Doctoral Dissertations

Date of Award

12-2018

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Chemical Engineering

Major Professor

Brian J. Edwards

Committee Members

Steven M. Abel, Mark Dadmun, Bamin Khomamin

Abstract

Individual molecule dynamics have been shown to influence significantly the bulk rheological and microstructural properties of polymeric liquids undergoing high strain-rate flows. The objective of this study was to perform equilibrium and Nonequilibrium Molecular Dynamics (NEMD) simulations for monodisperse polyethylene liquids over a wide range of deformation rates under steady shear and planar elongational flows in an attempt to understand the underlying physical processes that shape the dynamical responses of these complex liquids.Under steady shear conditions, the rheological and dynamical responses exhibited different behavior as functions of shear rate, which could be categorized within four shear rate regions. For shear rates smaller than the inverse disengagement time, the topological properties of the liquid remained relatively unperturbed from quiescent conditions and the rheological characteristic functions remained constant throughout. For shear rates between the inverse disengagement and inverse Rouse times, chain orientation became the dominant dynamical system response with only a mild degree of chain stretching and disentanglement being evident. Rheological characteristic functions displayed shear-thinning behavior, and a plateau in the shear stress profile was observed. For shear rates between the inverse Rouse and inverse entanglement times, significant chain stretching became apparent which led to a reduction in the number of entanglements, thereby enabling a rotational motion of the individual molecules in response to the vorticity of the shear field. At higher shear rates, the rotational motion of the chains became the sole relaxation mode of the system as the number of entanglements was gradually reduced to a low level. The analysis of the transient responses revealed that the stress overshoot and undershoot commonly observed at high shear rate can likely be attributed to tube orientation rather than tube stretching.Under planar elongational flow, a coil-stretch transition, with an associated hysteresis in the configurational flow profile, was observed over a specific range of strain rates. Steady state results revealed bimodal distribution functions in which configurational states were simultaneously populated by relatively coiled and stretched molecules. The realization of this bi-phasic coil-stretch transition was an unanticipated microphase separation into a heterogeneous liquid composed of regions of either highly stretched or tightly coiled macromolecules.

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