A method for constructing a model for evaluating tensile properties of polyelectrolyte grafted nanoparticles

Abstract

A model construction method for evaluating tensile mechanical properties of polyelectrolyte grafted nanoparticle composite materials by molecular dynamics simulation belongs to the technical field of nanocomposite materials. The method obtains the internal stress tensor of the nanocomposite material by applying one-dimensional tensile strain to the polyelectrolyte nanocomposite material. In the simulation process, the strain and stress values of the system are output within a certain time interval, thereby ensuring the continuity and accuracy of the data. The core advantage of the application lies in that under the condition of accurately introducing long-range electrostatic interactions of the polymer system, not only can the one-dimensional stretching process of the polymer nanocomposite material be efficiently simulated, but also the key mechanical property parameters in the deformation process can be accurately extracted, greatly enriching the research means of material mechanical properties. Compared with the traditional experimental method, the present technology significantly reduces the research cost, and at the same time, avoids the inevitable external interference and uncontrollable factors in the experimental operation, thereby providing a more reliable, economical and efficient alternative solution for the mechanical property evaluation of nanocomposite materials. Therefore, the present application has significant value in promoting the scientific research and technical application of nanocomposite materials.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of polymer-based nanocomposites, and relates to a method for one-dimensional stretching deformation of polyelectrolyte grafted nanoparticle composites based on molecular dynamics simulation and evaluation of mechanical properties of the composites. TECHNICAL BACKGROUND

[0002] Polymer-based nanomaterials are innovative functional materials formed by adding nanoparticles with a particle size of 1-100 nm to a polymer matrix, and are an important part of new materials in the national strategic emerging industry. Due to the combination of the properties of nanoparticles and polymers, polymer-based nanomaterials exhibit unique mechanical, electromagnetic, catalytic and other excellent properties, and have wide applications in the fields of aerospace, clean energy and precision industry. Therefore, the research on the performance of nanocomposites has attracted attention from the industry and academia.

[0003] The polyelectrolyte grafted nanoparticle composites have a nanoparticle size and a polyelectrolyte chain size belonging to the nanoscale, and long-range electrostatic interactions exist between the building units, so that the structure and dynamic properties of the polyelectrolyte grafted nanoparticle composites have multi-scale coupling characteristics. The research on the mechanical properties of the polyelectrolyte grafted nanoparticle composites is limited by the loading experimental device, experimental environment and characterization analysis instrument, and the experiment consumes a lot and has a high cost. It is difficult to fully understand the mechanical properties of the polyelectrolyte grafted nanoparticle composites through experimental methods.

[0004] Molecular dynamics simulation as a scientific research paradigm of "computer experiment" calculates the energy or interaction force between molecules through the change of the coordinates of "molecules" in the system. In particular, the test of mechanical properties involves non-equilibrium behavior, and the molecular dynamics method can more conveniently study the experimental stretching process. Due to its high efficiency and cost-saving characteristics, the molecular dynamics method is widely used in the fields of composites, life science, precision machinery and the like.

[0005] The research on polymer-based nanocomposites by molecular dynamics simulation method has been continuously developed and improved. However, there are still some problems: first, the all-atom potential energy function (OPLS potential energy, COMPASS potential energy, etc.) model of the polyelectrolyte grafted nanoparticle involves more degrees of freedom of the system, and a large amount of computer time is required for stretching simulation calculation. In addition, the strain rate range of the molecular dynamics simulation obtained in the literature is 106-10 10 s -1 , which is much higher than the typical experimental value of 0.01-1 s -1(J. Chem. Phys. 2017, 147, 134901), the coarse-graining method can realize the strain rate stretching simulation corresponding to the experimental conditions within a limited machine time length, and obtain simulation results that can be compared with experiments. Secondly, existing simulation experiments of polymer-based nanocomposites all consider neutral systems, but the synthesis and preparation process of composites often makes each component charged (Progress in Polymer Science, 2023, 143, 101710). The introduction of electrostatic Coulomb interaction makes the simulation system be able to reproduce the real experiment. Therefore, how to efficiently and accurately simulate the behavior of charged polymer nanocomposite systems under laboratory strain rate, while ensuring the equilibrium state and effective loading of the simulation system, to better match the experimental conditions and improve the reliability of the simulation results? It is a technical problem that needs to be solved in the research and development process of polymer-based nanocomposites. SUMMARY

[0006] In order to solve the above problems, the present application provides a method for testing the mechanical properties of polyelectrolyte grafted nanoparticle composites based on molecular dynamics simulation, specifically a strain loading and statistical internal stress tensor method suitable for molecular dynamics simulation is used, which avoids the controversy of the stress calculation method based on the Green-Kubo formula, and by appropriately selecting the coarse-grained force field between the nanoparticles, polyelectrolyte monomers, counterions, salt anions and salt cations, setting reasonable simulation parameters and steps, the mechanical property parameters of polyelectrolyte grafted nanoparticle composites under different stretching rates can be accurately calculated, such as stress-strain curve, elastic modulus, maximum yield stress, etc.

[0007] In order to achieve the above purpose, the technical scheme of the present application is as follows:

[0008] A model construction method for evaluating the tensile mechanical properties of polyelectrolyte grafted nanoparticle composites using molecular dynamics method, comprising the following steps:

[0009] The application realizes the tensile deformation of the nanocomposite material by loading the polyelectrolyte grafted nanoparticle composite material in the 'deform' (deformation) command of Lammps, and records the internal stress tensor in the tensile deformation process, and statistically analyzes the mechanical properties. More specifically, on the basis of obtaining the equilibrium structure of the polyelectrolyte grafted nanoparticles under different parameters, the simulation equilibrium system is loaded with tensile deformation, the simulation system is uniformly expanded along the X-axis direction of the two sides of the boundary, so that the simulation system is subjected to one-dimensional tensile deformation, and at the same time, since the volume of the material remains unchanged, the Y and Z direction boundaries are gradually shrunk, realizing the control of the overall tensile deformation of the simulation system. According to Newton's third law, the negative value of the internal stress obtained by statistics is the stress load of the nanocomposite material. The molecular dynamics method and the coarse-grained force field used in the application are more close to the real experiment, and the mechanical properties of the nanocomposite material obtained are more accurate.

[0010] The molecular dynamics simulation comprises the following steps:

[0011] (1) Construct a polyelectrolyte grafted nanoparticle composite material model, and compile the program to save the coordinate file and topological structure information of the nanocomposite material model as a data file recognizable by Lammps. Use Lammps to write a program to realize the non-equilibrium molecular dynamics simulation calculation of the polyelectrolyte grafted nanoparticle composite material. The model comprises nanoparticles of different shapes and sizes, grafted polyelectrolyte chains of different topological structures, and salt ions of different valence states.

[0012] (2) Set the initial simulation parameters, use a three-dimensional simulation mode, select the LJ dimensionless format, set the mass of the nanoparticles, the mass of the polyelectrolyte chain monomers, the mass of the counterions, the mass of the salt anions, and the mass of the salt cations, and set the boundary conditions of the simulation space in three directions as periodic boundary conditions;

[0013] (3) Define the interaction potential between particles, the bond effect mainly includes bond length and bond angle, the bond length is described by FENE potential energy, and the bond angle is described by Hamonic potential energy to control the rigidity of the ligand chain. The non-bonding effect mainly includes electrostatic and volume repulsion interaction, the electrostatic interaction is described by Coulomb potential energy, and the volume repulsion is described by Lennard-Jones potential. Set the time step Δt of the simulation to 0.005, because the Velocity-Verlet algorithm is used to solve the motion equation in the simulation, a reasonable time step can increase the accuracy of the simulation and prolong the time scale of the simulation; set the dielectric constant of the simulation system;

[0014] (4) Set the nanoparticles to meet the Rigid-Body Constraint requirement, and the shape remains unchanged during the program running;

[0015] (5) Apply random initial velocity satisfying Gaussian distribution to the whole model, and optimize the initial conformation by energy minimization;

[0016] (6) Relax the optimized model to stable state by NPT ensemble, corresponding to actual pressure P=1.0 atm and temperature T=300 K, and the pressure control in the relaxation adopts Nose-Hoover method, and the temperature control adopts Langevin method, because the Langevin temperature control method is more suitable for nano-mesoscopic system;

[0017] (7) Relax the whole model to stable equilibrium state by NVT ensemble, ensure that the system is in a stress-free state, and control the temperature at 1.0 tau, and the temperature control in the relaxation adopts Langevin method;

[0018] (8) Set the stretching deformation amplitude, load one-dimensional stretching deformation on the whole polymer nanocomposite material model by non-equilibrium molecular dynamics simulation (NEMD), and complete the molecular dynamics simulation of the tensile mechanical properties of the polyelectrolyte grafted nanocomposite material;

[0019] (9) Obtain corresponding data files and graphic files of the stretching process, real-time dynamic images of the stretching process can be obtained by using VMD software, and the output files are analyzed and processed according to the needs, so that the stress-strain curve, elastic modulus and other mechanical property parameters are obtained.

[0020] The present application mainly has the following advantages:

[0021] a) The model construction method for testing the tensile mechanical properties of the polyelectrolyte grafted nanoparticle composite material provided by the present application can not only effectively reduce the cost and loss caused by experiments, but also can reproduce the experiments that cannot be accurately performed and completed due to the limitation of related equipment and materials.

[0022] b) The present application can load stretching strain on the simulation box in non-equilibrium molecular dynamics simulation to obtain the tensile stress tensor, so that the mechanical property characteristics of the nanocomposite material can be obtained conveniently and quickly, the limitation of high calculation cost of full-atom simulation can be broken through, a larger space and time scale can be realized, and the mechanical properties of the nanocomposite material system can be tested with higher calculation efficiency.

[0023] c) In the calculation process of the particles in the system, the average value of the sampling interval is set as the stress value, the thermodynamic disturbance of the simulation system is reduced, the problem of too large sampling value deviation is effectively avoided, and the sample point value is more stable.

[0024] d) By flexibly controlling the stretching deformation strain amplitude and the total duration of the stretching process, the stretching rate of the polyelectrolyte nanocomposite material can be flexibly regulated, and the mechanical property parameters under different stretching rates can be conveniently counted.

[0025] e) By the above steps and setting parameters of the polyelectrolyte nanocomposite material, only by changing the stretching deformation amplitude in the above step (8), the stretching, compression and other non-equilibrium molecular dynamics simulation of the polyelectrolyte nanocomposite material can be realized, and the stretching stress-strain curve and elastic modulus can be obtained.

[0026] f) In addition, by compiling programs, different shape nanoparticles, different grafting density and grafting mode, different topological structure polyelectrolyte chain, different valence counter ion, different salt ion strength model can be established, and the mechanical property related parameters can be obtained by the non-equilibrium molecular dynamics method of the application. BRIEF DESCRIPTION OF DRAWINGS

[0027] Figure 1 Snapshot of polyelectrolyte grafted nanoparticle established by compiling program (123 and the like are marked);

[0028] Figure 2 Snapshot of polyelectrolyte grafted nanoparticle composite material obtained after relaxation;

[0029] Figure 3 Molecular dynamics simulation flowchart of polyelectrolyte grafted nanoparticle composite material;

[0030] Figure 1 In the figure, 1 represents a nanoparticle; 2 represents a grafting point; 3 represents a polyelectrolyte monomer; 4 represents a counter ion; and 5 represents a simulation system composed of polyelectrolyte grafted nanoparticles and counter ions;

[0031] Figure 2 In the figure, 1 represents a stretching deformation loading direction. DETAILED EMBODIMENT

[0032] The method of the application is further described in detail in combination with the drawings and embodiments:

[0033] The specific embodiment of the application is the stretching molecular dynamics simulation of the polyelectrolyte grafted nanoparticle composite material, such as Figure 3 The molecular dynamics simulation flowchart of the polyelectrolyte grafted nanoparticle composite material is as follows:

[0034] 1) The polyelectrolyte grafted nanoparticle composite material model is established by the compiling program of the inventors, such as Figure 1As shown, the compiler will save the coordinate file of the nanocomposite model and the topological structure information as a data file recognizable by Lammps. The non-equilibrium molecular dynamics simulation of the polyelectrolyte grafted nanoparticle composite is realized by using the program written by Lammps. The model includes nanoparticles, grafted polyelectrolyte chains, and counterions.

[0035] 2) Set the initial parameters of the simulation, use the three-dimensional simulation mode, set the unit to LJ format, set the mass of the nanoparticles, the mass of the polyelectrolyte chain monomer, the mass of the counterion, and the mass of the salt ion, and set the boundary conditions of the simulation space in three directions to be periodic.

[0036] 3) Define the interparticle potential, the bonding interaction mainly includes bond length and bond angle, the bond length is described by FENE potential, and the bond angle is described by Hamonic potential to control the rigidity of the ligand chain. The non-bonding interaction mainly includes electrostatic and volume repulsion interaction, the electrostatic interaction is described by Coulomb potential, and the volume repulsion is described by Lennard-Jones potential. Set the time step of the simulation; set the dielectric constant of the simulation system;

[0037] 4) Set the nanoparticles to meet the Rigid-Body Constraint requirement, and the shape remains unchanged during the program running;

[0038] 5) Apply random initial velocity satisfying Gaussian distribution to the overall model, and optimize the initial conformation by energy minimization;

[0039] 6) After optimization, use NPT ensemble to relax to the stable state, the corresponding actual pressure P = 1.0 atm and temperature T = 300 K, the pressure control in the relaxation uses Nose-Hoover method, and the temperature control uses Langevin method, because the Langevin temperature control method is more suitable for nanometer mesoscopic system;

[0040] 7) Relax the overall model to a stable equilibrium state using NVT ensemble, ensure that the system is in a stress-free state, and control the temperature at 1.0 tau, the temperature control in the relaxation uses Langevin method;

[0041] 8) Set the stretching deformation amplitude, load one-dimensional stretching deformation on the entire polymer nanocomposite model through non-equilibrium molecular dynamics simulation (NEMD), and complete the molecular dynamics simulation of the tensile mechanical properties of the polyelectrolyte grafted nanocomposite;

[0042] 9) Finally, the corresponding data file and stretching process graphic file are obtained, the real-time dynamic images of the stretching process can be obtained using VMD software, the output file is analyzed and processed according to the needs, and the stress-strain curve, elastic modulus and other mechanical property parameters are obtained.

[0043] In conclusion, only the specific embodiment of the present application, but the scope of protection of the invention is not limited thereto, any of the researchers and engineers in the technical field of the invention within the scope of the technology, can be done some changes, should be as an infringement of the scope of protection of the invention. Therefore, the scope of protection of the present application should be subject to the scope of protection of the claims.

Claims

1. A method for constructing a model to evaluate the tensile mechanical properties of polyelectrolyte-grafted nanoparticle composites using molecular dynamics simulations, characterized in that, In molecular dynamics simulations, a fixed stretching rate is applied to induce one-dimensional tensile deformation in the simulated system. The stress tensor within the system during this deformation process is recorded at regular simulation steps to describe the tensile mechanical properties of polyelectrolyte-grafted nanocomposites. The molecular dynamics simulation includes the following steps: (1) Construct a coarse-grained model of polyelectrolyte-grafted nanoparticle composite material, and compile a program to output a data file that can be read by Lammps for a fixed rate / fixed tensile force of the grafted nanoparticle composite material; use Lammps to write a program to realize the molecular dynamics simulation calculation of polyelectrolyte stretching; the model includes: spherical nanoparticles of different diameters; ligand polyelectrolyte chains of different conformations, the conformations including linear homopolymers, cyclic homopolymers, linear block copolymers and cyclic block copolymers; counterions; salt anions; salt cations; (2) Set the initial simulation parameters, adopt the three-dimensional simulation mode, set the measurement unit to LJ format, set the mass of nanoparticles, polyelectrolyte chain monomers, counterion mass, salt anion mass, and salt cation mass, and set the boundary conditions of the three directions of the simulation space to be periodic boundary conditions. (3) Define the interparticle interaction potential. Bonding mainly includes bond length and bond angle. Bond length is described by the FENE potential energy, and bond angle is described by the Hamonic potential energy, which can control the rigidity of the ligand chain. Non-bonding mainly includes electrostatic and volume repulsion interactions. Electrostatic interaction is described by the Coulomb potential energy, and volume repulsion is described by the Lennard-Jones potential energy. Set the simulation time step. Set the dielectric constant of the simulation system. (4) Set the nanoparticles to meet the Rigid-Body Constraint requirement so that their shape remains unchanged during program execution; (5) Apply a random initial velocity that satisfies a Gaussian distribution to the overall model and optimize the initial conformation by minimizing energy; (6) After optimization, the NPT ensemble is used to relax to a steady state, with the corresponding actual pressure P = 1.0 atm and temperature T = 300 K; (7) The model was relaxed to equilibrium using the NVT ensemble and the temperature was controlled at 1.0τ, i.e. 300K. (8) Apply one-dimensional tensile deformation to the entire polymer nanocomposite model to complete the molecular dynamics simulation of the tensile mechanical properties of polyelectrolyte grafted nanocomposite. (9) Post-process the saved information as needed to obtain mechanical performance parameters.

2. The model construction method for evaluating the tensile mechanical properties of polyelectrolyte-grafted nanoparticle composites using molecular dynamics simulation according to claim 1, characterized in that, Step (3) considers the electrostatic interactions between polymer ligands. The long-range electrostatic interaction calculation adopts the particle-particle / particle-grid method. The relative permittivity is set to describe the solvent properties, with ε being the value for water solvent. r =80, other solvent conditions are achieved by changing the dielectric constant.

3. The model construction method for evaluating the tensile mechanical properties of polyelectrolyte-grafted nanoparticle composites using molecular dynamics simulation according to claim 1, characterized in that, The step (8) of applying tensile loading to the polyelectrolyte grafted nanoparticle composite material is as follows: setting the one-dimensional tensile deformation amplitude in the X direction, adjusting the tensile rate by changing the duration of the tensile process, loading the nanocomposite material by applying deformation to the simulated box, keeping the total volume of the material constant during the tensile process (the polymer nanocomposite material is incompressible), being stretched in the X direction, and shrinking in the Y and Z directions with the stretch, corresponding to a Poisson's ratio μ≈0.

5.

4. The model construction method for evaluating the tensile mechanical properties of polyelectrolyte-grafted nanoparticle composites using molecular dynamics simulation according to claim 1, characterized in that, When averaging the particles in the simulation system in step (8), the sampling interval size is first set, and values ​​are taken at each sampling interval in the sampling interval, and the average value of the sampled values ​​is used as the value of the sampling interval.

5. The model construction method for evaluating the tensile mechanical properties of polyelectrolyte-grafted nanoparticle composites using molecular dynamics simulation according to claim 1, characterized in that, The temperature and pressure control methods in steps (6) and (7) are both Langevin methods; the non-equilibrium tensile deformation process in step (8) is described by the SLLOD equation of motion.

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