6G ultralow dielectric constant polyimide material and preparation method thereof

Abstract

The invention provides an ultra-low dielectric constant polyimide nanofiber material for 6G based on an electrostatic spinning technology and a preparation method of the ultra-low dielectric constant polyimide nanofiber material. Aiming at the technical bottleneck that the dielectric property of the existing high polymer material is difficult to meet the 6G high-frequency signal transmission requirement, fluorine-containing polyimide is innovatively adopted as a main chain matrix, and polyisocyanate is introduced into a polymer system as an in-situ foaming agent through molecular structure design. In the electrostatic spinning forming process, the polycondensation reaction process of tetracarboxylic dianhydride and polyisocyanate is accurately regulated and controlled through a programmed temperature control technology, micro-nano-scale foaming is achieved through carbon dioxide gas generated by an intermediate decarboxylation reaction, and finally the hollow fiber membrane of a spindle-shaped structure is formed. The dielectric constant (10GHz) of the obtained fiber membrane can reach 1.1-1.6, the dielectric loss is less than 0.003, and the fiber membrane has good flexibility.

Description

Technical Field

[0001] The present invention relates to the field of polyimide nanofiber materials, and in particular to an ultra-low dielectric constant polyimide nanofiber membrane based on electrostatic spinning technology and a preparation method thereof. Background Art

[0002] With the breakthrough progress of 5G and 6G technologies, more urgent demands have been placed on data transmission capacity and speed. When digital signals are transmitted at high speeds, they often face many signal integrity issues, including crosstalk, reflection, and attenuation. Therefore, as the signal base frequency continues to increase, the performance requirements for high-frequency printed circuit board substrates are increasing day by day. The dielectric constants of traditional inorganic materials are relatively high. Commonly used low-dielectric constant materials containing silicon oxide, whether crystalline or amorphous (ceramic or glass), have a dielectric constant that is difficult to reach below 4. Commonly used polymer low-dielectric materials, such as polytetrafluoroethylene, have a dielectric constant of 2.0 and polyethylene, which has a dielectric constant of 2.3. However, this type of polymer material has disadvantages such as high temperature resistance and low bonding strength with metal substrates, which greatly limits its scope of application. Therefore, how to prepare ultra-low dielectric constant materials has become the focus of current research.

[0003] The dielectric constant of a dielectric material can be expressed by the classic Clausius-Mossotti formula: r =[1+2(P m / V m )] / [1-2(P m / V m )], where P m is the molar polarizability, V m It is the molar volume of the atomic group. The size of the dielectric constant is related to the total polarizability inside the material and the material density. Therefore, there are two main ways to reduce the dielectric constant of the material: introducing fluorine elements and increasing the free volume fraction (FFV). However, for polyimide (PI), both methods have disadvantages: the reactivity of fluorinated monomers is low, and it is difficult to synthesize high molecular weight PI, which affects the overall performance of PI products; increasing FFV is conducive to reducing the dielectric constant, but it will lead to an increase in the coefficient of thermal expansion (CTE) of the material. Other methods of reducing the dielectric constant, such as introducing aliphatic rings or forming a microporous structure inside the material, will be accompanied by a decrease in the thermal or mechanical properties of the PI film.

[0004] Polyimide (PI) is one of the high-performance special engineering plastics and is considered to be the most promising substrate for flexible electronic devices. It has low loss, high breakdown strength, excellent mechanical properties, thermal stability and electrical insulation properties and is widely used in the microelectronics industry. However, traditional polyimide is formed by the condensation of a strong electron-donating diamine and a strong electron-accepting dianhydride. The degree of conjugation of the formed polyimide molecular chain is extremely high, and there are many easily polarized imide bonds. Polarization occurs under the influence of an external electric field, and the macroscopically exhibits a high dielectric constant, generally 3.4 to 3.9. The substrate material for 5G communications requires that the dielectric constant of the encapsulation film used is lower than 3.0 to ensure the frequency of signal transmission. In addition, with the widespread application of millimeter-wave radar and the upcoming research on 6G communications, the dielectric constant of insulating materials needs to be further reduced to below 2.0. Traditional polyimide materials cannot meet the needs of 5G and future 6G communication industries. Therefore, a polyimide material with an ultra-low dielectric constant (D k <2), which is of great significance for expanding the application field of polyimide.

[0005] Air has the lowest dielectric constant (D k =1), therefore, by appropriately introducing air into polyimide materials, their dielectric constant can be significantly reduced. This method, which eliminates the need for expensive monomers and effectively adjusts the dielectric constant of polyimide materials by adjusting the structure, ratio, and distribution of the pores, has attracted widespread attention and has become a hot topic in recent research on low / ultra-low dielectric polyimide films.

[0006] With the continuous development of the microelectronics and integrated circuit industries, the miniaturization and micro-miniaturization of electronic components have become the future development trend. However, the reduction in the size of metal interconnects and the increase in packing density have led to an increase in resistance (R) and capacitance (C) delays, and their impact has become comparable to the operating speed of the device. Ideally, R and C should be reduced simultaneously to achieve continuous expansion of the device (Nature, 2020, 582, 511-514). Therefore, it is urgent to develop a low dielectric constant material (D k <2), it has excellent electrical, mechanical and thermal properties, and is compatible with the current metal oxide semiconductor (CMOS) process, and acts as a diffusion barrier to prevent electromigration of metal interconnect structures in interconnects as an intermetallic and interlayer dielectric. Summary of the Invention

[0007] Based on the problems existing in the background technology, the present invention proposes a kind of ultra-low dielectric constant polyimide nanofiber material based on electrostatic spinning technology and its preparation method. When preparing nanofiber material, the inventor innovatively adopts isocyanate group as block molecule, and reacts with fluorine-containing dianhydride to generate the intermediate structure of seven-membered isoimide ring. Because the isoimide ring structure can release carbon dioxide gas by decarboxylation at high temperature, the original nanofiber bundle structure is expanded into a hollow structure such as spindle shape from the center. The existence of this hollow core-shell structure greatly reduces the density of polyimide material, and greatly reduces the dielectric constant of PI fiber film. Therefore, the polyimide nanofiber material of the present invention can be used as an interlayer insulating dielectric material with ultra-low dielectric constant, and has very good application prospects in the fields such as high-frequency and high-speed integrated circuit packaging, interlayer insulation, stress buffer and ray barrier, and can significantly improve the signal transmission rate of high-frequency and high-speed circuit, reduce signal transmission time delay and reduce signal propagation attenuation.

[0008] One aspect of the present invention provides a polyimide nanofiber material, characterized in that the polyimide nanofiber material is formed by an imidization reaction of a polyamic acid precursor composed of a fluorine-containing diamine compound, a fluorine-containing dianhydride compound, and a polyisocyanate compound, followed by in-situ foaming. The polyimide nanofiber material has a hollow structure formed by the in-situ foaming.

[0009] Furthermore, the hollow structure may include a spindle-shaped, fusiform and sponge-shaped structure. Preferably, the hollow structure is a spindle-shaped structure.

[0010] The polyamic acid precursor comprises at least one selected from the group consisting of a repeating unit represented by the following formula 1, a repeating unit represented by the following formula 2, and a repeating unit represented by the following formula 3:

[0011]

[0012] The polyimide comprises at least one selected from the group consisting of a repeating unit represented by the following formula 4, a repeating unit represented by the following formula 5, and a repeating unit represented by the following formula 6:

[0013]

[0014] In the above formulas 1 to 6, Ar 1 Represents the main chain structure of diisocyanate group, Ar 2 Represents the main chain structure of triisocyanate group, Ar 3 Represents the main chain structure of tetraisocyanate group, Ar 4 Represents the main chain structure containing fluorinated dianhydride groups, Ar 5 represents the main chain structure of a fluorinated diamine group.

[0015] Furthermore, the dielectric constant of the polyimide nanofiber material at 10 GHz is in the range of 1.1 to 1.6.

[0016] Furthermore, the dielectric constant of the polyimide nanofiber material at 10 GHz is in the range of 1.1 to 1.3.

[0017] Furthermore, in the polyimide nanofiber material, the size of the hollow structure is 1 to 8 μm.

[0018] Furthermore, in the above formulas 1 to 6, Ar 1 represents a diisocyanate group, and Ar 1 Each is the same or different and can be selected from one or more of the following:

[0019] Ar 1 :

[0020]

[0021] Ar 2 represents a triisocyanate group, and Ar 2 Each is the same or different and can be selected from one or more of the following:

[0022] Ar 2 :

[0023] Ar 3 represents a tetraisocyanate group, and Ar 3 It can be the following structure: Ar 3 :

[0024] Ar 4 represents a fluorinated dianhydride group, and Ar 4 Each is the same or different and can be selected from one or more of the following:

[0025] Ar 4 :

[0026] Ar 5 represents a fluorinated diamine group, and Ar 5 Each is the same or different and can be selected from one or more of the following:

[0027] Ar 5 :

[0028]

[0029] In the above Ar 1 to Ar5 In the figure, * represents a connecting bond, which, when appearing on an aromatic ring, can be connected to any carbon atom on the aromatic ring.

[0030] Furthermore, the polyisocyanate compound may be selected from one or more of the following:

[0031]

[0032] Furthermore, the fluorine-containing dianhydride compound can be selected from one or more of the following:

[0033]

[0034] Furthermore, the fluorine-containing diamine compound can be selected from one or more of the following:

[0035]

[0036] Another aspect of the present invention provides a method for preparing an ultra-low dielectric constant polyimide nanofiber material based on electrospinning technology, characterized in that it comprises the following steps:

[0037] 1) under the protection of anhydrous and oxygen-free inert gas, adding a fluorine-containing diamine compound, a polyisocyanate compound and an organic solvent to react with the fluorine-containing dianhydride compound to obtain a polyamic acid precursor solution;

[0038] 2) injecting the polyamic acid precursor solution obtained in step 1) into a syringe, and forming a polyamic acid nanofiber membrane on a collector substrate using electrospinning technology;

[0039] 3) The polyamic acid nanofiber membrane obtained in step 2) is subjected to programmed temperature increase for imidization reaction, carbon dioxide released by high-temperature decarboxylation is used for in-situ foaming, and the membrane is peeled off from the substrate after natural cooling to obtain the polyimide nanofiber material.

[0040] Electrospinning technology, capable of rapidly and efficiently producing micro- and nanoscale fibers with tunable physical properties, has recently gained popularity across a wide range of applications. Fiber membranes produced using electrospinning technology offer advantages such as large surface area, adjustable micro- and nanostructures, a wide range of materials, and high porosity. These micro- and nanofiber membranes hold significant appeal and advantages in the design and manufacture of lightweight, thin, breathable, and comfortable-to-wear flexible electronic devices.

[0041] In the method for preparing the polyimide nanofiber material of the present invention, the fluorine-containing diamine compound, the polyisocyanate-based compound and the fluorine-containing dianhydride compound are the same as those described above.

[0042] Furthermore, the organic solvent may be selected from at least one of N-methylpyrrolidone, dimethyl sulfoxide, N,N-dimethylformamide and N,N-dimethylacetamide.

[0043] Preferably, in step 1), the molar ratio of the fluorine-containing diamine compound, the polyisocyanate compound and the fluorine-containing dianhydride compound can be 0.7:0.3:1 to 0.99:0.01:1.

[0044] Preferably, the reaction conditions of step 1) are -20 to 10° C. in an organic solvent under anhydrous and oxygen-free conditions for 2 to 6 hours.

[0045] Preferably, described step 2) comprises: by step 1) the polyamic acid precursor solution obtained is added into the syringe syringe that range is 10mL and discharges bubble in pipe.Put on special spinning needle, aluminium foil is close to receiving roller, regulates spinning voltage, pushing speed, the rotating speed of receiving roller and the distance of needle to receiving roller, ensures that precursor solution slowly flows out from needle.Under high voltage electric field, the precursor solution at needle is polarized to form Taylor cone, arrives at the jet unstable region before receiving roller and is further stretched, and after volatilizing solvent, polyamic acid precursor falls on receiving roller as nanofiber.

[0046] In order to be suitable for electrospinning, preferably, in the step 2), the solid content of the polyamic acid precursor solution is 10-15 wt % and the viscosity is 8000-12000 CPs.

[0047] Preferably, in step 2), the specification of the special spinning needle is 19-22G.

[0048] Preferably, in step 2), the pushing speed is 0.0016 to 0.0030 mm / s.

[0049] Preferably, in step 2), the needle moving speed is 5 to 20 mm / s.

[0050] Preferably, in step 2), the spinning voltage is 10,000 to 40,000 V.

[0051] Preferably, in step 2), the distance from the needle to the receiver roller is 25 to 40 cm.

[0052] Preferably, in step 2), the spinning temperature is 25-40°C.

[0053] Preferably, in step 2), the high voltage electric field is 8000 to 20000 volts.

[0054] Preferably, the diameter of the nanofibers in the polyamic acid nanofiber membrane obtained in step 2) is 20 to 600 nm. In addition, the inventors have confirmed that the structures of different fluorinated diamine and fluorinated dianhydride compounds can lead to different degrees of curling of the fiber bundle.

[0055] Preferably, the programmed heating conditions in step 3) are: using gradient heating conditions with a heating rate of 1-3°C / min, heating to 100°C, keeping warm for 30-90min to remove most of the organic solvent; heating to 140-160°C, heating for 0.5-2h to remove the remaining organic solvent and possible trace water from the solvent, diamine, and dianhydride monomers; heating to 180-220°C, heating for 0.5-2h, at which time the isoimide ring decomposes to release carbon dioxide; heating to 260-280°C, heating for 0.5-1h, partial imidization reaction of the polyamic acid segment for dehydration; heating to 320°C, heating for 0.5-2h, and the imidization reaction is completely completed; heating to 340-360°C, heating for 0.5-1h, reaching a temperature above the glass transition temperature of the polymer for annealing; and naturally cooling to room temperature.

[0056] Preferably, the diameter of the nanofibers in the polyimide nanofiber material obtained in step 3) is 20 to 600 nm.

[0057] The inventors discovered that nanofiber membranes produced from different polyisocyanate compounds exhibit varying degrees of crosslinking and surface microstructure. Specifically, when the molar ratio of the polyisocyanate compound is 1% to 30% relative to the total molar ratio of the polyisocyanate compound and the fluorinated diamine compound, the resulting spindle-shaped hollow structures range in size from 1 to 8 μm. Therefore, by optimizing the selection of the polyisocyanate compound and the polymerization ratio and method of each reactive monomer, the present invention has produced a new material with a spindle-shaped hollow structure.

[0058] The present invention is beneficial in that:

[0059] Since 1937, when German chemist Otto Bayer synthesized a variety of linear, branched or cross-linked polymers using polyisocyanate-based polyaddition reactions, isocyanate-based polymer technology has been widely studied and rapidly developed. The present invention is based on the Bayer reaction, utilizing isocyanate and amino groups to react to generate polyisoimide (polyurea), which is then processed into a nanofiber membrane structure through electrostatic spinning technology. Decarboxylation releases carbon dioxide at high temperatures, and carbon dioxide is used to foam the nanofibers in situ to form a spindle-shaped hollow structure. Prior art has not yet reported the use of polyisocyanates for in-situ foaming of polyimide fiber systems, and due to the configurational adjustability of polyisocyanate-based compounds, fluorinated diamines and dianhydride structures, the size, morphology, spacing and compliance of the microstructure of the obtained foamed polyimide fiber membrane can be regulated, thereby achieving the regulation of dielectric constant, which can be applied to different D k High-frequency communication materials in demand.

[0060] The innovative points of the present invention are as follows:

[0061] (1) Different from the rosary structure caused by the non-evaporation of solvent in conventional methods, the hollow structure such as the spindle shape of the present invention is caused by the decomposition of isocyanate groups at high temperatures to release carbon dioxide. The polyamic acid before imidization has a normal fiber structure, and a spindle-shaped structure appears after imidization.

[0062] (2) The configuration of isocyanate molecules, fluorinated diamines and dianhydride structures can be adjusted. The nanofibers obtained using rod-shaped diamine monomers are needle-shaped; cross-linked porous fiber membranes are obtained using multifunctional isocyanate monomers; curved nanofiber membranes are obtained using flexible diamine monomers; the size, morphology, spacing and softness of the microstructure of the obtained foamed polyimide fiber membrane can be controlled.

[0063] (3) Different isocyanate group contents lead to different hollow structures of polyimide fiber bundles. For the same system, increasing the isocyanate group content will increase the porosity, thereby effectively reducing the dielectric constant. According to the classical composite dielectric theory:

[0064] ε eff =ε f V f +ε m (1-V f )#(1)

[0065] Where, ε eff is the effective dielectric constant of the composite material, ε f and ε m are the dielectric constants of fiber material and air, V f is the volume fraction of the fiber material. As the porosity (air ratio) increases, V fDecrease, making ε eff Significantly reduced.

[0066] The hollow structure has an enhancing effect on porosity. The porosity of the hollow core-shell structure can be estimated by the following equation:

[0067]

[0068] Among them, ρ hollow and ρ soild are the densities of hollow fibers and solid fibers, respectively. Theoretical calculations show that when the porosity exceeds 50%, the dielectric constant can be reduced to below 1.3, which is consistent with the experimental results.

[0069] (4) The effect of molecular structure on the intrinsic dielectric properties of polyimide: By introducing a trifluoromethyl group, the larger electronegativity of the fluorine atom in the trifluoromethyl group is used to cut off the conjugation of the electron cloud, inhibiting the formation of CTC (charge transfer complex), thereby reducing the generation of dipoles. By optimizing the combination ratio of the fluorinated structure, its effect on the dielectric properties of polyimide is studied, aiming to find a fluorinated polyimide material with a specific ratio to achieve extremely low dielectric constant and loss.

[0070] (5) Compared with the foamed material obtained by a foaming agent, the hollow structure of the present invention is smaller (micrometer level), and because the two-step molding method of first preparing a polyamic acid precursor solution and then performing imidization reaction molding is adopted, the molecular weight of the polymer material is higher, and the mechanical strength and toughness of the material are higher.

[0071] The polyimide material with an ultra-low dielectric constant obtained by the preparation method of the present invention can be used to manufacture interlayer dielectrics and packaging materials for large-scale antenna arrays and highly integrated chips for 6G communications. The polyimide material prepared by the present invention is particularly suitable for dielectric substrates for large-scale antenna arrays in 6G communication systems, interlayer dielectrics for high-frequency circuit boards, and high-density integrated circuit packaging, significantly reducing signal transmission loss and crosstalk. BRIEF DESCRIPTION OF THE DRAWINGS

[0072] Figure 1 The surface morphology of the polyimide nanofiber membrane of Example 1 before and after imidization is shown, wherein: Figure 1 a to Figure 1 c is the microscopic morphology and local magnification of the polyamic acid material before imidization. Figure 1 d to Figure 1 f is the microscopic morphology and local magnified image of the polyimide material after imidization.

[0073] Figure 2 The surface morphology of the polyimide nanofiber membranes of the embodiment of the present invention and the comparative example is shown, wherein: Figure 2 a corresponds to Comparative Example 1, Figure 2 b corresponds to Example 1, Figure 2 c corresponds to Comparative Example 2, Figure 2 d corresponds to Comparative Example 3, Figure 2 e corresponds to Example 2, Figure 2 f corresponds to Example 4. DETAILED DESCRIPTION

[0074] The present invention can be implemented by the following examples: The materials used in the examples and comparative examples of the present invention can all be purchased from the market.

[0075] Example 1:

[0076] Before the reaction, the diamine monomer was vacuum dried at 50°C for 12 hours, and the dianhydride monomer was vacuum dried at 110°C for 24 hours. Afterwards, the following steps were performed in an anhydrous, oxygen-free, nitrogen atmosphere: 1.5 g of 2,2'-bis(trifluoromethyl)diaminobiphenyl (CAS: No. 341-58-2) was added to a 100 mL three-necked flask equipped with a mechanical stirrer, a constant temperature low-temperature bath, a thermometer, and a constant pressure dropping funnel, and stirred at room temperature; 10 mL of anhydrous dimethylacetamide (DMAC) was added to dissolve, and after complete dissolution, the temperature was controlled to 10°C; 2.975 g of 4,4'-(hexafluoroisopropylene)diphthalic anhydride (CAS: No. 1107-00-2) was dissolved in 20 mL of anhydrous DMAC and added dropwise to the reaction system at a drop rate of 1 mL / min; after the addition was complete, 0.5 g of 4,4′-methylenebis(phenyl isocyanate) (CAS: No. 101-68-8) was added, and after the pole climbing phenomenon appeared, the reaction was continued for 2 hours to terminate the reaction, thereby obtaining a polyamic acid precursor solution with a solid content of 15 wt%.

[0077] The obtained polyamic acid precursor solution is added to a syringe with a range of 10mL and the bubbles in the tube are discharged. A special spinning needle (21G) is replaced, and aluminum foil (8μm) is placed close to the receiving roller. The needle movement speed (20mm / s), spinning voltage (1.5w V), pushing speed (0.0016mm / s), speed of the receiving roller (100RPM), distance from the needle to the receiving roller (20cm) and spinning temperature (36°C) are adjusted to ensure that the precursor solution flows slowly from the needle. Under a high voltage electric field of 15,000 volts, the glue at the needle is polarized to form a Taylor cone, which is further stretched at the unstable region of the jet before reaching the receiving roller. After evaporating the solvent, it falls on the receiving roller in the form of nano-scale fibers to obtain a polyamic acid nanofiber membrane.

[0078] The polyimide nanofiber membrane obtained by spinning was subjected to a programmed temperature increase for imidization reaction. The programmed temperature increase method is as follows: using a gradient heating condition, heating to 100°C and holding for 60 minutes to evaporate most of the solvent; then heating to 160°C and holding for 60 minutes to evaporate the remaining solvent; then heating to 220°C and holding for 60 minutes to decompose and release carbon dioxide; then heating to 280°C and holding for 40 minutes to dehydrate by imidization reaction; then heating to 320°C and holding for 30 minutes to complete the imidization reaction; finally, heating to 350°C and holding for 30 minutes. After cooling naturally to room temperature, it was peeled off from the substrate to obtain the polyimide nanofiber membrane.

[0079] Example 2:

[0080] Before the reaction, the diamine monomer was vacuum dried at 50°C for 12 hours, and the dianhydride monomer was vacuum dried at 110°C for 24 hours. Afterwards, the following steps were performed in an anhydrous, oxygen-free, nitrogen atmosphere: 1.5 g of 4,4′-[1,4-phenylbis(oxy)]bis[3-(trifluoromethyl)aniline] (CAS: No. 94525-05-0) was added to a 100 mL three-necked flask equipped with a mechanical stirrer, a constant temperature low-temperature bath, a thermometer, and a constant pressure dropping funnel, and stirred at room temperature; 10 mL of anhydrous dimethylacetamide (DMAC) was added to dissolve, and after complete dissolution, the temperature was controlled to 10°C; 2.57 g of 9,9-Bis(trifluoromethyl)-2,3,6,7-xanthenetetracarboxylic dianhydride (CAS: No. 139162-14-4) was dissolved in 20 mL of anhydrous DMAC and added dropwise to the reaction system at a rate of 1 mL / min. After the addition was complete, 0.25 g of isophorone diisocyanate (CAS: No. 4098-71-9) was added. After the pole climbing phenomenon appeared, the reaction was continued for 2 h to obtain a polyamic acid precursor solution with a solid content of 15 wt%.

[0081] The obtained polyamic acid precursor solution is added to a syringe with a range of 10mL and the bubbles in the tube are discharged. A special spinning needle (21G) is replaced, and aluminum foil (8μm) is placed close to the receiving roller. The needle movement speed (20mm / s), spinning voltage (1.5w V), pushing speed (0.0016mm / s), speed of the receiving roller (100RPM), distance from the needle to the receiving roller (20cm) and spinning temperature (35°C) are adjusted to ensure that the precursor solution flows slowly from the needle. Under a high voltage electric field of 15,000 volts, the glue at the needle is polarized to form a Taylor cone, which is further stretched at the unstable area of ​​the jet before reaching the receiving roller. After evaporating the solvent, it falls on the receiving roller in the form of nano-scale fibers to obtain a polyamic acid nanofiber membrane.

[0082] The polyimide nanofiber membrane obtained by spinning was subjected to a programmed temperature increase for imidization reaction. The programmed temperature increase method is as follows: using a gradient heating condition, heating to 100°C and holding for 60 minutes to evaporate most of the solvent; then heating to 160°C and holding for 60 minutes to evaporate the remaining solvent; then heating to 220°C and holding for 60 minutes to decompose and release carbon dioxide; then heating to 280°C and holding for 40 minutes to dehydrate by imidization reaction; then heating to 320°C and holding for 30 minutes to complete the imidization reaction; finally, heating to 350°C and holding for 30 minutes. After cooling naturally to room temperature, it was peeled off from the substrate to obtain the polyimide nanofiber membrane.

[0083] Example 3:

[0084] Before the reaction, the diamine monomer was vacuum dried at 50°C for 12 hours, and the dianhydride monomer was vacuum dried at 110°C for 24 hours. Afterwards, the following steps were performed in an anhydrous, oxygen-free, nitrogen atmosphere: 1.5 g of 2,2-bis(4-aminophenyl)hexafluoropropane (CAS: No. 1095-78-9) was added to a 100 mL three-necked flask equipped with a mechanical stirrer, a constant temperature low-temperature bath, a thermometer, and a constant pressure dropping funnel, and stirred at room temperature; 10 mL of anhydrous dimethylacetamide (DMAC) was added to dissolve, and after complete dissolution, the temperature was controlled to 10°C; 3.133 g of 4,4'-(Hexafluoroisopropylene) diphthalic anhydride (CAS: No. 1107-00-2) was dissolved in 25 mL of anhydrous DMAC and added dropwise to the reaction system at a rate of 1 mL / min. After the addition was complete, 0.087 g of toluene-2,4-diisocyanate (CAS: No. 584-84-9) was added. After the pole climbing phenomenon appeared, the reaction was continued for 2 hours to obtain a polyamic acid precursor solution with a solid content of 15 wt%.

[0085] The obtained polyamic acid precursor solution is added to a syringe with a range of 10mL and the bubbles in the tube are discharged. A special spinning needle (21G) is replaced, and aluminum foil (8μm) is placed close to the receiving roller. The needle movement speed (20mm / s), spinning voltage (1.5w V), pushing speed (0.0016mm / s), speed of the receiving roller (100RPM), distance from the needle to the receiving roller (20cm) and spinning temperature (35°C) are adjusted to ensure that the precursor solution flows slowly from the needle. Under a high voltage electric field of 15,000 volts, the glue at the needle is polarized to form a Taylor cone, which is further stretched at the unstable area of ​​the jet before reaching the receiving roller. After evaporating the solvent, it falls on the receiving roller in the form of nano-scale fibers to obtain a polyamic acid nanofiber membrane.

[0086] The polyimide nanofiber membrane obtained by spinning was subjected to a programmed temperature increase for imidization reaction. The programmed temperature increase method is as follows: using gradient heating conditions, heating to 100°C and holding for 60 minutes to evaporate most of the solvent; then heating to 160°C and holding for 60 minutes to evaporate the remaining solvent; then heating to 220°C and holding for 60 minutes to decompose and release carbon dioxide; then heating to 280°C and holding for 60 minutes to dehydrate by imidization reaction; then heating to 320°C and holding for 40 minutes to complete the imidization reaction; finally, heating to 350°C and holding for 30 minutes. After cooling naturally to room temperature, it was peeled off from the substrate to obtain the polyimide nanofiber membrane.

[0087] Example 4:

[0088] Before the reaction started, the diamine monomer was vacuum dried at 50°C for 12 hours, and the dianhydride monomer was vacuum dried at 110°C for 24 hours. Afterwards, the following steps were carried out in an anhydrous, oxygen-free, nitrogen atmosphere: 1.5 g of 2,2-bis[4-(4-aminophenoxy)phenyl]-1,1,1,3,3,3-hexafluoropropane (CAS: No. 437769-37-4) was added to a 100 mL three-necked flask equipped with a mechanical stirrer, a constant temperature low-temperature bath, a thermometer, and a constant pressure dropping funnel, and stirred at room temperature; 10 mL of anhydrous dimethylacetamide (DMAC) was added to dissolve, and after complete dissolution, the temperature was controlled to 10°C; 1.836 g of 4,4'-(Hexafluoroisopropylene) diphthalic anhydride (CAS No. 1107-00-2) was dissolved in 15 mL of anhydrous DMAC and added dropwise to the reaction system at a rate of 1 mL / min. After the addition was complete, 0.46 g of triphenylmethane triisocyanate (CAS No. 2422-91-5) was added. After the pole climbing phenomenon appeared, the reaction was continued for 2 hours to obtain a polyamic acid precursor solution with a solid content of 15 wt%.

[0089] The obtained polyamic acid precursor solution is added to a syringe with a range of 10mL and the bubbles in the tube are discharged. A special spinning needle (21G) is replaced, and aluminum foil (8μm) is placed close to the receiving roller. The needle movement speed (20mm / s), spinning voltage (1.5w V), pushing speed (0.0016mm / s), speed of the receiving roller (100RPM), distance from the needle to the receiving roller (20cm) and spinning temperature (35°C) are adjusted to ensure that the precursor solution flows slowly from the needle. Under a high voltage electric field of 15,000 volts, the glue at the needle is polarized to form a Taylor cone, which is further stretched at the unstable area of ​​the jet before reaching the receiving roller. After evaporating the solvent, it falls on the receiving roller in the form of nano-scale fibers to obtain a polyamic acid nanofiber membrane.

[0090] The polyimide nanofiber membrane obtained by spinning was subjected to a programmed temperature increase for imidization reaction. The programmed temperature increase method is as follows: using gradient heating conditions, heating to 100°C and holding for 60 minutes to evaporate most of the solvent; then heating to 160°C and holding for 60 minutes to evaporate the remaining solvent; then heating to 220°C and holding for 60 minutes to decompose and release carbon dioxide; then heating to 280°C and holding for 60 minutes to dehydrate by imidization reaction; then heating to 320°C and holding for 40 minutes to complete the imidization reaction; finally, heating to 350°C and holding for 30 minutes. After cooling naturally to room temperature, it was peeled off from the substrate to obtain the polyimide nanofiber membrane.

[0091] Example 5:

[0092] Before the reaction, the diamine monomer was vacuum dried at 50°C for 12 hours, and the dianhydride monomer was vacuum dried at 110°C for 24 hours. Afterwards, the following steps were performed in an anhydrous, oxygen-free, nitrogen atmosphere: 1.5 g of 4,4′-[1,4-phenylbis(oxy)]bis[3-(trifluoromethyl)aniline] (CAS: No. 94525-05-0) was added to a 100 ml three-necked flask equipped with a mechanical stirrer, a constant temperature low-temperature bath, a thermometer, and a constant pressure dropping funnel, and stirred at room temperature; 10 mL of anhydrous dimethylacetamide (DMAC) was added to dissolve, and after complete dissolution, the temperature was controlled to 10°C; 2.91 g of 5,5′-[(2,3,5,6-tetrafluoro-1,4-phenylene)bis(oxy)]bis[4,6,7-trifluoro-1,3-isobenzofurandione (CAS: No. 143363-91-1) was dissolved in 20 mL of anhydrous DMAC and added dropwise to the reaction system at a drop rate of 1 mL / min. After the addition was complete, 0.55 g of triphenylmethane triisocyanate (CAS: No. 2422-91-5) was added. After the pole climbing phenomenon appeared, the reaction was continued for 2 h to terminate the reaction, thereby obtaining a polyamic acid precursor solution with a solid content of 15 wt%.

[0093] The obtained polyamic acid precursor solution is added to a syringe with a range of 10mL and the bubbles in the tube are discharged. A special spinning needle (21G) is replaced, and aluminum foil (8μm) is placed close to the receiving roller. The needle movement speed (20mm / s), spinning voltage (1.5w V), pushing speed (0.0016mm / s), speed of the receiving roller (100RPM), distance from the needle to the receiving roller (20cm) and spinning temperature (35°C) are adjusted to ensure that the precursor solution flows slowly from the needle. Under a high voltage electric field of 15,000 volts, the glue at the needle is polarized to form a Taylor cone, which is further stretched at the unstable area of ​​the jet before reaching the receiving roller. After evaporating the solvent, it falls on the receiving roller in the form of nano-scale fibers to obtain a polyamic acid nanofiber membrane.

[0094] The polyimide nanofiber membrane obtained by spinning was subjected to a programmed temperature increase for imidization reaction. The programmed temperature increase method is as follows: using a gradient heating condition, heating to 100°C and holding for 60 minutes to evaporate most of the solvent; then heating to 160°C and holding for 60 minutes to evaporate the remaining solvent; then heating to 220°C and holding for 60 minutes to decompose and release carbon dioxide; then heating to 280°C and holding for 40 minutes to dehydrate by imidization reaction; then heating to 320°C and holding for 30 minutes to complete the imidization reaction; finally, heating to 350°C and holding for 30 minutes. After cooling naturally to room temperature, it was peeled off from the substrate to obtain the polyimide nanofiber membrane.

[0095] Comparative Example 1:

[0096] A polyimide nanofiber membrane was obtained using the same preparation method as in Example 1, except that no polyisocyanate compound was added when preparing the polyamic acid precursor solution, and 2.14 g of 2,2'-bis(trifluoromethyl)diaminobiphenyl was used.

[0097] Comparative Example 2

[0098] The polyimide nanofiber membrane was obtained using the same preparation method as in Example 4, except that no polyisocyanate compound was added when preparing the polyamic acid precursor solution, and 2.40 g of 2,2-bis[4-(4-aminophenoxy)phenyl]-1,1,1,3,3,3-hexafluoropropane was used.

[0099] Comparative Example 3

[0100] The polyimide nanofiber membrane was obtained using the same preparation method as in Example 4, except that 30.4 mg of triphenylmethane triisocyanate and 2.35 g of 2,2-bis[4-(4-aminophenoxy)phenyl]-1,1,1,3,3,3-hexafluoropropane were used in the preparation of the polyamic acid precursor solution.

[0101] Test example:

[0102] 1. Surface morphology of polyimide nanofiber membrane

[0103] The surface morphologies of the polyimide nanofiber membranes of the examples and comparative examples before and after imidization were characterized respectively.

[0104] Figure 1 The surface morphology of the polyimide nanofiber membrane of Example 1 before and after imidization is shown, wherein: Figure 1 a to Figure 1 c is the microscopic morphology and local magnification of the polyamic acid material before imidization. Figure 1 d to Figure 1f is the microscopic morphology and local magnification of the polyimide material after imidization. Figure 1 It can be seen that an obvious spindle structure appears after imidization.

[0105] Figure 2 The surface morphology of the polyimide nanofiber membranes of the embodiment of the present invention and the comparative example is shown, wherein: Figure 2 a corresponds to Comparative Example 1, Figure 2 b corresponds to Example 1, Figure 2 c corresponds to Comparative Example 2, Figure 2 d corresponds to Comparative Example 3, Figure 2 e corresponds to Example 2, Figure 2 f corresponds to Example 4. Figure 2 It can be seen that different fluorinated diamine and dianhydride monomer structures lead to different degrees of curling of the fiber bundles, and different isocyanate groups lead to different crosslinking degrees and surface microstructures of the nanofiber membranes. Specifically, Example 1 ( Figure 2 b) is a spindle-shaped structure, Example 2 ( Figure 2 e) is a shuttle-shaped structure, Example 4 ( Figure 2 f) is a sponge-like structure, and Comparative Example 1 ( Figure 2 a) is a linear structure, Comparative Example 2 ( Figure 2 c) is a porous structure, Comparative Example 3 ( Figure 2 d) is a cross-linked structure.

[0106] 2. Low dielectric properties

[0107] The polyimide nanofiber membranes prepared in Examples 1-5 and Comparative Examples 1-3 were cut into rectangular specimens and subjected to dielectric property testing. A vector network analyzer (VNA) was used to analyze the dielectric constant and dielectric loss at 10 GHz. The dielectric properties of the specimens were measured using international standards IEC 60250 and ASTM D150. The dielectric properties were measured at a frequency of 10 GHz and a temperature of 25°C for specimens 40 mm long and 3 mm wide. The results are shown in Table 1 below.

[0108] [Table 1]

[0109] product Dielectric constant Dielectric loss Example 1 1.27 0.0017 Example 2 1.19 0.0016 Example 3 1.28 0.0027 Example 4 1.29 0.0021 Example 5 1.26 0.0014 Comparative Example 1 2.02 0.0040 Comparative Example 2 1.87 0.0036 Comparative Example 3 1.63 0.0031

[0110] As can be seen from Table 1 above, the dielectric constant of the polyimide nanofiber membrane prepared by the present invention is less than 1.3, and the dielectric loss is less than 0.003, indicating excellent dielectric properties. Compared with the embodiment, Comparative Examples 1 and 2 do not use a polyisocyanate compound, and no hollow structure such as a spindle shape is formed, and the dielectric properties are not as good as those of the embodiment. The content of the polyisocyanate compound used in Comparative Example 3 is too small and is not within the scope of the present invention. No hollow structure such as a spindle shape is formed, and the dielectric properties are not as good as those of the embodiment.

[0111] The above description is only a preferred specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any technician familiar with the technical field, within the technical scope disclosed by the present invention, who makes equivalent replacements or changes based on the technical solution and inventive concept of the present invention, should be covered by the scope of protection of the present invention.

Claims

1. A polyimide nanofiber material, characterized in that: The polyimide nanofiber material is obtained by imidization reaction of a polyamic acid precursor and in-situ foaming. Wherein, the polyimide nanofiber material has a hollow structure formed by the in-situ foaming. The polyamic acid precursor comprises at least one selected from the group consisting of a repeating unit represented by the following formula 1, a repeating unit represented by the following formula 2, and a repeating unit represented by the following formula 3: The polyimide comprises at least one selected from the group consisting of a repeating unit represented by the following formula 4, a repeating unit represented by the following formula 5, and a repeating unit represented by the following formula 6: In the above formulas 1 to 6, Ar 1 Represents the main chain structure of diisocyanate group, Ar 2 Represents the main chain structure of triisocyanate group, Ar 3 Represents the main chain structure of tetraisocyanate group, Ar 4 Represents the main chain structure containing fluorinated dianhydride groups, Ar 5 represents the main chain structure of a fluorinated diamine group.

2. The polyimide nanofiber material according to claim 1, characterized in that: The size of the hollow structure is 1 to 8 μm.

3. The polyimide nanofiber material according to claim 1, characterized in that The Ar 1 Each is the same or different and is selected from one or more of the following: The Ar 2 Each is the same or different and is selected from one or more of the following: The Ar 3 for: The Ar 4 Each is the same or different and is selected from one or more of the following: The Ar 5 Each is the same or different and is selected from one or more of the following: In the Ar 1 To the Ar 5 In the figure, * represents a connecting bond. When it appears on an aromatic ring, it indicates that it is connected to any carbon atom on the aromatic ring.

4. The polyimide nanofiber material according to claim 1, characterized in that The polyamic acid precursor is obtained by polymerization of a fluorine-containing diamine compound, a fluorine-containing dianhydride compound and a polyisocyanate compound.

5. The polyimide nanofiber material according to claim 4, characterized in that: The polyisocyanate compound is selected from one or more of the following:

6. The polyimide nanofiber material according to claim 4, characterized in that: The fluorine-containing dianhydride compound is selected from one or more of the following:

7. The polyimide nanofiber material according to claim 4, characterized in that: The fluorine-containing diamine compound is selected from one or more of the following:

8. The polyimide nanofiber material according to claim 1, characterized in that: The microstructure is in the shape of a spindle, a fusiform or a sponge.

9. The polyimide nanofiber material according to claim 8, characterized in that: The hollow structure is a spindle-shaped structure.

10. A method for preparing a polyimide nanofiber material, characterized in that: The following steps are involved: 1) under the protection of anhydrous and oxygen-free inert gas, adding a fluorine-containing diamine compound, a polyisocyanate compound and an organic solvent to react with the fluorine-containing dianhydride compound to obtain a polyamic acid precursor solution; 2) injecting the polyamic acid precursor solution obtained in step 1) into a syringe, and forming a polyamic acid nanofiber membrane on a collector substrate using electrospinning technology; 3) The polyamic acid nanofiber membrane obtained in step 2) is subjected to programmed temperature increase for imidization reaction, carbon dioxide released by high-temperature decarboxylation is used for in-situ foaming, and the membrane is peeled off from the substrate after natural cooling to obtain the polyimide nanofiber material.

11. The preparation method according to claim 10, characterized in that: The organic solvent is selected from at least one of N-methylpyrrolidone, dimethyl sulfoxide, N,N-dimethylformamide and N,N-dimethylacetamide.

12. The preparation method according to claim 10, characterized in that In the step 1), the molar ratio of the fluorine-containing diamine compound, the polyisocyanate compound and the fluorine-containing dianhydride compound is 0.7:0.3:1 to 0.99:0.01:

1.

13. The preparation method according to claim 10, characterized in that The reaction conditions of step 1) are to react in the organic solvent at -20 to 10° C. under anhydrous and oxygen-free conditions for 2 to 6 hours.

14. The preparation method according to claim 10, characterized in that The step 2) includes: adding the polyamic acid precursor solution obtained in step 1) into a syringe and expelling bubbles in the tube, replacing a special spinning needle, placing aluminum foil tightly against a receiving roller, adjusting the spinning voltage, pushing speed, rotation speed of the receiving roller, and the distance from the needle to the receiving roller to ensure that the precursor solution slowly flows out of the needle; under a high-voltage electric field, the precursor solution at the needle is polarized to form a Taylor cone, and is further stretched when it reaches the jet instability area before the receiving roller. After the solvent is volatilized, the polyamic acid precursor falls on the receiving roller in the form of nanofibers.

15. The preparation method according to claim 10, characterized in that: The programmed temperature increase in step 3) includes: using gradient heating conditions, first heating to 100° C., holding for 30 to 90 minutes to remove most of the organic solvent; then heating to 140 to 160° C., holding for 30 to 90 minutes to remove the remaining organic solvent; then heating to 180 to 220° C., holding for 30 to 90 minutes to decompose and release carbon dioxide; then heating to 260 to 280° C., holding for 30 to 90 minutes to dehydrate by imidization reaction; then heating to 320° C., holding for 30 to 90 minutes to completely complete the imidization reaction; finally, heating to 340 to 360° C., holding for 5 to 60 minutes; and naturally cooling to room temperature.

16. The preparation method according to claim 10, characterized in that: The diameter of the nanofibers in the polyamic acid precursor nanofiber membrane obtained in step 2) is 20 to 600 nm.

17. The preparation method according to claim 10, characterized in that: The diameter of the nanofibers in the polyimide nanofiber material obtained in step 3) is 20 to 600 nm.

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