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Journal of RheologyVolume 45, Issue 5 (September-October 2001) |
Contents
Improved model of non-affine strain measureS.
T. Milner AbstractA 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 non-affinely 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 Q = C-a/tr(C-a) with a = 0.7, where C-l is the Finger tensor. The resulting second normal stress ratio Y = - N2/Nl is (1 - a)/2 = 0.15. The original Doi-Edwards strain measure is well described except for the second normal stress by this same function with a = 0.8. Influence of gelation on particle shape in sheared biopolymer blendsBettina Wolf, William J. Frith,
and Ian T. Norton AbstractThe morphology of biopolymer droplets 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 analysed and compared against models for affine deformation conditions (Janssen, 1997), and steady shear conditions (Maffetone & 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. The effect of branching on the rheological properties of solutions of aromatic etherimide copolymersIbrahim Sendijarevic, Matthew W. Liberatore, and Anthony
J. McHugh Larry J. Markoskib) and Jeffrey S. Moorea,b) AbstractA series of AB/ AB2 etherimide copolymers of nearly constant weight-averaged molecular weight, synthesized from starting co-monomer compositions ranging from 0 - 1 mole fraction AB (xAB), were characterized. Zero-shear viscosity of various concentration solutions in N-methyl pyrrolidinone (NMP) showed a slight increase with xAB in the range 0.00 £ xAB £ 0.80, followed by a sharp rise at higher xAB. Likewise, solution flow birefringence showed negligible response for xAB < 0.80, followed by arise with xAB at higher fractions. The dilute concentration, zero shear viscosity and intrinsic viscosity correlated directly with the calculated distance between branches (lAB). The zero-shear viscosity , h0, 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 xAB. The concentration dependence of h0 also scaled with the product of the concentration and intrinsic viscosity (c[h0]), indicating that, over the range of concentrations studied, the architectural dependence of the viscosity is well described by [h0]. Likewise, increased viscoelastic effects were observed with xAB in steady shear and oscillatory flows. Taken together, these results clearly indicate a transition in the dynamics from unentangled, hyperbranched polymer (HBP) behavior to an entangled, linear-like behavior occurs at a critical starting monomer composition, xAB ~ 0.80. Phase-separated biopolymer mixture rheology: Prediction using viscoelastic emulsion modelJ. R. Stokes, B. Wolf, and W. J. Frith AbstractThe relationship between the morphology and rheology of phase-separated biopolymer mixtures is investigated. Biopolymer mixtures, which are utilised 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 leads to products with novel microstructures and material properties (Wolf et al. 2000). An emulsion model (Palierne 1990), commonly used for the prediction of the linear viscoelastic properties 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, characterised 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 pre-shear rate such that the radius of droplets after a preshear of 10 s-1 is around 20 to 50 mm while after a preshear of 100 s-1 the droplets are typically less than 10 mm. 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 subject to preshear rates of between 1 and 100 s-1. The interfacial tension for the gelatin-maltodextrin system studied is approximately 50 mN/m at 60°C. c) To whom correspondence should be addressed. E-mail: tordjeman@ldv.univ-montp2.fr Micellar structure changes in aqueous mixtures of nonionic surfactantsLiang Quo and Ralph H. Colbya) Min Y. Lin Gregory P. Dadob) AbstractRheology and small-angle neutron scattering are used to probe the structure of nonionic surfactant mixtures in water. Small amounts of a C14 diol (SurfynolÒ 104) cause enonnous structural and rheological changes when added to aqueous solutions of an ethylene oxide-propylene oxide-ethylene oxide triblock copolymer (PluronicÒ P105). The C14 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 copolymer micelles and causes a cascade of changes in micelle structure, with resultant changes in rheology. Particularly striking is the spherical to wormlike micelle transition, where the viscosity changes by a factor of more than 104. a) To whom correspondence should be
addressed, e-mail: rhc@plmsc.psu.edu. Experimental observation and numerical simulation of transient 'stress fangs' within flowing molten polyethyleneK. Lee and M. R. Mackley T. C. B. McLeish and T. M. Nicholson O. G. Harlen AbstractWe report experimental observations and matching numerical simulation for the time-dependent start-up flow of two molten polyethylenes (PEs) within a slit entry and exit geometry. For the case of a low density polyethylene (LDPE), an unexpected transient, birefringence 'stress fang' was observed downstream of the slit exit. The stress fang consisted of a localised region of stress concentration. The stress fang, however, was not observed for a linear low density polyethylene (LLDPE) sample subjected to the same processing condition. A matching time-dependent numerical simulation of the flow is also presented. Using a split Lagrangian-Eulerian method for simulating transient viscoelastic flow with the multi-mode pom-pom constitutive equation, the general features of the stress fangs were predicted for the LDPE. In addition, the simulation did not predict stress fangs for the LLDPE. The paper demonstrates that for this particular case the pom-pom model can successfully discriminate the complex flow behaviour of different PEs, and shows that the presence (or otherwise) of a stress fang is sensitive to the particular rheology of the polymer that arises from long chain branching. Transient molecular orientation and rheology in flow aligning thermotropic liquid crystalline polymersVictor M. Ugaz, Wesley R. Burghardt* Weijun Zhou, Julia A. Kornfield AbstractQuantitative measurements of molecular orientation and rheology are reported for various transient shear flows of a nematic semi-flexible 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, 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. * Corresponding author: w-burghardt@northwestem.edu, 847-467-1401 (phone), 847-491-3728 (fax). The effects of interparticle interactions and particle size on reversible shear thickening: Hard-sphere colloidal dispersionsBrent J. Maranzano and Norman J. Wagner* AbstractA comparison between the effects of two colloidal stabilizing methods (electrostatic versus Brownian) on the reversible shear thickening transition in concentrated colloidal suspensions 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 (CHNO3 » .1 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. * Corresponding author. Email: wagner@che.udel.edu Thermorheological behavior of magnetic dispersionsB. Chae, A.
M. Lane, and J. M. Wiest AbstractWe examine the temperature dependence of the steady and oscillatory shear flow properties of magnetic dispersions. The dispersions ¾ characteristic of those used in the production of flexible magnetic data storage media ¾ 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 non-magnetic ¾ but otherwise identical ¾ particles we confirm that the temperature independence is a consequence of the rheological behavior being governed by temperature independent interparticle magnetic forces. Effect of humic acid adsorption on the rheological properties of sodium montmorillonite suspensionsM. M. Ramos-Tejadaa), J. de Vicenteb), A. Ontiverosa), and J. D. G. Duránb)* a)
Department of Applied Physics b)
Department of Applied Physics AbstractIn 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 z-potentials of edges and faces, zedge ´ zface, was demonstrated. This indicates that the rheological behavior of NaMt suspensions is largely controlled by electrostatic interactions. Adsorption of HA also changes significantly the viscoelastic properties of the suspensions, as determined by both oscillometric and creep-recovery tests. A trend was observed of NaMt suspensions from approximately solid-like when no HA is adsorbed, to almost liquid-like when HA concentration is in the range 50-100 mg/l. * Corresponding author: J. D. G. Durán, Departamento de Física Aplicada, Facultad de Ciencias, Universidad de Granada, 18071 Granada, Spain. Fax: +34 958 243214. Tel: +34 958 246103. E-mail: jdgarcia@ugr.es Rheology of aging, concentrated, polymeric suspensions: Application to pasty sewage sludgesJean-Christophe Baudez Philippe Coussot* AbstractThe 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 fairly well be 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. * To whom correspondence should be sent. A slotted plate device for measuring static yield stressLixuan Zhu,
Ning Sun, Kyriakos Papadopoulos, and Daniel De Kee* AbstractA 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 measuring surface 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 % TiO2) 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 non-uniform stress distribution along a virtual cylindrical surface. Yield stress values of TiO2 suspensions were compared with the values obtained via a variety of other methods, including indirect extrapolation from steady-shear data, vane creep testing, and vane stress-ramp measurements using an SR-5000 rheometer. Very small yield stress (up to ~ 10-4 Pa) measurements of low-concentration bentonite suspensions (2, 3, 5 wt %) could be determined only with the slotted-plate device. The vane method could not measure yield 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. * Corresponding author. Email: ddekee@tulane.edu Simulation of film blowing including flow-induced crystallizationAntonios K.
Doufasa) and Anthony J. McHughb) AbstractThe 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 "quasi-cylindrical" 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. a) Current address: The Dow Chemical
Company, M. E. Pruitt Research Center, 1702 Building, Midland MI 48674 Normal stresses and free surface deformation in concentrated suspensions of noncolloidal spheres in a viscoelastic fluidIsidro E. Zarragaa), Davide A. Hill, and David
T. Leighton, Jr.b) AbstractConcentrated suspensions of noncolloidal spheres in a constant viscosity elastic fluid were characterized rheologically using rotating plate viscometers and profilometry of the suspension surface 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 Nl was found to be positive and the second normal stress difference N2 negative. Although the magnitude of Nl and N2 increased with the solids volume fraction f, in general the ratio |Nl/N2| 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 (f ³ 0.3). The magnitude of N2 at high concentrations approached that measured for similar suspensions in Newtonian fluids, while the magnitude of Nl could be attributed to the viscoelasticity of the suspending fluid. Measurements of the surface deformation 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. a) Present address: 3M Center, Building
270-2N-14, St. Paul, MN 55144-1000 |
Please e-mail suggestions and comments to albertco@umche.maine.edu. Updated 25 January 2004 |