N. Fatkullin et R. Kimmich, The role of intermolecular interactions in the viscoelastic properties of polymer melts, MACRO SYMP, 146, 1999, pp. 103-108
The exact microscopic expression for the stress tensor in polymer liquids c
ontains a tensor product of the the segment position vector with the total,
intra- plus inter-chain, force acting on the segment. On the other hand, t
he widely accepted theory of viscoelasticity of polymer melts (1)) is based
on the assumption, that contributions from interchain interactions to the
viscosity of polymer melts is negligible relative to the effectively intrac
hain entropic interactions. Starting from the exact Green-Kubo formula for
the viscosity, the Rouse dynamic correlation functions, and Newton's second
law, we show that the intrachain assumption is inadequate. Rather, the int
rachain and interchain forces acting on polymer segments cancel each other
largely. The intrachain contribution therefore cannot be dominant as antici
pated in the usual treatment (1)), or, in other words, the interchain contr
ibution cannot be ignored. The main contribution to viscoelastic properties
of polymer melts can only arise from a part of the total stress tensor as
already suggested by M.Fixman based on a different argument (2)). It is con
cluded that the viscosity is of a purely interchain nature, and is determin
ed by the tensor product of the vector connecting the centers-of-mass of ne
ighboring macromolecules on the one hand, and the total force by which macr
omolecules interact, on the other, just in the case of simple liquids.