Joannes Mewis

Thixotropy - a general review

Abstract The time-dependent behaviour associated with thixotropy rather than with viscoelasticity is discussed. The evolution of the concept is traced back and a generalized definition is accepted. Subsequently, the various experimental methods are considered with which meaningful measurements of thixotropy can be performed. The specific experimental difficulties encountered with the systems under consideration are treated separately. It is impossible to enumerate all materials that show thixotropy or anti-thixotropy. Instead, they are classified in groups according to their origin or application. The resulting table is used to deduce the characteristics which accompany thixotropic phenomena. This leads to a discussion of the time effects in terms of the molecular or microscopic structure. An attempt is made to provide a systematic outline of the published models and possible constitutive equations for thixotropic materials. Both inelastic and viscoelastic descriptions are included.

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...

Rheology of concentrated dispersions

Abstract Concentrated dispersions constitute an important class of materials, not only in science but also in technology, yet complete analysis of their rheological behaviour is still lacking. Available experimental data clearly underline the very general nature of the mechanical response encountered in concentrated dispersions. Thus, dispersions could be useful in fundamental rheological studies of the various constitutive equations. The analysis of dispersion mechanics in terms of underlying phenomena is sufficiently advanced to show a strong interrelationship between rheology and colloidal stability. The variety of structures found links both. On the one hand, colloidal effects determine the interparticle forces during deformation and consequently the variable structure and the rheological character??? the other hand, suitable rheological measurements, which may usefully be ???mented with those obtained using other techniques, can elucidate forces and structures involved as shown by analyses in terms of variable spectra and other basic visco??? measurements.

The rheological properties of suspensions of fibres in Newtonian fluids subjected to extensional deformations

Data are presented for the extensional flow of suspensions containing 0.1−1 % of fibres by volume. The aspect ratio of the fibres was varied from 280 to 1260. The observed stress levels were between one and two orders of magnitude greater than in shearing flow, in agreement with the a priori predictions of Batchelor (1971).

Transient Behavior of Liquid Crystalline Solutions of Poly(benzylglutamate)

A solution of poly‐γ‐benzyl‐L‐glutamate (PBLG) in m‐cresol has been investigated. Viscoelastic properties during steady state shear flow and in oscillatory flow are presented. The existence of a stable negative first normal stress difference is confirmed. The dynamic Newtonian viscosity is considerably smaller than the corresponding steady state shear value. The transient behavior after cessation of flow is studied in detail using dynamic measurements. After cessation of flow, the dynamic moduli change over a time scale which is much larger than that for stress relaxation. Effects of frequency, temperature and previous shear rate are analyzed. The structural interpretation of the data is discussed. Available models for polymeric liquid crystals have to be developed further before a comparison with the present results becomes possible.

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...

Effect of compatibilization on the breakup of polymeric drops in shear flow

A block copolymer may be added as a compatibilizer during polymer processing in order to promote intimate mixing of thermodynamically immiscible homopolymers. The action of this compatibilizer can only partially be attributed to its effect on the interfacial tension between the immiscible homopolymers. Here the additional contributions of the compatibilizer are directly probed by measuring the capillary number during coalescence experiments. Model blends consisting of polyisobutylene (PIB) and polydimethylsiloxane (PDMS), compatibilized with various amounts of a PIB–PDMS diblock copolymer, are used for this purpose. The mean capillary number of the droplets is determined from the mechanical frequency response of the blends. With increasing amounts of compatibilizer, a systematic increase in steady shear capillary number is seen, to values well above the critical capillary number for droplet breakup of uncompatibilized systems. This indicates that a simple decrease in interfacial tension is not the only effect of adding the compatibilizer to these immiscible blends. Past simulations suggest that these results are associated with gradients in interfacial tension (Marangoni stresses) induced by the gradients of compatibilizer concentration due to flow. Direct evidence of the presence of such interfacial tension gradients along the surface of compatibilized drops was obtained by optical microscopy.A block copolymer may be added as a compatibilizer during polymer processing in order to promote intimate mixing of thermodynamically immiscible homopolymers. The action of this compatibilizer can only partially be attributed to its effect on the interfacial tension between the immiscible homopolymers. Here the additional contributions of the compatibilizer are directly probed by measuring the capillary number during coalescence experiments. Model blends consisting of polyisobutylene (PIB) and polydimethylsiloxane (PDMS), compatibilized with various amounts of a PIB–PDMS diblock copolymer, are used for this purpose. The mean capillary number of the droplets is determined from the mechanical frequency response of the blends. With increasing amounts of compatibilizer, a systematic increase in steady shear capillary number is seen, to values well above the critical capillary number for droplet breakup of uncompatibilized systems. This indicates that a simple decrease in interfacial tension is not the only ef...

Sorption and diffusion of chlorinated hydrocarbons in silicalite-filled PDMS membranes

Abstract Pervaporation performance of a membrane is determined as well by the sorption as by the diffusion characteristics of the permeating components in the membrane. In order to enhance the selectivity of PDMS membranes, e.g. for the separation of VOCs from water, a filler (silicalite) may be incorporated in the membrane. Vapour-sorption experiments of Cl-HCs in PDMS and silicalite showed that the sorption in PDMS is determined by the hydrophobicity of the organic component, whereas for the sorption in the zeolite the molecular sieving effect is the dominant factor. As a result of this, voluminous organics are prevented from entering into the pores of the zeolite. The sorption capacity of the (un) filled membrane strongly decreases by a temperature rise from 20° to 70°C, whereas the sorption in the zeolite is not significantly affected. In most cases, the sorption capacity of the zeolite exceeds the one of the PDMS. With respect to the diffusion, small and light molecules, as well as molecules with a double bond, are favoured in comparison with the heavy ones. This rule applies for the diffusion both in the zeolite and in the (un) filled PDMS. The diffusion coefficient in silicalite is however a factor of 1000 smaller than the one in PDMS so that the positive effect of the filler can mainly be attributed to a kind of reservoir function for the organics in the membrane.

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.

Structure and dynamics of a polymer solution subject to flow-induced phase separation

Experiments combining mechanical rheometry with polarimetry (birefringence and scattering dichroism) have been conducted on a 6% solution of polystyrene (1.86x106 molecular weight) in dioctyl phthalate. Birefringence is used to measure the extent of segmental orientation, whereas the dichroism is sensitive to orientation and deformation of concentration fluctuations associated with the process of flow-induced phase separation. The results indicate that these fluctuations grow predominately along the neutral (or vorticity axis) of a simple shear flow. At higher rates of shear, orientation in the flow direction is favored. The transition in orientation direction is accompanied by time-dependent behavior in the optical properties of the solution during shear and the onset of shear thickening of the viscosity and the first normal stress difference coefficient.