Precipitation of nanoparticles in submicron emulsions induced by droplet coalescence

The precipitation of BaSO₄ is an in literature thoroughly discussed precipitation reaction. Here the precipitation of BaSO₄ by applying a miniemulsion technique is introduced. This technique involves the preparation of two stable water-in-oil submicron sized emulsions (miniemulsions) via high pressure homogenisation. N-decane presents hereby the continuous phase. The dispersed phases are water based BaCl₂- and K₂SO₄-solutions. Each emulsion contains one of the reactants within the water droplets. After blending the two precursor miniemulsions, the mixture is high pressure homogenised again. During high pressure homogenisation droplets coalesce which induces the formation of nanoparticles by precipitation. The high pressure homogenisation process is chosen in order to allow for controlled droplet-droplet coalescence. Parameters influencing droplet coalescence and particle synthesis such as the homogenisation pressure, the dispersed phase fraction and the precursor concentration are presented.     

Encapsulation of magnetic nickel nanoparticles via inverse miniemulsion polymerization

In this work, the encapsulation of magnetic nickel nanoparticles in polyacrylamide particles was performed via inverse miniemulsion polymerization. The dispersion of nickel nanoparticles was characterized in polar solvents including water, ethanol, and dimethyl sulfoxide using different stabilizers. The best results were obtained when the nonionic stabilizer poly(ethylene glycol) octadecyl ether (Brij 76) was used to stabilize the nickel nanoparticles in dimethyl sulfoxide. In addition, the block copolymer poly(ethylene‐co‐butylene)‐b‐poly(ethylene oxide) was used as a surfactant to create inverse miniemulsions while minimizing the coalescence of the miniemulsion droplets. Different types of salts such as zinc, nickel, and sodium nitrates were tested as lipophobes to retard Ostwald ripening. Transmission electron microscopy images of polyacrylamide/nickel particles synthesized with zinc and nickel salts as lipophobes indicate that nickel nanoparticles are embedded in the polyacrylamide matrix. Magnetization curves show that the saturation magnetization of polyacrylamide/nickel particles is only slightly below that of the pure nickel nanoparticles. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Synthesis of polyvinylpyrrolidone/silver nanoparticles hybrid latex in non-aqueous miniemulsion at high temperature

Since emulsions tend to become unstable with increasing temperature, reactions in emulsions are usually not performed at high temperatures. We show that non-aqueous inverse miniemulsions are relatively stable at temperatures higher than 150 °C. As an example of this remarkable stability, composite particles consisting of Ag nanoparticles and a polymer were synthesized via the polyol process in the monomer droplets to reduce silver ions to silver followed by the polymerization of the monomer. Silver nanoparticles were hence embedded in polyvinylpyrrolidone matrix particles. We showed here that non-aqueous miniemulsion nanodroplets are suitable loci for reactions performed at high temperature.     

Particle nucleation and growth in seeded semibatch miniemulsion polymerization of hybrid CeO2/acrylic latexes

CeO₂/acrylic hybrid coatings with high solids content and with nanoparticle percentages up to 5 wt% have been successfully synthesized by seeded semibatch miniemulsion polymerization process. The droplet nucleation efficiency has been assessed by Capillary Hydrodynamic Chromatography and TEM analysis. The effect of the stability of the miniemulsion, the type of initiator and the number of particles of the seed on the efficiency of the nucleation of the nanodroplets fed has been investigated. It was found that the less stable the hybrid miniemulsion, the higher the diffusion of the monomer out of the droplets and hence, the seed latex particles grew in size. However, the CeO₂ nanoparticles did not diffuse out with the monomer and remained in very small droplets that eventually nucleate leading to a bimodal population. When stable miniemulsions were produced by using a polymer as hydrophobe, droplet size increased reducing the number of particles in the seed and monomer diffusion was minimized enhancing nucleation of droplets with larger sizes that produced broad PSDs. Coalescence of droplets was negligible because the size distribution of the nanoceria particles did not change from the seed particles to the final latex. The UV–Vis absorption capacity of the films prepared with increasing the amount of CeO₂ increased, but scattering effects were observed at high loading of CeO₂ due to the large size of the CeO₂ aggregates.     

Water-based hybrid zinc phosphate–polymer miniemulsion as anticorrosive coating

A water-based hybrid zinc phosphate–polymer miniemulsion was synthesized via a multiple miniemulsion process. In this process zinc phosphate was synthesized in situ using homogenization of two inverse miniemulsions containing suitable precursors dispersed within a polymerizable continuous phase. Converting the resultant inverse miniemulsion into a direct miniemulsion, followed by polymerization, resulted in a water-based hybrid zinc phosphate, organic solvent free, which was used to coat steel panels via autodeposition. The coated panels were subjected to salt spray testing which showed better anticorrosion performance in comparison to an equivalent polymer coated panel prepared with a separate corrosion resistance step.     

Seeded emulsion polymerization of n-butyl acrylate utilizing miniemulsions

Nucleation of polymer particles in the seeded emulsion polymerization of n-butyl acrylate (BuA) was studied through experiments designed to control the amount of new particles formed. The results show that for the batch and semicontinuous seeded polymerization of BuA, a small amount of new particles was formed in the system in which the monomer was added neat, whereas a singificant amount of new particles was formed when the monomer was added as a miniemulsion. This suggests that new particles formed in the miniemulsion process were from nucleation of the monomer droplets. These experiments also showed that monomer-droplet nucleation decreased with increasing seed concentration in the reactor. For the seeded semicontinuous polymerizations, monomer-droplet nucleation decreases with decreasing BuA miniemulsion feed rate. The results also show that monomerdroplet nucleation takes place whenever miniemulsion droplets exist in the reactor. This study suggests that miniemulsions can be used to control the particle size distribution of a polymer latex system.     

Miniemulsion and conventional emulsion copolymerization of styrene and butadiene: A comparative kinetic study

The kinetics of conventional and miniemulsion copolymerizations of styrene and butadiene were compared using the Mettler RC1 calorimeter. A two‐step homogenization procedure was applied to obtain miniemulsions of these monomers with hexadecane as the costabilizer. The results indicated that the miniemulsion polymerizations proceeded mainly by nucleation in the monomer droplets, while in the conventional emulsion polymerizations, particle formation occurred by a combination of micellar and homogeneous nucleation. The overall rate of miniemulsion polymerization was faster than the corresponding conventional emulsion system if the surfactant concentration was below the critical micelle concentration (cmc) and slower if the surfactant concentration was above the cmc. The homogenization process is important for making stable miniemulsion systems, but had no effect on the conventional emulsion system (without hexadecane), most likely because of the second stage addition of the butadiene monomer. The dependencies of the rate of polymerization (heat of reaction) and number of particles on the surfactant concentration differed for the two types of polymerization systems. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 2304–2312, 2006     

Facile synthesis of one‐to‐one copy of monomer droplet to latex particle via equilibrium stabilized miniemulsion polymerization

Miniemulsion polymerization has been traditionally used to synthesize latex particles with a high homogenization energy to prepare an oil/water miniemulsion followed by further polymerizations. However, the exact copy of monomer droplets to latex particles depends critically on emulsion formulation, homogenization condition as well as the stability of the miniemulsified droplets after homogenization. In this study, we demonstrated experimentally for the first time that one‐to‐one copy of monomer droplets to latex particles can be synthesized via polymerization of a miniemulsion prepared from a less stringent preparation process including formulation without costabilizer and low homogenization energy. The criterion to obtain narrow size distribution of monomer droplets was established by equilibration of a low energy homogenized emulsion for different keeping time and the bulk homogenized emulsion subsequently phase separated into two layers. Top layer is the polydispersed monomer‐rich phase. The bottom layer is the equilibrium‐stabilized monomer droplet emulsion, in which the size distribution of droplets is narrow. The equilibrium‐stabilized emulsion is stable for days and subsequent polymerization exhibits nearly 100% droplet nucleation. Furthermore, the effect of surfactant/costabilizer and initiator on the one‐to‐one feature for the synthesis of latex particles was investigated in details. POLYM. ENG. SCI., 2009. © 2009 Society of Plastics Engineers     

Disruptive burning of precursor/solvent droplets in flame‐spray synthesis of nanoparticles

Flame spray pyrolysis is an established technique for synthesizing nanoparticles in the gas phase through aerosol combustion of precursor/solvent droplets. The combustion characteristics of isolated micron‐sized precursor/solvent droplets are investigated experimentally. Pure solvent droplets burn uniformly and classically quasisteady, whereas precursor/solvent droplets manifest disruptive combustion behavior. The fast onset of droplet disruption, which occurs only for solutions with dissolved metal precursors, is not due to solid‐particle precipitation within the droplet. Instead, the mechanism of disruptive droplet burning is similar to that of slurry droplets, consisting of three main steps: (1) diffusion‐controlled burning of the high‐volatile solvent, (2) viscous‐shell formation due to decomposition of the low‐volatile metal precursor, and (3) subsequent disruption due to heterogeneous nucleation. The time sequence of the three steps depends on the concentration and decomposition characteristics of the metal precursor, shortening with increased concentration and higher incremental decomposition temperature. © 2013 American Institute of Chemical Engineers AIChE J, 59: 4553–4566, 2013     

One-step fabrication of multihollow polystyrene particles from miniemulsion system with nonionic surfactant

This paper presented a new facile approach to fabricate polystyrene (PS) multihollow particles from miniemulsion polymerization. The surfactant used in this miniemulsion system was OP-10, a nonionic surfactant of nonyl phenyl polyoxyethylene with an average of 10 ethylene oxide units per molecule (hydrophilic-lipophilic balance, HLB = 13.9). Due to the partition characteristic of OP-10 in miniemulsion, W/O/W structured monomer droplets could be formed after ultrasonic dispersion. Under irradiation by γ-ray, styrene polymerized through a droplet nucleation mechanism, which was the feature of miniemulsion polymerization. The formation of multihollow structure was affected by the content of OP-10 (W_OP) and pre-added PS (W_PS). It was found that when W_OP was above 2 wt% and W_PS was in the region of 2-10 wt% based on the monomer, multihollow PS particles could be obtained. The molecular weight of the PS latex particles synthesized was determined to be 10⁵ g/mol by GPC.     

In situ exfoliation of graphite oxide nanosheets in polymer nanocomposites using miniemulsion polymerization

Poly(styrene-co-butyl acrylate) (poly(St-co-BA)) nanocomposite latices based on graphene oxide (GO) were synthesized by miniemulsion polymerization. The polymerization procedure involved dispersing an aqueous solution of graphite oxide in a monomer phase, followed by emulsification in the presence of a hydrophobe and a surfactant into miniemulsions. The focus was to investigate the suitability of miniemulsion for the synthesis of polymer nanocomposites based on a graphene derivative (i.e., GO) with exfoliated structure in a one-step nano-incorporation technique. Poly(St-co-BA) nanocomposites containing the exfoliated GO nanoplatelets, which have improved mechanical and thermal properties were successfully synthesized by the miniemulsion process. The nanostructure of the nanocomposites was investigated by transmission electron microscopy (TEM) and X-ray diffraction (XRD). TEM and XRD indicated that the nanocomposites mainly showed exfoliated morphologies, except at relatively high GO content. TEM also revealed that the nanocomposite latices had the so-called ‘‘armored’’ structure, where the nanosized GO sheets are distributed around the edges of the copolymer particles.     

Study of the preparation and mechanism of formation of hollow monodisperse polystyrene microspheres by SPG (Shirasu Porous Glass) emulsification technique

In a previous study, it was found that monodisperse polystyrene (PSt) hollow particles can be prepared under special conditions by combining a Shirasu Porous Glass (SPG) emulsification technique and subsequent suspension polymerization process. That is, a mixture of styrene (St), N,N‐dimethylamino ethyl methacrylate (DMAEMA), hexadecane (HD), and initiator N, N′‐azobis(2,4‐dimethylvaleronitrile) (ADVN) was used as the dispersed phase in an aqueous phase containing poly(vinyl pyrrolidone) (PVP), sodium lauryl sulfate (SLS), and water‐soluble inhibitor. The dispersed phase was created by pushing the oil phase through the uniform pores of an SPG membrane into the continuous phase to form uniform droplets. Then, the droplets were polymerized at 70°C. It has been puzzling that hollow microspheres were obtained only when sodium nitrite (NaNO₂) was used as a water‐soluble inhibitor, while one‐hole particles were formed when hydroquinone (HQ) or diaminophenylene (DAP) was used. In this study, the mechanism of formation of the hollow microspheres was verified by measuring the variation of diameter, molecular weight distribution, and monomer conversion, and by observing morphological changes during the polymerization, as well as by changing the type and amount of hydrophilic monomer, and initiator. It was found that the diameter of the oil droplets decreased, and a large amount of secondary new particles formed immediately after polymerization started in the case of NaNO₂. However, there was no such apparent behavior to be observed when HQ or DAP was used. It was determined that the hollow particles formed due to the rapid phase separation between PSt and HD, and as a consequence, a large amount of monomer diffused into the aqueous phase to form the secondary particles. Rapid phase separation confined the HD inside the droplets, a nonequilibrium morphology. On the other hand, one‐hole particles, representing an equilibrium morphology, formed when the phase separation occurred slowly because a lot of monomer existed inside of the droplets to allow mobility of the PSt. The addition of DMAEMA allowed the hollow particles to be formed more easily by decreasing the interfacial tension between the copolymer and aqueous phase. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 85: 1530–1543, 2002     

Magnetite‐coated polystyrene hybrid microspheres prepared by miniemulsion polymerization

Hybrid microspheres with a polystyrene core coated with magnetite nanoparticles were prepared by miniemulsion polymerization. Acrylic acid was used as a comonomer to promote the anchoring of the magnetite nanoparticles onto the polymeric surface. The addition of a hydrophobic agent prevents effectively the monomer from diffusing into the aqueous phase. Magnetite was treated with a silane coupling agent in order to introduce some interactions with the polymers. The morphology and the structure of the hybrid microspheres were characterized using X‐ray diffraction, infrared spectroscopy, transmission electron microscopy and thermogravimetric analysis. The results show that the morphology of the hybrid microspheres was influenced by the concentrations of acrylic acid, hydrophobic agent and surfactant, and that the degree of coating can be tuned by changing these parameters. The miniemulsion polymerization technique is adaptable to the synthesis of magnetite‐coated polymer particles, and the synthesis can be scaled up. Copyright © 2008 Society of Chemical Industry     

Effect of cationic surfactant on homogenization and polymerization of methyl methacrylate miniemulsion modified by chitosan

Homogenization of monomer droplets and polymerization of methyl methacrylate (MMA) miniemulsion, stabilized by a combination of cationic surfactants including different amounts of n‐hexadecyltrimethylammonium chloride (HTMA) and chitosan 100, were investigated. With increasing HTMA concentration or decreasing concentration of chitosan 100, the required ultrasonication time for the miniemulsion to reach a critically stabilized state is reduced. After polymerization, for higher HTMA concentrations, droplet nucleation dominates because of higher surface coverage of HTMA on the droplets. However, for lower HTMA concentrations, the size distributions (SDs) of final latex particles are different from those of the droplets. Because surface coverage of HTMA on the droplets is lower in this condition, both droplet nucleation and homogeneous and/or micellar nucleation occur simultaneously in the miniemulsion polymerization. On increasing the concentration of chitosan 100 in the system, the SD of the final latex particles is far different from that of the droplets. This is due to a decrease of the effective surface coverage of chitosan 100 and HTMA on the droplets. Consequently, the opportunity of collision coalescence of the droplets increases, and MMA molecules can diffuse out more easily into the aqueous phase. Droplet nucleation is, therefore, no longer the major source for the latex particles. Copyright © 2006 Society of Chemical Industry     

Miniemulsion polymerization of styrene with polymeric costabilizers

The polymeric costabilizers poly(stearyl methacrylate‐co‐2‐hydroxyethylmethacrylate) (PSH) and poly(lauryl methacrylate‐co‐2‐hydroxyethylmethacrylate) (PLH), composed of a hydrophilic backbone and several hydrophobic alkyl (stearyl or lauryl) side chains, were prepared by the free‐radical copolymerization of stearyl methacrylate (SMA) or lauryl methacrylate (LMA) with 2‐hydroxyethylmethacrylate and evaluated in the miniemulsion polymerization of styrene (ST). For comparison, the reactive costabilizers SMA and LMA were also included in this work. The hydrophobicity of costabilizers in increasing order was PLH < PSH < LMA < SMA. Only a small amount of these comb‐like copolymers was capable of producing kinetically stable ST emulsion droplets. The more hydrophobic the costabilizer was, the more effective was the costabilizer in the retardation of Ostwald ripening. About 30–40% of the monomer droplets were successfully converted into latex particles during the polymerization. The degree of monomer droplet nucleation increased with increasing hydrophobicity of the costabilizer. The formation of particle nuclei in the continuous aqueous phase played a crucial role in the polymerization kinetics. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 1961–1969, 2004     

CONTINUOUS MINIEMULSION POLYMERIZATION: KINETIC INVESTIGATIONS

Experimental data on the dynamic behavior of the miniemulsion polymerization of methyl methacrylate in a CSTR are presented. These data are contrasted with those for similar conventional emulsion polymerizations. The dominance of nucleation in monomer droplets in miniemulsion systems is shown to be responsible for the absence of the decaying oscillatory transients observed in continuous emulsion polymerization. Rates of polymerization for the miniemulsion system are reported to be twice those of the equivalent emulsion system. In addition, the polymer produced in this study is of high molecular weight, indicating a high degree of segregation within the polymer particles. These last two observations are attributed to the extremely small monomer droplets produced by the combination of the addition of a cosurfactant and the high shear to which the unpolymerized emulsion is subjected     

Batch polymerization of methyl methacrylate in mini/macroemulsions

The kinetics of the isothermal batch macroemulsion and miniemulsion polymerizations of methyl methacrylate (MMA) at 50°C have been studied. Hexadecane was used as the cosurfactant or swelling agent. The nucleation mechanisms were observed to be different between macroemulsions and miniemulsions. The effect of surfactant, cosurfactant, initiator, shear, and hold time on droplet nucleation was studied. The miniemulsion particles were found to contain more radicals on average than the macroemulsion particles using certain recipes. This resulted in higher polymerization rates for the miniemulsions at identical particle numbers. The latex-particle-size distributions were similar even though the mini-emulsion droplets start out with a high polydispersity of around 1.5. Miniemulsion latexes were found to be more stable under shear. Conductance of emulsions during polymerization was found to be a valuable on-line tool for investigating particle nucleation and growth. © 1993 John Wiley & Sons, Inc.     

Comparison of Commercially Available and Synthesized Titania Nano‐Additives in Composite HTPB/AP Propellant

This study presents a comparison of commercially available titania nano‐particles produced using electric wire explosion with nano‐particles manufactured by the authors using a sol‐gel synthesis process. For the present study, 20‐nm titania was purchased off‐the‐shelf. It was determined to be rough spheres of 20 nm forming large, micron‐sized agglomerates, whereas particles synthesized using the sol‐gel process were found to be fundamentally 10 nm but with sub‐micron agglomerations thereof. The nano‐titania was added to 80% AP monomodal propellants at 0.3% and 1.0% by mass. Additional, 85% bimodal‐AP mixtures were made, comparing commercial titania to the laboratory‐synthesized particles at 1.0% by mass. Another set of samples compared a method of pre‐mixing the synthesized additives directly into the binder material at 0.3% by mass of laboratory titania; two additional pre‐mixed titania batches at 0.5% by mass were doped with either Fe or Cu into the nanocrystals. All propellants were tested up to 13.8 MPa at 3.8 MPa increments. Dry powder laboratory additives show a 60‐to‐100% increase in burning rate over the baseline samples with no catalyst and a 20‐to‐30% increase over the commercial nano‐particles. Pre‐mixed additives were found to produce similar burning rate increases but with lower concentrations required. This latest generation of particle synthesis techniques was further demonstrated in this study to have great potential for future propellant catalyst development.     

Convenient method for removing and concentrating hydrophobic organic compounds from water with amphiphilic polymers

A stable fluoroacrylate copolymer emulsion was successfully prepared by miniemulsion polymerization with fluoroacrylate, lauryl methylacrylate, and methyl methacrylate as monomers. Extremely hydrophobic fluoroacrylate, instead of conventional cosurfactants, was used as a reactive cosurfactant to stabilize the miniemulsions. The results indicated that fluoroacrylate retarded Ostwald ripening and allowed the production of stable miniemulsions. The chemical compositions of the copolymer were studied with Fourier transform infrared and ¹H‐NMR. The average composition of the copolymers prepared with miniemulsions was in good agreement with the feed ratio according to ¹H‐NMR from the integration ratios corresponding to typical protons of the individual monomers. The particle size distribution and morphology of the latex particles were determined with laser particle analysis and transmission electron microscopy. The particle size of the latex underwent no change in the process of miniemulsion polymerization, but the particle size distributions were broader than those of conventional emulsion polymerization. The effects of various reaction parameters, including the temperature and concentrations of the emulsifier and initiator, on the miniemulsion polymerization were also investigated, and the polymerization rate and conversion increased with increasing concentrations of nonylphenol polyethoxylate (with an average of 40 ethylene oxide units per molecule), cetyltrimethylammonium, and 2,2′‐azobisisobutyronitrile. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 104: 641–647, 2007     

Study of compartmentalization in the polymerization of miniemulsions of styrene and butyl methacrylate

Different surfactant and costabilization systems were studied for the miniemulsion polymerizations of styrene (St) and of butyl methacrylate (BMA). It was found that the combination of sodium dodecyl sulfate, Triton X‐405, and octadecyl acrylate yielded excellent results. With this system all of the droplets initially present in the reactor polymerized. This stabilization system was then used in the study of compartmentalization of the monomer droplets in miniemulsion polymerization. Blends of BMA and St miniemulsions were prepared separately and polymerized together to study mass transfer in this system. It was observed that no matter is exchanged between the different phases. This compartmentalization was also demonstrated and exploited by producing a bimodal molecular weight distributions latex, achieved with a semicontinuous process. The lower MWD was created by adding a second miniemulsion containing a transfer agent. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 91: 115–124, 2004