A bend-resistant transparent polyamide-imide film, and a method of making and using the same

Abstract

The application relates to the technical field of flexible display materials, and discloses a bend-resistant transparent polyamide-imide film as well as a preparation method and application thereof. The preparation method comprises the following steps: dissolving a dianhydride monomer in a diamine solution to perform a polymerization reaction, so as to obtain a polyamide acid solution; adding a monovalent alkali metal salt and an acid chloride monomer into the polyamide acid solution to perform copolymerization, so as to obtain a precursor solution; adding a dehydrating agent and a catalyst into the precursor solution to perform dehydration and imidization, so as to obtain a polyamide-imide solution; mixing the polyamide-imide solution with a precipitating agent to perform precipitation, so as to obtain a polyamide-imide solid; dissolving the polyamide-imide solid in a polar organic solvent, and then adding a multivalent alkali earth metal salt, so as to obtain a polyamide-imide solution with multiple coordination; coating the polyamide-imide solution with multiple coordination on the surface of a substrate; and performing demolding after temperature curing, so that the transparent polyamide-imide film is obtained. The transparent polyamide-imide film prepared by the application has a tensile strength of 160 MPa at most, a transmittance of > 88% at a wavelength of 550 nm, a yellowness value of < 3, and can be bent 200,000 times without breaking at a bending radius of 0.5 mm.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of flexible display material, in particular to a bend-resistant transparent polyamide-imide film and a preparation method and application thereof. BACKGROUND

[0002] With the rapid development of flexible electronic devices, the market demand for bendable, foldable and wearable display devices continues to grow. Traditional brittle glass is difficult to meet the needs of flexible display devices, and the flexible cover film prepared by combining polymer film and transparent protective coating can effectively solve the problems faced by flexible display devices and replace traditional brittle glass.

[0003] The flexible polymer film and protective coating of the flexible cover film need to meet the following key requirements: 1) excellent optical transparency (should be comparable to inorganic glass); 2) high tensile strength and modulus; 3) good heat resistance (high glass transition temperature Tg) and excellent dimensional stability (low coefficient of thermal expansion CTE); 4) excellent ductility and excellent folding reliability (can withstand more than 200,000 folding cycles); 5) easy to process and realize mass production. Transparent polyimide (CPI) film has become the most promising cover film candidate material for flexible display devices due to its excellent optical transparency, excellent mechanical properties, high thermal stability and good chemical resistance.

[0004] However, the conformational entropy or entanglement of CPI is very small, and the limited entropy elasticity and deformation space cannot withstand the molecular chain slip caused by long-term folding of the film, resulting in poor bend resistance, accumulation of plastic deformation, and formation of visible wrinkles, affecting the reliability of flexible electronic devices.

[0005] Building cross-linked structures in the film is considered an effective means to prevent plastic slip of macromolecular chains during cyclic bending, thereby avoiding the early formation of folds. Based on this, the present application develops a transparent polyamide-imide film with high bend resistance, thereby improving the reliability of flexible electronics. SUMMARY

[0006] The present application provides a bend-resistant transparent polyamide-imide film and a preparation method and application thereof, aiming to solve the problem of poor bend resistance of existing transparent polyamide-imide films.

[0007] In order to achieve the above-mentioned purpose, the technical scheme is adopted as follows.

[0008] The first aspect of the present application provides a preparation method of a bend-resistant transparent polyamide-imide film, comprising:

[0009] S1, dissolving a diamine monomer in a polar organic solvent to obtain a diamine solution; dissolving a dianhydride monomer in the diamine solution to perform a polymerization reaction, and obtaining a polyamic acid solution;

[0010] S2, adding a monovalent alkali metal salt to the polyamic acid solution, and then adding an acid chloride monomer to perform a copolymerization reaction, and obtaining a precursor solution;

[0011] S3, adding a dehydrating agent and a catalyst to the precursor solution to perform a dehydration imidization, and obtaining a polyamide-imide solution; mixing the polyamide-imide solution with a precipitating agent to perform a precipitation, and obtaining a polyamide-imide solid containing a monovalent alkali metal salt;

[0012] S4, dissolving the polyamide-imide solid containing a monovalent alkali metal salt in a polar organic solvent, and then adding a multivalent alkaline earth metal salt to stir uniformly; performing a filtration and a defoaming to obtain a polyamide-imide solution with multiple coordination; the multivalent alkaline earth metal salt includes at least one of CaCl2 or MgCl2;

[0013] S5, coating the polyamide-imide solution with multiple coordination on a surface of a substrate, and performing a temperature rising curing to obtain a transparent polyamide-imide film with bending resistance.

[0014] Preferably, the diamine monomer includes at least one of the compounds shown in 1-1a to 1-15a:

[0015]

[0016] Preferably, the dianhydride monomer includes at least one of the compounds shown in 1-1b to 1-12b:

[0017]

[0018] Preferably, the acid chloride monomer includes at least one of the compounds shown in 1-1c to 1-18c:

[0019]

[0020] Preferably, the polar organic solvent includes at least one of N-methyl pyrrolidone (NMP), N,N-dimethylformamide (DMF) or N,N-dimethylacetamide (DMAc);

[0021] The dehydrating agent is acetic anhydride;

[0022] The catalyst is pyridine, triethylamine or isoquinoline;

[0023] The precipitating agent is deionized water, ethanol or a mixture of deionized water and ethanol.

[0024] Preferably, the ratio of the number of moles of the diamine monomer to the total number of moles of the dianhydride monomer and the acid chloride monomer is 1:(1-1.03); wherein the molar ratio of the dianhydride monomer to the acid chloride monomer is 9:1-1:9;

[0025] The molar ratio of the dehydrating agent to the dianhydride monomer is (2-2.5):1;

[0026] The molar ratio of the dehydrating agent to the dianhydride monomer is (2-2.5):1;

[0027] Preferably, the monovalent alkali metal salt is LiCl; the addition amount of the monovalent alkali metal salt is 0.5-2.0wt% of the sum of the amounts of the diamine monomer and the dianhydride monomer;

[0028] The addition amount of the polyvalent alkaline earth metal salt is 0.5-10wt% of the mass of the polyamide-imide solid containing the monovalent alkali metal salt.

[0029] Preferably, the temperature of the dehydrating imidization in step S3 is 20-60℃, and the time is 1-24h;

[0030] The temperature of the precipitation is 10-40℃.

[0031] In a second aspect of the present application, a transparent polyamide-imide film resistant to bending is provided, which is prepared by the above preparation method.

[0032] In a third aspect of the present application, the above transparent polyamide-imide film resistant to bending is applied in a flexible display device.

[0033] Compared with the prior art, the present application has the following beneficial effects:

[0034] The preparation method of the present application controls the viscosity of the polyamide acid and the polymerization system after the introduction of the amide bond by introducing the monovalent alkali metal salt LiCl, and reduces the intermolecular force through the single coordination effect and the amide bond in the solution phase, which mainly plays a role of dissolving aid, improves the solubility of the polyamide acid-polyamide copolymer, and thus improves the casting film forming performance of the polymer glue solution, and at the same time, the thin film after solidification and molding participates in the multi-coordination synergistic effect; the introduction of the amide structure helps effective chain packing and orientation, which can effectively improve the mechanical properties of the thin film; the introduction of the colorless polyvalent alkaline earth metal ion Ca 2+ or / and Mg 2+ forms a multi-coordination structure with the amide bond, and forms a cation-π interaction with the benzene ring and the imide ring. The present application constructs a monovalent / polyvalent ion coordination non-covalent cross-linking network through the synergistic effect of the above strategies, enhances the intermolecular force and entropy elasticity to prevent plastic slip of the macromolecular chain in the cyclic folding, and thus obtains a transparent polyamide-imide film resistant to bending with an energy dissipation mechanism, which has excellent flexibility and high light transmittance.

[0035] The uniaxially stretched transparent polyamide-imide film with multi-coordination structure prepared in the application has a tensile strength of up to 160 MPa, a transmittance of > 88% at a wavelength of 550 nm, a yellowness value of < 3, and can be bent 200,000 times without breaking at a bending radius of 0.5 mm. BRIEF DESCRIPTION OF DRAWINGS

[0036] In order to more clearly illustrate the technical solutions of the embodiments of the application, the drawings needed in the embodiment description will be briefly introduced. Obviously, the drawings in the following description are only some embodiments described in the application, and for those skilled in the art, other drawings can also be obtained without creative labor on the basis of these drawings.

[0037] Figure 1 The real object graph and SEM spectrum of the transparent polyamide-imide film prepared for Example 1;

[0038] Figure 2 The infrared spectrum of the transparent polyamide-imide film prepared for Examples 1-3 and Comparative Example 1;

[0039] Figure 3 The SEM spectrum of the transparent polyamide-imide film prepared for Examples 1-3 and Comparative Example 1 after bending. DETAILED DESCRIPTION

[0040] The technical solutions in the embodiments of the application will be described clearly and completely in the following description of the embodiments of the application, combined with the drawings in the embodiments of the application. Obviously, the described embodiments are part of the embodiments of the application, not all the embodiments. Based on the embodiments in the application, all other embodiments obtained by those skilled in the art without creative labor are within the scope of protection of the application.

[0041] In the following description of the embodiments of the application, the terms "include", "contain", "have" and "contain" and the like are all open terms, that is, they mean to include but not limited to.

[0042] In the following description of the embodiments of the application, the term "and / or" is used to describe the association relationship of the associated objects, which means that there can be three kinds of relationships, for example, A and / or B, which can represent the following three cases: A exists alone, B exists alone and A and B exist at the same time. Wherein A, B can be singular or plural. The character " / " generally represents that the associated objects before and after are in an "or" relationship.

[0043] In the following description of the present embodiments, the term "at least one" means one or more and the term "multiple" means two or more. The phrase "at least one of the following (one)" or similar expressions refers to any combination of these items, including a single item or any combination of multiple items. For example, "at least one of a, b or c", or "at least one of a, b and c", can mean a, b, c, a-b (i.e. a and b), a-c, b-c, or a-b-c, where a, b, and c can be single or multiple.

[0044] The terms used in the present embodiments are merely for the purpose of describing specific embodiments and are not intended to limit the present application. The singular forms "a" and "the" used in the present embodiments and the appended claims are also intended to include the plural forms, unless the context clearly indicates otherwise.

[0045] Those skilled in the art should understand that in the following description of the present embodiments, the order of the serial numbers does not mean the order of execution, and some or all steps can be executed in parallel or in sequence, and the execution order of each process should be determined according to its function and inherent logic, and should not constitute any limitation on the implementation process of the present embodiments.

[0046] Those skilled in the art should understand that the numerical ranges in the present embodiments should be understood as each intermediate value between the upper limit and the lower limit of the range. Each smaller range between any stated value or stated range and any other stated value or intermediate value within the stated range is also included in the present application. The upper and lower limits of these smaller ranges can be independently included or excluded from the range.

[0047] Unless otherwise specified, the technical / scientific terms used herein have the same meaning as generally understood by those skilled in the art to which the present application belongs. Although only preferred methods and materials are described herein, any methods and materials similar or equivalent to those described herein can also be used in the implementation or testing of the present application. All documents mentioned in the specification are incorporated by reference to disclose and describe the methods and / or materials related to the documents. In the event of conflict between the content of the specification and any incorporated document, the content of the specification shall prevail.

[0048] In a first aspect, the present application provides a preparation method of a bend-resistant transparent polyamide-imide film, comprising:

[0049] S1, in an inert atmosphere, dissolving a diamine monomer in a polar organic solvent to obtain a diamine solution; dissolving a dianhydride monomer in the diamine solution to perform a polymerization reaction, to obtain a polyamide acid solution;

[0050] In the present application, the diamine monomer has non-planar or strong electronegativity, including at least one of the following compounds 1-1a to 1-15a:

[0051]

[0052] In the present application, the polar organic solvent is used to dissolve the above-mentioned diamine monomer, and the polar organic solvent includes at least one of N-methyl pyrrolidone (NMP), N,N-dimethylformamide (DMF) or N,N-dimethylacetamide (DMAc), preferably N,N-dimethylacetamide.

[0053] In the present application, the dianhydride monomer includes at least one of the compounds shown in 1-1b to 1-12b:

[0054]

[0055] In the present application, the diamine monomer is preferably 2,2'-bis(trifluoromethyl)-4,4'-diaminobiphenyl (TFMB); and the dianhydride monomer is preferably 2,2'-bis(3,4-dicarboxylic acid) hexafluoropropane dianhydride (6FDA) and / or 3,3',4,4'-biphenyl tetracarboxylic dianhydride (BPDA).

[0056] In the present application, the temperature of the polymerization reaction is preferably room temperature, and the reaction time is preferably 3-6h; the polymerization reaction is preferably carried out under stirring, and the stirring speed is preferably 350-500rpm.

[0057] S2, adding a monovalent alkali metal salt to the polyamic acid solution, and then adding an acid chloride monomer to carry out a copolymerization reaction to obtain a precursor solution;

[0058] In the present application, the acid chloride monomer includes at least one of the following compounds:

[0059]

[0060] The acid chloride monomer is preferably terephthaloyl chloride (TPC), isophthaloyl chloride (IPC) and / or 4,4'-biphenyl dicarboxylic chloride (BPDC). In the present application, the temperature for adding the acid chloride monomer is preferably room temperature or added in an ice bath. After adding the acid chloride monomer, the solid content of the obtained mixture is preferably 5-20wt%, more preferably 10-15wt%.

[0061] In the construction of polyamic acid (PAA) based polymer thin film, the coordination between ions and carboxylic acid groups can effectively improve the viscosity of polyamic acid. In the present application, monovalent alkali metal salt LiCl is selected as the solubilizing ion, and monovalent coordination can effectively reduce the hydrogen bond interaction and significantly improve the viscosity of the polymer. In the present application, LiCl solid or LiCl solution can be added to the polyamic acid solution, wherein the amount of LiCl added is 0.5-2.0wt% of the total amount of diamine monomer and dianhydride monomer, preferably 0.8wt%.

[0062] In the present application, the copolymerization reaction is preferably carried out at room temperature or in an ice bath, and the reaction time is preferably 12-24h; the copolymerization reaction is preferably carried out under stirring; the stirring speed is preferably 200-350rpm.

[0063] In the present application, the ratio of the number of moles of diamine monomer to the total number of moles of dianhydride monomer and acyl chloride monomer is 1:(1-1.03); the molar ratio of dianhydride monomer to acyl chloride monomer is 9:1-1:9.

[0064] S3, adding a dehydrating agent and a catalyst to the precursor solution to carry out dehydrating imidization to obtain a polyamide-imide solution; mixing the polyamide-imide solution with a precipitant to carry out precipitation to obtain a polyamide-imide solid containing monovalent alkali metal salt;

[0065] In the present application, the dehydrating agent is acetic anhydride; the catalyst is pyridine, triethylamine or isoquinoline, preferably pyridine. The molar ratio of the dehydrating agent to dianhydride monomer is (2-2.5):1; the amount-of-substance ratio of the dehydrating agent to the catalyst is 1:(0.5-2).

[0066] In the present application, the temperature of the dehydrating imidization is preferably 20-60℃, and the time is 1-24h; the precipitant is deionized water, ethanol or a mixture of deionized water and ethanol, wherein the volume ratio of water to ethanol in the mixture is preferably 1:1; the precipitation temperature is preferably 10-40℃, more preferably 25℃, and the precipitate is preferably washed after precipitation.

[0067] S4, dissolving the polyamide-imide solid containing monovalent alkali metal salt in a polar organic solvent at room temperature, and then adding a multivalent alkaline earth metal salt and stirring uniformly; filtering and defoaming to obtain a polyamide-imide solution with multiple coordination;

[0068] In the present application, the multivalent alkaline earth metal salt comprises at least one of CaCl2 or MgCl2, preferably CaCl2, MgCl2 or a mixture of CaCl2 and MgCl2. The multivalent alkaline earth metal salt is preferably added in an amount of 0.5-10 wt% of the mass of the polyamide-imide solid containing monovalent alkali metal salt, preferably 1 wt%.

[0069] In the present application, the stirring time is preferably 6-12 h, and the stirring speed is preferably 150-250 rpm.

[0070] The present application does not have special limitations on the operation of filtration and defoaming, and the filtration and defoaming technical solutions well known to those skilled in the art can be used. In the examples of the present application, a needle cylinder filter or a sand core filter is preferably used for filtration; the defoaming temperature is preferably room temperature, and the defoaming method is preferably vacuum defoaming or standing defoaming.

[0071] In the present application, the solid content of the obtained multi-coordinated polyamide-imide solution is preferably 5-20%, more preferably 10-15%.

[0072] S5, the multi-coordinated polyamide-imide solution is coated on the surface of a substrate, and after temperature curing and demolding, a bend-resistant transparent polyamide-imide film is obtained. The substrate is preferably a glass substrate.

[0073] In the present application, the solvent is evaporated by first being kept at 50-80℃ for 1-3 h, then being kept at 150-180℃ for 1-3 h, and finally being kept at 320-350℃ for 1-2 h, and then demolding in deionized water.

[0074] In the present application, the introduction of amide structure enhances the hydrogen bonding in the transparent polyimide CPI molecular chain, which can effectively promote the accumulation and orientation of the chain, thereby significantly improving the mechanical properties of the CPI film. At the same time, due to the resonance structure of the amide bond, when combined with aromatic groups, the electronic transition (such as n-π* transition) is constrained by symmetry, which has an advantage in maintaining the absorption characteristics of the molecular coplanarity, thereby effectively reducing light absorption and improving the light transmittance of the material. Therefore, the amide bond in the constructed polyamide-imide (CPAI) hardly absorbs visible light and can significantly improve the rigidity of the polymer chain.

[0075] The application can significantly improve the casting film forming performance of polymer glue solution by coordinating the viscosity of polymer through monovalent alkali metal salt. In the construction process of polyamic acid (PAA) based polymer film, the coordination between ions and carboxylic acid groups can effectively improve the viscosity of polyamic acid. With the advancement of chemical imidization reaction, the coordination ability of imide ring is significantly weakened, and ions do not participate in the reaction in the formation process of polyimide. At the stage of introducing amide bond, strong intermolecular hydrogen bond interaction may lead to gelation of polymer system. Therefore, the use of ions coordinated with amide groups, especially ions capable of forming coordination with specific carbon atoms of the group, can effectively weaken the intermolecular interaction, thereby playing a role in dissolving. Among various ions, monovalent alkali metal salt (such as LiCl) with weak coordination ability is an effective dissolving ion, which can effectively reduce the hydrogen bond interaction and significantly improve the viscosity of the polymer through monovalent coordination. While multivalent alkaline earth metal ions usually have strong coordination ability, which will lead to the increase of polymer viscosity, thereby adversely affecting the polymerization reaction. While multivalent alkaline earth metal ions usually have strong coordination ability, which will lead to the increase of polymer viscosity, thereby adversely affecting the polymerization reaction.

[0076] Specifically, the application introduces monovalent alkali metal salt LiCl to adjust the viscosity of the polymer after the completion of polyamic acid polymerization, and then adds acyl chloride for copolymerization to generate polyamide-polyamic acid copolymer. In the solution phase, LiCl as a monovalent salt can act on the amide bond through monodentate effect to reduce intermolecular forces, improve the solubility of polyamide-polyamic acid copolymer, thereby improving the casting film forming performance of polymer glue solution, and playing a role in dissolving. Subsequently, by maintaining a low temperature environment for the polymer during precipitation and washing, LiCl is retained in the polyamide-imide polymer solid; in the solid film, Li + is converted into coordination with amide bond, participates in the multi-coordination of alkaline earth metal ions (such as Ca 2+ , Mg 2+ ), and forms a network structure of various coordination non-covalent bonds centered on Li + , Ca 2+ and / or Mg 2+ . This network structure significantly enhances the mechanical properties, heat resistance and thermal dimensional stability of the film, and reduces the electron transfer recombination effect between macromolecular chains, thereby improving the light transmittance of the film. In addition, by adjusting the concentration and proportion of Li + , Ca 2+ and Mg 2+ , the comprehensive performance of polyamide-imide film can be further regulated to meet the specific needs of different application scenarios.

[0077] The present application prevents plastic slip of macromolecular chains in the process of cyclic bending by constructing cross-linking structure in the film, thereby avoiding the formation of early creases. Based on the presence of functional amide bonds, colorless alkaline earth metal ions are introduced to form non-covalent cross-linking network by coordination with amide bonds, which can jointly act with hydrogen bond network to form cross-linking network, effectively dissipating the energy generated by molecular chains in the process of bending. At the same time, benzene rings or imide rings can form cation-π interaction with alkaline earth metal cations, which is different from the electron sharing and orbital overlap in coordination bonds, and has the characteristics of colorless transparency, large bonding area and adjustable bond angle. In particular, Ca 2+ or Mg 2+ ions, due to their high valence and multi-coordination ability, make the use amount of Ca 2+ or Mg 2+ salt more controllable, and the cost is relatively low.

[0078] The present application constructs monovalent / multi-valence ion coordination non-covalent cross-linking network through the synergistic effect of the above strategies, enhances the interaction force and entropy elasticity between molecular chains to prevent plastic slip of macromolecular chains in the process of cyclic folding, and then obtains a bending-resistant transparent polyamide-imide film with energy dissipation mechanism, which has excellent flexibility and high light transmittance.

[0079] The bending-resistant transparent polyamide-imide film prepared by the present application has a tensile strength of up to 160 MPa, a transmittance of >88% at a wavelength of 550 nm, a yellowness value of <3, and can be bent 200,000 times without breaking at a bending radius of 0.5 mm. The bending-resistant transparent polyamide-imide film of the present application can be used for flexible display devices.

[0080] The present application is further illustrated by the following examples.

[0081] Example 1

[0082] The present embodiment provides a preparation method of a bending-resistant transparent polyamide-imide film, comprising:

[0083] S1, under the protection of nitrogen, 10 mmol of 2,2'-bis(trifluoromethyl)-4,4'-diaminobiphenyl TFMB is dissolved in 60 ml of N,N-dimethylacetamide to obtain a diamine solution; 4 mmol of 2,2'-bis(3,4-dicarboxylic acid) hexafluoropropane dianhydride is added to the above diamine solution, and mixed and stirred at a speed of 400 rpm for 3 h to obtain a polyamide acid solution;

[0084] S2, adding 50 mg of LiCl solid to the polyamic acid solution, mixing and stirring at 400 rpm for 1 hour to obtain a Li ion-coordinated polyamic acid mixed solution; adding 6 mmol of terephthaloyl chloride (TPC) to the Li ion-coordinated polyamic acid mixed solution, and adding DMAc dropwise to maintain the solid content of the resulting mixed solution after the addition of TPC at 10 wt %. After the addition is completed, stirring and reacting at a rate of 250 rpm at room temperature for 12 hours to obtain a polyamide-amic acid (PAA+PA) solution;

[0085] S3, adding 8 mmol of acetic anhydride and 4 mmol of anhydrous pyridine to the polyamide-amic acid solution, reacting at 60° C. for 4 h, and after the reaction, precipitating with a mixture of methanol and water (the volume ratio of methanol to water in the mixture is 1:1) as a precipitant. The precipitate is washed with deionized water to obtain a white fibrous polyamide-imide resin;

[0086] S4, 2g of polyamide-imide resin was dissolved in DMAC, and CaCl2 powder (1wt% of the mass of the polyamide-imide resin) was added, and the mixture was stirred at 200 rpm for 6h at room temperature, and then vacuum-degassed at room temperature to obtain a mixture containing 1wt% CaCl2. 2+ A polyamide-imide solution; wherein the solid content of the polyamide-imide solution is 10%;

[0087] S5 The above-mentioned 1wt% Ca 2+ The polyamide-imide solution was evenly coated on a glass bottom plate by a coating machine, and then placed in a vacuum drying oven, first kept at 80°C for 1 hour, then kept at 180°C for 1 hour, and finally kept at 250°C for 1 hour. After cooling to room temperature, it was taken out and placed in deionized water. After the film automatically fell off, it was placed in a vacuum drying oven at 80°C for 2 hours to obtain a transparent polyamide-imide film, which was recorded as CPAI-1%Ca.

[0088] The chemical structure of the transparent polyamide-imide film prepared in Example 1 is shown below.

[0089]

[0090] Example 2

[0091] The difference between Example 2 and Example 1 is that in step S4, CaCl2 is replaced by MgCl2, and the rest is the same as Example 1. The transparent polyamide-imide film prepared in Example 2 is denoted as CPAI-1%Mg.

[0092] Example 3

[0093] Example 3 differs from Example 1 in that 1 wt% CaCl2in step S4 is replaced by 0.5 wt% CaCl2and 0.5 wt% MgCl2, and the rest is the same as Example 1. The transparent polyamide-imide film prepared in Example 3 is denoted as CPAI-1%CaMg.

[0094] Example 4

[0095] Example 4 differs from Example 1 in that the amount of CaCl2in step S4 is changed to 0.5 wt%, and the rest is the same as Example 1. The transparent polyamide-imide film prepared in Example 4 is denoted as CPAI-0.5%Ca.

[0096] Example 5

[0097] Example 5 differs from Example 1 in that the amount of CaCl2in step S4 is changed to 2 wt%, and the rest is the same as Example 1. The transparent polyamide-imide film prepared in Example 5 is denoted as CPAI-2%Ca.

[0098] Example 6

[0099] Example 6 differs from Example 1 in that the amount of CaCl2in step S4 is changed to 5 wt%, and the rest is the same as Example 1. The transparent polyamide-imide film prepared in Example 6 is denoted as CPAI-5%Ca.

[0100] Example 7

[0101] Example 7 differs from Example 1 in that the amount of CaCl2in step S4 is changed to 10 wt%, and the rest is the same as Example 1. The transparent polyamide-imide film prepared in Example 7 is denoted as CPAI-10%Ca.

[0102] Example 8

[0103] Example 8 differs from Example 1 in that 1 wt% CaCl2in step S4 is replaced by 0.5 wt% MgCl2, and the rest is the same as Example 1. The transparent polyamide-imide film prepared in Example 8 is denoted as CPAI-0.5%Mg.

[0104] Example 9

[0105] Example 9 differs from Example 1 in that 1 wt% CaCl2in step S4 is replaced by 2 wt% MgCl2, and the rest is the same as Example 1. The transparent polyamide-imide film prepared in Example 9 is denoted as CPAI-2%Mg.

[0106] Example 10

[0107] Example 10 differs from Example 1 in that 1 wt% CaCl2 is replaced by 5 wt% MgCl2 in step S4, and the rest is the same as Example 1. The transparent polyamide-imide film prepared in Example 10 is denoted as CPAI-5%Mg.

[0108] Example 11

[0109] Example 11 differs from Example 1 in that 1 wt% CaCl2 is replaced by 10 wt% MgCl2 in step S4, and the rest is the same as Example 1. The transparent polyamide-imide film prepared in Example 11 is denoted as CPAI-10%Mg.

[0110] Example 12

[0111] Example 12 differs from Example 1 in that 1 wt% CaCl2 is replaced by 0.25 wt% CaCl2 and 0.25 wt% MgCl2 in step S4, and the rest is the same as Example 1. The transparent polyamide-imide film prepared in Example 12 is denoted as CPAI-0.5%CaMg.

[0112] Example 13

[0113] Example 13 differs from Example 1 in that 1 wt% CaCl2 is replaced by 1 wt% CaCl2 and 1 wt% MgCl2 in step S4, and the rest is the same as Example 1. The transparent polyamide-imide film prepared in Example 13 is denoted as CPAI-2%CaMg.

[0114] Example 14

[0115] Example 14 differs from Example 1 in that 1 wt% CaCl2 is replaced by 2.5 wt% CaCl2 and 2.5 wt% MgCl2 in step S4, and the rest is the same as Example 1. The transparent polyamide-imide film prepared in Example 14 is denoted as CPAI-5%CaMg.

[0116] Example 15

[0117] Example 15 differs from Example 1 in that 1 wt% CaCl2 is replaced by 5 wt% CaCl2 and 5 wt% MgCl2 in step S4, and the rest is the same as Example 1. The transparent polyamide-imide film prepared in Example 15 is denoted as CPAI-10%CaMg.

[0118] Comparative Example 1

[0119] The difference between the comparative example and Example 1 is that no multivalent alkali earth metal salt CaCl2 is added in S4, and the rest is the same as Example 1. The transparent polyamide-imide film prepared in the comparative example 1 is denoted as CPAI.

[0120] The actual object of the transparent polyamide-imide film prepared in Example 1 is shown in FIG. 1a, which presents a colorless transparent state and good bending resistance; the SEM spectrum thereof is shown in FIG. 1b, which does not present phase separation and crystallization phenomenon. Figure 1 Figure 1 The actual object of the transparent polyamide-imide film prepared in Example 1 is shown in FIG. 1a, which presents a colorless transparent state and good bending resistance; the SEM spectrum thereof is shown in FIG. 1b, which does not present phase separation and crystallization phenomenon.

[0121] The transparent polyamide-imide films CPAI-1%Ca, CPAI-1%Mg and CPAI-1%CaMg and CPAI prepared in Examples 1-3 and the comparative example 1 are subjected to infrared spectrum analysis, and the results are shown in FIG. 2. Figure 2 As can be seen from FIG. 2, the strong absorption peaks caused by asymmetric and symmetric stretching of imine group C=O are respectively present at 1786 and 1724 cm-1, the absorption peak caused by stretching of imine group C-N is present at 1370 cm-1; in addition, the absorption peak caused by stretching of amide group C=O is present at 1674 cm-1, and the absorption band of amide bond of polyamide acid (present at 1662 cm-1and 1713 cm-1) completely disappears, indicating that the CPAI film after heat treatment has been completely imidized. Figure 2 -1 -1 -1 -1 -1

[0122] The transparent polyamide-imide films prepared in Examples 1-15 and the comparative example 1 are subjected to optical performance test, and the test results are shown in Table 1.

[0123] Table 1 Optical performance data of transparent polyamide-imide film

[0124]

[0125] As can be seen from Table 1, the optical performance of the polyamide-imide film prepared in Examples 1-15 by introducing alkali earth metal is equivalent to that of the polyamide-imide film of the comparative example 1 without alkali earth metal, and a small amount of introduced metal can effectively ensure the optical performance of the CPAI film.

[0126] The transparent polyamide-imide films prepared in Examples 1-15 and the comparative example 1 are subjected to mechanical performance test, and the test results are shown in Table 2.

[0127] Table 2 Mechanical performance data of transparent polyamide-imide film

[0128] Embodiment CPAI Tensile strength / MPa Elastic modulus / GPa Elongation at break / % Comparative Example 1 CPAI 138.2±6.07 4.0±0.11 6.66±0.97 Example 4 CPAI-0.5% Ca 158.22±4.42 5.0±0.15 5.20±0.58 Example 1 CPAI-1% Ca 156.30±7.82 4.62±0.23 7.83±0.38 Example 5 CPAI-2% Ca 160.21±2.87 4.61±0.08 12.88±1.05 Example 6 CPAI-5% Ca 135.18±2.76 3.95±0.04 21.65±4.07 Example 7 CPAI-10% Ca 104.21±3.17 3.08±0.08 25.61±3.24 Example 8 CPAI-0.5% Mg 145.57±1.71 4.23±0.08 8.33±0.53 Example 2 CPAI-1% Mg 155.67±5.30 4.51±0.10 9.53±0.87 Example 9 CPAI-2% Mg 159.76±1.12 4.71±0.03 20.73±3.19 Example 10 CPAI-5% Mg 116.62±5.82 3.39±0.04 34.88±5.44 Example 11 CPAI-10% Mg 81.6±4.83 2.79±0.11 13.90±2.18 Example 12 CPAI-0.5% CaMg 140.32±2.18 4.04±0.07 6.89±0.58 Example 3 CPAI-1% CaMg 152.60±5.12 4.14±0.09 8.11±0.72 Example 13 CPAI-2% CaMg 166.92±4.28 4.54±0.12 15.51±1.07 Example 14 CPAI-5% CaMg 125.91±6.07 3.42±0.19 19.98±1.57 Example 15 CPAI-10% CaMg 129.37±2.77 3.16±0.10 41.40±2.88

[0129] ​​​​​​​From Table 2, it can be seen that the introduction of a small amount of alkaline earth metal coordination can effectively improve the mechanical properties of the film, wherein CPAI-2%Ca has the highest tensile strength (160.21 MPa), CPAI-0.5%Ca has the highest elastic modulus (5.0 GPa), and CPAI-10%CaMg has the highest elongation at break (41.40%). The above results show that a small amount of ion introduction (0.5-2%) can form a multi-coordination form to effectively improve the tensile strength of the film, and excessive ion introduction overflow forms a single coordination form to improve the elongation at break of the film.

[0130] The transparent polyamide-imide films CPAI, CPAI-1%Ca, CPAI-1%Mg and CPAI-1%CaMg prepared in Comparative Example 1 and Examples 1-3 were subjected to a bending resistance test, and the above transparent polyamide-imide films were respectively bent 200,000 times at a bending radius of 0.5 mm, and the morphology of the bending part was observed, and the SEM spectra are shown in Figure 3 Figure 3 Among them, Figure 3 a, b, c, d, respectively, are the SEM images of the bending parts of CPAI, CPAI-1%Ca, CPAI-1%Mg and CPAI-1%CaMg. It can be seen from

[0131] Although the present application has been described in detail in the general description and specific embodiments herein, modifications or improvements can be made to the present application on the basis of the present application, which will be apparent to those skilled in the art. Therefore, these modifications or improvements made on the basis of not deviating from the spirit of the present application, all belong to the scope of protection claimed by the present application.​

Claims

1. A method for producing a transparent polyamide-imide film resistant to bending, characterized by, The preparation method comprises the following steps: S1, dissolving a diamine monomer in a polar organic solvent to obtain a diamine solution; dissolving a dianhydride monomer in the diamine solution to perform a polymerization reaction, and obtaining a polyamic acid solution; S2, adding a monovalent alkali metal salt into the polyamic acid solution, and then adding an acyl chloride monomer to perform a copolymerization reaction, and obtaining a precursor solution; The monovalent alkali metal salt is LiCl; S3, adding a dehydrating agent and a catalyst into the precursor solution to perform a dehydration imidization, and obtaining a polyamide-imide solution; mixing the polyamide-imide solution with a precipitating agent to perform a precipitation, and obtaining a polyamide-imide solid containing a monovalent alkali metal salt; S4, dissolving the polyamide-imide solid containing the monovalent alkali metal salt in a polar organic solvent, and then adding a multivalent alkaline earth metal salt to stir uniformly; performing a filtration and a defoaming to obtain a multi-coordinated polyamide-imide solution; the multivalent alkaline earth metal salt comprises at least one of CaCl2 or MgCl2; S5, applying the multi-coordinated polyamide-imide solution to a surface of a substrate, and performing a temperature rising solidification to obtain a bend-resistant transparent polyamide-imide film.

2. The production method according to claim 1, characterized by, The diamine monomer comprises at least one of compounds shown in 1-1a to 1-15a:

3. The production method according to claim 1, characterized by, The dianhydride monomer comprises at least one of compounds shown in 1-1b to 1-12b:

4. The method of claim 1, wherein, The acyl chloride monomer comprises at least one of compounds shown in 1-1c to 1-8c:

5. The preparation method according to claim 1, characterized in that The polar organic solvent comprises at least one of N-methyl pyrrolidone (NMP), N,N-dimethylformamide (DMF) or N,N-dimethylacetamide (DMAc); The dehydrating agent is acetic anhydride; The catalyst is pyridine, triethylamine or isoquinoline; The precipitating agent is deionized water, ethanol or a mixture of deionized water and ethanol.

6. The method of claim 1, wherein, The ratio of the number of moles of the diamine monomer to the total number of moles of the dianhydride monomer and the acyl chloride monomer is 1:(1-1.03); the molar ratio of the dianhydride monomer to the acyl chloride monomer is 9:1-1:9; The molar ratio of the dehydrating agent to the dianhydride monomer is (2-2.5):1; The amount-of-substance ratio of the dehydrating agent to the catalyst is 1:(0.5-2).

7. The preparation method according to claim 1, characterized in that The addition amount of the monovalent alkali metal salt is 0.5-2.0 wt% of the sum of the use amounts of the diamine monomer and the dianhydride monomer; The addition amount of the multivalent alkaline earth metal salt is 0.5-10 wt% of the mass of the polyamide-imide solid containing the monovalent alkali metal salt.

8. The method of claim 1, wherein, In step S3, the dehydration imidization is performed at a temperature of 20-60 ℃ for 1-24 h; The precipitation is performed at a temperature of 10-40 ℃.

9. The bend-resistant transparent polyamide-imide film prepared by the preparation method in any one of claims 1-8.

10. The application of the bend-resistant transparent polyamide-imide film in claim 9 in a flexible display device.

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