The Study of the Behavior of Nanocomposites Including Polymer Grafted Nanoparticles with Dissipative Particle Dynamics Simulation
Authors: Chaki Kenta 
Published: February 2020

Recently nanocomposites attract a lot of interests in the industry and the academic because the materials have a possibility to make their function much higher. The candidates of compositions are so many that it is the hard task that one finds the objective function. The insertion of inorganic nanoparticles like silica into polymers is one of the ways to make the material function higher. Controlling their dispersion is the important and difficult theme and the state of dispersion will have an influence on the physical properties like viscoelasticity. If the dispersive structure is necessary, grafting polymers onto nanoparticles is one of the methods.
Computer simulation is a powerful tool to study the structure and physical properties of materials, but it is difficult to perform atomistic molecular dynamics simulations to understand the dispersion and slow dynamics of nanocomposites on account of the time and length scale. We therefore attempt to perform the Dissipative Particle Dynamics (DPD) simulation to study the behavior of nanocomposites including polymer grafted nanoparticles.
The fundamental setup of the DPD simulation follows the method of Groot and Warren [1]. We make a sphere nanoparticle by collecting DPD particles with bond and angle potentials. The angle potential is considered to reproduce rigidity of the particle. We also make a linear chain polymer for the matrix polymers and the polymers grafting onto the particles. To assume the particle like silica and the polymer like polystyrene and not to penetrate the polymers into the particles, the interaction between the particles and polymers are more repulsive than that between particles and between polymers. The graft polymers are grafted onto the all surface of the particle to ignore the interactions on the interface between particles and matrix polymers. The modeling procedures are achieved with J-OCTA [2].
Fig. 1 shows the dispersion of the grafted nanoparticles with volume fraction of 3% of themselves. The lengths of both matrix polymers and grafted polymers are the same and each of these polymers is composed of 10 DPD particles with bonding, which assumes to ignore the entanglement effect. If no grafted polymers are on the particle, the particles are easy to aggregate. Next, with using the dispersive structure, we obtained the linear relaxation moduli G(t) by Green-Kubo formula. Figure 2 shows the comparison of G(t) with nanocomposites and pure polymers. The polymer chain length in pure polymers is also 10. By inserting the nanoparticles, the modulus is increasing and the terminal relaxation time is longer compared to those of pure polymers. These results would indicate that DPD simulation reproduces the effect of reinforcement of nanoparticles in the polymer system.
If the molecular weight of polymers is larger than the molecular weight between entanglements, the entanglement effect would appear and have influence on the physical properties. Recently to reproduce the entanglement effect with DPD, the slip-spring model is suggested [3]. In the presentation, we would discuss the application of the model to the nanocomposites.

References:

[1] R. D. Groot and P. B. Warren, J. Chem. Phys. 107, 4423 (1997).
[2] http://www.j-octa.com
[3] M. Langeloth, Y. Masubuchi, M. C. Böhm, and F. Müller-Plathe, J. Chem. Phys. 138, 104907 (2013).

Figure 1