Flow-induced temperature change and anisotropic heat capacity of a linear short-chain polyethylene liquid

Chunggi Baig (University of Patras, Greece) and Brian J. Edwards (University of Tennessee, USA)

The anisotropic configurational changes induced in the linear, short-chain polyethylene liquid are 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) are 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 is calculated at various values of flow strength, and compared to the set point temperature of the simulation. The same idea is applied to NEMD, where the configurational temperature is compared to the set point kinetic temperature of the system. It is found that the configurational temperature of the system decreases with increasing flow strength, approaching the melting point of the liquid. This appears to correlate with experimental observations concerning flow-induced crystallization in seemingly isothermal liquids. Flow-induced changes in the heat capacity are 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 is shown to decrease with increasing flow strength. Finally, the simulation results lead to the concept of a nonequilibrium temperature in these systems undergoing flow, and a simple expression may be derived which quantifies this effect.