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By means of a stress imposed rheometer coupled with a “vibrating cell,” generating a Brownian motion at a macroscopic scale into the samples, we have shown that dense-phase vibrated powders exhibit rheological behaviors archetypal of non-Newtonian viscoelastic fluids. These behaviors have been accurately described through a free volume structural model based on simple “stick-slip” granular interactions. As a result, the evolution of the steady-state viscosity has been accurately expressed as a function of the shear rate, the frictional stress, the granular pressure, the mass of the samples, the vibration frequency, the vibration energy, the intergranular contact network mean life, and the free volume distribution. The model is consistent with Hookean, Coulombian, and Newtonian limits and is not only descriptive but also explicative and predictive of the encountered phenomena. In particular, a “time-granular temperature superposition principle,” theoretically predicted by the model, has been experimentally verified, the “granular temperature” being controlled through the vibration energy and frequency. Moreover, this superposition principle has been precisely described by a “Vogel-Fulcher-Tammann” law, leading to very close analogies with molecular systems near their glass transition point. [ABSTRACT FROM AUTHOR]

In inertialess suspensions of rigid particles, the rotational motion of each particle is governed by the so-called freely rotating condition, whereby the total torque acting on the particle must be zero. In this work, we study the effect of viscoelasticity of the suspending liquid on the rotation period of a sphere by means of three-dimensional finite element simulations, for conditions corresponding to a macroscopic shear flow. The simulation results capture the slowing down of the rotation, relative to the Newtonian case, which was recently observed in experiments. It is shown that such a phenomenon depends on the specific constitutive equation adopted for the viscoelastic liquid. Analysis of transients shows a clear correlation between rotation rate and the development of first normal stress difference. [ABSTRACT FROM AUTHOR]

The inertialess three-dimensional (3D) flow of viscoelastic shear-thinning fluids in a 4:1 sudden square-square contraction was investigated experimentally and numerically and compared with the flow of inelastic fluids. Whereas for a Newtonian fluid the vortex length remains unchanged at low Reynolds numbers, with the non-Newtonian fluid there is a large increase in vortex length with fluid elasticity leading to unstable periodic flow at higher flow rates. In the steady flow regime the vortices are 3D and fluid particles enter the vortex at the middle plane, rotate towards its eye, drift sideways to the corner-plane vortex, rotate to its periphery, and exit to the downstream duct. Such dynamic process is reverse of that observed and predicted with Newtonian fluids. Numerical predictions using a multimode Phan-Thien–Tanner viscoelastic model are found to match the visualizations accurately and in particular are able to replicate the observed flow reversal. The effect of fluid rheology on flow reversal, vortex enhancement, and entry pressure drop is investigated in detail. [ABSTRACT FROM AUTHOR]

Characterizing purely viscous or purely elastic rheological nonlinearities is straightforward using rheometric tests such as steady shear or step strains. However, a definitive framework does not exist to characterize materials which exhibit both viscous and elastic nonlinearities simultaneously. We define a robust and physically meaningful scheme to quantify such behavior, using an imposed large amplitude oscillatory shear (LAOS) strain. Our new framework includes new material measures and clearly defined terminology such as intra-/intercycle nonlinearities, strain-stiffening/softening, and shear-thinning/thickening. The method naturally lends a physical interpretation to the higher Fourier coefficients that are commonly reported to describe the nonlinear stress response. These nonlinear viscoelastic properties can be used to provide a “rheological fingerprint” in a Pipkin diagram that characterizes the material response as a function of both imposed frequency and strain amplitude. We illustrate our new framework by first examining prototypical nonlinear constitutive models (including purely elastic and purely viscous models, and the nonlinear viscoelastic constitutive equation proposed by Giesekus). In addition, we use this new framework to study experimentally two representative nonlinear soft materials, a biopolymer hydrogel and a wormlike micelle solution. These new material measures can be used to characterize the rheology of any complex fluid or soft solid and clearly reveal important nonlinear material properties which are typically obscured by conventional test protocols. [ABSTRACT FROM AUTHOR]

A new method for measurement of the viscoelastic properties of the liquid with the torsion resonator on forced resonance is presented in this paper. Both theoretical analysis and experiments of different glycerol-water mixtures with our homemade torsion pendulum apparatus demonstrate that this new method avoids the restriction of the common method that the resonator should have the high quality factor both in air and in liquid sample and has the prominent advantage of measuring the concentrated and large-viscous liquid in the low frequency range (≤100 Hz) with a better precision, which is usually unavailable by the common method. Employment of the new method can thus extend the accessible ranges of viscosity and frequency of the torsion resonator apparatus and close the gaps between conventional rheometers and the torsion resonators. [ABSTRACT FROM AUTHOR]

The second normal stress difference N2 experienced by non-Newtonian fluids flowing in a pipe may give rise to secondary flows in the transverse direction. As a result, one component tends to encapsulate the other in stratified flows. In multilayer coextrusion, such secondary flows tend to distort the interface and affect layer uniformity. This paper presents numerical simulations of the elastically driven encapsulation in two-component stratified viscoelastic fluids. The simulations are based on a phase-field theoretical model and use finite elements with adaptive meshing to resolve the moving interfaces. The results suggest two mechanisms for elastic encapsulation: One due to the mismatch of N2 between the components and the other due to noncircular geometry of the cross section. In circular pipes, the more elastic fluid tends to protrude into the other component in the center of the pipe and become encapsulated. This produces the curtate cycloid interface shape commonly seen in experiments. If the cross section is noncircular, both the geometric effect and the elastic stratification are at work, and the interfacial motion is determined by the competition of these two mechanisms. This understanding provides an explanation for the anomalous encapsulation of the less elastic component by the more elastic one observed in multilayer coextrusion. [ABSTRACT FROM AUTHOR]

We survey the linear viscoelasticity of a new type of transient network: bridged wormlike micelles, whose structure has been characterized recently by Ramos and Ligoure [Macromolecules 40, 1248–1251 (2007)]. This composite material is obtained by adding telechelic copolymers (water-soluble chains with hydrophobic stickers at each extremity) to a solution of entangled wormlike micelles (WM). For comparison, naked WM and WM decorated by amphiphilic copolymers are also investigated. While these latter systems exhibit almost a same single ideal Maxwell behavior, solutions of bridged WM can be described as two Maxwell fluids component blends, characterized by two markedly different characteristic times, τfast and τslow, and two elastic moduli, Gfast and Gslow, with Gfast>Gslow. We show that the slow mode is related to the viscoelasticity of the transient network of entangled WM, and the fast mode to the network of telechelic active chains (i.e., chains that do not form loops but bridge two micelles). The dependence of the viscoelasticity with the surfactant concentration, [lowercase_phi_synonym], and the sticker-to-surfactant molar ratio, β, is discussed. In particular, we show that Gfast is proportional to the number of active chains in the material, [lowercase_phi_synonym]β. Simple theoretical expectations then allow evaluation of the bridges/loops ratio for the telechelic polymers. [ABSTRACT FROM AUTHOR]

This paper reports finite-element simulations of drop deformation in converging flows in an axisymmetric conical geometry. The moving interface is captured using a diffuse-interface model and accurate interfacial resolution is ensured by adaptive refinement of the grid. We have explored the effects of viscoelasticity on drop deformation when either the drop or the matrix is a Giesekus fluid. Contrary to the popular belief that viscoelasticity in the drop hinders deformation and that in the matrix enhances deformation, we predict a more complex picture in which viscoelasticity in either component may suppress or promote drop deformation depending on the capillary number Ca and the drop-to-matrix viscosity ratio β. Smaller Ca and β are conducive to the behavior mentioned above, while large Ca and β may produce the opposite effect. Both trends are explained by the reaction of the polymer stress to the inhomogeneous and transient deformation in the converging flow field. Finally, this understanding reconciles contradictory results in the literature as opposite limits in the parameter space. [ABSTRACT FROM AUTHOR]

This paper reports initial experimental flow birefringence studies of cross-slot flow for a range of molten polyethylenes using a multipass rheometer (MPR). An MPR was modified to accommodate a cross-slot center section insert thereby enabling small quantities of molten polymer to be evaluated in a near two-dimensional pure shear extensional flow. The results show that it was possible to obtain precise processing data for cross-slot flow using 10 g quantities of polymer. Experimental observations on different polyethylenes showed that in all cases the flow commenced from a Newtonian situation where the optical birefringent fringes around the central stagnation point were essentially symmetric. As the flow developed, the fringe pattern became progressively more asymmetric about the exit symmetry plane and the extent of the viscoelasticity of the polymer could be quantified by the observed level of anisotropy between the birefringence fringes on the entry and exit symmetry planes of the flow. The progressive development of anisotropy and stress with increasing extensional flow rate and type of polymer is presented for a number of flow conditions and it was found that increasing viscoelasticity of the polymer resulted in a progressive increase in viscoelastic center-line extensional stresses. The data provide benchmark large strain extensional flow experiments for certain polymer melts that can in the future be used to compare with numerical simulation. [ABSTRACT FROM AUTHOR]

The transient and steady-state elongational viscosity data of three bidisperse polystyrene blends were investigated recently by Nielsen et al. [J. Rheol. 50, 453–476 (2006)]. The blends contain a monodisperse high molar mass component (ML=390 kg/mol) in a matrix of a monodisperse small molar mass component (either MS=103 kg/mol or MS=52 kg/mol at two different weight fractions). The experimental data are analyzed in the framework of the molecular stress function model of Wagner et al. [J. Rheol. 49, 1317–1327 (2005)], which is based on the assumption of a strain-dependent tube diameter and the interchain pressure term of Marrucci and Ianniruberto [Macromolecules 37, 3934–3942 (2004)]. The dilution of the long chains in the matrix of the short chains is identified as the origin of a drastic increase in the tube-diameter relaxation time of the long chains, leading to a large stretching potential of the long-chain component and an increasing steady-state elongational viscosity with increasing strain rate. In addition, in the dilution regime, a transition from affine chain stretch to nonaffine tube squeeze with decreasing strain rate is identified. The dilution regime ends at a critical strain rate, when the tube diameter of the supertubes created by the interaction of the long chains among themselves, is reduced by deformation to the tube diameter of the bulk. A nonlinear extension of the basic double reptation concept is developed comprising all of these different phenomena, and allowing (albeit by use of empirical linear-viscoelastic shift factors to correct the linear-viscoelastic predictions) for a quantitative description of the transient and steady-state elongational viscosities of the bidisperse polystyrene blends. [ABSTRACT FROM AUTHOR]

The breakdown of finite element (FEM) computations for the steady symmetric two-dimensional flow of dilute and ultradilute Oldroyd-B fluids around a cylinder in a channel, at Weissenberg numbers Wi=O(1), is shown to arise due to a coil-stretch transition experienced by polymer molecules in the wake of the cylinder in the vicinity of the location of the stress maximum on the centerline. In dilute Oldroyd-B fluids, due to the modification of the flow caused by the presence of the polymer, the coil-stretch transition leads to the stress maximum diverging toward infinity at a finite value of Wi≈0.7. On the other hand, in ultradilute solutions, the stress maximum approaches infinity only as Wi→∞. In FENE-P fluids, the coil-stretch transition leads to the mean extension of the molecules saturating to a value close to the fully-extended length, with the maximum stress remaining bounded with increasing Wi. An estimation of the number of finite elements required to achieve convergence for ultradilute Oldroyd-B fluids reveals that obtaining solutions at Wi>1 is not feasible. [ABSTRACT FROM AUTHOR]

Several internal parameters are studied for a 3D simple shear flow of soft polydispersed granular materials consisting of viscoelastic spheres. These internal parameters include the contact time, the multiple collision group size, and the coordination number. It is found that the contour plots of the contact time and the coordination number in the plane defined by the concentration (solid fraction) and dimensionless stiffness are similar. These contours are qualitatively the same as the regime chart/flowmap proposed in earlier studies. The resulting constitutive relation shows different rate dependency at different concentrations and shear rates. Based on a simple dimensional analysis and a power law formulation, the rate dependency may be expressed by an index. The two extreme values of this index, 0 and 1, correspond to solidlike and gaslike granular materials, respectively. The contour map of this rate index (the power of the dimensionless shear rate in the constitutive relation) resembles those of the λ-shaped curve typical of a phase diagram for ordinary materials. [ABSTRACT FROM AUTHOR]

Single-step and three types of reversing double-step experiments were conducted on concentrated solutions of a linear and two dendritic (hyperbranched or arborescent) polyisobutylenes (PIBs). The results in reversing double-step experiments further confirm our previous findings on polyethylene melts that the K-BKZ theory provides better predictions for long chain branched polymers than linear ones. The short chain branched material looks more like the linear material in the present experiments. Isochronal derivatives of the strain potential function (W1 and W2) were calculated from torque (T) and normal force (N) responses in single-step parallel plate experiments. The damping function was extracted from W1 and W2 for both linear and branched PIBs. The damping function of the linear PIB falls between two versions of the Doi–Edwards model. The arborescent PIB with longer branches and lower branching frequency shows weak damping behavior, which is a specific behavior for long chain branched polymers. The arborescent PIB with shorter branches, higher branching frequency and low entanglement density, on the other hand, exhibits the same damping behavior as that of the linear PIB, which is consistent with the findings in the reversing double-step experiments in which it behaves more like a linear polymer. [ABSTRACT FROM AUTHOR]

The viscoelastic behavior of very concentrated and electrostatically stabilized suspensions of kaolinite particles has been investigated in the linear and nonlinear regime as a function of volume fraction, ionic strength and in the presence of polymer at various concentrations. Material properties such as linear viscoelastic moduli and cohesive energy density are extensively enhanced by either increasing volume fraction or decreasing ionic strength. Attention has been paid to the large amplitude oscillatory shear behavior of concentrated suspensions of plate-like particles, characterized by a hump in G″ curves. Rheological investigation shows the extreme sensitivity of the intensity of the strain hardening in G″ to excluded volume, electrostatic and steric interactions. A physical interpretation of this nonlinear behavior has been proposed. [ABSTRACT FROM AUTHOR]

We investigate the link between particle interactions and induced flow patterns around two identical spheres sedimenting along their centerline in a polymeric fluid. The fluid is strongly shear thinning and, in agreement with previous results, the spheres are observed to chain even at large initial separation distances. The wake of a single particle displays an upward motion of fluid, i.e., a “negative wake” that is commonly observed in fluids with low extensional viscosities. We show that the features of this negative wake vary only weakly with the Deborah number. In the two-sphere case, the pattern of the induced flow depends on the sphere separation distance. The change in the flow pattern does not, however, induce any significant qualitative change in the sphere interactions. Upstream of the leading sphere and downstream of the trailing one along the sedimentation axis, the variations of the fluid velocity are well described by a single master curve for different values of the sphere separation distance. The existence of such a curve indicates that non-Newtonian effects near each particle are dominated by local conditions near the sphere surfaces, and are only weakly influenced by the presence of a second sphere. [ABSTRACT FROM AUTHOR]

When coal is heated in the absence of oxygen it softens at approximately 400 °C, becomes viscoelastic, and volatiles are driven off. With further heating, the viscous mass reaches a minimum viscosity in the range of 103–105 Pa s and then begins to resolidify. A high-torque, high-temperature, controlled-strain rheometer with parallel plates has been used to study the rheology during this process. Under shear, the viscosity of the softening mass decreases with increasing shear rate. During resolidification, the viscosity increases as C-C bond formation and physical interactions gives rise to an aromatic network, but, under shear, the network breaks apart and flows. This is viewed as a yielding of the structure. The higher the shear rate, the earlier the yielding occurs, such that if the shear rate is low enough, the structure is able to build. Also, further into resolidification lower shear rates are able to break the structure. It is proposed that resolidification occurs through the formation of aromatic clusters that grow and become cross-linked by non-covalent interactions. As the clusters grow, the amount of liquid surrounding them decreases and it is thought that the non-covalent interactions between clusters and liquid could decrease and the ability of growing clusters to move past each other increases, which would explain the weakening of the structure under shear. This work is part of a program of work aimed at attaining a greater understanding of microstructural changes taking place during carbonization for different coals, in order to understand the mechanisms that give rise to good quality cokes and coke oven problems such as excessive wall pressure. [ABSTRACT FROM AUTHOR]

Viscoelastic properties are followed during the formation of gels by moderately concentrated solutions of atactic polystyrene (aPS) in a mixed solvent of carbon disulfide and tri-m-tolyl phosphate (TCP). This mixture is a thermodynamically “good” solvent for aPS and with solution glass transition temperatures far below the temperatures used in the study. Gel formation is driven by the CS2 component, at temperatures and polymer concentrations similar to those observed in solutions of aPS in pure CS2. The TCP adjusts the viscoelastic response to a convenient range. The results show appreciable hysteresis in the melting and formation temperatures of gels, with the gel formation being a slow process, permitting dynamic mechanical or creep and creep recovery studies as a function of ageing during gel formation, as well as on the solutions prior to permit gel formation. These data reveal systematic behavior analyzed using the linear viscoelastic constitutive relation to obtain the viscosity and a measure of the steady-state recoverable compliance prior to gel formation and the equilibrium compliance after gel formation. This behavior is discussed in terms of models in which segments of the backbone possess a helical conformation stabilized by interactions with CS2 and gel crosslink loci formed via these sequences. [ABSTRACT FROM AUTHOR]

We investigated the linear melt viscoelasticity and the crystallization kinetics of a series of model poly(ethylene oxide) stars with different functionalities (f=4–32 arms) and moderately entangled arms (their molecular masses ranging from 5.5 to 12 kg/mol). The limited data in the homogeneous state indicated that the zero-shear viscosity η0 was adequately described by the Milner–McLeish model for functionalities f<32, where the core effect is insignificant; a similar behavior was observed for the recoverable compliance Je0 which depended on both the molecular weight and the number of the arms. Below the melting point, the isothermal crystallization was measured with differential scanning calorimetry and rheology, and analyzed in terms of the Avrami theory, expanding over a wide range of temperatures. The results were supported by additional polarizing optical microscopy data and indicated a slower crystallization kinetics of the stars compared to their linear analogues. They showed a strong dependence of the crystallization rate on the arm molecular weight, whereas the available experimental evidence is suggestive of some functionality dependence as well. [ABSTRACT FROM AUTHOR]

We have investigated the intentional temporary degradation and the subsequent recovery of the drag-reducing properties of surfactant solutions. Degradation was achieved by exposing the fluid to mechanical stresses after which it showed significantly reduced drag reduction capability. The recovery time varies from a fraction of second to several minutes, depending on the surfactant concentration, the counter-ion concentration and the temperature. The recovery process generally showed a linear increase of the drag reduction level with time, with the rate of recovery being essentially independent of the flow velocity and the initial level of degradation, though the recovery at rest is considerably slower than the recovery under turbulent flow conditions. The recovery time decreases sharply with increasing concentration and temperature, and with increasing counter-ion concentration for the cationic surfactant. The mechanical degradation apparently affects not only the shear-induced structure but also the rod-like micelles themselves. [ABSTRACT FROM AUTHOR]

A filament stretching extensional rheometer and capillary breakup extensional rheometer are used to measure the extensional rheology of a series of wormlike micelle solutions experiencing a uniaxial elongational flow. The experiments are performed using a series of wormlike micelle solutions of both cetylpyridinium chloride and sodium salicylate (NaSal) in an aqueous sodium chloride solution and cetyltrimethylammonium bromide and NaSal in de-ionized water. The linear viscoelasticity of all the wormlike micelle solutions is well described by a Maxwell model with just one or two relaxation times while the steady shear measurements all demonstrate characteristics of shear banding at large shear rates. In transient homogeneous uniaxial extension imposed by a filament stretching rheometer, each of the wormlike micelle solutions demonstrate significant strain hardening. At large extension rates, the wormlike micelle solution filaments are all found to fail through a dramatic rupture near the axial midplane at a constant stress independent of imposed extension rate. The result is an extensional viscosity that decays linearly with increasing extension rate. This filament failure likely stems from the local scission of individual wormlike micelle chains. For the more concentrated solutions, as the imposed extension rate is reduced, a critical extension rate is found below which the filament does not rupture, but instead elastocapillary pinch off is recovered and the elastic tensile stresses achieved in the fluid filament grow far beyond the value observed at rupture. This dramatic upturn in the elastic tensile stress and the extensional viscosity at low extension rates is not intuitively expected and is most likely a result of structural changes to the entangled wormlike micelle solution. Strain hardening is also observed in capillary breakup rheometry experiments, however, when the results of filament stretching and capillary breakup rheometry measurements at nominally the same extension rate are superimposed, the results do not agree; the extensional viscosity measurements from filament stretching are in some instances more than an order of magnitude larger. This result calls into question the use of capillary breakup rheometry for quantitatively measuring the extensional viscosity of wormlike micelle solutions. [ABSTRACT FROM AUTHOR]

Two-step reversing flows are severe tests of constitutive equations for polymer melts and have attracted interest for testing the response of branched polymers. Here we report on single-step and three types (types B, C, and D) of reversing torsional flows on branched and linear polyethylenes (PEs). The responses were analyzed using Kaye–Bernstein–Kearsley–Zapas (K-BKZ) theory. Comparisons between data and theory in type B and C flows for the two branched PEs (LD 103 and LD 146) and a linear HDPE are presented. For the LD 146 material and to strains higher than previously examined, the results agree with prior studies, i.e., the K-BKZ theory provides a good description of the response of branched polymers in reversing flows while the opposite is the case for the linear polymers. Surprisingly, it is found that the other branched polymer (LD 103) behaves more like a linear polymer and not a branched polymer in that it does not follow the K-BKZ theory. In the type D flow, one which was not previously investigated, the K-BKZ theory describes the normal force (NF) responses for branched LD 146 but not for the linear PE. Also, isochronal derivatives of the strain potential function (W1 and W2) were calculated from torque (T) and NF responses in single-step tests. The damping function fits to a modified sigmoidal formula and is close to the BKZ limit as reported by Wagner (2004a) for branched polystyrenes. The normal stress difference ratio and damping function were extracted from W1 and W2. Finally, the Cox–Merz rule was examined for one of the branched PEs. [ABSTRACT FROM AUTHOR]

Rheological measurements are used to compare clay nanocomposites prepared through melt mixing using two different polypropylene matrices. Steady state and transient nonlinear rheological experiments are employed to separate the contributions of flow induced orientation of the tactoids and particulate network build-up. The conditions under which the rheological properties are dominated by the aggregate network are subsequently identified. Under these conditions, the low frequency linear viscoelastic behavior is analyzed using scaling concepts for fractal networks to determine the degree of network formation by exfoliation. Moreover, the high frequency behavior of the moduli can be used to quickly assess the dispersion quality. The results from the analysis of the linear viscoelastic data are compared to structural features extracted from electron microscopy and small angle X-ray scattering data. [ABSTRACT FROM AUTHOR]

A recent, nonlinear viscoelastic theory for predicting the thermomechanical response of glassy polymers has been shown to predict behaviors from enthalpy relaxation to temperature-dependent mechanical yield in various modes of deformation quite well. The foundation of this theory rests on a “material clock” that depends on the potential energy of the system. The molecular basis for the clock and the Rational Mechanics framework for the constitutive equation are briefly reviewed. The theory is then used to predict and explain much more complicated behavior of glassy polymers: the change in compressive yield stress during physical aging at different temperatures, the peculiar enthalpic response of glassy polymers previously compressed to different strains, “volumetric implosion” on samples subjected to tensile strains, and the dependence of the shift factor on aging time and applied stress. [ABSTRACT FROM AUTHOR]

Dispersions of multiarm star polymers in an athermal solvent are studied as a model system to explore the effects of soft colloidal interactions on the dynamics of colloidal glasses. Linear viscoelastic measurements in the glassy state are congruent with Mode Coupling Theory predictions for hard sphere glasses at moderate frequencies, indicating similarities in the relaxation processes of hard and soft colloidal glasses near equilibrium. On the other hand, distinct features of the star relaxation (related to arm disengagement) are observed to affect the nonlinear behavior associated with shear melting of the glass, which exhibits transitions previously unreported for hard sphere systems. Whereas a single maximum in G″(γ) is evident under large amplitude oscillatory shear at frequencies near the beta relaxation, secondary transitions between yielding and the onset of macroscopic flow are observed at higher and lower frequencies. The latter are annealed out upon complete fluidization of the star polymer suspension. The overall nonlinear behavior of the soft glassy suspension is interpreted in terms of two primary mechanisms: (i) deformation and yielding of the cage-like glassy microstructure, as in the hard sphere case, and (ii) local stretching and disengagement of interpenetrating arms of the colloidal star polymers. [ABSTRACT FROM AUTHOR]

Our previous, extensively validated, nonlinear viscoelastic formalism for glassy polymers is extended to include the effects of chemical reaction. Two modifications are necessary. First, the extent of reaction represents an additional degree of freedom that must be included in the free energy series expansion. Second, simultaneous reaction and deformation can lead to “compression set” that is represented by an evolving stress-free configuration in the cross-linked solid. The material clock, which is based on potential energy and must also incorporate these modifications, naturally predicts the increase in glass transition with cure. [ABSTRACT FROM AUTHOR]

We investigate the concentration dependence of the characteristic relaxation time of dilute polymer solutions in transient uniaxial elongational flow. A series of monodisperse polystyrene solutions of five different molecular weights (1.8×106<=M<=8.3×106 g/mol) with concentrations spanning five orders of magnitude were dissolved in two solvents of differing solvent quality (diethylphthalate and oligomeric styrene). Optical measurements with a capillary breakup extensional rheometer of the rate of filament thinning and the time to breakup in each fluid are used to determine the characteristic relaxation time. A criterion for a lower sensitivity limit is introduced, in the form of a minimum concentration cmin necessary for experimental resolution of the effects of polymeric viscoelasticity. This criterion is validated by experiment and comparison to numerical calculations with a multimode bead-spring dumbbell model. These calculations also rationalize previous paradoxical observations of extensional thinning in fluid threads of ultradilute polymer solutions in which stress relaxation apparently occurred faster than predicted by the Zimm theory. Above this minimum sensitivity limit we show that the effective relaxation time of moderately dilute solutions (0.01<=c/c*<=1) in transient extensional flow rises substantially above the fitted value of the relaxation time extracted from small amplitude oscillatory shear flow and above the Zimm relaxation time computed from kinetic theory and intrinsic viscosity measurements. This effective relaxation time exhibits a power-law scaling with the reduced concentration (c/c*) and the magnitude of the exponent varies with the thermodynamic quality of the solvent. The scaling of this “self-concentration” effect appears to be roughly consistent to that predicted when the dynamics of the partially elongated and overlapping polymer chains are described within the framework of blob theories for semi-dilute solutions. [ABSTRACT FROM AUTHOR]

This work is devoted to the study of the rheological properties of sodium montmorillonite suspensions in aqueous media containing polyelectrolytes in solution. Two different polyelectrolytes are employed: polyacrylic acid (PAA) and polyethyleneimine (PEI). PAA can bear negative charge, thus acting as a polyanion, while PEI can be considered as a polycation, although the charge of both polymers is strongly dependent on pH of the solution. The rheological behavior of clay suspensions is essentially determined by the electric potential of the faces and edges of the laminar clay particles. In order to analyze the changes in the interfacial electric potential of clay surfaces, the zeta potential of clay particles was estimated from electrophoresis measurements for different solution compositions. The yield stress and the storage modulus of the suspensions were determined demonstrating that only in some cases the storage modulus can be correlated with the changes in electrostatic interactions between particles. In particular, in clay/PEI suspensions at neutral-basic pH the changes in the viscoelastic properties do not match with those in surface-to-surface electrostatic interactions. Different mechanisms are proposed to explain the wide variety of rheological phenomena observed. [ABSTRACT FROM AUTHOR]

A magnetic resonance elastography (MRE) based method that consists in measuring dynamic viscoelastic parameters of soft matter by analysis of propagating shear waves at different frequencies is proposed. Dynamic shear tests were performed on soft gels with a dedicated magnetic resonance imaging system at low field (0.1 T) and were compared with results obtained with a mechanical rotational rheometer. Storage and loss moduli were plotted against frequency with both methods and good agreement was found between their results in the shared frequency range. Therefore, it is shown that the described method could be a reliable and accessible tool for three-dimensional dynamic mechanical analysis on soft matter able to overcome dynamic range, sample dimensions and directional limitations of mechanical rheometers. Advantages and limits of the method are discussed. [ABSTRACT FROM AUTHOR]

The linear viscoelastic properties of binary blends of monodisperse linear polymers of various polymers, including polyisoprene, polybutadiene, and polystyrene, are compared with the predictions of the model of Park and Larson (2004), which is an extension of the Milner-McLeish model [Milner and McLeish (1998)]. For each polymer system the model parameters needed in the predictions of the binary blend are obtained from experimental data for monodisperse polymers. For well entangled blends of nearly monodisperse polymers, the model predictions are in good agreement with experimental data for a wide range of values of the Graessley parameter Gr, which is proportional to the ratio of the reptation time of the long chain in the undilated tube to the constraint-release Rouse time. Our results show that for polymers of multiple chemical types, the terminal behavior of the long chain is controlled by reptation in an undilated tube for small GrGrc, where Grc is around 0.06. For values of Gr near Grc, behavior intermediate between reptation in a dilated and an undilated tube is found. [ABSTRACT FROM AUTHOR]

The coalescence in air of two polymeric drops into a single drop (also referred to as sintering) was investigated for two thermotropic liquid crystalline polymers. Initial coalescence via elastic contact was ruled out based on the magnitude of the equilibrium compliance values and the process was, therefore, believed to be driven by surface tension and resisted by means of viscous flow. Remarkably the viscous coalescence model developed for Newtonian fluids (an extension of the Frenkel and Eshelby approach) agreed well under some conditions of temperature with coalescence data (i.e., observation of neck growth under a microscope). On the other hand, the extension of the Newtonian model to the viscoelastic case by incorporating the upper convected Maxwell model (UCM) assuming steady state stresses always underpredicted the rate of coalescence. The viscous neck growth model using the UCM constitutive equation was extended to the transient stress case in order to incorporate the slow growth of viscosity at the startup of flow. The unsteady state UCM approach represented a qualitative improvement over the Newtonian and steady state UCM formulations because it predicted accelerated coalescence, relative to the Newtonian model, by increasing the relaxation time. However, the model was unable to quantitatively predict the experimental coalescence rates, as it overpredicted the acceleration of coalescence. [ABSTRACT FROM AUTHOR]

We have developed a sensitive method to detect very low levels of long chain branching in sparsely branched polymers and applied it to high-density polyethylene samples with broad molar mass distribution obtained by Ziegler–Natta, Phillips, and metallocene catalysis. We compare experimental dynamic moduli with predicted values. The predicted moduli, which are only valid for truly linear chains, are computed from a known molar mass distribution by the application of a modified time-dependent diffusion reptation model described in [van Ruymbeke et al., Macromolecules 35, 2689 (2002)]. Discrepancies between experimental and predicted moduli are observed at branching levels below the usual detection limit of 13C nuclear magnetic resonance (about 1 branch per 10000 carbon atoms). Our method is easier to implement and the results more clearcut than those obtained by the zero-shear viscosity method. The sensitivity is comparable with the activation energy spectrum method described in [Wood-Adams et al., Macromolecules 33, 7489 (2000)]. The use of our method is however not restricted to thermorheologically complex polymers. The interest of polymer fractionation is also demonstrated for the detection of extremely low levels of long chain branching. [ABSTRACT FROM AUTHOR]

The creep compliance and relaxation functions used in characterizing the mechanical response of linear viscoelastic solids are traditionally found by conducting two separate experiments. Alternatively, one of the functions may be determined from a single experiment while the other is obtained through interconversion. All direct interconversion methods, however, require the solution of an ill-posed problem. The goal of this paper is to present the theoretical framework for developing a new apparatus, based on “spring loading,” which facilitates the determination of both creep and relaxation functions from a single experiment. There is no need for interconversion. Questions of stability with respect to the measured data are discussed and a stable numerical algorithm is presented. [ABSTRACT FROM AUTHOR]

We study the motion of a Brownian probe particle subjected to a small amplitude oscillatory external force and immersed in a colloidal dispersion, as a theoretical benchmark for particle-tracking microrheology experiments. The application of an external force on the probe drives the microstructure of the dispersion out of equilibrium; opposing this is the Brownian diffusion of the probe and colloidal “bath” particles. The degree to which the microstructure is driven away from equilibrium is given by the nondimensional external force on the probe, or Péclet number, Pe. The nonequilibrium microstructure of the dispersion is calculated for small departures from equilibrium, i.e., to first order in Pe, and to leading order in the bath particle volume fraction, accounting for excluded volume and hydrodynamic interactions between particles. The nonequilibrium microstructure is used to calculate the average velocity of the probe, from which one may infer the complex microviscosity (or modulus) of the dispersion via application of Stokes drag law. The microviscosity is computed over the entire range of oscillation frequencies, thereby determining the linear viscoelastic response of the dispersion. After appropriate scaling, our results are in qualitative agreement with traditional (macro-) rheology studies, suggesting that oscillatory-probe microrheology can be a useful tool to examine the viscoelasticity of hard-sphere colloidal dispersions and perhaps other complex fluids. [ABSTRACT FROM AUTHOR]

We investigate active particle-tracking microrheology in a colloidal dispersion by Brownian dynamics simulations. A probe particle is dragged through the dispersion with an externally imposed force in order to access the nonlinear viscoelastic response of the medium. The probe’s motion is governed by a balance between the external force and the entropic “reactive” force of the dispersion resulting from the microstructural deformation. A “microviscosity” is defined by appealing to the Stokes drag on the probe and serves as a measure of the viscoelastic response. This microviscosity is a function of the Péclet number (Pe=Fa/kT)—the ratio of “driven” (F) to diffusive (kT/a) transport—as well as of the volume fraction of the force-free bath particles making up the colloidal dispersion. At low Pe—in the passive microrheology regime—the microviscosity can be directly related to the long-time self-diffusivity of the probe. As Pe increases, the microviscosity “force-thins” until another Newtonian plateau is reached at large Pe. Microviscosities for all Péclet numbers and volume fractions can be collapsed onto a single curve through a simple volume fraction scaling and equate well to predictions from dilute microrheology theory. The microviscosity is shown to compare well with traditional macrorheology results (theory and simulations). [ABSTRACT FROM AUTHOR]