We develop a hierarchical approach that combines atomistic and mesoscopic simulations which can generally be applied to vinyl polymers. As a test case the approach is applied to atactic polystyrene (PS), probably the most widely studied among all amorphous polymers. First a specific model for atactic PS is chosen. The bonded parameters in the coarse-grained (CG) force field, based on data obtained from atomistic simulations of isolated PS dimers, are chosen in a way which allows differentiating between meso- and racemic dyads. This approach in principle allows to study isotactic and syndiotactic melts as well. Nonbonded interactions between coarse-grained beads were chosen as purely repulsive.

Then mesoscopic dynamic simulations in the CG level are performed. The proposed CG is tested along a different number of properties. Structure of short PS chains on the monomeric level (distribution function of bonds, bending and dihedral angles) as well as on the level of the whole chain (internal distances, radius of gyration, end-to-end distance) is found in excellent agreement with the one predicted by very long atomistic MD runs. Also the size of the longer chains (radius of gyration, characteristic ratio) is found in very good agreement with experimental data. An important advantage of the present methodology is the capability to obtain well-equilibrated atomistic configurations of long PS melts. To achieve this, a rigorous approach for reinserting the atomistic detail, that combined minimization and short MD runs, has been developed. The methodology has been successfully tested for short PS chains and the structure was found to be exactly similar with the one obtained directly from very long atomistic MD runs as well as from experimental measurements (V. Harmandaris, N. Adhikari, N. van der Vegt, K. Kremer, Macromolecules 2006).

The main advantage of the new model is its dynamical efficiency: the overall speed up, comparing with atomistic simulations, is more than 3 orders of magnitude. Therefore, one of the main goals of the present model is to be used in the study of the long time dynamics of high molecular length PS. For this reason a direct investigation of the dynamics in the coarse-grained description has been made by comparing the mesoscopic with the atomistic simulations in the level of mean square displacements for short PS chains. Current work considers a detailed study of the time mapping as a function of both molecular length and temperature.