SCALING BEHAVIOUR OF THE SHEAR VISCOSITY OF SHORT FLEXIBLE LENNARD-JONES CHAINS

摘要

In this work, we propose a simple modification to two scaling approaches aiming at relating viscosity to thermodynamic quantities in dense liquids, namely the so called “excess entropy” scaling and “the scaled dynamic” approach. This modification, which corresponds to the use of the residual viscosity instead of the total viscosity in the scaling procedure, is dedicated to extent these approaches to a range of thermodynamic conditions not restricted to the dense phase. These modified approaches are then applied to molecular dynamics simulation results of the fully flexible Lennard-Jones chain fluid model for short chains, i.e. below the entanglement threshold. Interestingly, both modified scaling schemes are suitable for the Lennard-Jones chain fluid model whatever the thermodynamic conditions and the chain length. Concerning the excess entropy scaling, it has been noticed that, when the monomer is considered, the increase of the reduced residual/excess viscosity is directly proportional, with a slope of one, to the loss of available configurations. In addition, as expected, it has been found that the slope was decreasing when the length of the chain increases because of possible rotational motions. Concerning the scaled dynamic approach, it has been noticed that the longer the chain the larger the influence of density on the viscosity relatively to that of the temperature.