VISCOSITY OF LIQUIDS

Abstract

Polymer solutions and melts which are polydisperse with respect to molecular weight behave differently in various ways than monodisperse ones under conditions of unidirectional longitudinal flow. Inasmuch as polydispersions are mixtures of monodisperse fractions, each of a different molecular weight, the differences in the mechanical properties are due to causes which ought to be sought in the dependence of the mechanical properties of monodisperse polymers on their molecular weight. When monodisperse polymers are deformed longitudinally at a constant rate until a certain stress level Oct has been reached, it is possible that a condition of steady longitudinal flow will be reached with both the axial tension and the axial strain rate remaining constant in time. When the stress exceeds Ocr , the material responds to uniaxial tension as one of high elasticity, i.e., loses its ability to flow [1-4]. The magnitude of Ocr is somewhat above 105 dyn/cm ~ and is almost independent of the molecular weight of the monodisperse polymer, depending only on the macromolecular structure. The flow of melts of monodisperse polymers is characterized by a constant longitudinal viscosity until rupture occurs. In the case of polydisperse polymers, on the other hand, the longitudinal viscosity has been found to increase with the strain rate [5, 6].