Entanglement dynamics at flat surfaces: Investigations using multi-chain molecular dynamics and a single-chain slip-spring model

Abstract

The dynamics of an entangled polymer melt confined in a channel by parallel plates is investigated by Molecular Dynamics (MD) simulations of a detailed, multi-chain model. A primitive path analysis predicts that the density of entanglements remains approximately constant throughout the gap and drops to lower values only in the immediate vicinity of the surface. Based on these observations, we propose a coarse-grained, single-chain slip-spring model with a uniform density of slip-spring anchors and slip-links. The slip-spring model is compared to the Kremer-Grest MD bead-spring model via equilibrium correlation functions of chain orientations. Reasonably good agreement between the single-chain model and the detailed multi-chain model is obtained for chain relaxation dynamics, both away from the surface and for chains whose center of mass positions are at a distance from the surface that is less than the bulk chain radius of gyration, without introducing any additional model parameters. Our results suggest that there is no considerable drop in topological interactions for chains in the vicinity of a single flat surface. We infer from the slip-spring model that the experimental plateau modulus of a confined polymer melt may be different to a corresponding unconfined system even if there is no drop in topological interactions for the confined case.

Surface Disentanglement and Slip in a Polymer Melt: A Molecular Dynamics Study

“…We define a free chain Weissenberg number using the bulk shear-rate: Wi F =γ bulk τ F , where τ F = 2.5 × 10 5 τ LJ is the rotational relaxation time of the free chains (n = 256), which is defined from the longest Maxwell mode of a spectrum fitted to the end-to-end vector correlation function of free chains. The mean rotational relaxation time of all free chains in the channel is very similar to that of chains in the center of a channel without tethered chains [14], or that of a system of free chains using periodic boundaries. An alternative definition of the longest relaxation time, as used in previous work with which we may compare results [54], fits a single stretched exponential to the normalized end-to-end vector correlation function and then calculates the area under the curve.…”

“…In section III we introduce the MD model and methods. Z1 Primitive Path Analysis (PPA) [11][12][13][14] is employed to characterize the topological state of the fluid both at equilibrium and at high Weissenberg numbers.…”

Effect of flow-induced molecular alignment on welding and strength of polymer interfaces

“…17 Small deviations from equilibrium behavior are present close to the walls, which is consistent with reports in the literature. 26,27 We also equilibrated melts with M = 2400 chains using the same boundary conditions but double the length in the z−direction.…”

Coupling between Polymer Conformations and Dynamics Near Amorphous Silica Surfaces: A Direct Insight from Atomistic Simulations

“…Generic bead-spring and Monte Carlo models, which do not embody any specific chemical characteristics of the material, have been implemented for examining universal trends in properties of hybrid materials. These models are able to capture basic polymer features such as polymer flexibility [ 17 , 18 ] or conformational changes at the interface [ 19 , 20 ], and by setting up an explicit type of interaction between the polymer and surface, they were used to examine dynamical processes such as the chain dynamics in melts [ 21 , 22 , 23 ] or the adsorption dynamics [ 24 , 25 , 26 ] near flat surfaces. The roughness was introduced in generic models by attaching fixed obstacles to a flat surface [ 27 , 28 , 29 ] or to the walls of a slit [ 30 ], which in all cases resulted in reduced lateral mobility of adsorbed polymers with respect to the neat surface.…”