Model construction method for evaluating tensile property of polyelectrolyte grafted nanoparticles

Abstract

The invention discloses a model construction method for simulating and evaluating the tensile mechanical property of a polyelectrolyte grafted nanoparticle composite material by adopting molecular dynamics, and belongs to the technical field of nano composite materials. According to the method, the internal stress tensor of the nano composite material is obtained by applying one-dimensional tensile strain to the polyelectrolyte nano composite material. In the simulation process, strain and stress values of the system are output within a certain time interval, so that continuity and accuracy of data are ensured. The method has the core advantages that under the condition that long-range electrostatic interaction of a polymer system is accurately introduced, the one-dimensional stretching process of the polymer nano composite material can be efficiently simulated, key mechanical property parameters in the deformation process can be accurately extracted, and the research means of the mechanical property of the material are greatly enriched. Compared with a traditional experiment method, the technology has the advantages that the research cost is remarkably reduced, external interference and uncontrollable factors which are difficult to avoid in experiment operation are avoided, and a more reliable, economical and efficient alternative scheme is provided for mechanical property evaluation of the nano composite material. Therefore, the method disclosed by the invention has a remarkable value in promoting scientific research and technical application of nano composite materials.

Description

Technical Field

[0001] The invention belongs to the technical field of polymer-based nanocomposite materials, and relates to a method for evaluating the mechanical properties of a composite material by performing one-dimensional tensile deformation on a polyelectrolyte grafted nanoparticle composite material based on molecular dynamics simulation. Technical Background

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

[0003] Polyelectrolyte grafted nanoparticle composites have nanoscale particle sizes and polyelectrolyte chain sizes, and there are long-range electrostatic interactions between the building units. Their structures and dynamic properties have multi-scale coupling characteristics. Experimental studies on their mechanical properties are easily limited by loading experimental equipment, experimental environment, and characterization and analysis instruments. In addition, the experiments are labor-intensive and costly. It is difficult to fully understand the mechanical properties of polyelectrolyte grafted nanocomposites through experimental methods.

[0004] As a scientific research paradigm of "computer experiment", molecular dynamics simulation calculates the energy or interaction force between molecules by changing the coordinates of "molecules" in the system. In particular, the test of mechanical properties involves non-equilibrium behavior. The molecular dynamics method can more conveniently study the experimental stretching process. Due to its high efficiency and cost-saving experimental research, it is widely used in composite materials, life sciences, precision machinery and other fields.

[0005] The molecular dynamics simulation method for studying polymer-based nanocomposites has been continuously developed and improved. However, there are still some problems: First, since the all-atom potential energy function (OPLS potential energy, COMPASS potential energy, etc.) model of polyelectrolyte grafted nanoparticles involves more degrees of freedom of the system, the tensile simulation calculation requires a lot of computer time, and the strain rate range of molecular dynamics simulation obtained in the literature is 106~10 10 s -1 , much higher than the typical experimental value of 0.01 to 1 s -1(J.Chem.Phys.2017,147,134901), the coarse-grained method can realize the strain rate stretching simulation corresponding to the experimental conditions within a limited machine time, and obtain simulation results that can be compared with the experiment. Secondly, the existing simulation experiments of polymer-based nanocomposites all consider neutral systems, but the synthesis and preparation process of composite materials often make each component charged (Progress in Polymer Science, 2023,143,101710), and the introduction of electrostatic Coulomb interaction enables the simulation system to reproduce the real experiment. Therefore, how to efficiently and accurately simulate the behavior of charged polymer nanocomposite systems under laboratory strain rates, while ensuring the equilibrium state and effective loading of the simulation system, so as 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 of the invention

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

[0007] In order to achieve the above object, the technical solution of the present invention is as follows:

[0008] A method for constructing a model for evaluating the tensile mechanical properties of a polyelectrolyte-grafted nanoparticle composite material using a molecular dynamics method comprises the following steps:

[0009] The present invention loads the polyelectrolyte grafted nanoparticle composite material through the "deform" command in Lammps to achieve tensile deformation of the nanocomposite material, and records the internal stress tensor of the system during 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 simulated equilibrium system is loaded with tensile deformation, and the simulated system is uniformly expanded along the boundaries of both sides of the X-axis direction, so that the simulated system undergoes one-dimensional tensile deformation. At the same time, since the volume of the material remains unchanged, the boundaries in the Y and Z directions gradually shrink, and the control of the overall tensile deformation of the simulated system is achieved. According to Newton's third law, the negative value of the internal stress obtained by statistics is the stress load of the nanocomposite material. Among them, the molecular dynamics method and coarse-grained force field adopted by the present invention are closer to real experiments, and the mechanical properties of the obtained nanocomposite material are more accurate.

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

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

[0012] (2) Setting the initial simulation parameters, using the three-dimensional simulation mode, selecting the LJ dimensionless format, setting the mass of the nanoparticles, the mass of the polyelectrolyte chain monomer, the mass of the counterion, the mass of the salt anion, and the mass of the salt cation, and setting the boundaries of the three directions of the simulation space to periodic boundary conditions;

[0013] (3) Define the interaction potential between particles. The bonding interaction 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 interaction mainly includes electrostatic and volume repulsion interactions. The electrostatic interaction is described by Coulomb potential energy, and the volume repulsion effect is described by Lennard-Jones potential. Set the simulation time step Δt = 0.005. This is because the Velocity-Verlet algorithm is used to solve the equation of motion in the simulation. A reasonable time step can both increase the accuracy of the simulation and extend 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 requirements and keep their shape unchanged during program execution;

[0015] (5) Applying a random initial velocity that satisfies the Gaussian distribution to the overall model, and optimizing the initial conformation by minimizing energy;

[0016] (6) After optimization, the NPT ensemble is used to relax to a stable state, corresponding to an actual pressure of P = 1.0 atm and a temperature of T = 300 K. The pressure control during relaxation is carried out by the Nose-Hoover method, and the temperature control is carried out by the Langevin method, because the Langevin temperature control method is more suitable for nano-mesoscopic systems;

[0017] (7) The NVT ensemble is used to relax the entire model to a stable equilibrium state to ensure that the system is in a state without prestress, and the temperature is controlled at 1.0τ. The Langevin method is used for temperature control during relaxation;

[0018] (8) setting the tensile deformation amplitude, loading one-dimensional tensile deformation on the entire polymer nanocomposite model through non-equilibrium molecular dynamics simulation (NEMD), and completing the molecular dynamics simulation of the tensile mechanical properties of the polyelectrolyte grafted nanocomposite;

[0019] (9) Obtain the corresponding data files and graphic files of the stretching process. Use VMD software to obtain real-time dynamic images of the stretching process. Analyze and process the output files as needed to obtain mechanical performance parameters such as stress-strain curves and elastic modulus.

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

[0020] a) The present invention provides a model construction method for testing the tensile mechanical properties of polyelectrolyte grafted nanoparticle composite materials, which can not only effectively reduce the cost and loss caused by the experiment, but also reproduce through simulation the experiments that cannot be accurately conducted and completed due to the limitations of related equipment and materials.

[0021] b) By using the method of the present invention, tensile stress tensors are obtained by loading tensile strain on a simulation box in a non-equilibrium molecular dynamics simulation, so that the mechanical property characteristics of the nanocomposite material can be obtained quickly and conveniently, and the high computational cost limitation of the all-atom simulation can be broken through, and a larger spatial and temporal scale can be achieved, thereby testing the mechanical properties of the nanocomposite material system with higher computational efficiency.

[0022] c) In the calculation process of particles in the system, the sampling interval is set to take the average value as the stress value, which reduces the thermodynamic disturbance of the simulation system. This effectively avoids the problem of excessive deviation of the sampling value and makes the sample point value more stable.

[0023] d) By flexibly controlling the tensile deformation strain amplitude and the total duration of the stretching process, the stretching rate of the polyelectrolyte nanocomposite material can be flexibly adjusted, which facilitates the statistical analysis of the mechanical performance parameters at different stretching rates.

[0024] e) Through the steps and setting parameters of the above polyelectrolyte nanocomposite material, only by changing the tensile deformation amplitude in the above step (8), non-equilibrium molecular dynamics simulations such as stretching and compression of the polyelectrolyte nanocomposite material can be achieved to obtain the tensile stress-strain curve, elastic modulus, etc.

[0025] f) In addition, by compiling the program, models of nanoparticles of different shapes, different grafting densities and grafting methods, polyelectrolyte chains of different topological structures, counterions of different valence states, and different salt ion strengths can be established, and the parameters related to their mechanical properties can be obtained by the non-equilibrium molecular dynamics method of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 Snapshot of polyelectrolyte grafted nanoparticles created by the compiler (marked 123, etc.);

[0027] Figure 2 Snapshot of the polyelectrolyte-grafted nanoparticle composite obtained after relaxation;

[0028] Figure 3 Flowchart of molecular dynamics simulation of polyelectrolyte grafted nanoparticle composites;

[0029] Figure 1 In the figure: 1 represents nanoparticles; 2 represents grafting points; 3 represents polyelectrolyte monomers; 4 represents counterions; 5 represents a simulation system consisting of polyelectrolyte-grafted nanoparticles and counterions;

[0030] Figure 2 Middle: 1 indicates the tensile deformation loading direction. Specific embodiments

[0031] The method of the present invention is further described in detail with reference to the accompanying drawings and embodiments:

[0032] The specific embodiment of the present invention is a stretching molecular dynamics simulation of a polyelectrolyte-grafted nanoparticle composite material, such as Figure 3 Flowchart of molecular dynamics simulations for polyelectrolyte-grafted nanoparticle composites:

[0033] 1) The polyelectrolyte grafted nanoparticle composite material model was established by the inventor's compiler program, such as Figure 1As shown, the compiler saves the coordinate file and topological structure information of the nanocomposite model as a data file recognizable by Lammps. Lammps is used to write a program to realize the non-equilibrium molecular dynamics simulation calculation of the polyelectrolyte grafted nanoparticle composite material. The model includes nanoparticles, grafted polyelectrolyte chains, and counterions.

[0034] 2) Set the initial simulation parameters, use the three-dimensional simulation mode, set the measurement unit to LJ format, set the mass of the nanoparticles, the mass of the polyelectrolyte chain monomer, the mass of the counter ion, and the mass of the salt ion, and set the boundaries of the three directions of the simulation space to periodic boundary conditions.

[0035] 3) Define the interaction potential between particles. Bonding interaction mainly includes bond length and bond angle. Bond length is described by FENE potential energy, and bond angle is described by Hamonic potential energy to control the rigidity of ligand chain. Non-bonding interaction mainly includes electrostatic and volume repulsion interaction. Electrostatic interaction is described by Coulomb potential energy, and volume repulsion is described by Lennard-Jones potential. Set the simulation time step; set the dielectric constant of the simulation system;

[0036] 4) Set the nanoparticles to meet the Rigid-Body Constraint requirements and keep their shape unchanged during program execution;

[0037] 5) Apply a random initial velocity that satisfies the Gaussian distribution to the overall model and optimize the initial conformation by minimizing energy;

[0038] 6) After optimization, the NPT ensemble is used to relax to a stable state, corresponding to the actual pressure P = 1.0 atm and the temperature T = 300 K. The pressure control during relaxation adopts the Nose-Hoover method, and the temperature control adopts the Langevin method, because the Langevin temperature control method is more suitable for nano-mesoscopic systems;

[0039] 7) The whole model was relaxed to a stable equilibrium state using the NVT ensemble to ensure that the system was in a state without prestress, and the temperature was controlled at 1.0 tau. The Langevin method was used for temperature control during relaxation;

[0040] 8) setting the tensile deformation amplitude, loading one-dimensional tensile deformation on the entire polymer nanocomposite model through non-equilibrium molecular dynamics simulation (NEMD), and completing the molecular dynamics simulation of the tensile mechanical properties of the polyelectrolyte grafted nanocomposite;

[0041] 9) Finally, the corresponding data files and graphic files of the stretching process are obtained. The real-time dynamic images of the stretching process can be obtained using VMD software. The output files are analyzed and processed as needed to obtain mechanical performance parameters such as stress-strain curves and elastic modulus.

[0042] In summary, the above is only a specific implementation of the present invention, but the protection scope of the invention is not limited thereto. Any changes that researchers and engineers in the technical field can make within the technical scope of the present invention should be regarded as infringing the protection scope of the present invention. Therefore, the protection scope of the present invention should be based on the protection scope of the claims.

Claims

1. A method for constructing a model for evaluating the tensile mechanical properties of polyelectrolyte grafted nanoparticle composites using molecular dynamics simulation, characterized in that: In the molecular dynamics simulation, a fixed stretching rate is applied to make the simulation system undergo one-dimensional tensile deformation; and the stress tensor in the system during this deformation process is recorded at regular simulation step lengths to describe the tensile mechanical properties of the polyelectrolyte grafted nanocomposite material; The molecular dynamics simulation comprises the following steps: (1) Construct a coarse-grained model of a polyelectrolyte grafted nanoparticle composite material, and compile the program to output a data file that can be read by Lammps. Use Lammps to write a program to implement molecular dynamics simulation calculations of fixed rate / fixed tensile force stretching of polyelectrolyte grafted nanocomposites; the model includes spherical nanoparticles of different diameters, ligand polyelectrolyte chains of different conformations (linear, cyclic homopolymers / block copolymers), counterions, salt anions, and salt cations; (2) Setting the initial simulation parameters, using the three-dimensional simulation mode, setting the measurement unit to the LJ format, setting the mass of the nanoparticles, the mass of the polyelectrolyte chain monomer, the mass of the counterion, the mass of the salt anion, and the mass of the salt cation, and setting the boundaries of the three directions of the simulation space to periodic boundary conditions; (3) Define the potential for particle interaction. Bond interaction mainly includes bond length and bond angle. Bond length is described by FENE potential energy, and bond angle is described by Hamonic potential energy to control the rigidity of the ligand chain. Non-bonded interaction mainly includes electrostatic and volume repulsion interactions. Electrostatic interaction is described by Coulomb potential energy, and volume repulsion is described by Lennard-Jones potential. Set the simulation time step; set the dielectric constant of the simulation system; (4) Set the nanoparticles to meet the Rigid-Body Constraint requirements and keep their shape unchanged during program execution; (5) Applying a random initial velocity that satisfies the Gaussian distribution to the overall model, and optimizing the initial conformation by minimizing energy; (6) After optimization, the NPT ensemble was used to relax to a stable state, corresponding to an actual pressure of P = 1.0 atm and a temperature of 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) Loading one-dimensional tensile deformation on the entire polymer nanocomposite model to complete the molecular dynamics simulation of the tensile mechanical properties of the polyelectrolyte grafted nanocomposite; (9) Post-process the saved information as needed to obtain mechanical property parameters.

2. The method for constructing a model for evaluating the tensile mechanical properties of polyelectrolyte grafted nanoparticle composites using molecular dynamics simulation according to claim 1, characterized in that: In the step (3), the electrostatic interaction between polymer ligands is taken into account. The long-range electrostatic interaction calculation adopts the particle-particle / particle-grid method. The relative dielectric constant is set to describe the solvent properties. The water solvent is ε r =80, and other solvent conditions are achieved by changing the dielectric constant.

3. The method for constructing a model 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 stretching and loading the polyelectrolyte grafted nanoparticle composite material is specifically as follows: setting the one-dimensional stretching deformation amplitude in the X direction, adjusting the stretching rate by changing the duration of the stretching process, and applying deformation to the simulation box to achieve loading of the nanocomposite material, keeping the total volume of the material unchanged during the stretching process (the polymer nanocomposite material is incompressible), being stretched in the X direction, and shrinking in the Y and Z directions with the stretching, and the corresponding material Poisson's ratio μ≈0.

5.

4. The method for constructing a model for studying 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 every 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 method for constructing a model for studying 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 using the SLLOD motion equation.

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