Modeling and thermal conductivity optimization method for polymer material series-parallel semi-crystal model

Abstract

The invention provides a modeling and thermal conductivity optimization method of a polymer material series-parallel semi-crystal model. Compared with a pure two-phase or three-phase series model, the method has the advantages that the complicated model conformation treatment process is not needed, the related physical properties of the semi-crystal polymer can be better represented, the treatment is simple and convenient, the process applicability is wide, and the design requirements of various types of semi-crystal polymer models are met. And meanwhile, a treatment method for further improving the heat-conducting property of the semi-crystal polymer model by utilizing an electric field is provided, and compared with a method for improving the heat-conducting property by directly applying external force to cause the polymer to generate strain, a polymer material with excellent heat-conducting property can be better obtained on the premise of ensuring the mechanical property of the material force. According to the invention, the polymer model under the microscopic scale is constructed, the experiment cost is reduced, the influence factors of the heat-conducting property of the polymer material are explored by fully utilizing a molecular dynamics method, and a theoretical mechanism is provided for preparing a high-performance radiator.

Description

Technical Field

[0001] The invention belongs to the technical field of computer simulation of polymer materials, and in particular relates to a modeling method and a thermal conductivity optimization method of a series-parallel semi-crystalline model of a polymer material. Background Art

[0002] With the vigorous development of aerospace technology, the electronic components of its equipment are developing towards high integration and miniaturization. With the rapid increase in circuit density and load and the reduction in geometric size, it is more difficult to dissipate heat from the structure, and a large amount of heat is generated, which seriously affects the working performance and stability of the equipment. The "10°C rule" in the electronics industry points out that for every 10°C increase in the operating temperature of electronic devices, the failure rate will increase by an order of magnitude. Statistics show that the failure of electronic products caused by poor heat dissipation has reached 55%, which puts more stringent requirements on the thermal conductivity of thermal management materials. In electronic components, the heat generated by the integrated circuit is first transferred to the thermal interface material, and then through the electronic packaging material, the external thermal interface material and the high thermal conductivity material, the heat sink dissipates the heat to the external environment through passive or active heat dissipation. Its main dissipation path can be understood as transmission in the thermal interface material with good insulation. Therefore, it is necessary to improve the thermal conductivity of the thermal interface material to solve the severe challenges faced by the high-performance chip thermal management system.

[0003] In today's material science field, polymer materials have attracted widespread attention due to their unique physical and chemical properties (insulation, high toughness, low density, low price, good corrosion resistance, etc.). Their thermal conductivity is a key material property and is of great significance for many applications, especially when a balance is required between thermal conductivity and insulation. However, due to the low inherent thermal conductivity of polymer materials, which is about 0.1 to 0.5 W / (m·K) at room temperature, the application of polymers in electronic equipment heat dissipation is severely limited. In addition, the low contact thermal conductivity between the polymer-based thermal interface material and the heterogeneous interface between metals further limits its application. Therefore, it is necessary for us to explore the thermal conductivity mechanism of polymer-based thermal interface materials and provide a microscopic theoretical mechanism for the preparation of high thermal conductivity polymer thermal interface materials.

[0004] In order to fully explore the influencing mechanism of thermal conductivity of polymer-based materials, Asegun Henry et al. studied the thermal conductivity of a single polyethylene chain in the paper "Anomalous heat conduction in polyethylene chains: Theory and molecular dynamics simulations" published in the journal "Physical Review B". They found that the abnormally high thermal conductivity of polyethylene single chains along the chain direction is 300W / (m·K), which far exceeds the intrinsic thermal conductivity of the polymer block. However, the theoretical and simulation methods only explored the ideal single-chain polymer molecular chain, and did not conduct a deeper exploration of the factors affecting the thermal conductivity of the polymer block. Jun Liu et al. studied the effect of strain on the thermal conductivity of bulk amorphous polyethylene in the paper "Tuning the thermal conductivity of polymers with mechanical strains" published in the journal "Physical Review B". They found that with the orderly arrangement of some molecular chains in the polymer block, the thermal conductivity increased by 5 times, but the direct application of strain will seriously affect the mechanical properties of the polymer material. In addition, Jixiong He et al. published a paper titled "Molecular dynamics simulation of thermal transport in semicrystalline polyethylene: Roles of strain and the crystalline amorphous interphase region" in the Journal of Applied Physics. They used molecular dynamics simulation to explore the effect of interchain coupling between the crystalline and amorphous regions of an ideal series semicrystalline polyethylene model on thermal conductivity, and found that the topological structure of the interlayer chain determines the dependence of thermal conductivity on strain. These studies used molecular dynamics simulation to explore some of the influencing mechanisms of thermal conductivity of polymer materials at the microscopic scale. In addition, there is a state of local crystallization in polymers, but the current research on semicrystalline polymer models is still limited to two-phase or three-phase series models, and polymer models that have both series and parallel characteristics still need to be further explored.

[0005] Ferroelectric polymer - polyvinylidene fluoride (PVDF), which is a thermoplastic polymer, is composed of -CH2CF2- repeating units. This simple chemical structure gives its molecular chain a high degree of flexibility. Under different conditions, PVDF can present different molecular chain conformations and crystal structures. In addition, due to the CF2 group contained in it, its molecular chain is sensitive to electric fields. When the applied electric field strength is greater than the coercive electric field, the molecular chain segments will change, such as Figure 1-2 As shown in the figure, it compresses in the direction of the electric field and expands perpendicular to the electric field, which has an electrostrictive effect. Based on this, we can obtain PVDF with stretched molecular chains by controlling the coupling effect of the electric field. Compared with other traditional polymers, PVDF has the advantages of high temperature resistance, aging resistance, good insulation, and high crystallinity. Therefore, it is an ideal thermal interface material matrix. Therefore, it is necessary for us to explore the influencing factors of thermal interface materials with PVDF as a polymer matrix to improve the theoretical mechanism for the preparation of high-performance heat sinks.

[0006] The research on high thermal conductivity polymer-based thermal interface materials and the factors affecting the thermal conductivity of their contact with metals is of great strategic significance for achieving global energy conservation and emission reduction, improving energy utilization efficiency and solving electronic thermal packaging problems. Summary of the invention

[0007] The purpose of the present invention is to avoid the shortcomings of the prior art and provide a modeling and thermal conductivity optimization method for a series-parallel semi-crystalline model of a polymer material.

[0008] To achieve the above object, the technical solution adopted by the present invention is: a modeling and thermal conductivity optimization method of a series-parallel semi-crystalline model of a polymer material, characterized in that it includes the following steps:

[0009] Step 1: First, a fully crystalline polymer model of corresponding size is established according to the type of polymer required;

[0010] Step 2: According to the simulation requirements, the central crystalline area is selected as the 'permanent crystalline area', and the other areas are selected as the 'temporary crystalline area', and the covalent bonds between the atoms in the 'permanent crystalline area' and the 'temporary crystalline area' are broken;

[0011] Step 3: Name the main chain atom at the end of the molecular chain in the 'permanent crystalline region' as R1, and name the main chain atom at the end of the molecular chain in the 'temporary crystalline region' as R2, and then randomly form covalent bonds between R1 and R2 atoms according to simulation requirements to obtain Model I with different interchain couplings;

[0012] Step 4: Use the open source software Lammps as the simulation software to perform molecular dynamics simulation on Model I, and obtain a semi-crystalline polymer model that conforms to the series morphology in the three directions of X, Y, and Z, which is called the series-parallel semi-crystalline polymer model IV;

[0013] Step 5: Using Model IV as the initial model, explore the effect of applying equal strong electric fields in the X and Y directions on the morphology of series-parallel semi-crystalline polymers, and use the non-equilibrium molecular dynamics method (NEMD) to perform heat transfer analysis to obtain the temperature variation curve of each layer position and the heat flow variation curve of each layer position;

[0014] Step 6: Use Python software to perform linear fitting on the temperature change curves of each layer position and the heat flow change curves over time to obtain the temperature gradient value and the rate of change of heat flow over time. Calculate the thermal conductivity of the material according to Fourier's law, and obtain the effect of changes in electric field strength on the thermal conductivity of polymer materials.

[0015] Furthermore, the 'permanent crystalline region' in step 2 is completely surrounded by the 'temporary crystalline region' to ensure that amorphous polymer chains exist at the ends of the crystalline region in the interchain direction or along the chain direction during the subsequent simulation of local melting.

[0016] In step 3, the model I with different interchain couplings is obtained, and the interchain coupling in the model I is set according to the simulation requirements. Specifically, it is determined according to the bond length of the end atoms, and the distance exceeding the bond length is not connected to prevent the unreasonable structure of the model I.

[0017] In step 4, the open source software Lammps is used as the simulation software to perform molecular dynamics simulation on model I according to the following sub-steps:

[0018] (1) According to the predicted size of the deformed polymer by model I, a simulation box is created and the corresponding size is set, and all the model atoms are located inside the box, thereby obtaining model II;

[0019] (2) Visualizing the model through the ovito software, rendering model II in color according to the molecular chain number, and then observing whether the polymer molecular chain is divided correctly, and correcting the molecular chain that is divided incorrectly, so that the molecular number of the permanent crystalline region can be correctly extracted in the later stage, or the atomic number can be further determined by determining the z coordinate after the molecular number is determined;

[0020] (3) Recording the molecular chain numbers of the ‘permanent crystalline region’ where there is no interchain coupling between the molecular chains of the ‘permanent crystalline region’ and the ‘temporary crystalline region’; further determining the atomic numbers of the ‘permanent crystalline region’ where there is interchain coupling between the molecular chains of the ‘permanent crystalline region’ and the ‘temporary crystalline region’ through the coordinate positions;

[0021] (4) Setting the potential function describing the interaction between atoms in Model II and setting the initial simulation conditions and parameters, wherein the set temperature must exceed the melting point of the crystalline polymer;

[0022] (5) For the potential function, if the long-range Coulomb force interaction is considered, in order to avoid errors in obtaining the initial series-parallel semi-crystalline polymer model, the setting of the long-range Coulomb force interaction is first canceled, and initially only the spatial position distribution of the molecular chain is considered for model II;

[0023] (6) After setting the boundary conditions in the X, Y, and Z directions to non-periodic boundary (f) conditions, in order to prevent the polymer atoms from moving out of the simulation box during the local melting of Model II and causing simulation errors, reflective walls are set on each side of the simulation box; the atoms in the "permanent crystalline region" are divided into the same crystalline group according to the atomic number in sub-step (3) of step 4, and the force and velocity of the atoms in the crystalline group are set to 0, forming a fixable crystalline region;

[0024] (7) Model II was locally melted using a canonical ensemble (NVT). When the temporary crystalline region was melted at high temperature, the permanent crystalline region was fixed by simulation commands. After 1-2 ns, Model III was obtained. At this time, the permanent crystalline region still maintained a crystalline conformation, while the temporary crystalline region had transformed into an amorphous state.

[0025] (8) Use Model III as the model file, set the long-range Coulomb force interaction in the potential function, change the boundary conditions in the X, Y, and Z directions to periodic boundary (p) conditions, cancel the reflection wall set in sub-step (6) of step 4, cancel the fixed "permanent crystalline zone" setting in sub-step (7) of step 4, and set the simulation temperature below the melting point of the crystalline polymer to avoid structural damage in the subsequent crystalline zone;

[0026] (9) Model III is relaxed under an isothermal and isobaric ensemble (NPT). After 1-2 ns, a semi-crystalline polymer model that conforms to the series morphology in the X, Y, and Z directions is obtained, which is called the series-parallel semi-crystalline polymer model IV.

[0027] In sub-step (1) of step 4, a symmetrically distributed vacuum layer is added to the model box to prevent more amorphous chains from concentrating on one side.

[0028] In sub-step (2) of step 4, the polymer molecular chain is observed as follows: open the Model II file in text mode, observe the second column of data under "Atoms", and judge whether the molecular number division is correct based on the total number of molecules that meet the model.

[0029] In sub-step (4) of step 4, the initial simulation conditions and parameters include model size, atom type, unit system, boundary conditions, neighbor list radius, time step, pressure, and temperature; the melting point of the crystalline polymer is determined according to literature or by using Lammps to simulate the polymer heating process.

[0030] Sub-step (8) of step 4 also includes: if the polymer chains are distributed in the simulation box, then proceed directly to the next simulation; if the polymer chain clusters in the simulation box are around the "permanent crystalline region" and there is a large vacuum layer, then the vacuum layer range is reduced in the model file to prevent errors due to exceeding the long-range Coulomb force interaction range during relaxation under the isothermal and isobaric ensemble (NPT).

[0031] In step 5, when investigating the effect of simultaneously applying equal-strength electric fields in the X and Y directions on the morphology of series-parallel semi-crystalline polymers, the applied electric field strength does not exceed the breakdown electric field strength value of the polymer; the simulation temperature is set above the melting temperature of the amorphous polymer and below the melting point of the crystalline polymer; after the electric field is applied, the model is cooled to room temperature, and then the effect of applying electric fields of different intensities on the thermal conductivity of the model is measured.

[0032] In step 5, the non-equilibrium molecular dynamics method is specifically as follows: the model is evenly divided into N layers along the heat conduction direction, the temperature gradient of each layer under stable heat flow is counted and output, and the thermal conductivity is calculated using Fourier's law, where N depends on the model length and N≥8.

[0033] The beneficial effects of the present invention are:

[0034] 1. The present invention proposes a new and more reasonable semi-crystalline polymer modeling method based on the molecular dynamics simulation method for the first time, namely, a series-parallel semi-crystalline polymer modeling method. Moreover, the invention does not need to go through a complicated model conformation processing process, the processing is simple, the process is widely applicable, and it meets the design requirements of various types of semi-crystalline polymer models.

[0035] 2. The series-parallel polymer simulation unit proposed for the first time in this invention can better characterize the relevant physical properties of semi-crystalline polymers compared with a simple two-phase or three-phase series model, and provides a new idea for the subsequent exploration of the influencing mechanism of the physical properties of polymers and their composites based on molecular dynamics methods at the microscopic scale.

[0036] 3 The present invention proposes for the first time a treatment method for further improving the thermal conductivity of a semi-crystalline polymer model by using an electric field. Compared with directly applying external force to cause polymer strain and thus improve thermal conductivity, it can better obtain a polymer material with excellent thermal conductivity while ensuring the mechanical properties of the material. The present invention constructs a polymer model at a microscopic scale, reduces experimental costs, and makes full use of molecular dynamics methods to explore the factors affecting the thermal conductivity of polymer materials, thereby improving the theoretical mechanism for preparing high-performance heat sinks. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Figure 1 It is a flow chart of the present invention;

[0038] Figure 2The model diagram of β-type pure crystalline PVDF in the embodiment of the invention, wherein (a) is the β-type unit cell of PVDF, and (b) is the model diagram after the cell is expanded;

[0039] Figure 3 This is a diagram of Model I in the embodiment of the invention, the middle green part is the 'permanent crystalline region', and the rest is the 'temporary crystalline region';

[0040] Figure 4 Rendering color diagram of Model II in the embodiment of the present invention, wherein (a) is a model diagram with an incorrect molecular number, (b) is a model diagram after the molecular number is corrected, and (c) is a cross-sectional diagram of the model after the molecular number is corrected;

[0041] Figure 5 The model diagrams of the 'temporary crystalline region' after melting in the embodiment of the present invention, wherein (a) is the model diagram after direct melting, (b) is the cross-sectional diagram of the model after melting, and (c) is the model diagram after adjusting the size of the vacuum layer;

[0042] Figure 6 It is a graph showing the change of total energy of the system according to the embodiment of the present invention with the relaxation time;

[0043] Figure 7 This is a schematic diagram of the morphological changes of the model after an electric field is applied according to an embodiment of the present invention;

[0044] Figure 8 The cross-sectional diagram of the model after applying electric fields of different strengths for 2 ns according to the embodiment of the present invention;

[0045] Fig. 9 It is a block diagram of the equilibrium molecular dynamics (NEMD) method not in the embodiment of the present invention;

[0046] Fig.10 The electric field strength of the embodiment of the present invention is The heat flow of the cold and hot sources of the series-parallel semi-crystalline PVDF polymer model varies with time;

[0047] Fig.11 The electric field strength of the embodiment of the present invention is Axial temperature gradient diagram of series-parallel semi-crystalline PVDF polymer model;

[0048] Fig.12 This is a graph showing the variation of thermal conductivity of series-parallel semi-crystalline PVDF polymers with applied electric field strength in an embodiment of the present invention. DETAILED DESCRIPTION

[0049] The principles and features of the present invention are described below in conjunction with the accompanying drawings. The examples given are only used to explain the present invention and are not used to limit the scope of the present invention.

[0050] Embodiment 1:

[0051] Take the β-type crystalline polymer of polyvinylidene fluoride (PVDF) as an example.

[0052] A modeling method and thermal conductivity optimization method for a series-parallel semi-crystalline model of a polymer material, comprising the following steps:

[0053] Step 1: Establish a β-type crystalline polymer model of polyvinylidene fluoride (PVDF):

[0054] The lattice parameters of its unit cell are The space group is Cm2m, and one unit cell contains two PVDF monomers (-CH2-CF2-), each containing 4 C, H, and F atoms, such as Figure 2 (a); then expand the cell by 4, 5, and 50 in the X, Y, and Z directions respectively to obtain a crystalline polymer model, remove the periodicity, and add hydrogen atoms to the end C atoms of each main chain to obtain a pure crystalline polymer model, as shown in Figure 2 (b)

[0055] Step 2: According to the crystalline mass fraction, some atoms in the central part are selected as the 'permanent crystalline region', and then all the end atoms of the 'permanent crystalline region' and the 'temporary crystalline region' are broken. Here, 12 molecular chains in the central part are selected as the 'permanent crystalline region', each of which contains 26 PVDF monomers;

[0056] Step 3: Name the main chain atom at the end of the molecular chain in the 'permanent crystalline region' as R1, and name the main chain atom at the end of the molecular chain in the 'temporary crystalline region' as R2. Then, according to the simulation requirements, all R1 and R2 atoms are connected to form a bridge-like link to obtain Model I, as shown in Figure 3 As shown;

[0057] Step 4: Use the open source software Lammps as the simulation software and perform molecular dynamics simulation on Model I according to the following sub-steps:

[0058] (1) If model I is initially periodic, in order to avoid the disorder of bond distribution caused by directly setting the vacuum layer, the 'Unwrap trajectories' function in the visualization software ovito can be used to display the unfolded coordinates of each atom and then re-export it as a Lammps model file. Then, according to the predicted size of the deformed polymer according to model I, a simulation box is set up and the corresponding size of the simulation box is adjusted so that all the model atoms are located inside the box, thus obtaining model II.

[0059] (2) Visualize the model through the Ovito software. Render the color of Model II according to the molecular chain serial number, and then observe whether the division of polymer molecular chains is correct. Correct the wrongly divided molecular chains so as to correctly extract the molecular serial numbers of the permanent crystalline region in the later stage or further determine the atomic serial numbers by judging the z - coordinate after determining the molecular serial numbers. In this example, the initial arrangement of molecular serial numbers is incorrect. As shown in Figure 4 (a), where the color distribution of the same molecular chain is inconsistent. Therefore, the correct model is obtained after correction through relevant codes, as shown in Figure 4 (b);

[0060] (3) Draw the Figure 4 central sectional view of (b), as shown in Figure 4 (c). It can be seen that the 'permanent crystalline region' is distributed in the middle area of the molecular chains in the central blue part, which is marked by a black dashed box in the figure. However, in this model, since some atoms in the 'permanent crystalline region' and the 'temporary crystalline region' are on the same molecular chain, the 'permanent crystalline region' cannot be directly grouped by the molecular chain serial number (in this example, the molecular serial numbers where the atoms in the crystalline region are located are 11 - 14, 19 - 22, 27 - 30). Therefore, first extract all the atoms on the molecular chains where the 'permanent crystalline region' is located, and then determine and record the atomic serial numbers of the 'permanent crystalline region' according to the coordinate values of the atoms in the Z - direction (37.5 < z < 90.19);

[0061] (4) Here, the PCFF potential function is selected to describe the interaction between atoms in Model II. Set the atomic type to the full form, the simulation unit to real, the neighbor list to 2.0 bin, the time step to 0.5 fs, the pressure to 0 atm, and then set the simulation temperature to 800 K. This value is obtained by directly simulating the heating and melting of the model as shown in Figure 2 by Lammps;

[0062] (5) The "pair_style" potential in the PCFF force field is "lj / class2 / coul / long", where 'long' represents considering the long - range Coulomb interaction. Therefore, it is necessary to set "kspace_style pppm 1.0e - 6". However, to avoid the error of "exceeding the Coulomb interaction", initially only consider the spatial distribution of atoms in Model II, that is, change 'long' to 'cut' and cancel the setting of "kspace_style pppm 1.0e - 6";

[0063] (6) After the boundary conditions in the X, Y, and Z directions are all set to non-periodic boundary (f) conditions, in order to prevent the polymer atoms from moving out of the simulation box during the local melting of Model II and causing simulation errors, reflection walls are set on all surfaces of the simulation box, and then the atoms in the "permanent crystalline region" are divided into the same crystalline group according to the atomic number in step (3);

[0064] (7) Use the command “velocity crystal set 0 0 0, fix crystal setforce 0 0 0” to fix the “permanent crystalline region”, perform local melting simulation on model II under the canonical ensemble (NVT), and obtain model III after relaxation for 1 ns, as shown in Figure 5 (a) and draw a cross-sectional diagram, as shown in Figure 5 As shown in (b), at this time, the 'permanent crystalline region' in the model still maintains a crystalline conformation, while the 'temporary crystalline region' has transformed into an amorphous state;

[0065] (8) After resetting the long-range Coulomb force interaction in the potential function, change the boundary conditions in the X, Y, and Z directions to periodic boundary (p) conditions, cancel the reflection wall set in sub-step (6) of step (4), cancel the fixed setting of the "permanent crystalline region" in sub-step (7) of step (4), set the simulation temperature to 300K; and observe Figure 5 (a) It is found that there is no vacuum layer in the X and Y directions, but there is a large vacuum layer in the Z direction. To avoid model errors when considering long-range Coulomb forces, the size of the vacuum layer is manually adjusted to an appropriate position, such as Figure 5 (c)

[0066] (9) After model III is relaxed for 1 ns under the isothermal and isobaric ensemble (NPT), model IV is obtained, in which the three directions of X, Y, and Z all conform to the semi-crystalline polymer model of series morphology, which is called the series-parallel semi-crystalline polymer model. At this time, by observing the change of the total energy of the system with time during the relaxation process, it can be found that when the time reaches 1 ns, the total energy of the system has reached a dynamically stable state, such as Figure 6 As shown, the rationality of the obtained model is further proved; and if other molecular dynamics simulation software is needed for subsequent simulation, the data file can be converted into an xyz file through ovito, or converted into a cif file using atomsk software, and then different types of model files can be converted into each other through visualization software such as Materials Studio or VMD.

[0067] Step 5: Explore the changes in the morphology of Model IV after applying equal electric fields (Ex=Ey) in the X and Y directions:

[0068] First, the simulation temperature is set above the melting temperature of the amorphous polymer and below the melting point of the crystalline polymer. Then, the simulation is conducted by applying the The electric field of equal strength (E x =E y )2ns later, the morphology of the series-parallel semi-crystalline polymer changes. Figure 7 The figure shows the morphological changes of the model after applying an electric field. After applying an electric field of equal strength to the model, the PVDF series-parallel semi-crystalline polymer will shrink in the X and Y directions and stretch in the Z direction. Applying electric fields of different strengths, removing the electric field after 2ns of electro-induced deformation, the cross-section of the model is shown in Figure 8 As shown, it can be seen that the "permanent crystalline region" still maintains a relatively stable structure. Then the temperature is cooled to room temperature 300K, and the NPT, NVT, and NVE ensembles are relaxed for 2ns to obtain a dynamic stable model at room temperature. The non-equilibrium molecular dynamics method is then used to perform heat transfer analysis, and the model is divided into multiple layers along the heat transfer direction (Z direction), and the thickness of each layer is approximately Then fix the two outermost layers of atoms on both sides as fixed ends, and use the two adjacent layers of atoms as heat source and heat sink respectively. The simulation box is set up as follows Fig. 9 As shown; then the Langevin method is used to control the temperature of the heat source and the heat sink respectively to generate heat flow in the system until the heat flow basically maintains a stable growth and then the heat flow curve over time is statistically calculated, as shown Fig.10 As shown, finally the temperature of each layer is counted, and the temperature variation curve of each layer position is plotted along the heat transfer direction, as shown in Fig.11 shown.

[0069] Step 6: Calculate the effect of electric field strength on the thermal conductivity of PVDF series-parallel semi-crystalline polymers:

[0070] The temperature gradient value and the rate of change of heat flow over time were obtained by linearly fitting the curves of temperature change with each layer position and heat flow change with time through Python software, and then the cross-sectional dimensions of the model were extracted, and the thermal conductivity of the material was calculated according to Fourier's law, and the effect of the change of electric field intensity on the thermal conductivity of the polymer material was obtained, such as Fig.12 As shown;

[0071] The above is only one of the embodiments of the present invention. Researchers in this technical field can make various modifications and use the present invention without departing from the research ideas and technical processes of the present invention. Therefore, simple modifications and equivalent changes to the present invention belong to the claims of the present invention and still belong to the protection scope of the technical solution of the present invention.

[0072] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. A modeling and thermal conductivity optimization method for a series-parallel semi-crystalline model of a polymer material, characterized in that: The following steps are involved: Step 1: First, a fully crystalline polymer model of corresponding size is established according to the type of polymer required; Step 2: According to the simulation requirements, the central crystalline area is selected as the 'permanent crystalline area', and the other areas are selected as the 'temporary crystalline area', and the covalent bonds between the atoms in the 'permanent crystalline area' and the 'temporary crystalline area' are broken; Step 3: Name the main chain atom at the end of the molecular chain in the 'permanent crystalline region' as R1, and name the main chain atom at the end of the molecular chain in the 'temporary crystalline region' as R2, and then randomly form covalent bonds between R1 and R2 atoms according to simulation requirements to obtain Model I with different interchain couplings; Step 4: Use the open source software Lammps as the simulation software to perform molecular dynamics simulation on Model I, and obtain a semi-crystalline polymer model that conforms to the series morphology in the three directions of X, Y, and Z, which is called the series-parallel semi-crystalline polymer model IV; Step 5: Using Model IV as the initial model, explore the effect of applying equal strong electric fields in the X and Y directions on the morphology of series-parallel semi-crystalline polymers, and use the non-equilibrium molecular dynamics method (NEMD) to perform heat transfer analysis to obtain the temperature variation curve of each layer position and the heat flow variation curve of each layer position; Step 6: Use Python software to perform linear fitting on the temperature change curves of each layer position and the heat flow change curves over time to obtain the temperature gradient value and the rate of change of heat flow over time. Calculate the thermal conductivity of the material according to Fourier's law, and obtain the effect of changes in electric field strength on the thermal conductivity of polymer materials.

2. The method for modeling and optimizing thermal conductivity of a polymer material series-parallel semi-crystalline model according to claim 1, characterized in that: The 'permanent crystalline region' in step 2 is completely surrounded by the 'temporary crystalline region' to ensure that amorphous polymer chains exist at the ends of the crystalline region in the interchain direction or along the chain direction during the subsequent simulation of local melting.

3. The modeling and thermal conductivity optimization method of a polymer material series-parallel semi-crystalline model according to claim 1, characterized in that: In step 3, the model I with different interchain couplings is obtained, and the interchain coupling in the model I is set according to the simulation requirements. Specifically, it is determined according to the bond length of the end atoms, and the distance exceeding the bond length is not connected to prevent the unreasonable structure of the model I.

4. The method for modeling and optimizing thermal conductivity of a polymer material series-parallel semi-crystalline model according to claim 1, characterized in that: Step 4 uses the open source software Lammps as the simulation software, and performs molecular dynamics simulation on model I according to the following sub-steps: (1) According to the predicted size of the deformed polymer by model I, a simulation box is created and the corresponding size is set, and all the model atoms are located inside the box, thereby obtaining model II; (2) Visualizing the model through the ovito software, rendering model II in color according to the molecular chain number, and then observing whether the polymer molecular chain is divided correctly, and correcting the molecular chain that is divided incorrectly, so that the molecular number of the permanent crystalline region can be correctly extracted in the later stage, or the atomic number can be further determined by determining the z coordinate after the molecular number is determined; (3) Recording the molecular chain numbers of the ‘permanent crystalline region’ where there is no interchain coupling between the molecular chains of the ‘permanent crystalline region’ and the ‘temporary crystalline region’; further determining the atomic numbers of the ‘permanent crystalline region’ where there is interchain coupling between the molecular chains of the ‘permanent crystalline region’ and the ‘temporary crystalline region’ through the coordinate positions; (4) Setting the potential function describing the interaction between atoms in Model II and setting the initial simulation conditions and parameters, wherein the set temperature must exceed the melting point of the crystalline polymer; (5) For the potential function, if the long-range Coulomb force interaction is considered, in order to avoid errors in obtaining the initial series-parallel semi-crystalline polymer model, the setting of the long-range Coulomb force interaction is first canceled, and initially only the spatial position distribution of the molecular chain is considered for model II; (6) After setting the boundary conditions in the X, Y, and Z directions to non-periodic boundary (f) conditions, in order to prevent the polymer atoms from moving out of the simulation box during the local melting of Model II and causing simulation errors, reflective walls are set on each side of the simulation box; the atoms in the "permanent crystalline region" are divided into the same crystalline group according to the atomic number in sub-step (3) of step 4, and the force and velocity of the atoms in the crystalline group are set to 0, forming a fixable crystalline region; (7) Model II was locally melted using a canonical ensemble (NVT). When the temporary crystalline region was melted at high temperature, the permanent crystalline region was fixed by simulation commands. After 1-2 ns, Model III was obtained. At this time, the permanent crystalline region still maintained a crystalline conformation, while the temporary crystalline region had transformed into an amorphous state. (8) Use Model III as the model file, set the long-range Coulomb force interaction in the potential function, change the boundary conditions in the X, Y, and Z directions to periodic boundary (p) conditions, cancel the reflection wall set in sub-step (6) of step 4, cancel the fixed "permanent crystalline zone" setting in sub-step (7) of step 4, and set the simulation temperature below the melting point of the crystalline polymer to avoid structural damage in the subsequent crystalline zone; (9) Model III is relaxed under an isothermal and isobaric ensemble (NPT). After 1-2 ns, a semi-crystalline polymer model that conforms to the series morphology in the X, Y, and Z directions is obtained, which is called the series-parallel semi-crystalline polymer model IV.

5. The method for modeling and optimizing thermal conductivity of a polymer material series-parallel semi-crystalline model according to claim 4, characterized in that: In the step (1), a symmetrically distributed vacuum layer is added to the model box to prevent more amorphous chains from concentrating on one side.

6. The method for modeling and optimizing thermal conductivity of a polymer material series-parallel semi-crystalline model according to claim 4, characterized in that: In the step (2), observing the polymer molecular chain specifically includes: opening the Model II file in text mode, observing the second column of data under "Atoms", and judging whether the molecular number division is correct or not based on the total number of molecules that conform to the model.

7. The method for modeling and optimizing thermal conductivity of a polymer material series-parallel semi-crystalline model according to claim 4, characterized in that: In the step (4), the initial simulation conditions and parameters include model size, atom type, unit system, boundary conditions, neighbor list radius, time step, pressure, and temperature; the melting point of the crystalline polymer is determined according to literature or by using Lammps to simulate the polymer heating process.

8. The method for modeling and optimizing thermal conductivity of a polymer material series-parallel semi-crystalline model according to claim 4, characterized in that: The step (8) also includes: if the polymer chains are distributed in the simulation box, directly proceed to the next simulation; if the polymer chain clusters in the simulation box are around the "permanent crystalline region" and there is a large vacuum layer, the vacuum layer range is reduced in the model file to prevent an error message due to exceeding the long-range Coulomb force interaction range during relaxation under an isothermal isobaric ensemble (NPT).

9. A method for modeling and optimizing thermal conductivity of a polymer material series-parallel semi-crystalline model according to any one of claims 1 to 8, characterized in that: In step 5, when investigating the effect of simultaneously applying equal-strength electric fields in the X and Y directions on the morphology of series-parallel semi-crystalline polymers, the applied electric field strength does not exceed the breakdown electric field strength value of the polymer; the simulation temperature is set above the melting temperature of the amorphous polymer and below the melting point of the crystalline polymer; after the electric field is applied, the model is cooled to room temperature, and then the effect of applying electric fields of different intensities on the thermal conductivity of the model is measured.

10. The method for modeling and optimizing thermal conductivity of a polymer material series-parallel semi-crystalline model according to claim 9, characterized in that: In step 5, the non-equilibrium molecular dynamics method is specifically as follows: the model is evenly divided into N layers along the heat conduction direction, the temperature gradient of each layer under stable heat flow is counted and output, and the thermal conductivity is calculated using Fourier's law, where N depends on the model length and N≥8.

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