摘要:

The flow of isotropic and liquid‐crystalline hydroxypropylcellulose‐water solutions is studied in a transparent slit die with several geometries including divergent and convergent configurations. The flow is observed between crossed polars. In addition to the two different flow regimes that are known in shear (zones I and II), a new flow behavior is observed (zone III). Zone I is characterized by a large macroscopic orientation, as seen between crossed polars. It is localized where either shear rates or elongation rates are large. The extinction angle is a function of the position in the die. It is similar to the flow‐aligning regime observed at high shear rates. Zone II is much less oriented, due probably to a large density of defects. It is typical of the low shear rate region observed in simple shear. Zone III exhibits a peculiar behavior with long‐range orientation instabilities. It is always localized after a diverging zone. Flow around obstacles and colliding streams are also studied.

ISSN:0148-6055    

DOI:10.1122/1.550564

出版商:The Society of Rheology    

年代:1994

数据来源: AIP

摘要:

A common rheological hypothesis, that the stress in a fluid element is only a function of its own deformation history, is rendered questionable in liquid‐crystalline polymers (LCPs) due to the presence of distortional elasticity, through which neighboring fluid elements may directly influence one another. However, the fine defect texture in LCPs has led to the suggestion that fluid properties may be averaged over amesoscopiclength scale, intermediate between the molecular and macroscopic, so that averaged measures of fluid structure and stress at this scale depend only on their own deformation history [R. G. Larson and M. Doi, J. Rheol.35, 539 (1991)]. We describe an experimental test of this hypothesis. If true, it should be possible to use rheological and rheo‐optical data obtained in simple shear flow to predict the velocity and molecular orientation fields in a nonhomogeneous shear flow. Quantitative flow birefringence experiments are conducted on a liquid‐crystalline solution of poly(benzyl glutamate), in plane Poiseuille flow. At low flow rates, the data support the local response hypothesis. As flow rate is increased, however, a profound instability occurs that is unanticipated based on behavior reported in homogeneous simple shear flow. The instability is characterized by large wavelength disturbances in structure oriented perpendicular to the flow direction that are clearly visible to the naked eye. With increasing flow rate, these structures decrease in size and become increasingly chaotic. Despite the onset of the instability, time‐averaged measurements of average orientation may be qualitatively predicted based on simple shear flow data.

ISSN:0148-6055    

DOI:10.1122/1.550565

出版商:The Society of Rheology    

年代:1994

数据来源: AIP

摘要:

We have investigated experimentally the behavior of a water soluble associating polymer system, hydrophobically modified (hydroxypropyl)guar, with very few randomly distributed hydrophobic substituents along the chains. We focus mainly on the rheological effects due to the superposition of the reversible hydrophobic interaction network on the physical entanglement network in dense macromolecular systems of that kind. Both linear and nonlinear response to transient, steady, and oscillatory shear flow prove that, in the semidilute and moderately concentrated regime, the hydrophobically associating polymer behaves like a classical dense macromolecular system whose long‐time dynamics can be described using only one long relaxation time, identified as a retarded disengagement time, much larger than the association lifetime. The temporary hydrophobically associating network can be destroyed when applying a critical shear stress τc, which is studied as a function of polymer concentration.

ISSN:0148-6055    

DOI:10.1122/1.550566

出版商:The Society of Rheology    

年代:1994

数据来源: AIP

摘要:

The deformation in uniaxial elongational flow of dispersed droplets in immiscible molten polymer blends was studied for viscosity ratios 0.005

ISSN:0148-6055    

DOI:10.1122/1.550568

出版商:The Society of Rheology    

年代:1994

数据来源: AIP

摘要:

Composite particles consisting of a polymer core and an inorganic shell were formed by suspension polymerization. For suspensions in a silicone oil, the steady‐shear viscosity and creep behavior were measured in electric fields up to 2.0 kV mm−1. Although the polymer core is not ER active, the suspensions of composite particles show a striking increase in the steady‐shear viscosity and the flow curve changes from Newtonian to Bingham profiles. The ER effects can be attributed to the shell layers on the polymer surfaces. The creep curves at low stresses are composed of instantaneous elastic, retarded elastic, and viscous regions. With increasing stress the retarded elastic and viscous components decrease. At some critical stress the strain almost instantaneously increases and reaches the equilibrium without viscous flow. After the removal of the critical stress, the suspensions show no elastic recovery. Therefore the creep and recovery behavior is purely plastic and the critical stress corresponds to the static yield value. The application of stresses above the static yield value causes the suspensions to flow. The development of yield stress (plateau value) in steady shear can be derived from the ideal chain model in which the particles all align into chains of single particle width and equal spacing. However, the model cannot predict the instantaneous deformation without recovery below the yield stress. The thick column formed by several chains may be responsible for purely plastic responses.

ISSN:0148-6055    

DOI:10.1122/1.550601

出版商:The Society of Rheology    

年代:1994

数据来源: AIP

摘要:

A stable ‘‘island’’ has been discovered in the slip‐stick region of a linear low‐density polyethylene. Within this region, which exists only in a narrow temperature range, pressure oscillations cease, the pressure drop falls by a substantial amount, and the extrudate becomes reasonably smooth. Unstable oscillating flow persists at throughputs on both sides of this island. The existence of this intermediate stable flow challenges all current theories of extrusion instabilities. The phenomenon resembles observations of a drop in extrusion pressure for which a mesophase transition has been suggested.

ISSN:0148-6055    

DOI:10.1122/1.550523

出版商:The Society of Rheology    

年代:1994

数据来源: AIP

摘要:

We perform numerical simulations of rods and spheroids undergoing Jeffery orbits in a variety of shear flows. The numerical simulations are based on the boundary element method, which allows for the accurate modeling of the problem geometry. We compare the period of rotation for spheroids and rods, both far from walls and very close to walls. We find that the wall effects in three dimensions are minimal, even for flow in gaps not much larger than the longest dimension of the particle. We also show that two‐dimensional simulations grossly overpredict the wall effects seen in three dimensions. Results are similar for both linear and nonlinear shear flows. We also briefly look at the orbital motion of a particle in close proximity to another particle, and show that, again, there is very little effect on the period of rotation, although the resulting centroid trajectories are very different from that of an isolated particle.

ISSN:0148-6055    

DOI:10.1122/1.550604

出版商:The Society of Rheology    

年代:1994

数据来源: AIP

摘要:

The singular nature of the hard sphere potential combined with lubrication stresses near contact poses interesting issues with respect to the high frequency viscoelastic behavior. Dilute theories demonstrate clearly that soft potentials and/or lubrication stresses that reduce the relative mobility to zero at contact lead to a well defined plateau inG’as ω→∞, whereas a hard sphere potential without hydrodynamic interaction producesG’≊ω1/2in this limit. The former follows from a small deformation of the equilibrium structure due solely to the oscillatory convection and the latter from a diffusional boundary layer near contact required to satisfy the no‐flux boundary condition. Two sets of data that delineate the high frequency response for colloidal hard spheres at high volume fraction appear to differ in this regime, suggesting different physics for the interactions at small separations. Here we apply our nonequilibrium theory to extend the existing treatments to high volume fractions to predict both limits quantitatively and provide a possible interpretation for the experimental results. The two experimental systems only differ in the surface modification of the particles and the high frequency modulus is the only rheological property sensitive to this difference. The predictions of our theory with varying extent of hydrodynamic interaction illustrate the link between the behavior of the high frequency modulus and the hydrodynamic properties very near the particle surface.

ISSN:0148-6055    

DOI:10.1122/1.550530

出版商:The Society of Rheology    

年代:1994

数据来源: AIP

摘要:

Two different consistency criteria are considered for the stress tensor expressions of two different kinetic theory models for dilute polymer solutions. The criteria are material objectivity and thermodynamic consistency. Two separate approaches for checking thermodynamic consistency are considered, the first proposed by Grmela [Phys. Lett.111A, 41 (1985)] and the second by Jongschaap [LectureNotesinPhysics(Springer, Berlin, 1991)]. The two models considered are a Hookean dumbbell model with internal viscosity (no linearization or rotation matrix approximations), and an inertial, elastic bead‐spring chain. Both sets of criteria may be applied to these models without finding either a closed‐form expression for the stress tensor, or solving any of the equations. We find that the Giesekus form for the stress tensor expression for a Hookean dumbbell with internal viscosity is both thermodynamically consistent and objective, whereas the Kramers form is not thermodynamically consistent. The Kramers form for the stress tensor is found to be thermodynamically consistent for an inertial, elastic bead‐spring chain, but is not materially objective. We also find that the approaches of Grmela and Jongschaap are the same, although each requires a differentansatzfor the relationship between thermodynamic quantities and polymer conformations.

ISSN:0148-6055    

DOI:10.1122/1.550531

出版商:The Society of Rheology    

年代:1994

数据来源: AIP

摘要:

The nonhomogeneous nature of the flow in the parallel disks rheometer necessitates the differentiation of the measured quantities (torque and normal force) with respect to the shear rate at the disk rim. Commercial instruments do not calculate true material functions online, rather they report apparent Newtonian values, i.e., ones obtained assuming the material functions are constants. In this work, we present a single‐point correction technique to obtain approximate values for material functions without numerical differentiation. The advantage of the single‐point correction method is that it gives more accurate results than the apparent Newtonian values and it takes less time than numerical differentiation. This can, therefore, be useful in quality control laboratories and for process‐line measurements where reasonably accurate data are needed in a short time. A single‐point correction is applied to the parallel disks device for the shear viscosity and a new correction method for the normal stress coefficients is also developed. The accuracy of these approximate methods is tested with experimental results for a polymer melt and a polymer solution. The correction for both shear viscosity η and normal stress coefficient Ψ≡Ψ1−Ψ2avoids the numerical differentiation of the data and can be easily implemented in software that provide online material functions.

ISSN:0148-6055    

DOI:10.1122/1.550532

出版商:The Society of Rheology    

年代:1994

数据来源: AIP