Steady shearing flow of a moderately entangled polyethylene liquid

Abstract

The rheological properties and dynamical responses of a monodisperse polyethylene (PE) liquid, C 700 H 1402 , 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 (s d ), Rouse time (s R ), entanglement time, (s 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,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 C 700 H 1402 liquid with those of the mildly entangled PE liquid C 400 H 802 revealed how the properties of the liquids scaled with chain length. V C 2016 The Society of Rheology. [http://dx.

Flow-induced crystallization of a polyethylene liquid above the melting temperature and its nonequilibrium phase diagram

“…A detailed description of the NEMD simulation methodology used in this study can be found in preceding work, including the SKS potential parameters employed as the united-atom potential model of PE [50,65,66] (also see the Supplemental Material of Ref. [49] for a complete description).…”

From intermediate anisotropic to isotropic friction at large strain rates to account for viscosity thickening in polymer solutions

“…14,23 The underlying reason for the appearance of this damping behavior was demonstrated to be caused by the shear-induced rotational motion of chains, 23,28 further supported by non-equilibrium atomistic simulations. [29][30][31] For this reason, similar undershoots are not expected to occur in elongational flows that are free from rotational contributions to the velocity gradient tensor. 24 We are going to calculate elongational flow properties using the parameters we employed earlier in a quantitative comparison with shear rheological data.…”

Nonlinear shear of entangled polymers from nonequilibrium molecular dynamics

“…Unentangled C50 and slightly entangled C178 chains have been simulated in bulk and in confined systems. MD has been used to explore the structural, conformational, rheo‐optical, and topological properties of an entangled C400 chains in the linear and highly nonlinear regimes; the dynamics and rheology of supercooled melts of C10 C20 and C150, and C1000 chains; the steady and transient behavior of a C100 melt; the steady and transient behavior of a C400 melt; and steady and startup shear of a C700 melt …”

“…MD has been used to explore the structural, conformational, rheo-optical, and topological properties of an entangled C400 chains in the linear and highly nonlinear regimes 30,31 ; the dynamics and rheology of supercooled melts of C10 32 C20 and C150, 33 and C1000 28 chains; the steady 34 and transient 34 behavior of a C100 melt; the steady and transient 31,35 behavior of a C400 melt; and steady and startup shear of a C700 melt. 36,37 In polymer fluids, long-chain entanglement leads to the slow dynamics that makes polymer processing phenomena rich and difficult to predict empirically. These slow dynamics mean that MD of long polymers is extremely difficult without cutting-edge hardware and code.…”