MOLECULAR DYNAMICS SIMULATION OF NANO-SCALE POLYMERIC RHEOLOGICAL PROPERTIES ...

MOLECULAR DYNAMICS SIMULATION OF NANO-SCALE POLYMERIC RHEOLOGICAL PROPERTIES AND EXTRUSION FLOWS

Rong-Yeu Chang*, Jenn-Jye Wang
Department of Chemical Engineering
National Tsing Hua University, HsinChu, Taiwan 30043, ROC

Abstract
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 is confined between two structured gold atomic walls. Isothermal simulations (350K) of 4:1 unsteady extrusion flow with various extrusion rates are conducted.

Introduction
Rheological properties of polymer are critical for polymer process. The extrusion molding is important polymer process of nowadays. In this work the rheological properties of polymer have been studied by molecular dynamics simulation. We also develop the MD numerical analysis to simulate the polymer process. It can efficient provide more micro phenomena, such as the structure of molecule chain,  orientation…etc.

Analysis Approach
The short chain PE (C50H102) has selected as the simulated model. Shifted Lennard-Jones potential enable us to describe Van der Waals Force.

1# irenechen

These parameters[4] are listed in Table 1. Regarding to the analysis model of extrusion flow, the below governing equation is adopted.

The 5-order Gear predictor-corrector method is used to solve Newton’s law of motion.


Results and Discussion
Isothermal simulations (350K) of 4:1 unsteady extrusion flow with various extrusion rates are conducted.
The simulated model is shown in Figure 1. Fluid is composed of 576 linear C50H102 molecules. The number of particle is 28,800. The initial density is 2.35

-3. Wall molecules are composed of 3 BCC lattice layers. The
time step is 0.0005 m/
, and the direction of depth is period boundary.
The analysis results of extrusion speed 35.17 (m/s) is shown in Figure 2. The figure 3 shows the density distribution. There are fountain effects when the fluid flows through the narrow channel. The orientation of molecule chain near melt front is shown in  Figure 4. The orientations of molecule chains are perpendicular to the flow direction. However, the molecule chains near mold wall are parallel to the flow direction. The velocity field is shown in Figure 5. The flow path is shown in Figure 6. 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. No vertex is found in the simulation result.

Conclusions
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. No vertex is found in the simulation result. Besides, the orientations of molecule chains are perpendicular to the flow direction. However, the molecule chains near mold wall are parallel to the flow direction.

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