by Dr. Sathish K. Sukumaran, Yamagata University, Japan

Abstract: 

In the linear regime, the dynamics and rheology of unentangled
polymeric liquids can be described by the Rouse model. The model
however fails miseraly in the nonlinear regime. To understand
phenomena observed under fast flows, such as shear thinning and
stress overshoot, we have performed extensive coarse-grained
molecular dynamics (MD) simulations of unentangled polymer melts and
solutions. The models used were: (1) standard Kremer-Grest (KG)
model (short chains), (2) Rouse+XV model (Rouse-like model with
excluded volume interaction between different chains), and (3) "soft
core" models (weak excluded volume). Using models (2) and (3) we can
simulate long unentangled chains as chain crossings are permitted.
We found that the "soft core" models (3) showed very weak shear
thinning, while models (1) and (2), exhibited shear thinning in
agreement with experiments. This suggested that excluded volume
interactions, which are believed to be effectively screened in
melts, are crucial for shear thinning. The results of the
simulations will be discussed from two perspectives: (1)
flow-induced molecular friction reduction; (2) shear blobs. I  will
argue that the two perspectives might be complementary in the sense
that while each one can explain only certain of the observations,
together they can account for all of them.