Characterization of polymer dispersions by Fourier transform rheology (original) (raw)
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Rheological model to predict the thixotropic behaviour of colloidal dispersions
Colloids and Surfaces A: Physicochemical and Engineering Aspects, 2004
We propose a rheological structural model to describe the thixotropic behaviour of colloidal dispersions. The model shows a certain connection between the structural changes of colloidal dispersions with viscosity. Viscosity depends on the size, shape and orientation of colloidal aggregates. Instantaneous shear rate changes cause instantaneous changes in aggregate shape and orientation, but non-instantaneous changes in aggregate size. The kinetic process in changing aggregate size determines the thixotropic behaviour of colloidal dispersions. The model differentiates two characteristic thixotropic times: one for structure formation and other for structure destruction. The elastic effects due to non-instantaneous changes of aggregate shape and orientation are also included in the model. The model has the following parameters: (i) three parameters to determine equilibrium or steady-state rheology; (ii) two parameters to set the viscosity for a given aggregate size at a given shear rate; (iii) two characteristic thixotropic times, which depend on shear rate; (iv) a characteristic elastic modulus. Each group of parameters are fitted independently with appropriate rheological tests and coupling among them is prevented. The model has been tested with consecutive lineal increases and decreases of the shear rates with time levels of maximum shear rates. Carbopol solutions of different molecular weights have been analysed with good agreement between experimental and predicted viscosity.
Rheological Characterization of Concentrated Nanoclay Dispersions in an Organic Solvent
2009
Nanoclay dispersions in organic solvents are widely used in cosmetics for a variety of gels and creams, whose properties depend on the powder content and the processing method. The control of the shear applied during processing is therefore essential for achieving the required properties. This study demonstrates the utility of applying rheological measurements for characterizing cosmetic products based on nanoclays and relating their viscoelastic properties to end-use performances. In particular, a rheological characterization of bentonite dispersions in isododecane at different clay content and shear history is presented. For each inorganic content, both mixed samples and samples subjected to several calendering runs were studied. The effect of shear and clay content on the viscoelastic properties was investigated by a combination of oscillatory shear experiments under small-deformation conditions and by X-Ray diffraction. The tested samples showed a gel-like behaviour with a final structure depending on the applied shear stress. By increasing the inorganic content in the dispersion, a reduction in the gel stability to a further shear application was observed. Two models, developed for colloidal gels, were used to fit the rheological results enabling to evaluate the microstructure and the degree of dispersion of the tested samples and to relate the colloidal structure to the elastic properties.
Rheology and diffusion in concentrated sterically stabilized polymer dispersions
Colloids and Surfaces A: Physicochemical and Engineering Aspects, 1995
Two monodisperse latices of different size (226 and 350 nm) were synthesized by the emulsion polymerization of styrene. Subsequent absorption of poly(viny1 alcohol) (PVA) gave two sterically stabilized latices' of different size and thickness of the adsorbed layer, but the same PVA concentration in similar volume fractions. Particle size and size distribution were determined by transmission electron microscopy and dynamic light scattering from dilute dispersions. Polymer layer thickness was determined by capillary viscometry and by the difference of hydrodynamic radii from quasielastic light scattering (QELS) in dilute solution. Concentrated dispersions up to 30% (w/w) were studied with regard to their microscopic dynamics with fiber-optical quasielastic light scattering (FO-QELS). The concentration dependence of the diffusion coefficients of the covered latices showed deviations from theoretical predictions for the self-diffusion of hard spheres. Moderate deviation from theory was also observed for the uncovered latices. Various rheometrical tests were performed on the sample. The viscoelastic transition region extended over a relatively broad concentration range. Curves for G' at different particle compositions in the viscoelastic region were incorporated into a mastercurve by including the longest relaxation time. The concentration dependence of the diffusion processes, as derived from FO-QELS, was different from that of the rheological phenomena.
Rheologica Acta, 2001
Market competition, cost reduction, and environmental driving forces are pushing industrial researchers to formulate polymer dispersions with a particle loading as high as possible. The bene®t will be an increased timespace yield during production, reduced transportation costs, and less energy consumption for the removal of solvent in the application process. At the same time the viscosity has to be kept low enough in order to ensure a sucient heat transfer during polymerization as well as reasonable stir-, pump-, and sprayability during transport and application. Particle loadings beyond a volume fraction of 0.60 can only be achieved if the particle size distribution is either broad or bi-or multimodal. Consequently, the rheology and the packing possibilities of highly concentrated suspensions have been discussed intensively by academic as well as industrial researchers. The literature strongly focuses on suspensions of non-
Rheological Characterization of A Waterborne Organic Nanoparticle Dispersion
Applied Rheology, 2015
Organic nanoparticles of poly(styrene-co-maleimide) or SMI were synthesized in aqueous dispersion with a maximum concentration of 35 wt.% and are favorably applied in industrial coating processes. In order to evaluate the further processability and flow behavior of these nanoparticle dispersions, general rheological characterization under creep, oscillatory and rotational testing was done by applying various shear stresses, shear rates and frequencies on an air-bearing cylindrical rheometer. Creep tests at different stresses show that the nanoparticle dispersions behave like a viscous material. The crossover of G’ and G” according to oscillatory experiments also demonstrates a transition to viscoelastic behavior at high frequency. The sensitivity of shear-viscosity behavior to concentration and temperature of the dispersions has been evaluated. In parallel, the influences of gap size, repeatability and water evaporation have been statistically evaluated and could be successfully con...
Linear viscoelastic behavior of aggregated colloidal dispersions
Physical Review E, 1997
The viscoelastic behavior of a depletion-flocculated dispersion of colloidal spheres is investigated at different volume fractions of the spheres, using a controlled stress and a dynamic rheometer. Combining the results, we obtain the storage GЈ and loss GЉ moduli over a frequency range of 0.02ϽϽ200 rad/s. The measured GЈ gradually increases with increasing frequency, while GЉ almost remains constant, indicating a broad spectrum of relaxation times. To describe and explain the observed behavior of the moduli as a function of frequency and volume fraction in terms of microscopic parameters, a microrheological model based on the fractal concept is proposed. Comparing experimental results with model calculations, we find a good agreement between the two, with physically plausible parameter values.
Rheology and surface properties of filled dispersions
Proceedings of the Estonian Academy of Sciences. Engineering, 2006
Rheology and wetting properties of commercial water-born dispersion of the copolymer of vinyl ester of a synthetic saturated monocarboxylic acid and acrylate, obtained by emulsion polymerization, have been investigated. For composition modification, 0.8 phr of dibutylglycol as plasticizer and from 0 to 10 phr of TiO 2 and Al 2 O 3 particles as filler were added. It has been shown that dispersion behaviour under shear between two coaxially placed cylinders depends not only on the dispersion composition, but on the shear rate as well. At low shear rates (up to 40 s-1), an increase of the viscosity was found, while at higher shear rates dispersion shows pseudoplastic behaviour. Effective amount of TiO 2 decreases the intensity of determined anomalies. For pure, plasticized and TiO 2 modified dispersions, a tendency to restore viscosity value by decreasing the shear rate was found. This effect for Al 2 O 3 filled dispersion was insignificant. The results of advancing contact angle measurements showed that an addition of up to 3 phr of mineral filler to the dispersion does not change its wetting properties. Higher amounts of fillers worsen wettability properties.
Effect of repulsive interactions on structure and rheology of sheared colloidal dispersions
Soft Matter, 2012
A previously developed Smoluchowski theory for concentrated hard-sphere suspensions in shear flow is extended to study structure and rheology of colloidal suspensions with soft repulsive interactions. Accelerated Stokesian Dynamics simulations are carried out to provide insight and to enable direct comparison with theoretical predictions. The effect of extended range repulsive interactions is studied by considering repulsive interactions with different steepness, using identical potentials in simulation and theory, for varying shear rates characterized in dimensionless form as 0.1 # Pe # 100; here, Pe ¼ 6ph _ ga 3 /k b T is the ratio of hydrodynamic to Brownian forces and h is the fluid viscosity, _ g is the shear rate, a is the particle radius and k b T is the thermal energy. Examples of predicted microstructures and the equivalent simulated results for hard-sphere suspensions at f ¼ 0.40 are also presented for comparison. The predicted pair distribution function is in good agreement with simulations before the onset of a shear-induced ordering transition in simulations of the soft colloids. The calculations of shear viscosity based on the predicted microstructure were also in general agreement with simulation results. The role of hydrodynamic interactions on flow-induced structures is discussed in the context of the proposed theory.
Physical Review E, 2006
A comprehensive experimental study of the dynamics and rheology of concentrated aqueous dispersions of poly͑ethylene glycol͒-grafted colloidal spheres is reported. The study focuses on good solvent conditions, for which excluded-volume interactions dominate. At high concentrations a glass transition is evident from the nondecaying component of the intensity correlation function measured with three-dimensional dynamic light scattering. Results for the linear viscoelastic and steady shear rheology on approaching the glass transition correlate well with the slowing of the diffusive dynamics; in particular, at, or close to, the concentration where the dynamics becomes nonergodic, the dispersions acquire a low-frequency plateau in the elastic shear modulus as well as a yield stress. The overall behavior of the dispersions conforms to that of hard-sphere dispersions; however, some qualitative differences are observed in the evolution of the dynamics and rheology with increasing concentration near the glass transition.