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     

苯乙烯-有机硅烷共聚物作为助稳定剂的细乳液聚合研究

以苯乙烯(St)与甲基丙烯酰氧基丙基三乙氧基硅烷(TEPM)的共聚物P(St-TEPM)为助稳定剂,用于不同亲水性单体的细乳液聚合.考察了P(St-TEPM)助稳定剂在不同亲水性单体(St、甲基丙烯酸甲酯MMA、醋酸乙烯酯VAc)的细乳液聚合过程中对单体转化率、聚合物粒子粒径的影响及成核机理.结果表明,P(St-TEPM)单独作为助稳定剂用于不同单体(St、MMA、VAc)的细乳液聚合,亲油性较好的St和亲水性较好的VAc的聚合转化率分别为90.6%和63.8%,聚合物粒子的最终数目和单体液滴的起始数目(N pf/Nmi )分别为1.06和0.10.通过以上分析认为P(St-TEPM)可以作为细乳液聚合的助稳定剂使用,亲油性单体St聚合机理以单体液滴成核为主体.     

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.     

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.     

The influence of monomer types on the colloidal stability in the miniemulsion copolymerization involving alkoxysilane monomer

Unlike conventional emulsion polymerization, monomer droplet nucleation becomes dominant in miniemulsion polymerization, offering the miniemulsion polymerization a great advantage over conventional emulsion polymerization when incorporating alkoxysilane monomer, which can easily undergo premature hydrolysis and condensation reactions, into polymer latex. The extensive premature hydrolysis and condensation can lead to the issue of the colloidal instability. In this article, the influence of monomer types on the colloidal stability in the miniemulsion co-(or ter-)polymerization was investigated when incorporating alkoxysilane monomer into styrene or acrylate latex. In the cases of butyl acrylate (BA)/γ-methacryoxypropytrimethoxysilane (MPMS), BA/methyl methyacrlate (MMA)/MPMS, and BA/styrene (St)/MPMS miniemulsion polymerization, nearly no coagulum was observed. The obtained latex had a long shelf life. However, the coagulum was formed in the late stage of MMA/MPMS and St/MPMS miniemulsion copolymerization. The shelf life of the corresponding latex was short. The selection of the main monomer, which can fast consume alkoxysilane comonomer, was critical to obtain the stable latex. In this way, the alkoxysilane groups were completely buried in particles thus the coagulation caused by condensation reactions derived from the alkoxysilane hydrolysis among particles was suppressed.     

Particle nucleation and growth mechanisms in miniemulsion polymerization of styrene

Styrene miniemulsion polymerizations stabilized by sodium lauryl sulfate in combination with a reactive costabilizer, lauryl methacrylate (LMA) or stearyl methacrylate (SMA), were studied. A small amount of extremely hydrophobic dye was incorporated into monomer droplets (10² nm in diameter) to investigate particle nucleation and growth mechanisms. In addition to monomer droplet nucleation, particle nuclei generated in the aqueous phase (homogeneous nucleation) also play an important role in both LMA‐ and SMA‐containing polymerization systems. The way that these two nucleation mechanisms compete with each other is closely related to the water solubility of the costabilizer (LMA > SMA). The fraction of latex particles originating from homogeneous nucleation increases with decreasing hydrophobicity of the costabilizer. Zeta potential data of latex particles and the molecular weight and molecular weight distribution of emulsion polymers provide supporting evidence for the proposed competitive particle nucleation and growth mechanisms. © 2002 Society of Chemical Industry     

Miniemulsion polymerization of vegetable oil macromonomers

The Thames Research Group developed vegetable oil macromonomer (VOMM) technology to combine the advantages of oil-modified polyesters and waterborne systems, and reduce volatile organic compounds in waterborne coatings. VOMMs offer the advantage of temporary plasticization with the potential for crosslinking after film formation. However, incorporating VOMMs into emulsions is challenging because the highly hydrophobic nature of VOMMs restricts their diffusion through the water phase. Miniemulsion polymerization has been used to incorporate highly hydrophobic monomers in waterborne systems. Diffusion limitations are avoided by polymerizing inside the monomer droplets, and to ensure this, droplet stabilization is required. In our study, a soybean oil-based VOMM was used as a copolymerizable hydrophobe in miniemulsion polymerization. Monomer droplets were stabilized prior to polymerization via catastrophic phase inversion to form stable and small droplets (100 nm). Dynamic light scattering analysis was used to confirm miniemulsion stability. A coagulum-free latex was obtained after polymerization. Surface tension studies and light scattering techniques were used to confirm that monomer droplet nucleation was the dominant mechanism. Gel content studies indicated the formation of a highly branched or crosslinked network upon film application. The miniemulsion technique permitted VOMM incorporation as high as 35 wt% into the polymer backbone.     

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.     

Kinetics of miniemulsion polymerizations of ethylene glycol dimethacrylate and/or methyl methacrylate

Two types of reactive acrylic microgel particles of methyl methacrylate (MMA)/ethylene glycol dimethacrylate (EGDMA) (66.4/33.6 and 0/100 (mol/mol)) were prepared by miniemulsion polymerization with 2,2′‐azobisisobutyronitrile as the initiator in the temperature range 60–80 °C. Ostwald ripening occurred during the very early stage of polymerization. In addition to the predominant droplet nucleation, homogeneous nucleation was also observed. The polymerization rate for MMA/EGDMA miniemulsion was higher than that for EGDMA miniemulsion. By contrast, comparable apparent limiting conversions were observed for the polymerizations of MMA/EGDMA and EGDMA. The mole fractions of MMA and EGDMA units incorporated into the copolymer product were estimated to be 0.62 and 0.38, respectively, for the polymerization of MMA/EGDMA at 70 °C. The mole fraction of the EGDMA unit containing one ? C?C? within the MMA/EGDMA microgel particle product was estimated to be 0.23, which was comparable to that (0.22) of the EGDMA unit containing one ? C?C? within the EGDMA microgel particle product. © 2015 Society of Chemical Industry     

Relationship between droplet size and fluid flow characteristics in miniemulsion polymerization of methyl methacrylate

Static mixer (SM) can be applied for emulsification, but the fundamental understanding of the nature of fluid flow and mixing in static mixers, is however poor. Droplet size is a very important parameter in miniemulsion systems and affects strongly the mechanism of particle formation in polymerization reactions. In this study, static mixer was used as homogenization device for emulsification of methyl methacrylate (MMA). Re number (Re) was obtained for SM inserted tube in different flow rates. It was demonstrated the nature of fluid flow was turbulent under our experimental conditions. The relationship between droplet size—the most important variable in our study—and Weber number (We) was investigated. The results showed that the ratio of the droplet size to the pipe diameter was fit as an exponential function with an order of −0.35. The polymerization of created droplets under certain We values by SM showed that it is possible to obtain a reasonable 1 : 1 copy of droplets to the particles. All these, indicate that using relationship between We and droplet size allow one to obtain acceptable condition of droplet nucleation in miniemulsion polymerization. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Miniemulsion copolymerizations of styrene and methyl methacrylate in the presence of reactive costabilizer

Miniemulsion stability of three‐component disperse phase systems comprising styrene [ST (1)], methyl methacrylate [MMA (2)], and stearyl methacrylate [SMA (3)] was investigated. The Ostwald ripening rate (ω) increases with increasing MMA content in the monomer mixture. The empirical equation 1 /ω = k(φ₁/ω₁ + φ₂/ω₂) + φ₃/ω₃ was proposed to adequately predict the miniemulsion stability data. The empirical parameter k was determined to be 555.77, and the Ostwald ripening rate (ω₃) and water solubility of SMA were estimated to be 8.77 × 10^?21 cm³/s and 1.90 × 10^?9 mL/mL, respectively. A water‐insoluble dye was used as a molecular probe to study particle nucleation mechanisms in the miniemulsion copolymerizations. In addition to the primary monomer droplet nucleation, homogeneous nucleation also plays an important role in the formation of particle nuclei, and this mechanism becomes more important for the polymerization systems with higher MMA contents as a result of the enhanced aqueous phase polymer reactions. The polymer composition data suggest that, during the early stage of polymerization, MMA is consumed more rapidly by free radical polymerization compared with ST. The final latex particle surface potential data also support this conclusion. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Acrylic latices stabilized by polymerizable non-ionic surfactant

The final latex particle size is controlled by the concentration of polymerizable non-ionic surfactant NE-40 in the emulsion copolymerization of methyl methacrylate (MMA) and butyl acrylate (BA). The particle size decreases with increasing NE-40 concentration and increases with increasing persulphate initiator concentration. The dependence of particle size on the initiator concentration does not follow conventional Smith–Ewart theory, which is attributed to the bridging flocculation process during the particle nucleation period. The differences in the particle nucleation and growth stages and colloidal stability observed in the NE-40 and nonyl phenol-40 mol ethylene oxide adduct (NP-40) stabilized systems can be attributed to the different distribution patterns of surfactant molecules in the particles. Experimental data also indicate that the particle size decreases with increasing electrolyte concentration, or agitation speed. The total scrap, presumably caused by the bridging flocculation process, increases rapidly with increase in the NaCl concentration The amount of large flocs formed during polymerization is generally greater for the run operated at higher agitation speed. As expected, the latex products stabilized by non-ionic surfactants show excellent stability toward added sodium salt.     

Predominant droplet nucleation in emulsion polymerization

Emulsions stabilized against diffusional degradation by incorporating a polymeric cosurfactant have been produced and polymerized. The presence of large numbers of small droplets shifts the nucleation mechanism from micellar or homogeneous nucleation, to droplet nucleation. When an efficient cosurfactant is used, this process is referred to as miniemulsion polymerization. Polymer, however, is known to be a poor cosurfactant. Its advantage is that, unlike most cosurfactants, it is innocuous in the recipe. Results indicate that even a poor cosurfactant (polymer) is adequate to stabilize small droplets against diffusional degradation long enough to nucleate them into polymer particles. The dependence of the concentration and molecular weight of the cosurfactant on the droplet size and distribution is investigated. Droplet diameters range from 19.5 to 141.2 nm with polydispersities of about 1.023. The polymeric cosurfactant is found to affect the mechanism of nucleation. On-line conductance measurements are used to successfully differentiate between nucleation mechanisms. The observed reaction rates are dependent on the amount of polymeric cosurfactant present. In addition, the latexes prepared with the polymeric cosurfactant have lower polydispersities (1.006) than either latexes prepared from classical emulsions (1.049) or from alkane-stabilized miniemulsions (1.037). © 1996 John Wiley & Sons, Inc.     

Nucleation mechanism and morphology of polystyrene/Fe3O4 latex particles via miniemulsion polymerization using AIBN as initiator

In this study, oil‐based magnetic Fe₃O₄ nanoparticles were first synthesized by a coprecipitation method followed by a surface modification using lauric acid. Polystyrene/Fe₃O₄ composite particles were then prepared via miniemulsion polymerization method using styrene as monomer, 2,2′‐azobisisobutyronitrile (AIBN) as initiator, sodium dodecyl sulfate (SDS) as surfactant, hexadecane (HD) or sorbitan monolaurate (Span20®) as costabilizer in the presence of Fe₃O₄ nanoparticles. The effects of Fe₃O₄ content, costabilizer, homogenization energy during ultrasonication, and surfactant concentration on the polymerization kinetics (e.g., conversion), nucleation mechanism, and morphology (e.g., size distributions of droplets and latex) of composite particles were investigated. The results showed that at high homogenization energy, an optimum amount of SDS and hydrophobic costabilizer was needed to obtain composite particles nucleated predominately by droplet nucleation mechanism. The morphology of the composite particles can be well controlled by the homogenization energy and the hydrophobicity of the costabilizer. The magnetic composite particles can be made by locating Fe₃O₄ inside the latex particles or forming a shell layer on their PS core surface depending on the aforementioned polymerization conditions. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Monte Carlo Simulation of Droplet Nucleation in RAFT Free Radical Miniemulsion Polymerization

Abstract

The early stages of the reversible addition/fragmentation transfer (RAFT) miniemulsion polymerization were simulated, focusing on the effect of the RAFT agent on droplet nucleation. For highly reactive RAFT agents, a large number of free radicals (N_c ) needed to be captured by a droplet in order to initiate polymerization in the droplet, which was totally different from the behavior of regular miniemulsion polymerization. More interestingly, it was found that droplet size had a significant influence on N_c value. It was shown that the RAFT agent has a significant influence on miniemulsion polymerization, leading to long induction periods and retardation of polymerization. In addition, miniemulsion droplets with different sizes are nucleated at different times, which could lead to very low nucleation efficiency. The results would be very helpful in understanding and designing a RAFT miniemulsion polymerization system.     

Effects of compatibilizing agents in poly(n-butyl acrylate)/poly(methyl methacrylate) composite latexes

Graft copolymers with poly(n-butyl acrylate) (PBA) backbones and poly(methyl methacrylate) (PMMA) macromonomer side chains are used as compatibilizing agents for PBA/PMMA composite latexes. The composite latexes are prepared by seeded emulsion polymerization of methyl methacrylate (MMA) in the presence of PBA particles. Graft copolymers were already incorporated into the PBA particles prior to using these particles as seed via miniemulsion (co)polymerization of n-butyl acrylate (BA) in the presence of the macromonomers. Comparison between size averages of composite and seed particles indicates no secondary nucleation of MMA during seeded emulsion polymerization. Transmission electron microscopy (TEM) observations of composite particles show the dependence of particle morphologies with the amount of macromonomer (i.e., mole ratio of macromonomer to BA and molecular weight of macromonomer) in seed latex. The more uniform coverage with the higher amount of macromonomer suggests that graft copolymers decrease the interfacial tension between core and shell layers in the composite particles. Dynamic mechanical analysis of composite latex films indicates the existence of an interphase region between PBA and PMMA. The dynamic mechanical properties of these films are related to the morphology of the composite particles, the arrangement of phases in the films, and the volume of the interphase polymer. © 1997 John Wiley & Sons, Inc.     

Miniemulsion copolymerization of styrene and butyl acrylate initiated by redox system at lower temperature: Reaction kinetics and evolution of particle‐size distribution

The kinetics of the miniemulsion copolymerization of styrene (St) and butyl acrylate (BA) initiated by redox initiators, (NH₄)₂S₂O₈/NaHSO₃, at lower temperature (45°C) was studied. The polymerization rate in miniemulsion copolymerization is lower than that of the corresponding conventional emulsion copolymerization. In regard to the rate of polymerization, the initiator concentration plays a more important role in miniemulsion copolymerization than in conventional emulsion polymerization, while the surfactant concentration has a more important role in conventional emulsion polymerization than in miniemulsion polymerization. These are attributed to their different nucleation mechanisms, which are the same as those found in the miniemulsion polymerization carried out at higher temperatures. While by eliminating nucleation via micelle and ensuring against homogeneous nucleation, miniemulsion polymerization can be carried out by the sole nucleation mechanism—monomer droplet nucleation—at lower temperature. Because of this, the particles become narrower during the polymerization and, finally, monodisperse polymer particles are obtained. The result of the particle numbers indicated that a continuous nucleation will cease at about 60% conversion. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 73: 315–322, 1999     

Tracking the fate of seed particles in dispersion polymerization: Preparation and application of fluorescent polymer particles

The mechanism of seeded dispersion polymerization of methyl methacrylate (MMA) was investigated by employing submicron fluorescent polymer particles as seed. These poly(methyl methacrylate) latex particles, containing fluorescent material, were synthesized by a two‐step miniemulsion polymerization process and then applied in the seeded dispersion polymerization of MMA. The seed particles were located by tracking the fluorescent signal in the micron‐size final particles. The analysis of the final particles showed that most of them contained more than two seed particles. On average, there were 3.7 seed particles in each final particle as obtained under the given conditions of the seeded dispersion polymerization. The location of the seed within the particles being well‐separated from each other was considered to indicate that the aggregation of the particles did not occur immediately, but took place after some particle growth had first taken place. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Identification of nucleation loci in emulsion polymerization processes. I. New information from spectroscopy studies

Particle nucleation is the forcing function in emulsion polymerization reactions and as such it plays a significant role in the development of most of the properties of the final latex. The locus of nucleation in emulsion polymerization remains a contentious issue. Recent developments in the spectroscopy of emulsions strongly suggest that the locus of particle nucleation is a population of small nano‐droplets of size range between 30–100 nanometers in diameter. These nano‐droplets are generated independently of the rate of initiator decomposition and appear to be functions only of the emulsification conditions. In this paper the simulation studies leading to the identification of the nano‐droplet population are described. The theoretical evidence suggesting that the nano‐droplets are the main loci of particle nucleation is presented and along with the recommendations for the experimental work. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 2847–2857, 2006     

Synthesis and characterization of poly(methyl methacrylate–butyl acrylate–acrylic acid)/polyaniline core–shell nanoparticles in miniemulsion media

Conductive polymer particles, polyaniline (PANI)‐coated poly(methyl methacrylate–butyl acrylate–acrylic acid) [P(MMA–BA–AA)] nanoparticles, were prepared. The P(MMA–BA–AA)/PANI core–shell complex particles were synthesized with a two‐step miniemulsion polymerization method with P(MMA–BA–AA) as the core and PANI as the shell. The first step was to prepare the P(MMA–BA–AA) latex particles as the core via miniemulsion polymerization and then to prepare the P(MMA–BA–AA)/PANI core–shell particles. The aniline monomer was added to the mixture of water and core nanoparticles. The aniline monomer could be attracted near the outer surface of the core particles. The polymerization of aniline was started under the action of ammonium persulfate (APS). The final product was the desired core–shell nanoparticles. The morphology of the P(MMA–BA–AA) and P(MMA–BA–AA)/PANI particles was characterized with transmission electron microscopy. The core–shell structure of the P(MMA–BA–AA)/PANI composites was further determined by Fourier transform spectroscopy and ultraviolet–visible measurements. The conductive flakes made from the core–shell latexes were prepared, and the electrical conductivities of the flakes were studied. The highest conductivity of the P(MMA–BA–AA)/PANI pellets was 2.05 S/cm. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012