Molecular Rheology of Entangled Polymeric Fluids: A molecular simulation perspective

The temporal and spatial evolution of different nonlinear phenomena during the startup of shear and elongational flow of various entangled, linear polyethylene melts and solutions has been studied. The high-fidelity coarse-grained dissipative particle dynamics method is developed and evaluated based on previous NEMD simulations. The molecular level simulations of polymer melts and solutions indicate that shear banding is an elastic instability due to local orientation and segmental stretching and the commensurate disentanglement of the polymer network, which leads to the formation of regions with a low and high induced flow rate. Flow inhomogeneity begins after shear stress experiences a maximum as the first normal stress N1 reaches its maximum. We observed strain banding for liquids having both monotonic and non-monotonic flow curves. Further, The planar elongational flow simulations demonstrate a flow-induced coil-stretch transition (CST) and its associated hysteresis caused by configurational microphase separation. Results indicate that the breadth of the CST hysteresis loop is enlarged for the longer molecule liquid relative to the shorter one. Furthermore, relaxation simulations reveal that reducing the applied flow Deborah number (De) from a high value corresponding to a homogeneous phase of highly stretched molecules to a De within the biphasic region results in a two-stage relaxation process.