Coarse-grained molecular-dynamics simulations of nanoparticle diffusion in polymer nanocomposites

Abstract

Molecular-dynamics simulations have emerged as an effective tool to characterize polymer systems. Molecular level effects (even on microsecond time scales) are nowadays well reproduced by atomistically detailed models. Beyond this, further insights into the properties of the polymer system at a mesoscopic level can be gained by resorting to simulations based on appropriate coarse-grained models. However, reducing the number of degrees of freedom during the coarse-graining procedure may have a significant impact on atomistic level effects. A common example is the overall enhancement of the diffusive motion of polymer chains in coarse-grained simulations, which arises from the reduced friction of the coarse-grained beads. In the present work we investigate this well-known effect and study how the diffusive properties of the nanoparticle are affected by the coarse-graining procedure. To this end, we apply iterative Boltzmann inversion to develop two coarse-grained models of a nanocomposite based on the thermoplastic polyimide R-BAPB, containing a single fullerene C60 nanoparticle. By changing the size and, correspondingly, the total number of coarse-grained beads in each polymer chain, we can control the effect of chemical detalization on various phenomena. We exploit this idea to study the influence of the degree of detalization of polymer chains on their structural properties as well as on the diffusive properties of the fullerene nanoparticle, whose detalization is kept fixed. Although the structural properties of the coarse-grained systems are in good agreement with those of the fully atomistic system, the nanoparticle diffusion is significantly affected by the local chain structure. In particular, we find that the coarse-graining of the polymer chains on the length scale of the nanoparticle size leads to a full suppression of the subdiffusive regime observed in the fully atomistic system.