Previous Experimental Polymer Rheology Versus Flow Induced Crystallization

Abstract

Experimental results, which have mostly been obtained with the aid of flow birefringence, are quoted. For a series of polystyrenes insights are obtained into the adjustment times for steady flow. For shear rates of practical interest these times are extremely short compared with the times showing up in the relaxation time spectrum. The question arises, whether this spectrum can still be of influence. Measurements on polyethylenes are quoted, which are in agreement. But this means that, fortunately, in capillary rheometry mostly the steady state viscosity is measured. This statement also holds for duct flow, which has often been used for the investigation of shear induced crystallization, in particular on polypropylenes. The linear stress optical rule is recalled. It reflects the persistence of an internal equilibrium, which breaks down only at high enough shear rates. This equilibrium seems to hold for the random flight statistics of chain segments connecting entanglements. The mentioned adjustment times are already short, before the said break down occurs.