Molecular process of stress relaxation for sheared polymer melts

While the stress relaxation has been one of the most useful protocols in polymer rheology, the underlying molecular processes have not yet been fully analyzed with direct quantification and have appeared to be somewhat ambiguous or even controversial among the existing experimental and numerical results. Here we analyzed in detail the basic molecular characteristics behind the stress relaxation phenomena of entangled linear polymer melts upon cessation of steady shear flow using atomistic nonequilibrium molecular dynamics simulations. Two separate relaxation mechanisms were identified: fast structural relaxation via the entropic chain retraction force and the induced orientational relaxation, and slow thermally-driven orientational relaxation. These processes were directly confirmed and quantified, and the physical origins were elucidated for several well-known experimental observations of stress relaxation behaviors. Mesoscopic structural analysis accounting for individual chain dynamics further reveals the conformational transition of polymer chains during relaxation. The present findings can be a useful basis for comprehending the general characteristics of the nonequilibrium relaxation phenomena exhibited by flowing polymeric materials with various molecular architectures and flow types.