A polyamide-imide film with high glass transition temperature, low thermal expansion coefficient and low retardation value, and a preparation method and application thereof

Abstract

The application discloses a polyamide-imide film with high glass transition temperature, low thermal expansion coefficient and low retardation value and a preparation method and application thereof, and the structure of the polyamide-imide is as follows: wherein n is an integer greater than 0, m is an integer greater than 0, m:n=(5-1):1, R1 and R2 are residues of diamines, and the R1 and R2 are a combination of at least one of structures shown in formula I-1, I-2 and structures shown in formula I-3, I-4. The application is obtained through matching of molecular structures among hydrogenated pyromellitic dianhydride, diamines containing amides and diamines containing fluorene structures, and the polyamide-imide film has low thermal expansion coefficient, high glass transition temperature, low optical retardation, high transparency and good tensile modulus, and has excellent comprehensive performance and can be applied to the fields of electronics and flexible display.

Description

Technical Field

[0001] This invention relates to the field of polymer materials technology, specifically to a polyamide-imide film with high glass transition temperature, low coefficient of thermal expansion, and low retardation value, as well as its preparation method and applications. Background Technology

[0002] Polyimide (PI) films hold promise as a replacement for rigid glass covers and other substrates currently used in flexible display applications, such as liquid crystal displays (LCDs) and organic light-emitting diode (OLEDs). Aromatic polyimides possess very high thermal stability, with glass transition temperatures (Tg) exceeding 300°C. However, tuning the optical parameters of PI films, including optical transparency, refractive index, birefringence, and thickness retardation (Rth), is crucial for optimal optical device performance. In recent years, polymer films used as cover windows and touch sensor panels in flexible AMOLED displays have shown promise in applications with low Rth values. th High thermal stability is a critical requirement. Optical retardation value is also a key factor in the selection of flexible LCD substrates. To realize flexible TFT-LCDs, polymer substrates must possess low Rth, high Tg (approximately 400°C), and high dimensional stability, characterized by a low coefficient of thermal expansion. These substrate properties are essential for the manufacturing processes involved in fabricating the alignment layer and thin-film transistors (TFTs).

[0003] Transparent polyimide (CPI) is chosen for flexible electronic devices due to its excellent optical transparency and superior thermal and mechanical properties. To obtain CPI, various structural modifications are introduced into the molecular chain, such as alicyclic units, fluorine substitution, and non-coplanar groups, to suppress the formation of intramolecular and intermolecular charge-transfer complexes (CTCs) and improve optical transparency. In CPI films, semi-alicyclic PI generally exhibits better optical transparency and lower optical retardation than fully aromatic PI. Polyimide films (trade name Neopulim) Neopulim L-3430, derived from alicyclic dianhydrides such as hydrogenated pyromellitic dianhydride (HPMDA) and aromatic diamine 4,4'-diaminodiphenyl ether (ODA), has an optical retardation of 20 nm and an optical transparency of 90%. However, due to the easy movement of alicyclic segments at high temperatures, Neopulim L-3430 films exhibit a high CTE (58 ppm / K). This mismatch with the CTE of TFT layer materials can lead to layer deformation with temperature changes. Furthermore, Neopulim L-3430 has a Tg of 303°C, lower than the 400°C required for TFT technology. Therefore, the development of CPI flexible display substrates requires further improvements in heat resistance and reductions in CTE to surpass the performance of Neopulim films.

[0004] It is well known that fluorene-based polymers typically exhibit good organic solubility and low optical retardation due to their large molecular size and fused ring structure. Chinese patent application CN116348296 discloses a CPI film prepared from dihydrofluorene and an aromatic diamine. The fluorene groups on the polymer backbone reduce Rth to <100 nm, increase Tg at >400 °C, and dramatically increase the CTE (>70 ppm / K) of the PI film.

[0005] Chinese patent application CN105073851A reports CPI films of 3,3',4,4'-biphenyltetracarboxylic acid dianhydride (BPDA) and 9,9-bis(3,4-dicarboxyphenyl)fluorene dianhydride (BPAF), with CTE < 35 ppm / k, R th <200nm, Tg<300℃. CN112708134A discloses a CPI film with low CTE<20ppm / K and Tg>350℃, copolymerized from alicyclic dianhydrides and aromatic dianhydrides with diamide-bonded diamines. However, due to the rigid bonding of the amides, the Rth value is >500nm.

[0006] Therefore, it is urgent to develop polymer films that can achieve both low Rth and low CTE without sacrificing high Tg. Summary of the Invention

[0007] This invention addresses the problem in existing technologies where high glass transition temperature, low coefficient of thermal expansion, and low optical retardation are mutually exclusive in polyimide film materials. It provides a transparent polyamide-imide film with high glass transition temperature, low coefficient of thermal expansion, and low retardation value. Through matching of molecular structures, the transparent polyamide-imide film achieves a glass transition temperature higher than 400°C, a coefficient of thermal expansion lower than 40.0 ppm / K (100-200°C), and an optical retardation value in the thickness direction of less than 255 nm.

[0008] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0009] A polyamide-imide film with high glass transition temperature, low coefficient of thermal expansion, and low retardation value, wherein the polyamide-imide structure is as follows:

[0010]

[0011] Where n is greater than 0, m is greater than 0; m:n = (5-1):1; R1 and R2 are residues of diamine;

[0012] R1 is at least one of the structures shown in formulas I-1 and I-2:

[0013]

[0014] R2 is at least one of the structures shown in formulas I-3 and I-4:

[0015]

[0016] In this invention, hydrogenated pyromellitic acid is used as the dianhydride, and 9,9-bis(4-aminophenyl)fluorene (FDA) and 9,9-bis(3-fluoro-4-aminophenyl)fluorene (FFDA) are selected as partial diamine structures. Their rigid fluorene-containing cardo structure is used to increase the glass transition temperature of polyamide-imide and reduce the optical retardation value in the thickness direction. However, since their structure will also increase the coefficient of thermal expansion of the polymer material, this invention adds diamines containing amide structures to the polymer to reduce the coefficient of thermal expansion of the material and increase the tensile modulus. Finally, an excellent thin film with high glass transition temperature, low coefficient of thermal expansion and low optical retardation value in the thickness direction is obtained.

[0017] Since 9,9-bis(4-aminophenyl)fluorene (FDA) and 9,9-bis(3-fluoro-4-aminophenyl)fluorene (FFDA) are diamine components, although they can increase the glass transition temperature of the material, they also lead to a significant increase in the coefficient of thermal expansion. By selecting different structures and adjusting the proportions, a transparent polyamide-imide film that simultaneously achieves a high glass transition temperature, a low coefficient of thermal expansion, and a low optical retardation value in the thickness direction is finally obtained.

[0018] Preferably, R1 has the structure shown in I-1, and R2 has the structure shown in I-3.

[0019] The glass transition temperature (T) of the polyamide-imide film g At temperatures above 400℃, the coefficient of thermal expansion (CTE) is below 40ppm / K (under test conditions of 100-200℃), and the optical retardation (Rth) in the thickness direction is below 255nm. These are excellent overall properties for polyamide-imide films.

[0020] The polyamide-imide film has a tensile modulus of 3 GPa or higher and a total transmittance of 80% or higher. More preferably, the polyamide-imide film can achieve a total transmittance of 88% or higher.

[0021] The present invention also provides a method for preparing the polyamide-imide film with high glass transition temperature, low coefficient of thermal expansion, and low retardation value, comprising the steps of: under the protection of an inert gas, using diamine and dianhydride as monomers in a reaction solvent to carry out a polymerization reaction to obtain a polyamide resin solution, which is then precipitated, filtered, and dried to obtain a polyamide-imide resin.

[0022] The polyamide-imide resin is dissolved in a solvent, and the polyamide-imide resin solution is coated onto a substrate and cured by heating to obtain the polyamide-imide film.

[0023] Preferably, the dianhydride is hydrogenated pyromellitic dianhydride (HPMDA), and the diamine is a combination of diamine A and diamine B; wherein the molar ratio of diamine A to diamine B is (1-5):1; and the total molar ratio of the dianhydride to the diamine is (0.95-1.05):1.

[0024] The diamine A is at least one of 4,4'-diaminobenzoyl aniline and N,N'-bis(4-aminophenyl)terephthalamide; the diamine B is at least one of 9,9-bis(4-aminophenyl)fluorene and 9,9-bis(3-fluoro-4-aminophenyl)fluorene.

[0025] Preferably, the reaction solvent includes one or more of γ-butyrolactone, m-cresol, N,N-dimethylformamide, N,N-dimethylacetamide, N-methylpyrrolidone, and dimethyl sulfoxide;

[0026] The solvent used for dissolution is one or more of γ-butyrolactone, N,N-dimethylformamide, and N-methylpyrrolidone.

[0027] The solid content of the reaction solution during polymerization is 5-25%, preferably 10-20%; the polymerization temperature is 160-200℃, and the polymerization time is 5-10h.

[0028] Preferably, the heat curing includes the following steps: curing at 60-100℃ for 1-2 hours, curing at 100-160℃ for 1-2 hours, curing at 160-250℃ for 1-2 hours, and curing at 260-350℃ for 1-2 hours.

[0029] More preferably, the heat curing is performed at 80°C for 1 hour to allow the solvent to evaporate slowly, followed by heat curing at 100°C, 150°C, 200°C, and 230°C for 1 hour each. This step facilitates complete curing and ensures that the solvent is completely removed.

[0030] The present invention also provides the application of the polyamide-imide film as a substrate material in the fields of electronic displays or flexible displays.

[0031] Compared with the prior art, the present invention has the following beneficial effects:

[0032] In this invention, hydrogenated pyromellitic acid is selected as the dianhydride. By introducing a rigid fluorene-containing diamine structure, its glass transition temperature is increased, and its optical retardation value in the thickness direction is reduced. The introduction of an amide structure reduces the coefficient of thermal expansion of the film through intramolecular hydrogen bonding. By increasing its tensile modulus through intermolecular forces, and through continuous adjustment of different structures and proportions, this molecular structure simultaneously possesses the advantages of high glass transition temperature, low coefficient of thermal expansion, and low retardation value, resulting in a glass transition temperature (T0). gAt temperatures above 400℃, the coefficient of thermal expansion (CTE) is less than 40.0ppm / K (100-200℃), the optical retardation value in the thickness direction is less than 255nm, the tensile modulus is greater than 3.0GPa, and the total transmittance is above 88%, making it suitable for applications in electronics and flexible displays. Attached Figure Description

[0033] Figure 1 The linear thermal expansion coefficient is the polyamide-imide film obtained in Examples 1-4.

[0034] Figure 2 The transmittance is the polyamide-imide film obtained in Examples 1-4.

[0035] Figure 3 The DMA curves are for the polyamide-imide films obtained in Examples 1-4. Detailed Implementation

[0036] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention. Modifications or equivalent substitutions made by those skilled in the art based on their understanding of the technical solutions of this invention, without departing from the spirit and scope of the invention, should be covered within the protection scope of this invention.

[0037] The raw materials used in the following specific embodiments were all purchased from the market, and the evaluation methods for the performance of the polyamide-imide films obtained in each embodiment are described.

[0038] (1) Dynamic thermomechanical analysis

[0039] The testing instrument was a dynamic thermomechanical analyzer, model TA Instruments DMA Q800. The test sample was 15 mm long, 8 mm wide, and approximately 0.03 mm thick. Tensile testing was conducted under N2 protection, with a load frequency of 1 Hz and a heating rate of 3 °C / min, within the range of 30–450 °C.

[0040] (2) Thermomechanical analysis

[0041] Thermal analysis was performed using a Shimadzu TMA-60 with a fixed load of 1.0 g, a heating rate of 5 °C / min, and a nitrogen flow rate. The film size measured by TMA was 5 mm wide and 20 mm long. Data from a second heating was collected after heating to 200 °C / min in the TMA chamber and then cooling to room temperature to eliminate the effects of moisture absorption and residual stress.

[0042] (3) Mechanical property testing

[0043] The testing instrument was an Instron Model 5567 universal testing machine, using tensile mode at a stretching rate of 5 mm / min. The primary tests were for the tensile modulus, tensile strength, and elongation at break of the film. The sample dimensions were 50 mm long and 10 mm wide.

[0044] (4) Birefringence analysis

[0045] The refractive index of the PI thin film was measured using a Metricon Model 2010 / M prism coupler. The in-plane and out-of-plane refractive indices (nn) at a wavelength of 632.8 nm were directly obtained. TE and n TM The birefringence (Δn) and retardation (Rth) are calculated by the following formula;

[0046] Δn=n TE -n TM (1)

[0047] R th =Δn×d (2)

[0048] Abbreviation Explanation:

[0049] DABA: 4,4′-diaminobenzilide

[0050] FDA: 9,9-bis(4-aminophenyl)fluorene

[0051] HPMDA: Hydropyromellitic dianhydride

[0052] DMAc: N,N-dimethylacetamide

[0053] Example 1

[0054] DABA (6.0 mmol, 1.3636 g), FDA (1.5 mmol, 0.5226 g), and HPMA (7.5 mmol, 1.6812 g) were placed in a 50 mL three-necked flask equipped with a stirrer, thermometer, and water separator. Then, γ-butyrolactone (14.269 g) was added, resulting in a solid content of 20% by weight. The reaction mixture was stirred at room temperature for 1 h to dissolve the monomers. A small amount of isoquinoline was added to the homogeneous reaction solution as a catalyst, and the mixture was heated to 180 °C and reacted for 8 h. The viscous solution was cooled to 100 °C, diluted with γ-butyrolactone to a solid content of 10%, and slowly poured into an ethanol / water mixture (vol / vol = 1 / 1) while stirring vigorously. The precipitate was collected by filtration, thoroughly washed twice with boiling ethanol / water (vol / vol = 1 / 1), and dried in a vacuum oven at 100 °C for 6 h to obtain polyamide-imide powder.

[0055] A suitable amount of dried polyamide-imide powder was added to N,N-dimethylacetamide (DMAc) to prepare a 10wt% polyamide-imide solution. This solution was then cast onto a clean glass substrate and cured in a vacuum oven at 80°C for 1 hour to allow the DMAc to evaporate slowly. Subsequently, the solution was cured at 100°C, 150°C, 200°C, and 230°C for 1 hour each. After naturally cooling to room temperature, the glass substrate was immersed in water to detach the film. The film was then dried in a 100°C oven to obtain the polyamide-imide film.

[0056] The main properties of the polyamide-imide film prepared in this embodiment are shown in Tables 1 and 2.

[0057] Example 2

[0058] DABA (5.25 mmol, 1.1934 g), FDA (2.25 mmol, 0.7839 g), and HPMA (7.5 mmol, 1.6812 g) were placed in a 50 mL three-necked flask equipped with a stirrer, thermometer, and water separator. Then, γ-butyrolactone (14.634 g) was added, resulting in a solid content of 20% by weight. The reaction mixture was stirred at room temperature for 1 h to dissolve the monomers. A small amount of isoquinoline was added to the homogeneous reaction solution as a catalyst, and the mixture was heated to 180 °C and reacted for 8 h. The viscous solution was cooled to 100 °C and diluted with γ-butyrolactone to a solid content of 10%. This solution was then slowly poured into an ethanol / water mixture (vol / vol = 1 / 1) while stirring vigorously. The precipitate was collected by filtration, thoroughly washed twice with boiling ethanol / water (vol / vol = 1 / 1), and dried in a vacuum oven at 100 °C for 6 h to obtain polyamide-imide powder.

[0059] A suitable amount of dried polyamide-imide powder was added to N,N-dimethylacetamide (DMAc) to prepare a 10wt% polyamide-imide solution. This solution was then cast onto a clean glass substrate and cured in a vacuum oven at 80°C for 1 hour to allow the DMAc to evaporate slowly. Subsequently, the solution was cured at 100°C, 150°C, 200°C, and 230°C for 1 hour each. After naturally cooling to room temperature, the glass substrate was immersed in water to detach the film. The film was then dried in a 100°C oven to obtain the polyamide-imide film.

[0060] The main properties of the polyamide-imide film prepared in this embodiment are shown in Tables 1 and 2.

[0061] Example 3

[0062] DABA (4.5 mmol, 1.0227 g), FDA (3 mmol, 1.0452 g), and HPMA (7.5 mmol, 1.6812 g) were placed in a 50 mL three-necked flask equipped with a stirrer, thermometer, and water separator. Then, γ-butyrolactone (14.9964 g) was added, resulting in a solids content of 20% by weight. The reaction mixture was stirred at room temperature for 1 h to dissolve the monomers. A small amount of isoquinoline was added to the homogeneous reaction solution as a catalyst, and the mixture was heated to 180 °C and reacted for 8 h. The viscous solution was cooled to 100 °C, diluted with γ-butyrolactone to a solids content of 10%, and slowly poured into an ethanol / water mixture (vol / vol = 1 / 1) while stirring vigorously. The precipitate was collected by filtration, thoroughly washed twice with boiling ethanol / water (vol / vol = 1 / 1), and dried in a vacuum oven at 100 °C for 6 h to obtain polyamide-imide powder.

[0063] A suitable amount of dried polyamide-imide powder was added to N,N-dimethylacetamide (DMAc) to prepare a 10wt% polyamide-imide solution. This solution was then cast onto a clean glass substrate and cured in a vacuum oven at 80°C for 1 hour to allow the DMAc to evaporate slowly. Subsequently, the solution was cured at 100°C, 150°C, 200°C, and 230°C for 1 hour each. After naturally cooling to room temperature, the glass substrate was immersed in water to detach the film. The film was then dried in a 100°C oven to obtain the polyamide-imide film.

[0064] The main properties of the polyamide-imide film prepared in this embodiment are shown in Tables 1 and 2.

[0065] Example 4

[0066] DABA (3.75 mmol, 0.8522 g), FDA (3.75 mmol, 1.3065 g), and HPMA (7.5 mmol, 1.6812 g) were placed in a 50 mL three-necked flask equipped with a stirrer, thermometer, and water separator. Then, γ-butyrolactone (15.3596 g) was added, resulting in a solid content of 20% by weight. The reaction mixture was stirred at room temperature for 1 h to dissolve the monomers. A small amount of isoquinoline was added to the homogeneous reaction solution as a catalyst, and the mixture was heated to 180 °C and reacted for 8 h. The viscous solution was cooled to 100 °C, diluted with γ-butyrolactone to a solid content of 10%, and slowly poured into an ethanol / water mixture (vol / vol = 1 / 1) while stirring vigorously. The precipitate was collected by filtration, thoroughly washed twice with boiling ethanol / water (vol / vol = 1 / 1), and dried in a vacuum oven at 100 °C for 6 h to obtain polyamide-imide powder.

[0067] A suitable amount of dried polyamide-imide powder was added to N,N-dimethylacetamide (DMAc) to prepare a 10wt% polyimide solution. This solution was then cast onto a clean glass substrate and cured in a vacuum oven at 80°C for 1 hour to allow the DMAc to evaporate slowly. Subsequently, the solution was cured at 100°C, 150°C, 200°C, and 230°C for 1 hour each. After naturally cooling to room temperature, the glass substrate was immersed in water to detach the film. The film was then dried in a 100°C oven to obtain the polyamide-imide film.

[0068] The main properties of the polyamide-imide film prepared in this embodiment are shown in Tables 1 and 2.

[0069] Table 1. Main properties of the polyamide-imide films prepared in Examples 1-4

[0070]

[0071] a It was obtained from DMA at a heating rate of 5 °C / min at 1 Hz.

[0072] b The temperature range was recorded using TMA (secondary heating analysis) at 50–200℃ with a heating rate of 5℃ / min, and the CTE value was taken in the temperature range of 100–200℃.

[0073] c For a CPI film with a thickness of 10 μm.

[0074] d Transmittance at 450 nm with an average film thickness of 30 μm.

[0075] Table 2. Mechanical properties of polyamide-imide films prepared in Examples 1-4

[0076] Example 1 3.9 95 7.8 Example 2 3.4 73 5.3 Example 3 3.1 64 5.2 Example 4 3.2 91 7.9

[0077] From Table 1, Table 2 and Figure 1-3 It can be seen that with the increase of fluorene diamine content, the polyamide-imide film generally shows a trend of gradually increasing glass transition temperature, increasing thermal expansion coefficient, decreasing optical retardation value, and increasing transmittance; in terms of mechanical properties, it also generally shows the characteristics of decreasing tensile modulus, decreasing tensile strength and then increasing tensile strength, and decreasing elongation at break and then increasing tensile strength.

[0078] Somewhat surprisingly, the applicant found that Example 3 exhibited relatively outstanding performance, and showed the same results after repeated testing.

Claims

1. A polyamide-imide film with high glass transition temperature, low coefficient of thermal expansion, and low retardation value, characterized in that, The polyamide-imide has the following structure: Where n is greater than 0, m is greater than 0; m:n = (5-1):1; R1 and R2 are residues of diamine; R1 has the structure shown in Equation I-1: R2 has the structure shown in Equation I-3: The polyamide-imide film has a glass transition temperature above 400°C, a coefficient of thermal expansion below 40 ppm / K, and an optical retardation value in the thickness direction below 255 nm; the polyamide-imide film has a tensile modulus above 3 GPa and a total transmittance above 88%.

2. The method for preparing a polyamide-imide film with high glass transition temperature, low coefficient of thermal expansion, and low retardation value according to claim 1, characterized in that, The process includes the following steps: under the protection of an inert gas, a polymerization reaction is carried out in a reaction solvent using diamine and dianhydride as monomers to obtain a polyimide resin solution, which is then precipitated, filtered, and dried to obtain polyamide-imide resin. The polyamide-imide resin is dissolved in a solvent, and the polyamide-imide resin solution is coated onto a substrate and cured by heating to obtain the polyamide-imide film.

3. The method for preparing a polyamide-imide film with high glass transition temperature, low coefficient of thermal expansion, and low retardation value according to claim 2, characterized in that, The dianhydride is hydrogenated pyromellitic dianhydride, and the diamine is a combination of diamine A and diamine B; wherein diamine A is 4,4'-diaminobenzoylaniline, and diamine B is 9,9-bis(4-aminophenyl)fluorene; the molar ratio of diamine A to diamine B is (1-5):1; the total molar ratio of the dianhydride to the diamine is (0.95-1.05):

1.

4. The method for preparing a polyamide-imide film with high glass transition temperature, low coefficient of thermal expansion, and low retardation value according to claim 2, characterized in that, The reaction solvent includes γ-butyrolactone, m-cresol, N , N -Dimethylformamide, N , N -Dimethylacetamide, N One or more of methylpyrrolidone and dimethyl sulfoxide; The solvent used for dissolution is γ-butyrolactone. N , N -Dimethylformamide, N One or more of methylpyrrolidone.

5. The method for preparing a polyamide-imide film with high glass transition temperature, low coefficient of thermal expansion, and low retardation value according to claim 2, characterized in that, The solid content of the reaction solution during polymerization is 5-25%, the polymerization temperature is 160-200℃, and the polymerization time is 5-10 hours.

6. The method for preparing a polyamide-imide film with high glass transition temperature, low coefficient of thermal expansion, and low retardation value according to claim 2, characterized in that, The heat curing process includes the following steps: curing at 60-100℃ for 1-2 hours, curing at 100-160℃ for 1-2 hours, curing at 160-250℃ for 1-2 hours, and curing at 260-350℃ for 1-2 hours.

7. The application of the polyamide-imide film according to claim 1 as a substrate material in electronic displays.

8. The application of the polyamide-imide film according to claim 1 as a substrate material in the field of flexible displays.

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