حسین اسلامی

Large scale atomistic molecular dynamics simulation for a nanoparticle in oligomeric poly(methyl methacrylate), composed of 20 repeating units, for a long time, up to 100 ns, are performed. Simulations are done for systems up to 87500 atoms, each containing a single bare or surface-grafted nanoparticle of various diameters and grafting densities. The effect of surface area, surface curvature, grafting density, and hydrogen bonding on the alteration of local structural and dynamical properties of the polymer is studied in details. Although atomistic simulations are only feasible for oligomeric chains in contact with surfaces, the results of the present simulation still discriminate the interphase thickness, defined in terms of local and global chain properties. In the case of structural properties, a minimum interphase thickness, ? 2 nm, is associated with local properties such as layering of individual polymer monomers. However, when probed in terms of global chain properties like the extension of chains from the interface to the polymer phase, a thicker interphase, three times the radius of gyration of the unperturbed chain (Rg ? 1 nm), is observed. Our results on the chain structures are shown to be in good agreement with experiment where available. An examination of the dynamical properties shows that the surface influence on the polymer dynamics depends on the length and time scale of the corresponding bulk property. The change in time scales, in a 0.5 nm thick spherical shell, around the nanoparticle, is shown to cover a broad range from a few tens of a percent (for a short-time dynamical property, like the hydrogen bond formation) to 15? 20 orders of magnitude (for a long-time dynamical property, such as the relaxation of end-to-end vector in grafted chains). Therefore, the influence and the range of surface effects on dynamical properties (interphase thickness) depend on the inherent time scale of those properties. In all cases, a thicker interphase is observ