Volume 27, Issue 1, February 1983
Index of content:
27(1983); http://dx.doi.org/10.1122/1.549697View Description Hide Description
The solution of the equations of motion for a Casson model non‐Newtonian fluid in a rotational Couette viscometer is presented. Proper equations for determination of rheological parameters from data obtained from commercial viscometers are derived and presented. Sample curves illustrating the influence of viscometer geometry on the rheogram are presented.
Binary Blends of Monodisperse Polymers: Use of a Kinetic Network Model for Nonlinear Shear Stress Predictions in Entangled Polymer Fluids27(1983); http://dx.doi.org/10.1122/1.549696View Description Hide Description
A molecular network theory incorporating the concept of entanglement disruption and regeneration was previously developed to describe not only steady‐state but also transient rheological properties of monodisperse polymer melts and concentrated solutions. The viscoelastic responses of these systems are dominated by a rapid segmental motion at short times and a more sluggish, structure‐dependent network relaxation process at long times. In a binary blend of monodisperse fractions, the entangled network is composed of chains of two different lengths. The chain dynamics, and thus the characteristic response time, of each component in such a system is influenced by the presence of the coexistent component. This interaction is accommodated by a simple mixing assumption, which leads to accurate predictions of composition‐dependent linear viscoelasticproperties, such as zero‐shear viscosity. In addition, the model gives an excellent description of the shear‐rate‐dependent viscosity curves of binary blends of various compositions and concentrations. Time‐dependent viscoelasticproperties, such as shear stress growth and relaxation functions, have also been predicted. Comparison of these results with experimental data indicates good agreement, reinforcing confidence in both the basic model and the mixing rule selected here for blends. The latter provides the critical step to predict ultimately the nonlinear rheological properties of polydisperse polymers from knowledge of their molecular weight distribution, as the established mixing rule can be easily generalized to account for multiple interactions among the constituent chains.
27(1983); http://dx.doi.org/10.1122/1.549721View Description Hide Description
Okra mucilage F, a product of the fruits of Hibiscus esculentus (okra), is a water-soluble, 1.7‐million‐molecular‐weight glycoprotein which produces viscous, shear‐thinning, and viscoelasticsolutions in water. Power‐law exponent for okra solutions decreases and viscosity at unit shear rate increases as polymer concentration increases. Limiting viscosity number of okra in water decreases tenfold as the shear rate at which viscosity is measured increases from 1 to
27(1983); http://dx.doi.org/10.1122/1.549698View Description Hide Description
When subject to moderate shear rates in a cone‐plate and parallel plate rheometer, viscoelastic samples tend to fracture at the edge of the sample thus preventing the measurement of viscosity and normal stresses at high shear rates. An explanation of this behavior has been given in terms of a critical elastic strain energy by Hutton and we offer an alternative explanation based on fracture mechanics. It is shown here that fracture occurs if where is the second normal stress difference, Γ is the surface tension coefficient, and a is the size of the fracture. The experimental data presented here show that this equation correctly predicts the occurrence of shear fracture and also the shear rate at which it occurs. At high shear rates, Newtonian liquids and some viscoelasticliquids are ejected from the cone‐plate and parallel plate rheometer due to the effect of centrifugal forces. For a Newtonian liquid a simple consideration of the centrifugal and surface tension forces at the sample edge provides an estimate of the critical rotation rate for ejection of the sample. For a viscoelasticliquid, if sample ejection will occur at high shear rates and the critical shear rate may also be estimated from the Newtonian case.
27(1983); http://dx.doi.org/10.1122/1.549695View Description Hide Description
Nucleate boiling curves for aqueous solutions of drag‐reducing polymers have been measured experimentally. The polymers examined are a galactomannan polysaccharide (Galactasol 211), a polyacrylamide (Separan MGL), two polymers consisting of both acrylamide and acrylic acidmonomers (Separan NP‐10P and Separan AP‐30), two polyethylene oxides (Aldrich No. 18202‐8 and No. 18946‐4), and three hydroxyethyl celluloses (Natrosol 250MR, 250HR, and 250HHR). The boiling curves have been measured using an electrically heated platinum wire submerged in a saturated pool of liquid at atmospheric pressure.Polymer concentrations corresponding to relative viscosities of 1.01, 1.04, 1.08, 1.16, and 1.32 have been tested. The observed changes in the nucleate boiling curves for polymer solutions are in qualitative agreement with those predicted by the Rohsenow pool boiling correlation. In particular, the temperature shifts in the boiling curves are predicted adequately based only on how the polymers change the solutionsurface tension and, more importantly, the solutionviscosity.Drag reduction effects appear to be unimportant; hence, polymer type, molecular weight, and concentration are important only insofar as they affect solution properties. These results differ from those reported by earlier investigators for pool boiling of dilute polymer solutions on heated flat plates.
27(1983); http://dx.doi.org/10.1122/1.549699View Description Hide Description
Apparent viscosity over a range of in shear rate and first normal stress difference functions for dilute drag‐reducing solutions of Separan AP‐30 in distilled water were measured for both freshly prepared and shear‐degraded solutions, for concentrations from 100 to 500 wppm. A nonlinear integral constitutive equation is proposed which accurately represents these data, and the effects of concentration and shear degradation on the properties (e.g., the model parameters) is shown. Shearing has a greater effect on the low shear viscosity of these solutions than it has on the elastic properties, resulting in a greater degree of drag reduction for the sheared than the fresh solutions.
27(1983); http://dx.doi.org/10.1122/1.549734View Description Hide Description