Abstract
We investigate the molecular origin of shear-thinning in melts of flexible, semiflexible and rigid oligomers with coarse-grained simulations of a sheared melt. Entanglements, alignment, stretching and tumbling modes or suppression of the latter all contribute to understanding how macroscopic flow properties emerge from the molecular level. In particular, we identify the rise and decline of entanglements with increasing chain stiffness as the major cause for the non-monotonic behaviour of the viscosity in equilibrium and at low shear rates, even for rather small oligomeric systems. At higher shear rates, chains align and disentangle, contributing to shear-thinning. By performing simulations of single chains in shear flow, we identify which of these phenomena are of collective nature and arise through interchain interactions and which are already present in dilute systems. Building upon these microscopic simulations, we identify by means of the Irving–Kirkwood formula the corresponding macroscopic stress tensor for a non-Newtonian polymer fluid. Shear-thinning effects in oligomer melts are also demonstrated by macroscopic simulations of channel flows. The latter have been obtained by the discontinuous Galerkin method approximating macroscopic polymer flows. Our study confirms the influence of microscopic details in the molecular structure of short polymers such as chain flexibility on macroscopic polymer flows.
Highlights
Understanding the relation between viscosity and structure and its implications on macroscopic flow is of prime importance, for semiflexible polymers, which are omnipresent in nature (DNA, actin filaments, microtubules), and synthetic polymers
We investigate the molecular origin of shear-thinning in melts of flexible, semiflexible and rigid oligomers with coarse-grained simulations of a sheared melt
Non-equilibrium molecular dynamics (NEMD) simulations have been applied for forty years [5,6,7,8,9,10,11,12,13,14,15,16] to study how macroscopic flow properties, such as shear-thinning, emerge from dynamics of microscopic structures
Summary
Understanding the relation between viscosity and structure and its implications on macroscopic flow is of prime importance, for semiflexible polymers, which are omnipresent in nature (DNA, actin filaments, microtubules), and synthetic polymers (polyelectrolytes, dendronised polymers). The underlying models tend to be more involved in these cases and include bending and even torsion terms absent in the early studies mentioned above It comes as a bit of a surprise that the specific influence of stiffness on shear-thinning in polymers has only recently come into the focus of attention. We apply a hybrid multiscale method that couples atomistic details obtained by molecular dynamics with a continuum model approximated by the discontinuous Galerkin method.
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