Chlorinated polyamide aqueous salt diffusion coefficient calculation method and system based on molecular simulation

Abstract

The application belongs to the field of membrane separation technology, and relates to a chlorinated polyamide water salt diffusion coefficient calculation method based on molecular simulation, which comprises the following steps: establishing a molecular model of m-phenylenediamine monomers and isophthaloyl chloride monomers and cross-linking into a polyamide chain, folding the optimized chain model into an amorphous cell structure in a cubic lattice; designing a secondary cross-linking scheme of m-phenylenediamine monomers and the polyamide chain, performing cross-linking reaction, and forming a stable cross-linked polyamide membrane model; designing a chlorination scheme, performing chlorination treatment on the cross-linked polyamide model, and forming a chlorinated polyamide model; using software to simulate the water salt diffusion characteristics of the cross-linked polyamide model and the chlorinated polyamide model under normal temperature conditions, and finally calculating the water salt diffusion coefficient through the mean square displacement curve. The application effectively solves the problem that a stable diffusion coefficient cannot be obtained in the polyamide material water salt separation experiment process, and reduces the test cost.

Description

Technical Field

[0001] This invention belongs to the field of membrane separation technology and relates to a method and system for calculating the water-salt diffusion coefficient of chlorinated polyamide based on molecular simulation. Background Technology

[0002] The reaction of m-phenylenediamine with trimesoyl chloride forms a polyamide material with a three-dimensional cross-linked network. Polyamides possess excellent properties such as high selectivity, convenient preparation, and stable chemical properties, and are therefore widely used in membrane separation, especially in seawater desalination and wastewater treatment, where they are currently the most commonly used separation layer materials for nanofiltration and reverse osmosis membranes. However, with the development of membrane separation technology, polyamide membranes have encountered a "trade-off" bottleneck in flux and retention performance, and industrial treatment environments are placing increasingly stringent requirements on the chlorine resistance of polyamide materials.

[0003] The water-salt diffusion coefficient characterizes the permeation and retention performance of membrane materials. A higher diffusion coefficient for water and salt particles within the membrane material indicates better permeation performance and a higher treatment flux. Conversely, a lower diffusion coefficient indicates a higher retention rate for that substance. In practical water treatment applications, separation membranes are often exposed to active chlorine added to inhibit biofouling. Chlorination alters the structure and properties of polyamides, affecting the diffusion of water and salt particles within the membrane, which is further reflected in the membrane's treatment capacity and separation performance. Therefore, studying the changes in the water-salt diffusion coefficient within chlorinated polyamides is particularly important.

[0004] Traditional water-salt diffusion tests are often affected by factors such as preparation process, test materials, and test conditions, making it difficult to measure the results. Furthermore, traditional water-salt diffusion tests consume a lot of materials and take a long time, which reduces the membrane's service life and consumes significant electrical and material resources. In addition, the requirements for membrane module fabrication are very stringent, and it is impossible to maintain the temperature of the test material at the same level as the ambient temperature, since the diffusion coefficient is affected by temperature. Therefore, the water-salt diffusion coefficient within the membrane in a real-world environment is difficult to measure and cannot directly reflect the flux and retention performance of the tested membrane material.

[0005] Therefore, based on existing technology, it has been found that relying solely on traditional water-salt diffusion tests to study the separation characteristics of polyamides and chlorinated polyamides cannot yield stable and reliable diffusion coefficient measurement results, and will also consume a lot of time, pharmaceuticals, and electricity resources. Summary of the Invention

[0006] The purpose of this invention is to provide a method and system for calculating the water-salt diffusion coefficient of chlorinated polyamide based on molecular simulation, so as to overcome the problems existing in the above-mentioned background art.

[0007] The technical problem solved by this invention is achieved through the following technical solution:

[0008] The method for calculating the water-salt diffusion coefficient of chlorinated polyamide based on molecular simulation is characterized by the following steps:

[0009] S1. In Material Studio software, construct molecular models of m-phenylenediamine monomer MPD and trimesoyl chloride TMC, and perform preprocessing.

[0010] S2. Polymerize m-phenylenediamine monomer MPD and trimesoyl chloride TMC, and bond the reaction sites of the two monomer molecules to obtain a polyamide chain with a predetermined degree of polymerization.

[0011] S3. Mix several polyamide chains obtained in S2 with m-phenylenediamine monomer MPD and add them into the unit cell to obtain an initial model of polyamide with low crosslinking degree.

[0012] S4. Under the COMPASS II force field, the initial polyamide model obtained in S3 is subjected to structural optimization and energy minimization. At the same time, the reaction site spacing of the m-phenylenediamine monomer MPD and the polyamide chain is measured, and further crosslinking is performed to obtain a crosslinked polyamide model that meets the preset crosslinking degree and actual density.

[0013] S5. Perform NVT dynamic calculations on the crosslinked polyamide model obtained in S4 based on the reactive force field to eliminate the residual stress in the crosslinked polyamide model;

[0014] S6. Based on the cross-linked polyamide model obtained in S5 to eliminate residual stress, chlorination treatment is performed to obtain a chlorinated polyamide model, and the chlorinated polyamide model is subjected to structural optimization and energy minimization treatment under the COMPASS II force field.

[0015] S7. Based on the cross-linked polyamide model and chlorinated polyamide model obtained in S4 and S6, water molecules and sodium chloride ions are filled into the unit cell, and NVT dynamic calculations are performed on the cross-linked polyamide model and chlorinated polyamide model filled with solution under COMPASS II force field to simulate the water-salt diffusion process and generate a water-salt diffusion trajectory file.

[0016] S8. Based on the water-salt diffusion trajectory file generated by the NVT kinetic calculation in S7, track the H2O and salt ion substances in the water-salt diffusion process, use analysis tools to calculate and plot the mean square displacement curve, and use the diffusion coefficient formula to calculate the water-salt diffusion coefficient of the chlorinated polyamide model.

[0017] Preferably, in step S1, the molecular models of m-phenylenediamine monomer MPD and trimesoyl chloride monomer TMC are constructed, and the construction method includes the following steps:

[0018] S1.1 Use the SketchAtom and SketchRing tools in the software toolbar to draw the monomer molecular structure, and use the Clean tool to correct the drawn structure;

[0019] S1.2 labels the N atom at the amino terminus of m-phenylenediamine (MPD) as R1, and labels the C atom of the acyl chloride group of trimesoyl chloride (TMC) as R2. R1 and R2 are the reaction sites of the two monomers.

[0020] Preferably, in step S2, the method for bonding the reaction sites of the two monomer molecules to obtain a polyamide chain with a predetermined degree of polymerization is as follows:

[0021] The H atom on the amino group of the m-phenylenediamine monomer MPD and the Cl atom on the acyl chloride group in the polyamide chain were removed, and then the amino N was connected to the acyl chloride C.

[0022] Preferably, in step S4, the initial model undergoes structural optimization and energy minimization, while simultaneously measuring the MPD of the m-phenylenediamine monomer and the reaction site spacing of the polymer chain, followed by further crosslinking to obtain a crosslinked polyamide model that conforms to the preset crosslinking degree and actual density. This includes the following steps:

[0023] S4.1 Set the pressure to 0.0001 GPa, perform NPT kinetic calculations on the initial polyamide model obtained in S3, and compress the volume of the initial polyamide model until it meets the actual density requirements; after the compression process is completed, a cross-linked polyamide model that meets the actual density requirements is obtained.

[0024] S4.2 Anneal the cross-linked polyamide model obtained in S4.1 that meets the actual density requirements at 298K to 500K to cause the molecular chains to fold and shrink; after the annealing process, a cross-linked polyamide model with folded and shrunken molecular chains is obtained.

[0025] S4.3 The distance between the H and Cl reaction sites of the m-phenylenediamine monomer MPD and the polymer chain was measured, and was less than [a certain value]. Remove the H atom from the amino group of the m-phenylenediamine monomer MPD and the Cl atom from the acyl chloride group in the polyamide chain. Then connect the amino N to the acyl chloride C. Repeat steps S4.1, S4.2, and S4.3 until the crosslinked polyamide model meets the preset crosslinking degree and actual density. After crosslinking is completed, remove the excess m-phenylenediamine monomer and replace the remaining acyl chloride group with a carboxyl group.

[0026] Preferably, in step S5, NVT kinetic calculations are performed on the crosslinked polyamide model based on the reactive force field to eliminate residual stress in the crosslinked polyamide model. This process includes:

[0027] The cross-linked polyamide model with molecular chain folding and shrinkage obtained in S4 was subjected to NVT kinetic calculations for 1 ns, and the density and energy fluctuations during the calculation process were checked. When the density and energy fluctuations were less than 10% of the mean, the cross-linked polyamide model entered a stable state and the residual stress was eliminated.

[0028] Preferably, in step S6, the cross-linked polyamide model obtained in step S5 with residual stress eliminated is subjected to chlorination to obtain a chlorinated polyamide model, and the chlorinated film model is subjected to multiple structural optimizations and energy minimizations under the COMPASS II force field, including the following steps:

[0029] S6.1 Based on the number of Cl atoms to be introduced, randomly select the H of the free-terminal amino group or the H of the amide bond on the polyamide chain and modify it to Cl;

[0030] S6.2 Similarly, randomly select several amide bonds or acyl chloride bonds, break the connecting bond between C and N, add a hydroxyl group to the carbonyl end to make it a carboxyl group, and add H to the free end containing N.

[0031] Preferably, in step S7, based on the cross-linked polyamide model and the chlorinated polyamide model obtained in steps S4 and S6, a number of water molecules and sodium chloride ions are filled into the unit cell, and NVT kinetic calculations are performed on the cross-linked polyamide model and the chlorinated polyamide model filled with solution under the COMPASS II force field to simulate the water-salt diffusion process, including the following steps:

[0032] S7.1 Create a Connolly surface in the cross-linked polyamide model and chlorinated polyamide model obtained in S4 and S6, thereby obtaining a free volume, and fill the free volume of the cross-linked polyamide model and chlorinated polyamide model with a certain number of water molecules and salt ions of a predetermined concentration or mass fraction;

[0033] S7.2 Based on the solution-immersed membrane crosslinked polyamide model and chlorinated polyamide model obtained in S7.1, NVT kinetic calculations are performed on the solution-filled crosslinked polyamide model and chlorinated polyamide model under the COMPASS II force field for 1 ns, simulating the water-salt diffusion process while optimizing the structure and energy.

[0034] Preferably, based on the water-salt diffusion trajectory file generated by the NVT kinetic calculation in S7, the H2O and salt ion substances in the water-salt diffusion process are tracked, the mean square displacement curve is calculated and plotted using analysis tools, and the water-salt diffusion coefficient is calculated using the diffusion coefficient formula, the specific form of which is:

[0035] S8.1 Selects the water molecules to be tracked by charge and sets them as a set; similarly, salt ions are set as another set.

[0036] S8.2 In the water-salt diffusion trajectory file obtained in S7, use the analysis tool to calculate the mean square displacement of the tracked set and output tabular data and graphs;

[0037] S8.3 Based on the calculation formula, the diffusion coefficient of the tracking water salt particles is calculated using the slope of the mean square displacement curve:

[0038]

[0039] Where r i (t) is the termination position of particle i at time t; r i (0) represents the initial position of particle i; <> indicates calculating the average value; when the mean square displacement-time curve is linear over a long range, the diffusion coefficient of the target particle can be calculated using Einstein's equation based on the slope of the mean square displacement curve. The calculation formula can be simplified to the following form:

[0040]

[0041] Where k is the slope of the mean square displacement curve.

[0042] On the other hand, the water-salt diffusion coefficient calculation system based on molecular simulation of chlorinated polyamide includes: a molecular model building module, which builds molecular models of m-phenylenediamine monomer MPD and trimesoyl chloride monomer TMC in Materials Studio software and performs preprocessing; polymerizes an appropriate amount of m-phenylenediamine monomer MPD and trimesoyl chloride TMC, and bonds the reaction sites of the two monomer molecules to obtain a polyamide chain with a preset degree of polymerization;

[0043] The lattice building module mixes several polyamide chains obtained from S2 and m-phenylenediamine monomer MPD and adds them into the unit cell to obtain an initial model of polyamide with low crosslinking degree.

[0044] The first molecular model construction module performs multiple structural optimizations and energy minimization processes on the initial polyamide model under the COMPASS II force field. At the same time, it measures the reaction site spacing of the m-phenylenediamine monomer MPD and the polyamide chain, and further crosslinks it to obtain a crosslinked polyamide model that meets the preset crosslinking degree and actual density. Based on the reaction force field, the obtained crosslinked polyamide model for water-salt diffusion simulation is subjected to NVT kinetic calculations to eliminate the residual stress of the crosslinked polyamide model.

[0045] The second molecular model construction module performs chlorination on the cross-linked polyamide model obtained from the first molecular model construction module to obtain a chlorinated polyamide model, and performs multiple structural optimizations and energy minimizations on the chlorinated polyamide model under the COMPASS II force field.

[0046] The third molecular model construction module, based on the cross-linked polyamide model and chlorinated polyamide model obtained from the first molecular model construction module and the second molecular model construction module, fills the unit cell with a number of water molecules and sodium chloride ions.

[0047] The water-salt diffusion process simulation module performs NVT kinetic calculations on the cross-linked polyamide model and chlorinated polyamide model obtained for water-salt diffusion simulation based on the reactive force field to simulate the water-salt diffusion process; and tracks H2O and salt ion substances in the water-salt diffusion process based on the trajectory file generated by the NVT kinetic calculation.

[0048] The calculation module uses analysis tools to calculate and plot the mean square displacement curve in the obtained trajectory file, and uses the diffusion coefficient formula to calculate the water-salt diffusion coefficient in each polyamide model.

[0049] The advantages and beneficial effects of this invention are as follows:

[0050] Compared with existing technologies, this invention provides a method for calculating the water-salt diffusion coefficient of chlorinated polyamide based on molecular simulation. This method can simulate and analyze the water-salt diffusion characteristics of cross-linked polyamide and chlorinated polyamide with lower time and material costs. It can be used for preliminary research on polyamide materials and further development of subsequent experiments, greatly improving the efficiency of experimental research on water-salt particle diffusion characteristics. This is of great significance for the study of the chlorination mechanism and chlorine resistance of polyamide materials. At the same time, the invention uses molecular dynamics simulation to solve the difficulty in obtaining stable and reliable diffusion coefficient measurement results in the study of the separation characteristics of cross-linked polyamide and chlorinated polyamide by water-salt diffusion experiments in existing technologies. Attached Figure Description

[0051] Figure 1 This is a reaction mechanism diagram of the intermediate phenylenediamine monomer and the pyromellitic trimethylol chloride monomer of the present invention;

[0052] Figure 2 This is the cross-linked polyamide model with a cross-linking degree of 76.5% in this invention;

[0053] Figure 3 This is the chlorinated polyamide model with a chlorination concentration of 4.3% in this invention;

[0054] Figure 4 This is the cross-linked polyamide model filled with water and salt in this invention;

[0055] Figure 5 This is the chlorinated polyamide model filled with water and salt in this invention;

[0056] Figure 6 This is a flowchart of the method of the present invention. Detailed Implementation

[0057] The present invention will be further described in detail below through specific embodiments. The following embodiments are merely descriptive and not limiting, and should not be used to limit the scope of protection of the present invention.

[0058] To achieve rapid analysis of the water-salt diffusion performance of polyamide materials and to explore its chlorination mechanism at the microscopic atomic level, this invention uses MS software to model polyamide materials. Under the COMPASS II force field, the Dynamics tool in the Forcite module of MS software is used to simulate the water-salt diffusion characteristics of cross-linked polyamide and chlorinated polyamide materials under solution immersion conditions. Finally, the diffusion coefficient of the water-salt diffusion process is calculated by the slope of the mean square displacement curve.

[0059] This invention discloses a method for calculating the water-salt diffusion coefficient of chlorinated polyamide based on molecular simulation:

[0060] S1. Using the SketchAtom and SketchRing tools in the Materials Studio software toolbar, we drew molecular models of the m-phenylenediamine monomer MPD and the pyromellitic tricarboxylic acid chloride monomer TMC. We then used the Clean tool to modify the drawn structures and used the Forcite module to perform geometric optimization on the two molecular models to obtain the lowest energy configuration. Finally, we labeled the N atom at the amino terminus of m-phenylenediamine MPD as R1 and the C atom of the acyl chloride group of pyromellitic tricarboxylic acid chloride TMC as R2.

[0061] S2, using the Repeat Unit tool in the Build Polymers module, the optimized m-phenylenediamine monomer MPD and pyromellitic tricarboxylic acid chloride monomer TMC molecular models are polymerized into a polyamide chain with a degree of polymerization of 10 in a 1:1 ratio according to the order of the R1 and R2 reaction sites connected end to end. In this example, 10 m-phenylenediamine monomer molecules and 10 pyromellitic tricarboxylic acid chloride monomer molecules are connected.

[0062] S3, using an Amorphous Cell module, several polymerized polyamide chains and several optimized m-phenylenediamine monomer molecules are placed into a cubic lattice and mixed uniformly. In this example, 20 polyamide chains and 100 m-phenylenediamine monomer molecules are added separately, with a concentration of 0.1 g / cm³. 3 The initial density was determined, and periodic boundary conditions were applied to the cube to eliminate boundary effects;

[0063] In molecular dynamics simulations, the reaction conditions are simplified by using a case where no accelerator is present.

[0064] S4. Under the COMPASS II force field, the Forcite module was used to perform geometry optimization on the initial cubic lattice model of the low-crosslinked polyamide to obtain its molecular structure model with the lowest energy. See also Figure 1 Based on the cyclic reaction mechanism of the amino group of m-phenylenediamine monomer and the acyl chloride group of trimesoyl chloride monomer, a distance measuring tool was used to measure the distance between the reaction sites of the two monomers while optimizing energy and geometry, with the preset bonding distance being less than or equal to 1. If the distance is less than the bonding distance, the cross-linking reaction is carried out first, followed by a second geometry optimization. The distance is measured and cross-linking is performed multiple times while compressing the lattice until the preset cross-linking degree is reached. In this example, 60 m-phenylenediamine monomers were cross-linked by distance measurement, and the preset cross-linking degree of the cross-linked polyamide model is 76.5%.

[0065] S4, with the pressure set to 0.0001 GPa, NPT kinetic calculations were performed on the initial polyamide model to compress its volume until it met the actual density requirements. The compression process was accompanied by annealing of the initial polyamide model at 298 K–500 K using the Anneal annealing tool. The purpose was to eliminate internal stress and allow the molecular chains to fold and shrink. After annealing, the geometry was adjusted using the Geometry Optimization tool.

[0066] During the cross-linking reaction, care should be taken to avoid excessive bonding of reacting atoms. After the cross-linking is completed, the excess m-phenylenediamine monomer is removed, and the remaining acyl chloride groups are replaced with carboxyl groups.

[0067] S5. NVT kinetic calculations were performed on the crosslinked polyamide model with molecular chain folding and contraction at 298 K for 1 ns to relax the molecular system and examine density and energy fluctuations during the calculation process. When the density and energy fluctuations were below 10% of the mean, the crosslinked polyamide model reached a stable state, yielding the crosslinked polyamide model for water-salt diffusion simulation. The results are shown in [link to results]. Figure 3 .

[0068] S6. Based on the required number of Cl atoms, randomly select the H atoms in the free-terminal amino groups or amide bonds of the polyamide chain and modify them to Cl. Then, similarly, randomly select several amide or acyl chloride bonds, break the bonds between C and N, adding a hydroxyl group to the carbonyl end to make it a carboxyl group, and adding a H group to the free N-containing end. Then, use Anneal and Geometry Optimization tools for geometric and energy optimization to obtain a model of the chlorinated polyamide with the target chlorination degree. The results are shown in [link to documentation]. Figure 4 .

[0069] S7. The free volume is calculated using the Connolly Surface tool in both the crosslinked polyamide and chlorinated polyamide models. A predetermined concentration or mass fraction of water molecules and salt ions is then added to the free volume of both models. In this example, each lattice is filled with 1000 water molecules and a minimum amount of 1 Na ion. + and 1 Cl - See results Figure 5 and 6 .

[0070] S7. Water-salt diffusion simulations were performed on cross-linked polyamide and chlorinated polyamide models immersed in solution under the COMPASS II force field. The time step was set to 1 fs, the duration was set to 1 ns, and the temperature and pressure were controlled by the Anderson and Berenden methods, respectively. When calculating nonbonded interactions, the van der Waals force was set to Atombased, and the Coulomb force was set to Eward.

[0071] S8, the trajectory files obtained from the water-salt diffusion simulation include cross-linked polyamide and chlorinated polyamide models at different simulation times. The Charge tool in the Atom Selection function is used to select water molecules and salt ions to be tracked based on their charge, and these are set into different sets in the Set for subsequent mean square displacement analysis. The Mean Square Displacement tool in the ForciteAnalysis module is used to calculate the mean square displacement of the selected sets, and tables and graphs are output frame by frame from the trajectory file.

[0072] S8, calculate the water-salt diffusion coefficient D for the cross-linked polyamide model and the chlorinated polyamide model using the following formula:

[0073]

[0074] Where r i (t) is the termination position of particle i at time t; r i (0) represents the initial position of particle i; <> indicates calculating the average value;

[0075] When the mean square displacement-time curve is linear over a long range, the diffusion coefficient of the target particle can be calculated using Einstein's equation based on the slope of the mean square displacement curve. The calculation formula can be simplified to the following form:

[0076]

[0077] Where k is the slope of the mean square displacement curve.

[0078] When performing statistical analysis of the diffusion coefficients of the cross-linked polyamide model and the chlorinated polyamide model, it is necessary to calculate each model at least three times and take their average value to reduce the error.

[0079] Another embodiment discloses a water-salt diffusion coefficient calculation system based on a molecular simulation model of chlorinated polyamide, comprising:

[0080] In the Materials Studio software, the molecular model building module is used to build molecular models of m-phenylenediamine monomer MPD and trimesoyl chloride monomer TMC, and pre-process them. Appropriate amounts of m-phenylenediamine monomer MPD and trimesoyl chloride TMC are polymerized, and the reaction sites R1 and R2 of the two monomers are bonded to obtain a polyamide chain with a preset degree of polymerization.

[0081] The lattice building module mixes several polyamide chains obtained from S2 and m-phenylenediamine monomer MPD and adds them into the unit cell to obtain an initial model of polyamide with low crosslinking degree.

[0082] The first molecular model construction module performs multiple structural optimizations and energy minimization processes on the initial polyamide model under the COMPASS II force field. At the same time, it measures the MPD of the m-phenylenediamine monomer and the reaction site spacing of the polymer chain, and further crosslinks it to obtain a crosslinked polyamide model that meets the preset crosslinking degree and actual density. Based on the reaction force field, the obtained crosslinked polyamide model for water-salt diffusion simulation is subjected to NVT kinetic calculations to eliminate residual stress in the model.

[0083] The second molecular model construction module performs chlorination on the cross-linked polyamide model obtained from the first molecular model construction module to obtain a chlorinated polyamide model, and performs multiple structural optimizations and energy minimizations on the chlorinated polyamide model under the COMPASS II force field.

[0084] The third molecular model construction module, based on the cross-linked polyamide model and chlorinated polyamide model obtained from the first and second molecular model construction modules, fills the unit cell with a number of water molecules and sodium chloride ions.

[0085] The water-salt diffusion process simulation module performs NVT kinetic calculations on the cross-linked polyamide model and chlorinated polyamide model obtained for water-salt diffusion simulation based on the reactive force field to simulate the water-salt diffusion process; and tracks H2O and salt ion substances in the water-salt diffusion process based on the trajectory file generated by the NVT kinetic calculation.

[0086] The calculation module uses analysis tools to calculate and plot the mean square displacement curve in the obtained trajectory file, and uses the diffusion coefficient formula to calculate the water-salt diffusion coefficient in the cross-linked polyamide model and the chlorinated polyamide model.

[0087] The various embodiments in this specification are described in a progressive manner, with each embodiment focusing on its differences from other embodiments. Similar or identical parts between embodiments can be referred to interchangeably. For the apparatus disclosed in the embodiments, since they correspond to the methods disclosed in the embodiments, the description is relatively simple; relevant parts can be referred to the method section.

[0088] Although embodiments and drawings of the present invention have been disclosed for illustrative purposes, those skilled in the art will understand that various substitutions, variations and modifications are possible without departing from the spirit and scope of the present invention and the appended claims. Therefore, the scope of the present invention is not limited to the contents disclosed in the embodiments and drawings.

Claims

1. A method for calculating the water-salt diffusion coefficient of chlorinated polyamide based on molecular simulation, characterized in that: The steps include the following: S1. In Material Studio software, construct molecular models of m-phenylenediamine monomer MPD and trimesoyl chloride TMC, and perform preprocessing. S2. Polymerize m-phenylenediamine monomer MPD and trimesoyl chloride TMC, and bond the reaction sites of the two monomer molecules to obtain a polyamide chain with a predetermined degree of polymerization. S3. Mix several polyamide chains obtained in S2 with m-phenylenediamine monomer MPD and add them into the unit cell to obtain an initial model of polyamide with low crosslinking degree. S4. Under the COMPASS II force field, the initial polyamide model obtained in S3 is subjected to structural optimization and energy minimization. At the same time, the reaction site spacing of the m-phenylenediamine monomer MPD and the polyamide chain is measured, and further crosslinking is performed to obtain a crosslinked polyamide model that meets the preset crosslinking degree and actual density. S5. Perform NVT dynamic calculations on the crosslinked polyamide model obtained in S4 based on the reactive force field to eliminate the residual stress in the crosslinked polyamide model; S6. Based on the cross-linked polyamide model obtained in S5 to eliminate residual stress, chlorination treatment is performed to obtain a chlorinated polyamide model, and the chlorinated polyamide model is subjected to structural optimization and energy minimization treatment under the COMPASS II force field. S7. Based on the cross-linked polyamide model and chlorinated polyamide model obtained in S4 and S6, water molecules and sodium chloride ions are filled into the unit cell, and NVT dynamic calculations are performed on the cross-linked polyamide model and chlorinated polyamide model filled with solution under COMPASS II force field to simulate the water-salt diffusion process and generate a water-salt diffusion trajectory file. S8. Based on the water-salt diffusion trajectory file generated by the NVT kinetic calculation in S7, track the H2O and salt ion substances in the water-salt diffusion process, use analysis tools to calculate and plot the mean square displacement curve, and use the diffusion coefficient formula to calculate the water-salt diffusion coefficient of the chlorinated polyamide model. In step S5, NVT kinetic calculations are performed on the crosslinked polyamide model based on the reactive force field to eliminate residual stress in the crosslinked polyamide model. This process includes: The cross-linked polyamide model with molecular chain folding and shrinkage obtained in S4 was subjected to NVT kinetic calculations for 1 ns, and the density and energy fluctuations during the calculation process were checked. When the density and energy fluctuations were less than 10% of the mean, the cross-linked polyamide model entered a stable state and the residual stress was eliminated. In step S7, based on the cross-linked polyamide model and the chlorinated polyamide model obtained in steps S4 and S6, a number of water molecules and sodium chloride ions are filled into the unit cell, and NVT kinetic calculations are performed on the cross-linked polyamide model and the chlorinated polyamide model filled with solution under the COMPASS II force field to simulate the water-salt diffusion process, including the following steps: S7.1 Create a Connolly surface in the cross-linked polyamide model and chlorinated polyamide model obtained in S4 and S6, thereby obtaining a free volume, and fill the free volume of the cross-linked polyamide model and chlorinated polyamide model with a certain number of water molecules and salt ions of a predetermined concentration or mass fraction; S7.2 Based on the solution-immersed membrane crosslinked polyamide model and chlorinated polyamide model obtained in S7.1, NVT kinetic calculations are performed on the solution-filled crosslinked polyamide model and chlorinated polyamide model under the COMPASSII force field for 1 ns, simulating the water-salt diffusion process while optimizing the structure and energy. Based on the water-salt diffusion trajectory file generated by the NVT kinetic calculation in S7, the H2O and salt ions in the water-salt diffusion process are tracked. The mean square displacement curve is calculated and plotted using analytical tools. The water-salt diffusion coefficient is calculated using the diffusion coefficient formula, the specific form of which is: S8.1 Selects the water molecules to be tracked by charge and sets them as a set; similarly, salt ions are set as another set. S8.2 In the water-salt diffusion trajectory file obtained in S7, use the analysis tool to calculate the mean square displacement of the tracked set and output tabular data and graphs; S8.3 Based on the calculation formula, the diffusion coefficient of the tracking water salt particles is calculated using the slope of the mean square displacement curve: in r i (t) yes i Particles in t The end position under time; r i (0) yes i The initial position of the particle; <> This indicates the calculation of the average value. When the mean square displacement-time curve is linear over a long range, the diffusion coefficient of the target particle can be calculated using Einstein's equations based on the slope of the mean square displacement curve. The calculation formula can be simplified to the following form: in k This represents the slope of the mean square displacement curve.

2. The method for calculating the water-salt diffusion coefficient of chlorinated polyamide based on molecular simulation according to claim 1, characterized in that: In step S1, molecular models of m-phenylenediamine monomer MPD and trimesoyl chloride monomer TMC are constructed. The construction method includes the following steps: S1.1 Use the Sketch Atom and Sketch Ring tools in the software toolbar to draw the monomer molecular structure, and use the Clean tool to correct the drawn structure; S1.2 labels the N atom at the amino terminus of m-phenylenediamine (MPD) as R1, and labels the C atom of the acyl chloride group of trimesoyl chloride (TMC) as R2. R1 and R2 are the reaction sites of the two monomers.

3. The method for calculating the water-salt diffusion coefficient of chlorinated polyamide based on molecular simulation according to claim 1, characterized in that: In step S2, the method for bonding the reaction sites of the two monomer molecules to obtain a polyamide chain with a predetermined degree of polymerization is as follows: The H atom on the amino group of the m-phenylenediamine monomer MPD and the Cl atom on the acyl chloride group in the polyamide chain were removed, and then the amino N was connected to the acyl chloride C.

4. The method for calculating the water-salt diffusion coefficient of chlorinated polyamide based on molecular simulation according to claim 1, characterized in that: In step S4, the initial model undergoes structural optimization and energy minimization. Simultaneously, the MPD of the m-phenylenediamine monomer and the reaction site spacing of the polymer chains are measured. Further crosslinking is then performed to obtain a crosslinked polyamide model that meets the preset crosslinking degree and actual density. This includes the following steps: S4.1 Set the pressure to 0.0001 GPa, perform NPT kinetic calculations on the initial polyamide model obtained in S3, and compress the volume of the initial polyamide model until it meets the actual density requirements; after the compression process is completed, a cross-linked polyamide model that meets the actual density requirements is obtained. S4.2 Anneal the cross-linked polyamide model obtained in S4.1 that meets the actual density requirements at 298K to 500K to cause the molecular chains to fold and shrink; after the annealing process, a cross-linked polyamide model with folded and shrunken molecular chains is obtained. S4.3 Measure the distance between the H atom on the m-phenylenediamine monomer MPD and the reaction site Cl on the polymer chain. If it is less than 5 Å, delete the H atom on the amino group of the m-phenylenediamine monomer MPD and the Cl atom on the acyl chloride group in the polyamide chain. Then connect the amino N to the acyl chloride C. Repeat S4.1, S4.2, and S4.3 until the crosslinked polyamide model meets the preset crosslinking degree and actual density. After crosslinking is completed, excess m-phenylenediamine monomer is removed, and the remaining acyl chloride groups are replaced with carboxyl groups.

5. The method for calculating the water-salt diffusion coefficient of chlorinated polyamide based on molecular simulation according to claim 1, characterized in that: In step S6, the cross-linked polyamide model obtained in step S5 with residual stress eliminated is subjected to chlorination to obtain a chlorinated polyamide model. The chlorinated polyamide model is then subjected to multiple structural optimizations and energy minimizations under the COMPASS II force field, including the following steps: S6.1 Based on the number of Cl atoms to be introduced, randomly select the H of the free-terminal amino group or the H of the amide bond on the polyamide chain and modify it to Cl; S6.2 Similarly, randomly select several amide bonds or acyl chloride bonds, break the connecting bond between C and N, add a hydroxyl group to the carbonyl end to make it a carboxyl group, and add H to the free end containing N.

6. A system for calculating the water-salt diffusion coefficient based on a molecular simulation model of chlorinated polyamide, characterized in that: The calculation system is implemented based on the calculation method described in any one of claims 1-5, including: a molecular model building module, which builds molecular models of m-phenylenediamine monomer MPD and trimesoyl chloride monomer TMC in MaterialsStudio software and performs preprocessing; polymerizes an appropriate amount of m-phenylenediamine monomer MPD and trimesoyl chloride TMC, and bonds the reaction sites of the two monomer molecules to obtain a polyamide chain with a preset degree of polymerization; The lattice building module mixes several polyamide chains obtained from S2 and m-phenylenediamine monomer MPD and adds them into the unit cell to obtain an initial model of polyamide with low crosslinking degree. The first molecular model construction module performs multiple structural optimizations and energy minimization processes on the initial polyamide model under the COMPASS II force field. At the same time, it measures the reaction site spacing of the m-phenylenediamine monomer MPD and the polyamide chain, and further crosslinks it to obtain a crosslinked polyamide model that meets the preset crosslinking degree and actual density. Based on the reaction force field, the obtained crosslinked polyamide model for water-salt diffusion simulation is subjected to NVT kinetic calculations to eliminate the residual stress of the crosslinked polyamide model. The second molecular model construction module performs chlorination on the cross-linked polyamide model obtained from the first molecular model construction module to obtain a chlorinated polyamide model, and performs multiple structural optimizations and energy minimizations on the chlorinated polyamide model under the COMPASS II force field. The third molecular model construction module, based on the cross-linked polyamide model and chlorinated polyamide model obtained from the first molecular model construction module and the second molecular model construction module, fills the unit cell with a number of water molecules and sodium chloride ions. The water-salt diffusion process simulation module performs NVT kinetic calculations on the cross-linked polyamide model and chlorinated polyamide model obtained for water-salt diffusion simulation based on the reactive force field to simulate the water-salt diffusion process; and tracks H2O and salt ion substances in the water-salt diffusion process based on the trajectory file generated by the NVT kinetic calculation. The calculation module uses analysis tools to calculate and plot the mean square displacement curve in the obtained trajectory file, and uses the diffusion coefficient formula to calculate the water-salt diffusion coefficient in each polyamide model.

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