Abstract:
A copolyimide nano-fiber non-woven fabric, a process for producing the same and the use thereof. The process comprises the following steps: tetracid dianhydride monomer and diamine monomer are polycondensated in the reaction medium of a high polar solvent under mechanical agitation to form a solution of a copolyamic acid; the copolyamic acid solution is electrostatically spinned under a high voltage electric field to give a non-woven fabric of a nano-fiber of the copolyamic acid; and then the non-woven fabric is imidized. The copolyimide nano-fiber non-woven fabric has features of a strong tear resistance, a high porosity, high/low-temperature resistances and excellent mechanical properties, etc., and can be used in battery membrane and capacitor membrane.

Description:
SUMMARY OF THE UTILITY MODEL 
       [0001]    The present invention relates to a copolyimide, a process for producing the same and the use thereof, and in particular to a copolyimide nano-fiber non-woven fabric, a process for producing the same and the use thereof as battery membrane. 
       BACKGROUND ART 
       [0002]    Chemical power source is an extremely important part of modern life, and products, such as mobile phone batteries, automobile power batteries in development, etc., are indispensable for human to pursue high quality life. The safety of the batteries is important scientific and technological issues and social responsibility issue concerned by human, and the development of safe battery membranes is a technology key to solve the battery safety issues. Currently, the battery membrane of polyethylene (PE), polypropylene (PP), etc., used in battery industry cannot ensure the integrity thereof under a relatively high temperature due to a low melting temperature and an over-high heat shrink ratio, thus leading to breakage of the battery membrane due to thermal shrinkage even melting, under the conditions of overheat, overcharge, etc., and resulting in serious accidents, such as thermal runaway and explosion, due to the internal short circuit of batteries. Therefore, it is critical to develop materials with good heat resistance and thermal shrinkage resistance and to apply the material in battery membranes to solve the safety issues with respect to chemical power source. 
       CONTENT OF THE INVENTION 
       [0003]    An object of the present invention is to provide a copolyimide nano-fiber non-woven fabric with the features, such as strong tear resistance, a high porosity, high/low-temperature resistances, excellent mechanical performances, etc. 
         [0004]    Another object of the present invention is to provide a method for producing the polyimide nano-fiber non-woven fabric of the present invention. 
         [0005]    Yet another object of the present invention is use of the polyimide nano-fiber non-woven fabric in battery membrane. 
         [0006]    To achieve the above objects, in the present invention, the following technical solution is used: 
         [0007]    A copolyimide nano-fiber non-woven fabric of the present invention, is formed by the copolymerization of more than three of the four monomers of (I), (II), (III) and (IV) to form a copolyamic acid, then electrostatical spinning and imidization: 
         [0000]    
       
                 
         
             
             
         
       
     
         [0008]    wherein the copolyimide has the following chemical structure formula: 
         [0000]    
       
                 
         
             
             
         
       
     
         [0009]    n is a natural number between 50-300; m is a natural number between 50-300, R 1  and R 3  are C 4 -C 30  tetracid dianhydride monomer residue structures; R 2  and R 4  are C 6 -C 30  diamine monomer residue structures, and the ratio of the total mole number of the tetracid dianhydride monomers to the total mole number of the diamine monomers is always kept at 1:1. 
         [0010]    Preferably, a copolyimide nano-fiber non-woven fabric, is formed by the copolymerization of one tetracid dianhydride monomer and two diamine monomers, i.e., the above three monomers of (I), (III) and (IV) or (II), (III) and (IV). Wherein the molar ratio of the three monomers is (I):(III):(IV) or (II):(III):(IV)=[1]:[0.05-0.95]:[0.05-0.95]. 
         [0011]    Preferably, a copolyimide nano-fiber non-woven fabric, also can be formed by the copolymerization of two tetracid dianhydride monomers and one diamine monomers, i.e., the above three monomers of (I), (II) and (III) or (I), (II) and (IV), wherein the molar ratio of the three monomers is (I):(II):(III) or (I):(II):(IV)=[0.05-0.95]:[0.05-0.95]:[1]. 
         [0012]    Preferably, a copolyimide nano-fiber non-woven fabric, also can be formed by the copolymerization of two tetracid dianhydride monomers and two diamine monomers, i.e., the above four monomers of (I), (II), (III) and (IV), wherein the molar ratio relationship of the four monomers is [(I)+(II)]:[(III)+(IV)]=1:1. 
         [0013]    Preferably, R 1  and R 3  are selected from one of the following tetracid dianhydride residue structures, respectively: 
         [0000]    
       
                 
         
             
             
         
       
       
                 
         
             
             
         
       
     
         [0014]    Preferably, R 2  and R 4  are selected from one of the following diamine residue structures, respectively: 
         [0000]    
       
                 
         
             
             
         
       
       
                 
         
             
             
         
       
     
         [0015]    The chemical components of the copolyimide nano-fiber of the present invention can be the copolymerization product of one dianhydride monomer and two diamine monomers, or the copolymerization product of two dianhydride monomers and one diamine monomer, or the copolymerization product of two dianhydride monomers and two diamine monomers. In particular, R 1  and R 3  therein can be the same residues and also can be different residues, and R 2  and R 4  can be the same residues and also can be different residues; when R 1  and R 3  are the same, R 2  and R 4  are different; and when R 2  and R 4  are the same, R 1  and R 3  must be different, to ensure that the chemical components of said copolyimide nano-fiber is formed by the copolymerization of at least three monomers. 
         [0016]    The copolyimide nano-fiber non-woven fabric of the present invention has a thickness of 10-60 μm and breaking elongation not lower than 20%, is completely insoluble in a common organic solvent, has a glass transition temperature not lower than 210° C., a thermal decomposition temperature not lower than 510° C. and a melting temperature higher than 350° C., even does not melt at a temperature lower than the decomposition temperature, and has a porosity higher than 80%, mechanical strength higher than 20 MPa and electrical breakdown strength greater than 1×10 7  V/m. The electrostatically spun copolyimide nano-fiber non-woven fabric with such features has tear resistance, thermal shrinkage resistance, high temperature resistance and high-voltage high-current overcharge resistance, and the imide nano-fiber non-woven fabric of the present invention has huge potential market for use in various high-capacity and high power battery membranes and capacitor membranes, such as industries, like automobile power batteries and supercapacitors. 
         [0017]    Another object of the present invention is to provide a process for producing the copolyimide nano-fiber non-woven fabric, and the steps of the process comprise: 
         [0018]    (1) purifying more than three monomers, adding into a polymerization reaction kettle together with an appropriate amount of a solvent, reacting under agitation for a time period to obtain a solution of a copolyamic acid (polyimide precursor), electrostatically spinning the copolyamic acid solution in a high voltage electric field, and collecting using a stainless steel roller as a collector to obtain a non-woven fabric of a nano-fiber of the copolyamic acid. 
         [0019]    Wherein the solvent used is a high polar solvent, preferably one of N,N-dimethyl formamide (DMF) and N,N-dimethyl acetamide (DMAC); the time of the reaction under agitation is 1-10 h, preferably 5-10 h; the reaction temperature is 0-30° C., preferably 5-10° C.; the electric field intensity for the electrostatic spinning is preferably 250-300 Kv/m; and the diameter of the stainless steel roller collector is 0.3 m. 
         [0020]    (2) placing the obtained non-woven fabric of a nano-fiber of the copolyamic acid in a high temperature furnace, and heating for imidization. 
         [0021]    In this situation, the imidization is performed in a nitrogen gas atmosphere, and the temperature rise program of the heating process includes heating from room temperature to 200-250° C. at a temperature rise rate of 20° C./min, and maintaining at the temperature for 30 min, heating to 330-370° C. at a temperature rise rate of 5° C./min, and maintaining at the temperature for 30 min, and shutting off the power source. 
         [0022]    (3) Property characterization, comprising measuring the absolute viscosity of the copolyamic acid solution and the spinning solution, the diameter of the electrostatically spun copolyamic acid nano-fiber, the thermal decomposition temperature of the copolyimide nano-fiber non-woven fabric, the mechanical properties (strength, breaking elongation, etc.) of the copolyimide nano-fiber non-woven fabric, the glass transition temperature of the copolyimide nano-fiber non-woven fabric, the specific surface area of the copolyimide nano-fiber non-woven fabric, and the electrical breakdown strength of the copolyimide nano-fiber non-woven fabric. 
         [0023]    In the present invention, a NDJ-8S viscometer (Shanghai Precision &amp; Scientific Instrument Company) is used to measure the absolute viscosity of the polyamic acid solution and the spinning solution; the diameter of the electrostatically spun polyamic acid nano-fiber is measured by a scanning electron microscope VEGA 3 SBU (Czech Republic); a WRT-3P thermogravimetric analyzer (TGA) (Shanghai Precision &amp; Scientific Instrument Co., Ltd.) is used to measure the thermal decomposition temperature of the copolyimide nano-fiber non-woven fabric; a CMT8102 electronic universal testing machine (Shenzhen SANS Materials Testing Co., Ltd.) is used to measure the mechanical properties (strength, breaking elongation, etc.) of the copolyimide nano-fiber non-woven fabric; a Diamond dynamic mechanical analyzer (DMA) (Perkin-Elmer, America) is used to measure the glass transition temperature of the copolyimide nano-fiber; the specific surface area of the copolyimide nano-fiber porous membrane or non-woven fabric of the present invention is measured by a JW-K pore distribution and specific surface area measuring instrument (Beijing JWGB Sci &amp; Tech Co., Ltd.); and the electrical breakdown strength of the copolyimide nano-fiber non-woven fabric is measured by a dielectric breakdown tester DJD-20 KV (Beijing crown measurement test instrument Co., LTD). 
         [0024]    The porosity of the copolyimide nano-fiber non-woven fabric of the present invention is calculated by the following formula: 
         [0000]      porosity β=[1−(ρ/ρ 0 )]×100
 
         [0000]    wherein ρ is the density (g/cm 3 ) of the copolyimide nano-fiber non-woven fabric, and ρ 0  is the density (g/cm 3 ) of a copolyimide body film (produced by a solution casting method). 
         [0025]    Yet another object of the present invention is use of the copolyimide nano-fiber non-woven fabric in battery membrane. 
         [0026]    In the present invention, dianhydride and diamine are used as the reaction raw materials and a high polar solvent as the reaction medium, and polycondensed under mechanical agitation, to form a solution of copolyamic acid (co-PI precursor polymer). In this situation, the total number of the dianhydride monomers and the diamine monomers is more than three, and the total number of the dianhydride functional groups is equal to or substantially equal to the total number of the diamine functional groups. The obtained solution is processed into a non-woven fabric of a nano-fiber of the copolyamic acid by high voltage electrostatic spinning technology, and the non-woven fabric is imidized at a high temperature higher than 300° C., to form a high temperature resistant nano-fiber non-woven fabric battery membrane capable of isolating the electrodes in chemical power source. The copolyimide nano-fiber non-woven fabric has features, such as strong tear resistance, a high porosity, high/low-temperature resistances, excellent mechanical performances, etc., and has good heat resistance and thermal shrinkage resistance when used in battery membranes, not leading to the breakage of the battery membrane due to thermal shrinkage even melting, under the conditions of overheat, overcharge, etc., and thus the phenomena, such as thermal runaway, etc., due to the internal short circuit of the batteries. In addition, the copolyimide nano-fiber non-woven fabric has huge potential market for use in various high-capacity and high power battery membranes and capacitor membranes, such as industries, like automobile power batteries and supercapacitors. 
         [0027]    The present invention is further described in detail in conjunction with examples. 
     
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0028]    A process for producing the copolyimide nano-fiber non-woven fabric of the present invention, the steps of the process comprise: 
         [0029]    (1) On the basis of an equal total molar amount of the dianhydride functional groups to the total molar amount of the diamine functional groups, mixing appropriate amounts of one dianhydride monomer and two diamine monomers or mixing appropriate amounts of two dianhydride monomers and one diamine monomer or mixing appropriate amounts of two dianhydride monomers and two diamine monomers, adding into a polymerization reaction kettle together with an appropriate amount of a solvent, reacting under agitation for a time period to obtain a solution of a copolyamic acid (polyimide precursor), electrostatically spinning the copolyamic acid solution in a high voltage electric field, and collecting using a stainless steel roller as a collector to obtain a porous membrane or non-woven fabric of a nano-fiber of the copolyamic acid. In this situation, the solvent is preferably one of N,N-dimethyl formamide (DMF) and N,N-dimethyl acetamide (DMAC); the temperature of the reaction kettle is 0-30° C.; the time of the reaction under agitation is preferably 1-10 h; the electric field intensity of the high voltage electric field is 250-300 Kv/m; and the diameter of the stainless steel roller collector is 0.3 m. 
         [0030]    (2) Placing the obtained non-woven fabric of a nano-fiber of the copolyamic acid in a high temperature furnace, and heating for imidization. In this situation, the temperature rise program of the heating process includes heating from room temperature to 200-250° C. at a temperature rise rate of 20° C./min, and maintaining at the temperature for 30 min, heating to 330-370° C. at a temperature rise rate of 5° C./min, and maintaining at the temperature for 30 min, and shutting off the power source. 
         [0031]    (3) Property characterization: comprising measuring the absolute viscosity of the copolyamic acid solution and the spinning solution, the diameter of the electrostatically spun copolyamic acid nano-fiber, the thermal decomposition temperature of the copolyimide nano-fiber non-woven fabric, the mechanical properties (strength, breaking elongation, etc.) of the copolyimide nano-fiber porous membrane or non-woven fabric, the glass transition temperature of the copolyimide nano-fiber non-woven fabric, the specific surface area of the copolyimide nano-fiber non-woven fabric, and the electrical breakdown strength of the copolyimide nano-fiber non-woven fabric. 
       Embodiment 1 
       [0032]    One tetracid dianhydride monomer and two diamine monomers were selected as the comonomers. Purified biphenyl dianhydride (BPDA), p-phenylenediamine (PPD) and oxydianiline (ODA) were mixed at a molar ratio of 1:0.5:0.5, and reacted in N,N-dimethyl formamide (DMF) as a solvent following the above steps. In reaction step (1), the temperature of the reaction kettle of the example is 10° C., the time of the reaction under agitation is 6 h, and the electric field intensity of the high voltage electric field for the electrostatic spinning is 300 Kv/m; and in reaction step (2), the temperature rise program includes heating from room temperature to 200° C. at a temperature rise rate of 20° C./min, and maintaining at the temperature for 30 min, heating to 350° C. at a temperature rise rate of 5° C./min, and maintaining at 350° C. for 30 min, shutting off the power source, and naturally cooling to room temperature. 
         [0033]    Property characterization: the mass concentration of the copolyamic acid (polyimide precursor) solution is 7% and the absolute viscosity thereof is 5.2 Pa·s, the diameter of the copolyamic acid nano-fiber is 100-400 nm and is mainly distributed at about 250 nm, and the copolyimide nano-fiber non-woven fabric has tensile strength of 25 MPa, breaking elongation of 24%, glass transition temperature of 285° C., thermal decomposition temperature of 530° C., a porosity of 84.2%, a specific surface area of 37.4 m 2 /g and electrical breakdown strength of 1.2×10 5  V/cm or 12 V/μm. 
       Embodiment 2 
       [0034]    One tetracid dianhydride monomer and two diamine monomers were selected as the comonomers. Purified pyromellitic dianhydride (PMDA), oxydianiline (ODA) and Benzidine (Bz) were mixed at a molar ratio of 1:0.6:0.4, and reacted in N,N-dimethyl formamide (DMF) as a solvent following the above steps; in reaction step (1), the temperature of the reaction kettle of the example is 5° C., the time of the reaction under agitation is 6 h, and the electric field intensity of the high voltage electric field for the electrostatic spinning is 250 Kv/m; and in reaction step (2), the temperature rise program includes heating from room temperature to 250° C. at a temperature rise rate of 20° C./min, and maintaining at the temperature for 30 min, heating to 370° C. at a temperature rise rate of 5° C./min, and maintaining at 370° C. for 30 min, shutting off the power source, and naturally cooling to room temperature. 
         [0035]    Property characterization: the mass concentration of the copolyamic acid (polyimide precursor) solution is 5% and the absolute viscosity thereof is 4.8 Pa·s, the diameter of the copolyamic acid nano-fiber is 100-300 nm and is mainly distributed at about 200 nm, and the copolyimide nano-fiber non-woven fabric has tensile strength of 24 MPa, breaking elongation of 23%, a glass transition temperature of 298° C., a thermal decomposition temperature of 560° C., a porosity of 82.0%, a specific surface area of 38.8 m 2 /g and an electrical breakdown strength of 1.3×10 5  V/cm or 13 V/μm. 
       Embodiment 3 
       [0036]    One tetracid dianhydride monomer and two diamine monomers were selected as the comonomers. Purified pyromellitic dianhydride (PMDA), methylene dianiline (MDA) and oxydianiline (ODA) were mixed at a molar ratio of 1:0.5:0.5, and reacted in N,N-dimethyl formamide (DMF) as a solvent following the above steps; in reaction step (1), the temperature of the reaction kettle of the example is 5° C., the time of the reaction under agitation is 10 h, and the electric field intensity of the high voltage electric field for the electrostatic spinning is 250 Kv/m; and in reaction step (2), the temperature rise program includes heating from room temperature to 250° C. at a temperature rise rate of 20° C./min, and maintaining at the temperature for 30 min, heating to 370° C. at a temperature rise rate of 5° C./min, and maintaining at 370° C. for 30 min, shutting off the power source, and naturally cooling to room temperature. 
         [0037]    Property characterization: the mass concentration of the copolyimide precursor (copolyamic acid, co-PAA) solution is 6% and the absolute viscosity thereof is 4.8 Pa·s, the diameter of the copolyamic acid nano-fiber is 100-400 nm and is mainly distributed at about 250 nm, and the copolyimide nano-fiber non-woven fabric has tensile strength of 20 MPa, breaking elongation of 21%, a glass transition temperature of 296° C., a thermal decomposition temperature of 510° C., a porosity of 85.1%, a specific surface area of 36.9 m 2 /g and electrical breakdown strength of 1.1×10 5  V/cm or 11 V/μm. 
       Embodiment 4 
       [0038]    One tetracid dianhydride monomer and two diamine monomer were selected as the comonomers. Purified diphenyl sulfone dianhydride (DSDA), bis(aminophenoxyphenyl) sulfone (BAPS) and oxydianiline (ODA) were mixed at a molar ratio of 1:0.3:0.7, and reacted in N,N-dimethyl formamide (DMF) as a solvent following the above steps; in reaction step (1), the temperature of the reaction kettle of the example is 5° C., the time of the reaction under agitation is 10 h, and the electric field intensity of the high voltage electric field for the electrostatic spinning is 250 Kv/m; and in reaction step (2), the temperature rise program includes heating from room temperature to 200° C. at a temperature rise rate of 20° C./min, and maintaining at the temperature for 30 min, heating to 330° C. at a temperature rise rate of 5° C./min, and maintaining at 330° C. for 30 min, shutting off the power source, and naturally cooling to room temperature. 
         [0039]    Property Characterization: the mass concentration of the copolyimide precursor (copolyamic acid, co-PAA) solution is 8% and the absolute viscosity thereof is 4.2 Pa·s, the diameter of the copolyamic acid nano-fiber is 100-300 nm and is mainly distributed at about 180 nm, and the copolyimide nano-fiber non-woven fabric has tensile strength of 20 MPa, breaking elongation of 25%, a glass transition temperature of 238° C., a thermal decomposition temperature of 520° C., a porosity of 81.3%, a specific surface area of 36.9 m 2 /g and electrical breakdown strength of 1.4×10 5  V/cm or 14 V/μm. 
       Embodiment 5 
       [0040]    Two tetracid dianhydride monomers and one diamine monomer were selected as the polymerization monomers. Purified biphenyl dianhydride (BPDA), pyromellitic dianhydride (PMDA) and oxydianiline (ODA) were mixed at a molar ratio of 0.5:0.5:1, and reacted in N,N-dimethyl formamide (DMF) as a solvent according to the above steps. In this situation, in reaction step (1), the temperature of the reaction kettle of the example is 5° C., the time of the reaction under agitation is 10 h, and the electric field intensity of the high voltage electric field for the electrostatic spinning is 250 Kv/m; and in reaction step (2), the temperature rise program includes heating from room temperature to 250° C. at a temperature rise rate of 20° C./min, and maintaining at the temperature for 30 min, heating to 370° C. at a temperature rise rate of 5° C./min, and maintaining at 370° C. for 30 min, shutting off the power source, and naturally cooling to room temperature. 
         [0041]    Property characterization: the mass concentration of the copolyamic acid solution is 6% and the absolute viscosity thereof is 5.5 Pa·s, the diameter of the copolyamic acid nano-fiber is 150-400 nm and is mainly distributed at about 280 nm, and the copolyimide nano-fiber non-woven fabric has a tensile strength of 23 MPa, a breaking elongation of 22%, a glass transition temperature of 295° C., a thermal decomposition temperature of 550° C., a porosity of 85.0%, a specific surface area of 36.9 m 2 /g and electrical breakdown strength of 1.1×10 5  V/cm or 11 V/μm. 
       Embodiment 6 
       [0042]    Two tetracid dianhydride monomers and one diamine monomer were selected as the polymerization monomers. Purified hydroquinone diphthalic anhydride (HQDPA), pyromellitic dianhydride (PMDA) and oxydianiline (ODA) were reacted at a molar ratio of 0.5:0.5:1 in an appropriate amount of solvent of N,N-dimethyl formamide (DMF) following the above steps. In this situation, in reaction step (1), the temperature of the reaction kettle of the example is 10° C., the time of the reaction under agitation is 5 h, and the electric field intensity of the high voltage electric field for the electrostatic spinning is 300 Kv/m; and in reaction step (2), the temperature rise program includes heating from room temperature to 200° C. at a temperature rise rate of 20° C./min, and maintaining at the temperature for 30 min, heating to 350° C. at a temperature rise rate of 5° C./min, and maintaining at 350° C. for 30 min, shutting off the power source, and naturally cooling to room temperature. 
         [0043]    Property characterization: the mass concentration of the copolyamic acid solution is 8% and the absolute viscosity thereof is 4.2 Pa·s, the diameter of the copolyamic acid nano-fiber is 80-300 nm and is mainly distributed at about 150 nm, and the copolyimide nano-fiber non-woven fabric has tensile strength of 23 MPa, breaking elongation of 24%, a glass transition temperature of 278° C., a thermal decomposition temperature of 540° C., a porosity of 81.4%, a specific surface area of 41.8 m 2 /g and electrical breakdown strength of 1.4×10 5  V/cm or 14 V/μm. 
       Embodiment 7 
       [0044]    Two tetracid dianhydrides and two diamines were selected as the comonomers. Purified benzophenonetetracarboxylic dianhydride (BTDA), pyromellitic dianhydride (PMDA), benzidine (Bz) and oxydianiline (ODA) were mixed at a molar ratio of 1:1:1:1 and reacted in an appropriate amount of N,N-dimethylacetamide (DMAc) as a solvent following the above steps. In this situation, in reaction step (1), the temperature of the reaction kettle of the example is 5° C., the time of the reaction under agitation is 6 h, and the electric field intensity of the high voltage electric field for the electrostatic spinning is 250 Kv/m; and in reaction step (2), the temperature rise program includes heating from room temperature to 250° C. at a temperature rise rate of 20° C./min, and maintaining at the temperature for 30 min, heating to 370° C. at a temperature rise rate of 5° C./min, and maintaining at 370° C. for 30 min, shutting off the power source, and naturally cooling to room temperature. 
         [0045]    Property characterization: the mass concentration of the copolyamic acid solution is 6% and the absolute viscosity thereof is 4.3 Pa·s, the diameter of the copolyamic acid nano-fiber is 100-300 nm and is mainly distributed at about 150 nm, and the copolyimide nano-fiber non-woven fabric has tensile strength of 22 MPa, breaking elongation of 24%, a glass transition temperature of 288° C., a thermal decomposition temperature of 540° C., a porosity of 80.5%, a specific surface area of 41.8 m 2 /g and electrical breakdown strength of 1.5×10 5  V/cm or 15 V/μm. 
       Embodiment 8 
       [0046]    Two tetracid dianhydrides and two diamines were selected as the comonomers. Purified biphenyl dianhydride (BPDA), hydroquinone diphthalic anhydride (HQDPA), p-phenylenediamine (PPD) and oxydianiline (ODA) were mixed at a molar ratio of 1:1:1:1, and reacted in an appropriate amount of N,N-dimethyl formamide (DMAc) as a solvent following the above steps. In this situation, in reaction step (1), the temperature of the reaction kettle of the example is 10° C., the time of the reaction under agitation is 10 h, and the electric field intensity of the high voltage electric field for the electrostatic spinning is 300 Kv/m; and in reaction step (2), the temperature rise program includes heating from room temperature to 250° C. at a temperature rise rate of 20° C./min, and maintaining at the temperature for 30 min, heating to 350° C. at a temperature rise rate of 5° C./min, and maintaining at 320° C. for 30 min, shutting off the power source, and naturally cooling to room temperature. 
         [0047]    Property characterization: the mass concentration of the copolyamic acid solution is 8% and the absolute viscosity thereof is 4.0 Pa·s, the diameter of the copolyamic acid nano-fiber is 50-250 nm and is mainly distributed at about 150 nm, and the copolyimide nano-fiber non-woven fabric has tensile strength of 21 MPa, breaking elongation of 23%, a glass transition temperature of 284° C., a thermal decomposition temperature of 530° C., a porosity of 80.2%, a specific surface area of 42.0 m 2 /g and electrical breakdown strength of 1.5×10 5  V/cm or 15 V/μm. 
         [0048]    The above examples should not be understood to limit the scope of the present invention. Those skilled in the art can make some non-intrinsic modification and adjustment to the present invention according to the above contents of the present invention, which still belong to the scope of the present invention.