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Journal of RheologyVolume 43, Issue 2 (March-April 1999) |
Contents
Determination Of Shear Rate And Viscosity From Batch Mixer DataM. Bousmina*, A. Ait-Kadi and J. B. Faisant†, *Corresponding author. E-mail: bousmina@gch.ulaval.ca AbstractA general analysis allowing the determination of shear rate and viscosity from batch mixer rotor speed and torque data is presented. The batch mixer was represented by two effective adjacent sets of concentric cylinders exerting the same torque as that obtained from the batch mixer. The effective internal radius was determined through a general procedure for calibration using non- Newtonian fluid. The effective equivalent internal radius, Ri, was determined for different polymers and processing conditions. The results revealed that Ri is a universal quantity practically insensitive to the nature and to the rheological behavior of the fluid under mixing. In the case of small gaps, it was found that there is a special position in the gap where the effective internal radius, the shear rate and viscosity are independent of rheological characteristics of the fluid under mixing. This validates the Newtonian approximation previously used by Goodrich and Porter to extract the shear rate-viscosity dependence from batch mixer data. The technique was tested on 7 different amorphous and semi-crystalline polymers and the results were found to be in reasonable agreement with the data obtained independently with cone-and-plate and capillary rheometers. Contributions of both shear stress between the two cylinders and the stress generated at the wall were evaluated. The latter was found predominant. Steady-State Behavior Of Dilute Polymers In Elongational Flow. Dependence Of The Critical Elongational Rate On Chain Length, Hydrodynamic Interaction And Excluded VolumeJ. G. Hernández Cifre and J. García de la Torre AbstractThe steady-state properties of flexible polymer chains in solutions undergoing elongational flow have been studied using Brownian dynamics simulation. The coil-stretch transition is observed when the elongational rate, e, exceeds a certain critical value ec. In this work, we describe in detail the simulation procedure and how to extract polymer dimensions, solution viscosity and birefringence from the trajectories. Preliminary simulations involving no hydrodynamic interaction (HI) are used to check the simulation procedures by comparing their results with theoretical predictions for such an (unphysical) case. Afterwards, simulations with fluctuating non-averaged HI are carried out to provide results comparable with experiments. After simulations with and without intramolecular potential, we arrive at a most important conclusion: the chain length dependence of ec is the same in theta conditions as in good solvent conditions. Combining ec with other solution properties such as the longest relaxation time, the intrinsic viscosity and the radius of gyration, dimensionless compound quantities can be formulated. From our simulation results, we obtain numerical values for such quantities, which include the HI effect, and which are therefore useful for analysing experimental data. Disappearance Of Extrusion Instabilities In Brass Capillary Dies†Venu G. Ghanta, Brian L. Riise*, and Morton M. Denn** *Present address: MBA Polymers, 500 West Ohio Avenue, Richmond, CA 94804 **Corresponding author. E-mail: denn@socrates.berkeley.edu AbstractWe have extruded a linear low-density polyethylene through capillary dies fabricated from stainless steel and brass. We confirm a result first reported by Ramamurthy: sharkskin can be eliminated by the use of a brass die. We also find a substantially enhanced throughput from the brass die relative to the stainless-steel die at stresses in the range where sharkskin is observed with the latter. Finally, the large pressure oscillations and periodic extrudate distortions observed in the "slip-stick" region with stainless steel are absent with brass, where the transition to the "upper branch" of the flow curve is more gradual. †The editorial procedure for this manuscript was carried out by Editorial Board member Kurt Wissbrun. Singular Behavior Of Rouse-Like SpectraA. I. M. Denneman*, R. J. J. Jongschaap, and J. Mellema *Corresponding author AbstractIn this study an extended Rouse formalism is used to obtain results belonging to (i) linear chains (K beads; K – 1 Hookean springs), (ii) K × K cubic networks or (iii) K × K × K cubes immersed in a Newtonian fluid. These three systems are subjected to small-amplitude oscillatory shear flow of frequency w and the resulting dynamic moduli G' (w) and G" (w) show significant differences. For large K, the corresponding relaxation spectra H(l) appear to be smooth functions of time l, in which some obvious discontinuities are observed, i.e. the so-called Van Hove singularities (well-known in solid state physics). Actually, for these three systems a relation is obtained between H(l) and the frequency spectra g(w) used in solid state physics. A Phenomenological Model Of An Elastomer With An Evolving Molecular Weight DistributionStephen B. Smeulders* and Sanjay Govindjee† *E-mail: lextern@aol.com AbstractWe present a model of an elastomeric material whose network microstructure is characterized by a Molecular Weight Distribution (MWD). The MWD is allowed to evolve in time and the evolution is governed by an integro-differential equation that is based on chemo-kinetic mechanisms of crosslink degradation and main chain scission. With respect to main chain scission, we differentiate between random and centrally weighted events. We further introduce a particular network free energy function that is a generalization of the eight-chain model and a finite deformation viscoelasticity model. For this combination, we develop closed form solutions to two experimentally imposable strain histories. These results permit the calibration of the MWD evolution model. A sample calibration of the developed formalism is shown against hypothetical "experimental data." State Diagrams Of Soy GlobulinsAlfredo Morales and Jozef L. Kokini* *Corresponding author. AbstractState transitions of 7S and 11S soy globulins were studied as a function of moisture by monitoring their rheological and calorimetric properties. Small amplitude oscillatory rheometry and differential scanning calorimetry (DSC) were used to characterize their denaturation and complexing reactions within a temperature range of 40-200°C and moistures contents of approximately 20%, 30% and 40%. Major increases in rheological properties showed that the 7S and 11S fractions with 40% moisture underwent structure forming reactions above 70°C and 100°C, respectively, while they reacted at higher temperatures for lower moisture. DSC helped to identify the individual complexing behavior of each fraction. The above, along with the information from their glass transition, previously reported, allowed to develop state diagrams for each protein, where glassy, rubbery, entangled polymer, reaction and free flow states were identified. Rheological Properties Of Two Polypropylenes With Different Molecular StructureS. Kurzbeck*, F. Oster, H. Münstedt T.Q. Nguyen, R. Gensler *Corresponding author AbstractTwo polypropylenes with different molecular structure were investigated in shear and elongational flow. One resin is an ethylene-propylene copolymer with linear molecules and the other is a long-chain branched propylene homopolymer. The elongational flow behaviour was investigated in stressing and creep experiments. For the linear material no strain hardening can be observed whereas the strain-hardening behaviour of the long-chain branched material is more pronounced than it was found for any other polyolefin up to now. The branched resin also exhibits high recoverable strains. The strain-hardening behaviour is analysed with respect to its strain and strain rate dependence. The results are discussed with regard to the molecular structure. The coexistence of a long-chain branched structure and a high molecular weight component is assumed to be the reason for the outstanding elongational flow behaviour of the branched polypropylene. Brownian Dynamics Simulations Of A DNA Molecule In An Extensional Flow FieldR. G. Larson, Hua Hu D. E. Smith, S. Chu AbstractThe unraveling dynamics of long, isolated, molecules of DNA subjected to an extensional flow in a crossed-slot device (Perkins et al. 1997; Smith and Chu 1998) are predicted by Brownian dynamics simulations using measured elastic and viscous properties of the DNA as the only inputs. Quantitative agreement is obtained both in the percentages of various unraveling states, such as "folds," "kinks", "dumbbells," half-dumbbells," and "coils," and in the ensemble-averaged stretch and rate of stretch. Under fast flows (De ³ 10), unraveling is initially nearly affine, but for fractional stretch greater than » 1/3, stretching is delayed to an extent that varies widely from molecule to molecule by flow-induced folded states, which are far-from-equilibrium kinetic hindrances not predicted by dumbbell models. From the computer simulations, the source of the high molecule-to-molecule heterogeneity in the experiments is traced to variability in the initial polymer configuration, which sets the unraveling path the molecule must take at De ³ 10. Formation of folds and kinks during unraveling can be predicted fairly reliably just by examining the initial state. The high-De unraveling behavior is consistent with the predictions of one-dimensional "kink dynamics" simulations. The Application Of The Multipass Rheometer To Time Dependent Capillary Flow Measurements Of A Polyethylene MeltM. Ranganathan, M. R. Mackley, and P. H. J. Spitteler AbstractThis communication presents experimental data on the time dependent capillary flow measurements for a high density polyethylene (HDPE) using a newly developed Multipass Rheometer (MPR). The experimental data consist of precision pressure measurements on the MPR which has well defined fully constrained boundary conditions. A model developed previously by Molenaar and Koopmans (1994) and incorporating melt compressibility has been adapted to suit the MPR boundary conditions and is shown to give good agreement with the experimental data without necessitating the use of any arbitrary adjustable parameters. The results in particular show that the experimentally observed pressure relaxation on cessation of the piston movement can be almost entirely attributed to the compressibility of the melt alone and this conclusion is consistent with the findings reported by Hatzikiriakos and Dealy (1994) for related time dependent extrusion experimental observations using a conventional capillary rheometer. Shear Elasticity And Yield Stress Of Silica-Silicone Physical Gels: Fractal Approach J-M. Piau*, M. Dorget+, J-F. Palierne++ A. Pouchelon * Corresponding author ** Université Joseph Fourier Grenoble 1 + Present address: Rhône-Poulenc Recherche, 52 rue de la Haie CoqF-93308 Aubervilliers Cedex ++ Present address: ENS, 46 allée d'Italie, 69364 Lyon Cedex 07AbstractThe elastoplastic behaviour of silica-silicone compounds has been characterised by a yield stress ss and an elastic modulus Go. Scaling laws have been established for the changes in rheometrical parameters with the volume fraction Fn of silica: ss µ (Fn)3.3 and Go µ (Fn)4.2 and single master curves have been obtained whatever the silica type used. The mesoscopic structure of the compounds has been studied using X-ray and light scattering and a semi-dilute fractal structure has been observed, with a fractal dimension D » 1.8 and a characteristic length scale 4 mm. In this paper, a formulation of the non fluctuating semi-dilute fractal concept allows us to relate the fractal dimension to the rheological scaling laws: ss µ (Fn)4/(3-D) and Go µ (Fn)5/(3-D). Self Oscillations In Capillary Flow Of Entangled PolymersS. Q. Wang* and N. Plucktaveesak *E-mail: sxw13@po.cwru.edu AbstractA well-established "co-extrusion" technique is applied to visualize and document a newly found self-oscillation due to an unstable boundary condition in capillary flow of high density polyethylene. In contrast to the other types of self-oscillations of interfacial origin reported in the literature, the phenomenon can only occur in a die where the upstream portion of the capillary die wall has a surface condition different from that of the downstream portion. The self-oscillation phenomenon takes place under a constant pressure when one half of the die wall (coated with a layer of fluoropolymer) assumes steady slip due to adhesive breakdown, and the boundary condition of the other half switches between no-slip and slip states through the cohesive chain disentanglement mechanism. A New Transient Network Model For Associative Polymer NetworksR. H. W. Wientjes, R. J. J. Jongschaap, M. H. G. Duits, and J. Mellema AbstractA new model for the linear viscoelastic behaviour of polymer networks is developed. In this model the polymer system is described as a network of spring segments connected via sticky points (as in the Lodge model). An important extension (with respect to previous models) is that chain connectivity is taken into account. All segments that are located in between connected stickers are supposed to carry stress. The attachment and detachment of stickers is described with kinetic equations in which activation energies play a role. Simultaneous transitions involving groups of stickers are allowed. The model shows a strong dependence upon the number of segments per chain. Broad relaxation spectra can be obtained. The storage modulus can have more than one plateau corresponding with the fact that stress relaxation may need the breakup of several bonds. |
Please e-mail suggestions and comments to albertco@umche.maine.edu. Updated 25 January 2004 |