Volume 12, Issue 4, December 1968
Index of content:
12(1968); http://dx.doi.org/10.1122/1.549096View Description Hide Description
Hysteresis loops on a shear stress versus shear rate plot have been reported by numerous investigators studying polymeric fluids. Both clockwise and counterclockwise loops have been observed. Currently available nonlinear viscoelastic models can be used to derive expressions for these hysteresis loops as well as for the corresponding normal stress hysteresis loops. It is suggested that the hysteresis loop experiment may be particularly useful for the testing of nonlinear viscoelastic models.
12(1968); http://dx.doi.org/10.1122/1.549093View Description Hide Description
Several types of viscoelastic shear stress hysteresis loops were obtained for three forms of time‐dependent shear rate inputs and four different polymer solutions. A correlation plot based on two dimensionless groups from the Bird‐Carreau nonliner viscoelastic model demonstrates the experimental conditions required to obtain the various types of loops. The data were used to evaluate the predictive capabilities of the Bird‐Carreau model and a “slowly varying flow” approximation to the same model. The “slow flow” model predicted the hysteresis response for experiments where the time of the experiment was larger than a characteristic time of the fluid. The model was qualitatively correct for a much wider range of experimental conditions. Five types of primary normal stress difference hysteresis loops are reported along with a demonstration of a method to calculate the “zero shear rate” limiting value of the primary normal stress material function
12(1968); http://dx.doi.org/10.1122/1.549094View Description Hide Description
The objective of this study was to gain a better understanding of the mechanisms responsible for rheological dilatancy in an aqueous pigment suspension. Of particular interest was the relationship between the extent of dilatancy and colloidal stability. This relationship was studied using iron oxide suspensions which were dispersed by addition of sodium polymethacrylate having a molecular weight of 4000. The level of colloidal stability was varied by changing the degree of dissociation of the carboxyl groups on the polymer.Measurements of the extent of adsorption of the dispersant were made and the adsorbed charge was found to correlate very well with low‐shear viscosity measurements. Additional experiments were made to determine the effect of pigment volume concentration on dilatancy over the range of 40–47%. A cup‐and‐bob viscometer was used to measure the shear stress over a shear rate range of The extent of rheological dilatancy was found to increase with (a) decreasing colloidal stability, and (b) increasing pigment concentration. Based on these findings, it was postulated that rheological dilatancy results from a progressive increase in flocculation due to shear. The fact that a suspension can exhibit both pseudoplasticity and dilatancy was explained by considering floc formation and floc disruption as competitive rate processes.
12(1968); http://dx.doi.org/10.1122/1.549095View Description Hide Description
Experimental non‐Newtonian viscosity, primary normal stress difference, dynamic viscosity, and dynamic storage modulus data were obtained for 16 viscoelastic fluids. The fluids investigated were solutions of almost monodisperse polystyrene in Aroclor (a mixture of chlorinated diphenyls). Several nonlinear rheological equations of state were fit to the experimental data, and the Bird‐Carreau model was found to fit the data within 15% for every solution investigated. Bird‐Carreau model parameters were determined by use of a nonlinear least‐squares computer analysis, and these parameters were related to polymer molecular weight and concentration. Experimental values of the steady‐state compliance were obtained from primary normal stress difference and shear stress data. These values agreed with the Rouse theory prediction.
12(1968); http://dx.doi.org/10.1122/1.549124View Description Hide Description
The steady‐state compliance has been derived from low‐frequency dynamic viscoelastic measurements on solutions of several sample of polystyrene, poly‐α‐methyl styrene, polyvinyl acetate, and cellulose tributyrate in several different solvents including a Θ solvent for polystyrene. The molecular weight (M) distributions were reasonably sharp. Maximum concentrations (c) ranged from to In general, changes with increasing concentration from a value close to the Zimm theory prediction to a value close to the Rouse theory prediction, the transition occurring in the neighborhood of where [η] is the intrinsic viscosity. However, for polystyrenes of molecular weights 19,800 and 51,000, values of were abnormally small, and some of those for cellulose tributyrate appeared to be abnormally large. Otherwise, there is no marked influence of the chemical nature of either polymer or solvent. The change in behavior with increasing concentration near is attributed to an overlapping of molecular domains which is sufficient to alter the hydrodynamic interaction. A further change, observed by other investigators at higher concentrations, where the dependence of on c and M becomes quite different, is attributed to domination of the behavior by entanglements which develop at a higher degree of overlapping.
12(1968); http://dx.doi.org/10.1122/1.549125View Description Hide Description
An all‐glass rising column capillary viscometer for the measurement of the lower Newtonian viscosity of polymer melts and concentrated solutions has been designed and tested. Interchangeable capillaries of radii 0.500–3.500 mm permit a wide range of viscosities to be covered. Temperature is controlled by means of a refluxing liquid and vapor jacket, which permits operation at temperatures up to 300°C. A somewhat unconventional method of operation is described, in which the applied external pressure is either very low or zero. Suitable treatment of the experimental data permits both the viscosity and density of the fluid to be determined with moderate, but generally acceptable, precision and accuracy. In addition, the surface tension of the fluid may be estimated, this estimate being more accurate when the smaller diameter capillaries are used. The validity of this approach has been confirmed experimentally by measurements on a standard oil having a viscosity of 213 P at 25°C and on a polyethylene melt at 190°C, having a lower Newtonian viscosity of