Computer Simulations of nonlinear dynamics of entangled polymer liquids

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Using molecular dynamics simulations, we study dynamics of a model polymer melt composed of short chains with bead number N=10 in supercooled states. In quiescent conditions, the stress relaxation function is calculated, which exhibits a stretched exponential relaxation on the time scale of the n relaxation time and ultimately follows the Rouse dynamics. In sheared steady states, shear-thinning and elongation of chains into ellipsoidal shapes take place for a strong shear regime. In such strong shear, we find that the chains undergo random tumbling motion taking stretched and compact shapes alternatively.We next visualized entanglements in polymer melts. In quiescent states, a particle at an entanglement interacts with those belonging to a different chain persistently for long times. Entangled regime on each chain can be identified if the inter-chain interaction energy supported by each particle is averaged over a time interval much longer than monomeric relaxation time. We then apply a shear flow with rate much faster than the entanglement motion. With increasing strain the chains take zigzag shapes with bending taking place at the entangled regions. The chains arc first stretched as a network but disentanglement events subsequently occur, resulting in stress overshoot observed experimentally.