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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
ORAL PRESENTATION
Session: 3 Non-equilibrium thermodynamics and Molecular Dynamics
(scheduled: Monday, 15:40 )
A Generalized Hamiltonian-Based Algorithm for Rigorous Equilibrium Molecular Dynamics Simulation in the NVT, NpT, and μVT Ensembles
J. Santiago, D.J. Keffer, B.J. Edwards, C. Baig
University of Tennessee, Knoxville, TN 37996-2200, USA
We provide a methodical procedure for generating equations of motion for rigorous simulation in three different statistical ensembles, the canonical ensemble (NVT), the isothermal-isobaric ensemble (NpT), and the grand canonical ensemble (μVT) under equilibrium conditions. The procedure begins with a Hamiltonian in terms of laboratory coordinates in a mathematical frame of reference where time and/or mass is dilated. The equations of motion are derived relying on the symplectic relationship between the Hamiltonian and the equations of motion. We define a non-canonical transformation from the laboratory coordinates in the mathematical frame of reference to laboratory coordinates in the physical frame of reference, in much the same way as the original NVT development of Nose and Hoover. However, the new equations are completely general, unlike their predecessors, in that they are valid whether or not an external force field is present. Several illustrations of simulations involving these ensembles will be presented which validate the new algorithms. © IWNET 2006
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and Kleanthi for IWNET 2006