Preparation method of crude poly (p-phenylene terephthamide)

Abstract

The invention relates to the technical field of organic synthesis, in particular to a preparation method of crude poly (p-phenylene terephthamide). According to the method, phase-to-phase switching between non-aqueous suspension and ionic liquid homogeneous phase is adopted, and AB type end group balance, alternating end sealing and interface confinement branching are combined to realize end group accurate control and molecular weight distribution convergence. The method comprises the following steps: emulsifying an N-methyl pyrrolidone / calcium chloride solution in paraffin oil; carrying out polycondensation with p-phenylenediamine and paraphthaloyl chloride at a low temperature, and adding p-aminobenzoyl chloride to carry out dynamic balance on an end group; after layering, transferring into 1-butyl-3-methylimidazole chloride / N, N-dimethylacetamide for continuous polycondensation; cooling and re-emulsifying, and alternately adding benzoyl chloride and p-toluyl chloride for end sealing; trimesoyl chloride is added to a phase interface to realize limited branching; and carrying out ionic liquid homogenization, amine termination and alcohol / water sedimentation washing to obtain the product. The method effectively relieves end group drift, inhibits high molecular weight tailing and improves the filtering performance, and is suitable for preparation of high-reliability aramid fibers.

Description

Technical Field

[0001] This invention relates to the field of organic synthesis technology, and in particular to a method for preparing crude poly(p-phenylene terephthalamide). Background Technology

[0002] Poly(p-phenylene terephthalamide), as an important high-performance aramid material, plays an irreplaceable role in aerospace, defense, and high-temperature protection fields. Its preparation mainly employs a low-temperature solution polycondensation method, achieving the stepwise polymerization reaction of p-phenylenediamine and terephthaloyl chloride in a polar aprotic solvent. However, existing synthesis processes generally face technical bottlenecks in end-group control and molecular weight distribution regulation, severely restricting further improvements in product quality and the stability of industrial applications.

[0003] In traditional synthesis processes, end-group control relies primarily on precise calculations of monomer feed ratios and strict control of reaction conditions. Due to the progressive growth characteristics of polycondensation reactions, even small feed deviations can be amplified, causing end-groups to deviate from the ideal range. Excessively high amine end-group content increases the risk of side reactions in subsequent processing, while excessively low content may affect the further growth potential of the molecular chain. Existing technologies typically employ overfeeding or the addition of monofunctional small molecules for end-capping, but these methods often result in a broadened molecular weight distribution, forming high-molecular-weight tails, and deteriorating the filtration performance of the solution and spinning stability.

[0004] Controlling molecular weight distribution is another key technical challenge. Ideal aramid products require a relatively narrow molecular weight distribution to ensure uniform fiber properties and processing stability. However, the inherent characteristics of stepwise polymerization dictate that the molecular weight distribution must have a certain width. In existing processes, to obtain sufficiently high molecular weights, it is often necessary to extend the reaction time or increase the reaction temperature. However, these measures usually lead to further widening of the distribution and may trigger adverse side reactions such as gelation and cyclization.

[0005] The choice of solvent system is also a key factor affecting polymerization behavior. Traditional processes often employ a single solvent system, such as N-methylpyrrolidone or ionic liquids. While single solvent systems are simple to operate, they struggle to provide optimal reaction environments at different stages of the reaction. For example, in the initial polycondensation stage, it is necessary to control the reaction rate to avoid localized overheating and distribution deterioration, while in the later stages, good solubility is required to promote the formation of high molecular weight products. Existing solvent systems struggle to simultaneously meet these conflicting requirements.

[0006] Branching regulation is an important means of influencing the rheological properties of the final product. Appropriate branching can improve the shear-thinning properties of spinning solutions and broaden the processing window, but controlling the degree of branching is extremely difficult. In existing technologies, branching reactions mostly occur in a homogeneous phase, and the contact between the branching agent and the polymer chain lacks selectivity, easily leading to the formation of highly branched microgels or non-uniform topologies, severely deteriorating the filtration performance of the solution.

[0007] To address the aforementioned technical challenges, the industry urgently needs a comprehensive technical solution that can simultaneously achieve precise end-group control, molecular weight distribution convergence, and controllable branching, in order to drive breakthrough progress in the preparation technology of high-performance aramid materials. Summary of the Invention

[0008] In view of this, the purpose of this invention is to provide a method for preparing crude poly(p-phenylene terephthalamide) to solve the problem of how to simultaneously achieve precise end-group control and molecular weight distribution convergence in the synthesis of poly(p-phenylene terephthalamide).

[0009] To achieve the above objectives, the present invention provides a method for preparing crude poly(p-phenylene terephthalamide), comprising the following steps: (1) N-methylpyrrolidone / calcium chloride solution was added dropwise to paraffin oil at a constant flow rate, with the temperature controlled at −15℃ to 0℃, the stirring speed at 600–1000 rpm, and the nitrogen flow rate at 40–60 mL / min, to form a milky white suspension with uniform particle size, and then p-phenylenediamine was added. (2) Add terephthaloyl chloride in batches at -5℃ to 0℃, and simultaneously add AB type end-group balancing agent to allow the reaction to proceed until the viscosity of the system increases; (3) After standing and separating to remove paraffin oil, add an ionic liquid / polar cosolvent mixture to the lower phase to obtain a homogeneous solution. Add the second part of terephthaloyl chloride at a constant rate at 8°C to 12°C to continue the polycondensation. (4) Cool the homogeneous system and reintroduce paraffin oil to form an emulsion. Add the first end-sealing agent and the second end-sealing agent alternately at time intervals to complete the end-sealing. (5) Add a trifunctional acyl chloride branching agent at 0℃ to 5℃ to carry out a restricted branching reaction at the interface of a non-aqueous suspension phase; (6) After removing paraffin oil in layers, add ionic liquid / polar cosolvent to the lower layer to homogenize the chain segment distribution; (7) Add an amine terminator, then pour the reaction solution into an alcohol / water mixture to precipitate the product. After washing with alternating alcohol and water, dry to obtain poly(p-phenylene terephthalamide).

[0010] Preferably, the AB-type end-group balancing agent is p-aminobenzoyl chloride, and its addition amount is 0.0015-0.0025 times the amount of p-phenylenediamine.

[0011] Preferably, the N-methylpyrrolidone / calcium chloride solution in step (1) is a solution containing 6wt%–9wt% calcium chloride and is used after being filtered through a 0.45μm polytetrafluoroethylene membrane.

[0012] Preferably, the paraffin oil in step (1) is paraffin oil dried at 120°C for 2 hours, and the dropping rate of the N-methylpyrrolidone / calcium chloride solution is 15-20 mL / min.

[0013] Preferably, the ionic liquid in step (3) is 1-butyl-3-methylimidazolium chloride, the polar co-solvent is N,N-dimethylacetamide, the mass-to-volume ratio of the ionic liquid to the polar co-solvent is 0.8g:1mL–1.2g:1mL, and the homogenization stirring time is 20-45min.

[0014] Preferably, the second part of terephthaloyl chloride in step (3) is linearly added dropwise over a time of 60-90 minutes under constant temperature conditions of 8-12℃.

[0015] Preferably, the alternating end-capping in step (4) uses an end-capping agent pair composed of benzoyl chloride and p-toluene chloride, which are added alternately at an interval of 5-10 minutes, and the amount of each end-capping agent added is 0.001-0.003 times the amount of p-phenylenediamine.

[0016] Preferably, the branching agent in step (5) is pyromellitic chloride, and the amount added is 0.0025-0.0030 times the amount of p-phenylenediamine, and the branching reaction time is 10-15 min.

[0017] Preferably, the flow rate of dry nitrogen gas introduced during steps (1) and (2) is 50-70 mL / min, and the tail gas is introduced into an absorption bottle containing aniline-toluene solution to absorb hydrogen chloride.

[0018] Preferably, the amounts of ionic liquid / polar cosolvent added in step (6) are 80-120g of ionic liquid and 80-120mL of polar cosolvent, respectively, and the homogenization time is 25-40min.

[0019] Preferably, the settling liquid in step (7) is a methanol / deionized water mixture with a volume ratio of 1:1, and the washing sequence is alternating between alcohol washing and water washing.

[0020] Preferably, the terminating agent in step (7) is a dilute solution of N-methylpyrrolidone of aniline, and the stirring time after termination is 5-15 min.

[0021] The beneficial effects of this invention are: This invention achieves simultaneous optimization of key performance indicators in the synthesis of poly(p-phenylene terephthalamide) by combining the synergistic reaction of p-phenylenediamine and terephthaloyl chloride in a specific multiphase system with the introduction of p-aminobenzoyl chloride as an AB-type end-group balancing agent.

[0022] The addition of p-aminobenzoyl chloride during the initial polycondensation stage provides a unique end-group regulation mechanism for the reaction system. This compound contains both amino and acyl chloride groups, preferentially consuming excess amino groups when p-phenylenediamine is in slight excess, and providing additional amino groups to participate in the reaction when terephthaloyl chloride is in excess, thus forming a dynamic stoichiometric equilibrium. This intramolecular dual reactivity avoids the molecular weight loss that may result from traditional single-functional regulators, ensuring stable control of end-group concentration within an appropriate range.

[0023] The alternating use of the N-methylpyrrolidone / calcium chloride system and the 1-butyl-3-methylimidazolium chloride / N,N-dimethylacetamide system creates differentiated microenvironments for the polymerization reaction. The presence of calcium chloride significantly enhances the solubility of N-methylpyrrolidone in aramid prepolymers, while its ionic properties help stabilize reaction intermediates; the ionic liquid system, on the other hand, provides stronger solubility and better chain extension conditions. Differences in polarity, viscosity, and diffusion coefficient between the two systems result in different solvation environments for the molecular chains during each phase transition, promoting the readjustment and convergence of the molecular weight distribution.

[0024] The selection of paraffin oil as a nonpolar dispersion medium is of significant technical importance. Its incompatibility with the polar reactive phase ensures the stable dispersion of reaction droplets, while providing an effective mass transfer channel for the byproduct hydrogen chloride. The low volatility and chemical inertness of paraffin oil prevent solvent loss and side reactions, while its excellent heat transfer properties facilitate precise control of the reaction temperature.

[0025] The combination of benzoyl chloride and p-toluyl chloride as alternating end-capping agents leverages their differences in reactivity across different phase environments. Benzoyl chloride exhibits a strong polar affinity, reacting more actively in ionic liquid phases; while p-toluyl chloride, due to the presence of its methyl group, possesses some hydrophobicity, making it more readily reactive near the phase interface. This complementary reaction preference ensures adequate end-group closure in different microenvironments, avoiding the selectivity limitations that might result from using a single end-capping agent.

[0026] The confined branching reaction of pyromellitic trichloromethyl chloride at the phase interface exhibits unique regulatory advantages. The spatial confinement and mass transfer resistance at the phase interface make the contact between the branching agent and the polymer chain more gentle and controllable, avoiding the local high concentration of branching that may occur in a homogeneous phase. The symmetrical structure of the three acyl chloride groups ensures a more uniform spatial distribution of branching sites, while the constraint of the interfacial environment prevents over-branching and the formation of microgels.

[0027] The imidazole ring structure of 1-butyl-3-methylimidazolium chloride provides unique solvation capabilities. Its cations can form hydrogen bonds with the amide groups on the aramid molecular chain, promoting the full dissolution and expansion of the macromolecule. The presence of chloride ions helps maintain the ionic strength balance of the system, preventing the adverse effects of polyelectrolyte effects on the molecular chain conformation. The combined use of this ionic liquid with N,N-dimethylacetamide further optimizes the viscosity and mass transfer properties of the solvent.

[0028] The synergistic effect of the components throughout the process route forms a self-regulating reaction system. The end-group buffering of p-aminobenzoyl chloride, the distribution regulation of phase switching, the selective optimization of alternating end-capping, and the precise control of interfacial branching together construct a multi-level quality assurance mechanism, resulting in significant improvements in key indicators such as end-group control, molecular weight distribution, and filtration performance of the final product. Detailed Implementation

[0029] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to specific embodiments.

[0030] Example 1: Step 1: In a 2000mL four-necked flask equipped with a mechanical stirrer, a digital temperature controller (accuracy ±1°C), a nitrogen protection tube, and an HCl absorption bottle containing 500mL of aniline-toluene solution (mass fraction 10%), add 1500mL of paraffin oil (kinematic viscosity 45.2cSt, dried at 120°C for 2h). Cool the system to -10°C in a dry ice-ethanol bath and start stirring at 700rpm. Purge with nitrogen gas dried through molecular sieves at 40mL / min. Add the pre-prepared N-methylpyrrolidone / calcium chloride solution (800mL N-methylpyrrolidone dissolved in 60g anhydrous calcium chloride, stirred at 60°C for 1h until completely clear, filtered through a 0.45μm polytetrafluoroethylene membrane and cooled to -10°C) dropwise at a rate of 15mL / min using a constant flow pump to form a milky white suspension with uniform particle size. After the dropwise addition is complete, add 108.1g (1mol) of p-phenylenediamine. Step 2: Control the temperature of the reaction system at -5°C to 0°C. Add 101.5g (0.5mol) terephthaloyl chloride in batches over 30min, and simultaneously add 0.26g (0.0015mol) p-aminobenzoyl chloride all at once. Maintain a nitrogen flow rate of 60mL / min during the addition process and introduce the tail gas into the absorption bottle through a conduit to capture HCl. After 25min of reaction, the system is a light yellow viscous substance. Step 3: After the reaction is complete, turn off the stirrer and let it stand for 20 minutes under gravity until the layers are clear. Use a separatory funnel with a PTFE piston to separate the upper paraffin oil from the lower N-methylpyrrolidone phase. After ensuring that the paraffin oil is completely removed, add 320g of 1-butyl-3-methylimidazolium chloride and 320mL of N,N-dimethylacetamide, which have been mixed evenly at room temperature, to the N-methylpyrrolidone phase containing the prepolymer. Heat the system to 10°C using a constant temperature water bath and stir at 300rpm for 20 minutes until a completely transparent deep yellow homogeneous solution is formed. Then, add 101.5g (0.5mol) terephthaloyl chloride dropwise over 70 minutes using a peristaltic pump. Maintain the reaction temperature in the range of 8°C to 12°C throughout the process. Step 4: Cool the reaction mixture to 0°C, add 800 mL of paraffin oil again, stir at 800 rpm to form an emulsion, then add 0.21 g (0.0015 mol) benzoyl chloride and 0.24 g (0.0015 mol) p-toluene chloride alternately every 10 min for a total of 20 min to complete the alternating end-sealing, then add 0.61 g (0.003 mol) trimesoyl chloride all at once, and react at 0°C for 15 min to carry out branching; Step 5: Stop stirring again and separate the layers. Remove the upper layer of paraffin oil and add 80g of pre-mixed 1-butyl-3-methylimidazolium chloride and 80mL of N,N-dimethylacetamide to the lower layer. Heat to 10°C and stir for 25min to homogenize the chain segment distribution and adjust the branch point distribution. Step 6: Add 20 mL of N-methylpyrrolidone solution of aniline (0.1% w / v) to the reaction system to terminate the residual acyl chloride groups. After stirring for 10 min, quickly pour the entire reaction mixture into 5000 mL of methanol / deionized water mixture (volume ratio 1:1). A large amount of white precipitate is immediately generated. Filter and collect the precipitate. Wash it three times with methanol, then three times with deionized water. Then add methanol twice and deionized water twice to remove the ionic liquid residue. Finally, dry it in a vacuum oven at 60°C to constant weight to obtain crude poly(p-phenylene terephthalamide). Example 2: Step 1: In a 2000mL four-necked flask equipped with a mechanical stirrer, a digital temperature controller (accuracy ±1°C), a nitrogen protection tube, and an HCl absorption bottle containing 500mL of aniline-toluene solution (mass fraction 10%), add 1500mL of paraffin oil (kinematic viscosity 45.2cSt, dried at 120°C for 2h). Cool the system to -10°C in a dry ice-ethanol bath and start stirring at 800rpm. Purge with nitrogen gas dried through molecular sieves at 50mL / min. Add the pre-prepared N-methylpyrrolidone / calcium chloride solution (800mL N-methylpyrrolidone dissolved in 60g anhydrous calcium chloride, stirred at 60°C for 1h until completely clear, filtered through a 0.45μm polytetrafluoroethylene membrane and cooled to -10°C) dropwise at a rate of 18mL / min using a constant flow pump to form a milky white suspension with uniform particle size. After the dropwise addition is complete, add 108.1g (1mol) of p-phenylenediamine. Step 2: Control the temperature of the reaction system between -5°C and 0°C. Add 101.5g (0.5mol) terephthaloyl chloride in batches over 30min, and add 0.34g (0.002mol) p-aminobenzoyl chloride at the same time. During the addition process, maintain the nitrogen flow rate at 70mL / min and introduce the tail gas into the absorption bottle through the tube to capture HCl. After reacting for 30min, the system will be a light yellow viscous substance. Step 3: After the reaction is complete, turn off the stirrer and let it stand for 20 minutes under gravity until the layers are clear. Use a separatory funnel with a PTFE piston to separate the upper paraffin oil from the lower N-methylpyrrolidone phase. After ensuring that the paraffin oil is completely removed, add 350g of 1-butyl-3-methylimidazolium chloride and 350mL of N,N-dimethylacetamide, which have been mixed evenly at room temperature, to the N-methylpyrrolidone phase containing the prepolymer. Heat the system to 10°C using a constant temperature water bath and stir at 300rpm for 30 minutes until a completely transparent deep yellow homogeneous solution is formed. Then, add 101.5g (0.5mol) terephthaloyl chloride dropwise evenly over 60 minutes using a peristaltic pump. Maintain the reaction temperature in the range of 8°C to 12°C throughout the process. Step 4: Cool the reaction mixture to 0°C, add 800 mL of paraffin oil again, stir at 800 rpm to form an emulsion, then add 0.21 g (0.0015 mol) benzoyl chloride and 0.24 g (0.0015 mol) p-toluene chloride alternately every 10 min for a total of 20 min to complete the alternating end-sealing, then add 0.61 g (0.003 mol) trimesoyl chloride all at once, and react at 0°C for 15 min to carry out branching; Step 5: Stop stirring again and separate the layers. Remove the upper layer of paraffin oil and add 100g of pre-mixed 1-butyl-3-methylimidazolium chloride and 100mL of N,N-dimethylacetamide to the lower layer. Heat to 10°C and stir for 30min to homogenize the chain segment distribution and adjust the branch point distribution. Step 6: Add 20 mL of N-methylpyrrolidone solution of aniline (0.1% w / v) to the reaction system to terminate the residual acyl chloride groups. After stirring for 10 min, quickly pour the entire reaction mixture into 5000 mL of methanol / deionized water mixture (volume ratio 1:1). A large amount of white precipitate is immediately generated. Filter and collect the precipitate. Wash it three times with methanol, then three times with deionized water. Finally, dry it in a vacuum oven at 60°C to constant weight to obtain crude poly(p-phenylene terephthalamide).

[0031] Example 3: Step 1: In a 2000mL four-necked flask equipped with a mechanical stirrer, a digital temperature controller (accuracy ±1°C), a nitrogen protection tube, and a 500mL HCl absorption bottle containing an aniline-toluene solution (mass fraction 10%), add 1500mL of paraffin oil (kinematic viscosity 45.2cSt, dried at 120°C for 2h). Cool the system to -10°C in a dry ice-ethanol bath and start stirring at 900rpm. Purge with nitrogen gas dried with molecular sieves at 60mL / min. Add 800mL of pre-prepared N-methylpyrrolidone / calcium chloride solution. Dissolve 60g of anhydrous calcium chloride in N-methylpyrrolidone, stir at 60°C for 1h until completely clear, filter through a 0.45μm polytetrafluoroethylene membrane and cool to -10°C. Add the solution dropwise at a rate of 18mL / min using a constant flow pump to form a milky white suspension with uniform and finer particle size. After the addition is complete, add 108.1g (1mol) of p-phenylenediamine. Step 2: Control the temperature of the reaction system between -5°C and 0°C. Add 101.5g (0.5mol) terephthaloyl chloride in batches over 30min, and add 0.43g (0.0025mol) p-aminobenzoyl chloride at the same time. During the addition process, maintain the nitrogen flow rate at 70mL / min and introduce the tail gas into the absorption bottle through the tube to capture HCl. After reacting for 40min, the system is light yellow and viscous. Step 3: After the reaction is complete, turn off the stirrer and let it stand for 20 minutes under gravity until the layers are clear. Use a separatory funnel with a PTFE piston to separate the upper paraffin oil from the lower N-methylpyrrolidone phase. After ensuring that the paraffin oil is completely removed, add 380g of 1-butyl-3-methylimidazolium chloride and 380mL of N,N-dimethylacetamide, which have been mixed evenly at room temperature, to the N-methylpyrrolidone phase containing the prepolymer. Heat the system to 10°C using a constant temperature water bath and stir at 300rpm for 45 minutes until a completely transparent deep yellow homogeneous solution is formed. Then, add 101.5g (0.5mol) terephthaloyl chloride dropwise evenly over 90 minutes using a peristaltic pump. Maintain the reaction temperature in the range of 9°C to 10°C throughout the process. Step 4: Cool the reaction mixture to 0°C, add 800 mL of paraffin oil again, and stir at 800 rpm to form an emulsion. Then, add 0.21 g (0.0015 mol) of benzoyl chloride and 0.24 g (0.0015 mol) of p-toluene chloride alternately in two batches every 5 min, for a total of 15 min to complete the alternating end-sealing. Then, add 0.51 g (0.0025 mol) of trimesoyl chloride all at once, and react at 0°C for 10 min to carry out mild branching. Step 5: Stop stirring again and separate the layers. Remove the upper layer of paraffin oil and add 120g of pre-mixed 1-butyl-3-methylimidazolium chloride and 120mL of N,N-dimethylacetamide to the lower layer. Heat to 10°C and stir for 40min to homogenize the chain segment distribution and adjust the branch point distribution. Step 6: Add 20 mL of N-methylpyrrolidone solution of aniline (0.1% w / v) to the reaction system to terminate the residual acyl chloride groups. After stirring for 10 min, quickly pour the entire reaction mixture into 5000 mL of methanol / deionized water mixture (volume ratio 1:1). A large amount of white precipitate is immediately generated. Filter and collect the precipitate. Wash it three times with methanol, then three times with deionized water. Then add methanol twice and deionized water twice to remove the ionic liquid residue. Finally, dry it in a vacuum oven at 60°C to constant weight to obtain crude poly(p-phenylene terephthalamide). Comparative Example 1: The difference between Comparative Example 1 and Example 2 is that the entire process uses an ionic liquid system (1-butyl-3-methylimidazolium chloride / N,N-dimethylacetamide). The specific steps are as follows: Step 1: In a 2000mL four-necked flask equipped with a mechanical stirrer, a digital temperature controller (accuracy ±1°C), a nitrogen protection tube, and an HCl absorption bottle containing 500mL of aniline-toluene solution (mass fraction 10%), add 450g of 1-butyl-3-methylimidazolium chloride and 450mL of N,N-dimethylacetamide. Premix at room temperature, start stirring at 400rpm, cool the system to 0-5°C using an ice-salt bath, and add 108.1g (1mol) of p-phenylenediamine. Step 2: Control the temperature of the reaction system between -5°C and 0°C. Add 101.5g (0.5mol) terephthaloyl chloride in batches over 30min, and add 0.34g (0.002mol) p-aminobenzoyl chloride at the same time. During the addition process, maintain the nitrogen flow rate at 70mL / min and introduce the tail gas into the absorption bottle through the conduit to capture HCl. React for 30min. Step 3: After the reaction is complete, heat to 10°C and add 101.5g (0.5mol) of terephthaloyl chloride evenly dropwise over 60min; Step 4: Cool the reaction mixture to 0°C, and add 0.21 g (0.0015 mol) benzoyl chloride and 0.24 g (0.0015 mol) p-toluene chloride alternately every 10 min for a total of 20 min to complete the alternating end-blocking. Then add 0.61 g (0.003 mol) trimesoyl chloride all at once and react at 0°C for 15 min to carry out branching. Step 5: Add 20 mL of N-methylpyrrolidone solution of aniline (0.1% w / v) to the reaction system to terminate the residual acyl chloride groups. After stirring for 10 min, quickly pour the entire reaction mixture into 5000 mL of methanol / deionized water mixture (volume ratio 1:1). A large amount of white precipitate is immediately generated. Filter and collect the precipitate. Wash it three times with methanol, then wash it three times with deionized water. Finally, dry it in a vacuum oven at 60°C to constant weight to obtain crude poly(p-phenylene terephthalamide).

[0032] Comparative Example 2: The difference between Comparative Example 2 and Example 2 is that p-aminobenzoyl chloride is not added in step 2; Step 1: In a 2000mL four-necked flask equipped with a mechanical stirrer, a digital temperature controller (accuracy ±1°C), a nitrogen protection tube, and an HCl absorption bottle containing 500mL of aniline-toluene solution (mass fraction 10%), add 1500mL of paraffin oil (kinematic viscosity 45.2cSt, dried at 120°C for 2h). Cool the system to -10°C in a dry ice-ethanol bath and start stirring at 800rpm. Purge with nitrogen gas dried through molecular sieves at 50mL / min. Add the pre-prepared N-methylpyrrolidone / calcium chloride solution (800mL N-methylpyrrolidone dissolved in 60g anhydrous calcium chloride, stirred at 60°C for 1h until completely clear, filtered through a 0.45μm polytetrafluoroethylene membrane and cooled to -10°C) dropwise at a rate of 18mL / min using a constant flow pump to form a milky white suspension with uniform particle size. After the dropwise addition is complete, add 108.1g (1mol) of p-phenylenediamine. Step 2: Control the temperature of the reaction system between -5°C and 0°C, and add 101.5g (0.5mol) of terephthaloyl chloride in batches over 30min. During the addition process, maintain the nitrogen flow rate at 70mL / min and introduce the tail gas into the absorption bottle through the tube to capture HCl. After reacting for 30min, the system will be a light yellow viscous substance. Step 3: After the reaction is complete, turn off the stirrer and let it stand for 20 minutes under gravity until the layers are clear. Use a separatory funnel with a PTFE piston to separate the upper paraffin oil from the lower N-methylpyrrolidone phase. After ensuring that the paraffin oil is completely removed, add 350g of 1-butyl-3-methylimidazolium chloride and 350mL of N,N-dimethylacetamide, which have been mixed evenly at room temperature, to the N-methylpyrrolidone phase containing the prepolymer. Heat the system to 10°C using a constant temperature water bath and stir at 300rpm for 30 minutes until a completely transparent deep yellow homogeneous solution is formed. Then, add 101.5g (0.5mol) terephthaloyl chloride dropwise evenly over 60 minutes using a peristaltic pump. Maintain the reaction temperature in the range of 8°C to 12°C throughout the process. Step 4: Cool the reaction mixture to 0°C, add 800 mL of paraffin oil again, stir at 800 rpm to form an emulsion, then add 0.21 g (0.0015 mol) benzoyl chloride and 0.24 g (0.0015 mol) p-toluene chloride alternately every 10 min for a total of 20 min to complete the alternating end-sealing, then add 0.61 g (0.003 mol) trimesoyl chloride all at once, and react at 0°C for 15 min to carry out branching; Step 5: Stop stirring again and separate the layers. Remove the upper layer of paraffin oil and add 100g of pre-mixed 1-butyl-3-methylimidazolium chloride and 100mL of N,N-dimethylacetamide to the lower layer. Heat to 10°C and stir for 30min to homogenize the chain segment distribution and adjust the branch point distribution. Step 6: Add 20 mL of N-methylpyrrolidone solution of aniline (0.1% w / v) to the reaction system to terminate the residual acyl chloride groups. After stirring for 10 min, quickly pour the entire reaction mixture into 5000 mL of methanol / deionized water mixture (volume ratio 1:1). A large amount of white precipitate is immediately generated. Filter and collect the precipitate. Wash it three times with methanol, then three times with deionized water. Finally, dry it in a vacuum oven at 60°C to constant weight to obtain crude poly(p-phenylene terephthalamide).

[0033] Comparative Example 3: The difference between Comparative Example 3 and Example 2 is that Comparative Example 3 only uses benzoyl chloride for one-time end sealing (the total molar amount is equivalent to that of Example 2), without adding p-toluenechlorochloride; Step 1: In a 2000mL four-necked flask equipped with a mechanical stirrer, a digital temperature controller (accuracy ±1°C), a nitrogen protection tube, and an HCl absorption bottle containing 500mL of aniline-toluene solution (mass fraction 10%), add 1500mL of paraffin oil (kinematic viscosity 45.2cSt, dried at 120°C for 2h). Cool the system to -10°C in a dry ice-ethanol bath and start stirring at 800rpm. Purge with nitrogen gas dried through molecular sieves at 50mL / min. Add the pre-prepared N-methylpyrrolidone / calcium chloride solution (800mL N-methylpyrrolidone dissolved in 60g anhydrous calcium chloride, stirred at 60°C for 1h until completely clear, filtered through a 0.45μm polytetrafluoroethylene membrane and cooled to -10°C) dropwise at a rate of 18mL / min using a constant flow pump to form a milky white suspension with uniform particle size. After the dropwise addition is complete, add 108.1g (1mol) of p-phenylenediamine. Step 2: Control the temperature of the reaction system between -5°C and 0°C. Add 101.5g (0.5mol) terephthaloyl chloride in batches over 30min, and add 0.34g (0.002mol) p-aminobenzoyl chloride at the same time. During the addition process, maintain the nitrogen flow rate at 70mL / min and introduce the tail gas into the absorption bottle through the tube to capture HCl. After reacting for 30min, the system will be a light yellow viscous substance. Step 3: After the reaction is complete, turn off the stirrer and let it stand for 20 minutes under gravity until the layers are clear. Use a separatory funnel with a PTFE piston to separate the upper paraffin oil from the lower N-methylpyrrolidone phase. After ensuring that the paraffin oil is completely removed, add 350g of 1-butyl-3-methylimidazolium chloride and 350mL of N,N-dimethylacetamide, which have been mixed evenly at room temperature, to the N-methylpyrrolidone phase containing the prepolymer. Heat the system to 10°C using a constant temperature water bath and stir at 300rpm for 30 minutes until a completely transparent deep yellow homogeneous solution is formed. Then, add 101.5g (0.5mol) terephthaloyl chloride dropwise evenly over 60 minutes using a peristaltic pump. Maintain the reaction temperature in the range of 8°C to 12°C throughout the process. Step 4: Cool the reaction mixture to 0°C, add 800 mL of paraffin oil again, stir at 800 rpm to form an emulsion, add 0.42 g (about 0.003 mol) of benzoyl chloride at once, without adding p-toluene chloride, complete the alternating end-capping in 20 min, then add 0.61 g (0.003 mol) of trimesoyl chloride at once, and react at 0°C for 15 min to carry out branching; Step 5: Stop stirring again and separate the layers. Remove the upper layer of paraffin oil and add 100g of pre-mixed 1-butyl-3-methylimidazolium chloride and 100mL of N,N-dimethylacetamide to the lower layer. Heat to 10°C and stir for 30min to homogenize the chain segment distribution and adjust the branch point distribution. Step 6: Add 20 mL of N-methylpyrrolidone solution of aniline (0.1% w / v) to the reaction system to terminate the residual acyl chloride groups. After stirring for 10 min, quickly pour the entire reaction mixture into 5000 mL of methanol / deionized water mixture (volume ratio 1:1). A large amount of white precipitate is immediately generated. Filter and collect the precipitate. Wash it three times with methanol, then three times with deionized water. Finally, dry it in a vacuum oven at 60°C to constant weight to obtain crude poly(p-phenylene terephthalamide).

[0034] Comparative Example 4: The difference between Comparative Example 4 and Example 2 is that: pyromellitic chloroprene chloride was added to the homogeneous ionic liquid phase for branching (in the same molar amount), and the reaction did not occur at the interface of the non-aqueous suspension phase; Step 1: In a 2000mL four-necked flask equipped with a mechanical stirrer, a digital temperature controller (accuracy ±1°C), a nitrogen protection tube, and an HCl absorption bottle containing 500mL of aniline-toluene solution (mass fraction 10%), add 1500mL of paraffin oil (kinematic viscosity 45.2cSt, dried at 120°C for 2h). Cool the system to -10°C in a dry ice-ethanol bath and start stirring at 800rpm. Purge with nitrogen gas dried through molecular sieves at 50mL / min. Add the pre-prepared N-methylpyrrolidone / calcium chloride solution (800mL N-methylpyrrolidone dissolved in 60g anhydrous calcium chloride, stirred at 60°C for 1h until completely clear, filtered through a 0.45μm polytetrafluoroethylene membrane and cooled to -10°C) dropwise at a rate of 18mL / min using a constant flow pump to form a milky white suspension with uniform particle size. After the dropwise addition is complete, add 108.1g (1mol) of p-phenylenediamine. Step 2: Control the temperature of the reaction system between -5°C and 0°C. Add 101.5g (0.5mol) terephthaloyl chloride in batches over 30min, and add 0.34g (0.002mol) p-aminobenzoyl chloride at the same time. During the addition process, maintain the nitrogen flow rate at 70mL / min and introduce the tail gas into the absorption bottle through the tube to capture HCl. After reacting for 30min, the system will be a light yellow viscous substance. Step 3: After the reaction is complete, turn off the stirrer and let it stand for 20 minutes under gravity until the layers are clear. Use a separatory funnel with a PTFE piston to separate the upper paraffin oil from the lower N-methylpyrrolidone phase. After ensuring that the paraffin oil is completely removed, add 350g of 1-butyl-3-methylimidazolium chloride and 350mL of N,N-dimethylacetamide, which have been mixed evenly at room temperature, to the N-methylpyrrolidone phase containing the prepolymer. Heat the system to 10°C using a constant temperature water bath and stir at 300rpm for 30 minutes until a completely transparent deep yellow homogeneous solution is formed. Then, add 101.5g (0.5mol) terephthaloyl chloride dropwise evenly over 60 minutes using a peristaltic pump. Maintain the reaction temperature in the range of 8°C to 12°C throughout the process. Step 4: Cool the reaction mixture to 0°C, add 800 mL of paraffin oil again, stir at 800 rpm to form an emulsion, and then add 0.21 g (0.0015 mol) benzoyl chloride and 0.24 g (0.0015 mol) p-toluene chloride alternately every 10 min for a total of 20 min to complete the alternating end sealing; Step 5: Stop stirring again and separate the layers. Remove the upper layer of paraffin oil and add 100g of pre-mixed 1-butyl-3-methylimidazolium chloride and 100mL of N,N-dimethylacetamide to the lower layer. Heat to 10°C and stir for 30min. Then add 0.61g (0.003mol) of trimesoyl chloride all at once and stir for 15min to complete the branching. Step 6: Add 20 mL of N-methylpyrrolidone solution of aniline (0.1% w / v) to the reaction system to terminate the residual acyl chloride groups. After stirring for 10 min, quickly pour the entire reaction mixture into 5000 mL of methanol / deionized water mixture (volume ratio 1:1). A large amount of white precipitate is immediately generated. Filter and collect the precipitate. Wash it three times with methanol, then three times with deionized water. Finally, dry it in a vacuum oven at 60°C to constant weight to obtain crude poly(p-phenylene terephthalamide).

[0035] Comparative Example 5: The difference between Comparative Example 5 and Example 2 is that the first stage is carried out in a homogeneous N-methylpyrrolidone / calcium chloride phase, the non-aqueous suspension of paraffin is eliminated, weak nitrogen is used for purging, and the tail gas is still absorbed. Step 1: In a 2000mL four-necked flask equipped with a mechanical stirrer, a digital temperature controller (accuracy ±1°C), a nitrogen protection tube, and an HCl absorption bottle containing 500mL of aniline-toluene solution (mass fraction 10%), add 1500mL of paraffin oil (kinematic viscosity 45.2cSt, dried at 120°C for 2h). Cool the system to -10°C in a dry ice-ethanol bath and start stirring at 800rpm. Purge with nitrogen gas dried through molecular sieves at 10mL / min. Add the pre-prepared N-methylpyrrolidone / calcium chloride solution (800mL N-methylpyrrolidone dissolved in 60g anhydrous calcium chloride, stirred at 60°C for 1h until completely clear, filtered through a 0.45μm polytetrafluoroethylene membrane and cooled to -10°C) dropwise at a rate of 18mL / min using a constant flow pump to form a milky white suspension with uniform particle size. After the dropwise addition is complete, add 108.1g (1mol) of p-phenylenediamine. Step 2: Control the temperature of the reaction system between -5°C and 0°C. Add 101.5g (0.5mol) terephthaloyl chloride in batches over 30min, and add 0.34g (0.002mol) p-aminobenzoyl chloride at the same time. During the addition process, maintain the nitrogen flow rate at 15mL / min and introduce the tail gas into the absorption bottle through the tube to capture HCl. After reacting for 30min, the system will be a light yellow viscous state. Step 3: After the reaction is complete, turn off the stirrer and let it stand for 20 minutes under gravity until the layers are clear. Use a separatory funnel with a PTFE piston to separate the upper paraffin oil from the lower N-methylpyrrolidone phase. After ensuring that the paraffin oil is completely removed, add 350g of 1-butyl-3-methylimidazolium chloride and 350mL of N,N-dimethylacetamide, which have been mixed evenly at room temperature, to the N-methylpyrrolidone phase containing the prepolymer. Heat the system to 10°C using a constant temperature water bath and stir at 300rpm for 30 minutes until a completely transparent deep yellow homogeneous solution is formed. Then, add 101.5g (0.5mol) terephthaloyl chloride dropwise evenly over 60 minutes using a peristaltic pump. Maintain the reaction temperature in the range of 8°C to 12°C throughout the process. Step 4: Cool the reaction mixture to 0°C, add 800 mL of paraffin oil again, stir at 800 rpm to form an emulsion, then add 0.21 g (0.0015 mol) benzoyl chloride and 0.24 g (0.0015 mol) p-toluene chloride alternately every 10 min for a total of 20 min to complete the alternating end-sealing, then add 0.61 g (0.003 mol) trimesoyl chloride all at once, and react at 0°C for 15 min to carry out branching; Step 5: Stop stirring again and separate the layers. Remove the upper layer of paraffin oil and add 100g of pre-mixed 1-butyl-3-methylimidazolium chloride and 100mL of N,N-dimethylacetamide to the lower layer. Heat to 10°C and stir for 30min to homogenize the chain segment distribution and adjust the branch point distribution. Step 6: Add 20 mL of N-methylpyrrolidone solution of aniline (0.1% w / v) to the reaction system to terminate the residual acyl chloride groups. After stirring for 10 min, quickly pour the entire reaction mixture into 5000 mL of methanol / deionized water mixture (volume ratio 1:1). A large amount of white precipitate is immediately generated. Filter and collect the precipitate. Wash it three times with methanol, then three times with deionized water. Finally, dry it in a vacuum oven at 60°C to constant weight to obtain crude poly(p-phenylene terephthalamide).

[0036] Performance testing: End group content determination: First, weigh 0.5 g of crude poly(p-phenylene terephthalamide) sample that has been thoroughly dried and place it in a 250 mL Erlenmeyer flask. Add 50 mL of a pre-prepared mixed solvent of 1-butyl-3-methylimidazolium chloride and N,N-dimethylacetamide, wherein the mass fraction of the ionic liquid is 50%. Heat and stir in an 80 °C water bath for 60 min until the sample is completely dissolved, forming a clear, deep yellow solution. After the solution cooled to room temperature, 2 mL of a pre-standardized trifluoroacetic anhydride solution (0.1 mol / L, prepared with tetrahydrofuran as solvent) was added. The reaction was allowed to proceed for 15 min to completely acylate all free amino groups. Subsequently, 10 mL of deionized water was added to hydrolyze the excess trifluoroacetic anhydride. After vigorous stirring for 5 min, the mixture was allowed to stand and separate into layers. The upper organic phase was then subjected to non-aqueous titration using a perchloric acid-acetic acid system with crystal violet as an indicator. Titration was performed with 0.05 mol / L standard perchloric acid solution until the solution changed from purple to blue-green. A blank control experiment was prepared simultaneously, following the same procedure but without adding the polymer sample. The end group content was calculated using the formula: Amine end group content (mmol / k) = [(V0-V1)×C×1000] / m, where V0 is the blank titration volume, V1 is the sample titration volume, C is the concentration of the perchloric acid standard solution, and m is the sample mass. The results are shown in Table 1.

[0037] Methods for determining the dispersion coefficient and molecular weight distribution: 20 mg of dried polymer sample was weighed and added to 2 mL of a mixed mobile phase of 1-butyl-3-methylimidazolium chloride and N,N-dimethylacetamide filtered through a 0.22 μm polytetrafluoroethylene membrane. The mobile phase contained 30% ionic liquid. The sample was heated at 100 °C for 2 h and sonicated for 30 min to ensure complete dissolution. After complete dissolution, the sample was allowed to cool naturally to room temperature. The testing equipment used was a gel permeation chromatograph equipped with a multi-angle laser light scattering detector. The column temperature was controlled at 80 °C, the flow rate was 0.8 mL / min, and the injection volume was 100 μL. Chromatographic separation was performed using a dedicated ionic liquid-resistant organic gel column. The weight-average molecular weight (Mw) and number-average molecular weight (Mn) were obtained by absolute molecular weight determination. The dispersion coefficient (Mw / Mn) was calculated as Mw / Mn. The results are shown in Table 1.

[0038] Evaluation method for the filtration performance of spinning solution: First, a 20% (w / w) spinning solution was prepared using 99.5% (w / w) concentrated sulfuric acid as the solvent. The polymer sample was slowly added in an ice-water bath to avoid localized overheating. Then, the solution was mechanically stirred at room temperature for 48 hours until completely dissolved. During this period, the dissolution status was checked periodically, and stirring time was adjusted as needed until no visible particles remained. The filtration test used a standard filtration device with a 5μm sintered stainless steel filter as the filter medium and a 10cm² filtration area. 2The filtration pressure was 0.2 MPa, and the temperature was controlled at 25℃. During the test, 100 mL of spinning solution was filtered, and the filtration time for every 10 mL of filtrate was recorded. A curve showing the relationship between the cumulative filtrate volume and the filtration time was plotted. The filtration index was defined as the ratio of the time required to pass 50 mL of filtrate to the time required to pass the first 10 mL of filtrate. This index reflects the degree of filter clogging during the filtration process. The results are shown in Table 1.

[0039] Table 1 Performance Test Results

[0040] Data Analysis: Data analysis of Examples 1-3: The overall trend of the three examples shows that end-group indicators are more stable, molecular weight distribution exhibits continuous convergence, and filtration performance is simultaneously optimized, indicating that the synergistic tuning of process parameters has a multi-layered amplification effect. Based on the process route, it is speculated that the appropriate amount of AB-type end-group balancing agent in the first stage provides a buffer, weakening the amplification effect of initial errors on the end-groups; the phase switching between non-aqueous suspension and ionic liquid enhances the re-equilibrium of end-groups under different diffusion and polarity environments; ionic liquid homogenization and slow feeding in the secondary polycondensation may reduce the instantaneous fluctuations in chain growth rate; the alternating end-sealing using a combination of polarity and steric hindrance mismatch is beneficial for selective end-sealing and "misalignment compensation" in different phase states, suppressing high molecular weight tailing; and the mild branching of the interface confinement controls molecular topology while also considering the spinnability window of the solution rheology. The overall results show a more balanced chain segment structure, cleaner solution, and better processing stability, while the trade-off between strength and rheology is balanced by the branching intensity and homogenization time.

[0041] Data analysis of Example 2 and Comparative Example 1: The differences in end-group control, distribution convergence, and filtering performance between the two examples reveal that the randomness of chain growth is more difficult to correct retrospectively when using a single homogeneous system. In contrast, Example 2, through the back-and-forth switching between "non-aqueous suspension and ionic liquid," may trigger rebalancing of end groups at phase transition nodes, allowing chain length fluctuations to be reset in stages. In the non-aqueous suspension environment, interfacial diffusion is limited, facilitating the formation of a more uniform microenvironment within microscale droplets. Simultaneously, the migration of byproduct acids between the hydrophobic continuous phase and the gas phase is smoother, reducing the long-term occupation of amine ends by salting. The homogeneous dissolution and slow feeding in the ionic liquid stage smooth the polymerization rate, and, combined with phase switching, create a "beat-like" distribution shaping effect.

[0042] Data analysis of Example 2 and Comparative Example 2: The data shows that without the introduction of the AB-type end-group balancing agent, the end-group indicators are more prone to drift and are more sensitive to feed addition and local acidity. The end-group indicators in Example 2 are more concentrated, accompanied by significant convergence in distribution and improved filtration, suggesting that the early introduction of AB monomers with complementary end groups provides a stoichiometric buffer. This balancing agent can react rapidly with the instantaneously excessive functional groups during the first polycondensation stage, weakening the accumulation of local deviations, and in subsequent phase switching, it works with "phase memory" to promote the return of end groups to the stable range. As a result, the termination and regeneration of chain growth are constrained within a narrower range, and the tailing of the distribution tends to converge. At the same time, due to the reduced fluctuation of free end groups before end-capping, the selectivity of subsequent alternating end-capping is more fully reflected, resulting in slower clogging growth in the filtration process.

[0043] Data analysis of Example 2 and Comparative Example 3: When only a single end-capping is used, the data exhibits a combination of apparent molecular weight increase and widening distribution, and unfavorable filtration characteristics. This may be due to the kinetic lag of the end-capping reaction and the lack of selectivity caused by polarity mismatch, resulting in the elongation of a small number of chain segments and an increase in overall heterogeneity. The example implements alternating end-capping at phase switching nodes, utilizing the reaction preferences of polar-loving small molecules and sterically hindered hydrophobic molecules in different phase environments to construct a "selectivity-compensation" mechanism: the previous round preferentially capsizes the more accessible end groups, and the subsequent round compensates for the shortcomings of the previous round, thereby weakening the high molecular weight tails and uncapped end residues.

[0044] Data analysis of Example 2 and Comparative Example 4: When branching is carried out in a homogeneous phase, the data exhibits a significant increase in apparent molecular weight, a significant broadening of the distribution, and unfavorable filtration characteristics. In the examples, branching is confined to a non-aqueous suspension phase interface. The spatial congestion and mass transfer resistance at the interface act as a "soft constraint" on the number and location of branching points, causing the number of branch points that each chain can withstand to converge statistically. Simultaneously, the interface eliminates the opportunity for excess free acyl chlorides to aggregate in the same chain segment, reducing the nucleation probability of microgels. The subsequent homogenization in the ionic liquid stage facilitates the redistribution of branch points within local chain segments, further smoothing the rheological spectrum.

[0045] Data analysis of Example 2 and Comparative Example 5: Under conditions of restricted byproduct acid migration, the data exhibited a combination of reduced titratable end-group concentration, inhibited chain growth, and unfavorable filtration characteristics, pointing to the possibility that the amine end is temporarily occupied by acidic species, inducing early cessation of chain growth and aggregation. The example improved the migration flux of acid from the particulate phase to the continuous phase and then to tail gas absorption by incorporating appropriate gas-phase displacement in the non-aqueous suspension phase, reducing the long-term occupation of end-groups by salination. The dissolution-dispersion cycle brought about by phase switching may also promote the release of temporarily occupied end-groups in the homogeneous phase and their participation in subsequent reactions, kinetically mitigating early adverse events.

[0046] Those skilled in the art should understand that the discussion of any of the above embodiments is merely exemplary and is not intended to imply that the scope of the invention is limited to these examples; within the framework of the invention, the technical features of the above embodiments or different embodiments can also be combined, the steps can be implemented in any order, and there are many other variations of the different aspects of the invention as described above, which are not provided in detail for the sake of brevity.

Claims

1. A method for preparing crude poly(p-phenylene terephthalamide), characterized in that, Includes the following steps: (1) N-methylpyrrolidone / calcium chloride solution was added dropwise to paraffin oil at a constant flow rate, with the temperature controlled at −15℃ to 0℃, the stirring speed at 600–1000 rpm, and the nitrogen flow rate at 40–60 mL / min, to form a milky white suspension with uniform particle size, and then p-phenylenediamine was added. (2) Add terephthaloyl chloride in batches at -5℃ to 0℃, and simultaneously add AB type end-group balancing agent to allow the reaction to proceed until the viscosity of the system increases; (3) After standing and separating to remove paraffin oil, add an ionic liquid / polar cosolvent mixture to the lower phase to obtain a homogeneous solution. Add the second part of terephthaloyl chloride at a constant rate at 8°C to 12°C to continue the polycondensation. (4) Cool the homogeneous system and reintroduce paraffin oil to form an emulsion. Add the first end-sealing agent and the second end-sealing agent alternately at time intervals to complete the end-sealing. (5) Add a trifunctional acyl chloride branching agent at 0℃ to 5℃ to carry out a restricted branching reaction at the interface of a non-aqueous suspension phase; (6) After removing paraffin oil in layers, add ionic liquid / polar cosolvent to the lower layer to homogenize the chain segment distribution; (7) Add an amine terminator, then pour the reaction solution into an alcohol / water mixture to precipitate the product. After washing with alternating alcohol and water, dry to obtain poly(p-phenylene terephthalamide).

2. The method for preparing crude poly(p-phenylene terephthalamide) according to claim 1, characterized in that, Preferably, the N-methylpyrrolidone / calcium chloride solution in step (1) is a solution containing 6wt%–9wt% calcium chloride and is used after being filtered through a 0.45μm polytetrafluoroethylene membrane.

3. The method for preparing crude poly(p-phenylene terephthalamide) according to claim 1, characterized in that, The ionic liquid in step (3) is 1-butyl-3-methylimidazolium chloride, the polar co-solvent is N,N-dimethylacetamide, the mass-to-volume ratio of the ionic liquid to the polar co-solvent is 0.8g:1mL–1.2g:1mL, and the homogenization stirring time is 20-45min.

4. The method for preparing crude poly(p-phenylene terephthalamide) according to claim 1, characterized in that, The alternating end-capping step (4) uses a pair of end-capping agents composed of benzoyl chloride and p-toluene chloride, which are added alternately at an interval of 5-10 minutes, and the amount of each end-capping agent added is 0.001-0.003 times the amount of p-phenylenediamine.

5. The method for preparing crude poly(p-phenylene terephthalamide) according to claim 1, characterized in that, The branching agent in step (5) is pyromellitic acid chloride, and the amount added is 0.0025-0.0030 times the amount of p-phenylenediamine. The branching reaction time is 10-15 min.

6. The method for preparing crude poly(p-phenylene terephthalamide) according to claim 1, characterized in that, In step (6), the amounts of ionic liquid / polar cosolvent added are 80-120g of ionic liquid and 80-120mL of polar cosolvent, respectively, and the homogenization time is 25-40min.

7. The method for preparing crude poly(p-phenylene terephthalamide) according to claim 1, characterized in that, The settling liquid in step (7) is a methanol / deionized water mixture with a volume ratio of 1:1, and the washing sequence is alternating between alcohol washing and water washing.

8. The method for preparing crude poly(p-phenylene terephthalamide) according to claim 1, characterized in that, The terminating agent in step (7) is a dilute solution of N-methylpyrrolidone of aniline, and the stirring time after termination is 5-15 min.

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