A nanoporous polyimide film with a helical icosahedral structure and a preparation method thereof

Abstract

The present invention discloses a nanoporous polyimide film with a spiral 24-hedron structure and a preparation method thereof, which belongs to the technical field of organic film preparation. The preparation method is: coating a block copolymer, allowing the block copolymer to self-assemble to form a film with an ordered spiral 24-hedron microstructure, and then heat-treating the film with the ordered spiral 24-hedron microstructure to crack off the degradable polyester component to obtain the nanoporous polyimide film with a spiral 24-hedron structure; wherein the block copolymer is a block copolymer of polyamic acid and degradable polyester, or a block copolymer of polyimide and degradable polyester. The nanoporous polyimide film with a spiral 24-hedron structure obtained by the preparation method of the present invention has high pore size uniformity, excellent three-dimensional continuous penetration, uniform thickness, good mechanical properties; at the same time, it has good adsorption properties.

Description

Technical Field

[0001] The present invention relates to the technical field of organic thin films, in particular to a nanoporous polyimide film with a spiral icosahedral structure and a preparation method thereof. Background Art

[0002] Polyimide (PI) is widely used in high-tech fields due to its high modulus and strength, low water absorption, high and low temperature resistance, hydrolysis resistance, and radiation resistance. Porous polymer materials have recently seen rapid development, with increasing research and application of porous polyimide membranes. Porous PI films are gaining increasing attention in lithium batteries, filtration, separation, and absorption applications due to their high and low temperature resistance, flame retardancy, radiation resistance, low smoke emission, and minimal toxic gas release during decomposition.

[0003] At present, there are mainly the following methods for producing porous PI films: (1) porogen addition method, in which a porogen is added to a polyamic acid solution or a soluble PI solution, and the porogen is removed during or after the PI film formation process to obtain a porous PI film; (2) self-reaction pore-forming method, in which small molecules (such as H2O, CO2 or ROH) generated during the imidization process of the PI precursor are foamed to form pores; (3) supercritical CO2 method, in which a conventional method is used to prepare a PI precursor polyamic acid solution or a soluble PI solution. Membrane, then the prepared PI membrane is treated in supercritical CO2, and CO2 is released through pressurization and thermal imidization to obtain a porous PI film; (4) Unstable block removal method, unstable blocks are added to PI by copolymerization or grafting, and the unstable blocks are removed under appropriate conditions to form pores in the PI matrix; (5) Solution-induced phase separation method, PI precursor polyamic acid solution or soluble polyimide is immersed in a coagulation bath, the solvent is removed, dried, and thermally imidized to form a porous structure in the PI matrix. (6) Breathing diagram method, water vapor condenses into spherical droplets on the surface of the polyamic acid solution, and self-assembles to form an ordered arrangement through capillary force. After the solvent and water are completely evaporated, the ordered structure is replicated and fixed to form an ordered porous structure, mostly in the form of a micron-scale hexagonal honeycomb. Among the above methods, the method of adding porogens is simple to operate and the pore size and porosity are adjustable, but the prepared pores are not through and have irregular shapes. Other methods have problems such as complex porous film preparation process and technology, high cost, and large pore size.

[0004] In summary, existing preparation methods cannot precisely control the pore distribution of porous PI films, resulting in uneven pore size, poor porosity uniformity, and poor permeability, making them unsuitable for applications with higher requirements. Therefore, it is highly desirable to provide a porous polyimide film with a uniform pore size and a three-dimensional permeable structure, and a preparation method thereof. Summary of the Invention

[0005] The purpose of the present invention is to provide a nanoporous polyimide (PI) film with uniform pore size and three-dimensional penetration of spiral icosahedral structure and its preparation method, so as to solve the problems of uneven pore size, large size and poor continuous penetration of the PI film in the above-mentioned prior art.

[0006] To achieve the above object, the present invention provides the following solutions:

[0007] One of the technical solutions of the present invention: a method for preparing a nanoporous polyimide film with a spiral icosahedral structure, comprising coating a block copolymer, allowing the block copolymer to self-assemble to form a film with an ordered spiral icosahedral microstructure, and then heat-treating the film with an ordered spiral icosahedral microstructure to obtain the nanoporous polyimide film with a spiral icosahedral structure; wherein the block copolymer is a block copolymer of polyamic acid and degradable polyester, or a block copolymer of polyimide and degradable polyester.

[0008] Furthermore, the block copolymer of polyamic acid and degradable polyester is a block copolymer of single-end amino polyamic acid and single-end isocyanate-based L-polylactic acid, and the block copolymer of polyimide and degradable polyester is a block copolymer of single-end hydroxymethyl polyimide and L-polylactic acid, or a block copolymer of single-end maleimide polyimide and single-end furyl-based L-polylactic acid.

[0009] Furthermore, the preparation method of the block copolymer of the single-terminal amino polyamic acid and the single-terminal isocyanate-based L-polylactic acid is as follows: the single-terminal isocyanate-based L-polylactic acid and the single-terminal amino polyamic acid are subjected to a graft reaction; the structural formula of the single-terminal isocyanate-based L-polylactic acid is Wherein, n=70-210; the structural formula of the single-terminated amino polyamic acid is Wherein, m=45-135; the structural formula of the block copolymer of the single-terminal amino polyamic acid and the single-terminal isocyanate-terminal L-polylactic acid is

[0010]

[0011] Furthermore, the molecular weight of the single-ended isocyanate-containing L-polylactic acid is 10,000 to 30,000, and the molecular weight of the single-ended amino-containing polyamic acid is 18,600 to 55,800.

[0012] Furthermore, the molar ratio of the single-ended isocyanate-based L-polylactic acid to the single-ended amino-based polyamic acid is 1:1.

[0013] Furthermore, the single-ended isocyanate-based L-polylactic acid is prepared by reacting single-ended hydroxyl-based L-polylactic acid and 4,4′-diphenylmethane diisocyanate. The structural formula of the single-ended hydroxyl-based L-polylactic acid is

[0014] Among them, n=70~210.

[0015] Furthermore, the single-end hydroxyl-L-polylactic acid is obtained by polymerization reaction of benzyl alcohol and L-lactide under the action of a catalyst.

[0016] Furthermore, the structural formula of the block copolymer of the single-terminal hydroxymethyl polyimide and L-polylactic acid is

[0017] Among them, n=59~87, m=140~210.

[0018] Furthermore, the molecular weight of the block copolymer of single-end hydroxymethyl polyimide and L-polylactic acid is 55600-83400, wherein the molecular weight of the single-end hydroxymethyl polyimide is 35600-53400, and the molecular weight of the L-polylactic acid is 20000-30000.

[0019] Furthermore, the preparation method of the block copolymer of the single-terminal hydroxymethyl polyimide and L-polylactic acid is as follows: dissolving the catalyst and the single-terminal hydroxymethyl polyimide in a solvent, adding L-lactide, stirring evenly, freezing through liquid nitrogen → vacuuming → passing N2 → thawing, and then heating the reaction to obtain the block copolymer of the single-terminal hydroxymethyl polyimide and L-polylactic acid; the structural formula of the single-terminal hydroxymethyl polyimide is Wherein, n=59-87; the molecular weight of the single-ended hydroxymethyl polyimide is 35600-53400.

[0020] Furthermore, the heating reaction temperature is 110° C. and the time is 12 hours.

[0021] Furthermore, the molar ratio of the single-terminated hydroxymethyl polyimide to L-lactide is 1:140-210.

[0022] Furthermore, the single-terminated hydroxymethyl polyimide is obtained by polymerization reaction of hexafluorodianhydride, 3,4′-diaminodiphenyl ether, p-aminobenzyl alcohol, and phthalic anhydride.

[0023] Furthermore, the preparation method of the block copolymer of the single-end maleimide polyimide and the single-end furanyl L-polylactic acid is as follows: single-end furanyl L-polylactide and single-end maleimide polyimide are subjected to a graft reaction; the structural formula of the single-end furanyl L-polylactide is The structural formula of the single-ended maleimide polyimide is The structural formula of the block copolymer of the single-ended maleimide polyimide and the single-ended furanyl L-polylactic acid is: Among them, n=33~66, m=70~140.

[0024] Furthermore, the molecular weight of the single-ended maleimide polyimide is 17,000 to 34,000, and the molecular weight of the single-ended furanyl L-polylactic acid is 10,000 to 20,000.

[0025] Furthermore, the molar ratio of the single-ended maleimide polyimide to the single-ended furanyl L-polylactic acid is 1:1.

[0026] Furthermore, the single-end furanyl L-polylactide is obtained by polymerization of furan methanol, triethylaluminum toluene solution, and L-lactide.

[0027] Furthermore, the single-ended maleimide polyimide is obtained by polymerization of hexafluorodianhydride, m-phenylenediamine, maleic anhydride and aniline.

[0028] Furthermore, the process of coating the block copolymer to allow the block copolymer to self-assemble to form a film having an ordered helical tetrahedral microstructure comprises coating the block copolymer in the form of a solution and initiating the self-assembly of the block copolymer by heating to volatilize the solvent.

[0029] The solvent can be introduced during the preparation of the block copolymer, that is, the solvent used in the reaction process, and the solvent of the reaction system is not removed after the reaction is completed, and the product in the form of a solution is directly coated; or it can be introduced after the pure block copolymer is obtained, that is, the pure solid block copolymer is first obtained, and then the solid block copolymer is dissolved in the solvent for coating.

[0030] Furthermore, the conditions for heating and volatilizing the solvent are first heating and drying at 60° C. for 1 hour, and then heating and drying at 120° C. for 1 hour.

[0031] During heating, as the solvent evaporates from the solution, the block copolymers self-assemble to form an ordered helical tetrahedral microstructure. The principle of self-assembly is as follows: although the different blocks are chemically bonded, due to thermodynamic incompatibility, they tend to separate from each other to reduce contact area and thus interfacial energy; while longer macromolecular chains tend to curl and entangle together. These two processes are in equilibrium under certain conditions. When the temperature, concentration, block ratio, and degree of polymerization are changed, microscopic phase separation occurs between the two polymer blocks due to thermodynamic incompatibility (heating to evaporate the solvent changes the polymer concentration and temperature). The presence of chemical bonds between the blocks inhibits macroscopic phase separation, resulting in a phase structure with an average microdomain size of around tens of nanometers.

[0032] Furthermore, the solvent is N,N′-dimethylacetamide, N-methylpyrrolidone or γ-butyrolactone.

[0033] Furthermore, the solvent is preferably N,N′-dimethylacetamide.

[0034] Furthermore, the heat treatment procedure is 100°C for 1 hour → 200°C for 1 hour → 300°C for 1 hour → 350°C for 2 hours.

[0035] When the coating material is a block copolymer of polyamic acid and a biodegradable polyester, the subsequent heat treatment not only cleaves the biodegradable polyester component in the block polymer, but also undergoes an imidization reaction to form polyimide. However, when the coating material is a block copolymer of polyimide and a biodegradable polyester, the subsequent heat treatment only cleaves the biodegradable polyester component in the block polymer, forming a nanoporous PI film with a helical tetrahedral structure.

[0036] The technical concept of the present invention is: a degradable polyester is grafted onto the end of a PI precursor polyamic acid (PAA) to form a block copolymer of polyamic acid and degradable polyester, the block copolymer of polyamic acid and degradable polyester is coated, the solvent is heated to volatilize and induce the block copolymer of polyamic acid and degradable polyester to self-assemble to form a film having an ordered spiral 24-hedral microstructure, and then the film having an ordered spiral 24-hedral microstructure is heat-treated. During the heat treatment process, the polyamic acid part of the block copolymer undergoes an imidization reaction to form polyimide, and the degradable polyester component in the block polymer is cleaved to form pores, thereby obtaining a nanoporous PI film with a spiral 24-hedral structure. Alternatively, a degradable polyester is grafted onto the end of a polyimide to form a block copolymer of polyimide and degradable polyester, the block copolymer of polyimide and degradable polyester is coated, and the solvent is heated to volatilize, causing the block copolymer of polyimide and degradable polyester to self-assemble into a film having an ordered helical 24-hedron microstructure. The film having an ordered helical 24-hedron microstructure is then heat-treated. During the heat treatment, the degradable polyester component in the block copolymer is cleaved to form pores, resulting in a nanoporous PI film with a helical 24-hedron structure. In both of the above processes, the self-assembly process of the block copolymer and the structure of the porous polyimide film obtained after heat treatment are regulated by controlling the content of the degradable polyester in the block copolymer. The content of the degradable polyester in the block copolymer is in turn regulated by controlling the molecular weight and amount ratio of the degradable polyester to the polyamic acid, or the molecular weight and amount ratio of the degradable polyester to the polyimide.

[0037] The second technical solution of the present invention: a nanoporous polyimide film with a spiral icosahedral structure prepared according to the preparation method of the nanoporous polyimide film with a spiral icosahedral structure.

[0038] The nanoporous polyimide film obtained by the preparation method of the present invention has a spiral icosahedron structure. The spiral icosahedron is a continuous minimal surface that exhibits a highly three-dimensional periodic structure. In a sense, it is the opposite of a sphere: a sphere has a uniform positive curvature, while a spiral icosahedron structure has a uniform negative curvature. This structure divides the space into two maze-like twin channels separated by a thin wall, which form a perfect mirror relationship with each other. Thanks to the continuity of its network, this type of structure has considerable potential application value in the fields of proton conductors and batteries. In addition to ensuring mechanical properties, the three-dimensional periodic minimal surface structure of the spiral icosahedron structure has obvious advantages in anti-collision performance compared to other structures. It also has a smooth, continuous, well-connected, three-dimensional through-pore structure. Porous materials with a spiral icosahedral structure can be used as photonic materials. Their three-dimensional ordered bicontinuous phase regions are particularly suitable for separating and extracting free charges, thereby maximizing the efficiency of solar cells. Compared with materials with hexagonal close-packed porous structures, their huge porosity and specific surface area have good solvent resistance, high-temperature chemical stability and dimensional stability, and can be widely used in separation, catalysis and photonic crystal fields.

[0039] The present invention discloses the following technical effects:

[0040] The nanoporous PI film with a helical icosahedral structure prepared by the method of the present invention has high pore size uniformity, excellent three-dimensional continuous penetration, uniform thickness, good mechanical properties, and good adsorption performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0042] Figure 1 This is a transmission electron microscope image of the film before heat treatment in Example 1 of the present invention;

[0043] Figure 2 This is a scanning electron microscope image of the film after heat treatment in Example 1 of the present invention;

[0044] Figure 3 This is the thermal weight loss curve of the film in air atmosphere before heat treatment in Example 1 of the present invention;

[0045] Figure 4 This is a transmission electron microscope image of the film before heat treatment in Example 2 of the present invention;

[0046] Figure 5This is a scanning electron microscope image of the film after heat treatment in Example 2 of the present invention;

[0047] Figure 6 This is the thermogravimetric curve of the film before heat treatment in air atmosphere in Example 2 of the present invention;

[0048] Figure 7 This is a transmission electron microscope image of the film before heat treatment in Example 3 of the present invention;

[0049] Figure 8 This is a scanning electron microscope image of the film after heat treatment in Example 3 of the present invention;

[0050] Figure 9 This is the thermogravimetric curve of the film before heat treatment in air atmosphere in Example 3 of the present invention;

[0051] Figure 10 This is a transmission electron microscope image of the film before heat treatment in Comparative Example 1 of the present invention;

[0052] Figure 11 This is a scanning electron microscope image of the film after heat treatment in Comparative Example 1 of the present invention;

[0053] Figure 12 This is the thermogravimetric curve of the film before heat treatment in air atmosphere in Comparative Example 1 of the present invention;

[0054] Figure 13 This is a transmission electron microscope image of the film before heat treatment in Comparative Example 2 of the present invention;

[0055] Figure 14 This is a scanning electron microscope image of the film after heat treatment in Comparative Example 2 of the present invention;

[0056] Figure 15 This is the thermogravimetric curve of the film before heat treatment in air atmosphere in Comparative Example 2 of the present invention;

[0057] Figure 16 This is a transmission electron microscope image of the film before heat treatment in Comparative Example 3 of the present invention;

[0058] Figure 17 This is a scanning electron microscope image of the film after heat treatment in Comparative Example 3 of the present invention;

[0059] Figure 18 This is the thermogravimetric curve of the film before heat treatment in air atmosphere in Comparative Example 3 of the present invention;

[0060] Figure 19 This is the adsorption equilibrium diagram of the porous PI films prepared in Examples 1-3 of the present invention and Comparative Examples 1-3 for water and toluene. DETAILED DESCRIPTION

[0061] Various exemplary embodiments of the present invention will now be described in detail. This detailed description should not be considered as limiting the present invention, but rather as a more detailed description of certain aspects, features, and embodiments of the present invention.

[0062] It should be understood that the terms described herein are intended only to describe particular embodiments and are not intended to limit the present invention. In addition, for numerical ranges herein, it should be understood that each intermediate value between the upper and lower limits of the range is also specifically disclosed. Each smaller range between any intermediate value within a stated value or stated range and any other stated value or intermediate value within the stated range is also encompassed by the present invention. The upper and lower limits of these smaller ranges may be independently included or excluded within the scope.

[0063] Unless otherwise indicated, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the art. Although only preferred methods and materials are described herein, any methods and materials similar or equivalent to those described herein may also be used in the practice or testing of the present invention. All documents mentioned in this specification are incorporated by reference to disclose and describe the methods and / or materials associated with the documents. In the event of any conflict with any incorporated document, the contents of this specification shall prevail.

[0064] It will be apparent to those skilled in the art that various modifications and variations may be made to the specific embodiments of the present invention without departing from the scope or spirit of the invention. Other embodiments will be apparent to those skilled in the art from the present invention. The present description and examples are intended to be illustrative only.

[0065] The words “include,” “including,” “have,” “contain,” etc. used in this document are open-ended terms, meaning including but not limited to.

[0066] The raw materials used in the following examples and comparative examples are all common commercial products, among which the purity of stannous octoate is 97%, and the purity of other reagents is ≥98%; pyromellitic dianhydride, hexafluorodianhydride, 4,4′-diaminodiphenyl ether, 3,4′-diaminodiphenyl ether, and m-phenylenediamine were treated in a vacuum oven at 140°C for 4 hours before use and used immediately after cooling to room temperature; L-lactide was recrystallized three times in anhydrous toluene before use; benzyl alcohol and furfuryl alcohol were dried with calcium oxide at room temperature for 24 hours and then distilled under reduced pressure, and added Treat with molecular sieves for later use; reflux N,N'-dimethylformamide and N,N'-dimethylacetamide with calcium hydride for 1 hour, distill under reduced pressure and add Treat with molecular sieves for later use. The remaining reagents were used directly.

[0067] Example 1

[0068] (1) 0.12 g (0.0003 mol) of stannous octoate (Sn(Oct)2) and 0.108 g (0.001 mol) of benzyl alcohol were dissolved in 100 g of N,N′-dimethylformamide and poured into a 250 mL polymerization bottle with a magnetic rotor. Dry nitrogen was passed through the bottle at room temperature and stirred for 12 h. 14.4 g (0.1 mol) of L-lactide was added and stirred evenly. The mixture was frozen in liquid nitrogen → vacuum → passed through N2 → thawed three times and then reacted at 110°C for 12 h. The product was precipitated with methanol and dried in vacuum to obtain 13.02 g (0.001 mol) of single-end hydroxyl L-polylactic acid (PLLA-OH, molecular weight 13120), the structural formula of which is as follows:

[0069] n=92;The reaction process of this step is:

[0070]

[0071] (2) 0.25 g (0.001 mol) of 4,4′-diphenylmethane diisocyanate (MDI) was dissolved in 75 g of N,N′-dimethylformamide and poured into a 250 mL three-necked flask with a magnetic rotor; 13.02 g of PLLA-OH prepared in step (1) was dissolved in 100 g of N,N′-dimethylformamide and slowly added dropwise to the three-necked flask, and stirred at 80° C. for 4 h to obtain a N,N′-dimethylformamide solution of single-ended isocyanate-based L-polylactic acid (PLLA-NCO, molecular weight 13370) (containing 0.001 mol of PLLA-NCO). The structural formula of PLLA-NCO is as follows:

[0072] n = 92;

[0073] The reaction process of this step is:

[0074]

[0075] (3) 5.45 g (0.025 mol) of pyromellitic dianhydride was dissolved in 50 g of N,N′-dimethylformamide and poured into a 250 mL three-necked flask with a magnetic rotor. 5 g (0.025 mol) of 4,4′-diaminodiphenyl ether and 0.04 g (0.00043 mol) of aniline were dissolved in 50 g of N,N′-dimethylacetamide and then added to the three-necked flask. N2 was passed through the flask and stirred at room temperature for 24 h to obtain a viscous solution of single-terminated amino polyamic acid (PAA-NH2, molecular weight 24400) (containing 0.00043 mol of PAA-NH2). The structural formula of PAA-NH2 is as follows:

[0076] m=59;The reaction process of this step is:

[0077]

[0078] (4) 80.55 g of the N,N′-dimethylformamide solution of PLLA-NCO prepared in step (2) (containing 0.00043 mol PLLA-NCO) was taken out and slowly added to the viscous single-terminal amino polyamic acid (PAA-NH2) solution (containing 0.00043 mol PAA-NH2) in the three-necked flask in step (3), and stirred at 80° C. for 2 h to obtain a viscous block copolymer (molecular weight 37700) solution of single-terminal amino polyamic acid and single-terminal isocyanate-based L-polylactic acid. The structural formula of the block copolymer is:

[0079]

[0080] n=92,m=59;The reaction process of this step is:

[0081]

[0082] The viscous block copolymer solution of single-terminal amino polyamic acid and single-terminal isocyanate-based L-polylactic acid was cooled to room temperature and coated on a clean glass plate. The solution was then placed in a forced air drying oven and heated at 60°C for 1 hour and then at 120°C for 1 hour to volatilize the solvent N,N′-dimethylformamide in the block copolymer solution, thereby initiating the self-assembly of the block copolymer to form an ordered helical tetrahedral microstructure. The film was then peeled off from the glass plate (the transmission electron microscope image of the film at this time is as shown in FIG. 1 ). Figure 1 As shown), the film was placed in a muffle furnace and heat-treated at 100°C for 1h → 200°C for 1h → 300°C for 1h → 350°C for 2h to cause an imidization reaction of the block copolymer of the single-terminal amino polyamic acid and the single-terminal isocyanate-based L-polylactic acid. At the same time, the L-polylactic acid component was cleaved to obtain a nanoporous PI film with a helical tetrahedral structure (molecular weight of 22280). The structural formula is m=59;The reaction process of this step is:

[0083]

[0084] The scanning electron microscope image of the nanoporous PI film with helical tetrahedral structure is shown in Figure 2. Figure 2 As shown. Figure 1 and Figure 2It can be seen that the block copolymer of single-terminal amino polyamic acid and single-terminal isocyanate-based L-polylactic acid self-assembles into a nanostructure with ordered helical icosahedral features before heat treatment, in which the light-colored part is L-polylactic acid with a size of 30 to 40 nm. After heat treatment to remove the L-polylactic acid, a helical icosahedral nanoporous structure is obtained with a pore size of about 58 nm. The thermal weight loss curve of the film in air atmosphere before heat treatment is shown in Figure 2. Figure 3 As shown by Figure 3 It can be seen that the L-polylactic acid in the block copolymer of single-terminal amino polyamic acid and single-terminal isocyanate L-polylactic acid begins to decompose at 246°C and is completely decomposed at 544°C, with a decomposition mass percentage of about 36.93%.

[0085] Example 2

[0086] (1) 8.88 g (0.02 mol) of hexafluorodianhydride was dissolved in 50 g of N,N′-dimethylformamide and poured into a 250 mL three-necked flask with a magnetic rotor. 4 g (0.02 mol) of 3,4′-diaminodiphenyl ether and 0.042 g (0.00034 mol) of p-aminobenzyl alcohol were dissolved in 30 g of N,N′-dimethylformamide and then added to the three-necked flask. After stirring at room temperature for 24 h, 1.48 g (0.01 mol) of phthalic anhydride was added to the reaction system and continued to stir at room temperature for 24 h to obtain a viscous solution. 10 g of xylene was added to the three-necked flask and refluxed at 180°C using an oil-water separator until anhydrous product was generated. The product was precipitated with ethanol and then vacuum dried to obtain 12.08 g of single-terminal hydroxymethyl polyimide (PI-CH2OH, molecular weight 35900), the structural formula of which is as follows: n = 59;

[0087] The reaction process of this step is:

[0088]

[0089] (2) 0.012 g (0.03 mmol) of stannous octoate and 3.59 g (0.0001 mol) of PI-CH2OH prepared in step (1) were dissolved in 100 g of N,N′-dimethylformamide, added to a 250 mL polymerization bottle with a magnetic rotor, passed through dry N2 at room temperature, stirred for 12 h, added 2.19 g (0.0152 mol) of L-lactide, stirred evenly, and subjected to three cycles of liquid nitrogen freezing → vacuum → passing N2 → thawing. The reaction was then carried out at 110°C for 12 h. The product was precipitated with methanol and vacuum dried to obtain 5.48 g of a block copolymer of single-end hydroxymethyl polyimide and L-polylactic acid (molecular weight 56050, wherein the molecular weight of the L-polylactic acid part is 20150) solid. The structural formula of the block copolymer is as follows:

[0090] n=59,m=140;The reaction process of this step is:

[0091]

[0092] (3) The block copolymer solid of single-terminal hydroxymethyl polyimide and L-polylactic acid prepared in step (2) was dissolved in 50 g of N, N′-dimethylformamide, poured into a 250 mL three-necked flask with a magnetic rotor, and stirred at room temperature until dissolved to obtain a viscous solution of the block copolymer of single-terminal hydroxymethyl polyimide and L-polylactic acid. The solution was coated on a clean and dry glass plate, placed in a blast drying oven, and heated and dried at 60° C. for 1 h, and then heated and dried at 120° C. for 1 h to volatilize the solvent N, N′-dimethylformamide in the block copolymer solution, thereby inducing the self-assembly of the block copolymer to form an ordered spiral tetrahedral microstructure. The film was peeled off from the glass plate (at this time, its transmission electron microscope image is as shown in FIG. Figure 4 As shown), placed in a muffle furnace, and heat treated at 100℃ for 1h→200℃ for 1h→300℃ for 1h→350℃ for 2h to crack the L-polylactic acid component in the block copolymer of single-end hydroxymethyl polyimide and L-polylactic acid to obtain a nanoporous PI film with a helical tetrahedral structure (molecular weight of 35900), the structural formula of which is n=59; the reaction process of this step is:

[0093]

[0094] The scanning electron microscope image of the nanoporous PI film with helical tetrahedral structure is shown in Figure 2. Figure 5 As shown. Figure 4 and Figure 5 It can be seen that the block copolymer of single-terminal hydroxymethyl polyimide and L-polylactic acid self-assembles into a nanostructure with ordered helical icosahedral features before heat treatment, in which the light-colored part is L-polylactic acid with a size of about 30nm. After heat treatment to remove L-polylactic acid, a helical icosahedral nanoporous structure with a pore size of about 85nm is obtained. The thermal weight loss curve of the film in air atmosphere before heat treatment is shown in Figure 2. Figure 6 As shown by Figure 6 It can be seen that the L-polylactic acid in the block copolymer of single-end hydroxymethyl polyimide and L-polylactic acid begins to decompose at 248°C and is completely decomposed at 556°C, with a decomposition mass percentage of about 38.74%.

[0095] Example 3

[0096] (1) 8.88 g (0.02 mol) of hexafluorodianhydride was dissolved in 45 g of N,N′-dimethylacetamide and poured into a 250 mL three-necked flask with a magnetic rotor. 2.16 g (0.02 mol) of m-phenylenediamine and 0.044 g (0.00045 mol) of maleic anhydride were dissolved in 50 g of N,N′-dimethylacetamide and then added to the three-necked flask. After stirring at room temperature for 24 h under N2, 1.86 g (0.02 mol) of aniline was added. Stirring at room temperature for 24 h under N2 was continued to obtain a viscous solution. 10 g of xylene was added to the three-necked flask and refluxed at 180°C using an oil-water separator until anhydrous solution was generated. The product was precipitated with methanol and then vacuum dried to obtain 9.9 g of a single-ended maleimide polyimide (molecular weight 23100) solid, the structural formula of which is as follows:

[0097] n=45;The reaction process of this step is:

[0098]

[0099] (2) 0.098 g (0.001 mol) of furanol was dissolved in 50 g of anhydrous toluene and added to a 250 mL polymerization bottle with a magnetic rotor. Dry nitrogen was passed through the bottle at room temperature for protection. 2 ml of a 0.94 M triethylaluminum toluene solution was injected with a syringe and stirred at room temperature for 12 h. 14.4 g (0.1 mol) of L-lactide was dissolved in 50 g of toluene and injected into the polymerization bottle with a syringe and stirred evenly. The mixture was frozen with liquid nitrogen → vacuum → passed through nitrogen → thawed three times and then reacted at 110 ° C for 12 h. The product was precipitated with methanol and dried in vacuum to obtain 12.95 g of single-end furanyl L-polylactic acid (molecular weight 13000), the structural formula of which is as follows: m=91;The reaction process of this step is:

[0100]

[0101] (3) 3.75 g (0.00016 mol) of the single-ended maleimide polyimide solid prepared in step (1) was dissolved in 50 g of N, N′-dimethylformamide and poured into a 250 mL three-necked flask with a magnetic rotor. 2.11 g (0.00016 mol) of the single-ended furanyl-L-polylactide prepared in step (2) was dissolved in 50 g of N, N′-dimethylformamide and then added to the three-necked flask. The mixture was stirred at 55° C. for 24 h to obtain a viscous block copolymer (molecular weight 36100) solution of the single-ended maleimide polyimide and the single-ended furanyl-L-polylactic acid. The structural formula of the block copolymer is:

[0102] n = 45, m = 91;

[0103] The reaction process of this step is:

[0104]

[0105] The viscous block copolymer solution of single-terminal maleimide polyimide and single-terminal furanyl L-polylactic acid was cooled to room temperature and then coated on a clean and dry glass plate. The solution was placed in a blast drying oven and heated to 60°C for 1 hour and then heated to 120°C for 1 hour to volatilize the solvent N,N′-dimethylformamide in the block copolymer solution, thereby inducing the self-assembly of the block copolymer to form an ordered helical tetrahedral microstructure. The film was peeled off from the glass plate (the transmission electron microscope image of the film at this time is as shown in FIG. Figure 7 The obtained product was placed in a muffle furnace and heat treated at 100°C for 1 hour → 200°C for 1 hour → 300°C for 1 hour → 350°C for 2 hours to cleave the L-polylactic acid component in the block copolymer of single-end maleimide polyimide and single-end furanyl L-polylactic acid to obtain a nanoporous PI film with a helical tetrahedral structure (molecular weight of 23100), the structural formula of which is:

[0106] n=45;The reaction process of this step is:

[0107]

[0108] The scanning electron microscope image of the nanoporous PI film with helical tetrahedral structure is shown in Figure 2. Figure 8 As shown. Figure 7 and Figure 8 It can be seen that the block copolymer of single-terminal maleimide polyimide and single-terminal furanyl L-polylactic acid self-assembles into a nanostructure with ordered helical icosahedral features before heat treatment, in which the light-colored part is L-polylactic acid with a size of about 30nm. After heat treatment to remove L-polylactic acid, a helical icosahedral nanoporous structure with a pore size of about 51nm is obtained. The thermal weight loss curve of the film in air atmosphere before heat treatment is shown in Figure 2. Figure 9 As shown by Figure 9 It can be seen that the L-polylactic acid in the block copolymer of single-end maleimide polyimide and single-end furanyl L-polylactic acid begins to decompose at 251° C. and is completely decomposed at 548° C., with a decomposition mass percentage of about 38.20%.

[0109] Comparative Example 1

[0110] (1) 0.12 g (0.0003 mol) of stannous octoate and 0.108 g (0.001 mol) of benzyl alcohol were dissolved in 100 g of N,N′-dimethylformamide and added to a 250 mL polymerization bottle with a magnetic rotor. Dry nitrogen was passed through the bottle at room temperature and stirred for 12 h. 10.66 g (0.074 mol) of L-lactide was added and stirred evenly. The mixture was frozen with liquid nitrogen → vacuum → passed through nitrogen → thawed three times and then reacted at 110°C for 12 h. The product was precipitated with methanol and dried in vacuum to obtain 9.58 g of PLLA-OH (molecular weight 9590).

[0111] (2) 0.25 g (0.001 mol) of MDI was dissolved in 75 g of N,N′-dimethylformamide and poured into a 250 mL three-necked flask with a magnetic rotor; 9.58 g of PLLA-OH prepared in step (1) was dissolved in 100 g of N,N′-dimethylformamide and slowly added dropwise to the above three-necked flask, and stirred at 80° C. for 4 h to obtain a N,N′-dimethylformamide solution of PLLA-NCO (molecular weight 9840) (containing 0.001 mol of PLLA-NCO).

[0112] (3) Dissolve 5.45 g (0.025 mol) of pyromellitic dianhydride in 50 g of N,N′-dimethylformamide and pour the solution into a 250 mL three-necked flask equipped with a magnetic rotor. Dissolve 5 g (0.025 mol) of 4,4′-diaminodiphenyl ether and 0.04 g (0.00043 mol) of aniline in 50 g of N,N′-dimethylacetamide and add the solution to the three-necked flask. Pass nitrogen through the flask and stir at room temperature for 24 h to obtain a viscous PAA-NH2 (molecular weight 24400) solution (containing 0.00043 mol of PAA-NH2).

[0113] (4) 79.40 g of the N,N′-dimethylformamide solution of PLLA-NCO prepared in step (2) (containing 0.00043 mol PLLA-NCO) was taken out and slowly added to the PAA-NH2 solution (containing 0.00043 mol PAA-NH2) in the three-necked flask in step (3), and stirred at 80°C for 2 h to obtain a viscous block copolymer (molecular weight of 34240) of single-terminal amino polyamic acid and single-terminal isocyanate-based L-polylactic acid; the obtained block copolymer solution was cooled to room temperature and then coated on a clean and dry glass plate, placed in a blast drying oven, heated and dried at 60°C for 1 h, and then heated and dried at 120°C for 1 h, and the film was peeled off from the glass plate (the transmission electron microscope image at this time is as shown in FIG. Figure 10 As shown), it was placed in a muffle furnace and heat treated at 100℃ for 1h→200℃ for 1h→300℃ for 1h→350℃ for 2h to obtain a porous PI film (molecular weight of 22280). The scanning electron microscope image is shown as follows: Figure 11 As shown. Figure 10 and Figure 11 It can be seen that the block copolymer of single-terminal amino polyamic acid and single-terminal isocyanate-group L-polylactic acid self-assembles into independent circular nanostructures before heat treatment, of which the light-colored part is L-polylactic acid with a size of 30 to 50 nm. After heat treatment to remove L-polylactic acid, the circular nanopore structure is retained with a pore size of about 37 nm. The thermal weight loss curve of the film in air atmosphere before heat treatment is shown in Figure 2. Figure 12 As shown by Figure 12 It can be seen that the L-polylactic acid in the block copolymer of single-terminal amino polyamic acid and single-terminal isocyanate L-polylactic acid begins to decompose at 247°C and is completely decomposed at 550°C, with a decomposition mass percentage of about 28.86%.

[0114] Comparative Example 2

[0115] (1) 8.88 g (0.02 mol) of hexafluorodianhydride was dissolved in 50 g of N,N′-dimethylformamide and poured into a 250 mL three-necked flask with a magnetic rotor. 4 g of 3,4′-diaminodiphenyl ether (0.02 mol) and 0.042 g of p-aminobenzyl alcohol (0.00034 mol) were dissolved in 30 g of N,N′-dimethylformamide and then added to the three-necked flask. After stirring at room temperature for 24 h under N2, 1.48 g of phthalic anhydride (0.01 mol) was added to the reaction system and continued to stir at room temperature for 24 h to obtain a viscous solution. 10 g of xylene was added to the three-necked flask and refluxed at 180°C using an oil-water separator until anhydrous product was generated. The product was precipitated with ethanol and then dried in vacuo to obtain 12.08 g of PI-CH2OH (molecular weight 35900).

[0116] (2) 0.012 g (0.03 mmol) of stannous octoate and 3.59 g (0.0001 mol) of PI-CH2OH prepared in step (1) were dissolved in 100 g of N,N′-dimethylformamide, added to a 250 mL polymerization bottle with a magnetic rotor, passed through dry N2 at room temperature, stirred for 12 h, 1.40 g (0.0097 mol) of L-lactide was added, stirred evenly, and subjected to three cycles of liquid nitrogen freezing → vacuum → passing N2 → thawing, and then reacted at 110°C for 12 h. The product was precipitated with methanol and vacuum dried to obtain 4.77 g of a block copolymer of single-end hydroxymethyl polyimide and L-polylactic acid (molecular weight 48470, of which the molecular weight of the L-polylactic acid part is 12570) solid.

[0117] (3) The block copolymer solid of single-terminal hydroxymethyl polyimide and L-polylactic acid prepared in step (2) was dissolved in 50 g of N,N′-dimethylformamide, poured into a 250 mL three-necked flask with a magnetic rotor, and stirred at room temperature until dissolved to obtain a viscous solution of the block copolymer of single-terminal hydroxymethyl polyimide and L-polylactic acid. The solution was coated on a clean and dry glass plate, placed in a blast drying oven, and heated and dried at 60° C. for 1 h, then heated and dried at 120° C. for 1 h, and the film was peeled off from the glass plate (the transmission electron microscope image of the solution at this time is as shown in FIG. 1 ). Figure 13 As shown), it was placed in a muffle furnace and heat treated at 100℃ for 1h→200℃ for 1h→300℃ for 1h→350℃ for 2h to obtain a porous PI film (molecular weight of 35900). The scanning electron microscope image is shown as follows: Figure 14 As shown. Figure 13 and Figure 14 It can be seen that the block copolymer of single-terminal hydroxymethyl polyimide and L-polylactic acid self-assembles into independent irregular nanostructures before heat treatment, in which the light-colored part is L-polylactic acid with a size of 50-65nm. After heat treatment to remove L-polylactic acid, a circular nanopore structure is formed with a pore size of about 42nm. The thermal weight loss curve of the film in air atmosphere before heat treatment is shown in Figure 2. Figure 15 As shown by Figure 15 It can be seen that the L-polylactic acid in the block copolymer of single-terminal hydroxymethyl polyimide and L-polylactic acid begins to decompose at 250°C and is completely decomposed at 546°C, with a decomposition mass percentage of about 33.26%.

[0118] Comparative Example 3

[0119] (1) 8.88 g (0.02 mol) of hexafluorodianhydride was dissolved in 45 g of N,N′-dimethylacetamide and poured into a 250 mL three-necked flask with a magnetic rotor. 2.16 g of m-phenylenediamine (0.02 mol) and 0.044 g of maleic anhydride (0.00045 mol) were dissolved in 50 g of N,N′-dimethylacetamide and then added to the three-necked flask. After stirring at room temperature for 24 h under N2, 1.86 g of aniline (0.02 mol) was added. Stirring at room temperature for 24 h under N2 was continued to obtain a viscous solution. 10 g of xylene was added to the three-necked flask and refluxed at 180°C using an oil-water separator until anhydrous product was generated. The product was precipitated with methanol and then vacuum dried to obtain 9.9 g of single-end maleimide polyimide (molecular weight 23100) as a solid.

[0120] (2) 0.098 g (0.001 mol) of furanol was dissolved in 50 g of anhydrous toluene and added to a 250 mL polymerization bottle with a magnetic rotor. Dry nitrogen was passed through the bottle at room temperature for protection. 2 ml of 0.94 M triethylaluminum toluene solution was injected with a syringe and stirred at room temperature for 12 h. 8.64 g (0.06 mol) of L-lactide was dissolved in 50 g of toluene and injected into the polymerization bottle with a syringe and stirred evenly. The mixture was frozen with liquid nitrogen → vacuum → passed through nitrogen → thawed three times and then reacted at 110 ° C for 12 h. The product was precipitated with methanol and dried in vacuum to obtain 7.75 g of single-end furanyl L-polylactic acid (molecular weight 7700).

[0121] (3) 3.75 g (0.00016 mol) of the single-end maleimide polyimide solid prepared in step (1) was dissolved in 50 g of N, N′-dimethylformamide and poured into a 250 mL three-necked flask with a magnetic rotor. 1.25 g (0.00016 mol) of the single-end furanyl levorotatory polylactide prepared in step (2) was dissolved in 50 g of N, N′-dimethylformamide and then added to the three-necked flask and stirred at 55° C. for 24 h to obtain a viscous block copolymer (molecular weight of 30800) solution of the single-end maleimide polyimide and the single-end furanyl levorotatory polylactic acid. The obtained block copolymer solution was cooled to room temperature and then coated on a clean and dry glass plate. The film was placed in a blast drying oven and heated and dried at 60° C. for 1 h and then at 120° C. for 1 h. The film was peeled off from the glass plate (the transmission electron microscope image of the film at this time is as shown in FIG. 2 ). Figure 16 As shown), it was placed in a muffle furnace and heat treated at 100℃ for 1h→200℃ for 1h→300℃ for 1h→350℃ for 2h to obtain a porous PI film. The scanning electron microscope image is shown in FIG. Figure 17 As shown. Figure 16 and Figure 17 It can be seen that the block copolymer of single-end maleimide polyimide and single-end furanyl L-polylactic acid self-assembles into independent irregular spindle-shaped nanostructures before thermal decomposition, of which the light-colored part is L-polylactic acid with a size of about 30nm×80nm. After heating to remove the L-polylactic acid, a circular nanopore structure with a pore size of about 59nm is formed. The thermal weight loss curve of the film in air atmosphere before heat treatment is shown in Figure 2. Figure 18 As shown by Figure 18 It can be seen that the L-polylactic acid in the block copolymer of single-end maleimide polyimide and single-end furanyl L-polylactic acid begins to decompose at 247°C and is completely decomposed at 552°C, with a decomposition mass percentage of about 28.52%.

[0122] Effect verification

[0123] The physical properties of the PI porous films prepared in Examples 1-3 of the present invention and Comparative Examples 1-3 are shown in Table 1:

[0124] Table 1

[0125]

[0126] As shown in Table 1, the nanoporous PI films with helical tetrahedral structures prepared in Examples 1-3 of the present invention have high pore size uniformity, excellent three-dimensional continuous penetration, uniform thickness, and good mechanical properties.

[0127] Test of adsorption performance of PI porous film for water and toluene

[0128] The porous PI films prepared in Examples 1-3 and Comparative Examples 1-3 were immersed in water and toluene at 25°C until adsorption equilibrium was reached, and the absorption percentage was measured. The adsorption equilibrium diagram of the porous PI films prepared in Examples 1-3 and Comparative Examples 1-3 for water and toluene is shown in FIG. Figure 13 As shown by Figure 13 It can be seen that the absorption percentage of water and toluene by the nanoporous PI film with a spiral icosahedral structure prepared in Examples 1-3 is significantly higher than that of the porous PI film prepared in Comparative Documents 1-3, proving that the nanoporous PI film with a spiral icosahedral structure prepared in Examples 1-3 has a larger porosity.

[0129] The embodiments described above are merely descriptions of preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Without departing from the spirit of the present invention, various modifications and improvements made to the technical solutions of the present invention by persons skilled in the art should fall within the scope of protection defined by the claims of the present invention.

Claims

1. A method for preparing a nanoporous polyimide film with a helical tetrahedral structure, characterized in that: The block copolymer is coated to allow the block copolymer to self-assemble to form a film having an ordered helical icosahedral microstructure, and then the film having the ordered helical icosahedral microstructure is heat-treated to obtain the helical icosahedral structure nanoporous polyimide film; wherein the block copolymer is a block copolymer of polyamic acid and degradable polyester, or a block copolymer of polyimide and degradable polyester; The block copolymer of polyamic acid and degradable polyester is a block copolymer of single-terminal amino polyamic acid and single-terminal isocyanate-based L-polylactic acid; the block copolymer of polyimide and degradable polyester is a block copolymer of single-terminal hydroxymethyl polyimide and L-polylactic acid, or a block copolymer of single-terminal maleimide polyimide and single-terminal furyl-based L-polylactic acid; The structural formula of the block copolymer of the single-terminal amino polyamic acid and the single-terminal isocyanate-based L-polylactic acid is: Wherein, n=70~210, m=45~135; The structural formula of the block copolymer of single-terminal hydroxymethyl polyimide and L-polylactic acid is , where n = 59 to 87, m = 140 to 210; The structural formula of the block copolymer of the single-end maleimide polyimide and the single-end furanyl L-polylactic acid is Among them, n=33~66, m=70~140.

2. The method for preparing the nanoporous polyimide film with a helical icosahedral structure according to claim 1, wherein: The preparation method of the block copolymer of the single-terminal amino polyamic acid and the single-terminal isocyanate-based L-polylactic acid is as follows: the single-terminal isocyanate-based L-polylactic acid and the single-terminal amino polyamic acid are subjected to a graft reaction; the structural formula of the single-terminal isocyanate-based L-polylactic acid is Wherein, n=70-210; the structural formula of the single-terminated amino polyamic acid is Among them, m=45~135.

3. The method for preparing the nanoporous polyimide film with a helical icosahedral structure according to claim 1, wherein: The preparation method of the block copolymer of the single-terminal hydroxymethyl polyimide and L-polylactic acid is as follows: dissolving a catalyst and the single-terminal hydroxymethyl polyimide in a solvent, adding L-lactide, stirring evenly, freezing with liquid nitrogen → vacuuming → passing N2 → thawing, and then heating for reaction to obtain the block copolymer of the single-terminal hydroxymethyl polyimide and L-polylactic acid; the structural formula of the single-terminal hydroxymethyl polyimide is Among them, n=59~87.

4. The method for preparing a nanoporous polyimide film having a helical icosahedral structure according to claim 1, wherein: The preparation method of the block copolymer of the single-end maleimide polyimide and the single-end furanyl L-polylactic acid is as follows: single-end furanyl L-polylactide and single-end maleimide polyimide are subjected to graft reaction; the structural formula of the single-end furanyl L-polylactide is Wherein, m=70-140; the structural formula of the single-ended maleimide polyimide is Among them, n=33~66.

5. The method for preparing the nanoporous polyimide film with a helical icosahedral structure according to claim 1, wherein: The process of coating the block copolymer to allow the block copolymer to self-assemble to form a film with an ordered helical tetrahedron microstructure comprises coating the block copolymer in the form of a solution and initiating the self-assembly of the block copolymer by heating to volatilize the solvent.

6. The method for preparing a nanoporous polyimide film having a helical icosahedral structure according to claim 5, wherein: The solvent is N,N ′ -dimethylacetamide, N-methylpyrrolidone or gamma-butyrolactone.

7. The method for preparing a nanoporous polyimide film having a helical icosahedral structure according to claim 1, wherein: The heat treatment procedure is 100°C for 1 hour → 200°C for 1 hour → 300°C for 1 hour → 350°C for 2 hours.

8. A nanoporous polyimide film with a helical icosahedral structure prepared according to the method for preparing a nanoporous polyimide film with a helical icosahedral structure according to any one of claims 1 to 7.

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