Shear-Thinning in Oligomer Melts—Molecular Origins and Applications

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.

Molecular origin of driving-dependent friction in fluids

“…( 1)), we have validated this condition for all considered fluids and pulling velocities. As a consequence, the nonequilibrium process is completely described by a two-time autocorrelation function and thus exactly modeled by a generalized Langevin equation (9). As the cumulant approximation represents the only assumption underlying dcTMD, this result is remarkable because it suggests that dcTMD should be a valuable approach to characterize the friction behavior of even complex fluids.…”

“…As another effect of the numerous internal degrees of freedom of C 40 H 82 undergoing conformational dynamics on multiple timescales, we have identified a stretched exponential long-time decay of the friction memory kernel. Indeed, the timescales of the conformational dynamics of polymer chains have been linked to shear thinning [6], with fast shearing leading to disentanglement of side chains and decreasing fluid internal friction [9]. Since the kernel decays too slow to be completely sampled by fast pulling velocities, the lubricant moreover exhibits a generic velocity dependence as discussed by Eq.…”

“…In the equilibrium limit of very slow pulling, we expect that the memory kernel (11) can be compared to the memory function obtained from an unbiased simulation. The latter can be calculated, e.g., by iteratively solving the Volterra equation [41] m ẍẍ(t) = K(t) ẋ2 + t 0 dτ K(τ ) ẍ(t − τ ) ẋ(t) , (12) which can be derived from the generalized Langevin equation (9).…”

“…The latter is of great practical importance, e.g., for the design of lubricants which aim to minimize friction and wear [5]. Non-Newtonian behavior is believed to result from structural changes within the liquid, such as the alignment of polymers along the shearing direction [6][7][8][9] or from the formation of molecule clusters [10][11][12].…”

On a hybrid continuum-kinetic model for complex fluids

“…A prototype of hybrid multiscale methods is the heterogeneous multiscale method proposed by E, Enquist et al [14-17, 19, 34], see also [8,36,[38][39][40] for its application for complex polymeric fluids. In this context let us also refer to triple-decker atomistic-mesoscopic-continuum method [20], the seamless multiscale methods [18,35], the equation-free multiscale methods [25,26] or the internal-flow multiscale method [4,5].…”