Orientation of phenyl rings and methylene bisectors at the free surface of atactic polystyrene

Atomistically detailed models of free-standing thin films and the bulk structure of amorphous atactic polystyrene have been produced by reverse mapping from equilibrated coarse-grained models. The bridging technique employed in the simulation allows the generation of a moderate sized atomistic system (six independent parent chains Of C400H402, 4812 atoms) with a more reasonable computational effort than is required when all of the construction is performed on chains expressed with atomistic detail. Reverse mapping from the coarse-grained model to the atomistically detailed model is found to be straightforward, without ring piercing or concatenation. The calculated surface energy (38 +/- 10 erg/cm(2)) is in reasonable agreement with prior experimental findings. The surface of the thin films is enriched in phenyl rings. The rings at the surface tend to be oriented so that they are pointing outward, but rings in the middle of the thin film show no prefer-red orientation. In contrast with the phenyl rings, the bisectors for the methylene groups show little tendency for orientation, even when the methylene groups are close to the surface. These observations in the simulation are in qualitative agreement with conclusions reported recently (Gautam et al. Phys. Rev. Lett. 2000, 30, 3854, and Briggman et al. J. Phys. Chem. B 2001, 105, 2785), based on the application of new spectroscopic techniques to the characterization of polymer surfaces.