Intrinsic atomic-level stresses in polymer melts and networks

Abstract

By means of the virial stress formula, the macroscopic stress tensor in a polymer system may be expressed as a sum of atomic-level stress tensors, with each of the latter associated with a single atom and with all tensors, macroscopic and atomic, referred to the same fixed laboratory reference frame. In order to gain insight into the interplay between the covalent and noncovalent interactions in such systems, we refer the atomic-level stress tensor associated with a given atom to a moving local frame which maintains a fixed orientation with respect to the covalent structure attached to that atom; we term this the intrinsic atomic-level stress. We compute, by the method of molecular dynamics, the intrinsic stresses for model polymer melts and networks based on systems of freely rotating chains. The noncovalent interactions are through a truncated Lennard-Jones potential which is taken as either purely repulsive or has an attractive portion. We observe that (i) the intrinsic atomic-level stresses are highly anisotropic, even in an equilibrium melt in which the macroscopic stress is isotropic; (ii) the intrinsic atomic-level stress is the same in a polymer melt and in the corresponding polymer network model, if the latter is only moderately deformed; (iii) the intrinsic stresses in dense polymer melts are very different in the presence of noncovalent interactions from those in ideal systems; (iv) the addition of an attractive portion to the noncovalent potential has large effects on the deviatoric part of the intrinsic stress, thus casting doubt on the utility of the van der Waals’ picture for such systems.