Showing papers in "Journal of Rheology in 2004"
TL;DR: In this article, it was shown that slip is characterized by universal scaling properties, which depend on solvent viscosity, bulk shear modulus, and particle size, and a model based on elastohydrodynamic lubrication between the squeezed particles and the shearing surface.
Abstract: Microgel pastes and concentrated emulsions are shown to exhibit a generic slip behavior at low stresses when sheared near smooth surfaces. The magnitude of slip depends on the applied stress. Well above the yield stress, slip is negligible compared to the bulk flow. Just above the yield stress, slip becomes significant and the total deformation results from a combination of bulk flow and slip. At and below the yield stress, the bulk flow is negligible and the apparent motion is entirely due to wall slip. By directly imaging the deformation of pastes and from rheological measurements, we show that slip is characterized by universal scaling properties, which depend on solvent viscosity, bulk shear modulus, and particle size. A model based on elastohydrodynamic lubrication between the squeezed particles and the shearing surface explains these properties quantitatively.
251 citations
TL;DR: In this paper, the authors applied two models to describe strain hardening of the viscosity during the course of elongation of long chain branched polypropylene and found that the model parameters vary systematically with the degree of branching and therefore their values can be used as a measure of this.
Abstract: In order to study the rheology of long chain branched polymers, branches have been added on linear polypropylene precursors in varying amounts using reactive modification with peroxydicarbonates. The branched polypropylene samples show distinct strain hardening, something absent from the linear melt; this considerably improves the melt strength of the modified polymer. The zero shear viscosity and the elasticity measured by the relaxation spectrum also increase with the number of branches per molecule. Two models are applied to describe strain hardening of the viscosity during the course of elongation. The model parameters were found to vary systematically with the degree of branching and, therefore, their values can be used as a measure of this. Consequently, fluidity, elasticity, strain hardening, and melt strength are all related to the degree of long chain branching.
150 citations
TL;DR: In this paper, the linear viscoelastic and steady shear flow properties of high phase volume suspensions of a range of agar microgel particles have been measured and are found to depend upon the deformability (or modulus) of the particles.
Abstract: The linear viscoelastic and steady shear flow properties of high phase volume suspensions of a range of agar microgel particles have been measured and are found to depend upon the deformability (or modulus) of the particles. Agar concentrations in the range 0.5-5 wt % are utilized, giving a range of particle modulus spanning 2.4-185 kPa. On increasing the particle modulus, in suspensions with phase volumes above maximum packing, the storage modulus increases by two orders of magnitude although the loss tangent (tan delta) also increases due to increasing viscous dissipation. The flow properties of the suspensions at high shear stresses also showed significant differences due to changing particle rigidity. The suspensions containing the hardest particles are found to display limited evidence of shear-thickening behavior at high stresses, while those containing the softest particles continue to shear thin. A high-shear plateau in the viscosity is observed for suspensions with particles of medium rigidity. The suspensions containing the stiffer particles also have a considerably higher viscosity for the same degree of space filling. Empirical expressions linking the viscoelastic and flow properties to the particle modulus have been derived. (C) 2004 The Society of Rheology.
147 citations
TL;DR: In this article, the role of interactions between close particles in the shear thickening of concentrated colloids is examined by using a Stokesian dynamics simulation of model systems, where the interactions are repulsive thermodynamic forces and lubrication forces.
Abstract: The role of interactions between close particles in the shear thickening of concentrated colloids is examined by using a Stokesian dynamics simulation of model systems. The interactions are repulsive thermodynamic forces and lubrication forces. Three different models are contrasted in their thickening behavior: Brownian spheres, polymer coated spheres, and Hookian spheres. Respectively, they show: a “mild” continuous thickening, a “strong” continuous thickening, and a strain thickening with loss of steady state. The relationship of order-disorder transitions and thickening is examined. Depending on the volume fraction and range of repulsive forces, thickening can be observed with or without an order-disorder transition at its onset. The different thickening responses arise from the dependence of the relaxation time of close particle contacts on interparticle gap. A time-scale based criterion for strong thickening is proposed and supported by the simulations. A simple theoretical model based the motion of a pair of particles leads to this criterion, but also predicts the mild thickening of Brownian spheres. It gives a simple fitting of flow curves which includes the details of the interparticle interactions.
141 citations
TL;DR: In this paper, the authors used the Palierne model with an interfacial shear modulus to analyze the dynamic response of polymer blends in which interfacial tension gradients induce an extra relaxation mechanism besides droplet relaxation.
Abstract: Shear-induced coalescence was studied in immiscible blends of polydimethylsiloxane (PDMS) and polyisoprene (PI) with a droplet-matrix morphology, using both rheology and scanning electron microscopy. Dynamic moduli of the blends compatibilized with different amounts of a PDMS–PI diblock were measured. The experimental results indicate that the blend response is characterized by two relaxation mechanisms. The general Palierne model with an interfacial shear modulus was used to analyze the data, since this model can describe the dynamic response of polymer blends in which interfacial tension gradients induce an extra relaxation mechanism besides droplet relaxation. Scanning electron microscopy was used to investigate the droplet size evolution in the blends during coalescence. For systems with a high amount of compatibilizer, it is shown that coalescence is completely suppressed under the conditions studied here.
139 citations
TL;DR: In this paper, the viscoelastic properties of coacervates were studied at various pH values, and it was shown that the viscosity of the WP/GA co-acervate exhibited a surprisingly low shear-rate dependence, especially at pH 4.0.
Abstract: Complex coacervation in whey protein (WP)/gum arabic (GA) mixtures occurred in a specific pH window between 2.5 and 4.8. After phase separation, a concentrated polymer (also called coacervate) phase was obtained, whose viscoelastic properties were studied at various pH values. The viscosity of the WP/GA coacervate exhibited a surprisingly low shear-rate dependence, especially at pH 4.0, from 0.3 to 30 s−1 and a shear thinning above 30 s−1, indicating a structural change. Hysteresis in the flow curve was measured at the pH values at which the electrostatic interactions were the strongest. Hysteresis was due to a slow structural rearrangement of the coacervate phase and, with time, the initial viscosity was completely recovered, showing that structural changes were reversible. In frequency sweep experiments, the values of the viscous modulus G″ were up to ten times higher than the values of the elastic modulus G′, indicating the highly viscous character of the coacervates. pH 4.0 appeared to be the pH at wh...
137 citations
TL;DR: In this paper, the melt rheology of ultrahigh molecular weight polymeric materials characterized by a narrow molecular weight distribution has been analyzed and the characteristic value of the plateau modulus (∼2 MPa) is in close agreement with those reported for a model high molecular weight monodisperse polyethylene.
Abstract: The melt rheology of ultrahigh molecular weight polymeric materials characterized by a narrow molecular weight distribution has been analyzed. Ultrahigh molecular weight polyethylene obtained from a metallocene catalyst shows a well-developed “plateau” modulus in a range of angular frequency of more than 3 decades. The characteristic value of the plateau modulus (∼2 MPa) is in close agreement with those reported for a model high molecular weight monodisperse polyethylene. From this value one can determine a characteristic molecular weight between entanglements of 1200 g mol−1. The molecular weight dependency of different, experimentally based relaxation times obtained from the linear viscoelastic response exhibits an exponent power law close to 3.0 for these materials. This seems to contradict the 3.4 dependence observed in the usual molecular weight range, which is based on the chain contour length fluctuation approach, but is in agreement with the latest reptation-based models. These models predict a cr...
120 citations
TL;DR: In this paper, a series of model suspensions containing the same glass fibers in a Newtonian polybutene and in a Boger fluid was investigated, and both types of fiber suspensions exhibited non-negligible normal stresses.
Abstract: The rheological behavior of two series of model suspensions containing the same glass fibers in a Newtonian polybutene and in a Boger fluid has also been investigated. The steady-state shear viscosity of both supensions increased with fiber content, but the suspensions in the Boger fluid became shear thinning. Both types of suspension exhibited non-negligible normal stresses. The steady-state viscosity and normal stress difference of the supensions in the polybutene are well predicted by the Lipscomb (1987) equation coupled with the Folgar–Tucker (1984) model. Both types of fiber suspensions were shown to exhibit shear and normal stress overshoots in stress growth experiments. Under flow reversal, a shear stress overshoot was observed at a larger deformation compared to the primary overshoot. The reverse overshoot has been attributed to tumbling of fibers that are not totally aligned in the flow direction even after a very long time. When the flow was reversed, the normal stress difference took initially mimimum values (negative values in the polybutene case) and then depicted a smaller positive overshoot before reaching a steady-state value. The normal stress undershoot has been attributed to a transient fiber-oriented structure. The shape and the magnitude of these overshoots depend on the fiber content, nature of the matrix, and time delay between consecutive experiments.
115 citations
TL;DR: In this paper, a nucleation model based on the concept that heterogeneous nucleation originates from pre-existing microvoids on the solid particles is proposed to predict the evolution of the bubble size distribution.
Abstract: The cell size distribution in a thermoplastic foam to a large extent determines its mechanical and thermal properties. It is difficult to predict because of the many physical processes involved, each affected in turn by an array of factors and parameters. The two most important processes are bubble nucleation and diffusion-driven bubble growth. Neither has been thoroughly understood despite intensive and long-standing research efforts. In this work, we consider foaming by a physical blowing agent dissolved in a polymer melt that contains particulate nucleating agents. We propose a nucleation model based on the concept that heterogeneous nucleation originates from pre-existing microvoids on the solid particles. The nucleation rate is determined by a bubble detachment time. Once nucleated, the bubbles grow as the dissolved gas diffuses through the polymer melt into the bubbles, a process that couples mass and momentum transport. By using the Oldroyd-B constitutive equation, we explore the role of melt viscoelasticity in this process. Finally, we integrate the nucleation and growth models to predict the evolution of the bubble size distribution. A cell model is employed to simulate the effects of neighboring bubbles and the depletion of blowing agents. The latter also causes the nucleation rate to decline once growth of older bubbles is underway. Using the physical and operating parameters of a recent foam extrusion experiment, we are able to predict a cell size distribution in reasonable agreement with measurements.
109 citations
TL;DR: In this paper, a Stokesian dynamics simulation of model systems of polymer coated particles is used to examine the body effects that occur in continuous shear thickening of concentrated colloid suspensions.
Abstract: Many body effects that occur in continuous shear thickening of concentrated colloid suspensions are examined by using a Stokesian dynamics simulation of model systems of polymer coated particles. The shear thickening state is probed in a number of ways: computed scattering intensities, imaging of density variation using Voronoi constructions, examination of the distribution of interparticle forces, and computation of the fabric of contacts. The shear thickening transition in these systems is found to be associated with the growth of a network of close particle contacts and shear induced density variations. This paper focuses on the network of contacts. The distributions of force magnitude are found to be exponential. The network directly relates to the normal stress differences. Both the data and simple physical argument suggest that thickening can be viewed as an approach to “jamming.”
107 citations
TL;DR: In this paper, the authors investigated the effect of the strength of interparticle attraction on shear thickening in dense suspensions and found that the shear rate characterizing the onset of hydrocluster formation is a decreasing function of strength of attraction.
Abstract: Shear thickening in dense suspensions is investigated as the strength of interparticle attraction is increased. Starting with hard spheres, attractions are induced through depletion interactions up to and beyond the point where the suspensions gel and the zero shear rate viscosity diverges. For a range of volume fractions we find that the stress at the onset of shear thickening is a weakly increasing function of the strength of attraction but the extent of shear thickening is diminished. For attractions of sufficient strength to gel the suspension, shear thickening is completely absent. These observations are discussed in terms of a time scale argument for hydrocluster formation and how the shear rate where hydroclusters are formed is influenced by interparticle attractions. For this purpose we develop methods to deconvolute hydrodynamic and thermodynamic contributions to the viscosity and find that the shear rate characterizing the onset of hydrocluster formation is a decreasing function of strength of a...
TL;DR: In this paper, more general features of associative behavior are investigated for both associative hydrophobically modified ethoxylated urethanes and more complex HASE systems can be seen in some other nonlinear properties, such as failure of the Cox-Merz rule for the viscosities.
Abstract: Shear thickening is very common in telechelic associative polymers. Yet, it is not a universal characteristic of the whole class of associative polymers. Here, more general features of associative behavior are investigated. In particular, nonlinear rheological properties have been studied for both associative hydrophobically modified ethoxylated urethanes and hydrophobically modified alkali-soluble emulsions (HASE) polymers. A known common characteristic is the failure of the Cox-Merz rule for the viscosities. The equivalent relation for the first normal stress is examined here, a parallelism with the results for the viscosities has been observed. More pronounced similarities between telechelics and the more complex HASE systems can be seen in some other nonlinear properties. The dynamic moduli display strain hardening for both moduli at intermediate strains at all but the lowest frequencies, even when no shear thickening can be detected in steady state shear flow. The linear relaxation functions differ quite strongly among associative polymers. However, they all display a specific non-linear relaxation. It includes short time strain hardening followed by strain softening starting at long relaxation times. The results indicate that the same microstructural mechanisms are responsible for the rheological behavior of both classes of associative polymer.
TL;DR: In this paper, a FENE-P dumbbell model was used to describe the molecular chain conformation and the orientation evolution for the amorphous phase and the semi-crystalline phase, respectively.
Abstract: The aim of this work is to model the flow-enhanced crystallization and the flow-induced morphological changes of semi-crystalline materials during and after shearing flow. A FENE-P dumbbell model and a rigid dumbbell model are used to describe the molecular chain conformation and the orientation evolution for the amorphous phase and the semi-crystalline phase, respectively. The effect of flow on crystallization is considered by relating excess free energy and flow-induced orientation to crystallization kinetics. The crystallization of the material couples back to influence the solidification rheology of the crystallizing system. An isotactic polypropylene is used as an example to illustrate model predictions. We predict a pronounced effect of short-term shear treatments in accelerating nucleation and changing rheological behavior. Results are compared with available experimental data.
TL;DR: In this paper, the effect of addition of a small amount of poly(D-lactic acid) (PDLA) on the melt rheology of PLLA was investigated.
Abstract: The effect of addition of a small amount of poly(D-lactic acid) (PDLA) on the melt rheology of poly(L-lactic acid) (PLLA) was investigated. PDLA added to PLLA produces a stereocomplex which stays unmelted even above Tm of PLLA. Because of the imbalanced contents of component polymers, the stereocomplex does not grow into a large crystallite but rather acts as a crosslinking point of PLLA chains resulting in the apparent introduction of the long chain branching as well as in the apparent increase in molecular weight. Such changes in the molecular structure give rise to the change in the melt rheology depending on the content and the molecular weight of PDLA. The addition of low Mw PDLA significantly affects the shear rheology of PLLA melts while high Mw PDLA does not give such a significant effect. However, the addition of PDLA with high and low Mw’s gives a strong strain hardening character to PLLA melt even at a very low PDLA content.
TL;DR: In this article, an extensional rheometer for creep experiments has been presented, which enables the application of defined portions of mechanical work to the polymer sample in its state of undercooled melt.
Abstract: A novel extensional rheometer for creep experiments has been presented in part I. The device can be used also for an investigation of the influence of short term elongational flow on the structure development in crystalline polymers. It enables the application of defined portions of mechanical work to the polymer sample in its state of undercooled melt. For the purpose short-term elongational creep is used. With increasing mechanical work the number density of nuclei in iPP increases by decades. Results, as previously obtained with shear flow, are introduced for comparison. Prospects of a theoretical description are discussed.
TL;DR: In this paper, a phenomenological model for the dynamics of a single drop immersed in an immiscible matrix is proposed with the two incompressible component liquids being in general viscoelastic.
Abstract: A phenomenological model for the dynamics of a single drop immersed in an immiscible matrix is proposed with the two incompressible component liquids being in general viscoelastic. The model is formulated by assuming that the drop is always ellipsoidal, and the model parameters are determined once and for all in the small deformation limit. The model is thereafter applicable to whatever flow condition is imposed at infinity, and for whatever intensity of flow field. Predictions of steady state deformation, drop breakup, and drop relaxation display the effects of constitutive elasticity on the drop dynamics.
TL;DR: In this paper, a cone-partitioned plate shear rheometer was used to measure the shear viscosity and the first and second normal stress coefficients for polystyrene melt with Mw=200 kg/mol (PS 200 k).
Abstract: Measurements of the shear viscosity and the first and second normal stress coefficients are shown at 175 °C for a nearly monodisperse polystyrene melt with Mw=200 kg/mol (PS 200 k). Tests are performed on a cone-partitioned plate shear rheometer and cover a range of Weissenberg numbers (τdγ) from 0.13 to 40. Experimental problems encountered are the axial compliance of the rheometer and the normal force capacity of the transducer. The later limits the maximum shear rate to τdγ=40. Experimental data are compared with the models of Ottinger [termed thermodynamically consistent reptation model (TCR), Ottinger, H. C., J. Rheol. 43, 1461–1493 (1999)] for the convective constraint release parameter δ2=0, 1, and 2 and Mead, Larson, and Doi [termed Mead, Larson, and Doi (MLD), Mead, D. W., R. G. Larson, and M. Doi, Macromolecules 31, 7895–7914 (1998)] for δ2=1. The steady state and transient values of p21, N2, and N1 agree qualitatively well between both models and the experiment. The predicted normal stress ra...
TL;DR: In this article, the rheological properties of surface treated and untreated CaCO3-high-density polyethylene composites were studied and related to particle-particle and particle-matrix interactions.
Abstract: The rheological properties of surface treated and untreated CaCO3-high-density polyethylene composites were studied and related to particle–particle and particle-matrix interactions. Steady shear rheological measurements on composites with different loading (0–30 vol %) were carried out with preshear treatment prior to the measurements, duration of kneading during compounding, and surface treatment (stearic acid) of the filler as variable parameters. The steady state shear viscosity massively increased with increasing filler volume fraction due to the presence of a small number of agglomerates. Although no polymer was entrapped within the agglomerates, their presence led to massive increase in shear viscosity. The rheological response proved to be a more sensitive test for filler dispersion than scanning electron microscopy and shear thinning beyond that of the polymer matrix was observed, due to deagglomeration of the filler. The presence of agglomerates also led to a stress overshoot in the step shear r...
TL;DR: In this article, the authors examined the stability of the fountain flow using the extended Pom-Pom constitutive equations and showed that it is subject to a viscoelastic instability.
Abstract: We numerically investigate the stability problem of the injection molding process. It was indicated by Bulters and Schepens [Bulters and Schepens (2000)] that surface defects of injection molded products may be attributed to a flow instability near the free surface during the filling stage of the mold. We examine the stability of this flow using the extended Pom–Pom constitutive equations. The model allows for controlling the degree of strain hardening of the fluids without affecting the shear behavior considerably. To study the linear stability characteristics of the injection molding process we use a transient finite element algorithm that is able to efficiently handle time dependent viscoelastic flow problems and includes a free surface description to take perturbations of the computational domain into account. It is shown that the fountain flow, which is a model flow for the injection molding process, is subject to a viscoelastic instability. If the various rheologies are compared, we observe that the...
TL;DR: In this article, the hydrodynamic interaction parameter h* is obtained a priori by matching the drag force from a fully extended bead-spring model in extensional flow with that from Batchelor's theory for a cylindrical rod.
Abstract: The nonlinear transient extensional and steady-state shear rheological properties of dilute polystyrene solutions of molecular weight 3.9 and 10.2 million in a theta solvent are predicted using Brownian dynamics (BD) simulations with the bead-spring model. Full hydrodynamic interaction is incorporated into BD simulations using the Rotne–Prager tensor. The hydrodynamic interaction parameter h* is obtained a priori by matching the drag force from a fully extended bead-spring model in extensional flow with that from Batchelor’s theory for a cylindrical rod [Hsieh et al. (2003)]. The agreement between experimental data [Gupta et al. (2000)] and simulation results for the transient Trouton ratio versus strain is good from low to medium strains. However, the plateaus at high strains predicted by the simulations are higher than measured. We also find that hydrodynamic interaction hinders the unraveling of a polymer chain in strong extensional flow (Wi≫1) due to the hydrodynamic clustering of beads. In steady she...
TL;DR: In this paper, the authors explore the nonlinear rheological response of a soft gel formed by a crowded colloidal star polymer and focus on the occurrence of a stress plateau, marked hysteresis, and yield stress in its flow curve.
Abstract: We explore the nonlinear rheological response of a soft gel formed by a crowded colloidal star polymer and focus on the occurrence of a stress plateau, marked hysteresis, and yield stress in its flow curve. With the aid of nuclear magnetic resonance velocimetry we find evidence for fluctuations in the flow behavior across the gap of the concentric cylindrical Couette device, in association with a degree of apparent slip at the inner wall. The time scale of the these fluctuations appears rapid (with respect to the measurement time per shear rate in the flow curve), on the order of tens to hundreds of milliseconds, with the speed of fluctuations associated with the flow history of the sample. Our velocity profile analysis suggests a qualitative model in which intermittent changes due to jamming/unjamming transitions occur, analogous to cage dynamics in colloidal glasses.
TL;DR: In this article, the deformation of a spherical drop suspended in a homogeneous viscoelastic matrix is investigated and the corresponding stress is calculated by extending Batchelor's approach for both small and large deformations.
Abstract: The deformation of a viscoelastic drop suspended in a homogeneous viscoelastic matrix is investigated. The drop and the matrix fluids are assumed to obey linear viscoelastic constitutive equations. Small-deformation analysis is carried out on the basis of the general solution for creeping flow around a spherical drop. The corresponding stress is calculated by extending Batchelor’s approach to viscoelastic media for both small and large deformations. Good agreement was found between the model predictions and experimental results available in the literature.
TL;DR: In this paper, the effect of the compatibilization of polyisobutylene (PIB) droplets in a polydimethylsiloxane (PDMS) matrix with a diblock copolymer of PIB and PDMS was investigated.
Abstract: Block copolymers may be added as surface-active compatibilizers in order to control the morphology of blends of immiscible polymers. The effects of such added compatibilizers on the rheological properties of droplet–matrix blends are investigated experimentally. Model blends composed of polyisobutylene (PIB) droplets in a polydimethylsiloxane (PDMS) matrix, compatibilized with a diblock copolymer of PIB and PDMS, are studied here. The viscosity ratio of the blends, i.e., the ratio of the viscosity of the droplets to that of the matrix, is varied from 0.1 to 2.7. The viscosity and the first normal stress difference under steady shear conditions, and complex moduli after cessation of shear are measured. It is found that addition of the compatibilizer slightly raises the magnitude of the terminal complex viscosity of blends at all ratios of viscosity. Furthermore, with addition of the compatibilizer, the terminal relaxation time is found to increase sharply at high viscosity ratios, whereas the steady shear ...
TL;DR: In this paper, the authors investigate the effects of polymer molecular structure on the solution rheology of a hydrophobically modified associative poly(ethyl acrylate-co-methacrylic acid) backbone.
Abstract: We investigate the effects of polymer molecular structure on the solution rheology of a hydrophobically modified associative polymer comprised of macromonomers with alkyl hydrophobes attached to a poly(ethyl acrylate-co-methacrylic acid) backbone. In particular, the effect of polymer backbone composition with variable proportions of methacrylic acid (MAA) and ethyl acrylate (EA) are examined. We find that the concentration of the MAA monomer has a large impact on polymer viscoelasticity. Polymers with low MAA content have smaller hydrodynamic size that result in lower viscosities and dynamic elastic moduli compared to polymers with high MAA content. Moreover, the balance between the polymer hydrodynamic size, the chain flexibility, and the aggregation of the EA blocks yield maxima in these material functions with respect to the MAA concentration. The scaling of shear viscosity, high frequency elastic modulus, and creep compliance with polymer concentration exhibits power-law behavior with different exponents. In all cases, three power-law regimes, regardless of the MAA content, are observed that can be attributed to the presence of different modes of hydrophobic interaction. However, the transitions shift to lower concentrations as the MAA content increases. With regards to the effects of the macromonomer side-chain concentration, we observe a substantial increase in viscosity at intermediate macromonomer content (1 mol %), possibly due to an increase in the number of intermolecular junctions as the number of hydrophobes per chain increases. This is in contrast to (i) low macromonomer concentration (0.3 mol %) behavior that reveals low viscosity due to weak hydrophobic associations, and (ii) high macromonomer concentration (1.9 mol %) behavior that favors more intramolecular association resulting in lower viscoelastic properties compared to intermediate macromonomer concentrations.We investigate the effects of polymer molecular structure on the solution rheology of a hydrophobically modified associative polymer comprised of macromonomers with alkyl hydrophobes attached to a poly(ethyl acrylate-co-methacrylic acid) backbone. In particular, the effect of polymer backbone composition with variable proportions of methacrylic acid (MAA) and ethyl acrylate (EA) are examined. We find that the concentration of the MAA monomer has a large impact on polymer viscoelasticity. Polymers with low MAA content have smaller hydrodynamic size that result in lower viscosities and dynamic elastic moduli compared to polymers with high MAA content. Moreover, the balance between the polymer hydrodynamic size, the chain flexibility, and the aggregation of the EA blocks yield maxima in these material functions with respect to the MAA concentration. The scaling of shear viscosity, high frequency elastic modulus, and creep compliance with polymer concentration exhibits power-law behavior with different expone...
TL;DR: In this article, a method for refining the predictions of a finitely extensible bead-spring chain model with hydrodynamic interactions is introduced, which involves the representation of a long but finite macromolecule of NK Kuhn steps with a coarse-grained bead spring chain model, and subsequently "fine graining" the representation by progressively increasing the number of beads, N. Extrapolating the results obtained using exact Brownian dynamics simulations for several values of N to the limit N−1→NK, leads to improved estimates for the behavior of the actual polymer
Abstract: A new method for refining the predictions of a finitely extensible bead-spring chain model with hydrodynamic interactions is introduced. The suggested successive fine-graining procedure involves the representation of a long but finite macromolecule of NK Kuhn steps with a coarse-grained bead-spring chain model, and subsequently "fine graining" the representation by progressively increasing the number of beads, N. Extrapolating the results obtained using exact Brownian dynamics simulations for several values of N to the limit N−1→NK, leads to improved estimates for the behavior of the actual polymer chains. The extrapolated results for the extensional viscosity are found to be in good agreement with the recent experimental observations of Gupta et al. (2000). The scaling of the extensional viscosity with molecular weight is also examined.
TL;DR: In this paper, the effect of confining walls on the rheology and dynamics of dilute polymeric solutions using a self-consistent multiscale simulation technique is presented.
Abstract: We present the effect of confining walls on the rheology and dynamics of dilute polymeric solutions using a self-consistent multiscale simulation technique In Part I we formulate the mathematical problem necessary to understand and model these dynamics Various polymer models (the Kramers’ freely jointed bead-rod chain, FENE, worm-like, and inverse Langevin chains) are used to describe the polymer chain dynamics based on their success in previous studies [HP Babcock et al, Phys Rev Lett 85, 2018 (2000); J S Hur et al, J Rheol 44, 713 (2000); 45, 421 (2001)] Theoretical arguments suggest that the main consequences of confinement on chains are (a) the entropic force law is altered due to the loss of chain configurational space and (b) the viscous drag on the chain is increased due to hydrodynamic interactions with the wall In this study, the correct entropic spring force law in the presence of confining walls is developed In addition, an efficient multiscale simulation technique for modeling
TL;DR: In this paper, a new model for the viscosity of miscible polymer blends is described, which combines a mixing rule for chain relaxation, based on the double reptation concept as adopted by Tsenoglou (1989), with a calculation of the concentration dependence of the monomeric friction factors.
Abstract: A new model for the viscosity of miscible polymer blends is described. It combines a mixing rule for chain relaxation, based on the double reptation concept as adopted by Tsenoglou (1989), with a calculation of the concentration dependence of the monomeric friction factors, based on the self-concentration model of Lodge and McLeish (2000). The model is successful in anticipating the experimentally observed curvatures in plots of viscosity versus composition, and the predictions are in promising agreement with the data for several systems. The model introduces no freely adjustable parameters, and all the necessary parameters may be determined from measurements on pure components.
TL;DR: In this paper, the authors compare the predictions of several suspension temperature models of particle migration to laser Doppler velocimetry measurements in a concentrated suspension of noncolloidal spheres.
Abstract: This study compares the predictions of several “suspension temperature” models of particle migration to laser Doppler velocimetry measurements in a concentrated suspension of noncolloidal spheres. We compare the shear rate, concentration, and suspension temperature profiles in narrow-gap Couette flow. The models predict the observed macroscopic shear rate and concentration profiles well at moderate bulk particle concentration but diverge from one another and from the data at high concentrations. In addition, the predictions of the models compare poorly with suspension temperature measurements. Most of the models greatly underpredict the magnitude of the scalar temperature, capturing instead only the magnitude of the smaller two diagonal components of the temperature tensor. Also, the models do not predict the observed variation of the suspension temperature with particle concentration. Our investigation shows that both neglect of suspension temperature anisotropy and qualitative choices of model coefficie...
TL;DR: The soft glassy rheology (SGR) model has been used to describe diverse nonergodic materials exhibiting strong local disorder and slow mesoscopic rearrangement as discussed by the authors.
Abstract: In recent years, the paradigm of soft glassy matter has been used to describe diverse nonergodic materials exhibiting strong local disorder and slow mesoscopic rearrangement. As so far formulated, however, the resulting soft glassy rheology (SGR) model treats the shear stress in isolation, effectively scalarizing the stress and strain rate tensors. Here we offer generalizations of the SGR model that combine its nontrivial aging and yield properties with a tensorial structure that can be specifically adapted, for example, to the description of fluid film assemblies or disordered foams.
TL;DR: In this paper, an elongational rheometer is used to study the rheological behavior of gelled propellant simulants in uniaxial elongational flow, and the results of the tested fluids measured in simple shear and in uniao-ial elongation agree fairly closely.
Abstract: An elongational rheometer is used to study the rheological behavior of gelled propellant simulants in uniaxial elongational flow. In simple shear such fluids typically exhibit a shear thinning behavior which could be described by a power-law constitutive equation. Knowledge of the elongational behavior of these fluids is important for understanding the processes of their atomization and spray formation. The results of the present work demonstrated that the three-dimensional power-law model permits description of uniaxial elongation of these fluids as well. Moreover, the values of the rheological parameters of the tested fluids measured in simple shear and in uniaxial elongation agree fairly closely. Therefore the elongational behavior of gelled propellant simulants can be inferred from their shear behavior.