Polymer rheology: Influence of molecular weight and polydispersity

Literature data on the non-Newtonian flow of bulk polymer and of polymer solutions are correlated on the basis of a four-parameter equation, η = η∞ + (η0 − η∞)/[1 + (τD)m], η being the viscosity at shear rate D, and η0 and η∞ limiting values at D = 0 and D = ∞, respectively. The parameters η0, η∞, and τ all show dependence on molecular weight, and in general there is good correlation between τ and η0. There is evidence that τ is related to a molecular weight higher than the weight-average. The exponent m shows dependence on molecular weight distribution and approaches an upper limit of unity for a monodisperse linear polymer. For linear unblended polymers it may be expressed empirically by m = (Mn/Mw)1/5.