摘要:

The fiber‐windup technique, developed by Macosko and Lorntson [SPE Tech. Papers19, 461–467 (1973)] for rotational shear rheometers, was re‐examined to obtain transient uniaxial extensional viscosity data. Macosko and Lorntson used a special force transducer to measure the tension in the filament being pulled. In contrast, in this study commercial rotational shear rheometers (Rheometrics Mechanical Spectrometer‐800 and Rheometrics Fluids Spectrometer‐II) with torque transducers were used to obtain the extensional viscosity. One end of the sample is clamped, while the other end is wound around a drum at a constant rotational speed, to achieve a given extension rate. Tests were conducted with a polyisobutylene sample at room temperature to verify reproducibility and were compared with published studies. Results for polyisobutylene and IUPAC‐X low density polyethylene were found to be in good agreement with published results. Hencky strains of up to 6 could be achieved. The technique can provide valuable extensional viscosity data for high viscosity liquids.

ISSN:0148-6055    

DOI:10.1122/1.550792

出版商:The Society of Rheology    

年代:1996

数据来源: AIP

摘要:

Using a Couette viscometer, we investigated the dependence of the steady‐shear viscosity of simulated neutralized current acid waste onpH and different acids. For a fixed weight percent of waste solids, the relative viscosity decreased aspH was lowered. The magnitude of the relative viscosity varied for the different acids. The values of random maximum packing for these different slurries were obtained using a floc‐agglomerate structural model proposed by Tsutsumietal. (1994). As the weight percent of waste solids is normalized by the value of maximum packing, the relative viscosity data approximately fall onto a single master curve for a specific shear rate. A direct relationship between the macroscopic rheological properties (steady shear viscosity) and the suspension microstructure was established through the packing behavior of a suspension. A comparison of the strongly flocculated and noncolloidal suspensions indicates that besides the normalized weight fraction, the dependence of shear rate must be considered to estimate the relative viscosity for strongly flocculated suspensions.

ISSN:0148-6055    

DOI:10.1122/1.550756

出版商:The Society of Rheology    

年代:1996

数据来源: AIP

摘要:

The transient elongational behavior of a highly‐aligned 60% fiber volume fraction long‐discontinuous‐fiber filled poly‐ether‐ketone‐ketone (PEKK) melt system was investigated with a computer controlled extensional rheometer at 370 °C. Long discontinuous graphite fibers, 7 μm in diameter, and PEKK thermoplastic constituted the melt system. Experiments conducted under conditions to approximate controlled strain rate and controlled stress produced τ̄+E(t,ε̇̄) and ε̇̄−(t,τE) similar to a shear dominated flow of a non‐linear viscoelastic fluid. Increasing strain rate resulted in an increasing peak stress and in an increasing ratio between peak stress and plateau stress. Increasing controlled stress level resulted in decreasing elongational viscosity and increasing post‐elastic‐step strain rate. The ratio of apparent ηE/ηPEKKwas 3.67×108to 3.94×1010for average strain rates from 6.81×10−7to 1.49×10−3s−1. A variable taper shear cell analysis provided the range of expected ηE/η, which agreed well at high strain rates with the experimental data.

ISSN:0148-6055    

DOI:10.1122/1.550787

出版商:The Society of Rheology    

年代:1996

数据来源: AIP

摘要:

The flow behaviour of Phillipsetal.’s suspension model [Phys. Fluids A4, 30 (1991)] in channel and pulsatile flows is investigated by a combination of analytical and numerical methods (using an unstructured finite volume method). In the channel flow, we found that the kinematics develop faster than the concentration, with an entrance length of 0.2H3/a2, whereHis the channel half width, andais the particles’ radius. The corresponding entrance length for the concentration is 0.5H3/a2. In time‐periodic flows, the steady‐state kinematics and the stresses are periodic in time, and yet the steady‐state concentration (and thus the viscosity) is time‐independent. This feature is perhaps a critical test of the model, and indeed of any model in which the flux is quasi‐linear in the generalized strain rate.

ISSN:0148-6055    

DOI:10.1122/1.550757

出版商:The Society of Rheology    

年代:1996

数据来源: AIP

摘要:

The onset of shear thickening (dilatancy) has been studied in submicron model suspensions of sterically stabilized spherical particles. Stress controlled rheometers have been used for this purpose, so that measurements can be performed beyond the onset of sudden shear thickening. Systematic data are presented for the effect of particle size, particle concentration, and the nature of the suspending medium on the onset of shear thickening. As a first approximation, the critical shear rate changes inversely proportional with the medium viscosity. The changing solvency of the medium for the stabilizer polymer introduces additional changes through effects on the thickness and stiffness of the steric barrier. Thinner, stiffer barriers cause lower critical shear rates. In the softer systems the critical shear stress becomes independent of particle concentration in dense suspensions. This does not seem to be the case for the harder systems. The effect of particle radiusacould shed some light on the underlying mechanisms of shear thickening. The data do not, however, enable us to decide between a scaling for the critical shear rate witha2or one witha3. The latter seems more appropriate for harder systems, the former for softer ones.

ISSN:0148-6055    

DOI:10.1122/1.550791

出版商:The Society of Rheology    

年代:1996

数据来源: AIP

摘要:

We report new evidence of secondary flow during plane Couette shearing of entangled polystyrene solutions and an entangled polyisobutylene melt. The secondary flow is shown to be driven by normal stress imbalances at the free edges of the materials, and is accompanied by spatial variations in birefringence. In the polymer solutions, the secondary flow, though unmistakable, is weak and is only apparent from tracer particle visualization experiments using a video microscopy technique. In the polymer melt, however, secondary flow is much stronger and causes gross shape changes in sheared samples that are easily visualized with the naked eye. We also show that several details of the plane Couette secondary flow are correctly predicted by a simple differential constitutive equation in the narrow gap approximation. Our findings suggest that the secondary flow’s effect on sliding plate rheological measurements could be minimized by (a) using large plates with narrow gaps, i.e., small sample aspect ratios; (b) performing measurements at low Weissenberg numbers (Wi≡τLγ̇); and (c) employing measuring techniques such as laser birefringence that permit stresses to be determined close to the center of the shearing surfaces of the plane Couette cell. In addition to secondary flow, we find significant levels of slip during steady shearing of entangled polystyrene solutions. The slip, though qualitatively similar to theentangledslippredictions of Brochard and deGennes [Langmuir8, 3033–3037 (1992)], is unusual because at low slip velocitiesVsthe slip lengthsb≡Vs/γ̇ are of the order of 150 μm, which is much larger than expected for entangled slip. Furthermore, slip in the solutions is shown to be nonisotropic with the slip velocity manifesting a similar shear rate dependence to the birefringence. These findings suggest that the slip law for entangled polymers may be more complex than previously thought.

ISSN:0148-6055    

DOI:10.1122/1.550758

出版商:The Society of Rheology    

年代:1996

数据来源: AIP

摘要:

Some colloidal suspensions with a thixotropic behavior and yield stress also display a minimum stress in their flow curves. One of these is an aqueous suspension of synthetic clay, consisting of anisotropic particles of nanometric size, forming a transparent thixotropic gel. Four flow regimes corresponding to four different states of strain field were defined by combining visualization techniques and rheometric measurements in cone‐plate geometry. Links with the microstructure of the suspension are suggested. Flow instabilities such as localized shear and stick slip were identified in the falling branch of the flow curve and in the minimum stress area. Two regimes involving the bulk properties of the sample were identified. Consequently, proper procedures for characterizing the thixotropy of the suspensions are proposed.

ISSN:0148-6055    

DOI:10.1122/1.550759

出版商:The Society of Rheology    

年代:1996

数据来源: AIP

摘要:

A formulation is given for a bead‐spring model which possesses the following features simultaneously: (i) the Hookean springs may have different spring moduli, (ii) the friction coefficients belonging to the beads may be different, (iii) preaveraged hydrodynamic interaction may be included and (iv) the geometry of the bead‐spring structure may contain cycles. The possible presence of cycles implies that one may use, in principle, three different representations for describing the geometry: one bead position representation and two different connector vector representations: one related to the springs of the entire bead‐spring structure and the second related to the springs of a particular substructure (the spanning tree) of the entire bead‐spring structure. For these three representations it was possible to derive the equations of motion and the expressions for the corresponding modified Rouse‐ and Kramers matrices. In case of a dilute solution of equal Hookean bead‐spring structures (the spring moduli and friction coefficients may be non‐equal, preaveraged hydrodynamic interaction may be included and the geometry may contain cycles) an explicit constitutive equation is obtained. The model is formulated in such a way that further extensions and modifications are relatively simple to incorporate.

ISSN:0148-6055    

DOI:10.1122/1.550760

出版商:The Society of Rheology    

年代:1996

数据来源: AIP

摘要:

Immiscible polymer blends display a complex flow behavior caused by the coupling between morphology and rheology. The flow induced microstructure has been studied on model systems of nearly inelastic polymers. For these systems, the elastic properties of the blend are mainly governed by the interface. Measurements of the storage modulus and of the first normal stress difference, both reflecting this enhanced elasticity, have been used to probe the blend morphology. From oscillatory measurements after cessation of flow the mean diameter of the disperse phase, as generated by the previous flow, has been calculated using the model of Palierne [Rheol. Acta29, 204 (1990)]. A procedure based on a direct fitting of the dynamic moduli with the model is compared with one that uses a weighted relaxation spectrum. The steady state normal stress data, on the other hand, have been related to the morphology of the blend by means of the model of Doi and Ohta [J. Chem. Phys.95, 1242 (1991)]. Since this model predicts a direct proportionality between the contribution of the interface to the normal stress and the specific interfacial area, the size of the droplets can be calculated once the proportionality constant is known. The conditions for which the model is valid have been determined and the required constant is obtained by comparing the results with those from the dynamic moduli. The resulting droplet sizes have been used to develop a data reduction scheme: The specific interfacial area is found to be inversely proportional to the ratio of interfacial tension over shear stress for several blends with various concentrations and viscosity ratios.

ISSN:0148-6055    

DOI:10.1122/1.550800

出版商:The Society of Rheology    

年代:1996

数据来源: AIP

摘要:

General, experimentally verifiable predictions of individual component behavior of linear flexible polymers in arbitrary molecular weight distributions are derived from the double reptation mixing rule. The distribution of stress/orientation among the different components of the molecular weight distribution during steady deformation and constrained elastic recovery are developed for the case of a single exponential monodisperse relaxation function. The des Cloizeaux ‘‘double reptation’’ model and the Tsenoglou network model are shown to be equivalent and the precise relationship between the model parameters is determined. Distinct contributions to the relaxation process from the separate mechanisms of reptation and matrix relaxation (constraint release) are identified. Experimental relaxation spectra of bidisperse blends of nearly monodisperse polybutadienes reveal a cascade of discrete peaks in the terminal zone that are in qualitative agreement with theoretical predictions from the double reptation mixing rule with a Doi–Edwards monodisperse relaxation function. The ability to accurately calculate experimental relaxation spectra for complex blends of nearly monodisperse materials is a powerful tool in developing, discriminating, and evaluating new mixing rules.

ISSN:0148-6055    

DOI:10.1122/1.550786

出版商:The Society of Rheology    

年代:1996

数据来源: AIP