treatment is an imidization heat treatment, and the specific heat treatment process is as follows: First, the wet film is kept at 80°C for 60 minutes, then the temperature is increased from 80°C to 150°C at a rate of 5°C / minute, and kept at 150°C for 60 minutes, then the temperature is increased from 150°C to 350°C at a rate of 10°C / minute, and finally kept at 350°C for 90 minutes.

[0180] A highly transparent polyimide film is prepared using the method described above.

[0181] The thin film obtained in this embodiment has a transmittance of 89.9% at 550 nm, which is 2.2 percentage points higher than that of the control sample obtained in Comparative Example 16 (the preparation method of Comparative Example 16 is to remove the application of an external electric field to the wet film in step S3 of the method of Example 16, and the other processes are the same as those of Example 16. The transmittance of the control sample obtained by the preparation method of Comparative Example 16 is 87.7%).

[0182] Example 17 Steps S1 and S2 are the same as in Example 8.

[0183] S3. During the heat treatment of the wet film, an external electric field is applied to the wet film to solidify it into a polyimide film. The external electric field is formed by connecting an electric field generating device to a conductive substrate and electrodes. The electrodes are located above the wet film and have a gap between them (i.e., the electrodes do not contact the wet film). The electric field generating device is a high-voltage power supply. The electric field strength is 1000 V / mm, and the external electric field uses an alternating current field with a frequency of 1000 Hz. The electrodes are mesh-like with a mesh diameter of 5 μm, and the distance between the electrodes and the surface of the wet film is 200 μm. The external electric field causes uneven distribution of electric force within the wet film, driving the flow of the polyimide film-forming solution and synergistically inducing spatial orientation of the molecular chains. The conductive substrate 2 can be placed on the heating stage 1, and the conductive substrate 2 and the wet film 4 on the conductive substrate 2 can be heated by the heating stage 1 to perform heat treatment on the wet film 4. An insulating pad 3 is placed on the heating stage 1, and the insulating pad 3 supports the electrode 5. The electrode 5 does not contact the wet film 4. An electric field generating device 6 is connected between the electrode 5 and the conductive substrate 2 to form an external electric field.

[0184] The heat treatment is an imidization heat treatment, and the specific heat treatment process is as follows: First, the wet film is kept at 80°C for 90 minutes, then the temperature is increased from 80°C to 200°C at a rate of 6°C / minute, and kept at 200°C for 60 minutes, then the temperature is increased from 200°C to 350°C at a rate of 10°C / minute, and finally kept at 350°C for 90 minutes.

[0185] A highly transparent polyimide film is prepared using the method described above.

[0186] The thin film obtained in this embodiment has a transmittance of 90% at 550 nm, which is 2.4 percentage points higher than that of the control sample obtained in Comparative Example 17 (the preparation method of Comparative Example 17 is to remove the application of an external electric field to the wet film in step S3 of the method of Example 17, and the other processes are the same as those of Example 17. The transmittance of the control sample obtained by the preparation method of Comparative Example 17 is 87.6%).

[0187] A system for implementing the method of improving transparency by inducing polyimide molecular alignment with an external electric field according to any of the above embodiments includes a heating stage 1, a conductive substrate 2, an insulating pad 3, an electrode 5, and an electric field generating device 6, wherein: The conductive substrate 2 is placed on the heating platform 1; The conductive substrate 2 supports the insulating pad 3, the insulating pad 3 supports the electrode 5, and there is a gap between the electrode 5 and the conductive substrate 2; The electric field generating device 6 is connected to the conductive substrate 2 and the electrode 5 respectively.

[0188] Samples from some of the more typical embodiments and comparative examples were selected, and wavelength-transmittance curves were obtained by spectrophotometry. (See also...) Figures 3-6 In each of the attached figures, the curves (red) of the embodiment with an applied external electric field are significantly higher than the curves (black) of the comparative embodiment without an applied external electric field. Moreover, the enhancement effect is not limited to a single wavelength, but shows an overall increase in transmittance across the entire visible light region, resulting in improved transmittance across the entire wavelength range.

[0189] Those skilled in the art will readily understand that the above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for improving transparency by inducing the molecular arrangement of polyimide under an external electric field, characterized in that, Includes the following steps: S1. Prepare a polyimide film-forming solution, wherein the polyimide film-forming solution is selected from a polyimide precursor solution or a soluble polyimide solution; S2. The polyimide film-forming solution is coated onto a conductive substrate to form a wet film on the conductive substrate. S3. During the heat treatment of the wet film, an external electric field is applied to the wet film to solidify it into a polyimide film. The external electric field is formed by connecting an electric field generating device to a conductive substrate and an electrode. The electrode is located above the wet film and there is a gap between the electrode and the wet film. The external electric field causes uneven distribution of electric field force in the wet film, which induces the polyimide molecular chains to spatially align.

2. The method for improving transparency by inducing polyimide molecular alignment with an external electric field according to claim 1, characterized in that, In step S1, the polyimide film-forming solution is a polyimide precursor solution, and the polyimide precursor solution is a polyamic acid solution; In step S3, the heat treatment is an imidization heat treatment, and the specific heat treatment process is as follows: First, the wet film is kept at 80℃~100℃ for 30 minutes to 60 minutes, then the temperature is increased to 150℃~200℃ at a rate of 5℃~10℃ / minute, and kept at 150℃~200℃ for 30 minutes to 60 minutes, then the temperature is increased to 300℃~350℃ at a rate of 5℃~10℃ / minute, and finally kept at 300℃~350℃ for 30 minutes to 90 minutes.

3. The method for improving transparency by inducing polyimide molecular alignment with an external electric field according to claim 2, characterized in that, The polyamic acid solution is prepared by reacting polyamic acid diamine monomer and dianhydride monomer in a polar aprotic solvent; the diamine monomer is selected from 4,4'-diaminodiphenyl ether, p-phenylenediamine or fluorinated diamine; the dianhydride monomer is selected from pyromellitic dianhydride, biphenyl dianhydride or hexafluorodianhydride.

4. The method for improving transparency by inducing polyimide molecular alignment with an external electric field according to claim 1, characterized in that, In step S1, the polyimide film-forming solution is a soluble polyimide solution; In step S3, the heat treatment is a hot annealing treatment, and the specific heat treatment process is as follows: First, hold at 80℃~120℃ for 30 minutes~90 minutes, and then raise the temperature at a rate of 5℃~10℃ / minute to (T g +10℃) ~ (T g +30℃), in (T g +10℃) ~ (T g Keep warm at a temperature of +30℃ for 30 to 60 minutes, where T g This is the glass transition temperature of the soluble polyimide.

5. The method for improving transparency by inducing polyimide molecular alignment with an external electric field according to claim 1, characterized in that, In step S1, the soluble polyimide solution is formed by dissolving soluble polyimide in a solvent. The soluble polyimide is a chemically modified polyimide, wherein the chemical modification includes at least one of introducing fluorine-containing groups, introducing alicyclic structures, introducing bulky side groups, or introducing distorted non-coplanar structures.

6. The method for improving transparency by inducing polyimide molecular alignment with an external electric field according to claim 5, characterized in that, When the chemical structure is modified to introduce fluorine-containing groups, the soluble polyimide with introduced fluorine-containing groups is a fluorinated polyimide, which is obtained by polymerization of a monomer containing hexafluorodianhydride.

7. The method for improving transparency by inducing polyimide molecular alignment with an external electric field according to claim 5, characterized in that, When the chemical structure is modified to introduce an alicyclic structure, the soluble polyimide with the introduced alicyclic structure is an alicyclic polyimide.

8. The method for improving transparency by inducing polyimide molecular alignment with an external electric field according to claim 5, characterized in that, When the chemical structure is modified to introduce bulky side groups, the monomer used for the soluble polyimide with the introduced bulky side groups is triphenylmethyldiamine.

9. The method for improving transparency by inducing polyimide molecular alignment with an external electric field according to claim 5, characterized in that, When the chemical structure is modified to introduce a distorted non-coplanar structure, the monomer used for the soluble polyimide with the distorted non-coplanar structure is a non-coplanar dianhydride.

10. The method for improving transparency by inducing polyimide molecular alignment with an external electric field according to claim 1, characterized in that, In step S1, the solid content of the polyimide film-forming solution is 5% to 25% by weight.

11. The method for improving transparency by inducing polyimide molecular alignment with an external electric field according to claim 1, characterized in that, The solvent of the polyimide film-forming solution is selected from one or more of N-methylpyrrolidone, N,N-dimethylacetamide, N,N-dimethylformamide, γ-butyrolactone, or cyclopentanone.

12. The method for improving transparency by inducing polyimide molecular alignment with an external electric field according to claim 1, characterized in that, In step S3, the electric field strength of the external electric field is 400V / mm to 2000V / mm.

13. The method for improving transparency by inducing polyimide molecular alignment with an external electric field according to claim 1, characterized in that, In step S3, the external electric field is a DC electrostatic field or an AC electric field with a frequency not greater than 10000Hz.

14. The method for improving transparency by inducing polyimide molecular alignment with an external electric field according to claim 1, characterized in that, In step S3, the electrode is a mesh.

15. The method for improving transparency by inducing polyimide molecular alignment with an external electric field according to claim 14, characterized in that, The mesh diameter of the electrode is 0.5μm to 10μm, and the distance between the electrode and the wet film surface is 10μm to 500μm.

16. A system for implementing the method of improving transparency by inducing polyimide molecular alignment with an external electric field as described in any one of claims 1 to 15, characterized in that, It includes a heating platform, a conductive substrate, insulating pads, electrodes, and an electric field generating device, wherein: The conductive substrate is placed on the heating stage; The conductive substrate supports the insulating pad, the insulating pad supports the electrode, and there is a gap between the electrode and the conductive substrate; The electric field generating device is connected to the conductive substrate and the electrode, respectively.

17. A high-transparency polyimide film, characterized in that, It is prepared by the method of improving transparency by inducing the molecular arrangement of polyimide with an external electric field as described in any one of claims 1 to 15.

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