Improved predictions of nonlinear uniaxial tensile stress in polymer melts by accounting for microscopic conformation and entanglement changes

We investigate the nonlinear response behaviors of polymer melts with molecular dynamics simulation, during which the generalized Kraynik-Reinelt boundary condition is employed to uniaxially stretch the equilibrium polymer melts reaching steady states. By examining the changes in the stretching ratio R(n)nb, the orientational order parameter P2(n), and the entanglement during stretching, we find that the stress predicted based on the initial average entanglement state of the system has obvious deviation under a large strain. Thus, we propose that both the loss of entanglement and the heterogeneous distribution of entanglement points among different chains are essential to account for the nonlinear rheological behaviors. Incorporating these factors leads to a theoretical prediction that better aligns with the simulated stress observed under large strains and reveals the heterogeneity of stress distribution associated with entanglement heterogeneity.