 |
 |
|
|
 |
Journal of RheologyVolume 42, Issue 5 (September-October 1998) |
Contents
Gel Transition Studies On Non-Ideal Polymer Networks Using Small Amplitude Oscillatory Rheometry D. J. Power1, A. B. Rodd1, L. Paterson2, and D. V. Boger1a 1Department of Chemical Engineering, The University of Melbourne 2CSIRO Division of Petroleum Resources Melbourne, Australia aCorresponding Author AbstractHydroxypropylguar (HPG) readily forms a physical gel in the presence of borate ions. A discussion of gel properties is given, with emphasis on the rheological properties of borate crosslinked HPG at the structural transition point. A gel point characterisation method developed by Winter and Chambon (1986) maybe used to define the exact point of transition from liquid behavior to that of a true chemical network with permanent crosslink sites. This technique has been applied to the HPG system described above whose crosslink sites (or junction zones) are at a pH activated dynamic equilibrium. Analysis of the loss modulus at different stages of the solution transition presents a new method for establishing the solution/gel transition of borate crosslinked HPG. A maximum in the loss modulus is observed to correspond very closely to the gel transition determined using the method of Winter and Chambon. The steady gel properties of the fluid are studied as a function of the concentration of both the polymer and the crosslinking species. The gel point relaxation exponent n varies between 0.14 and 0.5, depending on the concentration of the species which constitute the gel network. Results from both rheological techniques are presented as well as a discussion of their respective limitations. Small amplitude oscillatory measurements are performed using a constant strain Weissenberg Rheogoniometer for all rheological measurements. Rheology And Dynamics Of Sheared Arrays Of Colloidal Particles Jeffrey J. Gray* and Roger T.
Bonnecaze† *E-mail: jeff@che.utexas.edu AbstractConcentrated suspensions of colloidal particles undergo dynamical microstructural transitions under shear. During the transition from oscillating face-centered cubic twin structures to sliding layer structures, the system can exhibit hysteretic and discontinuous rheology as the shear rate is varied. We capture this behavior with a dynamic simulation of a sheared lattice of non-Brownian spherical particles with screened electrostatic interactions and hydrodynamic interactions determined using the Stokesian dynamics approximation. Rheological data are determined for a range of volume fractions, electrostatic screening lengths and shear rates or shear stresses. In controlled stress simulations, static yield stresses are observed. In controlled shear rate simulations of certain lattice orientations, plateau viscosities are observed at high and low shear rates with a high to low shear rate plateau viscosity ratio ranging from 1.4 to 2.2. Large viscosity transitions with hysteretic-like rheology are observed only in controlled shear rate simulations of face-centered cubic (111) layers sheared parallel to the (211) direction with full representation of the hydrodynamic particle interactions. Rheological curves collapse when stresses are scaled by the elastic modulus and shear rates by the elastic modulus divided by the high-shear-rate limiting viscosity. The magnitude of the hysteretic viscosity jump and the scaled critical stresses match experimental values. Closure Approximations For The Doi Theory: Which To Use In Simulating Complex Flows Of LCPs J. Feng, C. V. Chaubal, and L. G. Leal AbstractThe goal of this paper is to determine which closure model should be used in simulating complex flows of liquid-crystalline polymers (LCPs). We examine the performance of six closure models: the quadratic closure, a quadratic closure with finite molecular aspect ratio, the two Hinch-Leal closures, a hybrid between the quadratic and the first Hinch-Leal closures and a recently proposed Bingham closure. The first part of the paper studies the predictions of the models in homogeneous flows. We generate their bifurcation diagrams in the (U,Pe) plane, where U is the nematic strength and Pe is the Peclet number, and place special emphasis on the effects of the flow type. These solutions are then compared with the "exact solutions" of the unapproximated Doi theory. Results show the Bingham closure to give the best approximation to the Doi theory in terms of reproducing transitions between the director aligning, wagging and tumbling regimes at the correct values of U and Pe and predicting the arrest of periodic solutions by a mildly extensional flow. In the second part of the paper, we employ the closure models to compute a complex flow in an eccentric cylinder geometry. All the models tested predict the same qualitative features of the LCP dynamics. Upon closer inspection of the quantitative differences among the solutions, the Bingham closure appears to be the most accurate. Based on these results, we recommend using the Bingham closure in simulating complex flows of LCPs. The Dynamics Of Ultradilute Polymer Solutions In Transient Flow: Comparison Of Dumbbell-Based Theory And Experiment Graham M. Harrison, Johan Remmelgas, and L.
Gary Leal AbstractPredictions of the Chilcott-Rallison FENE dumbbell model for polymer configuration in start-up flow of a co-rotating two-roll mill are compared with birefringence measurements in a 40 ppmw solution of high molecular weight polystyrene. Two versions of the Chilcott-Rallison model are considered, one with a constant bead friction and the other with bead friction that is assumed to increase in proportion to the end-to-end dimension of the dumbbell. Parameters for the model were determined via independent experiments. We show that the flow, at this concentration level, is unchanged from the form for a Newtonian fluid. Measurements and predictions of the birefringence are compared at the stagnation point of the flow, where a high degree of polymer stretch is possible. The Chilcott-Rallison model with constant bead friction, and an approximation to the inverse Langevin spring function, gives excellent agreement with the experimental results. Polymer Modified Asphalts As Viscoelastic Emulsions Didier Lesueur*, Jean-François Gérard Pierre Claudy Jean-Marie Létoffé Didier Martin, Jean-Pascal Planche *Present Address: Laboratoire
Central des Ponts et Chaussées AbstractLinear viscoelastic properties of Polymer Modified Asphalts (PMAs) were studied at various temperatures and frequencies. The materials consisted of blends of paving grade Asphalt Cements (ACs) and diblock poly(styrene-b-butadiene) (SB) or triblock poly(styrene-b-butadiene-b-styrene) (SBS) copolymer up to 6 wt.% concentrations, which yielded heterogeneous PMAs with an emulsion-like morphology a polymer rich phase dispersed within an asphalt phase. In addition, the 6% SB modified binder was studied before and after dynamic vulcanization (i.e. in-situ crosslinking of the polymer-rich inclusions to increase the PMA stability). The rheological response of the blends was calculated using the Palierne emulsion model, knowing the mechanical properties of each phase, the volume fraction of dispersed phase and the capillary number of the dispersed droplets. The interfacial tension then acted as an adjustable parameter and was estimated to be of order of 10-5 N/m.As a consequence of the colloidal nature of ACs (a solid phase — the so-called asphaltenes — dispersed in a liquid phase, the maltenes), their mechanical properties were highly improved after polymer modification for two reasons. (1) Swelling of the polymer caused a decrease in the maltene content of the matrix, leading to an increase in its asphaltene content when compared to the initial material composition and a subsequent increase in modulus. (2) The presence of the dispersed phase modifies the rheology of the materials as described by the Palierne emulsion model. Thus, a highly swollen polymer will create a very hard matrix with a high volume fraction of soft inclusions, whereas a lightly swollen polymer will generate a blend with a matrix almost similar to the original AC with a low volume fraction of harder polymer particles. Therefore, the swelling extent of the polymer should be controlled to optimize the properties of PMAs. Evaluation Of Models Combining Rheological Data With The Molecular Weight Distribution D. Maier 1*, A. Eckstein 1, Chr. Friedrich 1†, J. Honerkamp 1,2 1Freiburger Materialforschungszentrum, Stefan-Meier-Straße 21D-79104 Freiburg im Breisgau, Germany 2 Universität Freiburg, Fakultät für Physik, Herrmann-Herder-Straße 3D-79104 Freiburg im Breisgau, Germany *E-mail: dima@fmf.uni-freiburg.de AbstractThe aim of this article is to test and to improve existing models combining rheological data with the molecular weight distribution (MWD). This process of testing and improving was separated into two distinct steps: First, the mixing behavior of the relaxation modulus obtained from a polymer blend was investigated and compared with the proposed mixing behavior derived from the linear and the quadratic mixing rules with a mixing parameter b equal to 1 and 2. Second, for the first time, kernel functions weighted with the MWD are estimated directly from experimental data and compared to the weighted kernel functions published in the literature. This procedure was performed with polystyrene blends composed of two monodisperse polymers with narrow MWD determined using size exclusion chromotography (SEG). It was shown that the existing mixing rules are not able to adequately describe the mixing behavior of the polystyrene blends. An improved mixing rule was derived with b of 3.84. Furthermore, only for the improved model, a kernel function exists which is able to describe the experimental data over a wide molecular range. It is shown how improved estimates of the MWD of a polymer can be determined using the new model.Transient Response Of Electrorheological Effect To A Step Field In An Immiscible Polymer Blend · First Mode In Type I Blend Kozo Tajiri, Hiroshi Orihara, Yoshihiro
Ishibashi, Masao Doi Akio Inoue AbstractWe have investigated the initial transient response of an immiscible polymer blend electrorheological fluid to a step electric field by means of microscopic observations and measurements of the transmitted light intensity and the shear stress. In the absence of shear flow, the transmitted light intensity increased steeply in the early stage after applying a step field, corresponding to the elongation and coalescence of droplets of one polymer dispersed in the other to form bridges along the field. In the presence of shear flow, on the other hand, the increase of transmitted light intensity was followed by that of shear stress. From this fact it was clarified that the stretched structure consisting of elongated droplets and bridges tilt from the direction of the field in addition to the elongation under shear flow, and the electrorheological effect in the early stage should be ascribed to the formation of elongated structures followed by the tilt of them. Shear Thickening In Low-Concentration Solutions Of Worm-Like Micelles I: Direct Visualization Of Transient Behavior And Phase Transitions Y. T. Hu and Philippe Boltenhagen1 Department of Chemical Engineering, University of California Santa Barbara, CA 93106-5080 D.J. Pine2 1 Present address: Laboratoire d'Ultrasons et de Dynamique des Fluides Complexes,URA au CNRS no. 851, Université Louis Pasteur - rue Blaise Pascal, 67070 Strasbourg, France2 Corresponding author, E-mail: pine@engineering.ucsb.eduAbstractShear thickening of low-concentration solutions of worm-like micelles is investigated using simultaneous rheological and visualization measurements. Shear-induced structures (SIS) are directly visualized in transparent Couette cells using a laser light scattering technique similar to dark-field microscopy. From these measurements, four different regimes of behavior are identified. In regime I, which occurs below a critical shear stress sc, the shear rate increases monotonically with stress and no shear-thickening or SIS are observed. In regime II, which occurs for stresses greater than sc but less than ss, SIS nucleate inhomogeneously and grow from the inner cylinder of the Couette cell. In this regime, the steady state shear rate initially decreases with increasing stress and then increases again as the stress is raised. The steady state in regime II is characterized by two coexisting states separated by a cylindrical interface (concentric with the Couette cylinders). Near the inner cylinder, viscous SIS are observed, while near the outer cylinder, a much less viscous fluid similar to the original micellar solution is observed. The steady state in regime II is observed only under conditions of controlled stress. In regime III, which occurs for stresses above ss but below sf, SIS nucleate homogeneously throughout the shear cell and appear to fill the gap. Regime IV is characterized by fracture of SIS and is observed at sf . The shear-thickening transition bears many similarities to a phase transition as opposed to a simple hydrodynamic instability.Shear-Thickening In Low-Concentration Solutions Of Worm-Like Micelles II: Slip, Fracture, And Stability Of The Shear-Induced Phase Y. T. Hu and Philippe Boltenhagen1 Department of Chemical Engineering, University of California, Santa Barbara, CA 93106-5080 Eric Matthys D.J. Pine2 1 Present address; Laboratoire d'Ultrasons et de Dynamique des Fluides Complexes, URA au CNRS no. 851, Université Louis Pasteur - rue Blaise Pascal, 67070 Strasbourg, France2 Corresponding author, E-mail: pine@engineering.ucsb.eduAbstractThe rheology of the shear-thickened state is investigated in low-concentration solutions of worm-like micellar solutions using mechanical, optical, and velocity profile measurements. The zero-shear-rate viscosity of the solutions increases by more than a factor of 1000 as the concentration of surfactant is increased from 1 mM to 10 mM. By contrast, the apparent viscosity of the shear thickened state of these same solutions is observed to be remarkably independent of concentration over a wide range of shear rates. This is shown to be a consequence of the development of slip layers between the very viscous gel-like shear-induced structures (SIS) which form in the bulk of the surfactant solution and the walls of the Couette devices in which the measurements are made. As the applied shear stress is increased even further, there is evidence that the SIS fractures, giving rise to a shear-rate-independent stress and an apparent viscosity which decreases with increasing shear rate. After the SIS fractures, large fluctuations in the shear rate are observed to result from an SIS which alternately fractures, slips, heals, and refractures. The fracture stress is proportional to the concentration and inversely proportional to the cell radius. Probing Magnetic Paints Through Magnetorheological And Susceptibility MeasurementsAndrei A. Potanin, Ronald J. Hirko Viktor T. Peikov, Alan M. Lane AbstractLinear viscoelastic measurements with and without a superimposed magnetic field are used to characterize the structure of model magnetic paints comprised of metal particles, cyclohexanone and polyvinylchloride wetting resin. Simple network modeling is used to derive a formula for the field-induced increase of G' with the single parameter, critical field Hcrit, which characterizes dispersion quality by how easily particles align under the magnetic field. Milling weakens the structure of the paint, provided sufficient resin concentration is maintained, as revealed by a decrease of G' and Hcrit with milling time. At low resin concentration G' increases with milling time indicating that milling induces structural reorganization which, being unsupported by resin adsorption, results in forming a stronger network. Magnetorheological measurements are correlated with susceptibility measurements in a small alternating magnetic field which probes particle interactions without breaking the structure. As the resin concentration increases, rheological spectra shift to shorter relaxation times due to structure transformation from a network to small aggregates. Susceptibility spectra shift to longer times due to weakening of particle interactions. Shear-Induced Mixing And Demixing In Poly(Styrene-Co-Maleic Anhydride)/Poly(Methyl Methacrylate) Blends Divya Chopra*, Dimitris Vlassopoulos Savvas G. Hatzikiriakos *Present address: Department of Chemical Engineering AbstractThe effects of shear flow on the phase behavior of a polymer blend with high glass transition temperature (Tg) constituents possessing large viscosity difference were investigated using shear and capillary rheometry, complemented by differential scanning calorimetry and analysis of the extrudates with scanning electron microscopy. The blend is a lower critical solution temperature polymer mixture of a random copolymer of styrene and maleic anhydride, SMA (Tg=178 °C), and poly(methyl methacrylate), PMMA, (Tg=105°C). Depending on temperature, both shear-induced mixing, typically at very high shear rates, and shear-induced demixing, typically at moderate shear rates, were observed. In the former case, extrudates were optically transparent, yielding one Tg and were thermorheologically simple at all temperatures below and up to the capillary extrusion one. On the other hand, extrudates related to shear-induced demixing were opaque, yielding two distinct Tg's and were thermorheologically complex. The experimental methodology presented here for the determination of the shear-phase diagram in a flowing polymer blend should be applicable to any industrial mixture, and it is of particular value for assessing the effects of strong shear flow, relevant in polymer processing applications. Finally, the method of blend preparation, i.e., solution-cast versus melt-mixed samples, affected slightly the rheologically determined demixing temperatures.Extrudate Swell Behavior Of Polyethylenes: Capillary Flow, Wall Slip, Entry/Exit Effects And Low Temperature Anomalies Xiaoping Yang, Shi-Qing Wang*, C. Chai† Department of Macromolecular Science Case Western Reserve University Cleveland, Ohio 44106-7202 *Corresponding Author, e-mail: sxw13@po.cwru.edu †BP Chemicals s.n.c., Boîte postale n° 6, 13117 Lavéra, France AbstractThis paper describes a comprehensive experimental study of the extrudate swell behavior of high density and linear low density polyethylenes. The contributions from bulk melt elasticity, entry extensional flow and exit boundary discontinuity are each explicitly investigated as a function of molecular weight and its distribution. The elusive apparent molecular weight dependence of the transient extrudate swell ratio (ESR) is elucidated to arise from the different molecular relaxation rates. A master curve is obtained at a fixed stress for the time-dependent ESR of different molecular weights when represented in terms of the rescaled time, t/ t, where t is the actual time allowed for growth of ESR and t is related to the overall molecular relaxation time of each HDPE. The intricate influence of the exit boundary discontinuity on the extrudate swell is illustrated on the basis of the recently accumulated precise knowledge of wall slip behavior of linear polyethylenes (LPE). Wall slip localized at the die exit wall is demonstrated to reduce the ESR of LPE under all stresses below an interfacial stick-slip transition. The entry effect is characterized in two ways: by comparing the ESR values below and above the stick-slip transition of the HDPE at the critical stress, and by comparing the ESR from a long die of aspect ratio L/D=15 with that from an orifice die of L/D=1/4. The HDPE samples are found to exhibit an anomalously larger ESR above a critical stress at low temperatures (from 150 to 160°C).The full content of this work has been presented at the 69th Annual Society of Rheology Meeting (October 21, 1997). Thermodynamics Of Viscoelastic Fluids: The Temperature EquationPeter Wapperom, Martien A. Hulsen AbstractFrom the thermodynamics with internal variables we will derive the temperature equation for viscoelastic fluids. We consider the type of storage of mechanical energy, the dissipation of mechanical energy, the compressibility of the fluid, the non-equilibrium heat capacity and thermal expansion, and deformation induced anisotropy of the heat conduction. The well known stress differential models that fit into the thermodynamic theory will be treated as an example. Adapting a power-law scaling of the shear moduli on temperature and density, as is usual in rubber elasticity, we will derive an approximation of the temperature equation in measurable quantities. This equation will be compared with experimental results. |
Please e-mail suggestions and comments to albertco@umche.maine.edu. Updated 25 January 2004 |