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Journal of RheologyVolume 44, Issue 4 (July-August 2000) |
Contents
Influence of fluid elasticity on drops impacting on dry surfacesRegan Crooks and David V. Boger AbstractThe impact of Newtonian and non-Newtonian fluids on surfaces of varying and known roughness is studied experimentally using a high-speed drum camera to make observations at 1,000 frames per second. A new technique was devised to establish when splash occurs on a hydrophilic surface. Experimental results for the onset of splash for Newtonian fluids agree with those of previous authors. The influence of elasticity on the onset of splash was examined using a carefully constructed group of elastic fluids of constant and equal viscosity. These constant viscosity elastic liquids were constructed by varying the polymer concentrations and Newtonian solvent concentrations to maintain a nearly constant shear viscosity and equilibrium surface tension. The apparent extensional viscosities for these materials increased dramatically with molecular weight. Great care was taken to eliminate the influence of dynamic surface tension. Photographs of the elastic fluid drops impinging on metal surfaces showed that the dilute polymer solutions exhibited splash and that the splash threshold increased with increasing Rouse relaxation time. All data obtained for the onset of splash can be collapsed onto a single functional relationship between the threshold for splashing and a dimensionless roughness parameter. Scaling behavior: Effect of precursor concentration and precursor molecular weight on the modulus and swelling of polymeric networksKavitha Sivasailarn and Claude Cohen* *Corresponding Author AbstractEnd-linked poly(dimethylsiloxane) (PDMS) networks, containing a minimal number of defects, are prepared from vinyl-terminated PDMS chains in the presence of a theta (θ) solvent. Unreactive tri-methyl terminated PDMS of the same molecular weight distribution as the reactive PDMS precursors were chosen as the θ-solvent. Four precursor molecular weights ranging from 10,000 to 100,000 daltons were studied. The dependence of the initial modulus after cure, the dry modulus after solvent extraction and the degree of equilibrium swelling on precursor concentration during cure were compared to recent scaling predictions. The experimental scaling exponents were found to be strong functions of the molecular weight of the precursor chains and their values approach the theoretical predictions for high molecular weight precursors. The relationship between the modulus of the fully swollen network and the degree of equilibrium swelling is found to be independent of precursor molecular weight and precursor concentration during cure and in better agreement with the corresponding, more robust, theoretical prediction. Network structure dependence of volume and glass transition temperatureJeffry J. Fedderly*, Gilbert F. Lee, John D. Lee,
Bruce Hartmann Karel Duek, Miroslava Duková-Smr_ková, Ján
omvársky AbstractA series of polyurethanes was used to determine the molar contributions of chain ends (CE) and branch points (BP) to free volume and glass transition temperature (Tg). The polyurethanes were copolymers of diphenylmethane diisocyanate (MDI) and poly(propylene oxide) (PPO), with hydroxyl functionalities of one, two, and three. The equivalent weights of all the PPOs were equal, such that the chemical composition of the chain segments was essentially identical. Therefore, the only distinctions among polymers were differences in CE and BP concentration. Theory of branching processes computer simulations were used to determine the concentration of CE due to imperfect network formation. Other CE contributions were from the monofunctional PPO. Polymer volumes and Tg's were correlated to CE and BP concentrations, and the contributions of these species were determined from least squares fits. The molar volume and Tg contributions were then used to determine free volume thermal expansion coefficients. These values were compared to thermal expansion coefficients obtained from WLF parameters obtained from the measurement of dynamic moduli as a function of temperature. Effect of interparticle forces on shear thickening of oxide suspensionsGeorge V. Franks*, Zhongwu Zhou, Nanda J. Duin,
and David V. Boger *Corresponding Author. Present address: AbstractThe rheological behavior of concentrated nearly monodisperse, nearly spherical silica and narrow particle size distribution alumina suspensions was measured in Couette geometry. Attractive forces between the particles (suspensions flocculated at or near the iso-electric point) produce high viscosities. Repulsive forces between the particles produce dispersed suspensions and decrease the viscosity at low and intermediate shear rates. The viscosity of the dispersed suspensions shear thickens (abruptly increases and approaches that of the flocculated suspensions) at a critical shear rate (and therefore stress) provided the volume fraction of solids of the suspension is sufficiently high. As the pH is adjusted farther from the iso-electric point the critical shear rate (and stress) increase. At pH away from the iso-electric point the addition of salt decreases the critical shear rate (and stress). Thus shear thickening is not only dependent upon hydrodynamic interactions but also depends on surface forces, in particular repulsive interparticle forces. Increasing the magnitude and range of the repulsive forces increases the shear stress (and rate) at which shear thickening starts. Reducing the magnitude of the repulsive force allows the particles to be more easily forced into clusters resulting in shear thickening. Recoverable creep compliance properties of associative model polymer and polyoxyethylene solutionsDonald J. Plazek Zane N. Frund AbstractHydrophobically-modified polymers are composed of water soluble and water insoluble (hydrophobic alkyl end-groups) components. They are referred to as associative polymers since their aqueous solutions exhibit enhanced viscosities due to the aggregation of their alkyl end-groups into micelle-like structures. The aggregation is attributed to the rejection of the hydrophobic end-groups by the water. The rheological properties including the creep recovery behavior of associative model polymer solutions were determined as a function of molecular weight, solution concentration, shear rate, temperature and solvent type in this investigation. These polymers are composed of a linear water soluble polyoxyethylene backbone chain extended with isophorone diisocyanate and end-capped with hexadecyl groups, and have molecular weights ranging from 1.7×104 to 8.4×104 grams/mole. For comparison purposes, poly(oxyethylene) resins of various molecular weights were also evaluated. A previously obscured mechanical terminal dispersion, which diminishes with increasing polymer concentration and decreasing temperature, has been clarified. Structure vs rheological properties in fibrillar thermoreversible gels from polymers and biopolymersJean-Michel Guenet AbstractIn this paper is discussed the evolution of the elastic modulus as a function of polymer concentration in light of the molecular structure for a series of thermoreversible gels prepared from synthetic polymers and biopolymers. The gels under consideration all possess fibrillar morphology. It will be shown that Jones and Marques' theory derived for rigid networks is quite suitable for casting some light on the structure-property relation rather than the percolation approach. Simulations of fiber flocculation: Effects of fiber properties and interfiber frictionChristian F. Schmid, Leonard H. Switzer, and
Daniel J. Klingenberg AbstractNon-Brownian fibers commonly flocculate in flowing suspensions at relatively low concentrations (< 1% by weight). We have developed a particle-level simulation technique modeling fibers as chains of rods connected by hinges to probe fiber flocculation. The model incorporates fiber flexibility, irregular fiber equilibrium shapes, and frictional fiber interactions. Model fibers reproduce known orbits of isolated rigid and flexible fibers in shear flow. Simulation predictions of first normal stress differences in homogeneously dispersed, dilute flexible fiber suspensions agree with experimental data. Fiber features such as flexibility and irregular equilibrium shapes strongly impact single fiber and suspension behavior. Fibers aggregate in simulations with interfiber friction, in the absence of attractive forces between fibers. Strong flocculation is observed in suspensions of stiff fibers with irregular equilibrium shapes. Flocs contain many fibers with three or more contact points, and derive cohesiveness from elastic energy held in fibersconsistent with the elastic interlocking mechanism of flocculation. At higher concentrations (nL3 ť 100, where n is the fiber number density and L is the fiber length), coherent fiber networks form in simulations. Average numbers of contacts per fiber and contact force magnitudes in sheared and static networks are compared with existing fiber network theory predictions. Rheological models based on the double reptation mixing rule: The effects of a polydisperse environmentFrédéric Léonardi, Jean-Charles Majesté, Ahmed Allal,
and Gérard Marin* *Corresponding Author. E-mail: gerard.marin@univ-pau.fr AbstractWe present a comparison of various rheological models based on the double reptation concept, which relate linear viscoelastic data to molecular weight distribution (MWD), in order to determine the most efficient way to describe polydispersity effects. We have used for this study a two-step process. First, we have performed a systematic comparison of the predictions of various models with experimental data already published in the literature for binary blends of polystyrene (PS) and polymethylmethacrylate (PMMA). Then, we have compared the values of the complex shear modulus derived from these molecular models with rheological data obtained on "commercial" polydisperse polymers (PS, PMMA and HDPE). It is shown that only the two models which explicitly take into account the effects of the polydisperse surrounding of a macromolecular chain through "tube renewal" effects, are able to describe correctly the polydispersity effects on zero-shear viscosity and steady state compliance. A simple strain measure for entangled polymersGiuseppe Marrucci, Francesco Greco, and Giovanni Ianniruberto AbstractIn the classical theory of Doi and Edwards for entangled polymers the strain measure Q arises from the assumption of affine deformation of the tube segments. On the other hand, we have recently argued [G. Marrucci, F. Greco, and G. Ianniruberto, "Possible role of force balance on entanglements", Macromolecular Symposia (in press)] that force balance on the nodes of the entangled network generates departures from affinity. An ad hoc 'lattice' model of the network was then proposed to fulfil force balance automatically, at least for single step deformations. The corresponding strain measure is linked to the square root of the Finger tensor, and is therefore easy to use. Predictions obtained with the new strain measure are here compared with a variety of step strain data. Rheology of F-actin solutions determined from thermally-driven tracer motionT. G. Mason1, T. Gisler2, K. Kroy3, E. Frey4, and D. A. Weitz4 1
Corporate Strategic Research, ExxonMobil Research and Engineering Co. 2
Universität Konstanz, Fakultät für Physik 3
Ecole Sup. de Physique et Chimie Industrielles 4
Department of Physics and DEAS, Harvard University AbstractWe report measurements of the frequency-dependent complex shear modulus of semidilute F-actin solutions based on optical observations of the thermally-excited motion of monodisperse tracer microspheres. Because the tracer spheres cause incident laser light to be strongly scattered, we determine their average motion using diffusing wave spectroscopy (DWS). From the measured mean square displacement, we extract the retardation spectrum of the actin solution using a regularized fit based on a discretized model involving a linear superposition of harmonically bound Brownian particles. At an actin concentration of C = 1.2 mg/ml and for microspheres of radius a = 0.8 μm, we find that the complex modulus exhibits a dominant low frequency plateau modulus and a high frequency rise with the loss modulus dominating above a crossover frequency. Over a limited range of frequencies well above the crossover frequency, the magnitude of the high frequency storage modulus, G'(ω), is consistent with the power law scaling ω3/4. The observed gradual crossover appears to be at odds with previous theoretical predictions, but it corresponds to a simple structure of the retardation spectrum. A comparison of the rheology of reactive filled systems using lubricated squeezing flowJ. A. Walberer and A. J. McHugh AbstractLubricated squeezing flow rheometry has been used to study the rheological behavior of highly filled polymer composites that stiffen as they are processed. Reactive systems consisting of a crosslinking polymer phase or a polymerizing polymer phase filled with glass beads up to 65% by volume were prepared in a banbury mixer and compared with non-reactive systems of up to 65% filled polydimethylsiloxanes (PDMS) of differing molecular weights. The torque time profile from banbury mixing shows a linear rise in torque with time for the crosslinking system while the polymerizing system shows a linear followed by power law rise in torque with time. The relaxation modulus following a step strain was measured for the reactive systems at various mixing times. Comparisons of the normalized relaxation moduli of the reactive and non-reactive materials suggest similarities between the effect of filler amount and crosslinking amount and between the combined effects of molecular weight and filler amount and the degree of polymerization. The viscosity in biaxial extension was also measured for each system. The biaxial extensional viscosity behavior of the filled PDMS materials with extension rate was similar to that reported in shear while the behavior of both of the reactive systems was similar. A plastic-viscoelastic constitutive equation developed by White and Tanaka was used to quantify trends in the viscosity data. A flow-induced phase inversion in immiscible polymer blends containing a liquid-crystalline polymer studied by in-situ optical microscopyMarianne Astruc and Patrick Navard* *Corresponding Author AbstractThe phase inversion from a morphology of hydroxypropylcellulose in water (HPC50%) droplets in polydimethylsiloxane (PDMS) matrix to a morphology of PDMS droplets in HPC50% matrix can be induced by a change of shear rate, due to a viscosity ratio inversion. Such a process passing through four different transient morphological stages was studied by optical microscopy in a transparent shear device. In a certain concentration region, at a fixed shear rate, after sheets of PDMS were formed, the transition "hesitates" between phase inversion and refined starting morphology. The influence of PDMS concentration, shear rate, elasticity and phase dimension on the final morphology was investigated and compared with different models given in the literature. The influence of concentration and shear rate on the duration of the transient flow was also studied. We propose a simplified model of breakage of filaments to explain a part of our results. Generalization of the deformation field method to simulate advanced reptation models in complex flowE. A. J. F. Peters, A. P. G. van Heel, M. A.
Hulsen, and B. H. A. A. van den Brule AbstractIn this paper we will show how the recently introduced deformation field method [Peters et al. (1999) and van Heel et al. (1999)] can be generalized to complex flow simulations of non-time-strain separable integral constitutive models. To illustrate this generalization we start with the time-strain separable Doi-Edwards model and we show how the approach can be generalized to non-time-strain separable models As an example of such model we consider the reptation model which was recently introduced by Mead, Larson, and Doi [Mead et al. (1998)]. In this reptation model also tube-stretch and convective constraint release are taken into account. We use both models to simulate start-up of two-dimensional flow past a cylinder positioned between two parallel plates. Don't cry for me Charlie Brown or With compliance comes comprehensionDonald J. Plazek1 and Isabel
Echeverria2 AbstractThe different viscoelastic functions are, in theory, equivalent to each other, although the information that they emphasize is different. However, since viscoelastic measurements are never available over the entire time or frequency scale, and the results are not known with infinite precision, it is worthwhile to look at various of the otherwise equivalent viscoelastic functions. We present experimental data which show what appears to be an entanglement network plateau in the modulus, in non-entangled samples of Selenium and a low molecular weight polystyrene with a narrow distribution of molecular weights. We show how the confusion can be easily avoided by inspecting the compliances and the retardation spectrum. We also present data on the molecular weight dependence of the length of the plateau as determined from the dynamic storage modulus and from the retardation spectrum. We find that the retardation spectrum identifies the presence of an entanglement network of polymer chains, whereas at low molecular weights the storage modulus does not. The origin of the false entanglement plateaus is shown to arise from the reciprocal of steady-state recoverable creep compliance, Js, which is convoluted in the modulus functions but is additive in the compliances. This paper is dedicated to the memory of Charles Schulz in the name of all good souls searching for the truth. Effect of solvent quality and ions on the rheology and gelation of k-carrageenanSrividya Ramakrishnan and Robert K. Prud'homme AbstractThe rheology and conformational helix transition of κ-carrageenan in aqueous solutions of glycerol and sorbitol at low water activity are studied. Carrageenan forms a hydrated gel in water but a weakly flocculated network of partially hydrated particles in glycerol. The melting transitions of carrageenan in glycerol/water solutions show a single peak in the complex moduli at low glycerol concentrations, and two peaks when the glycerol concentration is higher than 40% (w/w). These are thought to be due to the breakage of hydrogen bonds (low temperature peak) and calcium ion dissociation (high temperature peak). The helix-coil transition is shifted to higher temperatures in solutions with higher glycerol or sorbitol content. The temperature of the conformational transition corresponds with that of gelation for all samples. However, there is more hysteresis between the cooling and heating curves (for both OR and rheology) at high water content. This suggests that gelation occurs through helix aggregation in water but the lower solubility of carrageenan results in a more random network of helices in low water activity solvents. Theory of linear viscoelasticity of cholesteric liquid crystalsAlejandro D. Rey AbstractThe theory of linear viscoelasticity of rod-like cholesteric liquid crystals subjected to small amplitude oscillatory shear flow is formulated and applied to the cholesteric helix along the flow, velocity gradient, and vorticity directions. Expressions for the zero and infinite frequency viscosities are derived and their ordering is predicted. Based on the classical ordering of the Miesowicz shear viscosities and anisotropies of torque coefficients, it is found that the largest (smallest) zero frequency viscosity obtains with the helix along the flow (gradient) direction. In addition, the difference between the zero and infinite frequency viscosities is found to be sensitive to the helix orientation, such that it is largest (smallest) when the helix is along the flow (gradient) direction. The complex viscosity corresponds to a viscoelastic material with a single relaxation time. The relaxation time depends on the Frank elastic constants involved in the deformation, such that when the helix is along the vorticity it is twist dependent, and splay-bend otherwise. The strength of the viscoelasticity is largest (smallest) when the helix is along the flow (gradient) direction. The hard-rod theory of Doi is used to confirm the predicted dependence of the strength of the viscoelastic response on the cholesteric helix orientation. Viscoelasticity of thermoreversible gelatin gels from mammalian and piscine collagensPaula M. Gilsenan and Simon B. Ross-Murphy* *Corresponding Author: AbstractThis paper describes the characterisation of a number of gelatin samples extracted from both mammalian (bovine and porcine) and fish (cod) collagens using oscillatory shear. The regime for gel formation, and characterisation is discussed, and subsequent gel melting behaviour is considered in the light of both Eldridge-Ferry and Takahashi models for the concentration dependence of the melting temperature. Frequency sweep data for some of the latter samples suggest that, although the junctions are more complex than simple entanglements, they have some features in common with entanglement networks compared to true gel networks (those with an equilibrium modulus). This has consequences on the longer-term behaviour of the gels, and suggests, for example, the possibility of gel healing. Brownian dynamics simulations of single DNA molecules in shear flowJoe S. Hur, Eric S. G. Shaqfeh Ronald G. Larson AbstractWe present the results of Brownian dynamics simulations of a series of different polymer models which have been used to examine the recent experimental facings of Smith et al. (l999) who studied the dynamics of a single DNA molecule in steady shear flow. The steady average extension at various Weissenberg numbers (Wi) is shown to be well predicted by multi-mode nonlinear models. Quite surprisingly, the normalized average extension (x/L) asymptotes to less than 1/2 even for extremely large Wi and we discuss this result on a physical basis. The probability density function of molecular extension at various values of Wi using the Kramer's chain and the FENE dumbbell suggests that the number of internal modes is important in a model designed to capture the dynamics of a real DNA molecule. Three different frequency regimes in the PSD observed at finite Wi in the experiments are shown to arise from the coupling of the Brownian fluctuations in the gradient direction and the convection in the streamwise direction. Our simulation results, especially in light of the excellent agreement with experiment, demonstrate the basic physical elements necessary for any rheological model to capture the dynamics of single polymer chains in flow. |
Please e-mail suggestions and comments to albertco@umche.maine.edu. Updated 25 January 2004 |