Influence of nanoparticle inclusion on the cavity size distribution and accessible volume in polystyrene - Silica nanocomposites

摘要

Confinement effects on the accessible volume and the cavity size distribution in polystyrene - silica nanocomposites are studied by atomistic molecular dynamics simulations. Atomistically derived properties of pure polystyrene are compared against coarse-grained data. Microstructures for various nanoparticle diameters and grafting densities are analyzed using a geometrical procedure to determine the accessible volume and cavities which provide sufficient space for a hard-sphere probe particle with a radius between 0 and 0.25 nm. The spatial distribution of the accessible volume depends weakly on the size and grafting density of the nanoparticle. Contrary to intuitive expectations, variations of the accessible volume are not directly related to changes of the specific volume. The separation from the nanoparticle to achieve bulk behavior of the accessible volume depends also weakly on the size of the probe particle. For a zero-radius probe particle, the spatial variation of the unoccupied volume is shorter than the one of the specific volume. Average cavity sizes in the vicinity of a nanoparticle are smaller than in the bulk. They are of ellipsoidal shape and inclined from the nanoparticle surface by approximately 30. Both the size of the nanoparticle and its grafting density induce minor effects on the cavity properties. Peculiarities in the spatial distribution of the cavity sizes persist for larger separations from the nanoparticle than modifications in the specific volume. The validity of approximating the accessible volume and the cavity size distribution with the help of quantities derived by a Voronoi tessellation is tested. It is shown that the volume of a Voronoi cell does not provide such a description. Nevertheless, it is possible to correlate the rather abstract Voronoi related properties to structural details of the polymer matrix. We demonstrate that the volume of an atomic Voronoi cell is reduced with an increasing number of bonded neighbors. Its shape becomes elongated when the number of bonds per atom is raised. (C) 2016 Elsevier Ltd. All rights reserved.