Viscoelastic Plateau Research Articles

The “solvent” effect of short arms on linear and nonlinear shear rheology of entangled asymmetric star polymers

This study examines the influence of the short arms in model asymmetric 3-arm star polymers, in which the short arm is unentangled while the other two long arms are entangled, on linear and nonlinear shear rheological behavior. As increasing the short arm length of the asymmetric star polymers, the linear viscoelastic rubber plateau decreases, indicating the relaxed short arms act as “solvent” that dilute the star polymers. This idea is examined by the relationship between the plateau moduli of polymer solutions GN(ϕ) and the corresponding polymer melts GN(1) for entangled linear polymers, GN(ϕ) = GN(1)ϕ1+α, where ϕ is the volume fraction of the polymer. The “solvent” effect of the short arms is also supported by the nonlinear rheology results showing weaker damping function and larger peak strain at stress overshoot. With the above clarification of the short-arm contributions, this work provides a guidance of molecular design for tuning linear and nonlinear rheology of star polymers.

Rheology and Phase Behavior of Surfactant–Oil–Water Systems and Their Relationship with O/W Nano-Emulsion’s Characteristics Obtained by Dilution

In order to study the relationship between the rheology of a surfactant’s concentrated dispersions and the oil and water liquid crystals from which O/W nanoemulsions (NEs) can be produced by water dilution, the phase diagram of a model SOW (surfactant–oil–water) system was constructed. The dispersion’s compositions to be characterized by rheology were chosen in the diagram’s regions that contain liquid crystal phases. For this, the dilution lines S/O = 25/75, 55/45, and 70/30 with a water content of 20 and 40 wt% (corresponding to surfactant concentrations between 15 and 55 wt%) were chosen. By adding these dispersions to a water pool, NEs were obtained, and it was shown that droplet size distribution depends on the amount of the liquid crystal phase in the initial dispersion and its rheology. The study of the oscillatory amplitude of the dispersion showed a linear viscoelastic plateau (G’ > G”) and a softening deformation region (G” > G’), indicating a viscoelastic behavior of the dispersions. The study was carried out at a constant temperature of 30 °C, and the results show that rheological characterization by itself is not enough to predict that monomodal droplet distributions are obtained. However, the presence and quantity of lamellar liquid crystal phase are important to obtain monodisperse and kinetically stable NEs.

Rheological properties of artificial boluses of cereal foods enriched with legume proteins

The properties of artificial food bolus are studied by dynamic oscillatory and capillary rheometry as functions of bolus water content (WC), in the usual range of saliva hydration, for four cereal products: sponge cake, extruded flat bread and their counterpart enriched in legume proteins. All boluses followed the same rheological behavior characterized by (1) solid -like in the linear viscoelastic domain and (2) Herschel-Bulkley model for large shear strain. Hence, four characteristic rheological properties are determined: modulus at viscoelastic plateau, characteristic stress at transition to flow, yield stress and consistency in the flow regime. The decrease of these properties with WC was fitted by an exponential decay function, from which was extracted a coefficient α (5 = α ≤ 30), defined as a coefficient of interaction of the food with water. The values of α are of the same order of magnitude as the plasticization coefficient of starch by water. They were larger for the extruded pea based (EFP, α ≥ 15), and were lower for the sponge cake (SC, α ≤ 15). The variations for the different rheological properties are discussed in terms of matter state, envisioning bolus as a suspension of soft swellable particles. The comparison of these values with those encountered for real boluses from similar foods suggests that these results contribute to define a coefficient of interaction of food with saliva.

Nonaffinity and Fluid-Coupled Viscoelastic Plateau for Immersed Fiber Networks.

We employ a matrix-based solver for the linear rheology of fluid-immersed disordered spring networks to reveal four distinct dynamic response regimes. One regime-completely absent in the known vacuum response-exhibits coupled fluid flow and network deformation, with both components responding nonaffinely. This regime contains an additional plateau (peak) in the frequency-dependent storage (loss) modulus-features that vanish without full hydrodynamic interactions. The mechanical response of immersed networks such as biopolymers and hydrogels is thus richer than previously established and offers additional modalities for design and control through fluid interactions.

Formation of liquid crystal/gel emulsions to nano-emulsions constructed by polyalkoxylated fatty alcohol (PAFA)-based mixed surfactant systems

In the construction of ternary phase diagrams, the polyalkoxylated fatty alcohol (PAFA)-based mixed surfactant systems including PAFA-AS (alkyl sulfonate), PAFA-CB (cocamidopropyl betaine) and PAFA-APG (alkyl polyglucosides) were used to develop self-standing liquid crystal/gel emulsions containing rapeseed oil methyl esters (ROME) and water. The formation of liquid crystal/gel emulsions are observed at semi-dilute regions of the phase diagrams. A pre-emulsion was chosen from each of PAFA-AS, PAFA-CB and PAFA-APG systems for minor modification with sodium silicate. Upon aqueous dilution of the modified pre-emulsions to weight fractions (Φw) of 0.8 and 0.6 and with an isothermal shaker agitation, the samples demonstrate dramatic increases in apparent viscosity with flow resistance and shear thinning behaviour. In oscillatory amplitude study, the emulsions show linear viscoelastic (LVE) plateau (G’>G”) and strain softening region (G”>G’) indicating the samples promote a viscoelastic behaviour. Further affirmation by Cole-Cole plots reveal the emulsion samples behave as a Maxwell fluid. The optical microscope study verifies the emulsions of PAFA-AS, PAFA-CB and PAFA-APG systems comprising of multilamellar vesicles, bicontinuous cubic phase and multilamellar phase, respectively. Upon aqueous dilution of the liquid crystal/gel emulsions with an isothermal agitation, the formation of nano-emulsion droplets is confirmed by transmission electron microscopy (TEM) and dynamic light scattering studies. The nano-emulsions display spherical and elliptical shapes with mean droplet sizes are in the range of 158.37 to 206.43 nm and zeta potential values are in the range of –12.07 to –32.79 mV.

Systematic bottom-up theory for the viscoelastic response of worm-like chain networks

Using the frequency-dependent force-extension response functions of single worm-like chains as the only input, the linear complex viscoelastic modulus of a polymeric network with given connectivity is derived from a systematic bottom-up theory via iterative coarse-graining. Choosing a cubic connectivity and accounting for random network orientation, we find excellent agreement with experimental data for actin networks under shear over the entire frequency range with the strength of the osmotic pressure that acts within the polymeric network as the only fitting parameter. In particular, we obtain a viscoelastic plateau regime at low frequencies and a crossover to an intermediate-frequency regime characterized by a power law behavior and an inhomogeneous shear deformation field.

Rheological characterisation of gluten from triticale (x Triticosecale Wittmack).

Triticale gluten still remains very poorly characterised rheologically. In this study the mechanical spectra of gluten isolated from four triticale cultivars were registered and fitted with Cole-Cole functions yielding the visco-elastic plateau parameters. Master spectra were calculated. A retardation test was performed and used to calculate the composite mechanical spectra and the width of visco-elastic plateau l. Protein fractional composition of triticale flour and gluten was studied using capillary zone electrophoresis. Differentiated HMW-GS/SS compositions were identified in the triticale cultivars studied. The rheological parameters reached the following values: JN0 1.05·10-3 to 2.69·10-3 Pa-1 , GN0 372 to 956 Pa, ω0 0.003 to 0.06 rad s-1 , l 169 to 3121, Je0 1.57·10-3 to 5.03·10-3 Pa-1 , Ge0 199 to 637 Pa and η0 1.06·107 to 3.93·107 Pa s. Visco-elastic properties of triticale gluten correspond to the lower end of medium visco-elasticity shown by common wheat gluten. Master spectra and the composite mechanical spectra prove that four triticale glutens exhibit practically an identical type of visco-elastic behaviour of a biopolymeric visco-elastic liquid similar to wheat gluten. The visco-elastic plateau parameters GN0 , JN0 , ω0 and l appeared significantly correlated with the contents of prolamins and secaloglutenins in triticale flours and glutens. © 2017 Society of Chemical Industry.

Transition from Linear to Nonlinear Viscoelasticity for Bimodal Magnetic Elastomers under a Magnetic Field

We have investigated the transition from linear viscoelasticity to nonlinear viscoelasticity for bimodal magnetic elastomers under a magnetic field. For bimodal magnetic elastomers, the transition strain increased remarkably with the volume fraction of nonmagnetic particles. For monomodal magnetic elastomers, the transition strain also increased with the volume fraction of magnetic particles; however, the increment was not significant. A possible mechanism of the widening of the linear viscoelastic plateau by nonmagnetic particles is discussed.

Foam-like compression behavior of fibrin networks.

The rheological properties of fibrin networks have been of long-standing interest. As such there is a wealth of studies of their shear and tensile responses, but their compressive behavior remains unexplored. Here, by characterization of the network structure with synchronous measurement of the fibrin storage and loss moduli at increasing degrees of compression, we show that the compressive behavior of fibrin networks is similar to that of cellular solids. A nonlinear stress-strain response of fibrin consists of three regimes: (1) an initial linear regime, in which most fibers are straight, (2) a plateau regime, in which more and more fibers buckle and collapse, and (3) a markedly nonlinear regime, in which network densification occurs by bending of buckled fibers and inter-fiber contacts. Importantly, the spatially non-uniform network deformation included formation of a moving "compression front" along the axis of strain, which segregated the fibrin network into compartments with different fiber densities and structure. The Young's modulus of the linear phase depends quadratically on the fibrin volume fraction while that in the densified phase depends cubically on it. The viscoelastic plateau regime corresponds to a mixture of these two phases in which the fractions of the two phases change during compression. We model this regime using a continuum theory of phase transitions and analytically predict the storage and loss moduli which are in good agreement with the experimental data. Our work shows that fibrin networks are a member of a broad class of natural cellular materials which includes cancellous bone, wood and cork.

Relations between Stereoregularity and Melt Viscoelasticity of Syndiotactic Polypropylene

A set of syndiotactic polypropylene samples with tailored degree of stereoregularity and uniform distribution of stereodefects, obtained via organometallic catalysis, was used to study the relation between the syndiotactic pentad concentration and the viscoelastic plateau modulus in the melt state. The plateau modulus was found to increase with increasing the degree of stereoregularity, indicating that syndiotactic polypropylenes of different stereoregularity produce a different entanglement density of the amorphous phase. The change in plateau modulus was highly nonlinear for large values of syndiotactic pentad contents, suggesting that the presence of even a relatively small number of defects along the syndiotactic backbone significantly alters the space filling attitude of the polymer chain in the melt. This result, which confirms and extends those already obtained from experiments and numerical simulations, points out to the concept that the dynamics of macromolecular chains is largely controlled by the relative configuration of consecutive stereoisomeric centers along the chain, thus providing a link between chain dynamics and molecular architecture.

Random Block Copolymers: Structure, Dynamics, and Mechanical Properties in the Bulk and at Selective Substrates

Using computer simulations of a soft, coarse-grained model and a Lennard-Jones bead–spring model, we investigate the behavior of random block copolymer (RBCP) blends in the bulk and in the vicinity of a solid substrate. The RBCP is comprised of six random sequences of A and B blocks and adopts a microemulsion-like morphology as we increase the incompatibility between A and B segment species. The soft, coarse-grained model is used to efficiently equilibrate the morphology and molecular conformations in the melt, and its explicit molecular conformations are used to generate equilibrated starting configurations for the bead–spring model. Since the pure A and B melts are structurally asymmetric, A-rich and B-rich domains differ in their density and viscosity in the melt or mechanical properties in the glassy state, respectively. We demonstrate that the self-assembled morphology in the melt gives rise to a viscoelastic transient plateau in the stress autocorrelation function. While an analysis of the entanglem...

Effect of wheat dietary fibres on bread dough development and rheological properties

Several fractions of wheat fibres were isolated from starchy endosperm, aleurone layer and bran, and characterized for their hydration properties and arabinoxylans (AX) content. The influence of their addition, up to 10%, to standard flour was studied through mixing tests, and rheological tests at small and large deformations. The effect of insoluble AX on dough development was accounted for by their capacity to retain water, whatever their origin and percentage of addition. The addition of insoluble AX increased the viscoelastic plateau modulus. The addition of soluble and insoluble AX to the dough did not modify the overall dough flow behaviour in shear, characterized by a Newtonian plateau at low shear rates followed by shear-thinning behaviour at larger shear rates. This behaviour could be fitted by the Cross model. The addition of water soluble AX modified the Newtonian viscosity value. Conversely, the addition of insoluble ones increased dough consistency, probably through a filler-like effect in the dough matrix.

Nonlinear, time-dependent shear flow behaviour, and shear-induced effects in wheat flour dough rheology

Retardation test, step-shear rate experiments, low-amplitude and large-amplitude dynamic measurements have been combined to study the nonlinear and time-dependent viscosity of dough and shear-induced effects of flow on dough structure. Despite large quantitative differences in linear viscoelastic constants, doughs from different flours or with different water contents display the same type of flow behaviour. Shear-induced structural changes cause flow to shift from a high viscosity steady-state regime to a low viscosity one. The process, irreversible, is responsible for the time-dependent character of dough viscosity and seems to be controlled by the mechanical energy absorbed. Nevertheless, the two steady-state viscosities follow the same shear-thinning flow curve, fitted by a Cross equation with an exponent close to 1; the Newtonian plateau is approached at very low shear rate values. Viscosity data obtained on different doughs yield a unique flow master curve in reduced coordinates. Shear-induced structural changes cause also the linear viscoelastic plateau modulus of dough to decrease; this progressive weakening of the network structure is irreversible and seems governed by the accumulated strain. These characteristics of dough rheology are discussed with reference to the behaviour of concentrated suspensions.

Comparison of viscoelastic properties of gluten from spelt and common wheat

Rheological properties of gluten from spelt and common wheat were studied. The mechanical spectra of gluten samples were registered over a frequency range of 0.001–200 rad/s. Retardation tests were performed to keep all measurements within a linear regime. The mechanical spectra were fitted with Cole–Cole functions to calculate the viscoelastic plateau modulus GN0, the central frequency of the upper dissipative loss peak ω0, and the spread parameter n. Steady state compliance Je0 and Newtonian viscosity η0 were determined from the retardation tests results. Recovery data were converted from time to frequency domain using the Kaschta method and combined with dynamic data; this enabled the extension of the gluten mechanical spectra down to 10−6 rad/s, revealing the lower dissipative peak loss. The width of the viscoelastic plateau τm0/τ0 was calculated, and substantial qualitative and quantitative differences were found in spelt and common wheat gluten. All differences in gluten rheological properties were related to spelt and common wheat flour baking quality and protein composition.

The pattern of the linear viscoelastic behaviour of wheat flour dough as delineated from the effects of water content and high molecular weight glutenin subunits composition

Abstract Dynamic measurements and retardation tests were combined to characterise the linear viscoelastic behaviour of wheat flour dough in the 10 −5 –10 2 rad/s frequency range. Analysis of the data provided the Newtonian steady-state viscosity, the steady-state compliance, the terminal relaxation time, the viscoelastic plateau compliance and a measure of the upper frequency limit of the viscoelastic plateau. The influence of dough water content and composition of high molecular weight glutenin subunits on dough viscoelasticity was studied. Both factors affected dough viscoelasticity in a similar and remarkable way. In particular, the same inverse relationship between steady-state viscosity and compliance, and the same power law relationship between steady-state and plateau compliances, was found to hold whether the variability was due to high molecular weight glutenin subunits or to dough water content.

Polymer time constants during low temperature nanoimprint lithography

We investigate low temperature nanoimprint into polymers, where adequate pressure choice helps to increase the imprint velocity of larger patterns und thus decreases pattern size effects by reduction of the effective viscosity. In order to make use of shear rate effects, the imprint has to be performed within the viscoelastic plateau region and not in the viscous flow regime of the polymer. Therefore elastic effects play an important role and may lead to shape recovery of the polymer after imprint. We address the counter play of elastic effects and viscous flow by conducting experiments very near to the glass transition temperature. The elastic behavior at the beginning of the imprint is simulated for different pattern sizes and thus different aspect ratios of the stamp. The investigations show, that a reduction of temperature has to be compensated by an increased imprint time and this time increase has to consider the reduction of viscosity on the one hand and the extension of the polymer flow time constants on the other hand. The experiments are suitable to define a lower limit for utilization of shear thinning effects.

Mixing rheometry for studying the manufacture of lubricating greases

The overall objective of this work was to evaluate the manufacture of lubricating greases through the mixing rheometry technique, by studying the effect of some processing variables such as rotation speed, intensity and duration of the homogenization treatment and thermal profile applied, in terms of the power-draw characterization and rheological behaviour of the final product. With this aim, lithium lubricating greases were prepared by inducing the saponification reaction between 12-hydroxystearic acid and hydrated lithium hydroxide within a naphtenic lubricating oil medium in an open vessel. The saponification reaction occurred until neutralization by stirring with a controlled-rotational speed mixing rheometer using an anchor impeller. Different rotational speeds were selected. Finally, a highly intensive homogenization treatment was applied using a rotor-stator turbine in order to reduce crystal sizes. Different homogenization treatments and cooling profiles were applied on the incipient greases. The manufacture of lubricating greases was always followed through the evolution of torque with processing time. The experimental results obtained demonstrate that the mechanical behaviour of a lubricating grease strongly depends on some of the processing variables studied. Some exponential models have been proposed to evaluate the influence of the processing variables on both the consistency index and the linear viscoelastic plateau modulus of the lubricating greases.

A study of the effect of locust bean gum on the rheological behaviour and microstructure of a β-lactoglobulin gel at pH 7

The effect of locust bean gum (LBG), a non-gelling polysaccharide, on the thermal gelation of β-lactoglobulin, at 80 °C, and on the gel properties after quenching to 20 °C was studied by small deformation rheology and by confocal laser scanning microscopy (CLSM). The concentration of β-lactoglobulin was kept constant at 10 wt% and that of LBG varied from 0 to 0.78 wt%. For all the concentrations studied, the presence of LBG enhanced the aggregation rate and the strength of the protein gel, but the magnitude of these effects depended on the β-lactoglobulin/LBG ratio: 0.35 wt% LBG resulted only in a very slight increase of G′, whereas 0.45 wt% LBG caused a ~fivefold jump; for higher LBG concentrations, the differences between the systems were quite small. The linear viscoelastic behaviour, at 20 °C, was characterized over the 10−5 to 100 rad/s frequency range by combining the dynamic and retardation tests. Compliance data were converted from the time to the frequency domain. The viscoelastic plateau was seen to extend down in the 0.001–0.0001 rad/s range and its lower limit seemed not to vary much with LBG concentration. Its upper limit was visibly beyond 100 rad/s. The observed microstructure of the gels showed that they were two-phase and that the state of aggregation of β-lactoglobulin was influenced by the β-lactoglobulin/LBG ratio.

A phenomenological analysis of wheat gluten viscoelastic response in retardation and in dynamic experiments over a large time scale

Dynamic measurements and retardation tests (creep and creep recovery) were combined systematically to characterise the different aspects of gluten rheology. Gluten flow behaviour exhibits extreme shear thinning and time-dependent viscosity. The viscoelastic contribution to the total strain behaves linearly over large strain and stress ranges; however, the material shows physical ageing and does not exhibit time translational invariance. In spite of these difficulties, it was possible, in the case of fresh gluten samples or after the material has been submitted to mechanical loading for a period long enough, to obtain the linear viscoelastic response over seven logarithmic decades of time or frequency by combining the data from dynamic measurements and from retardation tests. The method of Kaschta was used to calculate the discrete retardation spectrum from creep recovery data; the spectrum was then used to convert recovery data into storage and loss compliance and modulus. Converted recovery data match dynamic data satisfactorily in the case of fresh gluten. Some discrepancy was observed in the case of gluten submitted to previous mechanical history; it has been attributed to the effect of physical ageing. The storage and nonviscous loss compliance functions show that the extension of the viscoelastic plateau of gluten is very large. Their shape suggests a bimodal spectrum; however, this point would need further confirmation.

Tensile stress and recoverable strain measurements on polystyrene melts. Interpretation of results in terms of rubberlike elasticity

AbstractExtensional tests at constant strain rate \documentclass{article}\pagestyle{empty}\begin{document}$ \dot \varepsilon $\end{document} have been carried out on polystyrene melts with different molecular weight distributions at various temperatures and strain rates. The true tensile stress is found to be well approximated by the sum of two contributions: (1) a neo‐Hookean expression involving the recoverable strain and (2) a contribution rapidly reaching a steady‐state value. Two experimental parameters can be defined: an elasticity modulus \documentclass{article}\pagestyle{empty}\begin{document}$ G(\dot \varepsilon ) $\end{document} from (1) and a viscosity \documentclass{article}\pagestyle{empty}\begin{document}$ \eta _{\rm v} (\dot \varepsilon ) $\end{document} from (2). It is further shown that time‐temperature equivalence applies not only to the stress but also to the recoverable strain and to G and ηv. The dependence of G and ηv on strain rate is then discussed. For high strain rates, G is close to the linear viscoelastic plateau modulus of PS melts and decreases with decreasing strain rate. The value of ηv is found to a good approximation to be equal to three times the shear viscosity taken at a shear rate equivalent to the elongational strain rate.