摘要:

Polyisobutylene (PIB) was the first polymer whose viscoelastic behavior was thoroughly studied. The measurements made worldwide on the PIB sample distributed from the National Bureau of Standards were compiled and analyzed by Bob Marvin. The superposition of the data substantiated the time–temperature reduction process, which was suggested by Herbert Leaderman, first put into practice by Arthur Tobolsky, and given a theoretical foundation by John Ferry. Polymers that behaved in a like manner were called thermorheologically simple by A. J. Staverman and F. Schwarzl. Subsequently the behavior of PIB has been found to be the exception rather than the rule. The various deviations for this simplicity are briefly reviewed and PIB is reexamined.

ISSN:0148-6055    

DOI:10.1122/1.550776

出版商:The Society of Rheology    

年代:1996

数据来源: AIP

摘要:

Tensile stress studies were made on dilute solutions of poly(ethylene oxide) and polyacrylamide using a fiber spinning apparatus to investigate the interaction of polymer molecules with the flow field. The addition of dyes or metal ions to the solutions has a considerable influence on the tensile stress. These ingredients normally cause contraction of the polymer coils and stiffen the molecular chains. One additive, Congo Red, markedly increased the tensile stress of poly(ethylene oxide) solutions.

ISSN:0148-6055    

DOI:10.1122/1.550771

出版商:The Society of Rheology    

年代:1996

数据来源: AIP

摘要:

The rheological behavior of Brownian electrorheological (ER) fluids is studied as a model for flocculated colloidal dispersions. The ER fluid has the advantages that the interparticle potential energy can be varied by simply changing the applied field strength, and the microstructure consists of essentially linear chains of particles aligned with the field direction. Under simple shear flow, the suspension has a high‐shear‐rate Newtonian viscosity and a shear thinning viscosity at lower shear rates. For moderate attractive potential well depths,Umin/kT, the suspension has a low‐shear viscosity that scales as exp(Umin/kT). Furthermore, the low‐shear limiting behavior is seen at shear rates that scale as exp(−Umin/kT). A theory is proposed that makes use of the time scale of diffusion for aggregated particles out of their mutual potential well, τ∼(a2/D)(kT/Umin)exp(Umin/kT), much in the spirit of the Eyring theory. Hereais the particle radius andDis the diffusivity of an isolated particle. When the shear rate is nondimensionalized by τ, the reduced viscosity data for all field strengths collapse onto a single universal curve. Although we use a relatively small monolayer suspension, our simulation results compare well to the limited experimental and theoretical work on Brownian ER suspensions. The scaling relationship for the low‐shear viscosity has also been evidenced in other studies of flocculated dispersions.

ISSN:0148-6055    

DOI:10.1122/1.550772

出版商:The Society of Rheology    

年代:1996

数据来源: AIP

摘要:

This paper reports experiments on particle migration in viscoelastic fluids used in hydraulic fracturing. It is found that particle migration in such fluids is controlled by the elastic properties of the suspending fluid and the shear rate gradient. In fluids with low but measurable normal stresses and dominant shear‐thinning properties, particles migrate to regions of lower shear rate. Migration is fast initially, but slows down rapidly over a short distance. For these fluids the bulk migration velocity correlates with the product of the Weissenberg number and the mean shear rate gradient. In contrast, highly elastic fluids with relaxation times well above one second and shear‐thickening properties at low shear rates, flow with a central plug region or slip at the wall, producing little or no migration.

ISSN:0148-6055    

DOI:10.1122/1.550773

出版商:The Society of Rheology    

年代:1996

数据来源: AIP

摘要:

The influence of molecular weight distribution and extrusion processing variables on the morphological features and orientation of high density polyethylene (HDPE) uniaxially extruded tubular films was investigated. In order to gain a better understanding of the orientation–crystallization behavior occurring during extrusion processing, the melt flow properties of the two HDPE resins with identicalM̄n(14 600) values but different molecular weight distributions (M̄w/M̄n=10.3, 15.1), utilized in our previous study, were characterized by dynamic rheological experiments over the temperature range from 150 °C to 230 °C within the angular frequency range from 0.1 to 100 rad/s. The experimental data were shifted to produce master flow curves. The flow activation energy calculated from the shifting process was found to be 25.9 kJ/mol for resin 1 and 29.1 kJ/mol for resin 2. The characteristic relaxation time at 190 °C obtained by use of a Carreau–Yasuda analysis for resin 2 having the broader molecular weight distribution was found to be 6.5 times greater than that of resin 1. This observation further supports our previous conjecture that the prominence of the fibril nuclei in resin 2 is due to its longer melt relaxation time behavior. The extrusion processing variables of melt temperature at the die exit, quench height (which is the distance from the exit of the die to the cooling ring), flow rate of the air through the cooling ring, film line speed, and die gap were varied to control the melt relaxation time of HDPE resins and the processing time frame for cooling. The results show that a longer melt relaxation time and a shorter cooling processing time can enhance the formation of fibril nuclei. The importance of melt relaxation behavior in influencing the final morphological structure in HDPE extruded films and their associated properties is clearly made evident in this paper.

ISSN:0148-6055    

DOI:10.1122/1.550801

出版商:The Society of Rheology    

年代:1996

数据来源: AIP

摘要:

The kinetics of structure development in a moderately concentrated liquid–liquid dispersion under the hydrodynamic conditions of simple shear flow has been analyzed using differential population balance equations. The existing models of the breakup and coalescence phenomena have been examined, modified, and included in the population balances to the extent necessary. It was found that in the present range of drop sizes and viscosity ratios the temporal evolution of the average drop size is primarily determined by drop breakup rather than by coalescence. It was further shown that the evolution of the size distribution function as well as the average drop size are strong functions of the viscosity ratio, exhibiting sharp changes at the critical capillary number. The role of the initial drop size distribution on the time‐dependent state of the dispersion was examined, including the path leading to a bimodal distribution.

ISSN:0148-6055    

DOI:10.1122/1.550774

出版商:The Society of Rheology    

年代:1996

数据来源: AIP

摘要:

Nuclear magnetic resonance imaging has been used to investigate the flow of 0.2% aqueous solutions of xanthan gum. Apparent slip was observed in solutions made from the material supplied by UNAM but not in that supplied by Aldrich or Kelco. The apparent slip velocity was a constant fraction of the maximal velocity for a given contraction ratio. The apparent slip velocity also appeared to be independent ofL/Dbut was strongly influenced by the wall stress, the observed apparent slip velocities being comparable with those determined using the Mooney analysis on capillary flows. After exposure to the action of a syringe pump, which reduced the mean molecular weight of the xanthan, the UNAM xanthan solution no longer exhibited apparent slip. Slip appears to be a function of molecular weight, possibly through sensitivity to the aspect ratio of the molecule.

ISSN:0148-6055    

DOI:10.1122/1.550775

出版商:The Society of Rheology    

年代:1996

数据来源: AIP

摘要:

A simple relation is presented for the viscosity of isotropic solutions of flexible, semiflexible and rodlike chains over a range of concentration from dilute solutions to undiluted polymer, chain contour lengthLand persistence lengthâ. The expression is designed to account for the separate effects of screening of thermodynamic and hydrodynamic interactions, and the onset of intermolecular chain entanglements under Flory theta solvent conditions, or in ‘‘good’’ solvents, for which intramolecular excluded volume is important in dilute solutions. The relation includes a revised (dimensionless) Fox parameterX̃cthat determines the onset of intermolecular entanglements, whereX̃c=πNAρLcâ/3MLfor semiflexible polymers withâ≪L, with ρ the polymer density,MLthe mass per unit length of the chain, andLcthe value ofLfor the onset of entanglement effects in undiluted polymer;X̃c≊100 for a number of systems, including rodlike chains.

ISSN:0148-6055    

DOI:10.1122/1.550777

出版商:The Society of Rheology    

年代:1996

数据来源: AIP

摘要:

Viscometric data on moderately concentrated solutions of the semiflexible macromolecule sodium hyaluronate over a range of molecular weightMand solute concentrationcin aqueous 0.1MNaCl are analyzed using a generalized expression for the viscosity of isotropic polymers and their solutions. Owing to its semiflexible nature, the effects of excluded volume interactions are effectively screened for sodium hyaluronate under the conditions studied, and intramolecular hydrodynamic interactions are largely screened, even at infinite dilution. The consequences of this for scaling of the viscosity relative of solutions of sodium hyaluronate with the parametersc[η] andcMis discussed. The dependence of the viscosity oncMis in accord with the generalized relation, and leads to an estimate for the persistence length in reasonable accord with that found by light scattering measurements on dilute solutions. The temperature dependence of the viscosity suggests that the dependence of the persistence length on temperature may be partially suppressed with increasing polymer concentration.

ISSN:0148-6055    

DOI:10.1122/1.550778

出版商:The Society of Rheology    

年代:1996

数据来源: AIP

摘要:

The shear stress τ–shear strain γ behavior of suspensions of carbonyl iron particles in silicone oil was determined at a shear rate γ̇=0.042 s−1as a function of applied magnetic inductionB0(0.5–6.0 kG), volume fraction of particles φ(0.05–0.30), and roughness of the shearing surfaces. At low fields, the steady‐state shear stress τSwas significantly higher for a roughened surface compared to a smooth surface, but became only slightly dependent on roughness at high fields. In accord with theoretical considerations, the ratio τS/φ was proportional to the square of the calculated magnetic fieldH0inside the suspension at low fields, but its dependence onH0became smaller as the field increased. The smaller dependence may result from the magnetic saturation of the particles or the slipping boundary condition.

ISSN:0148-6055    

DOI:10.1122/1.550779

出版商:The Society of Rheology    

年代:1996

数据来源: AIP