Molecular dynamics results on the pressure tensor of polymer films

Abstract

Polymeric thin films of various thicknesses, confined between two repulsive walls, have been studied by molecular dynamics simulations. Using the anisotropy of the perpendicular, P N (z), and parallel components, P T (z), of the pressure tensor the surface tension of the system is calculated for a wide range of temperature and for various film thicknesses. Three methods of determining the pressure tensor are compared: the method of Irving and Kirkwood (IK), an approximation thereof (IK1), and the method of Harasima (H). The 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 component P N (z). When evaluated by MD (or MC)-simulations 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 methods give the same expression for the total pressure when integrated over the whole system, and thus the same surface tension, whereas the so-called surface of tension, z s , depends on the applied method. The difference is small 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 the interface.

Importance of Force Decomposition for Local Stress Calculations in Biomembrane Molecular Simulations

“…This physical requirement has been invoked to assess the quality of simulation protocols 49 or stress calculation methods. 50 Its violation in atomistic simulations of lipid bilayers 26,29,30,33 has sparkled some controversy about the role of constraints, as discussed in the previous section.…”

Local dynamic mechanical properties in model free-standing polymer thin films

“…In thin films, however, the local stresses depend strongly on position ͑e.g., near the free surfaces or at the middle of the film͒ and direction ͑e.g., tangential or normal to the free surfaces͒. 21 To consider such a heterogeneous stress distribution explicitly, we have applied a onedimensional version of the Irving-Kirkwood stress tensor: 21…”

Particle stress in suspensions of soft objects

“…Generally, the 'particle-based' total stress reflects oscillations of the particle density (data not shown), while the new approach is free of this artefact. Indeed, such an ad hoc definition of the local stress must reflect, at least to some extent, the concentration profile of particles [12].…”

“…Equation (2.3) gives the stress on a plane at z parallel to the walls (figure 1). The above approach is inspired by the so-called 'MOP', first introduced by Todd et al [11] for the case of a simple liquid and further examined by Varnik et al [12] in the case of a polymer melt. To the best of our knowledge, an extension of the MOP to the case of extended particles immersed in a fluid has not been proposed so far.…”