Entangled-to-packed crossover in nonlinear extensional rheology of entangled polymers

The tube model, which reduces the interaction of many-chain entanglement to a single-chain picture, has enabled quantitative descriptions of the mechanical behaviors of polymeric materials and underpins applications from polymer processing to life sciences. In strong extensional flows, however, this theory systematically fails. Herein, through molecular dynamics simulations of entangled polymers, we reveal a strain-induced crossover from the entangled state to a packed state in the inter-chain configuration. Following the saturation of primitive chain stretch, convective disentanglement and chain alignment reorganize the system into a packed configuration of aligned segments. This crossover makes an explicit inter-chain contribution necessary for describing tensile stress beyond the intra-chain conformational stress, explaining the breakdown of tube-based descriptions and quantitatively capturing the mechanical response at large extensional strains. These findings clearly show how the inter-chain reorganization governs the nonlinear extensional behaviors. Entanglements among long-chain macromolecules dictate the mechanical performance of polymeric materials and are described with the tube model but the validity of this model has been questioned for nonlinear extension. Here the authors reveal the key mechanism that governs the mechanical response during extension by elucidating the evolution of entanglements using computer simulations and theoretical analysis.