Abstract
Abstract It is shown that the strain rates leading to shear flow instability in the form of polymer melts and solutions squeezing out of the working gap, occurrence of visible eddies on the free surfaces of test samples in cone-and-plate rheometers, at parallel superposition of steady and oscillatory shears, and break of practically monodisperse linear polymers forced through a nozzle, are determined by the second shear rate derivative of the elastic energy accumulated at a steady shear flow. For polydisperse polymers, stable and instable flow conditions are associated with polymer segregation according to molecular weights (M > M c) into clusters which migrate toward the nozzle axis, if incipient instability originates inside the nozzle. Instability originating at the nozzle inlet is analysed, consideration being given to pre-stationary flow conditions. Clusters are treated as dissipative structures according to Prigozhin.
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