Nonionic latices in aqueous media part 1. Preparation and characterization of polystyrene latices

A series of non-ionic polystyrene latices in aqueous media containing particles with a narrow size distribution have been prepared using a nonyl phenol poly(ehylene glycol) condensate as the surfactant, methoxy poly(ethylene glycol methacrylate) as the comonomer/stabilizer, and ascorbic acid/hydrogen peroxide as the initiator system. As a control synthesis for comparison with the above latex, a charge stabilized polystyrene latex was prepared, using an anionic surfactant and potassium persulphate as the initiator. Latices employing a combination of charge plus steric stabilization mechanisms were also prepared, in order to investigate the effect of the non-ionic surfactant and the comonomer/stabilizer. The particle size of the latices was measured by transmission electron microscopy, the surface charge density by conductimetric titration and the glass transition temperature of the polymer by differential scanning calorimetry. The latex prepared using non-ionic ingredients, showed no titratable charge and exhibited a profound lowering of the glass transition temperature, with respect to the charge stabilized latex. On the basis of these results, schematic models for the polymerization mechanism and the morphology of the latex particles are proposed.     

Studies on the preparation and characterization of monodisperse polystyrene latices. VI. Preparation of zwitterionic latices

The preparation of zwitterionic latex particles is reported by using a mixed anionic and cationic initiator system without requiring surface-active agents. Isoelectric points were found from microelectrophoresis experiments and were in the pH range of 3.5-5. Close to the isoelectric point, the latices coagulated as expected, and good stability was achieved outside this narrow range. This range of stability was in good agreement with predictions from current theory. Redispersion after coagulation was found to be difficult as was expected for a hydrophobic colloid. The electrokinetic behavior did not result in the maximum in zeta potential at an electrolyte concentration of 1 mM unlike the situation for other hydrophobic polystyrene latex particles, and hence these systems may be even better models for other colloidal studies.     

Studies on the preparation and characterization of monodisperse polystyrene latices V.: The preparation of cationic latices

Summary Methods are described for the preparation of monodisperse polystyrene latices using as initiators, 2-azo-bis-(2-methylpropamidinium) dichloride and 2-azo-bis-(2-isopropyliminazolium) dichloride. The preparations were carried out in the absence of any added surface active agents. The effect on particle size of varying initiator concentration, monomer concentration, temperature and ionic strength was examined and the conditions defined for preparing monodisperse latices, with particles in the size range 0.2 to 1m. Electrophoretic measurements confirmed that the latex particles were cationic. The evidence obtained suggested that the charged sites on the particle surface were amidinium groupings. The latices were not stable at pH values greater than 11 and prolonged heating at 90° also caused coagulation.

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Zusammenfassung Es wurden Methoden zur Darstellung monodisperser Polystyrol-Latices unter Verwendung von 2-azo-bis-(2-methylpropamidinium)-dichlorid und 2-azo-bis-(2-isopropyliminazolium)-dichlorid als Initiator beschrieben. Die Darstellung erfolgte in Abwesenheit von grenzflächenaktiven Verbindungen. Der Einfluß unterschiedlicher Initiator- und Monomerkonzentrationen, der Temperatur und der lonenstärke auf die Teilchengröße wurde untersucht und die Bedingungen festgelegt, unter denen monodisperse Latices mit Teilchen im Größenbereich von 0,2-1m erhalten werden. Elektrophoretische Messungen bestätigten, daß die Latexteilchen kationisch sind. Die Ladungen auf der Teilchenoberfläche sind Amidiniumgruppen. Die Latices waren bei pH-Werten über 11 nicht stabil; längeres Erhitzen auf 90° führte ebenfalls zur Koagulation.

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With 6 figures and 9 tables     

Preparation of monodisperse polystyrene latices

Summary A series of monodisperse polystyrene latices was prepared by using the direct and the seeding emulsion polymerization methods. The recipes for their preparation are given in full detail. Best results were obtained when using Aerosol MA (a surfactant of the sulfosuccinate series) as the emulsifier. The particle size distributions were evaluated from electron microscopic data. The average size of latex particles ranged from 200 to 950 m. The polydispersity ratios being very near unity and modal frequencies about 95% indicate a very high monodispersity of latex preparations. The reproducibility of the particle size distribution parameters was remarkably good. Significant deviation from theSmith-Ewart mechanism was found. A possible mechanism of the emulsifier action in the formation of monodisperse latices is postulated.

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Zusammenfassung Es wurde eine Serie von monodispersen Polystyrollatices dargestellt. Die experimentellen Bedingungen, unter welchen monodisperse Latices entstehen, wurden untersucht. Die Vorschriften sind genau angegeben. Mit Aerosol MA (einem oberflächenaktiven Mittel der Sulfosuccinatserie) als Emulgator wurden die besten Ergebnisse erhalten. Mittels Elektronenmikroskopie wurde die Verteilung der Teilchengrößen ermittelt. Die durchschnittliche Größe der Latexteilchen lag zwischen 200 und 950 m. Die Polydispersitätsquotienten lagen in unmittelbarer Nähe von 1, und die modalen Frequenzen um 95% indizierten eine sehr hohe Monodispersität der Latexproben. Die Reproduzibilität der Teilchengröße-Verteilung-Parameter war besonders ausgeprägt. Es wurde eine Abweichung vomSmith-Ewart-Mechanismus beobachtet. Der mögliche Mechanismus der Emulgatorwirkung bei Entstehung der monodispersen Latices wurde diskutiert.

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With 7 figures and 8 tables     

Preparation and characterization of detergent-free monodisperse polystyrene latices of small particle size

Summary The preparation has been described of detergent-free monodisperse polystyrene latices with particle diameters in the order of 200 nm. Conductometric titration shows the surface charge of the dispersed particles to consist mainly of carboxyl groups. Sulphate groups, if present, can be eliminated by oxidation with persulphate. The surface charge can be varied over a rather large range.Coagulation concentrations for K⁺ are found to be pH-dependent, unlike those for Mg²⁺. Coagulation with Ba²⁺ follows a somewhat capricious pattern, possibly due to specific interaction of these ions with the stabilizing surface entities.From the coagulation concentrations for K⁺ and Mg²⁺ and the corresponding-potentials, Hamaker constants for mutual attraction,A, have been calculated according to the DLVO-theory. TheseA-values show but little variation. Their average value,A = 9.9 × 10^–21 J, compares favourably with the outcome of calculations based on the Lifshits theory and with experimental values obtained for latices with larger particles that have been prepared in different ways.

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Zusammenfassung Es wird beschrieben, wie man tensidfreie Polystyroldispersionen mit Teilchendurchmessern in der Größenordnung von 200 nm herstellen kann. Konduktometrische Titrationen zeigen, daß die Oberflächenladung hauptsächlich von Carboxylgruppen gebildet wird. Oberflächensulphatgruppen können durch Oxidierung mit Persulphat beseitigt werden. Die Größe der Oberflächenladung kann ziemlich breit variiert werden.Die Flockungskonzentration für K⁺ ist pH-abhängig; die für Mg²⁺ aber nicht. Die Flockung mit Ba²⁺ benimmt sich ein wenig kapriziös, vielleicht weil spezifische Interaktion mit Oberflächengruppen stattfindet.Die Flockungskonzentrationen für K⁺ und Mg²⁺ ermöglichen, zusammen mit den entsprechenden-Potentialen, die Berechnung der Hamakerkonstanten für gegenseitige Attraktion,A, aus der DLVO-Theorie. DieseA-Werte sind sich ziemlich gleich. Ihr Mittelwert,A = 9.9 × 10^–21 J, verhält sich günstig zu denen, welche auf Grund der Lifshits-Theorie berechnet wurden, und zu experimentellen Werten für Latices mit größeren Teilchendurchmessern, die in anderer Weise hergestellt wurden.

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With 2 figures and 2 tables     

Compression studies on aqueous polystyrene latices

An apparatus, that allows simultaneous measurements to be made of excess osmotic pressure and optical diffraction of polymer colloid dispersions, has been constructed. Results are reported for monodisperse polystyrene latices at several salt concentrations. An interesting feature of the results is a clear indication of a co-existence region occurring with particles of 182 nm diameter in sodium chloride concentrations of 10^–4 mol dm^–3     

Differential thermal analysis of negatively charged polystyrene latices

To study vicinal water structure around polystyrene latex particles differential thermal analysis (DTA) has been utilized. Measurements of programmable water thermodesorption from polystyrene surfaces under dynamic and quasiisothermal conditions were made. Therefore, the adsorption potential distribution function was calculated and the hydrophobicity of polystyrene surfaces determined. DTA is presented as a useful method to study the interfacial properties of polymer colloids.     

Adsorption of different amphiphilic molecules onto polystyrene latices

In order to know the influence of the surface characteristics and the chain properties on the adsorption of amphiphilic molecules onto polystyrene latex, a set of experiments to study the adsorption of ionic surfactants, nonionic surfactants and an amphiphilic synthetic peptide on different latex dispersions was performed. The adsorbed amount versus the equilibrium surfactant concentration was determined. The main adsorption mechanism was the hydrophobic attraction between the nonpolar tail of the molecule and the hydrophobic regions of the latex surface. This attraction overcame the electrostatic repulsion between chains and latex surface with identical charge sign. However, the electrostatic interactions chain-surface and chain-chain also played a role. General patterns for the adsorption of ionic chains on charged latex surfaces could be established. Regarding the shape, the isotherms presented different plateaus corresponding to electrostatic effects and conformational changes. The surfactant size also affects the adsorption results: the higher the hydrophilic moiety in the surfactant molecule the lower the adsorbed amount.     

A study of some of the rheological properties of concentrated polystyrene latices

Monodisperse latices form highly ordered but liquid-like structured dispersions at moderate to high concentrations. The rheological measurements show such systems to be viscoelastic. Shear wave propagation and creep compliance measurements have been carried out as a function of volume fraction using polystyrene latices in dilute electrolyte. These measurements enabled the high frequency limiting modulus and the zero shear rate viscosity to be obtained.The data are compared with predictions from models based on the perturbation of ordered particle structures, the magnitude of the particle pair potentials and the restricted diffusive motion of particles in the structures.     

Studies of particle-particle interactions using polystyrene latices and time-average light scattering

Dilute dispersions of monodisperse negatively-charged polystyrene latex particles, radii 161 Å, have been examined by time-average light scattering at various latex volume fractions and electrolyte concentrations. The latter were varied from the low value produced by maintaining mixed bed ion-exchange resin beads in the systems (ca. 10^–5 mol dm^–3) to the value of 5×10^–3 mol dm^–3 obtained by the addition of sodium chloride. From angular scattering measurements determinations of the structure factors were made; these were produced as a consequence of the particle-particle interactions in the system. By extrapolation of the structure factor to zero scattering angle, values were obtained for the osmotic compressibility and hence the osmotic pressure of the systems as a function of the latex volume fraction. It was found that the experimental data obtained could be interpreted in terms of a hard-sphere model for the particle-particle interaction. Good agreement was obtained provided that the particles were assigned a hard-sphere radius which was determined by the electrostatic repulsion between the particles.     

Colloid chemical studies of polystyrene latices polymerized without any surface-active agents

Summary The influence of secondary minimum for the stability of spherical colloid has been studied using a series of soap-free polystyrene latices from 350 m to 1400 m in diameter.The critical flocculation concentration (c. f. c.) and the zeta-potential at the c. f. c. for each latex were determined with several salts of different ionic valencies. The values of c. f. c. varied with the particle size, reaching a maximum value at 750 m and then decreasing gradually with the diameter.Another phenomenon detected in the experiment was the complete reversibility of coagulation which was observed only in the cases of large size latices, i. e., the coagulates of large size particles were fully redispersed in the solution by stirring. The time-dependence of their sedimentation curves was reproducible.These experimental results could not be explained by the simple theoretical consideration for stability of suspension. The satisfactory explanation is given only using the term of secondary minimum of interaction potential, i. e., the origin of coagulation was shifted from the ordinary primary minimum to the shallow secondary minimum at about 750 m of the particle diameter.

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Zusammenfassung Der Einfluß des Sekundär-Minimum auf die Stabilität von Kolloiden wurde an seifenfreien Polystyrol-Latex-Suspensionen mit einem Durchmesser von 350 m bis 1400 m untersucht. Die kritische Flockungskonzentration (c. f. c.) und das Zeta-Potential bei c. f. c. wurde mit verschiedenen Salzen bestimmt.Der Wert von c. f. c. ändert sich mit der Teilchengröße.Ein weiterer wichtiger experimenteller Befund ist die vollständige Reversibilität der Flockung bei größeren Latices.Derartige Versuchsergebnisse können durch das Sekundär-Minimum befriedigend erklärt werden.

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With 7 figures in 10 details and 2 tables     

Dynamic light scattering for characterization of latices

With photon correlation spectrometry the diffusion coefficients of colloid particles in highly diluted aqueous suspensions can be measured and average diameters and polydispersities of the samples can be determined. Electrokinetic and electroviscous effects caused by polarization of the electrostatic double layer influence the diffusion of the particles. The adsorption of macromolecules at the interfaces of the particles results in an increase of the hydrodynamic diameter and a decrease of the diffusion and sedimentation coefficients. The hydrodynamic thicknesses of the polymer layers can be evaluated. The thickness values and their dependences on adsorbed amount and molar mass can only be interpreted by the existence of long tails of the adsorbed macromolecules dangling from the interface into the solution. The resulting conformation model is supported by the new theory of Scheutjens-Fleer. Special importance have those tails for the interaction of particles and their stability and flocculation in disperse systems.     

Titanium compounds as coatings on polystyrene latices and as hollow spheres

Sub-micron-sized anionic polystyrene latices have been coated with uniform layers of amorphous titanium dioxide by hydrolysis of titanium tetrabutoxide in ethanolic solutions containing the polymer cores. The thickness of the coating layer could be altered by adjusting the concentration of titanium tetrabutoxide and the amount of polymer latex added to the system. Hollow colloidal spheres of crystal titanium dioxide were obtained by calcination of the so-coated polystyrene latices at an elevated temperature. Received: 29 July 1999/Accepted in revised form: 15 October 1999     

Preparation and characterization of polystyrene latexes bearing disaccharide surface groups

Emulsifier-free polystyrene latexes covered with disaccharide species have been prepared by a seed (co)polymerization method in the presence of a hexylmethacrylate-terminated oligosaccharide 6-(2-methylpropenoyloxy)hexyl -d-cellobioside (CHMA). First, the surface activity of this macromonomer was studied using ring and drop volume methods, respectively. Then, its polymerization onto polystyrene seed latexes was carried out upon varying experimental conditions: particle size, solid contents of the seed and type of comonomer. Analysis of the macromonomer distribution between aqueous and polymer phases suggested that an optimal surface yield can be reached upon increasing the overall surface area of particle and or using methylmethacrylate as comonomer. Finally, the presence of the disaccharide residues at the particle surface was shown, either directly using NMR and ESCA analysis, or indirectly through the comparison of the electrophoretic mobility behavior of the seed polystyrene and functionalized latexes.