Volume 50, Issue 4, July 2006
Index of content:
50(2006); http://dx.doi.org/10.1122/1.2209558View Description Hide Description
Constitutive equations for concentrated suspensions that explicitly account for the development of anisotropy in the microstructure are not generally available, even for relatively simple systems of hard spheres suspended in a Newtonian medium. Here, we use a directionally dependent mean-free path length and a truncated Cartesian tensor expansion to define a second-order structuretensor for systems of suspended particles. This tensor captures the principal nature of the microstructure. A semiempirical differential equation is developed for the structuretensor, with representation theorems being used to insure frame indifference. A separate equation is proposed to relate the stress tensor to the structure and rate of strain tensors. These coupled equations model structure and stress in both steady and time-dependent viscometric flows. Results from Stokesian dynamics simulations are used to demonstrate the utility of this modeling approach. The simulations were for monodisperse suspensions in an infinite shear field, and included hydrodynamic and short-ranged repulsive forces between the particles. Steady-state results and model fit are emphasized.
50(2006); http://dx.doi.org/10.1122/1.2209090View Description Hide Description
Several compounds were prepared by melt compounding a metallocene linear low density polyethylene with a graft-modified polyethylene and an organically modified clay in different proportions in a twin screw extruder. X-ray diffraction,transmission electron microscopy, and rheology were used to characterize the extent of intercalation of the silicate galleries. Incorporation of minimum weight fraction of 2% clay and 50% maleated polyethylene was found to introduce radical changes in the rheological behavior of the nanocomposites. Broadening peaks in x-ray diffraction indicated increasing dispersion of silicate galleries into polymer as a function of graft modification. With higher incorporation of maleated polyethylene, possible networking was seen with solidlike response beginning to appear at low frequencies. Flow activation energy was found to decrease with incorporation of clay. Relaxation spectra determined using a parsimonious model were found to extend to higher relaxation time scales with exfoliation. Finally, the Cox-Merz rule was found to fail over the whole range of shear rates for exfoliated compounds, indicating increased interactions in the matrix resulting in highly anisotropic distribution of individual silicate layers.
Evolution of rheological properties of highly concentrated emulsions with aging —Emulsion-to-suspension transition50(2006); http://dx.doi.org/10.1122/1.2206712View Description Hide Description
The effect of aging on the rheological properties and physical structure of highly concentrated water-in-oil emulsions with dispersed phase of is the subject of this study. It was proven by various experimental techniques that aging leads to the emulsion-to-suspension transition. Significant shift of rheological properties to the solid-like behavior is the result of the emulsionsaging, which shows itself as an increase of the storage modules with time as well as Newtonian viscositymeasured in the upward sweeping shear rate mode. Comparison of flow curves measured in the upward and downward sweeping shear rate modes shows that the rheopectic effect at low stresses is observed for both fresh and aged emulsions.Viscosity measurements in the downward mode demonstrate transition to the elastic-like behavior at low stresses with appearance of strongly pronounced yield stress. Dependencies of the characteristic rheological parameters on aging have been investigated by using different analytical methods such as optical microscope studies, differential scanning calorimetry measurements, and x-ray diffraction. All these methods showed that aging is associated with the process of slow crystallization of the dispersed droplets that comprise of overcooled highly concentrated aqueous solution, which can be considered as the main mechanism for the evolution of the emulsion's rheological properties. In this sense, the observed emulsion-to-suspension transition is the consequence of the phase transition. The kinetics of the transition process is described by the Johnson-Mehl-Avrami-Kolmogorov equation and correlates with the evolution of the rheological properties of the dispersion.
50(2006); http://dx.doi.org/10.1122/1.2206711View Description Hide Description
The start-up and steady uniaxial elongational viscosity have been measured for two monodisperse polystyrene melts with molecular weights of 52 and , and for three bidisperse polystyrene melts. The monodisperse melts show a maximum in the steady elongational viscosity vs. the elongational rate, , of about two times the limiting value of expected for a Newtonian fluid, whereas the bidisperse melts have a maximum of up to a factor of seven times the Trouton limit of . The Wiest model which incorporates anisotropic drag and finite extensibility may be used to interpret the results in molecular terms.
50(2006); http://dx.doi.org/10.1122/1.2208366View Description Hide Description
We describe a full-chain stochastic tube model for entangled melts and solutions of linear polymers. The model incorporates two forces that result from chain confinement: a tensile force along the chain that prevents chain collapse and a transverse force that keeps the chain within the virtual tube. The model tracks conjugate constraint pairs and utilizes a consistent approach to constraint release and renewal. Chain loops within the tube are permitted. A single time-scaling adjustable parameter is required. Model predictions are compared with published transient and steady-state data on two entangled polystyrene solutions.
50(2006); http://dx.doi.org/10.1122/1.2206706View Description Hide Description
We propose a model for the stress softening of isotropic, incompressible rubberlike materials. The model is derived from a micromechanical scheme of a polymericnetwork reinforced with fine filler particles, idealized as rigid, and connected by two different types of chains: elastic and breakable. The fraction of breakable chains, assigned through an appropriate distribution function, is responsible for the network alteration. This prototypical system is then extended to a three-dimensional model with isotropic stress softening. In order to illustrate this model, we discuss two explicit examples: the homogeneous deformation of uniaxial extension and the inhomogeneous deformation of azimuthal shear.
50(2006); http://dx.doi.org/10.1122/1.2206713View Description Hide Description
We have developed a new spring force law which can be used in bead-spring chain models of the worm-like chain. The bead-spring chain has no bending potentials between the springs, and so differs from the current models only in the functional form of the force law. This new model can accurately represent a chain that contains many persistence lengths even if each spring represents only a few persistence lengths. We also discuss the assumptions in the model and other sources of possible error. The new force law significantly reduces the error compared with using the Marko-Siggia interpolation formula.
Dynamic viscoelastic shear properties of soft matter by magnetic resonance elastography using a low-field dedicated system50(2006); http://dx.doi.org/10.1122/1.2207187View Description Hide Description
A magnetic resonanceelastography (MRE) based method that consists in measuring dynamic viscoelastic parameters of soft matter by analysis of propagating shear waves at different frequencies is proposed. Dynamic shear tests were performed on soft gels with a dedicated magnetic resonance imaging system at low field and were compared with results obtained with a mechanical rotational rheometer. Storage and loss moduli were plotted against frequency with both methods and good agreement was found between their results in the shared frequency range. Therefore, it is shown that the described method could be a reliable and accessible tool for three-dimensional dynamic mechanical analysis on soft matter able to overcome dynamic range, sample dimensions and directional limitations of mechanical rheometers. Advantages and limits of the method are discussed.
50(2006); http://dx.doi.org/10.1122/1.2206716View Description Hide Description
In this work, the effect of three different additives (oleic acid, aluminum stearate, and silicananoparticles) on the aggregation,sedimentation, and redispersibility of concentrated iron-based magnetorheological fluids was investigated. With this aim, the sedimentation behavior was analyzed using an electromagnetic induction method, which is suitable for studying the sedimentation of opaque magnetic suspensions. The redispersibility was studied, in a quantitative way, by means of rheological measurements, both in the presence and in the absence of external magnetic field. For this purpose, samples were subjected to a constant shear stress at different moments (steps) of the settling process. The time evolution of the corresponding shear rate was measured at each step. Interestingly, it was found that although the addition of oleic acid or aluminum stearate does not avoid particle settling, the redispersibility of the suspensions is considerably enhanced. On the contrary, silicananoparticles behave as a gel-forming agent capable of preventing particle settling under rest conditions. Unfortunately, when iron-silica suspensions are sheared compact sediments are progressively formed, making the redispersion extremely difficult.
50(2006); http://dx.doi.org/10.1122/1.2206715View Description Hide Description
The Gaussian Approximation, proposed originally by Öttinger [J. Chem. Phys., 90, 463–473 (1989)] to account for the influence of fluctuations in hydrodynamic interactions in Rouse chains, is adapted here to derive a new mean-field approximation for the FENE spring force. This FENE-PG force law approximately accounts for spring-force fluctuations, which are neglected in the widely used FENE-P approximation. The Gaussian Approximation for hydrodynamic interactions is combined with the FENE-P and FENE-PG spring force approximations to obtain approximate models for finitely extensible bead-spring chains with hydrodynamic interactions. The closed set of ordinary differential equations governing the evolution of the second moments of the configurational probability distribution in the approximate models is used to generate predictions of rheological properties in steady and unsteady shear and uniaxial extensional flows, which are found to be in good agreement with the exact results obtained with Brownian dynamics simulations. In particular, predictions of coil-stretch hysteresis are in quantitative agreement with simulations’ results.