Oscillatory Shear Measurements Research Articles

Preparation and rheological investigation of tough PAAm hydrogel by adding branched polyethyleneimine

ABSTRACTIn this article, tough hydrogels are prepared by introducing the polyethyleneimine (PEI) with branched structure and a large number of NH2 and NH groups into permanently crosslinked polyacrylamide (PAAm) hydrogels matrix. To investigate the effects of B‐PEI and chemical crosslinking agent (Bis) on the strength and toughness of hydrogels, a series of B‐PEI/PAAm hydrogels with different mass percentage of Bis and B‐PEI were manufactured and the rheological and swelling properties were compared. For all hydrogels, the storage modulus (G′) was much higher than the loss modulus (G″) in the linear viscoelastic region through the whole frequency range. The solid‐like behavior and elastic nature (G′ > G″) are attributed to the permanent covalence crosslinking. Therefore, G′ increased when more Bis was added. For the nonlinear oscillatory shear measurement, hydrogels with B‐PEI broke at larger γ than the pure PAAm hydrogels, indicating that the toughness of B‐PEI/PAAm hydrogels has been improved by introducing B‐PEI. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020, 137, 48541.

Drying kinetics and viscoelastic properties of concentrated thin films as a model system for spray drying

Controlling the development of the particle morphology during spray drying is of large importance to obtain high quality powders. During drying of skin-forming materials it is still unknown how the rheological properties of the skin develop as a function of time, moisture content and temperature. We here studied films prepared from whey protein – maltodextrin mixtures as a model system for spray drying. The rheological properties were assessed by oscillatory shear measurements at relevant high dry matter contents (66–82 w/w%). During drying, the films high in whey protein became brittle and had slower evaporation compared to films high in maltodextrin. Rheological analysis showed that for whey protein rich systems (with higher ratio than 25:75 WP:MD) the films were in structural arrest at the dry matter contents measured. Maltodextrin films on the other hand showed typical viscoelastic polymer behavior, although as little as 1% addition of whey protein altered its viscoelastic properties drastically. The viscoelastic properties could be related to vacuole formation during single droplet drying: samples that undergo structural arrest at a lower dry matter content (high in whey protein), form less and larger vacuoles compared to samples that undergo structural arrest only at high concentration (high in maltodextrin).

Physicochemical characterization and emulsifying properties of a novel exopolysaccharide produced by haloarchaeon Haloferax mucosum

The physicochemical characterization and emulsifying functional properties of a novel exopolysaccharide (EPS) produced by haloarchaea Haloferax mucosum (DSM 27191) were investigated. This biopolymer has a high molecular weight of 152 kDa and important protein content of 10%. Different culture media compositions were investigated taking the ATCC 2185 medium as a base and supplementing with varying concentrations of yeast extract and glucose or sucrose as carbon sources to produce the EPS in a liquid medium. The highest EPS production (7.15 ± 0.44 g/L) was obtained at 96 h. EPS aqueous dispersions showed a non-Newtonian rheological behavior which was well fitted to the Cross equation. The EPS (at 0.32% w/w) was capable of stabilizing water-in-oil emulsions with different nonpolar solvents, including n-hexane, kerosene, chloroform, castor oil and mineral oil. EPS retained its emulsifying activity after to be incubated for one hour in a wide range of temperatures (25, 40, 70 and 100 °C), pH (4, 6.5, 7 and 12) and NaCl concentrations (0, 2.0 and 4.0 M). The viscoelastic behavior and stability of hexane-in-water emulsion were examined through oscillatory shear measurements.

Rheology of polymer multilayers: Slip in shear, hardening in extension

The nonlinear rheology of multilayer stacks of alternating isotactic polypropylene (PP) and polyethylene (PE) films with N layers was measured in shear and uniaxial extension. We show that the force to extend N − 1 interfaces can lead to strain hardening. We studied three PE/PP pairs: a Ziegler–Natta catalyzed pair and two metallocene pairs, one with high density PE and one with linear low density PE. Interfacial tension was measured on matrix/droplet blends of each pair using small amplitude oscillatory shear measurements fitted with the Palierne model. Multilayer coextrusion was used to fabricate bilayers and sheets with hundreds of layers for each polyolefin pair. Under shear deformation, disentangling of chains in the N − 1 interfaces led to a decrease in the overall shear viscosity, which is interpreted as an interfacial slip velocity and is in agreement with previous studies on multilayer films. Under extensional flows, the multilayers exhibited an increased transient extensional viscosity (tensile stress growth coefficient, η E , M +) and pronounced strain hardening, even when the constituent homopolymers exhibited no strain hardening behavior. A model based on a force summation was developed to predict this behavior with interfacial tension as a fitting parameter; the extracted interfacial tensions agreed reasonably well with those from the Palierne model. The “slip in shear” and “hardening in extension” behaviors highlight the role of interfacial polymer physics during rheological measurements and polymer processing.

Rheological properties of silicone-surfactant-based wormlike micellar solution

One of the major drawbacks of ionic-surfactant-based wormlike micelles (WLMs) is their poor low-temperature stability, which severely limits their application. Generally, an ionic surfactant with a linear C12–C16 alkyl chain has a Krafft point around or above room temperature. Therefore, there is a demand for ionic-surfactant-based WLMs with greater lowtemperature stability. Silicone surfactants have flexible hydrophobic chain structures. Hence, they are expected to be suitable candidates for WLMs that can be stable at low temperatures. Sodium trimethylsilyl tetra(dimethylsiloxane) decyl sulfate (Si5C10SO4Na) and benzyltrimethylammonium chloride (BTAC) were used as surfactant and electrolyte, respectively, to obtain the WLM system with desired properties. No precipitation of surfactant crystals was observed in the WLM system obtained in this study when temperature was decreased to as low as 0 °C, indicating excellent low-temperature stability. Steady rheological measurements on the viscous solutions show shear thinning corresponding nearly to a power law relation (viscosity ∝ [shear rate]−1). Zero shear viscosity (η0) increased with increase in R, reaching a maximum at around R = 0.3 and decreasing thereafter. Oscillatory shear measurements for the viscoelastic samples, formed around the maximum-viscosity composition, show that the storage modulus (G′) and the loss modulus (G′′), with respect to the oscillation frequency (ω), cross each other and fit the Maxwell model very well in the low-ω region. The normalized Cole-Cole plot of G′′ / G′′max against G′ / G′′max was obtained as a semicircle centered at G′ / G′′max = 1, as is typical for WLM systems.

Effect of cinnamaldehyde on interfacial rheological properties of proteins adsorbed at O/W interfaces

The dynamics of heterogeneous food products such as emulsions can be affected significantly by the interfacial properties of their interfaces. Proteins are widely used to increase the stability of these food products. This work compares the interfacial properties of a model protein (whey protein isolate, WPI) and silkworm pupae (SLW) adsorbed at the O/W interface. A natural aldehyde (cinnamaldehyde, CNM) was used for both protein systems in order to promote protein-protein interactions. Interfacial properties were characterised during protein adsorption and after reaching a quasi-equilibrium state by means of oscillatory and step dilatational, and oscillatory interfacial shear measurements. The results obtained from dilatational and interfacial shear tests showed that the use of CNM resulted in the development of stronger interfaces, with higher values for the dilatational and surface shear storage moduli, and a lower loss tangent. Step-dilatation tests indicated that the addition of CNM also resulted in more homogeneous interfaces. Our results show that CNM addition can enhance the surface properties of SLW, to a level which is close to the properties of un-modified WPI stabilized interfaces.

Influence of the first normal stress differences on model hydrophobically modified ethoxylated urethane-thickened waterborne paints brush drag

Commercial paints are complex formulations and their application properties are typically explained based on their steady shear viscosity profiles. Hydrophobically modified ethoxylated urethane (HEUR) polymers are widely used as rheology modifiers (RM) in waterborne paints. In this work we formulated a series of HEUR-based model paints with matching high-shear viscosity (0.12 Pa-s at 10,000 s−1). Compositionally these paints were similar, differing only in the HEUR end-hydrophobe length (Cn, n = 10–18) and the amount used to obtain the targeted high-shear viscosity. Their linear viscoelastic response was dominated by a transient network of HEUR-bridged latex particles as revealed from oscillatory shear measurements. The low (steady) shear viscosities were related to the HEUR end-hydrophobe length while the high-shear viscosities tend to approach the targeted value. The first normal stress difference (N1) at high shear changed sign from negative for low Cn (n = 10, 12) to positive for high Cn (n = 16, 18) paints. Further, brush drag was measured in real time where the typical application shear rate was ≈ 10,000 s−1. Paint brush drag was found to increase monotonically with HEUR Cn and this variation/trend could not be explained through their high-shear viscosity. It is proposed that the observed trend can be explained based on the paint N1 sign, where a negative N1 provides a compressive force pulling the brush towards the surface and facilitates paint transfer at relatively lower drag. In contrast, a positive N1 exerts a tensile force tending to pull the brush out from the surface and extra force is required to overcome this effect. This work emphasizes the importance of normal stress (along with viscosity) to understand paint properties, especially for high-shear applications.

Heat-induced gel formation of a protein-rich extract from the microalga Chlorella sorokiniana

Abstract Microalgae proteins are an underutilized biomass source for foods with unknown technofunctionalities. Heat-induced gel formation was used in this study to investigate the gelation properties of a protein-rich extract from the microalga Chlorella sorokiniana. Gel formation was achieved at a minimum protein concentration of 9.9 g/100 mL and a temperature of 61 °C was necessary to produce a stable gel network. Small-amplitude oscillatory shear measurements revealed that a weak gel was formed with tan δ > 0.1, which was also evident during backward extrusion measurements. The elasticity of the gels increased when the final Tset was increased and a maximum gel elasticity was obtained at Tset = 80 °C and a heating rate of 1 °C min−1. Increasing the heating rate to 5 and 15 °C min−1 negatively affected the gel strength. Additionally, increasing the ionic-strength and increasing or decreasing the pH negatively affected the gel network, which was related to a change in electrostatic interactions. The results obtained showed that it is possible to produce protein gels from water-soluble protein-rich extracts from Chlorella sorokiniana, which may simplify application of protein extracts from this microalga in foods.

Yield stress measurement of a thixotropic colloid

Yield stress measurements have long been considered to be inconsistent and difficult, especially for a thixotropic colloid, since the thixotropic behavior gives rise to time-dependent rheological properties. In this paper, we attempted measuring the yield stress of aqueous xanthan gum (XG) solutions in different XG concentration. Our rheological measurements showed that XG in aqueous solutions, which can be characterized as weak gels, were thixotropic for 0.4, 0.6, 0.8, and 1 wt% XG concentrations. To ensure the accuracy of our rheological measurement data, detailed wall slip and stress relaxation time were first investigated prior to the actual yield stress measurement. A standard sample loading procedure was used to ensure the consistency of the loaded sample. Various methods were employed such as steady shear, oscillatory shear, rheological model fitting, and creep test to determine the yield stress of aqueous XG solutions. A sigmoidal model was also proposed to obtain a yield stress from our steady shear measurement unambiguously. In our steady shear measurements, a resistance to an avalanche of flow is observed and it is likely due to a conformational change of XG polymers under shear. As compared to other measurement methods, it was found that the most reliable way to obtain a yield stress is via small amplitude oscillatory shear measurements, in which a yield stress can be obtained conveniently at the maximum storage modulus, and it is well matched with our steady shear and creep test results.

Development of an online rheometer for simultaneous measurement of shear and extensional viscosity during the polymer extrusion process

Abstract The demand for valid and reliable online process surveillance control of modern production plants in extrusion and compounding has considerably increased in recent years. We present an online rheometer for simultaneously measuring the extensional and shear viscosity of polymer melts and filled compounds during processing. The unique die design features a hyperbolic slit contraction that enables constant extensional rates along the die. Further, pressure transducers can be incorporated directly into the two slit sections, which prevents material from accumulating in pressure holes. The rheometer was implemented to a compounding line containing a Leistritz ZSE27 MAXX co-rotating twin-screw extruder and strand pelletizing. We demonstrated the applicability for condition monitoring based on several examples. The rheological behavior of a polypropylene random copolymer is characterized during twelve successive extrusion cycles. We compared our results to oscillatory shear measurements with a plate-plate rheometer. Decrease of extensional viscosity were less severe than the decrease of shear viscosity during multiple extrusion runs. Blending a high and a low viscous polypropylene produced similar behavior. The Trouton ratio which is based on both the extensional and shear viscosity was introduced as an additional quality parameter. For distinguishing between different PP/PS compounds or PP glass fiber compounds, the Trouton ratio is a valid and beneficial parameter. Additionally, different methods for evaluating the melt flow rate (MFR) from shear viscosity data were compared.

Camphene/polystyrene solutions: A rheological approach for material processing industry

ABSTRACTRheological properties of polymer–camphene solutions are very relevant as they affect their stability, and they have many implications in operation costs. This work elucidates the rheological properties of polystyrene (PS–camphene) solutions with industrial applications. Unfortunately, the assessment of the very low shear viscosities of camphene‐based materials (below 0.005 Pa s) is still a challenge with commercial rheometers. Flow curves, stress growth tests, and small amplitude oscillatory shear (SAOS) measurements were carried out as a function of PS concentration (0, 2.5, 5.0, 10, and 20 wt % PS) to understand the rheological behavior of these systems. Results indicate that liquid camphene has a shear‐thinning behavior (flow index = 0.99) when the increase of polymer concentration involves changes in the structure of these systems (flow index = 0.10 for 20 wt % PS). An increase in PS concentration leads to a forwarding in the shear‐thinning zone, and the Newtonian region (from ∼1 s−1, for 0 wt % PS, to 10−3 s−1 for 20 wt % PS). Eventually, these results evidence the high dependence of η∞ on PS concentration (from ∼1.16 ± 0.01 Pa s, for 0 wt % PS, to 631.5 ± 0.2 Pa s for 20 wt % PS). © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019, 136, 47953.

Tailor‐made controlled rheology polypropylenes from metallocene and Ziegler–Natta resins

Production of controlled rheology polypropylenes (CRPPs) is practiced industrially by modifying existing commodity Ziegler–Natta resins through peroxide‐induced β‐scission reactions, resulting in materials with controlled rheological properties and accompanying narrower molecular weight distributions (MWDs). In this work, this methodology was studied using both metallocene‐based polypropylenes (mPPs) and Ziegler–Natta‐based polypropylenes (ZN‐PPs). Numerical simulations based on a previously proposed kinetic model indicated that the nature of the starting resin has a significant effect on the control of MWD polydispersity index (PDI) and weight‐average molecular weight () of the resulting CRPP. Based on these observations, experiments were carried out to demonstrate the feasibility of producing CRPP with targeted molecular and rheological characteristics. Commercial mPP and ZN‐PP resins were selected to produce CRPP with similar or melt flow rates (MFRs) but varying PDIs. The rheological properties and MWDs of these materials were evaluated through oscillatory shear and gel permeation chromatography (GPC) measurements and their extrusion behavior was briefly studied and assessed with respect to these properties. POLYM. ENG. SCI., 59:1114–1121 2019. © 2019 Society of Plastics Engineers

A Comparison of the Effect of Temperature on the Rheological Properties of Diutan and Rhamsan Gum Aqueous Solutions

The rheological properties exhibited by gums make its use in applications interesting, such as foods, cosmetics, enhanced oil recovery, or constructions materials. Regardless of application field, the effect of temperature on these properties is of great importance, since these properties can be modified and cause the gum not to be useful for those conditions. Diutan and rhamsan gums are biopolymers, belonging to the sphingans, with similar structures which differ in the substituents of their side chains. It is known that both gums exhibit suitable viscoelastic properties and flow behavior when used as a stabilizer, gelling agent, or thickener. Both gums are widely used in food industry, personal care products, construction materials, oil operations, etc. For this reason, to know the effect of the temperature on their rheological properties is very helpful. For this purpose, small amplitude oscillatory shear measurements and flow curves, as a function of the temperature from 10 °C to 60 °C, have been performed, and the results obtained for both gums compared. The obtained results provide interesting information from an industrial point of view, since they reveal that the rheological properties remained almost unaltered in the temperature range assessed with diutan gum aqueous solutions, being slightly more viscous and viscoelastic than rhamsan gum solutions.

Agonistic and antagonistic roles of fibroblasts and cardiomyocytes on viscoelastic stiffening of engineered human myocardium

Cardiomyocyte and stroma cell cross-talk is essential for the formation of collagen-based engineered heart muscle, including engineered human myocardium (EHM). Fibroblasts are a main component of the myocardial stroma. We hypothesize that fibroblasts, by compacting the surrounding collagen network, support the self-organization of cardiomyocytes into a functional syncytium. With a focus on early self-organization processes in EHM, we studied the molecular and biophysical adaptations mediated by defined populations of fibroblasts and embryonic stem cell-derived cardiomyocytes in a collagen type I hydrogel. After a short phase of cell-independent collagen gelation (30 min), tissue compaction was progressively mediated by fibroblasts. Fibroblast-mediated tissue stiffening was attenuated in the presence of cardiomyocytes allowing for the assembly of stably contracting, force-generating EHM within 4 weeks. Comparative RNA-sequencing data corroborated that fibroblasts are particularly sensitive to the tissue compaction process, resulting in the fast activation of transcription profiles, supporting heart muscle development and extracellular matrix synthesis. Large amplitude oscillatory shear (LAOS) measurements revealed nonlinear strain stiffening at physiological strain amplitudes (>2%), which was reduced in the presence of cells. The nonlinear stress-strain response could be characterized by a mathematical model. Collectively, our study defines the interplay between fibroblasts and cardiomyocytes during human heart muscle self-organization in vitro and underscores the relevance of fibroblasts in the biological engineering of a cardiomyogenesis-supporting viscoelastic stroma. We anticipate that the established mathematical model will facilitate future attempts to optimize EHM for in vitro (disease modelling) and in vivo applications (heart repair).

Rheological and textural insights into the blending of sweet potato and cassava starches: In hot and cooled pastes as well as in fresh and dried gels

Starches from different origins are often blended with the aim of lowering cost and modulating product texture. However, the law governing the blending consequence is not well established. To this end, the pastes, fresh and dried gels from the blends of sweet potato starch (SPS) with cassava starch (CS) at different ratios were investigated. The results revealed that SPS and CS largely differ in terms of pasting behavior, rheological properties of hot and cooled paste as well as the textural parameters of fresh and dried gels. Generally, there are two categories, namely additive and non-additive for the blending effects of different starches. The present study shows that the blending effect of SPS and CS highly depends on the ratios of individual components and is also property specific. The pasting behavior during peak viscosity development is of non-additive nature but the remaining part is governed by an additive law. In hot and cooled blend pastes, the individual starches behave independently in steady shear measurements while they interact more or less in dynamic oscillatory shear measurements, thus demonstrating additive and non-additive effects, respectively. As for fresh and dried blend gels, additive effects were concluded for most tested textural parameters and physicochemical properties except for the enthalpy of dried ones and the elasticity of the cooked ones. This study provides basis to formulate the blend of SPS and CS with particular functionalities.

Mobility of carrageenan chains in iota- and kappa carrageenan gels

Gels of kappa carrageenan (κ-car), iota carrageenan (ι-car) and their mixtures at a polysaccharide concentration of C = 10 g/L were prepared by cooling solutions in the presence of different concentrations of KCl or CaCl2. The mobility of carrageenan (Car) chains at 20 °C was determined using fluorescence recovery after photobleaching (FRAP). Slow recovery of the fluorescence was observed for the gels showing that a fraction of the Car chains remained mobile. The fraction of mobile chains in the gels varied between 25% and 75% depending on the type of Car and the type and concentration of salt. The fraction of mobile chains in gels of different Car or in different types of salt was not correlated to the gel stiffness determined by oscillatory shear measurements. The release of Car from gel fragments into excess solvent at the same salt concentration was probed as a function of time with light scattering. It was found that the released Car chains were on average smaller than those remaining in the gel.

Advances in rheological measurements of food products

The measurement of food rheology has improved significantly with time and the advent of instrumentation. Flow behavior, its prediction, and industrial applications are today more realistic. Evolution of tribology synchronized the measurement of food texture with the dynamic of oral processing and opened a new area of rheological study. Importance of small and large amplitude oscillatory shear measurements for food products have been highlighted in this review.

Molecular Weight Dependence of Methylcellulose Fibrillar Networks

Gelation of aqueous methylcellulose (MC) solutions upon heating has been shown to result from the formation of a network of semiflexible fibrils, with diameters of 15 ± 2 nm. Here, we investigate the impact of MC molecular weight on the elasticity and structure of aqueous gels at concentrations between 0.1 and 3 wt %. Small-amplitude oscillatory shear measurements conducted at a fixed concentration reveal that the gel modulus increases monotonically by a factor of 5 for weight-average molecular weights (Mw) between 22 and 550 kg/mol. Small-angle X-ray scattering data, fit to a semiflexible cylinder model, demonstrate that the fibril radius, Kuhn length, and volume fraction are approximately constant throughout this molecular weight range. Small-angle light scattering shows that the fibrillar-rich and fibrillar-depleted domains within the gel are associated with an essentially invariant heterogeneity correlation length. Direct visualization by cryo-TEM reveals that lower molecular weight MC forms fibrils o...

(Invited) Phase Separation in Mixed Polysaccharide Gels

Phase separation behavior has been studied for the mixed polysaccharides of kappa carrageenan (k-carr) and iota carrageenan (i-carr). Temperature dependence of G’ and G” for the mixture solution measured by dynamic rheological measurement showed two step increases in cooling process, suggesting the individual network formation of k-carr chain and i-carr chain at gelling temperatures (Tg) corresponding to Tg for pure each solution of k-carr and i-carr. The particle tracking method using fluorescent particles with diameter of 100nm was carried out for the mixtures indicating the formation of phase separated structure in the mixture. It was also indicated that the local viscosity of k-carr rich phase was lower than that of pure k-carr solution with the same concentration and the local viscosity of i-carr rich phase was higher than that of pure i-carr solution, suggesting that each phase contions other carrageenan chain. The large amplitude oscillatory shear (LAOS) measurements for the mixtures with various mixing ratios were also measured to investigate the effect of the phase separated structures on the physical property. The response of strain against the large deformation showed characteristic behaviors corresponding to the change of mixing ratio, suggesting a transitional change of phase separation structure with the change of volume ratio of each phase.

Rheological nature and dropping performance of sweet potato starch dough as influenced by the binder pastes

Abstract Starch dough is a biphasic system in which the solid starch granules are dispersed in a continuous liquid matrix of binder paste. Experiencedly, the rheological nature of starch dough is vital to starch noodle production and heavily depends on the binder paste applied. The starch dough processing in the manufacture of starch noodle is widely spread but lacks scientific evidence. In the present study, the rheological behaviours of nine binder pastes and their corresponding sweet potato starch doughs were investigated by both steady shear and dynamic oscillatory shear measurements. The results showed that the processing characteristics of starch doughs were highly dependent on the binder paste applied, i.e., the doughs with binder pastes from sweet potato, cassava, potato and waxy maize starch displayed better dropping performance than that with binder pastes from pea, mung bean, wheat, rice or maize starch. Rheologically, both binder pastes and starch dough exhibit overall shear-thinning and thixotropic properties. A good binder paste could be defined by a lower thixotropy, consistency and loss modulus, as well as a higher loss tangent. For the first time, the present study performed a rheological characterization of starch dough and cast a new light on the dependence of its processing suitability on the binder paste involved. The findings herein successfully merge the experiences of starch dough processing into science.