Oscillatory Shear Measurements Research Articles

Phase transition kinetics and rheology of gelatin-alginate mixtures

The present study reports systematic rheological data of binary aqueous mixtures of gelatin and alginate up to a concentration ratio of 1:1. The mixtures were prepared and rheologically studied with respect to the gelation point, melting point, and kinetics. The effect of the polysaccharide concentration on the sol–gel and gel–sol transitions is presented for concentrations up to 5% (w/w). Under small amplitude oscillatory shearing conditions, both transitions were shifted to higher temperatures. The kinetic study showed that the storage modulus will continue to increase in time under isothermal conditions, even if the rate of increase is considerably reduced. Furthermore, all binary gels exhibited a higher storage modulus than the pure gelatin gels at 5 °C. The increase was 9 and 24% for 5 and 2.5% gelatin, respectively. Assuming that the storage modulus is directly related to the amount of the triple helixes, all isothermal data could be successfully described by the kinetic gelation model proposed by Djabourov, Leblond, & Papon (1988a, 1988b). The excellent agreement between the rheological data and the gelation model confirm that this model can effectively predict the long-term elastic properties of the binary mixtures. Using isolated prototype modules, discrete gel specimens were formulated at 5 °C. Large amplitude oscillatory shear measurements (LAOS) showed that these specimens were still intact after the tests. The gels that were prepared directly on the rheometer plate exhibited strain hardening behavior during LAOS deformations. A nonlinear stress analysis of the Lissajous plots confirmed this finding.

PH- and Thermoresponsive Self-Assembly of Cationic Triblock Copolymers with Controlled Dynamics

Transient hydrogels formed by cationic BAB triblock copolymers consisting of a hydrophilic poly(dimethylaminoethyl methacrylate) (P(DMAEMA)) A block and amphiphilic B blocks composed of randomly distributed DMAEMA and n-butyl methacrylate (nBMA) units were investigated. Oscillatory shear measurements revealed formation of dynamic networks with terminal relaxation times that can be controlled by tuning the ionization degree (α) of the DMAEMA units or the temperature up until 50 °C. A sol–gel transition could be induced by increasing the pH. Above 50 °C irreversible aggregation was observed. The behavior of these pH-sensitive cationic copolymers is compared with that of pH-sensitive anionic copolymers, revealing that incorporating stimuli-responsive hydrophilic units within the hydrophobic blocks of amphiphilic block copolymers may be a general way to control the exchange dynamics of the latter.

The effect of the pH on thermal aggregation and gelation of soy proteins

Thermal aggregation and gelation of soy protein globulins was studied over a wide range of protein concentrations (1–95 g/L), pH values (5.8–6.8) and heating temperatures (65–95 °C). The size and structure of the aggregates was characterized by light scattering techniques and cryo-transmission electron microscopy (Cryo-TEM). At low concentrations or short heating times irregular spherical particles were formed with a hydrodynamic radius that increased with decreasing pH from 30 to 40 nm. The density of these particles increased with decreasing pH from 0.1 to 0.2 g/cm3. At longer heating times or at higher protein concentrations, these particles randomly aggregated into self-similar aggregates. At a critical heating time, gelation was observed for C = 40 g/L, which was characterized by oscillatory shear measurements. The gel stiffness increased sharply with increasing protein concentration, but was almost independent of the pH. The aggregation and gelation rate increased with increasing protein concentration and decreasing pH. The temperature dependence (65–85 °C) of the gelation rate was characterized by an activation energy of 180 kJ/mol independent of the pH. The effect of aggregation on the relationship between the pH and the charge density was determined by potentiometric titration.

Telechelic Polymer Hydrogels: Relation between the Microscopic Dynamics and Macroscopic Viscoelastic Response.

Telechelic polymers, that is, hydrophilic polymers with hydrophobic end-groups, spontaneously form hydrogels consisting of interconnected micelles. Here we investigate the relation between the microscopic dynamics determining the connectivity, that is, the lifetime of the physical bonds and the resulting rheological properties. This is achieved by quantitatively relating the chain exchange kinetics measured by time-resolved small-angle neutron scattering (TR-SANS) and the mechanical response obtained from linear oscillatory shear measurements. The results show that the characteristic relaxation time obtained from rheology coincides exactly with TR-SANS at intermediate concentrations. The activation energy, Ea, is concentration-independent and remain exactly the same as for TR-SANS. Upon crossing the melting point, a discrete change in activation energy is observed showing the contribution from the enthalpy of fusion to the release/debridging process. The results clearly show that the mechanical response and connectivity indeed are controlled by molecular exchange processes. The relaxation time at the lowest concentration is found to be faster in rheology as compared to TR-SANS, which can be quantitatively attributed to entropic forces arising from conformational deformation of bridging chains.

Formation of porous hydrogels by self-assembly of photo-cross-linkable triblock copolymers in the presence of homopolymers

Hydrogels with average pore sizes ranging from 10−2–103 μm were prepared using photo-cross-linkable self-assembling amphiphilic triblock copolymers based on poly(ethylene oxide) (PEO) mixed with homopolymers of dextran or PEO. Above a critical concentration, the triblock copolymers self-assembled into a transient network, which could be rendered permanent by UV-irradiation. Phase separation occurred for mixtures with either homopolymer above a critical concentration, which was arrested by UV-irradiation before it became macroscopic. The morphology of the hydrogels was probed by confocal laser scanning microscopy or light scattering, whereas their mechanical properties were studied using oscillatory shear measurements. The porosity of the hydrogels could be varied over orders of magnitude depending on the conditions. Hydrogels with pore sizes below 1 μm were obtained in the single phase regime, whereas gels with larger pore sizes could be obtained by irradiating phase separating mixtures. Microphase separation also led to an increase of the elastic modulus of the hydrogels.

Conjugated dual-phase transitions in crystalline/crystalline blend of poly(vinylidene fluoride)/poly(ethylene oxide)

Blends of two semi-crystalline polymers, poly(vinylidene fluoride) (PVDF) and poly(ethylene oxide) (PEO), were prepared via solution mixing method over the whole composition range. The research focuses on the liquid–liquid phase separation (LLPS) and miscibility window in the PVDF/PEO blends by employing several techniques. Small-amplitude oscillatory shear measurements were carried out to detect LLPS and determine spinodal and binodal decomposition temperatures in its early stages. An asymmetrical lower critical solution temperature (LCST) phase diagram was observed from dynamic temperature sweep experiments in which the system became immiscible after the solid–liquid phase transition (SLPS) temperatures in the PEO-rich blends. In fact, a one-way partial miscibility of PEO in the PVDF-rich phase blends was observed. The rheologically determined phase diagram was also verified by means of optical microscopy and cloud point technique. The results revealed that the PVDF/PEO blends undergo a viscoelastic phase separation (VPS). This was attributed to the large viscoelastic asymmetry of the components in the blend. A compressible regular solution free energy model was employed to determine the spinodal temperatures theoretically. The comparison between the experimental results and the mathematical modeling indicated that the model is able to predict the phase behavior qualitatively.

The rheo-Raman microscope: Simultaneous chemical, conformational, mechanical, and microstructural measures of soft materials.

The design and performance of an instrument capable of simultaneous Raman spectroscopy, rheology, and optical microscopy are described. The instrument couples a Raman spectrometer and optical microscope to a rotational rheometer through an optically transparent base, and the resulting simultaneous measurements are particularly advantageous in situations where flow properties vary due to either chemical or conformational changes in molecular structure, such as in crystallization, melting, gelation, or curing processes. Instrument performance is demonstrated on two material systems that show thermal transitions. First, we perform steady state rotational tests, Raman spectroscopy, and polarized reflection microscopy during a melting transition in a cosmetic emulsion. Second, we perform small amplitude oscillatory shear measurements along with Raman spectroscopy and polarized reflection microscopy during crystallization of a high density polyethylene. The instrument can be applied to study structure-property relationships in a variety of soft materials including thermoset resins, liquid crystalline materials, colloidal suspensions undergoing sol-gel processes, and biomacromolecules. Official contribution of the National Institute of Standards and Technology; not subject to copyright in the United States.

Measurements and Comparison to Predictions of Viscosity of Heavily Filled HDPE with Natural Fibers

AbstractComposites of rice hulls at various levels of loadings in recycled HDPE were characterized rheologically using a conical extrusion die and small amplitude oscillatory shear (SAOS) measurements. Although the results do not match, due to the invalidity of the Cox–Merz rule for such composites, both types of measurements indicate that increasing the filler concentration results in significantly higher viscosities and increased shear thinning. The errors associated with the use of a conical die due to viscous dissipation are assessed through computer simulations. They are small for relatively small throughputs (small pressure drops). The experimental results for viscosity are compared to predictions of the Einstein–Batchelor equation for suspensions of spherical particles, the Guth–Simha equation, and the empirical Krieger–Dougherty correlation, for filler loadings of up to 70% by weight. The comparison indicates that reasonable approximations of the consistency index of the power law viscosity model are possible through using the above expressions for loadings up to perhaps 40–50% by weight.

Physicochemical, microstructure and bioactive characterization of gels made from crayfish protein

Crayfish proteins are valuable active ingredients for food products, mainly due to its protein quality and antioxidant activity. A highly soluble crayfish protein concentrate (CF2L) obtained from crayfish surpluses was used to evaluate gelling properties at three different pH values (2.0, 6.5 and 8.0). Thermal gelation processes were monitored by Small Amplitude Oscillatory Shear (SAOS) measurements. Subsequently, gels were characterized by viscoelastic properties, water-holding capacity (WHC) and a selective solubility. All the systems exhibited a gel-like behavior, showing a strong dependence on pH.Antioxidant measurements were performed by using three different reagents (DPPH, ABTS and Folin-Ciocalteu) and revealed an interesting potential for human food. The pH effect was evaluated, showing a remarkable dependence of ABTS on its value.The results show that gels made from crayfish surpluses have potential for use in human nutrition, not only based on the physical properties, but also on its protein quality and antioxidant ability.

Microstructure and rheology of particle stabilized emulsions: Effects of particle shape and inter-particle interactions

Using fumed and spherical silica particles of similar hydrodynamic size, we investigated the effects of particle shape and inter-particle interactions on the formation, stability and rheology of bromohexadecane-in-water Pickering emulsions. The interparticle interactions were varied from repulsive to attractive by modifying the salt concentration in the aqueous phase. Optical microscope images revealed smaller droplet sizes for the fumed silica stabilized emulsions. All the emulsions remained stable for several weeks. Cryo-SEM images of the emulsion droplets showed a hexagonally packed single layer of particles at oil-water interfaces in emulsions stabilized with silica spheres, irrespective of the nature of the inter-particle interactions. Thus, entropic, excluded volume interactions dominate the fate of spherical particles at oil-water interfaces. On the other hand, closely packed layers of particles were observed at oil-water interfaces for the fumed silica stabilized emulsions for both attractive and repulsive interparticle interactions. At the high salt concentrations, attractive inter-particles interactions led to aggregation of fumed silica particles, and multiple layers of these particles were then observed on the droplet surfaces. A network of fumed silica particles was also observed between the emulsion droplets, suggesting that enthalpic interactions are responsible for the determining particle configurations at oil-water interfaces as well as in the aqueous phase. Steady shear viscosity measurements over a range of shear stresses, as well as oscillatory shear measurements at 1Hz confirm the presence of a network in fumed silica suspensions and emulsions, and the lack of such a network when spherical particles are used. The fractal structure of fumed silica leads to several contact points and particle interlocking in the water as well as on the bromohexadecane-water interfaces, with corresponding effects on the structure and rheology of the emulsions. The attenuation of droplet motion due to the formation of a particle network can be exploited for stabilizing emulsions and for modulating their rheology.

Flexibility of three differential constitutive models evaluated by large amplitude oscillatory shear and Fourier transform rheology

Abstract Using Fourier transform rheology the Giesekus, exponential Phan-Tien and Tanner, and modified eXtended Pom-Pom constitutive models were examined in the non-linear regime represented by large amplitude oscillatory shear (LAOS) measurements. For the experiments a 5 wt% solution of poly(ethylene oxide) (molecular weight 1000 kg/mol) solved in dimethyl sulfoxide was used. The minimal number of harmonics required for an adequate reconstruction of the original raw time-stress signal was shown. An emphasis was paid to simultaneous consideration of the normalised stress magnitudes and the phases of individual harmonics in the fitting procedure adjusting the non-linear parameters of the studied constitutive models.

Effects of Partial Replacement of Gelatin in High Sugar Gels with Gellan on their Textural, Rhelogical, and Thermal Properties

Effects of partial replacement of gelatin in simulated gummy confections with either high acyl or deacylated gellan on their textural, rheological, and thermal properties were investigated. Atomic force microscopy (AFM) images of high acyl and deacylated gellan revealed that both gellan types formed finely stranded networks as a result from air-drying of dilute aqueous solutions, the strand thickness of which was approximately 0.5–1 nm. Simulated gummy confections containing 5.025–7.1 % w/w gelatin, 0–0.075 % w/w high acyl or deacylated gellan, and 73–75 % w/w corn syrup and sucrose combined were prepared and analyzed using texture profile analysis (TPA) and small amplitude oscillatory shear measurements. The principal component analysis (PCA) of textural attributes obtained from TPA identified a cluster in the first quadrant formed by samples containing 7.1 % w/w gelatin but no gellan and those containing 6.025 % w/w gelatin and 0.075 % w/w high acyl or deacylated gellan. All simulated gummy confections showed storage modulus (G′) values greater than loss modulus (G″) values at 0.1 rad/s, G″ increasing more steeply with increasing angular frequency, and G′-G″ crossovers within the examined angular frequency range (0.1–100 rad/s), typical of high solid biopolymer gels. Furthermore, increasing gellan concentration at a total concentration of the gelling agents (i.e., gelatin and gellan) of 6.1 % w/w increased the melting temperature. These results attest the feasibility of improving the heat stability of gummy confections by the partial replacement of gelatin with either high acyl or deacylated gellan with maintaining textures characteristics of gummy confections containing gelatin as the only gelling agent.

Effect of LBG on the gel properties of acid-induced SPI gels

The effect of the addition of locust bean gum (LBG) on the rheological properties, microstructure and fractal dimension of glycono-δ-lactone induced soy protein isolate (SPI) gels were studied (LBG concentration = 0.01%–0.04%, SPI concentration = 6%). Both the small and large amplitude oscillatory shear measurements were carried out to characterize the rheological properties of SPI and SPI-LBG gels. The G′ of SPI-LBG gels was highly frequency dependent within the frequency range of 6.28 rad/s to 62.8 rad/s. Fourier transform rheology was used to analyze the higher harmonic signals from nonlinear stress waveform. SPI-LBG gels showed typical Type I network behavior similar to that of SPI gel at strain amplitude >2% in LAOS tests. Creep/recovery tests showed that the addition of LBG increased both the elastic and viscous moduli of SPI-LBG gels. The image of the microstructure captured by confocal laser scanning microscopic (CLSM) was used to determine the fractal dimension. The protein network structure of SPI-LBG gels showed greatly bigger pore volume, better scale behavior than SPI gels, and the chaos and complexity of the protein aggregates in SPI-LBG gels were much less than that in SPI gel. LBG was found to play an important role in a SPI-LBG gel system.

Scaling law, fractal analysis and rheological characteristics of physical gels cross-linked with sodium trimetaphosphate

The scaling behavior and fractal analysis of basil seed gum (BSG) cross-linked with sodium trimetaphosphate (STMP) have been investigated by rheological small amplitude oscillatory shear measurements. Storage modulus and critical strain (γo) of the gels exhibited power law relationships with BSG concentration. Based on the power-law exponent values, the fractal dimension (df) of gels was estimated using scaling models, revealed the weak-link regime of BSG. The df values lied well within the range of fractal dimension values (1.5–2.8) reported for protein gels. However, they slightly differed from df for diffusion-limited and reaction limited cluster-cluster aggregation processes. Stress sweep test was shown that STMP addition to BSG made a stronger gel than that of BSG lacking STMP. Mechanical spectrum of gels was also revealed that adding STMP can improve the elasticity of gels. BSG had a tan δ of >0.1, indicating paste-like weak gel, while tan δ of BSG-STMP has approached to 0.1 exhibited the character of a cross-linked network near to “true gel”. BSG-STMP was also recognized as a thermo-reversible physical gel, which gelation and thermal properties did not affect by STMP. Therefore, the scaling behavior can be applied for hydrocolloids gels to extract structural information through rheological measurements. Moreover, the rheological characteristics of BSG-STMP showed it can be used as a proper hydrogel in food and pharmaceutical applications.

First normal stress difference and in-situ spectral dynamics in a high sensitivity extrusion die for capillary rheometry via the ʽhole effect’

The development of a high sensitivity polymer melt extrusion flow instability detection die capable of estimating the normal stress differences of polymer melts during capillary rheometry tests via the 'hole effect' is presented here as proof-of-principle. Two polymer melts, a low density polyethylene (branched topology) and a high density polyethylene (linear topology), were tested in a variety of experimental configurations with the purpose of determining optimal conditions for performing normal stress difference measurements. The data was compared with rotational rheometry oscillatory shear measurements analyzed via the Laun rule. It is shown that it is possible to estimate the first normal stress difference in the capillary die, whereas the second normal stress difference cannot be determined within the experimental errors using the current configuration design. Furthermore, the influence of the induced streamline curvature via the 'hole effect' on the onset and development of melt flow instabilities is simultaneously assessed. It is shown that the hole depth has a stabilizing influence, i.e. the onset of instabilities occurs at higher shear rates, in the long chain branched polymer tested, whereas for the linear polymer tested it has a destabilizing effect on the stick-slip instability i.e. the onset of stick-slip occurs at lower shear rates.

Heat-induced gelation of casein micelles in aqueous suspensions at different pH

Heat-induced gelation of casein micelles in aqueous solution was investigated between pH 5.2 and pH 6.7 over a wide range of protein concentrations (C=25–160gL−1). For C≥40gL−1 the casein micelles rapidly formed a self-supporting gel above a critical temperature (Tc). At C=160gL−1, Tc decreased from 90°C at pH 6.5 to 30°C at pH 5.4 and increased with decreasing protein concentration. Oscillatory shear measurements during heating showed that the elastic modulus (Gel) of the gels increased strongly with increasing protein concentration, but was insensitive to the pH and the heating temperature except close to Tc where Gel decreased sharply with decreasing temperature. The microstructure of the gels was observed by confocal scanning laser microscopy. Heat-induced gelation of casein micelles was compared with that of sodium caseinate solutions free of calcium phosphate.

Rheological behavior of highly loaded cellulose nanocrystal/poly(vinyl alcohol) composite suspensions

Recent emphasis on the pilot scale production of cellulosic nanomaterials has increased interest in the effective use of these materials as reinforcements for polymer composites. An important, enabling step to realizing the potential of cellulosic nanomaterials in their applications is the materials processing of CNC/polymer composites through multiple routes, i.e. melt, solution, and aqueous processing methods. Therefore, the objective of this research is to characterize the viscoelastic behavior of aqueous nanocomposite suspensions containing cellulose nanocrystals (CNCs) and a water-soluble polymer, poly(vinyl alcohol) (PVA). Specifically, small amplitude oscillatory shear measurements were performed on neat PVA solutions and CNC-loaded PVA suspensions. The experimental results indicated that the methods used in this study were able to produce high-quality nanocomposite suspensions at high CNC loadings, up to 67 wt% with respect to PVA. Additionally, the structure achieved in the nanocomposite suspensions was understood through component attributes and interactions. At CNC loadings near and less than the percolation threshold, a polymer mediated CNC network was present. At loadings well above the percolation threshold, a CNC network was present, indicated by limited molecular weight dependence of the storage modulus. Overall, these results provide increased fundamental understanding of CNC/PVA suspensions that can be leveraged to develop advanced aqueous processing methods for these materials.

Viscoelastic Properties of Fresh Cement Paste to Study the Flow Behavior

During concrete pumping, the migration and redistribution of particles occur in a pipe and the lubrication layer that forms between the bulk concrete and the pipe wall is the governing factor determining the flow behavior. In order to identify flow behavior of pumping, in this study, the viscoelastic properties related to the microstructural behavior of a flocculated suspension were examined by using dynamic oscillatory measurements. Cement paste is assumed to be a constituent material of the lubrication layer and ten cases of mixing design are employed by changing the proportions of mineral admixtures. The relationship between the yield stress obtained from the steady shear test and the dynamic modulus resulted from the oscillatory shear measurement was derived and the implications of the correlation are discussed. Moreover, based on the investigation of the viscoelastic properties with oscillatory measurements, the initial behavior of pumped concrete was analyzed systematically.

Tuning of the rheological properties of concentrated silica suspensions using carbon nanotubes

In this study, the rheological behavior of fumed silica nanoparticles suspended in polyethylene glycol (PEG) and those containing multi-walled carbon nanotubes (MWNTs) was measured at steady and oscillatory shear stress using a stress-controlled rheometer. The obtained results showed that the critical viscosity of the concentrated silica suspension, decreased with the addition of MWNTs. Moreover, the onset of shear thickening for the suspension containing MWNTs appeared at higher shear rate than those without MWNTs. Oscillatory shear measurement shows that the storage modulus and loss modulus of silica suspension were higher than the suspension containing MWNTs. The higher complex viscosity of silica suspensions than the suspension containing MWNTs was also evident over the entire range of the frequency studied. The proposed mechanism of nanotube incorporation in dispersion revealed that the observed shear-thickening behavior of a suspension containing MWNTs could be attributed to the increased interaction force between MWNTs and PEG, as a result of increase in the number of hydrogen bonds. This finding suggests that MWNTs could be used as an efficient tool for tuning of the onset of shear thickening and rheological behavior of concentrated silica suspensions.

Preparation, characterization and rheological behavior of chitosan nanocapsule emulsion encapsulated tuberose fragrance

Abstract In this paper, ionic gelation method was adopted to produce nanocapsules (CNs) encapsulated tuberose fragrance with chitosan (CS) and sodium tripolyphosphate (TPP) as wall materials. The effects of CS/TPP mass ratio, pH value of CS solution, molecular mass of CS and tuberose fragrance (TF) concentration on particle size and particle size distribution (PDI) of chitosan nanocapsules encapsulated tuberose fragrance (CNTs) were investigated systematically. CNTs were characterized by dynamic light scattering (DLS), scanning electron microscopy (SEM), Fourier transforms infrared spectroscopy (FTIR), X-ray diffraction (XRD), thermogravimetric analyzer (TGA) and differential scanning calorimetry (DSC). The results showed that CNTs were successfully prepared. The optimum preparation conditions were as follows: CS/TPP mass ratio 5:1, pH of CS solution 4.0, and molecular mass of CS 150 kda. CNTs emulsions were also systematically investigated by steady-state shear and oscillatory shear measurements respectively. The rheological behaviors of CNTs were obtained.