Polyimide resin with controllable molecular weight and preparation method of polyimide film

Abstract

The invention relates to the technical field of chemical materials, in particular to polyimide with controllable molecular weight and a preparation method of a film containing the polyimide, which comprises the following steps: taking specific diamine and dianhydride as raw materials, under a composite solvent and assistant system, utilizing a solution polycondensation method and a secondary polymerization process, and introducing the dianhydride with a heterocyclic structure, thereby obtaining the polyimide with controllable molecular weight. The polyimide resin with controllable molecular weight is prepared by replacing traditional aromatic dianhydride, and compared with a traditional full-aromatic polyimide film, the film prepared from the polyimide resin prepared by the method not only keeps higher mechanical strength and good thermal stability, but also has the advantages of light transmittance, low dielectric, low thermal expansion and low water absorption.

Description

Technical Field

[0001] The invention relates to the technical field of chemical materials, and in particular to a method for preparing a polyimide resin with controllable molecular weight and a polyimide film. Background Art

[0002] Polyimide refers to a type of high molecular polymer containing an imide ring on the main chain. It has a highly conjugated molecular chain structure, which gives it excellent heat resistance and stability, as well as good mechanical and electrical properties. It can be used to manufacture films, coatings, advanced composite materials, fibers, engineering plastics, electro-optical materials, photoresists, etc. Traditional polyimide films are fully aromatic structures. Due to the presence of symmetrical and highly polar groups in the molecular chain, they have high heat resistance, but also lead to problems such as poor solubility and non-melting that are difficult to process. In addition, due to the formation of intramolecular and intermolecular charge transfer complexes between aromatic dianhydrides and diamines, traditional aromatic polyimides will cause the film to absorb blue-violet light in the visible light range and appear yellow to brownish yellow, affecting its application in the optoelectronic field. Therefore, improving the transparency of polyimide films has become a research focus for researchers in this field.

[0003] Patent CN112079743B discloses a diamine, polyimide and polyimide film. The polyimide film has the performance advantages of high heat resistance, low thermal expansion and high light transmittance, and can be widely used in various electrical and optical components, but the water absorption has not been studied. If the water absorption is high, the film will be easily aged and the service life will be shortened. Patent CN114891210B discloses a polyimide, a polyimide film and a preparation method thereof. The polyimide obtained by the invention contains aliphatic side chains, which hinder the close stacking of molecular chains and increase its solubility. At the same time, the aliphatic structure destroys the conjugated structure and weakens the electron complex transfer in the main chain, achieving a good transparent effect. The introduced fluorine element can further destroy the conjugated structure and improve the thermal stability, but the molecular weight of the polyimide prepared by the invention is uncontrollable, which may affect its application range.

[0004] Therefore, there is an urgent need to develop a method for preparing a polyimide resin with low water absorption and controllable molecular weight on the market. Summary of the invention

[0005] In view of the problems existing in the prior art, the purpose of the present invention is to obtain a method for preparing a polyimide with controllable molecular weight, which has a high yield, and a film prepared from the polyimide resin obtained by the method has good mechanical strength, thermal stability, high light transmittance, low dielectric properties, thermal expansion and water absorption.

[0006] In order to achieve the above object, the technical solution adopted by the present invention is as follows:

[0007] In one aspect, the present invention provides a method for preparing a polyimide resin with controllable molecular weight, comprising the following steps:

[0008] (1) adding a diamine monomer, a first solvent, a catalyst and an auxiliary agent to a reaction kettle in sequence, premixing, heating to 50-100° C. under nitrogen protection, adding a dianhydride monomer, reacting for 1-4 hours, then continuing to add the dianhydride monomer at 95-105° C., stopping heating, using the reaction heat to continue to raise the temperature in the reaction kettle, and simultaneously turning on cooling water to control the temperature at 100-140° C. and the pressure at 0.5-2 MPa to continue reacting for 1-3 hours to obtain a reaction slurry;

[0009] (2) Cooling the reaction slurry obtained in step (1), filtering to obtain a polyimide solution, dropping the polyimide solution into anhydrous ethanol to obtain a filamentous solid, and drying the filamentous solid to obtain a polyimide resin.

[0010] Preferably, a method for preparing a polyimide resin with controllable molecular weight comprises the following steps:

[0011] (1) adding a diamine monomer, a first solvent, a catalyst and an auxiliary agent to a high-pressure reactor in sequence, premixing, heating to 60-80° C. under nitrogen protection, adding dianhydride monomers in an amount of 73-76% of the total molar amount of dianhydride monomers, and performing a prepolymerization reaction for 2-3 hours, then continuing to add the remaining dianhydride monomers at 95-105° C., stopping heating, and using the reaction heat to continue to raise the temperature in the reactor, while turning on cooling water, controlling the temperature to 120-130° C., and the pressure to 0.8-1.5 MPa, and continuing to reflux for polycondensation for 1.5-2.5 hours to obtain a reaction slurry;

[0012] (2) Cooling the reaction slurry obtained in step (1), filtering to obtain a polyimide solution, dropping the polyimide solution into anhydrous ethanol to obtain a filamentous solid, and drying the filamentous solid to obtain a polyimide resin.

[0013] The invention uses diamine and dianhydride as raw materials, utilizes a solution polycondensation method and a secondary polymerization process in the presence of a composite solvent and an auxiliary agent system, and adopts a "batch addition, multi-point segmentation" mode to add dianhydride in the entire reaction process. The reaction progress is controlled by adjusting the feed amount and temperature at each stage of the prepolymerization (linear polycondensation) and polycondensation (bulk cross-linking polycondensation) reaction, and the degree of chain growth of the polyimide precursor is regulated to achieve the purpose of controllable molecular weight. The obtained polyimide has high mechanical strength and good thermal stability, and has the advantages of high light transmittance, low dielectric, low thermal expansion and low water absorption. In addition, the invention utilizes the reaction heat to continue to increase the temperature in the reaction kettle for the next reaction, and can effectively save energy and cost.

[0014] In some embodiments, the polyimide resin has a melt viscosity of 500-570 Pa·s at 350-430° C., a product weight yield of 86-93%, a weight average molecular weight of 30,000-45,000, and a molecular weight distribution index of 2.2-2.5.

[0015] In some embodiments, the diamine monomer in step (1) includes at least one of cis-3,4-tetrahydrofuran diamine, 3,6-diaminonitrogen anthracene, N,N'-diphenyldiphenyl diamine, 3,3-bistrifluoromethyl-4,4-diphenyl diamine, tetramethyl paraphenylenediamine, 1,4-phenylenediamine, 1,5-naphthalene diamine, naphthalene-2,6-diamine, naphthalene-1,4-diamine, naphthalene-2,7-diamine, trans-1,4-cyclohexane diamine, trans-1,2-cyclohexane diamine, 2,3-diaminopyridine, 2,6-diaminopyridine, 3,5-diaminopyridine, 2,6-diaminopyrazine, and 2,5-diaminopyrazine.

[0016] In some embodiments, the dianhydride monomer in step (1) includes at least one of 1,2,3,6-tetrahydrophthalic anhydride, 3,4,5,6-tetrahydrophthalic anhydride, 1,2-cyclopentanedicarboxylic anhydride, cyclobutane-1,2-dicarboxylic anhydride, 1,8-naphthalene dicarboxylic anhydride, 2,3-pyridine dicarboxylic anhydride, tetrafluorophthalic anhydride, pyridine-3,4-dicarboxylic anhydride, 2,2'-biphenyl dicarboxylic anhydride, and 4,5-dichloro-1,8-naphthalene dicarboxylic anhydride.

[0017] In some embodiments, the molar ratio of the diamine monomer to the dianhydride monomer is 1:(1.05-1.25).

[0018] The present invention selects diamines and dianhydrides of specific types and proportions to cooperatively control the polymerization degrees of linear polycondensation and bulk cross-linking polycondensation, thereby promoting the formation of "bulk" organic small molecules from a microscopic perspective, improving the degree of polycondensation, weakening the conjugated characteristics of polyimide molecular chains, blocking the flow of electron clouds, reducing the formation of CTCs, reducing the absorption of visible light, and enhancing the transparency of polyimide films. In addition, diamines and dianhydrides of specific types and proportions can effectively reduce the polarity of molecular chains and reduce the dielectric constant of polyimide resins.

[0019] In some embodiments, the catalyst in step (1) is any one or more of methylamine, dimethylamine, triethylamine, and tetrabutylammonium bromide, and the molar ratio of the catalyst to the diamine monomer is (0.005-0.01):1.

[0020] In some embodiments, in step (1), the first solvent is any one or more of N-methylpyrrolidone, N,N-dimethylacetamide, pyridine, hexamethylphosphoric triamide, 1,3-dimethyl-2-imidazolidinone or N-methylmorpholine, and the molar ratio of the first solvent to the diamine monomer is (2-3):1.

[0021] Preferably, the first solvent is N-methylmorpholine and N-methylpyrrolidone, and the molar ratio of the two is 1:(0.5-2).

[0022] In some embodiments, the preparation method of the auxiliary agent comprises the following steps:

[0023] 1) stirring 1-vinyl-3-methylimidazolium chloride, alkali metal or alkaline earth metal chloride at 75-85° C. for 36-38 hours, extracting to obtain a metal ion liquid;

[0024] 2) Add silica microspheres and silane coupling agent KH550 to anhydrous methanol, react at 50-60° C. for 5-6 hours, then add the metal ion liquid and sodium ethoxide obtained in step 1), continue to react for 5-6 hours, filter, wash and dry to obtain the auxiliary agent.

[0025] Preferably, the alkali metal or alkaline earth metal chloride is any one or more of lithium chloride, calcium chloride, aluminum chloride, potassium chloride, sodium chloride, and magnesium chloride; and the molar ratio of 1-vinyl-3-methylimidazole chloride to the alkali metal or alkaline earth metal chloride is 1:(1-4).

[0026] Preferably, the silica microspheres are amino silica microspheres.

[0027] In some embodiments, the mass ratio of the silica microspheres to the metal ion liquid is 1:(0.1-0.5).

[0028] The invention loads double bonds on the chloride of alkali metal or alkaline earth metal, and then loads the double bonds on silica microspheres to obtain an auxiliary agent, which can be filtered and collected in step (2), so that the chloride of alkali metal or alkaline earth metal can be recycled, reducing the discharge of industrial waste liquid containing metal ions, saving costs, protecting the environment, and improving the yield and the controllability of molecular weight. The possible reason is that the presence of amino groups consumes the acidic substances generated in the reaction process, promoting the reaction to proceed in the forward direction, and silica increases the contact area between the catalyst and the alkali metal chloride and the reactant, thereby improving the catalytic effect. Furthermore, by further limiting the ratio of silica microspheres and metal ion liquid, the auxiliary agent can adjust the polymerization degree of the reaction, increase the uniformity of polyimide molecules, and improve the mechanical strength of the polyimide resin.

[0029] In some embodiments, the amount of the additive added is 0.6-1 times the mass of the catalyst.

[0030] The second aspect of the present invention provides a method for preparing a polyimide film, comprising the following steps: adding a polyimide resin to a second solvent for dissolution, wherein the amount of the second solvent added is 2-3 times the mass of the polyimide resin, to obtain a polyimide solution, uniformly coating the polyimide solution on a clean glass plate, heating the temperature to 140-150°C at 3-5°C / min and keeping the temperature for 8-10h, then cooling to room temperature, placing the glass plate coated with the polyimide solution in deionized water at 35-45°C to peel off the film, and drying to obtain a polyimide film.

[0031] In some embodiments, the second solvent is one or more of N,N-dimethylformamide, N,N-dimethylacetamide, N-methylpyrrolidone, dimethyl sulfoxide and acetone.

[0032] Compared with the prior art, the present invention has the following beneficial effects:

[0033] (1) The present invention uses diamine and dianhydride as raw materials, and utilizes a solution polycondensation method and a secondary polymerization process in the presence of a composite solvent and an auxiliary agent system. The dianhydride is added in a "batch addition, multi-point segmentation" manner throughout the reaction process, and the reaction progress is controlled by adjusting the feed amount and temperature at each stage of the prepolymerization (linear polycondensation) and polycondensation (bulk cross-linking polycondensation) reaction to regulate the extent of polyimide precursor chain growth and achieve the purpose of controllable molecular weight. The obtained polyimide has high mechanical strength and good thermal stability, and has the advantages of high light transmittance, good solubility, low dielectric, low thermal expansion, and low water absorption. In addition, the present invention utilizes the reaction heat to continue to raise the temperature in the reactor for the next step of reaction, which can effectively save energy and cost.

[0034] (2) The present invention selects specific types and ratios of diamines and dianhydrides to synergistically control the degree of polymerization of linear condensation and bulk cross-linking condensation, thereby promoting the formation of "bulk" organic small molecules from a microscopic perspective, improving the degree of condensation, and weakening the conjugated properties of the polyimide molecular chain, blocking the flow of electron clouds, reducing the formation of CTC, reducing the absorption of visible light, and enhancing the transparency of the polyimide film.

[0035] (3) The present invention loads double bonds on the chloride of alkali metal or alkaline earth metal, and then loads it on silica microspheres to obtain an additive, so that the chloride of alkali metal or alkaline earth metal can be recycled, reducing the discharge of industrial waste liquid containing metal ions, saving costs, protecting the environment, and at the same time improving the yield and increasing the controllability of molecular weight. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 It is a process flow chart. DETAILED DESCRIPTION

[0037] The present invention will be described below in conjunction with specific embodiments. It should be noted that the following embodiments are examples of the present invention and are only used to illustrate the present invention, but not to limit the present invention. Other combinations and various modifications within the concept of the present invention may be performed without departing from the spirit or scope of the present invention.

[0038] In the following examples and comparative examples, the compounds and related reagents used can be purchased from the market, wherein the silica microspheres are amino-modified monodisperse mesoporous silica microspheres purchased from Shaanxi Xingbei Aike Biotechnology Co., Ltd., with a particle size of 5 μm.

[0039] The test methods for melt viscosity, product weight yield, weight average molecular weight, molecular weight distribution index, etc. are routine operations for those skilled in the art, and conventional test methods can be selected, wherein the test temperature for melt viscosity is 400°C.

[0040] Preparation Example 1

[0041] The preparation method of auxiliary agent-1 comprises the following steps:

[0042] 1) 2 mol of 1-vinyl-3-methylimidazolium chloride and 3 mol of aluminum chloride were stirred at 80° C. for 37 h and extracted to obtain a metal ion liquid;

[0043] 2) Add 3 g of silica microspheres and 1.5 g of silane coupling agent KH550 to 200 ml of anhydrous methanol, react at 55° C. for 5.5 h, then add 0.9 g of the metal ionic liquid obtained in step 1) and 0.04 g of sodium ethoxide, continue to react for 5.5 h, filter, wash and dry to obtain additive-1.

[0044] Preparation Example 2

[0045] The preparation method of auxiliary agent-2 is the same as that of Preparation Example 1, except that the amount of metal ion liquid added is 1.8 g.

[0046] Preparation Example 3

[0047] The preparation method of auxiliary agent-3 is the same as that of Preparation Example 1, except that the amount of metal ion liquid added is 0.15 g.

[0048] Example 1

[0049] See the process flow chart Figure 1 .

[0050] A method for preparing a polyimide resin with controllable molecular weight comprises the following steps:

[0051] (1) 1 mol of cis-3,4-tetrahydrofuran diamine, 2.5 mol of a mixed solution of N-methylmorpholine and N-methylpyrrolidone (the molar ratio of N-methylmorpholine to N-methylpyrrolidone is 1:1.5), 0.008 mol of methylamine and 0.2 g of auxiliary agent-1 are sequentially added to a high-pressure reactor for premixing. Under nitrogen protection, the temperature is raised to 70° C., 0.9 mol of 1,2,3,6-tetrahydrophthalic anhydride is added, and a prepolymerization reaction is carried out for 2.5 hours. Then, 0.3 mol of 1,2,3,6-tetrahydrophthalic anhydride is continuously added at 100° C., heating is stopped, and the temperature in the reactor is continuously raised by the reaction heat. At the same time, cooling water is turned on to control the temperature at 125° C. and the pressure at 1 MPa to continue reflux for condensation reaction for 2 hours to obtain a reaction slurry;

[0052] (2) The reaction slurry obtained in step (1) is cooled and filtered to obtain a polyimide solution, the polyimide solution is added dropwise to 3 times the volume of anhydrous ethanol to obtain a filamentous solid, and the filamentous solid is dried to obtain a polyimide resin. The melt viscosity of the polyimide resin is 528 Pa·s, the product weight yield is 93%, the weight average molecular weight is 42198, and the molecular weight distribution index is 2.2.

[0053] Example 2

[0054] A method for preparing a polyimide resin with controllable molecular weight comprises the following steps:

[0055] (1) 1 mol of cis-3,4-tetrahydrofuran diamine, 2 mol of a mixed solution of N-methylmorpholine and N-methylpyrrolidone (the molar ratio of N-methylmorpholine to N-methylpyrrolidone is 1:0.5), 0.005 mol of methylamine and 0.124 g of auxiliary agent-1 were added to a high-pressure reactor in sequence for premixing. Under nitrogen protection, the temperature was raised to 60° C., 0.78 mol of 1,2,3,6-tetrahydrophthalic anhydride was added, and a prepolymerization reaction was carried out for 2 hours. Then, 0.27 mol of 1,2,3,6-tetrahydrophthalic anhydride was added at 95° C., heating was stopped, and the temperature in the reactor was continued to rise by using the reaction heat. At the same time, cooling water was turned on to control the temperature to 120° C. and the pressure to 0.8 MPa to continue reflux for 1.5 hours of polycondensation reaction to obtain a reaction slurry;

[0056] (2) The reaction slurry obtained in step (1) is cooled and filtered to obtain a polyimide solution, the polyimide solution is added dropwise to 3 times the volume of anhydrous ethanol to obtain a filamentous solid, and the filamentous solid is dried to obtain a polyimide resin. The melt viscosity of the polyimide resin is 519 Pa·s, the product weight yield is 91%, the weight average molecular weight is 30340, and the molecular weight distribution index is 2.3.

[0057] Example 3

[0058] A method for preparing a polyimide resin with controllable molecular weight comprises the following steps:

[0059] (1) 1 mol of cis-3,4-tetrahydrofuran diamine, 3 mol of a mixed solution of N-methylmorpholine and N-methylpyrrolidone (the molar ratio of N-methylmorpholine to N-methylpyrrolidone is 1:2), 0.01 mol of methylamine and 0.25 g of auxiliary agent-1 were added to a high-pressure reactor in sequence for premixing. Under nitrogen protection, the temperature was raised to 80° C., 0.94 mol of 1,2,3,6-tetrahydrophthalic anhydride was added, and a prepolymerization reaction was carried out for 3 hours. Then, 0.31 mol of 1,2,3,6-tetrahydrophthalic anhydride was added at 105° C., heating was stopped, and the temperature in the reactor was continued to rise by using the reaction heat. At the same time, cooling water was turned on to control the temperature to 130° C. and the pressure to 1.5 MPa, and reflux was continued for condensation reaction for 2.5 hours to obtain a reaction slurry;

[0060] (2) The reaction slurry obtained in step (1) is cooled and filtered to obtain a polyimide solution, the polyimide solution is added dropwise to 3 times the volume of anhydrous ethanol to obtain a filamentous solid, and the filamentous solid is dried to obtain a polyimide resin. The melt viscosity of the polyimide resin is 567 Pa·s, the product weight yield is 92%, the weight average molecular weight is 44950, and the molecular weight distribution index is 2.5.

[0061] Example 4

[0062] A method for preparing a polyimide resin with controllable molecular weight, the specific implementation method is the same as that of Example 1, except that an equal amount of auxiliary agent-1 is replaced by auxiliary agent-2, and the melt viscosity of the polyimide resin is tested to be 508 Pa·s, the product weight yield is 81%, the weight average molecular weight is 28756, and the molecular weight distribution index is 2.3.

[0063] Example 5

[0064] A method for preparing a polyimide resin with controllable molecular weight, the specific implementation method is the same as Example 1, except that an equal amount of auxiliary agent-1 is replaced by auxiliary agent-3, and the melt viscosity of the polyimide resin is tested to be 641 Pa·s, the product weight yield is 85%, the weight average molecular weight is 59874, and the molecular weight distribution index is 3.0.

[0065] Example 6

[0066] A method for preparing a polyimide resin with controllable molecular weight, the specific implementation manner is the same as that of Example 1, except that the total amount of 1,2,3,6-tetrahydrophthalic anhydride added is 1.4 mol, the melt viscosity of the polyimide resin is 597 Pa·s, the product weight yield is 91%, the weight average molecular weight is 53950, and the molecular weight distribution index is 3.0.

[0067] Example 7

[0068] A method for preparing a polyimide resin with controllable molecular weight, the specific implementation manner is the same as that of Example 1, except that the amount of methylamine added is 0.015 mol, the melt viscosity of the polyimide resin is tested to be 699 Pa·s, the product weight yield is 92%, the weight average molecular weight is 68578, and the molecular weight distribution index is 3.2.

[0069] Example 8

[0070] A method for preparing a polyimide resin with controllable molecular weight. The specific implementation method is the same as that of Example 1, except that the amount of additive-1 added is 0.3 g. The melt viscosity of the polyimide resin is 785 Pa·s, the product weight yield is 80%, the weight average molecular weight is 65971, and the molecular weight distribution index is 3.5.

[0071] Example 9

[0072] A method for preparing a polyimide resin with controllable molecular weight, the specific implementation method is the same as that of Example 1, except that an equal amount of auxiliary agent-1 is replaced by aluminum chloride, and the melt viscosity of the polyimide resin is tested to be 401 Pa·s, the product weight yield is 83%, the weight average molecular weight is 28159, and the molecular weight distribution index is 3.6.

[0073] Example 10

[0074] A method for preparing a polyimide resin with controllable molecular weight, the specific implementation manner is the same as that of Example 1, except that the diamine monomer is N,N'-diphenylbiphenyl diamine, the dianhydride monomer is cyclobutane-1,2-dicarboxylic anhydride, and the polyimide resin melt viscosity is 516 Pa·s, the product weight yield is 91%, the weight average molecular weight is 35678, and the molecular weight distribution index is 2.3.

[0075] Embodiment 11

[0076] A method for preparing a polyimide resin with controllable molecular weight, the specific implementation manner is the same as that of Example 1, except that the diamine monomer is 5-naphthalene diamine, the dianhydride monomer is 2,3-pyridine dicarboxylic anhydride, and the polyimide resin melt viscosity is 538 Pa·s, the product weight yield is 91%, the weight average molecular weight is 39678, and the molecular weight distribution index is 2.3.

[0077] Example 12

[0078] A method for preparing a polyimide resin with controllable molecular weight, the specific implementation manner is the same as that of Example 1, except that the diamine monomer is trans-1,4-cyclohexanediamine, the dianhydride monomer is tetrafluorophthalic anhydride, and the polyimide resin melt viscosity is 548 Pa·s, the product weight yield is 89%, the weight average molecular weight is 42340, and the molecular weight distribution index is 2.4.

[0079] Embodiment 13

[0080] A method for preparing a polyimide resin with controllable molecular weight, the specific implementation manner is the same as that of Example 1, except that the diamine monomer is 2,6-diaminopyridine, the dianhydride monomer is 2,2'-biphenyldicarboxylic anhydride, and the polyimide resin melt viscosity is 553 Pa·s, the product weight yield is 87%, the weight average molecular weight is 43420, and the molecular weight distribution index is 2.5.

[0081] Embodiment 14

[0082] A method for preparing a polyimide resin with controllable molecular weight, the specific implementation manner is the same as that of Example 1, except that the diamine monomer is 2,5-diaminopyrazine, the dianhydride monomer is 4,5-dichloro-1,8-naphthalene dicarboxylic anhydride, and the polyimide resin melt viscosity is 563 Pa·s, the product weight yield is 89%, the weight average molecular weight is 44490, and the molecular weight distribution index is 2.5.

[0083] Comparative Example 1

[0084] A method for preparing a polyimide resin with controllable molecular weight, the specific implementation method is the same as Example 1, except that no auxiliary agent-1 is added. The melt viscosity of the polyimide resin is 523 Pa·s, the product weight yield is 75%, the weight average molecular weight is 29415, and the molecular weight distribution index is 3.8.

[0085] Performance Testing

[0086] The molecular weight controllable polyimide resin obtained in the above embodiments and comparative examples was prepared into a film, comprising the following steps: 10 g of polyimide resin was dissolved in 30 g of N,N-dimethylformamide, the polyimide solution was evenly coated on a clean glass plate with a coating thickness of 40 μm, the temperature was raised to 145°C at 4°C / min and kept for 9 hours, then cooled to room temperature, the glass plate coated with the polyimide solution was placed in deionized water at 40°C to peel off the film, and dried to obtain a polyimide film. The obtained film was subjected to the following tests:

[0087] 1. Linear thermal expansion coefficient

[0088] The prepared film was cut into samples of 4 mm × 30 mm. The sample was hung on a quartz hook and 0.010 N was applied. It was then heated from 30 ° C to 420 ° C at a rate of 10 ° C / min in a nitrogen atmosphere. The thermal expansion coefficient value was obtained in the range of 50 ° C-200 ° C, α = ΔL / (L × ΔT), where ΔL is the change in length, L is the original length, and ΔT is the change in temperature.

[0089] 2. Mechanical strength

[0090] The bending resistance was tested according to JIS C5016.

[0091] 3. Water absorption

[0092] A certain mass of film is placed at 35°C, 100% RH for 48 hours, then weighed and the mass is recorded as m0. It is then heated from 35°C to 250°C at a heating rate of 10°C / min, weighed and the mass is recorded as m1. Moisture absorption rate = (m0-m1) / m1×100%.

[0093] 4. Light transmittance

[0094] The light transmittance at 450nm was tested according to ASTM-D882.

[0095] 5. Dielectric properties

[0096] A broadband dielectric constant tester was used. First, the thickness of the polyimide film was measured with a thickness gauge. The conductive silver paste was evenly coated on the surface of the polyimide film. The thickness of the conductive silver paste was 1 mm. Then, the polyimide film coated with the conductive silver paste was cut into discs with a diameter of 10 mm as a sample. The impedance analyzer was adjusted to the capacitance-loss mode to obtain the capacitance-frequency and loss-frequency curves and data. The relative dielectric constant was calculated according to the formula.

[0097] Relative dielectric constant = 4C p d / πε0D 2

[0098] C p -Capacitance value of the sample;

[0099] d-thickness of the specimen;

[0100] ε0-vacuum electromechanical constant;

[0101] D-diameter of the specimen.

[0102] The test results are shown in Table 1:

[0103] Table 1

[0104]

[0105] It can be seen from the experimental data of Examples 1-3 and Examples 10-14 in Table 1 that the film prepared by the polyimide prepared by the present invention has good mechanical strength, low thermal expansion, low water absorption, dielectric properties, and good transparency; from the comparison between Example 1 and Examples 4, 5, 6, 7, and 8, it can be seen that the ratio of silica microspheres and metal ion liquids, the molar ratio of 1,2,3,6-tetrahydrophthalic anhydride to cis-3,4-tetrahydrofuran diamine, the molar ratio of methylamine to cis-3,4-tetrahydrofuran diamine, and the amount of additives added are changed, resulting in changes in the melt viscosity, weight-average molecular weight, and molecular weight distribution index of the polyimide resin, thereby causing the thermal expansion, water absorption, dielectric properties, and transparency of the polyimide film to decrease to varying degrees; from the comparison between Example 1 and Example 9, it can be seen that when aluminum chloride is directly used as an additive, the various properties of the polyimide film are relatively poor; from the comparison between Example 1 and Comparative Example 1, it can be seen that the yield is low without adding an additive, and the various properties of the polyimide film are reduced.

[0106] The above implementation modes are only for illustrating the technical concept and features of the present invention, and their purpose is to enable people familiar with this technology to understand the content of the present invention and implement it, and they cannot be used to limit the protection scope of the present invention. Any equivalent changes or modifications made according to the spirit of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for preparing a polyimide resin with controllable molecular weight, characterized in that: The following steps are involved: (1) Adding diamine monomer, first solvent, catalyst and auxiliary agent to a reaction kettle in sequence, premixing, heating to 50-100° C. under nitrogen protection, adding dianhydride monomer, reacting for 1-4 hours, then continuing to add dianhydride monomer at 95-105° C., stopping heating, using reaction heat to continue to increase the temperature in the reaction kettle, turning on cooling water, controlling the temperature at 100-140° C., continuing the reaction for 1-3 hours, and obtaining a reaction slurry; (2) Cooling the reaction slurry obtained in step (1), filtering to obtain a polyimide solution, dropping the polyimide solution into anhydrous ethanol to obtain a filamentous solid, and drying the filamentous solid to obtain a polyimide resin.

2. The method for preparing a polyimide resin with controlled molecular weight according to claim 1, wherein: The diamine monomer in step (1) includes at least one of cis-3,4-tetrahydrofuran diamine, 3,6-diaminonitrogen anthracene, N,N'-diphenylbenzene diamine, 3,3-bistrifluoromethyl-4,4-biphenyl diamine, tetramethyl paraphenylenediamine, 1,4-phenylenediamine, 1,5-naphthalene diamine, naphthalene-2,6-diamine, naphthalene-1,4-diamine, naphthalene-2,7-diamine, trans-1,4-cyclohexane diamine, trans-1,2-cyclohexane diamine, 2,3-diaminopyridine, 2,6-diaminopyridine, 3,5-diaminopyridine, 2,6-diaminopyrazine, and 2,5-diaminopyrazine.

3. The method for preparing a polyimide resin with controlled molecular weight according to claim 1, characterized in that: The dianhydride monomer in step (1) includes at least one of 1,2,3,6-tetrahydrophthalic anhydride, 3,4,5,6-tetrahydrophthalic anhydride, 1,2-cyclopentanedicarboxylic anhydride, cyclobutane-1,2-dicarboxylic anhydride, 1,8-naphthalene dicarboxylic anhydride, 2,3-pyridine dicarboxylic anhydride, tetrafluorophthalic anhydride, pyridine-3,4-dicarboxylic anhydride, 2,2'-biphenyl dicarboxylic anhydride and 4,5-dichloro-1,8-naphthalene dicarboxylic anhydride.

4. The method for preparing a polyimide resin with controlled molecular weight according to claim 1, characterized in that: The molar ratio of the diamine monomer to the dianhydride monomer is 1:(1.05-1.25).

5. The method for preparing a polyimide resin with controllable molecular weight according to claim 1, characterized in that: The catalyst in step (1) is any one or more of methylamine, dimethylamine, triethylamine, and tetrabutylammonium bromide, and the molar ratio of the catalyst to the diamine monomer is (0.005-0.01):

1.

6. The method for preparing a polyimide resin with controllable molecular weight according to claim 1, characterized in that: In step (1), the first solvent is any one or more of N-methylpyrrolidone, N,N-dimethylacetamide, pyridine, hexamethylphosphoric triamide, 1,3-dimethyl-2-imidazolidinone or N-methylmorpholine, and the molar ratio of the first solvent to the diamine monomer is (2-3):

1.

7. The method for preparing a polyimide resin with controllable molecular weight according to claim 1, characterized in that: The preparation method of the auxiliary agent comprises the following steps: 1) stirring 1-vinyl-3-methylimidazolium chloride, alkali metal or alkaline earth metal chloride at 75-85° C. for 36-38 hours, extracting to obtain a metal ion liquid; 2) Add silica microspheres and silane coupling agent KH550 to anhydrous methanol, react at 50-60° C. for 5-6 hours, then add the metal ion liquid and sodium ethoxide obtained in step 1), continue to react for 5-6 hours, filter, wash and dry to obtain the auxiliary agent.

8. The method for preparing a polyimide resin with controllable molecular weight according to claim 7, characterized in that: The mass ratio of the silica microspheres to the metal ion liquid is 1:(0.1-0.5).

9. A method for preparing a polyimide film, characterized in that: The following steps are involved: The polyimide resin is added into a second solvent to dissolve, and the amount of the second solvent added is 2-3 times the mass of the polyimide resin to obtain a polyimide solution, and the polyimide solution is evenly coated on a clean glass plate, and the temperature is increased to 140-150° C. at 3-5° C. / min and kept for 8-10 hours, and then cooled to room temperature, and the glass plate coated with the polyimide solution is placed in deionized water at 35-45° C. to peel off the film, and dried to obtain a polyimide film; The polyimide resin is obtained by the preparation method according to any one of claims 1 to 8.

10. The method for preparing a polyimide film according to claim 9, characterized in that: The second solvent is one or more of N,N-dimethylformamide, N,N-dimethylacetamide, N-methylpyrrolidone, dimethyl sulfoxide and acetone.

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