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Journal of RheologyVolume 43, Issue 4 (July-August 1999) |
Contents
Assessment of compliance and stress-relaxation data for polystyrene C. A. Hieber AbstractConsideration is given to available experimental results in the literature for the compliance and stress relaxation of polystyrene. It is shown that shear-compliance data from three independent sources correlate very well over the entire glass-rubber transition. On the other hand, available elongational stress-relaxation data from four independent sources indicate a lack of agreement, with an overall discrepancy on the order of four decades in time for E(t) over the entire transition region. By making use of the Knoff-Hopkins algorithm, the Leaderman approximation and a new procedure based on a least-squares collocation method, the above correlation for J(t) is used to generate G(t) which, in turn, is used to obtain E(t), assuming a constant compressibility. The resulting correlation indicates the need for further experimental work in this area.Predicting low density polyethylene melt rheology in elongational and shear flows with "pom-pom" constitutive equations N. J. Inkson*, T. C. B. McLeish*, O. G. Harlen**, and D. J. Groves* *Interdisciplinary Research Centre in Polymer Science and Technology **Department of Applied Mathematics AbstractA recent constitutive equation derived from molecular considerations on a model architecture containing two branch points the "pom-pom" captures the qualitative rheological behaviour of LDPE in shear and extension for the first time [McLeish and Larson (1998)]. We use a hypothetical melt of pom-poms with different numbers of arms to model the behaviour of LDPE. The linear relaxation spectra for various LDPE samples is mapped to the backbone relaxation times of the pom-pom modes. Data from start-up flow in uniaxial extension fixes the non-linear parameters of each mode giving predictions for shear and planar extension with no free parameters. This process was carried out for data in the literature and our own measurements. We find that multi-mode versions of the pom-pom equation, with physically reasonable distributions of branching, are able to account quantitatively for LDPE rheology over 4 decades in deformation rate in three different geometries of flows. The method suggests a concise and functional method of characterising long chain branching in polymer melts. Transition to shear-banding in pipe and Couette flow of worm-like micellar solutionsM. M. Britton, R. W. Mair, R. K. Lambert, and P. T. Callaghan AbstractWe used both conventional rheometry and NMR velocimetry to study shear banding in a solution of 200mM cetylpyridinium chloride and 120mM sodium salicylate in 0.5M sodium chloride. The solution behaved as a Maxwell fluid up to frequencies of 10 Hz. Theoretical predictions of critical strain rate and shear stress were in good agreement with measurements obtained using controlled strain rate rheometry. Using NMR velocimetry, we observed convincing evidence of shear-banding in capillary flow with a band of very high, approximately constant, shear rate next to the wall that grew in thickness with increasing apparent shear rate. We believe that the shear rate in this band (~600 s-1) marks the beginning of the hypothesized high shear rate limb of the flow curve. We also observed shear banding in both the cylindrical Couette and cone-and-plate geometries. Shear banding started at shear rates that were approximately the same as the critical shear rate measured with the mechanical rheometer. With increasing shear rate in the fluid, more than two shear bands were sometimes evident although they exhibited dynamical instabilities. That is, the highest shear rate band was variable in both magnitude and position. Fiber-fiber interaction in concentrated suspensions: disperse fibersColin Servais and Jan-Anders E. Månson*, Staffan Toll *Corresponding author AbstractA hypothesis for fiber-fiber interaction in planar randomly oriented concentrated fiber suspensions is proposed and tested. The idea is that at sufficiently high fiber concentrations, friction and lubrication at fiber-fiber contact points are the dominant interaction mechanisms. A fiber pull-out technique is introduced to measure the force per unit fiber length on a single longitudinally moving fiber embedded in a volume of bulk suspension. By varying both the fiber velocity and the fiber volume fraction, the lubrication and frictional components of the force are identified. Furthermore, the corresponding bulk shear viscosity resulting from the same mechanisms is derived and compared with experimental data. The results support the hypothesis. Fiber-fiber interaction in concentrated suspensions: dispersed fiber bundlesColin Servais, André Luciani, and Jan-Anders E. Månson* *Corresponding author AbstractA model is proposed to describe the rheology of planar randomly oriented concentrated fiber bundle suspensions in a shear-thinning matrix. The approach is that, at high-fiber contact points. A fiber pull-out technique is used to measure the force per unit length exerted on a single fiber tow of elliptical cross section embedded in a bulk suspension. By varying the tow velocity, fiber volume fraction, resin viscosity, and suspension structure, the factors affecting the lubrication and frictional components of the interaction forces were analyzed. The lubrication force is related to the flow behavior of the neat resin. The theoretical equations derived in this work allow for the computation of a shear viscosity of the suspension, which is in good agreement with experimental evidence. It is shown that dispersed fiber and dispersed fiber bundles suspensions are yield stress fluids. The relationship between steady state and oscillatory shear viscosity in planar randomly oriented concentrated fiber suspensionsColin Servais and Jan-Anders E. Månson* *Corresponding author AbstractA micromechanical model for the flow of in-plane randomly oriented concentrated fiber suspensions in molten polypropylene has been combined with the Rutgers-Delaware model for Herschel-Bulkley materials. At high fiber volume fractions Coulombic friction forces and hydrodynamic lubrication forces generated at the contact points between fibers are the dominant fiber-fiber interaction mechanisms. This feature has been shown here in both steady state and oscillatory shear. The complex viscosity and the steady state viscosity of the suspensions were measured as a function of an effective strain rate, which was defined as the strain rate in steady state shear and the product of the strain and the frequency in oscillatory shear. Four distinct strain regions were identified: viscoelastic behavior below an apparent yield stress, pseudo-solid plasticity at low effective shear rates above the yield stress, a viscous Newtonian plateau at medium effective shear rates, and a shear thinning region at high rates. The two latter regions were related to the viscosity-shear rate curves of the neat resins. The testing method, new for such fiber suspensions, allowed simple and rapid access to the shear parameters of the materials. Effects of mat structure, resin viscosity, and differences between different materials may be obtained. Rheological monitoring of long-term degrading polymer hydrogelsT. K. L. Meyvis, S. C. De Smedt, J. Demeester W. E. Hennink AbstractThis paper reports on the rheological characterization of long-term degrading hydrogel slabs using a rotational rheometer. A rheological methodology was developed and validated using hydrogel slabs made of cross-linked methacrylated dextran. The method was based on the identification of the optimal amount of compression of the hydrogel slabs between the plates of the geometry, needed for reliable G' measurements. A G'-plateau (G'p) could be identified for each hydrogel slab as a leveling off of G' upon further compression. This G'p was reproducible and of the same order of magnitude as G' of hydrogels prepared in situ, between the plates of the geometry. The method allowed doing rheological measurements without prior knowledge of the exact dimensions of the hydrogels. Using this method the enzymatic degradation of dex-ma hydrogel slabs with incorporated dextranase was characterized rheologically. It was proven that the observed changes in the rheological behavior were enzyme linked. Being able to reproducibly measure the rheological properties of hydrogel slabs prepared outside the rheometer has as great advantage that the influence of swelling, degradation or any treatment can be rheologically studied. Rheology of ABS polymer melts and viscoelastic constitutive modelsStanislav E. Solovyov* Terry L. Virkler Chris E. Scott** AbstractExperimental shear and tensile stress growth coefficients for start-up of steady shear and uniaxial elongation for a commercial grade acrylonitrile-butadiene-styrene (ABS) polymer melt are presented. One differential (PTT) and two K-BKZ type single integral (Wagner and PSM) nonlinear viscoelastic constitutive models were fit to shear and extensional experimental data. Comparison of fit quality was performed, and the PTT model was found to result in the best overall quality fit, although deviations from linear viscoelastic behavior at small extensional strains could not be adequately described by the linear viscoelasticity theory without a time correction. Experimental data for stress relaxation after cessation of both steady shear and uniaxial elongation were compared with three constitutive model predictions in order to evaluate their anticipated accuracy in prediction of residual stresses. The PTT model provided better predictions of ABS relaxational behavior than K-BKZ models except for shear stress relaxation in nonlinear regime. Qualitative differences between experimental data and model predictions were found to be present for all tested models at large shear and extensional strains.Shape recovery of a dispersed droplet phase and stress relaxation after application of step shear strains in a polystyrene/polycarbonate blend meltKenzo Okamoto, Masaoki Takahashi*, Hideki Yamane Hisahiko Kashihara Hiroshi Watanabe Toshiro Masuda AbstractWe observed the stress relaxation and shape recovery of a dispersed droplet phase after application of step shear strains in a polystyrene/polycarbonate blend melt. A polystyrene makes a droplet phase in a polycarbonate matrix of higher viscosity. The orientation angle of the droplet is independent of the initial radius. The angle does not change during stress relaxation and is nearly equal to the angle given by the affine deformation. The shape recovery of the droplets leads to the decay of the relaxation modulus at long times. The stress relaxation slows down at long times for large strains, reflecting the retarded shape recovery of the droplets. Calculated time dependences of the relaxation modulus based on the rate equations by Doi and Ohta do not agree with the observed slowing down of the stress relaxation. A force balance equation developed by Cohen and Carriere explains the retarded shape recovery of the droplet from a prolonged ellipsoid of revolution to a sphere. Low temperature rheology of lubricating mineral oils: Effects of cooling rate and wax crystallization on flow properties of base oilsRichard M. Webber AbstractThe formation of wax crystals in mineral oils at low temperatures due to poor solubility of n-paraffinic components profoundly affects oil rheology. For example, under quiescent cooling, wax crystals nucleate and grow to form macroscopic structures that cause development of a yield stress and produce rheology that is stress history dependent. The crystal formation and growth process is temperature history dependent. We have explored the effects of temperature history on the flow properties of lubricating mineral oils that do not contain performance chemical additives. The evolution of viscosity with decreasing temperature (T) is characterized by a strong increase in viscosity that occurs over a narrow 3 to 5 K range where the onset temperature (Tc) corresponds to that at which wax crystals become microscopically visible. Increasing cooling rate depresses Tc and causes the activation energy in the transition region to increase. These changes correlate to a decrease in average crystal size and a strong increase in the apparent steady state viscosity of the wax crystal dispersions at T << Tc. Despite strong effects of stress history, we show that T << Tc steady state flow properties are determined by cooling rate and its apparent effects on the process occurring at the onset of crystal nucleation and growth. These results are discussed in the context of a homogeneous nucleation model for wax crystallization. Viscosity and morphology of the two-phase system PDMS/P(DMS-ran-MPS)Verena Ziegler and Bernhard A. Wolf* *Bernhard.Wolf@Uni-Mainz.de AbstractStationary state viscosities h were measured at 50°C for two-phase blends consisting of poly(dimethylsiloxane) [PDMS] and poly(dimethylsiloxane-ran-methylphenylsiloxane) [COP] at different compositions as a function of shear rate up to 100 s-1. All mixtures exhibit shear-thinning behavior in contrast to the pure components; the sensitivity of h towards shear varies with composition in a characteristic manner reflecting the morphology of the blends. On the basis of these rheological results we infer that the concept of a single composition of co-continuity should be replaced by a composition range of co-continuity. For blends consisting of droplets of one phase suspended in the matrix of the complementary phase pictures were taken after rapid transfer from the shear cell (100 s-1) into the light microscope. The average dimensions of the droplets are approximately 13 mm (PDMS matrix) and 14 mm (COP matrix); this observation is in contrast with the data calculated according to Wu from the interfacial tension, which we have measured as a function of temperature, and from the viscosity ratios of the two phases. For the PDMS matrix this prediction (24 mm) is reasonable; however, if the matrix consists of COP, the theoretical result exceeds the measured dimension by approximately a factor of 14. Micrographs of a blend containing 60 vol. % PDMS taken in situ at 1 s-1 under stationary conditions disclose thread-like structures. The correlation between the prevalence of certain morphologies and the extent of shear thinning is discussed. Rheology and microstructure of dense suspensions of plate-shaped colloidal particlesS. M. Jogun and C. F. Zukoski AbstractThe flow properties and microstructure of dense kaolin clay suspensions are explored for volume fractions, f, as large as 0.39. To avoid flocculation these particles were suspended in pH 10 buffer solutions where edge-face interactions are negligible. These plate-shaped particles have an aspect ratio of 10 to 12 and, thus, are likely to show alignment above a critical volume fraction f* > 0.10. At low f, the suspensions are Newtonian but show substantial orientation with increased shear rate. As f is increased, the suspensions develop a yielding type of behavior. In the concentrated region f > f*, elastic moduli are a function of the previously applied shear rate, g-dot, decreasing from a low shear rate plateau of G' (0) to a high shear rate plateau of G' (¥). We interpret the modulus behavior as being a consequence of changes in alignment of domains in the suspension. Conductivity and X-ray scattering measurements confirm that particle alignment increases with increasing shear rate. The normalized modulus value, G'norm = [G' (g -dot)-G' (¥)]/[G' (0)- G' (¥)], is independent of f indicating that the characteristic alignment shear rate is independent of volume fraction. Conductivities and X-ray scattering intensities normalized in the same manner are also independent of the volume fraction, and superimpose on the modulus data. Surprisingly, these data indicate that fractional particle alignment at a given shear rate is independent of f for 0.05 < f < 0.39. |
Please e-mail suggestions and comments to albertco@umche.maine.edu. Updated 25 January 2004 |