Aldehyde monomer, special polyimine foam material with hydrophobic property, preparation method and recovery method

Abstract

The invention discloses an aldehyde monomer, a special polyimine foam material with recoverability and bulk-phase hydrophobic property, a preparation method and a recovery method, and belongs to the technical field of synthesis and preparation of high polymer materials. The polyimine foam material is prepared from an aldehyde monomer and an amine monomer through a foam preparation process including ball-milling mixing and prepolymerization, hot press molding and network curing, and salt template removal and drying. The core characteristic of the polyimine foam material is that the polyimine foam material has a porous structure with a self-similar body phase, that is, the morphology and the distribution height of pores from the surface to the interior are consistent; the polyimine foam material has excellent bulk phase hydrophobicity, that is, any fracture surface of the material has hydrophobicity consistent with that of the original outer surface. Based on recoverability endowed by dynamic imine bonds, the polyimine foam material has chemical recoverability and physical reprocessability at the same time, and has wide application prospects in the fields of long-acting waterproofness, self-cleaning, energy conservation, heat preservation, microfluidics and the like.

Description

Technical Field

[0001] This invention belongs to the field of polymer material synthesis and preparation technology, specifically relating to an aldehyde monomer and a special polyimide foam material with hydrophobic properties, its preparation method and recycling method. Background Technology

[0002] Hydrophobic polymers have demonstrated significant application value in areas such as waterproofing, self-cleaning, corrosion resistance, and anti-icing. Currently, most hydrophobic polymers are mainly prepared by constructing micron-nano-level rough structures on the material surface and modifying them with low surface energy materials (such as fluorine- or silicon-containing compounds). However, this type of surface engineering-dependent strategy has inherent limitations: its hydrophobic properties are strictly limited to the thin outermost surface layer of the material. In practical applications, the surface micro- and nano-structures are easily damaged by mechanical wear, leading to a rapid decline or even complete loss of hydrophobic properties. Furthermore, these surface treatment processes often involve complex steps, use large amounts of organic solvents or corrosive aqueous solutions, and typically require specialized equipment, resulting in high costs and environmental and safety concerns.

[0003] On the other hand, from the perspective of materials lifecycle management, most existing hydrophobic polymers do not adequately consider their post-disposal treatment in their design, and generally lack effective recycling and reuse pathways, making it difficult to meet the development requirements of green chemistry and the circular economy. It is worth noting that most hydrophobic polymers rely on biohazardous perfluoroalkyl and polyfluoroalkyl substances (PFAS) in their preparation process. Without effective recycling, these persistent compounds will inevitably accumulate as waste, leading to ecological risks and regulatory issues.

[0004] Dynamic covalent chemistry, especially imine chemistry, provides a promising molecular platform for constructing polymer networks with both superior performance and intrinsic recyclability. Based on the reversibility of dynamic imine bonds, various remodelable, repairable, and even chemically depolymerizable dynamic covalent polymer materials have been developed. However, combining the properties of dynamic polyimides with stable and durable hydrophobic functions, especially overcoming the limitations of traditional "surface hydrophobicity" to prepare intrinsic bulk hydrophobic materials with hydrophobic properties throughout the entire three-dimensional volume, still faces significant challenges. This requires materials to not only possess a uniform hydrophobic chemical composition from the surface to the interior, but also to achieve the construction and maintenance of full-scale hydrophobic characteristics at the microstructure level. Simultaneously, developing matching, simple, green, and scalable preparation processes, and achieving efficient closed-loop recycling of materials after use (including physical reprocessing and chemical recycling), are key issues that urgently need to be addressed to promote the practical application of such advanced functional polymer materials. Summary of the Invention

[0005] The purpose of this invention is to provide an aldehyde monomer and a special polyimide foam material with hydrophobic properties, a preparation method and a recycling method. The polyimide foam material has a bulk hydrophobicity from the surface to the interior, can be prepared by a solvent-free process, and can achieve physical and chemical closed-loop recycling.

[0006] The structural formula of the aldehyde monomer described in this invention is shown in (I):

[0007]

[0008] (I)

[0009] In equation (I), n is a positive integer representing the number of repeating units;

[0010] In formula (I), R represents benzyl, C2~C 12 Alkyl, C2~C 12 Perfluoroalkyl, C1~C 12 Ester group or long-chain siloxane with a number average molecular weight of 4000-6000.

[0011] The hydrophobic special polyimide foam material of this invention is obtained from one or more aldehyde monomers and one or more amine monomers through the foam preparation process proposed in this invention. The core feature of the polyimide foam material is its self-similar porous structure, meaning that the pore morphology and distribution are highly consistent from the surface to the interior. The polyimide foam material also possesses bulk hydrophobicity, meaning that any fracture surface of the material exhibits hydrophobic properties consistent with the original outer surface. Based on the recyclability conferred by dynamic imine bonds, the polyimide foam material simultaneously possesses chemical recyclability and physical reprocessing capability.

[0012] The preparation method of a special polyimide foam material with hydrophobic properties according to the present invention comprises the following steps:

[0013] (1) Ball milling and prepolymerization: Weigh one or more aldehyde monomers with a molar ratio of aldehyde to amine of 1:1 and place one or more amine monomers in a ball milling jar. Add 2 to 10 times the total mass of organic matter and water-soluble inorganic salt as a pore-forming template for solventless ball milling to achieve full mixing of organic and inorganic matter. Then, activate the polymerization reaction with mechanical force to obtain a uniform prepolymer / inorganic salt composite powder.

[0014] (2) Hot pressing and network curing: The prepolymer / inorganic salt composite powder obtained in step (1) is hot pressed to promote the complete cross-linking of the polymer network and achieve macroscopic shaping of the material, resulting in a composite block in which the polymer and water-soluble inorganic salt are evenly distributed.

[0015] (3) Salt template removal and drying: Soak the composite block obtained in step (2) in water to remove the inorganic salt pore template, and dry it to obtain a special polyimide foam material with a through-porous structure, recyclability and hydrophobic properties.

[0016] Preferably, the water-soluble inorganic salt selected in step (1) can be one or more of the following: calcium chloride, sodium chloride, potassium chloride, sodium sulfate, copper sulfate, magnesium sulfate, sodium carbonate, sodium bicarbonate, potassium carbonate, sodium nitrate, ammonium chloride, etc.

[0017] Preferably, in step (1), the ball milling speed is 50~500 rpm and the ball milling time is 10~120 minutes;

[0018] Preferably, in step (2), the hot pressing temperature is 80~150 ℃, the hot pressing pressure is 1~10 MPa, and the hot pressing time is 30~120 minutes;

[0019] Preferably, in step (3), the water is soaked and filtered for 1 to 10 days, and the water is changed every 3 to 12 hours.

[0020] The amine monomers described in this invention are one or more of the following: p-phenylenediamine, 1,4-butanediamine, tris(2-aminoethyl)amine, 4-[(4-aminocyclohexyl)methyl]cyclohexyl-1-amine, tris(4-aminophenyl)amine, 1,4-bis(4-amino-2-trifluoromethylphenoxy)benzene, 4,4'-(1,3-phenylenedioxy)diphenylamine, and 1,8-octanediamine.

[0021] The present invention also provides a polyimide foam material with recyclability and bulk hydrophobic properties, which is prepared by the above method.

[0022] This invention also provides a method for recycling the above-mentioned polyimide foam material with recyclability and bulk hydrophobic properties, including a physical reprocessing method and a chemical recycling method:

[0023] Physical reprocessing and recycling method: After crushing the polyimide foam material, it is mixed with water-soluble inorganic salts again. The process of "ball milling and prepolymerization, hot pressing and network curing, salt template removal and drying" in the above preparation steps can be repeated to achieve closed-loop physical reprocessing of polyimide foam material. The mechanical properties, chemical structure and hydrophobic properties of the resulting recycled material are comparable to those of the original material.

[0024] Chemical recovery method: The polyimide foam material is placed in a mixed solvent of tetrahydrofuran and dilute hydrochloric acid, and stirred at room temperature to promote the hydrolysis of imine bonds and achieve depolymerization of the polymer network. In this acidic system, the amine monomers will precipitate out of the depolymerization solution in the form of hydrochloride, while the aldehyde monomers will dissolve in the depolymerization solution. The two components can be separated by filtration, and the original aldehyde monomers and amine monomers can be recovered by concentration or neutralization steps respectively.

[0025] Preferably, the concentration of dilute hydrochloric acid in the mixed solvent is 0.005~0.5 mol / L, the volume of tetrahydrofuran is 10~1000 times the volume of dilute hydrochloric acid, and the polymer depolymerization time is 5~72 hours.

[0026] This invention provides a solvent-free, environmentally friendly, and scalable hydrophobic polyimide foam preparation strategy. The prepared polyimide foam has a self-similar structure throughout, ensuring stable performance and functional consistency of newly exposed surfaces after damage. The prepared hydrophobic polyimide foam exhibits bulk hydrophobicity from the surface inwards, and even after physical damage, the newly exposed surfaces remain hydrophobic. Thanks to dynamic imine bonds, the polyimide foam material can achieve a zero-waste physical and chemical dual closed-loop cycle, which conforms to the concept of circular economy and has broad application prospects in fields such as long-lasting waterproofing, self-cleaning, energy-saving insulation, and microfluidics.

[0027] The core principle of the polyimide foam material preparation and physical reprocessing method proposed in this invention lies in utilizing the reversible recombination ability of the polymer network endowed by dynamic covalent bonds. Through an integrated process of ball milling and prepolymerization, hot pressing and network curing, and salt template removal and drying, the polymer and inorganic salt template are uniformly composited and subsequently removed under solvent-free conditions, thereby constructing a porous foam material with structural consistency and interconnected channels. This foam material preparation and physical reprocessing method has good system applicability and scalability, and is not only applicable to polyimide systems but can also theoretically be extended to any other dynamic covalent polymer system, such as the "phenol-acetylene polymer system" and the "amine-acetylene polymer system." Attached Figure Description

[0028] Figure 1 The 1H NMR spectrum of an aldehyde monomer (PF-DBA) prepared in Example 1 of this invention (… 1 H NMR);

[0029] Figure 2 The 1H NMR spectrum of an aldehyde monomer (Bn-DBA) prepared in Example 1 of this invention (… 1 H NMR);

[0030] Figure 3 : Flowchart of the preparation process of the hydrophobic polyimide foam material described in this invention;

[0031] Figure 4 Fourier transform infrared (FT-IR) spectrum of the superhydrophobic polyimide foam obtained in Example 2;

[0032] Figure 5 Scanning electron microscope (SEM) images of the top surface, side surface, and internal cross-section of the superhydrophobic polyimide foam obtained in Example 2;

[0033] Figure 6 High-speed photograph of the process by which a water droplet bounces completely off the surface of the superhydrophobic polyimide foam obtained in Example 2 after contacting it;

[0034] Figure 7 Comparison photos of the original state of the superhydrophobic polyimide foam obtained in Example 2 and the surface water droplet morphology after sandpaper abrasion, as well as contact angle data;

[0035] Figure 8 Statistical analysis of contact angle and roll-off angle data of superhydrophobic polyimide foam with different cut sections obtained in Example 2;

[0036] Figure 9 : Process flow diagram of physical reprocessing of the superhydrophobic polyimide foam obtained in Example 2;

[0037] Figure 10 The process flow diagram of chemical recovery of the superhydrophobic polyimide foam obtained in Example 2 and the comparison of the NMR spectra of the recovered monomer and the original monomer are shown. Detailed Implementation

[0038] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of this application. Any modifications, equivalent substitutions, and improvements made within the core ideas and principles of this invention should be included within the protection scope of this invention.

[0039] Example 1

[0040] The synthetic route for a perfluoroalkyl-modified aldehyde monomer designed in this invention is shown in formula (II). The specific steps are as follows:

[0041] 1. Vanillin (compound R1, 10.0 g, 65.7 mmol), 1,2-dibromoethane (compound R2, 37.0 g, 197.2 mmol), and K2CO3 (13.6 g, 98.6 mmol) were added to acetonitrile (180 mL), and the mixture was stirred at 80 °C for 15 hours under nitrogen protection. After the reaction solution was cooled to room temperature, K2CO3 was removed by filtration, and the filter cake was washed with acetonitrile (50 mL). The organic phases were combined, and the solvent was removed by rotary evaporation under reduced pressure. The crude product was purified by silica gel column chromatography to obtain 4-(2-bromoethoxy)-3-methoxybenzaldehyde (compound R3, 15.4 g, yield 90.5%).

[0042] 2. 2,5-Dihydroxyterephthalic acid (compound R4, 5.0 g, 25.2 mmol) was added to ethanol (EtOH, 200 mL) and stirred at 80 °C until completely dissolved. Then, concentrated sulfuric acid (20 mL, 98% by mass) was slowly added dropwise over 30 minutes, and the reaction was continued at 80 °C with stirring for 12 hours. After the reaction solution cooled to room temperature, it was neutralized to neutral with saturated sodium bicarbonate aqueous solution. The precipitate was collected by filtration, and the filter cake was dried and purified by silica gel column chromatography to obtain diethyl 2,5-dihydroxyterephthalate (compound R6, 4.9 g, yield 76.4%).

[0043]

[0044] (II)

[0045] 3. Compound R3 (15.0 g, 58.0 mmol), compound R6 (5.9 g, 23.2 mmol), potassium iodide (KI, 1.0 g, 6.0 mmol), tetrabutylammonium bromide (TBAB, 0.6 g, 1.9 mmol), and potassium carbonate (K2CO3, 8.0 g, 58.0 mmol) were placed in N,N-dimethylformamide (DMF, 200 mL) and stirred at 80 °C for 15 hours under a nitrogen atmosphere. After the reaction solution was cooled to room temperature, it was poured into vigorously stirred water (400 mL) to precipitate a pale yellow precipitate. The precipitate was filtered through a Buchner funnel and washed with water to obtain the crude product. The crude product was dispersed in ethyl acetate (EtOAc, 100 mL), stirred at room temperature for 12 hours, filtered, and the filter cake was dried under vacuum to obtain Et-DBA molecules.

[0046] 4. Add 5.0 g (8.2 mmol) of Et-DBA molecules to a mixed solvent of DMF (200 mL) and EtOH (150 mL), and stir at 90 °C until completely dissolved. Separately, dissolve potassium hydroxide (KOH, 2.3 g, 41.0 mmol) in water (30 mL), and add this solution dropwise to the above reaction system at 90 °C. After stirring at 90 °C for 10 hours, separate the white precipitate by hot filtration. Dissolve the precipitate in a mixed solution of water (100 mL) and ethanol (20 mL), adjust the pH of the system to 2 with hydrochloric acid, filter the precipitate through a Buchner funnel, wash with water (50 mL), and dry under vacuum to obtain COOH-DBA molecules;

[0047] 5. COOH-DBA (5.0 g, 9.0 mmol), 1H,1H,2H,2H-perfluorooctanol (compound R7, 6.6 g, 18.0 mmol), and 4-dimethylaminopyridine (DMAP, 0.9 g, 7.2 mmol) were dissolved in dichloromethane (DCM, 250 mL). After cooling the reaction system in an ice-water bath for 30 min, a dichloromethane solution (150 mL) of 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride (EDCI, 6.9 g, 36.1 mmol) was slowly added dropwise to the reaction solution over 30 min. After removing the ice bath, the reaction solution was stirred at room temperature for 12 h. The organic phase was then washed with saturated sodium chloride aqueous solution, dried over anhydrous magnesium sulfate, filtered, and concentrated under reduced pressure. The perfluoroalkyl-modified dialdehyde monomer (PF-DBA, 6.2 g, yield 54.9%) was purified by silica gel column chromatography. The 1H NMR spectrum of the obtained PF-DBA monomer is as follows: Figure 1 As shown.

[0048] The remaining aldehyde monomers can be obtained using similar synthetic routes and preparation methods.

[0049] For example, a benzyl-modified dialdehyde monomer (Bn-DBA) can be obtained by reacting COOH-DBA with benzyl alcohol (denoted as compound R8), as shown in formula (III):

[0050]

[0051] (III)

[0052] The specific synthesis method is similar to that of PF-DBA. COOH-DBA (5.0 g, 9.0 mmol), benzyl alcohol (compound R8, 1.9 g, 18.0 mmol), and 4-dimethylaminopyridine (DMAP, 0.9 g, 7.2 mmol) were dissolved in dichloromethane (DCM, 250 mL). After cooling the reaction system in an ice-water bath for 30 minutes, a solution of 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride (EDCI, 6.9 g, 36.1 mmol) in dichloromethane (150 mL) was slowly added dropwise to the reaction solution over 30 minutes. After removing the ice bath, the reaction solution was stirred at room temperature for 12 hours. The organic phase was then washed with saturated sodium chloride aqueous solution, dried over anhydrous magnesium sulfate, filtered, and concentrated under reduced pressure. The benzyl-modified dialdehyde monomer (Bn-DBA, 3.4 g, yield 51.5%) was purified by silica gel column chromatography. The 1H NMR spectrum of the obtained Bn-DBA monomer is as follows: Figure 2 As shown.

[0053] Example 2

[0054] The hydrophobic foam preparation process of this invention is as follows: Figure 3 As shown. All aldehyde and amine monomers synthesized and involved in this invention can be used to prepare hydrophobic foam materials using this preparation process.

[0055] Taking the perfluoroalkyl-modified aldehyde monomer PF-DBA and tris(2-aminoethyl)amine (TREN) synthesized in Example 1 as an example, the following steps were taken: First, the synthesized PF-DBA aldehyde monomer (0.935 g, 0.75 mmol), TREN amine monomer (0.073 g, 0.5 mmol), and sodium chloride (4.03 g) four times the total mass of organic matter were placed in a ball mill jar and milled at 200 rpm for 1 hour using a planetary ball mill. The mixture was then removed and dried in a vacuum oven at 80 °C for 24 hours. Subsequently, the dried sample was milled again under the same conditions (200 rpm, 1 hour) to obtain a uniform powder. The powder was transferred to a hot press mold and hot-pressed at 120 °C and 5 MPa for 1 hour. Then, the surface of the hot-pressed polyimide-inorganic salt composite was sanded and then immersed in water for a total soaking time of 3 days (with water changed every 12 hours) to remove the inorganic salt pore-forming template. After soaking, the sample was allowed to air dry at room temperature, and then the surface was sanded again to obtain the final polyimide foam material (Foam-PFPI). Testing showed that the obtained polyimide foam Foam-PFPI surface had a contact angle of approximately 151° and a roll-off angle of approximately 2°, exhibiting superhydrophobicity.

[0056] like Figure 4As shown, Fourier transform infrared spectroscopy results indicate that the stretching vibration peak corresponding to the aldehyde group in the Foam-PFPI material (1678 cm⁻¹) is present. -1 ) and the stretching vibration peak of the amino group (3355 cm) -1 With 3282 cm -1 All peaks disappeared completely, while a stretching vibration peak (1645 cm⁻¹) belonging to the imine bond appeared. -1 This proves that the polymerization reaction was completed and a polyimide network was formed.

[0057] Other aldehyde and amine monomers can also be used to prepare hydrophobic foam materials using similar preparation processes. For example, the benzyl-modified aldehyde monomer Bn-DBA and the amine monomer TREN synthesized in Example 1 can also be used to prepare hydrophobic foam materials using similar preparation processes.

[0058] The synthesized Bn-DBA aldehyde monomer (0.551 g, 0.75 mmol), TREN amine monomer (0.073 g, 0.5 mmol), and sodium chloride (2.5 g), four times the total mass of organic matter, were placed in a ball mill jar and milled at 200 rpm for 1 hour using a planetary ball mill. The mixture was then removed and dried in a vacuum oven at 80 °C for 24 hours. The dried sample was then milled again under the same conditions (200 rpm, 1 hour) to obtain a uniform powder. This powder was transferred to a hot press mold and hot-pressed at 120 °C and 5 MPa for 1 hour. The hot-pressed polyimide-inorganic salt composite was then sanded and immersed in water for a total soaking time of 3 days (water changed every 12 hours) to remove the inorganic salt pore-forming template. After soaking, the sample was allowed to air dry at room temperature, and finally, the surface was sanded again to obtain the final polyimide foam material (Foam-BnPI). Tests showed that the obtained Foam-BnPI surface has a contact angle of approximately 132°, exhibiting excellent hydrophobicity.

[0059] Example 3

[0060] The microstructure of the superhydrophobic polyimide foam Foam-PFPI obtained in Example 2 was observed. Figure 5 As shown, scanning electron microscope (SEM) images of its upper surface, side surface, and internal cross-section reveal that the material exhibits a uniform porous network throughout its volume, with a high degree of consistency between the surface and internal morphology, directly demonstrating its bulk self-similar structural characteristics. Using density methods, the porosity of the Foam-PFPI material is calculated to be approximately 81%.

[0061] Example 4

[0062] like Figure 6As shown, a high-speed camera was used to record the process of a water droplet (approximately 28 μL in volume) impacting the surface of superhydrophobic polyimide foam (Foam-PFPI) (Weber number We ≈ 30). The water droplet rapidly spreads and retracts upon contact with the surface, completely bouncing off the surface within approximately 23 milliseconds without any liquid residue or spreading, demonstrating that the material surface has extremely low water adhesion and excellent dynamic water repellency.

[0063] Example 5

[0064] Mechanical abrasion resistance: The surface of the superhydrophobic polyimide foam Foam-PFPI obtained in Example 2 was subjected to an abrasion test using 400-grit sandpaper (pressure 10 kPa). Figure 7 As shown, when the sample was worn down to 1 / 2 and 1 / 4 of its original height, the water droplets on the worn sample surface still remained spherical, and the static water contact angle measurement was still about 151°, which was not significantly different from the original sample.

[0065] Example 6

[0066] Demonstration of bulk superhydrophobicity: To confirm that the superhydrophobicity permeates the entire superhydrophobic polyimide foam (Foam-PFPI) obtained in Example 2, the sample was cut along a uniaxial direction at 10% intervals of its original length, thus exposing multiple fresh internal cross-sections. The static water contact angle and droplet roll-off angle data of these cross-sections were systematically measured. (See attached data.) Figure 8 As shown in the statistical chart, the contact angle of all tested sections remained stable at around 151°, and the roll-off angle was approximately 2°. This result is basically consistent with the measured values ​​of the original outer surface, proving that Foam-PFPI possesses bulk superhydrophobicity from the surface inwards.

[0067] Example 7

[0068] Based on the dynamic covalent bonds in the polymer network of materials, we established such as Figure 9 The physical reprocessing flow shown. The foam waste ( Figure 9 a) Mix with sodium chloride in a predetermined ratio, and then repeat the ball milling and prepolymerization process in Example 2. Figure 9 b) Hot pressing and network curing ( Figure 9 c) Salt template removal and drying steps ( Figure 8 f) enables the physical reprocessing of foam materials. Furthermore, even if cutting is required during the recycling process ( Figure 9 d) Polishing Figure 9 e) and other finishing operations on the material shape can also generate scraps that can be reused in the physical cycle through hot pressing (e) Figure 9(As shown in g). This means the entire physical reprocessing cycle produces no waste. This process requires no organic solvents or reagents to regenerate waste foam into a new product with the same porous structure as the original material, thus realizing the physical reprocessing cycle of foam materials.

[0069] Example 8

[0070] The foam material synthesized in this invention can achieve monomer-level closed-loop chemical recycling at room temperature, as follows: Figure 10 As shown in the schematic diagram, the Foam-PFPI sample was placed in a mixed solvent of tetrahydrofuran / dilute hydrochloric acid. Figure 10 a) Stirring at room temperature. An acidic environment catalyzes the hydrolysis of imine bonds, causing complete depolymerization of the entire polymer network within 24 hours. After depolymerization, the triamine monomer precipitates from the depolymerization solution as hydrochloride, while the dialdehyde monomer dissolves in the solution. The two components are separated by filtration, yielding a filtrate containing dissolved aldehyde monomers and a filter residue of amine monomer hydrochloride (TREN·HCl). Pouring the filtrate into water precipitates the aldehyde monomer PF-DBA, which can be collected by filtration (recovery >90%). TREN·HCl is added to a methanol solution with an equal mass of potassium hydroxide and stirred for 5 minutes. Undissolved potassium hydroxide is then removed by filtration, and the amine monomer TREN is collected by distillation (recovery >85%). Figure 10 b and Figure 10 As shown in c, the recovered monomers were verified by NMR spectroscopy to have no structural changes and can be directly used for a new round of polymerization, realizing a chemical cycle from polymer to monomer.

[0071] As shown in the above embodiments, this invention provides a bulk hydrophobic polymer foam material based on polyimide and its solvent-free preparation and recycling methods. The preparation process of this invention is green and simple, requires no organic solvents, and operates under mild and controllable conditions. Experimental verification shows that the obtained foam material possesses a self-similar porous structure from the surface to the interior and intrinsic hydrophobicity throughout, making it suitable for applications such as long-lasting waterproofing, energy-saving and heat-insulating buildings, and microfluidics. The material can also be reused through physical reprocessing or regenerated through mild chemical depolymerization, providing a new paradigm for the design of sustainable polymer materials. Furthermore, the preparation / physical reprocessing process involved in this invention has system universality and can be further extended to other dynamic covalent polymer systems for the preparation of porous materials and their physical recycling.

[0072] The embodiments described above are merely illustrative of the implementation methods of the present invention, but should not be construed as limiting the scope of the present invention. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these modifications and improvements all fall within the protection scope of the present invention.

Claims

1. An aldehyde monomer, characterized in that: Its structural formula is shown in (I). ; （I） In equation (I), n is a positive integer representing the number of repeating units; In formula (I), R represents benzyl, C2~C 12 Alkyl, C2~C 12 Perfluoroalkyl, C1~C 12 Ester group or long-chain siloxane with a number average molecular weight of 4000-6000.

2. A method for preparing a polyimide foam material with recyclability and bulk hydrophobic properties, comprising the following steps: (1) Ball milling and prepolymerization: Weigh one or more of the aldehyde monomers described in claim 1 with a molar ratio of aldehyde group to amine group of 1:1 and place one or more of the amine monomers in a ball milling jar. Add 2 to 10 times the total mass of organic matter of water-soluble inorganic salt as a pore-forming template and perform solventless ball milling to achieve full mixing of organic matter and inorganic matter. Then, activate the polymerization reaction with mechanical force to obtain a uniform prepolymer / inorganic salt composite powder. (2) Hot pressing and network curing: The prepolymer / inorganic salt composite powder obtained in step (1) is hot pressed to promote the complete cross-linking of the polymer network and achieve macroscopic shaping of the material, resulting in a composite block in which the polymer and water-soluble inorganic salt are evenly distributed. (3) Salt template removal and drying: Soak the composite block obtained in step (2) in water to remove the inorganic salt pore template, and dry it to obtain a polyimide foam material with a through-porous structure, recyclability and bulk hydrophobic properties.

3. The method for preparing a polyimide foam material with recyclability and bulk hydrophobic properties as described in claim 2, characterized in that: The water-soluble inorganic salt selected in step (1) is one or more of the following: calcium chloride, sodium chloride, potassium chloride, sodium sulfate, copper sulfate, magnesium sulfate, sodium carbonate, sodium bicarbonate, potassium carbonate, sodium nitrate, and ammonium chloride.

4. The method for preparing a polyimide foam material with recyclability and bulk hydrophobic properties as described in claim 2, characterized in that: In step (1), the ball milling speed is 50~500 rpm and the ball milling time is 10~120 minutes.

5. The method for preparing a polyimide foam material with recyclability and bulk hydrophobic properties as described in claim 2, characterized in that: In step (2), the hot pressing temperature is 80~150 ℃, the hot pressing pressure is 1~10 MPa, and the hot pressing time is 30~120 minutes.

6. The method for preparing a polyimide foam material with recyclability and bulk hydrophobic properties as described in claim 2, characterized in that: In step (3), soak and filter the product in water for 1 to 10 days, and change the water every 3 to 12 hours.

7. The method for preparing a polyimide foam material with recyclability and bulk hydrophobic properties as described in claim 2, characterized in that: The amine monomer is one or more of p-phenylenediamine, 1,4-butanediamine, tris(2-aminoethyl)amine, 4-[(4-aminocyclohexyl)methyl]cyclohexyl-1-amine, tris(4-aminophenyl)amine, 1,4-bis(4-amino-2-trifluoromethylphenoxy)benzene, 4,4'-(1,3-phenylenedioxy)diphenylamine, and 1,8-octanediamine.

8. A polyimide foam material with recyclability and bulk hydrophobic properties, characterized in that: It is prepared by the preparation method described in any one of claims 2 to 7.

9. A method for recycling a polyimide foam material with recyclability and bulk hydrophobic properties as described in claim 8, characterized in that: The physical reprocessing and recycling method involves crushing the polyimide foam material and then remixing it with water-soluble inorganic salts, repeating the "ball milling and prepolymerization, hot pressing and network curing, salt template removal and drying" process in the preparation steps of claim 1, thereby achieving closed-loop physical reprocessing of the polyimide foam material.

10. A method for recycling a polyimide foam material with recyclability and bulk hydrophobic properties as described in claim 8, characterized in that: For chemical recovery, polyimide foam material is placed in a mixed solvent of tetrahydrofuran and dilute hydrochloric acid, and stirred at room temperature to promote the hydrolysis of imine bonds, thereby achieving polymer network depolymerization. In this acidic system, amine monomers precipitate from the depolymerization solution in the form of hydrochloride salts, while aldehyde monomers dissolve in the depolymerization solution. The two components are separated by filtration, and then subjected to concentration or neutralization steps respectively to recover the original aldehyde monomers and amine monomers. The concentration of dilute hydrochloric acid in the mixed solvent is 0.005~0.5 mol / L, and the volume of tetrahydrofuran is 10~1000 times that of dilute hydrochloric acid. The polymer depolymerization time is 5~72 hours.

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