(639b) Flow-Induced Configuration Microphase Separation and Crystallization of Entangled Polyethylene Under Uniaxial Extensional Flows

Recent nonequilibrium molecular dynamics (NEMD) simulations of planar extensional flows (PEF) of a linear C₁₀₀₀H₂₀₀₂ polyethylene melt [M. H. Nafar Sefiddashti, B. J. Edwards, and B. Khomami, J. Chem. Phys. 148, 141103 (2018); Phys. Rev. Lett. 121, 247802 (2018)] demonstrated that entangled polymer melts can undergo a coil-stretch transition and configurational microphase separation within an intermediate range of extension rate, where the coil and stretched molecules coexist in separate domains. PEF is, however, hard to reproduce in experimental settings, and hence most of the extensional flow experiments of polymeric liquids have been performed in uniaxial extension flow geometries. This makes the comparison of PEF simulation findings and experimental observations difficult and debatable.

In this work, we studied the same entangled PE melt under uniaxial extensional flow (UEF) via NEMD simulations and directly compared the flow response of C₁₀₀₀H₂₀₀₂ under PEF and UEF at similar extension rates (expressed in terms of dimensionless Rouse Deborah number, De.) These simulations revealed that under UEF, the entangled melts undergo a qualitatively similar coil-stretch transition and configurational microphase separation, within the roughly same range of flow strength as that in PEF. Furthermore, the melt experienced flow-induced crystallization (FIC) at De > 9 and a temperature of 450 K, fully 50 K above the quiescent melting point. The onset of FIC was slightly lower than that observed for PEF. Overall, the comparison of PEF and UEF simulations suggests that entangled melts exhibit qualitatively similar responses to these extensional flows. This is especially important from a simulation perspective, as the computational costs of UEF simulations for long polymer chains are significantly higher than PEF simulations.