Rheologica Acta最新文献

The rheological behavior of immiscible polymer blends is significantly influenced by their interfacial structure. In this study, the low-frequency tail behavior observed in the Cole–Cole plot of bi-polymer blends was analyzed to establish a theoretical model. The model introduces key parameters, including the effective bulk modulus at the interface, interfacial length, and diffusion interruption coefficient, to describe the tail behavior. The validity of the theoretical prediction was ascertained through rheological experiments of polypropylene and polystyrene blends prepared under varying processing conditions, accompanied by complex nonlinear least squares analysis. The results of the study indicate a correlation between the slope and extent of the tail in the Cole–Cole plot and interfacial morphology, diffusion characteristics, and mixing parameters. A more pronounced tail was observed in co-continuous morphologies and the case of enhanced interfacial interactions and diffusion. This study proposes a novel rheological criterion for characterizing interfacial structures in polymer blends.

Nanostructured epoxy composite resins have broad usage in adhesives, coatings, composites, and 3D printing. With these materials, careful control of the rheological properties is critical to ensuring that the properties meet their required performance targets. However, it can be difficult to accurately measure the rheological properties. In this work, we establish a method to develop a reliable pre-shear (PS) procedure to repeatably measure the apparent yield stress of the resins, which is critical to ensure the accurate understanding of the material behavior. The resins in this study consisted of an epoxy resin with nanoclay as a shear thinning agent, ionic liquid (1-ethyl-3-methylimidazolium dicyanamide) as a latent curing agent, and poly(ethylene oxide-b-propylene oxide-b-ethylene oxide) block copolymer (BCP) as a nanostructured component. We establish a methodology to evaluate the effectiveness of a pre-shear protocol and evaluate several methods to identify a pre-shear procedure that resulted in repeatable transient creep results on a rheometer. We identified that large amplitude oscillatory shear was the most effective method for these materials, and the optimal magnitude of the shear was dependent on the composition of the epoxy resins. Through the consistent application of this approach, we were able to use transient creep testing to identify the phase boundaries in the epoxy/BCP resins when the BCP micelles undergo an order-order transition from spherical to hexagonal micelles through changes in the yield stress of the material. This work adds to the new growing body of literature demonstrating the importance of establishing rigorous pre-shear conditions to improve the accuracy of structured yield stress fluids.

Strain hardening of polymer melts in extensional flows is considered a desirable rheological feature because it stabilizes the homogeneity of free surface flows which is of importance, e.g., in film blowing, blow molding, and fiber spinning. Relating strain hardening to molecular characteristics, specifically topology in homopolymer melts, has been a long-standing challenge in rheology. While long-chain branching is known to be a decisive feature to enhance strain hardening, a general, quantitative relation between strain hardening and molecular topology is still missing. We propose a novel Strain Hardening Index (SHI) that can be used to assess the strain hardening behavior and to compare strain hardening of polymer melts with different topology and different chemistry, and we discuss its correlation with the steady-state compliance (J_s^0). We consider the strain hardening characteristics of model polystyrene comb and pom-pom systems as well as of model poly(( ±)-lactide) graft copolymers and several polydisperse low-density polyethylene melts. We show that the proposed SHI of typical low-density polyethylene melts is equivalent to that of polystyrene pom-poms and combs with specific topologies. This finding might pave the way to rheologically informed topological tailoring of the strain hardening of industrially important polymers such as, e.g., polyethylene.

Graphical Abstract

Numerous industrial, biological and geophysical fluids display the time-dependent rheological property known as thixotropy, in which the viscosity evolves over time and in response to changes in stress or strain rate. A wide range of phenomenological behaviour is associated with this property, and numerous models have been proposed and used to capture this. The aim of this paper is to classify systematically how modelling choices correspond to predicted behaviour, and, conversely, how observed behaviour can inform modelling choices. To this end, ‘ideal’ thixotropic models (without elasticity) are considered from a theoretical standpoint and the range of behaviour that different models can predict are explored. The approach is illustrated by considering the steady and transient responses to simple shear, with particular emphasis given to the role of the steady-state flow curve for a given model construction. The requirements for models to capture complex rheological phenomena like yield-stress ageing and “viscosity bifurcations” are outlined, and the implications of different modelling choices are discussed. The importance of carefully analysing the type of behaviour that a given thixotropic model can exhibit is highlighted.

Measurements of elongational properties of polymer melts with counter-rotating drums according to Sentmanat have become very popular over recent years. However, some results contrary to those obtained from classical elongational rheometers are published, particularly at higher elongations at which the uniformity of sample deformation plays a decisive role. In this paper, it is reviewed how the uniformity of sample geometry was determined in pioneering devices, and experimental results were verified by applying various deformation modes and different types of rheometers. Concerning the commercial counter-rotating drums insets, uniform sample deformation is often assumed but rarely verified. To overcome this deficiency, a video equipment synchronized with the rheometer functions was installed. On commercial products of a high density polyethylene and a linear polypropylene, it was shown that maxima of tensile stress or elongational viscosity, respectively, are accompanied by non-uniform elongations. An algorithm is presented for the numerical description of the sample edges. From a comparison of the width averages as a function of experimental time with the corresponding data for the uniform deformation, conclusions with respect to the uniformity of a sample can be drawn and the range assessed, in which an evaluation is based on reliable measurements.

Graphical Abstract

Capillary thinning of polymeric fluids is central to biological and industrial processes, yet the mechanisms governing thinning dynamics remain unresolved, especially for semi-flexible polymers. Using ideal solutions of semi-flexible DNA, we validate a phenomenological model for exponential capillary thinning that accounts for each polymer in the molecular weight distribution. For semi-flexible polymers, self-selection of the exponential time constant occurs by a fundamentally different mechanism than for highly flexible systems, and is not simply governed by the longest polymers in the molecular weight distribution.

An inter-laboratory comparison was performed to set a baseline for how the properties of difficult materials vary based on location and measurement tool. These tests focused on rheology of a Newtonian fluid, a viscous silicone oil, and two colloidal gels with yield stress behavior: a commercially available milk-based cream and an aging aluminum oxide hydroxide gel. Rheological data were collected on these materials using an array of rheometric test geometries including a cone and plate, parallel plates, a cup and bob, a 4-arm vane, and 12- and 24-arm vanes having fractal cross section that were fabricated independently by each lab and for which accurate torque and rotation conversion factors have been established. Characterization by the 3D-printed fractal vanes agree between the two laboratories and agree with reference data obtained with cone-and-plate, parallel-plate, and cup-and-bob measurement tools. The viscous oil exhibited predominantly Newtonian behavior in shear while weak viscoelastic effects emerged at high frequency and can be accurately described by a fractional Maxwell model. The colloidal gels exhibited a more intricate thixo-elastoviscoplastic (TEVP) rheological behavior, including thixotropy, as well as distinct dynamic and static yield stresses. To explore the elastoviscoplastic character of these systems, we show how the fractal vane geometry can be readily utilized with such materials to measure creep and partial elastic recoil without concern about slip or shear banding.

Several models are available in the literature describing the viscosity of suspensions, but only a few incorporate the complex nature of the matrix. When they do so, they use the power law model. We propose a viscosity model based on the Cross model for non-Newtonian suspensions, consisting of non-Brownian particles in aqueous xanthan gum (XG) solutions. Aqueous solutions of xanthan gum with and without calcium carbonate particles (CC) were tested. We prepared mixtures with tap water at XG wt% in the semidilute regime, and CC vol% between 5% and 30%. Two different mixing protocols were used, differing in whether or not solutions were prepared via dilution with the stock XG solution. The base xanthan gum viscosities were collapsed into a master curve based on the Cross model. The shear-thinning behaviour is described as a function of polymer concentration by scaling laws for the Cross parameters, the zero-shear viscosity (eta _{0}), infinite shear viscosity (eta _{infty }), consistency index K and the m index. Protocol 1 mixtures could give rise to suspensions viscosities lower than the base XG fluid. This behaviour was attributed to the mixing procedure and named the “dilution effect”. Protocol 2 was developed to correct the mixing procedure. A predictive model was formulated by deriving an effective viscosity equation using the Cross model. The results show the relative effective viscosity to be roughly independent of the shear rate between (1 mathrm{s^{-1}}) and (1000 mathrm{s^{-1}}). A collapse of the average relative effective viscosities was achieved using an existing model for Newtonian suspensions.

Despite many attempts, the molecular mechanism of the nonlinear viscoelastic response of polymeric liquids in fast shear flows has not yet been fully elucidated. In this study, we examined the viscosity growth curves for a few well-characterized, nearly monodisperse, densely branched pom-pom polystyrene melts. The viscosity growth curves exhibit double peaks rather than the widely reported (for most polymer melts) single peak. To investigate the underlying molecular mechanism responsible for the observed behavior, we conducted primitive chain network (multi-chain sliplink) simulations, and found that the first and second peaks correspond to arm orientation and backbone stretch, respectively. We further observed that backbone stretch is reduced by coherent molecular tumbling at the flow start-up, and hence the second peak intensity comes out comparable to that of the orientation-induced first peak. In our sample, the number of backbone entanglements is small, so the mechanism of branchpoint withdrawal does not play a significant role.

Viscoelastic fluid flows are widely described by elastic dumbbell models such as Oldroyd-B and FENE-P. However, these constitutive equations can yield significantly different predictions, which may contradict experimental observations. In this work, we analyze the fully developed flow of a viscoelastic fluid in a straight channel and present a theory based on the lubrication approximation for calculating the elastic stresses and flow rate for four elastic dumbbell models, including various microstructurally inspired terms. We compare the predictions of different models and elucidate the impact of (i) the finite extensibility, (ii) conformation-dependent friction coefficient, and (iii) conformation-dependent non-affine deformation on the polymer stresses, velocity, and flow rate. We demonstrate that including all three microstructurally inspired terms in a constitutive equation can significantly affect the response of a viscoelastic fluid even in a fully developed flow, thus highlighting their potential necessity for accurate modeling of viscoelastic channel flows with mixed kinematics.