Poly(styrene-b-2-vinylpyridine-1-oxide) and poly(dimethylsiloxane-b-2 vinylpyridine-1-oxide) diblock copolymers. 1. Preparation and characterisation

Sequential anionic polymerisation routes have been used to prepare AB diblock copolymers, where A is either polystyrene or polydimethylsiloxane, and B is poly(2-vinylpyridine-1-oxide). The latter block, which is water-soluble, was obtained from the oxidation of poly(2-vinylpyridine) using peroxyacetic acid (giving 100% yield).The resultant diblock copolymers were characterised by gel-permeation chromatography, proton nuclear magnetic resonance and gravimetric microanalysis to give relative block lengths and polydispersity indices. For both types of block copolymersM _w /M _n values <1.25 could be readily obtained under carefully controlled conditions.     

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.     

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     

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.     

Preparation of micron-size monodisperse polymer particles having highly crosslinked structures and vinyl groups by seeded polymerization of divinylbenzene using the dynamic swelling method

Micron-size monodisperse polystyrene/polydivinylbenzene (PS/PDVB) composite particles having highly crosslinked structures and vinyl groups were prepared as follows. First, 1.9 m-size monodisperse PS seed particles produced by dispersion polymerization were dispersed in ethanol/water (70/30, w/w) solution which dissolved divinylbenzene (DVB) monomer, benzoyl peroxide as an initiator and poly(vinyl alcohol) as a stabilizer. The PS seed particles were swollen with a large amount of DVB monomers to 4.3 m in diameter while keeping good monodispersity by the dynamic swelling method, where water was slowly added, continuously, with a micro feeder into the dispersion. And then, the seeded polymerization of the absorbed DVB was carried out.Part CXXXV of the series Studies on Suspension and Emulsion     

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.     

Microscopic observation of ordered colloids in sedimentation equilibrium and the importance of Debye-screening length. 8. Unsymmetrical ordering and inclusion of anisotropic particles

Unsymmetrical crystal-like ordering of anisotropic-shaped particles are observed directly by a metallurgical microscope in sedimentation equilibrium and in deionized suspensions. The anisotropic colloids are polydispersed gold sols, rectangular tungstic acid, ellipsoidal poly(tetrafluoroethylene)(PTFE), aggregated (dimer, trimer, ...) polystyrene spheres, and bowl-shaped polymers. The distribution of the particles is analyzed by the two-dimensional distance distribution function. The interparticle distance continues to decrease as the initial concentration of the particles increases. Young's elastic modulus for the crystal-like structures is estimated to be 3.1 Pa for PTFE particles. The transformation from the unsymmetrical ordering to the symmetrical (hexagonal) array takes place as the concentration of diffusible ions decreases, which shows that the electrostatic interparticle repulsion and the elongated Debye-screening length around the particles play a key role. Furthermore, a bowl-shaped particle is observed to include a sphere in its cavity. This inclusional association is influenced greatly by the delicate changes in the effective sizes of the host and guest molecules containing the electrical double layers. These results show that electrostatic interparticle repulsion and the elongated Debye-screening length around the particles are essential for the ordering.     

Estimation of distribution of vinyl groups in micron-sized monodisperse polymer microspheres by bromine titration method

The distribution of vinyl groups in polystyrene/poly(styrene-divinylbenzene) (PS/P(S-DVB)) composite microspheres produced by seeded copolymerization of S and DVB in the presence of 2.1-m monodisperse PS seed particles was estimated from the amount detected with bromine titration method as a function of the reaction time under various conditions.Part CXXIII of the series Studies on Suspension and Emulsion.     

A new biodegradable polydepsipeptide microsphere for application in drug delivery systems

A sequential polydepsipeptide containing a tripeptide sequence L-alanyl-Lalanyl-ethyl L-glutamyl and an-hydroxy acid L-lactic acid, poly(Ala-Ala-Glu(OEt)-Lac), was synthesized to prepare the microspherical particles by the solvent evaporation process. In this case, the solvents play the most important role for the preparation of polydepsipeptide microspheres and, as an example, when 200 mg of the polydepsipeptide dissolved in 10 ml of 98/2% chloroform/dichloroacetic acid mixture was stirred at 400 rpm and 30 C, the microspherical particles with mean diameter of 58m were formed after pouring into 200 ml of 1% (w/v) poly(vinyl alcohol) solution. 17-Estradiol was incorporated into the particles, and the resulting particles were found to contain 5 mg of drug per 25 mg of the particle. The in vivo release of drug from the microspherical formulation was evaluated by measuring the pharmacological influence on rat prostate. It was found that the sufficient amount of drug, keeping the effective pharmacological influence, is supplied during the first 12-week period, followed by an incomplete supplying of drug intil the implant is perfectly degraded in vivo in the 25th week from the start of implantation.     

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.     

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.     

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.     

The influence of surfactants on latex flocculation with poly(diallyldimethyl ammonium chloride)

Aqueous latex was flocculated by mixtures of poly(diallyldimethyl ammonium chloride), PDADMAC, and anionic surfactants. Sodium dodecyl sulfate, (SDS), and Aerosol OT influenced flocculation whereas nonionic Tergitol NP-10 did not. The flocculation domains were correlated with properties of the polymer-surfactant complexFlocculation was never observed above the CMC of the corresponding surfactant solution without polymer or latex. At SDS concentrations greater than 10^–3.6 M the flocculation boundary corresponded to the first appearance of insoluble polymer-surfactant complex which was characterized by dynamic light scattering and microelectrophoresis. Under these conditions latex (diameter 570 nm) and dispersed polymer-surfactant complex particles (diameters between 30 and 2 000 nm) displayed simultaneous homo and heteroflocculation. The boundaries of the flocculation domains at low surfactant concentration were determined by the ratio of polymer to latex and by the net electrostatic charge of the soluble polymer-surfactant complex. On the other hand, the mechanisms controlling flocculation boundaries in the dispersed polymer-surfactant domain require further clarification.     

Non-ionic emulsion polymerisation

A process, similar to conventional emulsion polymerisation, is described by which sterically stabilised polymer dispersions are made in water alone. Ionic initiators and surfactants are replaced by non-ionic species, and a copolymerisable stabilising macromonomer is also used. The dispersions have good stability to solvent addition, high shear rates and freeze-thaw cycling; however, the effectiveness of stabilisation depends on the nature of both the monomers and the macromonomers.     

Aqueous dispersion of polyurethane ionomers from hexamethylene diisocyanate and trimellitic anhydride

Polyurethane (PU) ionomers were prepared from trimellitic anhydride (TMA), poly(tetramethylene adipate) glycol (PTAd), and hexamethylene diisocyanate (HDI) in acetone. Upon neutralizing the carboxylic groups with a tertiary amine (TEA), and adding water to PU ionomer solution, followed by removing the acetone, stable aqueous PU dispersions were obtained.Effects of interionic molecular weight and nonionic hydrophilic segment, viz. monofunctional ethylene-propylene oxide ether on particle, size, emulsion viscosity, mechanical, and viscoelastic properties of the emulsion cast films were examined.     

Estimation of polydispersity in polymers and colloids by field-flow fractionaion

Various methodologies of sedimentation, thermal, and steric field-flow fractionation for the estimation of the polydispersity in polymers and colloids are presented. These are based either on retention and/or on zone-spreading data. The reference materials used are nearly monodisperse and polydisperse submicron polystyrene and PVC latex beads, nearly monodisperse spherical particles of hematite, and polydisperse irregular particles of strengite (FePO₄·2H₂O) and PS polymers of various molecular weights. The results found are compared with those determined by other techniques or given by the manufacturers.     

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.     

Use of porous biodegradable polymer implants in meniscus reconstruction. 1) Preparation of porous biodegradable polyurethanes for the reconstruction of meniscus lesions

Porous biodegradable poly(urethanes) for reconstructing menisci have been prepared using two different combinations of techniques: freeze-drying/salt-leaching and in-situ polymerization/salt-leaching. Using these methods, homogenous porous materials with a controllable and reproducible morphology can be prepared. The materials were made of three different poly(urethanes): a methylenediphenyldiisocyanate-based polyurethane, a lysine diisocyanate-based poly(urethane), and a poly(-caprolactone)-based poly(urethane). The compressive stress-strain behavior of the Estane foams was determined. Foams made by the freeze-drying/salt-leaching technique implanted in dogs showed healing and good ingrowth of fibrocartilaginous tissue.     

The properties and behavior of supermolecular formations in aging aqueous poly (vinyl alcohol) solutions

The structure of supermolecular formations and the interparticle interaction in aqueous poly(vinyl alcohol) solutions aging at different concentrations were investigated. Integral light scattering was used to determine the single particle scattering functionP(K), the structure functionS(K), and hence the radial distribution functiong(r). Nonrandom arrangement of the supermolecular formations in solutions under study is much less intensive than that of particles in polymer latices.