Rheology and structural changes of polymer melts via nonequilibrium molecular dynamics

Abstract

Results of nonequilibrium molecular dynamics computer simulations of a planar Couette flow are presented for the multibead anharmonic-spring model. The finitely extensible nonlinear elastic force law is used to connect the up to 100 beads of a chain molecule. Rheological data (shear viscosity, normal pressure differences) are discussed and compared with quantities describing the chain conformation (e.g., alignment tensor, static structure factor). This renders possible a test of the theoretical approaches which connect these quantities. In agreement with recent experiments, the static strucure factor exhibits characteristic elliptical distortions of the polymer coil whose magnitude depends on the distance from the gyration center. In our simulations the zero-shear-rate viscosity is found to scale linearly with the number of beads N up to chains with N=60. A weak upturn of the viscosity per bead for N=100 is found which may indicate the onset of the reptation regime.