Volume 21, Issue 1, March 1977
Index of content:
21(1977); http://dx.doi.org/10.1122/1.549456View Description Hide Description
Flow patterns around agitators of varying design have been determined for the systems: polystyrene/styrene, polybutadiene/styrene, and (butadiene‐styrene)‐copolymer/styrene at several concentrations. Normal stress coefficients and shear viscosities have been determined in the same concentration range. Three regimes of flow are observed for most agitator types: (1) centrifugal force dominated at low concentrations, (2) vortex patterns or stagnant regions near the agitator of intermediate concentrations, (3) normal stress dominated at high concentrations. It is argued that this behavior may be represented by the dimensionless group where is the zero shear normal stress coefficient, R the radius of the agitator, and ρ the solution density. This is the ratio of viscoelastic to inertial forces. It is found that the transformations of regimes (1) to (2) and (2) to (3) occur in agitators of the same diameter at the same concentrations in any particular solution. The critical concentrations for different solutions correlates with the same value of
21(1977); http://dx.doi.org/10.1122/1.549462View Description Hide Description
This paper describes modifications of a Weissenberg Rheogoniometer (Model R‐16) which were made in order to diminish machine motions during stress transient measurements.Piezoelectric crystal load cells were installed in the torque and axial force measuring systems, and a cross‐beam support was added to increase axial rigidity. Start‐up and relaxation tests were performed with the modified instrument, the original torsion bar system, and the Rheometrics Mechanical Spectrometer, with polystyrene solutions ranging in zero shear viscosity from 700 to 56,000 poise. The shear stress transient data were practically independent of cone angle, platen diameter, and machine stiffness. After rigidification the normal stress transients showed relatively small variations with platen diameter and axial stiffness, but the effect of cone angle was still appreciable, as had been noted earlier by Meissner for melts. However, the normal stress transients appeared to approach an asymptotic behavior for the 4° and 8° gap angles. Flow visualization studies were made using a clear platen and dye-impregnated tracer solutions. The edge instability characterized by fluid expulsion from the gap did not appear to be associated with any nonuniform flow inside the gap. The edge instability occurred in a characteristic range of platen angular velocities for each solution.
21(1977); http://dx.doi.org/10.1122/1.549463View Description Hide Description
Stress relaxationmeasurements were made as a function of temperature and hydrostaticpressure on two lightly filled elastomers (Hypalon 40 and Viton B), one highly filled elastomer (Neoprene WB), and on an EPDM rubber. The latter was not piezorheologically simple. The lightly filled elastomers showed piezorheologically simple behavior, i.e., their response curves under different hydrostaticpressures could be superposed empirically by a simple horizontal shift along the logarithmic axis. The filled elastomer was piezorheologically simple only in the rubbery region and in the beginning of the transition region. The dependence of the empirical shift distances, on P could not be described by either the Ferry‐Stratton or the Bueche‐ equation. By considering the bulk modulus to be linearly related to pressure, a new equation has been developed for which describes the pressure dependence well and contains the WLF equation as a limiting case. Published data on the response of poly(vinyl chloride) under superposed hydrostaticpressure are shown to obey the new equation also. The theoretical importance of the new equation lies in the fact that combination of the usual isobaric measurements at atmospheric pressure as function of temperature with isothermal measurements as function of pressure allows, in principle, all the molecular parameters required by the free volume theory to be determined unambiguously.
A Study on the Interfacial Instability in the Stratified Flow of Two Viscoelastic Fluids Through a Rectangular Duct21(1977); http://dx.doi.org/10.1122/1.549457View Description Hide Description
A hydrodynamic stability analysis was carried out to better understand the phenomenon of interfacialinstability in the stratified flow of two viscoelastic fluids through a rectangular duct, using the Coleman‐Noll second‐order fluid as a constitutive equation. Because of the complexity of the equations to be dealt with, we considered, instead of a full three‐dimensional treatment of the disturbance, the two sets of two‐dimensional disturbances, namely (a) disturbances spatially dependent only upon the and coordinates, and (b) disturbances spatially dependent only upon the and coordinates. For each set of disturbances, approximate solutions of the governing system differential equations were obtained by using the perturbation technique. It has been found that the occurrence of interfacialinstability in the plane depends only on the viscosity ratio of the two fluids (i.e., independent of the fluid elasticity), whereas the occurrence of interfacialinstability in the plane depends on both the viscosity and elasticity ratios of the two fluids concerned. Regions of stability were constructed in terms of the viscosity and elasticity ratios of the two fluids. The stability regions determined may be used for guidance in judiciously choosing the viscosity and elasticity ratios, and the composition ratio of the two fluids flowing side by side in order to avoid an irregular interface.
21(1977); http://dx.doi.org/10.1122/1.549464View Description Hide Description
Almost all basic work in rheology, both experimental and theoretical, has dealt with isothermal flows, with the temperature being held constant during any one measurement. The present paper presents systematic studies of the nonisothermal behavior of two polystyrenes (a commercial sample and a sample of sharp molecular weight distribution). The experiments involve stress growth and stress relaxation during elongation in an Instron tensile tester, with rapid changes of temperature being imposed during the pulling and after it. Generalized non‐isothermal viscoelastic theory is proposed and tested. The generalized time‐temperature superposition involved there works well in simple stress relaxation (when the temperature changes are imposed after the pulling) but in the simultaneous type of experiments (when the temperature changes are imposed during the pulling), the accommodation to the new state seems to be more rapid than expected from theory.
Non‐Uniqueness of Determination of Molecular Weight Distribution Dependence of Properties by Blending Experiments21(1977); http://dx.doi.org/10.1122/1.549458View Description Hide Description
It is shown that in general blending experiments alone do not determine unique relations for the dependence of polymerproperties upon molecular weight distribution. In particular, two theoretical expressions for the steady shear elastic compliance of molten polymer lead to identical property blending rules.