Inge Vinckier

Relationship between rheology and morphology of model blends in steady shear flow

Immiscible polymer blends display a complex flow behavior caused by the coupling between morphology and rheology. The flow induced microstructure has been studied on model systems of nearly inelastic polymers. For these systems, the elastic properties of the blend are mainly governed by the interface. Measurements of the storage modulus and of the first normal stress difference, both reflecting this enhanced elasticity, have been used to probe the blend morphology. From oscillatory measurements after cessation of flow the mean diameter of the disperse phase, as generated by the previous flow, has been calculated using the model of Palierne [Rheol. Acta 29, 204 (1990)]. A procedure based on a direct fitting of the dynamic moduli with the model is compared with one that uses a weighted relaxation spectrum. The steady state normal stress data, on the other hand, have been related to the morphology of the blend by means of the model of Doi and Ohta [J. Chem. Phys. 95, 1242 (1991)]. Since this model predicts a...

Transient rheological response and morphology evolution of immiscible polymer blends

The response of semi-concentrated model blends (10% disperse phase), consisting of slightly viscoelastic polymers, on a stepwise increase in shear rate is investigated. During the initial stage of the response droplets deform into fibrils. The shear and normal stress transients during the deformation process are modeled by combining the approach of Doi and Ohta with the affine deformation theory for single droplet behaviour. In the proposed equations the scaling relations of Doi and Ohta for transient stresses are preserved. They do not contain any fitting parameter. First, the model predictions are compared with experimental results on model blends. Good agreement is found under conditions for which affine deformation is expected. Second the applicability of the scaling relations of the Doi–Ohta theory is verified experimentally. Although the scaling laws should only apply for 50:50 mixtures of Newtonian liquids with equal viscosity, the experiments show that they hold as well for semiconcentrated system...

Stress relaxation as a microstructural probe for immiscible polymer blends

Relaxation has been investigated in immiscible blends that consist of slightly viscoelastic components. Both the shear and normal stresses have been measured after cessation of steady shear flow as well as after transient shear histories. The latter can generate a fibrillar structure which can relax by either retraction or break-up via end-pinching or Rayleigh instabilities. Each of these three relaxation mechanisms is reflected in the shape of the stress curves, from which also the corresponding structural time scales can be deduced. The experimental results have been used to evaluate the Doi-Ohta and Lee-Park models for immiscible blends. The scaling relations by Doi-Ohta are confirmed by the experimental results, but none of the existing models can correctly predict the complex relaxation behaviour observed for a highly deformed droplet phase. In the present study an alternative approach has been proposed. The stress relaxation due to fibril break-up via Rayleigh instabilities has been predicted successfully by combining physical models for the structural changes with the basic approach of the Doi-Ohta model.

Transient stresses in immiscible model polymer blends during start-up flows

Abstract The transient rheological behaviour of immiscible polymer blends is studied during start-up flows. The stresses can be divided in a contribution from the component polymers and a contribution caused by the presence of an interface. By means of relaxation measurements, the component stresses after inception of flow can be determined. In this manner, the different contributions to the stress can be compared separately with model predictions. For the contribution of the components, one normally uses mixing rules that do not take into account the morphology of the blend. As expected, a slight morphology effect can be detected. The stress contributions from the interface are calculated combining the Doi–Ohta approach with different models that describe the droplet deformation. For flows at small capillary numbers, the Maffettone–Minale model leads to accurate stress predictions. With larger capillary numbers, the normal stress profiles become more complex. The initial part is rather well described by the model of Almusallam et al. [J. Reol. 44 (2000) 1055]. It is demonstrated that the morphological changes in the blend can be deduced in quite some detail from the measured stress profiles.

Rheology of semi-dilute emulsions: viscoelastic effects caused by the interfacial tension

Abstract The steady state rheological properties of viscous emulsions are discussed in the dilute and semi-dilute concentration regions. In these systems the first normal stress differences can be measured as well. Such data have been collected over a wide range of ratios of droplet over matrix viscosity. In this manner data became available to evaluate the Choi-Schowalter model. Application of the latter to the normal stresses requires that the droplet diameter be known. At high shear rates the droplet diameter changes nearly inversely proportional to the shear rate. This results in a first normal stress difference proportional to shear rate and hence a ‘normal viscosity’ can be defined. This is used to compare the data with the available theoretical predictions. At low shear rates deviations from a constant normal viscosity can be observed. They are associated with a hysteresis region, where no single steady state droplet size can be defined anymore. Slightly viscoelastic components have been used as well to investigate whether this would result in deviations from the behaviour observed for mixtures of Newtonian fluids.

Rheology and Morphology of Immiscible Polymer Blends

Blending immiscible polymers provides an economic alternative to synthesizing new polymers. The properties of a blend depend on the characteristics of the constituent polymers as well as on the microstucture which is generated during the processing. If one wants to optimize the final structure of a blend it is thus essential to understand the interplay between rheology and morphology development during flow. The kinematic conditions during processing can however be complex and morphological changes such as droplet deformation, break-up, retraction and coalescence can all occur simultaneously. Moreover it is common practice to derive the structural information from specimens that have been cooled from the melt after e.g extrusion (Utracki, 1989).