Higher Elongational Viscosity Research Articles

Improving the barrier properties of a biodegradable polyester for packaging applications

Miscible and thermally stable blends of biodegradable poly(butylene adipate-co-terephthalate) (PBAT) with poly(hydroxy ether of bisphenol A) (PH) are investigated to obtain membranes for packaging applications. Previously the miscibility and adequate degradability of these blends was proven, therefore this system is a good candidate for packaging applications. The crystallinity degree and the free volume, both of crucial importance in transport properties, are analysed as a function of blend composition. The transport properties to different gases and vapours are greatly reduced with the addition of PH. The blends show high elongational viscosity values, which allows expecting good film processability. Overall, this work sheds light on the factors involved in the reduction of permeability which would allow to broaden this strategy to other promising biodegradable materials.

Physicochemical properties of jet milled wheat flours and doughs

The aim of this study was to evaluate the physicochemical properties of white wheat flour pulverized by jet mill and the rheological behavior of the resultant doughs. A commercial coarse (control) wheat flour with d50 < 75 μm (CON) was micronized by jet-mill under two different feed rates, resulting in fine and ultrafine flour preparations with d50 < 21 μm (JM1) and d50 < 12 μm (JM2), respectively; d50 value was reduced with decreasing feed rate. The levels of damaged starch (5.3–11.5% as is) and water soluble arabinoxylans (0.4–0.5% d.b.) increased (p < 0.05) with reduction of flour particle size, while no changes in apparent peak molecular weight, determined by HPSEC–RI, and primary structure of the AXs (e.g. degree of branching) estimated by 1H NMR, were noted upon flour jet-milling. Starch gelatinization properties and dough rheological behavior of CON, JM1, JM2 flours and a mixture (CON:JM1 1:1), covering a particle size range applicable to actual breadmaking processes, were studied. Calorimetry of aqueous flour dispersions (flour:water 30:70 w/w) showed a decrease in starch gelatinization enthalpy of the flours containing jet mill fractions compared to control flour. For the jet mill flours, the creep-recovery test of doughs (flour:water 56:44) revealed an increase (p < 0.05) in resistance to deformation, elasticity and zero shear rate viscosity. Additionally, Texture Profile Analysis (TPA) and compression - stress relaxation testing after lubricated squeezing flow (LSF) revealed that with decreasing flour particle size, the doughs become harder with higher consistency, more sticky and gummy and exhibit longer half relaxation time, lower relaxation rate and higher elongational viscosity.

Direct numerical simulation of the drag-reducing turbulent boundary layer of viscoelastic fluid

Direct numerical simulation of a zero-pressure gradient drag-reducing turbulent boundary layer of homogeneous viscoelastic fluids was performed using constitutive equation models such as the Oldroyd-B and Giesekus models. Mean velocity profiles and turbulence statistics at the different streamwise locations were discussed using both inner and outer scaling. The maximum drag reduction ratio for the Oldroyd-B model, which has the higher elongational viscosity, is larger than for the Giesekus model. The distinct difference in turbulence statistics near the wall between the Oldroyd-B model and Newtonian fluid is observed, as reported in the drag-reducing turbulent channel flow, while in the outer region, distributions of turbulence statistics for the Oldroyd-B model with a drag reduction ratio of about 40% are similar to those for Newtonian fluid. The production term for the turbulent boundary layer does not correspond to the amount of drag reduction, which is consistent with the fact that the streamwise turbulence intensity profile is not a direct indication of the drag reduction. The contribution of the advection term to the budget of streamwise Reynolds normal stress, which does not appear for the turbulent channel flow, is not negligible near the wall for the Oldroyd-B model. For the Oldroyd-B model with a maximum drag reduction ratio of 42%, we can see that quasi-streamwise vortices are weakened and become larger in the streamwise direction, compared to Newtonian fluid. On the other hand, quasi-streamwise vortices for the Giesekus model with a maximum drag reduction ratio of 16% are slightly larger than those for Newtonian fluid. These modifications of near-wall coherent structures are explained by the profile of the trace of mean viscoelastic stress and the elastic energy theory presented by Min et al. [J. Fluid Mech. 486, 213 (2003)] for the drag-reducing turbulent channel flow.

An unsymmetric constitutive equation for anisotropic viscoelastic fluid

A continuum constitutive theory of corotational derivative type is developed for the anisotropic viscoelastic fluid–liquid crystalline (LC) polymers. A concept of anisotropic viscoelastic simple fluid is introduced. The stress tensor instead of the velocity gradient tensor D in the classic Leslie–Ericksen theory is described by the first Rivlin–Ericksen tensor A and a spin tensor W measured with respect to a co-rotational coordinate system. A model LCP-H on this theory is proposed and the characteristic unsymmetric behaviour of the shear stress is predicted for LC polymer liquids. Two shear stresses thereby in shear flow of LC polymer liquids lead to internal vortex flow and rotational flow. The conclusion could be of theoretical meaning for the modern liquid crystalline display technology. By using the equation, extrusion–extensional flows of the fluid are studied for fiber spinning of LC polymer melts, the elongational viscosity vs. extension rate with variation of shear rate is given in figures. A considerable increase of elongational viscosity and bifurcation behaviour are observed when the orientational motion of the director vector is considered. The contraction of extrudate of LC polymer melts is caused by the high elongational viscosity. For anisotropic viscoelastic fluids, an important advance has been made in the investigation on the constitutive equation on the basis of which a series of new anisotropic non-Newtonian fluid problems can be addressed.

Metallocene ethylene-1-octene copolymers: Influence of comonomer content on thermo-mechanical, rheological, and thermo-oxidative behaviours before and after melt processing in an internal mixer

The influence of the comonomer content in a series of metallocene-based ethylene-1-octene copolymers (m-LLDPE) on thermo-mechanical, rheological, and thermo-oxidative behaviours during melt processing were examined using a range of characterisation techniques. The amount of branching was calculated from 13C NMR and studies using differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA) were employed to determine the effect of short chain branching (SCB, comonomer content) on thermal and mechanical characteristics of the polymer. The effect of melt processing at different temperatures on the thermo-oxidative behaviour of the polymers was investigated by examining the changes in rheological properties, using both melt flow and capillary rheometry, and the evolution of oxidation products during processing using infrared spectroscopy. The results show that the comonomer content and catalyst type greatly affect thermal, mechanical and oxidative behaviour of the polymers. For the metallocene polymer series, it was shown from both DSC and DMA that (i) crystallinity and melting temperatures decreased linearly with comonomer content, (ii) the intensity of the β-transition increased, and (iii) the position of the tan δ max peak corresponding to the α-transition shifted to lower temperatures, with higher comonomer content. In contrast, a corresponding Ziegler polymer containing the same level of SCB as in one of the m-LLDPE polymers, showed different characteristics due to its more heterogeneous nature: higher elongational viscosity, and a double melting peak with broader intensity that occurred at higher temperature (from DSC endotherm) indicating a much broader short chain branch distribution. The thermo-oxidative behaviour of the polymers after melt processing was similarly influenced by the comonomer content. Rheological characteristics and changes in concentrations of carbonyl and the different unsaturated groups, particularly vinyl, vinylidene and trans-vinylene, during processing of m-LLDPE polymers, showed that polymers with lower levels of SCB gave rise to predominantly crosslinking reactions at all processing temperatures. By contrast, chain scission reactions at higher processing temperatures became more favoured in the higher comonomer-containing polymers. Compared to its metallocene analogue, the Ziegler polymer showed a much higher degree of crosslinking at all temperatures because of the high levels of vinyl unsaturation initially present.

Stretching of a straight electrically charged viscoelastic jet

A charged polymer jet may be accelerated and stretched by an external electric field, and this process is relevant to electrospinning for making nanofibers. The stretching of an electrified jet is governed by the interplay among electrostatics, fluid mechanics and rheology, and the role of viscoelasticity has not been systematically explored before. This paper presents a slender-body theory for the stretching of a straight charged jet of Giesekus fluid. Results show strain-hardening as the most influential rheological property. It causes the tensile force to rise at the start, which enhances stretching of the jet. Further downstream, however, the higher elongational viscosity tends to suppress jet stretching. In the end, strain-hardening leads to thicker fibers. This confirms the main result of a previous study using empirical rheological models. The behavior of the electrically driven jet forms an interesting contrast to that in conventional fiber spinning.

Improvement of processability of poly(?-caprolactone) by radiation techniques

Improvement of processability of Poly(ε-caprolactone) (PCL) was achieved by introduction of a branch structure using gamma-irradiation from a 60Co source. Irradiated PCL has higher molecular weight by producting a branch structure. Hence, the irradiation at a lower dose, such as 3 Mrad, leads to a higher melt viscosity. The branched structure gave improved properties for dynamic viscoelasticity and elongational viscosity. High elongational viscosity was observed by entanglement due to branch chain formed during irradiation, and the elongational viscosity for 3 Mrad is higher than 1.5 Mrad. Due to a higher elongational viscosity, PCL foam can be produced by a molding process. Foam produced from irradiated PCL pellets at 3 Mrad has honeycomb-like structure, and the foam showed higher enzymatic degradation compared to film samples. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 74: 1815–1820, 1999

Preparation and Characterization of a Branched Bacterial Polyester

The preparation, rheological behavior, and solution properties of branched poly(3-hydroxy butyrate)-co-(3-hydroxy valerate), PHBV, are described. Branching is produced by the use of dicumyl peroxide (DCP) during the reactive extrusion of PHBV. The shear viscosities of reactively extruded PHBV and the straight-chain polymer of similar molecular weight are nearly identical, but the reactively extruded PHBV's elongational viscosity is considerably higher. High elongational viscosity of these polymers results from a branched structure produced when extrusion is carried out in the presence of small amounts of DCP (0.025−0.3 wt %). Strain hardening is observed in the elongational response of the branched PHBV at all deformation rates and increases with increasing concentrations of DCP. Study of the solution properties of the linear and branched PHBV indicates larger hydrodynamic volume compared to that of polystyrene of comparable molecular weight. Mark−Houwink plots, observed for PHBV, exhibited increasing nonlinearity with increasing DCP content. Trifunctional branching frequencies computed on the basis of intrinsic viscosity ratios indicated explosive growth in branching, possibly due to the generation of microgel, when branching reactions are initiated with a DCP level exceeding 0.2 wt %.

On the mechanism of turbulent drag reduction in dilute polymer solutions: dynamics of vortex filaments

The localized-induction approximation (LIA) is generalized in the framework of the quasi-one-dimensional approach to include the effect of polymer additives on stretching of thin vortex filaments. It is shown that in dilute polymer solutions vorticity diffusion is negligible for vortices with longwave perturbations, and their strength remains constant in spite of the fact that high longitudinal stress and elongational viscosity may be generated in the course of vortex stretching in shear flow. General governing equations for vortex filaments in a background flow of dilute polymer solution are proposed. Their numerical solution is obtained. It shows that interaction between a spanwise perturbed vortex filament and a background shear flow leads to strong vortex stretching and to emergence of configurations similar to horseshoe or hairpin vortices. In dilute polymer solutions, strong stretching of vortex filaments yields high longitudinal stress (high elongational viscosity). The latter retards or completely arrests vortex stretching, which is similar to the findings of [1] that frequency of turbulence generation in ‘bursts’ decreases in dilute polymer solutions. Therefore, a more gentle mean velocity profile is expected in the boundary layer, which results in drag reduction.

Effects of fluid elasticity on the static and dynamic settling of a spherical particle

The results of an experimental investigation of the settling velocity of spherical particles in viscoelastic fluids are presented. As well as being fundamentally interesting, this problem is also of relevance to the under- standing of particle transport by well-technology fluids used in the oil industry. It is shown that the settling velocity is reduced by elastic effects in the fluid (e.g., presence of normal stress differences and high elongational viscosity), and that this effect becomes significantly higher with increasing shear rates experienced by the falling sphere, i.e. the ratio of settling velocity to sphere diameter. Imposing an orthogonal shear-flow field on the settling particle causes a further dramatic reduction of the settling velocity in viscoelastic fluids.

Film casting of a modified Giesekus fluid: a steady-state analysis

A theoretical analysis of the isothermal steady-state film casting process is conducted. A modified Giesekus rheological model is used in the study. The coupled effects of the rheological model parameters on the velocity profile, the stress profiles, and the film tension are found to be related to the elongational viscosity curve. A positive (negative) slope of the elongational viscosity curve corresponds to a velocity profile above (below) the Newtonian profile. The stress profiles are related to both the magnitude and shape of the elongational viscosity curve. The normalized film tension and the elongational viscosity exhibit strikingly similar characteristics. At high deformation rates, fluids with higher chain extensibility have higher elongational viscosity and require higher film tension; fluids with larger mobility factor have lower elongational viscosity and require lower film tension.

An analysis of unsteady one‐dimensional stretching of a viscoelastic fluid

AbstractThe time‐dependent response of a viscoelastic liquid to unsteady one‐dimensional stretching deformations was examined. Oldroyd's three‐constant model for a viscoelastic fluid was used. Two cases representing two different stretching histories were analyzed: a sine stretching pulse and a step stretching pulse. The results show that high elongational viscosity may be easily reached in both cases. As the relaxation time of the liquid becomes comparable to the pulse width, elongational viscosity increases with the increase in maximum stretching rates. Conditions to maintain high levels of elongational viscosity at a subsequently reduced stretching rate were given as functions of the relaxation time and initial stretching rates. In view of recent turbulent boundary layer data, the results were used to discuss possible explanations of turbulent drag reduction in polymer solutions. It was concluded that the basic mechanisms for drag reduction in polymer soluations. It was concluded that the basic mechanisms for drag reducation may be related to the effects of high elongational viscosity and local stabilization of small shear disturbances.

A Burgers-Type Model of Turbulent Decay in a Non-Newtonian Fluid

To study the effects of the high elongational viscosity on the free (“grid-generated”) turbulence of polymer solutions, a Burgers-type model equation is proposed and formed by replacing the Newtonian viscosity of the original Burgers equation with a non-Newtonian viscosity. By using the proposed model equation, numerical experiments were carried out to compute the energy decays, the changes in the energy spectra, and the variations of the mean square of the velocity gradient with time. Comparisons between the Newtonian and the non-Newtonian cases were based on the same Reynolds numbers and the same initial random velocity fields generated by a Gaussian process with two types of spectrum. Measurements on real grid turbulence are discussed in the light of these calculations. The computational results show, in general, the suppression of “shock waves” because of the non-Newtonian viscosity effect, but the turbulent decay rate at large Reynolds number is not always increased, which is surprising. It appears that the calculations support qualitatively the idea of vortex stretching inhibition in polymer turbulence due to high elongational viscosity.