A nonlinear rheological model which accounts for the time-dependent elastic, viscous and yielding phenomena is developed in order to describe the flow behavior of thixotropic materials which exhibit yield stress. A key feature of the formulation is a smooth transition from an ‘elastically’ dominated response to a ‘viscous’ response without a discontinuity in the stress–strain curve. The model is phenomenological and is based on the kinetic processes responsible for structural changes within the thixotropic material. As such, it can predict thixotropic effects, such as stress overshoot during start-up of a steady shear flow and stress relaxation after cessation of flow. Thus this model extends a previously proposed viscoplastic model [J. Rheol. 34 (1991) 647] to include thixotropy. An analysis and comparison to experimental data involving oscillatory shear flow are provided to evaluate the accuracy of the model and to estimate the model parameters in a prototype concentrated suspension. The experiments were conducted using a series of concentrated suspensions of silicon particles and silicon carbide whiskers in polyethylene. The data obtained with this experimental system indicated much better agreement between the theory and experiments that obtained in earlier work.

Transient phenomena in thixotropic systems

The origins, uses and evaluation of constitutive equations for the stress tensor of polymeric liquids are discussed.

Constitutive equations for polymeric liquids

From a thermodynamic theory, a new model for elastoviscoplastic fluid flow is presented It extends the Bingham viscoplastic model and the Oldroyd viscoelastic model Fundamental flows are studied: simple shear flow, uniaxial elongation and large amplitude oscillatory shear The complex moduli (G',G'') are founded to be in qualitative agreement with experimental data for materials that present microscopic network structures and large scale rearrangements Various fluids of practical interest, such as liquid foams, droplet emulsions or blood, present such elastoviscoplastic behavior: at low stress, the material behaves as a viscoelastic solid, whereas at stresses above a yield stress, the material behaves as a fluid

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A new constitutive equation for elastoviscoplastic fluid flows

The transport properties and, specifically, the rheological behavior of real and complex materials such as polysaccharide systems can be significantly affected by several factors, mainly related to molecular and supermolecular features. Most of these factors are common to all polymeric systems, as they have universal character; others are peculiar to carbohydrate polymers. If we bear in mind the concepts discussed in §1.4, we can easily imagine for polysaccharides a variety of structural conditions much wider than that generally observed for synthetic polymers. On both molecular and supermolecular scales, polysaccharides possess special characteristics that reflect specific behavior so that different classes of materials can be identified.

Rheology of polysaccharide systems

The linear rheology of nanoparticle filled polymer (NPFP) melts has been a quite charming but controversial topic of long standing. This article reviews recent research advances to provide a general understanding of its universal appearance and underlying mechanism. This work summarizes the rheological criteria for determining the so-called liquid-to-solid transition with increasing filler content, the contradictory ideas of four kinds of time-concentration superposition principles proposed for constructing master curves of linear rheology, and a wide range of constitutive and phenomenological models focused on creating rheological contributions of the polymer, filler, and interface region from different perspectives. Controversies about microstructures of NPFPs including filler structure and chain dynamics of the matrix are briefly described. Several open questions are highlighted to outline the most likely general framework for the further investigation of the linear rheology of NPFP melts.

Linear rheology of nanofilled polymers

A three‐dimensional viscoelastic‐plastic model for flow of particle filled polymer melts has been formulated. The approach is based upon a modification of the Leonov model by introducing a structure function describing stress generated by the presence of the dispersed phase under the assumption that the material obeys the von Mises yielding criterion before flow takes place. The model has been applied to steady simple shear and to transient shear flows, and equations have been derived for the components of stress tensor for each flow situation. A verification of the model has been made against limited experimental data available in the literature, indicating that the model is in fair agreement with experiments.

XY Fan

Papers

Viscoelastic plastic constitutive equation for flow of particle filled polymers