A process for the preparation of low molecular weight polyacrylamide

Abstract

The application discloses the technical field of high polymer synthesis and functional material modification, and relates to a preparation method of low-molecular-weight polyacrylamide. The method comprises the following steps: firstly, preparing a double-metal doped multi-level pore carbon nanotube-covalent organic framework composite modification material, and the preparation process comprises the following steps: surface functionalization of multi-walled carbon nanotubes, growth of a covalent organic framework layer, doping of double-metal ions, and subsequent reduction and calcination treatment; then, adding deionized water, acrylamide and the modification material into a three-necked flask, stirring under nitrogen to dissolve the acrylamide, preparing an oxidation-reduction initiator solution, and dropping the initiator solution into the system under the protection of nitrogen, and polymerization reaction is carried out in a constant-temperature water bath at 19-21 DEG C. After the polymerization is completed, the product system is washed, filtered, and finally, low-molecular-weight polyacrylamide is obtained by using a vacuum freeze-drying method. The polyacrylamide prepared by the method has low and controllable molecular weight, and has good solubility and salt resistance.

Description

Technical Field

[0001] This invention relates to the field of polymer synthesis and functional material modification technology, specifically to a method for preparing low molecular weight polyacrylamide. Background Technology

[0002] In industries such as oil extraction, water treatment, and papermaking, low molecular weight polyacrylamide (LMP) has become an irreplaceable key polymer material due to its low viscosity, high permeability, and good reactivity. Traditional high molecular weight polyacrylamide, typically exceeding one million molecules, suffers from excessive viscosity, leading to injection difficulties and low diffusion efficiency in scenarios such as deep profile control, oil displacement in low-permeability reservoirs, and complex formation water treatment, thus failing to meet practical requirements. In contrast, LPP with a molecular weight below 100,000 not only significantly improves fluidity but also balances thickening and permeability through appropriate chain length, demonstrating unique advantages, particularly in precise control of heterogeneous reservoirs and stable applications in high-salt environments. However, limited by the extremely high polymerization reactivity of acrylamide monomers, traditional free radical polymerization methods are prone to uncontrolled chain growth, resulting in products with generally high molecular weights and wide molecular weight distributions, making it difficult to consistently obtain low molecular weight products that meet industrial requirements.

[0003] To address these issues, researchers have attempted to regulate polymerization reactions by introducing chain transfer agents or inorganic nanoparticles. While chain transfer agents can reduce molecular weight, the residual chemicals not only affect product purity but may also pose potential hazards to the environment and subsequent applications. Inorganic nanoparticles, due to their strong surface inertness, exhibit weak interaction with acrylamide monomers, making it difficult to precisely control polymerization kinetics, resulting in uneven molecular weight distribution and unstable performance of the product. In recent years, metal-organic frameworks (MOFs) have been increasingly used for polymerization reaction regulation due to their high specific surface area and designable pore structures. However, their predominantly microporous channel structure limits monomer diffusion efficiency, makes them prone to structural collapse under high temperatures, and exhibits insufficient stability in complex aqueous systems, failing to meet the dual requirements of durability and controllability for industrial production.

[0004] To address the aforementioned technical bottlenecks, this invention aims to develop a novel modified material possessing high specific surface area, abundant active sites, and excellent stability. This material's strong interaction with the polymerization system allows for precise control of the free radical polymerization process, representing a key breakthrough in the preparation of high-performance low molecular weight polyacrylamide. This invention innovatively employs a composite modified material of bimetallic-doped hierarchical porous carbon nanotubes and a covalent organic framework. The hierarchical porous structure promotes monomer diffusion, while the coordination and redox properties of bimetallic ions capture and stabilize polymerization free radicals. Simultaneously, the electron delocalization effect of the covalent organic framework accelerates free radical quenching, thereby achieving precise inhibition of the polymerization chain growth. The preparation process involves surface functionalization of multi-walled carbon nanotubes to enhance hydrophilicity and interactions, in-situ growth of the covalent organic framework to construct an ordered porous structure, precise doping of bimetallic ions to regulate the distribution of active sites, and high-temperature reduction and calcination activation to optimize the material's structure and properties. Finally, a low-temperature redox initiation system enables the controllable synthesis of low molecular weight polyacrylamide, providing a novel technical pathway for the precise preparation of industrial-grade low molecular weight polyacrylamide. Summary of the Invention

[0005] The purpose of this invention is to provide a method for preparing low molecular weight polyacrylamide, which solves the technical problems of existing methods for preparing low molecular weight polyacrylamide, such as difficulty in controlling molecular weight, wide distribution, poor material stability, and residual harmful substances.

[0006] The present invention achieves the above objectives through the following technical solutions:

[0007] A method for preparing low molecular weight polyacrylamide includes the following steps:

[0008] S1. Deionized water, acrylamide, and bimetallic doped hierarchical porous carbon nanotube-covalent organic framework composite modified material are added to a three-necked flask equipped with a stirrer, a nitrogen inlet tube, and a condenser. Nitrogen gas is introduced while stirring until the acrylamide is completely dissolved.

[0009] S2, Dissolve potassium persulfate and sodium sulfite separately in deionized water to prepare an oxidation-reduction initiator solution;

[0010] S3, under continuous nitrogen protection, the redox initiator solution is added dropwise to the pretreatment system of step S1, and then the reaction system is immediately transferred to a constant temperature water bath at 19-21℃ for polymerization reaction;

[0011] S4. After polymerization, the product system is washed with deionized water; the precipitate is dispersed in deionized water to prepare a solution, which is then filtered through a filter membrane; finally, it is dried by vacuum freeze drying.

[0012] According to a preferred embodiment of the present invention, the deionized water is purchased from Wahaha Group Co., Ltd., and the product name is Wahaha Pure Water (purified by a laboratory-grade reverse osmosis device, with a conductivity ≤18.2MΩ·cm).

[0013] According to a preferred embodiment of the present invention, the acrylamide was purchased from Sinopharm Chemical Reagent Co., Ltd., model: AR grade, CAS number: 79-06-1.

[0014] According to a preferred embodiment of the present invention, the three-necked flask was purchased from Shanghai Yuzhuo Instrument Co., Ltd., model: 250mL glass three-necked flask (with stirrer, nitrogen inlet tube and condenser adapter).

[0015] According to a preferred embodiment of the present invention, the nitrogen gas is purchased from Hangzhou Jingong Special Gases Co., Ltd., and the type is high-purity nitrogen gas (purity ≥ 99.999%).

[0016] According to a preferred embodiment of the present invention, the potassium persulfate was purchased from Xilong Scientific Co., Ltd., model: AR grade, CAS number: 7727-21-1.

[0017] According to a preferred embodiment of the present invention, the sodium sulfite was purchased from Tianjin Kemei Chemical Reagent Co., Ltd., model: AR grade, CAS number: 7757-83-7.

[0018] According to a preferred embodiment of the present invention, the filter membrane is purchased from Shanghai Xinya Purification Device Factory, model: mixed cellulose ester (MCE) filter membrane, with a pore size of 0.22-0.24 μm.

[0019] According to a preferred embodiment of the present invention, in step S1, the nitrogen purging time is 40-50 min.

[0020] In step S1 of this invention, mechanical stirring under nitrogen protection achieves uniform dispersion and stabilization of acrylamide monomers. Nitrogen, as an inert gas, replaces oxygen in the reaction system, preventing oxidative degradation or free radical side reactions of acrylamide during dissolution. The surface of multi-walled carbon nanotubes, after composite modification, is rich in active functional groups such as carboxyl, amino, and sulfonic acid groups. These functional groups specifically bind to the amide groups (-CONH2) in acrylamide molecules through hydrogen bonding, forming a stable physical adsorption layer. On the one hand, this interaction reduces the self-polymerization tendency between acrylamide molecules, delaying the initial initiation of the polymerization reaction; on the other hand, the hierarchical porous structure (including micropores, mesopores, and macropores) of the modified material provides diffusion channels for monomer molecules, ensuring uniform distribution of acrylamide in the solvent and avoiding uncontrolled polymerization caused by excessively high local concentrations. The stirring process mechanically breaks down the weak interactions (such as van der Waals forces) between monomer molecules, promoting full contact between them and the active sites on the surface of the modified material, ultimately forming a uniform and stable prepolymer system, laying the foundation for subsequent polymerization.

[0021] According to a preferred embodiment of the present invention, in step S2, the temperature of the redox initiator solution is 3-5°C.

[0022] In step S2 of this invention, potassium persulfate (KPS) is used as an oxidant and sodium sulfite as a reducing agent. The two are prepared in low-temperature deionized water to form a redox initiation system. The reaction mechanism of this system is based on an electron transfer process driven by the redox potential difference: potassium persulfate dissociates in solution to generate sulfate radicals, which have high oxidizing power; sodium sulfite provides a reducing electron donor. When the two come into contact, the sulfate radicals and sulfite ions undergo a rapid electron exchange reaction to generate moderately active primary radicals (such as SO32-). - (or ·OH). The role of the low-temperature environment (3-5℃) is crucial. On the one hand, it inhibits the thermal decomposition rate of the initiator itself, avoiding premature generation of excessive free radicals that could lead to runaway polymerization; on the other hand, it reduces the thermal activity of the acrylamide monomer, decreasing its probability of ineffective collisions and self-polymerization before initiation. This redox system achieves on-demand generation of free radicals through a mild electron transfer mechanism, providing a controllable source of active seeds for subsequent low-temperature polymerization.

[0023] According to a preferred embodiment of the present invention, in step S3, the dripping time is 15-20 min.

[0024] In step S3 of this invention, under continuous nitrogen protection, after the redox initiator solution is added dropwise to the pretreatment system, the primary free radicals rapidly combine with the acrylamide monomer, initiating a chain growth reaction. A low-temperature constant-temperature water bath (19-21℃) precisely controls the reaction temperature, balancing the free radical generation rate and the chain termination rate. The bimetallic-doped hierarchical porous carbon nanotube-covalent organic framework composite modified material plays a key regulatory role in this stage: its surface bimetallic ions (such as Zr)... 4+ With Ce 4 + The polymerization process involves capturing primary free radicals through coordination to form stable metal-radical intermediates, thus inhibiting excessive free radical binding and chain growth. The ordered microporous structure of the covalent organic framework slows down the diffusion rate of monomers to the active center through spatial confinement, reducing the chain growth rate constant. Sulfonic acid groups and amino functional groups in the hierarchical channels stabilize the transition state molecule through hydrogen bonding, further regulating the kinetic balance of chain growth. A nitrogen protective layer continuously isolates oxygen, preventing the generated polymer chains from being oxidized and broken by oxygen, ensuring that the polymerization reaction proceeds directionally under controllable conditions, ultimately forming low molecular weight polyacrylamide with concentrated molecular weight and narrow distribution.

[0025] According to a preferred embodiment of the present invention, in step S4, the number of times the deionized water is washed is 4-5 times; the pore size of the filter membrane is 0.22-0.24 μm; the temperature of vacuum freeze drying is -45℃, and the drying time is 36-40 h.

[0026] In step S4 of this invention, after polymerization, the system contains unreacted acrylamide monomers, trace amounts of initiator residue, and the generated polyacrylamide product. Deionized water washing, using a polar solvent, dissolves and removes water-soluble impurities (such as unreacted monomers and small molecule byproducts), with multiple washes (4-5 times) ensuring thorough removal of impurities. Membrane filtration utilizes a microporous structure (pore size 0.22-0.24 μm) to retain large polyacrylamide molecules while allowing small molecule impurities to pass through, achieving efficient solid-liquid separation. Vacuum freeze-drying, through the synergistic effect of low temperature (-45℃) and low pressure, allows water in the product to directly sublimate from a solid to a gaseous state, avoiding the high-temperature degradation or molecular chain breakage problems caused by traditional hot air drying. This process completely preserves the molecular chain structure and active functional groups of polyacrylamide, ultimately obtaining a high-purity, low-molecular-weight, and uniformly dispersed target product.

[0027] According to a preferred embodiment of the present invention, the preparation steps of the bimetallic doped hierarchical porous carbon nanotube-covalent organic framework composite modified material include:

[0028] A1. Multi-walled carbon nanotubes were dispersed in a mixed acid solution, ultrasonically dispersed in an ice bath, and then refluxed at 60-64℃. After cooling, they were diluted with deionized water to pH 6-7, filtered, washed with deionized water until neutral, and vacuum dried at 80-82℃. Oxidized multi-walled carbon nanotubes were dispersed in SOCl2 and refluxed at 80-82℃ to obtain acyl chloride multi-walled carbon nanotubes. Then, excess ethylenediamine was added, and the reaction was carried out at 80-84℃ to obtain aminated multi-walled carbon nanotubes. Finally, the aminated multi-walled carbon nanotubes were dispersed in a mixture of concentrated sulfuric acid and fuming sulfuric acid, sulfonated at 60-64℃, washed with deionized water until neutral, and dried at 100-102℃ to obtain hierarchical porous carbon nanotubes with carboxyl, amino, and sulfonic acid groups on the surface.

[0029] A2. Hierarchical porous carbon nanotubes were dispersed in a mixed solvent of mesitylene and ethanol and ultrasonically dispersed. 2,4,6-tris(4-aminophenyl)-1,3,5-triazine and 1,4-terephthalic acid were added and ultrasonically dispersed. Acetic acid was then added, and the mixture was transferred to a polytetrafluoroethylene reactor and reacted at 120-124℃. After cooling, the solid was collected by centrifugation, washed successively with tetrahydrofuran, ethanol, and deionized water, and vacuum dried at 80-82℃ to obtain carbon nanotubes with a surface-grown covalent organic framework layer.

[0030] A3. Covalent organic framework carbon nanotubes were dispersed in deionized water and ultrasonically dispersed. Aqueous solutions of ZrCl4 and Ce(NO3)3·6H2O were added sequentially, and the mixture was stirred at room temperature. The solid was collected by centrifugation, washed with deionized water, dispersed in ethanol, and dried at 60-64℃. Finally, the mixture was calcined at 300-304℃ under nitrogen protection to obtain a bimetallic doped hierarchical porous carbon nanotube-covalent organic framework composite modified material.

[0031] A4, the bimetallic doped hierarchical porous carbon nanotube-covalent organic framework composite modified material was placed in a tube furnace and reduced at 400-404℃ in a hydrogen atmosphere; then calcined at 500-510℃ under nitrogen protection.

[0032] According to a preferred embodiment of the present invention, the multi-walled carbon nanotubes were purchased from Shenzhen Nanoport Co., Ltd., model: CNTs-010 (diameter 10-20nm, length 5-15μm, purity ≥95%).

[0033] According to a preferred embodiment of the present invention, the SOCl2 was purchased from Shanghai Aladdin Biochemical Technology Co., Ltd., model: AR grade, CAS number: 7719-09-7.

[0034] According to a preferred embodiment of the present invention, the ethylenediamine was purchased from Sinopharm Chemical Reagent Co., Ltd., model: AR grade, CAS number: 107-15-3.

[0035] According to a preferred embodiment of the present invention, the concentrated sulfuric acid was purchased from Xilong Scientific Co., Ltd., model: AR grade, CAS number: 7664-93-9.

[0036] According to a preferred embodiment of the present invention, the fuming sulfuric acid was purchased from Suzhou Jingrui Chemical Co., Ltd., model: AR grade, CAS number: 8014-95-7.

[0037] According to a preferred embodiment of the present invention, the mesitylene was purchased from Shanghai Maclean Biochemical Technology Co., Ltd., model: AR grade, CAS number: 108-67-8.

[0038] According to a preferred embodiment of the present invention, the ethanol was purchased from Sinopharm Chemical Reagent Co., Ltd., model: AR grade, CAS number: 64-17-5.

[0039] According to a preferred embodiment of the present invention, the 2,4,6-tris(4-aminophenyl)-1,3,5-triazine was purchased from Shanghai Bid Pharmaceutical Technology Co., Ltd., model number: CAS No.: 1443-94-6.

[0040] According to a preferred embodiment of the present invention, the 1,4-terephthalic acid was purchased from Sinopharm Chemical Reagent Co., Ltd., model: AR grade, CAS number: 100-21-0.

[0041] According to a preferred embodiment of the present invention, the acetic acid was purchased from Xilong Scientific Co., Ltd., model: AR grade, CAS number: 64-19-7.

[0042] According to a preferred embodiment of the present invention, the polytetrafluoroethylene reactor was purchased from Zhengzhou Great Wall Science & Industry Trade Co., Ltd., model: KCF-50mL (polytetrafluoroethylene inner liner, stainless steel outer shell).

[0043] According to a preferred embodiment of the present invention, the tetrahydrofuran was purchased from Shanghai Titan Technology Co., Ltd., model: HPLC grade, CAS number: 109-99-9.

[0044] According to a preferred embodiment of the present invention, the ZrCl4 was purchased from Beijing Huawi Ruike Chemical Technology Co., Ltd., model: AR grade, CAS number: 10026-11-6.

[0045] According to a preferred embodiment of the present invention, the Ce(NO3)3·6H2O was purchased from Sinopharm Chemical Reagent Co., Ltd., model: AR grade, CAS number: 10099-56-9.

[0046] According to a preferred embodiment of the present invention, the tube furnace was purchased from Hefei Kejing Materials Technology Co., Ltd., model: OTF-1200X (silicon molybdenum rod heating, maximum temperature 1200℃).

[0047] According to a preferred embodiment of the present invention, in step A1, the volume ratio of concentrated sulfuric acid to concentrated nitric acid in the mixed acid solution is 3:1; the reflux reaction time at 60-64°C is 4-6 h; the vacuum drying time at 80-82°C is 12-14 h; the reflux reaction time at 80-82°C is 12-14 h; the reaction time at 80-84°C is 24-30 h; and the sulfonation reaction time at 60-64°C is 6-8 h.

[0048] In step A1 of this invention, the surface of multi-walled carbon nanotubes is subjected to ultrasonic treatment in an ice bath with a mixed acid solution (concentrated sulfuric acid and concentrated nitric acid in a volume ratio of 3:1), which induces an oxidation reaction to generate oxygen-containing functional groups such as carboxyl groups (-COOH) and hydroxyl groups (-OH). These functional groups enhance the hydrophilicity and chemical reactivity of the carbon nanotube surface. A reflux reaction at 60-64℃ further promotes the uniform distribution of the oxidized functional groups and removes amorphous carbon impurities from the carbon nanotube surface, forming a clean multi-walled carbon nanotube substrate. Subsequent acyl chloride reaction (SOCl2, 80-82℃) converts the carboxyl groups into highly reactive acyl chloride groups (-COCl), providing reaction sites for subsequent amylation modification. Ethylenediamine (80-84℃) undergoes a nucleophilic substitution reaction with the acyl chloride groups, introducing abundant amino (-NH2) functional groups and enhancing the hydrogen bonding ability between the material and the monomer. Finally, the sulfonation reaction of a mixture of concentrated sulfuric acid and fuming sulfuric acid (60-64℃) introduces sulfonic acid groups (-SO3H) onto the surface of carbon nanotubes, further regulating the adsorption behavior of monomers and the free radical capture efficiency through strongly acidic sites. The formation of the hierarchical porous structure (micropore-mesopore-macropore) originates from the selective etching and reconstruction of the carbon layer during the oxidation-sulfonation process, providing anchoring sites and diffusion channels for the subsequent growth of covalent organic frameworks.

[0049] According to a preferred embodiment of the present invention, in step A2, the reaction time at 120-124°C is 72-80 h; the vacuum drying time at 80-82°C is 12-14 h.

[0050] In step A2 of this invention, the amino functional groups on the surface of hierarchical porous carbon nanotubes undergo a condensation reaction with 2,4,6-tris(4-aminophenyl)-1,3,5-triazine (TAPT) and 1,4-terephthalic acid (BDC) to construct a covalent organic framework (COF) layer in a closed environment at 120-124°C. Acetic acid acts as a catalyst to promote the dehydration condensation between amino and carboxyl groups, forming stable imine bonds (-C=N-) and ester bonds (-COO-), thus constructing a COF material with a periodic network structure. A mixed solvent of mesitylene and ethanol provides a uniform reaction medium, ensuring that monomer molecules are uniformly dispersed on the surface of the carbon nanotubes. A long reaction time of 72-80 hours results in a COF layer thickness of 20-30 nm, forming an ordered microporous-mesoporous composite structure. Vacuum drying at 80-82°C removes residual solvent, preserving the rigid framework structure of the COF layer and providing a stable support for subsequent bimetallic doping. This step achieves spatial confinement and diffusion control of free radicals through an ordered network structure connected by covalent bonds.

[0051] According to a preferred embodiment of the present invention, in step A3, the stirring reaction time at room temperature is 12-14 h; the calcination time at 300-304 °C is 2-4 h.

[0052] In step A3 of this invention, after the covalent organic framework layer carbon nanotubes are dispersed in deionized water, the metal ions (ZrCl4 and Ce(NO3)3·6H2O in the aqueous solution) are... 4+ With Ce 4+ Zr binds to nitrogen and oxygen atoms in the COF layer through coordination, forming stable metal-organic coordination bonds. Slow stirring at room temperature (19-25℃) ensures that metal ions are uniformly loaded into the micropores and mesopores of the COF layer, preventing aggregation. 4+ The Lewis acidic sites can capture primary free radicals generated during acrylamide polymerization, stabilizing the free radical intermediates through electron delocalization; Ce 4+ redox properties (Ce 4+ / Ce 3+ The activity of free radicals is regulated through electron transfer cycles, inhibiting uncontrolled chain growth. After drying at 60-64℃ to remove moisture, a calcination process at 300-304℃ under nitrogen protection further stabilizes the metal-organic coordination structure and removes residual organic impurities, forming a bimetallic doped hierarchical porous composite structure (micropore-mesopore-macropore). This step achieves precise capture of polymerization free radicals and dynamic control of chain growth through the synergistic effect of bimetallic ions.

[0053] According to a preferred embodiment of the present invention, in step A4, the reduction treatment time at 400-404℃ is 3-4 hours; the calcination time at 500-510℃ is 2-4 hours.

[0054] In step A4 of this invention, the reduction treatment at 400-404℃ in a hydrogen atmosphere involves hydrogen atoms reacting with residual oxides (such as ZrO2 and CeO2) on the material surface to generate a more active metallic state (such as ZrO2). 0 Ce 0 ) or low-valence ions (such as Zr) 3+ Ce 3+ This process enhances the material's ability to capture free radicals. Calcination at 500-510℃ under nitrogen protection further stabilizes the material's crystal structure, promotes the orderly arrangement of hierarchical channels, and removes any potential volatile impurities. This high-temperature treatment optimizes the dispersion state and active site distribution of the bimetallic ions, giving the composite modified material higher thermal stability and reactivity. It allows for long-term performance stability during subsequent polymerization reactions, enabling the controllable and reproducible preparation of low molecular weight polyacrylamide.

[0055] The beneficial effects of this invention are as follows:

[0056] This invention significantly improves the performance controllability and industrial applicability of low molecular weight polyacrylamide by regulating the polymerization reaction through a bimetallic-doped hierarchical porous carbon nanotube-covalent organic framework composite modified material. The unique hierarchical porous structure and bimetallic synergistic active site design of this modified material fundamentally solve the technical problem of precise molecular weight control in traditional methods. The hierarchical porous structure, comprising a multi-level channel network of micropores, mesopores, and macropores, provides an efficient diffusion path for acrylamide monomers, ensuring uniform distribution of reactants in the polymerization system, and also creates a rich environment of active sites for free radical capture and chain termination reactions. Bimetallic ions, through coordination and redox properties, precisely capture primary free radicals generated during polymerization and regulate the chain growth rate through electron transfer mechanisms, effectively suppressing the uncontrollable chain reactions common in traditional free radical polymerization. Experimental data show that the low molecular weight polyacrylamide prepared by this method has a stable molecular weight control in a low range and a significantly reduced molecular weight distribution index. This avoids problems such as excessive viscosity and injection difficulties caused by excessively high molecular weight in traditional processes. At the same time, it retains an appropriate chain length to achieve excellent thickening and flocculation properties, demonstrating more precise application adaptability in oil extraction profile control, water treatment, and the paper industry.

[0057] The composite structure design and multi-step preparation process of the modified material further enhance its environmental adaptability and reaction stability. After surface functionalization, the multi-walled carbon nanotubes introduce various active groups, significantly improving the interaction between the material and acrylamide monomers and enhancing the dispersion uniformity of the polymerization system. The covalent organic framework layer, with its ordered network structure linked by strong covalent bonds, maintains a stable physical morphology under high-temperature polymerization conditions, avoiding the active site failure problem caused by thermal expansion or structural collapse in traditional porous materials. The doping of bimetallic ions not only provides a high density of free radical capturing sites but also regulates the kinetic balance of the polymerization reaction through the synergistic effect between metal ions, making the reaction process more stable and controllable. The use of vacuum freeze-drying effectively preserves the original molecular structure and porosity characteristics of the polymerization product, avoiding product denaturation and performance loss caused by high-temperature drying. This modified material can be recovered through a simple separation step after the reaction, and after activation treatment, it can be reused with stable performance, significantly reducing material costs and waste emissions in industrial production, meeting the process requirements of green chemistry and sustainable development.

[0058] This method demonstrates significant advantages in process parameter optimization and industrial implementation, providing a reliable technical path for the large-scale production of low molecular weight polyacrylamide. By precisely controlling key process parameters such as nitrogen protection time, initiator solution temperature, dropping rate, and polymerization temperature, the polymerization reaction is ensured to proceed smoothly under mild conditions, avoiding molecular weight runaway or side reactions caused by drastic fluctuations in reaction conditions. Optimized washing and filtration steps effectively remove unreacted monomers and trace impurities, improving product purity and safety. Vacuum freeze-drying technology avoids damage to the polymer structure caused by high temperatures, maintaining the molecular chain integrity and functional activity of the low molecular weight polyacrylamide. The final product exhibits excellent solubility and rapid dispersion characteristics, maintaining stable thickening effects even in complex water environments. Compared to traditional processes, the low molecular weight polyacrylamide prepared by this method exhibits higher apparent viscosity and lower residual monomer content at the same concentration, while also possessing good salt resistance and shear resistance. It can be widely used in fields with stringent polymer performance requirements, such as profile control in high-temperature and high-salinity oil reservoirs, advanced water treatment, and specialty papermaking processes, demonstrating broad market application prospects and industrial promotion value. Detailed Implementation

[0059] The following detailed embodiments are only used to further illustrate this application and should not be construed as limiting the scope of protection of this application. Those skilled in the art can make some non-essential improvements and adjustments to this application based on the above application content.

[0060] Example 1

[0061] 100g of deionized water, 15g of acrylamide, and 0.12g of bimetallic-doped hierarchical porous carbon nanotube-covalent organic framework composite modified material were added to a three-necked flask equipped with a stirrer, a nitrogen inlet tube, and a condenser. The stirrer was turned on and stirred at 300 rpm. At the same time, high-purity nitrogen (purity ≥99.999%) was introduced through the nitrogen inlet tube at a flow rate of 200 mL / min for 40 min to displace oxygen in the system. Nitrogen introduction was stopped after no obvious bubbles were observed on the liquid surface. Stirring was continued for 30 min until the acrylamide was completely dissolved, forming a homogeneous and transparent solution. 0.006g of potassium persulfate and 0.0045g of sodium sulfite were dissolved in 5g of deionized water, and the solution temperature was controlled at 3°C ​​using an ice-water bath to prepare a redox initiator solution. Under continuous nitrogen protection (nitrogen flow rate 150 mL / min), the redox initiator solution was added dropwise to the pretreatment system of step S1 at a rate of 2 mL / min through a constant pressure dropping funnel. During the dropwise addition, the stirring speed was maintained at 300 rpm, and the dropwise addition time was strictly controlled at 15 min. Immediately after the dropwise addition was completed, the reaction system was transferred to a 20℃ constant temperature water bath. The reaction temperature was precisely maintained within ±0.5℃ using a water bath temperature controller. The polymerization reaction continued for 3 hours, and the system status was recorded every 30 min during this period. After polymerization, the product system was transferred to centrifuge tubes and washed four times with deionized water. Each wash was centrifuged at 8000 rpm for 5 min, and the supernatant was discarded. The precipitate was dispersed in 100 g of deionized water to prepare a 1% (w / w) solution, which was then filtered using a mixed cellulose ester filter membrane with a pore size of 0.22 μm. The filter membrane was pre-wetted with deionized water to avoid clogging. Finally, the filtrate was transferred to a vacuum freeze dryer and dried at -45 °C and 10 Pa vacuum for 36 h to obtain white flocculent low molecular weight polyacrylamide. The preparation steps of the bimetallic doped hierarchical porous carbon nanotube-covalent organic framework composite modified material are as follows: A1, 1g of multi-walled carbon nanotubes are dispersed in a mixed acid solution (the mixed acid is prepared by mixing concentrated sulfuric acid and concentrated nitric acid at a volume ratio of 3:1), ultrasonically dispersed in an ice bath (0-5℃) (ultrasonic power 200W, ultrasonic time 30min), and then transferred to a reflux reaction apparatus. The reaction is carried out at 60℃ for 4h, with gentle stirring during the reaction. After cooling to room temperature, the solution is diluted with deionized water to pH=6, and then filtered using a vacuum filtration apparatus (filter paper specification 0).Filter the 45 μm oxide multi-walled carbon nanotubes and wash them with deionized water until neutral (tested with pH paper). Dry them under vacuum at 80°C for 12 h (vacuum degree ≤100 Pa). Disperse the oxide multi-walled carbon nanotubes in sulfur dichloride and reflux at 80°C for 12 h to obtain acyl chloride multi-walled carbon nanotubes. Then add excess ethylenediamine (ethylenediamine to oxide multi-walled carbon nanotube mass ratio 5:1) and react at 80°C for 24 h to obtain aminated multi-walled carbon nanotubes. Finally, disperse the aminated multi-walled carbon nanotubes in concentrated... Sulfonation was carried out at 60°C for 6 hours in a mixture of sulfuric acid and fuming sulfuric acid (volume ratio of concentrated sulfuric acid to fuming sulfuric acid 1:1). After washing with deionized water until neutral, the mixture was dried at 100°C for 12 hours to obtain hierarchical porous carbon nanotubes with carboxyl, amino, and sulfonic acid groups on the surface. In step A2, 1 g of the hierarchical porous carbon nanotubes were dispersed in a mixed solvent of mesitylene and ethanol (volume ratio of mesitylene to ethanol 3:1) and ultrasonically dispersed (ultrasonic power 150 W, ultrasonic time 20 min). 0.08 g of [the mixture] was then added. 2,4,6-Tris(4-aminophenyl)-1,3,5-triazine and 0.04 g 1,4-terephthalic acid were ultrasonically dispersed (ultrasonic power 150 W, ultrasonic time 20 min); then acetic acid (acetic acid to mixed solvent volume ratio 1:10) was added, and the mixture was transferred to a polytetrafluoroethylene reactor and reacted at 120 °C for 72 h, maintaining a rotation speed of 50 rpm during the reaction; after cooling to room temperature, the solid was collected by centrifugation (centrifugation speed 5000 rpm, centrifugation time 10 min), and washed successively with tetrahydrofuran (washed 3 times, 20 mL each time), ethanol (washed 3 times, 20 mL each time), and deionized water (washed 3 times, 30 mL each time), and vacuum dried at 80 °C for 12 h to obtain surface-grown covalent organic framework layer carbon nanotubes; A3, 1 g of covalent organic framework layer carbon nanotubes were dispersed in deionized water and ultrasonically dispersed (ultrasonic power 100 W, ultrasonic time 15 min); 0.01 g ZrCl4 and 0.A 0.05g aqueous solution of Ce(NO3)3·6H2O (total mass of ZrCl4 and Ce(NO3)3·6H2O aqueous solution is 10mL) was stirred at room temperature for 12h (stirring speed 200rpm); the solid was collected by centrifugation (centrifugation speed 5000rpm, centrifugation time 10min), washed with deionized water (washed 3 times, 30mL each time), then dispersed in ethanol (ethanol volume 50mL), dried at 60℃ for 12h (vacuum degree ≤100Pa); finally, under nitrogen protection (nitrogen flow rate 100mL), the reaction was carried out. Under a hydrogen atmosphere (hydrogen flow rate 50 mL / min), calcination at 300 °C for 2 h (heating rate 5 °C / min) yielded a bimetallic-doped hierarchical porous carbon nanotube-covalent organic framework composite modified material; A4, 1 g of the bimetallic-doped hierarchical porous carbon nanotube-covalent organic framework composite modified material was placed in a tube furnace and reduced at 400 °C for 3 h (heating rate 3 °C / min) under a hydrogen atmosphere (hydrogen flow rate 50 mL / min); subsequently, under nitrogen protection (nitrogen flow rate 100 mL / min), calcined at 500 °C for 2 h (heating rate 3 °C / min).

[0062] Example 2

[0063] The specific implementation method is the same as in Example 1, except that 100g of deionized water, 15g of acrylamide, and 0.16g of bimetallic doped hierarchical porous carbon nanotube-covalent organic framework composite modified material are added to a three-necked flask equipped with a stirrer, a nitrogen inlet tube, and a condenser. Nitrogen gas is purged for 45 minutes while stirring until the acrylamide is completely dissolved. 0.007g of potassium persulfate and 0.005g of sodium sulfite are dissolved in 5g of deionized water to prepare a redox initiator solution. Under continuous nitrogen protection, the redox initiator solution is added dropwise to the pretreatment system in step S1 over a period of 18 minutes. The reaction system is then immediately transferred to a 20°C constant temperature water bath for polymerization for 3 hours. After polymerization, the product system is washed five times with deionized water. The precipitate is dispersed in deionized water to prepare a solution, which is then filtered through a 0.23μm pore size filter membrane. Finally, the product is dried at -45°C for 38 hours using a vacuum freeze-drying method to obtain low molecular weight polyacrylamide. The preparation steps of the bimetallic doped hierarchical porous carbon nanotube-covalent organic framework composite modified material are as follows: A1, 1g of multi-walled carbon nanotubes were dispersed in a mixed acid solution, ultrasonically dispersed in an ice bath, refluxed at 62℃ for 5h, cooled, diluted with deionized water to pH=6, filtered, washed with deionized water until neutral, and vacuum dried at 81℃; oxidized multi-walled carbon nanotubes were dispersed in sulfur dichloride, refluxed at 81℃ for 13h to obtain acyl chloride multi-walled carbon nanotubes; then excess ethylenediamine was added, and reacted at 82℃ for 27h to obtain aminated multi-walled carbon nanotubes; finally, aminated multi-walled carbon nanotubes were dispersed in a mixture of concentrated sulfuric acid and fuming sulfuric acid, sulfonated at 62℃ for 7h, washed with deionized water until neutral, and dried at 101℃ to obtain hierarchical porous carbon nanotubes with carboxyl, amino, and sulfonic acid groups on the surface; A2, 1g of hierarchical porous carbon nanotubes were dispersed in a mixed solvent of mesitylene and ethanol, ultrasonically dispersed; 0.08g of... 2,4,6-Tris(4-aminophenyl)-1,3,5-triazine and 0.04 g of 1,4-terephthalic acid were ultrasonically dispersed; then acetic acid was added, and the mixture was transferred to a polytetrafluoroethylene reactor and reacted at 122 °C for 76 h; after cooling, the solid was collected by centrifugation, washed successively with tetrahydrofuran, ethanol, and deionized water, and dried under vacuum at 81 °C to obtain carbon nanotubes with a surface-grown covalent organic framework layer; A3, 1 g of covalent organic framework layer carbon nanotubes were dispersed in deionized water and ultrasonically dispersed; then 0.01 g of... A solution of ZrCl4 and 0.005g of Ce(NO3)3·6H2O was stirred at room temperature for 13h. The solid was collected by centrifugation, washed with deionized water, dispersed in ethanol, and dried at 62℃. Finally, it was calcined at 302℃ for 3h under nitrogen protection to obtain a bimetallic doped hierarchical porous carbon nanotube-covalent organic framework composite modified material. A4, 1g of the bimetallic doped hierarchical porous carbon nanotube-covalent organic framework composite modified material was placed in a tube furnace and reduced at 402℃ for 3h under hydrogen atmosphere. Then, it was calcined at 505℃ for 3h under nitrogen protection.

[0064] Example 3

[0065] The specific implementation method is the same as in Example 1, except that 100g of deionized water, 15g of acrylamide, and 0.2g of bimetallic doped hierarchical porous carbon nanotube-covalent organic framework composite modified material are added to a three-necked flask equipped with a stirrer, a nitrogen inlet tube, and a condenser. Nitrogen gas is purged for 50 minutes while stirring until the acrylamide is completely dissolved. 0.008g of potassium persulfate and 0.006g of sodium sulfite are dissolved in 5g of deionized water to prepare a redox initiator solution. Under continuous nitrogen protection, the redox initiator solution is added dropwise to the pretreatment system in step S1 over a period of 20 minutes. The reaction system is then immediately transferred to a 21°C constant temperature water bath for polymerization for 3 hours. After polymerization, the product system is washed five times with deionized water. The precipitate is dispersed in deionized water to prepare a solution, which is then filtered through a 0.24μm pore size filter membrane. Finally, the product is dried at -45°C for 40 hours using a vacuum freeze-drying method to obtain low molecular weight polyacrylamide. The preparation steps of the bimetallic doped hierarchical porous carbon nanotube-covalent organic framework composite modified material are as follows: A1, 1g of multi-walled carbon nanotubes were dispersed in a mixed acid solution, ultrasonically dispersed in an ice bath, refluxed at 64℃ for 6h, cooled, diluted with deionized water to pH=7, filtered, washed with deionized water until neutral, and vacuum dried at 82℃; oxidized multi-walled carbon nanotubes were dispersed in sulfur dichloride, refluxed at 82℃ for 14h to obtain acyl chloride multi-walled carbon nanotubes; then excess ethylenediamine was added, and reacted at 84℃ for 30h to obtain aminated multi-walled carbon nanotubes; finally, aminated multi-walled carbon nanotubes were dispersed in a mixture of concentrated sulfuric acid and fuming sulfuric acid, sulfonated at 64℃ for 8h, washed with deionized water until neutral, and dried at 102℃ to obtain hierarchical porous carbon nanotubes with carboxyl, amino, and sulfonic acid groups on the surface; A2, 1g of hierarchical porous carbon nanotubes were dispersed in a mixed solvent of mesitylene and ethanol, ultrasonically dispersed; 0.08g of... 2,4,6-Tris(4-aminophenyl)-1,3,5-triazine and 0.04 g of 1,4-terephthalic acid were ultrasonically dispersed; then acetic acid was added, and the mixture was transferred to a polytetrafluoroethylene reactor and reacted at 124 °C for 80 h; after cooling, the solid was collected by centrifugation, washed successively with tetrahydrofuran, ethanol, and deionized water, and dried under vacuum at 82 °C to obtain carbon nanotubes with a surface-grown covalent organic framework layer; A3, 1 g of covalent organic framework layer carbon nanotubes were dispersed in deionized water and ultrasonically dispersed; then 0.01 g of... A solution of ZrCl4 and 0.005g of Ce(NO3)3·6H2O was stirred at room temperature for 14h. The solid was collected by centrifugation, washed with deionized water, dispersed in ethanol, and dried at 64℃. Finally, it was calcined at 304℃ for 4h under nitrogen protection to obtain a bimetallic doped hierarchical porous carbon nanotube-covalent organic framework composite modified material. A4, 1g of the bimetallic doped hierarchical porous carbon nanotube-covalent organic framework composite modified material was placed in a tube furnace and reduced at 404℃ for 4h under a hydrogen atmosphere. Then, it was calcined at 510℃ for 4h under nitrogen protection.

[0066] Comparative Example 1

[0067] The specific implementation method is the same as in Example 1, except that 100g of deionized water and 15g of acrylamide are added to a three-necked flask equipped with a stirrer, a nitrogen inlet tube, and a condenser. The stirrer is turned on and stirred at 300 rpm. At the same time, high-purity nitrogen (purity ≥99.999%) is introduced through the nitrogen inlet tube at a flow rate of 200 mL / min for 40 min to replace the oxygen in the system. Nitrogen is introduced and stopped after no obvious bubbles are observed on the liquid surface. Stirring continues for 30 min until the acrylamide is completely dissolved, forming a uniform and transparent solution. 0.006g of potassium persulfate and 0.0045g of sodium sulfite are dissolved in 5g of deionized water respectively. The solution temperature is controlled at 3°C ​​using an ice-water bath to prepare a redox initiator solution. Under continuous nitrogen protection (nitrogen flow rate 150 mL / min), the redox initiator solution was added dropwise to the pretreatment system of step S1 at a rate of 2 mL / min through a constant pressure dropping funnel. During the dropwise addition, the stirring speed was maintained at 300 rpm, and the dropwise addition time was strictly controlled at 15 min. Immediately after the dropwise addition was completed, the reaction system was transferred to a 20℃ constant temperature water bath. The reaction temperature was precisely maintained within ±0.5℃ using a water bath temperature controller. The polymerization reaction continued for 3 hours, and the system status was recorded every 30 min during this period. After polymerization, the product system was transferred to centrifuge tubes and washed four times with deionized water. Each wash was centrifuged at 8000 rpm for 5 min, and the supernatant was discarded. The precipitate was dispersed in 100 g of deionized water to prepare a 1% (w / w) solution, which was then filtered using a mixed cellulose ester filter membrane with a pore size of 0.22 μm. The filter membrane was pre-wetted with deionized water to avoid clogging. Finally, the filtrate was transferred to a vacuum freeze dryer and dried at -45 °C and 10 Pa vacuum for 36 h to obtain polyacrylamide.

[0068] Comparative Example 2

[0069] The specific implementation method is the same as in Example 1, except that 100g of deionized water, 15g of acrylamide, and 0.12g of activated carbon are added to a three-necked flask equipped with a stirrer, a nitrogen inlet tube, and a condenser. The stirrer is turned on and stirred at 300rpm. At the same time, high-purity nitrogen (purity ≥99.999%) is introduced through the nitrogen inlet tube at a flow rate of 200mL / min for 40min to replace the oxygen in the system. Nitrogen is introduced and stopped after no obvious bubbles are observed on the liquid surface. Stirring continues for 30min until the acrylamide is completely dissolved, forming a uniform and transparent solution. 0.006g of potassium persulfate and 0.0045g of sodium sulfite are dissolved in 5g of deionized water, and the solution temperature is controlled at 3°C ​​using an ice-water bath to prepare a redox initiator solution. Under continuous nitrogen protection (nitrogen flow rate 150 mL / min), the redox initiator solution was added dropwise to the pretreatment system of step S1 at a rate of 2 mL / min through a constant pressure dropping funnel. During the dropwise addition, the stirring speed was maintained at 300 rpm, and the dropwise addition time was strictly controlled at 15 min. Immediately after the dropwise addition was completed, the reaction system was transferred to a 20℃ constant temperature water bath. The reaction temperature was precisely maintained within ±0.5℃ using a water bath temperature controller. The polymerization reaction continued for 3 hours, and the system status was recorded every 30 min during this period. After polymerization, the product system was transferred to centrifuge tubes and washed four times with deionized water. Each wash was centrifuged at 8000 rpm for 5 min, and the supernatant was discarded. The precipitate was dispersed in 100 g of deionized water to prepare a 1% (w / w) solution, which was then filtered using a mixed cellulose ester filter membrane with a pore size of 0.22 μm. The filter membrane was pre-wetted with deionized water to avoid clogging. Finally, the filtrate was transferred to a vacuum freeze dryer and dried at -45 °C and 10 Pa vacuum for 36 h to obtain polyacrylamide.

[0070] Comparative Example 3

[0071] The specific implementation method is the same as in Example 1, except that 100g of deionized water, 15g of acrylamide, and 0.12g of silica are added to a three-necked flask equipped with a stirrer, a nitrogen inlet tube, and a condenser. The stirrer is turned on and stirred at 300rpm. At the same time, high-purity nitrogen (purity ≥99.999%) is introduced through the nitrogen inlet tube at a flow rate of 200mL / min for 40min to replace the oxygen in the system. Nitrogen is introduced and stopped after no obvious bubbles are observed on the liquid surface. Stirring continues for 30min until the acrylamide is completely dissolved, forming a uniform and transparent solution. 0.006g of potassium persulfate and 0.0045g of sodium sulfite are dissolved in 5g of deionized water, and the solution temperature is controlled at 3°C ​​using an ice-water bath to prepare a redox initiator solution. Under continuous nitrogen protection (nitrogen flow rate 150 mL / min), the redox initiator solution was added dropwise to the pretreatment system of step S1 at a rate of 2 mL / min through a constant pressure dropping funnel. During the dropwise addition, the stirring speed was maintained at 300 rpm, and the dropwise addition time was strictly controlled at 15 min. Immediately after the dropwise addition was completed, the reaction system was transferred to a 20℃ constant temperature water bath. The reaction temperature was precisely maintained within ±0.5℃ using a water bath temperature controller. The polymerization reaction continued for 3 hours, and the system status was recorded every 30 min during this period. After polymerization, the product system was transferred to centrifuge tubes and washed four times with deionized water. Each wash was centrifuged at 8000 rpm for 5 min, and the supernatant was discarded. The precipitate was dispersed in 100 g of deionized water to prepare a 1% (w / w) solution, which was then filtered using a mixed cellulose ester filter membrane with a pore size of 0.22 μm. The filter membrane was pre-wetted with deionized water to avoid clogging. Finally, the filtrate was transferred to a vacuum freeze dryer and dried at -45 °C and 10 Pa vacuum for 36 h to obtain polyacrylamide.

[0072] Performance testing

[0073] The multilayer decorative panels prepared using the processes described in Examples 1-3 and Comparative Examples 1-3 were subjected to performance testing according to the following methods.

[0074] The performance testing of low molecular weight polyacrylamide includes four indicators: molecular weight and its distribution, solubility, apparent viscosity, and thermal stability. Molecular weight and its distribution were determined using gel permeation chromatography (GPC). The test conditions were as follows: TSK-GEL G3000PWXL and TSK-GEL G2500PWXL columns in tandem; mobile phase: 0.1 mol / L sodium nitrate aqueous solution (containing 0.02% NaN3 as a stabilizer); flow rate: 0.8 mL / min; column temperature: 30℃; injection volume: 20 μL; and a narrow-distribution polyacrylamide standard (molecular weight range 1×10⁻⁶). 3 -1×10 7A standard curve was established, and the number-average molecular weight (Mn), weight-average molecular weight (Mw), and polydispersity index (PDI = Mw / Mn) were calculated using a universal calibration method. The solubility test method was as follows: 0.5 g of low molecular weight polyacrylamide sample was weighed and placed in a 250 mL stoppered conical flask, 100 mL of deionized water was added, and the mixture was mechanically stirred at 200 rpm in a 25°C constant temperature water bath. The system state was observed every 2 minutes, and the time required for complete dissolution was recorded (based on the absence of visible particles in the solution). Simultaneously, the clarity of the solution after dissolution was tested (by visually observing whether there was any turbidity or suspended matter). The apparent viscosity was tested using a rotational viscometer (Brookfield DV-II+Pro). The test conditions were: low molecular weight polyacrylamide was prepared into a 1 wt% aqueous solution, kept at a constant temperature of 25°C for 30 minutes, and then the apparent viscosity (mPa·s) was measured using an RV-3 rotor at 60 rpm. Thermal stability was tested using a thermogravimetric analyzer (TGA). The test conditions were as follows: sample mass 5-10 mg, under nitrogen protection atmosphere (flow rate 50 mL / min), the temperature was increased from 30 °C to 300 °C at a heating rate of 10 °C / min, and the curve of sample mass change with temperature was recorded. Thermal stability was evaluated by the initial decomposition temperature (the temperature corresponding to 5% weight loss, T5%).

[0075] Test results:

[0076] Table 1: Test results of each embodiment and comparative example

[0077]

[0078] As shown in Table 1, Examples 1-3 exhibit significant advantages over Comparative List 1-3 in several key performance indicators. This fully demonstrates that the method of preparing low molecular weight polyacrylamide using bimetallic doped hierarchical porous carbon nanotube-covalent organic framework composite modified materials solves the technical problems of existing low molecular weight polyacrylamide preparation methods, such as difficulty in controlling molecular weight, wide distribution, poor material stability, and residual harmful substances. In terms of molecular weight and distribution, the number average molecular weight (Mn) of the products obtained in Examples 1-3 is between 39,000 and 52,000 Da, far lower than the 286,000 to 321,000 Da of Comparative List 1-3, and the polydispersity index (PDI) is 1.8-2.1, significantly lower than the 6.0-6.5 of Comparative List 1-3. This indicates that the method of the present invention can precisely control the polymerization reaction, effectively inhibit chain growth, and achieve the goal of low molecular weight and narrow distribution, solving the problems of excessively high molecular weight and wide distribution that are difficult to control in traditional free radical polymerization. Regarding solubility, the complete dissolution time of Examples 1-3 was shortened to 6-8 minutes, while that of Comparative List 1-3 required 15-18 minutes. Furthermore, the solutions from the Examples were clear and free of turbidity after dissolution, while the Comparative List showed slight to significant turbidity. This indicates that the bimetallic-doped hierarchical porous carbon nanotube-covalent organic framework composite modified material promoted the uniform dispersion and reaction of acrylamide, making the product easier to dissolve and avoiding the dissolution difficulties and solution quality problems caused by uneven polymerization in traditional methods. As for apparent viscosity, the 1 wt% solutions prepared in Examples 1-3 had an apparent viscosity between 10.2-12.5 mPa·s, higher than the 8.9-9.5 mPa·s of Comparative List 1-3, while also having a shorter dissolution time. This demonstrates that the polyacrylamide prepared by this invention can maintain good thickening properties even at low molecular weights, overcoming the defect of insufficient viscosity caused by excessively low molecular weight in traditional low molecular weight products. From a thermal stability analysis, the initial decomposition temperature (T5%) of Examples 1-3 exceeded 200°C, while that of Comparative List 1-3 was below 190°C. This demonstrates that the bimetallic-doped hierarchical porous carbon nanotube-covalent organic framework composite modified material imparts better thermal stability to the product, solving the problem of easy decomposition and failure of traditional low molecular weight polyacrylamide under high temperature conditions. Considering all performance indicators, this invention solves the problems of difficult molecular weight control, wide molecular weight distribution, poor material stability, and residual harmful substances in the preparation of low molecular weight polyacrylamide using traditional methods. It achieves controllable and stable preparation of low molecular weight polyacrylamide, improving the overall performance and application value of the product.

[0079] The embodiments described above are merely examples of several implementations of the present invention, and while the descriptions are relatively specific and detailed, they 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 scope of protection of the present invention.

Claims

1. A method for preparing low molecular weight polyacrylamide, characterized in that, Includes the following steps: S1. Deionized water, acrylamide, and bimetallic doped hierarchical porous carbon nanotube-covalent organic framework composite modified material are added to a three-necked flask equipped with a stirrer, a nitrogen inlet tube, and a condenser. Nitrogen gas is introduced while stirring until the acrylamide is completely dissolved. S2, Dissolve potassium persulfate and sodium sulfite separately in deionized water to prepare an oxidation-reduction initiator solution; S3, under continuous nitrogen protection, the redox initiator solution is added dropwise to the pretreatment system of step S1, and then the reaction system is immediately transferred to a constant temperature water bath at 19-21℃ for polymerization reaction; S4. After polymerization, the product system was washed with deionized water; the precipitate was dispersed in deionized water to prepare a solution, which was then filtered through a filter membrane; finally, it was dried by vacuum freeze-drying. The preparation steps of the bimetal-doped hierarchical porous carbon nanotube-covalent organic framework composite modified material include: A1. Multi-walled carbon nanotubes were dispersed in a mixed acid solution, ultrasonically dispersed in an ice bath, and then refluxed at 60-64℃. After cooling, the solution was diluted with deionized water to pH 6-7, filtered, and washed with deionized water until neutral. The solution was then vacuum dried at 80-82℃. Oxidized multi-walled carbon nanotubes were dispersed in SOCl2 and refluxed at 80-82℃ to obtain acyl chloride multi-walled carbon nanotubes. Excess ethylenediamine was then added, and the solution was reacted at 80-84℃ to obtain aminated multi-walled carbon nanotubes. Finally, the aminated multi-walled carbon nanotubes were dispersed in a mixture of concentrated sulfuric acid and fuming sulfuric acid and sulfonated at 60-64℃. After washing with deionized water until neutral, the solution was dried at 100-102℃ to obtain hierarchical porous carbon nanotubes with carboxyl, amino, and sulfonic acid groups on the surface. A2. Hierarchical porous carbon nanotubes were dispersed in a mixed solvent of mesitylene and ethanol and ultrasonically dispersed. 2,4,6-tris(4-aminophenyl)-1,3,5-triazine and 1,4-terephthalic acid were added and ultrasonically dispersed. Acetic acid was then added, and the mixture was transferred to a polytetrafluoroethylene reactor and reacted at 120-124℃. After cooling, the solid was collected by centrifugation, washed successively with tetrahydrofuran, ethanol, and deionized water, and vacuum dried at 80-82℃ to obtain carbon nanotubes with a surface-grown covalent organic framework layer. A3. Covalent organic framework carbon nanotubes were dispersed in deionized water and ultrasonically dispersed. Aqueous solutions of ZrCl4 and Ce(NO3)3·6H2O were added sequentially, and the mixture was stirred at room temperature. The solid was collected by centrifugation, washed with deionized water, dispersed in ethanol, and dried at 60-64℃. Finally, the mixture was calcined at 300-304℃ under nitrogen protection to obtain a bimetallic doped hierarchical porous carbon nanotube-covalent organic framework composite modified material. A4, the bimetallic doped hierarchical porous carbon nanotube-covalent organic framework composite modified material was placed in a tube furnace and reduced at 400-404℃ in a hydrogen atmosphere; then calcined at 500-510℃ under nitrogen protection.

2. The method for preparing low molecular weight polyacrylamide according to claim 1, characterized in that, In step S1, the nitrogen gas is introduced for 40-50 minutes.

3. The method for preparing low molecular weight polyacrylamide according to claim 1, characterized in that, In step S2, the temperature of the redox initiator solution is 3-5℃.

4. The method for preparing low molecular weight polyacrylamide according to claim 1, characterized in that, In step S3, the dripping time is 15-20 minutes.

5. The method for preparing low molecular weight polyacrylamide according to claim 1, characterized in that, In step S4, the deionized water washing is performed 4-5 times; the pore size of the filter membrane is 0.22-0.24 μm; the vacuum freeze-drying temperature is -45℃ and the drying time is 36-40 h.

6. The method for preparing low molecular weight polyacrylamide according to claim 1, characterized in that, In step A1, the volume ratio of concentrated sulfuric acid to concentrated nitric acid in the mixed acid solution is 3:1; the reflux reaction time is 4-6 h at 60-64℃; the vacuum drying time is 12-14 h at 80-82℃; the reflux reaction time is 12-14 h at 80-82℃; the reaction time is 24-30 h at 80-84℃; and the sulfonation reaction time is 6-8 h at 60-64℃.

7. The method for preparing low molecular weight polyacrylamide according to claim 1, characterized in that, In step A2, the reaction time at 120-124℃ is 72-80h; the vacuum drying time at 80-82℃ is 12-14h.

8. The method for preparing low molecular weight polyacrylamide according to claim 1, characterized in that, In step A3, the stirring reaction time at room temperature is 12-14 hours; the calcination time at 300-304℃ is 2-4 hours.

9. The method for preparing low molecular weight polyacrylamide according to claim 1, characterized in that, In step A4, the reduction treatment at 400-404℃ takes 3-4 hours; the calcination at 500-510℃ takes 2-4 hours.

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