The coil–stretch transition and chain segregation in elongational flow of an entangled polydisperse polyethylene melt

Atomistic nonequilibrium molecular dynamics simulations of entangled polydisperse polyethylene melts possessing a realistic log-normal molecular-weight distribution under planar elongational flow demonstrate that polydispersity fundamentally modifies the coil–stretch transition (CST). In contrast to the sharp, bi-directional transition observed in monodisperse systems, the CST in polydisperse melts becomes distributed across the molecular ensemble due to chain-length-dependent disentanglement and heterogeneous coupling to the entanglement network. Steady-state chain statistics reveal concurrent populations of coiled and highly extended molecules spanning a wide range of deformation rates. This coexistence does not arise from bistability; rather, it reflects the decoupling of chain dynamics across the molecular-weight spectrum, with each chain population experiencing a nonlinear reduction in entanglement constraints at a distinct effective Deborah number. We refer to this phenomenon as a distributed coil–stretch transition (DCST), emphasizing the divergence of configurational dynamics between chains of different lengths. Long chains undergo early stretching and rapid disentanglement and consequently dominate the flow-induced ordering, while shorter chains remain partially coiled, retain a higher portion of their entanglement density, and continue to exhibit folding and retraction even at elevated deformation rates. Macroscopic elongational viscosity exhibits a nonmonotonic dependence on the deformation rate, with an upward shift near De ≈ 1 arising from the superposition of heterogeneous chain-length-dependent stress contributions. Independent energetic analyses reveal progressively more favorable intermolecular interactions accompanying sequential chain stretching and segregation, whereas flow-induced birefringence and the corresponding stress–optical response provide complementary optical evidence supporting the distributed coil–stretch transition.