ATOMISTIC SIMULATION OF A GLASSY POLYMER SURFACE

An atomistically detailed computer simulation of a glassy atactic polypropylene surface exposed to vacuum has been performed. Local structural features and interfacial thermodynamic properties have been calculated by averaging over sets of static model microstates. Each microstate was obtained by Monte Carlo generation, followed by total potential energy minimization. The internal energy contribution to surface tension, predicted from interatomic forces and distances on the basis of a new formulation, based on dilating the surface area, is within 7% of the experimental value. The density profile in the interfacial region is sigmoidal. Backbone bonds exhibit a tendency to orient parallel to the surface. The distribution of rotational states of skeletal bonds is perturbed relative to the bulk. All the above structural features are observed within a 10-Å-thick region at the vacuum/polymer interface. At the level of entire chains, structure deviates from its bulk characteristics over a region of thickness commensurate with overall chain dimensions. The distribution of chain centers of mass displays a maximum roughly one radius of gyration away from the edge of the polymer. The average chain width parallel to the surface is significantly larger near the edge of the polymer than in the bulk. This is because chains in the interfacial region display a tendency to orient with their longest span and their longest principal axis parallel to the surface. The dependence of structural features on polymer molecular weight is discussed. © 1990, American Chemical Society. All rights reserved.