Rheologica Acta最新文献

We present linear viscoelastic data with anionically synthesized and critically fractionated polybutadiene (rich in vinyl content) rings having about Z = 22 entanglements. These rings are experimentally as pure as currently possible. They exhibit a power-law stress relaxation G(t) that is well-described by the state-of-the-art fractal loopy globule (FLG) model (power-law exponent of − 3/7). Previously reported data with polystyrene rings, prepared by anionic synthesis in dilute solution and purified by liquid chromatography at the critical condition, having Z = 14 entanglements, showed a power-law G(t) as well. Recent developments with different synthetic methods yielding not so well-characterized rings with a very large number of entanglements (up to 300), suggest that a rubbery plateau emerges in the linear viscoelastic response for Z > 15. Our work confirms the power-law G(t) with the FLG exponent with another chemistry and contributes to the current discussion about different regimes of rheological behavior, indicating that a possible deviation from the power-law FLG type of behavior toward rubbery plateau may occur for Z > 22. To fully capture the experimental G(t) data, the FLG model is complemented by two additional relaxation modes which are attributed to ring-ring (RR) and ring-linear (RL) threading, in accordance with recent reports in the literature. The faster RR mode likely reflects a new mechanism of stress relaxation not described by FLG, and the slower RL mode is attributed to synthetic and material handling imperfections (for example, due to thermal treatment). However, it does not change the punchline of the work: no rubbery plateau for entangled rings with up to 22 entanglements.

Graphical Abstract

Stress relaxation modulus for entangled ring polybutadiene (exhibiting power-law decay) and its linear precursor (exhibiting rubbery plateau), along with fits to the data: tube model for linear chains, and fractal loopy globule (FLG) with slow modes (RR and Tsalikis et al.) for the ring.

We consider the elongational rheology of model polystyrene topologies with 2, 3 and 4 stars, which are connected by one (2-star or “Pom-Pom”), two (3-star) and three (4-star) linear backbone chains. The number of arms of each star varies from q_a = 3 to 24, the molecular weight of the arms from M_w,a = 25 kg/mol to 300 kg/mol, and the backbone chains from M_w,b = 100 kg/mol to 382 kg/mol. If the length of the arm is shorter than the length of the backbone, i.e. M_w,a < M_w,b, and despite the vastly different topologies considered, the elongational stress growth coefficient can be modeled by the Hierarchical Multi-mode Molecular Stress Function (HMMSF) model, based exclusively on the linear-viscoelastic characterization and a single nonlinear parameter, the dilution modulus. If the length of the arms of the stars is similar or longer than the length of the backbone chain (M_w,a ≥ M_w,b) connecting two stars, the impact of the backbone chain on the rheology vanishes and the elongational stress growth coefficient is dominated by the star topology showing similar features of the elongational stress growth coefficient as those of linear polymers.

Graphical Abstract

Food rheology is key to manage the swallowing safety of people affected by swallowing disorders (dysphagia). Simple approaches to assess the flow properties of texture-modified drinks are widely used, but relatively poorly understood. This study focuses on the International Dysphagia Diet Standardisation Initiative (IDDSI) flow test, adopted by caregivers worldwide. This test considers the gravity-driven flow in a vertical syringe. Newtonian liquids and non-Newtonian fluids obtained using a commercial starch-based thickener were considered in this study. An accurate theoretical description of the flow is proposed for Newtonian and power-law fluids considering the effect of fluid properties and of the syringe geometry. A rheological map is proposed, based on the results of several thousand simulations, to capture quantitatively the effect of rheological properties and density on the IDDSI classification, highlighting the important effect of the fluid density which is usually ignored. The sensitivity of the IDDSI results with respect to the syringe outlet diameter is discussed, as well as the different average shear rates at which different IDDSI levels are tested. The rheological map also shows quantitatively that different combinations of the fluid rheological properties and density can result in the same IDDSI classification, suggesting interesting directions for future clinical research.

The transition from concentric primary flow to non-tangential secondary flow of blood was investigated using experimental steady shear rheometry and numerical modelling. The aims were to: assess the difference in secondary flow in a Newtonian versus shear-thinning blood analogue; and measure the secondary flow in the blood. Both experiments and numerical modelling showed that the transition from primary to secondary flow was the same in a Newtonian fluid and a shear-thinning blood analogue. Experiments showed whole blood transitioned to secondary flow at lower modified Reynolds numbers than the Newtonian fluid; and transition was haematocrit dependent with higher RBC concentrations transitioning at lower modified Reynolds numbers. These results indicate that modelling blood as a purely shear-thinning fluid does not predict the correct secondary flow fields in whole blood; non-Newtonian effects beyond shear-thinning behaviour are influential, and incorporating effects such as multiphase contributions and viscoelasticity, yield stress and thixotropy is necessary.

In quasi-two-dimensional conditions, different invasion patterns can be observed when a fluid displaces another fluid with a higher viscosity. The transition from interfacial instability to fracture during gas invasion remained poorly understood, and classification criteria for different invasion patterns demanded further improvement. In this study, single-point compressed gas injection experiments were conducted in magnesium lithium phyllosilicate (MLPS) suspensions in a rectangular Hele-Shaw cell. Interestingly, with the increase of the injection pressure and the decrease of the concentration of MLPS suspension, the gas invasion pattern underwent a transition from viscous elastic fracture, elastic fracture, elastic viscous fingering to viscous fingering, in which viscous elastic fracture was observed for the first times. We detailly discuss the characteristics and occurrence conditions of each invasion pattern. Furthermore, by analyzing the velocity field of each invasion pattern, it is found that the relationship between the velocity direction around the gas and the gas growth direction varies with different invasion patterns. A simple and effective quantitative indicator is constructed to distinguish the different invasion patterns. Following the identification of invasion patterns, a further investigation was conducted into the relationship between invasion patterns and experimental conditions. By utilizing the relationships among injection conditions and material rheological properties, two dimensionless numbers, Bingham number and Weissenberg number, are conducted, which have an impact on the various invasion patterns and invasion process. A unified phase diagram based on the Bingham number and Weissenberg number was also proposed to incorporate the possible gas invasion patterns in the MLPS suspension.

Oscillatory shear tests are frequently used to determine viscoelastic properties of complex fluids. Both the amplitude and frequency of the input signal can be independently varied, allowing rheologists to probe a wide range of material responses. Historically, most oscillatory tests have focused on the measurement and application of the total strain. However, the total strain is a composite parameter consisting of recoverable and unrecoverable components. Use of only the total strain therefore provides an incomplete description of the rheology. In this work, we provide a mathematical derivation for the determination of the recoverable and unrecoverable components in steady-state linear viscoelastic oscillatory flows via a simple experimental procedure. The relationship between the total strain and its components is fully explored and challenged in the context of how rheologists define moduli and common rheological models. These relations are demonstrated via experimental measurements on model viscoelastic and viscoplastic materials: wormlike micelles and Carbopol 980. Additionally, we show how the derived mathematics fully details the conditions where the Cox-Merz rules are valid in terms of recovery rheology. Finally, we demonstrate how a thorough understanding of the strain components can be used to create a simple nonlinear model that reproduces all common amplitude sweep behaviors.

Polyurethane (PU) is a versatile polymer with many applications in a wide range of products. A novel 3D printing technology called liquid additive manufacturing (LAM) extended its possibilities by generating PU elastomers with gradient properties in continuous processing. LAM, being a relatively new technique, has not been extensively researched, particularly in terms of the curing behavior of the liquid resin. In this work, we investigated the effect of composition on gelation time t_GP as measured by time-resolved mechanical spectroscopy (TRMS) and analyzed using the Winter–Chambon criterion with the assistance of the IRIS software. This method is more accurate than the previous approach, which involved time sweeps with a constant frequency. It was found that the gel time t_GP first decreased and then increased with increasing polyol ratio, ranging from 231 to 378 min. Furthermore, the crosslink densities of the different PU elastomers measured from the rheological and tensile tests were calculated and compared based on the theory of rubber elasticity. The crosslink density decreased with an increasing polyol ratio in both methods. However, the crosslink density values obtained from the rheological measurements were higher than those from the tensile tests. These findings demonstrate that adjusting the polyol ratio is an effective means of achieving gradient properties. The composition effects we measured offer valuable insights for the design of LAM–PU elastomers.

Melt strain hardening is an interesting characteristic property of the elongational flow of polymers. While strain hardening of many unmodified polymer melts has been widely discussed, a comprehensive presentation of the influence of particles on this property is missing. Using literature data and own measurements, the effects of solid particles of various geometries are compared. Micro-sized particles generally reduce melt strain hardening and may even lead to strain thinning. This behavior is postulated to be due to shear flow components around the particles and resulting shear thinning of the polymer matrices that reduces the resistance to flow. More complex is the influence of nano-sized fillers and layered silicate nanoparticles, in particular. Weakly exfoliated particles show effects similar to micro-fillers, but for strongly exfoliated silicates distinct strain hardening is observed that increases with decreasing elongational rate. This behavior is particularly pronounced for polymers modified with maleic anhydrides and thought to be related to electrostatic forces between exfoliated platelets of the silicates and polymer molecules hindering molecular motions.

A variable-entanglement density constitutive model is developed for the description of the rheological properties of entangled polymer melts and concentrated polymer solutions using non-equilibrium thermodynamics (NET). It proposes two evolution equations: one for the average number of entanglements per chain and one for the orientation of entanglement strands. Direct comparison with non-equilibrium molecular dynamics simulation data shows that the model can accurately describe the loss of entanglements due to the applied flow for three molecular weights by using the same value for the convective constraint release (CCR) parameter. The CCR relaxation time depends on the trace of the inverse of the orientation tensor instead of an explicit dependency on the velocity gradient. Finally, the stress tensor contains an additional contribution inspired by the Curtiss-Bird or tumbling snake model. Overall, the model proposed here carefully derives via NET and builds upon the work of Ianniruberto-Marrucci when stretching is not considered.

Graphical abstract