High-heat-resistance low-birefringence colorless polyimide film and preparation and application thereof

Abstract

The application discloses preparation and application of high-heat-resistance low-birefringence colorless polyimide film. Specifically, based on a fluorine-containing side biphenyl m-phenylenediamine, homopolymerization (or copolymerization) is carried out with commercial dianhydride, diamine and the like, and high-performance colorless polyimide (CPI) film is prepared through casting, heat treatment and other processes. The fluorine-containing side biphenyl m-phenylenediamine is synthesized through two steps from commercial raw materials through Suzuki coupling reaction and reduction reaction, and the fluorine-containing side biphenyl m-phenylenediamine can be efficiently synthesized. The CPI film material prepared in the application exhibits excellent high-heat-resistance, low-birefringence, high-optical-transparency, low-dielectric performance and the like, and has potential application in the fields of flexible liquid crystal, organic light-emitting diode display device, wearable equipment, intelligent vehicle window and the like. In addition, the synthesis of the fluorine-containing side biphenyl diamine is convenient, the route is simple, the reaction is efficient, and the fluorine-containing side biphenyl diamine is suitable for large-scale production.

Description

Technical Field

[0001] This invention belongs to the field of optical thin films, specifically relating to a high heat-resistant, low birefringence colorless polyimide thin film and its preparation and application. Background Technology

[0002] For future displays, thinness, flexibility, and wearability have become irreversible development trends, especially for flexible liquid crystal displays (LCDs) and organic light-emitting diodes (OLEDs). Currently, the key optical cover and substrate for commercialization is flexible ultrathin glass (UTG), but its brittleness, high density, and difficulty in processing greatly limit its development into a new generation of large-size, multi-curved displays. In contrast, high-transparency optical polymer films are the best solution, with the following performance requirements: 1) high transparency; 2) high colorlessness and low yellowness; 3) high heat resistance, including high glass transition temperature and high thermal decomposition temperature; 4) high dimensional thermal stability; 5) low birefringence; and 6) easy processing and molding. Therefore, colorless polyimide (CPI) films, with their high heat resistance and dimensional stability, have been selected as the preferred film material for flexible displays.

[0003] Currently, the vast majority of published or publicly available CPI films exhibit large birefringence values ​​(Δn). This is because the rigid aromatic macromolecular chains in the casting process are more likely to align in-plane, resulting in a lower in-plane refractive index (n) of the film. TE The refractive index is much higher than that in the out-of-plane direction (n). TM For example, CN 113429785, CN 109348718, and Macromolecules 1998, 31, 2080-2086 all demonstrate high Δn values ​​(>0.03) for CPI films. High birefringence delays light, reducing color contrast and increasing color shift at viewing angles, thus negatively impacting color reproduction in optical displays. Therefore, designing and adjusting the molecular chain structure is crucial for achieving low birefringence in optical films. Furthermore, simultaneously achieving high transparency, high colorlessness, high glass transition temperature, and low birefringence in CPI films is even more challenging. Macromolecules (2007, 40, 7902-7909), Macromolecules (2015, 48, 3462-3474), and Polymer (2023, 265, 125579) have reported a series of excellent sulfur-containing CPI films with Δn values ​​much less than 0.01, as low as 0.002, and glass transition temperatures T0. g The relatively low Tg levels (Tg < 300℃, even < 250℃) limit its further application. CN 113429785 discloses a CPI film based on a fluorene-containing or polymethyl structure that can simultaneously satisfy high transparency (Tg < 300℃, even < 250℃). 550 >89%, YI≤3) and high glass transition (Tg The requirement is for temperatures above 350℃, but the birefringence (Δn value) is greater than 0.01; similar situations also appear in CN 109348718 and CN101959935, which adversely affect their expanded applications. Similarly, Polymer 2017,117,76-83 also discloses CPI thin films with fluorene side-group structures, which achieve ultra-low birefringence (Δn as low as 0.0005) through structural adjustment, but still have a low glass transition temperature (T). g <320℃). Summary of the Invention

[0004] In view of the key optical films in the background technology, especially meeting the requirements of high heat resistance (high glass transition temperature, high thermal decomposition temperature), low birefringence, and high optical transparency, this invention provides a high heat resistance, low birefringence, colorless polyimide film and its preparation and application. The high heat resistance, low birefringence, colorless polyimide film is characterized by being prepared by polymerization of the key fluorinated side biphenyl m-phenylenediamine.

[0005] A first aspect of the present invention is to provide a colorless polyimide film with high heat resistance and low birefringence, wherein the polyimide has the following general structural formula:

[0006]

[0007] Wherein, Ar1 is a residue of fluorinated biphenyl-m-phenylenediamine, R0 is independently selected from one or both of F and CF3, q is the number of substitutions of R0 group (1 to 2), and has the characteristic structure of fluorinated biphenyl-m-phenyl; R1 and R2 are residues of commercial dianhydrides; R3 is a residue of commercial diamine, n and m are the repeating number of the corresponding structural unit, and the ratio of n:m ranges from 1:0 to 5:95.

[0008] Preferably, the fluorinated side-biphenyl m-phenylenediamine is limited to one or more of the structural formulas I-1 to I-19:

[0009]

[0010] Preferably, the commercially available dianhydride is limited to one or more of structural formulas II-1 to II-9:

[0011]

[0012] Preferably, the commercially available diamine is limited to one or more of the structural formulas III-1 to III-6:

[0013]

[0014] R4 is independently selected from one or both of F and CF3, and p is the number of substitutions of the R4 base, with p ranging from 1 to 2.

[0015] Preferably, the glass transition temperature (Tg) of the high heat resistance, low birefringence, colorless polyimide film is 383–425°C, and the optical transmittance at 550 nm is T0. 550 The yellowness index (YI) ranges from 88.4% to 89.7%, and the corresponding thermogravimetric temperature (T) for 5 wt% is 1.25 to 2.77. d5％ The temperature range is 457–536℃, and the optical birefringence Δn is 0.0007–0.0048.

[0016] Preferably, the high heat-resistant, low birefringence colorless polyimide film has a dielectric constant D at a high frequency of 10 GHz. k The value is 2.64 to 3.05, and the dielectric loss D f The value ranges from 0.006 to 0.021.

[0017] The second aspect of the present invention is to provide a method for preparing the fluorinated side-biphenyl-m-phenylenediamine described in the first aspect, wherein the novel fluorinated side-biphenyl-m-phenylenediamine (structural formulas I-1 to I-19) is synthesized efficiently in only two steps using the classic Suzuki coupling reaction and reduction reaction.

[0018] Specifically, the following steps are included:

[0019] (1) Under an inert atmosphere, halo-dinitrobenzene compounds and fluorinated phenylboron compounds undergo Suzuki coupling catalyzed by a palladium catalyst in an alkaline environment to obtain fluorinated side-biphenyldinitrobenzene.

[0020] (2) Under an inert atmosphere, the fluorinated biphenyl dinitro group described in step (1) is reduced to obtain a novel fluorinated side-biphenyl m-phenylenediamine.

[0021] Preferably, the halodinitrobenzene compound in step (1) is one of the structural formulas IV-1 to IV-2.

[0022]

[0023] Preferably, the fluorinated phenylboronide in step (1) is one of the structural formulas V-1 to V-38.

[0024]

[0025] Preferably, the palladium catalyst in step (1) is one or more of Pd(PPh3)4, Pd(OAc)2, PdCl2, PdCl2(dppf), and PdCl2(PPh3)2, and the molar ratio of the palladium catalyst to the halodinitrobenzene is 0.001:1 to 0.1:1.

[0026] More preferably, the molar ratio of palladium catalyst to halodinitrobenzene is 0.01:1 to 0.05:1.

[0027] Preferably, the solvent used for Suzuki coupling in step (1) is one or more of dimethyl sulfoxide (DMSO), N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMAc), ethanol, isopropanol, tetrahydrofuran, ethyl acetate, dioxane, toluene, benzene, and water.

[0028] Preferably, the reagent used in the alkaline environment of step (1) is one or more of Cs2CO3, K2CO3, Na2CO3, NaHCO3, Li2CO3, K3PO4 and CsF.

[0029] Preferably, the reaction temperature for the Suzuki coupling in step (1) is 50–150 °C.

[0030] More preferably, the reaction temperature for the Suzuki coupling in step (1) is 80–120°C.

[0031] Preferably, the inert atmosphere in step (1) is either nitrogen or argon.

[0032] Preferably, the fluorinated biphenyl dinitro group in step (1) is one of the structural formulas VI-1 to VI-19.

[0033]

[0034] Preferably, the fluorinated side-biphenyl m-phenylenediamine in step (2) is one of the structural formulas I-1 to I-19.

[0035] Preferably, the inert atmosphere in step (2) is either nitrogen or argon.

[0036] Preferably, the solvent for the reduction reaction in step (2) is one or more of dimethyl sulfoxide (DMSO), N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMAc), ethanol, isopropanol, tetrahydrofuran, ethyl acetate, dioxane, toluene, benzene, and water.

[0037] Preferably, the temperature of the reduction reaction in step (2) is 50 to 150°C, and more preferably, the temperature of the reduction reaction is 70 to 90°C.

[0038] Preferably, the reduction system used in step (2) is a Pd / C and N2H4·H2O system or a SnCl2 and HCl system.

[0039] More preferably, in the Pd / C and N2H4·H2O system, the amount of Pd / C is 1 to 15 wt% of the fluorinated side-sided biphenyl dinitro group, and the amount of N2H4·H2O is 2 to 8 times the amount of the fluorinated side-sided biphenyl dinitro group.

[0040] More preferably, in the SnCl2 and HCl system, the amount of HCl added is 1 to 100 wt% of the fluorinated biphenyl dinitro group, and the amount of SnCl2 added is 2 to 16 times the amount of the fluorinated biphenyl dinitro group.

[0041] A third aspect of the present invention is to provide a method for preparing a colorless polyimide film with high heat resistance and low birefringence as described in the first aspect, wherein the colorless polyimide film is characterized by being prepared by polymerization of a fluorinated biphenyl side-group diamine: a homopolymerization (or copolymerization) reaction is carried out between a fluorinated biphenyl side-group diamine (I-1 to I-19) and commercially available dianhydrides (structural formulas II-1 to II-9) and commercially available diamines (structural formulas III-1 to III-6) under certain conditions (solvent, temperature, etc.), and a CPI optical film with high heat resistance and low birefringence is obtained by processes such as casting and heat treatment.

[0042] In this invention, given a fixed reactant, the reaction conditions can be freely selected by those skilled in the art under practical operating conditions. The homopolymerization (or copolymerization) reaction employs methods well-known in the art for the synthesis of polyimides, namely, one-step or two-step methods, with corresponding differences in reaction conditions (solvent, temperature, etc.).

[0043] A method for preparing a colorless polyimide film with high heat resistance and low birefringence specifically includes the following steps:

[0044] (1) The fluorinated side-biphenyl-m-phenylenediamine prepared by the method described in the second aspect of the present invention is subjected to homopolymerization / copolymerization reaction with commercial dianhydride and commercial diamine to form a polyimide (or polyamic acid precursor) solution.

[0045] (2) The polyimide (or polyamic acid precursor) solution described in step (1) is used to obtain a high heat resistance, low birefringence, colorless polyimide film through a film-forming process.

[0046] The commercial dianhydride mentioned in step (1) is limited to one or more of the structural formulas II-1 to II-9.

[0047] The diamine mentioned in step (1) is one or more of the structural formulas III-1 to III-6.

[0048] Preferably, the sum of the amounts of the fluorinated side-biphenyl m-phenylenediamine and the commercial diamine in step (1) is in the ratio of the amount of the commercial dianhydride to 1:(0.9-1.1).

[0049] Preferably, the homopolymerization / copolymerization reaction in step (1) is carried out in a one-step or two-step process.

[0050] More preferably, the one-step method involves polycondensation of fluorinated side-biphenyl m-phenylenediamine with commercial dianhydride and commercial diamine under an inert atmosphere, with or without a catalyst, in a high-boiling-point solvent at high temperature to obtain a homogeneous and viscous polyimide solution. The catalyst is one or both of benzoic acid and isoquinoline, the high-boiling-point solvent is one or more of m-cresol, p-chlorophenol, and m-chlorophenol, the reaction temperature is 150–200°C, the reaction time is 5–36 h, and the inert atmosphere is one of nitrogen or argon.

[0051] More preferably, the two-step method involves the low-temperature polycondensation of fluorinated side-biphenyl-m-phenylenediamine with commercial dianhydride and commercial diamine in an aprotic polar solvent under an inert atmosphere to obtain a homogeneous and viscous polyamic acid solution. The aprotic polar solvent is one or more of N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMAc), dimethyl sulfoxide (DMSO), and N-methylpyrrolidone (NMP). The reaction temperature is -10 to 50°C, the reaction time is preferably 10 to 120 h, and the inert atmosphere is either nitrogen or argon.

[0052] Preferably, the film-forming process in step (2) includes casting and heat treatment.

[0053] More preferably, the casting process includes one or more of the following: direct casting film formation, precision slot coating film formation, and continuous casting and stretching film formation.

[0054] More preferably, the heat treatment process includes a solvent removal process and an imidization process, the heating method is continuous heating or gradient heating, the temperature is 20-450°C, and the high-temperature atmosphere is one of nitrogen, argon, or vacuum.

[0055] More preferably, the imidization includes thermal imidization or chemical imidization methods.

[0056] A fourth aspect of the present invention is to provide a flexible liquid crystal display (LCD) device, the flexible liquid crystal display (LCD) device comprising a high heat-resistant, low birefringence colorless polyimide film as described in the first aspect of the present invention.

[0057] A fifth aspect of the present invention is to provide an organic light-emitting diode (OLED) display device, the organic light-emitting diode (OLED) display device comprising the high heat resistance, low birefringence, colorless polyimide film described in the first aspect.

[0058] A sixth aspect of the present invention is to provide a wearable device comprising the high heat resistance, low birefringence, colorless polyimide film described in the first aspect.

[0059] A seventh aspect of the present invention is to provide a smart window, the smart window comprising the high heat resistance, low birefringence, colorless polyimide film described in the first aspect.

[0060] Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0061] This invention provides a high-heat-resistant, low-birefringence colorless CPI film with a characteristic structure of fluorinated biphenyls substituted at positions 1, 3, and 5, exhibiting excellent thermal stability (T). g ≥383℃), extremely high colorless transparency (T 550 ≥88.4%, YI≤2.77), ultra-low birefringence (Δn≤0.0048, minimum 0.0007), and superior low dielectric properties (D). k As low as 2.64, D f As low as 0.006, it can simultaneously meet the high glass transition temperature T. g The overall solution outperforms existing patents and literature in terms of performance, meeting the requirements for low birefringence Δn, and has great application potential in key optical film fields such as flexible liquid crystal (LCD) or organic light-emitting diode (OLED) display devices, wearable devices, and smart car windows.

[0062] The method for preparing fluorinated biphenyl-m-phenylenediamine provided by this invention has a simple synthetic route, straightforward post-reaction processing, and high yield (reaction yield maintained at 80%-90%), making it suitable for large-scale synthesis. The high-heat-resistant, low-birefringence colorless CPI prepared by this invention employs an alicyclic backbone structure, with the introduction of an aromatic biphenyl side structure. Based on this chemical structure design, the CPI film prepared by this invention achieves a significantly reduced difference in in-plane and out-of-plane refractive indices, thus possessing low birefringence characteristics. Furthermore, the alicyclic and biphenyl structures effectively increase the glass transition temperature of the molecular chain, thereby achieving high heat resistance. Attached Figure Description

[0063] Figure 1 The DMA (glass transition temperature Tg) curves of the CPI films prepared in Examples 3 and 5 are shown.

[0064] Figure 2 The UV-Vis (optical transmittance) curves of the CPI films prepared in Examples 1 and 5 are shown.

[0065] Figure 3 The birefringence and yellowness values ​​of the CPI films prepared in Examples 1-5 are shown. Detailed Implementation

[0066] The specific implementation of the present invention will be further described below with reference to the accompanying drawings and examples, but the implementation and protection of the present invention are not limited thereto. It should be noted that any processes not specifically described in detail below are those that can be implemented or understood by those skilled in the art by referring to the prior art. Reagents or instruments whose manufacturers are not specified are considered to be conventional products that can be purchased commercially.

[0067] Example 1

[0068] 1. Synthesis of fluorinated benzidine

[0069] Under nitrogen protection, 1-bromo-3,5-dinitrobenzene (11.86 g, 48.04 mmol, formula IV-1), 2-trifluoromethylphenylboronic acid (9.58 g, 50.44 mmol, formula V-1), Pd(PPh3)4 (2.77 g, 2.40 mmol), K3PO4 (20.00 g, 94.22 mmol), 1,4-dioxane (80 mL), and deionized water (20 mL) were added to a 200 mL Schlenk flask and heated to 95 °C with stirring until the reaction was complete. The mixture was then extracted with ethyl acetate (EA) and separated by column chromatography to obtain the corresponding 2'-trifluoromethyl-3,5-dinitrobiphenyl (structural formula VI-1), a dark brown crystal. Yield: 81%. NMR results: 1 H NMR (500MHz, DMSO-d6) δ8.95-8.90(m,1H),8.65-8.55(m,2H),8.01-7.90(m,1H),7.88-7.81(m,1H),7.81-7.72(m,1H),7.63-7.56(m,1H).

[0070] Under argon protection, 10.00 g (32.03 mmol, formula VI-1), 54.20 g (240.35 mmol), and 40 mL of concentrated HCl solution were dissolved in 100 mL of ethyl acetate. The mixture was then stirred and heated to 70 °C until the reaction was complete. The solution was then neutralized to a weakly alkaline state (pH 7-8) with NaOH solution, extracted with dichloromethane (DCM), and recrystallized to obtain the target fluorinated diamine, 2'-trifluoromethyl-3,5-diaminobiphenyl (structural formula I-1). Yield: 84%. NMR results: 1 H NMR (500MHz, DMSO-d6) δ7.76-7.68(m,1H),7.66-7.58(m,1H),7.54-7.48(m,1H),7.33-7.26(m,1H),5.86-5.79(m,1H),5.71(s,2H),4.79(s,4H).

[0071]

[0072] 2. Synthesis and Thin Film Preparation of CPI

[0073] Under nitrogen protection, 2'-trifluoromethyl-3,5-diaminobiphenyl (2.52 g, 10 mmol, Formula I-1), cyclobutane dianhydride (0.588 g, 3 mmol, Formula II-1), cyclohexane dianhydride (1.57 g, 7.0 mmol, Formula II-3), benzoic acid (0.24 g, 2 mmol), and m-cresol (10 mL) were added to a 150 mL three-necked flask. After complete dissolution, the mixture was heated to 180 °C and maintained for 24 h. Toluene was used to remove small molecule byproducts to promote polymerization and complete imidization. Subsequently, a CPI solution with the corresponding structure (15 wt% solids content, intrinsic viscosity 1.211 dL / g, PI structure as shown below) was obtained by ethanol precipitation, drying, and DMF dissolution. The CPI solution was then coated through a precision slit, heated in a gradient of 20-150-250 °C under vacuum, and then peeled off to obtain a CPI film material.

[0074]

[0075] Example 2

[0076] 1. Synthesis of fluorinated benzidine

[0077] Under argon protection, 1-iodo-3,5-dinitrobenzene (14.12 g, 48.04 mmol, formula IV-2), 4-trifluoromethylphenylboronic acid pinacol ester (13.72 g, 50.44 mmol, formula V-6), PdCl2(PPh3)2 (2.51 g, 2.40 mmol), K2CO3 (20.00 g, 144.72 mmol), toluene (80 mL), and deionized water (20 mL) were added to a 200 mL Schlenk flask. The mixture was heated to 75 °C and stirred until the reaction was complete. The reaction was then extracted with dichloromethane (DCM) and separated by column chromatography to obtain the corresponding 4'-trifluoromethyl-3,5-dinitrobiphenyl (structural formula VI-3), which was a dark brown crystal. Yield: 83%. NMR results: 1 H NMR (500MHz, DMSO-d6) δ8.95-8.90(m,2H),8.89-8.83(m,1H),8.21-8.13(m,2H),7.95-7.88(m,2H).

[0078] Under nitrogen protection, 10.00 g (32.03 mmol, formula VI-3) of 4'-trifluoromethyl-3,5-dinitrobiphenyl and 1 g of Pd / C were dispersed and dissolved in 100 mL of tetrahydrofuran (THF), followed by the dropwise addition of 8.01 g (160 mmol) of N2H4·H2O. The entire system was stirred and refluxed for 8 h. After the reaction was complete, Pd / C was removed by diatomaceous earth filtration, followed by DCM extraction, column separation, and recrystallization to obtain the target 4'-trifluoromethyl-3,5-diaminobiphenyl (structural formula I-3). Yield: 84%. NMR results: 1 H NMR (500MHz, DMSO-d6) δ7.79-7.70(m,2H),7.69-7.63(m,2H),6.15-6.08(m,2H),5.93-5.84(m,1H),4.90(s,4H).

[0079]

[0080] 2. Synthesis and Thin Film Preparation of CPI

[0081] Under nitrogen protection, 2'-trifluoromethyl-3,5-diaminobiphenyl (0.76 g, 3 mmol, Formula I-1), 4'-trifluoromethyl-3,5-diaminobiphenyl (1.51 g, 6 mmol, Formula I-3), trifluorom-phenylenediamine (0.18 g, 1 mmol, Formula III-1), cyclohexane dianhydride (2.24 g, 10 mmol, Formula II-3), isoquinoline (0.237 g, 1.84 mmol), and p-chlorophenol (10 mL) were added to a 150 mL three-necked flask. After complete dissolution, the mixture was heated to 150 °C and maintained for 18 h. Continuous nitrogen was used to remove small molecule byproducts to promote polymerization and complete imidization. Subsequently, a CPI solution with the corresponding structure (18 wt% solids, intrinsic viscosity 0.744 dL / g, PI structure as shown below) was obtained by methanol precipitation, drying, and dissolution with DMAc. Subsequently, the CPI solution was cast into a film, and the temperature was continuously increased from 20 to 350°C under vacuum conditions. After peeling, the CPI film material was obtained.

[0082]

[0083] Example 3

[0084] 1. Synthesis of fluorinated benzidine

[0085] Under argon protection, 1-bromo-3,5-dinitrobenzene (11.86 g, 48.04 mmol, formula IV-1), 2,4-bis(trifluoromethyl)phenylboronic acid (13.00 g, 50.44 mmol, V-12), Pd(PPh3)4 (2.77 g, 2.40 mmol), K2CO3 (20.00 g, 144.72 mmol), ethyl acetate (80 mL), and deionized water (20 mL) were added to a 200 mL Schlenk flask. The mixture was heated to 100 °C and stirred until the reaction was complete. The reaction was then extracted with chloroform (CHCl3) and separated by column chromatography to obtain the corresponding 2',4'-bis(trifluoromethyl)-3,5-dinitrobiphenyl (structural formula VI-9), a dark brown crystal. Yield: 85%. NMR results: 1 H NMR (500MHz, DMSO-d6) δ8.99-8.92(m,1H),8.73-8.68(m,2H),8.29-8.21(m,1H),7.87-7.80(m,1H).

[0086] Under nitrogen protection, 10.00 g (26.30 mmol, formula VI-9), 54.20 g (240.35 mmol), and 40 mL of concentrated HCl solution were dissolved in 100 mL of dioxane. The mixture was then stirred and heated to 80 °C until the reaction was complete. The solution was then neutralized to a weakly alkaline state (pH 7-8) with KOH solution, extracted with ethyl acetate (EA), and recrystallized to obtain the target 2',4'-bis(trifluoromethyl)-3,5-diaminobiphenyl (structural formula I-9). Yield: 82%. NMR results: 1 H NMR (500MHz, DMSO-d6) δ8.08-7.96(m,2H),7.62-7.54(m,1H),5.92-5.85(m,1H),5.78-5.68(m,2H),4.90(s,4H).

[0087]

[0088] 2. Synthesis and Thin Film Preparation of CPI

[0089] Under argon protection, 2',4'-bis(trifluoromethyl)-3,5-diaminobiphenyl (25.62 g, 80 mmol, Formula I-9), bisfluorene diamine (3.92 g, 20 mmol, Formula III-4), dispirocycloanedihydric anhydride (26.90 g, 70 mmol, Formula II-6), cyclohexanedihydric anhydride (6.72 g, 30 mmol, Formula II-3), isoquinoline (2.58 g, 20 mmol), and p-chlorophenol (100 mL) were placed in a 250 mL three-necked flask. After complete dissolution, the mixture was heated to 190 °C and maintained for 30 h. Continuous argon gas was used to remove small molecule byproducts to promote polymerization and complete imidization. Subsequently, ethanol precipitation, drying, and DMAc dissolution were performed to obtain a CPI solution with the corresponding structure (solid content 18 wt%, intrinsic viscosity 1.137 dL / g, PI structure as shown below). Subsequently, the CPI solution was cast into a film, and the temperature was increased in a gradient of 50-100-250-400℃ under vacuum conditions. After peeling, the CPI film material was obtained.

[0090]

[0091] Example 4

[0092] 1. Synthesis of fluorinated benzidine

[0093] Under nitrogen protection, 1-bromo-3,5-dinitrobenzene (11.86 g, 48.04 mmol, formula IV-1), 2-[3,5-bis(trifluoromethyl)phenyl]boronipinol ester (17.15 g, 50.44 mmol, formula V-37), Pd(OAc)2 (2.85 g, 2.40 mmol), K2CO3 (20.00 g, 144.72 mmol), dimethyl sulfoxide (DMSO) (80 mL), and deionized water (20 mL) were added to a 200 mL Schlenk flask and heated to 110 °C with stirring until the reaction was complete. The mixture was then extracted with ethyl acetate (EA) and separated by column chromatography to obtain the corresponding 3',5'-bis(trifluoromethyl)-3,5-dinitrobiphenyl (structural formula VI-18), a dark brown crystal. Yield: 81%. NMR results: 1 H NMR (500MHz, DMSO-d6) δ9.15-9.08(m,2H),8.92-8.89(m,1H),8.65(s,2H),8.26(s,1H).

[0094] Under nitrogen protection, 10.00 g (26.30 mmol, formula VI-18) of 3',5'-bis(trifluoromethyl)-3,5-dinitrobiphenyl, 54.23 g (240.35 mmol) of SnCl₂·2H₂O, and 40 mL of concentrated HCl solution were dissolved in 100 mL of dimethyl sulfoxide. The mixture was then stirred and heated to 85 °C until the reaction was complete. The solution was then neutralized to a weakly alkaline state (pH 7-8) with K₂CO₃ solution, extracted with chloroform (CHCl₃), and recrystallized to obtain the target 3',5'-bis(trifluoromethyl)-3,5-diaminobiphenyl (structural formula I-18). Yield: 85%. NMR characterization results: 1 H NMR (500MHz, DMSO-d6) δ8.07(s,2H),8.02(s,1H),6.27-6.17(m,2H),6.01-5.92(m,1H),4.98(s,4H).

[0095]

[0096] 2. Synthesis and Thin Film Preparation of CPI

[0097] Under nitrogen protection, 2',4'-bis(trifluoromethyl)-3,5-diaminobiphenyl (1.76 g, 5.5 mmol, Formula I-9), 3',5'-bis(trifluoromethyl)-3,5-diaminobiphenyl (1.44 g, 4.5 mmol, Formula I-18), fluorene-containing aromatic dianhydride (4.58 g, 10 mmol, Formula II-9), and DMAc (20 mL) were placed in a 50 mL two-necked flask and stirred at 25 °C for 36 h until a high viscosity was reached (intrinsic viscosity 1.853 dL / g). Subsequently, this polyamic acid (PAA) precursor solution was poured onto a clean glass plate and directly cast into a film. The film was then continuously heated to 300 °C in an oven, and finally peeled off to obtain a CPI film material (structure shown below).

[0098]

[0099] Example 5

[0100] Under nitrogen protection, 1-iodo-2,4-dinitrobenzene (14.12 g, 48.04 mmol, formula IV-2), 4-fluoro-2-trifluoromethylphenylboronic acid (10.49 g, 50.44 mmol, formula V-14), Pd(PPh3)4 (2.77 g, 2.40 mmol), K3PO4 (20.00 g, 94.22 mmol), isopropanol (80 mL), and deionized water (20 mL) were added to a 250 mL Schlenk flask and heated to 120 °C with stirring until the reaction was complete. The mixture was then extracted with ethyl acetate (EA) and separated by column chromatography to obtain the corresponding 2'-trifluoromethyl-4'-fluoro-3,5-dinitrobiphenyl (structural formula VI-11), a dark brown crystal. Yield: 83%. Mass spectrometry results: HRMS (ESI) m / z calcd for C 13 H6F4N2O4 + [M+H] + :331.0358,found 331.0361.

[0101] Under nitrogen protection, 10.00 g (30.28 mmol, formula VI-11), 54.23 g (240.35 mmol), and 40 mL of concentrated HCl solution were dissolved in 100 mL of isopropanol. The mixture was then stirred and heated to 90 °C until the reaction was complete. The solution was then neutralized to a weakly alkaline state (pH 7-8) with KOH solution, extracted with chloroform (CHCl3), and recrystallized to obtain the target 2'-trifluoromethyl-4'-fluoro-3,5-diaminobiphenyl (structural formula I-11). Yield: 84%. NMR results: 1 H NMR (500MHz, DMSO-d6) δ7.54-7.56(m,1H),7.43-7.44(m,1H),7.41-7.42(m,1H),6.28-6.29(m,2H),6.05-6.06(m,1H),4.76-4.77(m,4H).

[0102]

[0103] Under nitrogen protection, 2'-trifluoromethyl-4'-fluoro-3,5-diaminobiphenyl (1.62 g, 6 mmol, Formula I-11), commercially available fluorofluorene diamine (1.83 g, 4 mmol, Formula III-6), dicyclohexane dianhydride (0.94 g, 4 mmol, Formula II-4), dispirocycloane dianhydride (2.30 g, 6 mmol, Formula II-6), isoquinoline (2.58 g, 20 mmol), and p-chlorophenol (30 mL) were placed in a 150 mL three-necked flask. After complete dissolution, the mixture was heated to 200 °C and maintained for 15 h. Toluene was used to remove small molecule byproducts to promote polymerization and complete imidization. Subsequently, CPI solutions with the corresponding structures were obtained by methanol precipitation, drying, and dissolution with DMAc (solid content 18 wt%, intrinsic viscosity 2.168 dL / g, PI structure as shown below). Subsequently, the CPI solution was cast into a film, and the temperature was increased in a gradient of 50-120-250-450℃ under vacuum conditions. After peeling, the CPI film material was obtained.

[0104]

[0105] The performance of the CPI films obtained in Examples 1-5 was tested, and the results are shown in Table 1. The intrinsic viscosity was measured using an Ubbelohde viscometer at 30°C. The polymer (PAA or PI) concentration was 0.5 g / dL, and the average value was taken from three measurements to reflect the molecular weight of the polymer. The glass transition temperature (Tg) of the CPI film is also shown. g The temperature was measured by DMA, and the characterization instrument was a TA Q800. The heating rate was 3℃ / min, and the atmosphere was nitrogen. The 5wt% thermal decomposition temperature (T) was measured by DMA. d5％ The optical transparency of the CPI film was measured by TGA, using a TA Q50 instrument at a heating rate of 10℃ / min under a nitrogen atmosphere. The optical transparency was measured by UV-Vis, using a Shimadzu 3600Plus instrument (range 200–800 nm). Yellowness (YI) and haze were measured by a thin film colorimeter, using a HunterLab D65 light source. The optical refractive index (out-of-plane n) was also measured. TM and in-plane n TE The birefringence value was measured by an edge-coupled device, characterized by a Model 2010 / M instrument, and the value was Δn=n. TE -n TM The corresponding wavelength is 637 nm. The dielectric properties of CPI thin films (dielectric constant D) k Dielectric loss D f The results were measured using a vector network analyzer, characterized by an Agilent N5244A at 10 GHz in resonant cavity mode. Key properties of the CPI thin film are shown in Table 1.

[0106] Table 1

[0107]

[0108] The above data indicate that the T of the CPI film prepared by this invention is... g The value is 383~425℃, T d5％ The value is 457–536℃; meanwhile, the optical transmittance T 550 The value is 88.4%–89.7%, the yellow index YI value is 1.25–2.77, and the ultra-low birefringence Δn is 0.0007–0.0048; in addition, the dielectric properties at a high frequency of 10 GHz are excellent. k Values ​​range from 2.64 to 3.05, D f Values ​​range from 0.006 to 0.021.

[0109] Figure 1 The DMA curves of the CPI films prepared in Examples 3 and 5 are shown, where the peak value of tanδ reflects its high glass transition temperature Tg value. Figure 2 The UV-Vis curves of the CPI films prepared in Examples 1 and 5 reflect their high optical transparency at wavelengths of 400-800 nm. Figure 3 The birefringence and yellowness values ​​of the CPI films prepared in Examples 1-5 are shown, indicating their low birefringence and low yellowness.

[0110] As can be seen from the above embodiments, the present invention provides a high heat resistance, low birefringence, colorless CPI film with a characteristic structure containing fluorinated biphenyl side, exhibiting excellent thermal stability, extremely high colorless transparency, ultra-low birefringence, and superior low dielectric properties. It can simultaneously meet the performance requirements of high glass transition temperature (383~425℃) and low birefringence (0.0007~0.0048), and has great application potential in key optical film fields such as flexible liquid crystal (LCD) or organic light-emitting diode (OLED) display devices, wearable devices, and smart car windows.

[0111] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A colorless polyimide film with high heat resistance and low birefringence, characterized in that, The general structural formula of the polyimide is as follows: ， Wherein, Ar1 is a residue of fluorinated biphenyl-m-phenylenediamine, R0 is independently selected from one or both of F and CF3, q is the number of substitutions of R0 group 1 to 2, and has the characteristic structure of fluorinated biphenyl-m-phenyl; R1 and R2 are residues of commercial dianhydrides; R3 is a residue of commercial diamine, n and m are the repeating number of the corresponding structural unit, and the ratio of n:m ranges from 1:0 to 5:95; The fluorinated side-biphenyl-m-phenylenediamine is limited to one or more of the structural formulas I-1 to I-19: ； The commercially available dianhydride is limited to one or more of structural formulas II-1 to II-9: ； The commercially available diamine is limited to one or more of the structural formulas III-1 to III-6: ， R4 is independently selected from one or both of F and CF3, and p is the number of substitutions of the R4 base, with p taking values ​​from 1 to 2; The glass transition temperature of the high heat resistance, low birefringence, colorless polyimide film T At a temperature of 383~425℃, the optical transmittance at 550nm is... T 550 The yellowness index (YI) ranges from 88.4% to 89.7%, and the corresponding thermogravimetric temperature at 5 wt% is 1.25 to 2.

77. T d5% Values ​​range from 457 to 536 °C, with an optical birefringence Δ. n The value ranges from 0.0007 to 0.0048.

2. The high heat resistance, low birefringence, colorless polyimide film according to claim 1, characterized in that, The high heat-resistant, low birefringence colorless polyimide film exhibits a dielectric constant of [value missing] at a high frequency of 10 GHz. D k The value is 2.64 ~ 3.05, dielectric loss D f The value is 0.006 ~ 0.

021.

3. The method for preparing a high heat-resistant, low birefringence colorless polyimide film according to any one of claims 1-2, characterized in that, Specifically, the steps include the following: (1) Fluorinated side-biphenyl m-phenylenediamine is homopolymerized or copolymerized with commercial dianhydride and commercial diamine to form a polyimide or polyamic acid precursor solution; (2) The polyimide or polyamic acid precursor solution described in step (1) is used to obtain a high heat resistance, low birefringence, colorless polyimide film through a film-forming process; The sum of the amounts of the fluorinated side-biphenyl-m-phenylenediamine and the commercial diamine in step (1) is in the molar ratio of the amount of the commercial dianhydride to 1:(0.9~1.1); The homopolymerization or copolymerization reaction in step (1) is carried out in one step or two steps. The one-step method involves the high-temperature polycondensation of fluorinated side-biphenyl m-phenylenediamine with commercial dianhydride and commercial diamine, with or without a catalyst, in a high-boiling-point solvent under an inert atmosphere to obtain a homogeneous and viscous polyimide solution. The catalyst is one or both of benzoic acid and isoquinoline, the high-boiling-point solvent is one or more of m-cresol, p-chlorophenol, and m-chlorophenol, the reaction temperature is 150-200 °C, the reaction time is 5-36 h, and the inert atmosphere is one of nitrogen or argon. The two-step method involves the low-temperature polycondensation of fluorinated side-biphenyl-m-phenylenediamine with commercially available dianhydride and commercially available diamine in an aprotic polar solvent under an inert atmosphere to obtain a homogeneous and viscous polyamic acid solution. The aprotic polar solvent is... N, N -Dimethylformamide (DMF) N,N -Dimethylacetamide (DMAc), dimethyl sulfoxide (DMSO) N One or more of methylpyrrolidone (NMP) are used in the reaction, which takes place at a temperature of -10 to 50 °C and a time of 10 to 120 h. The inert atmosphere is either nitrogen or argon. The film-forming process in step (2) includes casting and heat treatment. The casting process includes one or more of the following: direct casting film formation, precision slot coating film formation, casting and stretching continuous film formation. The heat treatment process includes a solvent removal process and an imidization process. The heating method is continuous heating or gradient heating, with a temperature range of 20~450 °C. The high-temperature atmosphere is one of nitrogen, argon, or vacuum.

4. A flexible liquid crystal display device, characterized in that, Includes the colorless polyimide optical film as described in claim 1.

5. An organic light-emitting diode display device, characterized in that, Includes the colorless polyimide optical film as described in claim 1.

6. A wearable device, characterized in that, Includes the colorless polyimide optical film as described in claim 1.

7. A smart vehicle window, characterized in that, Includes the colorless polyimide optical film as described in claim 1.

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