The effect of metallic cations on the electrophoretic properties of a pretreatedHevea latex

High-ammonia latex concentrate prepared from doubly-centrifuged fieldHevea latex was exhaustively dialysed to remove any residual water-soluble non-rubber constituents. The electrophoretic mobilities of the dialysed latex in the presence of various metallic cations were investigated as a function of electrolyte concentration. The mobility decreased with increasing concentration of the cations Na⁺, Ba²⁺, Mg²⁺ and Ca²⁺ in a manner consistent with the effect of simple electrolyte on compression of the electric double layer. Anomalous behaviour was noted for the divalent ion copper, in that it reversed the charge of the latex particles at a concentration even lower than that of uranyl ions. Multivalent cations (lanthanum, cerium and thorium) had a profound influence on the latex particles where very low charge reversal concentrations were observed. It is believed that strong adsorption of hydrolysed species from the metallic ions was responsible for reversing the charge of the originally negative latex particles. These experiments indicated that the efficiency with which the cations reversed the charge of the latex particle surface was in the order: lanthanum > cerium > copper > thorium > uranyl > calcium > magnesium > barium > sodium. The number of cation binding sites on the latex particle surface and the chemical free energies of cation adsorption were calculated. It was found that the interaction of the latex particle with the hydrolysable metallic cations was much more stronger than that with the simple divalent cations and that this intercation was comparable to that of biological surfaces.     

Surfactant-free emulsion polymerization of styrene using crosslinked seed particles

The aim of this research was to prepare a monodisperse polystyrene latex without surfactants adsorbed at the particle surface. Conventional polymerization formulations usually lead to large amounts of oligomers. Furthermore, they are characterized by a low reproducibility with respect to particle size. This was overcome by using a seed latex that was crosslinked in order to overcome dissolution in the monomer phase. By adjusting the seed concentration, any desired particle size in the range 0.5–1.2 m could be obtained. The monodispersity was very good.     

Chemical heterogeneity in emulsion copolymers of carboxylic monomers

Copolymer latices of butylacrylate (BA) with acrylic and methacrylic acid (AA and MAA) were prepared by batch type emulsion polymerization, and, for comparison, copolymers with identical monomer composition were prepared by batch type solution polymerization.The distribution of the carboxylic monomers in the latex particles and the serum was studied by density gradient and sedimentation experiments with the analytical ultracentrifuge. Dynamic mechanical measurements of films of these copolymers were used to determine the storage and loss moduli as a function of temperature. From these measurements the position and extension of the glass transition range on the temperature scale is obtained. For heterogeneous emulsion copolymers with two glass transition temperatures the distribution of the carboxylic monomer units in the different copolymer phases can be determined. Electron microscopy of ultra thin cross-sections of stained films gave further insight into the film morphology.The combination of the results obtained with the different methods gives rise to the following clues: In the BA/AA latices about 40% (by weight) of the total AA used in the recipe are found in the serum as a water soluble polymer, about 50% are found to increase the glass transition temperatureT _g of the bulk of the BA copolymer and, therefore, are thought to be incorporated into the interior of the latex particles, and the remaining 10% are, conclusively, located on the particle surface.In the BA/MAA latices no water soluble copolymer could be detected in the serum, about 90% of the MAA used is found in the bulk of the copolymer, and about 10% form a second hard phase on the surface of the latex particles.Dynamic mechanical measurements on the copolymer latex films show at least two phases with different glass transition temperatures: the bulk of the copolymer with a relatively low content of (M)AA units and a glass transition range at low temperatures, and a second (M)AA rich phase with a highT _g.The latter phase forms a honeycomb-like structure surrounding the packed latex particles. That results in a three-dimensional network of polymer with a highT _g extending throughout the latex film. In spite of the fact that this phase is built from a small fraction of the total copolymer only, it has a very pronounced influence on the performance behaviour of latex films.Dedicated to Professor Dr. R. Manecke on the occasion of his 70th birthday.     

Conformational changes of polyglutamic acid adsorbed on cationic polystyrene as characterized by microelectrophoresis technique

Polypeptides (polyglutamic acid in sodium form) of three different polymerization degrees have been adsorbed on cationic polystyrene latex. The electrokinetic behavior of the complex PG/PS has been studied by microelectrophoretic mobility. The effect of ionic strength and pH on electrokinetic properties of the complex PG/PS have also been studied. It has been found that the appearance of maximum inu _e -logc curves is related to coverage degree of polystyrene surface and the polymerization degree of polypeptides chains. Likewise, the form of such curves depends markedly on pH. An attempt has been made to explain the data in terms of conformational changes of the adsorbed polypeptides chains.Presented at the NATO Advanced Study Institut on Polymer Colloid, Strasbourg (France), July 1988.     

Molecular properties of acetone and some of its chloroderivatives adsorbed at the surface of water

The relations between the electric surface potential (V) and the surface tension () of aqueous solutions of acetone, chloroacetone, 1,3-dichloroacetone, and their concentration were investigated. The vertical components of dipole moments of the above mentioned compounds were determined using the Helmholtz equation. The calculations were carried out on the basis of surface excess values, which were obtained from surface tension measurements and surface potential changes. Once the vertical component of dipole moments were found and the orientation of adsorbed molecules was assumed, the local dielectric permittivities of the surface monolayer were estimated.     

Some aspects of polymer colloids I. Preparation and properties of different types of latex particles

The boundary region separating a latex particle from the surrounding medium has a great influence on the properties of latex dispersions. Four types of polystyrene and polystyrene/comonomer latices differing greatly in the structure of the boundary region were prepared. The first part of a series of papers reports on the preparation of the various latex dispersions. Mean particle sizes were obtained from simple turbidity measurements, quasi-elastic light scattering, and electron micrographs. The behavior of the particles in the centrifugal force field is a simple tool for detecting aggregation tendencies that are not directly related to salt stability. The BET-surface area agrees with the area calculated from the mean particle size when a sharp boundary and smooth surface is developed between the particle and the surrounding medium. In the case of particles with extended boundary regions (core/shell particles or particles with hairy envelopes), film formation reduces the specific surface area. Removal of soluble oligomers and polymers from the boundary region during subsequent treatments (purification and centrifugation before freeze-drying) can increase the surface area considerably.     

Adsorption of anionic surfactants on positively charged polystyrene particles II

In the case of cationic polystyrene latex, the adsorption of anionic surfactants involves a strong electrostatic interaction between both the particle and the surfactant, which may affect the conformation of the surfactant molecules adsorbed onto the latex-particle surface. The adsorption isotherms showed that adsorption takes place according to two different mechanisms. First, the initial adsorption of the anionic surfactant molecules on cationic polystyrene surface would be due to the attractive electrostatic interaction between both ionic groups, laying the alkyl-chains of surfactant molecules flat on the surface as a consequence of the hydrophobic interaction between these chains and the polystyrene particle surface, which is predominantly hydrophobic. Second, at higher surface coverage the adsorbed surfactant molecules may move into a partly vertical orientation with some head groups facing the solution. According to this second mechanism the hydrophobic interactions of hydrocarbon chains play an important role in the adsorption of surfactant molecules at high surface coverage. This would account for the very high negative mobilities obtained at surfactant concentration higher than 5×10^–7 M. Under high surface-coverage conditions, some electrophoretic mobility measurements were performed at different ionic strength. The appearance of a maximum in the mobility-ionic strength curves seems to depend upon alkyl-chain length. Also the effects of temperature and pH on mobilities of anionic surfactant-cationic latex particles have been studied. The mobility of the particles covered by alkyl-sulphonate surfactants varied with the pH in a similar manner as it does with negatively charged sulphated latex particles, which indicates that the surfactant now controls the surface charge and the hydrophobic-hydrophilic character of the surface.Dedicated to the memory of Dr. Safwan Al-Khouri IbrahimPresented at the Euchem Workshop on Adsorption of Surfactants and Macromolecules from Solution, Åbo (Turku), Finland, June 1989     

Functionalization of colloidal silica and silica surfaces via silylation reactions

A series of trialkoxysilane compounds tipped with primary amine groups were used to functionalize the surfaces of glass and colloidal silica. Streaming potential and microelectrophoretic mobility measurements were used to monitor the stability of the functionalized surfaces.Hydrolytic breakdown of the surface-to-silane coupling was induced by either successively increasing and decreasing the pH of the solution in contact with the surface, or by aging the derivatised surfaces in aqueous solution over prolonged periods of time. The chemistry of the spacer units between the trialkoxysilane group and the primary amine tip had a major influence on the subsequent hydrolytic stability. Large hydrophobic spacer groups showed small changes in the electrokinetic properties on storage, but large changes when successively titrated with acid and base through the pH range. The behavior observed with small hydrophobic spacer groups was that large changes in electrokinetic properties were obtained on storage and with pH titration.     

Surfactant monolayers on a clean mercury surface: Apparatus and experiments

A Langmuir trough for studying monolayers on a mercury surface was constructed usingT. Smith's design. The surfactant (long-chain alkyl-trimethylammonium compounds) in aqueous solution were spread on a clean mercury surface in an atmosphere of helium, and the surface pressure re-areaA and thickness of surface filmd-areaA curves were obtained. The-A curves were characterized by the appearance of multiple inflection points and plateaus, being explained as stepwise dense surface packing of molecules, and the formation of multilayers by film compression with long axes of molecules lying flat on the mercury surface.     

Preparation of composite fine particles by heterocoagulation

To prepare regular composite particles comprised of organic and inorganic compounds, based on heterocoagulation theory, the properties of the mixture of small amphoteric latices (2a=250 nm) and large spherical silica (2a=240–1590 nm) were investigated as a function of pH, particle number ratio, particle size ratio and electrolyte concentration in the medium. It is apparent that under suitable conditions, we may prepare a stable mixed suspension comprising uniform composite particles, which are made up of many latices regularly adsorbed on silica surfaces, and each composite particle is undergoing Brownian motion as an isolated unit. This new composite particle is very stable for electrolyte, base and acid medium, and its surface charges (sign and magnitude) can be controlled by changing the pH of the medium.     

Dielectric properties of cleaned and monodisperse polystyrene latexes in microwaves

The dielectric behavior (, ) of three well-cleaned monodisperse polystyrene latexes having the same particle size and the same number of chemically-bound surface groups has been studied at a fixed microwave frequency (9.4 GHz), as a function of temperature and surface group (SO ₄ ^– , COO^–, OH).A large dielectric relaxation was observed in the sulfate-stabilized latex, which has the most polar surface end-group. The anomalous behavior in the thermal dependence of the hydroxyl and carboxyl-stabilized latexes (the OH latex being more pronounced than the COO^– latex) may originate from differences in the experimental conditions used for the preparation of such polymer colloids, or due to the presence of ionic species.On the basis of various dielectric models, the apparent volume fractions of the latexes were calculated. The amount of bound water around the latex particle was quantitatively correlated to the polarity of surface end-group (SO ₄ ^– > COO^– > OH). The differences between the calculated and actual values were not only a reflection of the thickness of vicinal water, but could also be indicative of the presence of oligomeric species in the suspension's medium (serum) of the latex. The permittivities of hydrated particle and of bound water were obtained with a non-linear iterative procedure.     

Characterization of wood fibers modified by phenol-formaldehyde

Wood-fiber phenol-formaldehyde-resin (PFR) modified surfaces, obtained from the adsorption of a PFR/water solution, are investigated as a function of the nature and the amount of PFR adsorbed. Surface are measurements are performed by using krypton adsorption at 77 K. Chemical modification is monitored by the electron spectroscopy for chemical analysis (ESCA) technique and the surface energy by the inverse phase gas chromatography (IPGC) method at infinite dilution. The London dispersive componentγ _S ^L of the surface energy shows a relationship to the concentration of carbon and oxgen at the fiber surface.γ _S ^L increases from 27.5 mN·m⁻¹ for the untreated fiber to 42.5 mN·m⁻¹ for the fibers treated with 20% high molecular-weight-grade phenol-formaldehyde. The surface atomic ratio O/C determined using the ESCA technique exhibits a decrease from 44% for untreated to 31% for treated samples. Surface area also decreases from 2.09 m²/g to 1.50 m²/g. The PFR adsorbed by wood fibers is observed as the dispersive component of surface energy starts to increase, as the surface oxygen concentration decreases, and on the surface area of the wood fiber.     

Electrokinetic study on concentrated suspensions using colloid vibration potential measurements

-potentials of a silica suspension and three types of polystyrene latex suspensions with different surface charge groups were measured, as a function of the particle concentration () in the suspension over a wide range, using the colloid vibration potential (CVP) technique. The concentration dependence of the-potential in silica suspension is explained well by Levine et al.s [1] cell model theory, verifying the applicability of the cell model to the CVP in silica suspension. However, the-potential of latex suspensions ordinarily decreases as the particle concentration increases, even after being corrected by the term of (1-). This tendency is especially noticeable in the systems that have particles with high surface charge densities. Furthermore, the conductivity measurements of these suspensions reveal that the conductivity of these systems, especially in their highly charged state, increases as the particle concentration is increased; opposite in tendency to silica suspensions. These new findings can be explained as follows: on the highly charged surface of a latex particle, a polyelectrolyte-like (hairy) layer is present, which overlaps at some point. This permits interparticle surface conduction and results in the abnormal behavior of CVP in these systems.     

Polystyrene(1)/poly(butyl acrylate-methacrylic acid)(2) core-shell emulsion polymers. Part I. Synthesis and colloidal characterization

Polystyrene (PS) (1)/Poly(n-butyl acrylate (BA)-methacrylic acid (MAA)) (2) structured particle latexes were prepared by emulsion polymerization using monodisperse polystyrene latex seed (118 nm) and different BA/MAA ratios. Three main aspects have been investigated: i) the polymerization kinetics; ii) the particle morphology as a function of reaction time; iii) the distribution of MAA units between the water phase and the polymer particles.The amount of MAA in the shell copolymer was found to be the main factor controlling the particle shape and morphology. The shape of the structured particles was, generally, non-spherical, and the shape irregularities increased as a particles was, generally, non-spherical, and the shape irregularities increased as a function of reaction time. At the beginning of the second stage reaction, new small particles were observed, which coalesced onto the PS seed as the polymerization proceeded. The distribution of the MAA groups in the latex particles and the serum was analyzed by alkali/back-acid titration, using ionic exchanged latexes. No MAA groups were detected in the latex serum. Due to the lowTg of the BA-MAA copolymers, alkali conductimetric titrations accounted for all the MAA groups on and within the polymer particles. Therefore, for these systems, this method is not only limited to a thin surface layer, as it is often assumed.     

Distribution of emulsified monomers with different water solubility

The location and distribution of acrylic acid and styrene in emulsions made with a cationic surfactant, cetyltrimethylammonium bromide (CTAB), or an anionic surfactant, sodium dodecylsulfate (SDS), were determined with ultra-violet spectroscopy, conductivity, and potentiometry. In these systems, the acrylic acid remains in the aqueous phase near the micelle surface, whereas the styrene is located in the micelles or in emulsified droplets. In the absence of acrylic acid, some of the styrene is solubilized in the micelle interior and some is adsorbed at the micelle inner surface. Upon addition of acrylic acid, all the styrene is displaced to the center of the micelles. The interaction between acrylic acid and CTAB micelles is stronger than that between acrylic acid and SDS micelles. With CTAB, acrylic acid is adsorbed at the micelle surface, whereas with SDS, acrylic acid remains in the intermicellar solution. These differences can account for the differences reported in the emulsion copolymerization of acrylic acid and styrene using CTAB or SDS.     

Participation of electrolyte cations in albumin adsorption onto negatively charged polymer latices

The participation of electrolyte cations in the adsorption of bovine serum albumin (BSA) onto polymer latices was investigated. The latices used were hydrophobic polystyrene (PS), and hydrophilic copolymers, i.e., styrene (St)/2-hydroxyethyl methacrylate(HEMA) copolymer [P(St/HEMA)] and styrene/acrylamide (AAm) copolymer [P(St/AAm)]. Three kinds of electrolyte cations (Na⁺, Ca²⁺, Mg²⁺) were used as the chloride. The amount of BSA adsorbed in every cation medium showed a maximum near the isoelectric point (iep, pH about 5) of the protein. The amounts of BSA adsorbed onto copolymer latices (except in the acidic pH region lower than the iep) were considerably smaller than that onto PS latex because of the steric repulsion and the decrease in the hydrophobic interaction between BSA and copolymer latices. In the acidic pH region, there was little difference in the amount of BSA adsorbed in every cation medium. However, in the pH region higher than the iep, the amounts of BSA adsorbed (particularly onto PS latex) in divalent cations (Ca²⁺ and Mg²⁺) media were relatively greater compared with that in a monovalent (Na⁺) one. This result was interpreted on the basis of the differences in such effects of electrolyte cations as dehydration power, suppression of the electrostatic repulsion, and binding affinity to BSA molecule. Ion Chromatographic estimation of the amounts of electrolyte cations captured upon BSA adsorption (in pH > 5) revealed that divalent cations were incorporated into the contact interface between the latex and BSA molecule so as to prevent the accumulation of anion charge and facilitate the protein adsorption.     

Viscoelastic properties of polyacrylonitrile particles stabilised by a poly-2-vinylpyridine/polytert butylstyrene block copolymer

The viscoelastic properties of a dispersion of polyacrylonitrile particles stabilised by a block copolymer poly-2-vinylpyridine/polytert butylstyrene dispersed in solvesso have been measured as a function of particle concentration and frequency at ambient temperatures. At low volume fraction of particles it was found that the loss modulus of the dispersions was larger than the storage modulus, whilst at volume fractions > 0.40 the storage modulus dominates the rheology. This is attributable to there being a steric repulsion between the particles as a result of an increasing concentration of particles and the resultant reduction in interparticle separation in the dispersion. In addition the observed exponential increase of the storage modulus with increasing particle volume fraction mirrors the exponential increase in force with decreasing surface separation of the same type of polymers adsorbed to mica.     

The effect of particle concentration on zeta potential in extremely dilute solutions

The electroporetic mobility of hexadecane particles in water and in very dilute CTAB solutions was measured. The technique of microelectrophoresis was applied. Zeta potential was calculated according to the Hückel formula, applying Henry's correction factors. Electrokinetic charge density was calculated according to the formula used by Delgado et al., and previously discovered empiricaly by Loeb et al. and derived mathematicaly by Ohshima et al. It was found that the particle concentration in emulsion limits their charge in solutions of very low ion concentration (10^–7:10^–6 M), because the greater the particle concentration, the smaller the extent of the ions adsorbed at the surface of one particle. The zeta potential was found to be independent of particle concentration if the ratio of the number of bulk ions to particle number is not lower than 1.6×10⁶. This ratio depends on particle size, and the value 1.6×10⁶ relates to particles of diameter 1.6 m.     

Mixed monolayers of polymethacrylic esters containing aromatic and aliphatic groups

The interfacial properties of mixtures of polymethacrylic polymers containing either aromatic or aliphatic side groups were studied at water-air interface in order to define the role of geometrical orientation on surface compatibility and the effect of aromatic interactions on ordered bidimensional systems. Two binary systems were studied: polyphenylmethacrylate/ polyhexylmethacrylate and polyphenylmethacrylate/polybenzylmethacrylate.Surface pressure and surface potential measurements were performed in the 288–303 K temperature range on the mixtures at different polymer concentrations. Further information was obtained from ellipsometric measurements and scanning electron microscopy of the collapsed material. The experimental results allow for the conclusion that both polymers containing aromatic groups are almost ideally miscible whereas mixtures of aliphatic and aromatic polymers are completely immiscible.     

SAXS study of core-shell colloids

Polymeric core-shell systems were produced by a two-stage emulsion polymerization technique under fixed conditions: i) monodisperse seed latex with a sufficiently high particle number; ii) starved monomer-II addition; iii) water-soluble initiator; iv) incompatibility of core and shell polymer. From electron micrographs, it is not possible to determine where the second polymer is located within these two-stage emulsion polymers. The internal structure of the particles can be detected by small-angle x-ray scattering. The results indicate that; i) the emulsion polymerization process takes place in a small surface layer region of the seed particles, and ii) a small interfacial layer exists between the core and shell polymer.Part 6 of Polymerizations in the Presence of Seeds