Modeling the shear-induced structural changes in polymeric fluids

A simple general thermodynamic model is presented for predicting the shear- induced phase changes in polymer fluids. It is based on the established con cept of generalized Gibbs free energy of mixing, with an extra entropic sto rage term due to flow. The latter term is related to the conformational cha nges of the polymer chains due to flow, via the first normal stress differe nce or the viscoelastic storage modulus. The general energy analysis is car ried out within the framework of Flory's statistical mechanical lattice mod el. As such, this approach provides some insights with respect to the confi gurational changes of a polymer fluid subject to shear flow, and thus to th e molecular origins of shear-induced structural changes. Moreover, the mode l is universal with respect to its applicability to different kinds of poly meric fluids, such as solutions and blends. Theoretical predictions of the phase diagrams under shear agree reasonably well with selected experiments with the systems polystyrene/dioctyl phthalate and poly(styrene-co-maleic a nhydride)/poly(methyl methacrylate). (C) 1999 Elsevier Science B.V. All rig hts reserved.