Dispersions Of Latex Particles Research Articles

Effects of sucrose fatty acid ester addition on the structural, rheological and retrogradation behavior of high amylose starch-based wood adhesive

In this study, sucrose fatty acid ester (SE) was used to evaluate the characteristics of a high amylose starch-based wood adhesive. Fourier transform infrared (FT-IR) spectroscopy confirmed the occurrence of graft copolymerization reactions whereas X-rays diffraction (XRD) analysis verified the presence of amylose-SE complexes. Scanning electron microscopy (SEM) a revealed uniform distribution of spaces whereas transmission electron microscopy (TEM) showed good dispersion of latex particles with SE addition as evident by small and poly-dispersed particles in the wood adhesive. The water dynamics in the starch adhesive system was studied using low-field nuclear magnetic resonance (LF-NMR). Furthermore, the addition of SEs resulted in enhanced shear-thinning and solid like-behaviors and anti-retrogradation properties of the adhesive with incorporation of SE. These results showed that SEs could improve the rheological and anti-retrogradation properties of the wood adhesive and offers a major step forward to prepare bio-based adhesives as an alternative for petroleum-based wood adhesives.

Simple statistical thermodynamic model of the heteroaggregation and gelation of dispersions and emulsions

The heteroaggregation behaviour of mixtures of equal-sized particles (type A + type B) exhibiting short-ranged attractive interactions is investigated using the sticky hard-sphere model. The average cluster size is calculated as a function of the total particle volume fraction, the binary mixture composition, and the A–B stickiness interaction parameter τ AB - 1 . We show that a value of τ AB - 1 = 10 2, equivalent to an attractive well depth of ∼5 kT in a realistic continuous pair potential, leads to a state of heteroaggregation just below the gelation threshold of the equimolar mixture of volume fraction 0.1. We discuss the conditions under which the assumptions of this statistical thermodynamic model are satisfied experimentally, with particular reference to recent data on the heteroaggregation behaviour of protein-stabilized emulsions and latex particle dispersions.

Miniemulsion Polymerization with Arrested Ostwald Ripening Stabilized by Amphiphilic RAFT Copolymers

We describe a method and conditions for the preparation of monodisperse latex particle dispersions using low molecular-weight, amphiphilic styrene-b-acrylic acid macro-RAFT diblock copolymers as sole stabilizers. The macro-RAFT copolymers are soluble in styrene monomer droplets when the acrylic acid is in its neutral, protonated form, but adsorb irreversibly at the interface between the monomer droplets and the aqueous phase to form an insoluble monolayer when conditions favor deprotonation. Droplets thus stabilized exhibited no coarsening by Ostwald ripening over at least several days. Polymerization yielded a one-to-one correspondence between the size of the initial droplets and the resultant latex particles, consistent with ideal miniemulsion polymerization, when the least labile macro-RAFT stabilizer was used. GPC analysis of molecular weights showed that in all cases the polymerization remained under RAFT control. The approach described enables true miniemulsion polymerization to proceed in the absen...

Numerical simulation of colloidal dispersion filtration: description of critical flux and comparison with experimental results

During filtration via membrane processes, colloids accumulate at the porous surface leading to fouling phenomena. In this study, a rigorous simulation of momentum and mass transfer using CFD modelling has been developed to describe such an accumulation during cross flow filtration. These simulations integrate detailed modeling of physicochemical properties specific to colloidal dispersions (because of the surface interactions (repulsive and attractive) occurring between the colloids particles). These interactions are accounted for via the experimental variation of the colloidal osmotic pressure with volume fraction (associated with a variation in the diffusion coefficient) which are fitted by a relationship integrated into the CFD code. It contains a description of the colloidal phase transition leading to the formation of a condensed phase (deposit or gel layer) from the accumulated dispersed phase (concentration polarization). It is then possible to determine the critical flux which separates filtration conditions below which mass accumulation is reversible (in the dispersed phase) and above which it is irreversible (in the condensed phase). The computed value of critical flux is compared with that determined experimentally for a dispersion of latex particles.

Cake Collapse in Pressure Filtration

Aqueous dispersions of latex particles have been aggregated by addition of Ca2+ ions and then filtered in a pressure filtration cell. The filtration cakes have been examined through small-angle neutron scattering, void volume fraction measurements, and permeate flux measurements. At all filtration pressures (20−400 kPa), the cakes were extensively collapsed (latex volume fractions φ = 0.27−0.46), and the remaining porosity had a tenuous structure (fractal dimensions df = 1.3−1). However, the local structures remained the same as in the aggregates of the original suspensions. The mechanisms that produce this collapse are made of very small relative motions of the particles, which leave the local coordination of the latex particles unchanged but allow large voids to be reduced. These motions could be inhibited by using particles with nonspherical shapes and by increasing the friction forces that act between particle surfaces, thereby reducing the extent of collapse and increasing the permeability of the cakes.

The Effect of Many-Body Interactions on the Sedimentation of Monodisperse Particle Dispersions

An experimental investigation was made of the sedimentation rate of low-charged monodisperse silica and polystyrene latex particle dispersions as a function of the particle volume fraction. It was found that the normalized sedimentation velocityU/U0, corrected for the effect of the two-body hydrodynamic interaction, increases with the particle volume fraction, which indicates that the degree of particle aggregation inside the dispersions increases with the particle volume fraction. This phenomenon results from attractive many-body hydrodynamic interactions between colloidal particles. It is reported for the first time that the many-body hydrodynamic interaction becomes important at the particle concentration of 6.5 vol% in monodisperse dispersions, and the many-body thermodynamic interaction is negligible at a low particle concentration, i.e., less than 15 vol%. The effect of many-body hydrodynamic interaction on the particle microstructure was also experimentally examined by using a nondestructive Kossel diffraction technique based on the principle of back-light scattering. It was found that the particle packing structure inside the dispersion initially becomes more ordered with the increase of the particle volume fraction. However, there is less increase in the particle ordering structure after 6 vol%. Furthermore, after the particle concentration reaches 10 vol%, the particle packing structure decreases to a value lower than that of 6 vol% due to the increased particle aggregation, as found in the sedimentation experiments. Predictions of a statistical thermodynamic model were compared with the experimental data on structure factors. It is found that particle dimerization occurs around 10 vol%, which agrees with the sedimentation results.

Effects of Functionalized Latex Particles and Anionic Surfactants on the Flow Behavior of Aqueous Gelatin Dispersions

The flow behavior of aqueous gelatin dispersions containing latex particles with different surface characteristics is studied as a function of the concentration of anionic surfactants. The work describes the nature and strength of interactions occurring in the system. The study of the effects of individual components may help to gain information on the flow behavior of commercial photographic color coupler dispersions. Due to interactions with the gelatin, the flow behavior of a dispersion of latex particles in 4.6% (w/w) aqueous gelatin solution depends strongly on charge and surface characteristics of the particles. For hard particles with nonionic hydrophilic shells, the flow behavior can be described by the theory of hard spheres. If anionic surfactants such as sodiumi-dodecylbenzenesulfonate (DBS) or sodium di-sec-butylnaphthalenesulfonate (BXG) are added, the rheological behavior is mainly determined by the interactions between the gelatin and the surfactants in the continuous phase. If soft particles with ionic shells are dispersed in the aqueous gelatin, the viscosity is more strongly increased than predicted by the hard sphere model. Upon further addition of a surfactant, the primarily pseudoplastic flow behavior becomes more and more Newtonian, presumably due to depletion of originally surface-adsorbed gelatin molecules. The stabilization of the latex is changed from steric to electrostatic. If large amounts of surfactant are added, depletion flocculation is most likely to occur. A model of the gelatin–latex–surfactant interactions is presented which takes into account the surface characteristics of the particles.

Latex film formation

Modern microscopy and spectroscopic techniques have made possible deep insights into the process of film formation from aqueous dispersions of latex particles and the evolution of the mechanical properties of these films. This knowledge is important for the design of high performance coatings that are friendly to the environment. Although all aspects of the mechanism of film formation have received active attention over the past several years, some of the most important recent advances have occurred in three areas. First, there have been detailed studies of the drying process, from which we have a deeper understanding of the ways in which water evaporates from a wet latex dispersion. Second, further information is available about the compaction process, leading to the formation of a void free film. Finally, there have been several new studies of the polymer diffusion process, particularly in films formed from structured latex.

Degree of dispersion of latex particles in cement paste, as assessed by electrical resistivity measurement

The degree of dispersion of latex particles in latex-modified cement paste was assessed by measurement of the volume electrical resistivity and modeling this resistivity in terms of latex and cement phases that were partly in series and partly in parallel. The assessment was best at low values of the latex-cement ratio; it underestimated the degree of latex dispersion when the latex cement ratio was high, especially > 0.2.

Microemulsions: A Medium for Polymerization Reactions

AbstractAqueous and nonaqueous microemulsions containing either of the monomers styrene or hexyl methacrylate polymerize when initiated with potassium persulfate. Reactions in both types of microemulsions lead to stable, bluish dispersions of latex particles with diameters ranging from 40 to 90 nm. By substituting formamide for water, one can investigate the effect of the monomer solubility in the continuous domain on the microemulsion polymerization mechanism in a way independent of the monomer type. When the monomer solubility in the polar domain is high, the latex particles are larger and have a larger number of polymer chains than when the monomer solubility is low. If the monomer solubility in the continuous domain is greater than the overall monomer concentration in the microemulsion, the reaction mechanism shifts from that of a microemulsion polymerization to one of a solution polymerization in the presence of empty micelles.

Dispersion of Latex Particles in a Nematic Solution. 2. Phase Diagram and Elastic Properties

We have prepared colloidal suspensions of 600 Å diameter latex polyballs in a lyotropic liquid crystal. The phase diagrams of the mixtures have been determined for a few values of the surfactant to water ratio. In the nematic phase, the particles remain suspended at high temperatures but undergo a gas−liquid transition at lower temperatures. The elastic constants and rotational viscosity of the mixtures in the nematic phase have been measured using the Freedericksz transition technique. They are found to be insensitive to the presence of the particles up to a few percent in the particle concentration.

Degree of Dispersion of Latex Particles in Cement Paste, as Assessed by Electrical Resistivity Measurement

The degree of dispersion of latex particles in latex-modified cement paste was assessed by measurement of the volume electrical resistivity and modeling this resistivity in terms of latex and cement phases that are partly in series and partly in parallel. The assessment was best at low values of the latex-cement ratio; it underestimated the degree of latex dispersion when the latex/cement ratio was high, especially > 0.2.

On the dispersion of latex particles in a nematic solution. I. Experimental evidence and a simple model

Latex polyballs are suspended in an isotropic micellar solution which exhibits a nematic phase at low temperatures. At the transition, we observe that the particles are either retained or expelled from the structured phase depending on their size. A mean field theory with two coupled order parameters is proposed to account for this behavior at the transition. The experimental and theoretical phase diagrams are in qualitative agreement.

Consistency of voltammetric metal speciation in latex particle dispersions

Differential pulse polarography (DPP) and, especially, differential pulse anodic stripping voltammetry (DPASV) were applied to the study of the binding of Cd(II) by polystyrene latex particles. Some complications that arose in previous voltammetric studies of different heavy metal + polystyrene latex systems at low concentrations of the metal ion were considered. The problem consisted basically of noticeable differences between the stability constants computed from current data and those obtained from potential data. The results show that the metal-to-ligand ratio is a key parameter for understanding such problems, and that these could be related to the presence of a small minority of strong complexing sites. Complications derived from association-dissociation kinetics and/or from non-excess of ligand also might be involved. The various possibilities are discussed and some qualitative considerations and general guidelines are given, especially to define criteria to test the reliability of voltammetric measurements on heavy metal + colloid systems.

Viscosity Behavior and Electroviscous Effect of Ionic Polymer Solutions.

Viscosity behaviors of aqueous solutions of sodium polystyrenesulfonate (NaPSS) and of latex dispersions were investigated with utmost care. Substantial shear-thinning effect was noted. The reduced viscosity of NaPSS solution (extrapolated to zero shear rate) increased with decreasing polymer concentration but after passing through a maximum it decreased. Considering this maximum, the intrinsic viscosity was estimated. The exponent α of the relation, [η]=K Mα, thus obtained was not two as widely believed, but was found to be 1.2-1.6, which implies that the conformation of the ionic polymer at infinite dilution is not rod-like, which contradicts to the widely accepted view. The maximum in viscosity curve was also observed for dispersions of latex particles, which do not undergo conformational change. Hence, the characteristic behavior of the viscosity of ionic polymer solutions is concluded to be not due to the conformational change of the polymer. The most important factors in the viscosity are the distortion of the ionic cloud around the polymer and/or the electrostatic interaction between ionic solutes.

Ordered micelle structuring in thin films formed from anionic surfactant solutions: I. Experimental

Reflected light micro-interferometry was used to observe stratification (i.e., the kinetics of layered structuring) in thinning foam films formed from micellar solutions of sodium n-alkyl sulfates at concentrations much lower than those at which liquid crystalline structures form in bulk solutions. The effects of surfactant concentration and chain length, added electrolyte, and capillary pressure on the step-wise jump transition thicknesses have been investigated. We have further conducted a model study with films formed from aqueous dispersions of latex particles. In similar fashion to those formed from micellar solutions, the thinning films changed thickness with regular step-wise jump transitions, and the films exhibited a number of metastable equilibria. These observations verify that the stratification of thin liquid films can be explained as a layer-by-layer thinning of ordered structures of micelles or colloidal particles formed inside the film. The stratification-nonstratification phase diagram is presented for anionic micelles and is similar to the order-disorder phase diagram for latexes. In the accompanying paper (Part II) we show that the phenomenon of stratification and ordered micellar structures is governed by the long range electrostatic repulsion beteen the ionic micelles and the restricted volume of the film. This present study provides evidence, for the first time, for the presence of structural forces in thin films with thicknesses on the order of 100 nm and strongly suggests that the free liquid film dynamics may be used as a probe to study the ordering in, and stability of, microheterogeneous systems.