IWNET

2006

4th International workshop on nonequilibrium thermodynamics and complex fluids
3-7 september 2006, Rhodes, Greece

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ORAL PRESENTATION
Session: 3 Non-equilibrium thermodynamics and Molecular Dynamics
(scheduled: Monday, 14:00 )

A molecular dynamics study of the stress-optical behavior of a linear short-chain polyethylene melt under shear

C. Baig, B.J. Edwards, D.J. Keffer
Department of Chemical Engineering, University of Tennessee, Knoxville, TN 37996-2200, USA

In this study, we investigate details of the stress-optical behavior of a linear polyethylene melt under shear using a realistic potential model. We demonstrate the existence of the critical shear stress, above which the stress-optical rule (SOR) begins to fail. The critical shear stress of the SOR of this melt turns out to be approximately 5.5 MPa, which is fairly higher than 3.2 MPa at which shear thinning starts. This indicates that the SOR is valid up to a point well beyond the incipient point of shear thinning. Furthermore, contrary to conventional wisdom, the breakdown of the SOR turns out not to be exactly correlated with the saturation of chain extension and orientation: it is observed to occur at shear rates well before maximum chain extension is obtained. In addition to the stress and birefringence tensors, we also compare two important coarse-grained second-rank tensors, the conformation and orientation tensors. The birefringence, conformation, and orientation tensors display nonlinear relationships to each other at high values of the shear stress, and the deviation from linearity begins at approximately the critical shear stress for the breakdown of the SOR. © IWNET 2006

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