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

Atomistically detailed models of free-standing thin films and the bulk stru cture of amorphous atactic polystyrene have been produced by reverse mappin g from equilibrated coarse-grained models. The bridging technique employed in the simulation allows the generation of a moderate sized atomistic syste m (six independent parent chains Of C400H402, 4812 atoms) with a more reaso nable 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 su rface energy (38 +/- 10 erg/cm(2)) is in reasonable agreement with prior ex perimental findings. The surface of the thin films is enriched in phenyl ri ngs. 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 orien tation. 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 qual itative agreement with conclusions reported recently (Gautam et al. Phys. R ev. Lett. 2000, 30, 3854, and Briggman et al. J. Phys. Chem. B 2001, 105, 2 785), based on the application of new spectroscopic techniques to the chara cterization of polymer surfaces.