Polyamide-imide as well as preparation method and application thereof

Abstract

The invention discloses polyamide-imide as well as a preparation method and application thereof, belongs to the technical field of flexible display, and solves the problem that related materials in the prior art cannot have excellent light transmittance, low thermal expansion and excellent bending resistance at the same time. The polyamide-imide has the following structure: # imgabs0 #. Based on the molecular design of an AB-TFMB diamine monomer and gradient activity synergistic process innovation, a polyamide-imide film system with ultrahigh mechanical properties and application stability is successfully constructed.

Description

Technical Field

[0001] The present invention relates to the technical field of flexible display, and more particularly to a polyamide-imide, a preparation method thereof, and an application thereof. Background Art

[0002] With the accelerated evolution of the fifth-generation flexible display technology towards ultra-thinness and foldability, the colorless transparent polyimide (CPI) film, as the core substrate material of flexible display devices, faces challenges in multi-dimensional performance limits. Under ultra-thin working conditions, the material needs to simultaneously meet the following requirements: ① withstand more than 200,000 dynamic bends with a curvature radius of 1 mm without plastic deformation accumulation; ② maintain ultra-high mechanical strength (tensile strength > 150 MPa) and anti-creep characteristics; ③ maintain dimensional stability (CTE < 12 ppm / K) during a high-temperature process at 350 °C; ④ achieve a visible light transmittance > 88% (yellowness index YI < 3). However, the existing CPI systems have always been trapped in the inherent contradiction of "rigidity-flexibility imbalance" - although the rigid main-chain structure can reduce the thermal expansion coefficient, it causes stress whitening or wrinkling due to molecular chain slippage; while the flexible-chain design improves the bend resistance, but the CTE exceeds the standard and the mechanical properties drop sharply, severely restricting its industrial application in the high-end flexible OLED field.

[0003] There are three key technical bottlenecks in the existing technology system at the molecular engineering level: First, the antagonistic effect between the molecular enhancement mechanism and optical properties - although constructing a hydrogen bond network through amide bonds can increase the tensile strength to 130 MPa, the strong intermolecular force causes solution gelation, and the amide bond-containing system often requires the introduction of an acyl chloride process, which brings the problem of HCl corroding equipment. Second, the kinetic barrier of structure-activity synergy regulation - although using rigid monomers such as biphenyl dianhydride (BPDA) can reduce the CTE to 15 ppm / K, its low solubility parameter (δ = 23.5 MPa^1 / 2) results in insufficient molecular chain orientation, and more than 5% of residual stress is generated during the film-forming process. Third, the differential effect of the reaction activity of functional monomers - although the -CF3 group of the fluorine-containing monomer 6FDA can increase the light transmittance to 85%, its low reaction rate constant (k = 0.05 L / mol·min) leads to uneven polymerization degree distribution (PDI > 1.8), and the number-average molecular weight of the product < 5 × 10^4 g / mol. In addition, the traditional step-by-step film-making process has a risk of phase separation, and the chain breakage and yellowing (YI increase > 4) caused by direct thermal imidization and the high-cost defect of chemical imidization (dehydrating agent dosage > 30 wt%) form a process paradox. Summary of the Invention

[0004] The present invention provides a polyamide-imide, a preparation method thereof, and an application thereof, so as to solve the problem that the related materials in the existing technology cannot have excellent light transmittance, low thermal expansion, and excellent bend resistance at the same time.

[0005] In a first aspect, the present invention provides a polyamide-imide having a structure shown in Formula I:

[0006]

[0007] In a second aspect, the present invention provides a method for preparing the polyamide-imide according to any possible implementation manner of the first aspect, comprising the steps of: dissolving a diamine monomer in a polar organic solution containing a LiCl co-solvent under a nitrogen atmosphere to obtain a diamine solution, wherein the diamine monomer contains an amide bond; adding 2,2`-bis(3,4-dicarboxylic acid) hexafluoropropane dianhydride monomer, and adding 3,3',4,4'-biphenyltetracarboxylic dianhydride monomer after stirring at room temperature for 1 h; and repeating the operation of adding the two dianhydride monomers in sequence 3 to 5 times to carry out a polymerization reaction to obtain a polyamide-acid solution; adding a dehydrating agent and a catalyst to the polyamide-acid solution to carry out partial dehydration cyclization to obtain a partially preimidized polyamide-imide solution; coating the partially preimidized polyamide-imide solution, and then gradually raising the temperature to remove the solvent, demold, and perform high-temperature treatment to obtain the polyamide-imide film.

[0008] As a possible implementation manner, the molar ratio of the 2,2`-bis(3,4-dicarboxylic acid) hexafluoropropane dianhydride monomer to the 3,3',4,4'-biphenyltetracarboxylic dianhydride monomer is 9:1 to 81.

[0009] As a possible implementation manner, the amount of substance of the diamine monomer is m + n; and / or, the total amount of substance of the 2,2`-bis(3,4-dicarboxylic acid) hexafluoropropane dianhydride monomer added is m; and / or, the total amount of substance of the 3,3',4,4'-biphenyltetracarboxylic dianhydride monomer added is n; and / or, the molar ratio of the dehydrating agent to the catalyst is 2:1; and / or, the ratio of the amount of substance of the dehydrating agent to the sum of the amounts of substance of the two dianhydride monomers is 1.4 to 1.8:1.

[0010] As a possible implementation manner, the diamine monomer is N,N'-(2,2'-bis(trifluoromethyl)-[1,1'-biphenyl]-4,4'-diyl)bis(4-aminobenzamide); and / or, the mass concentration of the LiCl co-solvent is 0.5% to 2%; and / or, for the polar organic solution containing the LiCl co-solvent, the solvent is one or a combination of N-methylpyrrolidone, N,N-dimethylformamide, and N,N-dimethylacetamide; the solid content of the polymer solution is 5% to 25%, preferably 15% to 20%; and / or, the dehydrating agent is acetic anhydride; and / or, the catalyst is any one of pyridine, triethylamine, and isoquinoline.

[0011] As a possible implementation, the conditions for the dehydration cyclization are a temperature of 20 to 60 °C and a duration of 1 to 24 hours; and / or, the conditions for the polymerization reaction are: a stirring condition of 350 to 500 rpm and a reaction duration of 3 to 6 h.

[0012] As a possible implementation, the gradient heating for solvent removal, film peeling, and high-temperature treatment includes the steps of: treating at 50 to 100 °C for 10 to 30 min, then treating at 120 to 180 °C for 20 to 60 min, and then performing the film peeling; subjecting the film obtained by film peeling to high-temperature treatment at 200 to 280 °C for 0.5 to 1.5 h to obtain the polyamide-imide film.

[0013] As a possible implementation, it further includes the steps of: filtering and degassing the partially pre-imidated polyamide-imide solution and then performing the coating operation; and / or, after performing the film peeling operation, performing high-temperature treatment to obtain the polyamide-imide film.

[0014] As a possible implementation, the degassing method is any one of vacuum degassing or static degassing; and / or, the degassing temperature is room temperature.

[0015] In a third aspect, the present invention provides an application of the polyamide-imide according to any one of the possible implementations of the first aspect or the polyamide-imide prepared by the preparation method according to any one of the possible implementations of the second aspect in a flexible display.

[0016] Based on the molecular design of AB-TFMB diamine monomer and the innovation of gradient activity synergy process, the present invention successfully constructs a polyimide-amide film system with both ultra-high mechanical properties and processing stability, and its technical advantages are reflected in the following three dimensions:

[0017] (1) Molecular engineering innovation: Synergistic enhancement mechanism of AB-TFMB diamine monomer

[0018] The present invention introduces an AB-TFMB diamine monomer containing an amide bond, breaking through the technical shackles of the traditional hydrogen bond enhancement system; compared with the conventional acyl chloride process route, this monomer realizes: ① directly forming an intramolecular hydrogen bond network, increasing the tensile strength to 180 MPa; ② avoiding the corrosion risk of HCl by-products to the reaction equipment; ③ retaining the coordination active sites of the amide group and LiCl, providing a basis for the subsequent dynamic solubilization system.

[0019] (2) Process synergy innovation: Synergistic effect of gradient active polymerization and dynamic solubilization

[0020] ①Gradient active polymerization process: Based on the kinetic differences of monomers (6FDA: k = 0.05 L / mol·min; BPDA: k = 0.18 L / mol·min), a stepwise feeding strategy of "prioritize low-activity - enhance high-activity" is adopted. When 6FDA is initially added, the electron shielding effect of its -CF3 group reduces the formation rate of CTC complexes; later, BPDA is introduced to construct a rigid backbone, and the molecular chain orientation is regulated through the π-π stacking of the biphenyl structure, precisely controlling the CTE at 9.8 ppm / K.

[0021] ②LiCl dynamic solubilization system: Innovatively adopt LiCl additive, and inhibit hydrogen bond-induced gelation through the dynamic coordination of Cl- with amide groups, reducing the rotational viscosity of the solution to an easy-to-film viscosity; after film formation, the residual Li + forms an ionic coordination cross-linked network with carbonyl groups, significantly improving the toughness and creep resistance of the film.

[0022] (3) Breakthrough in forming process: Synergistic optimization of partial pre-imide dynamic forming and cost efficiency

[0023] The partial pre-imide process developed in this invention can, on the one hand, improve the solubility of the polymer and thus increase the solid content of the solution; on the other hand, through the synergistic effect of precisely controlling the imidization process and the forming process, it can significantly reduce production costs while improving production efficiency. Compared with the traditional direct thermal method (which easily causes molecular chain breakage and yellowing), chemical imidization (requiring a large amount of expensive dehydrating agents and catalysts), and low solid content, this technology implements thermoforming at the 70% - 90% imidization stage of the prepolymer, combines gradient heating to simultaneously complete solvent removal and residual reactions, reduces the usage of chemical reagents, increases the solid content, and shortens the process cycle. Pre-imide formation pre-constructs a stable molecular backbone, inhibits high-temperature yellowing, and optimizes the uniformity of molecular chain arrangement, improving the mechanical properties of the material while reducing the interfacial stress.

[0024] Based on this, the multi-gradient active polymerization process and dynamic imidization technology proposed in this invention can prepare polymers with high molecular weight and narrow distribution. The transparent polyamide-imide film prepared in this invention has a transmittance > 88% at a wavelength of 550 nm, a yellowness value < 3, and can be bent 200,000 times without breaking at a bending radius of 0.5 mm. Its highest tensile strength is 180 MPa, and its thermal expansion coefficient is < 10 ppm / K.

[0025] This invention discloses a method for preparing a bend-resistant, transparent polyamide-imide (CPAI) film for flexible display applications, achieving performance breakthroughs through the coordinated design of molecular structure and process. A ternary copolymer system is constructed using AB-TFMB diamine containing an amide bond, 6FDA dianhydride containing an electronegative -CF3 group, and BPDA dianhydride with a rigid biphenyl structure. The amide bonds of AB-TFMB diamine form an intermolecular hydrogen bond network, improving the film's mechanical strength while avoiding the HCl corrosion problem associated with the acyl chloride process. 6FDA inhibits charge transfer complexes, increasing transparency, while BPDA enhances molecular chain orientation and reduces CTE. A gradient active polymerization strategy was proposed as an innovative process. The raw materials were added in reverse order of reaction activity (BPDA>6FDA) and supplemented with 1wt% LiCl to prevent gelation, controlling the system viscosity to 8000±500cP. A dynamic imidization stage was used to form the film by thermal method at a prepolymer conversion rate of 70% to 90%. This ensured that the amount of chemical imidization reagent used was controlled to reduce costs while preventing yellowing during high-temperature treatment. Solvent removal and thermal imidization were simultaneously completed through gradient temperature increase, ultimately achieving a low CTE, high light transmittance, and bend-resistant flexible film by improving production efficiency.

[0026] The present invention achieves breakthroughs through three-dimensional innovations in gradient active polymerization process, LiCl dynamic solubilization system and pre-imide dynamic molding technology: ① The new process of "active gradient controlled polymerization" is pioneered. Based on the transition state theory, a step-by-step feeding sequence of rigid monomers is designed. Initially, low-activity 6FDA (conversion rate > 92%) is added to inhibit the formation of CTC complexes by utilizing the electron shielding effect of its -CF3 group; later, high-activity BPDA (k = 0.18L / mol·min) is introduced to construct a rigid skeleton to achieve synergistic optimization of CTE and transmittance. ② The development of LiCl dynamic coordination solubilization technology solves the problem of hydrogen bond-induced gelation through the reversible coordination of Cl- and amide groups. The addition of LiCl (0.5wt% to 1.2wt%) reduces the viscosity of the solution, and the residual Li+ forms an ionic coordination cross-linking network with the carbonyl group, significantly improving the toughness and creep resistance of the film. ③ Construct a dynamic imidization molding system, and implement thermoforming when the prepolymer reaches 70% to 90% imidization degree. Coupled with a gradient temperature ramp (80→250℃ / controllable rate), the solvent removal and residual imidization are simultaneously completed, eliminating interfacial stress defects and optimizing the uniformity of molecular chain arrangement.

[0027] This technology breaks through the "performance seesaw" effect of traditional CPI materials. The prepared polyamide-imide film has a transmittance (T550) of 88.21% at a wavelength of 550nm, a yellowness value YI = 2.7, no cracks after being bent 200,000 times with a curvature of 0.5mm, and a tensile strength of 180.3MPa, an elastic modulus of 5.7GPa and a CTE of 10.4ppm K. -1The equalization performance indicators such as these exceed the prior art. Through the multi-level collaboration of molecular design - process innovation - structure regulation, the integration of high light transmittance, low expansion, and ultra-high bend resistance characteristics is achieved for the first time in a single material system, providing a revolutionary material solution for the development of ultra-thin flexible electronic devices. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for the description of the embodiments or the prior art. Obviously, the drawings in the following description are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0029] Figure 1 Optical and SEM images of CPAI-5 / 5 provided by the embodiments of the present invention, where a is the optical image and b is the SEM image.

[0030] Figure 2 SEM image of the film surface of CPAI-5 / 5 provided by the embodiments of the present invention after being bent 200,000 times at a bending radius of 0.5 mm. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0031] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, rather than all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts belong to the scope of protection of the present invention.

[0032] To solve the problem that related materials in the prior art cannot simultaneously possess excellent light transmittance, expansion, and bend resistance performance, the embodiments of the present invention provide an experiment on the preparation of polyamide-imide, an experiment on performance measurement, and an application experiment.

[0033] The present invention provides a polyamide-imide film. In its preparation method, diamines containing amide bonds, BPDA and 6FDA are used as dianhydride monomers; the solubility of the monomers in the solvent is increased through a gradient living polymerization process while improving the light transmittance of the film, and LiCl is added as a co-solvent and remains in the film to form a coordination crosslinked network. Finally, a high-performance film is prepared through a dynamic imidization technique.

[0034] The preparation method provided by the present invention includes the steps of: dissolving a diamine monomer containing an amide bond in a polar organic solution containing a co-solvent under a nitrogen atmosphere to obtain a diamine solution; adding 6FDA monomer, and adding BPDA monomer after stirring at room temperature for 1 h; repeating the operation of adding 6FDA monomer and BPDA monomer 3 to 5 times, the total addition amount of 6FDA monomer is m mol, and the total addition amount of BPDA monomer is n mol to obtain a polyamide-acid solution; adding a dehydrating agent and a catalyst to the obtained polyamide-acid solution for dehydration cyclization to obtain a pre-imide polyamide-imide solution; coating the obtained pre-imide polyamide-imide solution, and then performing gradient heating, and obtaining a polyamide-imide film after demolding, which has the characteristics of low CTE value, bend resistance and transparency.

[0035] In the present invention, the preparation method of the diamine solution is preferably: dissolving the diamine monomer in a polar organic solvent under a protective gas condition, and the polar organic solvent is N-methylpyrrolidone (NMP), N,N-dimethylformamide (DMF) or N,N-dimethylacetamide (DMAc); preferably DMAc.

[0036] In the present invention, the diamine monomer in the diamine solution is preferably N,N'-(2,2'-bis(trifluoromethyl)-[1,1'-biphenyl]-4,4'-diyl)bis(4-aminobenzamide) (AB-TFMB).

[0037] In the present invention, the molar ratio m:n of the two dianhydride monomers added is 9:1 to 81.

[0038] In the present invention, the temperature of the polymerization reaction after adding the dianhydride monomer is preferably room temperature; the duration of the polymerization reaction is preferably 3 to 6 hours; the polymerization reaction is preferably carried out under stirring conditions at a rate of 350 to 500 rpm.

[0039] In the present invention, the polar organic solution containing LiCl co-solvent, its solvent is one or a combination of several of N-methylpyrrolidone, N,N-dimethylformamide and N,N-dimethylacetamide; the solid content of the polymer solution is 5% to 25%, preferably 15% to 20%.

[0040] After the copolymerization reaction, a dehydrating agent and a catalyst are added for cyclization dehydration to obtain a polyamide-imide solution. In the present invention, the dehydrating agent is preferably acetic anhydride; the catalyst is preferably one of pyridine, triethylamine or isoquinoline, more preferably anhydrous pyridine; the molar ratio of the dehydrating agent and the catalyst is more preferably 2:1; the molar ratio of the dehydrating agent to the dianhydride monomer is more preferably 1.4 to 1.8:1.

[0041] In the present invention, the cyclodehydration temperature condition is 20 to 60 °C; the dehydration duration is 1 to 24 hours, more preferably 6 to 12 hours.

[0042] Preferably, the preimidized polyamide-imide solution is filtered and degassed in sequence to obtain the polyamide-imide solution in the present invention. There are no special limitations on the operations of filtration and degassing in the present invention, and the conventional technical solutions of filtration and degassing in the art are adopted. In the present invention, the equipment for filtration is preferably a syringe filter or a sintered filter. In the present invention, the temperature for degassing is preferably room temperature; the degassing method is preferably vacuum degassing or static degassing.

[0043] After obtaining the preimidized polyamide-imide solution, the polyamide-imide solution is coated in the present invention, then the temperature is increased gradually, and then after demoulding and high-temperature treatment, a polyamide-imide film is obtained.

[0044] In the present invention, the gradual temperature increase is preferably: treating at 50 to 100 °C for 10 to 30 min, then treating at 120 to 180 °C for 20 to 60 min, and then demoulding; the film obtained by demoulding is subjected to high-temperature treatment at 200 to 280 °C for 0.5 to 1.5 h to obtain the polyamide-imide film.

[0045] The preparation method provided by the present invention is simple to operate, and is beneficial to obtaining a low-CTE bend-resistant transparent polyamide-imide film with excellent bend resistance, mechanical properties and light transmittance.

[0046] There are no special limitations on the application of the obtained polyamide-imide film in flexible displays in the present invention, and the application of the film in displays well-known to those skilled in the art can be adopted.

[0047] In the examples of the present invention, unless otherwise specified, all raw materials are commercially available products well-known to those skilled in the art.

[0048] The technical solutions of the present invention will be further described below in conjunction with specific examples.

[0049] Example 1

[0050] This example provides a preparation method of polyamide-imide.

[0051] Under nitrogen protection, N,N'-(2,2'-bis(trifluoromethyl)-[1,1'-biphenyl]-4,4'-diyl)bis(4-aminobenzamide) (AB-TFMB) (5.5847 g, 10 mmol) was dissolved in a polar organic solution [the solvent was N,N-dimethylacetamide (DMAc), and this solution contained a cosolvent of LiCl with a mass concentration of 1%] to obtain a diamine solution; the diamine solution was mixed and stirred at a speed of 400 rpm for 1 hour, 2,2`-bis(3,4-dicarboxylic acid) hexafluoropropane dianhydride (6FDA, 0.4442 g, 1 mmol) was added, and after stirring at room temperature for 1 h, 3,3',4,4'-biphenyltetracarboxylic dianhydride (BPDA, 0.2942 g, 1 mmol) was added. 1 mmol of 6FDA and 1 mmol of BPDA were repeatedly added four times to obtain a polyamide-acid solution.

[0052] Acetic anhydride (1.6334 g, 16 mmol) and anhydrous pyridine (0.6328 g, 8 mmol) [wherein, the molar ratio of acetic anhydride to anhydrous pyridine was 1:0.5, and the molar ratio of acetic anhydride to dianhydride monomer (BPDA\6FDA) was 1.6:1] were added to the obtained polyamide-acid solution, and the reaction was carried out at 60 °C for 4 h to obtain a preimidized polyamide-imide solution.

[0053] The obtained preimidized polyamide-imide solution was filtered through a fritted filter, and then degassed under vacuum at room temperature to obtain a pure polyamide-imide solution; among them, the solid content of the polyamide-imide solution was 10%.

[0054] The obtained polyamide-imide solution was kept warm for 20 min, then treated at 150 °C for 40 min, and then demolded; the film obtained by demolding was treated at 240 °C for 1 h to obtain a polyamide-imide film, denoted as CPAI-5 / 5.

[0055] The optical picture and SEM picture of CPAI-5 / 5 are as Figure 1 shown, and the SEM picture of the film surface of CPAI-5 / 5 after bending 200,000 times with a bending radius of 0.5 mm is as Figure 2 shown. It can be seen that CPAI-5 / 5 presents a colorless and transparent state and has good anti-bending property; moreover, no phase separation and crystallization phenomenon occur.

[0056] Example 2

[0057] This example provides a preparation method of polyamide-imide.

[0058] The difference from Example 1 was that the molar ratio m:n of BPDA to 6FDA was 9:1, and a polyamide-imide film was obtained, denoted as CPAI-9 / 1.

[0059] Example 3

[0060] This example provides a method for preparing polyamide-imide.

[0061] The difference from Example 1 is that the molar ratio m:n of BPDA to 6FDA is 7:3, and a polyamide-imide film is obtained, denoted as CPAI-7 / 3.

[0062] Example 4

[0063] This example provides a method for preparing polyamide-imide.

[0064] The difference from Example 1 is that the molar ratio m:n of BPDA to 6FDA is 3:7, and a polyamide-imide film is obtained, denoted as CPAI-3 / 7.

[0065] Example 5

[0066] This example provides a method for preparing polyamide-imide.

[0067] The difference from Example 1 is that the molar ratio m:n of BPDA to 6FDA is 1:9, and a polyamide-imide film is obtained, denoted as CPAI-1 / 9.

[0068] Comparative Example 1

[0069] This comparative example provides a method for preparing polyamide-imide.

[0070] The difference from Example 1 is that 6FDA is not added, and a polyamide-imide film is obtained, denoted as CPAI-A.

[0071] The transparent polyamide-imide film prepared in this comparative example has a chemical structure as shown in Formula II:

[0072]

[0073] Comparative Example 2

[0074] This comparative example provides a method for preparing polyamide-imide.

[0075] The difference from Example 1 is that BPDA is not added, and a polyamide-imide film is obtained, denoted as CPAI-B.

[0076] The transparent polyamide-imide film prepared in this comparative example has a chemical structure as shown in Formula III:

[0077]

[0078] Comparative Example 3

[0079] This comparative example provides a method for preparing polyamide-imide.

[0080] The difference from Example 1 is that the feeding order of the two dianhydride monomers is to first add BPDA and then add 6FDA, and 1 mmol of BPDA and 1 mmol of 6FDA are repeatedly added in sequence four times to obtain a polyamide-imide film, denoted as CPAI-C.

[0081] Comparative Example 4

[0082] This comparative example provides a method for preparing polyamide-imide.

[0083] The difference from Example 1 is that a sufficient amount of 6FDA is added, and a sufficient amount of BPDA is added after stirring at room temperature for 1 h without repeating the addition, to obtain a polyamide-imide film, denoted as CPAI-D.

[0084] Comparative Example 5

[0085] This comparative example provides a method for preparing polyamide-imide.

[0086] The difference from Example 1 is that the molar ratio of acetic anhydride to anhydrous pyridine is 1:0.5, and the molar ratio of acetic anhydride to dianhydride monomers (BPDA\6FDA) is 2.4:1, to obtain a polyamide-imide film, denoted as CPAI-E.

[0087] Comparative Example 6

[0088] This comparative example provides a method for preparing polyamide-imide.

[0089] The difference from Example 1 is that pre-imideization is not carried out (dehydrating agent and catalyst are not added), to obtain a polyamide-imide film, denoted as CPAI-F.

[0090] Example 6

[0091] This example provides a performance test experiment.

[0092] Various performance tests were carried out on the polyamide-imide films obtained in Examples 1 to Comparative Example 2, and the results shown in Table 1 were obtained.

[0093] Table 1 Performance test results

[0094]

[0095] As can be seen from Table 1, as the content of BPDA increases, the CTE value decreases; while as the content of 6FDA increases, the transmittance at 550 nm (T550) increases and the yellowness decreases. Among them, the comprehensive performance of CPAI-5 / 5 is the best, ensuring 10.4 ppm K-1 In addition to a CTE of Figure 2 As can be seen from

[0096] In Comparative Example 1, the copolymerized BPDA exhibits a linear molecular chain arrangement, with a tensile strength of 208.0 Mpa, an elastic modulus of 6.2 GPa, and a Tg of 420 °C while maintaining a CTE value of 3.2 ppm K -1 However, its strong conjugation effect results in a decline in the optical properties of the film (YI value is 7.9). In Comparative Example 2, the introduction of electronegative groups in the copolymerized 6FDA leads to a decrease in CTC and an improvement in optical properties (YI value is only 1.9), but the low linearity results in a CTE value as high as 38.0 ppm K -1 and poor mechanical properties.

[0097] Compared with Example 1 in Comparative Example 3, adding the highly active BPDA monomer first and then the less active 6FDA monomer easily leads to a lower molecular weight and a wide molecular weight distribution, thereby causing the mechanical properties of the film to decrease by about 15%, and both the CTE value (19.8) and the yellowness value (4.1) increase.

[0098] Compared with Example 1 in Comparative Example 4, non-stepwise addition will also result in a wide molecular weight distribution of the polymer solution. The properties of the obtained film are not only relatively worse in terms of the CTE value (28.1) and the yellowness value (5.6), but also the mechanical properties decrease by 20%, failing to meet the commercial requirements.

[0099] Compared with Example 1 in Comparative Example 5, in addition to the high cost due to the large amount of chemical reagents required for complete chemical imidization, the inability to eliminate interfacial stress defects and optimize the uniformity of molecular chain arrangement leads to a decline in all properties. And in Comparative Example 6, thermal imidization results in a serious increase in yellowness and fails to meet the requirements.

[0100] The technical features of the above-described embodiments can be combined arbitrarily. For the sake of brevity of description, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, it should be considered as the scope described in this specification.

[0101] Although the preferred embodiments of the present invention have been described, those skilled in the art can make additional changes and modifications once they know the basic creative concept. Therefore, the appended claims are intended to be construed to include the preferred embodiments as well as all changes and modifications falling within the scope of the present invention.

[0102] Obviously, those skilled in the art can make various modifications and variations to the present invention without departing from the spirit and scope of the present invention. Thus, if these modifications and variations of the present invention fall within the scope of the claims of the present invention and their equivalent technologies, the present invention also intends to include these modifications and variations.

Claims

1. A polyamide-imide, characterized in that, It has the structure shown in Formula I:

2. The preparation method of the polyamide-imide according to claim 1, characterized in that, It includes the steps: Under a nitrogen atmosphere, dissolve the diamine monomer in a polar organic solution containing LiCl cosolvent to obtain a diamine solution, and the diamine monomer contains an amide bond; Add 2,2`-bis(3,4-dicarboxylic acid) hexafluoropropane dianhydride monomer, and after stirring at room temperature for 1 h, add 3,3',4,4'-biphenyltetracarboxylic dianhydride monomer; and repeat the operation of adding the two dianhydride monomers in sequence 3 to 5 times to carry out the polymerization reaction to obtain a polyamide-acid solution; Add a dehydrating agent and a catalyst to the polyamide-acid solution to carry out partial dehydration cyclization to obtain a partially preimidized polyamide-imide solution; Coat the partially preimidized polyamide-imide solution, and then gradually increase the temperature to remove the solvent, demold, and perform high-temperature treatment to obtain the polyamide-imide film.

3. The preparation method according to claim 2, characterized in that, The molar ratio of the 2,2`-bis(3,4-dicarboxylic acid) hexafluoropropane dianhydride monomer to the 3,3',4,4'-biphenyltetracarboxylic dianhydride monomer is 9:1 to 81.

4. The preparation method according to claim 2, characterized in that, The amount of substance of the diamine monomer is m + n; And / or, the total amount of substance of the 2,2`-bis(3,4-dicarboxylic acid) hexafluoropropane dianhydride monomer added is m; And / or, the total amount of substance of the 3,3',4,4'-biphenyltetracarboxylic dianhydride monomer added is n; And / or, the molar ratio of the dehydrating agent to the catalyst is 2:1; And / or, the ratio of the amount of substance of the dehydrating agent to the sum of the amounts of substance of the two dianhydride monomers is 1.4 to 1.8:

1.

5. The preparation method according to claim 2, characterized in that, The diamine monomer is N,N'-(2,2'-bis(trifluoromethyl)-[1,1'-biphenyl]-4,4'-diyl)bis(4-aminobenzamide); And / or, the mass concentration of the LiCl cosolvent is 0.5% to 2%; And / or, for the polar organic solution containing LiCl cosolvent, its solvent is one or a combination of several of N-methylpyrrolidone, N,N-dimethylformamide, and N,N-dimethylacetamide; the solid content of the polymer solution is 5% to 25%; And / or, the dehydrating agent is acetic anhydride; And / or, the catalyst is any one of pyridine, triethylamine, and isoquinoline.

6. The preparation method according to claim 2, characterized in that, The conditions for the dehydration cyclization are a temperature of 20 to 60 °C and a duration of 1 to 24 hours; And / or, the conditions for the polymerization reaction are: a stirring condition of 350 to 500 rpm and a reaction duration of 3 to 6 h.

7. The preparation method according to claim 2, wherein The gradual temperature increase to remove the solvent, demold, and perform high-temperature treatment includes the steps: Treat at 50 to 100 °C for 10 to 30 min, then treat at 120 to 180 °C for 20 to 60 min, and then perform the demolding; Perform high-temperature treatment on the film obtained by demolding at 200 to 280 °C for 0.5 to 1.5 h to obtain the polyamide-imide film.

8. The preparation method according to claim 2, characterized in that, It also includes the steps: Filter and defoam the partially preimidized polyamide-imide solution and then perform the coating operation.

9. The preparation method according to claim 8, characterized in that, The defoaming method is any one of vacuum defoaming or static defoaming; And / or, the defoaming temperature is room temperature.

10. Use of the polyamide-imide according to claim 1 or the polyamide-imide prepared by the preparation method according to any one of claims 2 to 9 in a flexible display.

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