The rheological properties of liquids composed of flexible chain molecules: A molecular dynamics computer simulation study

Abstract

Nonequilibrium molecular dynamics simulations of planar Couette flow have been performed for dense liquids composed of model linear chain molecules each containing six interaction centers. For one liquid the potential parameters were chosen so that it resembles n‐hexane. For the other liquid, referred to as flexane, the energy barriers directly opposing internal rotations are set to zero. The rheological properties of these liquids have been examined at 200 and 300 K at constant density. Torsional flexibility gives rise to a dynamic coupling which is associated with a significant reduction (by a factor of 2) in the equilibrium viscosity. A coupling mechanism is proposed based on large angle fluctuations without necessarily involving conformational transitions. This coupling disappears at high shear rates when there are significant structural changes in the liquid associated with shear thinning. Flexibility also causes changes in the shear rate dependent steady‐state distribution of conformers and at low temperatures this results in reduced shear dilatancy.