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Stress overshoot is one of the most important nonlinear rheological phenomena exhibited by polymeric liquids undergoing start-up shear at sufficient flow strengths. Despite considerable previous research, the fundamental molecular characteristics underlying stress overshoot remain unknown. Here, we analyze the intrinsic molecular mechanisms behind the overshoot phenomenon using atomistic nonequilibrium molecular dynamics simulations of entangled linear polyethylene melts under shear flow. Through a detailed analysis of the transient rotational chain dynamics, we identify an intermolecular collision angular regime in the vicinity of the chain orientation angle 73952; ≈ 20° with respect to the flow direction. The shear stress overshoot occurs via strong intermolecular collisions between chains in the collision regime at 73952; = 15°-25°, corresponding to a peak strain of 2-4, which is an experimentally well-known value. The normal stress overshoot appears at approximately 73952; = 10°, at a corresponding peak strain roughly equivalent to twice that for the shear stress. We provide plausible answers to several basic questions regarding the stress overshoot, which may further help understand other nonlinear phenomena of polymeric systems. [ABSTRACT FROM AUTHOR]

A comprehensive understanding of chain-branching effects, essential for establishing general knowledge of the structure-property-phenomenon relationship in polymer science, has not yet been found, due to a critical lack of knowledge on the role of short-chain branches, the effects of which have mostly been neglected in favor of the standard entropic-based concepts of long polymers. Here, we show a significant effect of short-chain branching on the structural and dynamical properties of polymeric materials, and reveal the molecular origins behind the fundamental role of short branches, via atomistic nonequilibrium molecular dynamics and mesoscopic Brownian dynamics by systematically varying the strength of the mobility of short branches. We demonstrate that the fast random Brownian kinetics inherent to short branches plays a key role in governing the overall structure and dynamics of polymers, leading to a compact molecular structure and, under external fields, to a lesser degree of structural deformation of polymer, to a reduced shear-thinning behavior, and to a smaller elastic stress, compared with their linear analogues. Their fast dynamical nature being unaffected by practical flow fields owing to their very short characteristic time scale, short branches would substantially influence (i.e., facilitate) the overall relaxation behavior of polymeric materials under various flowing conditions. [ABSTRACT FROM AUTHOR]

We present a detailed analysis of the configurational temperature (Tconf) for its application to polymeric materials using nonequilibrium molecular dynamics (NEMD) and nonequilibrium Monte Carlo (NEMC) methods. Simulations were performed of linear polyethylene liquid C78H158 undergoing shear and elongational flows. At equilibrium, Tconf is equal to the set point temperature of the simulation. An aphysically large decrease in Tconf is observed in the NEMD simulations for both flows, especially at strong flow fields. By analyzing separately the individual contributions of the different potential interaction modes to the configurational temperature, it is found that the bonded modes (which constitutes almost 99.5% of the total) dominate the total Tconf over the nonbonded ones; i.e., bond-stretching (≈86.5%), bond-bending (≈11.8%), bond-torsional (≈1.2%), nonbonded intermolecular (≈0.4%), and intramolecular (≈0.1%) Lennard-Jones. The configurational temperature of the individual modes generally exhibits a nonmonotonic behavior with the flow strength and a dramatic change beyond a critical value of flow strength; this is mainly attributed to the dynamical effect of strong molecular collisions occurring at strong flow fields. In contrast, no such behavior is observed in the NEMC simulations where such dynamical effects are absent. Based on the principal physical concept of the configurational temperature, which represents the large-scale structural characteristics of the system, we propose to exclude the dynamical effects exhibited by the individual interaction modes, in obtaining a physically meaningful Tconf as the configurational entropy of the system should not be affected by such factors. Since (a) the main difference between equilibrium and nonequilibrium states lies in the change in the overall (global) structure (represented by the bond torsional and nonbonded modes), and (b) the local, very short structure (represented by the bond-stretching and bond-bending modes) is barely changing between equilibrium and nonequilibrium states and its contribution to the total system configurational entropy is negligible compared to the large-scale structural changes, in order to accurately describe the structural changes occurring at nonequilibrium states by use of the configurational temperature, we further propose that only the contributions from the bond-torsional and nonbonded modes to ΔTconf between equilibrium and nonequilibrium states should be taken into account to generate a physically meaningful ΔTconf. Applying the above hypothesis to the analysis of the simulation data, good agreement between the NEMD and NEMC simulations (and between NEMD simulations for different flows) is observed. Furthermore, the configurational temperature obtained in such way is found to match remarkably well with the heat capacity of amorphous polyethylene liquids and the flow-enhanced melting-point elevation reported in experiment. [ABSTRACT FROM AUTHOR]

Interfacial slip plays a crucial role in a variety of fluid dynamics problems occurring in practical polymer processing, lubrication, adhesion, nanocomposites, etc. Despite many research efforts, a fundamental understanding of the underlying molecular mechanisms and dynamics is still lacking. Here, we present the intrinsic molecular characteristics of the slip phenomena by using atomistic nonequilibrium molecular dynamics simulations of polyethylene melts under shear flow. Our results identify three distinctive characteristic regimes with regard to the degree of slip and reveal the underlying molecular mechanisms for each regime: (i) the z-to-x chain rotation mechanism in the vorticity plane in the weak flow regime, which effectively diminishes the wall friction against chain movement along the flow direction, (ii) the repetitive chain detachment-attachment (out-of-plane wagging) and disentanglement mechanism in the intermediate regime, and (iii) irregular (chaotic) chain rotation and tumbling mechanisms in the strong flow regime. The second and third regimes can be classified as dynamically stable and unstable, respectively. The present findings would greatly help us comprehend the general characteristics of the interfacial rheological phenomena of polymeric materials. [ABSTRACT FROM AUTHOR]

We present detailed results for the structural and rheological properties of unknotted and unconcatenated ring polyethylene (PE) melts under shear and elongation flows via direct atomistic nonequilibrium molecular dynamics simulations. Short (C78H156) and long (C400H800) ring PE melts were subjected to planar Couette flow (PCF) and planar elongational flow (PEF) across a wide range of strain rates from linear to highly nonlinear flow regimes. The results are analyzed in detail through a direct comparison with those of the corresponding linear polymers. We found that, in comparison to their linear analogs, ring melts possess rather compact chain structures at or near the equilibrium state and exhibit a considerably lesser degree of structural deformation with respect to the applied flow strength under both PCF and PEF. The large structural resistance of ring polymers against an external flow field is attributed to the intrinsic closed-loop configuration of the ring and the topological constraint of nonconcatenation between ring chains in the melt. As a result, there appears to be a substantial discrepancy between ring and linear systems in terms of their structural and rheological properties such as chain orientation, the distribution of chain dimensions, viscosity, flow birefringence, hydrostatic pressure, the pair correlation function, and potential interaction energies. The findings and conclusions drawn in this work would be a useful guide in future exploration of the characteristic dynamical and relaxation mechanisms of ring polymers in bulk or confined systems under flowing conditions. [ABSTRACT FROM AUTHOR]