Polymer dynamics

Abstract

Many of the practical applications of macromolecules, such as the increase of the viscosity of an oil by polymeric additives or the shear-thinning effect in a paint are related to their very unusual hydrodynamic and rheological behaviour. Polymeric liquids (i.e. macromolecular solutions, or molten polymers in a liquid state without any solvent) do not behave as classical Newtonian liquids which can be characterized by a viscosity independent of the applied velocity gradient, they are non-Newtonian viscoelastic fluids: after the application of an external deformation (such as stretching a chewing-gum) a polymeric liquid first behaves as an elastic solid or a rubber, the stress does not vanish and is proportional to the deformation; after a finite relaxation time, the stress relaxes to zero thus showing a liquid behaviour. Moreover, macromolecules, being large objects (the individual size might be as large as 1 μm), their motions are very slow and the relaxation times involved can be as large as seconds. This, in turn, leads to viscosities which can be as high as 10⁵ poise for polymer melts in the limit of vanishing shear rates.