(163bx) A Comparison of Rheological and Structural Properties of Linear Polyethylene Melts under Shear and Elongation Flow Using Nonequilibrium Molecular Dynamics Simulations | AIChE

(163bx) A Comparison of Rheological and Structural Properties of Linear Polyethylene Melts under Shear and Elongation Flow Using Nonequilibrium Molecular Dynamics Simulations

Authors 

Kim, J. M. - Presenter, The University of Tennessee
Keffer, D. J. - Presenter, University of Tennessee, Knoxville
Edwards, B. J. - Presenter, University of Tennessee
Baig, C. - Presenter, The University of Patras


1. Introduction

Nonequilibrium molecular
dynamics (NEMD) simulations play a significant role in our understanding of
rheological and structural behaviors of polymeric materials in flowing systems,
which is important not only in practical polymer processing, but also in
advancing our knowledge of fundamental characteristics of chain molecules, i.e.,
viscoelasticity.1 In
our previous paper2, we have developed so-called proper-SLLOD (or
p-SLLOD) algorithm. Furthermore, more recently using the p-SLLOD algorithm, we have recently performed
NEMD simulations of short-chain alkanes, such as C10H22
(decane), C16H34 (hexadecane) and C24H50
(tetracosane) under planar elongation flow (PEF) 3 extending work done by Cui et al. for
steady-state shear flow to steady-state PEF4. Many interesting results have been
observed there.
We also extended our NEMD simulations of PEF from the short-chain alkanes
performed in the previous paper5
to the more complex linear polyethylene melts of C50H102
up to C128H258. In this study, we perform NEMD
simulations of complex linear polyethylene melt of C50H102,
C78H158 and C128H258 under PCF and
compare it to our previous results of short-chain and long-chain alkanes in
both PCF and PEF.

 

2. Technical approach

In the present work, the same temperature, T=450 K, was
used for all the systems. A different density, however, was employed for each
system: r=0.7426 g/cm3 for C50H102,
r=0.7640 g/cm3 for C78H158,
and r=0.7754 g/cm3 for C128H258.
Specifically, we employed 120 molecules for C50H102
(total 6000 interaction sites), 160 molecules for C78H158 (total
12480 interaction sites), and 256 molecules for C128H258
(total 32768 interaction sites). The
reduced shear rate employed in this study
ranges from =0.0002 to 1.0. The box dimensions (x•y•z in unit of Å) of 93.02•45•45 were employed for C50H102, 130.5•54•54 for C78H158, and 212.7•68•68 for C128H258.

 3. Results and Discussion

The longest relaxation times,
the so-called Rouse time tRouse, were determined by the time
correlation function of the end-to-end vector of chains using equilibrium
molecular dynamics simulations: tRouse=500 ps for C50H102,
tRouse =1.4 ns for C78H158,
and tRouse =5.5 ns for C128H258.
The corresponding reduced critical elongation rates are found to be =0.0047
for C50H102, =0.0016 for C78H158,
and =0.00043
for C128H158. Approximately, above the critical shear rate for each system, the shear
viscosity, h, showed shear-thinning
behavior as shear rate
increased for all the systems in this study. In our
previous study of short-chain alkanes under PCF and PEF, we observed
tension-thinning behavior. Linear
polyethylene melts under PEF also showed this behavior.
The minimum behavior in the hydrostatic pressure was
observed for all the systems. In the linear polymer
melt under PEF, this behavior appeared to originate from the change in the
intermolecular LJ potential energy with shear rate through the two competing factors of chain alignment (static
factor) and intermolecular collision (dynamic factor). As for two important
structural quantities, the mean square end-to-end distance of chains, <Rete2>,
and the mean square radius of gyration of chains, <Rg2>,
it was observed and compared with other systems. Further details of chain conformation have been represented well by
investigating the conformation tensor, which is considered an important
physical quantity in polymer rheology.   Several interesting
differences between the polyethylene molecules under shear and elongational
flows will also be discussed.

 

4. References

1F. A. Morrison, Understanding
Rheology
(Oxford University Press, New York, 2001).

2C. Baig, B. J. Edwards, D. J. Keffer,
and H. D. Cochran, J. Chem. Phys. 122, 114103 (2005).

3C. Baig, B. J. Edwards, D. J. Keffer,
and H. D. Cochran, J. Chem. Phys. 122, 184906 (2005).

4S. T. Cui, S. A. Gupta, and P. T. Cummings, J. Chem. Phys. 105, 1214 (1996).

5C. Baig, B. J. Edwards, D. J. Keffer,
and H. D. Cochran, J. Chem. Phys. in press.