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Journal of RheologyVolume 44, Issue 5 (September-October 2000) |
Contents
Shear and extensional rheology of sparsely branched metallocene-catalyzed polyethylenesSusan E. Bin Wadud and Donald
G. Baird AbstractThe purpose of this study was to identify any rheological effects that are consistent with the presence of sparse levels of long chain branching (LCB) in three commercial metallocene-catalyzed polyethylenes (MCPE) all of the same melt flow index (MI) of 1.0. Two Dow INSITE MCPEs with apparently varying levels of LCB of approximately 0.17 and 0.57/10,000 carbon atoms and one Exxon EXXPOL MCPE with no LCB were studied. The breadth of distribution as determined by Mw/Mn of the three samples was 2.11 for the Exxon and one of the Dow samples, and 2.42 for the other Dow sample that had the highest degree of LCB. The MCPE with the highest branching seemed to have a slightly higher molecular weight tail in the distribution. Both the Dow samples had significantly higher flow activation energies than the Exxon sample, consistent with the presence of LCB, but this method could not distinguish between the two branched polymers. The differences in Mw could also not account for the appreciably higher zero-shear viscosities of the branched samples relative to the linear sample. Despite the differences in Mw and LCB content in the two Dow samples, they exhibited almost identical shear flow curves at temperatures between 120 and 170 °C. They also exhibited very similar shear stress growth behavior. Under constant extension rate deformation, the two samples with LCB showed a significant degree of strain hardening relative to the linear sample. Comparison between the two Dow samples revealed that the sample with the higher degree of LCB showed a greater degree of strain-hardening behavior. The extensional behavior is consistent with the LCB content determined by means of solution light scattering. A Theory for Flowing Nematic Polymers with Orientational DistortionJ. J. Feng1, G. Sgalari2, and L. G. Leal2 1 The Levich Institute for
Physicochemical Hydrodynamics 2 Department of Chemical Engineering AbstractUsing a non-local nematic potential, we generalize the Doi theory for nematic polymers to include distortional elasticity. We derive an evolution equation for the configuration tensor and a constitutive equation for a non-local stress tensor which is consistent with the long-range order in nematic polymers. One of the interesting effects of distortional elasticity is the appearance of a mean-field torque on the molecules as they are forced away by flow from their preferred orientation. This torque gives rise to an anti-symmetric part of the stress tensor. With a few molecular parameters, the complete system of equations is capable, we believe, of describing the evolution of the texture and the dynamics of disclinations in flowing nematic polymers. Thus, for the first time, a suitable platform for exploring complex flows of nematic polymers is established. In the limit of weak flows and small distortions, the theory properly reduces to the Leslie-Ericksen theory. The Leslie viscosities are derived in terms of molecular parameters. Rheological Properties of Dibenzylidene Sorbitol Networks in Poly(propylene oxide) MatricesM. Fahrlander, K. Fuchs, and Chr. Friedrich* * Corresponding author: chf@fmf.uni-freiburg.de AbstractWe have systematically investigated gels of dibenzylidene sorbitol (DBS) in amorphous poly(propylene oxide) (PPO) matrices by temperature, strain, and frequency dependent rheological measurements. The concentration and the substitution pattern of DBS as well as the molecular weight of the matrix have been varied. Even very small amounts below 0.3 wt.% of the low molecular weight gelators lead to a significant increase in the viscosity and elasticity as compared with the neat matrix. This reinforcing effect has been attributed to the formation of an internal DBS network, also proven by our morphological investigations. By means of temperature dependent tests, the temperature Tf of the thermoreversible gel formation have been detected. Strain dependent measurements establish a comparatively small range of linear viscoelastic response and show the extreme deformation sensibility of the gels above a critical strain. Because the principle of time temperature superposition (TTS) is unexpectedly valid for these heterogeneous systems, mastercurves can be constructed from frequency dependent measurements at temperatures well below Tf. In the case of higher DBS concentrations above 0.4 wt.%, a power law behavior with a small exponent instead of an equilibrium modulus Ge has been observed in the low frequency range of the mastercurves. We relate this peculiar feature to the dynamic break and reformation of the weak network junctions. Characterization of elastomer sliding behavior across crosslinked polydimethylsilexane and silica surfacesLars H. Genieser1*, Kees C. P. Hendriks2,
Frank T. P. Baaijens, and Han E. H. Meijer 1 Present address: Union Carbide Corporation 2 Present address: Royal Philips Electronics,
CFT AbstractA novel combination of experimental techniques including a Lateral Force Apparatus was developed for the purpose of characterizing the slip of polymer materials over hard surfaces. Quantitative shear stress data were obtained by sliding a crosslinked polydimethylsiloxane elastomer across smooth flat-tipped styli (mesas). A first set of mesas had a native silica (SiO) surface; a second set was functionalized with linear hydroxyl-terminated polydimethylsiloxane (PDMS-OH) polymers. The shear stress associated with sliding across the SiO mesas was as much as a factor of ten greater than that for sliding across the PDMS-OH functionalized mesas. Possible mechanisms underlying this difference in behavior are discussed. The extensional viscosity of a thermotropic liquid crystalline polymerA.D. Gotsis* and M.A. Odriozola** * To whom correspondence should be addressed ** Present address: Aiscondel S.A., Ctra Tarragona San
Sebastian
AbstractThe transient extensional viscosity of the nematic melt of thermotropic liquid crystalline co-polyester Vectra A950 was measured in uniaxial extensional flow in a commercial constant strain rate rheometer. For extension rates between 0.005 and 1 s-1 the extensional viscosity of Vectra A950 does not reach steady state for Hencky strains up to 3 to 4 units, where most samples break. If the viscosity at a certain value of strain is plotted vs. the strain rate, a rate-thinning curve is obtained. The extensional viscosity is much higher than 3 times the shear viscosity over a broad range of accumulated strain, while injection molded samples always show higher viscosity than compressed samples. The theory developed by Larson and Doi (1991) to describe the flow of polydomain LCPs has been adapted for its use in elongational flow. The trends of the predictions agree with the measured stress growth curves. The flow seems to be dominated by the effect of the texture. The variation in the viscosity measured in samples prepared by different methods is attributed to the variation in the liquid crystalline structure present in the samples and in particular to the initial domain size. Healing of confined polymer films following deformation at high shear rateYingxi Zhu and Steve Granick AbstractRecovery of equilibrated linear viscoelastic response of confined polymer melts, following cessation of large-amplitude shear in a surface forces apparatus, was found to be a single exponential process. The most extensive experiments concerned a polydimethylsiloxane (PDMS) of narrow molecular weight distribution and weight-average molecular weight Mw = 8,330 g-mol-1, for which recovery times were in the range 2-12 hr when the film thickness (D) was D/RG = 0.5 to 6 (RG is radius of gyration). Initially, to produce the deformed state, the films were sheared with effective shear rate » 104 sec-1. Recovery was probed by the subsequent application of small-amplitude sinusoidal shear forces at 256 Hz. Surprisingly, the nonlinear and linear shear moduli evaluated at the input frequency nearly coincided just before and just after cessation of large-amplitude shear. Recovery time constants, τR, increased linearly with prior shear rate at a given thickness (D). But at a given shear rate and variable D, τR passed through a maximum at D/RG » 3.5 - thinner films recovered more quickly. This contrasts with relaxation times in films that were at rest prior to shear. Owing to slip, these thinner films (D/RG < 3.5) may have been less uniformly deformed than thicker ones. We conjecture that chains in very thin films were separated by large-amplitude shear into two distinct populations, each moving preferentially with each of the sliding surfaces. Recovery kinetics would then reflect interdiffusion during which chain configurations lose memory of the distinction between top and bottom surfaces. Linear stability of viscoelastic Taylor-Couette flow: Influence of fluid rheology and energeticsU.
A. Al-Mubaiyedh, R. Sureshkumar, and B. Khomami AbstractIn this study we have theoretically investigated the effect of detailed fluid rheology (e.g., spectrum of relaxation times, shear thinning of first normal stresses, finite second normal stresses and ratio of solvent to total viscosity) and energetics on the purely elastic instability of Taylor-Couette flow. The isothermal analysis showed that irrespective of the details of the fluid rheology, solvent to total viscosity ratio and gap width, the secondary flow is time-dependent and non-axisymmetric. However, as the number of relaxation times is increased, the critical Deborah number reaches an asymptotic value which is approximately half of the critical Deborah number predicted by the single-mode constitutive equation. These results strengthen the conclusions of Al-Mubaiyedh et al. (1999) that have attributed the existence of a stationary secondary flow in the experiments of Baumert and Muller (1995, 1997) to the effect of energetics. The non-isothermal analyses predicted, for experimentally realizable values of Peclet and Brinkman numbers, the instability to be caused by stationary and axisymmetric disturbances as observed in the experiments of Baumert and Muller (1995, 1997). Moreover, it has been shown that although the details of the fluid rheology can influence the onset conditions in the non-isothermal viscoelastic Taylor-Couette flow, the mechanism of the instability as well as the structure of the bifurcated solutions are relatively insensitive to the details of the fluid rheology. Transient behavior of an ER fluid in shear flow modeSheila L. Vieira Luiz B. Pompeo Neto* and Antonio Celso F. Arruda * Corresponding author AbstractElectrorheological (ER) fluids made of starch particles in silicone fluid were studied under different electric field strength, particle concentration, water content, and shear rate. The ER fluids were sheared under constant shear rates during at least 210 minutes and the shear stress was measured. According to the results, the shear stress increased with time until a maximum was reached, decreased a little and then stabilized. Some samples did not show a point of maximum, but after increasing with time, they stabilized at a given shear stress. The influence of the shear rate on the shear stress depended on the time the sample was sheared, on the electric field strength, on concentration and on water content. The results were analyzed in terms of changes of lamellar formations in the direction of shear. A constitutive model for the prediction of ellipsoidal droplet shapes and stresses in immiscible blendsAbdulwahab
S. Almusallam, Ronald G. Larson, and Michael J. Solomon AbstractWe report a phenomenological constitutive model with no adjustable parameters appropriate for the transient behavior of droplets and blends. The time evolution of the droplet anisotropy tensor during droplet relaxation under quiescent conditions is described using a frame-invariant formulation that approximately imposes constancy of droplet volume. The Doi-Ohta theory [J. Chem. Phys. 95: 1242 (1991)] is then adapted to transient flows in which break-up and coalescence do not occur by replacing the Doi-Ohta relaxation terms with this relaxation description. Model predictions are compared to results of visualization of single droplets in step-shear and start-up of steady shear and to measurement of concentrated blend rheology in step-shear and start-up of steady shear. The model quantitatively described the relaxation after step strain of single droplets to axisymmetric and then to isotropic shapes. With the inclusion of the rational ellipsoidal closure for affine deformation [Int. J. Multiphase Flow 25: 35 (1999)], single droplet response in start-up of steady shear was also well predicted. For concentrated (φ=0.2) blends, good agreement between experiment and theory was obtained for step-strain, while in start-up of steady-shear the model increasingly failed as the shear rate was increased. Exponential shear flow of linear, entangled polymeric liquidsJesper
Neergaard, Kyungho Park, David C. Venerus, and Jay D. Schieber
AbstractA previously proposed reptation model is used to interpret exponential shear flow data taken on an entangled polystyrene solution. Both shear and normal stress measurements are made during exponential shear using mechanical means. The model is capable of explaining all trends seen in the data, and suggests a novel analysis of the data. This analysis demonstrates that exponential shearing flow is no more capable of stretching polymer chains than is inception of steady shear at comparable instantaneous shear rates. In fact, all exponential shear flow stresses measured are bounded quantitatively by stress measurements taken during inception of steady shear. Information taken from the model about chain stretching suggests that normal stress measurements are strong indications of stretching, whereas shear stress measurements are indicative of both chain stretching and segment orientation. Ultrasonic and microscopic investigation of blends of polydimethylsilexane and polyisobutylene at all concentrationsP.-Y. Longin, C. Verdier, and M. Piau AbstractPDMS/PIB blends are investigated using ultrasonic techniques (MHz range), in the complete concentration range [0-100%]. The velocities of propagation and attenuations of shear and longitudinal waves are determined experimentally together with simultaneous optical microscopy observations. From the knowledge of the acoustical and physical properties of the separate polymers, the acoustical longitudinal parameters of the blend can be predicted successfully up to 40% volume concentration of the dispersed phase, with a model developed by the authors and especially adapted to wave propagation in such viscoelastic emulsions. For shear waves, two emulsion models (Palierne and Lee-Park) are used for predicting the viscoelastic moduli associated with the acoustical data at ultrasonic frequencies, with a good agreement. The case of composite droplets [40-60%] has required to consider Friedrich's extension of Palierne's model. The ultrasonic method appears to be a very interesting tool for predicting phase inversion for polymer blends, as well as the evolution of the microstructure of the blend. Finally, this work provides an extension of the validity of emulsion models in the high frequency range. Complex transients in the capillary flow of linear polyethyleneL. Robert1,2, B. Vergnes2, and Y. Demay1,2 1
Institut Non Linéaire de Nice, UMR CNRS 129 2
CEMEF, Ecole des Mines de Paris, UMR CNRS 7635 AbstractThe spurt flow behavior of a linear high-density polyethylene was carefully studied in capillary experiments. Depending on the flow conditions, new types of complex transients were observed. The study pointed out a second oscillating area, described by a second hysteresis cycle, close to or inside the classical one. In one case, the second oscillating area was situated between two parts of the stable second branch, and pressure oscillation patterns for the two distinct oscillating areas were clearly reported. In another case, the smaller secondary oscillations occur within the cycle of the primary oscillations, showing double instability pressure patterns. The transients in the capillary flow of linear polyethylene put in evidence a complexity, which cannot be explained only in terms of melt compressibility associated with a simple relationship between slip and shear stress. Stress relaxation in dense and slow granular flowsD. Z. Zhang and R. M. Rauenzahn AbstractIn a dense granular system, particles interact in networks containing many particles and interaction times are long compared with the particle binary collision time. In these systems, the streaming part of the granular stress is negligible. We only consider the collisional stress in this paper. The average behavior of particle contacts is studied. By following the statistical method developed recently by the authors (J. Rheol, 41(6), 1997), we derive an evolution equation for the collisional stress. This equation provides guidance to collateral numerical simulations, which show that the probability distribution of particle contact times is exponential for long contact times. This can be explained by network interactions in a dense granular system. In general, the relaxation of the collisional stress is a combined effect of the decay of the contact time probability and the relaxation of collisional forces among particles. In the numerical simulations, the normal force between a pair of particles is modeled as parallel connect of a spring and a dashpot. In this case, the relaxation of the force magnitude conditionally averaged given a specific contact time is negligible, and the major contribution to the stress relaxation is from the exponential decay of the contact time probability. We also note that the probability decay rate is proportional to the imposed strain rate. Consequently, in a simple shear flow with a constant particle volume fraction, as the shear rate approaches zero, the shear stress approaches a finite value. This value is the yield stress for that particle volume fraction. Hence, the evolution equation of the collisional stress predicts viscoplasticity of dense granular systems. |
Please e-mail suggestions and comments to albertco@umche.maine.edu. Updated 25 January 2004 |