A thermally induced cross-linked phenolphthalein-based polybenzoxazole gas separation membrane material and a preparation method thereof

Abstract

This invention belongs to the field of membrane separation and relates to a thermally induced crosslinked phenolphthalein-based polybenzoxazole gas separation membrane material and its preparation method. Its structural formula is shown in Formula I: where R1, R2, and R3 are each independently H or C1-C4 alkyl groups; m1, m2, m, and n represent the number of repeating units, where m1, m2, and m are between 20 and 500, and n is 0 or between 20 and 500; Ar is a dianhydride linking unit, wherein the dianhydride group is derived from a dianhydride monomer, and the dianhydride monomer is at least one of pyromellitic dianhydride, phenylene dianhydride, biphenyltetracarboxylic dianhydride, hexafluoroisopropylphthalic anhydride, benzophenone tetracarboxylic dianhydride, and oxodiphthalic anhydride. This invention selects inexpensive and readily available phthaloyl-containing monomers. Below Tg, the lactone ring decomposes into free radicals to undergo thermally induced crosslinking, which improves the plasticization resistance of the polymer membrane and avoids the pore collapse problem caused by high-temperature crosslinking.

Description

Technical Field

[0001] This invention belongs to the field of membrane separation, specifically, it relates to a thermally induced crosslinked phenolphthalein-based polybenzoxazole gas separation membrane material and its preparation method. Background Technology

[0002] Gas separation membranes, as a green separation technology, offer advantages over traditional separation methods, including low cost, low energy consumption, simple operation, high selectivity, high efficiency, and easy scale-up. Polyimide gas separation membranes, with their high gas selectivity, excellent thermal stability, good mechanical properties, and good film-forming properties, are considered a promising gas separation material. However, traditional polyimide membranes suffer from low gas permeability and are prone to plasticization when separating gases containing high concentrations of soluble gases (such as CO2 and CH4). Plasticization refers to a phenomenon where, with increasing feed-side pressure, polymer chain segment mobility and free volume increase, leading to increased permeability and a sharp decrease in selectivity in the gas separation membrane. To improve the gas separation performance of polyimides, rigid large side-chain cardo groups can be introduced into the polymer backbone, reducing the polymer's bulk density, increasing free volume, and thus increasing gas permeability.

[0003] A novel phenolphthalein diamine reacts with 6FDA to form the polyimide Ac-6FDA-DAP. This Cardo-group polymer can undergo thermal decarboxylation and self-crosslinking at 300-350℃, or undergo thermal rearrangement at 375-450℃ to form a polybenzoxazole structure. Thermally induced crosslinking prevents plasticization at 30 atm, and the thermally induced crosslinking (TR) significantly improves gas permeability by 3.5-20 times. (Du P, Wang Z, Zhang T, et al. Journal of Membrane Science. 2022, 662:120934.)

[0004] Methods for synthesizing polyimides include one-step, two-step, and three-step methods. Two-step methods are further divided into chemical imidization and thermal imidization. Chemical imidization involves adding a dehydrating agent and catalyst after the formation of polyamic acid to achieve ring closure; thermal imidization involves heating the polyamic acid solution to 180-200°C and adding an azeotropic drying agent, such as toluene, to help remove the water formed during the ring-closure reaction. (Omole IC, Miller SJ, Koros W J. Macromolecules. 2008, 41(17): 6367-6375.)

[0005] Copolymers of polyimides offer the possibility of preparing membranes with gas permeability and selectivity unattainable by homopolymers. Furthermore, copolymers of polyimides can be prepared directly from known monomers, and their physical or chemical properties can be altered by changing the ratio of comonomers. Even more remarkably, by copolymerizing a highly selective but low-permeability polyimide with a highly permeable polyimide, it is possible to create a polyimide gas separation membrane material that possesses both high permeability and high selectivity. (Wang L, Cao Y, Zhou M, et al. Journal of Membrane Science. 2007, 305(1-2):338-346.)

[0006] Benzyl-induced crosslinking, a common method for crosslinking polymer membranes, aims to improve the gas separation performance of polymers and enhance membrane stability. This method utilizes aromatic methyl substituents in benzyl polyimide as crosslinking reaction sites. The crosslinking reaction occurs in air at 250°C, far below the decomposition temperature of most polyimides (typically above 430°C), ensuring the integrity of the polymer backbone structure. The phenyl-induced thermo-oxidative crosslinking reaction gives the prepared membrane high CO2 / CH4 separation selectivity and high permeability. (Zhu S, Wang Z, Shi Y, et al. Macromolecules. 2022, 55(15): 6890-6900.)

[0007] The presence of numerous benzene rings in the phenolphthalein group endows these polymers with excellent thermal stability, while the presence of lactone rings provides them with good solvent properties. The aromatic heterocyclic structures of the side groups reduce the polymer density and increase the free volume, which is beneficial for improving the gas separation performance of polyimide membrane materials. Benzyl polyimides generally exhibit high permeability, and the benzyl group can also achieve thermal oxidative crosslinking. Summary of the Invention

[0008] The purpose of this invention is to address some shortcomings in the preparation process of polyimide gas separation membrane materials and the problem of unsatisfactory gas separation performance. Phenolic diamine is selected, which contains large Cardo side groups, is inexpensive, and readily available. After obtaining the polyimide membrane, thermally induced self-crosslinking treatment is performed below the glass transition temperature, resulting in an excellent three-dimensional network structure while avoiding damage to the polymer membrane structure. This invention uses 3,3'-diaminophenolphthalein as the phenolphthalein diamine and employs azeotropic distillation to retain the hydroxyl groups on the phenolphthalein diamine, resulting in numerous hydrogen bonds on the polyimide polymer chain, improving its resistance to plasticization. Furthermore, the low gas permeability of homopolymer phenolphthalein-based polyimide can be further improved by adding benzyl diamine for copolymerization.

[0009] To achieve the above objectives, a first aspect of the present invention provides a thermally induced crosslinked phenolphthalein-based polybenzoxazole gas separation membrane material, the structural formula of which is shown in Formula I:

[0010]

[0011] R1, R2, and R3 are each independently H or C1-C4 alkyl groups;

[0012] m1, m2, m and n represent the number of repeating units, where m1, m2 and m are between 20 and 500, and n is 0, or n is between 20 and 500;

[0013] Ar is the linking unit of the dianhydride group, which is derived from the dianhydride monomer, and the dianhydride monomer is at least one of pyromellitic dianhydride, tetramethyl dianhydride, biphenyl dianhydride, hexafluoroisopropylphthalic anhydride, benzophenone tetracarboxylic anhydride, and oxobisphthalic anhydride.

[0014] The structures of each dianhydride monomer are as follows:

[0015]

[0016]

[0017] According to the present invention, preferably, the thermally induced crosslinked phenolphthalein-based polybenzoxazole gas separation membrane material is a PBO polymer synthesized by thermal imidization, thermally induced crosslinking, and thermal rearrangement reactions of diamine monomers and dianhydride monomers.

[0018] According to the present invention, preferably, the diamine monomer is 3,3'-diaminophenolphthalein, or the diamine monomer is 3,3'-diaminophenolphthalein and benzyldiamine.

[0019] According to the present invention, preferably, n is between 20 and 500, and the thermally induced crosslinked phenolphthalein-based polybenzoxazole gas separation membrane material is selected from any one of the following:

[0020] (i) In formula I, R1 = CH3, R2 = H, and R3 = H;

[0021] (ii) In formula I, R1 = CH3, R2 = CH3, R3 = H or R1 = H, R2 = CH3, R3 = CH3;

[0022] (iii) In formula I, R1 = CH3, R2 = CH3, and R3 = CH3;

[0023] (iv) In formula I, R1 = CH3, R2 = CH2CH3, R3 = H or R1 = CH2CH3, R2 = CH3, R3 = H;

[0024] (v) In formula I, R1=CH3, R2=CH2CH3, R3=CH2CH3 or R1=CH2CH3, R2=CH2CH3, R3=CH3.

[0025] According to the present invention, preferably, when R1 = CH3, R2 = H, and R3 = H in Formula I, the thermally induced crosslinked phenolphthalein-based polybenzoxazole gas separation membrane material is a PBO polymer synthesized by a thermal imidization, thermally induced crosslinking, and thermal rearrangement reaction of monomer (a), 3,3'-diaminophenolphthalein, and dianhydride monomer.

[0026] 2-Methyl-1,3-phenylenediamine.

[0027] When R1 = CH3, R2 = CH3, R3 = H or R1 = H, R2 = CH3, R3 = CH3 in Formula I, the thermally induced crosslinked phenolphthalein-based polybenzoxazole gas separation membrane material is a PBO polymer synthesized by a thermal imidization, thermally induced crosslinking, and thermal rearrangement reaction of monomer (b), 3,3'-diaminophenolphthalein, and dianhydride monomer.

[0028] 2,4-Dimethyl-1,3-phenylenediamine or 2,4-dimethyl-1,5-phenylenediamine.

[0029] When R1 = CH3, R2 = CH3, and R3 = CH3 in Formula I, the thermally induced crosslinked phenolphthalein-based polybenzoxazole gas separation membrane material is a PBO polymer synthesized from monomer (c), 3,3'-diaminophenolphthalein and dianhydride monomer through thermal imidization, thermally induced crosslinking, and thermal rearrangement reactions.

[0030] 2,4,6-Trimethyl-1,3-phenylenediamine.

[0031] When R1 = CH3, R2 = CH2CH3, R3 = H or R1 = CH2CH3, R2 = CH3, R3 = H in Formula I, the thermally induced crosslinked phenolphthalein-based polybenzoxazole gas separation membrane material is a PBO polymer synthesized from monomer (d), 3,3'-diaminophenolphthalein and dianhydride monomer through thermal imidization, thermally induced crosslinking and thermal rearrangement reaction;

[0032] 2-Methyl-4-ethyl-1,3-phenylenediamine or 4-methyl-2-ethyl-1,5-phenylenediamine.

[0033] When R1 = CH3, R2 = CH2CH3, R3 = CH2CH3 or R1 = CH2CH3, R2 = CH2CH3, R3 = CH3 in Formula I, the thermally induced crosslinked phenolphthalein-based polybenzoxazole gas separation membrane material is a PBO polymer synthesized from monomer (e), 3,3'-diaminophenolphthalein and dianhydride monomer through thermal imidization, thermally induced crosslinking and thermal rearrangement reaction;

[0034] Diethyltoluenediamine.

[0035] A second aspect of the present invention provides a method for preparing a thermally induced crosslinked phenolphthalein-based polybenzoxazole gas separation membrane material, the method comprising the following steps:

[0036] (1) Under the protection of an inert gas, a diamine monomer, a first solvent, and a dianhydride monomer are contacted to carry out an imidization reaction to obtain a polyimide; the dianhydride monomer is selected from at least one of pyromellitic dianhydride, pyrimidine dianhydride, biphenyl dianhydride, hexafluoroisopropylphthalic anhydride, benzophenone tetracarboxylic anhydride, and oxobisphthalic anhydride; the diamine monomer is 3,3'-diaminophenolphthalein, or the diamine monomer is 3,3'-diaminophenolphthalein and benzyldiamine; the benzyldiamine is at least one of 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;

[0037] (2) Preparation of polyimide dense film

[0038] The polyimide obtained in step (1) is dissolved in the first solvent and filtered to form a dense polyimide membrane;

[0039] (3) Thermally induced crosslinking treatment to form a dense film

[0040] In an air atmosphere, the polyimide dense film dried in step (2) is heated to 300-400°C and kept at that temperature to obtain the thermally induced crosslinked dense film.

[0041] (4) Thermal rearrangement

[0042] Under a nitrogen atmosphere, the thermally induced crosslinked dense membrane prepared in step (3) is heated to 400-450°C and kept at that temperature to obtain the thermally induced crosslinked phenolphthalein-based polybenzoxazole gas separation membrane material.

[0043] According to the present invention, preferably, step (1) includes: under the protection of a low-temperature ice bath and nitrogen, dissolving the diamine monomer in a first solvent, stirring evenly, adding dianhydride monomer in batches so that the concentration of the diamine monomer and dianhydride monomer solution is 25-35 wt%, and stirring continuously for 20-30 h until a high-viscosity polyamic acid solution is formed. The ice bath is removed, and then an azeotropic drying agent is added and stirred until evenly mixed. The mixture is heated to 180-200°C for thermal imidization reaction for 4-8 h to form a high-viscosity polyimide solution. Heating is stopped and the first solvent is added for dilution. Finally, the obtained polyimide solution is poured into methanol to precipitate. The precipitate is crushed, filtered, washed, and the residual solvent is removed. The obtained polyimide is then subjected to programmed temperature drying.

[0044] In this invention, the diamine monomer is dissolved in a first solvent, and the stirring time is 20-40 min under low-temperature ice bath and nitrogen protection. An azeotropic drying agent is added and the stirring time is 20-40 min. The amount of the first solvent added is such that the concentration of the diamine monomer and dianhydride monomer solution in the system is 25-35 w.

[0045] According to the present invention, preferably, the first solvent is selected from at least one of N-methyl-2-pyrrolidone, N,N-dimethylformamide and N,N-dimethylacetamide.

[0046] According to the present invention, preferably, the molar ratio of the diamine monomer and the dianhydride monomer is 1:1, and the molar ratio of 3,3'-diaminophenolphthalein and benzyldiamine is 2:1-1:2.

[0047] According to the present invention, preferably, the azeotropic drying agent is at least one selected from toluene, xylene, and o-dichlorobenzene; preferably, toluene.

[0048] According to the present invention, preferably, the programmed temperature rise is 50-70℃ for 20-28h, 80-100℃ for 10-16h, 110-130℃ for 10-16h, and 140-160℃ for 20-28h.

[0049] According to the present invention, the gradient temperature program is 60℃ for 24 hours, 90℃ for 12 hours, 120℃ for 12 hours, and 150℃ for 24 hours.

[0050] According to the present invention, preferably, step (2) includes: dissolving the polyimide obtained in step (1) in a first solvent, stirring until completely dissolved, filtering with a polytetrafluoroethylene filter membrane of 0.45 to 1 μm, injecting the filtered solution into a flat glass petri dish, and then placing it in a vacuum oven, setting a gradient heating program for drying, the gradient heating program being the same as in step (1), to obtain a dense polyimide film with uniform thickness and a smooth and flat surface.

[0051] According to the present invention, preferably, the thickness of the polyimide dense film is 50-100 μm, more preferably 70-80 μm.

[0052] According to the present invention, preferably, in step (3), the heating rate of the programmed heating is 4 to 6 °C / min, and the holding time is 1 to 3 h.

[0053] According to the present invention, preferably, in step (4), the heating rate of the programmed heating is 4 to 6 °C / min, and the holding time is 1 to 3 h.

[0054] A third aspect of the present invention provides a thermally induced crosslinked phenolphthalein-based polybenzoxazole gas separation membrane material prepared by the aforementioned preparation method.

[0055] This invention utilizes inexpensive and readily available monomers containing phthaloyl groups. Below the Tg (temperature gradient), the lactone ring decomposes into free radicals, undergoing thermally induced crosslinking. This improves the polymer membrane's resistance to plasticization while avoiding pore collapse caused by high-temperature crosslinking. Furthermore, because these monomers contain phenolic hydroxyl groups, the polyimide is an ortho-hydroxy polyimide, allowing for further thermal rearrangement after thermally induced crosslinking of the polyimide membrane, resulting in a more twisted and rigid polybenzoxazole membrane. In addition, copolymerizing a benzyl diamine based on phenolphthalein-based polyimide improves the gas separation performance of homopolymer polyimide and allows for thermal oxidative crosslinking of the benzyl group, further increasing the degree of crosslinking and improving structural stability. Using this type of PBO membrane for gas separation significantly improves gas permeability; copolymerization alone increases CO2 flux by 20 times, while the thermally rearranged copolymer polyimide increases CO2 flux by more than 120 times compared to homopolymer polyimide.

[0056] Other features and advantages of the present invention will be described in detail in the following detailed description section. Attached Figure Description

[0057] Exemplary embodiments of the present invention will be described in more detail with reference to the accompanying drawings.

[0058] Figure 1 The equations for synthesizing polyimides from diamine monomers and dianhydride monomers in the embodiments of the present invention are shown.

[0059] Figure 2 The three polyimides in the embodiments of the present invention 1 1H-NMR spectrum (DMSO-d6), where a is PI-A 1 1H-NMR spectrum (DMSO-d6), b is PI-B 1 H-NMR spectrum (DMSO-d6), c is PI-C 1 H-NMR spectrum (DMSO-d6).

[0060] Figure 3 is a flowchart of the thermally induced crosslinking reaction of polyimide in an air atmosphere and the thermal rearrangement reaction in a nitrogen atmosphere in an embodiment of the present invention.

[0061] Figure 4 The polyimide film prepared in the embodiments of the present invention: a) shows the flexibility of the PI film; b) shows the PI film.

[0062] Figure 5 This is a diagram illustrating the reaction mechanism of thermally induced crosslinking of the polyimide film in a high-purity air atmosphere, as described in this embodiment of the invention.

[0063] Figure 6 The polyimide films in the embodiments of the present invention are obtained by thermally induced crosslinking treatment of polyimide films at 300℃, 325℃, 350℃, and 400℃ in a high-purity air atmosphere, respectively, and polybenzoxazole films synthesized by thermal rearrangement reaction of films after thermally induced crosslinking treatment at 300℃ or 325℃ at 400℃, 425℃, and 450℃, respectively. a represents PI-A-300-air film, PI-A-325-air film, PI-A-350-air film, PI-A-400-air film, PI-A-300-air-400-N2, PI-A-325-air-425-N2 film, and PI-A-325-air-450-N2; b represents PI-B-300-air film, PI-B-325-air film, PI-B-350-air film, PI-B-400-air film, and P I-B-300-air-400-N2, PI-B-325-air-425-N2 membrane and PI-B-325-air-450-N2; c is PI-C-300-air membrane, PI-C-325-air membrane, PI-C-350-air membrane, PI-C-400-air membrane, PI-C-300-air-400-N2, PI-C-325-air-425-N2 membrane and PI-C-325-air-450-N2.

[0064] Figure 7 This is a comparison chart of the anti-plasticization properties of three types of untreated polyimide films in the embodiments of the present invention.

[0065] Figure 8 is a comparison of the infrared spectra of the polymer film before and after heat treatment at different temperatures in the embodiments of the present invention. Detailed Implementation

[0066] The specific embodiments of the present invention will be described in detail below. It should be understood that the specific embodiments described herein are for illustration and explanation only and are not intended to limit the present invention. The present invention will be further described below with reference to the embodiments, but the scope of the present invention is not limited to these embodiments.

[0067] Example 1

[0068] 4.18 g (348.32 g / mol, 12 mmol) of 3,3'-diaminophenolphthalein was added to a 100 mL four-necked flask. 22 mL of NMP was added, and the mixture was stirred for 30 min under a low-temperature ice bath and nitrogen protection. 5.33 g (444.24 g / mol, 12 mmol) of 6FDA was added in two portions, resulting in a solid content of 30 wt%. The mixture was stirred continuously for 12 h to form a high-viscosity polyamic acid solution. 15 mL of toluene was added, and the ice bath was removed. The mixture was heated to 180 °C and reacted for 6 h to obtain a very viscous polyimide solution. This solution was diluted and precipitated in methanol. The precipitate was washed with methanol during filtration to remove residual solvent. The temperature was then increased in a gradient: 60 °C for 24 h, 90 °C for 12 h, 120 °C for 12 h, and 150 °C for 24 h, yielding 7.81 g of dried polyimide. The ¹H NMR spectrum (DMSO-d6) of polyimide synthesized by the reaction of 3,3'-diaminophenolphthalein with 6FDA is shown below. Figure 2 As shown in a. Regarding the 1H NMR spectrum of polyimide, it should be noted that the ortho-hydroxyl group of phenolphthalein diamine is retained during the thermal imidization process, which can be clearly observed in the 1H NMR spectrum.

[0069] Dissolve 1.00g of dried polyimide powder in DMF and stir until completely dissolved. The solid content of the casting solution is 8.0wt%. Filter the solution using a 0.45 or 1μm 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 for polyimide in step (1) to obtain a dense polyimide film with uniform thickness and a smooth surface, with a film thickness between 70 and 80μm. The flowchart of the thermally induced crosslinking and thermal rearrangement reaction of polyimide in an air atmosphere is shown in Figure 3.

[0070] Will as Figure 4 The dried polyimide film shown was placed in a carbonization furnace and heated to 300℃, 325℃, 350℃, and 400℃ respectively at a heating rate of 5℃ / min under air atmosphere, and held at that temperature for 2 hours. The resulting thermally induced crosslinked films were named PI-A-300, PI-A-325, PI-A-350, and PI-A-400, respectively. The reaction mechanism of thermally induced crosslinking of the polyimide film under air atmosphere is shown in Figure 5.

[0071] The dense membranes after thermally induced crosslinking treatment at 300℃ or 325℃ for 2 hours were placed in a carbonization furnace and heated to 400℃, 425℃, and 450℃ at a heating rate of 5℃ / min under a nitrogen atmosphere, and held for 2 hours. The polybenzoxazole membranes obtained after thermally induced crosslinking were named PI-A-300-400, PI-A-325-425, and PI-A-325-450, respectively.

[0072] Comparison of polymer film images before and after processing, for example Figure 6 As shown in Table 1, the gas permeability and gas selectivity of polyimide membranes synthesized by DAP and 6FDA, membranes subjected to thermally induced crosslinking treatment at 300℃, 325℃, 350℃ and 400℃ in air atmosphere for 2h, and PI-A-300-400 membrane, PI-A-325-425 membrane and PI-A-325-450 membrane are shown.

[0073] Table 1

[0074]

[0075] As shown in Table 1, the permeability increased after thermally induced crosslinking. This is mainly because the lactone ring on the phenolphthalein group began to decompose into free radicals to form a crosslinked structure, increasing the interchain spacing. The increased selectivity is due to the formation of a more rigid polymer backbone after thermal crosslinking, resulting in better size sieving performance of the polymer membrane. The CO2 permeability of PI-A-300-400 did not increase significantly, and was even lower than that of PI-A-400. This is because thermal crosslinking at 300℃ formed a stable crosslinked structure, and at a relatively lower thermal rearrangement temperature (400℃), its thermal rearrangement conversion rate was low, and the limited number of polybenzoxazole structures had little impact on its gas separation performance. The polybenzoxazole gas separation membrane obtained after thermal rearrangement at higher temperatures showed a significant improvement in gas permeability, with the CO2 flux reaching as high as 963.91 barrer after treatment at 450℃.

[0076] For condensable gas CO2, the permeation isotherm of the glassy polymer may show an upward trend due to the plasticizing effect caused by the swelling of the polymer membrane under high pressure. Figure 7 It can be seen that when the CO2 pressure is 30 atm, the untreated polyimide film (PI-A) still does not show plasticization. This is because the azeotropic distillation method retains a large number of hydroxyl groups on phenolphthalein diamine, which increases the intermolecular forces and improves the resistance to plasticization.

[0077] Example 2

[0078] 2.09 g (348.32 g / mol, 6 mmol) of 3,3'-diaminophenolphthalein and 0.90 g (150.22 g / mol, 6 mmol) of 2,4,6-trimethylm-phenylenediamine were added to a 100 mL four-necked flask. 19 mL of NMP was added, and the mixture was stirred for 30 min under a low-temperature ice bath and nitrogen protection. 5.33 g (444.24 g / mol, 12 mmol) of 6FDA was added in two portions, resulting in a solid content of 30 wt%. The mixture was stirred continuously for 12 h to form a high-viscosity polyamic acid solution. 15 mL of toluene was added, and the ice bath was removed. The mixture was heated to 180 °C and reacted for 6 h. The highly viscous polyimide solution was diluted and precipitated in methanol. The precipitate was washed with methanol during filtration to remove residual solvent. The temperature was increased in a gradient: 60 °C for 24 h, 90 °C for 12 h, 120 °C for 12 h, and 150 °C for 24 h, yielding 7.23 g of dried polyimide. The ¹H NMR spectrum (DMSO-d6) of the copolyimide synthesized by reacting 3,3'-diaminophenolphthalein and 2,4,6-trimethylm-phenylenediamine with 6FDA is shown below. Figure 2 As shown in b.

[0079] Dissolve 1.00g of dried polyimide powder in DMF and stir until completely dissolved. The solid content of the casting solution is 8.0wt%. Filter the solution using a 0.45 or 1μm 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 for polyimide in step (1) to obtain a dense polyimide film with uniform thickness and a smooth surface, with a film thickness between 70 and 80μm. The flowchart of the thermally induced crosslinking and thermal rearrangement reaction of polyimide in an air atmosphere is shown in Figure 3.

[0080] Will as Figure 4 The dried polyimide film shown was placed in a carbonization furnace and heated to 300℃, 325℃, 350℃, and 400℃ respectively at a heating rate of 5℃ / min under air atmosphere, and held at that temperature for 2 hours. The resulting thermally induced crosslinked films were named PI-B-300, PI-B-325, PI-B-350, and PI-B-400, respectively. The reaction mechanism of thermally induced crosslinking of the polyimide film under air atmosphere is shown in Figure 5.

[0081] The dense membranes after thermally induced crosslinking treatment at 300℃ or 325℃ for 2 hours were placed in a carbonization furnace and heated to 400℃, 425℃, and 450℃ at a heating rate of 5℃ / min under a nitrogen atmosphere, and held for 2 hours. The polybenzoxazole membranes obtained after thermally induced crosslinking were named PI-B-300-400, PI-B-325-425, and PI-B-325-450, respectively.

[0082] Comparison of polymer film images before and after processing, for example Figure 6As shown in Table 2, the gas permeability and gas selectivity of polyimide membranes synthesized from DAP and DAM with 6FDA, membranes subjected to thermally induced crosslinking treatment at 300℃, 325℃, 350℃ and 400℃ in air atmosphere for 2h, and PI-B-300-400 membrane, PI-B-325-425 membrane and PI-B-325-450 membrane are shown.

[0083] Table 2

[0084]

[0085] As shown in Table 2, the permeability increased after thermally induced crosslinking. This is mainly because the lactone ring on the phenolphthalein group began to decompose into free radicals to form a crosslinked structure, while the benzyl group also underwent thermal oxidative crosslinking, increasing the interchain spacing. The increased selectivity is due to the rigid polymer skeleton formed after thermal crosslinking, which gives the polymer membrane better size sieving performance. The polybenzoxazole gas separation membrane obtained after thermal rearrangement showed a significant improvement in gas permeability, with a CO2 flux of up to 749.54 barrer after treatment at 450℃, but lower than the CO2 flux of PI-A-325-450. Because 2,4,6-trimethylm-phenylenediamine (DAM) accounts for a certain proportion in the polymer and cannot undergo thermal rearrangement, the polybenzoxazole structure in PI-B is less than that in PI-A, and the effect of the polybenzoxazole structure on improving the permeability of the former is less than that of the latter.

[0086] Although the number of hydroxyl groups on phenolphthalein diamine was reduced in Example 2 compared to Example 1, DAM still has a rigid structure. Figure 7 It can be seen that when the CO2 pressure is 30 atm, the untreated polyimide film (PI-B) still does not show plasticization, indicating that the phenolphthalein polyimide after DAM copolymerization still has excellent anti-plasticization properties.

[0087] Example 3

[0088] 2.09 g (348.32 g / mol, 6 mmol) of 3,3'-diaminophenolphthalein and 1.07 g (178.28 g / mol, 6 mmol) of diethyltoluene diamine were added to a 100 mL four-necked flask. 20 mL of NMP was added, and the mixture was stirred for 30 min under a low-temperature ice bath and nitrogen protection. 5.33 g (444.24 g / mol, 12 mmol) of 6FDA was added in two portions, resulting in a solid content of 30 wt%. The mixture was stirred continuously for 12 h to form a high-viscosity polyamic acid solution. 10 mL of toluene was added, and the ice bath was removed. The mixture was heated to 180 °C and reacted for 6 h. The highly viscous polyimide solution was diluted and precipitated in methanol. The precipitate was washed with methanol during filtration to remove residual solvent. The temperature was then increased in a gradient: 60 °C for 24 h, 90 °C for 12 h, 120 °C for 12 h, and 150 °C for 24 h, yielding 7.23 g of dried polyimide. The ¹H NMR spectrum (DMSO-d6) of the copolyimide synthesized by reacting 3,3'-diaminophenolphthalein and diethyltoluenediamine with 6FDA is shown below. Figure 2 As shown in c.

[0089] Dissolve 1.00g of dried polyimide powder in DMF and stir until completely dissolved. The solid content of the casting solution is 8.0wt%. Filter the solution using a 0.45 or 1μm 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 for polyimide in step (1) to obtain a dense polyimide film with uniform thickness and a smooth surface, with a film thickness between 70 and 80μm. The flowchart of the thermally induced crosslinking and thermal rearrangement reaction of polyimide in an air atmosphere is shown in Figure 3.

[0090] Will as Figure 4 The dried polyimide film was placed in a carbonization furnace and heated to 300℃, 325℃, 350℃, and 400℃ respectively at a heating rate of 5℃ / min under air atmosphere, and held at these temperatures for 2 hours. The resulting thermally induced crosslinked films were named PI-C-300, PI-C-325, PI-C-350, and PI-C-400, respectively. The reaction mechanism of the thermally induced crosslinking of the polyimide film under air atmosphere is shown in Figure 5.

[0091] The dense membranes after thermally induced crosslinking treatment at 300℃ or 325℃ for 2 hours were placed in a carbonization furnace and heated to 400℃, 425℃, and 450℃ at a heating rate of 5℃ / min under a nitrogen atmosphere, and held for 2 hours. The polybenzoxazole membranes obtained after thermally induced crosslinking were named PI-C-300-400, PI-C-325-425, and PI-C-325-450, respectively.

[0092] Comparison of polymer film images before and after processing, for example Figure 6As shown in Table 2, the gas permeability and gas selectivity of polyimide membranes synthesized from DAP and DETDA with 6FDA, membranes subjected to thermally induced crosslinking treatment at 300℃, 325℃, 350℃ and 400℃ in air atmosphere for 2h, and PI-C-300-400 membrane, PI-C-325-425 membrane and PI-C-325-450 membrane are shown.

[0093] Table 3

[0094]

[0095]

[0096] As shown in Table 3, the permeability decreased after thermally induced crosslinking, contrary to the results in Examples 1 and 2. This is mainly because the benzyl group in diethyltoluene diamine (DETDA) undergoes thermal oxidative crosslinking, reducing its free volume due to the ethyl group. This effect simultaneously influences the crosslinking structure formed by the decomposition of the lactone ring on the phenolphthalein group. At 400°C, the small molecules begin to degrade, resulting in higher CO2 permeability than the original membrane; simultaneously, due to the size sieving effect of the crosslinked structure, its selectivity is also superior to the original membrane. The polybenzoxazole gas separation membrane obtained after thermal rearrangement showed significantly improved gas permeability, with a CO2 flux reaching 642.41 barrer after treatment at 450°C. However, this result is lower than both the CO2 flux of PI-B-325-450 and PI-A-325-450. The reason is the same as in Example 2, and the larger volume proportion of DETDA in the polymer compared to DAM further limits the improvement in flux caused by the polybenzoxazole structure.

[0097] from Figure 7 It can be seen that when the CO2 pressure is 30 atm, the untreated polyimide film (PI-C) does not exhibit plasticization, but its anti-plasticization performance is significantly worse than that of PI-A and PI-B. This is because the ethyl group on DETDA improves the polymer's flexibility to some extent compared to the methyl group on DAM, but it is undeniable that PI-C still has excellent anti-plasticization performance.

[0098] Example 4

[0099] 1.39 g (348.32 g / mol, 4 mmol) of 3,3'-diaminophenolphthalein and 1.20 g (150.22 g / mol, 8 mmol) of 2,4,6-trimethylm-phenylenediamine were added to a 100 mL four-necked flask. 18 mL of NMP was added, and the mixture was stirred for 30 min under a low-temperature ice bath and nitrogen protection. 5.33 g (444.24 g / mol, 12 mmol) of 6FDA was added in two portions, resulting in a solid content of 30 wt%. The mixture was stirred continuously for 12 h to form a high-viscosity polyamic acid solution. 10 mL of toluene was added, and the ice bath was removed. The mixture was heated to 180 °C and reacted for 6 h. The highly viscous polyimide solution was diluted and precipitated in methanol. The precipitate was washed with methanol during filtration to remove residual solvent. The temperature was increased in a gradient: 60 °C for 24 h, 90 °C for 12 h, 120 °C for 12 h, and 150 °C for 24 h, yielding 7.23 g of dried polyimide. The ¹H NMR spectrum (DMSO-d6) of the copolyimide synthesized by reacting 3,3'-diaminophenolphthalein and 2,4,6-trimethylm-phenylenediamine with 6FDA is shown below. Figure 2 As shown in b.

[0100] Dissolve 1.00g of dried polyimide powder in DMF and stir until completely dissolved. The solid content of the casting solution is 8.0wt%. Filter the solution using a 0.45 or 1μm 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 for polyimide in step (1) to obtain a dense polyimide membrane with uniform thickness and a smooth surface, with a membrane thickness between 70 and 80μm. Table 4 shows its gas separation performance.

[0101] Table 4

[0102]

[0103] Example 5

[0104] 1.39 g (348.32 g / mol, 4 mmol) of 3,3'-diaminophenolphthalein and 1.43 g (178.28 g / mol, 8 mmol) of diethyltoluenediamine were added to a 100 mL four-necked flask. 19 mL of NMP was added, and the mixture was stirred for 30 min under a low-temperature ice bath and nitrogen protection. 5.33 g (444.24 g / mol, 12 mmol) of 6FDA was added in two portions, resulting in a solid content of 30 wt%. The mixture was stirred continuously for 12 h to form a high-viscosity polyamic acid solution. 10 mL of toluene was added, and the ice bath was removed. The mixture was heated to 180 °C and reacted for 6 h. The highly viscous polyimide solution was diluted and precipitated in methanol. The precipitate was washed with methanol during filtration to remove residual solvent. The temperature was increased in a gradient: 60 °C for 12 h, 90 °C for 12 h, 120 °C for 24 h, and 150 °C for 24 h, yielding 6.80 g of dried polyimide. The ¹H NMR spectrum (DMSO-d6) of the copolyimide synthesized by reacting 3,3'-diaminophenolphthalein and diethyltoluenediamine with 6FDA is shown below. Figure 2 As shown in c.

[0105] Dissolve 1.00g of dried polyimide powder in DMF and stir until completely dissolved. The solid content of the casting solution is 8.0wt%. Filter the solution using a 0.45 or 1μm 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 for polyimide in step (1) to obtain a dense polyimide membrane with uniform thickness and a smooth surface, with a membrane thickness between 70 and 80μm. Table 5 shows its gas separation performance.

[0106] Table 5

[0107]

[0108] Example 6

[0109] The only difference between this comparative example and Example 2 is that after forming a high-viscosity polyamic acid solution, 5 mL of acetic anhydride and 5 mL of anhydrous pyridine were added, and the ice bath was removed. The reaction was continued at room temperature for 24 h. The high-viscosity polyimide solution was then poured into a methanol solution to precipitate the polyimide. During filtration, the polyimide was washed with methanol to remove residual solvent. The polyimide was then dried under vacuum in an oven at 80 °C for 24 h to obtain 8.25 g of dried polyimide. The remaining steps were the same. PI-D-300, PI-D-325, PI-D-350, PI-D-400, PI-D-300-400, PI-D-325-425, and PI-D-325-450 were obtained, respectively. Table 6 shows their gas separation performance:

[0110] Table 6

[0111]

[0112] As shown in Table 6, chemical imidization in Example 6 increased the CO2 flux compared to Example 2, but resulted in lower CO2 / CH4 selectivity. For thermally induced crosslinked phenolphthalein-based polybenzoxazole gas separation membrane materials, increasing flux is relatively easy, but improving selectivity is difficult due to low selectivity. Furthermore, the thermally induced crosslinked phenolphthalein-based polybenzoxazole gas separation membrane material obtained through chemical imidization also exhibits poor performance in later applications, showing defects such as the spinning of hollow fibers.

[0113] The various embodiments of the present invention have been described above. These descriptions are exemplary and not exhaustive, nor are they limited to the disclosed embodiments. Many modifications and variations will be apparent to those skilled in the art without departing from the scope and spirit of the described embodiments.

[0114] The endpoints and any values ​​of the ranges disclosed herein are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed herein.

Claims

1. A method for preparing a thermally induced crosslinked phenolphthalein-based polybenzoxazole gas separation membrane material, characterized in that, The method includes the following steps: (1) Under inert gas protection, the diamine monomer, the first solvent and the dianhydride monomer are brought into contact to form a high-viscosity polyamic acid solution. The ice bath is removed, and then an azeotropic desiccant is added and stirred until the mixture is uniform. A thermal imidization reaction is carried out to obtain polyimide. The dianhydride monomer is selected from at least one of pyromellitic dianhydride, pyromellitic dianhydride, biphenyl dianhydride, hexafluoroisopropylphthalic anhydride, benzophenone tetracarboxylic anhydride and oxobisphthalic anhydride. The diamine monomer is 3,3'-diaminophenolphthalein, or the diamine monomer is 3,3'-diaminophenolphthalein and benzyl diamine. (2) Preparation of polyimide dense film The polyimide obtained in step (1) is dissolved in the first solvent and filtered to form a dense polyimide membrane; (3) Thermally induced crosslinking treatment to form dense film In an air atmosphere, the polyimide dense film dried in step (2) is heated to 300-400°C and kept at that temperature to obtain the thermally induced crosslinked dense film. (4) Thermal rearrangement Under a nitrogen atmosphere, the thermally induced crosslinked dense membrane prepared in step (3) is heated to 400-450°C and kept at that temperature to obtain the thermally induced crosslinked phenolphthalein-based polybenzoxazole gas separation membrane material.

2. The preparation method according to claim 1, wherein, The benzyldiamine is at least one selected from 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.

3. The preparation method according to claim 1, wherein, Step (1) includes: Under low-temperature ice bath and nitrogen protection, the diamine monomer is dissolved in the first solvent and stirred evenly. The dianhydride monomer is added in batches to make the concentration of the diamine monomer and dianhydride monomer solution 25~35w%. The reaction is continued to be stirred for 20~30h until a high-viscosity polyamic acid solution is formed. The ice bath is removed, and then an azeotropic drying agent is added and stirred until evenly mixed. The mixture is heated to 180~200℃ for thermal imidization reaction for 4~8h to form a high-viscosity polyimide solution. Heating is stopped and the first solvent is added for dilution. Finally, the obtained polyimide solution is poured into methanol to precipitate. The precipitate is crushed, filtered, washed, and the residual solvent is removed. The obtained polyimide is then dried by programmed temperature rise.

4. The preparation method according to any one of claims 1-3, wherein, The first solvent is selected from N 2-Methyl-2-pyrrolidone, N,N -dimethylformamide and N,N At least one of dimethylacetamide; The molar ratio of diamine monomer to dianhydride monomer is 1:1, and the molar ratio of 3,3'-diaminophenolphthalein to benzyldiamine is 2:1-1:

2.

5. The preparation method according to claim 3, wherein, In step (1), the azeotropic drying agent is at least one of toluene, xylene, and o-dichlorobenzene; The programmed temperature rise is 50~70 ℃ for 20-28 h, 80~100 ℃ for 10-16 h, 110~130 ℃ for 10-16 h, and 140~160 ℃ for 20-28 h.

6. The preparation method according to claim 5, wherein, The azeotropic drying agent is toluene.

7. The preparation method according to claim 1, wherein, Step (2) includes: Dissolve the polyimide obtained in step (1) in the first solvent and stir until completely dissolved. Filter the solution with a 0.45~1 μm polytetrafluoroethylene filter membrane. Pour the filtered solution into a flat glass petri dish and then place it in a vacuum oven. Set a gradient heating program to dry the solution and obtain a dense polyimide film with uniform thickness and a smooth surface.

8. The preparation method according to claim 1, wherein, The thickness of the polyimide dense film is 50~100 μm.

9. The preparation method according to claim 8, wherein, The thickness of the polyimide dense film is 70~80 μm.

10. The preparation method according to claim 1, wherein, In step (3), the heating rate of the programmed heating is 4~6℃ / min, and the holding time is 1~3h; In step (4), the heating rate of the programmed heating is 4~6℃ / min, and the holding time is 1~3h.

11. A thermally induced crosslinked phenolphthalein-based polybenzoxazole gas separation membrane material prepared by the preparation method of claim 1.

12. The thermally induced crosslinked phenolphthalein-based polybenzoxazole gas separation membrane material according to claim 11, wherein, Its structural formula is shown in Formula I: Formula I R1, R2, and R3 are each independently H or C1-C4 alkyl groups; m1, m2, m and n represent the number of repeating units, where m1, m2 and m are between 20 and 500, and n is 0, or n is between 20 and 500; Ar is the linking unit of the dianhydride group, which is derived from the dianhydride monomer, and the dianhydride monomer is at least one of pyromellitic dianhydride, tetramethyl dianhydride, biphenyl dianhydride, hexafluoroisopropylphthalic anhydride, benzophenone tetracarboxylic anhydride, and oxobisphthalic anhydride.

13. The thermally induced crosslinked phenolphthalein-based polybenzoxazole gas separation membrane material according to claim 12, wherein, The diamine monomer is 3,3'-diaminophenolphthalein, or the diamine monomer is 3,3'-diaminophenolphthalein and benzyldiamine.

14. The thermally induced crosslinked phenolphthalein-based polybenzoxazole gas separation membrane material according to claim 12 or 13, wherein, n is between 20 and 500, and the thermally induced crosslinked phenolphthalein-based polybenzoxazole gas separation membrane material is selected from any of the following: In Equation I, R1 = CH3, R2 = CH3, and R3 = CH3.

15. The thermally induced crosslinked phenolphthalein-based polybenzoxazole gas separation membrane material according to claim 12, wherein, When R1=CH3, R2=CH3, and R3=CH3 in Formula I, the thermally induced crosslinked phenolphthalein-based polybenzoxazole gas separation membrane material is a PBO polymer synthesized from monomer (c), 3,3'-diaminophenolphthalein and dianhydride monomer through thermal imidization, thermally induced crosslinking, and thermal rearrangement reactions. （c）。

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