Stress Growth Experiments Research Articles

Measuring the transient extensional rheology of polyethylene melts using the SER universal testing platform

We use a new extensional rheology test fixture that has been developed for conventional torsional rheometers to measure the transient extensional stress growth in a number of different molten polyethylene samples including a linear low density polyethylene (Dow Affinity PL 1880), a low density polyethylene (Lupolen 1840H), and an ultrahigh molecular weight polyethylene (UHMWPE). The transient uniaxial extensional viscosity functions for the linear low density polyethylene (LLDPE) and low density polyethylene (LDPE) samples have both been reported previously in the literature using well-established instruments and this allows us to benchmark the performance of the new test fixture. Transient stress growth experiments are carried out over a range of Hencky strain rates from 0.003 to 30s−1 and the data show excellent agreement with the published material functions. At deformation rates greater than 0.3s−1 a true steady state extensional viscosity is not obtained in the LDPE samples due to the onset of neckin...

Rheological properties of short fiber model suspensions

The rheological behavior of two series of model suspensions containing the same glass fibers in a Newtonian polybutene and in a Boger fluid has also been investigated. The steady-state shear viscosity of both supensions increased with fiber content, but the suspensions in the Boger fluid became shear thinning. Both types of suspension exhibited non-negligible normal stresses. The steady-state viscosity and normal stress difference of the supensions in the polybutene are well predicted by the Lipscomb (1987) equation coupled with the Folgar–Tucker (1984) model. Both types of fiber suspensions were shown to exhibit shear and normal stress overshoots in stress growth experiments. Under flow reversal, a shear stress overshoot was observed at a larger deformation compared to the primary overshoot. The reverse overshoot has been attributed to tumbling of fibers that are not totally aligned in the flow direction even after a very long time. When the flow was reversed, the normal stress difference took initially mimimum values (negative values in the polybutene case) and then depicted a smaller positive overshoot before reaching a steady-state value. The normal stress undershoot has been attributed to a transient fiber-oriented structure. The shape and the magnitude of these overshoots depend on the fiber content, nature of the matrix, and time delay between consecutive experiments.

Rheological properties of short fiber filled polypropylene in transient shear flow

The behavior of a commercial short glass fiber filled polypropylene has been studied. It is well known that a thermoplastic polymer filled with short fibers with an initial isotropic orientation state exhibits a viscosity overshoot when sheared in the parallel plate geometry of a rheometer in the molten state. In this work, following a first deformation, the sample has been sheared in the reverse direction and a small viscosity overshoot has been measured. Similarly under stress growth experiments a large normal stress overshoot was observed. When the flow was reversed the normal stresses were initially negative and then exhibited a small positive overshoot. A model has been used to simulate these viscosity and normal stress overshoots. It is based on the Folgar–Tucker equation for fiber motion and the Lipscomb constitutive equation. This equation has been empirically modified to correctly predict the overshoots.

Influence of the Geometry on the Transient and Steady Flow of Lubricating Greases

The objective of this paper is to investigate the influences that geometry, roughness of the sensor system and gap size exert on the rheological measurements of a commercial lubricating grease. With this aim, creep and stress growth experiments, stepped strain rate ramps and dynamic measurements were performed using different geometries and roughness. A significant influence of gap size was found in transient and steady-state experiments in a certain range of strain rate. Roughening the surface of the geometry increases considerably the values of the stress in comparison to the measurements performed with smooth surfaces. The experimental results are discussed taking into account the wall slip phenomenon. In addition to this, different grooves made in the surfaces of the tools seem to eliminate or considerably decrease these wall slip effects. However, the influence of geometry or roughness was not detected in oscillatory tests performed inside the linear viscoelastic range.

Chemical and rheological properties of an extracellular polysaccharide produced by the cyanobacteriumAnabaena sp. ATCC 33047

The cyanobacterium (blue-green alga) Anabaena sp. ATCC 33047 produces an exopolysaccharide (EPS) during the stationary growth phase in batch culture. Chemical analysis of EPS revealed a heteropolysaccharidic nature, with xylose, glucose, galactose, and mannose the main neutral sugars found. The infrared (IR) spectrum of EPS showed absorption bands of carboxylate groups. The average molecular mass of the polymer was 1.35 MDa. Aqueous dispersions at EPS concentrations ranging from 0.2% to 0.6% (w/w) showed marked shear-thinning properties (power-law behavior). Linear dynamic viscoelastic properties showed that the elastic component was always higher than the viscous component. Viscous and viscoelastic properties demonstrated the absence of conformational changes within the concentration range studied. Stress-growth experiments revealed that 0.4% and 0.6% (w/w) EPS dispersions showed thixotropic properties. A detailed comparison of the linear dynamic viscoelasticity, transient flow, and decreasing shear rate flow curve properties was made for 0.4% (w/w) dispersions of xanthan gum (XG), Alkemir 110 (AG), and EPS. Viscoelastic spectra demonstrated that the EPS dispersion turned out to be more "fluidlike" than the AG and XG dispersions. The flow indexes indicated that the EPS dispersion was less shear-sensitive than that of XG, showing essentially the same viscosity, that is, >50 s(-1). The fact that viscosities of EPS and AG dispersions were not substantially different within the shear-rate range covered must be emphasized, in relation to EPS potential applications. The rheological behavior of EPS dispersions indicates the formation of an intermediate structure between a random-coil polysaccharide and a weak gel.

Chemical and rheological properties of an extracellular polysaccharide produced by the cyanobacterium Anabaena sp. ATCC 33047

The cyanobacterium (blue-green alga) Anabaena sp. ATCC 33047 produces an exopolysaccharide (EPS) during the stationary growth phase in batch culture. Chemical analysis of EPS revealed a heteropolysaccharidic nature, with xylose, glucose, galactose, and mannose the main neutral sugars found. The infrared (IR) spectrum of EPS showed absorption bands of carboxylate groups. The average molecular mass of the polymer was 1.35 MDa. Aqueous dispersions at EPS concentrations ranging from 0.2% to 0.6% (w/w) showed marked shear-thinning properties (power-law behavior). Linear dynamic viscoelastic properties showed that the elastic component was always higher than the viscous component. Viscous and viscoelastic properties demonstrated the absence of conformational changes within the concentration range studied. Stress-growth experiments revealed that 0.4% and 0.6% (w/w) EPS dispersions showed thixotropic properties. A detailed comparison of the linear dynamic viscoelasticity, transient flow, and decreasing shear rate flow curve properties was made for 0.4% (w/w) dispersions of xanthan gum (XG), Alkemir 110 (AG), and EPS. Viscoelastic spectra demonstrated that the EPS dispersion turned out to be more “fluidlike” than the AG and XG dispersions. The flow indexes indicated that the EPS dispersion was less shear-sensitive than that of XG, showing essentially the same viscosity, that is, >50 s−1. The fact that viscosities of EPS and AG dispersions were not substantially different within the shear-rate range covered must be emphasized, in relation to EPS potential applications. The rheological behavior of EPS dispersions indicates the formation of an intermediate structure between a random-coil polysaccharide and a weak gel. © 2000 John Wiley & Sons, Inc. Biotechnol Bioeng 67: 283–290, 2000.

Rheological modeling of concentrated colloidal suspensions

The use of concentrated colloidal suspensions is common in several industries such as paints, foodstuffs and pulp and paper. These suspensions are generally composed of strongly interactive particles. If the attractive forces dominate the repulsion and Brownian forces, the particles aggregate to form a three-dimensional network yielding a gel structure. Under flow, the micro-structure of suspensions can be drastically modified and the rheological properties are then governed by structure breakdown and build-up. In this work, we propose a structural network model based on a modified upper convected Jeffreys model with a single relaxation time and a kinetic equation to describe the flow-induced micro-structure evolution. Three distinct kinetic equations are tested for this purpose. The proposed model describes yield and thixotropic phenomena, nonlinear viscoelastic behavior and output signal distortions observed for relatively small strain amplitude during oscillatory measurements, and overshoots observed in stress growth experiments. A comparison of model predictions and experimental data for fumed silica and coating colors is also presented. However, different model parameters must be used to correctly predict the different flow properties indicating that a more versatile or generalized kinetic equation must be proposed.

Morphological evolution of immiscible polymer blends in simple shear and elongational flows

The evolution of the morphology of the dispersed phase of immiscible polymer blends is studied in both shear and extensional flows. In the simple shear flow, predictions of the Lee and Park [H.M. Lee, O.O. Park, Rheology and dynamics of immiscible polymer blends, J. Rheol. 38 (1994) 1405–1425] and of the modified Lee and Park as well as the modified Grmela and Ait-Kadi [M. Grmela, A. Ait-Kadi, Comments on the Doi–Ohta theory of blends, J. Non-Newtonian Fluid Mech. 55 (1994) 191–195] models ([C. Lacroix, M. Grmela, P.J. Carreau, Relationships between rheology and morphology for immiscible molten blends of polypropylene and ethylene copolymers under shear flow, J. Rheol. 42 (1998) 41–62] are compared to stress growth data. The size of the dispersed phase is strongly affected by moderately finite strain imposed during the stress growth experiments. The morphological changes after stress relaxation are well predicted by the models for a polypropylene (PP), ethylene vinylacetate (EVA) and ethylene methylacrylate (EMA) blend [C. Lacroix, M. Grmela, P.J. Carreau, Relationships between rheology and morphology for immiscible molten blends of polypropylene and ethylene copolymers under shear flow, J. Rheol. 42 (1998) 41–62]. The prediction is less satisfactory for a polystyrene (PS)/polyethylene (PE) blend. The morphological evolution for the elongation flows of the PP/EVA/EMA blends has also been investigated. The morphologies of samples extracted before and after extrusion through an hyperbolic shaped (nozzle) die show that the elongational flow induces fibrillar structures. The extensional viscosity data, needed to solve the interface governing equations of the Lee and Park model, have been obtained from entrance pressure drop measurements. These equations are shown to qualitatively describe the transition from a spherical to a fibrillar morphology.

Transient shear and extensional properties of biodegradable polycaprolactone

AbstractTransient shear and extensional properties of two grades of partially crystalline biodegradable aliphatic polyesters, poly ε caprolactone (PCL), were investigated at three different temperatures. Uniaxial extensional viscosity at a constant strain rate was obtained using the Meissner apparatus. The magnitudes of the stress‐over‐shoot during stress growth experiments were smaller than those typically observed for other polymers. Higher melt temperature and higher strain led to faster relaxation, while the lower molecular weight PCL 767 relaxed faster than the higher molecular weight PCL 787. The relaxation moduli are independent of strain for strain values below 0.1. Transient extensional measurements were conducted at strain rates of 0.01 to 1.0 s−1. At small stresses the extensional viscosity has the threefold value of shear viscosity as predicted by Trouton. There appeared to be no steady state regime for either grade of PCL studied and as a result ηe(ε) could not be determined. The departure from the linear limit is fastest for the highest extensional rate. Extension thickening behavior is observed at Hencky strains ranging from 1.0 to 2.0. PCL 767 displayed greater extension thickening than PCL 787 at the same temperatures. The Wagner integral constitutive equation was found to give an acceptable fit to the stress growth data in shear and extension, with the fit being better for PCL 767 than for PCL 787.

Relationships between rheology and morphology for immiscible molten blends of polypropylene and ethylene copolymers under shear flow

The linear and nonlinear viscoelastic properties of immiscible polymer blends polypropylene/ethylenevinylacetate–ethylenemethylacrylate [PP/(EVA–EMA)] have been investigated. The transient shear flow experiments reflect the structural changes of the blends during the flow. Overshoots in stress growth experiments are observed when the dispersed phase is deformable. In this case, a good description of these transient rheological data is obtained using modified versions of the Lee and Park [J. Rheol. 38, 1405 (1994)] and the Grmela and Ait-Kadi [J. Non-Newtonian Fluid Mech. 55, 191 (1994)] models. Predictions of the morphological evolution of the blend under transient shear flows were calculated from the modified models which are shown to describe the breakup and coalescence phenomena under moderately large deformation shear flow. When the dispersed phase is undeformable, these models, which are either based on the original Doi and Ohta [J. Chem. Phys. 95, 1242 (1991)] theory or derived to retrieve an extens...

Visualization of the flow of linear low density polyethylene/low density polyethylene blends through sudden contractions

Visualization of the flow of linear low density polyethylene/low density polyethylene blends through sudden contractions into a capillary has confirmed the leading role of the elongational stresses upstream of a contraction in determining the size of the upstream vortices. The two polymers (LLDPE and LDPE) from which the blends were prepared had very different behavior in elongational stress growth. The flow of LDPE, which showed the highest positive deviation (non-linearity) in elongational stress growth experiments, generated the largest vortices while the flow of LLDPE, which showed no deviation, generated very small vortices. A complete range of deviation in elongational stress growth was obtained by blending LLDPE with LDPE. Vortex length was correlated, for a complete series of LLDPE/LDPE blends, to the ratio of the elongational stresses (difference between the axial and radial components) along the flow axis to the shear stress on the wall of the capillary.

Flow‐induced anisotropy and its decay in polymeric liquid crystals

Flow induces anisotropy in polymeric liquid crystals. This can be demonstrated by comparing the stress transients during a sudden increase in shear rate with those during flow reversal: the damped oscillations of the shear stress resulting from these two experiments are shifted by nearly 180°. Flow‐induced anisotropy decays after the flow stops. Its evolution in time is followed by stress growth experiments in the flow direction and in the opposite one, with systematic changes in the rest period. The phase shift only disappears after several thousands of seconds. This time is much longer than the time necessary for the shear stress to relax. Other rheological characteristics, such as the variation of the dynamic moduli after stopping the flow, occur on the same time scale as the anisotropy decay. The anisotropy decay is not affected by temperature and consequently viscosity can be ruled out as a governing factor. The results agree in part with recent polydomain models.

A model for the thermorheological behavior of viscoelastic fluids

Using a power-law ansatz for the temperature dependence of the shear modulus on the level of internal variables, the thermorheological behavior is modeled for viscoelastic fluids of a special group of rheological constitutive equations (rate-type models). The model parameter introduced characterizes thermoelastic contributions. The relation between the model parameter and the physical quantities appearing in deformation processes is discussed. Based on the chosen temperature dependence of the shear modulus, thermodynamically consistent equations like the nonlinear rheological constitutive equation and the temperature equation are derived. The special cases of entirely entropy and energy elastic fluids are also considered. The thermorheological behavior (exo-, α- or endothermal processes) of a viscoelastic fluid in a stress-growth experiment followed by relaxation is analyzed with respect to the model parameter.

MEASURING MILK GELIFICATION BY MEANS OF AN INSTRON UNIVERSAL TESTING MACHINE OPERATING IN DYNAMIC MODE1

ABSTRACTThis note describes a simple method to measure the kinetics of milk coagulum formation using an Instron Universal Testing Machine operating in dynamic mode. Coaxial cylinder geometry is used, applying to the internal cylinder a vertical periodical motion of 2 mm amplitude and constant speed of 1 cm/min. The flow conditions are similar to that applied in a sliding rheometer. The experiment can be equated to a periodical stress growth experiment as coagulation proceeds. Data produced according to this technique provides information on the kinetic parameters which characterize the kinetic model currently used to describe milk coagulation

Foam Rheology: III. Measurement of Shear Flow Properties

The experimental study of foam is complicated by its inherently unstable nature and by the presence of a liquid film slip layer at the wall. A method is described for generating reproducible and stable foam capable of retaining its structure for prolonged times. An experimental technique has also been devised which eliminates problems associated with wall slippage and allows measurement of material functions without use of any empiricisms for the wall region. In a steady‐shear flow, foam behaves like a Bingham plastic with a viscosity inversely proportional to shear rate indicating the presence of a yield stress τ0. The value of the viscosity, which is significantly higher than the parent liquid viscosity, is an increasing function of gas volume fraction φ. Yield stress values obtained by extrapolating viscosity versus shear stress data agree with direct τ0 measurements obtained by using a stress relaxation technique. The yield stress is also found to increase with φ. Small amplitude oscillatory shearing experiments show foam to behave as an elastic solid for small deformations. This is evident from its frequency independent moduli, very small phase shift between strain input and stress output, and the elastic modulus's being much larger than the loss modulus. Stress growth experiments at the start‐up of steady‐shear flow reveal stress overshoots not larger than 30%. The transient viscosity divided by the steady‐state viscosity is found to be insensitive to shear rate, but is an increasing function of φ.

Rheological Properties of Rodlike Particles in a Newtonian and a Non‐Newtonian Fluid

The macroscopic rheological behavior of suspensions of nearly monodisperse glass fibers having a mean aspect ratio, ār, of 24.3 and a mean length, L̄, of 267 μm, and commercial ground glass fibers, ār=7.6 and L̄=84 μm, were studied. Volume fractions of 0.02, 0.05, and 0.08 were used. For Newtonian suspending fluids, the shear viscosities and the dynamic linear viscoelastic properties of the suspension showed Newtonian behavior. In a stress growth experiment, the shear stress obtained a maximum value before reaching steady state. Upon reversal of shearing, a similar stress growth pattern was retraced. The non‐Newtonian suspending fluid, a polyisobutylene in cetane solution, was found to behave as a second‐order fluid at low shear‐rates and frequencies and shear‐thinned at higher values. Suspensions in this fluid also behaved as second‐order fluids at low shear‐rates and frequencies. The dependency of two of the second‐order fluid constants upon the volume fraction of particles was determined.

Studies on the Transient Shear Flow Behavior of Liquid Crystalline Polymers

An understanding of the flow behavior of liquid crystalline polymers (LCP's) is of immense practical value because of the potential to form high modulus materials from these polymers. These fluids exhibit a high degree of structure even in the quiescent state, as evidenced by their ability to transmit polarized light. In an effort to understand how the structure changes during flow, we have carried out a study on the transient shear flow properties of two thermotropic copolyesters of 60‐ and 80‐mole % para‐hydroxybenzoic acid (PHB) and polyethyleneterephthalate (PET) and a lyotropic system of poly‐p‐phenyleneterephthalamide (PPT) in 100% sulfuric acid. In one of the first theories concerned with the flow behavior of liquid crystalline fluids, which was proposed by Ericksen, the transient flow properties are all predicted to be due to changes in orientation of a director which describes the orientation of packets of rod‐like molecules. Stress growth, interrupted stress growth, and stress relaxation experiments are carried out on the three LCP's and at first sight are in qualitative agreement with the predictions of Ericksen's theory. However, wide angle X‐ray scattering analysis of quenched samples subjected to shear flow along with annealing experiments on oriented samples indicate that these materials do not orient readily in shear flow. Furthermore, orientation generated during extensional flow relaxes at a rate much faster than is indicated by the interrupted stress growth experiments. It is concluded that the stress growth response of LCP's is due to a disruption of a domain structure which exists within the fluid rather than to orientation changes of the domains of rod‐like molecules.

Non-linear behaviour of asphalts in steady and transient shear flow

Steady-state and transient shear stress and normal stress data were obtained for four asphalts with a modified Weissenberg Rheogoniometer. Interest was specially related to non-linear behaviour at high shear-rates. The time-temperature superposition principle was found to hold in non-linear behaviour. Moreover, steady-state and transient data could be plotted as master curves irrespective of the nature of the asphalts. In particular, the master curve of steady-state viscosity could be extended to results published in the literature. In the nonlinear region the shear stress relaxation after cessation of a steady shear rate\(\dot \gamma\) becomes a function of\(\dot \gamma\)t only and is related to the primary normal-stress coefficient, as predicted by the Yamamoto equation. In the shear stress growth experiment an overshoot is obtained at a constant strain close to 1.5, independent of the rate of strain.

Combined constant strain rate and stress relaxation test for linear viscoelastic studies

AbstractThe strain history of the combined test is formed by a constant strain rate during the period 0 <t < t1 followed by a constant strain for t > t1. This test includes as limiting cases the relaxation test (t1 → 0), the constant strain rate or stress growth experiment (t1 → ∞), and the stress relaxation after cessation of constant strain rate flow (t1 sufficiently large). The constant t1 can be chosen arbitrarily such that the test conditions of the combined test can be adjusted according to the problem investigated and to the capability of the testing device. From this, practical advantages follow for linear viscoelastic studies of polymeric solids and liquids, e.g., the zero shear viscosity η0 for unstable polymer melts can be determined at least approximately. In order to obtain linear viscoelastic material functions from the measured stress record, a simple procedure is given which avoids the difficulties with the “factor‐of‐10 rule” for short relaxation times.

An extensiometer for molten plastics

AbstractA new rheometer has been developed to measure the extensional viscosity of molten thermoplastics at typical extrusion temperatures. It is at the same time versatile and easy to operate. One end of the test specimen is held in a fixed position by an oil‐cooled chimp mounted on the end of a force transducer. The other end of the specimen is held by, a moving, oil‐cooled chimp. The moving clamp is suspended from a carriage mounted on a ball nut which travels on a long lead screw. The lead screw is driven by an electric motor operating through a gear box, and its speed can be controlled to produce either a constant stress (creep) or a constant strain‐rate (stress growth) experiment. Experimental results have verified that in the “constant strain rate” mode the strain rate does rise rapidly to a constant level. Measurements on several polyethylene resins indicated that the extensional viscosity rises with strain rate at low strain rates.