| Summary: | 碩士 === 國立清華大學 === 化學工程學系 === 91 === In this work the rheological properties of polymer have been studied by molecular dynamics simulation. Couette flow with various shear rates are used to investigate the degree of slip, shear viscosity and normal stress difference. The fluid consists of chains of n-hexadecane and they are confined between two structured gold atomic walls. Isothermal simulations (350K) of 4:1 unsteady extrusion flow with various extrusion rates and 4:1:4 steady contraction-expansion flows with various external driving forces (g=0.2~0.3) are conducted to reveal the effects of flow rate.
(1) Couette flow: The degree of slip - decrease with the increase of wall absorptive energy. Although the walls are composited of strong absorbing gold atoms, large degree of slip can be found because of non-roughness of the slabs. The degree of the slip increases as with the shear rate increases ( ). Moreover, the viscosity shows shear thinning phenomena.
(2) Unsteady extrusion: For the short chain PE (C50H102), we can find the die swelling phenomena from our simulation, and the density of fluid is not uniform, which indicates that the fluid is compressible. Fountain effect can be found near the front of flow in the channel, but no vertex is found in the simulation result.
(3) Steady contraction-expansion flow: we can find that the chains are extended and the end to end distances are large as passing through the contraction region. The parabolic velocity profile can be found at the narrow section and large slip occurs near the wall. Furthermore vacuoles can be found around divergent regions and the size of vacuoles increase with the flow rate and decrease with the average fluidic density.
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