STRESS-RELAXATION IN A DIATOMIC LIQUID

By the use of nonequilibrium molecular dynamics we have studied the st ress relaxation following imposition of a constant-volume elongation i n the x(1) direction on a model diatomic liquid. Three consecutive mod es of relaxation of the stress difference tau=t(11)-1/2(t(22)+t(33)) a re found, each governed by exponentials e(-alpha it) with alpha(1) > a lpha(2) > alpha(3). Each mode is shown to correspond to the return to isotropy of a different characteristic of the liquid structure that ha s been rendered anisotropic by the deformation, namely (1) (r) over ba r (theta), the mean distance from a generic atom of interacting atoms in the theta direction, with the angle theta measured from the stretch axis x(1); (2) (N) over bar(theta), the mean number density of intera cting atoms in the theta direction; and (3) [[P-2(theta(b))]], a measu re of the orientation theta(b) of molecular axes with respect to x(1). The first two modes are identical in form to those studied previously [R. C. Picu and J. H. Weiner, J. Chem. Phys. 107, 7214 (1997)] for st ress relaxation in a monatomic liquid, but their rates of decay differ because of differences in packing and in atomic mobility. During the third mode of relaxation it is found that tau=C[[P-2(theta(b))]], wher e C may be understood in terms of the intrinsic stress tensor, a tenso r referred to a coordinate system fixed to the molecule [J. Gao and J. Il. Weiner, J. Chem. Phys. 90, 6749 (1989)]. The relevance of these r esults to stress relaxation in polymer melts is discussed. (C) 1998 Am erican Institute of Physics.