Polymer interface mechanical property simulation method based on molecular dynamics

Abstract

The invention discloses a molecular dynamics-based polymer interface mechanical property simulation method, which comprises the following steps of: setting the polymerization degree and the number of molecular chains according to a molecular formula of a polymer material, and constructing a double-layer molecular chain interface model of a polymer in a fused deposition forming process, performing quick compression simulation on the double-layer molecular chain interface model of the polymer, calculating simulation density, and optimizing the double-layer molecular chain interface model of the polymer subjected to quick compression simulation according to an error between the simulation density and the actual density of the polymer; performing boundary condition setting, force field setting, polymer forming temperature setting and molecular dynamics relaxation setting on the optimized double-layer molecular chain interface model of the polymer to obtain a polymer interface fusion model; and performing uniaxial stretching on the polymer interface fusion model under different process parameters to simulate the mechanical properties of the polymer interface. The mechanical property of the fused deposition formed polymer interface can be simulated and analyzed on the microscopic scale, and the accuracy is improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of additive manufacturing, and in particular, relates to a method for simulating polymer interface mechanical properties based on molecular dynamics. Background Art

[0002] Polymer materials are known as the cornerstone of modern industry. According to different material properties, polymers are divided into thermoplastic polymers and thermosetting polymers. As researchers continue to explore the excellent properties of polymer materials such as heat resistance, impact resistance, corrosion resistance and wear resistance, their application range has expanded to multiple industries such as automobiles, mechanical processing, aerospace, and electronic appliances. Especially in the field of aerospace, it has successfully replaced some metal materials such as aluminum alloys to manufacture parts, showing excellent strength and lightweight effects. With the increasing demand for complex and precision parts in these industries, traditional forming methods have limitations in manufacturing complex polymer structural parts, which limits the application range of polymer materials.

[0003] Additive manufacturing is a technology that constructs parts by adding materials layer by layer based on a three-dimensional digital model. There are many types of additive manufacturing technologies, which are applicable to almost all types of materials. Among them, fused deposition modeling, as one of the mainstream technologies in the field of additive manufacturing, is regarded as a potential solution for manufacturing complex polymer parts because it does not require molds and has high design freedom. However, when using fused deposition modeling technology to process polymer materials, the formed parts are prone to insufficient bonding strength between layers, causing the layers to become the weak link of the structure, which in turn affects the overall mechanical properties of the parts. This has limited the application scope of fused deposition modeling technology in the manufacture of high-performance polymer parts to a certain extent.

[0004] Theoretical research based on molecular dynamics simulation is an effective means to study interface properties and local physical phenomena. As an effective research tool, molecular dynamics simulation has been widely used to optimize various properties of polymers, including mechanics, thermal conductivity, electrical conductivity, etc. By simulating the microscopic behavior and mechanical properties of molecular chains in a specific environment, the characteristics of molecular chains are designed to improve the target performance. However, when using molecular dynamics to study the strength of polymer interfaces, due to the obvious limitations of time and space scales, the simulation time is only in nanoseconds to microseconds, and the scale of polymers is limited to hundreds of thousands of atoms. It cannot cover the long time scale of the actual polymer material interface destruction process and the large spatial range of the macroscopic interface. There are differences between the simulation conditions and the actual conditions. The simplified boundary conditions and idealized loading methods are inconsistent with the actual interface conditions, resulting in inaccurate simulation of the mechanical properties of the polymer interface. Summary of the invention

[0005] In response to the problems existing in the prior art, the present invention provides a method for simulating the mechanical properties of polymer interfaces based on molecular dynamics, which simulates and analyzes the mechanical properties of polymer interfaces formed by molten deposition at a microscopic scale, and promotes the use of molten deposition technology to manufacture complex structural parts that meet mechanical standards.

[0006] To achieve the above technical objectives, the present invention adopts the following technical solution: a method for simulating polymer interface mechanical properties based on molecular dynamics, specifically comprising the following steps:

[0007] Step S1, according to the molecular formula of the polymer material, the degree of polymerization and the number of molecular chains are set to construct a double-layer molecular chain interface model of the polymer during the fused deposition modeling process;

[0008] Step S2, performing a rapid compression simulation on the double-layer molecular chain interface model of the polymer by using the deform command in the molecular dynamics simulation software, calculating the simulated density, and optimizing the double-layer molecular chain interface model of the polymer by the rapid compression simulation according to the error between the simulated density and the actual density of the polymer;

[0009] Step S3, setting boundary conditions, force field, polymer forming temperature and molecular dynamics relaxation for the optimized polymer double-layer molecular chain interface model to obtain a polymer interface fusion model;

[0010] Step S4: subjecting the polymer interface fusion model to uniaxial stretching under different process parameters to simulate the mechanical properties of the polymer interface.

[0011] Furthermore, the double-layer molecular chain interface model of the polymer consists of an upper extrusion layer model and a lower deposition layer model, and the extrusion layer model and the deposition layer model have the same or different numbers of molecular chains.

[0012] Further, when the number of molecular chains set in the extrusion layer model and the deposition layer model is different, the number of molecular chains in the extrusion layer model is greater than the number of molecular chains in the deposition layer model.

[0013] Furthermore, the specific process of constructing the double-layer molecular chain interface model of the polymer in the molten deposition modeling process is: keeping the construction temperature of the double-layer molecular chain interface model of the polymer at the forming temperature of the polymer, setting the step size of the simulation time to 0.1 femtoseconds, calculating the total energy of the double-layer molecular chain interface model of the polymer under the PCFF force field during the simulation time, and updating the energy of the double-layer molecular chain interface model of the polymer under the PCFF force field by the steepest descent method until the total energy is minimized, thereby obtaining a stable double-layer molecular chain interface model of the polymer.

[0014] Furthermore, the energy under the PCFF force field includes angle-angle out-of-plane vibration energy, angle bending energy, bond stretching energy, bond-angle bending energy, bond-bond bending energy and dihedral angle torsion energy within the polymer molecules.

[0015] Furthermore, based on the error between the simulated density and the actual density of the polymer, the process of optimizing the double-layer molecular chain interface model of the polymer under rapid compression simulation is as follows: if the error between the simulated density and the actual density of the polymer exceeds 1%, the z-axis direction of the double-layer molecular chain interface model is rapidly compressed at a time step of 0.1 femtoseconds until the error between the simulated density and the actual density of the polymer is within 1%.

[0016] Furthermore, the process of setting the boundary conditions is: applying periodic boundary conditions p in the three directions of x, y and z of the optimized double-layer molecular chain interface model of the polymer respectively.

[0017] Furthermore, the process of setting the boundary conditions is: applying periodic boundary conditions p in the x and y directions of the optimized polymer double-layer molecular chain interface model respectively, and applying a vacuum layer in the z direction of the optimized polymer double-layer molecular chain interface model.

[0018] Furthermore, the force field is set to be a PFCC force field.

[0019] Furthermore, the molecular dynamics relaxation setting process is: using a canonical system and an isothermal and isobaric system to control the temperature and pressure of the optimized polymer double-layer molecular chain interface model according to the material properties of the polymer.

[0020] Compared with the prior art, the present invention has the following beneficial effects: the scale of the double-layer molecular chain interface model of the polymer constructed by the molecular dynamics-based polymer interface mechanical property simulation method of the present invention is at the atomic level, and the scale of the simulation time reaches femtoseconds, which can simulate and analyze the polymer interface mechanical properties formed by molten deposition at a microscopic scale. Compared with the first principle simulation method, the spatial scale of the simulation of the present invention is significantly increased to 10 4 Atoms, and by applying periodic boundary conditions, the error caused by the size effect of the polymer interface model can be overcome to a certain extent. The polymer interface mechanical properties simulation method based on molecular dynamics of the present invention simulates and analyzes the polymer interface mechanical properties of fused deposition forming at a microscopic scale, promotes the fused deposition forming technology to manufacture complex structural parts that meet mechanical standards, and makes product design no longer constrained by traditional processes, thereby accelerating the research and development process in the fields of innovative medical devices, aircraft parts, etc., filling the gap in related basic research in the field of additive manufacturing, and providing new ideas and methods for the development of various high-performance thermoplastic materials. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1It is a flow chart of the method for simulating polymer interface mechanical properties based on molecular dynamics of the present invention;

[0022] Figure 2 Schematic diagram of the double-layer molecular chain interface model of polyetheretherketone;

[0023] Figure 3 It is a schematic diagram of the simulation density after the rapid compression simulation of the double-layer molecular chain interface model of polyetheretherketone;

[0024] Figure 4 is the mean square displacement curve of polyetheretherketone at different temperatures;

[0025] Figure 5 Schematic diagram of interface fusion of the double-layer molecular chain interface model of polyetheretherketone after rapid compression simulation;

[0026] Figure 6 Schematic diagram of the interfacial tensile properties of the polyetheretherketone interfacial fusion model at different temperatures. DETAILED DESCRIPTION

[0027] The technical solution of the present invention is further explained below in conjunction with the accompanying drawings.

[0028] like Figure 1 The flowchart of the polymer interface mechanical properties simulation method based on molecular dynamics of the present invention comprises the following steps:

[0029] Step S1. According to the molecular formula of the polymer material, the degree of polymerization and the number of molecular chains are set to construct a double-layer molecular chain interface model of the polymer in the molten deposition forming process. The double-layer molecular chain interface model of the polymer in the present invention is composed of an upper extrusion layer model and a lower deposition layer model. The extrusion layer model and the deposition layer model have the same or different numbers of molecular chains. When the numbers of molecular chains set in the extrusion layer model and the deposition layer model are different, the number of molecular chains in the extrusion layer model is greater than the number of molecular chains in the deposition layer model.

[0030] The specific process of constructing the double-layer molecular chain interface model of the polymer during the molten deposition modeling process is as follows: the construction temperature of the double-layer molecular chain interface model of the polymer is maintained at the forming temperature of the polymer, the step length of the simulation time is set to 0.1 femtoseconds, the sum of the energy of the double-layer molecular chain interface model of the polymer under the PCFF force field during the simulation time is calculated, and the energy of the double-layer molecular chain interface model of the polymer under the PCFF force field is updated by the steepest descent method until the sum of the energy is minimized, thereby obtaining a stable double-layer molecular chain interface model of the polymer. Among them, the energy under the PCFF force field includes: angle-angle out-of-plane vibration energy, angle bending energy, bond stretching energy, bond-angle bending energy, bond-bond bending energy and dihedral torsion energy within the polymer molecules. The scale of the double-layer molecular chain interface model of the polymer constructed by the present invention is at the atomic level, and the scale of the simulation time reaches femtoseconds. It can simulate and analyze the mechanical properties of the polymer interface formed by molten deposition from a microscopic scale. Compared with the first-principles simulation method, the spatial scale of the simulation of the present invention is significantly increased to 10 4 atom.

[0031] Step S2, through the deform command in the molecular dynamics simulation software, the double-layer molecular chain interface model of the polymer is quickly compressed and simulated, the simulated density is calculated, and according to the error between the simulated density and the actual density of the polymer, the double-layer molecular chain interface model of the polymer that is quickly compressed and simulated is optimized. Specifically, if the error between the simulated density and the actual density of the polymer exceeds 1%, the z-axis direction of the double-layer molecular chain interface model is quickly compressed according to a time step of 0.1 femtoseconds until the error between the simulated density and the actual density of the polymer is within 1%. The present invention reduces the error between the double-layer molecular chain interface model of the polymer and the real polymer through rapid compression simulation, and the operation is simple.

[0032] The double-layer molecular chain interface model of the polymer is verified using molecular dynamics simulation software: the simulated density is calculated based on the rapid compression simulation, the glass transition temperature of the polymer is calculated based on the mean square displacement simulation, and the melting temperature of the polymer is calculated based on the normalized volume simulation. It can be seen that the simulated density, glass transition temperature and melting temperature calculated by this method are in good agreement with the actual density, glass transition temperature and melting temperature of the polymer, indicating that the double-layer molecular chain interface model of the polymer constructed by the present invention is correct.

[0033] Step S3, setting boundary conditions, force field, polymer forming temperature and molecular dynamics relaxation of the optimized polymer double-layer molecular chain interface model to obtain a polymer interface fusion model.

[0034] In the present invention, the force field is set to a PFCC force field, and the molecular dynamics relaxation setting process is: according to the material properties of the polymer, the canonical system and the isothermal and isobaric system are used to control the temperature and pressure of the double-layer molecular chain interface model of the optimized polymer, thereby simulating the setting parameters under different processes.

[0035] In one technical solution of the present invention, the process of setting the boundary conditions is: applying periodic boundary conditions p in the three directions of x, y, and z of the optimized polymer double-layer molecular chain interface model. By applying periodic boundary conditions, the error caused by the size effect of the polymer interface model can be overcome to a certain extent.

[0036] In another technical solution of the present invention, the process of setting the boundary conditions is: applying periodic boundary conditions p in the x and y directions of the optimized polymer double-layer molecular chain interface model respectively, and applying a vacuum layer in the z direction of the optimized polymer double-layer molecular chain interface model.

[0037] Step S4, subjecting the polymer interface fusion model to uniaxial stretching under different process parameters, simulating the mechanical properties of the polymer interface, drawing a stress-strain curve, and evaluating the interface tensile strength and elastic modulus of the polymer according to the stress-strain curve. The fused deposition modeling process parameters involved in the uniaxial stretching simulation include temperature, pressure, and strain rate.

[0038] The polymer interface mechanical properties simulation method based on molecular dynamics of the present invention simulates and analyzes the polymer interface mechanical properties of fused deposition modeling at a microscopic scale, promotes the fused deposition modeling technology to manufacture complex structural parts that meet mechanical standards, and makes product design no longer constrained by traditional processes, thereby accelerating the research and development process in the fields of innovative medical devices, aircraft parts, etc., filling the gap in related basic research in the field of additive manufacturing, and providing new ideas and methods for the development of various high-performance thermoplastic materials.

[0039] Example

[0040] This embodiment provides a systematic molecular dynamics simulation method for the fused deposition modeling printing process of a thermoplastic semi-crystalline polymer material, polyetheretherketone, which uses molecular dynamics modeling software EMC and simulation software LAMMPS for simulation:

[0041] (1) Construction of the double-layer molecular chain interface model of polyetheretherketone

[0042] Firstly, based on the unit chemical structure of polyetheretherketone, the smiles string of the molecular structure was established. Then, the polyetheretherketone single chain with a degree of polymerization of 5 was randomly generated through the EMC software. The number of molecular chains of polyetheretherketone in the extrusion layer model was set to 63, and the number of molecular chains of polyetheretherketone in the deposition layer model was set to 58. The energy under the PCFF force field was obtained, including: angle-angle out-of-plane vibration energy, angle bending energy, bond stretching energy, bond-angle bending energy, bond-bond bending energy and dihedral angle torsion energy within the polyetheretherketone molecule.

[0043] The temperature was kept at 300K, the simulation time was set to be long enough, the time step was set to 0.1 femtoseconds, the total energy of the double-layer molecular chain interface model of the polymer under the PCFF force field was calculated during the simulation time, and the energy of the double-layer molecular chain interface model of the polymer under the PCFF force field was updated by the steepest descent method until the total energy was minimized. After 10,000 iterations of optimization, a stable double-layer molecular chain interface model of polyetheretherketone was obtained, as shown in Figure 2 shown.

[0044] (2) Verification of the double-layer molecular chain interface model of polyetheretherketone

[0045] In order to verify the accuracy of the double-layer molecular chain interface model of polyetheretherketone, the density of the double-layer molecular chain interface model of polyetheretherketone was first calculated. Under the constant temperature and pressure ensemble, the system was cooled to 300K after 100 picoseconds at 0 atmosphere, and the density of the double-layer molecular chain interface model of polyetheretherketone was close to 1.14 g / cm 3 Then, the double-layer molecular chain interface model of polyetheretherketone was rapidly compressed, and the isothermal relaxation was performed for 250 picoseconds. The relaxation was performed for 20 picoseconds and 50 picoseconds under the canonical and constant temperature and pressure ensembles, respectively. Figure 3 As shown, the final simulated density is 1.27 g / cm 3 , close to its actual density.

[0046] The calculation of glass transition temperature is characterized by the mean square displacement of different system temperatures. The mean square displacement of polyetheretherketone changes suddenly near the glass transition temperature, such as Figure 4 As shown, the glass transition temperature of polyetheretherketone is 415K, which is close to the glass transition temperature of the actual material 416K. The calculation formula of the mean square displacement is:

[0047]

[0048] Among them, S MSD represents mean square displacement; R i (t) and R i (0) represents the displacement vector of any atom i in the system at time t and the initial moment, respectively, and N represents the total number of atoms.

[0049] The melting temperature is calculated using the solid-liquid coexistence method, using the keyword aniso in LAMMPS. Under ideal conditions and in infinite simulation time, the state evaluation criteria for polyetheretherketone are as follows:

[0050] When the temperature of the double-layer molecular chain interface model of polyetheretherketone is lower than the melting temperature, it means that polyetheretherketone is completely crystallized;

[0051] When the temperature of the double-layer molecular chain interface model of polyetheretherketone is greater than the melting temperature, it means that the polyetheretherketone is completely melted.

[0052] In the melting temperature simulation, due to the limitation of time scale, the double-layer molecular chain interface model of polyetheretherketone is regarded as partially melted. The melting temperature of polyetheretherketone material can be obtained by the slope change of normalized relative temperature of volume at room temperature. In this example, the melting temperature of polyetheretherketone is in the range of 613-633K, which is consistent with the actual melting temperature.

[0053] (3) Interface fusion simulation of polyetheretherketone

[0054] The double-layer molecular chain interface model of rapidly compressed polyetheretherketone was subjected to constant temperature heating treatment under the standard canonical ensemble to simulate the interlacing and bonding state of two adjacent layers of polyetheretherketone molecular chains during the molten deposition forming process. Figure 5 As shown:

[0055] At the initial moment, the polyetheretherketone molecular chains of the extruded layer and the deposited layer are in a state of cross-linking;

[0056] When the time is less than 50 picoseconds, the polyetheretherketone molecular chains gradually fuse at the interface;

[0057] When the time is greater than 50 picoseconds, the PEEK molecular chains are completely fused;

[0058] Subsequently, under the constant temperature and pressure ensemble, the entire system reached a state of equilibrium, realizing the gradual fusion of the polyetheretherketone material formed by molten deposition from the interlaced molecular chain state to the interface, and obtaining the interface fusion model of polyetheretherketone.

[0059] (4) Tensile simulation of the interfacial fusion model of polyetheretherketone

[0060] The interface tensile simulation experiment was carried out under constant temperature and constant pressure ensemble conditions, and the average stress of the polyetheretherketone material system was obtained by the following Virial formula:

[0061]

[0062] Where V0 is the volume of the interface fusion model of polyetheretherketone; m i is the mass of the ith atom; v i is the velocity vector of the ith atom; r ijis the distance vector between the i-th atom and the j-th atom; f ij is the force exerted on the i-th atom by the j-th atom.

[0063] For the study of the effects of different temperatures, as the temperature increases, e.g. Figure 6 As shown in the figure, the interfacial tensile strength and elastic modulus of polyetheretherketone both showed a significant decrease.

[0064] For the study of the effects of different pressures, different system pressures (1 to 100 atmospheres) were set and stress-strain curves of different pressures were plotted.

[0065] For the study of the effect of different strain rates, different strain rates (5×10 7 s- 1 ~5×10 11 s- 1 ), the stress-strain curves at different strain rates were obtained, and the strain cloud diagrams at different strain rates were obtained by OVITO software.

[0066] The above are only preferred embodiments of the present invention, and the protection scope of the present invention is not limited to the above embodiments. All technical solutions under the concept of the present invention belong to the protection scope of the present invention. It should be pointed out that for ordinary technicians in this technical field, some improvements and modifications without departing from the principle of the present invention should be regarded as the protection scope of the present invention.

Claims

1. A method for simulating polymer interface mechanical properties based on molecular dynamics, characterized in that: The specific steps include: Step S1, according to the molecular formula of the polymer material, the degree of polymerization and the number of molecular chains are set to construct a double-layer molecular chain interface model of the polymer during the fused deposition modeling process; Step S2, performing a rapid compression simulation on the double-layer molecular chain interface model of the polymer by using the deform command in the molecular dynamics simulation software, calculating the simulated density, and optimizing the double-layer molecular chain interface model of the polymer by the rapid compression simulation according to the error between the simulated density and the actual density of the polymer; Step S3, setting boundary conditions, force field, polymer forming temperature and molecular dynamics relaxation for the optimized polymer double-layer molecular chain interface model to obtain a polymer interface fusion model; Step S4: subjecting the polymer interface fusion model to uniaxial stretching under different process parameters to simulate the mechanical properties of the polymer interface.

2. The method for simulating polymer interface mechanical properties based on molecular dynamics according to claim 1, characterized in that: The double-layer molecular chain interface model of the polymer consists of an upper extrusion layer model and a lower deposition layer model, and the extrusion layer model and the deposition layer model have the same or different molecular chain numbers.

3. The method for simulating polymer interface mechanical properties based on molecular dynamics according to claim 2, characterized in that: When the molecular chain numbers set in the extrusion layer model and the deposition layer model are different, the molecular chain number of the extrusion layer model is greater than the molecular chain number of the deposition layer model.

4. The method for simulating polymer interface mechanical properties based on molecular dynamics according to claim 1, characterized in that: The specific process of constructing the double-layer molecular chain interface model of the polymer in the molten deposition modeling process is as follows: the construction temperature of the double-layer molecular chain interface model of the polymer is maintained at the forming temperature of the polymer, the step size of the simulation time is set to 0.1 femtoseconds, the total energy of the double-layer molecular chain interface model of the polymer under the PCFF force field during the simulation time is calculated, and the energy of the double-layer molecular chain interface model of the polymer under the PCFF force field is updated by the steepest descent method until the total energy is minimized, thereby obtaining a stable double-layer molecular chain interface model of the polymer.

5. The method for simulating polymer interface mechanical properties based on molecular dynamics according to claim 4, characterized in that: The energy under the PCFF force field includes angle-angle out-of-plane vibration energy, angle bending energy, bond stretching energy, bond-angle bending energy, bond-bond bending energy and dihedral angle torsion energy within the polymer molecules.

6. The method for simulating polymer interface mechanical properties based on molecular dynamics according to claim 1, characterized in that: According to the error between the simulated density and the actual density of the polymer, the process of optimizing the double-layer molecular chain interface model of the polymer for rapid compression simulation is as follows: if the error between the simulated density and the actual density of the polymer exceeds 1%, the z-axis direction of the double-layer molecular chain interface model is rapidly compressed at a time step of 0.1 femtoseconds until the error between the simulated density and the actual density of the polymer is within 1%.

7. The method for simulating polymer interface mechanical properties based on molecular dynamics according to claim 6, characterized in that: The process of setting the boundary conditions is: applying periodic boundary conditions p in the three directions of x, y and z of the optimized double-layer molecular chain interface model of the polymer respectively.

8. The method for simulating polymer interface mechanical properties based on molecular dynamics according to claim 6, characterized in that: The process of setting the boundary conditions is: applying periodic boundary conditions p in the x and y directions of the optimized polymer double-layer molecular chain interface model respectively, and applying a vacuum layer in the z direction of the optimized polymer double-layer molecular chain interface model.

9. The method for simulating polymer interface mechanical properties based on molecular dynamics according to claim 6, characterized in that: The force field is set to be a PFCC force field.

10. The method for simulating polymer interface mechanical properties based on molecular dynamics according to claim 6, characterized in that: The molecular dynamics relaxation setting process is: according to the material properties of the polymer, the canonical system and the isothermal and isobaric system are used to control the temperature and pressure of the double-layer molecular chain interface model of the optimized polymer.

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