722d

The anisotropic configurational changes induced in the linear, short-chain polyethylene liquid C78H158 were simulated using the Siepmann-Karaboni-Smit united atom model under steady-state uniaxial elongation and shear flow. The complementary simulation techniques of Nonequilibrium Monte Carlo (NEMC) and Nonequilibrium Molecular Dynamics (NEMD) were used to determine independently the flow-induced structural changes in these two flow fields, and to calculate physical properties of the system under these conditions. Using the extended Gibbs ensemble of the NEMC method, the configurational temperature of the system was calculated at various values of flow strength, and compared to the set point temperature of the simulation. The same idea was applied to NEMD, where the configurational temperature was compared to the set point kinetic temperature of the system. It was found that the configurational temperature of the system decreased with increasing flow strength, approaching the melting point of the liquid. This seemed to correlate with experimental observations concerning flow-induced crystallization in seemingly isothermal liquids. Flow-induced changes in the heat capacity were also observed, as calculated according to the standard thermodynamic definition of the heat capacity; i.e., the change in internal energy with temperature at constant density and average chain configuration. The heat capacity calculated in this manner was shown to decrease with increasing flow strength. Finally, the simulation results led to the concept of a nonequilibrium temperature in these systems undergoing flow, and a simple expression was derived which quantified this effect. The implications of these findings will be discussed and analyzed in this presentation.

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