Start-up Shear Research Articles

Consequences of Delayed Chain Retraction on the Rheology and Stretch Dynamics of Entangled Polymer Liquids under Continuous Nonlinear Shear Deformation

We generalize our recent new ideas for the continuous startup shear rheology of entangled polymer liquids in the transient stress overshoot regime to formulate a theory for the full constitutive response and nonequilibrium dynamics over all time scales, deformation rates, and degrees of entanglement. The convective constraint release (CCR) idea that chain retraction locally triggers disentanglement in a shear-rate-dependent manner is significantly modified to be physically consistent with our nonclassical treatment of delayed retraction due to an entanglement grip force. A detailed numerical study of the predictions of the theory is presented for the full stress–strain response, scaling behavior of the stress overshoot and undershoot features, orientational stress, primitive path contour length dynamics, and nonequilibrium steady-state properties spanning the slow and fast nonlinear deformation regimes. For deformations slow enough there is little or no chain stretch, our results are qualitatively the sam...

Numerical investigation of microstructural damage during kneading of wheat dough

Abstract The focus of this study is the kneading process of wheat dough. As an idealized kneader geometry, the eccentric cylinder benchmark geometry was considered. This work presents the first computational study in which the dough model developed by Tanner's group (R.I. Tanner, F. Qi, S.-C. Dai, Bread dough rheology and recoil: I. Rheology. J. Cereal Sci. 148 (1) (2008) 33–40) was solved for inhomogeneous flow. The values of the model parameter were obtained from small amplitude oscillatory shear, stress relaxation, start-up of simple shear, and compression experiments. To computationally evaluate the damage function, a time evolution equation had to be implemented for the Cauchy–Green strain tensor. The flow problem was numerically solved using a finite volume procedure available in the software OpenFOAM package rheoTool. As a result of the rapid decrease in accumulated strain, filaments of low damage were formed near the outer diverging part of the cylinder, convected downstream and further extended, and eventually broken down. Planar extension had the most destroying effect on the microstructure. In the future, we will investigate three-dimensional free-surface flows in industrial kneading geometries.

A fractional dashpot for nonlinear viscoelastic fluids

In this paper, we present a fractional dashpot that is capable of continuously adjusting the degree of dissipation between the whole dashpot and the Maxwell model (spring and dashpot in serial connection). It is related to the springpot by a frequency shift of the relaxation modulus in the complex domain. In contrast to the springpot, which is a viscoelastic solid-like model, the fractional dashpot is an intrinsically viscoelastic fluid model. We demonstrate that the fractional dashpot allows one to better model the polydispersity effect of typical viscoelastic fluids, overcoming undesired stationary predictions and reducing or even eliminating multiple modes in data fitting. The linear version of the fractional dashpot model is also scaled up to large deformation by incorporation of corotational tensor derivatives and a projected strain rate tensor for constructing an objective fractional constitutive equation. The resulting model having five model parameters (one for fractionality, two for linear viscoelasticity, one for straining, and the last one for rotation) is able to fit startup shear viscosity of a high molecular weight polystyrene solution in high accuracy, and yet using only a single mode.

Rheology of the cellulose nanocrystals filled poly(ε-caprolactone) biocomposites

The pristine and acetylated cellulose nanocrystal (CNC) particles were incorporated with poly(ε-caprolactone) to prepare two kinds of green nanocomposite systems: the former is incompatible, while the latter compatible thermodynamically. A rheological study was then performed systematically with various flow fields, including linear and nonlinear dynamic shear flow, creep and start-up flow. Some interesting results were shown then. The pristine CNC filled system shows far lower percolation threshold than the acetylated CNC filled one because the improved phase affinity yields diluent effect. The formation and rebuild-up of percolated network is driven by the Brownian motion of particles, which are nearly independent of altered phase affinity. However, the improved phase affinity makes the system more sensitive to the strain-responded deformation, leading to the reinforced Maxwell spring unit during creep and to a weak strain overshoot during dynamic flow. The structural evolution and relaxations of two systems were further evaluated from different perspectives.

Stress Undershoot of Entangled Polymers under Fast Startup Shear Flows in Primitive Chain Network Simulations

Stress Undershoot of Entangled Polymers under Fast Startup Shear Flows in Primitive Chain Network Simulations

Molecular characteristics of stress overshoot for polymer melts under start-up shear flow.

Stress overshoot is one of the most important nonlinear rheological phenomena exhibited by polymeric liquids undergoing start-up shear at sufficient flow strengths. Despite considerable previous research, the fundamental molecular characteristics underlying stress overshoot remain unknown. Here, we analyze the intrinsic molecular mechanisms behind the overshoot phenomenon using atomistic nonequilibrium molecular dynamics simulations of entangled linear polyethylene melts under shear flow. Through a detailed analysis of the transient rotational chain dynamics, we identify an intermolecular collision angular regime in the vicinity of the chain orientation angle θ ≈ 20° with respect to the flow direction. The shear stress overshoot occurs via strong intermolecular collisions between chains in the collision regime at θ = 15°-25°, corresponding to a peak strain of 2-4, which is an experimentally well-known value. The normal stress overshoot appears at approximately θ = 10°, at a corresponding peak strain roughly equivalent to twice that for the shear stress. We provide plausible answers to several basic questions regarding the stress overshoot, which may further help understand other nonlinear phenomena of polymeric systems.

Linking attractive interactions and confinement to the rheological response of suspended particles close to jamming

We study the response to simple shear start-up of an overdamped, athermal assembly of particles with tuneable attractive interactions. We focus on volume fractions close to the jamming point, where such systems can become disordered elastoplastic solids. By systematically varying the strength of the particle–particle attraction and the volume fraction, we demonstrate how cohesion and confinement individually contribute to the shear modulus and yield strain of the material. The results provide evidence for the influence of binding agents on the rheology of dense, athermal suspensions and describe a set of handles with which the macroscopic properties of such materials can be engineered.

Obtaining short-fiber orientation model parameters using non-lubricated squeeze flow

Accurate models of fiber orientation dynamics during the processing of polymer-fiber composites are needed for the design work behind important automobile parts. All of the existing models utilize empirical parameters, but a standard method for obtaining them independent of processing does not exist. This study considers non-lubricated squeeze flow through a rectangular channel as a solution. A two-dimensional finite element method simulation of the kinematics and fiber orientation evolution along the centerline of a sample is developed as a first step toward a fully three-dimensional simulation. The model is used to fit to orientation data in a short-fiber-reinforced polymer composite after squeezing. Fiber orientation model parameters obtained in this study do not agree well with those obtained for the same material during startup of simple shear. This is attributed to the vastly different rates at which fibers orient during shearing and extensional flows. A stress model is also used to try to fit to experimental closure force data. Although the model can be tuned to the correct magnitude of the closure force, it does not fully recreate the transient behavior, which is attributed to the lack of any consideration for fiber-fiber interactions.

A new stochastic simulation for the rheology of telechelic associating polymers

We here suggest a new Brownian dynamics algorithm for telechelic associating polymers with the aim of better understanding the complex rheological behavior of these systems and in particular of hydrophobically modified ethoxylated urethane solutions showing strain hardening in shear startup and significant deviations from the Cox–Merz rule (including shear thickening phenomena). Several molecular mechanisms have indeed been suggested to explain such a complex behavior, ranging from finite extensible nonlinear elasticity (FENE) effects, shear-induced increased association, and, very recently, flow-induced repulsive interactions between colliding flower-like micelles. The preliminary results here reported (only exploring a limited range of the wide parameter space) appear to suggest that FENE effects are mostly responsible for strain hardening, while deviations from the Cox–Merz rule are due to the persistence of bridge chains, as well as, in some cases, to the flow-induced formation of new bridges. In the explored parameter space, no significant role appears to be played by flow-induced collisions between micelles beyond that of preventing network collapse.

Enhancing the Rheological Performance of Wheat Flour Dough with Glucose Oxidase, Transglutaminase or Supplementary Gluten

The enzymes glucose oxidase and transglutaminase are frequently used to improve the breadmaking performance of wheat flours, as they have the ability to considerably alter the viscoelastic nature of the gluten network. To evaluate a flour’s breadmaking performance, rheological tests offer an attractive framework. In this study, the rheological impact of adding glucose oxidase or transglutaminase to wheat flour dough is investigated by means of linear oscillatory shear tests, creep-recovery shear tests and startup extensional tests. The former tests reveal that the enzymes render the dough stiffer and enhance its elastic character, until saturation is reached. In the breadmaking process, the use of excessive amounts of enzyme is known to be counterproductive. The strain-hardening index clearly reveals this overcross-linking effect. Besides enzymes, the gluten network can also be reinforced by adding supplementary gluten, which was indeed found to enhance the extent of strain-hardening.

Watching shear thinning in creep: Entanglement-disentanglement transition

We have carried out various rheometric experiments to probe the nonlinear rheological behavior of two well-entangled 1,4-polybutadiene solutions using both stress-controlled and rate-controlled startup shear. At a polymer concentration of 2% neither wall slip nor shear banding occurs, and there is no visible edge failure. At a moderate level of shear stress, the shear rate can take an induction time tind longer than the reptation time to rise toward an eventual steady-state value. The time tind for the entanglement-distanglement transition (EDT) shortens exponentially with increasing stress. The state of entanglement also changes in response to a sinusoidal stress σ0sinωt when its amplitude σ0 is beyond a threshold to produce a sufficiently high strain amplitude γ0 (>1). Moreover, the evolution of the state of chain entanglement during and after constant-stress creep has been studied using two different protocols. The first switches from creep to rate-controlled startup shear to determine the stress overshoot characteristic. In the second protocol, after creep to different stages, the sample is evaluated for its ability to undergo elastic recovery. We found that the overshoot response to the rate switching weakens and the ability to undergo elastic recoil deteriorates as the EDT progresses. These changes are clearly inherent because edge instability could not produce such effects. Thus, it is confirmed that an entanglement-disentanglement transition is a leading characteristic rheological response of these solutions.

Time evolution of agglomerate size of semisolid magnesium alloy AZ91D during a real isothermal shearing

Abstract The present study attempts to discuss the time evolution of the slurry microstructure of semisolid magnesium alloy AZ91D so as to control the forming process more effectively. We calculated the instantaneous shear stress and the time evolution of average agglomerate size of a real isothermal shearing process containing increasing shear rate, holding the maximal shear rate and decreasing shear rate. The calculated results demonstrate that we should adopt as great a maximal shear rate as possible to achieve a finer slurry microstructure and reduce the resulting larger startup shear stress by increasing the time interval required to increase the shear rate to maximum. Moreover, we also should shorten the time interval required to decrease the shear rate to zero as far as possible, so as to weaken agglomeration during decreasing shear rate and achieve a better final slurry microstructure.

Nonlinear Viscoelasticity and Generalized Failure Criterion for Polymer Gels

Polymer gels behave as soft viscoelastic solids and exhibit a generic nonlinear mechanical response characterized by pronounced stiffening prior to irreversible failure, most often through macroscopic fractures. Here, we describe this scenario for a model protein gel using an integral constitutive equation built upon the linear and the nonlinear viscoelastic properties of the gel. We show that this formalism predicts quantitatively the gel mechanical response in shear start-up experiments, up to the onset of macroscopic failure. Moreover, we couple the computed stress response with Bailey's durability criterion for brittle solids in order to predict the critical values of the stress σc and strain γc at failure. The excellent agreement between theory and experiments suggests that failure in this soft viscoelastic gel is a Markovian process and that Bailey's failure criterion extends beyond hard materials such as metals, glasses, or minerals.

Influence of in situ reactive interphase with graft copolymer on shear and extensional rheology in a model bilayered polymer system

Influence of an in situ reactive interphase composed of graft copolymer on the melt shear and extensional rheology was systematically studied with a model bilayer of polyamide-6 (PA6)/maleic anhydride grafted poly(vinylidene fluoride) (PVDF-g-MAH). Firstly, small-amplitude oscillatory shear was used in situ to probe the development of reactive interphase from the interfacial reactions by tracking the changes of viscoelastic responses. Secondly, shear (start-up shear, shear stress relaxation) and extensional rheology was comparatively performed on the healed bilayers to evaluate the effects of reactive interphase. Interestingly, shear stress relaxation and especially extensional rheology were pretty sensitive to the presence of reactive interphase, whereas start-up shear showed negligible sensitivity. Specifically, the reactive interphase retarded the stress relaxation of healed bilayers subjected to step strains. Particularly, extensional rheology provided a more direct and quantitative view of the contribution of reactive interphase to melt rheology in both linear and nonlinear regimes. The remarkable increase in transient extensional viscosity and enhanced strain hardening resulting from the interfacial reaction in bilayer were demonstrated in terms of the interfacial stress. Besides, effect of reaction extent on extensional rheology was further examined, where it was found that interfacial stress increased with reaction time. The observed changes in rheology were attributed to the in situ formed interphase with graft copolymers and the resulting increased entanglements between neighboring layers. This study highlights the remarkable sensitivity of shear stress relaxation and especially extensional rheology to a reactive interphase.

Communication: Appearance of undershoots in start-up shear: Experimental findings captured by tumbling-snake dynamics.

Our experimental data unambiguously show (i) a damping behavior (the appearance of an undershoot following the overshoot) in the transient shear viscosity of a concentrated polymeric solution, and (ii) the absence of a corresponding behavior in the transient normal stress coefficients. Both trends are shown to be quantitatively captured by the bead-link chain kinetic theory for concentrated polymer solutions and entangled polymer melts proposed by Curtiss and Bird, supplemented by a non-constant link tension coefficient that we relate to the nematic order parameter. The observed phenomena are attributed to the tumbling behavior of the links, triggered by rotational fluctuations, on top of reptation. Using model parameters deduced from stationary data, we calculate the transient behavior of the stress tensor for this "tumbling-snake" model after startup of shear flow efficiently via simple Brownian dynamics. The unaltered method is capable of handling arbitrary homogeneous flows and has the promising capacity to improve our understanding of the transient behavior of concentrated polymer solutions.

Effect of Chain Orientation and Stretch on the Stress Overshoot of Entangled Polymeric Materials under Start-Up Shear

One of the most important nonlinear rheological phenomena in flowing polymeric materials is the stress overshoot under start-up shear at sufficiently high shear rates γ, i.e., γ > τd–1, where τd is the terminal relaxation time of system. According to the well-known tube theory for entangled polymeric materials, a linear relationship between the anisotropy of the stress and that of the birefringence holds for strain rates in the τd–1 < γ < τR–1 range, where τR is the Rouse time. This implies that in such a flow range the stress overshoot behavior of entangled polymers is accurately described in terms of the chain orientation, as confirmed by previous experiments. However, there has been some recent debate on this issue, following the study of Lu et al. [ACS Macro Lett. 2014, 3, 569−573], whose coarse-grained molecular dynamics simulations produced results in apparent conflict with existing theoretical and experimental predictions, which had suggested that even for γ < τR–1, the stress overshoot is asso...

In search of physical meaning: defining transient parameters for nonlinear viscoelasticity

A complete set of model-independent viscoelastic functions for understanding responses to transient nonlinear rheological tests is presented, using large-amplitude oscillatory shear strain as a model nonlinear protocol. The derivation makes no assumptions about symmetries, and is therefore applicable to the responses to any input, allowing researchers to unambiguously define time-dependent moduli, viscosities, compliances, fluidities, and normal stress coefficients. A legend for interpreting the dynamic trajectories in modulus space is provided, along with explicit definitions of the rates at which the moduli change. These provide a quantitative mechanism to identify when, and by how much, a material response stiffens, softens, thickens, or thins while being deformed. In addition to providing analytical expressions for the moduli, the derivation requires the definition of a conceptually new term. This means there exist three, not two, time-dependent nonlinear viscoelastic functions by which any response can be fully described. The third function accounts for nonlinear properties such as yield stresses and the shifting of the strain equilibrium. This complete analysis scheme is unique in making a distinction between the strains in the lab and material frames. The quantitative sequence of physical process analysis, which is fully developed in this work, allows for comprehensive physical interpretations of responses to transient deformations of any kind to be made, including the steady alternance responses to large-amplitude oscillatory shear (LAOS), time-dependent oscillatory shear startup responses, and thixotropic and anti-thixotropic responses.

What Controls the Structure and the Linear and Nonlinear Rheological Properties of Dense, Dynamic Supramolecular Polymer Networks?

We investigated a series of telechelic polyisobutylenes, previously shown to exhibit self-healing, by means of small-angle X-ray scattering and rheology. All samples form a dense, dynamic network of interconnected micelles resulting from aggregation of the functional groups and leading to viscoelastic behavior. The dynamic character of this network manifests itself in the appearance of terminal flow at long time scales. While the elastic properties are distinctly molecular weight dependent, the terminal relaxation time is controlled by the functional end groups. The yielding properties under large deformation during startup shear experiments can be understood by a model of stress activation of the dynamic bonds. Stress relaxation experiments help to separate the nonlinear response into two contributions: a fast collapse of the network and a slow relaxation, happening on the time scale of the terminal relaxation. The latter is also known to control self-healing of the collapsed structure.

Retraction of "Evolution of Chain Conformation and Entanglements during Startup Shear".

ADVERTISEMENT RETURN TO ISSUEPREVRetractionNEXTORIGINAL ARTICLEThis notice is a retraction.Retraction of “Evolution of Chain Conformation and Entanglements during Startup Shear”Yuyuan Lu, Lijia An*, Shi-Qing Wang*, and Zhen-Gang Wang*Cite this: ACS Macro Lett. 2017, 6, 4, 344Publication Date (Web):March 15, 2017Publication History Received6 March 2017Published online15 March 2017Published inissue 18 April 2017https://doi.org/10.1021/acsmacrolett.7b00170Copyright © 2017 American Chemical SocietyRIGHTS & PERMISSIONSArticle Views998Altmetric-Citations1LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (119 KB) Get e-Alerts Get e-Alerts

Retraction of “Coupled Effect of Orientation, Stretching and Retraction on the Dimension of Entangled Polymer Chains during Startup Shear”

The theoretical description of how entangled polymers undergo large deformation has been at the heart of polymer dynamics. Over the years, the polymer physics community has adopted the tube model as the standard paradigm for describing polymer deformation and flow in both the linear and nonlinear regimes. The tube model envisions an ensemble of chains that undergo reptation and fluctuation inside smooth confining tubes on time scales of the Rouse relaxation time. It provides a reasonable and appealing description of the equilibrium and near equilibrium dynamics of entangled polymer melts and concentrated solutions. Some of the predictions based on this paradigm are also in apparent agreement with macroscopic rheological measurements for large deformations. However, some discrepancies with the tube model have been noted in the literature. For example, the time for the onset of time-strain superposibility in experiments has been shown to be much longer than the Rouse time anticipated based on the tube model. More recently, S.-Q. Wang’s group using particle-tracking velocimetry reported a number of nonlinear rheological phenomena that questioned the validity of the tube-model assumptions. The theoretical implications of the reported phenomena have been controversial.