Flow-Induced Friction Reduction in Shear Flows of Unentangled Polymer Melts

We here analyze very recent shear flow data of three unentangled polystyrene (PS) melts with different molar masses M (Macromolecules 2025, 58, 7062-7083) through single-chain Brownian dynamics simulations that account for finite chain extensibility and flow-induced friction reduction. It is confirmed here that in PS melts the monomeric friction coefficient ζ must significantly decrease as the Kuhn segment order parameter S increases, as also shown by existing many-chain molecular dynamics simulations. The resulting function ζ-(S) is almost indistinguishable for the two higher M samples and comparable to that previously extracted from uniaxial extension data of a PS melt with a similar M (Macromolecules 2019, 52, 4610-4616). A weaker dependence of ζ on S is, however, observed for the lowest M melt. This discrepancy, combined with the fact that (contrary to data) our single-chain simulations predict N ₂ ≈ 0, suggests the presence of many-chain effects other than just the ζ-(S) dependence. The fact that single-chain models still miss some physics is confirmed by the successful comparison between PS melt data and existing many-chain simulations. The agreement is, however, limited to shear flows since the same comparison fails in elongational flows.