Molecular dynamics results on the pressure tensor of polymer films

Polymeric thin films of various thicknesses, confined between two repulsive walls, have been studied by molecular dynamics simulations. Using the anis otropy of the perpendicular, P-N(z), and parallel components, P-T(z), of th e pressure tensor the surface tension of the system is calculated for a wid e range of temperature and for various film thicknesses. Three methods of d etermining the pressure tensor are compared: the method of Irving and Kirkw ood (IK), an approximation thereof (IK1), and the method of Harasima (H). T he IK- and the H-methods differ in the expression for P-T(z) (z denotes the distance from the wall), but yield the same formula for the normal compone nt P-N(z). When evaluated by molecular dynamics (or Monte Carlo)-simulation s P-N(z) is constant, as required by mechanical stability. Contrary to that , the IK1-method leads to strong oscillations of P-N(z). However, all metho ds give the same expression for the total pressure when integrated over the whole system, and thus the same surface tension, whereas the so-called sur face of tension, z(s), depends on the applied method. The difference is sma ll for the IK- and H-methods, while the IK1-method leads to values that are in conflict with the interpretation of z(s) as the effective position of t he interface. (C) 2000 American Institute of Physics. [S0021-9606(00)51334- 8].