CN115132286A - A method for constructing closed cell molecular model of foamed polymer - Google Patents

Abstract

The invention discloses a method for constructing a molecular model of foamed high polymer closed cells, and relates to the technical field of molecular simulation methods. The method comprises the following steps: constructing a high polymer initial molecular model, and adding an OPLSAA force field to the high polymer initial molecular model; performing energy minimization processing on the model; performing dynamic relaxation on the model; introducing cells into the model to generate a high polymer closed cell model with target density; releasing local stress due to cell restraint; and after the density and conformation of the high polymer closed cell model are stable, removing the restraint of the inner and outer cells to obtain a foamed high polymer closed cell molecular model with a stable configuration. The method realizes the construction of a molecular model of the closed cells of the foamed high polymer, is beneficial to the development of molecular simulation work of the foamed high polymer, defines the micro mechanism of deformation and mechanical property of the foamed high polymer and promotes the improvement of the performance of the foamed high polymer material.

Description

A method for constructing closed cell molecular model of foamed polymer

technical field

The invention relates to the technical field of molecular simulation methods, in particular to a method for constructing a closed cell molecular model of a foamed polymer.

Background technique

Foamed polymer has been widely used in practical engineering, especially in non-excavation and non-destructive repair due to its advantages of good impermeability, good durability, and environmental protection. Mechanical properties are the main factors that determine its repair effect. The main factors that affect the mechanical properties of foamed polymer materials are: density, strain rate, temperature, etc. Therefore, extensive experiments and finite element simulation studies have been carried out at home and abroad, revealing that the above main factors have an impact on foamed polymer materials. The general law of the influence of macroscopic mechanical properties has gained some useful understanding. The macroscopic performance of the mechanical properties of the material is determined by the microscopic molecular motion mechanism, and it is difficult to explain the deformation and destruction mechanism of the foamed polymer from the molecular scale by the existing macroscopic experiments. In response to the above problems, some scholars at home and abroad have used molecular dynamics simulation methods to study the micro-mechanical properties of polymer elastomers and the motion mechanism of molecular chains in the system when they are deformed by force, and explain the changes in macro-mechanical properties of polymers from the molecular scale. reason. Although related research work has been reported successively, there is still a lack of molecular dynamics simulation research on foamed polymer materials. The difference is unclear.

Molecular dynamics simulation is a numerical simulation technology that simulates the motion of material molecules. By solving the Newtonian equation of motion of the atoms in the system, information such as the position of the atoms in the system, the changes of the system structure, the energy of the system and the atoms, and the thermodynamic properties of the system can be obtained at different times. The mechanical properties of the material can be explained at the molecular scale, which makes up for the lack of experiments. With the rapid development of computer computing power and computing algorithms in recent years, molecular dynamics simulations have been widely used in polymer research. At this stage, there are no relevant reports on the construction of molecular models of closed cells of foamed polymers with different densities.

SUMMARY OF THE INVENTION

Based on this, the present invention provides a method for constructing a closed cell molecular model of a foamed polymer, so as to realize the construction of a closed cell molecular model of a foamed polymer.

A method for constructing a foamed polymer closed cell molecular model of the present invention comprises the following steps:

Step 1: Use the Amorphous Cell module in the Materials Studio software to build the initial molecular model of the polymer, and use Python programming to add the OPLSAA force field to the initial molecular model of the polymer;

Step 2: Use Lammps software to minimize the energy of the initial molecular model of the polymer after adding the force field;

Step 3: Use Lammps software to perform kinetic relaxation on the initial molecular model of the polymer after energy minimization;

Step 4: Introduce cells into the initial molecular model of the polymer after relaxation to generate a polymer closed cell model of the target density;

Step 5: Release the local stress due to cell constraints in the polymer closed cell model;

Step 6: After the density and conformation of the polymer closed cell model are stabilized, remove the internal and external cell constraints, and perform NPT relaxation on the polymer closed cell model at a temperature of 600K, and set the relaxation time to 100Ps. Set the pressure to 1Pa; cool down the polymer closed cell model after relaxation to 300K, continue to perform NPT relaxation on the polymer closed cell model at a temperature of 300K, set the relaxation time to 100Ps, set The constant pressure is 1Pa, and the closed-cell molecular model of foamed polymer with stable configuration is obtained.

Optionally, the use of Python programming to add the OPLSAA force field to the initial molecular model of the polymer includes:

Obtain the OPLSAA force field parameters corresponding to the initial molecular model of the polymer in LigParGen, and then write a Python script to modify the atom type and topology type of the polymer initial molecular model according to the atom type and topology type contained in the OPLSAA force field parameters, so that The atomic type and topological type of the initial molecular model of the polymer are consistent with the atomic type and topological type contained in the OPLSAA force field parameters.

Optionally, the use of Lammps software to perform energy minimization processing on the initial molecular model of the polymer after adding the force field includes:

Use the min_style command in the Lammps software to select the Polak-Ribiere version of the conjugate gradient algorithm, set the maximum number of iteration steps to 5000 steps, and set the time step to 0.5fs.

Optionally, cells are introduced into the initial molecular model of the polymer after relaxation, and the closed cell model of the polymer with the target density is generated, including:

A spherical cell is introduced into the initial molecular model of the polymer after relaxation, denoted as an internal cell, the center of the internal cell coincides with the center of the initial molecular model of the polymer, and the pore size of the internal cell is from gradually increase from zero;

A spherical cell is introduced outside the initial molecular model of the polymer after the introduction of the internal cell, denoted as external cell, the center of the external cell coincides with the center of the initial molecular model of the polymer, and the minimum initial value of the external cell is The pore size is calculated based on the standard of all atoms that completely enclose the initial molecular model of the polymer, and the pore size of the outer cell is continuously reduced;

The initial molecular model of the polymer introduced into the internal and external cells is recorded as the closed cell model of the polymer, and the rate of change of the internal and external cells of the closed cell model of the polymer is adjusted until the closed cell model of the polymer is reached. The target density is reached.

Optionally, cells are introduced into the initial molecular model of the polymer after relaxation, and the closed cell model of the polymer with the target density is generated, including:

Using the fix indent command in Lammps software, set the force constant to 10, the increase rate of internal cells and the decrease rate of external cells are set according to the size of the polymer closed cell model and the size of the target density, so The minimum initial pore size of the external cells is calculated by the following formula:

Wherein, _outside r is the minimum initial pore size of the outer cell, and L is the edge length of the initial molecular model of the polymer after relaxation.

Optionally, the dynamic relaxation of the initial molecular model of the polymer after the energy minimization treatment using Lammps software includes:

Perform 100Ps NVT kinetic relaxation on the initial molecular model of the polymer after the energy minimization treatment, so that each independent polymer chain in the initial molecular model of the polymer is fully relaxed; during the relaxation process, set the force The field is the OPLSAA force field, the temperature is controlled by the Nose thermal bath method, the pressure is controlled by the Berendsen constant pressure method, the van der Waals force is solved by the LJ interaction, and the electrostatic interaction is solved by the PPPM method;

Perform NPT molecular dynamics simulation on the initial molecular model of the polymer, and continue to relax the initial molecular model of the polymer at a pressure of 800Pa and a temperature of 600K. The relaxation time is set to 1000ps and the time step is set to 0.5fs ;

Control the temperature of the initial molecular model of the polymer to remain unchanged, and reduce the pressure to 1Pa. After the depressurization is completed, control the pressure and temperature to remain unchanged, and continue to relax the initial molecular model of the polymer under the conditions of 1Pa pressure and 600K temperature. , and set the relaxation time to 100 ps.

Optionally, the local stress generated due to cell confinement in the closed cell model of the released polymer includes:

After the closed cell model of the polymer reaches the target density, the inner cell diameter and the outer cell diameter are denoted as _rinend and _routend respectively; keeping the inner cell diameter _rinend unchanged, the outer cell diameter The pore size gradually decreases from the initial value r _{to the end} of r, and after repeated cycles, until the pore size of the _outer cell decreases from the initial value r to _{the end} of r, the closed cell model of the high polymer is in the process. No change in density and conformation.

Beneficial effects of the present invention:

The method for constructing a foamed polymer closed cell molecular model of the present invention makes up for the deficiencies of the existing polymer molecular models, and is helpful for the development of foamed polymer molecular simulation work. The model can be used to further To clarify the micro-mechanism of macro-deformation and macro-mechanical properties of foamed polymer, promote the improvement of foamed polymer material properties, and maximize the engineering benefits of foamed polymer.

Description of drawings

In order to illustrate the technical solutions in the embodiments of the present invention more clearly, the following briefly introduces the accompanying drawings that need to be used in the description of the embodiments or the prior art. Obviously, the drawings in the following description are only for the present invention. In some embodiments, for those of ordinary skill in the art, other drawings can also be obtained according to these drawings without any creative effort.

Fig. 1 is the flow chart of the construction method of a kind of foamed polymer closed cell molecular model provided by the invention;

Fig. 2 is the chemical structure diagram of a kind of long-chain polyurethane provided by the invention;

Fig. 3 is the polyurethane molecular model diagram after dynamic relaxation provided by the present invention;

Fig. 4 is the foaming process diagram of the polymer closed cell model provided by the present invention;

FIG. 5 is a cross-sectional view of a closed-cell molecular model of a 0.35 g/cm ³ foamed high polymer provided by the present invention.

Detailed ways

In the following description, for the purpose of illustration rather than limitation, specific details such as specific system structures and technologies are set forth in order to provide a thorough understanding of the embodiments of the present invention. In order to illustrate the technical solutions of the present invention, the following specific embodiments are used for description.

Referring to FIG. 1, it is a flow chart of a method for constructing a foamed polymer closed cell molecular model provided by the present embodiment, and the method may include the following steps:

Step 1: Use the Amorphous Cell module in the Materials Studio software to build the initial molecular model of the polymer, and use Python programming to add the OPLSAA force field to the initial molecular model of the polymer.

In this embodiment, the polymer selected by the initial molecular model of the polymer is non-aqueous reactive foaming polyurethane, and the main synthetic materials of non-aqueous reactive foaming polyurethane are polyphenylmethane polyisocyanate (MDI) and sucrose polyether polyol Alcohol, the structural formula is shown in Figure 2. The Amorphous Cell module in the Materials Studio software was used to build the initial molecular model of the polymer, and the OPLSAA force field was added to the initial molecular model of the polymer using Python programming.

The specific implementation method of adding the OPLSAA force field to the polymer initial molecular model is: obtain the OPLSAA force field parameters corresponding to the polymer initial molecular model in LigParGen, and then write Python according to the atom type and topology type contained in the OPLSAA force field parameters. The script modifies the atomic type and topological type of the initial molecular model of the polymer, so that the atomic type and topological type of the initial molecular model of the polymer are consistent with the atomic type and topological type contained in the OPLSAA force field parameters.

Step 2: Use Lammps software to perform energy minimization on the initial molecular model of the polymer after adding the force field.

In this embodiment, the initial molecular model of the polymer is optimized by performing an energy minimization process on the initial molecular model of the polymer after adding the force field. Among them, the specific implementation method of the energy minimization process is: use the min_style command in the Lammps software, select the Polak-Ribiere version of the conjugate gradient (cg) algorithm, set the maximum number of iteration steps to 5000 steps, and set the time step to 0.5fs.

Step 3: Use Lammps software to perform kinetic relaxation on the initial molecular model of the polymer after energy minimization treatment.

In this embodiment, the specific method for dynamic relaxation of the initial molecular model of the polymer after the energy minimization treatment using the Lammps software is as follows:

Step 301, performing 100Ps NVT kinetic relaxation on the initial molecular model of the polymer after the energy minimization treatment, so that each independent polymer chain in the initial molecular model of the polymer is fully relaxed; during the relaxation process, The force field is set as the OPLSAA force field, the temperature is controlled by the Nose thermal bath method, the pressure is controlled by the Berendsen constant pressure method, the van der Waals force is solved by the LJ interaction, and the electrostatic interaction is solved by the PPPM method.

Step 302, perform NPT molecular dynamics simulation on the initial molecular model of the polymer, continue to relax the initial molecular model of the polymer under the conditions of 800Pa pressure and 600K temperature, set the relaxation time to 1000ps, and set the time step is 0.5fs.

Step 303, control the temperature of the initial molecular model of the polymer to remain unchanged, reduce the pressure to 1Pa, and after the depressurization is completed, control the pressure and temperature to remain unchanged, and continue to analyze the initial molecular model of the polymer under the conditions of a pressure of 1Pa and a temperature of 600K. Relaxation was performed, and the relaxation time was set to 100 ps.

Step 4: Introduce cells into the initial molecular model of the polymer after relaxation to generate a closed cell model of the polymer with the target density.

Figure 4 shows the foaming process of the closed cell model of the polymer. A spherical cell is introduced into the initial molecular model of the polymer after relaxation, which is recorded as an internal cell. The center of the internal cell is the same as The centers of the initial molecular models of the polymer overlap, and the pore size of the internal cells gradually increases from zero;

In order to reflect the constraining effect of other cells and the surrounding polymer matrix on the expansion of the cell during the expansion process, a spherical cell is introduced outside the initial molecular model of the polymer after the introduction of the internal cell, which is recorded as the external cell , the center of the outer cell coincides with the center of the initial molecular model of the polymer, and the minimum initial pore size of the outer cell is calculated based on the standard of all the atoms that completely wrap the initial molecular model of the polymer. The pore size keeps decreasing.

The initial molecular model of the polymer introduced into the internal and external cells is recorded as the closed cell model of the polymer, and the rate of change of the internal and external cells of the closed cell model of the polymer is adjusted until the closed cell model of the polymer is reached. The target density is reached.

In this embodiment, the method for introducing internal and external cells is as follows: using the fix indent command in the Lammps software, setting the force constant to 10, and the increase rate of the internal cells and the reduction rate of the external cells are based on the closed cells of the polymer. The size of the pore model and the size of the target density are set, and the minimum initial pore size of the outer cells is calculated by the following formula (1):

Wherein, _outside r is the minimum initial pore size of the outer cell, and L is the edge length of the initial molecular model of the polymer after relaxation.

In order to obtain the closed cell model of the polymer at 0.35 g/cm3, both the increase rate of the inner cell size and the decrease rate of the outer cell size were set to be 0.1 nm/ps.

Step 5: Release the local stress due to cell confinement in the polymer closed cell model.

In this embodiment, the method for releasing the local stress due to the constraints of the inner and outer cells is: after the closed cell model of the polymer reaches the target density, the inner cell diameter and the outer cell diameter are recorded as _rinend and routrespectively _{. end} . Then keep the pore size of the _{inner cell at r and end} unchanged, and the pore size of the outer cell gradually decreases from the initial value r _outside to r _{outside end} , and repeats for many times until the outer cell pore size changes from the initial value r _outside During the process of decreasing to the _{outer end} of r, the density and conformation of the closed cell model of the polymer did not change significantly, and the closed cell model of the polymer had reached a stable state at this time.

Step 6: After the density and conformation of the polymer closed cell model are stabilized, remove the internal and external cell constraints, and perform NPT relaxation on the polymer closed cell model at a temperature of 600K, and set the relaxation time to 100Ps. Set the pressure to 1Pa; cool down the polymer closed cell model after relaxation to 300K, continue to perform NPT relaxation on the polymer closed cell model at a temperature of 300K, set the relaxation time to 100Ps, set The constant pressure is 1Pa, and the closed-cell molecular model of foamed polymer with stable configuration is obtained.

As an example, the cross-sectional view of the closed cell molecular model of the 0.35 g/cm 3 foamed high polymer obtained in this example is shown in FIG. 5 .

The construction method of the foamed polymer closed cell molecular model in this embodiment makes up for the deficiency of the existing polymer molecular model, and is helpful for the development of the foamed polymer molecular simulation work. The model can be used to further clarify The micro-mechanism of macro-deformation and macro-mechanical properties of the foamed polymer promotes the improvement of the properties of the foamed polymer material and maximizes the engineering benefits of the foamed polymer.

The above-mentioned embodiments are only used to illustrate the technical solutions of the present invention, but not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that: it is still possible to implement the foregoing implementations. The technical solutions described in the examples are modified, or some technical features thereof are equivalently replaced; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the spirit and scope of the technical solutions of the embodiments of the present invention, and should be included in the within the protection scope of the present invention.

Claims (7)

1. A method for constructing a molecular model of closed cells of a foamed high polymer is characterized by comprising the following steps:

step 1: constructing a high polymer initial molecular model by using an Amorphous Cell module in Materials Studio software, and adding an OPLSAA force field to the high polymer initial molecular model by adopting Python program programming;

and 2, step: performing energy minimization treatment on the high polymer initial molecular model added with the force field by using Lammps software;

and step 3: performing dynamic relaxation on the high polymer initial molecular model subjected to energy minimization by using Lammps software;

and 4, step 4: introducing cells into the relaxed high polymer initial molecular model to generate a high polymer closed cell model with target density;

and 5: releasing local stress generated by cell restriction in the high polymer closed cell model;

and 6: removing the restraint of inner and outer pores after the density and conformation of the high polymer closed cell model are stable, performing NPT relaxation on the high polymer closed cell model at the temperature of 600K, setting the relaxation time to be 100Ps, and setting the pressure to be 1 Pa; and cooling the relaxed high polymer closed cell model to 300K, continuing to perform NPT relaxation on the high polymer closed cell model at the temperature of 300K, setting the relaxation time to be 100Ps, and setting the pressure to be 1Pa, thus obtaining the foamed high polymer closed cell molecular model with a stable configuration.

2. The method for constructing the molecular model of foamed polymer closed cell pores according to claim 1, wherein the adding the OPLSAA force field to the initial molecular model of the polymer by Python programming comprises:

obtaining OPLSAA force field parameters corresponding to the high polymer initial molecular model from the LigParGen, writing Python script according to the atom type and the topology type contained in the OPLSAA force field parameters to modify the atom type and the topology type of the high polymer initial molecular model, and enabling the atom type and the topology type of the high polymer initial molecular model to be consistent with the atom type and the topology type contained in the OPLSAA force field parameters.

3. The method for constructing the molecular model of closed cells of foamed high polymer according to claim 1, wherein the energy minimization of the initial molecular model of foamed high polymer after the force field is added by using Lammps software comprises:

in Lammps software, a min _ style command is utilized, a Polak-Ribiere version of a conjugate gradient algorithm is selected, the maximum iteration step number is set to be 5000 steps, and the time step length is set to be 0.5 fs.

4. The method of claim 1 wherein introducing cells into the relaxed initial molecular model of the polymer to produce a model of closed cells of the polymer at a target density comprises:

introducing a spherical cell into the initial molecular model of the polymer after relaxation, wherein the spherical cell is marked as an inner cell, the center of the inner cell is coincided with the center of the initial molecular model of the polymer, and the pore diameter of the inner cell is gradually increased from zero;

introducing a spherical bubble outside the polymer initial molecular model after the introduction of the inner bubble, and recording the spherical bubble as an outer bubble, wherein the center of the outer bubble is superposed with the center of the polymer initial molecular model, the minimum initial pore diameter of the outer bubble is obtained by calculating by taking all atoms completely wrapping the polymer initial molecular model as a standard, and the pore diameter of the outer bubble is continuously reduced;

and recording the initial molecular models of the high polymer introduced into the inner foam pores and the outer foam pores as closed polymer foam pore models, and adjusting the change rate of the inner foam pores and the outer foam pores of the closed polymer foam pore models until the closed polymer foam pore models reach the target density.

5. The method of claim 4 wherein introducing cells into the relaxed initial molecular model of the polymer to produce a model of closed cells of the polymer at a target density comprises:

setting a force constant to be 10 by utilizing a fix index command in Lammps software, setting the increasing rate of inner cells and the decreasing rate of outer cells according to the size of a high polymer closed cell model and the size of a target density, and calculating the minimum initial pore diameter of the outer cells by the following formula:

wherein r is _{Outer cover} And L is the minimum initial pore diameter of the outer pores, and the side length of the initial molecular model of the polymer after relaxation.

6. The method for constructing a molecular model of closed cells of foamed polymer according to claim 1, wherein the dynamic relaxation of the initial molecular model of foamed polymer after energy minimization using Lammps software comprises:

NVT dynamic relaxation of 100Ps is carried out on the initial polymer molecular model subjected to the energy minimization treatment, so that each independent polymer chain in the initial polymer molecular model is fully relaxed; in the relaxation process, setting a force field as an OPLSAA force field, controlling the temperature by adopting a Nose hot bath method, controlling the pressure by adopting a Berendsen constant voltage method, adopting LJ interaction for van der Waals acting force, and solving the electrostatic interaction by adopting a PPPM method;

carrying out NPT molecular dynamics simulation on the high polymer initial molecular model, continuing relaxing the high polymer initial molecular model under the conditions of 800Pa pressure and 600K temperature, setting the relaxation time to be 1000ps, and setting the time step length to be 0.5 fs;

and controlling the temperature of the high polymer initial molecular model to be unchanged, reducing the pressure to 1Pa, controlling the pressure and the temperature to be unchanged after the pressure reduction is finished, and continuously relaxing the high polymer initial molecular model under the conditions of the pressure of 1Pa and the temperature of 600K, wherein the relaxation time is set to be 100 ps.

7. The method of claim 1, wherein said relieving local stresses in the polymer closed cell model due to cell restraint comprises:

after the high polymer closed cell model reaches the target density, the inner cell size and the outer cell size are respectively recorded as r _{Inner end} And r _{Outer end} (ii) a Maintaining internal cell diameter r _{Inner end} The pore diameter of the outer cells is unchanged from an initial value r _{Outer cover} Gradually decreases to r _{Outer end} A plurality of cycles until the outer cell diameter is from an initial value r _{Outer cover} Reduced to r _{Outer end} The density and conformation of the polymer closed cell model are unchanged.

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