Volume 44, Issue 1, January 2000
Index of content:
44(2000); http://dx.doi.org/10.1122/1.551072View Description Hide Description
The rheology and microstructure of concentrated colloidalsuspensions, within the shear thickening regime, are investigated using Stokesian dynamics. We consider systems stabilized by charge and/or polymer layers, at hard core volume fractions above 40%. At charge stabilized systems show transitions from ordered to disordered flow, with only a small increase in suspensionviscosity. At higher we observe larger jumps in viscosity with changes between order and disorder and vice versa. At high shear rates, interparticle gaps can become very small. This work investigates two ways in which these gaps may be controlled: by modifying the charge interaction potential [Dratler et al. (1997)], or by including a model for the presence of a polymer brush [Potanin and Russel (1995)]. The thickening observed is dependent on the gaps of closest approach of particles, but only weakly for hard sphere lubrication forces. Strong thickening is only observed with the presence of an enhanced lubricating force, in this article that due to a polymer coat. Models with polymer coats give examples of strong thickening within the disordered branch of flow. The effects of the number of particles used within these systems, the geometry of the computational box, and whether the simulation is two or three dimensional, can have a profound effect on the microstructure.
Nonlinear rheological response of phase separating polymer blends: Poly(styrene-co-maleic anhydride)/poly(methyl methacrylate)44(2000); http://dx.doi.org/10.1122/1.551079View Description Hide Description
We investigate the thermorheological properties of a partially misciblepolymer blend exhibiting lower critical solution temperature phase diagram and having high glass transition and molecular weight constituents, namely poly(styrene-co-maleic anhydride)/poly(methyl methacrylate), SMA/PMMA, in the nonlinear regime. By using parallel plate, sliding plate, and capillary rheometry, with the knowledge of the blend’s phase behavior, we address the following aspects of its dynamic response: (i) the sensitivity of nonlinear viscoelasticproperties to the phase behavior, which is reflected in the observed thermorheological complexity and a dual-peak stress response in large amplitude oscillatory shear; (ii) the effects of shear history on the morphological and rheological properties, which corroborate the picture of shear-induced fibrilar-type growth of the SMA-rich inclusions in the phase separated regime; and (iii) the evolution of shear and normal stresses and the relevant morphological changes in the phase separated blend during startup of simple shear flow. These results suggest that the main rheological features of complex partially misciblepolymer blends can be understood using ideas from the behavior of immiscible blends, provided that their phase state is precisely known.
44(2000); http://dx.doi.org/10.1122/1.551077View Description Hide Description
The linear viscoelastic behavior of a series of partially hydrogenated polybutadienes (PHPB) obtained by catalytic hydrogenation of an almost monodisperse polybutadiene (PB) was studied experimentally at temperatures above 110 °C. The molecular characterization of the polymers used in this study showed that the catalytic hydrogenation of PB produces materials with a bimodal distribution of polymer species for global conversions lower than approximately 90%. All the PHPBs, even the less hydrogenated, are a mixture of a highly hydrogenated PB (with approximately 89% saturation) and unreacted PB. The rheological behavior of the fluids was studied measuring the elastic and viscousmoduli at different temperatures in small-amplitude oscillatory shear flow. The two-phase polymer blends show thermorheological complex behavior and an increase in elasticity at low frequencies which may be associated with long relaxation time processes. The dynamic moduli of the pure components in the terminal region is modeled using a single relaxation time while the blends are described by a minimum of three relaxation modes. The two smaller relaxation times correspond to the relaxation of the blend components while the largest comes from the geometric relaxation of the shape of the interface. The zero-shear-rate viscosity of the PHPBs shows a positive deviation relative to log-additivity rule. But, when the predicted contribution of the interface is subtracted, the resulting zero-shear-rate viscosity as a function of the concentration follows this rule.
Measurement of pressure loss and observation of the flow field in viscoelastic flow through an undulating channel44(2000); http://dx.doi.org/10.1122/1.551080View Description Hide Description
The viscoelasticflow through undulating channels is experimentally studied by using 0.1 and 1.0 wt % aqueous solutions of polyacrylamide (PAA). In the measurement of the pressure loss, it is confirmed that the excess pressure loss occurs for both concentrations of PAA. However, the magnitude of the excess pressure loss for the 1.0 wt % solution of PAA is much smaller than that for the 0.1 wt % of PAA. The Deborah number at the occurrence of the excess pressure loss is within the range of 0.1–0.4 and it depends on the concentration of PAA and channel geometry. The flow pattern changes to an asymmetric one including a small disturbance when the excess pressure loss occurs. Flow unsteadiness observed in the 0.1 wt % solution of PAA is discussed in connection with the excess pressure loss. In the velocity measurement, an asymmetric character of measured profiles for the velocity provides evidence for its three-dimensional character. The variation of the stretch rate on the center line of an undulating channel indicates a significant phase shift beyond the occurrence of the excess pressure loss.
44(2000); http://dx.doi.org/10.1122/1.551074View Description Hide Description
One possible source of error in rheological characterization of concentrated suspensions using nonhomogeneous flows is the migration of particles from high to low shear rate regions to give rise to concentration gradients in the specimen. Numerical data are presented and simple experimental procedures are suggested for the assessment of the particle-migration effects in pressure-driven viscometers including capillary and rectangular slit viscometers.
44(2000); http://dx.doi.org/10.1122/1.551075View Description Hide Description
We compare the rheological behavior in a shear flow of two types of suspension of magnetic particles in the presence of a magnetic field. The first suspension is made of silica particles in a ferrofluid and the second one is made of carbonyl iron particles in silicone oil. The permeability curves of these two suspensions have been measured for different volume fractions as a function of the magnetic field in order to characterize the magnetic interactions. We show for both cases the existence of two different yield stresses: one associated with the solid friction of the particles on the plates of the rheometer and the second one with the rupture of the aggregates. This second yield stress presents a maximum with the volume fraction for the suspension of magnetic holes but grows faster than linearly with the volume fraction for the suspension of carbonyl iron. These features are explained by theoretical models based, respectively, on the deformation of aggregates of macroscopic size and on ruptures between particles at the particle scale. When plotted as a function of the Mason number, the curves for different fields well collapse on the same one but the slope of this master curve on a log–log plot is quite different from the one of the Bingham law, especially for the suspension of magnetic holes. For both suspensions the exponent depends on the volume fraction.
44(2000); http://dx.doi.org/10.1122/1.551076View Description Hide Description
Electrorheological (ER) properties of ER suspensions composed of silica particles and silicone oil under the squeezing flow have been investigated experimentally by changing experimental conditions such as applied electric field, volume concentration of silica particles, medium viscosity of silicone oil, and water content in silica. ER behavior in the squeeze flow is unique in several ways compared to that in the shear flow.Electric field and volume concentration increase as the squeezing proceeds, which increases the strength of chain structures formed between the electrodes. High yield stress and normal stress can be obtained in the case of high voltage, high volume fraction, low medium viscosity, and high water content. As the squeezing proceeds, the normal stress increases, showing some fluctuation due to the deformation of chain structures in the suspension. The fluctuation observed in the stress grows so violent under a certain condition, which seems to be caused partly by macroscopic rearrangement of chainlike structures and by the polydispersity of particles due to aggregation. As the serious fluctuation should be avoided for commercial application, an addition of additive or a surface treatment of particles is strongly recommended to enhance the stability of the dispersed phase.
44(2000); http://dx.doi.org/10.1122/1.551081View Description Hide Description
The “pom-pom” model of McLeish and Larson [J. Rheol. 42, 81–110 (1998)] provides a simple molecular theory for the nonlinear rheology of long chain branched polymer melts. A feature of this model is a maximum stretch for the branched molecules. Sharp transitions were predicted in the extensional viscosity at this maximum stretch. We introduce a simple treatment of the coupling between relaxed and unrelaxed polymer segments at branch points. The branch point is allowed to move in a quadratic localizing potential of unknown strength. Taking account of this effect smoothes the sharp transitions of the model and accounts for the extensional viscosity of “pom-pom” model polymers at their maximum stretch. The result is an improved multimode pom-pom fit for low-density polyethylene rheology. By fitting the nonlinear extensional viscosity, quantitative predictions are made for the nonlinear steady shear viscosity and transient first normal stress difference in shear.
44(2000); http://dx.doi.org/10.1122/1.551078View Description Hide Description
The end-capped telechelic HEUR associative polymer was reported to exhibit a single mode Maxwell typed relaxation behavior. suggested that the single mode Maxwellrelaxation time corresponded to the lifetime of the hydrophobe in the micellar junction. However, results from some recent publications suggested that a more complex relaxation behavior existed. In order to verify this, a hydrophobically modified urethane–ethoxylate (HEUR) polymer with of 26 000 and a hydrophobe was synthesized for the present study. Results from oscillatory measurements of a 2 wt % sample in distilled water indicates that two different relaxation modes are present. This is confirmed with a two-mode Maxwell model and the determination of the relaxation time spectrum from the dynamic moduli data. Further analysis of the data at different temperatures confirmed that one of the modes is possibly related to the lifetime (∼0.01 s at of the hydrophobe in the micellar junction and the other to the network relaxation (∼6 s at The lifetime is in the same order of magnitude as determined by NMR for other associative polymersystems (). This is the first experimental verification that the lifetime and network relaxation time can be differentiated in a model HEUR polymersystem.
44(2000); http://dx.doi.org/10.1122/1.551073View Description Hide Description
In this work the viscosity of a model blend, composed of poly-isobutene and polydimethyilsiloxane, has been measured at two different temperatures and for a wide range of blend compositions and shear rates. Cone and plate and capillary measurements showed a good overlap in the shear rate window accessible to both instruments. The experimental results have been compared to the predictions of two emulsion models [Frankel and Acrivos (1970), Choi and Schowalter (1975)], and of some simple, empirical mixing rules. Good quantitative agreement was found between the data and the Frankel–Acrivos model, once the correct dependency of the droplet radius upon shear rate is accounted for. To this end, droplet radii were independently determined by viscoelasticmeasurements upon cessation of steady shear flow. The Frankel–Acrivos model is also able to predict the change from a positive to a negative deviation from the ideal mixing rule for increasing shear rates, which is a typical feature of polymer blends.