Flow Effects on Melt Structure and Entanglement Network of Linear Polymers: Results from a Nonequilibrium Molecular Dynamics Simulation Study of a Polyethylene Melt in Steady Shear

Abstract

We present detailed results about the structural, conformational, rheo-optical, and topological properties of an entangled of C 400 H 802 linear polyethylene (PE) melt over a wide range of shear rates (covering both the linear and the highly nonlinear viscoelastic regimes) from direct nonequilibrium molecular dynamics (NEMD) simulations of a large system containing 192 chains (corresponding to 79200 interacting atomistic units). We discuss results for (i) the probability distribution of the mean-square chain end-to-end distance and its radius of gyration, (ii) the conformation tensor, (iii) the material functions in steady shear (viscosity, normal stress differences, nonequilibrium shear compliance, hydrostatic pressure), (iv) the flow birefringence, (v) the orientation angle and order parameter, (vi) the interaction energies and their relative importance, (vii) the intermolecular pair distribution function, and (viii) the intrinsic molecular shape of the chains (represented by the isosurface plots in terms of their monomer number density), all as a function of flow strength. A detailed primitive path (PP) analysis has allowed us to examine how the flow field alters the statistical properties of the underlying topological network of the melt (probability distribution functions and mean values of PP contour length, of the number and size of entanglement strands, etc.). Our results reveal significant distortions of all these distributions due to applied flow. One of the most important results of our work is that as the shear rate is increased, the average value of the contour length goes through a maximum and the number of entanglements per chain exhibits a shear-thinning behavior which bears many similarities with the corresponding behavior of the shear viscosity. Overall, most of the computed rheological properties of the C 400 H 802 melt change in a nonlinear way with the applied shear rate due to the simultaneous effect of (a) chain orientation and stretching, (b) chain rotation and tumbling under shear, and (c) chain disentanglement.

A force-level theory of the rheology of entangled rod and chain polymer liquids. I. Tube deformation, microscopic yielding, and the nonlinear elastic limit

“…The idea that entanglements are rare, long-lived, primarily binary dynamic interactions between pairs of chains has been suggested [34][35][36][37]. Much less such work exists under conditions of strong deformation [38].…”

Spatial distribution of entanglements in thin free-standing films

“…Of particular interest would be a more complete surface describing the dependence on film-averaged entanglement statistics on both h ef f /R ee and h ef f /L e . We also propose that new simulations and analyses be performed on entangled polymers under deformation; the entanglement dilution effect observed in steady-state continuous shear simulations [24] is very well captured by the theory [43], so in light of the present results a more critical evaluation of the theory, which would begin at the PP orientational scale, is called for. In closing, we reiterate that the connection between chain structure away from bulk, equilibrium conformations and either the statistics of the entanglement network or the dynamical properties of the system remains poorly understood (and not captured by either the classic Doi-Edwards tube model nor any recent extensions of it).…”

“…At the same time, it is known that the chain conformations are perturbed up to a distance ∼ R ee from a boundary, and a recent theory has been proposed connecting the orientational correlations that arise from such conformational perturbations to entanglement statistics [22,23]. Such correlations could be relevant for not only confined chain systems, but also bulk chains systems undergoing deformations, where it is known that chain-scale orientation occurs together with a decrease in the effective entanglement number [24,25].…”

Perspectives on the viscoelasticity and flow behavior of entangled linear and branched polymers

“…Baig et al [138] performed the first very large MD simulations of united-atom level polymers in shear flow for Z=14 using the p-SLODD algorithm, in which they demonstrated the loss of entanglements and the development of a bimodal distribution of chain-end separations, reflecting an excess of well-stretched chains. A modification of CCR to include the explicit dynamics entanglement loss was able to capture this effect [136].…”