Flow-induced alignment of rod-like and flexible polymers in the molten state

The flow-induced alignment of polymer chains in melts is calculated based on a Fokker–Planck equation for the single-link orientational distribution function of polymer segments. Analytic results, under the assumption of a vanishing sixth anisotropic moment of this function, are discussed for the cases (A) flexible polymers subjected to a stationary plane Couette flow and (B) rigid polymers subjected to a time-dependent shear flow. The results are expressed in terms of the orientational and reptational diffusion coefficients, the chain length, the shape of segments and the shear rate. In order to account for recent findings from computer simulations, the effect of flow-induced orientation of chain ends on the alignment of the polymer coil are analyzed. We also discuss the related rheological properties.