NONLINEARITIES IN POLYMER RHEOLOGY

Polymeric liquids often exhibit non-linear responses under flow condit ions in spite of the fact that inertia effects are usually negligible. Non-linearities arise when, as a consequence of flow, the distributio n of molecular conformations departs significantly from equilibrium. I n turn, this will occur whenever the strength of the velocity gradient becomes larger than the inverse of the longest relaxation time of the polymer molecule. Based on this general concept, various detailed int erpretations are given to the observed non-linearities (the non-Newton ian viscosity, for example) depending on the specific relaxation mecha nism which prevails under different conditions (e.g. dilute solutions or concentrated systems, and branched or linear chains). Here, cases w here new results were recently obtained are briefly discussed, includi ng a very curious non-linear effect which is encountered in liquid cry stalline polymers.