(243j) Structural and Rheological Responses of an Entangled Polyethylene Solution to Uniaxial Extensional Flows Via Nonequilibrium Molecular Dynamics Simulations

The complex flow response of entangled concentrated polymer solutions to extensional flow fields has been a subject of study and debate among polymer physicists and rheologists. This complexity arises from a diverse spectrum of flow-induced phenomena in these solutions ranging from extension thinning at low to intermediate or high extension rates to extension thickening at very high deformation rates to flow-induced phase separation and crystallization. Furthermore, experimental observations suggest that the type of solvent (i.e., oligomeric vs. small molecule solvent) could substantially influence the high-extension-rate response of entangled solutions, contradicting theoretical predictions. We have recently studied the planar elongational flow (PEF) of entangled solutions of C₁₀₀₀H₂₀₀₂ in n-hexadecane and benzene solvents [M. H. Nafar Sefiddashti, B. J. Edwards, and B. Khomami, Macromolecules, 53, 6432 (2020)] using nonequilibrium molecular dynamics (NEMD) simulations. These simulations demonstrated that both solutions undergo a chemical phase separation at extension rates higher than the inverse Rouse time of the systems and a flow-induced crystallization (FIC) at very high extension rates. Despite a difference in the onset of FIC, both the solutions exhibited qualitatively similar responses to the applied planar elongational flow field. Specifically, no significant extension thickening was observed in the extensional viscosities of the solutions at high deformation rates.

In this work, we studied a similar solution, i.e., C₁₀₀₀H₂₀₀₂ linear polyethylene solution in benzene at a concentration of 13.5 times that of overlap concentration, subject to uniaxial elongational flows (UEF). These simulations revealed that the solution experiences similar chemical phase separation and FIC as those observed under PEF, within the roughly same extension rate ranges expressed in terms of the dimensionless Rouse Deborah numbers. The structural and rheological responses of this entangled solution to UEF will be discussed, and a one-to-one comparison will be made to the response of the same solution subjected to PEF.