(200f) Nonequilibrium Molecular Dynamics Simulations of Entangled Polymer Melts and Solutions Undergoing Planar Elongational Flows

Tube based models predict that concentrated polymer solutions
behave roughly the same as entangled polymer melts, if they have the same
number of entanglements and if stress, strain rate and time are made
dimensionless appropriately. But there are a lot of experimental works that
show entangled polymer melts and solutions exhibit different behaviors when
they are subject to extensional flows at high extension rates. Specifically
there is an upturn in extensional viscosity of polymer solutions at extension
rates higher than the inverse of the Rouse time which has not been observed in
polymer melts. Recent experiments of Wingstrand et
al.[1], however,
have demonstrated that using oligomers as solvent, polymer solutions could
behave in a similar fashion as melts.

In this work we performed nonequilibrium molecular dynamics (NEMD)
simulations of entangled C₇₀₀H₁₄₀₂ and C₁₀₀₀H₂₀₀₂
linear polyethylene melts subject to planer elongational
flows. We also simulated concentrated solutions of C₁₀₀₀H₂₀₀₂
in C₁₆H₃₂ as an oligomeric solvent with the same monomeric
units in order to compare the behavior of polymer melts and solutions with
roughly the same number of entanglements. Simulation results show that polymer
solutions and melts exhibit roughly the same behavior at high extension rates.
Specifically, similar to the melt, no upturn was observed in primary
extensional viscosity of the solution, consistent with Wingstrand
et al. experiments. Other system properties including chain orientation,
chain stretch, and entanglement network properties and their potential
connection to this finding will be discussed in this presentation. Preliminary
results of extensional viscosity of concentrated solutions in small solvent
molecules with various molecular architectures such as benzene will also be
shown.