(255ar) Atomistic Simulation of Dynamics of Individual Molecules in Entangled Polymers Undergoing Homogenous Shear Flow

Nonequilibrium
molecular dynamics (NEMD) simulations of entangled linear C₄₀₀H₈₀₂
and C₇₀₀H₁₄₀₂ polyethylene melts were performed to
investigate the chain dynamics over a wide range of shear rates .
The rheological and dynamical responses of these liquids can be classified
roughly according to three shear rate regimes; namely, ,
,
and ,
where τ_d and τ_e
are the disengagement and entanglement time of the liquid. Under quiescent
conditions, the liquid dynamics are described well by the reptation theory of
Doi and Edwards. Over the intermediate shear rate range, shear-induced
rotational motion of the individual chain molecules became a dominant
relaxation mechanism as the number of chain entanglements decreased
dramatically with increasing
,
ultimately resulting
in a plateau in the shear stress profile. A new timescale became evident that
was associated with the period of the rotation/retraction cycles of the individual
molecules. In the third region, the rotational motion of the chains became the
sole relaxation mode of the system as the number of entanglements was reduced
to a level too low to support conventional reptation theory. The decorrelation (τ_d) and rotational (τ_rot)
relaxation times of the system were extracted by fitting a functional form of a
exp(-t/τ_d)cos(2πt/τ_rot)
to the end-to-end vector autocorrelation function data. Both the decorrelation
and rotational characteristic times exhibited a shear-thinning behavior that
scaled as Wi ^{- 0.76} at high shear rates, regardless of the chain length.
Wi is the dimensionless shear rate defined as . The number of entanglements was
also computed as a function
Wi. It is shown that this characteristic timescale has a
shear-thinning behavior which scaled as
Wi ^{- 0.35} at high shear rates for both liquids. The
implications of these findings will be presented with regard to convective
constraint release, Chain orientation, and chain stretch over a wide range of
shear rates.