Steady shearing flow of a moderately entangled polyethylene liquid

The rheological properties and dynamical responses of a monodisperse polyethylene (PE) liquid, C700H1402, were examined using equilibrium molecular dynamics and nonequilibrium molecular dynamics simulations of the atomistically detailed molecules. Equilibrium structural and dynamical properties of the PE liquid, such as the disengagement time (tau(d)), Rouse time (tau(R)), entanglement time, (tau(e)), reptation tube diameter, number of entanglements, and the distribution of the chain end-to-end vector, each followed very closely the predictions of the Doi and Edwards theory. Under steady shear conditions, the rheological and dynamical responses exhibited starkly different behavior as functions of shear rate, which could be categorized within four distinct shear rate regions; namely, gamma < tau(-1)(d), tau(-1)(d) < gamma < tau(-1)(R), tau(-1)(R) < gamma < tau(-1)(e) and gamma > tau(-1)(e). In the first region, the topological properties of the liquid remained relatively unperturbed from quiescent conditions and the rheological characteristic functions remained constant throughout. Little in the way of chain orientation or stretching occurred, and reptation theory described very well the system properties. Within the second range, chain orientation became the dominant dynamical system response with only a slight degree of chain stretching being evident. Rheological characteristic functions displayed shear-thinning behavior, and a plateau in the shear stress profile was observed. In the third range, significant chain stretching became apparent which led to a dramatic reduction in the number of entanglements, thereby enabling a rotational motion of the individual chain molecules in response to the vorticity of the shear field. A new timescale became evident that was associated with the period of the rotation/retraction cycles of the individual molecules. In the fourth region, the rotational motion of the chains became the sole relaxation mode of the system as the number of entanglements was gradually reduced to a level too low to support the conventional reptation theory. Furthermore, the individual molecular motions shared the same characteristics as those of similar chains in dilute and semidilute solution. Comparisons of the corresponding structural and dynamical properties of the C700H1402 liquid with those of the mildly entangled PE liquid C400H802 revealed how the properties of the liquids scaled with chain length. (C) 2016 The Society of Rheology.