Viscoelastic properties of liquid crystals of aqueous biopolymer solutions

The formation of aqueous, lyotropic phases of the biopolymers xanthan (M = 1.6·10⁶) and schizophyllan (M _W = 335 000) is investigated with stationary shear flow and oscillatory measurements, as well as with the aid of polarization microscopy, because these polymers show very different viscoelastic properties from coiled vinyl polymers. Xanthan and schizophyllan exhibit the same typical behavior observed in anisotropic solutions when the viscosity is plotted as a function of concentration and of shear rate. It has also been observed that the first normal stress difference for concentrated xanthan solutions shows a saturation effect at increasing shear rate. In oscillatory measurements only schizophyllan exhibits an maximum for the storage modulus. The absence of a such an elasticity maximum in the case of the xanthan solution may be attributed to the significantly higher flexibility of the xanthan helix. A comparison of the critical concentrations calculated according to Flory's theory and the experimentally determined values shows that the two-phase region is distinctly broader than the theory predicts. This deviation cannot be attributed to the flexibility of the polymer, but can, however, be explained by intermolecular interactions. In contrast to the non-charged schizophyllan the polyelectrolyte xanthan is affected in addition to the attractive interactions (H-bonds) by electrostatic repulsion forces.This paper was partly presented at the 198th ACS National Meeting in Miami Beach, Florida, Sept. 10–15, 1989 .     

Rheo-NMR study of a non-flow-aligning side-chain liquid crystal polymer in nematic solution

The flow behavior of a highly concentrated solution of a nematic side-chain liquid crystal polymer in a low molecular mass nematic solvent is investigated by deuterium nuclear magnetic resonance with simultaneous measurement of the shear viscosity in a cone-and-plate NMR viscometer. The director orientation under shear in the magnetic field is determined from the quadrupole splitting of the NMR spectra. The orientation as a function of shear rate is analyzed in terms of the Ericksen-Leslie-Parodi theory, yielding the Leslie coefficients ₂ and ₃ and thus the flow alignment parameter . From the combined analysis of orientation and viscosity as a function of shear rate a total of four independent viscosity parameters is obtained for the nematic solution. The value determined for the flow-alignment parameter, 0.2, and the analysis of the data based on Brochard's theory show that the polymer is of the non-flow-aligning type and has a slightly prolate shape.     

Viscoelastic properties of human cornea

Viscoelastic response of intact human cornea subjected to physiological intraocular pressure was determined from local deformations measured by a flying spot micrometer. One eye of a paired specimen was prepressurized at 15 mmHg for at least 8 hr while the other eye was left unpressurized before testing. Test results of five paired eyes showed that, in prepressurized enucleated eyes, the viscoelastic response was insignificant while significant viscoelastic response existed in the nonpressurized eyes. The latter viscoelastic properties were characterized by a five-element linear viscoelastic model and a nonlinear hereditary integral by Fung.     

Viscoelastic properties of laminated bodies

In this paper we define a class of two-phase laminate composites for which the constituent elements are either linearly elastic or exhibit viscoelastic effects. We determine the viscoelastic behaviour of the equivalent homogeneous transversely isotropic medium by a single time-independent memory function.     

Viscoelastic properties of fractal media

Temporal fractal sets for analysis of viscoelastic properties of nonhomogeneous media are considered. A fractional derivative directly related to fractal dimension is constructed. The relationship between the diffusion of the relaxation spectrum and the fractal dimension is established. Odessa State Polytechnical University, Odessa 270044, Ukraine. Institute of Molecular Physics, Polish Academy of Sciences, Poznan, Poland. Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 41, No. 1, pp. 162–172, January–February, 2000.     

Viscoelastic properties of polymer solutions

Summary Measurements were taken of the thrust of liquid jets ejecting from a long capillary into air for aqueous solutions of polyacrylamide (ET 597) of various concentrations. The measurements were then used to determine axial normal stresses for two capillary diameters: 1.52 mm (L/D = 201.5) and 2.44 mm (L/D = 207.2). The results show that the calculated values of the axial normal stress are higher for the larger capillary diameter than for the smaller, as recently reported byPowell andMiddleman. It has been further found that this diameter effect becomes more pronounced as the concentration of solute is increased. For the same materials, we also measured the primary normal stress difference by means of aWeissenberg rheogoniometer. Comparison between the two kinds of measurements shows that the magnitude of axial normal stresses is much smaller than that of normal stress differences over the range of shear rates studied (200 20,000 sec^–1) for the materials investigated. This result seems to point out the necessity of measuring the wall normal stresses, which are believed to depend not only on the capillary diameter, but also on the concentration. The authors therefore contend that, in general, the measurement of axial normal stresses alone is not sufficient to completely determine the elastic properties of viscoelastic solutions.     

Viscoelastic properties of magnetorheological fluids

We have studied the rheological properties of some magnetorheological fluids (MRF). MRF are known to exhibit original rheological properties when an external magnetic field is applied, useful in many applications such as clutches, damping devices, pumps, antiseismic protections, etc. While exploiting parameters such as magnetic field intensity, particle concentration and the viscosity of the suspending fluid, we highlighted the importance of each one of these parameters on rheology in the presence of a magnetic field. We made this study by conducting rheological experiments in dynamic mode at very low strain which facilitates the comprehension of the influence of the structure on MRF rheology. Our results confirmed the link between the magnetic forces which ensure the cohesion of the particles in aggregates, and the elastic modulus. Moreover, we found that the loss modulus varies with the frequency in a similar manner than the elastic modulus. The system, even with the smallest deformations, was thus not purely elastic but dissipates also much energy. Moreover, we demonstrated that this dissipation of energy was not due to the matrix viscosity. Actually, we attributed viscous losses to particle movements within aggregates.     

Viscoelastic properties in plane squeeze-film flows

In this paper, we consider the problem of plane squeeze-film flows in a kinematics based on the formalism of convected (moving and deforming) coordinates. The flows discussed are treated as instantaneous motions with proportional stretch histories (cf. [16]). Certain simplifications in the constitutive equations of an incompressible simple fluid (cf. [10]) have been achieved for moderately low Deborah numbers.Approximate solutions of plane flows are obtained either for slightly viscoelastic fluids or in a form valid in the vicinity of any arbitrarily chosen instant of time. The conditions of improved lubrication, leading to inequalities imposed on material constants or kinematic quantities, are discussed in detail. Also, the necessary conditions are discussed under which the time-dependent distance between the plates may decrease non-monotonically, showing some “bounces”.     

An equation of state and detonation properties of hydrazine–nitromethane liquid mixtures

Binary mixtures of hydrazine and nitromethane were found to be non-ideal associated solutions. An equation of state (EOS) of the hydrazine–nitromethane solutions has been developed. This EOS takes into account the possibility of the formation of associated molecules due to interactions between hydrazine and nitromethane molecules. EOS parameters, including a possible chemical formula for the associate and its standard heat of formation and entropy, have been determined. Thermodynamic calculations of detonation parameters of the hydrazine–nitromethane system have been done by means of the TDS code for a wide range of hydrazine content in the explosive mixture (0–80 wt.%). The reliability of the results is guaranteed by using both an accurate, theoretically justified EOS for detonation products, which is derived from first principles of statistical mechanics, and realistic potentials for intermolecular interactions. It was shown that the use of the proposed EOS of the hydrazine–nitromethane solutions considerably improves the accuracy of the predicted detonation properties of the solutions and, furthermore, allows one to evaluate their shock sensitivity. Received 25 October 1999 / Accepted 16 October 2000     

Viscoelastic properties of ultra-high viscosity alginates

Ultra-high viscosity alginates were extracted from the brown seaweeds Lessonia nigrescens (UHV_N, containing 61% mannuronate (M) and 2% guluronate (G)) and Lessonia trabeculata (UHV_T, containing 22% M and 78% G). The viscoelastic behavior of the aqueous solutions of these alginates was determined in shear flow in terms of the shear stress σ ₂₁, the first normal stress difference N ₁, and the shear viscosity η in isotonic NaCl solutions (0.154 mol/L) at T = 298 K in dependence of the shear rate [(g)\dot]\dot{\gamma} for solutions of varying concentrations and molar masses (3–10 × 10⁵ g/mol, homologous series was prepared by ultrasonic degradation). Data obtained in small-amplitude oscillatory shear (SAOS) experiments obey the Cox–Merz rule. For comparison, a commercial alginate with intermediate chemical composition was additionally characterized. Particulate substances which are omnipresent in most alginates influenced the determination of the material functions at low shear rates. We have calculated structure–property relationships for the prediction of the viscosity yield, e.g., η–M _w–c–[(g)\dot]\dot{\gamma} for the Newtonian and non-Newtonian region. For the highest molar masses and concentrations, the elasticity yield in terms of N ₁ could be determined. In addition, the extensional flow behavior of the alginates was measured using capillary breakup extensional rheometry. The results demonstrate that even samples with the same average molar mass but different molar mass distributions can be differentiated in contrast to shear flow or SAOS experiments.     

Viscoelastic properties of polymerlike reverse micelles

A dramatic increase in the viscosity of reverse micellar solutions of lecithin in a variety of organic solvents of up to a factor of 10⁶ upon the addition of a small amount of water can be observed. The formation of viscoelastic solutions can be explained by a water-induced aggregation of lecithin molecules into flexible cylindrical reverse micelles and the subsequent formation of a transient network of entangled micelles. The viscoelastic properties of these solutions are characterized as a function of water content and temperature for different organic solvents by means of dynamic shear viscosity measurements. The results are interpreted by making analogies to the behavior of semidilute polymer solutions and living polymers.Dedicated to Prof. Dr. J. Meissner on the occasion of his 60th birthday.     

Viscoelastic properties of butadiene-styrene block copolymers

Summary The viscous properties of SBS block copolymers (of Cariflex TR-1102 in this work) have been determined with the rate of shear being varied by 10⁸ times. It has been shown that in the region of low shear rates the polymer behaves as a structured highly concentrated disperse system which exhibits sharply pronounced thixotropy. In the region of high shear rates, the viscous properties of the polybutadiene matrix are of decisive importance. At low values of shear stress the compliance of the SBS block copolymer exceeds that of the polybutadiene matrix almost by a decimal order.At high shear stresses, just as in the case of viscous properties, the elastic behaviour of the SBS block copolymer approaches that of the polybutadiene matrix. Measurements of the dynamic characteristics at frequencies, which vary by a million times, most spectacularly point to the specificity of the viscoelastic properties over a wide temperature range. Under conditions of low temperatures the SBS block copolymer behaves as a rubber, while at temperatures above the glass transition temperature of polystyrene, at low loading intensities (low amplitudes and small deformations) the block copolymer shows low fluidity. This is most clearly seen from the frequency dependence of the loss modulus, especially in comparison with the relation typical of polybutadiene.

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Zusammenfassung Die viskosen Eigenschaften von SBS-Block-Copolymeren (hier Cariflex TR-1102) werden in einem Bereich von acht Zehnerpotenzen der Schergeschwindigkeit untersucht. Es wird gezeigt, daß im Bereich niedriger Schergeschwindigkeiten das Polymer sich wie ein strukturiertes hochkonzentriertes disperses System verhält, das eine ausgeprägte Thixotropie besitzt. Im Bereich hoher Schergeschwindigkeiten sind dagegen die viskosen Eigenschaften der Polybutadien-Matrix von entscheidender Bedeutung. Bei niedrigen Schubspannungswerten übersteigt die Komplianz des SBS-Block-Copolymeren diejenige der Polybutadien-Matrix höchstens um etwa 10%.Bei hohen Schubspannungen nähert sich das elastische Verhalten analog wie bei den viskosen Eigenschaften demjenigen der Polybutadien-Matrix an. Die Messung der dynamischen Charakteristiken über einem Frequenzbereich von sechs Zehnerpotenzen lassen die spezifischen viskoelastischen Eigenschaften in einem weiten Temperaturbereich sehr deutlich erkennen. Bei tiefen Temperaturen verhält sich das Polymer wie ein Gummi, während bei Temperaturen oberhalb der Glasübergangstemperatur des Polystyrols unter geringer Belastung (d.h. kleinen Amplituden bzw. Deformationen) das Polymer eine geringe Fluidität zeigt. Dies erkennt man am deutlichsten an der Frequenzabhängigkeit des Verlustmoduls, insbesondere durch Vergleich mit derjenigen bei Polybutadien.

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With 6 figures     

Viscoelastic stresses in the stagnation flow of a dilute polymer solution

In this paper, we consider viscoelastic stresses T₁₁, T₁₂ and T₂₂ arising in the stagnation flow of a dilute polymer solution; in particular, we consider an upper convected Maxwell (UCM) fluid. We present exact solutions to the coupled partial differential equations describing the viscoelastic stresses and deduce the results for the stress T₂₂ of Becherer et al. [P. Becherer, A.N. Morozov, W. van Saarloos, Scaling of singular structures in extensional flow of dilute polymer solutions, J. Non-Newtonian Fluid Mech. 153 (2008) 183–190]. As we considered the viscoelastic stresses over two spatial variables, we are able to study the effect of variable boundary data at the inflow. As such, our results are applicable to a wider range of fluid flow problems.     

The effect of liquid crystal structure on the rheological properties of nitrocellulose-dimethylacetamide solutions

Solutions of Pyro grade nitrocellulose (NC) in dimethyl acetamide (DMA), containing between 42.5% and 60% NC (w/w), have been studied by differential scanning calorimetry and polarised light microscopy. The results showed that NC forms a lyotropic liquid crystal structure in DMA. A transition from the liquid crystal phase to an isotropic phase occurred over the temperature range 27 °C to 67 °C, and the enthalpy of transition increased with NC concentration. Rheological properties were determined using an extrusion rheometer with a slit die. The solutions were shown to have a yield stress for flow which increased with increasing NC concentration. The solutions were also found to be thixotropic.     

Viscoelastic properties of styrene n-butyl methacrylate blends

Summary Viscoelastic properties of blends of styrene n-butyl methacrylate random copolymers, 65/35 in composition, varying in molecular weights, were determined using the Eccentric Rotating Disks device of the Rheometrics Mechanical Spectrometer. The complex modulus increased with increasing high molecular weight fraction, with relative differences diminishing at higher frequencies. The relaxation modulus gave maximum increase on the addition of 10% of high molecular-weight fraction. The various blending laws (including quadratic laws ofGraessley andBogue, Masuda, Einga andOnogi) were applied to the data and their limitations were shown.

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Zusammenfassung Es werden die viskoelastischen Eigenschaften der Mischungen von Styrol-n-Butyl-Methacrylat-Zufalls-Copolymeren der Zusammensetzung 65/35 in Abhängigkeit vom Molekulargewicht bestimmt, wobei ein Mechanisches Spektrometer der Fa. Rheometrics mit der Eccentric Rotating Disks-Einrichtung verwendet wird. Man findet, daß der komplexe Modul mit zunehmendem Anteil der höhermolekularen Fraktion ansteigt, wobei die relativen Differenzen zu hohen Frequenzen hin abnehmen. Der Relaxationsmodul zeigt ein Maximum des Anstiegs bei einer Zugabe von 10% des höhermolekularen Anteils. Die verschiedenen Mischungsgesetze (einschließlich der quadratischen Beziehungen vonGraessley, sowie vonBogue, Masuda, Einga undOnogi) werden zur Auswertung der Meßdaten herangezogen, und ihre Grenzen werden aufgezeigt.

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Paper presented at the Golden Jubilee Meeting of the Society of Rheology, Boston, Mass., October 29–November 2, 1979.

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With 7 figures and 1 table     

Diffuse-interface modeling of phase segregation in liquid mixtures

We simulate the phase separation of a binary mixture that is deeply quenched into the unstable range of its phase diagram. The mixture is described through the diffuse-interface model and the governing equations are integrated in 2D and 3D in a periodic box and in a channel using a pseudo-spectral method. Spinodal decomposition patterns for critical and off-critical mixtures are studied, revealing the scaling laws of the characteristic lengthscale and composition of single-phase microdomains, together with their dependence on the Peclet number. Comparison between 2D and 3D results reveals that 2D simulations capture, even quantitatively, the main features of the phenomenon. However, while the agreement between 2D and 3D simulations is excellent when the mixture is confined in a periodic box, it appears to be less pronounced in a channel-like geometry.