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 (R g ≈ 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 observed for global structural and long-time dynamical properties for chains in contact with a flatter and more densely grafted surface. Hydrogen-bond formation between polymer and surface decelerates the polymer dynamics. The effect is more pronounced at low temperatures.

A molecular dynamics simulation method with coupling to an external bath is used to simulate polyamide-6,6 trimers confined between graphite surfaces. In this simulation method, the temperature and the parallel component of pressure are kept fixed, and the distance between the confining graphite surfaces is changed to achieve equilibrium. The simulation results on the oscillatory behavior of solvation force, the number density of confined oligomers, and stepwise variation of the oligomer numbers as a function of distance between the confining graphite surfaces are reported and discussed. The hydrogen bonding in confined oligomers has also been studied in detail, and it is shown that hydrogen-bond formation depends on the layering effect and on the geometrical restrictions and reveals an oscillatory behavior like the solvation force oscillations. The autocorrelation functions for NH, CO, and end-to-end vectors are also studied, and it is concluded that the confined fluid has a considerably lower relaxation time than that of the bulk fluid, and the relaxation times for confined fluid show an oscillatory behavior with maxima corresponding to well-formed structures parallel to the surfaces. The study of local dynamics via calculating the autocorrelation functions for bins parallel to the surfaces reveals that the fluid layers close to the surfaces have higher relaxation times than the fluid in the central region. Our calculated center-of-mass diffusion coefficients also show oscillatory behavior with out-of-phase oscillations with respect to the solvation force oscillations.

A refined flexible all-atom model for a room temperature ionic liquid, 1-n-butyl 3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([bmim][Tf2N]), is reported here. Extensive molecular dynamics simulations were performed to check the validity of this model. We found that the thermodynamic and dynamic properties such as density, isobaric expansivity, isothermal compressibility, and self-diffusion coefficient described by the present model are in good agreement with experimental observations. Based on the calculated results, there has been a considerable improvement in the force field with respect to the previous model by Lopes et al. [15,16].