Aggregation phenomena in the suspension polymerization of VCM

In the suspension polymerization of VCM, insoluble polymer particles are formed inside the monomer droplets. The growth and aggregation of these particles are responsible for important polymer properties, such as porosity. It is well established that the most characteristic polymer particles, the primary particles, are of a narrow distribution with a size (diameter) ranging from 0.10–0.20 m. This work studied the formation of primary particles based on the aggregation phenomena that take place inside a monomer droplet. This was done by formulating a population balance equation, which was based on the following considerations: a) polymerization occurs in both the monomer and the polymer phases; b) there is continuous formation of the basic particles in the monomer phase; c) the growth of the polymer particles occurs as a result of both polymerization in the polymer phase and aggregation of the particles; d) the colloidal properties of the particles that are responsible for the aggregation phenomena were considered to be the net result of attraction and repulsion energies.It was shown that for particles carrying a constant charge it was not possible to predict the formation of primary particles of size 0.10–0.20 m. The particle size distribution had a mode diameter equal to the diameter of the basic particles. Consequently, the particle charge was allowed to vary in a way proportional to the particle radius raised to a power coefficient. For values of the coefficient greater than zero, i. e., when the particle charge increased during polymerization, the aggregation of the basic particles was efficient enough to result in the formation of large primary particles.     

Development of new hydrophilic polymer supports based on dimethylacrylamide

A survey of the development of hydrophilic resins based on dimethylacrylamide (DMA) is presented. It describes the preparation of DMA-based resins by suspension copolymerization of DMA and acrylamide-based crosslinking and functional monomers. Swellability data for a number of DMA-based resins are tabulated, demonstrating that the resins swell to approximately the same extent in dichloromethane as they do in water, methanol, dimethylformamide or dimethylsulphoxide, but they swell only slightly or not at all in tetrahydrofuran. Typical chemical reactions employed for quantitative estimation or derivatization of resin-bound functional groups, including those involving intra-resin crosslinking, are discussed. The development of composite polymer supports, in which the DMA-based polymer is supported on kieselguhr particles or an amphiphilic ion exchange polymer is supported on DMA-based resin beads, is also covered briefly.     

Formation of monodisperse poly(1-methacryloxybenzotriazole) particles by radiation-induced dispersion polymerization

Radiation-induced dispersion polymerization of 1-methacryloxybenzotriazole (MABt) in ethyl propionate starts as homogeneous mixture and the resulting polymer precipitates as spherical particles. Formation, size, and shape of polymer particles are strongly dependent on the initial monomer concentration. Three regions can be distinguished: formation of deformed particles at concentrations of 15 w/v-% MABt; formation of spherical particles at concentrations from 18 to 35 w/v-% MABt; no precipitation of polymer at concentrations of 40 w/v-% MABt. The spherical particles at 20 w/v-% MABt had a diameter of 0.54 ± 0.31 m for 3 kGy irradiation and 2.93±0.68 m for 30 kGy. The number distribution of the microspheres shows that the spherical particles with small sizes formed at low irradiation dose (low conversion) disappears with increasing irradiation dose because of multi-coating by newly produced polymer.     

Polymerization of acrylamide in nonionic microemulsions: Characterization of the microlatices and polymers formed

Free radical polymerization of acrylamide was carried out in nonionic microemulsions of water, an isoparaffinic oil, Isopar M and a blend of nonionic emulsifiers: a sorbitan sesquioleate and a polyoxyethylene sorbitol hexaoleate (HLB of the mixture: 9.3). The size and the stability of the latex particles formed after polymerization were studied as a function of monomer, emulsifier and electrolyte concentration. High emulsifier and high monomer contents favor obtaining high molecular weight polyacrylamides. It is shown that both the number of polymer chains contained in each latex particle and the size of the particles are essentially controlled by the acrylamide/emulsifier weight ratio.     

The preparation of monodisperse polyacrylonitrile particles in the presence of AB block co-polymers of poly-2-vinylpyridine/poly tertbutylstyrene

Radical dispersion polymerisation of acrylonitrile in cyclohexane was performed in the presence of the diblock copolymer poly-2-vinylpyridine/poly tertbutylstyrene, giving polymer latex particles stabilised by surface layers of chains of poly tertbutylstyrene. A seeded polymerisation route was employed and the addition of up to a further seven feeds of reactants resulted in the particle growth and the total volume fraction of particles increased to 0.2. The effect of varying the molecular weight of the stabiliser and stabiliser concentration was also investigated. Particle diameter in the range 40–150 nm were obtained depending on the condition of the polymerisation.     

Phase separation in colloidal dispersions with nonadsorbing polymer

The influence of the volume restriction effect on phase separation in colloidal dispersions consisting of a nonadsorbing polymer has been studied using statistical mechanical cell models and perturbation theories in combination with Monte Carlo experiments. The attractive interparticle potential proposed by Asakura and Oosawa is used to represent the effect of polymer depletion between the particles. The equilibrium properties are determined using the cell model for the solid-like phase (with Monte Carlo simulations for the structure of the solid phase) and a perturbation theory for the liquid-like phase. Both aqueous and nonaqueous dispersions are considered, and the phase separation behavior predicted is compared with available experimental and theoretical results.     

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     

Hydrogen bonds in unpolar matrix — Comparison of complexation in polymeric and low molecular-weight systems

By ene-reaction of 4-phenyl-1,2,4-triazoline-3,5-dione with double bonds, polar 4-phenyl-1,2,4-triazolidine-3,5-dione (phenyl urazole) groups are introduced into unpolar matrices. Hydrogen-bond complexes are formed between two amide-like units. The temperature dependence of complex formation in dilute hydrocarbon medium is obtained from remperature-dependent IR spectra in the region of the C=O stretching vibration. Results obtained for a low molecular-weight model system are compared with modified polybutadienes, where the groups are attached statistically along the polymer backbone. The enthalpy and the entropy of complex formation (ΔH _f =−28.6 kJ/mol; ΔS _f =−52 J/mol K⁻¹) obtained for the low molecular-weight model prove a dimeric chelate like complex involving two hydrogen bonds. The lower equilibrium constants observed for the groups attached to the polymer are related to additional topological constraints emerging from the polymer backbone.     

Polymer adsorption on fine particles; the effects of particle size and its stability

The effects of particle size on polyacrylamide (PAAm,M _w =59×10⁴, 500×10⁴) adsorption were investigated using a series of well-characterized hematite (-Fe₂O₃) dispersions. The -Fe₂O₃ particles with highly monodisperse and nearly spherical shape ranged in radius from 23 nm to 300 nm. the maximum amount of PAAm adsorption (M _m ) in each system, showed a steady increase with decreasing particle radius and was influenced strongly by particle concentrations in the medium. Furthermore, it was realized that the diameter of -Fe₂O₃ particles after treatment with PAAm under different particle concentrations decreased with increasing particle concentration. The relation between particle concentration in the medium and particle size after treatment was also influenced by the medium pH, i.e., at the medium pH close to the isoelectric point of -Fe₂O₃ particles (pH^o=9.2), the particle size after treatment increased with increasing particle concentration. All these results suggest that in the system of ultra-fine particles, the mixing process between particle-particle and polymerparticle will play an important role on the conformation of adsorbed polymer layer.     

On the collision efficiency approach to flocculation

The collision efficiency approach to flocculation is analyzed on the basis of a model describing the probability of aggregation by polymer bridging. The model allows for the effect of the randomness of distribution of polymer over particles as well as the effect of simultaneous coagulation and weak flocculation. It is argued that the collision efficiency approach is applicable to a rather limited range of experimental systems and provides a qualitative understanding rather than a quantitative theory of the relationship between flocculation and adsorption.     

Production of anomalously shaped carboxylated polymer particles by seeded emulsion polymerization

Anomalous polymer particles with a partial protuberance like octopus ocellatus were produced under alkaline conditions by seeded emulsion copolymerization for styrene and butyl acrylate, with styrene-butyl acrylate-methacrylic acid terpolymer emulsion as seed. The mechanism of production of the polymer particles was studied. By transmission electron microscopic observation of the particles at each conversion, it was observed that the anomalous polymer particles were produced by partial growth of each of the individual seed particles throughout polymerization. Ionization of the carboxyl groups and low viscosity in the growing particles during the process of polymerization were important factors for partial growth.Part CIII of the series Studies on Suspension and Emulsion.     

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     

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.     

Drug-carrying latices for prolonged release of drugs

Dichlorophene was used as a model drug. Drug-carrying latex particles were prepared by soap-free emulsion copolymerization of dichlorophene acrylate with some hydrophilic methacrylate comonomers. The comonomers used affected not only the structure and colloidal stability of particles, but also the drug-activity of particles. Preliminary study of the drug activity revealed that highly hydrophilic latex particles were very stable and too inactive to be engulfed by amoebae and kill them. On the contrary, less hydrophilic particles carrying a large amount of drug can be an effective device to release drugs at a moderate rate to fight against extracelluar targets such as parasites.     

Small-angle neutron scattering of colloidal silica dispersions in a non-polar solvent

Small-angle neutron scattering studies were performed on dilute dispersions of colloidal silica spheres in mixtures of h₁₂- and d₁₂-cyclohexane. The particles consisted of a SiO₂-core and a layer of stearyl alcohol molecules terminally attached with a chemical bond (Si-O) to the particle surface. The contrast variation method was applied to reveal the internal structure of the particles. The matchpoints determined with this method were in accordance with those calculated from the mass density of the particles and the atomic composition, as determined with elemental analysis.For a detailed interpretation of the scattering curves, we assumed that the particles were spherosymmetrical and consisted of two concentric layers. With the relation we derived between the radius of gyration and the reciprocal contrast for such a model, it was possible to determine all the parameters characterizing the particle in terms of this model. The model calculations performed using these parameters fitted very well to the experimental intensities for high contrasts. For lower contrasts, the fit was somewhat less good. This is probably due to random fluctuations in the scattering length density within the particle core.The different radii as found by neutron scattering, agreed very well with those determined using other techniques, such as light scattering (static and dynamic) and electron microscopy.     

Mechanical and morphological investigations of UHMWPE/Fe composites

Ultra-high molecular weight polyethylene/iron composites were investigated. The specimens were obtained by pressing in a steel die and sintering at different temperatures. By means of porosimetry, microscopy, microhardness, density, and partial volumes of the components it is shown that there are no microcavities. The microhardness does not depend upon the weight % content of the metal in the composites. It also neither depends on the pressure nor the temperature of sintering. For low metal content within the composites, microhardness Mayer equations are linear. For high metal content the dependence is nonlinear. With the increasing of the iron content tensile strength weakly decreases. However, plane-strain compression, dimension steadiness, Vicat softening temperature, and tribometric characteristics of the composites are improved. It is shown that the polymer is a well-dispersive medium. The particles of the components have a good mechanical compatibility. The polymer wets the surface of the iron; this is probably connected with the surface oxidation of the metal particles.     

The acceleration of decomposition of potassium persulfate in the presence of sodium dodecyl sulfate and polymer particles as a model of emulsion polymerization system

The decomposition rates of potassium persulfate (KPS) in aqueous solutions containing sodium dodecyl sulfate (SDS) in the presence of polystyrene or poly(methyl methacrylate) particles as models of emulsion polymerization systems were measured by isotachophoresis. Free SDS molecules dispersed in the monomolecular state had an ability to accelerate the KPS decomposition, but SDS molecules adsorbed onto the polymer particles did not accelerate it.Part CXX of the series of Studies on Suspension and Emulsion     

Double percolation effect on the electrical conductivity of conductive particles filled polymer blends

Electrical conductivity of carbon black (CB) filled polymer blends which are incompatible with each other was studied as a function of the polymer's blend ratio. Transmission electron microscope (TEM) analysis shows that CB distributes unevenly in each component of a polymer blend. TEM photographs of phase structure of solvent extracted HDPE/PMMA blend and solvent extraction experiments of PMMA/PP blend detect the blend ratio at which the structural continuity of filler rich phase is formed. The electrical conductivity of polymer blends is found to be determined by two factors. One is the concentration of CB in the filler rich phase and the other is the structural continuity of this phase. This double percolation affects the conductivity of conductive particle filled polymer blends.     

Interaction between an ion-penetrable particle and a solid particle. II. Criteria for heterocoagulation

The potential energy of the total interaction between two spherical colloidal particles of different nature is calculated, i. e., of an ion-penetrable particle and an ion-impenetrable solid particle having a constant surface potential or constant surface charge density. The criteria for heterocoagulation are derived. The obtained results suggest a possibility of selective coagulation in the mixed system.