ATOMISTIC SIMULATION OF A GLASSY POLYMER GRAPHITE INTERFACE

A computer simulation technique has been developed that is capable of probing the interface between an amorphous glassy polymer and a crystalline solid substrate in atomistic detail. The interface between bulk glassy atactic polypropylene and a graphite basal plane is used as a test case. The method requires the generation of a set of static model microstates that are in detailed mechanical equilibrium, each characterized by two-dimensional periodic boundary conditions and consisting of multiple chains of polymer sandwiched between two semiinfinite solid phases. To obtain a static model microstate, an initial guess microstate is first generated by a Monte Carlo procedure based on the rotational isomeric state model with corrections for long-range interactions. Subsequently, the total potential energy is minimized with respect to all microscopic degrees of freedom. From such a set of static model microstates, we have predicted the internal energy contribution to interfacial thermodynamic properties, as well as the local structural features. A quantitative estimate for the work of adhesion between atactic polypropylene and graphite has been obtained, which agrees well with experiment. The distribution of local internal stresses confirms that microstates are mechanically isotropic in their middle region (far from the solid surfaces), which is indistinguishable from unperturbed bulk polymer. The local structure of the polymer lying within 10 angstrom of a graphite surface is found to be different in many ways from that of the corresponding bulk. Near the solid, the local polymer density profile displays a maximum, the backbone bonds of chains develop considerable orientation parallel to the solid surface, and the usually perferred trans rotational state is suppressed. Adsorbed pendant hydrogens of the polymer concentrate preferentially on top of the centers of the hexagons in the graphite honeycomb. The polymer structure has been explored at the level of entire chains as well. The chain center of mass distribution displays a maximum approximately 1 unperturbed root-mean-squared radius of gyration away from each solid surface. Chains orient with their longest dimension parallel to the graphite phases. The intrinsic shape of the chain segment clouds, as characterized by spans and principal moments of inertia, is found not to be a strong function of position relative to the interface. All polymer/solid results presented have been screened for possible system size effects and compared to the corresponding results from a recent computer simulation of the free surface of glassy atactic polypropylene.