Dynamics and Rheology of a Supercooled Polymer Melt in Shear Flow.pdf

Dynamics and Rheology of a Supercooled Polymer Melt in Shear Flow Ryoichi Yamamoto and Akira Onuki Department of Physics, Kyoto University, Kyoto 606-8502, Japan (February 1, 2008) 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 2 relaxation function G(t) is calculated, which exhibits a stretched exponential relaxation on the time 0 scale of the α relaxation time τα and ultimately follows the Rouse dynamics characterized by the 0 time τR ∼ N 2τα . After application of shear γ˙ , transient stress growth σxy (t)/γ˙ first obeys the linear 2 growth t ′ ′ 0 dt G(t ) for strain less than 0.1 but saturates into a non-Newtonian viscosity for larger r strain. In steady states, shear-thinning and elongation of chains into ellipsoidal shapes take place a −1 for shear γ˙ larger than τR . In such strong shear, we find that the chains undergo random tumbling M motion taking stretched and compact shapes alternatively. We examine the validity of the stress- optical relation between the anisotropic parts of the stress tensor and the dielectric tensor, which 9 are violated in transient states due to the presence of a large glassy component of the stress. We ] furthermore introduce