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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: 7 Applications to complex materials: glasses, micelles, colloids, blends, interfaces
(scheduled: Thursday, 10:20 )
Flow of Polymer blends between Concentric Cylinders
M. Dressler1, B.J. Edwards2, E.J. Windhab1
1 Institute of Food Science and Nutrition, ETH Zurich, Switzerland
2 Department of Chemical Engineering, University of Tennessee, Knoxville, TN 37996-2200, USA
A thermodynamically consistent model for polymer blends with matrix phase viscoelasticity and break-up/coalescence is solved for laminar flows in the gap between concentric cylinders. The model has been developed using a Hamiltonian framework of non-equilibrium Thermodynamics. The model is solved for Poiseuille (axial), Couette (angular), and mixed Couette-Poiseuille (helical) flow to understand qualitatively viscometric material properties and droplet deformation in non-homogeneous shearing flows. Therefore, the model equations are formulated in terms of a two-point boundary value problem and a shooting algorithm is adopted to compute the non-linear flow fields together with the elastic stresses, the droplet shapes, and the break-up/coalescence rates in the gap. We investigate the profiles of the stress tensor components in connection with the droplet characteristics, we valuate viscometric material functions at the wall, and we discuss the narrow gap approximation. Moreover, we examine the influence of centrifugal forces on the flow behavior, and in particular on the break-up/coalescence rates in the gap for the three flows. © IWNET 2006
ORAL PRESENTATION
Session: 4 Complex fluid deformation and rheology: Theories and thermodynamic relationships
(scheduled: Tuesday, 11:10 )
Thermodynamics of Non-Isothermal Polymer Flows: Experiment, Theory and Simulation
T.C. Ionescu1, B.J. Edwards1, D.J. Keffer1, V.G. Mavrantzas2
1 Department of Chemical Engineering, University of Tennessee, Knoxville, TN 37996-2200, USA
2 Department of Chemical Engineering, University of Patras, Patras GR 26504, Greece
We provide a critical evaluation of the so-called ''Theory of Purely Entropic Elasticity'', which states that the free energy change of a flowing, non-isothermal viscoelastic fluid is entirely due to entropic effects, and contains no contributions due to elastic energy changes. Our investigation consists of both theoretical and experimental parts. In the theoretical part, we perform non-equilibrium Monte Carlo simulations to calculate both the energetic and entropic contributions to the free energy of the material under uniaxial elongational flow. This results in measurable energetic effects at higher strain rates, and these effects increase as temperature decreases. Experimentally, we measured the heat capacity at constant volume of LDPE under steady-state shear and uniaxial elongational flow conditions, and calculated the conformational contribution to this quantity. According to the Theory of Purely Entropic Elasticity, the conformational contribution to the heat capacity should be negligible, however, significant non-vanishing contributions are measurable at high strain rates. Results are qualitatively consistent between theory and simulation. © 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