(724b) Atomistic Simulation of Dynamics of Individual Molecules in Entangled Polymers Undergoing Homogenous Shear and Planer Elongational Flows

Nonequilibrium
molecular dynamics (NEMD) simulations of an entangled C₇₀₀H₁₄₀₂ linear polyethylene system were
performed to investigate the chain dynamics over a wide range of Weissenberg
numbers (Wi)
under steady shearing flow.
Similar to the unentangled (C₇₈H₁₅₈) and moderately entangled
(C₄₀₀H₈₀₂) melts examined in prior simulation studies, the
distribution of the chain end-to-end distance, |R_ete|, at high Wi was bimodal
with a peak at low |R_ete| which is associated with the dynamical
rotation/retraction cycles experienced by individual chains, and a peak at high |R_ete| which corresponds to the highly
stretched and oriented macromolecules. To understand the underlying physics,
the relevant system time scales including the entanglement time, Rouse time, and
disengagement time were determined using segmental mean square displacement
analysis of the chain molecules. The
longest (τ_d) and rotational
(τ_rot) relaxation times of the system
at high Wi were extracted by fitting a functional form of A*exp(-t/τ_d)cos(2πt/τ_rot)
to the end-to-end
vector autocorrelation data. The number of entanglements and other topological
features of the liquid were also computed as functions of Wi.

NEMD
simulations of linear polyethylene melts with different chain lengths (C₄₀₀H₈₀₂, C₇₀₀H₁₄₀₂, and C₁₀₀₀H₂₀₀₂) were also performed in planer
extensional flow and rheological and topological behavior of entangled systems
were studied in the nonlinear regime. Specifically, we studied the behavior of
extensional viscosity as a function of strain rate,
, to examine
the thinning exponents and determine if there is any signs of an upturn in the viscosity
for extensional rates on the order of the inverse Rouse time, as predicted by
standard theories. The effects of the chain stretch, entanglement density, and
chain disentangling on the system behavior will also be discussed.