Volume 25, Issue 1, February 1981
Index of content:
25(1981); http://dx.doi.org/10.1122/1.549612View Description Hide Description
The rotary clamp consisting of a pair of gears is a basic construction element for the design of various types of extensional rheometers for polymer melts. With the most recent model of the uniaxial extensional rheometer, built with rotary clamps, homogeneous deformations of Hencky strains up to have been performed with a low‐density polyethylene melt. For the evaluation and discussion of results from tests with large deformations, it is vital that parameters which characterize the quality of each test be defined and measured. Otherwise, the measured force and the rotational speed of the clamps might be of little relevance for the stress, strain, and strain rate within the sample. The biaxial extensional rheometer consists of eight rotary clamps in a circular arrangement. With this new instrument, extensional Hencky strains up to can be achieved for a polyisobutylene sample. Such a high deformation corresponds to a compression in the third direction of The material within the samples is deformed homogeneously. The samples are shaped as flat circular disks. During the test they are cut periodically at the rim between the clamps in order to keep the “active” diameter constant within a narrow range. Hence, from the forces measured at the clamps, the tensile stress can be determined.
A Comparison of Elongational Rheology as Measured in the Universal Extensional Rheometer and by the Bubble‐Collapse Method25(1981); http://dx.doi.org/10.1122/1.549637View Description Hide Description
Two polyethylene melts have been examined in elongational flow using two different experimental techniques: the rod‐stretching method of Münstedt and co‐workers, and the bubble‐collapse method of Middleman and co‐workers. Good agreement is observed in the case of a high density polyethylene. For the low‐density polyethylene the bubble‐collapse method appears to give incorrect results. It seems likely that for this highly elastic melt the time scale of bubble collapse is too short to permit attainment of a steady stress, even though the kinematics do reach constant stretch rate.
25(1981); http://dx.doi.org/10.1122/1.549636View Description Hide Description
Rheological measurements are reported of the behavior of a normally stiff paste of fine iron ore and 16% (w/w) water, flowing in a tube which was mechanically vibrated. The ranges of cyclic frequency and peak‐to‐peak amplitude were 15–30 Hz, and 0.508–1.27 mm, respectively. As is known, mechanical vibrations have the ability to “liquefy” pastes and in the present case the liquefied paste flowed through the tube under a very modest pressure head. The deductions that could be made depended on the assumptions, for on the one hand, if no slip was assumed to exist at the tube wall, then the calculated viscosity at any tube diameter could be correlated with the peak cyclic acceleration, viz., where A is the peak‐to‐peak amplitude and ω is the angular frequency. On the other hand, if slip at the wall was assumed, then the inference had to be made that the flow was more in the nature of rod flow, although the results were puzzling. While it is possible in principle to deduce, from suitable measurements, the relative values of the rheological parameters in slip flow of a Bingham fluid, in the present case this was not possible. Neither was it considered wise to use rheological measurements made earlier on a different apparatus, viz. the squeeze‐flow one, and that reported by Deysarkar and Turner elsewhere. In fact the work reported herein demonstrates that different apparatuses show different behavior. This possibly arises from the physical processes by which the mechanical vibrations of each apparatus were imparted to and transmitted in the paste.
25(1981); http://dx.doi.org/10.1122/1.549609View Description Hide Description
Samples of a long‐chain branched polyethylene were subjected to shear modification in a mixing extruder under various controlled shear conditions. The melt elasticity (time‐dependent recoverable strain) of the shear modified samples was found to decrease as the extrusion temperature was decreased or the rate of shear was increased, thereby demonstrating that the amount of shear modification can be controlled by these processing factors. The shear modification was found to be a reversible process. Reheating of the modified material resulted in the melt elasticity increasing back to the large recoverable strain exhibited by the unmodified material.
25(1981); http://dx.doi.org/10.1122/1.549608View Description Hide Description
The steady‐flow rheological properties of suspensions of mixtures of normal and glutaraldehyde‐hardened human red blood cells in albumin‐containing saline were determined in the shear rate range of 0.1–1500 sec−1, for total cell volume concentrations (hematocrits) between 30 and 43.5% and over the entire range of normal‐to‐hardened cell ratios. Suspensions of only hardened cells were Newtonian at lower shear rates, becoming dilatant at high shear rates.Suspensions of just normal cells were pseudoplastic, becoming Newtonian at high shear rates.Suspensions of mixtures of the two types of cells showed mixed behavior, dependent on the mixture composition. For suspensions with a constant total cell volume concentration, the apparent viscosity at a given shear rate is a nonlinear function of the fraction of the cells (f H ) which are hardened. Below a critical value of f H , viscosity is a linear function of f H , but the slope is low. Above the critical f H , viscosity increases rapidly with f H . The critical f H is associated with a critical hardened cell volume concentration where interactions between hardened cells become significant. It is expected that steady‐flow rheological measurements usually will not provide a sensitive method for determining changes in erythrocyte deformability distribution in blood or related suspensions, unless the abnormal cells are rigid and greatly distorted from the normal cell shape.
25(1981); http://dx.doi.org/10.1122/1.549610View Description Hide Description
A theory of stability for deformation of BKZ viscoelastic bars is developed. This theory is an extension of the one given by Ericksen for elastic bars. For appropriate strain potentials under certain loads in uniaxial creep and constant rate of extension, the bar will deform homogeneously for some time, and then become unstable and neck. An example is given simulating the behavior of a linear polyethylene bar in creep and in constant rate of extension. For the sake of illustration, the data used at small deformations were obtained from actual experiments, but at higher stretches were hypothesized so as to give the points of instability as actually observed.
Slip at the Wall and Extrudate Roughness with Aqueous Solutions of Polyvinyl Alcohol and Sodium Borate25(1981); http://dx.doi.org/10.1122/1.549613View Description Hide Description
Wall slip during flow of viscoelastic fluids through cylindrical tubes and the surface character of the extrudate have been studied using aqueous solutions of polyvinyl alcohol and sodium borate. Experiments with rough and smooth tubes of Plexiglas and aluminum show that the nature of the flow is highly dependent on the characteristics of the tube surface. For example, at apparent wall shear rate in rough and smooth Plexiglas tubes with inside diameter of 0.638 cm, a solution containing 3.5% polyvinyl alcohol and 3.5% registered a pressure drop through the rough tube more than twice that measured for the smooth tube. Also, distortion was severe for the extrudate from the rough tube at but comparable distortion from the smooth tube did not appear until A hot‐film anemometer has been adapted to study slip near the wall of a conduit. With hot‐film and reservoir pressure measurements we have shown that the microscopic nature of a tube wall can promote or inhibit slip, and that slip reduces extrudate swell and delays the onset of severe extrudate distortion. We have observed a transient effect termed slip‐initiated stress overshoot, and, with a Teflon tube, a region in which self‐oscillation occurs.
25(1981); http://dx.doi.org/10.1122/1.549611View Description Hide Description
An experimental and a theoretical study were carried out to determine the stress distributions of viscoelasticpolymeric melts in a converging channel. For the experimental study, two different types of experiment were conducted using converging channels: one was the measurement of wall normal stress with the aid of pressure transducers, and the other was the measurement of stress birefringence with the aid of a circular polariscope, which enabled us to determine both shear stress and normal stress distributions in the channel. It was found that the distribution of wall normal stress goes through a minimum and that the extensional stresses along the centerline of the converging channel determined from the stress birefringence patterns, go through a maximum. For the theoretical study, the Coleman‐Noll second‐order fluid was used to derive theoretical expressions for the wall normal stress, and shear and normal stresses in the converging flow field. It was found that the theoretical analysis corroborates qualitatively the experimentally determined stress distributions.
On Shear Stress at Wall and Mean Normal Stress Difference in Capillary Flow of Polymer Melts: Authors' Reply25(1981); http://dx.doi.org/10.1122/1.549641View Description Hide Description
25(1981); http://dx.doi.org/10.1122/1.549638View Description Hide Description