VISCOELASTICITY OF POLYMERIC MELTS AND CONCENTRATED-SOLUTIONS - THE EFFECT OF FLOW-INDUCED ALIGNMENT OF CHAIN ENDS

The viscous properties of polymeric liquids and the underlying microscopic distribution function for the segment orientations are investigated by a Fokker-Planck approach. The solution of the Fokker-Planck equation depends on the alignment of chain ends under shear flow, which is usually disregarded. However, it has been inferred from nonequilibrium molecular dynamics (NEMD) computer simulations that it is nonzero. Since the viscous properties depend critically on the magnitude of this end alignment it has been taken into account in the theoretical description. This turns out to be crucial for the plateau region and the high frequency behavior of the complex viscosity. The results are in good agreement with experimental data. Furthermore a significant chain length dependence of the viscous behavior is found.