Volume 45, Issue 5, September 2001
Index of content:
45(2001); http://dx.doi.org/10.1122/1.1378031View Description Hide Description
A new calculation of the strain measure for entangled polymers is presented, in which the entanglement network is modeled as a set of entanglement points to which are attached four entanglement strands, randomly oriented in equilibrium. The network deforms nonaffinely to maintain a net zero force on each entanglement point, following a recent suggestion of Marrucci. The resulting strain measure in the case of uniaxial and biaxial extension as well as simple shear is well described by with where is the Finger tensor. The resulting second normal stress ratio is The original Doi–Edwards strain measure is well described except for the second normal stress by this same function with
Transient molecular orientation and rheology in flow aligning thermotropic liquid crystalline polymers45(2001); http://dx.doi.org/10.1122/1.1389317View Description Hide Description
Quantitative measurements of molecular orientation and rheology are reported for various transient shear flows of a nematic semiflexible copolyether. Unlike the case of lyotropic liquid crystalline polymers(LCPs), whose structure and rheology in shear are dominated by director tumbling, this material exhibits flow aligning behavior. The observed behavior is quite similar to that seen in a copolyester that we have recently studied [Ugaz and Burghardt (1998)], suggesting that flow aligning dynamics may predominate in main-chain thermotropes that incorporate significant chain flexibility. Since the flow aligning regime has received little attention in previous attempts to model the rheology of textured, polydomain LCPs, we attempt to determine whether available models are capable of predicting the orientation and stress response of this class of LCP. We first examine the predictions of the polydomain Ericksen model, an adaptation of Ericksen’s transversely isotropic fluid model which accounts for the polydomain distribution of director orientation while neglecting distortional elasticity. This simple model captures a number of qualitative and quantitative features associated with the evolution of orientation and stress during shear flow inception, but cannot cope with reversing flows. To consider the possible role of distortional elasticity in the re-orientation dynamics upon reversal, we evaluate the mesoscopically averaged domain theory of Larson and Doi [Larson and Doi (1991)], which incorporates a phenomenological description of distortional elastic effects. To date, their approach to account for polydomain structure has only been applied to describe tumbling LCPs. We find that it captures the qualitative transient orientation response to flow reversals, but is less successful in describing the evolution of stresses. This is linked to the decoupling approximation adopted during the model’s development. Finally, a modified polydomain Ericksen model is introduced that provides some of the benefits of the Larson–Doi model while offering more realistic stress predictions.
Normal stresses and free surface deformation in concentrated suspensions of noncolloidal spheres in a viscoelastic fluid45(2001); http://dx.doi.org/10.1122/1.1396356View Description Hide Description
Concentrated suspensions of noncolloidal spheres in a constant viscosityelastic fluid were characterized rheologically using rotating plate viscometers and profilometry of the suspensionsurface deflection near a rotating rod. It was found that the relative viscosity was quantitatively consistent with a previously determined correlation for suspensions based on Newtonian fluids. Moreover, the first normal stress difference was found to be positive and the second normal stress difference negative. Although the magnitude of and increased with the solids volume fraction φ, in general the ratio decreased as loading increased. Analysis of the normal stress data suggests that the rheological contribution of the solids microstructure was in large part independent of that of the dissolved polymers at high solids loading The magnitude of at high concentrations approached that measured for similar suspensions in Newtonian fluids, while the magnitude of could be attributed to the viscoelasticity of the suspending fluid. Measurements of the surfacedeformation of the suspension near a rotating rod at different concentrations and shear rates exhibited three different types of deflection: pure rod climbing, pure rod dipping, and a combination of the two, with an upward climb near the rod and a downward deflection further away from the rod. These observations were found to be qualitatively consistent with the rheological measurements conducted in rotating plate viscometers.
45(2001); http://dx.doi.org/10.1122/1.1392300View Description Hide Description
The two-phase microstructural flow-induced crystallization model developed by the authors is applied to the simulation of film blowing. In order to isolate the effects due to crystallinity, a simplified “quasicylindrical” approximation is used for the momentum equations, which neglects the effect of axial curvature in the axial direction. The present simulations include the combined effects of flow-induced crystallization,viscoelasticity, and bubble cooling. In all cases studied, the location of the frost line is predicted naturally as a consequence of flow-induced crystallization. The effects of inflation pressure, melt extrusion temperature, and take-up ratio on the bubble shape are predicted to be in agreement with experimental observations. The combination of these processing conditions determines the shape of the bubble, i.e., whether the bubble contracts or expands. An important feature of our model is the prediction of the locked-in system stresses at the frost line that are related to the physical and mechanical properties of the film.
45(2001); http://dx.doi.org/10.1122/1.1392299View Description Hide Description
A slotted-plate device was constructed with a balance and a linear-motion platform to directly measure static yield stresses of suspensions by moving the plate in the suspension in a similar mode as is done in the well-known Wilhelmy-plate technique for measuringsurface tension. Wall effects associated with the original plate yield-stress instrument [De Kee et al. (1980)] were minimized by opening a series of slots on the plates. Yield-stress experiments were conducted on both high-concentration (40, 50, 60, and 70 wt % and low-concentration (2, 3, and 5 wt % bentonite) aqueous suspensions. The new setup avoids the disadvantages of the vane instrument, possible secondary flow between the blades as well as a nonuniform stress distribution along a virtual cylindrical surface. Yield stress values of suspensions were compared with the values obtained via a variety of other methods, including indirect extrapolation from steady-shear data, vane creeptesting, and vane stress-ramp measurements using an SR-5000 rheometer. Very small yield stress (up to Pa) measurements of low-concentration bentonite suspensions (2, 3, and 5 wt %) could be determined only with the slotted-plate device. The vane method could not measureyield stress values of bentonite suspensions of less than 7 wt % concentration. Relaxation tests on high-concentration suspensions indicated that these suspensions may not be purely elastic below yield stress.
45(2001); http://dx.doi.org/10.1122/1.1392298View Description Hide Description
The rheological behavior of pasty sewage sludges from different origins and at different ages of fermentation has been studied on the basis of careful rheometrical tests. These materials appear to be basically yield stress fluids whose flow curves can be fairly well represented by a Herschel–Bulkley model. The yield stress existence is evidenced by a clear transition between a viscoelastic and a viscous behavior at a critical shear stress. The rheological parameters are shown to mainly depend on the organic fraction and the time of fermentation. By extracting the main solids components (minerals, proteins,lipids, carbohydrates) we show that the behavior evolution is governed by the synthesis of volatile fatty acids.
45(2001); http://dx.doi.org/10.1122/1.1389312View Description Hide Description
The morphology of biopolymerdroplets gelled under shear as the dispersed phase of a two-phase mixture of biopolymers in aqueous solution (or “biopolymer blend”) was investigated experimentally. The process involves shearing the liquid biopolymer blend either at a steady stress till gelation, for creation of ellipsoidal gelled particles, or a stress step-up process for creation of high aspect ratio particles. The resulting particle shapes are analyzed and compared against models for affine deformation conditions [Janssen (1997)] and steady shear conditions [Maffetone and Minale (1998)]. Although they are only strictly valid for Newtonian fluid phases, application of these models to a situation where one phase is gelling, has provided insight into the morphological changes occurring in the shear-gelation process.
45(2001); http://dx.doi.org/10.1122/1.1392297View Description Hide Description
In this work we analyze the rheological behavior of Na-montmorillonite (NaMt) suspensions in the presence of humic acid. The analysis starts from the fact that the electric charges on plate and edge surfaces of clay particles largely determine the formation of three-dimensional structures in suspensions. Zeta potential data of NaMt as a function of humic acid (HA) concentration suggest that its adsorption takes place preferentially on the edges of the particles. A good correlation between the parameters describing the viscosity-shear rate or viscosity-shear stress dependencies and the product of the ζ potentials of edges and faces, is demonstrated. It indicates that the rheological behavior of NaMt suspensions is largely controlled by electrostatic interactions. Adsorption of HA also changes the viscoelastic properties of the suspensions significantly as determined by both oscillometric and creep-recovery tests. A trend of NaMt suspensions from approximately solid like when no HA is adsorbed, to almost liquid like when the HA concentration is in the range of 50–100 mg/l is observed.
45(2001); http://dx.doi.org/10.1122/1.1389314View Description Hide Description
The relationship between the morphology and rheology of phase-separated biopolymer mixtures is investigated. Biopolymer mixtures, which are utilized in the food industry for their textural and structuring properties, often phase separate and demix to form water-in-water emulsions.Controlling the morphology of biopolymer mixtures during flow processing and inducing gelation of one or both phases lead to products with novel microstructures and material properties [B. Wolf et al., Food Hydrocolloids 14, 217–225 (2000)]. An emulsion model [J. F. Palierne, Rheol. Acta 29, 204–214 (1990)], commonly used for the prediction of the linear viscoelasticproperties of polymer blends, is used here to relate the rheology to the morphology of water-in-water emulsions. The system under investigation is a gelatin–maltodextrin mixture which phase separates at 60 °C for particular concentrations, characterized by a binodal curve, into a gelatin-rich and maltodextrin–rich phase. Emulsions with phase volumes of 10% and 30% were examined with either phase as the dispersed phase. The morphology varies with the preshear rate such that the radius of droplets after a preshear of 10 s−1 is around 20–50 μm while after a preshear of 100 s−1 the droplets are typically less than 10 μm. Despite the low viscosity, elasticity, and interfacial tension of the gelatin–maltodextrin emulsion, the emulsion model is found to predict the rheology and morphology of the mixtures subjected to preshear rates of 1–100 s−1. The interfacial tension for the gelatin–maltodextrin system studied is approximately 50 μN/m at 60 °C.
45(2001); http://dx.doi.org/10.1122/1.1392296View Description Hide Description
We examine the temperature dependence of the steady and oscillatory shear flow properties of magnetic dispersions. The dispersions—characteristic of those used in the production of flexible magnetic data storage media—consist of acicular ferromagnetic metal oxide particles suspended in a solution of low molecular weight polymer and organic solvent. Although the viscosity of the suspending fluid is independent of shear rate and displays traditional temperature dependence, the viscosity of the dispersion is dramatically shear thinning and effectively independent of temperature. The small-amplitude oscillatory shear flow material functions are also nearly independent of temperature. By examining dispersions wherein part of the magnetic particles are replaced with nonmagnetic—but otherwise identical—particles we confirm that the temperature independence is a consequence of the rheological behavior being governed by temperature-independent interparticle magnetic forces.
The effects of interparticle interactions and particle size on reversible shear thickening: Hard-sphere colloidal dispersions45(2001); http://dx.doi.org/10.1122/1.1392295View Description Hide Description
A comparison between the effects of two colloidal stabilizing methods (electrostatic versus Brownian) on the reversible shear thickening transition in concentrated colloidalsuspensions is explored. Five suspensions of monodisperse silica are synthesized via the Stöber synthesis and dispersed in an index matched organic solvent to minimize van der Waals interactions. The residual surface charge is neutralized with nitric acid M) resulting in a near hard-sphere interaction that is confirmed by small angle neutron scattering measurements across a range of volume fractions. Rheological measurements demonstrate the effects of neutralization on the low shear and high shear rheology, which show that the onset of shear thickening moves to lower applied shear stresses and scales inversely with particle size cubed, in agreement with theory. Quantitative comparisons of both the low shear viscosity and the critical stress for shear thickening to predictions for hard spheres and literature data demonstrate the extreme sensitivity of high shear rheology to the surface properties in concentrated suspensions.
45(2001); http://dx.doi.org/10.1122/1.1389315View Description Hide Description
Rheology and small-angle neutron scattering are used to probe the structure of nonionic surfactant mixtures in water. Small amounts of a diol (Surfynol® 104) cause enormous structural and rheological changes when added to aqueous solutions of an ethylene oxide-propylene oxide-ethylene oxide triblock copolymer (Pluronic® P105). The diol is only soluble up to 0.1 wt % in pure water, but can be added in large quantities to aqueous solutions of the copolymer. The hydrophobic diol incorporates into the existing copolymermicelles and causes a cascade of changes in the micelle structure, with resultant changes in rheology. Particularly striking is the spherical to worm-like micelle transition, where the viscosity changes by a factor of more than
45(2001); http://dx.doi.org/10.1122/1.1389313View Description Hide Description
A series of etherimide copolymers of nearly constant weight-averaged molecular weight, synthesized from starting comonomer compositions ranging from 0 to 1 mole fraction were characterized. Zero-shear viscosity of various concentration solutions in N-methyl pyrrolidinone showed a slight increase with in the range of followed by a sharp rise at higher Likewise, the solution flowbirefringence showed negligible response for followed by a rise with at higher fractions. The dilute concentration, zero shear viscosity, and intrinsic viscosity correlated directly with the calculated distance between branches The zero-shear viscosity, exhibited a linear dependence on concentration in the dilute regime for the entire series of branched polymers and a power law dependence in the concentrated regime, with the coefficient increasing with The concentration dependence of also scaled with the product of the concentration and intrinsic viscosity indicating that, over the range of concentrations studied, the architectural dependence of the viscosity is well described by Likewise, increased viscoelastic effects were observed with in steady shear and oscillatory flows. Taken together, these results clearly indicate a transition in the dynamics from unentangled, hyperbranched polymer behavior to entangled, linear-like behavior occurs at a critical starting monomer composition,