A method for preparing a coaxial spun polyimide nanofiber membrane

Abstract

The present application relates to a coaxial polyimide nanofiber membrane preparation method, by the rigid anhydride containing ester group structure and rigid diamine is polymerized into the polyamide acid solution with crystallization characteristics, and the heat conduction filler is mixed in it as the core layer spinning solution, then it is with the thermoplastic polyimide solution as the sheath layer, by coaxial electrospinning-thermal imidization processing mode is prepared into composite polyimide nanofiber membrane, and then through high-temperature hot-pressing process, while the core layer still maintains the complete crystalline structure, the sheath layer structure occurs thermal bonding and produces crosslinking effect, thereby preparing the coaxial polyimide nanofiber membrane membrane with low dielectric properties and high thermal conductivity characteristics, which can be widely used in various fields such as communication base station, aerospace, microelectronic packaging and integrated circuit.

Description

Technical Field

[0001] This invention belongs to the field of polymer film technology, and specifically relates to a method for preparing coaxially spun polyimide nanofiber membranes. Background Technology

[0002] With the rapid development of fifth-generation (5G) mobile communication technology, the Internet of Things (IoT), artificial intelligence (AI), and high-performance computing chips, signal transmission delay, crosstalk, and energy loss, as well as the high heat generation and heat dissipation difficulties of highly integrated electronic components, have become bottlenecks restricting the performance of electronic devices. There is an urgent need to develop devices with low dielectric constants (Dk). k < 3.0) and low dielectric loss (D f Polyimide (PI) is an insulating material with a dielectric constant of < 0.005 and high thermal conductivity. Due to its excellent heat resistance, superior mechanical properties, good chemical stability, and outstanding dimensional stability, PI has long been considered an ideal dielectric and encapsulation material for high-performance electronic components. Dense, non-porous polyimide films prepared by the traditional thermal imidization method typically have a dielectric constant between 3.0 and 3.5 and a dielectric loss of around 0.01, which is insufficient to meet the stringent requirements of future communication technologies for ultra-low dielectric loss. Furthermore, its thermal conductivity does not offer a significant advantage over traditional polymer materials. While introducing large-volume side groups, fluorinated groups, or alicyclic structures to increase molecular chain spacing, reduce chain stacking density, and decrease polar groups can effectively reduce the dielectric constant, it faces a series of new challenges, including more complex processing techniques and higher monomer costs. Constructing porous structures can achieve similar effects, but it still faces serious problems such as uneven pore formation or damage to mechanical properties. Furthermore, the introduced pores disrupt the thermal conductivity network between materials, causing the original thermal conductivity pathways along the material matrix to be in a fragmented state of interconnection-blockage-connection, thus limiting the material's thermal conductivity. Therefore, there is an urgent need for a new strategy that can fundamentally change the microstructure of polyimide materials to achieve a significant reduction in dielectric constant and dielectric loss without significantly sacrificing their inherent superior properties.

[0003] Electrospinning technology offers a promising new approach for preparing low-dielectric polyimide materials. Through electrospinning, polyamic acid solutions can be directly and continuously processed into three-dimensionally interconnected nanofiber membranes. The "material-air" composite system, composed of numerous micro- and nano-sized pores and large inter-fiber interfaces, significantly reduces the overall dielectric constant of the material. Polymer membranes prepared solely by electrospinning exhibit poor mechanical properties, and polyimide nanofiber membranes prepared from a single polyimide matrix cannot achieve controllable morphological adjustment. Some studies have shown that polyimides containing ester groups can exhibit significant crystallinity, which has a significant inhibitory effect on dielectric loss (Industrial & Engineering Chemistry Research, 2022, 61(49): 17894-17903. , Communications Materials, 2024, 5(1): 1-11.). This is mainly attributed to the regular arrangement of the ester groups. Meanwhile, the crystalline structure formed by rigid ester functional groups promotes the orderly arrangement of molecular chains, which is beneficial to the formation of thermally conductive networks. In another study, researchers used this structure in conjunction with a flexible block structure to construct a polyimide film with intrinsically high thermal conductivity and low dielectric properties (CN119931339B). If the ester structure is presented in the form of nanofibers, the crystalline properties will be better maintained, and the thermal conductivity and dielectric properties will be optimized. Based on this, the following issues still need to be addressed: 1) How to further optimize the overall thermal conductivity while maintaining the nanofiber morphology and ensuring a low dielectric constant; 2) How to ensure that the key crystalline structure is maintained and not destroyed during the polyimide nanofiber film and its subsequent processing. Previously, researchers have prepared polyimide nanofiber films that can be bonded under high temperature treatment by introducing rigid imidazole units in combination with flexible structures through molecular-level structural design (CN103147253B). Because this type of nanofiber membrane with soft and hard segments relies on the softening of the overall molecular chain to complete the bonding, it cannot maintain the stable orientation characteristics of the internal molecular chain structure during high-temperature processing, and still shows shortcomings in the construction of the thermally conductive network and the control of dielectric loss. Summary of the Invention

[0004] The technical problem to be solved by the present invention is to provide a method for preparing coaxially spun polyimide nanofiber membranes, which overcomes the technical contradiction that traditional materials cannot simultaneously achieve high thermal conductivity and high strength.

[0005] This invention provides a method for preparing a coaxially spun polyimide nanofiber membrane, comprising the following steps:

[0006] (1) A rigid acid anhydride containing an ester group structure is polymerized with a rigid diamine to obtain a crystalline polyamic acid solution. The polyamic acid solution is then blended with a thermally conductive filler to obtain a crystalline polyamic acid mixed solution containing a thermally conductive filler, which is used as the core spinning solution.

[0007] (2) A thermoplastic polyimide solution was used as the sheath spinning solution and coaxially electrospun with the above core spinning solution to prepare a coaxial spun nanofiber membrane.

[0008] (3) The coaxial spun nanofiber membrane obtained above is subjected to thermal imidization treatment to obtain a polyimide nanofiber membrane;

[0009] (4) The above polyimide nanofiber membrane is processed by hot pressing to obtain a coaxial spun polyimide nanofiber membrane.

[0010] Preferably, the rigid anhydride containing an ester group in step (1) is one or more of the following structures:

[0011] .

[0012] Preferably, the rigid diamine in step (1) is one or more of the following structures:

[0013] .

[0014] Preferably, the molar ratio of rigid acid anhydride containing ester group structure to rigid diamine in step (1) is (1~1.05):1.

[0015] Preferably, the organic solvent used in the polymerization in step (1) is one or more of N-methylpyrrolidone, N,N-dimethylacetamide, N,N-dimethylformamide, and γ-butyrolactone.

[0016] Preferably, the polymerization temperature in step (1) is 0~40℃.

[0017] Preferably, the solid content of the crystalline polyamic acid in step (1) is 10~20wt%.

[0018] Preferably, in step (1), the proportion of ester functional groups in the total functional groups of the polyamic acid molecular chain is between 15% and 30%.

[0019] Preferably, the viscosity of the core spinning solution in step (1) is controlled at 500~3000P at 25°C.

[0020] Preferably, the thermally conductive filler in step (1) is a zero-dimensional thermally conductive filler, such as silver nanoparticles, nano-alumina, boron nitride nanospheres, or a one-dimensional thermally conductive filler, such as single-walled carbon nanotubes or silver nanowires; the solid content of the thermally conductive filler in the crystalline polyamic acid mixed solution is 0.1~1wt%. For zero-dimensional thermally conductive fillers, the filler diameter is controlled at 50~100nm; for one-dimensional thermally conductive fillers, the diameter is controlled at 0.8~2nm and the length at 100~500nm. The solvent used for the thermally conductive filler should be consistent with the solvent used for the polyamic acid solution.

[0021] Preferably, the thermoplastic polyimide solution in step (2) is prepared by a one-step method.

[0022] Preferably, the solid content of the thermoplastic polyimide solution in step (2) is 15~35wt%, the viscosity is maintained at 200~1000P at 25℃, and the viscosity of the sheath spinning solution is < the viscosity of the core spinning solution.

[0023] Preferably, in step (2), the inner diameter ratio of the outer needle to the inner needle in the coaxial electrospinning is (1.5~2.2):1; the volume ratio of the sheath spinning solution to the core spinning solution at the same solid content is (1.2~3.8):1; the extrusion push ratio of the sheath spinning solution to the core spinning solution is (1.5~4):1, and the extrusion push ratio is ≥ the volume ratio of the sheath spinning solution to the core spinning solution.

[0024] Preferably, in step (2), the coaxial electrospinning outer needle and inner needle are in an ultrasonic oscillation environment with a frequency of 20~30kHz.

[0025] Preferably, the coaxial electrospinning parameters in step (2) are as follows: the spinning voltage is 15~20kV, the spinning collection roller speed is controlled at 700~1000rpm to promote the formation of a network structure; the feed rate of the core spinning solution is 0.01~0.02ml / min, the feed rate of the sheath spinning solution is 0.015~0.08ml / min, and the distance between the spinning collection roller and the spinning needle is 15~25cm.

[0026] Preferably, the coaxial spun nanofiber membrane prepared in step (2) needs to be fixed flat and wrinkle-free in a stainless steel frame that meets the processing size requirements.

[0027] Preferably, the thermal imidization treatment parameters in step (3) are as follows: a stepped heating process is adopted, in the first stage the heating rate is 2℃ / min to a maximum of 120℃, and held for 60~80min; in the second stage the heating rate is 2℃ / min to a maximum of 220℃, and held for 60~70min; in the third stage the heating rate is 3℃ / min to a maximum of 350℃, and held for 20~30min; and the temperature is naturally cooled after the above heating process is completed.

[0028] Preferably, the hot pressing process parameters in step (4) are: the hot pressing temperature is between the glass transition temperature of the thermoplastic polyimide ±30℃ and the melting temperature of the crystalline polyimide prepared from the crystalline polyamic acid solution; the hot pressing pressure is 0.2~3 MPa.

[0029] The invention is characterized by using rigid acid anhydrides containing ester groups as raw materials to polymerize with rigid diamines to prepare a polyamic acid solution, a precursor for polyimide with crystalline properties. This solution is then thoroughly and uniformly mixed with nanoscale thermally conductive fillers to serve as the core spinning solution. A thermoplastic polyimide solution is used as the sheath spinning solution. Through reasonable control of the functional group ratio and molecular structure, the core spinning solution and sheath spinning solution exhibit crystalline orientation characteristics and thermoplastic properties, respectively. The two spinning solutions are then processed into coaxial spun nanofiber membranes using coaxial electrospinning, and finally, polyimide nanofibers are obtained through thermal imidization. The coaxial spun polyimide nanofiber membrane, processed by hot pressing, heats the thermoplastic polyimide sheath to its softening temperature, causing it to bond and crosslink under pressure, while the crystallization and orientation of the core layer remain unaffected, and the thermally conductive filler maintains its orientation along the fiber axis. Through this process, the outer layers are bonded and crosslinked, the core layers come into contact with each other, and the membrane is shaped after cooling, successfully constructing a stable thermally conductive network. This yields a coaxial spun polyimide nanofiber membrane with high crystallinity, low dielectric constant, and high thermal conductivity. This process simplifies the preparation process and avoids the complex design process of structural monomers, greatly improving the production efficiency of the industrialization process.

[0030] Beneficial effects

[0031] (1) This invention creatively constructs a coaxially spun polyimide nanofiber membrane with a "core-sheath" heterostructure through coaxial electrospinning technology. The highly crystalline and thermally conductive rigid core layer ensures excellent thermal conductivity and mechanical properties along the fiber axis; while the thermoplastic amorphous sheath layer achieves thermal bonding and cross-linking between fibers during subsequent hot pressing, forming a stable three-dimensional network. This unique structure of "combining rigidity and flexibility, and functional partitioning" essentially synergistically improves the thermal conductivity, mechanical strength, and toughness of the polyimide nanofiber membrane, overcoming the technical contradiction of traditional materials being unable to simultaneously achieve high thermal conductivity and high strength.

[0032] (2) On the one hand, the high shear rate spinning and programmed thermal imidization process effectively induces the crystallization of the core layer polyimide and ensures the uniform dispersion and directional arrangement of the nano-thermal conductive filler inside the fiber. On the other hand, by precisely controlling parameters such as the solid content of the core layer and sheath layer solution and the ratio of spinning needle size, the ratio of crystalline thermally conductive phase to bonding amorphous phase, the degree of bonding between fibers, and the porosity and density of the film in the final polyimide nanofiber membrane can be flexibly controlled, thereby realizing the on-demand design and optimization of the comprehensive performance of the polyimide nanofiber membrane. Attached Figure Description

[0033] Figure 1 This is a digital photograph of the coaxial spun polyimide nanofiber membrane prepared in Example 1.

[0034] Figure 2 This is a scanning electron microscope image of the coaxial spun polyimide nanofiber membrane prepared in Example 1.

[0035] Figure 3 The DSC curves are for the coaxial spun polyimide nanofiber membranes prepared in Examples 1 and 2.

[0036] Figure 4 The dielectric constant (A) and dielectric loss (B) of the coaxial spun polyimide nanofiber membranes prepared in Examples 1 and 2 are given.

[0037] Figure 5 The image shows a scanning electron microscope (SEM) image of the polyimide nanofiber membrane spun from the mixed solution prepared in Comparative Example 1.

[0038] Figure 6 The DSC curves are for the polyimide nanofiber membranes spun from the mixed solution prepared in Comparative Example 1.

[0039] Figure 7 Infrared thermography comparison of the thermal conductivity of the coaxial spun polyimide nanofiber membrane prepared in Comparative Example 2 with that of the samples in Comparative Example 1 and Example 1. Detailed Implementation

[0040] The present invention will be further illustrated below with reference to specific embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the invention. Furthermore, it should be understood that after reading the teachings of this invention, those skilled in the art can make various alterations or modifications to the invention, and these equivalent forms also fall within the scope defined by the appended claims.

[0041] Example 1

[0042] Step (1): Under a nitrogen atmosphere, take 99.5g of N,N-dimethylacetamide (DMAc), add 0.5g of boron nitride nanospheres (BNN) with a diameter of 50~100nm, mix thoroughly and place in an ultrasonic cell disruptor, sonicate the mixed solution for 20min at a working frequency of 20kHz and a working interval of 2s to obtain a thermally conductive filler dispersion solution (DMAc-mix-0.5) with a boron nitride nanosphere solid content of 0.5wt%.

[0043] Step (2): Under a nitrogen atmosphere, 20 mmol of 2,2'-di(trifluoromethyl)-(1,1'-diphenyl)-4,4'-diamine (TFMB) and 62.28 g of thermally conductive filler dispersion (DMAc-mix-0.5) were added to a 100 ml three-necked flask. After thorough stirring, 20 mmol of p-phenylene-bis(phenyl)trimethoxyester dianhydride (TAHQ) was slowly added. The mixture was reacted for 12 h under ice-water bath conditions to obtain a crystalline polyamic acid mixed solution (Crystal-PAA-mix-1) as the core spinning solution. The solid content of the crystalline polyamic acid was 20 wt%, the solid content of the boron nitride nanospheres was 0.4 wt%, and the viscosity of the core spinning solution at 25 °C was 2560 P.

[0044] Step (3): Under a nitrogen atmosphere, add 20 mmol of 4,4'-diaminodiphenyl ether (ODA) and 40.84 g of N-methylpyrrolidone (NMP) to a 100 ml three-necked flask. Stir thoroughly in an ice-water bath until completely dissolved, then slowly add 20 mmol of 4,4'-oxydiphthalic anhydride (ODPA). React for 12 h to obtain a clear and transparent thermoplastic polyamic acid (Thermo-PAA-1) solution with a solid content of 20 wt%. Add 10 ml of toluene and 1 ml of isoquinoline to the obtained Thermo-PAA-1 solution. React in an oil bath at 180 °C for 10 h. Then, precipitate the reaction solution in a large amount of deionized water, wash thoroughly, pulverize, and dry to obtain thermoplastic polyimide powder. 10g of powder was dissolved in 40g of N,N-dimethylacetamide (DMAc) to obtain a thermoplastic polyimide (Thermo-PI-1) solution with a solid content of 20wt% as the sheath spinning solution, and the viscosity was maintained at 950P at 25℃.

[0045] Step (4): Take 10 ml of Crystal-PAA-mix-1 solution and ultrasonically disperse it again for 15 min. Connect the treated solution and 10 ml of Thermo-PI-1 solution through a coaxial spinning needle. Position an ultrasonic oscillator at the connection point between the tubing and the syringe, maintaining a frequency of 20 kHz. Crystal-PAA-mix-1 solution is used as the core spinning solution, with a feed rate of 0.015 ml / min; Thermo-PI-1 is used as the sheath spinning solution, with a feed rate of 0.02 ml / min. The ratio of the inner diameter of the outer needle to the inner needle is 2:1, and the extrusion ratio of the sheath spinning solution to the core spinning solution is 3.5:1. The spinning voltage is 20 kV, the distance between the spinning needle and the spinning collection roller is 20 cm, and the spinning collection roller speed is 1000 rpm. Control the ambient temperature of the spinning equipment at 25℃ and the ambient humidity at 50%. A layer of aluminum foil is coated on the surface of the collecting roller as a receiving substrate to prepare a coaxial spun nanofiber membrane with a sheath core structure.

[0046] Step (5): Peel the prepared coaxial spun nanofiber membrane from the substrate, fix it to a stainless steel frame of a defined size with a pre-tightening force of 1.5 N / m, place it in a high-temperature vacuum oven, and use a stepped heating process. In the first stage, the temperature is raised to 100℃ at a heating rate of 2℃ / min and held for 60 min; in the second stage, the temperature is raised to 200℃ at a heating rate of 2℃ / min and held for 60 min; in the third stage, the temperature is raised to 320℃ at a heating rate of 3℃ / min and held for 20 min, and then cooled naturally to obtain a coaxial spun polyimide nanofiber membrane (Coaxial-PINF-1).

[0047] Step (6): After immersing the prepared coaxial spun polyimide nanofiber membrane in acetone to remove adhering impurities, it is placed in a Kapton mold with a limited thickness of 100 μm and hot-pressed at 2 MPa for 20 min at a temperature of 260℃ to finally obtain a coaxial spun polyimide nanofiber membrane (Crosslink-CPINF-H1) with cross-linked network structure, high crystallinity, low dielectric and high thermal conductivity. Figure 1 This is a digital photograph of the prepared coaxially spun polyimide nanofiber membrane. From... Figure 2 The SEM images show that the coaxial nanofibers have formed a cross-linked network state with each other overlapping. Figure 3 , 4 It can be seen that the coaxial spun polyimide nanofiber membrane exhibits good crystallization characteristics and excellent low dielectric constant and low dielectric loss characteristics.

[0048] Example 2

[0049] Step (1): Under a nitrogen atmosphere, take 99.7g of N,N-dimethylacetamide (DMAc), add 0.3g of single-walled carbon nanotubes (SWCNTs) with a diameter of 0.8~2nm and a length of 100~500nm, mix thoroughly and place in an ultrasonic cell disruptor, sonicate the mixed solution for 25min at a working frequency of 20kHz and a working interval of 2s to obtain a thermally conductive filler dispersion solution (DMAc-mix-0.3) with a solid content of 0.3wt% of single-walled carbon nanotubes.

[0050] Step (2): Under a nitrogen atmosphere, add 10 mmol of 2,2'-di(trifluoromethyl)-(1,1'-diphenyl)-4,4'-diamine (TFMB), 10 mmol of [4-(4-aminobenzoyl)oxyphenyl]4-aminobenzoate (ABHQ), and 63.41 g of thermally conductive filler dispersion (DMAc-mix-0.3) to a 100 ml three-necked flask. After stirring thoroughly, slowly add 20 mmol of p-phenylene-bisphenyltriterpenoid dianhydride (TAHQ). React thoroughly in an ice-water bath for 12 h to obtain a crystalline polyamic acid mixed solution (Crystal-PAA-mix-2) as the core spinning solution. The solid content of the crystalline polyamic acid is 20 wt%, the solid content of the single-walled carbon nanotubes is 0.24 wt%, and the viscosity of the core spinning solution at 25 °C is 1785 P.

[0051] Step (3): Under a nitrogen atmosphere, add 20 mmol of 2,2-bis[4-(4-aminophenoxy)phenyl]hexafluoropropane (HFBAPP) and 66.77 g of N-methylpyrrolidone (NMP) to a 100 ml three-necked flask. Stir thoroughly in an ice-water bath until completely dissolved. Then, slowly add 10 mmol of 4,4'-diphenyl ether dianhydride (ODPA). When the reaction solution becomes clear, slowly add 10 mmol of 3,3′,4,4′-benzophenone tetracarboxylic dianhydride (BTDA). React for 12 h to obtain a clear thermoplastic polyamic acid (Thermo-PAA-2) solution with a solid content of 20 wt%. Add 10 ml of toluene and 1 ml of isoquinoline to the obtained Thermo-PAA-2 solution. React in an oil bath at 180 °C for 10 h. Then, precipitate the reaction solution in a large amount of deionized water, wash thoroughly, pulverize, and dry to obtain thermoplastic polyimide powder. 10g of powder was dissolved in 40g of N,N-dimethylacetamide (DMAc) to obtain a thermoplastic polyimide (Thermo-PI-2) solution with a solid content of 20wt% as the sheath spinning solution, and the viscosity was maintained at 838P at 25℃.

[0052] Step (4): Take 10 ml of Crystal-PAA-mix-2 solution and ultrasonically disperse it again for 15 min. Connect the treated solution and 10 ml of Thermo-PI-2 solution through a coaxial spinning needle. Position an ultrasonic oscillator at the connection point between the catheter and the syringe, maintaining a frequency of 20 kHz. Crystal-PAA-mix-2 solution is used as the core spinning solution with a feed rate of 0.02 ml / min; Thermo-PI-2 is used as the sheath spinning solution with a feed rate of 0.023 ml / min. The ratio of the inner diameter of the outer needle to the inner needle is 2:1, and the extrusion ratio of the sheath spinning solution to the core spinning solution is 2.8:1. The spinning voltage is 18 kV, the distance between the spinning needle and the receiver is 15 cm, and the collecting roller speed is 800 rpm. Control the ambient temperature of the spinning equipment at 25℃ and the ambient humidity at 50%. A layer of aluminum foil is coated onto the surface of the collecting roller as the receiving substrate, thus preparing a coaxial spun nanofiber membrane with a sheath-core structure.

[0053] Step (5): Peel the prepared coaxial spun nanofiber membrane from the substrate, fix it to a stainless steel frame of a defined size with a pre-tightening force of 2 N / m, place it in a high-temperature vacuum oven, and use a stepped heating process. In the first stage, the temperature is raised to 110℃ at a heating rate of 2℃ / min and held for 60 min; in the second stage, the temperature is raised to 220℃ at a heating rate of 2℃ / min and held for 70 min; in the third stage, the temperature is raised to 340℃ at a heating rate of 3℃ / min and held for 20 min, and then cooled naturally to obtain a coaxial spun polyimide nanofiber membrane (Coaxial-PINF-2) with crystallization properties.

[0054] Step (6): After immersing the prepared coaxially spun polyimide nanofiber membrane in acetone to remove adhering impurities, it is placed in a Kapton mold with a limited thickness of 100 μm and hot-pressed at 280℃ and 2 MPa for 20 min to finally obtain a coaxially spun polyimide nanofiber membrane (Crosslink-CPINF-H2) with a cross-linked network structure, high crystallinity, low dielectric constant, and high thermal conductivity. Figure 3 , 4 It can be seen that the coaxially spun polyimide nanofiber membrane still exhibits good crystallization characteristics and excellent low dielectric constant and low dielectric loss characteristics.

[0055] Comparative Example 1

[0056] Step (1): Under a nitrogen atmosphere, take 99.5g of N,N-dimethylacetamide (DMAc), add 0.5g of boron nitride nanospheres (BNN) with a diameter of 50~100nm, mix thoroughly and place in an ultrasonic cell disruptor, sonicate the mixed solution for 20min at a working frequency of 20kHz and a working interval of 2s to obtain a thermally conductive filler dispersion solution (DMAc-mix-0.5) with a boron nitride nanosphere solid content of 0.5wt%.

[0057] Step (2): Under a nitrogen atmosphere, 20 mmol of 2,2'-di(trifluoromethyl)-(1,1'-diphenyl)-4,4'-diamine (TFMB) and 62.28 g of thermally conductive filler dispersion (DMAc-mix-0.5) were added to a 100 ml three-necked flask. After thorough stirring, 20 mmol of p-phenylene-bis(phenyl)trimethoxyester dianhydride (TAHQ) was slowly added. The mixture was reacted for 12 h under ice-water bath conditions to obtain a crystalline polyamic acid mixed solution (Crystal-PAA-mix-1) as the core spinning solution. The solid content of the crystalline polyamic acid was 20 wt%, the solid content of the boron nitride nanospheres was 0.4 wt%, and the viscosity of the core spinning solution at 25 °C was 2560 P.

[0058] Step (3): Under a nitrogen atmosphere, add 20 mmol of 4,4'-diaminodiphenyl ether (ODA) and 40.84 g of N,N-dimethylacetamide (DMAc) to a 100 ml three-necked flask. Stir thoroughly in an ice-water bath until completely dissolved, then slowly add 20 mmol of 4,4'-oxydiphthalic anhydride (ODPA). React for 12 h to obtain a clear and transparent thermoplastic polyamic acid (Thermo-PAA-1B) solution with a solid content of 20 wt%. Under a nitrogen atmosphere, take 20 ml of Crystal-PAA-mix-1 solution and mix it thoroughly with 20 ml of Thermo-PAA-1B solution by mechanical stirring to obtain a 1:1 mixed polyamic acid solution (Mix-PAA-1).

[0059] Step (4): Take 20 ml of Mix-PAA-1 solution and ultrasonically disperse it again for 15 min. Place the treated solution in a syringe and connect it to the spinning needle. Position an ultrasonic oscillator at the connection point between the tubing and the syringe, maintaining a frequency of 20 kHz and a feed rate of 0.015 ml / min. Set the spinning voltage to 20 kV, the distance between the spinning needle and the receiver to 20 cm, and the collecting roller speed to 1000 rpm. Control the ambient temperature of the spinning equipment to 25℃ and the ambient humidity to 50%. Cover the surface of the collecting roller with an aluminum foil as the receiving substrate to prepare a mixed-spun polyamic acid nanofiber membrane (Mix-PAANF-1).

[0060] Step (5): Peel the prepared mixed spun polyamic acid nanofiber membrane from the substrate, fix it to a stainless steel frame of a defined size with a pre-tightening force of 2 N / m, place it in a high-temperature vacuum oven, and use a stepped heating process. In the first stage, the temperature is raised to 100℃ at a heating rate of 2℃ / min and held for 60 min; in the second stage, the temperature is raised to 200℃ at a heating rate of 2℃ / min and held for 60 min; in the third stage, the temperature is raised to 320℃ at a heating rate of 3℃ / min and held for 20 min, and then cooled naturally to obtain the mixed spun polyimide nanofiber membrane (Mix-PINF-1).

[0061] Step (6): After immersing the prepared polyimide nanofiber membrane in acetone to clean off the adhering impurities, place it in a Kapton mold with a limited thickness of 100 μm, and hot press it at a temperature of 260℃ and a pressure of 2 MPa for 20 min to obtain a hot-pressed mixed polyimide nanofiber membrane (Mix-PINF-HP).

[0062] Step (7): The obtained Mix-PINF-HP-1 was detected by SEM and DSC. The results are as follows. Figure 5 The results showed that the nanofiber membrane obtained by spinning the blended polymer solution exhibited a significantly reduced crystallization melting peak on the DSC curve. Furthermore, after high-temperature hot pressing, significant inter-fiber bonding had occurred internally, and the nanofiber network with its clearly defined morphology and outline was no longer present. Figure 6 As shown. According to Figure 7 Infrared thermal imaging showed that even with the addition of thermally conductive filler, the thermal conductivity was not excellent because the thermal network could not be properly constructed.

[0063] Comparative Example 2

[0064] Step (1): Under a nitrogen atmosphere, add 20 mmol of 2,2'-di(trifluoromethyl)-(1,1'-diphenyl)-4,4'-diamine (TFMB) and 62.28 g of N,N-dimethylacetamide (DMAc) to a 100 ml three-necked flask. Stir thoroughly in an ice-water bath until completely dissolved, and then slowly add 20 mmol of p-phenylene-bis(phenyl)trimethoxyester dianhydride (TAHQ). React for 12 h to obtain a crystalline polyamic acid (Crystal-PAA-1) solution with a solid content of 20 wt% and a core spinning solution viscosity of 2583 P at 25 °C.

[0065] Step (2): Under a nitrogen atmosphere, add 20 mmol of 4,4'-diaminodiphenyl ether (ODA) and 40.84 g of N-methylpyrrolidone (NMP) to a 100 ml three-necked flask. Stir thoroughly in an ice-water bath until completely dissolved, then slowly add 20 mmol of 4,4'-oxydiphthalic anhydride (ODPA). React for 12 h to obtain a clear and transparent thermoplastic polyamic acid (Thermo-PAA-1) solution with a solid content of 20 wt%. Add 10 ml of toluene and 1 ml of isoquinoline to the obtained Thermo-PAA-1 solution. React in an oil bath at 180 °C for 10 h. Then, precipitate the reaction solution in a large amount of deionized water, wash thoroughly, pulverize, and dry to obtain thermoplastic polyimide powder. 10g of powder was dissolved in 40g of N,N-dimethylacetamide (DMAc) to obtain a thermoplastic polyimide (Thermo-PI-1) solution with a solid content of 20wt% as the sheath spinning solution, and the viscosity was maintained at 950P at 25℃.

[0066] Step (3): Take 10 ml of Crystal-PAA-1 solution and 10 ml of Thermo-PI-1 solution and connect them through a coaxial spinning needle. Position an ultrasonic oscillator at the connection point between the catheter and the syringe, maintaining a frequency of 20 kHz. Crystal-PAA-1 solution is used as the core spinning solution with a feed rate of 0.015 ml / min; Thermo-PI-1 is used as the sheath spinning solution with a feed rate of 0.02 ml / min. The ratio of the inner diameter of the outer needle to the inner needle is 2:1, and the extrusion ratio of the sheath spinning solution to the core spinning solution is 3.5:1. The spinning voltage is 20 kV, the distance between the spinning needle and the receiver is 20 cm, and the collecting roller speed is 1000 rpm. The ambient temperature of the spinning equipment is controlled at 25℃, and the ambient humidity at 50%. An aluminum foil layer is coated onto the surface of the collecting roller as the receiving substrate, thus preparing a coaxially spun nanofiber membrane with a sheath-core structure and no nanofillers.

[0067] Step (4): Peel the prepared coaxial spun polyamic acid nanofiber membrane from the substrate, fix it to a stainless steel frame of a defined size with a pre-tightening force of 1.5 N / m, place it in a high-temperature vacuum oven, and use a stepped heating process. In the first stage, the temperature is raised to 100℃ at a heating rate of 2℃ / min and held for 60 min; in the second stage, the temperature is raised to 200℃ at a heating rate of 2℃ / min and held for 60 min; in the third stage, the temperature is raised to 320℃ at a heating rate of 3℃ / min and held for 20 min, and then cooled naturally to obtain a coaxial spun polyimide nanofiber membrane (Coaxial-PINF-A) without thermally conductive filler.

[0068] Step (5): After immersing the prepared coaxial spun polyimide nanofiber membrane in acetone to clean off the adhering impurities, place it in a Kapton mold with a limited thickness of 100 μm, and hot press it at a temperature of 260℃ and a pressure of 2 MPa for 20 min to obtain a coaxial spun polyimide nanofiber membrane (Crosslink-CPINF-A) with a crosslinked network structure. Figure 7 Infrared thermal imaging results show that, when the obtained nanofiber membrane is compared with the Crosslink-CPINF-H1 prepared in Example 1 and the Mix-PINF-HP prepared in Comparative Example 1, the H1 sample containing thermally conductive filler has superior thermal conductivity.

Claims

1. A method for preparing a coaxially spun polyimide nanofiber membrane, characterized in that, Includes the following steps: (1) A rigid acid anhydride containing an ester group structure is polymerized with a rigid diamine to obtain a crystalline polyamic acid solution. The polyamic acid solution is then blended with a thermally conductive filler to obtain a crystalline polyamic acid mixed solution containing the thermally conductive filler, which is used as the core spinning solution. The rigid acid anhydride containing an ester group structure is one or more of the following structures: ； (2) A thermoplastic polyimide solution is used as the sheath spinning solution and coaxially electrospun with the core spinning solution to prepare a coaxial spun nanofiber membrane; wherein the solid content of the thermoplastic polyimide solution is 15~35wt%, the viscosity is maintained at 200~1000P at 25℃, and the viscosity of the sheath spinning solution is < the viscosity of the core spinning solution; the inner diameter ratio of the outer needle to the inner needle used in the coaxial electrospinning is (1.5~2.2):1; the volume ratio of the sheath spinning solution to the core spinning solution at the same solid content is (1.2~3.8):1; the extrusion push ratio of the sheath spinning solution to the core spinning solution is (1.5~4):1, and the extrusion push ratio is ≥ the volume ratio of the sheath spinning solution to the core spinning solution; (3) The coaxial spun nanofiber membrane obtained above is subjected to thermal imidization treatment to obtain a polyimide nanofiber membrane; (4) The above polyimide nanofiber membrane is processed by hot pressing to obtain a coaxial spun polyimide nanofiber membrane.

2. The preparation method according to claim 1, characterized in that, The rigid diamine in step (1) is one or more of the following structures: ； The molar ratio of the rigid acid anhydride containing the ester group structure to the rigid diamine is (1~1.05):

1.

3. The preparation method according to claim 1, characterized in that, The organic solvent used in the polymerization in step (1) is one or more of N-methylpyrrolidone, N,N-dimethylacetamide, N,N-dimethylformamide, and γ-butyrolactone; the polymerization temperature is 0~40℃; the solid content of the polyamic acid solution is 10~20wt%; and the viscosity of the core spinning solution is controlled at 500~3000P at 25℃.

4. The preparation method according to claim 1, characterized in that, In the polyamic acid molecular structure of step (1), the proportion of ester functional groups in the total functional groups of the polyamic acid molecular chain is between 15% and 30%.

5. The preparation method according to claim 1, characterized in that, The thermally conductive filler in step (1) is a zero-dimensional or one-dimensional thermally conductive filler; the solid content of the thermally conductive filler in the crystalline polyamic acid mixed solution is 0.1~1wt%.

6. The preparation method according to claim 1, characterized in that, The thermoplastic polyimide solution in step (2) is prepared directly in one step.

7. The preparation method according to claim 1, characterized in that, The coaxial electrospinning parameters in step (2) are as follows: the spinning voltage is 15~20kV, the spinning collection roller speed is controlled at 700~1000rpm to promote the formation of a network structure; the feed rate of the core spinning solution is 0.01~0.02ml / min, the feed rate of the sheath spinning solution is 0.015~0.08ml / min, and the distance between the spinning collection roller and the spinning needle is 15~25cm.

8. The preparation method according to claim 1, characterized in that, The thermal imidization treatment parameters in step (3) are as follows: a stepped heating process is adopted. In the first stage, the temperature is increased to a maximum of 120°C at a heating rate of 2°C / min and held for 60-80 min; in the second stage, the temperature is increased to a maximum of 220°C at a heating rate of 2°C / min and held for 60-70 min; in the third stage, the temperature is increased to a maximum of 350°C at a heating rate of 3°C / min and held for 20-30 min; and the temperature is allowed to cool naturally after the above heating process is completed.

9. The preparation method according to claim 1, characterized in that, The hot pressing process parameters in step (4) are as follows: the hot pressing temperature is between the glass transition temperature of thermoplastic polyimide ±30℃ and the melting temperature of crystalline polyimide prepared from crystalline polyamic acid solution; the hot pressing pressure is 0.2~3 MPa.

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