Abstract
In this work, we analyzed the individual chain dynamics for linear polymer melts under shear flow for bulk and confined systems using atomistic nonequilibrium molecular dynamics simulations of unentangled (C50H102) and slightly entangled (C178H358) polyethylene melts. While a certain similarity appears for the bulk and confined systems for the dynamic mechanisms of polymer chains in response to the imposed flow field, the interfacial chain dynamics near the boundary solid walls in the confined system are significantly different from the corresponding bulk chain dynamics. Detailed molecular-level analysis of the individual chain motions in a wide range of flow strengths are carried out to characterize the intrinsic molecular mechanisms of the bulk and interfacial chains in three flow regimes (weak, intermediate, and strong). These mechanisms essentially underlie various macroscopic structural and rheological properties of polymer systems, such as the mean-square chain end-to-end distance, probability distribution of the chain end-to-end distance, viscosity, and the first normal stress coefficient. Further analysis based on the mesoscopic Brightness method provides additional structural information about the polymer chains in association with their molecular mechanisms.
Highlights
Polymers undergo a variety of processing conditions in practical polymer processes, such as the plastic extrusion process
Through a detailed analysis of individual chain dynamics using atomistic nonequilibrium molecular dynamics (NEMD) simulations for unentangled (C50H102) and weakly entangled (C178H358) linear PE melts under shear flow in bulk and confined systems, we revealed and contrasted the characteristic molecular mechanisms with respect to the applied flow strength between the bulk and interfacial chains
In the weak flow regime, while both bulk and interfacial chains are aligned to the flow (x-)direction without a significant structural deformation, the interfacial chains of the confined system perform the z-to-x in-plane rotation at the wall
Summary
Polymers undergo a variety of processing conditions in practical polymer processes, such as the plastic extrusion process. Many unresolved rheological issues remain (especially from the microscopic viewpoint) for polymeric materials in bulk or confined systems, e.g., fundamental mechanisms underlying stress overshoot, interfacial slip, and melt instability for polymer melts under shear flow[2,3,4,5,6,7,8] To systematically control such rheological phenomena, it is essential to comprehend the intrinsic molecular dynamics of individual polymer chains separately in bulk and confined situations and how they compare to each other; such an understanding would greatly help to build general knowledge to accurately capture the physical aspects that underlie such complex macroscopic responses of polymer systems and tune the material properties in response to an arbitrary external flow field. We analyze the similarities and differences between the bulk and confined melt systems in the characteristic molecular mechanisms and rheological responses under various flow regimes
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