Nonlinear Viscoelastic Regime Research Articles

Interfacial rheological properties of pepsin-hydrolyzed lentil protein isolate at oil-water interfaces

Lentil protein isolate (LPI) was investigated for its potential as a plant protein-based emulsifier. LPI consists mainly of globulins with high molecular weights which are relatively slow to adsorb and stabilize oil-water interfaces. Smaller proteins, such as whey protein, can stabilize the interface much faster, quickly forming a viscoelastic film that slows down the rate of droplet recoalescence, leading to smaller droplets. To increase the rate of adsorption, LPI was enzymatically hydrolyzed by pepsin at 1.5% and 4.5% degree of hydrolysis (DH), to obtain small peptides. The behavior of these peptides was studied at the oil-water interface in comparison to whey protein isolate (WPI) and native lentil protein isolate (LPI). Interfacial properties were investigated using dilatational and interfacial shear rheology in the linear viscoelastic regime (LVE) and nonlinear viscoelastic regime (NLVE) and these were related to protein characteristics and emulsion formation. The 1.5% and 4.5%DH LPI showed high surface hydrophobicity, and consisted mainly of low molecular weight peptides, which contributed to faster adsorption kinetics and consequently covered the interface faster than LPI. In dilatation, the native and hydrolyzed LPI-stabilized interfaces had comparable stiffness and were clearly weaker than WPI-stabilized interfaces, which contrasts with the results in shear deformations, where the former both had higher stiffness than WPI. Both hydrolyzed LPI and WPI formed more brittle and less stretchable interfaces compared to native LPI in both dilatational and shear rheology. In emulsification tests, native LPI produces the largest droplets, and the 4.5% DH LPI and WPI have the smallest droplet size. But, bridging flocculation was observed when emulsions were prepared with both hydrolyzed LPI, which may be attributed to inter-droplet hydrophobic interactions between small polypeptides. This can be linked to an increase in the surface hydrophobicity of hydrolyzed LPI. Thus, the use of native LPI as a stabilizer in emulsion preparation is preferable over the hydrolyzed samples, despite its larger droplet size.

The effects of stem bromelain on the gelation behavior of kappa carrageenan under linear and nonlinear rheological regimes

Carrageenans have shown a versatile and wide range of applications in the food and biomedical fields. To tune the rheological properties and improve the biological functions of carrageenan gels, the effect of stem bromelain (SBr) on the intermolecular interaction and rheological property of κ-carrageenan (κC) was investigated. The viscoelastic properties and gel network structure of pure SBr, κC, and SBr/κC mixtures were studied using dynamic rheology in the linear and nonlinear viscoelastic regimes. Different SBr/κC mixtures were prepared with varying final pH levels (pH 6.06, 6.95, and 8.08). The time series and frequency dependence of SBr/κC mixtures in the linear viscoelastic regime showed that the elastic modulus is (3–10 times) greater than the viscous modulus, indicating network structure formation. The temperature dependence of the viscoelastic moduli SBr/κC mixtures exhibited a significantly improved physical property than the pure SBr and κC, demonstrating synergistic relation between SBr and κC via enhanced electrostatic interaction. By increasing the pH level of the mixtures, a decrease in the shear moduli was observed attributed to the increasing repulsive interaction between SBr and κC. The observed nonlinear rheological responses suggest a transiently crosslinked network. Accordingly, with the gentle imposition of increasing stress levels, the nonlinear responses suggest that SBr/κC is a stress-mediated, highly entangled complex network with a pH-controlled dynamic viscoelastic property. Therefore, our results provide structural insights into the microscopic origins of the rheological behavior of SBr/κC mixtures as a basis for precise material design.

Effect of fiber properties and orientation on the shear rheology and Poynting effect in meat and meat analogues

The development of plant-based meat analogues is an important aspect of the transition towards using more plant proteins. Production of such products with texture (and rheology) adequate for consumer acceptance like meats is a great challenge. However, there is a current lack in quantitative methods to compare whole cut meats and plant-based analogues that also consider its fibrous structures. We tackled this problem by combining non-linear shear rheology and shear-induced normal stress measurements on whole cut meats (beef and chicken) and plant-based fibrous model systems for meat analogues (soy and pea protein isolates combined with wheat gluten). We found different response in the linear viscoelastic regime for different samples, and the non-linear viscoelastic regime provided a deeper understanding into the role of structural components. Meat samples showed rearrangements at small strains and lower energy dissipation, higher intracycle strain stiffening, and a larger normal stress response at large strains than plant-based fibrous products, due to more elastic fibers. The plant-based fibrous products behave more solid-like until a larger strain, but the fibrous structures break down at large deformations resulting in higher energy dissipation, and show a lower normal stress response than meat. We therefore explore the importance of fiber properties and orientation on the nonlinear rheology of fibrous model systems for meat analogues, and provide a robust methodology to quantitatively compare different mechanical responses of meat and plant-based fibrous meat analogues. These results thereby allow comparison and tuning mechanical responses of plant-based meat analogue products towards real meat.

Modeling the rheological behavior of silica filled rubber compounds

The rheological behavior of styrene–butadiene rubber (SBR) compounds filled with silica is investigated as a function of silica volume fraction. To predict the mechanical response, a continuum model for entangled polymer melts filled with nanoparticles is herein introduced. This model is capable of describing the rheological response in both the linear and nonlinear viscoelastic regimes in the context of non-equilibrium thermodynamics to guarantee its thermodynamic admissibility. The constitutive model describes the polymer nanocomposite melts at a mesoscopic level of description by considering the conformation tensor between successive entanglement points, and the orientation tensor for the, in general, spheroidal nanoparticles that describes their average orientation. Evolution equations are developed for nanoparticles with an arbitrary shape but are eventually specified to the case of spherical ones. The multimode version of the new constitutive model provides a very accurate prediction of the rheological behavior of the processability range of SBR/silica nanocomposites. Thus, the new model is a tool able to provide answers to the several difficulties that rubber-producing manufacturers face when processing rubber compounds.

Molecular structure and linear-non linear rheology relation of rice starch during milky, dough, and mature stages

Immature rice has potential to be used as healthy food. The relation between molecular structure and rheological properties was investigated. The lamellar repeating distance (8.42–8.63 nm) and crystalline thickness (4.60–4.72 nm) were not different among stages indicating a complete lamellar structure even at early stage. The relative crystallinity was higher in dough (39.62 %) than milky (36.69 %) and mature starch (35.22 %) caused by molecular structure, amylose, and amylose-lipid complex. The short amylopectin branched chains (A and B1) in dough starch were easily entangled resulted in higher Payne effect and elastic dominant. Dough starch paste exhibited higher G′Max (738 Pa) than milky (685 Pa) and mature (645 Pa) starch. In a non-linear viscoelastic regime, small strain hardening was found in milky and dough starch. Mature starch showed the highest plasticity and shear thinning at high-shear strains as the long-branched chains (B3) microstructure was disrupted, disentangled, followed by chain orientation along shear.

Viscoelasticity of Low-Molecular-Weight Polyelectrolytes.

Shear-thickening fluids that absorb the impact energy of high-velocity projectiles are of great interest for aerospace and body-armor applications. In such a frame, we investigate transient states of neat and aqueous polyelectrolytes (PE) having low molecular weights and containing poly([2-(methacryloyloxy)ethyl]trimethylammonium) as polycations and poly(acrylamide-co-acrylic acid) as polyanions. We compare results with those of bulk water. We employ nonequilibrium molecular dynamics to simulate oscillatory shear, mainly in the linear viscoelastic regime. We find that neat PE exhibits properties of a viscoelastic solid, whereas water and the aqueous mixture of PE conform to viscoelastic liquids with Maxwellian behavior at low angular frequencies. Terminal relaxation times are ∼0.499 and ∼1.385 ps for water and the aqueous mixture of PE, respectively. At high angular frequencies, storage moduli show anomalous behaviors that correspond to transitions between shear thinning and shear thickening in complex shear viscosities. The change in potential energy with the increase of the angular frequency is mainly driven by intramolecular interactions for neat PE, whereas short-range Coulomb interactions are the major contributions for water and the aqueous mixture of PE. Upon observation of the molecular configurations, only the local polyionic structure in the aqueous mixture of PE shows improvement when increasing the angular frequency, whereas the rest remains barely affected. Thus, the water structure in the aqueous mixture of PE allows the storage of energy elastically through the hydrogen-bond network at large angular frequencies, whereas the mechanical contribution of polyions weakens and fully vanishes at the beginning of shear thinning, explaining the superimposed data with data of bulk water. Our method and findings set the path for future molecular simulations in the nonlinear viscoelastic regime with more complex underlying molecular mechanisms.

Rheology of mature fine tailings

Rheological characteristics of mature fine tailings (MFTs) were examined in both the linear and non-linear viscoelastic regimes. MFT samples exhibited thixotropic behavior as well as apparent slip, which was suppressed by using sandpaper of grit 80 (200 μm) at the wall of the parallel-plate geometry. The real yield stress of MFTs with shear-thinning behavior was retrieved after fitting to a Herschel–Bulkley equation. Creep and oscillatory shear tests are also used to verify the yield stress measurement. The yield stress of two MFT suspensions possessing similar volume fractions was found to be similar; however, a kaolinite suspension (formulated to mimic/match the MFT rheology) showed more shear thinning and less thixotropy. The presence of bitumen remaining in the MFTs (up to 2 wt. %) suppressed the apparent slip to a large extent due to the immobilization of the particles at the interface. Finally, the effect of temperature on MFT rheology was studied in detail over the range of 0–50 °C, showing a minimum viscosity and yield stress at about 20 °C (demonstrating a non-monotonic increase with the increase in temperature).

Acid-induced gels from soy and whey protein thermally-induced mixed aggregates: Rheology and microstructure

In this study, we explored how substituting whey protein isolate (WPI) with soy protein isolate (SPI) affects the linear and non-linear rheological behavior of acid-induced gels, and their microstructures. Commercial SPI and WPI dispersions (pH 7.0, 3.0 mS/cm) were preheated (95 °C, 30 min) at different protein concentrations (2%, 4%, 6%, and 8% w/w) and SPI: WPI ratios (0: 4, 1: 3, 2: 2, 3: 1 and 4: 0). The resultant thermally-induced aggregates were characterized before gelation was induced by glucono-δ-lactone (GDL). Small and large amplitude oscillatory shear (SAOS and LAOS) tests showed that replacing WPI with SPI decreased the strength (lower G′) and stretchability (lower γc) of acid-induced gels in the linear viscoelastic (LVE) regime. Gels containing SPI behaved more similar to pure SPI gels in the non-linear viscoelastic (NLVE) regime: displaying a relatively elastic response at large strain and a gradual transition to plastic behavior. The changes in rheological properties were explained by the differences in the gel microstructures, via fractal scaling theory, multiphoton laser scanning microscopy (MLSM) and scanning electron microscopy (SEM). WPI gels formed denser and homogenous gel networks with very strong inter-floc links, while hybrid gels and pure SPI gels formed coarser and more porous networks with intermediate inter-floc links. The constituent flocs in the latter were larger, with rougher, more elongated and branched structures. The present results provide useful information for future attempts to replace WPI with SPI in food products based on acid-induced gelation.

Towards a universal shear correction factor in filament stretching rheometry

Filament stretching rheometry is a prominent experimental method to determine rheological properties in extensional flow whereby the separating plates determine the extension rate. In literature, several correction factors that can compensate for the errors introduced by the shear contribution near the plates have been introduced and validated in the linear viscoelastic regime. In this work, a systematic analysis is conducted to determine if a material-independent correction factor can be found for non-linear viscoelastic polymers. To this end, a finite element model is presented to describe the flow and resulting stresses in the filament stretching rheometer. The model incorporates non-linear viscoelasticity and a radius-based controller for the plate speed is added to mimic the typical extensional flow in filament stretching rheometry. The model is validated by comparing force simulations with analytical solutions. The effects of the end-plates on the extensional flow and resulting force measurements are investigated, and a modification of the shear correction factor is proposed for the non-linear viscoelastic flow regime. This shows good agreement with simulations performed at multiple initial aspect ratios and strain rates and is shown to be valid for a range of polymers with non-linear rheological behaviour.

Rheology of complex biobased quaternary blends: Poly(lactic acid) [poly(ethylene oxide)]/poly(ether-b-amide)/poly(amide 11)

Linear and nonlinear viscoelastic approaches are used to study multiphase biobased blends of poly(lactic acid), poly(amide 11), poly(ether-b-amide) (PEBA), and poly(ethylene oxide) (PEO) in a wide range of compositions. The novelty of this work resides in (a) the study of hybrid quaternary blends with droplet-matrix as well as cocontinuous morphology, (b) the effect of the PEO and PEBA blend components on the rheological properties, and (c) the investigation of the nonlinear viscoelastic regime for complex hybrid blends. Correlations are established between blend morphology and rheological behavior. Among the different linear rheological approaches, i.e., elastic modulus, complex viscosity, yield stress, Cole–Cole plots, Han plots, and Mavridish–Shroff or Booij–Palmen plots, the latter is the most sensitive allowing extraction of a relaxation time from the plot. The results correlate reasonably well with the different morphologies of the blends. Both, linear and nonlinear methods lead to compatible results, distinguishing among blends with droplet-matrix, partially continuous dispersed phase and fully cocontinuous morphologies. Moreover, for the first time, the analysis of the nonlinear parameter, Q0, allows a qualitative comparison of the interfacial areas of blends with a dispersed phase-matrix morphology at different continuity levels and those with cocontinuous morphology.

Thermorheological properties of asphalt binders

AbstractThe rheological properties of different types of asphalt binders were studied and compared considering their constituents and physical characteristics. The saturate, aromatic, resin, and asphaltene (SARA) analysis and differential scanning calorimetry (DSC) have shown their individual constituents and two distinct glass transition temperatures, indicating the phase changes of the two main components of the asphalt binders, namely asphaltenes and maltenes. Rheological characterization was performed over a wide range of temperatures (−10°C‐60°C) showing that these materials may exhibit viscoelastic solid to viscous liquid behaviour. Master curves of complex viscosity, storage modulus, and loss modulus were constructed by applying the time‐temperature superposition principle, which was found applicable over the temperature range considered. Stress relaxation and steady‐shear test were applied to the samples in order to determine their rheological behaviour in the nonlinear viscoelastic regime (viscosity and nonlinear relaxation modulus). The rheological results were modelled and revealed that the Kaye‐Bernstein‐Kearsley‐Zappas (K‐BKZ) constitutive equation is suitable in representing the rheological behaviour of asphalts. The SARA analysis and rheological measurements were found to be compatible.

I-Rheo: determining the linear viscoelastic moduli of colloidal dispersions from step-stress measurements.

We report on the application of a Fourier transform-based method, 'i-Rheo', to evaluate the linear viscoelastic moduli of hard-sphere colloidal dispersions, both in the fluid and glass states, from a direct analysis of raw step-stress (creep) experimental data. We corroborate the efficacy of i-Rheo by comparing the outputs of creep tests performed on homogenous complex fluids to conventional dynamic frequency sweeps. A similar approach is adopted for a number of colloidal suspensions over a broad range of volume fractions. For these systems, we test the limits of the method by varying the applied stress across the materials' linear and non-linear viscoelastic regimes, and we show that the best results are achieved for stress values close to the upper limit of the materials' linear viscoelastic regime, where the signal-to-noise ratio is at its highest and the non-linear phenomena have not appeared yet. We record that, the range of accessible frequencies is controlled at the higher end by the relative weight between the inertia of the instrument and the elasticity of the complex material under investigation; whereas, the lowest accessible frequency is dictated by the extent of the materials' linear viscoelastic regime. Nonetheless, despite these constrains, we confirm the effectiveness of i-Rheo for gaining valuable information on the materials' linear viscoelastic properties even from 'creep ringing' data, confirming its potency and general validity as an accurate method for determining the material's rheological behaviour for a variety of complex systems.

The influence of protein/phospholipid ratio on the physicochemical and interfacial properties of biomimetic milk fat globules

The effect of protein/phospholipid ratio on the physicochemical and interfacial properties of biomimetic milk fat globules (BMFGs) was investigated. Butter milk phospholipid (BMP) vesicles prepared with ultrasonication were mixed with whey protein isolate (WPI) at different protein/phospholipid ratios (w/w). After emulsification, the Turbiscan stability index (TSI) was used to quantify the initial stability of BMFGs. At different pH values, visual observation and CLSM were performed to obtain the macro stability and microstructure of BMFGs. In addition, large amplitude oscillatory surface rheology was applied and analysed with Lissajous curves, to explore the oil/water interfacial properties of BMFGs in the nonlinear viscoelastic regime. The TSI indicated a faster creaming rate of BMP stabilized droplets compared to those stabilized by WPI. The macro stability of WPI/BMP co-stabilized BMFGs at different pH was dominated by WPI and BMFGs prepared at the WPI/BMP ratio of 20:80 phase separated from pH 3.0 to 5.0. At acidic pH, the WPI/BMP ratios strongly affected the coalescence and flocculation of BMFGs at the micro level. The surface elasticity indicated that WPI dominated the rheological properties of WPI/BMP co-stabilized oil/water interfaces. Besides, the significant dependence of surface elasticity on deformation amplitudes within the nonlinear regime of WPI stabilized oil/water interfaces indicated a clearly different structure from BMP stabilized interfaces. Conclusively, results of this study showed that the physicochemical and interfacial properties of BMFGs were significantly influenced by the protein/phospholipid ratio in the bulk, which needs to be considered in the design and application of biomimetic milk fat globules.

Piezo-Plunger Jetting Technology: An Experimental Study on Jetting Characteristics of Filled Epoxy Polymers

The droplet formation of Newtonian fluids and suspensions modified by spherical, non-colloidal particles has attracted much interest in practical and theoretical research. For the present study, a jetting technique was used which accelerates a geometrically defined plunger by a piezoelectric actuator. Changing rheological properties of materials and extending deformation rates towards nonlinear viscoelastic regimes created the requirement to extend dosage impulses towards larger magnitudes. To mimic the rheological characteristics of nonconductive adhesives we modified Newtonian epoxy resins by thixotropic additives and micro-scale glass spheres. Rheological analysis at steady shear and oscillatory shear ensured a differentiation between material and process-related factors. Evaluation of high-speed images allowed the investigation of drop dynamics and highlighted the dispense impulse reduction by material-specific dampening properties.

Effect of Filler Functionalization on the Thermo-Mechanical behavior of Polypropylene Nanocomposites

Present study is an experimental attempt to characterize the thermo-mechanical behavior of Thermoplastic polymer nanocomposites (PNC) surface functionalized with multi walled carbon nanotubes (MWCNT). Nanocomposites with two grades of MWCNT, pristine and –COOH functionalized were fabricated with isotactic polypropylene (iPP) and results were compared. Nanocomposites Films with 0%, 0.5%, 1% and 2% MWCNT loadings were prepared by conventional solution casting method followed by isothermal quasi static uniaxial tensile tests at constant strain rates of 0.1/min, cyclic tensile tests at strain rate of 0.1/min and isochronous creep characterization at stress levels of 0.2, 0.5, 1, 2, 5, 7, 10, 12 and 15 MPa performed on Tinius-Olsen Universal Testing machine to monitor strength and stiffness of the material and obtain the threshold of stress and time at room temperature to observe the transition from linear to nonlinear viscoelastic regime. Dynamic characterization at constant frequency of 1 Hz, temperature ramp 0-100°C, was performed to obtain storage modulus under dynamic loading conditions on Dynamic mechanical analyzer (DMA). It was reported that incorporation of functionalized MWCNT increases the strength, failure strain, reduces creep strain and increases stiffness of nanocomposites as compared to those with pristine-MWCNT up to 1% concentration. Moreover, it was also observed that loading under 1MPa and 5 seconds, the material behaves linearly. Prior to the testing, thermal stability of materials was monitored by Thermo-gravimetric analysis.

Fourier-transform rheology of unvulcanized styrene butadiene rubber filled with increasingly silanized silica

ABSTRACTSilica as one the most important fillers for rubber material is routinely modified by silane to improve its compatibility with the rubber matrix. Silanization of the silica particle affects both the linear and nonlinear rheological behaviors of the compounds. Their rheological nonlinearity, however, is mostly analyzed in an indirect way from linear rheological parameters, e.g. G′(ω1, γ0) and G″(ω1, γ0), which lose their physical meaning in the nonlinear viscoelastic regime. In the present work, the nonlinearity is directly quantified by the Fourier-transform rheology (FT-Rheology) technique in terms of I3/1(ω1, γ0), the third relative higher harmonic, for unvulcanized styrene butadiene rubber compounds filled with a fixed amount of silica, but varying dosages of silane. With the proposed model for I3/1(γ0), the contributions toward nonlinearity from the filler networks at a low strain amplitude and the one from the polymer networks at high strain amplitude can be successfully separated for filled systems. The utmost nonlinearity contribution from the filler networks decreases with the silane content, which is assigned to the weakening interparticle interaction of the filler. With increasing silanization of silica, the utmost nonlinearity contribution from the polymer networks is found to increase. This nonlinear mechanical response is attributed to the enhanced interfacial interaction between the filler and polymer.

Nonlinear viscoelasticity of a dilute suspension of Brownian spheroids in oscillatory shear flow

The nonlinear viscoelasticity of a dilute suspension of Brownian spheroids subject to an oscillatory shear deformation is calculated numerically. This is achieved by determining the suspension microstructure, parameterized via the orientation distribution function. Specifically, the long-time periodic orientation distribution function is obtained via a numerical solution to the Fokker–Planck equation by combining a finite-difference approximation in space with a Fourier series in time. From an ensemble average of the particle stresslet, weighted by the orientation distribution function, the entire stress tensor and relevant birefringence parameters, namely, the average orientation angle and linear dichroism, are calculated; this is done over a range of the Weissenberg number (Wi) and the Deborah number (De), or dimensionless strain-rate amplitude and oscillation frequency, respectively. Detailed calculations are presented for prolate spheroids of aspect ratio r=20; however, our methodology is general and can be applied to spheroids of arbitrary aspect ratio. We provide results in four viscoelastic regimes: linear viscoelastic (Wi≪1), quasilinear viscoelastic (Wi>1 and Wi/De≪1), quasisteady viscoelastic (De→0), and finally, the nonlinear viscoelastic regime (Wi ≳ 1 and Wi/De ≳ 1), which is our main emphasis. In this last regime, where the nonlinear and unsteady viscoelasticity of the material is probed, multiple overshoots are observed in the shear stress and first normal stress difference. The mechanistic origin of these overshoots can be understood from the periodic orientation dynamics (i.e., Jeffery orbits) of a particle under steady shear in the absence of Brownian rotation (Wi→∞). This is achieved by simultaneously analyzing the microstructure, shear stress, first normal stress difference, and birefringence parameters specifically at Wi=20 and De=1. For these values of Wi,De, and r, the period of a Jeffery orbit is comparable to the period of an oscillation cycle, allowing sufficient time for a single Jeffery orbit (and subsequent overshoot) to occur during an oscillation cycle. We contrast this behavior to recent work by Khair [J. Fluid. Mech. 791, R5 (2016)] on nearly spherical particles, for which many more overshoots are observed during an oscillation cycle, due to the shorter period of the Jeffery orbit. We briefly provide results for oblate spheroids of aspect ratio r=0.05 and compare to the results for r=20. Finally, we comment on the relevance of the present micro-mechanical analysis to the nonlinear viscoelasticity of other complex fluid materials.

Chain Retraction in Highly Entangled Stretched Polymer Melts.

We use computer simulations to study the relaxation of strongly deformed highly entangled polymer melts in the nonlinear viscoelastic regime, focusing on anisotropic chain conformations after isochoric elongation. The Doi-Edwards tube model and its Graham-Likhtman-McLeish-Milner (GLaMM) extension, incorporating contour length fluctuation and convective constraint release, predict a retraction of the polymer chain extension in all directions, setting in immediately after deformation. This prediction has been challenged by experiment, simulation, and other theoretical studies, questioning the general validity of the tube concept. For very long chains we observe the initial contraction of the chain extension parallel and perpendicular to the stretching direction. However, the effect is significantly weaker than predicted by the GLaMM model. We also show that the first anisotropic term of an expansion of the 2D scattering function qualitatively agrees to predictions of the GLaMM model, providing an option for direct experimental tests.

Large amplitude oscillatory shear behavior and gelation procedure of high and low acyl gellan gum in aqueous solution

The rheological properties of gellan fluid gels were investigated using large amplitude oscillatory shear (LAOS) technique in both linear and non-linear viscoelastic regimes, with consideration of high acyl (HA)/low acyl (LA) gellan ratio and Ca2+ concentration. The Lissajous curves and Chebyshev coefficients were used to analyze the LAOS data and successfully provided visual and quantitative representation of microstructural differences between HA and LA gellan gum, respectively. Temperature sweep measurements were performed to monitor the gelation procedure of gellan gum solution. The results show that HA gellan gum forms softer but more stable gels than LA gellan gum, especially at high temperature. And a synergistic interaction between HA and LA gellan gum may exist in the presence of abundant Ca2+ ions. The abovementioned study can allow a better understanding of structural characteristic of gellan gum and provide a practical approach to rheological assessment that facilitates the processing and diversified application of mixed gellan gum.

Primitive Path Analysis and Stress Distribution in Highly Strained Macromolecules.

Polymer material properties are strongly affected by entanglement effects. For long polymer chains and composite materials, they are expected to be at the origin of many technically important phenomena, such as shear thinning or the Mullins effect, which microscopically can be related to topological constraints between chains. Starting from fully equilibrated highly entangled polymer melts, we investigate the effect of isochoric elongation on the entanglement structure and force distribution of such systems. Theoretically, the related viscoelastic response usually is discussed in terms of the tube model. We relate stress relaxation in the linear and nonlinear viscoelastic regimes to a primitive path analysis (PPA) and show that tension forces both along the original paths and along primitive paths, that is, the backbone of the tube, in the stretching direction correspond to each other. Unlike homogeneous relaxation along the chain contour, the PPA reveals a so far not observed long-lived clustering of topological constraints along the chains in the deformed state.