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.     

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     

High pressure semibatch emulsion and miniemulsion copolymerization of vinyl acetate and ethylene

This study presents the experimental study of semibatch emulsion and miniemulsion copolymerization of vinyl acetate (VAc) and ethylene to vinyl acetate-ethylene (VAE) copolymer at 60°C and 80–300 psig. In the miniemulsion copolymerization, a water-soluble initiator (K₂S₂O₈) is used and VAc miniemulsion is prepared in presence of surfactant and cosurfactant using a sonicator or a high-shear homogenizer. Then, ethylene gas is supplied to the reactor at constant partial pressure. In a miniemulsion process, the mass transfer limitations of VAc from monomer droplets to the aqueous phase, and to micelles or polymer latex particles that are present in conventional macro-emulsion polymerization can be eliminated and the transfer of ethylene dissolved in the aqueous phase to the miniemulsion droplets is the major ethylene transport process for the polymerization. The experimental data show that the amount of ethylene incorporation into the copolymer is higher in miniemulsion polymerization than in emulsion polymerization. The ethylene pressure has been found to have a strong impact on the ethylene incorporation into the copolymer phase in both emulsion and miniemulsion copolymerizations but the increase is more pronounced in miniemulsion case. The VAE copolymer latex particles prepared by miniemulsion polymerization exhibited higher storage stability than those prepared by macro-emulsion polymerization.     

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     

Modeling the emulsion polymerization of amino-functionalized latex particles

A mathematical model for a semicontinuous seeded cationic emulsion polymerization was developed. The model includes the most distinctive features of the copolymerization of a cationic hydrophilic monomer with a hydrophobic one, including polymerization of the hydrophilic monomer in the outer shell of polymer particles and in the aqueous phase, and the possibility of having radical concentration profiles in the polymer particles.The reactions were carried out by means of a semicontinuous seeded cationic emulsion polymerization under starved conditions for styrene, which was the main monomer employed.The model predicts the evolution of the fractional overall conversions, the thickness of the outer shell, the total surface charge density and the partial conversions for the semicontinuous seeded cationic emulsion polymerization of styrene and aminoethyl methacrylate hydrochloride. Furthermore, the model can distinguish between the surface charge density provided by the cationic monomer than that given by the cationic initiator. Therefore, this model can predict the best conditions to obtain well-defined latexes with specific amounts of surface amino and amidine groups useful for immunoassays.     

Miniemulsion and macroemulsion copolymerization of vinyl acetate with vinyl versatate

The miniemulsion and macroemulsion polymerization of vinyl acetate with vinyl versatate in batch and semibatch systems was investigated. Vinyl versatate was added either as an emulsion with the vinyl acetate, or as a neat liquid stream. In the batch runs, there is a poor dispersion of vinyl versatate during the nucleation period for the runs in which the vinyl versatate was added neat at the beginning of the polymerization. This led to smaller particles, lower polymerization rate, and different polymer composition evolution when compared with runs in which the vinyl versatate was emulsified with the vinyl acetate. In seeded semibatch runs, residual surfactant in the seed latex, along with the propensity for homogeneous nucleation in vinyl acetate emulsions, resulted in continuing nucleation during the entire semibatch interval. The polymerization rate was primarily affected by monomer feed rate rather than the feeding mode. The effect of monomer feeding mode on copolymer composition was weak when the semibatch feed rate was low, indicating some level of vinyl versatate mass transfer resistance. In all runs, only one glass transition temperature was observed, indicating effective copolymerization. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 85: 2219–2229, 2002     

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.     

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.     

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     

Semibatch emulsion polymerization of butyl acrylate. I. Effect of monomer distribution

The effects of initial monomer charge on the particle formation and on the rate of polymerization were investigated for semibatch emulsion polymerization of butyl acrylate using sodium lauryl sulfate (SLS) as a surfactant and potassium persulfate (KPS) as an initiator. For the semibatch process with monomer (M) feed, it was found that by varying the monomer distribution ratio between the initial reactor charge and the feed it is possible to alter the contribution of monomer‐flooded and monomer‐starved nucleation mechanisms to the whole nucleation process. The number of particles increases as the initial monomer charge decreases, if the monomer concentration is below a critical value for any fixed system. The increase in number of particles is associated with a broad particle‐size distribution which might depict an emerging second peak on the particle‐size distribution curve. For low emulsifier concentration systems, a larger number of particles was obtained for a lower amount of monomer charge. Particle coagulation and emulsifier adsorption on the monomer droplets were counted as the main reasons for such behavior. For a semibatch process with monomer emulsion (ME) feed, the larger number of particles was formed at a lower initial monomer charge, similar to an M‐add semibatch process. However, the application of monomer charge to an ME‐add process was found to increase the possibility of secondary nucleation and led to the occurrence of a bimodal particle‐size distribution. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 74: 3094–3110, 1999     

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     

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.     

Swelling of poly(styrene-co-methacrylic acid) latex by isoprene

Carboxylated polystyrene latex was used as seed and isoprene as the second-stage monomer in an inhibited, seeded emulsion polymerization recipe for studies of monomer swelling kinetics at 80°C during interval III of an emulsion polymerization. The isoprene was added to the reactor in small portions using a syringe, and changes in the reactor pressure were continuously measured. Isoprene was added until a free liquid monomer phase was formed; that was, interval II was reached, as indicated by no further pressure increase upon the addition of more monomer. When the observed pressure increment, Op_i, per unit isoprene added was plotted as a function of the volume fraction of polymer in the latex particles, v_p, the graph could be divided into 3 domains. The break points in the Op_i curve could, in an analogous emulsion polymerization, be identified as the glass transition temperature for the polymer, the so-called gel point in interval III and the onset of interval III. In the second domain, where the v_p was between the glass transition temperature, T_g, for the seed polymer and the gel point, the value of Op_i decreased significantly with increasing monomer concentration in the latex particles. This was due to the entropy of mixing and the monomer acting as a plasticizer in the seed polymer. The rate of sorption of monomer to the latex particles was low at high values of v_p. It then increased rapidly with increasing monomer concentrations in the latex particles, [M]_p, and a maximum was observed in domain 2. At lower values of v_p the sorption rate decreased in domain 3 and finally became zero as the free liquid monomer phase started to form. Results from batch polymerization suggested that the rate of diffusion of adsorbed monomer and oligo radicals into the particles was retarded. A simplified form of the Vanzo equation was used to estimate the monomer partitioning. It predicted too high a value of [M]_p, especially in domain 2 of the swelling process. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 70: 2041–2051, 1998     

Batch and semicontinuous emulsion copolymerization of vinylidene chloride and butyl methacrylate. I. Kinetics in VDC–BMA emulsion polymerization and surface and colloidal properties of VDC–BMA latexes

Vinylidene chloride (VDC)—butyl methacrylate (BMA) comonomer mixtures with various composition (83 : 17, 60 : 40, 33 : 67 in mol %) were polymerized at 25°C using redox catalyst by batch and seeded semicontinuous emulsion copolymerization. The reactivity ratios determined in VDC (M₁)—BMA (M₂) emulsion copolymerization system were r₁ = 0.22 and r₂ = 2.41. Seven 35% solids (83 : 17 mol %) VDC–BMA copolymer latexes were prepared: one batch (G), one seeded batch (F), and 5 seeded semicontinuous polymerizations of 5 different monomer feed rates ranging from 0.27 (A) to 1.10 wt %/min (E). The kinetic studies of seeded semicontinuous polymerizations A-E showed that the rates of polymerizations (R_p) were controlled by the monomer addition rates (R_a). The conversion versus time curves for the polymerizations of 0 : 100–100 : 0 VDC–BMA mixtures by batch polymerization showed that the rate of polymerization (R_p) was a function of the number of particles, and that the rate of polymerization in a latex particle (R_pp) increased with increasing proportions of butyl methacrylate in the monomer mixture. All of the latexes had narrow particle size distributions. The greater particle number density in VDC polymerization and the greater water solubility of VDC suggest that the homogeneous nucleation mechanism is operative in VDC–BMA copolymerizations. The latex copolymers prepared by semicontinuous polymerization had lower number-and weight-average molecular weights than those of the corresponding batch copolymers, resulting from the monomer starvation occurring during the semicontinuous polymerization. The surface characterization study of the cleaned latexes showed that for the latexes by batch process, the surface charge density derived from strong-acid groups decreased with increasing proportion of VDC in the monomer mixture. On the other hand, for the latexes prepared by semicontinuous polymerization, the surface charge density derived from strong-acid groups did not depend on the monomer composition of the copolymers.     

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     

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.     

Chain transfer behavior of fractionated commercial mercaptans in emulsion polymerization of styrene

The chain transfer behavior of fractionated commercial tertiary mercaptans was investigated in batch and semicontinuous emulsion polymerization of styrene over the entire monomer conversion range. Four mercaptans were obtained by fractionation, which contained 9, 11, 12, and 13 carbons, respectively. The effect of the mercaptans was evaluated in terms of the consumption rates of the monomer and the chain transfer agents, the number average degree of polymerization, DP_n, and the polydispersity index, I, of the polymer. The batch experiments showed that the chain transfer efficiency decreases with increasing carbon number, which is due primarily to a lower diffusion rate of the chain transfer agent to the reaction sites (growing latex particles) through the aqueous phase. The partitioning ratio of the chain transfer agents between the aqueous phase and the monomer droplets also contributes, to a lesser extent, to the efficiency. The number average degree of polymerization and the polydispersity index are primarily controlled by the ratio of the mercaptan consumption rate over that of the monomer. In order to obtain a polymer with a constant DP_n and a narrow I, this ratio should be as close to unity as possible, as is the case for C₁₁. Otherwise, too high a ratio causes a severe increase in DP_n at the end of polymerization, and too low a ratio leads in the opposite direction. The semicontinuous experiments confirmed the batch results. © 1994 John Wiley & Sons, Inc.     

Secondary particle formation in seeded suspension polymerization

Seeded suspension polymerization can be applied to obtain core-shell particles with particle diameters larger than 1000 μm, which finds application in the rigid foam industry, for instance. However, depending on the operation conditions, the formation of secondary particles decreases drastically the efficiency of monomer incorporation in the seed particles. In the present work, the mechanism of secondary particles formation during seeded suspension polymerization was investigated using monomers (styrene, methyl acrylate and methyl methacrylate) and initiators (benzoyl peroxide and lauroyl peroxide) with different water solubilities and, in the case of the initiators, also different decomposition rates. Results showed that there was no seed polymer in the composition of the secondary particles but only pure polymer from the monomer feed, meaning that they were not formed by erosive breakage of the swelled seeds. The fraction of secondary particles increased when monomers with higher water solubility and initiators with decreasing water solubility were used. These results were consistent with the formation of secondary particles by homogeneous nucleation and monomer droplet nucleation.     

Kinetics and colloidal parameters of miniemulsion polymerization of butyl acrylate

BACKGROUND: The miniemulsion polymerization of butyl acrylate initiated by a macromonomeric azoinitiator (macroinimer, MIM) and stabilized by the non‐ionic emulsifier Tween 60 (TW‐60) was investigated. RESULTS: The monomer conversion and the polymerization rate increase with the amount of MIM and then decrease. The desorption rate constants were estimated using the Ugelstad/O'Toole, Gilbert and Nomura models. The Ugelstad/O'Toole and Gilbert models suggest an increase in the k′_des value with increasing emulsifier concentration at the highest MIM concentration while the Nomura model proposes no variation in k′_des with an increase in both TW‐60 and MIM concentrations. The polymerization rate increases in the following order with regard to initiator: MIM < ammonium persulfate < dibenzoyl peroxide < 2, 2′‐azobisisobutyronitrile. CONCLUSION: The increase in the rate of polymerization can be discussed in terms of both increased particle concentration and the gel effect. The size of the polymer particles decreases and the number of polymer particles increases with both TW‐60 and MIM concentrations. This behaviour is attributed to the formation of a larger number of smaller monomer and/or polymer particles and higher particle nucleation rate. The observed long nucleation period for the MIM‐initiated polymerization is attributed to the creation of a crosslinked structure and the immobilization of MIM chains. Copyright © 2009 Society of Chemical Industry