The central role of excluded volume in the production of anisotropic stress in deformed polymer melts

Nonequilibrium molecular dynamics computations are used to calculate the stress relaxation in an idealized "pearl necklace" polymer melt. The covalent bonds in the melt are approximated by stiff linear springs with potential u(b)(r), while nonbonded atoms interact through a Lennard-Jones potential u(nb)(r), which includes both a short-range repulsive portion, and a long-range attractive tail. The simulations are used to calculate the relaxation of difference stress t(11)-(t(22)+t(33))/2 in the melt following a constant volume extension in the x(1) direction. Results show that the dominant contribution to the difference stress arises from excluded volume interactions, through the repulsive portion of the Lennard-Jones potential, while the attractive tail gives rise to only a small portion of the difference stress. In contrast, attractive nonbonded interactions provide a dominant contribution to the mean stress. This behavior occurs because the difference stress is generated by deformation induced anisotropy in the orientation of nonbonded interactions acting on a representative atom in the melt. The anisotropy is localized to a small region surrounding the atom. The anisotropy in the orientation of these nonbonded pairs is generated by steric shielding: the deformation tends to orient the chain bonds parallel to the stretch direction, and so orients nonbonded pairs transverse to the stretch direction. (C) 2003 American Institute of Physics.