Phenolphthalein-based polybenzoxazole membrane as well as preparation method and application thereof

Abstract

The invention relates to the technical field of polymeric membrane separation, in particular to a phenolphthalein-based polybenzoxazole membrane as well as a preparation method and application thereof. The invention provides a phenolphthalein-based polybenzoxazole membrane which has a structural formula as shown in a formula I. A polymer is synthesized by adopting a one-step method, esterification crosslinking and thermal rearrangement methods, phenolphthalein-based diamine is selected as one of diamine monomers, phenolphthalein-based diamine contains a large number of benzene rings, and the thermal stability of the polymer can be improved; the monomer contains a-COOH group and can be crosslinked through an ester group, so that the polyimide film keeps good mechanical properties under the condition that the plasticizing resistance is improved; meanwhile, the monomer also contains phenolic hydroxyl groups, so that a thermal rearrangement reaction can be further carried out after esterification crosslinking of a polyimide film, and a polybenzoxazole film with a more distorted and more rigid structure is formed.

Description

Technical Field

[0001] This invention relates to the field of polymer membrane separation technology, and in particular to a phenolphthalein-based polybenzoxazole membrane, its preparation method, and its application. Background Technology

[0002] Polyimide and benzoxazole membranes are widely used in gas separation due to their high temperature resistance, solvent resistance, and excellent mechanical properties. However, traditional polyimide membranes are prone to segmental plasticization under high-pressure CO2 environments, which increases the permeability of the membrane material but decreases its selectivity, seriously affecting the stable operation of the membrane module.

[0003] To suppress plasticization, researchers often use diamines, diols, or trifunctional crosslinking methods. However, a large number of crosslinking points often lead to embrittlement of the membrane material and a significant decrease in mechanical properties, which is not conducive to the preparation of spiral wound membrane modules.

[0004] Phenolic phthaloline monomers, as rigid non-coplanar monomers, can significantly improve the conformational stability of polymer segments and synergistically enhance anti-plasticization ability with polyimide structures. Moreover, this monomer can undergo monoesterification crosslinking, which is milder than traditional crosslinking methods and can form stable crosslinking points without significantly reducing the flexibility of the chain segments, thereby obtaining a combination of "high anti-plasticization + high toughness".

[0005] Furthermore, phenolphthaloline-based polyimide can undergo thermal rearrangement (TR) at high temperatures, transforming into a PBO structure rich in free volume, which further enhances gas permeability. Therefore, developing a high-performance PBO membrane that can be prepared using a composite strategy of esterification crosslinking and thermal rearrangement is of great significance. Summary of the Invention

[0006] The purpose of this invention is to provide a phenolphthalein-based polybenzoxazole membrane, its preparation method, and its application, in order to solve the problems of unsatisfactory gas separation performance, anti-plasticization performance, and difficulty in maintaining mechanical properties of polyimide gas separation membrane materials.

[0007] To achieve the above-mentioned objectives, the present invention provides the following technical solution: This invention provides a phenolphthaloline-based polybenzoxazole membrane, the structural formula of which is shown in Formula I: Formula I; Where R1 is a C1-C6 alkylene group or a structural group. ; In the structure, x ranges from 4 to 45; R2, R3, R4, R5, R6, R7, R8, R9, R 10 Each is independently H or C1-C4 alkyl; R11 Each is independently a C1-C4 alkyl group; m1, m2, n1, and n2 represent the number of repeating units, m1 is 20~50, m2 is 20~500, n1 is 0 or 20~500, n2 is 0 or 20~500; s is 0~4; Ar is the linking unit between the anhydride groups of the dianhydride monomer, and each Ar may be the same or different.

[0008] The second technical solution of the present invention provides a method for preparing the above-mentioned phenolphthalein-based polybenzoxazole membrane, comprising the following steps: (1) Under an inert atmosphere, diamine monomer, dianhydride monomer and solvent 1 are mixed and polycondensation reaction is carried out. After the reaction is completed, imidization reaction is carried out to obtain polyimide solution. (2) Under an inert atmosphere, the polyimide solution, crosslinking agent and catalyst are mixed and subjected to a monoesterification reaction to obtain a monoesterified polyimide solution; (3) Mix the monoester polyimide solution and solvent 2, dry to form a film, and then heat-treat the film under a vacuum atmosphere to obtain a monoesterified crosslinked film. (4) Under a nitrogen or argon atmosphere, the monoesterified crosslinked membrane is subjected to a thermal rearrangement reaction to obtain a phenolphthalein-based polybenzoxazole membrane.

[0009] The third technical solution of the present invention provides a gas separation component comprising the above-mentioned phenolphthalein-based polybenzoxazole membrane.

[0010] Compared with the prior art, the present invention has the following beneficial effects: This invention provides a phenolphthalein-based polybenzoxazole membrane. Phenolphthalein, as a rigid non-coplanar monomer, significantly improves the conformational stability of polymer segments and synergistically enhances anti-plasticization ability with the polyimide structure. Furthermore, the phenolphthalein group contains a large number of benzene rings, giving the polymer excellent thermal stability. Simultaneously, the phenolphthalein monomer contains carboxyl groups, allowing for esterification and crosslinking. This achieves an excellent three-dimensional network structure while avoiding damage to the polymer membrane structure, maintaining the good mechanical properties of the polyimide gas separation membrane and improving its anti-plasticization performance. These monomers also contain phenolic hydroxyl groups, meaning the polyimide is an ortho-hydroxy polyimide, which can undergo further thermal rearrangement after esterification and crosslinking of the polyimide membrane, forming a more twisted and rigid polybenzoxazole membrane. In addition, copolymerizing a benzyl diamine-containing compound onto the phenolphthalein-based polyimide can further improve the gas separation performance of homopolymer polyimide. This invention improves the gas permeability and selectivity of the PBO membrane, enhances the anti-plasticization performance of the polymer membrane, and maintains its mechanical properties. Attached Figure Description

[0011] Figure 1This is a flowchart of the reaction process in Example 1 of the present invention, in which diamine monomer and dianhydride monomer undergo polycondensation polymerization, followed by monoesterification reaction with diol crosslinking agent to synthesize monoesterified polyimide.

[0012] Figure 2 This is a reaction mechanism diagram of the cross-linking of monoesterified polyimide chains under vacuum and thermal rearrangement under nitrogen atmosphere in Example 1 of the present invention.

[0013] Figure 3 The image shows the 1H-NMR spectrum (DMSO-d6) of phenolphthaloline diamine 2-(di(3-amino-4-hydroxyphenyl)methyl)benzoic acid (AHPBA) in Example 1 of this invention.

[0014] Figure 4 This is a comparison of the infrared spectra of the original un-monoesterified cross-linked polyimide film and the monoesterified cross-linked polyimide film in Example 1 of this application.

[0015] Figure 5 This is a comparison chart of the anti-plasticization properties of the original non-monoesterified crosslinked polyimide film and the monoesterified crosslinked polyimide film in Example 1 of this application. Detailed Implementation

[0016] Various exemplary embodiments of the present invention will now be described in detail. This detailed description should not be considered as a limitation of the present invention, but rather as a more detailed description of certain aspects, features, and embodiments of the present invention.

[0017] It should be understood that the terminology used in this invention is merely for describing particular embodiments and is not intended to limit the invention. Furthermore, with respect to numerical ranges in this invention, it should be understood that each intermediate value between the upper and lower limits of the range is also specifically disclosed. Every smaller range between any stated value or intermediate value within a stated range, and any other stated value or intermediate value within said range, is also included in this invention. The upper and lower limits of these smaller ranges may be independently included or excluded from the range.

[0018] Unless otherwise stated, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art. While only preferred methods and materials have been described herein, any methods and materials similar or equivalent to those described herein may be used in the implementation or testing of this invention. All references to this specification are incorporated by way of citation to disclose and describe methods and / or materials associated with those references. In the event of any conflict with any incorporated reference, the content of this specification shall prevail.

[0019] Various modifications and variations can be made to the specific embodiments described in this specification without departing from the scope or spirit of the invention, as will be apparent to those skilled in the art. Other embodiments derived from this specification will also be obvious to those skilled in the art. This application specification and embodiments are merely exemplary.

[0020] The terms “include,” “including,” “have,” “contain,” etc., used in this article are all open-ended terms, meaning that they include but are not limited to.

[0021] All raw materials used in this invention can be obtained commercially or prepared using existing technologies.

[0022] The room temperature involved in this invention is 25±2℃.

[0023] This invention provides a phenolphthaloline-based polybenzoxazole membrane, the structural formula of which is shown in Formula I: Formula I; Where R1 is a C1-C6 alkylene group or a structural group. ; In the structure, x ranges from 4 to 45; R2, R3, R4, R5, R6, R7, R8, R9, R 10 Each is independently H or C1-C4 alkyl; R 11 Each is independently a C1-C4 alkyl group; m1, m2, n1, and n2 represent the number of repeating units, m1 is 20~50, m2 is 20~500, n1 is 0 or 20~500, n2 is 0 or 20~500; s is 0~4; Ar is the linking unit between the anhydride groups of the dianhydride monomer, and each Ar may be the same or different.

[0024] The present invention also provides a method for preparing the above-mentioned phenolphthalein-based polybenzoxazole membrane, characterized by comprising the following steps: (1) Under an inert atmosphere, diamine monomer, dianhydride monomer and solvent 1 are mixed and polycondensation reaction is carried out. After the reaction is completed, imidization reaction is carried out to obtain polyimide solution. (2) Under an inert atmosphere, the polyimide solution, crosslinking agent and catalyst are mixed and subjected to a monoesterification reaction to obtain a monoesterified polyimide solution; (3) Mix the monoester polyimide solution and solvent 2, dry to form a film, and then heat-treat the film under a vacuum atmosphere to obtain a monoesterified crosslinked film. (4) Under a nitrogen or argon atmosphere, the monoesterified crosslinked membrane is subjected to a thermal rearrangement reaction to obtain a phenolphthalein-based polybenzoxazole membrane.

[0025] This invention employs a one-step, esterification-crosslinking, and thermal rearrangement method to synthesize polymers. Phenolphthaloline diamine is selected as one of the diamine monomers. Its phenolphthaloline group contains a large number of benzene rings, which improves the polymer's thermal stability. The monomer contains -COOH groups, which can crosslink through esterification, allowing the polyimide membrane to maintain good mechanical properties while improving its resistance to plasticization. Simultaneously, this type of monomer also contains phenolic hydroxyl groups, which can further undergo a thermal rearrangement reaction after the polyimide membrane is esterified and crosslinked, forming a more twisted and rigid polybenzoxazole membrane. Furthermore, the low gas permeability of homopolymer phenolphthaloline polyimide can be further improved by adding copolybenzyl diamine.

[0026] In step (1) of this invention, diamine monomer and dianhydride monomer are dissolved together in solvent 1 under an inert atmosphere, and polycondensation reaction is carried out by continuous stirring under programmed temperature rise. Then, imidization reaction is carried out by continued stirring to obtain a high-viscosity polyimide solution.

[0027] In this invention, the inert atmosphere is nitrogen.

[0028] In this invention, the dianhydride monomer includes one or more of pyromellitic dianhydride (PMDA), trimellitic dianhydride (MPDA), biphenyl dianhydride (BPDA), hexafluoroisopropylphthalic anhydride (6FDA), benzophenone tetracarboxylic anhydride (BTDA), and oxobisphthalic anhydride (ODPA). The structures of each anhydride monomer are as follows: (1) PMDA (2) MPDA (3) BPDA (4) 6FDA (5) BTDA (6) ODPA.

[0029] In this invention, the diamine monomer is 3,3'-diaminophenolphthalein substituted with or unsubstituted with C1-C4 alkyl groups; The 3,3'-diaminophenolphthalein substituted or unsubstituted with C1-C4 alkyl groups includes any one of the following structures: .

[0030] In this invention, the diamine monomer is a C1-C4 alkyl-substituted or unsubstituted 3,3'-diaminophenolphthalein and a C1-C4 alkyl-substituted or unsubstituted copolybenzyldiamine; the molar ratio of the C1-C4 alkyl-substituted or unsubstituted 3,3'-diaminophenolphthalein and the C1-C4 alkyl-substituted or unsubstituted copolybenzyldiamine is 0.5 to 2:1, for example, it can be 0.5:1, 1:1, 1.56:1 or 2:1, etc. The C1-C4 alkyl-substituted or unsubstituted copolybenzyl diamines include 1,3-phenylenediamine, 2-methyl-1,3-phenylenediamine, 2,4-dimethyl-1,3-phenylenediamine, 2,4-dimethyl-1,5-phenylenediamine, 2,4,6-trimethyl-1,3-phenylenediamine, 2-methyl-4-ethyl-1,3-phenylenediamine, 4-methyl-2-ethyl-1,5-phenylenediamine, and diethyltoluenediamine; The copolybenzyl diamine substituted with or unsubstituted with C1-C4 alkyl groups comprises any one of the following structures: .

[0031] In this invention, the 3,3'-diaminophenolphthalein substituted with or unsubstituted with C1-C4 alkyl groups is selected from any one of the following: (i) In formula I, R5-R 10 All are H; (ii) In formula I, R6 = CH3, R 10 =CH3, R5, R7, R8, and R9 are all H; (iii) In formula I, R6 = CH3, R9 = CH3, R5, R7, R8, R 10 All are H; In this invention, when R5-R in formula I 10 When both are H, the 3,3'-diaminophenolphthalein is 2-(bis(3-amino-4-hydroxyphenyl)methyl)benzoic acid, with the structure shown below: .

[0032] In this invention, when R6 = CH3, R in formula I 10 When CH3, R5, R7, R8, and R9 are all H, the structure of the 3,3'-diaminophenolphthalein is as follows: .

[0033] In this invention, when R6 = CH3, R9 = CH3, R5, R7, R8, R... 10 When both are H, the structure of the 3,3'-diaminophenolphthalein is as follows: .

[0034] In this invention, when n1 and n2 are between 20 and 500, the copolybenzyl diamine is selected from any one of the following: (i) In equation I, R2=H, R3=H, and R4=H; (ii) In formula I, R2=CH3, R3=H, and R4=H; (iii) In formula I, R2=CH3, R3=CH3, R4=H or R2=H, R3=CH3, R4=CH3; (iv) In formula I, R2=CH3, R3=CH3, R4=CH3; (v) In formula I, R2=CH3, R3=CH2CH3, R4=H or R2=CH2CH3, R3=CH3, R4=H; (ⅵ) In formula I, R2=CH3, R3=CH2CH3, R4=CH2CH3 or R2=CH2CH3, R3=CH2CH3, R4=CH3.

[0035] In this invention, when R2=H, R3=H, and R4=H in Formula I, the structure of the copolybenzyldiamine is as follows: .

[0036] In this invention, when R2=CH3, R3=H, and R4=H in Formula I, the structure of the copolybenzyldiamine is as follows: .

[0037] In this invention, when R2=CH3, R3=CH3, R4=H or R2=H, R3=CH3, R4=CH3 in Formula I, the structure of the copolybenzyldiamine is as follows: or .

[0038] In this invention, when R2=CH3, R3=CH3, and R4=CH3 in Formula I, the structure of the copolybenzyldiamine is as follows: .

[0039] In this invention, when R2=CH3, R3=CH2CH3, R4=H or R2=CH2CH3, R3=CH3, R4=H in Formula I, the structure of the copolybenzyl diamine is as follows: or .

[0040] In this invention, when R2=CH3, R3=CH2CH3, R4=CH2CH3 or R2=CH2CH3, R3=CH2CH3, R4=CH3 in Formula I, the structure of the copolybenzyldiamine is as follows: or .

[0041] In this invention, the preparation method of the 3,3'-diaminophenolphthalein includes the following steps: S1. Phenolphthalein monomer, nitrifying agent and solvent 3 are mixed and reacted to obtain dinitro monomer; S2. The dinitro monomer, solvent 4 and catalyst are mixed and reacted under hydrogen to obtain 3,3'-diaminophenolphthalein.

[0042] This invention utilizes inexpensive and readily available phenolphthalein monomers, which are reduced to phenolphthalein morphology via ring-opening reduction of the lactone ring. The reaction process of nitration and ring-opening reduction of phenolphthalein monomers to generate phenolphthalein morphology monomers is shown in the following equation: .

[0043] In this invention, the nitrating agent is yttrium nitrate or sodium nitrate; the solvent 3 is acetic acid or sulfuric acid; the molar ratio of the nitrating agent to the phenolphthalein monomer is 0.5~5:1, for example, it can be 0.5:1, 1:1, 2:1, 3:1, 4:1 or 5:1, etc.; the reaction temperature in step S1 is 10~50℃, for example, it can be 10℃, 20℃, 30℃, 40℃ or 50℃, etc., and the reaction time is 2~8h, for example, it can be 2h, 4h, 6h or 8h, etc. The solvent 4 is an alcohol or ether; the catalyst is a transition metal catalyst; the transition metal catalyst is palladium on carbon, platinum on carbon, or Raney nickel; the amount of catalyst used is 2-20 wt% of the dinitro monomer, for example, it can be 2 wt%, 5 wt%, 10 wt%, 15 wt%, or 20 wt%; the reaction temperature in step S2 is 20-80℃, for example, it can be 20℃, 30℃, 40℃, 50℃, 60℃, 70℃, or 80℃.

[0044] In this invention, the molar ratio of the diamine monomer to the dianhydride monomer is 1:1; the concentration of the solution after mixing the diamine monomer, the dianhydride monomer, and solvent 1 is 20~25wt%, for example, it can be 20wt%, 22wt%, 24wt%, or 25wt%, etc. The polycondensation reaction is carried out at a temperature of 80-90°C for 1-2 hours; the imidization reaction is carried out at a temperature of 180°C for 8 hours.

[0045] In some embodiments of the present invention, isoquinoline that promotes imidization may be added during the imidization reaction; after the addition of isoquinoline, the imidization reaction is first carried out at 120°C for 1 hour, and then the temperature is further increased to 200°C for 7 hours; the amount of isoquinoline added is 1 mL / 30 g membrane based on the final mass of the film.

[0046] Step (2) of the present invention involves adding a crosslinking agent and a catalyst to a cooled polyimide solution under an inert atmosphere to carry out a monoesterification reaction, thereby obtaining a highly viscous monoesterified polyimide solution.

[0047] In this invention, the crosslinking agent includes at least one selected from 1,3-ethylene glycol, 1,3-propanediol, 1,3-butanediol, PEG-200, PEG-400, PEG-600, PEG-1000, and PEG-2000; the catalyst is p-toluenesulfonic acid. The mass ratio of the catalyst to the crosslinking agent is 0.001~0.0011:1; the mass ratio of the crosslinking agent to the dianhydride is 3~4:1. The temperature of the monoesterification reaction is 130~140℃, for example, 130℃, 135℃ or 140℃, and the time is 18~24h, for example, 18h, 20h, 22h or 24h.

[0048] Step (3) of this invention is to first pour the monoesterified polyimide solution into methanol to precipitate, crush the precipitate, filter and wash it to remove residual solvent, dissolve the obtained monoesterified polyimide in solvent 2, stir until completely dissolved, filter it with a 0.45~1 μm polytetrafluoroethylene filter membrane, inject the filtered solution into a flat glass petri dish for programmed temperature rise drying to form a film, and continue to perform heat treatment under vacuum atmosphere to induce crosslinking of the monoesterified polyimide membrane to obtain a monoesterified crosslinked membrane.

[0049] In this invention, the thickness of the film is 50~150 μm, preferably 90~100 μm.

[0050] In this invention, the drying process involves first drying at 50-60°C, for example, 50°C, 55°C, or 60°C, for 36-48 hours, for example, 36 hours, 40 hours, 42 hours, or 46 hours, and then further heating to 100°C and drying for 12 hours. The heat treatment employs a programmed temperature rise; the programmed temperature rise includes: The first stage: the heating rate is 4~6℃ / min, preferably 5℃ / min, the temperature is raised to 160℃, and the holding time is 3~12h, for example, 3h, 5h, 10h or 12h, etc. The second stage: the heating rate is 4~6℃ / min, preferably 5℃ / min, the temperature is raised to 180℃, and the holding time is 3~12h, for example, 3h, 5h, 10h or 12h.

[0051] Step (4) of this invention involves thermally rearranging the monoesterified crosslinked membrane under a nitrogen or argon atmosphere to obtain a phenolphthalein-based polybenzoxazole membrane.

[0052] In this invention, the heating rate of the thermal rearrangement reaction is 4~6℃ / min, the temperature is 400~450℃ (e.g., 400℃, 425℃, 450℃, etc.), and the time is 1~3h (e.g., 1h, 2h, or 3h, etc.); the purpose of the thermal rearrangement reaction is to remove CO2 from the polyimide chain segment to generate a benzoxazole (PBO) structure.

[0053] In this invention, the benzoxazole structure after thermal rearrangement accounts for 10-70% of the main chain structural units and forms free volume microcavities with microphase separation in the membrane material, thereby significantly improving the permeation performance of the membrane material for CO2 and H2.

[0054] In this invention, the thermal rearrangement process preserves the monoesterification crosslinking points, enabling the membrane material to maintain excellent crosslinking network stability even under high temperature and high pressure gas environments.

[0055] In this invention, solvent 1 is at least one of N-methyl-2-pyrrolidone, N,N-dimethylformamide, N,N-dimethylacetamide and m-methylphenol, preferably m-methylphenol; Solvent 1 is at least one of N-methyl-2-pyrrolidone, N,N-dimethylformamide, N,N-dimethylacetamide and m-methylphenol; preferably N,N-dimethylformamide.

[0056] The present invention also provides a gas separation component comprising the above-described phenolphthaloline-based polybenzoxazole membrane.

[0057] In this invention, compared with conventional diamine and trifunctional crosslinking methods, monoesterification crosslinking is a flexible crosslinking method, and the tensile strength of the resulting gas separation membrane material decreases by no more than 5%, exhibiting good toughness retention.

[0058] The technical solutions provided by the present invention will be described in detail below with reference to the embodiments, but they should not be construed as limiting the scope of protection of the present invention.

[0059] Preparation Example 1 (1) Add 4.71 mmol of phenolphthalein monomer and 4.70 mmol of yttrium nitrate to 45 mL of acetic acid, react fully at room temperature for 8 h, add about twice the volume of deionized water and refrigerate for 24 h, filter, wash with deionized water, filter and dry in a vacuum oven at 60 °C for later use to obtain dinitro monomer. (2) Dissolve 29.39 mmol of dinitro monomer in 24 mL of methanol, add 1.2 g of 10% Pd / C, and purge with nitrogen to remove air. Then purge with hydrogen to carry out the reaction. When the hydrogen pressure in the reactor drops from 1.5 MPa to 0.5 MPa, open the gas valve and purge with hydrogen until the pressure reaches 1.5 MPa. Then close the gas valve and wait for the reaction to proceed. Repeat this process until the pressure in the reactor stops decreasing, ensuring the temperature does not exceed 70°C. After the reaction, filter out the catalyst, cool to allow crystallization, wash and dry to obtain 3,3'-diaminophenolphthalein.

[0060] Example 1 (1) Take 46 mmol of 6 FDA, 18 mmol of 3,3'-diaminophenolphthalein and 28 mmol of diethyltoluenediamine (DETDA) and add them to a 500 ml four-necked flask. Then add 92 ml of m-methylphenol and stir. The solid content is 25 wt%. Under nitrogen protection, heat to 80 °C and react for 2 h. After the reaction is completed, heat to 180 °C and continue stirring for 8 h to form a viscous polyimide solution.

[0061] (2) After the polyimide solution cooled to room temperature, 70 g of 1,3-propanediol and 75 mg of p-toluenesulfonic acid were added. Under nitrogen protection, the temperature was raised to 130 °C, and the monoesterification reaction was stirred for 18 h to form a highly viscous monoesterified polyimide solution. This solution was diluted and poured into a methanol solution to precipitate the polyimide. The precipitate was washed with methanol during filtration to remove residual solvent. A gradient heating process was set: 60 °C for 48 h and 100 °C for 12 h to obtain 30 g of dried monoesterified polyimide. The reaction flowchart is as follows: Figure 1 As shown.

[0062] Unmonoesterified crosslinked polyimide synthesized by reacting 3,3'-diaminophenolphthalein with 6FDA and DETDA 1 H-NMR spectrum (DMSO-d6) as shown Figure 3 As shown. Regarding the 1H NMR spectrum of polyimide, it should be noted that the ortho-hydroxyl group of phenolphthalein diamine is retained during the one-step reaction, and can be clearly observed in the 1H NMR spectrum.

[0063] (3) Take 1.00g of dried monoesterified polyimide powder and dissolve it in N,N-dimethylformamide (DMF). Stir until completely dissolved to obtain a casting solution with a solid content of 8.0 wt%. Filter it with a 0.45 or 1 μm polytetrafluoroethylene filter membrane. Pour the filtered solution into a flat glass petri dish and then place it in a vacuum oven. Repeat the vacuum drying procedure of polyimide in step (2) to obtain a dense monoesterified polyimide film with uniform thickness and smooth surface, with a film thickness between 90 and 100 μm.

[0064] (4) Place the dried monoesterified polyimide dense film in a vacuum oven and heat it to 160°C at a heating rate of 5°C / min under vacuum. Hold it for 3 hours and then heat it to 180°C at the same rate for 3 hours to obtain a monoesterified crosslinked film, named PI-180.

[0065] (5) The monoesterified crosslinked membrane was placed in a carbonization furnace and heated to 400 ℃ at a heating rate of 5 ℃ / min under a nitrogen atmosphere, and held at that temperature for 2 h. The phenolphthalein-based polybenzoxazole membrane obtained after thermally induced crosslinking was named PI-180-400-1. The flowchart of the crosslinking of the monoesterified polyimide membrane under vacuum and the thermogravimetric reaction under a nitrogen atmosphere is shown below. Figure 2 As shown.

[0066] Example 2 The only difference from Example 1 is that the dense film after monoesterification crosslinking treatment at 180°C was placed in a carbonization furnace and heated to 425°C at a heating rate of 5°C / min under a nitrogen atmosphere, and held for 2 hours. The phenolphthalein-based polybenzoxazole film obtained after thermal induction crosslinking was named PI-180-425.

[0067] Example 3 The only difference from Example 1 is that the dense film after monoesterification crosslinking treatment at 180°C was placed in a carbonization furnace and heated to 450°C at a heating rate of 5°C / min under a nitrogen atmosphere, and held for 2 hours. The phenolphthalein-based polybenzoxazole film obtained after thermal induction crosslinking was named PI-180-450.

[0068] Example 4 The only difference from Example 1 is that 3,3'-diaminophenolphthalein is replaced with 3,3'-diaminophenolphthalein having the following structure. All other additions are the same as in Example 1. The phenolphthalein-line-based polybenzoxazole membrane prepared is named PI-180-400-2.

[0069]

[0070] Example 5 The only difference from Example 1 is that DETDA is replaced with 1,3-phenylenediamine. All other additions are the same as in Example 1. The phenolphthalein-based polybenzoxazole membrane prepared is named PI-180-400-3.

[0071] Gas permeability and gas selectivity of polyimide dense membranes synthesized from 6FDA, DETDA, and AHPBA, as well as PI-180, PI-180-400, PI-180-425, and PI-180-450 membranes crosslinked under vacuum at 180°C, were tested according to the isovolume pressure rise method disclosed in [Macromolecules 2024, 57, 1370-1382]. The test results are shown in Table 1.

[0072] Table 1 shows the gas permeability and gas selectivity of polyimide dense membranes synthesized with 6FDA, DETDA, and AHPBA, as well as PI-180, PI-180-400, PI-180-425, and PI-180-450 membranes crosslinked under vacuum at 180°C.

[0073] Table 1 Results of Gas Permeability and Gas Selectivity Tests

[0074] The mechanical properties of the membranes were tested using an INSTRON 3343 electronic universal testing machine according to ASTM D882-12. After measurement, the tensile strength and elongation at break were calculated from the stress-strain curve data provided by the instrument. The mechanical properties of each membrane are the average of three independent experimental results.

[0075] Table 2 Mechanical property test results

[0076] As shown in Table 1, the permeability increases after monoesterification crosslinking. This is mainly because the carboxyl groups on the phenolphthalein group react with the glycol crosslinking agent to form a flexible crosslinked structure, increasing the interchain spacing. The increased selectivity is due to the excellent three-dimensional network structure obtained through monoesterification crosslinking, resulting in better size sieving performance of the polymer membrane. Furthermore, the polybenzoxazole gas separation membrane obtained through thermal rearrangement conversion exhibits significantly improved gas permeability.

[0077] Furthermore, as can be seen from Table 2, the polyimide film (PI-180) after monoesterification and crosslinking maintains good mechanical properties. This is because the crosslinking formed between polymer chains is a flexible crosslinking, which can prevent the polyimide film from becoming brittle due to crosslinking.

[0078] Under high-pressure CO2 environments, polymer membranes are prone to swelling, leading to plasticization, and the permeation isotherm of glassy polymers may show an upward trend. From... Figure 5 It can be seen that when the CO2 pressure is 40 atm, the monoesterified cross-linked polyimide film (PI-180) still does not show plasticization. This is because the cross-linked structure formed between the polyimide polymer chains makes it difficult for the structure to be destroyed under high pressure, thus improving the resistance to plasticization.

[0079] Comparative Example 1 The only difference from Example 1 is that 3,3'-diaminophenolphthalein is omitted.

[0080] Comparative Example 2 The only difference from Example 1 is that DETDA is omitted.

[0081] The mechanical properties and gas selectivity of the polybenzoxazole membranes prepared in Comparative Example 1 and Comparative Example 2 were tested.

[0082] Table 3. Gas selectivity test results of polybenzoxazole membranes prepared in Comparative Example 1 and Comparative Example 2.

[0083] Table 4. Mechanical property test results of polybenzoxazole films prepared in Comparative Example 1 and Comparative Example 2

[0084] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A phenolphthaloline-based polybenzoxazole membrane, characterized in that, Its structural formula is shown in Formula I: Equation I; Where R1 is a C1-C6 alkylene group or is structure; In the structure, x ranges from 4 to 45; R2, R3, R4, R5, R6, R7, R8, R9, R 10 Each is independently H or C1-C4 alkyl; R 11 Each is independently a C1-C4 alkyl group; m1, m2, n1, and n2 represent the number of repeating units, m1 is 20~50, m2 is 20~500, n1 is 0 or 20~500, n2 is 0 or 20~500; s is 0~4; Ar is the linking unit between the anhydride groups of the dianhydride monomer, and each Ar may be the same or different.

2. A method for preparing a phenolphthalein-based polybenzoxazole membrane as described in claim 1, characterized in that, Includes the following steps: (1) Under an inert atmosphere, diamine monomer, dianhydride monomer and solvent 1 are mixed and polycondensation reaction is carried out. After the reaction is completed, imidization reaction is carried out to obtain polyimide solution. (2) Under an inert atmosphere, the polyimide solution, crosslinking agent and catalyst are mixed and subjected to a monoesterification reaction to obtain a monoesterified polyimide solution; (3) Mix the monoester polyimide solution and solvent 2, dry to form a film, and then heat-treat the film under a vacuum atmosphere to obtain a monoesterified crosslinked film. (4) Under a nitrogen or argon atmosphere, the monoesterified crosslinked membrane is subjected to a thermal rearrangement reaction to obtain a phenolphthalein-based polybenzoxazole membrane.

3. The preparation method according to claim 2, characterized in that, The dianhydride monomers include one or more of pyromellitic dianhydride, trimellitic dianhydride, biphenyl dianhydride, hexafluoroisopropylphthalic anhydride, benzophenone tetracarboxylic anhydride, and oxobisphthalic anhydride. The diamine monomer is 3,3'-diaminophenolphthalein, which may be substituted with or unsubstituted with C1-C4 alkyl groups. The 3,3'-diaminophenolphthalein substituted or unsubstituted with C1-C4 alkyl groups includes any one of the following structures: 。 4. The preparation method according to claim 2, characterized in that, The diamine monomer is a 3,3'-diaminophenolphthalein substituted or unsubstituted with C1-C4 alkyl groups and a copolybenzyldiamine substituted or unsubstituted with C1-C4 alkyl groups; the molar ratio of the 3,3'-diaminophenolphthalein substituted or unsubstituted with C1-C4 alkyl groups and the copolybenzyldiamine substituted or unsubstituted with C1-C4 alkyl groups is 0.5~2:1; The copolybenzyl diamine substituted with or unsubstituted with C1-C4 alkyl groups comprises any one of the following structures: 。 5. The preparation method according to claim 3 or 4, characterized in that, The preparation method of the 3,3'-diaminophenolphthalein includes the following steps: S1. Phenolphthalein monomer, nitrifying agent and solvent 3 are mixed and reacted to obtain dinitro monomer; S2. The dinitro monomer, solvent 4 and catalyst are mixed and reacted under hydrogen to obtain 3,3'-diaminophenolphthalein.

6. The preparation method according to claim 5, characterized in that, The nitrating agent is yttrium nitrate or sodium nitrate; the solvent 3 is acetic acid or sulfuric acid; the molar ratio of the nitrating agent to the phenolphthalein monomer is 0.5~5:1; the reaction temperature in step S1 is 10~50℃ and the reaction time is 2~8 h; The solvent 4 is an alcohol or ether; the catalyst is a transition metal catalyst; the transition metal catalyst is palladium on carbon, platinum on carbon, or Raney nickel; the amount of catalyst used is 2-20 wt% of the dinitro monomer; the reaction temperature in step S2 is 20-80℃.

7. The preparation method according to claim 2, characterized in that, The molar ratio of the diamine monomer to the dianhydride monomer is 1:1; the concentration of the solution after mixing the diamine monomer, dianhydride monomer, and solvent 1 is 20-25 wt%. The polycondensation reaction is carried out at a temperature of 80-90°C for 1-2 hours; the imidization reaction is carried out at a temperature of 180°C for 8 hours.

8. The preparation method according to claim 2, characterized in that, The crosslinking agent includes at least one selected from 1,3-ethylene glycol, 1,3-propanediol, 1,3-butanediol, PEG-200, PEG-400, PEG-600, PEG-1000, and PEG-2000; the catalyst is p-toluenesulfonic acid. The mass ratio of the catalyst to the crosslinking agent is 0.001~0.0011:1; the mass ratio of the crosslinking agent to the dianhydride is 3~4:

1. The monoesterification reaction is carried out at a temperature of 130-140°C for 18-24 hours.

9. The preparation method according to claim 2, characterized in that, The drying process involves first drying at 50-60℃ for 36-48 hours, then continuing to raise the temperature to 100℃ and drying for another 12 hours. The heat treatment employs a programmed temperature rise; the programmed temperature rise includes: First stage: The heating rate is 4~6℃ / min, the temperature is raised to 160℃, and the holding time is 3~12h; Second stage: The heating rate is 4~6℃ / min, the temperature is raised to 180℃, and the holding time is 3~12h; The heating rate of the thermal rearrangement reaction is 4~6℃ / min, the temperature is 400~450℃, and the time is 1~3h.

10. A gas separation assembly, characterized in that, The membrane comprises the phenolphthaloline-based polybenzoxazole membrane of claim 1.

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