Polyaddition reaction in a dispersed medium: elaboration of new core-shell polyurethane particles

The elaboration in a dispersed organic medium of calibrated polyurethane particles with a core-shell structure is presented in this paper. The objective could be achieved by using a series of reactive steric stabilizers of the type ω-(OH)_x-poly(n-butyl acrylate), -polystyrene, -polysiloxane or -polybutadiene (x=1 or 2) that play the role of surfmers during the polyaddition reaction between ethylene glycol and tolylene-2,4-diisocyanate, in cyclohexane as a dispersant medium. The final size of the polyurethane particles (0,5-10 μm) was found to be a function of the steric stabilizer characteristics (nature, molar mass and concentration) and of the addition procedure of the different reactants. These novel particles constituted of a polyurethane core and various shells depending on the stabilizer used exhibit specific and original properties.     

Effects of trace water on charging of silica particles dispersed in a nonpolar medium

This paper presents an investigation of the effects of trace water on the charging of silica (SiO(2)) particles dispersed in a nonpolar medium. There are a growing number of applications that seek to use electrostatic effects in apolar media to control particle movement and aggregation stability in such systems. One factor that is often overlooked in the preparation of nonpolar colloidal dispersions is the amount of water that is introduced to the system by hygroscopic particles and surfactants. The amount and location of this water can have significant effects on the electrical properties of these systems. For nonpolar surfactant solutions it has been shown that water can affect the conductivity, and it has been speculated that this is due to swelling of the polar cores of inverse micelles, increasing the fraction of them that are charged. Some studies have suggested that particle surface charging may also be sensitive to water content, but a clear mechanism for the process has not been fully developed. The situation with particles is further complicated by the fact that it is often unclear whether the water resides on the particle surfaces or in the polar cores of inverse micelles. The current work explores not only the effect of water content on reverse micelle and particle charging but seeks to differentiate between water bound to the particles and water located in the micelles. This is accomplished by measuring the solution conductivity and the electrophoretic mobility of silicon dioxide particles dispersed in solutions of Isopar-L and OLOA 11000. The water content is determined for both the dispersion and the supernatant after centrifuging the particles out. It is found that at equilibrium the majority of the water in the system adsorbs to the surface of the hygroscopic silica particles. In addition, the effect of water on particle electrophoretic mobility is found to be dependent on surfactant concentration. At small OLOA concentrations, additional water results in an increase in particle mobility due to increased particle charging. However, at large OLOA concentrations, additional water leads to a decrease in particle mobility, presumably as a result of increased electrostatic screening or neutralization. Thus, particle charging and electrophoretic mobility in an apolar surfactant solution are found to be highly sensitive to both the total water content in the system and to its concentration relative to the amount of surfactant present.     

Semibatch emulsion polymerization of methyl methacrylate using different polyurethane particles

Aqueous acrylic‐polyurethane (AC–PU) hybrid emulsions were prepared by semibatch emulsion polymerization of methyl methacrylate (MMA) in the presence of four polyurethane (PU) dispersions. The PU dispersions were synthesized with isophorone diisocyanate (IPDI), 1000 and 2000 molecular weight (MW) poly(neopentyl) adipate, 1000 MW polytetramethyleneetherglycol, butanediol (BD), and dimethylol propionic acid (DMPA). MMA was added in the monomer emulsion feed. We studied the effect of the use of different PU seed particles on the rate of polymerization, the particle size and distribution, the number of particles, and the average number of radicals per particle. The PU rigidity was controlled by varying the polyol chemical structure, the polyol MW (M_n), and by adding BD. The monomer feed rate was varied to study its influence on the process. It was observed that the PU particles that had been prepared with a higher MW polyol swelled better with MMA before the monomer‐starved conditions occurred. There seemed to be no significant discrepancies between the series with different PU seeds in the monomer‐starved conditions. The overall conversion depended on the monomer addition rate, and the polymerization rate acquired a constant value that was comparable to the value of the monomer addition rate. The instantaneous conversion increased slightly. The average particle size increased, and the total particle number in the reactor was constant and similar to the number of PU particles in the initial charge. The average number of radicals per particle increased. The differences between the system with a constant particle number and average number of radicals per particle and the system with a fixed radical concentration are discussed. The semibatch emulsion polymerization of MMA in the presence of PU particles studied was better compared to the system with a fixed radical concentration. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 844–858, 2005     

Synthesis of polyhedral particles by dispersion polymerization in supercritical carbon dioxide

The novel synthesis of polyhedral particles was attained by the dispersion polymerization of styrene in supercritical carbon dioxide using a polydimethylsiloxane-based macroazoinitiator as a precursor of the surfactant. The macroazoinitiator, VPS-1001, composed of poly(dimethylsiloxane) and 6-8 molecules of the azo groups served as a precursor of the surfactant for the dispersion polymerization by azobisisobutylonitrile as an initiator to produce 0.8-4 μm polyhedral particles. The size of the particles decreased as a result of increasing the VPS-1001 concentration. Too high a concentration of VPS-1001 caused coagulation of the particles. A decrease in the temperature increased the particle size and size distribution, while a decrease in the pressure produced particles with nonspecific shapes. An increase in the stirring rotation speed tended to increase the size and size distribution. However, too high a speed of rotation also caused coagulation of the particles.     

Synthesis of hydrophilic polyurethane particles in non-aqueous inverse miniemulsions

It is shown that it is possible to synthesize high molecular weight hydrophilic polyurethane particles by reacting either tolylene-2,4-diisocyanate or isophorone diisocyanate and oligoethylene glycol (M _n ∼200 g mol⁻¹) in non-aqueous inverse emulsions. This procedure offers the advantage that the formation of polyurea can be prevented in consequence of the absence of water in the emulsion. Apparent molecular weights of hydrophilic polyurethane as high as 19,000 g mol⁻¹ (M _n) were obtained.     

Synthesis of quantum dot-tagged submicrometer polystyrene particles by miniemulsion polymerization

Submicrometer fluorescent polystyrene (PS) particles have been synthesized via miniemulsion polymerization using CdSe/ZnS core-shell quantum dots (QDs). The influence of QD concentration, QD coating (either trioctylphosphine oxide (TOPO)-coated or vinyl-functionalized), and surfactant concentration on the polymerization kinetics and the photoluminescence properties of the prepared particles has been analyzed. Polymerization kinetics were not altered by the presence of QDs, whatever their surface coating. Latexes exhibited particle sizes ranging from 100 to 350 nm, depending on surfactant concentration, and a narrow particle size distribution was obtained in all cases. The fluorescence signal of the particles increased with the number of incorporated TOPO-coated QDs. The slight red shift of the emission maximum was correlated with phase separation between PS and QDs, which occurred during the polymerization, locating the QDs in the vicinity of the particle/water interface. QD-tagged particles displayed higher fluorescence intensity with TOPO-coated QDs compared to those with the vinyl moiety. The obtained fluorescent particles open up new opportunities for a variety of applications in biotechnology.     

Synthesis of needle-like crystalline particles by chemical-oxidative dispersion polymerization of xylidine

 Chemical-oxidative dispersion polymerization of 3, 5-xylidine was carried out in an aqueous medium with ammonium persulfate as an oxidant and polyvinylalcohol as a stabilizer. The polymerization proceeded smoothly at room temperature, resulting in a stable polyxylidine (PXy) dispersion. The produced PXy particles had two types of shapes: needle-like and spherical. Results obtained by electron diffraction and X-ray diffraction measurements indicated that the PXy needle-like particles had a crystalline structure. Received: 14 January 1997 Accepted: 03 September 1997     

Segmented polyurethane synthesized by frontal polymerization

Frontal polymerization (FP) is a mode of converting a monomer into a polymer via a localized reaction zone that propagates through the monomer. In this study, segmented polyurethane was successfully prepared by FP. The reactants, poly (propylene oxide) glycol, 2, 4-toluene diisocyanate and 1,4-butanediol and the catalyst stannous caprylate, were mixed together at an initial temperature in the presence of dimethylbenzene (as the solvent). The reactions were thermally ignited at one end of the tubular reactor, and the resultant hot fronts propagated throughout the reaction reactor. No further energy was required for polymerization to occur. The effect factors of front velocity, stannous caprylate concentration and temperature on the FP, along with comparison of FP with bulk polymerization, were thoroughly investigated. Fourier transform infrared and differential scanning calorimetry were employed to characterize polyurethane (PU). The polymer materials obtained by FP displayed features similar to those obtained by batch polymerization. The reaction time of FP for preparing PU was lower than that of BP.     

Synthesis of highly dispersed potassium hexacobaltinitrite in the ionic liquid medium

A method for synthesizing highly dispersed potassium hexacobaltinitrite K₃[Co(NO₂)₆] in an ionic liquid (IL) is discussed. The crystalline products are characterized by scanning electron microscopy, X-ray powder diffraction, IR and NMR spectroscopy, and laser diffraction.     

Synthesis of ZnO/polystyrene composites particles by Pickering emulsion polymerization

ZnO/polystyrene composite particles were synthesized by Pickering emulsion polymerization. ZnO nanoparticles were first prepared by reaction of zinc acetate and sodium hydroxide in ethanol medium. Then different amount of styrene monomer was emulsified in water in the presence of ZnO nanoparticles either by mechanical stirring or by sonication, followed by polymerization of styrene. Two kinds of initiators were used to start the polymerization, azobisisobutyronitrile (AIBN) and potassium persulfate (KPS). The X-ray diffraction pattern verified the crystal structure of ZnO and FT-IR spectra evidenced the existence of ZnO and polystyrene (PS) in ZnO/polystyrene composite particles. Different morphologies were observed for the composite particles when using different initiators. From TEM photographs, AIBN-initiated system produced mainly core-shell composite particles with PS as core and ZnO as shell, while KPS-initiated system showed both composite particles and pure PS particles. Two schemes of reaction mechanism were proposed to explain the morphologies accordingly. Both systems of composite particles showed good pH adjusting ability.     

Click chemistry step growth polymerization of novel alpha-azide-omega-alkyne monomers

A novel step growth polymerization A-B strategy based on the click chemistry polyaddition of tailor-made alpha-azide-omega-alkyne low molar mass monomers was developed, leading to polytriazole (co)polymers with tunable structures and properties.     

Preparation of polyurethane dispersions by miniemulsion polymerization

With a two‐step miniemulsion polymerization, hydrophobic polyurethane (PU) dispersions were prepared with a cosurfactant, the costabilizer hexadecane (HD) in the oil phase, and sodium dodecyl sulfate (SDS) in the water phase. The first step involved the formation of NCO‐terminated prepolymers between isophorone diisocyanate and poly(propylene glycol) oligomer in toluene. Next, PU dispersions were produced by a miniemulsion method in which an oil phase containing NCO‐terminated prepolymers, HD, the chain extender 1,4‐butanediol (BD), the crosslinking agent trimethylol propane (TMP), and the catalyst dibutyltin dilaurate was dispersed in the water phase containing SDS. The influence of experimental parameters, such as the ultrasonication time, concentrations of SDS and HD, and TMP/BD and NCO/OH equivalent ratios, on the sizes of the miniemulsion droplets and polymer particles, as well as the molecular weights and thermal properties of the PU polymer, was examined. The chemical structure of the produced PU polymer was identified with a Fourier transform infrared spectrometer. The molecular weight distribution and average particle size were measured through gel permeation chromatography and dynamic light scattering, respectively. The thermal stability of the PU polymer was characterized with thermogravimetric analysis. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 4870–4881, 2005     

Coloured particles by dispersion polymerization

Coloured particles have been prepared by dispersion copolymerization of colourless vinyl monomers and polymerizable anthraquinone dyes. The dyes were synthesized by condensation of methacryloyl chloride and amino- or hydroxy-substituted anthraquinones. This paper describes the formation by dispersion polymerization of coloured copolymer particles of diameter greater than 5 microns. The coloured particles were characterized by size, visible absorption spectra, and solution properties of the copolymers.     

Synthesis of large-scale,narrowly dispersed,highly cross-linked,and spherical latex particles via one-step emulsion polymerization through particle coagulation

Particle coagulation technology is a facile approach to prepare large-scale and narrowly dispersed polymer particles. However, diverse shapes such as ellipsolid, snowman, dumbbell, and trimer among others were obtained if the cross-linker was directly added into the initial reaction mixtures due to the restriction of the highly cross-linking particle fusion process. In this study, we prepared sub-200?nm, narrowly dispersed, highly cross-linked, and spherical latex particles using particle coagulation technology by controlling the relation between the cross-linking net formation and particle coagulation. Depending on the addition time or feeding rate of the cross-linker (divinylbenzene, DVB), the particles with different sizes or shapes were obtained. The later the addition start time of DVB, the narrower the particle size distribution of the latex particles. Alternatively, the increase of the continuing feeding time could also be used to decrease the width of particle size distribution of the ultimate latex. In addition, narrowly dispersed and spherical latex particles also could be directly obtained by advancing the particle coagulation time using 2, 2′-Azobis (2-methylpropionamidine) dihydrochloride as a cationic initiator. Our study presents a new method that will further widen the fields of application of particle coagulation technology.     

Synthesis of monodisperse anionic submicron polystyrene particles by stabilizer-free dispersion polymerization in alcoholic media

In this work, monodisperse polystyrene (PS) particles were synthesized in ethanol/water medium using sodium salt of styrene sulfonic acid and 2,2′-azobis(isobutyronitrile) as ionic comonomer and nonionic initiator, respectively. The polymerization was carried out at low agitation speed, and no stabilizer (or surfactant) was added to the polymerization medium. This polymerization system (stabilizer-free dispersion polymerization) was initiated as a homogeneous solution of monomer, comonomer, medium, and initiator. With the production of free radicals, polymerization developed into a heterogeneous system. The effect of various polymerization conditions on the size and size distribution of the obtained particles was evaluated. The experimental results showed that with an increase in ethanol content, the size of the particles increased while no significant change was observed in particle size distribution. Furthermore, with increasing ionic comonomer content, the size of the particles decreased and their size distribution became broader. Moreover, it was observed that addition of an electrolyte to the polymerization medium also increased the particles’ size and broadened their size distribution. It is noteworthy to point out that the coagulation occurred in higher amounts of electrolyte. Finally, it is concluded that the polar component of Hansen solubility parameter of the polymerization medium affects the particle size and particle size distribution greatly.     

Connection of poly(methyl methacrylate) microgel particles dispersed in poly(vinyl alcohol) matrix by seed polymerization

Poly(methyl methacrylate) microgels covered with poly(hydroxyethyl methacrylate) thin layer was dispersed in poly(vinyl alcohol) matrix. Homogeneous and regular arrangement of the microgel particles was suggested by Bragg diffraction for the films prepared by varying the PVA/microgel ratio (from 6/4 to 3/7 (w/w)). It was proved that the regular arrangement and connection of the microgels by seeded polymerization in poly(vinyl alcohol) were possible. © 1996 John Wiley & Sons, Inc.     

Synthesis of monodisperse, highly cross-linked, fluorescent PMMA particles by dispersion polymerization

We describe a facile method to synthesize sterically stabilized monodisperse fluorescent poly(methyl methacrylate) (PMMA) colloids in the polar solvent mixture water/methanol with either a core-shell or a homogeneously cross-linked structure by dispersion polymerization. The particles were sterically stabilized by the polymer poly(vinylpyrrolidone) (PVP). The morphology of the particles was controlled by varying the moment at which the gradual addition of cross-linker and dye was started. The absence of these extra agents at a time when the particle nuclei formed reduced the negative effects on this important process to a minimum and produced a core-shell structure, whereas an essentially homogeneously cross-linked fluorescent polymer colloid structure could be obtained by reducing the starting time of the addition of dye and cross-linker to zero. Three different dyes were chemically incorporated into the polymer network. Such dyes are important for the use of the particles in confocal scanning laser microscopy studies aimed at characterizing concentrated dispersions quantitatively in real space. A series of PMMA particles with different sizes were obtained through the variation of the weight ratio of solvents and the content of cross-linker. Furthermore, the swelling properties of the cross-linked PMMA particles in a good solvent (tetrahydrofuran) were investigated. The particles were stable in polar solvents (water and formamide) but could also successfully be transferred to apolar solvents such as decahydronaphthalene (decalin). The PVP stabilizer also allowed the particles to be permanently bonded in flexible strings by the application of an external electric field.