The flow behavior of polystyrene melt in the nonsteady state

Non-Newtonian Flow Through Porous Media

“…Numerous investigators have reported the occurrence of "shear thickening behavior," but other descriptors such as "negative thixotropy," "anti thixotropy," and "rheopexy," have also been applied, depending upon whether the observed thickening action is reversible or accompanied by time-dependent effects. In the contemporary literature, one still occasionally finds shear thickening equated with the "dilatant" behavior observed in suspensions and slurries Chain unwinding and intensification of intermolecular interactions (33, 34) Formation of complete hydrophobic regions (35) Strong structures built up from quasicrystallites in solution (36,37) Inter-and intramolecular bridges (38) Intramolecular effects due to the isolated molecule (39,40) Competition between the formation of rheological units under shear and stress relaxation (41) Inability of the flowing volume element to respond to the shear field (42, 43) Temporary network formation during shearing (44,45) Intermolecular effects (46) Nonlinear behavior analogous to solid deformation (47) of particulate materials at high volume concentrations. Shear thickening behavior is of particular interest in connection with non-Newtonian flow through porous media because, as will be shown later, certain dilute polymeric solutions which exhibit shear thinning behavior in a viscometric flow seem to exhibit a shear thickening response under appropriate conditions of flow through porous media.…”

Influence of shearing history on the properties of polymer melt. I.

“…The magnitude of this maximum, however, depends on the length of recovery time. A complete recovery to the same level of maximum shear stress as in the first shearing run is obtainable.13- 16 As reported elsewhere, differences in the shear treatment also lead to differences in the morphology of the subsequently crystallized specimen. The different morphologies, in turn, lead to different mechanical properties of the solidified polymers.17.…”

“…Formation of molecular aggregates has been suggested as the reason for the observed time dependence of shear stress. [12][13][14][15][16] A polymer solution or melt which has been sheared until it reached equilibrium shear stress may be left to recover at the same experimental conditions. When sheared again, the sample may again show a maximum occurring at the same time interval from the beginning of the experiment.…”