Screened intermolecular forces and covalent bond forces in polymer melts

The intermolecular nonbonded forces exerted on a particular mer of a chain in a polymer melt are screened by adjacent mers on the same chain. As a result, the time average of this intermolecular force, referred to a coordinate system bearing a fixed relationship to the covalent bonds attached to the mer in question, is nonzero. These screened forces are studied here by molecular dynamics simulation of a model melt corresponding to the model for polyethylene (PE model) of Takeuchi and Roe. If the macromolecule in a particular conformation is visualized as a long curved cylinder, the screened forces are distributed over its surface, including its capped ends, directed inwards and approximately normal to the surface. These forces are relatively insensitive to the melt pressure p. They are present and have the same character, even when p=0. The character of these forces helps clarify the simulation result that the backbone covalent bonds are in compression at liquidlike densities.