MOLECULAR-DYNAMICS SIMULATION OF A GLASSY POLYMER SURFACE

The free surface of glassy atactic polypropylene is studied by molecular dynamics computer simulation. A model film system exposed to vacuum on both sides is examined at a temperature 22-degrees-C below the experimental glass transition temperature of the bulk polymer. The vinyl chains are represented as collections of backbone carbon atoms with united atom pendant methyl groups and explicit hydrogens. Bond lengths are held fixed, while bond angles are flexible. Simulations have been performed for durations of up to 400 ps and incorporate density profile dependent long-range contributions to the energy and stress. The spatial dependence of the short-time dynamical properties of the model film has been determined. Enhancements in the mean-squared displacement of atoms, in the orientational relaxation of backbone bonds, and in the rate of torsion angle fluctuations within a conformational state are observed near the free surface. One must penetrate into the film by approximately 15 angstrom from the extreme edge before observing a behavior in these dynamical features that resembles that of the bulk glassy polymer. This "dynamical interfacial thickness" is twice as large as the thickness over which the mass density profile varies. The center of mass motion of chains located as far as 2-3 times the unperturbed radius of gyration from the edge of the free surface is enhanced and highly anisotropic. Conformational transitions in the glassy polymer film have been analyzed. The glass is found to contain many "soft spots", where transitions occur with rates comparable to those observed in polymer liquids, as well as "stiff" regions, where the transitions are virtually arrested. An average rate for conformational isomerization in the polymer has been calculated by constructing a hazard plot of first passage times.