Individual chain dynamics of a polyethylene melt undergoing steady shear flow

Abstract

Individual molecule dynamics have been shown to influence significantly the bulk rheological and microstructural properties of short-chain, unentangled, linear polyethylene liquids undergoing high strain-rate flows. The objective of this work was to extend this analysis to a linear polyethylene composed of macromolecules of a much greater length and entanglement density; i.e., a liquid consisting of C 400 H 802 molecules, with approximately ten kinks per chain at equilibrium, as calculated by the Z1 code of Kr€ oger [Comput. Phys. Commun. 168, 209-232 (2005)]. To achieve this, we performed nonequilibrium molecular dynamics (NEMD) simulations of a model system using the well-established potential model of Siepmann et al. [Nature 365, 330-332 (1993)] for a wide range of Weissenberg numbers (Wi) under steady shear flow. A recent study by Baig et al. [Macromolecules 43, 6886-6902 (2010)] examined this same system using NEMD simulations, but focused on the bulk rheological and microstructural properties as calculated from ensemble averages of the chains comprising the macromolecular liquids. In so doing, some key features of the system dynamics were not fully elucidated, which this article aims to highlight. Specifically, it was found that this polyethylene liquid displays multiple timescales associated with not only the decorrelation of the end-to-end vector (commonly related to the Rouse time or disengagement time, depending on the entanglement density of the liquid), but also ones associated with the retraction and rotation cycles of the individual molecules. Furthermore, when accounting for these individual chain dynamics, the "longest" relaxation time of the system was higher by a factor of 1.7, independent of shear rate, when calculated self-consistently due to the coupling of relaxation modes. Brownian dynamics (BD) simulations were also performed on an analogous free-draining bead-rod chain model to compare the rotation and retraction dynamics of a single chain in dilute solution with individual molecular motions in the melt. These BD simulations revealed that the dynamics of the free-draining chain are qualitatively and quantitatively similar to those of the individual chains comprising the polyethylene melt at strain rates in excess of Wi % 50, implying a possible breakdown of reptation theory in the high shear limit. An examination of the bulk-average properties revealed the effects of the chain rotation and retraction cycles upon commonly modeled microstructural properties, such as the distribution function of the chain end-to-end vector and the entanglement number density. V

Modifying the pom-pom model for extensional viscosity overshoots

“…As noted in the introduction hints of ES are observed in molecular dynamics simulations of linear polymers [Sefiddashti et al (2015)] and in slip-link simulations [Masubuchi et al (2012[Masubuchi et al ( , 2014; Andreev et al (2013)]: These methods may provide corroboration of the assumptions in the present model for branched polymers. A more detailed tube model description of branched polymer dynamics might seek to capture the nested tube structure suggested by constraint release models, and to couple the dynamics of adjacent polymer segments in a branched molecule.…”

“…There are also indications of ES in recent computer simulation, e.g., the work of Sefiddashti et al (2015). However, in the context of branched polymers and the pom-pom model, the effects of ES are potentially very strong: (i) The onset of branch-point withdrawal within the model provides an occasion for a sudden and rapid increase in the amount of ES.…”

Stress Undershoot of Entangled Polymers under Fast Startup Shear Flows in Primitive Chain Network Simulations

“…Costanzo et al 7) proposed the tumbling motion of entangled polymers, an unexpected molecular mechanism just discovered in the nonequilibrium molecular dynamics simulations of Nafar Sefiddashti et al 9) . Inspired by the damped oscillations shown in the end-to-end vector autocorrelation function, as observed by Nafar Sefiddashti et al 9) at the steady state of fast shear flows, Costanzo et al 7) introduced a damped oscillating tumbling function into the equation for the chain stretch dynamics, while leaving unaltered the equation for the orientation dynamics.…”

“…Inspired by the damped oscillations shown in the end-to-end vector autocorrelation function, as observed by Nafar Sefiddashti et al 9) at the steady state of fast shear flows, Costanzo et al 7) introduced a damped oscillating tumbling function into the equation for the chain stretch dynamics, while leaving unaltered the equation for the orientation dynamics. Such a simplified approach, however, has been recently questioned in a second paper by Nafar Sefiddashti et al 10) , who analyzed nonequilibrium molecular dynamics simulations also under transient startup of fast shear flows, revealing undershoots.…”

Steady shearing flow of a moderately entangled polyethylene liquid

“…These simulations thus revealed that inclusion of constraint release events played an important role in the emergence of the plateau region and concomitant mitigation of the drop in stress, as predicted by the DE theory. Recent atomistic simulations of unentangled [15] and mildly entangled [16] polyethylene (PE) melts showed that individual chains experience retraction and rotation cycles within oriented tubelike structures formed by highly stretched surrounding chain molecules at large strain rates. The onset of chain rotation was attributed to the flowinduced disentanglement of polymer chains that occurs at high shear rates in steady shearing flow [16].…”