Volume 45, Issue 3, May 2001
Index of content:
45(2001); http://dx.doi.org/10.1122/1.1366717View Description Hide Description
This paper deals with concentrated suspensions(pastes) of particles at a solid concentration of 60% per weight in an acidic binder. This paste is used to manufacture petrochemical catalysts. First we show that in a squeezing experiment binder migration occurs in the suspension as long as the velocity at which the test is carried out does not exceed a critical value. Next, we develop a mathematical model of the test and a complete methodology to identify the rheological parameters of this model. We give an analytical expression of the critical velocity, which is related to the physical characteristics of the suspension. The exact identification of the rheological parameters requires the use of a numerical program which was developed for this application.
Application of large double-step shear strains to analyze deformation and shape recovery of a polymer droplet in an immiscible polymer matrix45(2001); http://dx.doi.org/10.1122/1.1357824View Description Hide Description
The deformation and shape recovery of a soft polymerdroplet in an immiscible polymer matrix with higher viscosity were directly observed from two directions after application of large double-step shear strains. After application of the first-step strain the deformeddroplet changes its shape, starting from a flat ellipsoid to a rodlike shape then to a dumbbell, an ellipsoid of revolution, and finally back to a sphere. When a large second-step strain is applied in the same or opposite direction to the first strain during the recovery, the shape of the droplet and the orientation angle depend on both and the shape during the recovery. Just after application of in the opposite direction to the first strain the droplet having a rod-like or dumbbell shape significantly deforms at both ends and stretches more than expected from the assumption of affine deformation. The deformeddroplet under strain has two orientation angles, and to the shear direction. The main angle of the inclined droplet is equal to the angle given by the total strain i.e., On the other hand, another angle observed at both ends of the deformeddroplet, is close to the angle given only by i.e., This difference in the orientation angle suggests a faster recovery at both ends and a slower recovery (remaining effects of the first strain) in the central part.
45(2001); http://dx.doi.org/10.1122/1.1366716View Description Hide Description
Electrorheological (ER) silicone elastomers containing particles based on silica and titania minerals and were prepared and characterized. An electrical field was applied to align the particles during the cure of the silicone prepolymer. For the silicone/silica compositions, a prominent Maxwell–Wagner dispersion in the dielectric response suggested that surface conductivity of the silica particles dominated the polarization. Alignment of the particles increased the overall dielectricpermittivity as well as the magnitude of the Maxwell–Wagner dispersion. Their ER response exhibited a negative deviation from a quadratic dependence on field intensity at high fields, and was accompanied by nonlinear conductivity. A highly nonlinear enhancement of the ER effect with increasing particle concentration was observed. For the silicone/titania elastomers as a class, the ER response increased with the particle’s permittivity. In the case of the elastomer, the ER effect increased with field frequency, as expected from high permittivity of relative to silicone, whereas the opposite dependence was observed for the silicone/silica ER elastomer, suggesting a conduction polarization mechanism for the latter.
45(2001); http://dx.doi.org/10.1122/1.1366714View Description Hide Description
A rheological model for the stress in liquid–liquid systems is developed based on unifying the theory of the present phenomenological models and by applying a description of the dispersed phase microstructure to remove most of the adjustable parameters present in the rheological models. However, this introduces new parameters in the model for the microstructure. The main features of the model are: (a) the stress related to the viscosity difference is not purely viscous, (b) a closure approximation is used for the stress contribution due to the interfacial stress, and (c) the stress relaxation time depends on the droplet deformation. Especially the latter is of importance to get the description of some characteristic rheological behavior of dispersive mixtures right. The interfacial area, the droplet stretch ratio, and the rate of change of the interfacial area show up explicitly in the equation for the stress evolution. The description of the spatial evolution of the dispersed phase microstructure is accomplished by coupling preexisting models of coalescence and breakup yielding a description of the evolution of the microstructure. Model predictions are compared with experimental results from literature [Vinckier et al. (1997); Vinckier (1998)]. It is shown that, by incorporating a structure dependent relaxation time, remarkably good agreement between the model and experiment is obtained, even for very different experimental conditions. Moreover, the complex rheological phenomena observed can now be understood in terms of the evolution of the microstructure.
45(2001); http://dx.doi.org/10.1122/1.1366715View Description Hide Description
We investigate linear viscoelastic properties and nonlinear stress relaxationdynamics in a series of bidisperse 1,4-polybutadiene blends. Blend systems comprising high- and low- molecular weight polymers with are formulated in which the short polymer component functions as an ideal nonvolatile solvent for the long polymer chains. In blends with a critical molecular weight is identified below whose terminal viscoelastic properties are independent of and vary with volume fraction of the long polymer molecules, in a manner consistent with expectations for entangled polymer solutions. In blends with several approximate scaling relationships between terminal rheological properties and can be determined from the experimental results, and The scaling exponents observed are consistent with earlier experimental reports, but disagree with theoretical predictions of the constraint release time in bidisperse melts. In agreement with previous step strain studies using entangled polystyrene solutions, nonlinear step shear measurements using blends with reveal an unusual short-time crossing pattern in shifted nonlinear relaxation moduli The unusual short-time dynamics are first observed at and are accompanied by a continuous transition from type A to type C damping at long times, Our findings are consistent with the idea that type C damping is a characteristic feature of well entangled polymer systems and suggest that type A damping is just a special case of type C, applicable only in the limit of weakly entangled polymer chains.
45(2001); http://dx.doi.org/10.1122/1.1357820View Description Hide Description
The flow properties of aqueous suspensions of thermosensitive latex particles are investigated as a function of volume fraction and temperature. The particles consist of a solid poly(styrene) core and a shell composed of crosslinked poly(N-isopropylacrylamide) (PNIPA) chains. The PNIPA network shrinks with increasing temperature leading to a denser layer of polymeric chains on the surface of the core particles. The shear viscosity obtained from suspensions of these particles at low shear is compared to the viscositymeasured in the high-frequency limit. In the limit of dilute suspensions the viscosity is modeled in terms of an effective hydrodynamic radius It is shown that of highly swollen particles depends markedly on frequency. The data indicate that the swollen network on the surface of the particles is partially drained at high frequencies. For shrunken networks measured in the low and high frequency limit coincides again. The high frequency shear modulusmeasured at high volume fractions demonstrates that the thermosensitive particles may be regarded as soft spheres. The repulsive interaction may be modeled in terms of a power law with an exponent of 9.
45(2001); http://dx.doi.org/10.1122/1.1357822View Description Hide Description
A wide variety of bead-spring kinetic theorymodels have been proposed to explain the stress growth and hysteretic behavior of dilute polymer solutions in uniaxial extension. We analyze the Kramers chain, a fine-scale model for polymer dynamics, in order to assess the validity of the coarser-grained bead-spring models in these deformations. Whereas the spring force is a simple function of the dumbbell length for the FENE spring, we find that the relationship between the ensemble-averaged end-to-end force and the extension for a Kramers chain depends on the kinematic history to which it has been subjected. In a quiescent fluid, the Kramers chain force–extension relationship is identical to the FENE force law. However, during start up of elongational flow, the ensemble-averaged end-to-end force for a given (end-to-end) length of the molecule increases with strain until steady state is reached. If the extensional flow is suddenly stopped, the Kramers chain force–extension relationship relaxes back to the FENE force–extension function. For all positive strains, the FENE dumbbell force law underpredicts the ensemble-averaged end-to-end force in the Kramers chain. For a Weissenberg number of 11.4, the end-to-end forces in the two models can differ by three to four orders of magnitude, indicating the unsuitability of the FENE dumbbell for modelingpolymers in strong transient extensional flows. This paper also presents a detailed analysis of the mechanisms causing stress–birefringence hysteresis. We find that it is essential for a dumbbell model to have an end-to-end force that depends upon the deformation history in order to capture configurational hysteresis.
45(2001); http://dx.doi.org/10.1122/1.1357821View Description Hide Description
Creep and dynamic mechanical measurements were carried out on a series of hyperbranched polyisobutylenes (PIBs), having a range of molecular weights daltons) and branching frequencies (3–57 branches/molecule). For all samples, the molecular weight of the branches was higher than the entanglement molecular weight of linear PIB, by as much as a factor of 10; nevertheless, only for molecular weights of approximately half-million daltons does the zero-shear viscosity exceed that of linear PIB. Both the viscosity and the length of the entanglement plateau are governed primarily by the branching frequency, rather than by the length of the branches. Such behavior is quite distinct from star-branched polymers. However, the magnitude of the plateau modulus and the temperature dependence of the terminal zone shift factors are the same for hyperbranched, star-branched, and linear PIB.
45(2001); http://dx.doi.org/10.1122/1.1359760View Description Hide Description
We investigated the response properties of immiscible polymer blendelectrorheological fluids to a pulse field. They were composed of liquid crystalline polymers and polydimethylsiloxane, and were classified into two blend types (type I and type II). It was found that the decay of shear stress after removing the field from both of the blends depended greatly on the pulse duration, the applied shear rate, and the temperature. The response properties are discussed on the basis of our model describing the structural change of the fluids.
45(2001); http://dx.doi.org/10.1122/1.1357823View Description Hide Description
Dispersion mechanisms in model fluid systems of different viscosity ratios were studied in a transparent Couette flow cell. The counter-rotating concentric cylinders were driven by two independent dc motors. Drops of the minor phase were then maintained at a constant position by fixing the inner and outer cylinders’ rotational speeds. The advantage of this new setup is that visualization can be made at high shear rates without any secondary flow effects, usually observed with cone-plate or parallel plates geometry. Constant viscosityviscoelasticdrops (Boger fluid) and Newtonian drops [high viscositypolydimethylsiloxane,(PDMS)],deformed at low shear rates in a Newtonian matrix (low viscosityPDMS), oriented along the flow field and dropdeformation increased with shear rate, as expected. However, when a critical shear rate (characteristic of the fluid system used) was reached, the deformeddrops began to contract in the flow direction. When increasing the shear rate over this critical value, drop contraction was followed by elongation perpendicular to the flow direction, i.e., parallel to the vorticity axis. This elongation increased with shear rate until the final breakup occurred. These deformation and breakup mechanisms were attributed to elastic normal forces present at high deformation rates.
45(2001); http://dx.doi.org/10.1122/1.1359761View Description Hide Description
The phenomenon of shear thickening has been studied in sterically stabilized colloidalsuspensions. Temperature has been used to vary the solvent quality of the suspending medium for the grafted polymer in the stabilizer layer. A decrease in temperature causes the polymer coat to shrink. This results in a lower viscosity in the shear thinning region but also in a lower critical shear rate and shear stress at the onset of shear thickening. The critical shear stress increases with increasing particle volume fraction, at least for the volume fractions studied here. In large amplitude oscillatory flow, strain hardening for both storage and loss moduli sets in at the same peak shear rate that causes shear thickening in steady state flow. Parallel superposition also has been used to probe the effect of the stabilizer layer. The frequency dependence of the superposition moduli changes drastically once shear thickening sets in. Surprisingly, this produces a simple and constant shape for the moduli-frequency curves with relaxation times that become nearly independent of the shear rate.
45(2001); http://dx.doi.org/10.1122/1.1357819View Description Hide Description