(33d) Influence of Polydispersity on Flow-Induced Configurational Microphase Separation and Crystallization of Well Entangled Polyethylene Melt Under Planar Elongational Flow

Understanding polymeric fluid dynamics is crucial for process design and optimization of net shape manufacturing of polymeric materials, as it involves complex non-linear coupling between fluid motion and microstructure evolution that, in turn, dictates crystallization dynamics. Despite substantial progress in theoretical and computational efforts that have paved the way for rational selection of essential process design variables, limited knowledge of the relevant physical micro-mechanisms under flow conditions continues to restrict the accuracy of models for microstructure evolution and spatio-temporal dynamics of crystallization. In particular, the study of flow-induced crystallization (FIC) that has significant industrial implications, presents experimental challenges due to its rapid dynamics at small spatio-temporal scales. Hence, nanoscale simulations, such as high-fidelity Nonequilibrium Molecular Dynamics (NEMD), coupled with innovative approaches for quantifying phase transition, are central to mechanistic understanding of FIC. To that end, building upon prior efforts by the MRAIL group at the University of Tennessee, recent atomistic simulations revealed a coil-stretch transition under planar elongational flow (PEF) in fully entangled polymer melts of monodisperse polymeric melts. These findings were facilitated by introducing an innovative approach for quantifying phase transitions and FIC in polymeric fluids, alongside developing a novel method for calculating nonequilibrium entropy in entangled polyethylene melts subjected to flow.

In this study, we build upon prior efforts by the MRAIL group at the University of Tennessee to unveil new insights on the conformational hysteresis and microphase separation in fully-entangled polydisperse polymer melts through NEMD simulations of a polydisperse linear polyethylene (PE) melt, C₁₀₀₀H_2002, with a polydispersity index (PDI) of 1.8. Specifically, NEMD simulations in planar extensional flow (PEF) over a broad range of Deborah numbers (De) were conducted to identify the critical De and strain for occurrence of flow-induced configurational microphase separation (FIMS), and FIC. In turn, we can construct a nonequilibrium phase diagram for the simulated polydisperse PE as well as its rheological response. Finally, we elucidated the molecular mechanisms of FIMS and FIC and calculated the kinetics of FIC as a function of flow strength to highlight the impact of polydispersity by employing a newly developed phase analysis method and clustering technique.