A method for simulating mechanical properties of polymer interfaces based on molecular dynamics

Abstract

The application discloses a polymer interface mechanics performance simulation method based on molecular dynamics, and comprises the following steps: setting the polymerization degree and the number of molecular chains according to the molecular formula of a polymer material, constructing a double-layer molecular chain interface model of the polymer in a fused deposition forming process, performing rapid compression simulation on the double-layer molecular chain interface model of the polymer, calculating the simulation density, and optimizing the double-layer molecular chain interface model of the polymer in the rapid compression simulation according to the error between the simulation density and the actual density of the polymer; setting the boundary conditions, the force field, the polymer forming temperature and the molecular dynamics relaxation of 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 polymer interface mechanics performance. The application can simulate and analyze the polymer interface mechanics performance of the fused deposition forming from the microscale, and improves the accuracy.

Description

Technical Field

[0001] This invention belongs to the field of additive manufacturing technology, specifically, it relates to a method for simulating the mechanical properties of polymer interfaces based on molecular dynamics. Background Technology

[0002] Polymer materials are hailed as the cornerstone of modern industry. Based on their material properties, polymers are classified into thermoplastic polymers and thermosetting polymers. As researchers continue to discover the superior properties of polymer materials, such as heat resistance, impact resistance, corrosion resistance, and wear resistance, their applications have expanded to multiple industries, including automotive, machining, aerospace, and electronics. Particularly in the aerospace field, they have successfully replaced some metal materials such as aluminum alloys in the manufacture of components, demonstrating excellent strength and lightweight performance. However, with the increasing demand for complex and precision components in these industries, traditional forming methods have limitations in manufacturing complex polymer structures, restricting the application range of polymer materials.

[0003] Additive manufacturing is a technology that constructs parts by building up materials layer by layer based on a three-dimensional digital model. There are many types of additive manufacturing technologies, applicable to almost all types of materials. Among them, fused deposition modeling (FDM), as one of the mainstream technologies in additive manufacturing, is considered a potential solution for manufacturing complex polymer parts due to its mold-free nature and high design freedom. However, when using FDM to process polymer materials, the resulting parts are prone to insufficient bonding strength between layers, making these interlayer spaces weak points in the structure and affecting the overall mechanical properties of the part. This, to some extent, limits the application scope of FDM in the manufacturing of high-performance polymer parts.

[0004] Molecular dynamics simulations are an effective tool for studying interfacial properties and local physical phenomena. As a powerful research tool, molecular dynamics simulations have been widely used to optimize various polymer properties, including mechanical, thermal, and electrical properties. By simulating the microscopic behavior and mechanical properties of molecular chains in specific environments, molecular chain characteristics can be designed to improve target performance. However, when using molecular dynamics to study polymer interfacial strength, significant limitations in time and space scale mean that simulation durations are limited to nanoseconds to microseconds, and the scale of polymers is confined to hundreds of thousands of atoms. This fails to cover the long timescales of actual polymer interfacial failure processes and the large spatial extent of macroscopic interfaces. Furthermore, simulation conditions differ from reality, and simplified boundary conditions and idealized loading methods do not match actual interfacial conditions, leading to inaccuracies in the simulation of polymer interfacial mechanical properties. Summary of the Invention

[0005] To address the problems existing in the prior art, this invention provides a method for simulating the mechanical properties of polymer interfaces based on molecular dynamics. This method simulates and analyzes the mechanical properties of polymer interfaces formed by fused deposition modeling at the microscale, thereby promoting the manufacture of complex structural parts that meet mechanical standards using fused deposition modeling technology.

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

[0007] Step S1: Based on the molecular formula of the polymer material, set the degree of polymerization and number of molecular chains, and construct a bilayer molecular chain interface model of the polymer during the melt deposition modeling process;

[0008] Step S2: Perform a rapid compression simulation of the polymer bilayer molecular chain interface model using the deform command in the molecular dynamics simulation software, calculate the simulated density, and optimize the polymer bilayer molecular chain interface model in the rapid compression simulation based on the error between the simulated density and the actual polymer density.

[0009] Step S3: Set boundary conditions, force field, polymer molding temperature, and molecular dynamics relaxation for the optimized polymer bilayer molecular chain interface model to obtain the polymer interface fusion model.

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

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

[0012] Furthermore, when the number 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.

[0013] Furthermore, the specific process of constructing the bilayer molecular chain interface model of the polymer during the fused deposition modeling process is as follows: the construction temperature of the polymer bilayer molecular chain interface model is maintained at the polymer forming temperature, the simulation time step is set to 0.1 femtoseconds, the total energy of the polymer bilayer molecular chain interface model under the PCFF force field is calculated under the simulation time, and the energy of the polymer bilayer molecular chain interface model under the PCFF force field is updated by the steepest descent method until the total energy is minimized, thus obtaining a stable polymer bilayer molecular chain interface model.

[0014] Furthermore, the energy under the PCFF force field includes: angular-angular out-of-plane vibrational energy, angular bending energy, bond stretching energy, bond-angular bending energy, bond-bond bending energy, and dihedral torsional energy within the polymer molecule.

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

[0016] Furthermore, the process of setting the boundary conditions is as follows: periodic boundary conditions p are applied in the x, y, and z directions of the optimized polymer bilayer molecular chain interface model.

[0017] Furthermore, the process of setting the boundary conditions is as follows: periodic boundary conditions p are applied in the x and y directions of the optimized polymer bilayer molecular chain interface model, and a vacuum layer is applied in the z direction of the optimized polymer bilayer molecular chain interface model.

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

[0019] Furthermore, the molecular dynamics relaxation setting process involves controlling the temperature and pressure of the optimized polymer bilayer molecular chain interface model using a canonical system and an isothermal-isobaric system based on the polymer's material properties.

[0020] Compared with existing technologies, the present invention has the following advantages: The polymer bilayer molecular chain interface model constructed by the polymer interface mechanical property simulation method based on molecular dynamics of the present invention is at the atomic level, and the simulation time scale reaches the femtosecond level. It can simulate and analyze the interface mechanical properties of polymers formed by melt deposition at the microscopic scale. Compared with the first-principles simulation method, the spatial scale of the simulation of the present invention is significantly increased, reaching 10^6 times. 4 Furthermore, by applying periodic boundary conditions, errors caused by the size effect in polymer interface models can be overcome to some extent. This invention, based on a molecular dynamics-based method for simulating the mechanical properties of polymer interfaces in fused deposition modeling (FDM), simulates and analyzes the mechanical properties of polymer interfaces at the microscopic scale. This promotes the fabrication of complex structural components that meet mechanical standards using FDM technology, freeing product design from the constraints of traditional processes. Consequently, it accelerates the R&D process in fields such as innovative medical devices and aircraft components, fills gaps in basic research in additive manufacturing, and provides new ideas and methods for developing various high-performance thermoplastic materials. Attached Figure Description

[0021] Figure 1This is a flowchart of the polymer interface mechanical property simulation method based on molecular dynamics of the present invention;

[0022] Figure 2 A schematic diagram of the bilayer molecular chain interface model of polyetheretherketone;

[0023] Figure 3 A schematic diagram of the simulated density after rapid compression simulation of the bilayer molecular chain interface model of polyetheretherketone (PEEK).

[0024] Figure 4 The mean square displacement curves of polyetheretherketone at different temperatures are shown.

[0025] Figure 5 A schematic diagram of interface fusion for the bilayer molecular chain interface model of polyetheretherketone after rapid compression simulation;

[0026] Figure 6 This is a schematic diagram of the interfacial tensile properties at different temperatures for a polyetheretherketone (PEEK) interfacial fusion model. Detailed Implementation

[0027] The technical solution of the present invention will be further explained and described below with reference to the accompanying drawings.

[0028] like Figure 1 This is a flowchart of the polymer interface mechanical property simulation method based on molecular dynamics of the present invention. The polymer interface mechanical property simulation method includes the following steps:

[0029] Step S1: Based on the molecular formula of the polymer material, set the degree of polymerization and number of molecular chains, and construct a bilayer molecular chain interface model of the polymer during the melt deposition modeling process. In this invention, the bilayer molecular chain interface model of the polymer consists 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 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.

[0030] The specific process for constructing a bilayer molecular chain interface model of polymers during fused deposition modeling is as follows: The construction temperature of the polymer bilayer molecular chain interface model is maintained at the polymer's forming temperature. The simulation time step is set to 0.1 femtoseconds. The total energy of the polymer bilayer molecular chain interface model under the PCFF force field is calculated for the simulation time. The energy of the polymer bilayer molecular chain interface model under the PCFF force field is updated using the steepest descent method until the total energy is minimized, resulting in a stable polymer bilayer molecular chain interface model. The energy under the PCFF force field includes: angular-angle-out-of-plane vibrational energy, angular bending energy, bond stretching energy, bond-angular bending energy, bond-bond bending energy, and dihedral torsional energy within the polymer molecules. The polymer bilayer molecular chain interface model constructed in this invention has an atomic scale and a simulation time scale reaching femtoseconds. It can simulate and analyze the mechanical properties of polymer interfaces formed by fused deposition modeling at a microscopic scale. Compared with first-principles simulation methods, the spatial scale of the simulation in this invention is significantly increased, reaching 10^- ... 4 atom.

[0031] Step S2: The polymer bilayer molecular chain interface model is rapidly compressed using the deform command in molecular dynamics simulation software. The simulated density is calculated, and the rapidly compressed simulation model is optimized based on the error between the simulated density and the actual polymer density. Specifically, if the error between the simulated density and the actual polymer density exceeds 1%, the z-axis of the bilayer molecular chain interface model is rapidly compressed with a time step of 0.1 femtoseconds until the error between the simulated density and the actual polymer density is within 1%. This invention reduces the error between the polymer bilayer molecular chain interface model and the real polymer through rapid compression simulation, and the operation is simple.

[0032] The bilayer molecular chain interface model of the polymer was verified using molecular dynamics simulation software: the simulated density was calculated based on rapid compression simulation, the glass transition temperature of the polymer was calculated based on mean square displacement simulation, and the melting temperature of the polymer was calculated based on 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 bilayer molecular chain interface model of the polymer constructed by this invention is correct.

[0033] Step S3: Set boundary conditions, force field, polymer forming temperature, and molecular dynamics relaxation for the optimized polymer bilayer molecular chain interface model to obtain the polymer interface fusion model.

[0034] In this invention, the force field is set as a PFCC force field, and the molecular dynamics relaxation setting process is as follows: based on the material properties of the polymer, the temperature and pressure of the optimized polymer bilayer molecular chain interface model are controlled by a canonical system and an isothermal and isobaric system, thereby simulating the setting parameters under different processes.

[0035] In one technical solution of the present invention, the process of setting boundary conditions is as follows: periodic boundary conditions p are applied in the x, y, and z directions of the optimized polymer bilayer molecular chain interface model. By applying periodic boundary conditions, errors 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 boundary conditions is as follows: periodic boundary conditions p are applied in the x and y directions of the optimized polymer bilayer molecular chain interface model, and a vacuum layer is applied in the z direction of the optimized polymer bilayer molecular chain interface model.

[0037] Step S4: Perform uniaxial tensile testing on the polymer interface fusion model under different process parameters to simulate the mechanical properties of the polymer interface, plot stress-strain curves, and evaluate the interfacial tensile strength and elastic modulus of the polymer based on the stress-strain curves. The uniaxial tensile simulation involves fused deposition modeling process parameters including temperature, pressure, and strain rate.

[0038] This invention provides a molecular dynamics-based method for simulating the mechanical properties of polymer interfaces at the microscale. This method simulates and analyzes the mechanical properties of polymer interfaces formed by fused deposition modeling (FDM), promoting the manufacture of complex structural parts that meet mechanical standards using FDM technology. This frees product design from the constraints of traditional processes, thereby accelerating the research and development process in fields such as innovative medical devices and aircraft components. It also fills the gaps in basic research related to additive manufacturing and provides new ideas and methods for developing various high-performance thermoplastic materials.

[0039] Example

[0040] This embodiment provides a systematic molecular dynamics simulation method for the melt deposition modeling printing process of a thermoplastic semi-crystalline polymer material, polyetheretherketone (PEEK). The simulation is performed using molecular dynamics modeling software EMC and simulation software LAMMPS.

[0041] (1) Construction of the bilayer molecular chain interface model of polyetheretherketone

[0042] First, based on the unit chemical structure of polyetheretherketone (PEEK), a SMILE string for the molecular structure is established. Then, using EMC software, PEEK single chains with a degree of polymerization of 5 are randomly generated. The number of PEEK molecular chains in the extrusion layer model is set to 63, and the number of PEEK molecular chains in the deposition layer model is set to 58. The energy under the PCFF force field is obtained, including: angular-angle-out-of-plane vibrational energy, angular bending energy, bond stretching energy, bond-angular bending energy, bond-bond bending energy, and dihedral torsional energy within the PEEK molecule.

[0043] The temperature was maintained at 300K, and a sufficiently long simulation duration was set with a time step of 0.1 femtoseconds. The total energy of the polymer bilayer molecular chain interface model under the PCFF force field was calculated for the simulation duration. The energy of the polymer bilayer molecular chain interface model under the PCFF force field was updated using the steepest descent method until the total energy was minimized. After 10,000 iterations of optimization, a stable polyetheretherketone (PEEK) bilayer molecular chain interface model was obtained. Figure 2 As shown.

[0044] (2) Verification of the bilayer molecular chain interface model of polyetheretherketone

[0045] To verify the accuracy of the bilayer molecular chain interface model of polyetheretherketone (PEEK), the density of the PEEK bilayer molecular chain interface model was first calculated. Under isothermal and isobaric ensemble conditions, the system was cooled to 300 K at 0 atmospheres over 100 picoseconds, yielding a density of approximately 1.14 g / cm³. 3 Then, the bilayer molecular chain interface model of polyetheretherketone was rapidly compressed, followed by 250 picoseconds of isothermal relaxation, and then 20 picoseconds and 50 picoseconds of relaxation under canonical and isothermal-isobaric ensembles, respectively. Figure 3 As shown, the final simulated density is 1.27 g / cm³. 3 It is close to its actual density.

[0046] The glass transition temperature is calculated by characterizing the mean square displacement at different temperatures. Polyetheretherketone (PEEK) exhibits abrupt changes in its mean square displacement near the glass transition temperature, such as... Figure 4 As shown, the glass transition temperature of polyetheretherketone (PEEK) is 415 K, which is close to the actual glass transition temperature of 416 K. The formula for calculating this mean square displacement is:

[0047]

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

[0049] The melting temperature was calculated using a solid-liquid coexistence method, employing the keyword `aniso` in LAMMPS. Under ideal conditions and within an infinite simulation time, the state evaluation criteria for polyetheretherketone (PEEK) are as follows:

[0050] When the temperature of the bilayer molecular chain interface model of polyetheretherketone is lower than the melting temperature, it indicates that polyetheretherketone is completely crystallized.

[0051] When the temperature of the bilayer molecular chain interface model of polyetheretherketone is higher than the melting temperature, it indicates that polyetheretherketone is completely melted.

[0052] In the melt temperature simulation, due to time scale limitations, the bilayer molecular chain interface model of polyetheretherketone (PEEK) is considered as partially melted. The melt temperature of PEEK can be obtained by the slope change of the normalized relative temperature of the volume at room temperature. In this example, the melt temperature of PEEK is found to be in the range of 613–633 K, which is consistent with the actual melt temperature.

[0053] (3) Simulation of interfacial fusion of polyetheretherketone

[0054] Under a standard canonical ensemble, a bilayer molecular chain interface model of rapidly compressed polyetheretherketone (PEEK) was subjected to isothermal heating to simulate the interleaving and bonding states of adjacent PEEK molecular chains during melt deposition modeling. Figure 5 As shown:

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

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

[0057] When the time exceeds 50 picoseconds, the polyetheretherketone molecular chains are completely fused.

[0058] Subsequently, under constant temperature and pressure ensemble, the entire system reached equilibrium, realizing the gradual fusion of polyether ether ketone (PEEK) material from the interlaced molecular chain state to the interface, thus obtaining the interface fusion model of PEEK.

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

[0060] Interfacial tensile simulation experiments were conducted under isothermal and isobaric ensemble conditions. The average stress of the polyetheretherketone (PEEK) material system was obtained using the following virial formula:

[0061]

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

[0063] Regarding the study of the effects of different temperatures, as the temperature increases, such as Figure 6 As shown, the interfacial tensile strength and elastic modulus of polyetheretherketone both decreased significantly.

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

[0065] The study on the effect of different strain rates was conducted by setting different strain rates (5×10). 7 s- 1 ~5×10 11 s- 1 Stress-strain curves at different strain rates were obtained, and strain contour plots at different strain rates were obtained using OVITO software.

[0066] The above are merely preferred embodiments of the present invention. The scope of protection of the present invention is not limited to the above embodiments. All technical solutions falling within the scope of the present invention's concept are within the scope of protection of the present invention. It should be noted that for those skilled in the art, any improvements and modifications made without departing from the principles of the present invention should be considered within the scope of protection of the present invention.

Claims

1. A method for simulating the mechanical properties of polymer interfaces based on molecular dynamics, characterized in that, Specifically, the steps include the following: Step S1: Based on the molecular formula of the polymer material, set the degree of polymerization and number of molecular chains, and construct a bilayer molecular chain interface model of the polymer during the melt deposition modeling process; the bilayer molecular chain interface model of the polymer consists of an upper extrusion layer model and a lower deposition layer model, wherein the extrusion layer model and the deposition layer model have the same or different numbers of molecular chains; The specific process of constructing a bilayer molecular chain interface model of polymer during fused deposition modeling is as follows: the construction temperature of the bilayer molecular chain interface model of polymer is maintained at the forming temperature of polymer, the simulation time step is set to 0.1 femtoseconds, the total energy of the bilayer molecular chain interface model of polymer under PCFF force field is calculated under the simulation time, and the energy of the bilayer molecular chain interface model of polymer under PCFF force field is updated by steepest descent method until the total energy is minimized, thus obtaining a stable bilayer molecular chain interface model of polymer. Step S2: Perform a rapid compression simulation of the polymer bilayer molecular chain interface model using the deform command in the molecular dynamics simulation software, calculate the simulated density, and optimize the polymer bilayer molecular chain interface model in the rapid compression simulation based on the error between the simulated density and the actual polymer density. Step S3: Set boundary conditions, force field, polymer molding temperature, and molecular dynamics relaxation for the optimized polymer bilayer molecular chain interface model to obtain the polymer interface fusion model. Step S4: Perform uniaxial stretching on the polymer interface fusion model under different process parameters to simulate the mechanical properties of the polymer interface.

2. The method for simulating the mechanical properties of polymer interfaces based on molecular dynamics according to claim 1, characterized in that, When the number of molecular chains is set differently in the extrusion layer model and the deposition layer model, the number of molecular chains in the extrusion layer model is greater than that in the deposition layer model.

3. The method for simulating the mechanical properties of polymer interfaces based on molecular dynamics according to claim 1, characterized in that, The energy under the PCFF force field includes: angular-angular out-of-plane vibrational energy, angular bending energy, bond stretching energy, bond-angular bending energy, bond-bond bending energy, and dihedral torsional energy within polymer molecules.

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

5. The method for simulating the mechanical properties of polymer interfaces based on molecular dynamics according to claim 4, characterized in that, The process of setting the boundary conditions is as follows: periodic boundary conditions p are applied in the x, y, and z directions of the optimized polymer bilayer molecular chain interface model.

6. The method for simulating the mechanical properties of polymer interfaces based on molecular dynamics according to claim 4, characterized in that, The process of setting the boundary conditions is as follows: periodic boundary conditions p are applied in the x and y directions of the optimized polymer bilayer molecular chain interface model, and a vacuum layer is applied in the z direction of the optimized polymer bilayer molecular chain interface model.

7. The method for simulating the mechanical properties of polymer interfaces based on molecular dynamics according to claim 4, characterized in that, The force field is set as a PFCC force field.

8. The method for simulating the mechanical properties of polymer interfaces based on molecular dynamics according to claim 4, characterized in that, The process of setting the molecular dynamics relaxation is as follows: based on the material properties of the polymer, the temperature and pressure of the optimized polymer bilayer molecular chain interface model are controlled using a canonical system and an isothermal and isobaric system.

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