Elucidating the flow-microstructure coupling in the entangled polymer melts. Part I: Single chain dynamics in shear flow

Tube-based theories have accurately predicted the rheological properties of entangled, linear, monodisperse polymer melts for slow flows since reptation is the dominant chain relaxation mechanism in this flow regime. However, at higher strain rates, the continuum level theories fail to describe the nonlinear properties precisely since all the relevant physical processes have not been incorporated self-consistently into the constitutive equation expression. Therefore, we have utilized dissipative particle dynamics (DPD) simulations to examine the flow microstructure coupling of moderately entangled polymer melts (13 ≤ 〈Z_k〉 ≤ 27, 〈Z_k〉: average entanglement density at equilibrium) undergoing simple shear flow. In so doing, not only do we gauge the fidelity of DPD simulations of flow of entangled polymeric fluids via a direct comparison with nonequilibrium molecular dynamics results, but also, we elucidate the intricate relationship between single chain dynamics and relaxation mechanisms in moderately entangled polymeric melts. Specifically, it is shown that at small shear rates, $γ̇$ ≤ $τd−1$⁠, chains reptate and flow align and as shear rate increases, at $τd−1$ ≤ $γ̇$ ≤ $τR−1$⁠, significant chain orientation and onset of chain extension lead to flow-induced chain disentanglement and a commensurate dilation of the “tubes” allowing chain rotation/retraction to occur. Ultimately, at very high shear rates, $τR−1$ ≤ $γ̇$⁠, the chains become significantly aligned with the flow leading to destruction of the entanglement network where the chain motion resembles that of dilute/semi-dilute polymer solutions under theta condition.