A method for preparing melamine-based polyimide by a two-step process

Abstract

This invention discloses a two-step method for preparing melamine-based polyimide, belonging to the field of polymer materials technology. The polyimide material is prepared from melamine and pyromellitic dianhydride via a two-step method of low-temperature prepolymerization followed by high-temperature thermal imidization: first, a polyamic acid precursor is prepared in a low-temperature solution, and then subjected to solid-phase thermal imidization for dehydration and cyclization. This invention utilizes the pre-arrangement of the precursor structure and the small molecule escape during the cyclization process to create pores, successfully constructing a triazine ring-crosslinked polyimide with a well-developed mesoporous structure. This solves the problems of particle agglomeration, pore blockage, and low specific surface area in traditional one-step solid-phase melting methods. The resulting polyimide exhibits a well-developed mesoporous structure, high specific surface area, and high thermal stability. 5% Its thermal decomposition temperature reaches 402.2℃, and its specific surface area is 38.8107 m² / g. It can be widely used in high-temperature resistant materials, adsorption separation, catalyst carriers and other fields.

Description

[0001] Technical Field This invention belongs to the field of polymer material synthesis technology, specifically relating to a two-step method for preparing melamine-based polyimide.

[0002] Background Technology Polyimide (PI) is a class of high-performance polymers containing imide rings in its main chain. Due to its excellent heat resistance, chemical corrosion resistance, mechanical properties, and electrical insulation, it has wide applications in aerospace, microelectronics, separation membranes, and adsorption materials. In recent years, porous polyimide materials have shown great potential in gas adsorption, catalyst supports, and energy storage materials due to their high specific surface area and unique pore structure.

[0003] Currently, the synthesis of polyimides from melamine and pyromellitic dianhydride mostly employs a one-step process, primarily the solid-state melt method and the solvothermal synthesis method. The solid-state melt one-step method involves grinding and mixing melamine and pyromellitic dianhydride uniformly in a solid state, followed by direct heating to a high temperature (usually above 300°C) under an inert atmosphere to carry out a polycondensation reaction. During this process, the monomers undergo melting, condensation, dehydration, and cyclization, ultimately producing polyimide in one step. This method has a short process flow, is simple to operate, and does not require large amounts of organic solvents, offering certain economic and environmental advantages. However, because the reaction takes place in a high-temperature solid-state system, the monomers and oligomers are prone to melting and sintering in the molten state, leading to densification of the product particles and closure or blockage of the pore structure. Furthermore, uneven local reaction rates under high-temperature conditions can easily generate internal stress and structural defects. Solvent thermal synthesis involves reactants in high-boiling-point polar solvents, which improves monomer dispersion and reaction uniformity. However, it still requires high-temperature reactions, has high solvent recovery costs, and is prone to environmental pollution. Furthermore, the products are still prone to problems such as excessive cross-linking, wide molecular weight distribution, and difficulty in controlling the porous structure, thus limiting its application in the field of functionalized porous materials.

[0004] Therefore, developing a method for preparing melamine-based polyimide with mild reaction conditions, controllable product structure, well-developed pores, and excellent thermal stability is of great significance for expanding the application range of polyimide materials. Summary of the Invention

[0005] To address the shortcomings of existing technologies, the present invention aims to provide a two-step method for preparing melamine-based polyimide. This is a method for preparing high specific surface area porous polyimide materials. The method employs a two-step process of low-temperature prepolymerization and high-temperature thermal imidization. By controlling the structural pre-arrangement of the precursor polyamic acid (PAA) and the release of small molecules during the thermal cyclization process, a polyimide material with abundant mesoporous channels is constructed.

[0006] To achieve the objectives of this invention, the specific technical solution adopted is as follows: A two-step method for preparing melamine-based polyimide includes the following steps: (1) Low-temperature solution prepolymerization: Under the protection of an inert atmosphere, melamine is uniformly dispersed or dissolved in a polar aprotic solvent, and triethylamine is added as a catalyst. Then, pyromellitic dianhydride is added. After stirring and reacting, the mixture is poured into a precipitant to precipitate. The precipitate is filtered, washed and dried to obtain polyamic acid precursor powder. (2) High-temperature solid-phase thermal imidization: The polyamic acid precursor powder obtained in step (1) is placed in a high-temperature furnace and undergoes a dehydration cyclization reaction under an inert atmosphere or an air atmosphere to obtain the desired melamine-based polyimide.

[0007] As a preferred embodiment of this application, the reaction formula for the two-step method for preparing melamine-based polyimide is as follows: .

[0008] As a preferred embodiment of this application, in step (1) of the two-step method for preparing melamine-based polyimide, the polar aprotic solvent is any one or a mixture of several of N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMAc), or N-methylpyrrolidone (NMP); the amount of solvent used is 5 to 10 times the total mass of the reactants; and the inert atmosphere is a nitrogen atmosphere.

[0009] As a preferred embodiment of this application, in step (1) of the two-step method for preparing melamine-based polyimide, the amount of triethylamine catalyst added is 1 to 10% of the total mass of the reactants.

[0010] As a preferred embodiment of this application, in step (1) of the two-step method for preparing melamine-based polyimide, the molar ratio of melamine to pyromellitic dianhydride is 1:1.0 to 1:2.0.

[0011] As a preferred embodiment of this application, in step (1) of the two-step method for preparing melamine-based polyimide, the temperature during the stirring reaction is 40-80°C and the time is 2-6 hours.

[0012] As a preferred embodiment of this application, in step (1) of the two-step method for preparing melamine-based polyimide, the precipitant is deionized water, methanol, or ethanol.

[0013] As a preferred embodiment of this application, in step (2) of the two-step method for preparing melamine-based polyimide, the conditions for the dehydration cyclization reaction are: heating to 300-350°C at a heating rate of 1-10°C / min, and maintaining the temperature at that temperature for 1-4 hours.

[0014] As a preferred embodiment of this application, the method for preparing melamine-based polyimide using a two-step method further includes a step of washing and drying the product by Soxhlet extraction after the reaction in step (2).

[0015] This invention also protects melamine-based polyimide prepared by the method described above.

[0016] As a preferred embodiment of this application, the prepared polyimide possesses a well-developed mesoporous structure, high specific surface area, and high thermal stability. 5% Its thermal decomposition temperature reaches 402.2℃, and its specific surface area is 38.8107 m². 2 / g, can be widely used in high-temperature resistant materials, adsorption separation, catalyst support and other fields.

[0017] Compared with the prior art, the main advantages of the present invention are as follows: (1) Controllable structure and well-developed pores: The present invention adopts a two-step process. First, a polyamic acid precursor with a specific order is formed at low temperature, avoiding hard agglomeration caused by direct melting. In the subsequent thermal imidization process, the amide bond and carboxyl group cyclize and dehydrate to form a rigid imide ring, while the escape of water molecules plays a role in in-situ pore formation.

[0018] (2) High product purity and regular structure: The prepolymerization process makes the monomer reaction more complete, the cyclization is complete, there are no obvious residual functional groups, and the chemical main chain structure is uniform.

[0019] (3) Excellent thermal stability: The thermal decomposition temperature of polyimide T5% prepared by the two-step method is 402.2℃, which is higher than that of the one-step method product, and the thermal stability is better.

[0020] (4) The process is mild and has good repeatability: the prepolymerization temperature is low, the reaction is controllable, the post-processing is simple, and it is suitable for large-scale preparation. Attached Figure Description

[0021] Figure 1 This is a flowchart of the two-step process for synthesizing polyimide according to the present invention.

[0022] Figure 2 The images show the FT-IR spectra of the raw materials, polyamic acid, and polyimide used in Example 1 of this invention.

[0023] Figure 3The XRD patterns of the raw materials, polyamic acid, and polyimide used in Examples 1 and 6 of this invention are shown.

[0024] Figure 4 The images show the SEM spectra of polyamic acid and polyimide in Examples 1 and 6 of this invention.

[0025] Figure 5 These are the TGA spectra of polyimide from Examples 1 and 6 of this invention.

[0026] Figure 6 The N2 adsorption-desorption isotherm and pore size distribution diagram of polyimide in Example 1 of the present invention are shown.

[0027] Figure 7 The N2 adsorption-desorption isotherm and pore size distribution diagram of polyimide in Example 6 of the present invention are shown. Detailed Implementation

[0028] A two-step method for preparing melamine-based polyimide includes the following steps: (1) Low-temperature solution prepolymerization: Under the protection of an inert atmosphere, melamine is uniformly dispersed or dissolved in a polar aprotic solvent, and triethylamine is added as a catalyst. Then, pyromellitic dianhydride is added to the system and stirred at 40-80°C for 2-6 hours. After the reaction is completed, it is poured into a precipitant to precipitate. After filtration, washing and drying, polyamic acid precursor powder is obtained. (2) High-temperature solid-phase thermal imidization: The polyamic acid powder obtained in step (1) is placed in a high-temperature furnace and heated to 300-350°C at a heating rate of 1-10°C / min under an inert atmosphere or air atmosphere. The temperature is kept constant for 1-4 hours to induce a dehydration cyclization reaction, thereby obtaining the polyimide material.

[0029]

[0030] In step (1), the molar ratio of melamine to pyromellitic dianhydride is 1:1.0 to 1:2.0, preferably 1:1.5.

[0031] In step (1), the polar aprotic solvent is any one or a mixture of several of N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMAc), or N-methylpyrrolidone (NMP).

[0032] In step (1), the amount of triethylamine used is 1 to 10 wt% of the total mass of the reactants.

[0033] In step (1), the precipitant is deionized water, methanol or ethanol.

[0034] In step (2), after the reaction is complete, the product is washed and dried using Soxhlet extraction.

[0035] The technical solution of the present invention will be clearly and completely described below with reference to the embodiments. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0036] It should be noted that the following detailed descriptions are illustrative and intended to provide further explanation of this application. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains.

[0037] It should be noted that the terminology used herein is for the purpose of describing particular implementations only and is not intended to limit the exemplary implementations according to this application. As used herein, the singular form includes the plural form unless the context clearly indicates otherwise. Furthermore, it should be understood that when the terms "comprising" and / or "including" are used in this description, they indicate the presence of features, steps, operations, devices, components, and / or combinations thereof.

[0038] In this invention, some conventional operating equipment, devices and components have been omitted or only briefly described.

[0039] Unless otherwise specified in the examples, the conditions shall be performed according to the standard conditions or the conditions recommended by the manufacturer.

[0040] In this invention, unless otherwise specified, all ratios are mass ratios, and % represents the mass percentage content.

[0041] Example 1: A two-step method for preparing melamine-based polyimide includes the following steps: (1) Under a nitrogen atmosphere, 100 mL of dried N,N-dimethylformamide was added to a reactor equipped with a mechanical stirrer and a condenser, followed by 5.04 g (0.04 mol) of melamine. The mixture was stirred to ensure uniform dispersion. Subsequently, 1.5 mL of triethylamine was added as a catalyst. Under stirring, 13.09 g (0.06 mol) of pyromellitic dianhydride was slowly added to the system, i.e., the molar ratio of MA to PMDA was 1:1.5. After the addition was complete, the system temperature was raised to 60 °C and the reaction was maintained at this temperature for 4 hours. After the reaction was completed, the reaction solution was slowly poured into a large amount of deionized water to precipitate the product. The precipitate was filtered to obtain a filter cake, which was washed three times alternately with deionized water and ethanol. The cake was then dried in a vacuum oven at 80 °C to constant weight to obtain polyamic acid (PAA) precursor powder.

[0042] (2) The polyamic acid (PAA) precursor powder obtained above was transferred to a crucible, and the crucible was placed in a muffle furnace and heated to 320°C at a heating rate of 5°C / min, and kept at 320°C for 2 hours. After naturally cooling to room temperature, the product was taken out, washed with deionized water as solvent using a Soxhlet extractor for 12 hours, and vacuum dried to obtain the target product, denoted as polyimide PI-1.

[0043] Example 2: A two-step method for preparing melamine-based polyimide includes the following steps: (1) Under a nitrogen atmosphere, 100 mL of dried N,N-dimethylacetamide was added to a reactor equipped with a mechanical stirrer and a condenser, followed by 5.04 g (0.04 mol) of melamine. The mixture was stirred to ensure uniform dispersion. Subsequently, 2.0 mL of triethylamine was added as a catalyst. Under stirring, 8.72 g (0.04 mol) of pyromellitic dianhydride was slowly added to the system, i.e., the molar ratio of MA to PMDA was 1:1. After the addition was complete, the system temperature was raised to 40 °C and the reaction was maintained at this temperature for 6 hours. After the reaction was completed, the reaction solution was slowly poured into a large amount of methanol to precipitate the product. The precipitate was obtained by filtration, washed three times with methanol, and then dried in a vacuum oven at 80 °C to constant weight to obtain polyamic acid (PAA) precursor powder.

[0044] (2) The polyamic acid (PAA) precursor powder obtained above was transferred to a crucible, and the crucible was placed in a tube furnace. Under a nitrogen atmosphere, the temperature was increased to 300°C at a heating rate of 1°C / min and kept at a constant temperature for 4 hours. After natural cooling, the product was taken out and subjected to Soxhlet extraction and washing with ethanol as solvent for 24 hours. After vacuum drying, the polyimide material was obtained, which was denoted as polyimide PI-2.

[0045] Example 3: A two-step method for preparing melamine-based polyimide includes the following steps: The preparation steps are basically the same as in Example 1, except that: in step (1), the amount of MA is 5.04 g (0.04 mol) and the amount of PMDA is 17.45 g (0.08 mol), that is, the molar ratio of MA to PMDA is 1:2; the amount of triethylamine catalyst is 2.5 mL; the reaction temperature is 80℃ and the reaction time is 2 hours.

[0046] In step (2), the thermal imidization heating rate is 10℃ / min, heated to 350℃, and kept at that temperature for 1 hour. The final product is denoted as polyimide PI-3.

[0047] Example 4: A two-step method for preparing melamine-based polyimide includes the following steps: The preparation steps are basically the same as those in Example 1, except that: in step (1), the molar ratio of MA to PMDA is 1:1.2 (MA 5.04g, PMDA 10.47g), the amount of triethylamine catalyst is 1.8 mL, the reaction temperature is 70℃, and the reaction time is 3 hours.

[0048] In step (2), the thermal imidization heating rate is 5℃ / min, and the temperature is raised to 310℃ and held constant for 2.5 hours. The final product is denoted as polyimide PI-4.

[0049] Example 5: A two-step method for preparing melamine-based polyimide includes the following steps: The preparation steps are basically the same as those in Example 1, except that: in step (1), the amount of triethylamine catalyst is 10 wt% of the total mass of the reaction monomers; the reaction temperature is 50°C and the reaction time is 5 hours.

[0050] In step (2), thermal imidization is carried out in an air atmosphere at a heating rate of 8°C / min, heated to 340°C, and held at that temperature for 1.5 hours. The final product is denoted as polyimide PI-5.

[0051] Example 6: (This example is a comparative example) To demonstrate the technical advantages of the two-step method of this invention compared to the one-step method, a comparative sample was prepared using a traditional solid-phase melting one-step method: 5.04 g (0.04 mol) of melamine and 13.09 g (0.06 mol) of pyromellitic dianhydride were ground evenly in a mortar, transferred to a crucible, and reacted at a constant temperature of 320°C at a rate of 5°C / min under a nitrogen atmosphere for 4 hours. The product was then subjected to Soxhlet extraction, washing, and drying to obtain the comparative sample, denoted as PI-7.

[0052] Taking Example 1 as an example, the structure and properties of the polyimide powders prepared according to the above preparation method were characterized. Among them, Figure 2 The images show the FT-IR spectra of the raw materials, polyamic acid, and polyimide used in Example 1.

[0053] Depend on Figure 2 It can be seen that PMDA is located at 1853.79cm -1 and 1768.87cm -1 The characteristic peak of COO disappeared, and the newly appearing characteristic peak of C=O(COOH) (1706.97 cm⁻¹) appeared. -1 The absorption peak of C=O(CONH) (amide I band: 1676.72 cm⁻¹) is observed. -1) and the bending vibration peak of C-NH (amide II band: 1583.34cm) -1 This demonstrates the successful synthesis of polyamic acid (PAA). After high-temperature treatment, the characteristic peak of PAA disappeared, replaced by the absorption peak of the imide ring at 1778.40 cm⁻¹. -1 1711.65cm -1 (Asymmetric and symmetric C=O of imide), and at 1373 cm -1 The appearance of a new absorption peak is attributed to the bending vibration of the CNC group of the imide group, indicating the successful synthesis of polyimide.

[0054] Figure 3 The images show the XRD patterns of the raw materials, polyamic acid, and polyimide used in Examples 1 and 6 of this invention. The images show that the crystallinity of PI-1 is significantly lower than that of PI-7, a result consistent with SEM analysis.

[0055] The PI-1 prepared in Example 1 and the PI-7 prepared in Example 6 were detected using scanning electron microscopy, respectively. Figure 4 As shown, a represents the two-step PAA method, b represents PI-1, and c represents PI-7. PI-1 has an irregular sheet-like / flocculent structure with a loose surface and abundant mesoporous channels; while PI-7 is a blocky crystal with layered stacking, exhibiting good crystallinity and a regular structure.

[0056] Figure 5 These are the TGA spectra of polyimide from Examples 1 and 6 of this invention. The data shows that PI-1 has a TGA content of... 5% The thermal decomposition temperature can reach 402.2℃, which is significantly better than PI-7 (342.0℃). This indicates that the loose porous structure formed by the two-step method can withstand thermal stress more uniformly, avoiding stress concentration and premature decomposition caused by defects due to the tight arrangement of molecular chains in highly crystalline materials.

[0057] Figure 6 and Figure 7 The figures show the N2 adsorption-desorption isotherms and pore size distributions of polyimide PI-1 in Example 1 and polyimide PI-7 in Example 6 of this invention. The nitrogen adsorption-desorption isotherm of PI-1 exhibits typical Type IV curve characteristics, with an H3-type hysteresis loop appearing in the relative pressure P / P0 range of 0.4–0.8. Calculations show that the product has a specific surface area of ​​38.81 m² / g, a total pore volume of 0.216 cm³ / g, and an average pore size of 20.33 nm, exhibiting a relatively well-developed slit-like mesoporous structure. In contrast, PI-7 has a specific surface area of ​​only 4.55 m² / g, a total pore volume of 0.024 cm³ / g, and an average pore size of 21.3 nm. The nitrogen adsorption-desorption isotherm shows that the adsorption amount is almost zero in the low-pressure region, indicating a lack of microporous structure.

[0058] In summary, the two-step method described in this invention successfully prepared a triazine ring cross-linked polyimide material with both good thermal stability and high specific surface area, as well as a well-developed mesoporous structure.

[0059] Obviously, the above embodiments are merely illustrative examples for clear explanation and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. A two-step method for preparing melamine-based polyimide, characterized in that, Includes the following steps: (1) Low-temperature solution prepolymerization: Under the protection of an inert atmosphere, melamine is uniformly dispersed or dissolved in a polar aprotic solvent, and triethylamine is added as a catalyst. Then, pyromellitic dianhydride is added. After stirring and reacting, the mixture is poured into a precipitant to precipitate. The precipitate is filtered, washed and dried to obtain polyamic acid precursor powder. (2) High-temperature solid-phase thermal imidization: The polyamic acid precursor powder obtained in step (1) is placed in a high-temperature furnace and undergoes a dehydration cyclization reaction under an inert atmosphere or an air atmosphere to obtain the desired melamine-based polyimide.

2. The method for preparing melamine-based polyimide using a two-step process according to claim 1, characterized in that, The reaction formula for this method is as follows: 。 3. The method for preparing melamine-based polyimide using a two-step process according to claim 1, characterized in that: In step (1), the polar aprotic solvent is any one or a mixture of several of N,N-dimethylformamide, N,N-dimethylacetamide or N-methylpyrrolidone; the amount of solvent used is 5 to 10 times the total mass of the reactants; and the inert atmosphere is a nitrogen atmosphere.

4. The method for preparing melamine-based polyimide using a two-step process according to claim 1, characterized in that: In step (1), the amount of triethylamine catalyst added is 1 to 10% of the total mass of the reactants.

5. The method for preparing melamine-based polyimide using a two-step process according to claim 1, characterized in that: In step (1), the molar ratio of melamine to pyromellitic dianhydride is 1:1.0 to 1:2.

0.

6. The method for preparing melamine-based polyimide using a two-step process according to claim 1, characterized in that: In step (1), the temperature during the stirring reaction is 40-80℃ and the time is 2-6 hours.

7. The method for preparing melamine-based polyimide using a two-step process according to claim 1, characterized in that: In step (1), the precipitant is deionized water, methanol or ethanol.

8. The method for preparing melamine-based polyimide using a two-step process according to claim 1, characterized in that, In step (2), the conditions for the dehydration cyclization reaction are: heating to 300-350°C at a heating rate of 1-10°C / min, and maintaining the temperature at this temperature for 1-4 hours.

9. The method for preparing melamine-based polyimide using a two-step process according to claim 1, characterized in that: After the reaction in step (2) is completed, the product is washed and dried using Soxhlet extraction.

10. Melamine-based polyimide prepared by the method according to any one of claims 1-9.

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