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Nonlinear rheology of Pickering emulsions is used to further investigate the nonlinear and unrecoverable transformation of inner structures, which is beyond the linear viscoelastic regime of tiny structural disturbances. Exploring various rheological methods plays a vital role in emulsion applications, such as simulating the macroscopic structural transformation between static and liquidlike flow states, strain overshoot, and regeneration for broken structures. According to our previous studies, cellulose nanofibers (CNFs) Pickering emulsions are a typical system for investigating polymer-based emulsions with the auxiliary surfactant [didodecyldimethylammonium bromide (DDAB)] for enhancing CNF absorption. To further study different rheological properties by varying CNF or DDAB contents, multiple interval thixotropic test, large amplitude oscillatory shear, and concentration-time-dependent superposition are employed to study the linear viscoelasticity and structural transformation of nonlinear range. This research was conducted based on the previous published works [Cui et al., Materials 15, 8285 (2022)] as a further characterization for the same sample series. [ABSTRACT FROM AUTHOR]

We use nonequilibrium atomistic molecular dynamics simulations of unentangled melts of linear and star oligomer chains ( C 25 H 52 ) to study the steady-state viscoelastic response under confinement within nanoscale hematite (α − F e 2 O 3) channels. We report (i) the negative (positive) first (second) normal stress difference and (ii) the presence of viscoelastic tension at low W i. With the aim of uncovering the molecular mechanism of viscoelasticity, we link these effects to bond alignment such that absorbed chains near the surface can carry the elastic force exerted on the walls, which decays as the chains become more aligned in the flow direction. This alignment is observed to be independent of the film thickness but enhanced as the shear rate increases or the surface attraction weakens. [ABSTRACT FROM AUTHOR]

This study discusses the feasibility of using a combined torsional-axial rheometer to indirectly measure the complex Poisson's ratio based on shear and Young's modulus. For this purpose, isothermal frequency sweeps in torsion and extension are performed sequentially on the same cylindrical specimen and under the same environmental conditions. The method is tested on two amorphous polymers, a semicrystalline polymer, a polymer blend, and a copolymer. The article includes an extensive literature review and an uncertainty assessment of the method to provide a basis for subsequent data comparison with existing research. The experimental data show a monotonic increase in the complex Poisson's ratio up to 0.5 as the temperature approaches α-relaxation for all samples, except for the amorphous polymer. The latter shows a local minimum in the complex Poisson's ratio observed near α-relaxation, which disappears after thermal annealing of the sample above the α-relaxation temperature. The real and imaginary parts of the complex Poisson's ratio are additionally determined by evaluating both phase shift angles from torsional and extensional measurements. All polymers show a certain offset between the torsional and extensional phase shift angles in the glassy state, which gradually decreases as the temperature approaches α-relaxation. The complex Poisson's ratio results are in good agreement with the literature data obtained by existing methods. This confirms that the method is applicable to polymers up to α-relaxation temperatures with significant time savings due to the nondestructive approach. This is of particular interest, given the limited availability of data in the literature. [ABSTRACT FROM AUTHOR]

Part I of this paper [Haward et al., J. Rheol. 67, 995–1009 (2023)] presents a three-dimensional microfluidic device (the optimized uniaxial and biaxial extensional rheometer, OUBER) for generating near-homogeneous uniaxial and biaxial elongational flows. Here, in Part II, the OUBER device is employed to examine the uniaxial and biaxial extensional rheology of model dilute polymer solutions, compared with measurements made under planar extension in the optimized-shape cross-slot extensional rheometer [OSCER, Haward et al. Phys. Rev. Lett. 109, 128301 (2012)]. In each case, micro-particle image velocimetry is used to measure the extension rate as a function of the imposed flow conditions, and excess pressure drop measurements enable estimation of the tensile stress difference generated in the fluid via a new analysis based on the macroscopic power balance for flow through each device. Based on this analysis, for the most dilute polymer sample tested, which is "ultradilute", the extensional viscosity is well described by Peterlin's finitely extensible nonlinear elastic dumbbell model. In this limit, the biaxial extensional viscosity at high Weissenberg numbers (Wi) is half that of the uniaxial and planar extensional viscosities. At higher polymer concentrations, although the fluids remain dilute, the experimental measurements deviate from the model predictions, which is attributed to the onset of intermolecular interactions as the polymer chains unravel in the extensional flows. Of practical significance (and fundamental interest), elastic instability occurs at a significantly lower Wi in uniaxial extensional flow than in either biaxial or planar extensional flow, thereby limiting the utility of this flow type for extensional viscosity measurement. [ABSTRACT FROM AUTHOR]

Programmable hydrogels, such as thiolated hydrogels, are frequently used for tissue engineering and drug delivery applications, because they offer the ability to control gelation, degradation, and adhesion. Understanding how the mechanical properties of these materials change during these processes is essential as they directly impact cell fate and delivery efficacy. The rheology of hydrogels has been quantified primarily via bulk rheological methods. While such methods are effective, they require large sample volumes and result in the destruction of the sample; therefore, responses to multiple stimuli must be recorded across many different samples. We have developed a magnetic microwire rheometer that can characterize the rheology of small sample volumes while maintaining the integrity of the sample, such that the material response to a range of stimuli can be recorded for a single sample. This capability enables insights into time-dependent rheological changes, such as gelation and degradation, and can be applied to characterize dynamic in situ systems that are the basis for tissue scaffolding, drug delivery vehicles, and other important biological applications. [ABSTRACT FROM AUTHOR]

Mud is a suspension of fine-grained particles (sand, silt, and clay) in water. The interaction of clay minerals in mud gives rise to complex rheological behaviors, such as yield stress, thixotropy, and viscoelasticity. Here, we experimentally examine the flow behaviors of kaolinite clay suspensions, a model mud, using steady shear rheometry. The flow curves exhibit both yield stress and rheological hysteresis behaviors for various kaolinite volume fractions (ϕ k). Further understanding of these behaviors requires fitting to existing constitutive models, which is challenging due to numerous fitting parameters. To this end, we employ a Bayesian inference method, Markov chain Monte Carlo, to fit the experimental flow curves to a microstructural viscoelastic model. The method allows us to estimate the rheological properties of the clay suspensions, such as viscosity, yield stress, and relaxation time scales. The comparison of the inherent relaxation time scales suggests that kaolinite clay suspensions are strongly viscoelastic and weakly thixotropic at relatively low ϕ k , while being almost inelastic and purely thixotropic at high ϕ k. Overall, our results provide a framework for predictive model fitting to elucidate the rheological behaviors of natural materials and other structured fluids. [ABSTRACT FROM AUTHOR]

Mixtures of polymers of varying topologies and stiffnesses display complex emergent rheological properties that often cannot be predicted from their single-component counterparts. For example, entangled blends of ring and linear polymers have been shown to exhibit enhanced shear thinning and viscosity, as well as prolonged relaxation timescales, compared to pure solutions of rings or linear chains. These emergent properties arise in part from the synergistic threading of rings by linear polymers. Topology has also been shown to play an important role in composites of flexible (e.g., DNA) and stiff (e.g., microtubules) polymers, whereby rings promote mixing while linear polymers induce demixing and flocculation of stiff polymers, with these topology-dependent interactions giving rise to highly distinct rheological signatures. To shed light on these intriguing phenomena, we use optical tweezers microrheology to measure the linear and nonlinear rheological properties of entangled ring-linear DNA blends and their composites with rigid microtubules. We show that linear viscoelasticity is primarily dictated by microtubules at lower frequencies, but their contributions become frozen out at frequencies above the DNA entanglement rate. In the nonlinear regime, we reveal that mechanical response features, such as shear thinning and stress softening, are mediated by entropic stretching, threading, and flow alignment of entangled DNA, as well as forced dethreading, disentanglement, and clustering. The contributions of each of these mechanisms depend on the strain rate as well as the entanglement density and stiffness of the polymers, leading to nonmonotonic rate dependences of mechanical properties that are most pronounced for highly concentrated ring-linear blends rather than DNA-microtubule composites. [ABSTRACT FROM AUTHOR]

We report a detailed experimental study of peculiar thixotropic dynamics of carbon black (CB, Vulcan XC-72) suspensions in mineral oil, specifically the observation of sequential stress increase then decrease at a fixed shear rate in a step-down test. We verify that such dynamics, though peculiar, come from a true material response rather than experimental artifacts. We also reveal how this long-time stress decay is associated with antithixotropic structural change rather than viscoelastic stress relaxation by using orthogonal superposition (OSP) rheometry to probe viscoelastic moduli during the step-down tests. The orthogonal storage and loss moduli are present, showing this two-timescale recovery then decay response, which demonstrates that this response is antithixotropic, and it involves shear-induced structuring. We further show a mechanical anisotropy in the CB suspension under shear using OSP. Based on the rheological results, a microstructural schematic is proposed, considering qualitatively thixotropic structure build-up, antithixotropic densification, and anisotropic structure evolution. Our observation for these CB suspensions is outside the standard paradigm of thixotropic structure-parameter models, and the elastic response provides us with new insight into the transient dynamics of CB suspensions. [ABSTRACT FROM AUTHOR]

We report the linear rheology for dense suspensions of sterically stabilized smooth and mesoscopically rough colloids interacting as hard particles. Small amplitude oscillatory measurements reveal that rough colloids at high volume fractions exhibit storage and loss moduli that are orders of magnitude greater than smooth colloids. Frequency-concentration superposition is used to collapse the viscoelasticity data onto a master curve, where shift factors suggest a more elastic microstructure and reduced cage volume for rough particles. A combination of the mode-coupling theory, hydrodynamic modeling, and the activated hopping theory shows that these rough particles with significantly reduced localization lengths tend to become trapped in their glassy cages for extended periods of time. High-frequency data show that rough colloids, but not smooth colloids, display a transition from a free-draining to a fully lubricated state above the crossover volume fraction and, furthermore, exhibit solidlike behavior. Scaling analyses support the idea that lubrication forces between interlocking asperities are enhanced, leading to rotational constraints and stress-bearing structures that significantly elevate the viscoelasticity of dense suspensions. The results provide a framework for how particle surface topology affects the linear rheology in applications such as coatings, cement, consumer products, and shock-absorbing materials. [ABSTRACT FROM AUTHOR]

We use molecular dynamics (MD) simulations in conjunction with the probe rheology technique to investigate the linear viscoelasticity of nanocolloidal suspensions. A particulate model of the solvent is used in which the hydrodynamics is governed by interparticle interactions. Active and passive probe rheology molecular simulations are performed on the colloidal suspensions of different volume fractions ranging from 0.30 to 0.45 to determine the linear viscoelastic properties of these systems. The viscoelastic modulus of the suspensions is obtained by analyzing the probe motion using continuum mechanics. In active rheology, the distribution of colloid particles around the probe is observed to be symmetric indicating that the system is in the linear regime at all conditions investigated. In passive rheology, the mean-squared displacement of the probe covers the range of motion from ballistic to diffusive regimes. The dynamic modulus and the reduced complex viscosity values obtained from probe rheology simulations are in good agreement with the results from the oscillatory nonequilibrium MD (NEMD) simulations and the literature theoretical predictions. At low frequency values, accounting for artificial hydrodynamic interactions between the probe and its periodic images improves the quantitative accuracy of the modulus values obtained from simulations. Simulations carried out using probes of different sizes indicate that only the probes that are larger than the colloids yield viscoelastic modulus values that are in good agreement with the NEMD values at all volume fractions investigated. [ABSTRACT FROM AUTHOR]

A thorough study is made of the dependences on salt concentration and polymer chain lengths of the low-frequency plateau of coacervates of poly (diallyl dimethyl ammonium chloride), PDADMAC, and poly (sodium 4-styrenesulfonate), PSS. The reliability and reproducibility of these measurements are carefully checked by determining the frequency-dependent stress limits of the rheometer through the use of reference fluids and by repeat experiments with coacervates. Long-time frequency sweeps show that coacervates with less salt are more repeatable than those with higher salt. A low-frequency plateau reliably appears only below a critical salt concentration, and the magnitude of the plateau depends strongly on salt concentration and chain lengths of both polycation and polyanion. It is only present for the molecular weight of the polycation, PDADMAC, higher than 100 kDa, but the magnitude of the plateau is more strongly influenced by the chain length of the polyanion, PSS. Possible causes of the low-frequency plateau are discussed. [ABSTRACT FROM AUTHOR]

In this work, a study of the linear viscoelastic properties of co-continuous polypropylene/polystyrene blends filled with multiwall carbon nanotubes (MWCNTs) is presented. The YZZ rheological model [W. Yu et al., Polymer 51, 2091–2098 (2010)] is employed to correlate the rheological behavior of the blends with their microstructure and electrical properties. A test design involving a sequence of small amplitude oscillatory shear and a time sweep (simulating thermal annealing) is used to evaluate the morphology and evolution of electrical properties. It was shown that the YZZ rheological model could be successfully modified to be able to quantify a co-continuous morphology of filled composites. The calculated characteristic domain size was found to be in good agreement with the experimental data obtained via scanning electron microscopy. Furthermore, it is shown that the characteristic domain size slightly decreased after 30 min of thermal annealing. It was shown, as well, that thermal annealing promoted a reduction in the electrical percolation threshold (wt. % MWCNT) from 0.28 to 0.06. [ABSTRACT FROM AUTHOR]

Polymers bearing associative groups (APs) are characterized by their fantastic viscoelastic behaviors. In a work recently published by our group [Jiang et al., Macromolecules53, 3438–3451 (2020)], a single chain sticky Rouse model (SRM) is proposed to describe the linear viscoelasticity of APs without the entanglement effect. In this work, equilibrium molecular dynamics simulation of an unentangled melt of an AP with uniformly distributed stickers is carried out, and the dynamic properties are simultaneously analyzed from the SRM. A chain model with capped stickers is proposed so that a well-defined association chemistry is promised in the simulation system. The relative effective frictional coefficient of stickers, which is the key parameter in the SRM, is extracted from the chain center-of-mass diffusion, and it is found to be consistent with the dynamics of associative reaction in the fully gelated network. Based on this, a linear relaxation modulus and segmental diffusion functions are predicted from the SRM without fitting parameters, and these are found to quantitatively agree with the simulation results, showing the effectiveness of the SRM in connecting the dynamic properties at different molecular levels. The change in relaxation modes and the definition of the effective chain center are found to be crucial in the scenario of the SRM. Finally, the above analysis from the SRM is successfully extended to the simulation system with asymmetric chains. All these simulation results strongly support the SRM as a molecular model for the linear rheology of AP. [ABSTRACT FROM AUTHOR]

Blood-derived products, particularly platelet-rich plasma (PRP), have received increased attention in the past several years due to their great potential as a therapy for osteoarthritis and tendon injuries. Therefore, characterizing the mechanical properties of PRP becomes important to better understand its therapeutic efficacy. This paper aims to investigate the rheological properties of PRP in order to provide further insight into its mechanism of action. The PRP samples in our study may have higher erythrocyte count than the average PRP reported in the literature. Flow-sweep, small amplitude oscillatory shear (SAOS), large amplitude oscillatory shear (LAOS), and thixotropy tests have been performed at room and physiological temperatures to characterize the non-Newtonian properties of PRP samples. Flow-sweep tests reveal shear-thinning behavior, with higher apparent viscosity observed at a lower temperature. Rheological models such as Carreau, Casson, power-law, and Herschel–Bulkley have been fitted to flow-sweep data with the latter showing the closest agreement. Over the acceptable narrow range of frequency in SAOS tests, the loss modulus appears to be slightly larger than the storage modulus with both moduli decreasing at higher temperature. The nonlinear viscoelastic properties are furthermore quantified through the static and dynamic LAOS analyses. For a given strain and angular frequency, the former identifies strain-softening and shear-thinning, while the latter discloses a range of other transient behavior within an oscillation cycle. Lastly, peak-hold tests have revealed consistent thixotropy in PRP solutions. [ABSTRACT FROM AUTHOR]

A particulate molecular model in which the solvent particles are considered explicitly is developed for studying the linear viscoelasticity of nanocolloidal suspensions using molecular dynamics simulations. Nanocolloidal systems of volume fractions ranging from 0.10 to 0.49 are studied. The hydrodynamics in these model systems are governed by interparticle interactions. The volume fraction dependence of the relative zero shear viscosity exhibited by this molecular model is consistent with that reported in the literature experiments and simulations. Over the range of frequencies studied, the relative dynamic viscosity values follow the same qualitative trend as that seen in the literature experiments. The time-concentration superposition (TCS) principle is successfully applied to construct the viscoelastic master curves that span nine decades of frequency in the case of the elastic modulus and more than four decades of frequency in the case of the loss modulus. The TCS principle was observed to fail at high volume fractions that are near the glass transition concentration; this finding is consistent with the literature experimental and simulation observations. The volume fraction dependence of the shift factors used in the construction of the viscoelastic master curves is in good quantitative agreement with that of the viscosity of the nanocolloidal systems. Our results demonstrate that molecular simulations in conjunction with an explicit solvent model can be used to quantitatively represent the viscosity and the viscoelastic properties of nanocolloidal suspensions. Such particulate models will be useful for studying the rheology of systems whose properties are governed by specific chemical interactions. [ABSTRACT FROM AUTHOR]

A new mathematical representation for nonlinear viscoelasticity is presented based on the application of the Volterra series expansion to the general nonlinear relationship between shear stress and shear strain history. This theoretical and experimental framework, which we call medium amplitude parallel superposition (MAPS) rheology, reveals a new material property, the third-order complex modulus, which describes completely the weakly nonlinear response of a viscoelastic material in an arbitrary simple shear flow. In this first part, we discuss several theoretical aspects of this mathematical formulation and new material property. For example, we show how MAPS measurements can be performed in strain- or stress-controlled contexts and provide relationships between the weakly nonlinear response functions measured in each case. We show that the MAPS response function is a superset of the response functions that have been previously reported in medium amplitude oscillatory shear and parallel superposition rheology experiments. We also show how to exploit inherent symmetries of the MAPS response function to reduce it to a minimal domain for straightforward measurement and visualization. We compute this material property for a few constitutive models to illustrate the potential richness of the datasets generated by MAPS experiments. Finally, we discuss the MAPS framework in the context of some other nonlinear, time-dependent rheological probes and explain how the MAPS methodology has a distinct advantage over these others because it generates data embedded in a very high-dimensional space without driving fluid mechanical instabilities, and is agnostic to the flow protocol. [ABSTRACT FROM AUTHOR]

In a recent article [Lee et al., J. Rheol. 63, 863–881 (2019)], the start-up of stress-controlled oscillatory flow both theoretically and experimentally was considered. In particular, they derive an analytical expression for the nonzero value around which the resulting strain (relative to the configuration before start-up) ultimately oscillates. Their derivation is based on the concept of recovery rheology, in which strain is decomposed into a sum of recoverable and unrecoverable components. Here, the same problem is treated based directly on the general theory of linear viscoelasticity. In this analysis, a transiently decaying contribution emerges that is not present in the Lee expression. This contribution is important since it gives an assessment of the time scale on which steady time-periodic oscillatory flow is established. [ABSTRACT FROM AUTHOR]

The rheological properties (viscosity, plasticity, viscoelasticity, and elasticity) of noncolloidal suspensions with varying concentrations of a solid phase (Al powder) in oligomeric polyethylene glycol have been studied in the shear mode of deformation. All typical types of rheological behavior have been observed. In the concentration range up to approximately 30%, suspensions are Newtonian liquids, while at a certain concentration, non-Newtonian and viscoelastic effects appear. With further increases in concentration, the flow curves become a power-law type, though the yield stress can be found only for suspensions with a high solid phase content. The elasticity of the suspensions can be detected at concentrations in the order of 55%. In a narrow range of concentrations, the rheological state of the suspensions can be treated as viscoplastic, wherein they preserve fluidity and become elastic. This concentration threshold, determined by the change in various rheological characteristics including the loss tangent, corresponds to the gelation or "colloid" glass transition. The most detailed experiments were devoted to the concentration range of 60–70 vol. %, which are treated as highly concentrated suspensions (HCSs). A concentration of 70% is the upper limit, because mixtures with higher concentrations do not keep their continuity and crumble. The rheological state of HCSs corresponds to the elastoplastic type of behavior. The separation of the total deformation into plastic and elastic components showed that plastic deformations (but not the flow) in the shearing of HCSs are small. Repeated shearing in the loading-unloading mode has been performed. In this case, a decrease in the elastic deformation and a remarkable growth of the shear elastic modulus was observed. This effect is presumably explained by the orientation of the particles and shear-induced anisotropy. A stress-concentration diagram of the physical states of suspensions has been proposed. The concentration range somewhat below that of the HCS state is of special interest for the powder injection molding technology, which is limited by the domain of plasticity. [ABSTRACT FROM AUTHOR]

Many industrial polymer melts are entangled and undergo crystallization during processing. Several recent studies have reported experimental data for the rheology of crystallizing entangled polymers. Meanwhile, over the past few years, slip-link models have been demonstrated to describe the rheology of entangled melts under a variety of nonlinear deformation conditions. In this work, we present a modification of the slip-link model to describe the rheology of an entangled melt undergoing crystallization. The partially crystallized melt is represented by a blend of linear chains with free ends and cross-linked, bridgelike chains with fixed ends that resemble the tie molecules between developing crystallites. Two new parameters are introduced: the fraction of cross-linked chains and a modulus shift factor, both of which are functions of the degree of crystallinity. The model captures the evolution of viscosity and elasticity simultaneously over the whole range of available frequencies in the linear regime. The model is validated using experimental datasets for isotactic polypropylene. [ABSTRACT FROM AUTHOR]

Material extrusion (ME), sometimes called Fused Deposition Modeling® or Fused Filament Fabrication, is an additive manufacturing technique that places order 300 μ m diameter molten polymer filaments sequentially onto a moving substrate to build an object. The feed material is a solid fiber that acts like a continuous piston in a heated barrel, which plasticates itself to push molten material through a nozzle. The barrel pressure is substantial, of order 30 MPa (≈ 4000 psi), and similar to that developed in contemporary polymer processing. The similarity does not end here with all the non-Newtonian and viscoelastic effects and heat transfer limitations that challenge extrusion operations coming to bear in the ME. These will be discussed in this review with suggestions of areas of study. [ABSTRACT FROM AUTHOR]

We characterize the yield behavior and flow curve of a commercially available, molten, tempered milk chocolate with a fat content of 30%, from rheological measurements performed with a rotational rheometer fitted with a four-bladed vane spindle in a large-gap setup. We show that the yield behavior can be captured by the viscoelastic White–Metzner model, which indicates that elastic deformation precedes flow as the shear stress is increased. Following an initial fluidization cycle, the shear modulus of the tempered chocolate increases by a factor of 60 to G = 4.4 × 10 4 Pa. We find that the Bingham model provides the best fit to our extensive flow curve data, where the yield stress is τ yield = 36.1 ± 0.6 Pa and the Newtonian plateau viscosity is μ B = 8.7 ± 0.02 Pa s. [ABSTRACT FROM AUTHOR]

From results published by Rudolf Hilfer for the similar problem involving dielectric permittivities, we derive an explicit analytic expression for the step-strain relaxation modulus implied by the Havriliak–Negami model for complex viscosities. [ABSTRACT FROM AUTHOR]

From results published by Rudolf Hilfer for the similar problem involving dielectric permittivities, we derive an explicit analytic expression for the step-strain relaxation modulus implied by the Havriliak–Negami model for complex viscosities. [ABSTRACT FROM AUTHOR]

We present results from a comprehensive investigation of the rheological responses of the gelled waxy crude oils during the yielding transition process by both stress-controlled and strain-controlled tests. If the deformation is within the linear viscoelastic region, G ′ increases slowly with increasing frequency following a power-law relationship. 80% of the total deformation accumulated during creeping is recoverable, and this recoverability remains largely independent of both imposed stress and testing time. The waxy gel relaxes the restored stress slowly with a time scale of ∼100 s. If the induced deformation exceeds the linear viscoelastic region, the elasticity of waxy structure starts to gradually decay with increasing creep stress and creep time, and the plastic contribution has come into play in the nonlinear region. A rapid reduction in the obtained time scale is observed with increasing applied strain. After the structure fractures, both G ′ and G ″ decrease with increasing strain following a power-law relationship. The ratio of the large amplitude shear-thinning exponents of G ′ and G ″ is around 2, indicating a more rapid decay of the elastic property than the viscous property. A brittleness index I is defined to quantitatively describe the brittleness of the waxy gel. The value of I for the waxy gel formed at lower temperatures is smaller, indicating that the waxy gel tends to have a more brittle fracture when yielding. The values of linear storage modulus G ′ ( γ 0 → 0 ) , critical linear strain γ 1 , critical linear stress σ 1 , and yield strain γ y obtained by different rheological testing methods are found to be well consistent. The yielding transition process is found to be strain determined rather than stress determined, and thereby γ 1 and γ y rather than yield stress are suggested to indicate the yielding transition. [ABSTRACT FROM AUTHOR]

A new polydisperse "toy" constitutive model is derived and developed from fundamental principles and ideas governing the nonlinear rheology of linear flexible polymers [Mead et al., J. Rheol. 59, 335-363 (2015)]. Specifically, the new model is comprised of four fundamental pieces. First, the model contains a simple differential description of the entanglement dynamics of discrete entanglement pairs. Second, the model contains a differential description of the ij entanglement pair orientation tensor dynamics. Third, following a similar development by Mead and Mishler [J. Non-Newtonian Fluid Mech. 197, 61-79 and 80-90 (2013).], a diluted stretch tube is constructed to describe the relative stretch of each component in the molecular weight distribution (MWD). Fourth, a description of configuration dependent friction coefficients is generated by generalizing the monodisperse formulation of Ianniruberto et al. [Macromolecules 45, 8058-8066 (2012)]. The polydisperse stress calculator is developed from the orientation, stretch and entanglement density and is fundamentally different from other molecular models that assume a constant entanglement density. The resulting model is comprised of three differential evolution equations and is simple to code and fast to execute. The model can simulate arbitrary fast nonlinear flows of arbitrary MWD's. In the slow flow linear viscoelastic limit, the model collapses to the double reptation model. This welcome result has positive implications with respect to our model parameter determination [Ye et al., J. Rheol. 47, 443-468 (2003); Ye and Sridhar, Macromolecules 38, 3442-3449 (2005)] for making quantitative calculations. [ABSTRACT FROM AUTHOR]

We studied the rheological and mechanical properties of a series of poly(vinyl alcohol) dual crosslink hydrogels, permanently crosslinked by glutaraldehyde and transiently crosslinked by borate ions, at various crosslinking densities. The contribution of the chemical and physical crosslinks to the dynamic moduli was first determined with linear oscillatory tests, and the additivity of the two contributions was validated. The single relaxation time of the system was interpreted as the physical bond breaking time which can be slowed down by reassociation of the open physical bonds before full relaxation. By comparing the Rouse relaxation time of the strand between two adjacent crosslinks with the healing time of the physical bonds, an increase in the relaxation time with decreasing density of physical crosslinks was explained. In all cases, the extensibility and stress at break of the gels were found to increase with the addition of physical crosslinks, but the extensibility increased the most for the smallest amount of chemical and physical crosslink densities while the stress at break increased the most for the most physically crosslinked system with a low chemical crosslink density. This strongly suggests that the toughening mechanism due to the presence of transient bonds can only be active on a network with a low density of chemical crosslinks. In large strain experiments, the tensile stress could be separated into a strain-dependent and a time-dependent term for all the crosslinking densities studied, and master curves of the reduced stress as a function of time were obtained. This showed that the chemical and physical crosslinks independently and additively contribute to the viscoelasticity of the dual crosslink gel. [ABSTRACT FROM AUTHOR]

The structure and the dynamics of supramolecular comblike polymers in the melt state is studied by a combination of linear rheology, dielectric spectroscopy, and small angle neutron scattering. The system consists of blends of 1,2-polybutyleneoxide (PBO) entangled backbones, randomly functionalized with thymine (thy) and barely entangled PBO graft chains--modified with 2,4-diamino-1,3,5-triazine (DAT) end groups. These bioinspired groups associate into a transiently branched comb architecture through heterocomplementary interaction involving the two different hydrogen bonding groups thy and DAT. In the present manuscript, we focus on the comparison of the macroscopic dynamics of the associating blends and permanent comb analogs. The viscoelastic and dielectric response of covalent and reversible combs are found to be comparable. The viscoelastic response of mixtures of thy-functionalized entangled backbones and DATend- modified barely entangled chains show a relaxation mechanism, which is mostly attributed to the association/breakage dynamics of the transient bonds with characteristic time ~1 s at T = -25 °C. In the parallel dielectric investigation, the reversible branched structure is still evident from the comparison with the corresponding permanent combs and allows the distinction between fixed arms relaxation and the lifetime. A α* process of the thy-thy association is likewise detected. The time scale of the supramolecular association makes the thy-DAT pair an ideal candidate for the development of responsive materials that combine permanent and transient linkages for novel applications and self-healing properties. [ABSTRACT FROM AUTHOR]

The nonlinear stress and microstructural response of a colloidal hard sphere suspension undergoing medium and large amplitude oscillatory simple shear have been studied using Accelerated Stokesian dynamics. The goal is to understand how nonlinearity arises and to link the structural effects to the resulting suspension stress. The imposed shear is given by the time-dependent rate γṀ(t) = γṀ0eiαt. Most results are shown for a hard-sphere suspension at a particle volume fraction ϕ = 0:4. These are freely flowing conditions far from either glassy or jammed conditions, but the concept of the particle cage from glass dynamics is used. The cage is defined here in a statistical manner as the surface of elevated nearest neighbor probability, a sphere at contact for equilibrium. The cage concept is used in interpreting the microstructural deformation: For sufficiently small strain amplitude γ0, the cage deforms negligibly due to flow and the suspension remains in the linear response regime, but this is found to require γ0 < 0:01 at ϕ = 0:4, as shown by a spectral decomposition of the microstructure in time, which discriminates rigorously between linear and nonlinear deformation. At larger γ0, termed medium amplitude and large amplitude in other studies, the material response is nonlinear. To preface the large amplitude oscillatory shear analysis, we use linear viscoelasticity theory to compare stress fluctuations at equilibrium to results obtained at finite Péclet number Pe and small γ0, as well as available experimental data and theoretical predictions; Pe = 6πηγṀ0a3/kT is the ratio of hydrodynamic to Brownian forces, where η is the viscosity of the suspending liquid, γṀ0 is the shear rate amplitude, a is the particle radius, k is the Boltzmann constant, and T is the absolute temperature. The shear stress σxy and the normal stress differences N1 and N2 are analyzed under oscillatory shear at amplitudes 0:01 ⩽ γ0 ⩽ 3:6 forarange of Pe. (The frequency α is related to Pe through α Ṁ0 = α α0 and the nondimensional frequency is given by the Deborah number De =Pe/α0 = 6πηγṀ0a3/kT .) Pipkin diagrams are shown for σxy, N1 and N2. When hydrodynamic forces dominate the flow of the suspension, the complex viscosity /η*/ has a nonmonotonic dependence on α 0, and Fourier-transform rheology shows the nonlinearity of the stress response to be maximized at an intermediate strain amplitude that depends on Pe. The elastic and viscous behavior of the suspension, as determined by a Chebyshev polynomial decomposition, is distinctly different for small and large Pe. The influence of the microstructure on the normal stress differences is discussed, noting that N1 is significant only when angular distortion of the microstructure is present, whereas N2 is formed with an accumulation of pair correlation at contact even at low oscillation amplitude. [ABSTRACT FROM AUTHOR]

We present unique nonlinear rheological data of well-defined symmetric Cayley-tree poly(methyl methacrylates) in shear and uniaxial extension. Earlier work has shown that their linear viscoelasticity is governed by the hierarchical relaxation of different generations, whereas the segments between branch points are responsible for their substantial strain hardening as compared to linear homopolymers of the same total molar mass at the same value of imposed stretch rate. Here, we extend that work in order to obtain further experimental evidence that will help understanding the molecular origin of the remarkable properties of these highly branched macromolecules. In particular, we address three questions pertinent to the specific molecular structure: (i) Is steady state attainable during uniaxial extension? (ii) What is the respective transient response in simple shear? and (iii) How does stress relax upon cessation of extension or shear? To accomplish our goal, we utilize state-of-the-art instrumentation, i.e., filament stretching rheometry and cone-partitioned plate shear rheometry for polymers with 3 and 4 generations, and complement it with state-of-the-art modeling predictions using the branch-on-branch (BoB) algorithm. The data indicate that the extensional viscosity reaches a steady state value, whose dependence on extensional rate is identical to that of entangled linear and other branched polymer melts. Nonlinear shear is characterized by transient stress overshoots and the validity of the Cox-Merz rule. Remarkably, nonlinear stress relaxation is much broader and slower in extension compared to shear. It is also slower at higher generation, and rateindependent for rates below the Rouse rate of the outer segment. For extension, the relaxation time is longer than that of the linear stress relaxation, suggesting a strong "elastic memory" of the material. These results are described by BoB semiquantitatively, both in linear and nonlinear shear and extensional regimes. Given the fact that the segments between branch points are less than three entanglements long, this is a very promising outcome that gives confidence in using BoB for understanding the key features. Moreover, the response of the segments between generations controls the rheology of the Cayley trees. Their substantial stretching in uniaxial extension appears responsible for strain hardening, whereas coupling of stretches of different parts of the polymer appears to be the origin of the slower subsequent relaxation of extensional stress. Concerning the latter effect, for which predictions are not available, it is hoped that the present experimental findings and proposed framework of analysis will motivate further developments in the direction of molecular constitutive models for branched and hyperbranched polymers. [ABSTRACT FROM AUTHOR]

Measuring the longest relaxation time of a viscoelastic liquid is an important step in its rheological characterization. In several instances, however, such measurement is unfeasible through conventional techniques, e.g., frequency response in linear viscoelasticity or measure of first normal stress difference. This difficulty is often found for very dilute macromolecular solutions. The case of polyelectrolyte solutions is especially important for their relevance in several bioengineering problems: Even theoretical predictions for this case can often not be tested in the absence of an experimental characterization. In this work, we succeed in measuring the relaxation time of two dilute/semidilute polyelectrolyte solutions as a function of added salt concentration. The relaxation time is measured through a novel microrheometrical technique based on the viscoelasticity-induced alignment of particles suspended in a microfluidic channel. We apply such technique to characterize two polyelectrolyte solutions, namely, hyaluronic acid and chitosan solutions. Relaxation times as small as ≈60 µs are detected, which are otherwise unmeasurable when using both conventional or other µ-rheometrical techniques. [ABSTRACT FROM AUTHOR]

This paper focuses on the relation between the structure and the rheological properties of non-Brownian ferrite particles suspended in a molten polypropylene matrix with and without the addition of a dispersant agent. The studied filler levels were 2, 15, and 20 vol.%. Under low deformation, the measurement of the complex shear modulus over time showed that the material structure was not stable. Such evolution was, on the one hand, more visible at concentrations near the percolation threshold (ɸc ≈ 0:25) and on the other hand, less visible when adding a dispersant agent. Oscillatory time sweep experiments were performed after a preshearing. It was shown that the kinetics of the evolution strongly depends on the applied deformation. Such evolution has also been highlighted by optical microscopy, in which an agglomeration phenomenon was clearly observed after few hours, again less visible for formulations with a dispersant agent. Under high deformations, no structure evolution was noticeable. Our results stress the important role of adding a dispersant into the formulation of highly filled materials. The dispersant agent acts therefore as a stabilizer and a lubricating agent. The origin of such structuring process has been attributed to a favorable entropy gradient from the polymer chains between the particles and the polymer chains in the bulk. [ABSTRACT FROM AUTHOR]

The nonlinear responses of olefin multiblock copolymers (OBCs) melts were investigated under large amplitude oscillatory shear (LAOS) using Fourier-Transform rheology in this work. Time-temperature superposition (TTS) was examined using linear viscoelastic functions, as well as nonlinear viscoelastic functions like the intrinsic Q factor (Q3,0) and the dynamic moduli of the third harmonic (G′ 33,0 and G″ 33,0). It is found that the intrinsic Q factor (Q3,0) exhibits similar TTS behavior as those of storage moduli and loss moduli. However, the deviation from typical liquidlike behavior of homogeneous melt in Q3,0 is much more evident than that in the storage moduli, indicating more sensitive of Q3,0 to the microstructure than linear viscoelastic functions. In addition, we suggested a new plot, the intrinsic phase angle at third harmonic (δ3,0) versus the intrinsic complex moduli at third harmonic (G*33,0|), which is named as the vGP3 plot. Using the vGP3 plot, the much weaker mesophase separation can be distinguished from the homogeneous state successfully. The high sensitivity of vGP3 to the failure of TTS makes it a new candidate in studying the thermorheological behaviors of complex fluids. Finally, the frequency dependency of intrinsic nonlinearity under LAOS was modeled by combining the molecular stress function (MSF) model and emulsion model. The MSF model can describe the experiment data of linear dynamic moduli and Q3,0 quite satisfactorily for homogeneous OBCs. For heterogeneous melt, the MSF model and the simple emulsion model account for the homogeneous contribution and the interfacial contribution, respectively. The predictions of the combined model agree quite well with the frequency dependency of the experimental data for both linear storage moduli and intrinsic Q factor (Q3,0). [ABSTRACT FROM AUTHOR]