Stock solution polymerization formula for optimizing thermal cyclization performance of polyacrylonitrile-based carbon fiber precursor and preparation method

Abstract

The invention discloses a stock solution polymerization formula for optimizing thermal cyclization performance of a polyacrylonitrile-based carbon fiber precursor and a preparation method, the stock solution polymerization formula comprises a first monomer, a second monomer, a solvent, an initiator and a trapping agent, and the polymerization behavior of the second monomer is regulated and controlled by introducing the behavior of the trapping agent into a polymerization system; the thermal cyclization performance of the PAN fiber is obviously improved. By adding the trapping agent rich in hydrogen bond sites, the concentration of the second monomer can be dynamically regulated and controlled, so that the reactivity ratio difference between the second monomer and AN is reduced, and the uniform distribution of the second monomer on a molecular chain is promoted. The uniformly distributed structure shows remarkable advantages in the subsequent thermal cyclization process, and the cyclization reaction can be gradually carried out in a wider temperature range due to the fact that carboxylic acid groups are distributed in a discrete state on a PAN molecular chain. In addition, due to uniform distribution of carboxylic acid groups, the catalytic efficiency of anion cyclization is also improved, the cyclization degree is improved, meanwhile, the half-peak width of a cyclization exothermic peak is increased, and the thermal stability of the fiber is remarkably improved.

Description

Technical Field

[0001] The present invention relates to the technical field of free radical solution polymerization, and in particular to a solution polymerization formula and a preparation method for optimizing the thermal cyclization performance of polyacrylonitrile-based carbon fiber precursor. Background Art

[0002] Carbon fiber is an inorganic non-metallic material with multiple excellent properties, including outstanding mechanical properties and chemical stability. PAN-based carbon fiber has the highest specific strength and highest specific modulus among currently produced high-performance fibers and is widely used in sports and leisure, pressure vessels, aerospace, and other fields. PAN-based precursor carbon fiber has always dominated carbon fiber production, accounting for approximately 90% of the world's total carbon fiber production. The technical bottleneck of carbon fiber in my country is mainly in the preparation of precursor fibers, and the key to determining the quality of the precursor fibers is the precursor polymer solution.

[0003] In the production process of PAN-based carbon fibers, the quality of the precursor has a decisive influence on the performance of the final product. The ideal carbon fiber precursor must meet a series of strict standards, including high purity, high orientation, suitable molecular weight distribution and excellent thermal stability. From a molecular level, pure acrylonitrile homopolymer is theoretically considered to be the best candidate material due to the high order of its molecular chain. This ordered structure is conducive to the formation of an ordered graphite microcrystalline structure during the high-temperature carbonization process, thereby achieving a high carbon conversion rate. However, in actual applications, it has been found that pure acrylonitrile homopolymer has some significant problems. The strong interaction between molecules leads to excessive cohesion, which makes the prepared fibers exhibit excessive rigidity. During the thermal stabilization stage, the homopolymer undergoes a violent and difficult-to-control free radical cyclization reaction process. The heat released in this process is concentrated, which can easily cause fiber breakage. The above factors have greatly limited the widespread application of pure acrylonitrile homopolymer in the field of carbon fiber manufacturing. Summary of the Invention

[0004] The purpose of the present invention is to remedy the defects of the existing technology and provide a solution polymerization formula and preparation method for optimizing the thermal cyclization performance of polyacrylonitrile-based carbon fiber precursor.

[0005] The present invention is achieved through the following technical solutions: A solution polymerization formula for optimizing the thermal cyclization performance of polyacrylonitrile-based carbon fiber precursor includes a first monomer, a second monomer, a solvent, an initiator, and a capture agent, wherein the capture agent is a polymer having the following structure: , Wherein, R is H or CH3.

[0006] After the capture agent is mixed with the second monomer, a hydrogen bond is formed between the two, and the hydrogen bond is expressed as follows: .

[0007] The first monomer is acrylonitrile; the second monomer is itaconic acid, acrylic acid, methacrylic acid, or a second monomer copolymer with a carboxyl group; the solvent is dimethyl sulfoxide or N,N-dimethylformamide; the initiator is azobisisobutyronitrile; and the capture agent is polyacrylamide or polymethacrylamide.

[0008] The solvent, the first monomer, the second monomer, the initiator and the capture agent are 100%, the solvent accounts for 72% to 84% by mass of the total mass of the system; the first monomer accounts for 16% to 28% by mass of the total mass of the system; the second monomer content is 0% to 3% by mass of the first monomer; the initiator accounts for 0.2% to 1.5% by mass of the first monomer; and the molar ratio of the carboxyl group contained in the capture agent to that contained in the second monomer is 1:1.

[0009] A method for preparing a stock solution for optimizing the thermal cyclization performance of polyacrylonitrile-based carbon fiber precursor comprises the following steps: (1) Measure the required solvent and store it temporarily for future use. Weigh the required second monomer, initiator and capture agent. Take part of the measured solvent and stir and dissolve the second monomer, initiator and capture agent in the solvent to prepare a second monomer-solvent mixed solution, an initiator-solvent mixed solution and a capture agent-solvent mixed solution with a ratio of 5%, 3% and 5% respectively. The stirring temperature is controlled between 20℃ and 27℃. After the dissolution is completed, store it for future use. (2) During the feeding stage, the feed valve of the polymerization kettle is opened and the agitator is started. Then, half of the solvent measured in advance is added into the polymerization kettle. Subsequently, the pre-prepared second monomer-solvent mixed solution, initiator-solvent mixed solution and capture agent-solvent mixed solution are sequentially added into the reactor. (3) Measure the required amount of acrylonitrile monomer and put it all into the polymerization kettle at one time. Then seal the polymerization kettle and start nitrogen replacement. After the nitrogen replacement is completed, increase the temperature in the polymerization kettle to the preset reaction temperature through the heating system of the polymerization kettle. The preset reaction temperature is in the range of 50-70°C. At the same time, increase the stirring speed to 60-80 rpm to start the polymerization reaction; (4) After the polymerization reaction is completed, the monomer removal process is started, and the unreacted acrylonitrile monomer is removed by vacuum distillation. The reaction temperature and stirring speed are maintained unchanged. The vacuum system is started to reduce the pressure in the polymerization kettle to -0.1Mpa, and the evaporated acrylonitrile monomer is recovered to obtain polyacrylonitrile stock solution.

[0010] In step (2), after each addition, the feeding container is repeatedly rinsed at least three times with part of the solvent; the temperature in the polymerization kettle is controlled during the entire feeding process, and the temperature in the polymerization kettle is adjusted through the jacket cooling water circulation system so that the temperature in the polymerization kettle and the jacket does not exceed 25°C.

[0011] In step (3), after the addition of acrylonitrile is completed, the addition container and pipeline are thoroughly flushed with all the remaining solvent to ensure that no acrylonitrile monomer remains.

[0012] In step (3), the nitrogen replacement is carried out by using a vacuum pump to evacuate the polymerization kettle to a vacuum state, the pressure reaching -0.1 MPa, and then high-purity nitrogen is introduced to restore the pressure to normal pressure. This cycle is repeated three times to reduce the oxygen content in the polymerization kettle to a safe range, thereby completing the nitrogen replacement.

[0013] In step (3), the polymerization reaction process lasts for 18 hours, and the temperature in the polymerization kettle is kept constant during the polymerization reaction, and the temperature fluctuation range is controlled within ±0.5°C; the de-monomerization process is continued until it is confirmed by online gas chromatography that the acrylonitrile monomer content in the polymerization kettle is lower than the quality standard of 0.1%; after the de-monomerization process is completed, the stirring system is first stopped, and the reaction solution is allowed to stand and degas for 1 hour to allow the tiny bubbles dissolved in the system to fully escape. Finally, the obtained polyacrylonitrile stock solution passes the quality inspection and is then transferred to the subsequent spinning process.

[0014] The spinning process is as follows: the obtained PAN stock solution is first delivered to the spinneret assembly at a constant pressure by a metering pump to form a uniform and stable stock solution stream, and the stock solution stream immediately enters a gradient coagulation bath with a temperature controlled at 40-60°C and a dimethyl sulfoxide concentration maintained at 50%-70%. Phase separation and solidification are achieved through double diffusion between the solvent and the non-solvent. The formed nascent fibers are first subjected to multi-stage water washing to remove residual solvent, and then to hot water drawing to strengthen molecular chain orientation. The fibers are then dried by hot rollers and subjected to relaxation and heat setting treatment to obtain PAN precursors. The PAN precursors are subsequently subjected to pre-oxidation and carbonization treatments to prepare carbon fiber products. During the entire spinning process, the temperature control accuracy is maintained within ±1°C, and the concentration fluctuation does not exceed ±2%.

[0015] The advantages of the present invention are that the performance of the polymer is optimized from multiple perspectives by introducing a second monomer (such as itaconic acid, acrylic acid and other carboxyl-containing compounds).

[0016] The present invention employs a capture agent containing an amide group that can form a stable hydrogen-bonded complex with a carboxylic acid-based second monomer. This hydrogen-bonded complex can temporarily lock the carboxylic acid group of the second monomer, effectively reducing its polymerization activity, making the consumption rate of different monomers more consistent, and enabling the second monomer to be evenly distributed across the molecular segments, improving the uniformity of the molecular chain, avoiding the formation of defects, and significantly broadening the polymer cyclization peak and achieving a higher degree of cyclization.

[0017] The present invention regulates the polymerization behavior of the second monomer by introducing a capture agent into the polymerization system, significantly improving the thermal cyclization performance of PAN fibers. By adding a capture agent rich in hydrogen bonding sites, the present invention can dynamically regulate the concentration of the second monomer, reducing the difference in reactivity with AN and promoting the uniform distribution of the second monomer on the molecular chain. This uniformly distributed structure exhibits significant advantages in the subsequent thermal cyclization process. Because the carboxylic acid groups are distributed in a discrete state on the PAN molecular chain, the cyclization reaction can proceed gradually over a wider temperature range. In addition, the uniform distribution of the carboxylic acid groups also improves the catalytic efficiency of anionic cyclization, increases the degree of cyclization, and increases the half-width of the cyclization exothermic peak, significantly improving the thermal stability of the fiber. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 This is an exothermic curve diagram of the itaconic acid polymerization reaction in Example 2 of the present invention; Figure 2 This is an exothermic curve diagram of the methacrylic acid polymerization reaction in Example 3 of the present invention; Figure 3 This is an exothermic curve diagram of acrylic acid polymerization reaction in Example 4 of the present invention; Figure 4 This is an exothermic curve of the itaconic acid polymerization reaction in Example 5 of the present invention; Figure 5 This is an exothermic curve diagram of the itaconic acid polymerization reaction in Example 6 of the present invention; Figure 6 This is an exothermic curve of the itaconic acid polymerization reaction in Example 7 of the present invention; Figure 7 This is an exothermic curve of the itaconic acid polymerization reaction in Example 8 of the present invention; Figure 8 This is an exothermic curve of the itaconic acid polymerization reaction in Example 9 of the present invention; Figure 9 This is the spectrum of the itaconic acid polymerization reaction of the present invention. DETAILED DESCRIPTION

[0019] A solution polymerization formula for optimizing the thermal cyclization performance of polyacrylonitrile (PAN)-based carbon fiber precursor includes a first monomer, a second monomer, a solvent, an initiator, and a capture agent. The capture agent is a polymer having the following structure, as shown below. The capture agent is used in conjunction with the second monomer to form hydrogen bonds between the two. During the polymerization process, the presence of hydrogen bonds limits the reactivity of the second monomer through steric hindrance, resulting in a more uniform distribution of the second monomer within the PAN polymer. This broadens the exothermic peak of the PAN polymer, thereby enhancing the oxidative cyclization of the PAN-based carbon fiber.

[0020] , Wherein R is H or CH3 capture agent includes polyacrylamide, polymethacrylamide.

[0021] ,

[0022] The picture shows the hydrogen bond.

[0023] The first monomer is acrylonitrile, the second monomer is itaconic acid, acrylic acid or methacrylic acid, the second monomer copolymer with carboxyl group, the solvent is dimethyl sulfoxide, N,N-dimethylformamide, the initiator is azobisisobutyronitrile, and the capture agent is polyacrylamide or polymethacrylamide.

[0024] The mass ratio of the feed materials is as follows: the solvent, the first monomer, the second monomer, the initiator, and the capture agent are 100%, with the solvent accounting for 72% to 84% by mass of the total mass of the system; the first monomer accounting for 16% to 28% by mass of the total mass of the system; the second monomer content is 0% to 3% by mass of the first monomer; the initiator accounts for 0.2% to 1.5% by mass of the first monomer; and the molar ratio of the carboxyl group contained in the capture agent to the carboxyl group contained in the second monomer is 1:1, preferably 0.75:1.

[0025] A method for preparing a stock solution for optimizing the thermal cyclization properties of polyacrylonitrile (PAN)-based carbon fiber precursor involves measuring the required solvent and temporarily storing it for later use. The required second monomer, initiator, and capture agent are then accurately weighed, with a weight error of no more than 0.5%. A portion of the measured solvent is then taken and stirred to dissolve the second monomer, initiator, and capture agent in the solvent to create solutions with ratios of 5%, 3%, and 5%, respectively. The stirring temperature should be controlled between 20°C and 27°C. After dissolution, the solution is stored for later use. Acrylonitrile is measured after the other preparations are added, delaying the measurement to minimize exposure risk.

[0026] The polymerization kettle must have the following conditions: an agitator to ensure that the reactants are fully mixed, a jacket heating / cooling system for accurately controlling the reaction temperature, and a nitrogen protection interface for maintaining an inert reaction environment. During the feeding stage, the solvent needs to be added in three times. The feed valve of the polymerization kettle is opened and the agitator is started. Then half of the solvent that has been measured in advance is added to the polymerization kettle, and the remaining solvent is temporarily stored for standby use. Subsequently, the pre-prepared second monomer-solvent mixed solution, initiator-solvent mixed solution, and capture agent-solvent mixed solution are added to the reactor in sequence. After each feeding, the feeding container needs to be repeatedly rinsed at least three times with part of the spare solvent to ensure that all components are completely transferred to the reactor and minimize errors caused by residues. The temperature in the kettle needs to be controlled throughout the feeding process. The jacket cooling water circulation ensures that the temperature of the material in the kettle and the jacket temperature do not exceed 25°C to prevent the initiator from decomposing prematurely during the feeding stage and triggering unnecessary pre-polymerization reactions.

[0027] After all reagents are added, the required amount of acrylonitrile monomer is measured and added all at once to the polymerization kettle. After the addition of acrylonitrile is complete, the addition container and piping are thoroughly flushed with any remaining solvent to ensure that no acrylonitrile monomer remains. After the addition process is complete, the polymerization kettle is sealed and nitrogen exchange begins. This nitrogen exchange utilizes a "vacuum-fill" cycle: a vacuum pump is used to evacuate the kettle to a vacuum state (pressure reaches -0.1 MPa), followed by the introduction of high-purity nitrogen to restore the pressure to atmospheric pressure (0 MPa). This cycle is repeated three times to ensure that the oxygen content in the kettle is reduced to a safe level, completing the nitrogen exchange. After exchange, the kettle temperature is gradually raised to the preset reaction temperature using the jacket heating system, typically within the range of 50-70°C depending on the specific process requirements. Simultaneously, the agitator speed is increased to 60-80 rpm, and the polymerization reaction officially begins. The reaction lasts for 18 hours, during which the kettle temperature must be maintained constant, with a temperature fluctuation within ±0.5°C, to ensure polymerization stability and uniform product molecular weight.

[0028] After the reaction is completed, the single-drying process is entered, and the unreacted acrylonitrile monomer is removed by vacuum distillation. The reaction temperature and stirring speed are maintained unchanged. The vacuum system is started to reduce the pressure in the kettle to -0.1Mpa, and the evaporated acrylonitrile monomer is recovered by the condensation system. The single-drying process needs to be continued until the acrylonitrile monomer content in the kettle is confirmed to be lower than the quality standard of 0.1% by online gas chromatography. After the single-drying is completed, the stirring system is stopped first, and the reaction solution is allowed to stand and degas for 1 hour to allow the tiny bubbles dissolved in the system to fully escape. The polyacrylonitrile stock solution finally obtained can be transferred to the subsequent spinning process after passing the quality inspection.

[0029] After degassing, the PAN stock solution is first delivered to the spinneret assembly at a constant pressure via a metering pump. Under controlled extrusion temperature, a uniform and stable stream of stock solution is formed. The stream immediately enters a gradient coagulation bath controlled at 40-60°C and a dimethyl sulfoxide concentration maintained at 50%-70%. Phase separation and solidification are achieved through double diffusion between the solvent and the non-solvent. The formed nascent fibers are first washed in multiple stages to remove residual solvent, then stretched with hot water to strengthen the molecular chain orientation. The fibers are then dried with hot rollers and subjected to relaxation and heat setting to obtain dense and uniform PAN precursors. After subsequent pre-oxidation and carbonization, the precursors can be prepared into carbon fiber products. Throughout the wet spinning process, temperature control accuracy must be maintained within ±1°C, and concentration fluctuations must not exceed ±2% to ensure the stability and repeatability of fiber performance.

[0030] By comparing the DSC test results of the polymerization liquid with the participation of the capture agent and the polymerization liquid without the participation of the capture agent in the polymerization process, the study found that the introduction of the capture agent significantly affected the exothermic behavior of the polymerization reaction. The test data showed that the exothermic peak width of the polymerization liquid with the participation of the capture agent showed a significant broadening phenomenon. This phenomenon was verified in the polymerization system of different second monomers, and the peak width could be widened by up to 99.7%. Further investigation of the changes in the same second monomer under different reaction times of the capture agent found that although the difference in reaction time would cause the degree of peak width broadening to fluctuate, all experimental groups showed a trend of increasing peak width. Among them, the sample with a reaction time of 12 hours showed the most significant peak width change, and its exothermic peak width increased by 160.7% compared with the control group.

[0031] [Example 1] The first monomer acrylonitrile 2.3760g, the second monomer itaconic acid 0.0240g, the solvent dimethyl sulfoxide 7.6g, the initiator azobisisobutyronitrile 0.0144g, under the protection of nitrogen at 64 ° C for 18h, the exothermic peak area and peak width are 4.985 ° C.

[0032] [Example 2] The first monomer is acrylonitrile 2.3640g, the second monomer is itaconic acid 0.0240g, the solvent is dimethyl sulfoxide 7.6g, the initiator is azobisisobutyronitrile 0.0144g, and polyacrylamide is added as a capture agent in an amount of 0.0120g. The reaction is carried out at 64℃ for 18h under the protection of nitrogen. The exothermic peak width after the capture agent and itaconic acid in the PAN-based spinning solution is 9.96℃, which is increased by 99.7%. Figure 1 shown.

[0033] [Example 3] The first monomer is acrylonitrile (2.3730 g), the second monomer is methacrylic acid (0.0159 g), the solvent is dimethyl sulfoxide (DMSO) (7.6 g), the initiator is azobisisobutyronitrile (0.0144 g), and polyacrylamide (0.0120 g) is added as a capture agent. The reaction is carried out at 64°C for 18 hours under nitrogen protection. The exothermic peak width after the capture agent and methacrylic acid in the PAN-based spinning solution is 8.93°C, which is increased by 79.1%. Figure 2 shown.

[0034] [Example 4] The first monomer is acrylonitrile (2.3740 g), the second monomer is acrylic acid (0.0132 g), the solvent is dimethyl sulfoxide (DMSO) (7.6 g), the initiator is azobisisobutyronitrile (0.0144 g), and polyacrylamide (0.0120 g) is added as a capture agent. The reaction is carried out at 64°C for 18 hours under nitrogen protection. The exothermic peak width after the capture agent and acrylic acid in the PAN-based spinning solution is 9.17°C, which is increased by 83.9%. Figure 3 shown.

[0035] [Example 5] The first monomer is acrylonitrile 2.3740g, the second monomer is itaconic acid 0.0132g, the solvent is dimethyl sulfoxide 7.6g, the initiator is azobisisobutyronitrile 0.0144g, and polyacrylamide is added as a capture agent in an amount of 0.0120g. The reaction is carried out at 64℃ for 3 hours under the protection of nitrogen. The exothermic peak width after the capture agent and the acrylic acid in the PAN-based spinning solution is 8.25, which is increased by 26.1%. Figure 4 shown.

[0036] [Example 6] The first monomer is acrylonitrile 2.3740g, the second monomer is itaconic acid 0.0132g, the solvent is dimethyl sulfoxide 7.6g, the initiator is azobisisobutyronitrile 0.0144g, and polyacrylamide is added as a capture agent in an amount of 0.0120g. The reaction is carried out at 64℃ for 6h under the protection of nitrogen. The exothermic peak width after the capture agent and the acrylic acid in the PAN-based spinning solution are 14.13, which is increased by 93.5%. Figure 5 shown.

[0037] [Example 7] The first monomer is acrylonitrile (2.3740 g), the second monomer is itaconic acid (0.0132 g), the solvent is dimethyl sulfoxide (DMSO) (7.6 g), the initiator is azobisisobutyronitrile (0.0144 g), and polyacrylamide (0.0120 g) is added as a capture agent. The reaction is carried out at 64°C for 12 hours under nitrogen protection. The exothermic peak width after the capture agent and the acrylic acid in the PAN-based spinning solution is 14.03, which increases by 160.7%. Figure 6 shown.

[0038] [Example 8] The first monomer is acrylonitrile 2.3740g, the second monomer is itaconic acid 0.0132g, the solvent is dimethyl sulfoxide 7.6g, the initiator is azobisisobutyronitrile 0.0144g, and polyacrylamide is added as a capture agent in an amount of 0.0120g. The reaction is carried out at 64℃ for 15h under the protection of nitrogen. The exothermic peak width after the capture agent and the acrylic acid in the PAN-based spinning solution are 10.95, which increases by 130.8%. Figure 7 shown.

[0039] [Example 9] The first monomer is acrylonitrile 2.3740g, the second monomer is itaconic acid 0.0132g, the solvent is dimethyl sulfoxide 7.6g, the initiator is azobisisobutyronitrile 0.0144g, and polyacrylamide is added as a capture agent in an amount of 0.0120g. The reaction is carried out at 64℃ for 18h under the protection of nitrogen. The exothermic peak width after the capture agent and the acrylic acid in the PAN-based spinning solution are 9.96, which is increased by 49.9%. Figure 8 shown.

[0040] By introducing a second monomer (such as itaconic acid, acrylic acid, or other carboxyl-containing compounds), this method optimizes polymer properties from multiple perspectives. First, the introduction of the second monomer breaks the excessive regularity of the molecular chain, promoting intersegmental motion and significantly enhancing fiber flexibility. Second, the carboxylic acid functional group alters the cyclization mechanism from free radical cyclization to a milder anionic cyclization. This shift not only lowers the cyclization temperature but also broadens the exothermic peak range, significantly improving the controllability of the thermal stabilization process. However, this copolymerization method also faces new technical challenges. Due to differences in reactivity ratios among different monomers, standard free radical polymerization processes are prone to uneven composition distribution. Specifically, in the initial polymerization stage, the more active acrylonitrile monomer is preferentially consumed, resulting in a product front end rich in the second monomer. By the late polymerization stage, when the second monomer is largely depleted, the resulting segments are almost entirely composed of acrylonitrile. This uneven molecular chain composition directly interferes with the subsequent thermal stabilization step, resulting in a decrease in the consistency of the cyclization reaction and ultimately affecting the overall fiber performance.

[0041] The present invention addresses this issue by proposing a solution using a capture agent. The capture agent contains an amide group that can form a stable hydrogen-bonded complex with a carboxylic acid-based second monomer. This hydrogen-bonded complex can temporarily lock the carboxylic acid group of the second monomer, effectively reducing its polymerization activity, aligning the consumption rates of different monomers, and enabling the second monomer to be evenly distributed across the molecular segments, improving the uniformity of the molecular chain and preventing the formation of defects. This significantly broadens the polymer cyclization peak and results in a higher degree of cyclization.

[0042] The capture agent introduced in this invention is a polymer rich in hydrogen bonding sites. This type of capture agent can interact with the second monomer through hydrogen bonding to form a stable hydrogen-bonded complex, thereby capturing the second monomer. The amide group in the capture agent can form hydrogen bonds with the carboxyl group of the second monomer, significantly reducing its participation in the copolymerization reaction and making the second monomer more evenly distributed in the product.

[0043] The present invention realizes precise control of the polymerization process through the dynamic equilibrium characteristics of the capture agent. In the acrylonitrile copolymerization system, after the capture agent forms a hydrogen bond complex with the second monomer, a "capture-release" dynamic equilibrium is established. The second monomer can maintain a relatively stable concentration state under the regulation of this dynamic equilibrium, which can ensure that the reactivity ratio ratio of the acrylonitrile monomer and the second monomer is within the ideal range. This precise control can make the second monomer more evenly distributed on the molecular chain, and make the sequence distribution of the copolymer finally formed more regular. The formation of the hydrogen bond complex can be confirmed by Fourier transform infrared spectroscopy, such as Figure 9 As shown, itaconic acid has two carboxyl groups, α-COOH and β-COOH. Due to the conjugation effect, α-COOH typically appears around 1635 cm⁻. When α-COOH forms a hydrogen bond, the C=O stretching vibration amplitude decreases due to the influence of the hydrogen bond. The α-COOH peak decreases until it disappears as the number of hydrogen bonds formed between α-COOH in itaconic acid and the amine groups in the hydrogen-bonded complex increases. β-COOH typically appears in its free state around 1720 cm⁻. It can be seen that with the addition of polyacrylamide, the β-COOH peak gradually decreases, indicating that the content of free carboxyl groups formed by hydrogen bonds with carboxyl groups decreases. This proves that the hydrogen-bonded complex can form stable hydrogen bonds with itaconic acid in DMSO solution.

[0044] The present invention provides a solution polymerization formula for optimizing the thermal cyclization performance of polyacrylonitrile. By introducing a capture agent into the polymerization system, the polymerization behavior of the second monomer is regulated, significantly improving the thermal cyclization performance of PAN fibers. In traditional copolymerization systems, carboxylic acid-based second monomers, due to their significant difference in reactivity with acrylonitrile (AN), tend to form localized enrichment or long block structures on the molecular chain, leading to intense reactions and structural defects during subsequent thermal cyclization. By adding a capture agent rich in hydrogen bonding sites, the present invention dynamically controls the concentration of the second monomer, minimizing the difference in reactivity with AN and promoting uniform distribution of the second monomer along the molecular chain. This uniform distribution exhibits significant advantages during the subsequent thermal cyclization process. Because the carboxylic acid groups are discretely distributed along the PAN molecular chain, the cyclization reaction can proceed gradually over a wider temperature range. Furthermore, the uniform distribution of the carboxylic acid groups enhances the catalytic efficiency of anionic cyclization, increases the degree of cyclization, and increases the half-width of the cyclization exothermic peak, significantly improving the thermal stability of the fiber. The exothermic peaks of PAN polymerization stock solutions without and with hydrogen bond capture agents were observed by DSC every 3 hours. It was found that the hydrogen bond capture agent always played a role in the entire polymerization process, broadening the cyclization exothermic peak and avoiding concentrated heat release.

[0045] The DSC curve of a pure PAN system without a capture agent and a second monomer exhibits a very sharp exothermic peak, indicating that the cyclization reaction is completed rapidly within a very narrow temperature window. This highly concentrated exothermic behavior is extremely detrimental to the fiber heat treatment process, so the heating rate should be minimized during the production process to slow the degree of chemical reactions within the fiber.

Claims

1. A solution polymerization formula for optimizing the thermal cyclization performance of polyacrylonitrile-based carbon fiber precursor, characterized by: The method comprises a first monomer, a second monomer, a solvent, an initiator and a capture agent, wherein the capture agent is a polymer having the following structure: ， Wherein, R is H or CH3.

2. The solution polymerization formula for optimizing the thermal cyclization performance of polyacrylonitrile-based carbon fiber precursor according to claim 1, characterized in that: After the capture agent is mixed with the second monomer, a hydrogen bond is formed between the two, and the hydrogen bond is expressed as follows: 。 3. The solution polymerization formula for optimizing the thermal cyclization performance of polyacrylonitrile-based carbon fiber precursor according to claim 1, characterized in that: The first monomer is acrylonitrile; the second monomer is itaconic acid, acrylic acid, methacrylic acid, or a second monomer copolymer with a carboxyl group; the solvent is dimethyl sulfoxide or N,N-dimethylformamide; the initiator is azobisisobutyronitrile; and the capture agent is polyacrylamide or polymethacrylamide.

4. The solution polymerization formula for optimizing the thermal cyclization performance of polyacrylonitrile-based carbon fiber precursor according to claim 1, characterized in that: The solvent, the first monomer, the second monomer, the initiator and the capture agent are 100%, the solvent accounts for 72% to 84% by mass of the total mass of the system; the first monomer accounts for 16% to 28% by mass of the total mass of the system; the second monomer content is 0% to 3% by mass of the first monomer; the initiator accounts for 0.2% to 1.5% by mass of the first monomer; and the molar ratio of the carboxyl group contained in the capture agent to that contained in the second monomer is 1:

1.

5. A method for preparing a stock solution for optimizing the thermal cyclization performance of polyacrylonitrile-based carbon fiber precursor, characterized in that: The following steps are involved: (1) Measure the required solvent and store it temporarily for future use. Weigh the required second monomer, initiator and capture agent. Take part of the measured solvent and stir and dissolve the second monomer, initiator and capture agent in the solvent to prepare a second monomer-solvent mixed solution, an initiator-solvent mixed solution and a capture agent-solvent mixed solution with a ratio of 5%, 3% and 5% respectively. The stirring temperature is controlled between 20℃ and 27℃. After the dissolution is completed, store it for future use. (2) During the feeding stage, the feed valve of the polymerization kettle is opened and the agitator is started. Then, half of the solvent measured in advance is added into the polymerization kettle. Subsequently, the pre-prepared second monomer-solvent mixed solution, initiator-solvent mixed solution and capture agent-solvent mixed solution are sequentially added into the reactor. (3) Measure the required amount of acrylonitrile monomer and put it all into the polymerization kettle at one time. Then seal the polymerization kettle and start nitrogen replacement. After the nitrogen replacement is completed, increase the temperature in the polymerization kettle to the preset reaction temperature through the heating system of the polymerization kettle. The preset reaction temperature is in the range of 50-70°C. At the same time, increase the stirring speed to 60-80 rpm to start the polymerization reaction; (4) After the polymerization reaction is completed, the monomer removal process is started, and the unreacted acrylonitrile monomer is removed by vacuum distillation. The reaction temperature and stirring speed are maintained unchanged. The vacuum system is started to reduce the pressure in the polymerization kettle to -0.1Mpa, and the evaporated acrylonitrile monomer is recovered to obtain polyacrylonitrile stock solution.

6. The method for preparing a stock solution for optimizing the thermal cyclization performance of polyacrylonitrile-based carbon fiber precursor according to claim 5, characterized in that: In step (2), after each addition, the feeding container is repeatedly rinsed at least three times with part of the solvent; the temperature in the polymerization kettle is controlled during the entire feeding process, and the temperature in the polymerization kettle is adjusted through the jacket cooling water circulation system so that the temperature in the polymerization kettle and the jacket does not exceed 25°C.

7. The method for preparing a stock solution for optimizing the thermal cyclization performance of polyacrylonitrile-based carbon fiber precursor according to claim 5, characterized in that: In step (3), after the addition of acrylonitrile is completed, the addition container and pipeline are thoroughly flushed with all the remaining solvent to ensure that no acrylonitrile monomer remains.

8. The method for preparing a stock solution for optimizing the thermal cyclization performance of polyacrylonitrile-based carbon fiber precursor according to claim 5, characterized in that: In step (3), the nitrogen replacement is carried out by using a vacuum pump to evacuate the polymerization kettle to a vacuum state, the pressure reaching -0.1 MPa, and then high-purity nitrogen is introduced to restore the pressure to normal pressure. This cycle is repeated three times to reduce the oxygen content in the polymerization kettle to a safe range, thereby completing the nitrogen replacement.

9. The method for preparing a stock solution for optimizing the thermal cyclization performance of polyacrylonitrile-based carbon fiber precursor according to claim 5, characterized in that: In step (3), the polymerization reaction process lasts for 18 hours, and the temperature in the polymerization kettle is kept constant during the polymerization reaction, and the temperature fluctuation range is controlled within ±0.5°C; the de-monomerization process is continued until it is confirmed by online gas chromatography that the acrylonitrile monomer content in the polymerization kettle is lower than the quality standard of 0.1%; after the de-monomerization process is completed, the stirring system is first stopped, and the reaction solution is allowed to stand and degas for 1 hour to allow the tiny bubbles dissolved in the system to fully escape. Finally, the obtained polyacrylonitrile stock solution passes the quality inspection and is then transferred to the subsequent spinning process.

10. The method for preparing a stock solution for optimizing the thermal cyclization performance of polyacrylonitrile-based carbon fiber precursor according to claim 9, characterized in that: The spinning process is as follows: the obtained PAN stock solution is first delivered to the spinneret assembly at a constant pressure by a metering pump to form a uniform and stable stock solution stream, and the stock solution stream immediately enters a gradient coagulation bath with a temperature controlled at 40-60°C and a dimethyl sulfoxide concentration maintained at 50%-70%. Phase separation and solidification are achieved through double diffusion between the solvent and the non-solvent. The formed nascent fibers are first subjected to multi-stage water washing to remove residual solvent, and then to hot water drawing to strengthen molecular chain orientation. The fibers are then dried by hot rollers and subjected to relaxation and heat setting treatment to obtain PAN precursors. The PAN precursors are subsequently subjected to pre-oxidation and carbonization treatments to prepare carbon fiber products. During the entire spinning process, the temperature control accuracy is maintained within ±1°C, and the concentration fluctuation does not exceed ±2%.

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