Characterization of particle surface and morphology in vinyl acetate-butyl acrylate emulsion copolymers — Influence of the copolymerization pathway

Surface characterization was investigated in vinyl acetate (VAc) butyl acrylate (BuA) copolymer latexes of various compositions and prepared with four different emulsion polymerization processes: conventionnal batch, composition-controlled batch, core-shell, emulsifier-free semi-continuous. Surface end-groups (sulfate or carboxylic) titration results were first compared and discussed according to the type of process and as a function of conversion. As previously shown [1], it was confirmed that batch latex particles present a heterogeneous structure with a rich VAc outlayer, as in core-shell particles. As expected, semi-continuous and composition-controlled batch particles exhibit surface end-group characteristics revealing a more homogeneous distribution of both monomers within the particles. These differences in particle morphology were corroborated by analyzing water-polymer interface in these latexes using the soap titration method, with the sodium dodecyl sulfate (SDS) or sodium hexadecyl sulfate (SHS) as emulsifier probes. When the BuA was batch-polymerized onto PVAc seed particles, the estimated surface composition seemed to show that probably phase rearrangement occurs in the particle during the synthesis or upon aging. It was also confirmed that SDS displays an abnormal adsorption due to complexation and solubilization in the rich-VAc shell of the particles.     

Rheology and particle interaction of thickened latex

The pseudoplastic rheological properties of concentrated monodisperse polystyrene latexes with known sodium lauryl sulfate and methylcellulose surface coverages have been studied. It was assumed that the flow units of a concentrated thickened latex subjected to mechanical shear are “flocs” which comprise many particles with immobilized medium in the interstices. During shearing, the particle-particle bonds within the flocs undergo compression and stretching, sometimes breaking and reforming, causing the energy dissipation measured as the yield stress. A model was developed to calculate the average number of bonds per floc and this model was applied to an empirical modification of Firth and Hunter's elastic floc model to correlate the yield stress with the particle-particle separation pressure (defined as a measure of the interaction strength). It was found that the yield stress of a thickened latex is affected by the particle-particle interaction and the morphology of the particle flocs. The particle-particle interaction is affected by the surface coverage of thickener and emulsifier, and their concentrations in the aqueous phase, as well as other factors. The morphology of the particle flocs is affected by the particle interaction and the mechanical treatment. The adsorption of emulsifier and thickener, the rheology of the thickened latexes, the morphology of the particle flocs, and the particle-particle interactions, as well as their interrelationships, are described.     

Preparation of monodisperse latexes and study of their rheological properties

The synthesis of (monodisperse) poly(styrene) latexes was reexamined using sodiumdodecyl sulfate as emulsifier and potassium persulfate as initiator. It has been shown, that at fixed emulsifier concentration c_E the variation of persulfate amount produces similar particle size variations as changes of electrolyte (potassium sulfate) concentration at a given low initiator level. For c_E << cmc a maximum of particle size versus initiator or electrolyte concentrations has been found. Concentrated monodisperse poly(styrene) dispersions and carboxylated latexes up to 50% have been prepared by a carefully controlled seeding process using monomer feed. Particle size influence on steady shear viscosity has been studied for different kinds of polymer dispersions: for electrostatic stabilized poly(styrene) latexes at high ionic strengths, for electrostatic and sterically stabilized carboxylated latexes at different pH values and for steric stabilized dispersions of poly(vinyl chloride) in plasticizer (dioctyl phthalate). All three kinds of dispersions give pronounced particle size effects on viscosity, which is different from hard-sphere-behaviour. Simple model equations have been formulated to describe the concentration and shear rate dependence on viscosity. Particle size influence may be discussed on the basis of the effective hydrodynamic particle diameter. Causes for the increase of the hydrodynamic particle size are given either by surface swelling or by the formation of particle clusters which are formed and destroyed within the shear field.     

Electroviscous effect of moderately concentrated colloidal suspensions: Stern-layer influence

A previous model for the viscosity of moderately concentrated suspensions has been extended. The influence of a dynamic Stern layer (DSL), which produces an additional surface conductance at the electrolyte-particle interface, is included. The theoretical treatment is based on Happel's cell model with Simha's boundary conditions for the interparticle hydrodynamic interactions and on a dynamic Stern-layer model for ionic conduction on the particle surface according to Mangelsdorf and White (ref 39). The results are valid for arbitrary zeta potentials and double-layer thickness. Extensive theoretical predictions are shown and interesting new behaviors are found. The comparison with the results in the absence of additional surface conductance shows a great influence of this mechanism in the energy dissipation during the laminar flow of these suspensions. We conclude that the inclusion of a dynamic Stern layer will be required to match the predictions with the experimental results.     

Characterization of surface endgroups in polystrene latexes

The stability against electrolytes for all-sulfate, all-carboxyl, and all-hydroxyl, polystyrene model latexes followed the order, sulfate > carboxyl > hydroxyl when determined from an IR light scattering technique. Two types of hydroxyl endgroups were identified by ¹³C-NMR (chemical shift correlations, and model compound comparisons), for the polystyrene model latexes. One type was due to termination of growing chain with an oligomer with a single monomer unit, and the other due to termination of growing chains. The surface hydroxyl groups of an all-hydroxyl latex were derivatized with hexafluoroacetone, and quantified using ¹⁹F-NMR techniques. Good agreement with the indirect conductometric titration values were obtained.     

Hydrodynamic and Colloidal Interactions in Concentrated Charge-Stabilized Polymer Dispersions

Hydrodynamic and colloidal interactions are explored in concentrated, charge-stabilized colloidal dispersions by measuring the dependence of rheology (e.g., low and high-shear viscosity, high-frequency viscosity, and modulus) and self-diffusivity on salt content, particle size, and concentration. Model, sulfonated polystyrene lactices of varying diameter are prepared and investigated by shear rheology, high-frequency torsional resonance, electrophoresis, titration, and dynamic light scattering. The high-frequency and high-shear viscosity both are dominated by hydrodynamic interactions, but are shown not to be identical, due to the microstructure distortion resulting from high shear rates. The short-time self-diffusion is also shown to be insensitive to direct particle interactions, but has a different concentration dependence than the high-frequency viscosity, further illustrating a predicted violation of a generalized Stokes-Einstein relationship for these properties. The apparent colloidal surface charge is extracted from the high-frequency elastic modulus measurements on concentrated dispersions. The surface charge is in good agreement with results from critical coagulation concentration measurements and perturbation theories, but disagrees with electrophoretic mobility experiments. This indicates that the effective surface charge determined by torsional high-frequency measurements is a more reliable predicter of the salt stability of charge-stabilized dispersions, in comparison to zeta-potentials determined from electrophoretic mobilities. Further, we demonstrate by direct comparison that measurements of the apparent plateau modulus by rotational rheometry underestimate the true, high-frequency modulus and provide unreliable estimates for the surface charge. Copyright 2000 Academic Press.     

Interaction of a Cationic Polymeric Flocculant with Anionic Stabilizers in Latex

Turbidimetric titration of aqueous solutions of sodium dodecyl sulfate and Leukanol and coagulation titration of polystyrene and polybutadiene latexes containing various amounts of these surfactants are performed.     

Rheological behavior of silica suspensions in aqueous solutions of associating polymer

Associating polymers are hydrophilic long-chain molecules containing a small amount of hydrophobic groups. The aqueous solutions show viscoelastic responses above some critical concentrations because a three-dimensional structure is formed by association of hydrophobic groups. When the associating polymers are added to silica suspensions at low concentrations, the flocculation is induced by bridging mechanisms, and the flow of suspensions become shear-thinning. For suspensions prepared with polymer solutions in which the associating network is developed, the viscosity decreases, shows a minimum, and then increases with increasing particle concentration. The viscosity decrease may arise from the breakdown of associating network due to adsorption of polymer chains onto the silica surfaces. As the particle concentration is increased, the polymer concentration in solution is decreased, and finally, all polymer chains are adsorbed on the surfaces. Beyond this point, the partial coverage of particle surfaces takes place and strong interactions are generated between particles by polymer bridging. Since the stable suspensions are converted to highly flocculated systems, the viscosity is increased and the flow becomes shear-thinning. The concentration effect of silica particles on the viscosity behavior of suspensions can be explained by a combination of viscosity decrease in solution due to polymer adsorption and viscosity increase due to flocculation.     

Sedimentation velocity and potential in a concentrated suspension of charged liquid drops

The sedimentation behavior of a concentrated suspension of charged liquid drops is analyzed theoretically at arbitrary surface potential and arbitrary double-layer thickness; that is, the effects of double-layer polarization and double-layer overlapping are taken into account. Kuwabara's unit cell model is employed to model the suspension system, and a pseudospectral method based on the Chebyshev polynomial is adopted to solve the governing electrokinetic equations numerically. Several interesting phenomena, which are of significant influence if the internal flow inside a liquid drop is taken into account, are observed. Key factors are examined such as the thickness of the electric double layer, the magnitude of the surface potential, the volume fraction of liquid drops, and the viscosity of the internal fluid. The results presented here add another dimension to the previous studies, which include concentrated suspensions of rigid particles and mercury drops under low zeta potential, with the consideration of the internal flow of liquid drops and double-layer polarization, characterized by its viscosity and the zeta potential respectively. It is found, among other things, that the smaller the viscosity of the internal fluid is, the higher the sedimentation velocity of liquid drops. The higher the zeta potential is, the larger the decrease in sedimentation velocity. In particular, the sedimentation velocity of an inviscid drop (gas bubble) is about three times higher than that of a rigid one. The decrease in sedimentation velocity resulting from the effect of double-layer polarization achieves about 50% if the zeta potential is sufficiently high.     

CONCERNING THE ORIGIN OF CHARGE AT THE POLYSTYRENE PARTICLE/WATER INTERFACE

The electrophoretic mobility behavior of well-characterized polystyrene latex particles, carrying one type of surface functional endgroups, has been studied as a function of pH. At low pH, the interaction of protons with the functional endgroups increased in the order: Hydroxyl > carboxyl > sulfate; at high pH the order of interaction was reversed; and at intermediate pH no interactions were observed. The particles of the polystyrene latexes in their different forms at the intermediate pH range, dispersed in deionized water, all exhibited the same mobility irrespective of the functional endgroup. The origin of charge in these systems is explained as being the result of either the preferential adsorption of hydroxyl ions or an electron - injection mechanism due to the overlap of local intrinsic molecular - ion states in polystyrene and water. At low concentrations of functional endgroups, the surface properties of the polystyrene latexes are largely dependent upon the hydrophobic nature of the surface.     

Stability of dispersions of colloidal alumina particles in aqueous suspensions

The colloidal stability of suspensions of alumina particles has been investigated by measuring particle size distribution, sedimentation, viscosity, and zeta potential. Alumina particles were found to be optimally dispersed at pH around 3 to 7.8 without dispersant and at pH 8.5 and beyond with dispersant. The above results corroborate zeta potential and viscosity measurement data well. The surface charge of alumina powder changed significantly with anionic polyelectrolyte (ammonium polycarboxylate, APC) and the iep shifted toward more acidic range under different dispersant conditions. It was found that the essential role played by pH and dispersant (APC) on the charge generation and shift in the isoelectric point of alumina manifests two features: (i) the stability decreases on approaching the isoelectric point from either side of pH, and (ii) the maximum instability was found at pH 9.1 for alumina only and at pH 6.8 for alumina/APC, which is close to the isoelectric points for both the system, respectively. Using the model based on the electrical double-layer theory of surfactant adsorption through shift in isoelectric points, the authors could estimate the specific free energy of interaction (7.501 kcal/mol) between particles and dispersant. The interaction energy, zeta potential, sedimentation, and viscosity results, were used to explain the colloidal stability of the suspension.     

Application of the coagulation titration method for quantitative determination of the content of anion-active emulsifiers in latex

Possibility of fast quantitative determination of the content of emulsifiers of the alkyl sulfate type in latexes by their titration in a strongly diluted (1: 10⁴) state with a solution of a cation-active surfactant (cetylpyridinium chloride) was demonstrated. The titration end point is determined as the abscissa of the point of a maximum (cm) in the curve describing the dependence of the “minute turbidity” of the latex on the cetylpyridinium chloride concentration. The influence exerted by the length of the hydrocarbon radical in the cation-active surfactant on the value of cm at the titration end point was studied.     

Study on the interaction between anionic and cationic latex particles and Portland cement

The interaction between organic latex polymers and the surface of hydrating cement was investigated by measuring the zeta potential and adsorbed amount of polymer on cement. First, differently charged model latex particles were synthesized in aqueous media by well-known emulsion polymerization technique. The latex polymers were characterized by differential scanning calorimetry (DSC), dynamic light scattering (DLS) and environmental scanning electron microscopy (ESEM). Electrokinetic latex surface properties were investigated by means of streaming potential measurements using a particle charge detector (PCD). It is shown that the anionic latexes adsorb a considerable amount of Ca²⁺ from the cement pore solution. Next, adsorption of the latex polymers on the surface of hydrating cement was confirmed by zeta potential measurements using the electroacoustic method. A water to cement ratio in the cement paste as low as 0.5 was studied, representing actual conditions in mortar and concrete. Additionally, adsorption isotherms were determined in a sedimentation test using the depletion method. For all latex polymers, Langmuir type adsorption isotherms were found. The latex dosages required to achieve saturated adsorption on the cement surface obtained from zeta potential measurements correspond well with those determined in the sedimentation test. Electron microscopy photographs confirm that the charged latex polymers adsorb selectively on surface areas of hydrating cement showing opposite charge. This way, domains of organic latex polymers exist on the cement surface. They provide adhesion between the inorganic cement matrix and the organic polymer film formed later on by particle coalescence as a result of cement hydration and drying.     

Efficient synthesis of sterically stabilized pH-responsive microgels of controllable particle diameter by emulsion polymerization

Emulsion polymerization of 2-vinylpyridine (2VP) in the presence of divinylbenzene (DVB) cross-linker, a cationic surfactant, and a hydrophilic macromonomer, monomethoxy-capped poly(ethylene glycol) methacrylate (PEGMA), at around neutral pH and 60 degrees C afforded near-monodisperse, sterically stabilized latexes at approximately 10% solids. Judicious selection of the synthesis parameters enabled the mean latex diameter to be varied over an unusually wide range for one-shot batch syntheses. Scanning electron microscopy studies confirmed near-monodisperse spherical morphologies, with mean weight-average particle diameters ranging from 370 to 970 nm depending on the initiator, polymeric stabilizer, and surfactant concentrations. Particle sizing studies were also conducted using disk centrifuge photosedimentometry and dynamic light scattering and gave similar data. These lightly cross-linked latexes acquired cationic microgel character at low pH, as expected. The critical pH for this latex-to-microgel transition was around pH 4.1 at 1.0 wt % DVB, which is significantly lower than the pKa of 4.92 estimated for linear P2VP homopolymer by acid titration. 1H NMR and aqueous electrophoresis studies indicated that substantial swelling occurred at low pH due to protonation of the 2VP groups, while dynamic light scattering (DLS) studies indicated volumetric swelling ratios of up to 3 orders of magnitude, depending on the initial latex diameter. Systematic variation of the degree of cross-linking led to a monotonic decrease in the pKa values of the P2VP latexes (as judged by acid titration) and also the critical swelling pH (as judged by visual inspection). This was attributed to the increasingly branched nature of the P2VP chains in their swollen microgel form. Preliminary studies of the kinetics of acid-induced swelling were also conducted using the pH jump method in conjunction with a stopped-flow apparatus. These P2VP latexes swell significantly faster than P2VP latexes described in the literature and the characteristic time scales observed in the present study are much closer to those predicted by the Tanaka equation.     

Particle size measurements of heterogeneous film-forming latexes

Two-phase latex particles consisting of mainly polystyrene (PS) and polyisoprene (PI) in ratios varying from 7030 to 2080 were prepared using different polymerization techniques. Methacrylic acid (MAA) was used in small amounts as a comonomer. The latexes had narrow size distributions, and showed different particle morphologies depending on the monomer composition and the polymerization conditions used. In most cases the latexes were filmforming at room temperature. Particle size distributions and average particle sizes of the latexes have been determined using different particle sizing methods.Size determination by TEM was performed after staining with osmium tetroxide (OsO₄) or uranyl acetate (UAc). The staining methods showed no significant differences in particle size averages and particle size distributions when the ratio between the PI and PS phase did not exceed 5050. At higher phase ratios OsO₄ staining was preferred. The glass transition temperature of the PI phase increased after OsO₄ staining, which prevented deformation of the latex particles. Particle morphologies for the heterogeneous latex particles were also determined after OsO₄ staining.Particle sizes measured by TEM were generally smaller than the corresponding sizes measured by quasielastic light scattering (QELS). The difference in the measured diameters increased with increasing PI and PMAA content in the latex particles. The larger sizes observed by QELS are results of the presence of an immobilized water layer surrounding the particles in the aqueous environment, water absorption and swelling due to the presence of carboxylic acid groups, and adsorption of soluble carboxylated polymers forming a hydrophilic corona around the particles. By TEM the hard sphere diameters of dehydrated particles are measured.     

THE PREPARATION AND SURFACE CHARACTERIZATION OF AN IDEAL MODEL COLLOID

ABSTRACT

Monodisperse polystyrene latexes were prepared with and without emulsifier using persulfate initiator. The latexes were ion-exchanged with purified Dowex 50W(H⁺)-Dowex 1(0H^?) mixed resin to remove solute electrolyte and adsorbed emulsifier and to convert acidic surface groups to the H⁺ form. Conductometric titration showed that all latexes contained only strong-acid surface groups, presumably the sulfate endgroups of the polymer chains introduced by the persulfate initiator. The sulfate-stabilized latexes (H⁺ form) were hydrolyzed to the hydroxyl form after 2–8 weeks at room temperature or 48–120 hours at 363°K. The rate of the acid-catalyzed hydrolysis is enhanced by increasing the glass contact surface area, by adding Pyrex glass beads. Conductometric titration of the hydrolyzed latexes after ion exchange showed no titratable groups, indicating that the latexes were stabilized by hydroxyl groups. The latexes were oxidized to the carboxyl form by heating the hydroxyl-stabilized latex with persulfate and traces of heavy metal ions or the sulfate-stabilized latex in contact with Pyrex glass beads. For example, latex 520′ prepared without emulsifier using persulfate initiator and bicarbonate buffer contained only strong-acid surface groups after cleaning, no titratable groups after hydrolysis, and only carboxyl groups in the same number as the original sulfate groups after oxidation. Thus the three forms of the latex provide ideal model colloids, rigid monodisperse spheres stabilized with the same number of chemically bound surface groups of three types —strong-acid sulfate, weak- acid carboxyl, and uncharged hydroxyl.     

含丙烯酸的共聚乳液的碱增稠过程与流动行为

合成了一系列苯乙烯、丙烯酸丁酯和丙烯酸共聚乳液样品，对乳液的加氢滴定过程及其流变行为进行了讨论。结果表明，氨的加入使含羧基乳液的增稠效应随共聚物中羧基含量的增加和柔性链段的增多而增大；随着氨的加入量的递增，乳液的ｐＨ值和粘度多呈两次突变；乳液因聚合物组成和ｐＨ值不同而具有不同的流动特性。     

Water redispersible cellulose nanofibrils adsorbed with carboxymethyl cellulose

Cellulose nanofibrils (CNFs) are difficult to redisperse in water after they have been completely dried due to the irreversible agglomeration of cellulose during drying. Here, we have developed a simple process to prepare water-redispersible dried CNFs by the adsorption of small amounts of carboxymethyl cellulose (CMC) and oven drying. The adsorption of CMC onto CNFs in water suspensions at 22 and 121 °C was studied, and the adsorbed amount of CMC was measured via conductimetric titration. The water-redispersibility of dried CNFs adsorbed with different amounts of CMC was characterized by sedimentation test. Above a critical threshold of CMC adsorption, i.e. 2.3 wt%, the oven dried CNF–CMC sample was fully redispersible in water. The morphology, rheological, and mechanical properties of water-redispersed CNF–CMC samples were investigated by field emission scanning electron microscopy, viscosity measurement, and tensile test, respectively. The water-redispersed CNFs preserved the original properties of never dried CNFs. This new method will facilitate the production, transportation and storage, and large-scale industrial applications of CNFs.     

The pH dependence of dispersion of TiO2 particles in aqueous surfactant solutions

The pH dependence of dispersion of titanium dioxide (TiO₂) particles has been examined in the presence of surfactant molecules in water. Whereas particles were dispersed in water at acid and alkaline regions rather than at neutral region, the dispersion was enhanced at neutral region in an aqueous sodium dodecyl sulfate (SDS) solution and at acid and alkaline regions in an aqueous dodecyldimethylamine oxide (C₁₂DAO) solution. Considering the pH dependence of zeta potential, the adsorption models of surfactant molecules on a particle were estimated on the basis of the modes of hemimicelle and double-layer compression. While the particles that adsorbed Al³⁺ were remarkably dispersed around pH 6, their dispersion does not largely depend on pH in the addition of SDS, indicating the adsorption of SDS molecules to form double-layer compression in the whole pH region. Dynamic light-scattering measurement and electron microscopic observation suggested that the particles were dispersed in water as small flocs.     

Preaparation of cationic latexes of poly(styrene-CO-butyl acrylate) and their properties evolution in latex dilution

Cationic latexes were prepared through emulsion copolymerization of styrene(St) and butyl acrylate(BA) with a cationic surfactant,cetyl trimethyl ammonium bromide(CTAB).Latex properties,including particle size,size distribution,ζpotential,surface tension and monomer conversion,were determined for latexes prepared with different CTAB amounts. Evolution of these properties during emulsion polymerization was followed in order to understand the mechanism of the particles formation.Results showed that both particle size andζpotential were function of polymerization time and latex solids.Parallel emulsion polymerizations with cationic,anionic charged initiator and charge-free initiators were also carried out,the latex properties were determined at different polymerization time.All these results were attentively interpreted based on the mechanisms of emulsion polymerization,surfactant adsorption and latex particle stabilization.