188 Rheologica Acta, Band 5, He]t 3 (1966) From the Institute o/Petrochemical Synthesis o/the Academy o/Sciences o/ Moscow (U.S.S.R.) Comparative Description of the Peculiarities of Deformation of Polymer and Plastic Disperse Systems By G. V. Vinogradov and A. I~a. Malkin With A most important characteristic of the rheological properties of fluid systems (in particular, polymer concentrated solutions and melts) is the dependence between the shear stress ~ and the shear rate 2- This dependence may be characterized by means of the flow function / (T, 2) = 0 or its graphic representation, the so-cMled flow curve. In many cases of experimental deter- mination of the function /(r,p)= 0 the steady value of one of the variables is measured while keeping the other variable constant at a preset value. Thus, if the experiments for determining / (v, 2) = 0 are performed with 2 = const. (and this is just the case we are going to consider in the following), the flow curve is plotted from the values of rs where ~s = lira ~ at t -+ co (t = time), if such a limit exists. By the apparent viscosity ~ of the system here is meant the ratio ~ = ~s/2; as a rule the dependence ~ (2) is unambiguous. The existence of a limit vs for polymer systems has usually not been questioned, though, as will be demonstrated below, cases are possible where no such limit exists, and others where this limit does not characterize the viscosity of the system because of the occurrence in the course of the experiment of a transition from internal to external friction owing to slippage of the system under consideration relative to the solid wall or the appearance of a rupture inside the system, the latter being accompanied by a velocity jump. The possibility of slippage of the polymer relative to the wall is usually taken into account when studying elastomers. Only recently Hutton (1) published studies in which attention has been paid to transitional test conditions at which internal friction passes into external in relatively low viscous polydimethylsiloxane. One of the most widespread methods of obtaining the flow curves of various systems 3 figures (l~eceived March 22, 1966) is by measuring the dependence of the volume flow rate on the pressure as the system under study flow through a capillary of known geometry, and treating the data obtained this way after Rabinowitsch-Weis- senberg with allowance for entrance cor- rections. This makes it possibles to obtain the so-cMled "true" flow curve which can be compared with the ~ (2) function obtained under different conditions of deformations e. g., during flow in an almost homogeneous stress field in a rotational viscometer. Depend- ing on the properties of the system studied and on the flow conditions, the experimental flow curves obtained in this way may coin- tide or not coincide with each other. A typical example of investigation of a system for which the flow curves obtained under various conditions of deformation coincide with each other a wide range of changes of the variables was described in (2). This correspondanee should be disturbed when elastic turbulence sets in, the latter phenomenon having been described repeated- ly for the case of flow of polymer melts in capillaries, but not having been studied so far in detail with rotational units. One of the reasons for this is the difficulty of carrying out investigations in rotational viscometers at elevated rates of deformation of highly viscous polymer melts. At the same time the peculiarities of flow in a capillary (nonhomogeneity of the stress field over the radius and the action of large and longitudin- ally variable pressures at high rate gradients) make it impossible to detect certain cha- racteristic features of the process which should probably be exhibited more distinctly in rotational viseometers with homogeneous shear stress fields. Indeed, when various polymer systems are studied in rotational viscometers at elevated shear rates, both tearoff of the polymer from the solid wall and the formation of discontinuities inside the flowing polymers are observed. This may be