Model construction method for researching homopolymer-homopolymer-diblock copolymer ternary entangled polymer mixture by dissipative particle dynamics

Abstract

The invention discloses a construction method for realizing a homopolymer-homopolymer-double block copolymer ternary entangled polymer mixture by adopting a dissipative particle dynamics method. The method comprises the following steps: acquiring a characteristic ratio and a balance bond length of a simulation system, and generating a series of homopolymers and double-block copolymers taking the bond length as a step length; according to the chain length, the chain number and the number density of a simulation system, putting a series of obtained random walking homopolymers and copolymers into a simulation box; introducing interaction potential among particles, introducing bond-forming interaction potential by using simple harmonic spring potential, and introducing bond-angle interaction potential by using a bending potential function; the rejection parameters are gradually increased, and the difference of the rejection parameters between the stages should be kept equal or approximately equal; setting a rejection parameter as a final use value; and detecting whether the conformation of the chain is balanced. The three-element entangled polymer mixture constructed by the invention can accurately describe the dynamic and rheological properties of an entangled polymer blend, and can be used for researching the structure and rheological properties of an entangled polymer interface.

Description

Technical Field

[0001] The invention relates to a model construction method for studying a homopolymer-homopolymer-diblock copolymer ternary entangled polymer mixture by dissipative particle dynamics, and belongs to the field of computer simulation. Background Art

[0002] Blending polymers with different chemical properties is an effective method for manufacturing new materials, but the incompatibility between different polymers will cause phase separation of the blend, and then a polymer-polymer interface with weak adhesion and mechanical properties will be generated inside the material. The rheological and molecular chain dynamics behavior of incompatible blends will become very complicated due to the presence of the interface, especially when there is an entanglement topological constraint effect at the interface, so the entangled polymer-polymer interface has become a hot topic of concern for materials scientists. In order to enhance the strength of the interface, additives such as block copolymers are usually added to the interface. Block copolymers that exceed the entanglement length not only reduce the interfacial tension and increase the interfacial viscosity, but also further improve the interfacial entanglement network and inhibit the generation of interfacial velocity slip. Therefore, ternary polymer blends are very valuable for industrial applications, and their interfacial properties are also complex.

[0003] Although the rheological behavior of polymer-polymer interfaces in rich ternary polymer blends has been experimentally discovered in the past decade, explanations at the molecular level are still relatively scarce. On the other hand, it is very difficult and time-consuming to obtain a balanced homopolymer-homopolymer-block copolymer ternary entangled polymer mixture with a flat interface through the approach of phase separation dynamics. Recently, Nikunen et al. improved the traditional dissipative particle dynamics method by increasing the conservative force from 25 to 200 to produce an entanglement effect. In addition, researchers have demonstrated that this method has a shorter entanglement length than the Kremer-Grest model, so the model can simulate polymers with a larger number of entanglements under the premise of the same chain length. Therefore, the present invention is intended to provide a dissipative particle dynamics study of a homopolymer-homopolymer-diblock copolymer ternary entangled polymer blend system. The method is used to study the properties and rheological behavior of the entangled polymer-polymer interface. Summary of the invention

[0004] The object of the present invention is to provide a method for studying the construction of a homopolymer-homopolymer-diblock copolymer ternary entangled polymer mixture using dissipative particle dynamics.

[0005] The present invention provides a method for constructing a homopolymer-homopolymer-diblock copolymer ternary entangled polymer mixture using a dissipative particle dynamics method, comprising the following steps:

[0006] S1. Obtaining the characteristic ratio and equilibrium bond length of the simulation system, and generating a series of homopolymers and diblock copolymers with the bond length as the step length according to the equilibrium bond length and the characteristic ratio, so that the generated chains have the correct end-to-end distance on a large scale;

[0007] S2, placing a series of random walk homopolymers and copolymers obtained in step S1 into a simulation box according to the chain length, number of chains and number density of the simulation system;

[0008] S3, using the dissipative particle dynamics method to introduce interaction potential between particles, using simple harmonic spring potential to introduce bonding interaction potential, and using bending potential function to introduce bond angle interaction potential;

[0009] S4. Increase the repulsion parameter gradually. The difference of the repulsion parameter between each stage should be kept equal or approximately equal, and the increase values ​​of the repulsion parameters of different particles should be kept equal;

[0010] S5, set the rejection parameter to the final use value and run for 1 to 2 Rouse time;

[0011] S6. Check whether the conformation of the chain is balanced. If the mean square intra-chain distance of each component no longer changes with time, the system is balanced. Otherwise, the system is not balanced. Repeat step S5 to obtain a balanced system.

[0012] In the above construction method, in step S1, if the characteristic ratio is known, it can be used directly; if the characteristic ratio of the simulated polymer is unknown, the equilibrium bond length can be obtained by directly simulating the short chain length polymer, and the bond angle cosine value can be obtained according to the bond vector correlation function, and then the theoretical value of the characteristic ratio can be obtained according to the theoretical prediction of the free rotation chain. The characteristic ratio and chain length used to generate the initial state of the diblock copolymer are the characteristic ratio and chain length of the homopolymer with the same chain length.

[0013] In the above construction method, in step S2, the simulation box is divided into an upper part and a lower part, and the homopolymer A N , B N The upper and lower parts of the simulation box are randomly placed, so that A is formed in the middle, upper boundary and lower boundary of the simulation box respectively. N -B N interface, randomly place the diblock copolymers into A N -B N The interface maintains the same surface density of block copolymers on the two interfaces, and the connection point of the two blocks is located at A N -B N The diblock copolymer is determined to be on the interface.

[0014] In the above construction method, in step S3, in the dissipative particle dynamics method, in order to better separate the particles from each other, the repulsive parameter of the conservative force between particles of the same kind or between particles with affinity is 25, that is, a AA =a BB =a CC =a DD =a AC =a BD = 25, the repulsion parameter between different copolymer particles is a AB =55, the repulsion parameter between the incompatible block copolymer and the homopolymer particles is a BC =a AD =40~55; in addition, the bonding potential coefficient is always twice the repulsion parameter between particles of the same kind.

[0015] In the above construction method, in step S3, in the dissipative particle dynamics method, the simulation time is 1 to 2 Rouse times, and the Rouse time is the Rouse relaxation time of a homopolymer of the same chain length corresponding to the longest polymer in the system.

[0016] In the above construction method, in step S4, the running time is 1 to 2 Rouse times.

[0017] In the above construction method, in step S5, the exclusion parameter is 200, that is, a AA =a BB =a CC =a DD =a AC =a BD =200, a AB =230,a BC =a AD =215~230.

[0018] The ternary entangled polymer mixture melt in equilibrium conformation obtained through the above steps can qualitatively describe the structure, dynamics and rheological properties of the polymer mixture, especially the structure and rheological properties of the interface, so as to explain the experimental phenomena at the molecular level and establish more accurate related theories.

[0019] The equilibrium system obtained by constructing the model using the above method also falls within the protection scope of the present invention.

[0020] The present invention has the following beneficial effects:

[0021] Through the construction method of the present invention, a ternary entangled polymer mixture with an entangled interface and close to an equilibrium conformation is established, and the homopolymer and copolymer molecules are placed in a pre-set phase region and the corresponding repulsion parameters are adjusted to quickly obtain the equilibrium conformation of the entangled polymer mixture. Compared with the method of directly obtaining a phase-separated entangled polymer mixture through phase separation kinetics, the equilibrium time used in the present invention is shorter, saving a lot of time and computing resources, and providing an effective way to efficiently and accurately study the structure and property relationship of entangled polymer mixtures and their entangled interfaces. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 The homopolymer (A N , B N ), diblock copolymer (C N D N )Model.

[0023] Figure 2 The dissipative particle dynamics model of the homopolymer-homopolymer-diblock copolymer ternary entangled polymer mixture system constructed in the present invention is shown.

[0024] Figure 3 The variation of the total number of particles and the entanglement point density along the normal direction of the interface is shown.

[0025] Figure 4 The time-varying displacement of the interfacial block copolymer along the velocity direction and the velocity gradient direction of the flow field under shear flow is shown.

[0026] Figure 5 A cross-sectional diagram showing the velocity of the particle under shear flow along the normal direction of the interface. DETAILED DESCRIPTION

[0027] Unless otherwise specified, the experimental methods used in the following examples are conventional methods.

[0028] Unless otherwise specified, the materials and reagents used in the following examples can be obtained from commercial sources.

[0029] Homopolymer A 125 -Homopolymer B 125 -Diblock copolymer C 125 D 125 Taking the balance of a ternary entangled polymer mixture as an example, the specific implementation steps of the balance method of the present invention are described:

[0030] 1) The entanglement dissipative particle dynamics method is used to directly simulate the equilibrium bond length of a homopolymer system with a chain length of 10, a number of chains of 10, and a number density of 1, and the equilibrium bond length is 0.964. The value of the bond angle cosine when the bending potential coefficient is 2 is obtained based on the potential function in the form of the bending potential cosine. When it is substituted into the intra-chain distance formula of the free rotating chain FRC model, a characteristic ratio of 3.32 can be obtained; the equilibrium bond length of the initial conformation is set to 0.95, and according to the relationship between the terminal distance of the free rotating chain and the characteristic ratio, a series of homopolymers and diblock copolymers with approximately correct terminal distances on a large scale are generated by random walk;

[0031] 2) Set the system number density to 1, homopolymer A 125 , B 125 The chain length is 125, the number of chains is 1440, and the diblock copolymer C 125 D 125 The length of each block is 125, the total chain length is 250, the number of chains is 160, and the simulation box size is set to 80r c ×50r c ×100r c , divide the box into upper and lower parts along the z direction, and take the homopolymer A produced in step (1) 125 , B 125 Randomly put them into the upper and lower parts of the box, so at z = 50r c and z = 0 (or 100r c ) generates an A at each 125 -B 125 Interface, then at z = 50r c 80 diblock copolymers C were placed on the interface of z = 0. 125 D 125 , i.e. block C 125 and D 125 The connection point is at z = 50r c , z = 0;

[0032] 3) The dissipative particle dynamics method is used to introduce the interaction potential between particles, where the repulsive parameter is a AA =a BB =a CC =a DD =a AC =a BD =25, a AB =55,a BC =a AD =55, use simple harmonic spring potential to introduce bonding interaction potential, use cosine bending potential to introduce bond angle potential, and simulate a Rouse time;

[0033] 4) Adjust the rejection parameters to a for every Rouse time. AA =a BB =a CC =a DD =a AC =a BD =50, a aB =80, a BC =a AD =80, a AA =a BB =a CC =a DD =a AC =a BD =100, a AB =130, a BC =a AD =130 and a AA =a BB =a CC =a DD =a AC =a BD =150, a AB =180, a BC =a AD =180, the bonding potential coefficient is always twice the repulsion parameter between particles of the same kind;

[0034] 5) Set the exclusion parameter to the final use value, that is, the exclusion parameter is a AA =a BB =a CC =a DD =a AC =a BD =200, a AB =230,a BC =a AD = 230, run a Rouse time, characterize the A 125 , B 125 and C 125 D 125 chain conformation, i.e., intrachain distances;

[0035] 6) Repeat step (5) until the distance within the chain no longer changes and an initial state with good conformational equilibrium is obtained, such as Figure 2 shown.

[0036] The system established above is analyzed. First, the particle density and normalized entanglement number density are statistically analyzed along the normal direction of the interface ( Figure 3), it was found that the particle number density and entanglement number density at the interface decreased, indicating that the intermolecular chain interaction at the interface is weaker than that in the bulk. If a shear flow with a velocity direction parallel to the interface and a velocity gradient perpendicular to the interface is applied to the system, the displacement of the block copolymer in each direction is statistically analyzed ( Figure 4 ), it can be found that the displacement of the molecular chain in the direction of the velocity gradient of the flow field is almost negligible compared to the direction of the velocity of the flow field, indicating that the block copolymer is fixed on the interface and cannot move along the normal direction of the interface. By counting the velocities of all particles along the shear field direction, it can be observed that there is a discontinuous change in velocity at the interface ( Figure 5 ), namely the velocity slip phenomenon, which is caused by the weak inter-chain interactions of molecules at the interface.

[0037] From the above analysis, the equilibrium conformation of the ternary entangled polymer mixture melt constructed by the present invention can qualitatively describe the structure, dynamics and rheological properties of the polymer mixture, especially the structure and rheological properties of the interface, so as to explain the experimental phenomena at the molecular level and establish more accurate related theories.

Claims

1. A method for constructing a homopolymer-homopolymer-diblock copolymer ternary entangled polymer mixture using a dissipative particle dynamics method, comprising the following steps: S1. Obtaining the characteristic ratio and equilibrium bond length of the simulation system, and generating a series of homopolymers and diblock copolymers with the bond length as the step length according to the equilibrium bond length and the characteristic ratio, so that the generated chains have the correct end-to-end distance on a large scale; S2, placing a series of random walk homopolymers and copolymers obtained in step S1 into a simulation box according to the chain length, number of chains and number density of the simulation system; S3, using the dissipative particle dynamics method to introduce interaction potential between particles, using simple harmonic spring potential to introduce bonding interaction potential, and using bending potential function to introduce bond angle interaction potential; S4. Increase the repulsion parameter gradually. The difference of the repulsion parameter between each stage should be kept equal or approximately equal, and the increase values ​​of the repulsion parameters of different particles should be kept equal; S5, set the rejection parameter to the final use value and run for 1 to 2 Rouse time; S6. Check whether the conformation of the chain is balanced. If the mean square intra-chain distance of each component no longer changes with time, the system is balanced. Otherwise, the system is not balanced. Repeat step S5 to obtain a balanced system.

2. The construction method according to claim 1, characterized in that: In step S2, the simulation box is divided into an upper part and a lower part, and the homopolymer A N , B N The upper and lower parts of the simulation box are randomly placed, so that A is formed in the middle, upper boundary and lower boundary of the simulation box respectively. N -B N interface, randomly place the diblock copolymers into A N -B N interface and keep the surface density of block copolymers on the two interfaces equal.

3. The construction method according to claim 1 or 2, characterized in that: In step S3, in the dissipative particle dynamics method, the repulsive parameter of the conservative force between particles of the same kind or between particles with affinity is 25, that is, a AA =a BB =a CC =a DD =a AC =a BD = 25, the repulsion parameter between different copolymer particles is a AB =55, the repulsion parameter between the incompatible block copolymer and the homopolymer particles is a BC =a AD =40~55.

4. The construction method according to claim 3, characterized in that: In step S3, in the dissipative particle dynamics method, the simulation time is 1 to 2 Rouse times.

5. The construction method according to any one of claims 1 to 4, characterized in that: In step S4, the running time is 1 to 2 Rouse times.

6. The construction method according to any one of claims 1 to 4, characterized in that: In step S5, the rejection parameter is 200.

7. The initial equilibrium state of the coarse-grained ternary entangled polymer mixture constructed by the method according to any one of claims 1 to 6.

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