Linear Rheological Behavior Research Articles

Large amplitude oscillatory shear (LAOS) for nonlinear rheological behavior of heterogeneous emulsion gels made from natural supramolecular gelators

The linear and nonlinear rheological behaviors of heterogeneous emulsions gels made from natural glycyrrhizic acid (GA) nanofibrils and sitosterol-oryzanol mixtures (sterols) were investigated using small amplitude oscillatory shear (SAOS) and large amplitude oscillatory shear (LAOS). The nonlinear rheological response was qualitatively analyzed using normalized Lissajous-Bowditch curves. The microstructure of the emulsion gels strongly depended on the concentration of sterols in the oil phase, and showed a percolated segregated network at 10–20 wt% sterols due to the partial coalescence of droplets, and a jamming transition without coalescence at higher sterols concentration of 30 wt%. The microstructure differences led to different linear and nonlinear viscoelastic behaviors of these emulsion gels. SAOS tests showed that the oil phase structuring by the sterols significantly enhance the viscoelasticity of GA nanofibril emulsion gels, and the percolating emulsion gels exhibited higher elasticity than the jammed emulsion gel, as evidenced by a lower damping factor and frequency power-law exponent. The data of crossover strain, phase angle, and the normalized Lissajous-Bowditch curves from LAOS tests further revealed that compared to the samples in a jammed state or without oil phase structuring, the emulsion gels with a percolating segregated network showed higher structural elasticity and thus were more resistant to large deformations, probably due to the slow relaxation of rigid, hydrodynamically interacting clusters of partially coalesced droplets. These findings could potentially aid in the design of novel emulsion gels, based on all-natural and sustainable building blocks, with specific textural and functional properties for foods, cosmetics, and pharmaceutical applications.

Linear and nonlinear rheology of oil in liquid crystal emulsions

While the majority of experimental and numerical studies focus on yield stress fluids with short-range repulsive interactions, the effect of long-range interaction is rarely known. In this work, we studied the linear and nonlinear rheological behavior of a model oil in nematic liquid crystal emulsions, which exhibit long-range droplet-droplet interaction. The characteristic of long-range interaction, attractive at large droplet separation and repulsive at a small surface distance, was inferred from the morphology and thixotropy of emulsions. We suggested a model accounting for the plateau modulus purely due to the long-range repulsive interaction. We further illustrated that neither the yield stress nor the relaxation time followed a simple power-law scaling with respect to the distance to jamming. The rescaled steady and dynamic flow curves could not collapse on master curves above jamming transition when the long-range and short-range repulsive interactions contribute simultaneously.

Rheological Properties of Covalent Adaptable Networks with 1,2,3-Triazolium Cross-Links: The Missing Link between Vitrimers and Dissociative Networks

Vitrimers, in stringent contrast to dissociative networks, involve associative covalent exchanges and remain fully cross-linked at all temperatures. However, we have previously reported on poly(1,2,3-triazolium) dynamic networks that exhibit linear rheological characterization indistinguishable from vitrimers although being based on a dissociative covalent exchange process, i.e., deN -alkylation and re-N-alkylation reactions. Herein, we highlight the main features of dissociative and associative covalent adaptable networks (CANs) and discuss their respective linear rheological behaviors. After giving a detailed overview of the extent and potential of dynamic polymer networks having 1,2,3-triazolium cross-links in particular and CANs involving trans-N-alkylation exchanges in general, we perform a detailed rheological comparative characterization of previously described aliphatic 1,2,3-triazolium-based dynamic networks and a new bicomponent thermosetting system issued from the mixing of linear polystyrene chains having complementary benzyl iodide and 1,2,3-triazole pendent groups. We propose a simple but comprehensive rheological characterization methodology that enables proper comparison between all types of CANs, i.e., vitrimers and dissociative networks. Owing to the combination of a high glass transition temperature dynamic network and fast trans-N-alkylation exchanges in this new dissociative network, it is possible to characterize both glass transition and network relaxation with single rheological experiments. We provide clear experimental evidence that although located within a narrow temperature range, the temperature dependences of these two transitions are distinct and follow WLF and Arrhenius models, respectively.

Effect of various surfactants on stability and thermophysical properties of nanofluids

The effect of Fe3O4 nanoparticles and carbon nanotubes (CNTs) on the viscosity of a nanofluid is experimentally investigated from 278 to 313 K by changing the nanoparticle volume fraction. These nanoparticles were put into distilled water with various surfactants, i.e., Colace (docusate sodium), trisodium citrate dihydrate (TSC), polyvinyl pyrrolidone, cetyl trimethylammonium bromide, tetramethylammonium hydroxide (TMAH), acacia senegal (GA), sodium dodecyl benzene sulfonate, sodium dodecyl sulfate (SDS), and sodium laurylsulfonate (SLS). Based on the present measurements, new empirical formulas are proposed for Fe3O4–water, CNT–water and Fe3O4–CNT–water nanofluids to provide accurate predictions for the nanofluid viscosity. Based on the viscosity testing, stabilities and thermal conductivities of Fe3O4–TMAH, Fe3O4–Colace, Fe3O4–TSC, CNT–SDS, CNT–GA, Fe3O4–CNT–SLS, and Fe3O4–CNT–TSC nanofluids with a volume concentration of 0.5% are investigated in the present research. Results indicate that better stability, smaller viscosity, and higher thermal conductivity are obtained, when the surfactants TMAH, SDS, and SLS are added into the Fe3O4–water, CNT–water, and the Fe3O4–CNT–water nanofluid, respectively. The CNT–water and Fe3O4–CNT–water nanofluids exhibit a shear-thinning behavior, whereas a linear rheological behavior can be observed by water-based Colace–Fe3O4, TMAH–Fe3O4, and TSC–Fe3O4 nanofluids.

Analysis of the ageing effect on the cyclic tension–compression loading behaviour of bitumen and mastics

Within the bituminous mixture the binder is subjected to different stress-state modes but the cyclic loading response is usually characterized on shear mode. This research investigates the evolution with ageing of the undamaged linear rheological behaviour and resistance to cyclic loading of bituminous materials using a tension–compression strain-sweep test. The results show that, similarly to the shear mode response, mineral fillers may induce a greater stiffening effect than ageing treatment, although the comparative evolution with ageing of materials may differ from that based on the shear mode response. The damage progression rate increases faster at lower temperatures and for the stiffer materials (mastics), where the flow deformation contribution to crack formation is smaller. The testing temperature is determinant for the evaluation of the effect of ageing and filler on fatigue resistance.

Melt rheology and interfacial properties of binary and ternary blends of PS, EOC, and SEBS

ABSTRACTThis works systematically investigates the interfacial properties of the binary and the ternary blends based on polystyrene (PS), ethylene octene copolymer (EOC), and styrene–ethylene–butylene–styrene (SEBS) by analyzing the melt linear rheological behavior of the blends and neat components. Moreover, the relationship between rheology, phase morphology, and mechanical properties of PS/EOC ternary blends with various quantities of SEBS were studied. The surface shear modulus (β) and interfacial tension values obtained by Palierne model indicated that the EOC/SEBS blend has the best interfacial properties, while the lowest interaction was found for PS/EOC blend. Based on the Palierne model and Harkin's spreading coefficients a core–shell type morphology with EOC phase encapsulated by the SEBS shell dispersed in the PS matrix was determined for the ternary blends. Scanning electron microscopy results revealed that both fibrillar and droplet forms of dispersed phase could be developed during the blending of PS and EOC in presence of SEBS. The extent of fibrillar morphology and interfacial interactions in PS/EOC/SEBS ternary blends was dependent on the SEBS content. The improvement of the mechanical properties of PS/EOC blends in the presence of SEBS was evidenced by the tensile and impact resistance experiments. The tensile strength reinforcement was more pronounced for the ternary blends with more fibrillar dispersed phase. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020, 137, 48791.

Effect of styrene-butadiene-styrene (SBS) on the rheological behavior of asphalt binders

The binders in pavement could support the loading in either linear or nonlinear regime. The linear viscoelasticity of asphalt binders was well established, however, a few of researches focused on the nonlinear viscoelasticity of asphalt binders. Here, this paper deals with the effects that SBS exerts on the linear and nonlinear rheological behavior of asphalt binders. The nonlinear rheological behavior of one kind of neat asphalt binder and the corresponding SBS modified asphalt binders under large amplitude oscillatory shear stress (LAOS stress) is analyzed by Fourier Transform (FT)-rheology and strain decomposition methods. The linear viscoelastic measurements show that the elastic proportion and the complexity of rheological behavior of the asphalt binder increases with increase in SBS content. The nonlinear viscoelastic measurements reveal that the relative nonlinearity of asphalt binders increases with increase in stress and decrease in frequency; the relationship between I3/I1 and stress amplitude can obey the sigmoidal function; the intrinsic nonlinearity Q0 decreases with increase in frequency and increases with the increase of SBS content. Both the FT analysis and decomposition results show that the nonlinearity of SBS modified asphalt binder is more obvious than neat asphalt binder and increases with the increase of SBS content. SBS could increase the nonlinearity and the elastic proportion of asphalt binder and decrease its fluidity. All the studied binders exhibit stress softening and stress thinning under all the studied test conditions.

Viscoelastic properties of suspensions of noncolloidal hard spheres in a molten polymer

We report an experimental study on suspensions of solid particles in a viscoelastic polymer matrix. A commercial entangled poly(ε-caprolactone) was used as the suspending fluid. Noncolloidal solid spheres (diameter = 15 μm) made of polymethylmethacrylate were dispersed in the polymer via a solvent casting method. The volume fraction of the spheres was varied from 5% to 30%, thus allowing to explore both dilute and concentrated regimes. Electron scanning microscopy demonstrated homogeneous dispersion of the spheres in the matrix. We measured the rheological properties of the suspensions both in linear and nonlinear regimes with both dynamic and transient tests. The experimental results demonstrate the reinforcement effect of the particles. Both viscous and elastic moduli increase as the concentration of the particles is increased. The results show good agreement with available theories, simulations, and previous experimental data. In particular, the second order parameter of the quadratic equation that describes the dependence of the shear viscosity of the suspension upon the volume fraction of particles is in agreement with the predicted value found by Batchelor [G. K. Batchelor and J. T. Green, “The hydrodynamic interaction of two small freely-moving spheres in a linear flow field,” J. Fluid Mech. 56, 375–400 (1972); G. K. Batchelor and J. T. Green, “The determination of the bulk stress in a suspension of spherical particles to order c2,” J. Fluid Mech. 56, 401–427 (1972); and G. K. Batchelor, “The effect of Brownian motion on the bulk stress in a suspension of spherical particles,” J. Fluid Mech. 83, 97–117 (1977)]. We probe experimentally that the linear rheological behavior of suspensions of particles in viscoelastic fluids is the same as for Newtonian fluids.

Effects of particle stiffness on the extensional rheology of model rod-like nanoparticle suspensions.

The linear and nonlinear rheological behavior of two rod-like particle suspensions as a function of concentration is studied using small amplitude oscillatory shear, steady shear and capillary breakup extensional rheometry. The rod-like suspensions are composed of fd virus and its mutant fdY21M, which are perfectly monodisperse, with a length on the order of 900 nm. The particles are semiflexible yet differ in their persistence length. The effect of stiffness on the rheological behavior in both, shear and extensional flow, is investigated experimentally. The linear viscoelastic shear data is compared in detail with theoretical predictions for worm-like chains. The extensional properties are compared to Batchelor's theory, generalized for the shear thinning nature of the suspensions. Theoretical predictions agree well with the measured complex moduli at low concentrations as well as the nonlinear shear and elongational viscosities at high flow rates. The results in this work provide guidelines for enhancing the elongational viscosity based on purely frictional effects in the absence of strong normal forces which are characteristic for high molecular weight polymers.

Characterization and phase‐transition behavior of thermoresponsive PVME nanogels in the presence of cellulose nanowhiskers: Rheology, morphology, and FTIR studies

The effects of cellulose nanowhiskers (CNWs) and irradiation dose on phase separation behavior as well as rheological and physical properties of Poly vinyl methyl ether (PVME) nanogels are investigated. Interactions between CNWs and polymer chains lead to the dehydration of PVME chains. Increasing the irradiation dose or CNW weight fraction enhances the VPTT, due to the decreased chain mobility. The phase separation behaviors of hybrid and nonhybrid nanogels are investigated at two different quench depths. At the shallow quench depth, the samples phase separate through nucleation and growth (NG) mechanism. However in the case of hybrid nanogels with a higher temperature, as a result of the enhanced dynamic asymmetry induced by the increased quench depth and the interaction of PVME chains with CNWs, the phase separation mechanism changes from NG to viscoelastic phase separation (VPS). The linear rheological behavior of nanogels was well correlated with the evolution of corresponding phase‐separating morphologies. At 40°C, a shoulder appeared in the storage modulus curve in the intermediate frequency range, due to shape relaxation of the droplets formed by NG mechanism. However, a solid‐like behavior was observed due to the existence of a percolated network structure induced by VPS at 60°C. POLYM. ENG. SCI., 59:899–912, 2019. © 2018 Society of Plastics Engineers

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.

Linear and nonlinear rheological behaviors of silica filled nitrile butadiene rubber

Nanoparticles significantly reinforce rubber and enhance nonlinear mechanical behavior while the underlining mechanisms are not clear. Herein the effects of crosslinking, filler content and filler surface chemistry on the linear and nonlinear rheological responses of silica filled nitrile butadiene rubber are investigated. A time-concentration superposition principle is employed for constructing rheological master curves. While the strain amplification and dynamics retardation contribute to the high-frequency linear rheology of the compounds, the former is crucial for the vulcanizates with matrix of similar crosslinking degree. In both the compounds and vulcanizates, the Payne effect is initiated at a similar microscopic strain of the rubber phase. A weak overshoot of loss modulus appears in compounds containing hydrophilic silica and vulcanizates containing hydrophobic silica. It is suggested that the viscoelastic matrix is highly important for the rheological responses of the filled compounds and vulcanizates.

Linear and nonlinear rheological behavior of native and debranched waxy rice starch gels

Abstract The rheological behaviours of native (WX) and debranched (DB) waxy rice starch gels in the linear viscoelastic (LVE) regime and non-linear (NL) regime were investigated using small amplitude oscillatory shear (SAOS), and large amplitude oscillatory shear (LAOS), respectively. Our results show that WX starch gels exhibited a wider LVE region than DB starch gels, and a scaling of G′ with frequency given by G’∼ω0.26. The G' and G” scaled nearly linear with concentration and were nearly independent of temperature in the tested range. The Lissajous plots showed a gradual yielding of the gels, followed by softening behavior. These results indicated that the WX gel has the characteristics of a weak particle gel (of non-entangled polymers). In contrast, the DB starch produced gels with a more stiff and brittle behavior. The shear moduli had a very strong concentration dependence (∼C10.30) and temperature dependence. The DB starch gel showed more abrupt yielding and strong softening behavior in the Lissajous curves. Creep measurements showed a lower compliance for these gels, and significant creep ringing. These observations suggest the DB starch gels consist of clusters of stiff polymers.

Effect of the chemical structure on the linear viscoelastic behavior of acrylic and styrenic polymer blends

It was investigated the influence of chemical structure on the linear viscoelastic rheological behavior of polymer blends comprising acrylic and styrenic polymers. Fourier transform infrared spectroscopy revealed a different bands characteristic of the chemical structures of the polymers. The chemical compositions of the styrenic copolymers contain acrylonitrile at levels of 25 and 21%, respectively, which are within the miscibility window for PMMA/SAN and PMMA/ABS blends. Differential scanning calorimetry revealed the glass transition temperature for the polymers and for the elastomeric phase present in an elastomeric-particle-containing poly (methyl methacrylate) that are different to that of the elastomeric phase of ABS. The rheological data obtained at low frequencies in the linear viscoelasticity region, revealed significant changes in the rheological behavior of acrylic homopolymer, when compared to an acrylic polymer containing elastomeric particles (core-shell type). It was possible to indicate the miscibility of polymer blends by means of the linear viscoelastic rheological behavior.

Simulation of the linear rheological behavior of silica-filled, star-shaped SBR compounds

The rheological behavior of star-shaped SSBR/silica 60 phr compounds with different filler surface areas was experimentally studied and simulated using constitutive modeling. Rheological behavior was characterized in small amplitude oscillatory shear (SAOS) and stress relaxation after a small step shear. Unfilled SBR and SBR filled with four different silica grades with BET surface areas of 55, 135, 160, and 195 m2/g were used. A clear trend in rheological behavior was observed with surface area. A frequency sweep in the SAOS regime indicated an increase in dynamic properties with surface area. Additionally, linear stress relaxation tests at a strain level of 0.05 showed an increase in relaxation modulus with surface area and the presence of a plateau in the relaxation modulus at large times in compounds containing silica with high surface areas. The Leonov and Simhambhatla-Leonov models, modified to incorporate multimode particle network relaxation, were successfully used to simulate the frequency dependence of the storage modulus and the time evolution of the linear relaxation modulus for all samples. However, simulations of the frequency dependence of the loss modulus showed poor results in comparison with experimental data for the filled compounds.

Viscoelastic phase separation of polystyrene/poly(vinyl methyl ether) blends with different dynamic asymmetry

Viscoelastic phase separation (VPS) can be induced in dynamically asymmetric polystyrene/poly(vinyl methyl ether) (PS/PVME) blend in which there is a large difference between glass transition temperature of the components (about 125 °C). In this study, the effect of molecular weight of the slower component, PS, on VPS mechanism is investigated. The correlation between the time evolution of the phase-separating morphologies and their linear and nonlinear rheological behavior is studied for blends with three different PSs. Rheological measurements and morphological observations are carried out at quench depth of 14 °C. While it was previously found that depending on composition of blend, three different types of VPS behavior can be observed due to self-induced stresses in the early stage of phase separation—(i) the transient gel induced VPS (TG-VPS), (ii) coalescence induced VPS (C-VPS), and (iii) aggregating nucleation and growth (ANG)—in this study, we show that different types of VPS can be observed for a single composition by changing the molecular weight of PS. In other words, dynamic asymmetry between PS-rich and PVME-rich phases decreases with a decrease of molecular weight of PS which results in weaker transient gel in the early stage of phase separation. Consequently, the phase separation mechanism changes for 10/90 blend from TG-VPS to C-VPS and, for 5/95 blend from C-VPS to ANG or NNG (normal nucleation and growth). Furthermore, the variation of fractal dimension by phase separation is studied for 10/90 blends containing PS with different molecular weights.

Effect of the aniline hydrochloride hydrotrope on the microstructure of SDS/water system: Linear rheological behavior

In this work, the linear viscoelastic properties of the sodium dodecylsulfate/aniline hydrochloride/water system were examined in detail as a function of aniline hydrochloride (AHC) to sodium dodecyl sulfate (SDS) molar ratio (R=[AHC]/[SDS]), SDS concentration and temperature. SDS aqueous solutions undergo a structural transition from sphere to rodlike micelles and subsequently to a viscoelastic network of wormlike micelles induced by the hydrotrope AHC at low and intermediate R molar ratios (i.e., R=0.1–0.6 for 5wt.% SDS), which screens the repulsions between the neighboring charged head groups. The rheological response indicates that at low R molar ratios and constant surfactant concentration, the rheological behavior of the system is predominantly viscous (G′<G″). However, upon increasing R, the elastic behavior predominates (G′>G″) and a maximum of the absolute value of the zero frequency complex viscosity (|η0*|) is observed, which suggest that the micellar length also goes through a maximum. The region of the elastic behavior diminishes at even greater R molar ratios and a predominantly viscous behavior appears again. This drop in |η0*| can be attributed to the transition from linear to branched wormlike micelles that conform the tridimensional viscoelastic network. The dynamic behavior of the system was analyzed in terms of the Granek-Cates theory, which indicates that the viscoelastic network is in the slow-breaking limit. Under these conditions, reptation is found to be the controlling relaxation mechanism for this system.

Linear rheology of carbon black filled polystyrene

Filling can cause polymer melt to undergo a so-called fluid-to-solid transition usually assigned to filler networking effect and heterogeneously retarded polymer dynamics while theories developed nowadays could not account for the several important aspects involving in the reinforcement and dissipation of the composites including the influence of molecular weight of the matrix. Herein linear dynamic rheological behaviors of carbon black filled polystyrene (PS) was investigated, disclosing significantly influences of weight-averaged molecular weight (Mw) of PS on the reinforcement and dissipation in and beyond the hydrodynamics regime. Attempts to create rheological master curves in hydrodynamics regime, taking account for both dynamics retardation in bulk polymer phase and strain amplification effect induced by filler, disclose time-concentration superpositions. The viscoelastic contributions from the dynamically retarded polymer phase and the viscoelastic “filler phase” are thus evaluated.

Nonlinear Rheology of Random Sulfonated Polystyrene Ionomers: The Role of the Sol–Gel Transition

The linear and nonlinear rheological behaviors of nonentangled sulfonated polystyrene (SPS) ionomers near the sol–gel transition were studied. When the degree of sulfonation, p, was below the gel point, the ionomer exhibited sol-like linear viscoelastic (LVE) behavior, and shear thinning was observed for steady shear flow. For p close to the gel point, the ionomer showed power-law-like LVE behavior over a wide frequency range. Strain hardening and shear thickening behavior were observed, and their magnitudes depended on the temperature, molecular weight of the PS precursor, and the Coulomb energy of the ion pair. Above the gel point, a distinct rubbery plateau was observed in the dynamic modulus. Melt fracture occurred upon start-up shear, which prevented quantitative examination of the nonlinear rheology. The possible mechanisms for strain hardening and shear thickening near the gel point are discussed with respect to formation of large clusters that nearly percolate in space.

Linear rheology of bituminous mastics modified with various polyolefins: a comparative study with their source binders

The effect of bituminous mastics on controlling the mechanical properties of the asphalt mixtures has been largely claimed. Theoretical models often fail to predict the rheological behavior of highly concentrated mastics because of the various interaction forces involved. This situation becomes much more complex when dealing with biphasic polymer modified bitumens (PMBs). This investigation explores the influence of various recycled polyolefins (LDPE, HDPE and PP) on the linear rheological behavior of modified mastics with a fixed filler/binder ratio of 65/35 (by weight). The filler was of siliceous-type, and all the PMBs contained 4 wt.% polymer. Oscillatory torsion/shear tests were carried out between 25 and 65 °C. It was found that the rheology of the HDPE-binder very much resembles its polymer-rich phase (PRP), which behaves like a concentrated polymer solution. In contrast, the LDPE-binder proved to have a bitumen-rich phase (BRP) still playing a large part in the PMB rheology. This result had a strong effect on the mastic rheology, as the filler shows more affinity for the bitumen. In consequence, the G′ plateau observed in the HDPE-mastic was similar to that found in its parent PMB. In contrast, if compared to its source binder, the LDPE-mastic manifested a rheological behavior better described by the filler acting as bridging points of PRP domains.