A method for predicting crystallinity and mechanical properties of polytetrafluoroethylene based on molecular dynamics simulation

Abstract

The application discloses a method for simulating crystallinity and mechanical properties of polytetrafluoroethylene based on molecular dynamics, which comprises the following steps: constructing a full-atom model of amorphous polytetrafluoroethylene; performing sintering simulation on the full-atom model of amorphous polytetrafluoroethylene, and constructing a coarse-grained model of polytetrafluoroethylene according to bond order information obtained from the sintering simulation; performing cooling crystallization simulation on the coarse-grained model of polytetrafluoroethylene, and calculating the crystallinity of polytetrafluoroethylene according to particle coordinate information obtained from the cooling crystallization simulation; and performing uniaxial tensile simulation on the coarse-grained model of polytetrafluoroethylene after the cooling crystallization simulation, and evaluating the mechanical properties of polytetrafluoroethylene according to the result of the uniaxial tensile simulation. The method can simulate the crystallinity and mechanical properties of polytetrafluoroethylene with different molecular weights, and the simulation result is consistent with the actual result, so that the test time and cost can be saved, and the method can provide certain guidance for actual production.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of high polymer materials, and particularly relates to a method for simulating crystallinity and mechanical properties of polytetrafluoroethylene based on molecular dynamics. BACKGROUND

[0002] Polytetrafluoroethylene (PTFE) is a spiral linear polymer with excellent corrosion resistance, good chemical inertness and good machining performance, and is widely used in the fields of semiconductor manufacturing, biomedical and chemical industry. The crystallinity of polytetrafluoroethylene powder is as high as 70-90%, the melting temperature and the melt viscosity are relatively high, and it is not easy to be extruded or injection molded. Therefore, a mode pressing sintering similar to “powder metallurgy” is usually used for production, that is, cold molding under a certain pressure, then free sintering by heating to a temperature above the melting point of 325℃, and finally forming a translucent solid by heat preservation and cooling. When heated, polytetrafluoroethylene changes from a crystal to an amorphous state, and the particles melt with each other. With the cooling, the amorphous state changes into a new banded crystal, and the crystallinity decreases to 25-40%. The mechanical properties of polytetrafluoroethylene products are affected by the molecular weight and the crystallinity. The molecular weight and the crystallinity of polytetrafluoroethylene powders of different brands are different, and change with the sintering temperature. In addition, polytetrafluoroethylene has different degrees of thermal decomposition, and the macromolecular chains are decomposed into short chains. Therefore, the molecular weight and the crystallinity of polytetrafluoroethylene after sintering and molding also exist differences. Generally, the lower the molecular weight, the stronger the crystallization power of the molecular chain, the higher the crystallinity, the greater the hardness and brittleness, but the worse the deformation ability and flexibility, and the lower the tensile strength and elongation at break.

[0003] Molecular dynamics (MD) simulation is a deterministic computer simulation method based on Newtonian mechanics. This method constructs an atomic model at the microscale, simulates the static structure and the behavior of dynamic evolution over time of the system. A force field is constructed by setting the interaction relationship between atoms. The initial state is determined by assigning the initial position and velocity of each basic unit in the system. The equilibrium system and the motion trajectory of each basic unit under long-time state are obtained by time integration. The physical quantities such as size, temperature and pressure are reflected by statistical physics methods.

[0004] At present, sintered materials are usually characterized by using various means to represent their sintering quality and mechanical properties. Scanning electron microscope (SEM) or transmission electron microscope (TEM) is used to observe the internal microstructure and crystal morphology. Differential scanning calorimeter (DSC) is used to measure the melting enthalpy to calculate the crystallinity. Gel permeation chromatograph is used to measure the molecular weight distribution. Uniaxial tension is used to measure the elastic modulus and tensile strength of the material. The above-mentioned experiments have a complicated process and a long cycle. SUMMARY

[0005] The application provides a method for simulating crystallinity and mechanical properties of polytetrafluoroethylene based on molecular dynamics.

[0006] The technical scheme adopted by the application is as follows:

[0007] One kind based on molecular dynamics simulation polytetrafluoroethylene crystallinity and mechanical properties of method

[0008] The method comprises the following steps:

[0009] 1) constructing an amorphous polytetrafluoroethylene all-atom model by using molecular modeling software;

[0010] In step 1), the amorphous polytetrafluoroethylene all-atom model is a visual three-dimensional model. The amorphous polytetrafluoroethylene all-atom model takes atoms as the basic unit and is mainly composed of a plurality of polytetrafluoroethylene molecular chains with the same or different molecular weight randomly stacked, and the polytetrafluoroethylene molecular chain is mainly composed of a plurality of tetrafluoroethylene monomers polymerized.

[0011] 2) sintering simulation is performed on the amorphous polytetrafluoroethylene all-atom model, and a polytetrafluoroethylene coarse-grained model is constructed according to the bond order information obtained by sintering simulation;

[0012] The step 2) specifically comprises the following steps:

[0013] 2.1) converting the amorphous polytetrafluoroethylene all-atom model into a text file and importing it into molecular dynamics simulation software, and initializing the ReaxFF force field in the molecular dynamics simulation software;

[0014] The text file of the amorphous polytetrafluoroethylene all-atom model includes the position coordinates, velocity information, chemical bonds, bond angles and dihedral angles of atoms, etc.

[0015] In step 2.1), initializing the ReaxFF force field specifically includes: setting the Reaxff force field parameters and boundary conditions, performing energy minimization optimization on the amorphous polytetrafluoroethylene all-atom model, giving the particles an initial velocity, and balancing the relaxation; the regular ensemble and Nosé-Hoover temperature control method are used for the balancing relaxation, and the relaxation temperature is 200K to avoid the chemical bond from being broken due to the high temperature;

[0016] 2.2) performing sintering simulation on the amorphous polytetrafluoroethylene all-atom model at a preset sintering simulation temperature higher than the melting point of polytetrafluoroethylene in a ReaxFF force field by using a canonical ensemble and a Berendson temperature control mode to obtain bond order information;

[0017] 2.3) obtaining broken chemical bonds, bond angles and dihedral angles by searching the bond order information, and deleting the broken chemical bonds, bond angles and dihedral angles from the amorphous polytetrafluoroethylene all-atom model after sintering simulation to obtain a reconstructed amorphous polytetrafluoroethylene all-atom model;

[0018] 2.4) converting the reconstructed amorphous polytetrafluoroethylene all-atom model into a polytetrafluoroethylene coarse-grained model by taking a CF2 group as a basic unit.

[0019] 3) performing cooling crystallization simulation on the polytetrafluoroethylene coarse-grained model, and calculating polytetrafluoroethylene crystallinity according to particle coordinate information obtained by the cooling crystallization simulation;

[0020] The step 3) specifically comprises the following steps:

[0021] 3.1) initializing a Dreiding force field in a molecular dynamics software;

[0022] 3.2) performing cooling crystallization simulation on the polytetrafluoroethylene coarse-grained model in the Dreiding force field by using a canonical ensemble, a Nosé-Hoover temperature control and a pressure control to obtain particle coordinate information;

[0023] The cooling crystallization simulation condition is that the pressure is 1 atm, the temperature is slowly decreased from the sintering simulation temperature to a crystallization relaxation temperature, and then relaxed at the crystallization relaxation temperature until the polytetrafluoroethylene coarse-grained model reaches a steady state, so that the polytetrafluoroethylene is fully crystallized;

[0024] The crystallization relaxation temperature is preferably 300 K;

[0025] 3.3) selecting middle particles of each molecular chain from all particles according to the particle coordinate information, calculating a local orientation factor of each middle particle, and judging whether the middle particle is in a crystallization zone according to the local orientation factor of each middle particle;

[0026] In the step 3.3), the specific process of calculating the local orientation factor of each middle particle is as follows:

[0027] 3.3.1) taking the middle particle as a target particle, screening local particles from the remaining middle particles, and all the local particles form a set P; the distance between the local particle and the target particle is less than a preset cutoff radius R;

[0028] 3.3.2) screen out the heterodesmic particles from the set P, all the heterodesmic particles constituting a set Q; the heterodesmic particles have no chemical bond with the target particle;

[0029] 3.3.3) calculate the bond vector of the target particle and each heterodesmic particle in the set Q respectively

[0030] the bond vector is obtained by the following formula:

[0031]

[0032] wherein n is the serial number of the particle whose bond vector is to be calculated, m and l are the serial numbers of two particles connected with the particle whose bond vector is to be calculated through a chemical bond, x is the coordinate value of the particle on the x-axis, y is the coordinate value of the particle on the y-axis, and z is the coordinate value of the particle on the z-axis;

[0033] 3.3.4) calculate the orientation factor of each heterodesmic particle in the set Q with the target particle;

[0034] the orientation factor of each heterodesmic particle with the target particle is obtained by the following formula:

[0035]

[0036] wherein i is the serial number of the target particle, j is the serial number of the heterodesmic particle, f ij is the orientation factor of the target particle i and the heterodesmic particle j, f ij is the angle between the bond vector and the bond vector ;

[0037] 3.3.5) calculate the average value of the orientation factor of all the heterodesmic particles in the set Q with the target particle, and take the average value as the local orientation factor of the intermediate particle.

[0038] In the step 3.3), the specific process of judging whether the intermediate particle is in the crystallization zone according to the local orientation factor of the intermediate particle is as follows: if the local orientation factor of the intermediate particle is greater than or equal to a preset threshold value, the intermediate particle is in the crystallization zone; if the local orientation factor of the intermediate particle is less than the preset threshold value, the intermediate particle is not in the crystallization zone.

[0039] 3.4) count the total number of particles in the crystallization zone and the total number of particles in the polytetrafluoroethylene coarse-grained model, calculate the ratio between the total number of particles in the crystallization zone and the total number of particles, and obtain the crystallinity of the polytetrafluoroethylene.

[0040] 4) perform uniaxial stretching simulation on the polytetrafluoroethylene coarse-grained model after the cooling crystallization simulation, and obtain the mechanical properties of the polytetrafluoroethylene according to the result of the uniaxial stretching simulation;

[0041] In the step 4), the uniaxial stretching simulation adopts a canonical ensemble, a Nosé-Hoover temperature control and a pressure control mode, the uniaxial stretching simulation temperature is 300 K, the pressure in the non-stretching direction is 1 atm, and a constant strain rate is applied in the stretching direction; the result of the uniaxial stretching simulation is the strain and stress information of the polytetrafluoroethylene coarse-grained model in the stretching direction after the uniaxial stretching simulation.

[0042] In the step 4), the specific process of obtaining the mechanical properties of the polytetrafluoroethylene according to the result of the uniaxial stretching simulation is as follows: a stress-strain curve is drawn according to the result of the uniaxial stretching simulation, and the elastic modulus and / or the tensile strength of the polytetrafluoroethylene are evaluated according to the stress-strain curve.

[0043] Two, a molecular dynamics simulation system applied to the above method

[0044] The molecular dynamics simulation system comprises:

[0045] A modeling module is configured to construct a full-atom model of amorphous polytetrafluoroethylene by using a molecular modeling software.

[0046] A molecular dynamics simulation module is configured to sequentially perform sintering simulation, cooling crystallization simulation and uniaxial stretching simulation on the full-atom model of amorphous polytetrafluoroethylene by using a molecular dynamics software.

[0047] A simulation module is configured to simulate the crystallinity of the polytetrafluoroethylene according to the result of the cooling crystallization simulation, and obtain the mechanical properties of the polytetrafluoroethylene according to the result of the uniaxial stretching simulation.

[0048] The molecular dynamics software is LAMMPS software.

[0049] The molecular modeling software is preferably Materials Studio software.

[0050] The present application has the following advantages:

[0051] 1. The method of the present application simulates the thermal decomposition behavior and microcrystallization process of polytetrafluoroethylene during sintering by means of molecular dynamics method and computer simulation, reflects the crystallinity and mechanical properties of polytetrafluoroethylene with different molecular weights through calculation of local orientation factor and uniaxial stretching simulation. The method of the present application can replace experiments to a certain extent, make reasonable prediction of sintering quality and mechanical properties of materials, and save experimental time and money cost, thereby providing certain guidance for actual production.

[0052] 2. The crystallinity obtained by the method of the present application is in good agreement with the actual test results, and the actual results of the mechanical properties obtained by the present application are in good agreement with the trend.

[0053] 3、The method of the present application is based on a coarse-grained (CG) model in the simulation of crystallization, that is, in the simulation of crystallization, the CF2 group is regarded as a whole based on the all-atom molecular dynamics simulation, and the interaction between the atoms in the group is not considered, thereby reducing the degrees of freedom of the system, improving the calculation efficiency, and not affecting the simulation results of the crystallinity and mechanical properties. BRIEF DESCRIPTION OF DRAWINGS

[0054] Figure 1 A flowchart for judging whether a particle is in a crystallization region in the method of the present application;

[0055] Figure 2 A plot of the change of crystallinity with time obtained in an embodiment of the present application;

[0056] Figure 3 A stress-strain curve obtained in an embodiment of the present application. DETAILED DESCRIPTION

[0057] The present application will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0058] The present application provides a method for simulating the thermal decomposition of polytetrafluoroethylene with different molecular weights in the sintering process and the crystallinity and mechanical properties after sintering. The process of the method of the present application mainly comprises: establishing an all-atom model of polytetrafluoroethylene; simulating the thermal decomposition behavior of polytetrafluoroethylene in the sintering process under the ReaxFF force field, reconstructing the coarse-grained model of polytetrafluoroethylene according to the chemical bond breaking information; simulating the crystallization process under the Dreiding force field, calculating the local orientation factor according to the atomic coordinates, judging whether the atom is in the crystallization region according to the local orientation factor, and calculating the crystallinity; simulating uniaxial tension under the Dreiding force field, drawing a stress-strain curve, and analyzing the mechanical properties.

[0059] The "system" and "simulation system" described in the present application both refer to the simulation object in the molecular dynamics software.

[0060] The method of the present application comprises the following steps:

[0061] 1) Constructing an all-atom model of amorphous polytetrafluoroethylene using modeling software; wherein the all-atom model of amorphous polytetrafluoroethylene takes atoms as the basic unit, and is mainly composed of a plurality of polytetrafluoroethylene molecular chains with the same or different molecular weights stacked randomly, and the polytetrafluoroethylene molecular chains are mainly composed of a plurality of tetrafluoroethylene monomers polymerized.

[0062] 2) Sintering simulation of the all-atom model of amorphous polytetrafluoroethylene using molecular dynamics software, and constructing a coarse-grained model of polytetrafluoroethylene according to the bond sequence information obtained by sintering simulation. Specifically, the following steps are included:

[0063] 2.1) Import the amorphous polytetrafluoroethylene full atom model into the molecular dynamics software, initialize the ReaxFF force field in the molecular dynamics software.

[0064] Wherein, initializing the ReaxFF force field is specifically: setting the Reaxff force field parameters and boundary conditions, performing energy minimization optimization on the amorphous polytetrafluoroethylene full atom model, giving the particle an initial velocity, and balancing the relaxation.

[0065] Wherein, the equilibrium relaxation adopts the canonical ensemble and the Nosé-Hoover temperature control method, and the relaxation temperature is 200K, so as to avoid the chemical bond from breaking due to too high temperature.

[0066] 2.2) In the initialized ReaxFF force field, the canonical ensemble (NVT ensemble) and Berendson temperature control method are used to perform sintering simulation on the amorphous polytetrafluoroethylene full atom model at a preset sintering simulation temperature, and the bond sequence information in the sintering simulation process is output through the fixreaxff / bonds instruction;

[0067] The sintering simulation temperature is higher than the melting point of polytetrafluoroethylene;

[0068] 2.3) By searching the bond sequence information obtained by sintering simulation, the broken chemical bonds, bond angles and dihedral angles are obtained, and the broken chemical bonds, bond angles and dihedral angles are deleted from the amorphous polytetrafluoroethylene full atom model after sintering simulation, to obtain a reconstructed amorphous polytetrafluoroethylene full atom model;

[0069] 2.4) Taking CF2 group as the basic unit and ignoring the interaction between atoms in the group, the reconstructed amorphous polytetrafluoroethylene full atom model is converted into a polytetrafluoroethylene coarse-grained model.

[0070] Wherein, the Reaxff force field simulates the generation and breaking of chemical bonds through bond sequence, and the total energy E of the system system is the sum of bond energy E bond , supersaturated bond energy E over , bond angle energy E val , dihedral angle energy E tors , van der Waals interaction energy E vdWals , coulomb interaction energy E coulomb , and special bond energy E speciflc .

[0071] Wherein, the boundary condition is a periodic boundary condition, that is, the simulation system is replicated in all directions, and when a particle moves out of the boundary, another particle enters the system from the opposite direction, so that the particle number and density of the simulation system remain unchanged.

[0072] Among them, the energy minimization optimization uses the conjugate gradient algorithm to calculate the interaction forces between particles, so that overlapping or closely spaced particles are rearranged to reduce the system energy and obtain a more reasonable geometric configuration.

[0073] In this context, assigning an initial velocity to a particle involves giving each particle in the system a random velocity at a specified temperature, with the velocity values ​​distributed according to a Gaussian distribution.

[0074] In this process, equilibrium relaxation under a ReaxFF force field transforms a non-equilibrium system into an equilibrium state, stabilizing physical quantities such as temperature and pressure. Depending on the complexity of the system, equilibrium relaxation can be performed in one or more steps. This invention employs a canonical ensemble (NVT) and Nosé-Hoover temperature control, with a relaxation temperature of 200K to prevent chemical bond breakage due to excessively high temperatures.

[0075] The sintering simulation uses the NVT ensemble and Berendson temperature control. The sintering simulation temperature must be higher than the PTFE melting point of 325℃. The bond sequence information is output through the fix reaxff / bonds command.

[0076] Preferably, in order to improve the reaction rate and cause chemical bond breaking within a finite simulation time, a sintering simulation temperature higher than the actual sintering temperature can be selected.

[0077] The process of reconstructing the polytetrafluoroethylene (PTFE) model and converting it into a coarse-grained model based on the output bond sequence information is as follows: Based on the bond sequence information output at the end of the simulation, the existence of corresponding chemical bonds before and after the simulation is checked, the PTFE all-atom model is modified, broken chemical bonds, bond angles and dihedral angles are deleted, the CF2 group is used as the basic unit, the interatomic interactions within the group are ignored, and the all-atom model is converted into a coarse-grained model for subsequent simulations.

[0078] 3) A cooling crystallization simulation of the coarse-grained polytetrafluoroethylene (PTFE) model was performed using molecular dynamics software. The crystallinity of PTFE was calculated based on the particle coordinate information obtained from the cooling crystallization simulation. For example... Figure 1 As shown, crystallinity calculation requires determining the degree of orderliness of particle arrangement within a local area, i.e., the local orientation factor. Microscopically, particles in a crystal are arranged in an ordered, locally oriented manner. By comparing the local orientation factor with a threshold, a particle is considered to be in a crystalline region if it is greater than or equal to this threshold. Finally, the ratio of the number of particles located in the crystalline region to the total number of particles is calculated to obtain the crystallinity of the entire system.

[0079] Specifically, the following steps are included:

[0080] 3.1) Initialize the Dreiding force field in the molecular dynamics software;

[0081] 3.2) In the Dreiding force field after initialization, the cooling crystallization simulation of the polytetrafluoroethylene coarse-grained model is carried out by using the NPT ensemble, the Nosé-Hoover temperature control and the pressure control mode, and the particle coordinate information after the cooling crystallization simulation is obtained;

[0082] In the cooling crystallization simulation condition: the pressure is 1 atm, the temperature is slowly decreased from the sintering simulation temperature to 300K, and then relaxed at 300K to make the polytetrafluoroethylene coarse-grained model reach a steady state, so that the polytetrafluoroethylene is fully crystallized;

[0083] 3.3) According to the particle coordinate information, the middle particles in each molecular chain are selected, the local orientation factor of each middle particle is calculated, and whether the middle particle is in the crystallization zone is judged according to the local orientation factor of each middle particle.

[0084] The specific process of calculating the local orientation factor of each middle particle is as follows:

[0085] 3.3.1) The middle particle is taken as a target particle, and the local particles are selected from the remaining middle particles except the target particle, and all the local particles form a set P; the distance between each local particle and the target particle is less than the preset cutoff radius R of the middle particle;

[0086] 3.3.2) The heterochain particles are selected from the set P, and all the heterochain particles form a set Q; there is no chemical bond between each heterochain particle and the target particle;

[0087] 3.3.3) The bond vector of the target particle and each middle particle in the set Q is calculated

[0088] The bond vector The bond vector is calculated by the following formula:

[0089]

[0090] In the formula, n is the serial number of the particle to be calculated, m and l are the serial numbers of the two particles connected by the chemical bond with the particle to be calculated, x is the coordinate value of the x-axis, y is the coordinate value of the y-axis, and z is the coordinate value of the z-axis;

[0091] 3.3.4) The orientation factor of each heterochain particle and the target particle in the set Q is calculated;

[0092] The orientation factor of each heterochain particle and the target particle is obtained by the following formula:

[0093]

[0094] In the formula, i is the serial number of the target particle, j is the serial number of the heterochain particle, and f ijis the orientation factor of target particle i and heteroparticle j, f ij is the bond vector is the angle between the bond vector is the bond vector of target particle i, is the bond vector of heteroparticle with serial number j;

[0095] 3.3.5) Calculate the average value of the orientation factor of all heteroparticles in set Q and target particles, and take the average value as the local orientation factor of the target particle, i.e. the intermediate particle;

[0096] The local orientation factor of the target particle is obtained by the following formula:

[0097]

[0098] In the formula, k is the number of particles in set Q, j is the serial number of the intermediate particle in set Q, i is the serial number of the target particle, f ij is the orientation factor of particles i and j, F is the local orientation factor of the target particle with serial number i.

[0099] In step 3.3), the specific process of judging whether the intermediate particle is in the crystallization zone according to the local orientation factor of the intermediate particle is as follows:

[0100] If the local orientation factor of the intermediate particle is greater than the preset threshold value, the intermediate particle is in the crystallization zone;

[0101] If the local orientation factor of the intermediate particle is less than the preset threshold value, the intermediate particle is not in the crystallization zone.

[0102] 3.4) Count the total number of particles in the crystallization zone and the total number of particles in the polytetrafluoroethylene coarse-grained model, calculate the ratio between the total number of particles in the crystallization zone and the total number of particles, and obtain the crystallinity of polytetrafluoroethylene;

[0103] The calculation formula of the crystallinity of polytetrafluoroethylene is as follows:

[0104]

[0105] In the formula, X is the crystallinity, N cr is the total number of particles in the crystallization zone, and N is the total number of particles in the polytetrafluoroethylene coarse-grained model.

[0106] Wherein, the Dreiding force field is the sum of the non-bond potential energy E non-bond , the bond potential energy E bond , the bond angle potential energy E angle and the dihedral angle potential energy E dihedral , which is as follows:

[0107] a) Non-bond potential energy E​non-bond , describing the inter-particle interaction force of non-chemical bond connection, the present application adopts Lennard-Jones 12-6 potential:

[0108]

[0109] wherein r is the distance between two coarse-grained particles, e is the L-J potential well depth, r c is the cut-off radius.

[0110] b) Bond length potential energy E bond , describing the inter-particle interaction force of chemical bond connection, the present application adopts harmonic potential:

[0111] E bond (r)=K b (r-r0) 2

[0112] wherein K b represents the chemical bond tensile elastic constant, and r0 is the length of the equilibrium chemical bond.

[0113] c) Bond angle potential energy E angle , describing the bond angle bending potential energy between two chemical bonds formed by three consecutive atoms, the present application adopts harmonic potential:

[0114] E angle (q)=Kq(q-q0) 2

[0115] wherein K q is the bond angle torsion elastic constant, and q0 is the degree of the equilibrium bond angle.

[0116] d) Dihedral angle potential energy E dihedral , describing the torsion potential of dihedral angle formed by four consecutive atoms, the present method adopts CHARMM potential:

[0117] E dihedral (f)=K f [[1+cos(nf-d)]]

[0118] wherein K f is the dihedral angle torsion constant, n is the dihedral angle coefficient, and d is the angle adjustment coefficient.

[0119] 4) performing uniaxial stretching simulation on the polytetrafluoroethylene coarse-grained model after the simulation of cooling crystallization by using the molecular dynamics software, and evaluating the mechanical properties of the polytetrafluoroethylene according to the result of the uniaxial stretching simulation;

[0120] The uniaxial stretching simulation adopts an NPT ensemble, a Nosé-Hoover temperature control and pressure control mode, the uniaxial stretching simulation temperature is 300K, the pressure in the non-stretching direction is 1atm, and a constant strain rate is applied in the stretching direction;

[0121] The uniaxial stretching simulation result is the strain and stress information of the polytetrafluoroethylene coarse-grained model in the stretching direction after the uniaxial stretching simulation.

[0122] The mechanical properties include the elastic modulus and / or the tensile strength.

[0123] The specific implementation of the present application is as follows:

[0124] Example 1

[0125] The molecular modeling software Materials Studio used in this embodiment is a full-scale material simulation platform, which can construct three-dimensional models of various crystals, amorphous and polymer materials.

[0126] The molecular dynamics simulation software LAMMPS used in this embodiment is an open-source classical molecular dynamics parallel simulator, which can simulate various atomic systems, has multiple functions built-in, and can set force fields and boundary conditions according to its own needs to simulate the trajectory and mechanical characteristics of the system in the micro-movement state. It is one of the commonly used tools in the current molecular dynamics method.

[0127] This embodiment specifically includes the following steps:

[0128] 1) Use the amorphous cell module of the Materials Studio software to establish a full-atom model of amorphous polytetrafluoroethylene; wherein the full-atom model of amorphous polytetrafluoroethylene takes atoms as the basic unit and is mainly composed of a plurality of polytetrafluoroethylene molecular chains with the same molecular weight randomly stacked, and the polytetrafluoroethylene molecular chain is mainly composed of a plurality of tetrafluoroethylene monomers polymerized. In this embodiment, the molecular weight of each polytetrafluoroethylene molecular chain is 20000.

[0129] 2) Import the full-atom model of amorphous polytetrafluoroethylene into the LAMMPS software, set the Reaxff force field parameters and boundary conditions in the LAMMPS software, optimize the full-atom model of amorphous polytetrafluoroethylene by energy minimization, give the particles initial velocity, and balance the relaxation; sintering simulation, according to the output bond order information, reconstruct the polytetrafluoroethylene model and convert it into a coarse-grained model.

[0130] Specific process and parameters are: under the ReaxFF force field, energy minimization optimization is carried out, NVT ensemble is set, and 500 ps is relaxed at 200 K. Set NVT ensemble, control the sintering simulation temperature at 700 K, simulate sintering for 2 ns, and output the bond sequence information of the process through the fix reaxff / bonds command. By comparing the bond sequence changes before and after sintering, deleting the broken chemical bonds and their corresponding bond angles and dihedral angles, reconstructing the polytetrafluoroethylene model, and converting the model into a coarse-grained model with CF2 groups as the basic unit.

[0131] 3) Set the Dreiding force field parameters, and balance the system and relax; cool and crystallize, output the particle coordinate information of the process, calculate the local orientation factor of each particle according to the particle coordinate information, and judge whether each particle is in the crystalline region according to the local orientation factor of each particle, and calculate the crystallinity of the system.

[0132] Specific process and parameters are: under the Dreiding force field, set NVT ensemble, and relax for 10 ns at 700 K; then set NPT ensemble, 700 K temperature, 1 atm pressure, and relax for 10 ns. Set NPT ensemble, simulate cooling and crystallization for 10 ns under 1 atm pressure as the temperature decreases from 700 K to 300 K; then set NPT ensemble, 300 K temperature, 1 atm pressure, and relax for 10 ns. According to the output particle coordinate information, the preset cutoff radius R is 0.4 nm, and the local orientation factor of each particle is calculated according to the flowchart shown in Figure 1 The threshold value is 0.7, and whether each particle is located in the crystalline region is judged by comparing the local orientation factor of each particle with the preset threshold value, and finally the crystallinity of the system is calculated.

[0133] 4) The polytetrafluoroethylene coarse-grained model after cooling and crystallization is subjected to uniaxial stretching simulation at a constant strain rate, the pressure and size information of the polytetrafluoroethylene coarse-grained model after cooling and crystallization in the uniaxial stretching simulation process are output, the stress-strain curve is drawn, and the elastic modulus and / or tensile strength of the polytetrafluoroethylene are evaluated according to the stress-strain curve.

[0134] Specific process and parameters are: under the Dreiding force field, set NPT ensemble, 300 K temperature, 1 atm pressure, and simulate uniaxial stretching at a strain rate of 0.00001, output the stress-strain information of the system.

[0135] In the above step 3), the crystallinity calculation flowchart is as shown in Figure 1 The specific steps are:

[0136] S1) Exclude all the first and last particles in the molecular chain, and only calculate the local orientation factor of the middle particles;

[0137] S2) Taking the selected particle as the center, the region with a preset cutoff radius R as the local region, screening all local particles with a distance less than the preset cutoff radius R from the selected particle and not being the head or tail of a molecular chain to obtain a set P.

[0138] S3) Further screening the inter-chain particles in set P located in different molecular chains by searching whether there is a chemical bond between each local particle in set P and the selected particle to obtain a set Q.

[0139] S4) Calculating the bond vector of the target particle and each intermediate particle in set Q

[0140] Bond vector The bond vector is calculated by the following formula:

[0141]

[0142] In the formula, n is the serial number of the particle to be calculated, m and l are the serial numbers of the two particles connected to the particle to be calculated through a chemical bond, x is the coordinate value of the x-axis, y is the coordinate value of the y-axis, and z is the coordinate value of the z-axis.

[0143] S5) Calculating the orientation factor of each inter-chain particle in set Q and the target particle; the calculation formula of the orientation factor of any inter-chain particle j and the target particle i in set Q is as follows:

[0144]

[0145] In the formula, i is the serial number of the target particle, j is the serial number of the inter-chain particle, f ij is the orientation factor of the target particle i and the inter-chain particle j, f ij is the angle between the bond vector and the bond vector is the bond vector of the target particle, is the bond vector of the inter-chain particle with serial number j;

[0146] S6) Calculating the average value of the orientation factors of all inter-chain particles in set Q and the target particle, and taking the average value as the local orientation factor of the target particle, i.e. the intermediate particle;

[0147] The local orientation factor of the target particle, i.e. the intermediate particle, is obtained by the following formula:

[0148]

[0149] In the formula, k is the number of particles in set Q, j is the serial number of the intermediate particle in set Q, i is the serial number of the target particle, f ij ​is the orientation factor of particle i and j, F is the local orientation factor of the target particle with serial number i;

[0150] S7) comparing the local orientation factor F of the target particle with a preset threshold value, if the local orientation factor F of the target particle is greater than the threshold value, the target particle is in the crystallization zone; otherwise, the target particle is not in the crystallization zone;

[0151] S8) calculating the local orientation factor of each intermediate particle according to steps S2) to S7), and judging whether each intermediate particle is in the crystallization zone according to the local orientation factor of each intermediate particle;

[0152] counting the number N of particles in the crystallization zone cr and the total number N of particles in the system, calculating the crystallinity according to the following formula:

[0153]

[0154] In the formula, X is the crystallinity, N cr is the total number of particles in the crystallization zone, and N is the total number of particles in the polytetrafluoroethylene coarse-grained model.

[0155] Example 2

[0156] In this embodiment, a polytetrafluoroethylene all-atom model with a molecular weight of 60000 of the polytetrafluoroethylene molecular chain is established according to step 1) in Example 1. Then the crystallinity and mechanical properties of polytetrafluoroethylene are obtained according to steps 2) to 4) in Example 1.

[0157] Example 3

[0158] In this embodiment, a polytetrafluoroethylene all-atom model with a molecular weight of 120000 of the polytetrafluoroethylene molecular chain is established according to step 1) in Example 1. Then the crystallinity and mechanical properties of polytetrafluoroethylene are obtained according to steps 2) to 4) in Example 1.

[0159] The crystallinity of polytetrafluoroethylene obtained in Examples 1 to 3 of the present application is shown in Figure 2 . Figure 2 is the crystallinity calculation result curve of the polytetrafluoroethylene coarse-grained model with a molecular weight of 20000, 60000 and 120000 over time in the cooling crystallization process. It can be seen that with the increase of the cooling crystallization simulation time, the crystallinity first increases and then tends to be stable, and the larger the molecular weight, the smaller the crystallinity. The final crystallinity of the models with a molecular weight of 20000 (Example 1), 60000 (Example 2) and 120000 (Example 3) is about 51%, 40% and 32% respectively.

[0160] The trend of the crystallinity obtained by the method of the present application with the change of the molecular weight is consistent with the actual situation that the lower the molecular weight, the higher the crystallinity.

[0161] The stress-strain curves of the PTFE sintered products with different molecular weights obtained in Embodiment 1 to Embodiment 3 are shown in the following figures. Figure 3 Figure 3 The stress-strain curves of the PTFE sintered products with different molecular weights obtained in Embodiment 1 to Embodiment 3 are shown in the following figures.

[0162] Comparative Example

[0163] The actual measured crystallinity of the PTFE sintered products with different molecular weights was calculated by a differential scanning calorimeter at a rate of 10 ℃ / min, heating the PTFE sintered products with different molecular weights from room temperature to 380 ℃, obtaining the melting enthalpy, and calculating the crystallinity according to the following formula:

[0164]

[0165] In the formula, X c is the crystallinity, ΔH f is the melting enthalpy of PTFE (J / g), and ΔH f0 is the melting enthalpy of PTFE with 100% crystallinity (J / g), and the specific value is 80 J / g.

[0166] The following table is a comparison of the simulated crystallinity obtained in Embodiment 1 to Embodiment 3 and the actual measured crystallinity obtained in the comparative example:

[0167]

[0168] It can be seen that the simulation results of the crystallinity are consistent with the experimental results.

[0169] The method of the present application can simulate the thermal decomposition behavior of molecular chains and the crystallinity after cooling during the sintering process of PTFE with different molecular weights, and also simulate the mechanical properties, so as to quickly evaluate the performance of PTFE through simulation calculation, without the need for complex experiments, and the conclusion is consistent with the actual results.

[0170] The above specific embodiments are used to explain and illustrate the present application, rather than limit the present application, and any modifications and changes made to the present application within the spirit and protection scope of the claims of the present application all fall within the protection scope of the present application.

[0171] ​The above description is only the preferred embodiment of the present application, and equivalent changes or modifications made according to the structure, features and principles described in the patent application scope of the present application are included in the patent application scope of the present application.

Claims

1. A method for simulating the crystallinity and mechanical properties of polytetrafluoroethylene based on molecular dynamics, characterized in that: Includes the following steps: 1) Construct an amorphous polytetrafluoroethylene (PTFE) all-atom model in a computer using molecular modeling software; 2) Perform sintering simulation on the amorphous polytetrafluoroethylene all-atom model, and construct a coarse-grained model of polytetrafluoroethylene based on the bond sequence information obtained from the sintering simulation. Step 2) specifically includes the following steps: 2.1) Import the amorphous polytetrafluoroethylene all-atom model into the molecular dynamics simulation software, and initialize the ReaxFF force field in the molecular dynamics simulation software; 2.2) In the ReaxFF force field, the amorphous polytetrafluoroethylene all-atom model was sintered at a preset sintering simulation temperature using a canonical ensemble and Berendson temperature control method to obtain bond sequence information. 2.3) By retrieving the bond sequence information, the broken chemical bonds, bond angles and dihedral angles are obtained. The broken chemical bonds, bond angles and dihedral angles are deleted from the amorphous polytetrafluoroethylene full-atom model after sintering simulation to obtain the reconstructed amorphous polytetrafluoroethylene full-atom model. 2.4) Using the CF2 group as the basic unit, the reconstructed amorphous polytetrafluoroethylene all-atomic model is converted into a polytetrafluoroethylene coarse-grained model; 3) Perform a cooling crystallization simulation on the coarse-grained polytetrafluoroethylene model, and calculate the crystallinity of polytetrafluoroethylene based on the particle coordinate information obtained from the cooling crystallization simulation. 4) Perform uniaxial tensile simulation on the coarse-grained model of polytetrafluoroethylene after cooling crystallization simulation, and obtain the mechanical properties of polytetrafluoroethylene based on the results of the uniaxial tensile simulation.

2. The method for simulating the crystallinity and mechanical properties of polytetrafluoroethylene based on molecular dynamics according to claim 1, characterized in that: In step 2.1), initializing the ReaxFF force field specifically involves: setting the ReaxFF force field parameters and boundary conditions, performing energy minimization optimization on the amorphous polytetrafluoroethylene all-atom model, assigning initial velocities to the particles, and achieving equilibrium relaxation; the equilibrium relaxation adopts a canonical ensemble and Nosé-Hoover temperature control method, with a relaxation temperature of 200K.

3. The method for simulating the crystallinity and mechanical properties of polytetrafluoroethylene based on molecular dynamics according to claim 1, characterized in that: Step 3) specifically includes the following steps: 3.1) Initialize the Dreiding force field in the molecular dynamics software; 3.2) In the Dreiding force field, the polytetrafluoroethylene coarse-grained model is subjected to cooling crystallization simulation using a canonical ensemble and Nosé-Hoover temperature and pressure control method to obtain particle coordinate information; The cooling crystallization simulation conditions are: pressure of 1 atm, temperature slowly decreasing from sintering simulation temperature to crystallization relaxation temperature, and then relaxing at crystallization relaxation temperature to reach steady state in the coarse-grained polytetrafluoroethylene model. 3.3) Based on the particle coordinate information, select the intermediate particles of each molecular chain from all particles, calculate the local orientation factor of each intermediate particle, and determine whether the intermediate particle is in the crystallization region based on the local orientation factor of each intermediate particle. 3.4) Count the total number of particles in the crystallization zone and the total number of particles in the polytetrafluoroethylene coarse-grained model, calculate the ratio between the total number of particles in the crystallization zone and the total number of particles, and obtain the crystallinity of the polytetrafluoroethylene.

4. The method for simulating the crystallinity and mechanical properties of polytetrafluoroethylene based on molecular dynamics according to claim 3, characterized in that: In step 3.3), the specific process for calculating the local orientation factor of each intermediate particle is as follows: 3.3.1) Using the intermediate particle as the target particle, select local particles from the remaining intermediate particles, and all local particles form a set P; the distance between the local particles and the target particle is less than the preset cutoff radius R. 3.3.2) Select heterochain particles from set P, and all heterochain particles constitute set Q; there are no chemical bonds between the heterochain particles and the target particles; 3.3.3) Calculate the bond vectors of the target particle and each heteroparticle in set Q. The bond vector It is obtained through the following formula: In the formula, n is the index of the particle whose bond vector is to be calculated, m and l are the indices of the two particles connected to the particle whose bond vector is to be calculated by chemical bonds, x is the coordinate value of the x-axis, y is the coordinate value of the y-axis, and z is the coordinate value of the z-axis. 3.3.4) Calculate the orientation factor of each heteroparticle in set Q relative to the target particle; The orientation factor of each heteroparticle relative to the target particle is obtained using the following formula: In the formula, i is the index of the target particle, j is the index of the heteroparticle, and f ij φ is the orientation factor for target particle i and heterochain particle j. ij bond vector With bond vector The angle between them; 3.3.5) Calculate the average value of the orientation factors of all heteroparticles and the target particle in set Q, and use the average value as the local orientation factor of the intermediate particle.

5. The method for simulating the crystallinity and mechanical properties of polytetrafluoroethylene based on molecular dynamics according to claim 3, characterized in that: In step 3.3), the specific process of determining whether the intermediate particle is in the crystallization region based on the local orientation factor of the intermediate particle is as follows: if the local orientation factor of the intermediate particle is greater than or equal to a preset threshold, then the intermediate particle is in the crystallization region; if the local orientation factor of the intermediate particle is less than the preset threshold, then the intermediate particle is not in the crystallization region.

6. The method for simulating the crystallinity and mechanical properties of polytetrafluoroethylene based on molecular dynamics according to claim 1, characterized in that: In step 4), the uniaxial tensile simulation adopts a canonical ensemble, Nosé-Hoover temperature and pressure control method, the uniaxial tensile simulation temperature is 300K, the pressure in the non-tensile direction is 1atm, and a constant strain rate is applied in the tensile direction; the result of the uniaxial tensile simulation is the strain and stress information of the coarse-grained polytetrafluoroethylene model in the tensile direction after the uniaxial tensile simulation.

7. The method for simulating the crystallinity and mechanical properties of polytetrafluoroethylene based on molecular dynamics according to claim 6, characterized in that: The specific process for obtaining the mechanical properties of polytetrafluoroethylene (PTFE) based on the results of uniaxial tensile simulation is as follows: plot the stress-strain curve based on the results of the uniaxial tensile simulation, and evaluate the elastic modulus and / or tensile strength of PTFE based on the stress-strain curve.

8. The method for simulating the crystallinity and mechanical properties of polytetrafluoroethylene based on molecular dynamics according to claim 1, characterized in that: In step 1), the amorphous polytetrafluoroethylene all-atom model is based on atoms as the basic unit and is mainly composed of several polytetrafluoroethylene molecular chains with the same or different molecular weights randomly stacked. The polytetrafluoroethylene molecular chains are mainly polymerized from several tetrafluoroethylene monomers.

9. A molecular dynamics simulation system applied to the method described in any one of claims 1 to 8, characterized in that: The molecular dynamics simulation system includes: The modeling module is used to construct an amorphous polytetrafluoroethylene all-atom model using molecular modeling software. The molecular dynamics simulation module is used to perform sintering simulation, cooling crystallization simulation and uniaxial tensile simulation on an amorphous polytetrafluoroethylene all-atom model using molecular dynamics software. The simulation module is used to simulate the crystallinity of polytetrafluoroethylene (PTFE) based on the results of cooling crystallization simulation, and to obtain the mechanical properties of PTFE based on the results of uniaxial tensile simulation.

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