Prediction method of polyimide stress threshold

Abstract

The invention provides a method for predicting a polyimide stress threshold. The method comprises the following steps: 1) constructing a polyimide full-atom molecular model by adopting molecular simulation software; 2) performing mechanical tensile simulation on the molecular model to calculate a stress-strain curve; 3) calculating a creep curve of the molecular model under different constant stresses according to a stress-strain curve calculation result, and determining a mechanical failure interval according to the creep curve; and 4) selecting different stress values in a mechanical failure interval by using a dichotomy to carry out creep curve iteration, and finally determining a stress threshold. The method provided by the invention not only can efficiently predict the stress thresholds of the polyimides with different structures, but also can simulate and analyze the creep behavior of the polyimides in the tensile stress process, and provides a theoretical basis for guiding the structural design and mechanical property optimization of polyimide materials.

Description

Technical Field

[0001] The present invention relates to a method for predicting the stress threshold of polyimide, belonging to the technical field of computer simulation of polymer material properties. Background Art

[0002] Polyimide is a kind of polymer material with excellent comprehensive properties, having characteristics such as high strength and toughness, high heat resistance, and low dielectric constant. In recent years, it has been widely used in the fields of electric power and electrical, microelectronics, flexible display, new energy, etc. With the continuous development of microelectronic devices and display terminal products towards flexibility, thinness, and wearability, polyimide film, as a high-performance polymer substrate, has become an indispensable key material in the manufacturing of flexible circuit boards, flexible radio frequency antennas, and flexible displays. However, in the application of flexible terminal products, polyimide film needs to withstand long-term repeated stretching, bending, extrusion, etc. It is crucial to deeply understand the failure behavior of the material under stress and improve the reliability of the material in the application of terminal products.

[0003] Currently, the research on the mechanical failure behavior of polyimide film during the stress process usually adopts mechanical stretching to measure the fracture ultimate tensile strength, uses dynamic mechanical analysis to evaluate the creep behavior of the material under constant stress or the stress relaxation behavior under constant strain, and uses a fatigue testing machine to conduct material fatigue tests under specific stress, strain, and power frequency. This traditional research method has high experimental costs, long cycles, and low efficiency from resin synthesis, sample preparation to performance testing, and it is difficult to meet the application requirements for the development of new high-performance polyimide materials due to the rapid iteration of flexible terminal product technology. Summary of the Invention

[0004] In view of the above problems, the present invention proposes a method for predicting the stress threshold of polyimide to solve the problems that the existing methods for studying the mechanical failure behavior of polyimide have high costs and long cycles, resulting in low screening efficiency of materials and difficulty in meeting the application requirements for the development of new high-performance polyimide materials due to the rapid iteration of flexible terminal product technology.

[0005] The object of the present invention is mainly achieved through the following technical solutions:

[0006] A method for predicting the stress threshold of polyimide, the method comprising the following steps:

[0007] (1) Construct a three-dimensional amorphous unit cell model of polyimide using molecular simulation software, and obtain a fully atomistic molecular model of polyimide in thermodynamic equilibrium through energy optimization and molecular dynamics simulation;

[0008] (2) Conduct mechanical stretching simulation on the fully atomistic molecular model of polyimide in thermodynamic equilibrium, and calculate the stress-strain curve;

[0009] (3) Select the stress values corresponding to the yield point and its adjacent strain points in the stress-strain curve, perform creep curve simulation, and determine the mechanical failure interval according to the creep curve;

[0010] (4) Select different stress values within the mechanical failure interval using the bisection method for creep curve iteration, and judge the stress threshold by analyzing the creep curve after iteration.

[0011] Optionally, in step (1), first construct a three-dimensional amorphous unit cell model of polyimide using molecular simulation software; secondly, perform energy optimization on the three-dimensional amorphous unit cell model of polyimide; then perform annealing-relaxation coupling treatment on the energy-optimized three-dimensional amorphous unit cell model of polyimide.

[0012] Optionally, the annealing-relaxation coupling treatment includes the following steps:

[0013] ① The annealing condition is to perform 15-25 cycle treatments in the temperature range of 298-798K;

[0014] ② Perform structural relaxation simulation using molecular dynamics to obtain a thermodynamically equilibrated all-atom molecular model of polyimide.

[0015] Optionally, in step (2), the mechanical stretching simulation process includes: first relax the all-atom molecular model of polyimide under zero stress until equilibrium, and then apply stress to the model to calculate the stress-strain curve.

[0016] Optionally, the stress-strain curve calculation includes: continuously apply stress at the same interval in the X direction to the all-atom molecular model of polyimide, control the stresses in the Y and Z directions to be 0, calculate the strain values under different stresses, and obtain the stress-strain curve.

[0017] Optionally, in step (3), the creep curve simulation includes: apply a certain stress in the X direction to the all-atom molecular model of polyimide, control the stresses in the Y and Z directions to be 0, calculate the change of strain with time under this stress, and obtain the creep curve.

[0018] Optionally, the certain stress is the stress value corresponding to the yield point in the stress-strain curve calculated in step (2), and the stress value corresponding to the strain point adjacent to the yield point.

[0019] Optionally, in step (3), the method for determining the mechanical failure interval according to the creep curve includes: analyze the creep curves obtained under different stresses, take the lowest stress value at which tertiary creep occurs as the upper limit of the mechanical failure interval, and take the maximum stress value at which secondary creep occurs but tertiary creep does not occur as the lower limit of the mechanical failure interval.

[0020] Optionally, in the step (4), the method of using the bisection method to select different stress values for creep curve iteration within the mechanical failure range includes: selecting the median of the maximum stress value and the minimum stress value in the mechanical failure range for creep curve simulation, continuously narrowing the stress value range for creep curve iterative calculation until the stress value interval between the upper and lower limits of the mechanical failure range is reduced to 1 MPa.

[0021] Optionally, in the step (4), the method for judging the stress threshold includes: analyzing the creep curve after iteration, and taking the upper limit stress value after reducing the stress value interval between the upper and lower limits of the mechanical failure range to 1 MPa as the stress threshold.

[0022] Compared with the prior art, the present invention can achieve at least one of the following beneficial effects:

[0023] (1) The present invention uses Materials Studio software for molecular dynamics simulation, and conducts simulation analysis on the stress-bearing process of polyimide through multiple steps such as energy optimization, molecular dynamics relaxation equilibrium, stress-strain curve simulation, and creep curve simulation. On this basis, the bisection method is used for creep curve iteration to narrow the mechanical failure range to 1 MPa, realizing the accurate prediction of the stress threshold of polyimides with different structures, and providing theoretical guidance for the molecular structure design and mechanical property optimization of polyimide materials.

[0024] (2) The method of using the all-atom molecular model of polyimide in the present invention to conduct creep curve simulation analysis under different constant stresses can achieve efficient prediction of the creep behavior of polyimides with different structures under stress, reveal the mechanical failure evolution process of polyimide under stress, and provide data support for the application of polyimide materials in flexible terminal products.

[0025] (3) Comparing the prediction results of the stress thresholds of polyimides with different structures obtained by using the method provided by the present invention with the maximum tensile stress values of the corresponding polyimide films in experimental verification tests, it is found that the stress threshold prediction results are consistent with the experimental test results, which proves the effectiveness of this method for predicting polyimide stress thresholds by computer simulation calculation. Compared with traditional experimental methods, it can achieve rapid screening of material structures and properties, significantly shorten the time and cost required for new material development, and has higher timeliness. Description of the Drawings

[0026] The drawings described herein are used to provide a further understanding of the present application, form a part of the present application, and the illustrative embodiments and descriptions thereof are used to explain the present application and do not constitute an improper limitation to the present application.

[0027] Figure 1 It is a flowchart of the method for predicting the stress threshold of polyimide provided by the present invention;

[0028] Figure 2 The all-atom molecular model of polyimide in thermodynamic equilibrium for Example 1;

[0029] Figure 3 The stress-strain curve of the polyimide molecular model for Example 1;

[0030] Figure 4(a) is the curve of the strain of the polyimide molecular model for Example 1 varying with time under a constant stress of 170 MPa;

[0031] Figure 4(b) is the curve of the strain of the polyimide molecular model for Example 1 varying with time under a constant stress of 180 MPa;

[0032] Figure 5 The creep curve simulated by the bisection method for the polyimide molecular model for Example 1 within the mechanical failure range;

[0033] Figure 6 The comparison between the stress threshold predicted by polyimide simulation and the maximum tensile stress value verified by experiments for Examples 1 - 4. Detailed implementation manners

[0034] The present invention is further elaborated below through specific examples, but the present invention is not limited to the following examples. All technologies implemented based on the present invention are covered within the protection scope of the present invention. In the following examples, the software, modules, and operation methods are, unless otherwise specified, those that can be obtained and implemented by those skilled in the art from commercial public channels.

[0035] The present invention aims to establish the structure-property relationship between the chemical structure and macroscopic properties of polyimide through molecular simulation, achieve efficient and accurate prediction of the mechanical properties and tensile failure behavior of polyimides with different structures, and provide theoretical guidance for the molecular structure design and performance optimization of polyimide materials. Specifically, the present invention uses Materials Studio software to optimize the polymer molecular model and simulate and predict the creep behavior and ultimate stress threshold of polyimide under stress.

[0036] The present invention provides a method for predicting the stress threshold of polyimide, more precisely, a method for quickly screening polyimide materials, as Figure 1 shown, the method includes the following steps:

[0037] (1) Construct a three-dimensional amorphous unit cell model of polyimide using molecular simulation software, and obtain the all-atom molecular model of polyimide in thermodynamic equilibrium through energy optimization and kinetic simulation;

[0038] (2) Conduct mechanical stretching simulation on the all-atom molecular model of polyimide in thermodynamic equilibrium, and calculate the stress-strain curve;

[0039] (3) Select the stress values corresponding to the yield point and its adjacent strain points in the stress-strain curve, perform creep curve simulation, and determine the mechanical failure interval according to the creep curve;

[0040] (4) Use the bisection method to select different stress values for creep curve iteration within the mechanical failure interval, and judge the stress threshold by analyzing the creep curve after iteration.

[0041] In step (1) of the above prediction method, first, use the Amorphous Cell Construction module in the molecular simulation software Materials Studio to construct a three-dimensional amorphous cell model of polyimide; secondly, call the Geometry Optimization module in Forcite to perform energy optimization on the three-dimensional amorphous cell model of polyimide; then, use the Anneal module and the Dynamics module to perform annealing-relaxation coupling treatment on the energy-optimized three-dimensional amorphous cell model of polyimide.

[0042] The annealing-relaxation coupling treatment method includes the following steps:

[0043] ① The annealing condition is to perform 15-25 cycles of treatment in the temperature range of 298-798K;

[0044] ② Use molecular dynamics to perform structural relaxation simulation to obtain a thermodynamically equilibrated all-atom molecular model of polyimide.

[0045] Specifically, the number of cycles in step ① can be 15 times, 18 times, 20 times, 23 times, or 25 times.

[0046] In step (2) of the above prediction method, the mechanical tensile simulation uses the Stress-Strain script in Scripting of Materials Studio;

[0047] The condition settings for the mechanical tensile simulation are: COMPASSⅡ force field, Souza-Martins pressure controller, Cauchy-controlled constant stress mode, NHL thermostat;

[0048] The mechanical tensile simulation process includes: first, relax the all-atom molecular model of polyimide under zero stress until equilibrium, and then apply stress to the model to calculate the stress-strain curve.

[0049] The stress-strain curve calculation process includes: continuously applying stresses at the same intervals in the X direction to the all-atom molecular model of polyimide, controlling the stresses in the Y and Z directions to be 0, with each stress interval being 5 - 20 MPa, for example, 5 MPa, 10 MPa, 15 MPa, 20 MPa, calculating the strain values under different stresses, and obtaining the stress-strain curve.

[0050] In step (3) of the above prediction method, the creep curve simulation includes: applying a certain stress in the X direction to the all-atom molecular model of polyimide, controlling the stresses in the Y and Z directions to be 0, and calculating the change of strain with time under this stress to obtain the creep curve;

[0051] The certain stress is the stress value corresponding to the yield point in the stress-strain curve calculated in step (2), and the stress values corresponding to the strain points adjacent to the yield point;

[0052] The method for determining the mechanical failure interval according to the creep curve is: analyzing the creep curves obtained under different stresses, taking the lowest stress value at which tertiary creep appears as the upper limit of the mechanical failure interval, and taking the maximum stress value at which secondary creep appears but tertiary creep does not appear as the lower limit of the mechanical failure interval.

[0053] In step (4) of the above prediction method, the method for performing creep curve iteration using the bisection method within the mechanical failure interval is: selecting the median of the maximum stress value and the minimum stress value in the mechanical failure interval for creep curve simulation, continuously narrowing the stress value range for creep curve iterative calculation until the stress value interval between the upper and lower limits of the mechanical failure interval is narrowed to 1 MPa;

[0054] The method for judging the stress threshold is: analyzing the creep curve after iteration, and taking the upper limit stress value after narrowing the stress value interval between the upper and lower limits of the mechanical failure interval to 1 MPa as the stress threshold.

[0055] Example 1

[0056] Taking the polyimide system of 4,4'-(hexafluoroisopropylidene) bisphthalic anhydride (6FDA) and 2,2'-bis(trifluoromethyl)-4,4'-diaminobiphenyl (TFDB) as an example, the method for predicting the stress threshold of the 6FDA / TFDB system is as follows:

[0057] (1) First, use the Amorphous Cell Construction module in the molecular simulation software Materials Studio to construct a three-dimensional amorphous unit cell model of 6FDA / TFDB polyimide. Set the number of single polyimide molecular chains placed in the molecular model to 10, and each molecular chain contains 20 repeating units;

[0058] Secondly, call the Geometry Optimization module in Forcite to perform energy optimization on the polyimide molecular model. The energy convergence criterion is set to 2×10 -5 kcal / mol using the smart method, and the maximum number of iterations is 10 5 . Obtain the global minimum energy configuration. If the convergence condition is not met, repeat the energy optimization until the convergence condition is satisfied.

[0059] Then, use Anneal in the Forcite module to find the lowest energy conformation. Set the annealing temperature range to 298 - 798K and perform 20 cycles of processing. Use the COMPASSⅡ force field, select Ultra-Fine for Quality, set the time step to 1fs, use the NPT ensemble, run 2,000,000 steps of annealing, save the molecular model structure every 10,000 steps, and extract the molecular model with the lowest potential energy; continue with the annealing of the NVT ensemble, with the simulation conditions the same as those of the NPT ensemble, and extract the molecular model with the lowest potential energy for dynamic simulation. Use Dynamics in the Forcite module to run the dynamics of the annealed polyimide molecular model for 2,000,000 steps each in the NPT and NVT ensembles to equilibrate the system, and obtain the final fully atomistic molecular model of polyimide after thermodynamic equilibrium, as Figure 2 shown.

[0060] (2) Perform a mechanical tensile stress-strain curve simulation on the fully atomistic molecular model of polyimide after thermodynamic equilibrium. Use the Stress-Strain script in Scripting of Materials Studio. During the simulation, use the COMPASSⅡ force field, set the pressure controller to Souza-Martins, and use the Cauchy control constant stress mode. The thermostat is NHL.

[0061] First, relax the fully atomistic molecular model of polyimide under zero stress until equilibrium. The number of cycles is 5, the time step is 1fs, and the number of steps is 500,000 steps to obtain the polyimide molecular model in equilibrium under zero stress;

[0062] Then, continuously apply the same interval of stress to the fully atomistic molecular model of polyimide in the X direction, control the stress in the Y and Z directions to be 0, with each stress interval being 10MPa. For each stress, first perform 50,000 steps of equilibrium, then perform a 50,000-step tensile simulation under that stress, calculate the strain values under different stresses, and extract each stress value and strain value to obtain the stress-strain curve, as Figure 3 shown.

[0063] (3) Select the stress value of 170 MPa corresponding to the yield point and the stress value of 180 MPa corresponding to the strain point adjacent to the yield point in the calculated stress-strain curve for creep curve simulation. Apply a stress of 170 MPa or 180 MPa in the X direction of the molecular model, control the stresses in the Y and Z directions to be 0, and perform a mechanical tensile simulation for 1,000,000 steps at a time step of 1 fs. Output a structural model every 10,000 steps, and calculate the change of strain with time under stresses of 170 MPa and 180 MPa respectively to obtain the creep curves, as shown in Figure 4;

[0064] Analyze the change trend of strain with time according to the creep curves shown in Figure 4(a) and Figure 4(b), and divide the creep intervals based on the initial linear increase of strain (primary creep), the entry into the plateau region in the middle (secondary creep), and the sharp rise in the later stage (tertiary creep). Only secondary creep but no tertiary creep occurred at 1000 ps under 170 MPa, while tertiary creep occurred after 240 ps under 180 MPa. The interval between the non-failure stress value of 170 MPa and the failure stress value of 180 MPa is the same as the stress interval value applied in the mechanical tensile curve, both being 10 MPa. Therefore, these two stress values are determined as the upper and lower limit stress values of the mechanical failure interval.

[0065] (4) Use the bisection method to perform creep curve iteration within the mechanical failure interval of 170 - 180 MPa until the interval between the upper and lower limit stress values of the mechanical failure interval is reduced to 1 MPa. As Figure 5 shown, simulate the creep curve under 175 MPa. Only primary and secondary creep occurred at 1000 ps. Therefore, the mechanical failure interval can be reduced to 175 - 180 MPa; select 178 MPa in this interval to simulate the creep curve. Tertiary creep still did not occur at 1000 ps, and further reduce the mechanical failure interval to 178 - 180 MPa; select 179 MPa in this interval to simulate the creep curve. Tertiary creep occurred after 380 ps, indicating that the stress threshold of the 6FDA / TFDB system polyimide is 179 MPa.

[0066] The comparison between the stress threshold predicted by simulating the 6FDA / TFDB system polyimide in Example 1 and the maximum tensile stress value verified by experiments is shown in Figure 6 .

[0067] Example 2

[0068] Taking the polyimide system of 4,4'-(hexafluoroisopropylidene) bisphthalic anhydride (6FDA) and 2,2'-bis(trifluoromethyl)-4,4'-diaminodiphenyl ether (6FODA) as an example, the method for predicting the stress threshold of the 6FDA / 6FODA system is as follows:

[0069] (1) First, use the Amorphous Cell Construction module in the molecular simulation software Materials Studio to construct a three-dimensional amorphous unit cell model of 6FDA / 6FODA polyimide. Set the number of single polyimide molecular chains placed in the molecular model to 10, and each molecular chain contains 20 repeating units;

[0070] Secondly, call the Geometry Optimization module in Forcite to perform energy optimization on the polyimide molecular model. The energy convergence criterion is the smart method of 2×10 -5 kcal / mol, and the maximum number of iterations is 10 5 . Obtain the global minimum energy configuration. If the convergence condition is not met, repeat the geometric optimization until the convergence condition is satisfied.

[0071] Then, use Anneal in the Forcite module to find the lowest energy conformation. Set the annealing temperature range to 298 - 798K and perform 20 cycles of processing. Use the COMPASSⅡ force field, select Ultra-Fine for Quality, the time step is 1fs, use the NPT ensemble, run the annealing for 2000000 steps, save the molecular model structure every 10000 steps, and extract the molecular model with the lowest potential energy; continue with the annealing of the NVT ensemble, with the simulation conditions the same as those of the NPT ensemble, and extract the molecular model with the lowest potential energy for dynamic simulation. Use Dynamics in the Forcite module to run the dynamics of the annealed polyimide molecular model for 2000000 steps in both the NPT and NVT ensembles to make the system reach equilibrium, and obtain the final all-atom molecular model of polyimide after thermodynamic equilibrium.

[0072] (2) Perform a mechanical tensile stress-strain curve simulation on the all-atom molecular model of polyimide after thermodynamic equilibrium. Use the Stress-Strain script in Scripting of Materials Studio. During the simulation, use the COMPASSⅡ force field, set the pressure controller to Souza-Martins, and use the Cauchy control constant stress mode, and the thermostat is NHL.

[0073] First, relax the all-atom molecular model of polyimide under zero stress until equilibrium. The number of cycles is 5, the time step is 1fs, and the number of steps is 500000 steps to obtain the polyimide molecular model in equilibrium under zero stress;

[0074] Then, the same stress intervals are continuously applied to the all-atom molecular model of polyimide in the X direction, while controlling the stresses in the Y and Z directions to be 0. Each stress interval is 10 MPa. For each stress, 50,000 steps of equilibration are first performed, followed by 50,000 steps of tensile simulation under that stress. The strain values under different stresses are calculated, and the stress-strain curve is obtained by extracting each stress value and strain value.

[0075] (3) Select the stress value of 170 MPa corresponding to the yield point and the stress value of 180 MPa corresponding to the strain point adjacent to the yield point from the calculated stress-strain curve for creep curve simulation. Apply a stress of 170 MPa or 180 MPa in the X direction of the molecular model, control the stresses in the Y and Z directions to be 0, and perform a mechanical tensile simulation of 1,000,000 steps under this stress with a time step of 1 fs. A structural model is output every 10,000 steps, and the creep curves showing the variation of strain with time under stresses of 170 MPa and 180 MPa are calculated respectively;

[0076] Based on the analysis of the trend of the variation of strain with time in the creep curve, the creep intervals are divided according to the initial stage of strain showing linear increase (primary creep), the middle stage entering the plateau region (secondary creep), and the late stage showing a sharp rise (tertiary creep). Only secondary creep but no tertiary creep occurs at 1000 ps under 170 MPa, while tertiary creep occurs after 480 ps under 180 MPa. The interval between the non-failure stress value of 170 MPa and the failure stress value of 180 MPa is the same as the stress interval value applied in the mechanical tensile curve, both being 10 MPa. Therefore, these two stress values are determined as the upper and lower limit stress values of the mechanical failure interval.

[0077] (4) Use the bisection method to perform creep curve iteration within the mechanical failure interval of 170 - 180 MPa until the interval between the upper and lower limit stress values of the mechanical failure interval is reduced to 1 MPa. Simulate the creep curve at 175 MPa, and tertiary creep occurs after 900 ps. Therefore, the mechanical failure interval can be reduced to 170 - 175 MPa; select 173 MPa in this interval to simulate the creep curve, and tertiary creep occurs after 930 ps, further reducing the mechanical failure interval to 170 - 173 MPa; select 172 MPa in this interval to simulate the creep curve, and only primary and secondary creep occur until 1000 ps, indicating that the stress threshold of the 6FDA / 6FODA system polyimide is 173 MPa.

[0078] The comparison between the stress threshold predicted by the simulation of the 6FDA / 6FODA system polyimide in Example 2 and the maximum tensile stress value verified by the experiment is shown in Figure 6 .

[0079] Example 3

[0080] Taking the polyimide system of 4,4'-(hexafluoroisopropylidene)diphthalic anhydride (6FDA) and 2,2-bis[4-(4-aminophenoxy)phenyl]hexafluoropropane (HFBAPP) as an example, the method for predicting the stress threshold of the 6FDA / HFBAPP system is as follows:

[0081] (1) First, use the Amorphous Cell Construction module in the molecular simulation software Materials Studio to construct a three-dimensional amorphous unit cell model of 6FDA / HFBAPP polyimide. Set the number of single polyimide molecular chains placed in the molecular model to 10, and each molecular chain contains 20 repeating units;

[0082] Secondly, call the Geometry Optimization module in Forcite to perform energy optimization on the polyimide molecular model. The energy convergence criterion is the smart method of 2×10 -5 kcal / mol, and the maximum number of iterations is 10 5 , to obtain the global minimum energy configuration. If the convergence condition is not met, repeat the energy optimization until the convergence condition is satisfied.

[0083] Then use Anneal in the Forcite module to find the lowest energy conformation. Set the annealing temperature range to 298 - 798K and perform 20 cycles of processing. Use the COMPASSⅡ force field, select Ultra-Fine for Quality, the time step is 1fs, use the NPT ensemble, run the annealing for 2000000 steps, save the molecular model structure every 10000 steps, and extract the molecular model with the lowest potential energy; continue with the annealing of the NVT ensemble, with the simulation conditions the same as those of the NPT ensemble, and extract the molecular model with the lowest potential energy for kinetic simulation. Use Dynamics in the Forcite module to perform kinetic runs of 2000000 steps for both the NPT and NVT ensembles on the annealed polyimide molecular model to make the system reach equilibrium, and obtain the final all-atom molecular model of polyimide after thermodynamic equilibrium.

[0084] (2) Perform a mechanical tensile stress-strain curve simulation on the all-atom molecular model of polyimide after thermodynamic equilibrium. Use the Stress-Strain script in Scripting of Materials Studio. During the simulation, use the COMPASSⅡ force field, set the pressure controller to Souza-Martins, and use the Cauchy control constant stress mode, and the thermostat is NHL.

[0085] First, relax the all-atom molecular model of polyimide until equilibrium under zero stress. The number of cycles is 5, the time step is 1 fs, and the number of steps is 500,000 steps, obtaining an equilibrium polyimide molecular model under zero stress;

[0086] Then, continuously apply stresses at the same intervals in the X direction to the all-atom molecular model of polyimide, controlling the stresses in the Y and Z directions to be 0. Each stress interval is 10 MPa. For each stress, first perform 50,000 steps of equilibration, and then perform a 50,000-step tensile simulation under this stress. Calculate the strain values under different stresses, and extract each stress value and strain value to obtain the stress-strain curve.

[0087] (3) Select the stress value of 150 MPa corresponding to the yield point and the stress value of 160 MPa corresponding to the strain point adjacent to the yield point in the calculated stress-strain curve for creep curve simulation. Apply a stress of 150 MPa or 160 MPa in the X direction of the molecular model, control the stresses in the Y and Z directions to be 0, and perform a 1,000,000-step mechanical tensile simulation under this stress. The time step is 1 fs, and output a structural model every 10,000 steps. Calculate the change of strain with time under stresses of 150 MPa and 160 MPa respectively to obtain the creep curves;

[0088] Analyze the change trend of strain with time according to the creep curves. Divide the creep intervals based on the initial linear increase of strain (primary creep), entering the plateau region in the middle (secondary creep), and the sharp rise in the later stage (tertiary creep). At 150 MPa, only secondary creep appears but not tertiary creep at 1000 ps, while at 160 MPa, tertiary creep appears after 680 ps. The interval between the non-failure stress value of 150 MPa and the failure stress value of 160 MPa is the same as the stress interval value applied in the mechanical tensile curve, both being 10 MPa. Therefore, determine these two stress values as the upper and lower limit stress values of the mechanical failure interval.

[0089] (4) Use the bisection method to perform creep curve iteration within the mechanical failure interval of 150 - 160 MPa until the interval between the upper and lower limit stress values of the mechanical failure interval is reduced to 1 MPa. Simulate the creep curve at 155 MPa, and tertiary creep appears after 600 ps. Therefore, the mechanical failure interval can be reduced to 150 - 155 MPa; select 153 MPa in this interval to simulate the creep curve, and tertiary creep appears after 690 ps, further reducing the mechanical failure interval to 150 - 153 MPa; select 152 MPa in this interval to simulate the creep curve. At 152 MPa, only secondary creep appears but not tertiary creep at 1000 ps, indicating that the stress threshold of the 6FDA / HFBAPP system polyimide is 153 MPa.

[0090] The comparison between the stress threshold predicted by the simulation of the 6FDA / HFBAPP system polyimide in Example 3 and the maximum tensile stress value verified by the experiment is shown in Figure 6 .

[0091] Example 4

[0092] Taking the polyimide system of 4,4'-(hexafluoroisopropylidene) bisphthalic anhydride (6FDA) and N,N'-(2,2'-bis(trifluoromethyl)-[1,1'-biphenyl]-4,4'-diyl) bis(4-aminobenzamide) (TFABMB) as an example, the method for predicting the stress threshold of the 6FDA / TFABMB system is as follows:

[0093] (1) First, use the Amorphous Cell Construction module in the molecular simulation software Materials Studio to construct a three-dimensional amorphous unit cell model of 6FDA / TFABMB polyimide. Set the number of single polyimide molecular chains placed in the molecular model to 10, and each molecular chain contains 20 repeating units;

[0094] Secondly, call the Geometry Optimization module in Forcite to optimize the energy of the polyimide molecular model. The energy convergence criterion is 2×10 -5 kcal / mol using the smart method, and the maximum number of iterations is 10 5 . Obtain the global minimum energy configuration. If the convergence condition is not met, repeat the energy optimization until the convergence condition is satisfied.

[0095] Then use Anneal in the Forcite module to find the lowest energy conformation. Set the annealing temperature range to 298 - 798K and perform 20 cycle treatments. Use the COMPASSⅡ force field, select Ultra-Fine for Quality, the time step is 1fs, use the NPT ensemble, run the annealing for 2000000 steps, save the molecular model structure every 10000 steps, and extract the molecular model with the lowest potential energy; continue with the annealing of the NVT ensemble, with the simulation conditions the same as those of the NPT ensemble, and extract the molecular model with the lowest potential energy for kinetic simulation. Use Dynamics in the Forcite module to run the dynamics of the annealed polyimide molecular model for 2000000 steps in both the NPT and NVT ensembles to make the system reach equilibrium, and obtain the final all-atom molecular model of polyimide after thermodynamic equilibrium.

[0096] (2) Perform mechanical tensile stress-strain curve simulation on the all-atom molecular model of polyimide in thermodynamic equilibrium. Use the Stress-Strain script in Scripting of Materials Studio. In the simulation process, adopt the COMPASSⅡ force field, set the pressure controller as Souza-Martins, and use the Cauchy control constant stress mode. The thermostat is NHL.

[0097] First, relax the all-atom molecular model of polyimide under zero stress until equilibrium. The number of cycles is 5, the time step is 1 fs, and the number of steps is 500,000 steps to obtain the polyimide molecular model in equilibrium under zero stress.

[0098] Then, continuously apply stresses with the same interval in the X direction to the all-atom molecular model of polyimide, control the stresses in the Y direction and Z direction to be 0. Each stress interval is 10 MPa. For each stress, first perform 50,000 steps of equilibrium, and then perform 50,000 steps of tensile simulation under this stress. Calculate the strain values under different stresses, and extract each stress value and strain value to obtain the stress-strain curve.

[0099] (3) Select the stress value of 200 MPa corresponding to the yield point and the stress value of 210 MPa corresponding to the strain point adjacent to the yield point in the calculated stress-strain curve for creep curve simulation. Apply a stress of 200 MPa or 210 MPa in the X direction of the molecular model, control the stresses in the Y direction and Z direction to be 0, and perform 1,000,000 steps of mechanical tensile simulation under this stress. The time step is 1 fs, and output a structural model every 10,000 steps. Calculate the change of strain with time under stresses of 200 MPa and 210 MPa respectively to obtain the creep curve.

[0100] Analyze the change trend of strain with time according to the creep curve. Divide the creep interval based on the initial linear increase of strain (primary creep), entering the plateau region in the middle (secondary creep), and a sharp rise in the later stage (tertiary creep). Only secondary creep appears but no tertiary creep at 1000 ps under 200 MPa, while tertiary creep appears after 30 ps under 210 MPa. The interval between the non-failure stress value of 200 MPa and the failure stress value of 210 MPa is the same as the stress interval value applied in the mechanical tensile curve, both being 10 MPa. Therefore, determine these two stress values as the upper and lower limit stress values of the mechanical failure interval.

[0101] (4) Use the bisection method to iterate the creep curve within the mechanical failure interval of 200 - 210 MPa until the stress value interval between the upper and lower limits of the mechanical failure interval is reduced to 1 MPa. Simulate the creep curve at 205 MPa, and tertiary creep occurs after 90 ps. Therefore, the mechanical failure interval can be reduced to 200 - 205 MPa; select 202 MPa in this interval to simulate the creep curve, and tertiary creep occurs after 210 ps, further reducing the mechanical failure interval to 200 - 202 MPa; select 201 MPa in this interval to simulate the creep curve, and only secondary creep but no tertiary creep occurs at 201 MPa after 1000 ps, indicating that the stress threshold of the 6FDA / TFABMB-based polyimide is 202 MPa.

[0102] The comparison between the stress threshold predicted by the simulation of the 6FDA / TFABMB-based polyimide in Example 4 and the maximum tensile stress value of the experimental verification test is shown in Figure 6 .

[0103] It should be noted that the experimental verification test is to prepare a film of the corresponding polyimide system by using the existing process, and then test it by using the existing method for testing the maximum tensile stress.

[0104] It can be seen from Figure 6 that the trend of the stress thresholds of different structural polyimide systems obtained by using the stress threshold prediction method of the present invention is consistent with the trend of the maximum tensile stress values of the experimental verification test, proving that the prediction method of the present invention can achieve rapid screening of material structures and properties, significantly shorten the time and cost required for the development of new materials, and has higher timeliness.

[0105] As mentioned above, the above is only the preferred specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily thought of by those skilled in the art within the technical scope disclosed by the present invention should be covered by the protection scope of the present invention.

Claims

1. A method for predicting the stress threshold of polyimide, characterized in that, The method includes the following steps: (1) Use molecular simulation software to construct a three-dimensional amorphous unit cell model of polyimide, and obtain a fully atomistic molecular model of polyimide in thermodynamic equilibrium through energy optimization and molecular dynamics simulation; (2) Conduct a mechanical stretching simulation on the fully atomistic molecular model of polyimide in thermodynamic equilibrium, and calculate the stress-strain curve; (3) Select the stress values corresponding to the yield point and its adjacent strain points in the stress-strain curve, conduct a creep curve simulation, and determine the mechanical failure interval based on the creep curve; (4) Use the bisection method to select different stress values within the mechanical failure interval for creep curve iteration, and judge the stress threshold by analyzing the creep curve after iteration.

2. The method for predicting the stress threshold of polyimide according to claim 1, wherein In step (1), first use molecular simulation software to construct a three-dimensional amorphous unit cell model of polyimide; second, perform energy optimization on the three-dimensional amorphous unit cell model of polyimide; then perform annealing-relaxation coupling treatment on the three-dimensional amorphous unit cell model of polyimide after energy optimization.

3. The method for predicting the stress threshold of polyimide according to claim 2, wherein The annealing-relaxation coupling treatment includes the following steps: ① The annealing condition is to perform 15-25 cycles of treatment in the temperature range of 298-798K; ② Use molecular dynamics to perform structural relaxation simulation to obtain a fully atomistic molecular model of polyimide in thermodynamic equilibrium.

4. The method for predicting the stress threshold of polyimide according to claim 1, wherein In step (2), the process of mechanical stretching simulation includes: first relax the fully atomistic molecular model of polyimide under zero stress until equilibrium, and then apply stress to the model to calculate the stress-strain curve.

5. The method for predicting the stress threshold of polyimide according to claim 4, characterized in that, The calculation of the stress-strain curve includes: continuously apply stress at the same interval to the fully atomistic molecular model of polyimide in the X direction, control the stress in the Y and Z directions to be 0, calculate the strain values under different stresses, and obtain the stress-strain curve.

6. The method for predicting the stress threshold of polyimide according to claim 1, characterized in that, In step (3), the creep curve simulation includes: apply a certain stress to the X direction of the fully atomistic molecular model of polyimide, control the stress in the Y and Z directions to be 0, calculate the change of strain with time under this stress, and obtain the creep curve.

7. The method for predicting the stress threshold of polyimide according to claim 6, wherein The certain stress is the stress value corresponding to the yield point in the stress-strain curve calculated in step (2), and the stress value corresponding to the strain point adjacent to the yield point.

8. The method for predicting the stress threshold of polyimide according to claim 1, characterized in that In step (3), the method for determining the mechanical failure interval based on the creep curve includes: analyze the creep curves obtained under different stresses, take the lowest stress value at which tertiary creep occurs as the upper limit of the mechanical failure interval, and take the maximum stress value at which secondary creep occurs but tertiary creep does not occur as the lower limit of the mechanical failure interval.

9. The method for predicting the stress threshold of polyimide according to any one of claims 1-8, characterized in that, In step (4), the method for using the bisection method to select different stress values within the mechanical failure interval for creep curve iteration includes: select the median of the maximum stress value and the minimum stress value in the mechanical failure interval for creep curve simulation, continuously narrow the stress value range for creep curve iteration calculation until the stress value interval between the upper and lower limits of the mechanical failure interval is reduced to 1MPa.

10. The method for predicting the stress threshold of polyimide according to claim 9, wherein In step (4), the method for judging the stress threshold includes: analyze the creep curve after iteration, and take the upper limit stress value after reducing the stress value interval between the upper and lower limits of the mechanical failure interval to 1MPa as the stress threshold.

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