Miniemulsion copolymerization of vinyl acetate and butyl acrylate. IV. Kinetics of the copolymerization

The copolymerization kinetics of conventional emulsions and miniemulsions of 50:50 and 25:75 molar ratios vinyl acetate–butyl acrylate monomer mixtures were studied using sodium hexadecyl sulfate as surfactant. Hexadecane was the cosurfactant used in the preparation of the miniemulsions, and ammonium persulfate was the initiator used in the polymerizations. The rate of polymerization showed four regions which extended to different conversions depending on the type of emulsion used (conventional or miniemulsion). The rate of polymerization for the miniemulsion process was always slower than for the conventional process. The dependence of the rate on the initiator concentration was higher for the miniemulsion process. The number of particles nucleated in the miniemulsion copolymerization process was lower than in the conventional emulsion copolymerization process. The initiator and surfactant concentration dependence of the number of particles were 0.8 and 0.25 for the miniemulsion copolymerization process and 0.0 and 0.68 for the conventional emulsion copolymerization process respectively. These effects were attributed to the different particle nucleation mechanism operating in each process.     

Effect of the presence of polymer in miniemulsion droplets on the kinetics of polymerization

Solution of polystyrene in styrene were dispersed in an aqueous gel phase comprising sodium lauryl sulfate, cetyl alcohol, and water using an emulsification process known to produce monomer droplet sizes inthe submicron size range (referred to as miniemulsion droplets). The shelf-life stabilities of these miniemulsions were studied to determine their relative droplet sizes, and the emulsions were concommitantly polymerized in an isothermal batch reaction calorimeter. The polymerization kinetics and final particle sizes produced were compared with miniemulsion and conventional emulsion polymerizations prepared using equivalent recipes without the addition of polystyrene. The results indicate that polymerization of miniemulsions prepared from polymer solutions produce significantly different kinetics than both miniemulsion and conventional emulsion polymerizations. In general, a small amount of polymer greatly increases the rate of polymerization and the final number of particles produced in the polymerization to the extent where even conventional polymerizations carried out above the critical micelle concentration of the surfactant polymerize more slowly. The results are explained by considering the system to be comprised of small, stable pre-formed monomer-swollen polymer particles which are able to efficiently capture aqueous phase radicals. This enables the system to produce a large final number of particles, similar to the initial number of pre-formed polymer particles, as opposed to miniemulsions and micelles in which only a relatively small fraction of the initial number of species (droplets or micelles) become polymer particles. © 1994 John Wiley & Sons, Inc.     

Miniemulsion polymerization of styrene with a block copolymer surfactant

A series of miniemulsion systems based on styrene/azobisisobutyronitrile in the presence of poly(methyl methacrylate‐b‐2‐(dimethylamino)ethyl methacrylate) as a surfactant and hexadecane (HD) as a cosurfactant were developed. For comparison, a series of pseudoconventional emulsions also were carried out with the same procedure used for the aforementioned series but without the cosurfactant (HD). Both the droplet size and shelf life were also measured. Experimental results indicate that it is possible to slow the effect of Ostwald ripening and thereby produce a stable miniemulsion with the block copolymer as the surfactant and HD as the cosurfactant. In addition, the extent to which varying the surfactant concentration and copolymer composition could affect both the polymer particle size during the polymerization and the polymerization rate was examined. Variation in the polymer particle sizes during polymerization indicates that droplet and aqueous (micellar or both homogeneous) nucleation occurs in the miniemulsion polymerization. With the same concentration of the surfactant used in the miniemulsion polymerization, the polymerization rates of systems with M₁₂B₃₆ are faster than those of systems with M₁₂B₁₂. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 1818–1827, 2000     

The effect of cosurfactants and the initiator concentration on the polymer to surfactant concentration in nanolatexes

The effect of cosurfactant and initiator concentration on the ab initio production of nanolatexes using low surfactant levels was investigated. While the use of cosurfactants (acrylic acid and pentanol) increased the amount of monomer that can be used in styrene‐SDS microemulsion formulations to 13 wt %, high surfactant concentrations are still required, resulting in polymer‐to‐surfactant ratios (Pol/Surf) <1. Latexes with particle size of 30 ± 5 nm were produced upon polymerization of these microemulsions. The Pol/Surf can be significantly increased by increasing the initiator concentration of emulsion polymerization recipes. Particle sizes are comparable with microemulsion latexes, however, less surfactant is required. The reduction in the particle size with higher initiator concentration is attributed to a higher efficiency of particle nucleation and to a higher nucleation rate relative to the rate of monomer transfer. Nanolatexes (particle size < 30 nm) were obtained with 19 wt % solids content and Pol/Surf of 3.6 in ab initio. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

Miniemulsion polymerization of styrene using alkyl methacrylates as the reactive cosurfactant

 Stable styrene miniemulsions were prepared by using alkyl methacrylates as the reactive cosurfactant. Like conventional cosurfactants (e.g., cetyl alcohol (CA) and hexadecane (HD)), alkyl methacrylates (e.g., dodecyl methacrylate (DMA) and stearyl methacrylate (SMA)) may act as a cosurfactant in stabilizing the homogenized miniemulsions. Furthermore, the methacrylate group may be chemically incorporated into latex particles in subsequent miniemulsion polymerization. The data of the monomer droplet size, creaming rate and phase separation of monomer as a function of time were used to evaluate the shelf-life of miniemulsions stabilized by sodium dodecyl sulfate in combination with various cosurfactants. Polystyrene latex particles were produced via both monomer droplet nucleation and homogeneous nucleation in the miniemulsion polymerization using CA or DMA as the cosurfactant, with the result of a quite broad particle size distribution. On the other hand, the miniemulsion polymerization with HD or SMA showed a predominant monomer droplet nucleation. The resultant particle size distribution was relatively narrow. In miniemulsion polymerization, the less hydrophobic DMA is similar to CA, whereas the more hydrophobic SMA is similar to HD. Received: 19 November 1996 Accepted: 20 February 1997     

Polymerization of styrene miniemulsions

Polymerization of styrene miniemulsions, prepared using a mixed emulsifier system comprising sodium lauryl sulfate and cetyl alcohol, was carried out using both water-soluble (potassium persulfate) and oil-soluble [2,2′-azobis-(2-methyl butyronitrile)] initiators. The effects of variation of initiator concentration, polymerization temperature, and added inhibitor on the kinetics and particle-size distributions were investigated to obtain more quantitative evidence concerning the locus of polymerization in miniemulsion systems. Experimental results for the kinetics and particle-size distributions clearly showed that monomer droplets became the main source of polymer particle formation. This was attributed to the fact that stable emulsions with droplet diameters in the range of 0.05 to 0.15 μm were produced using this mixed-emulsifier system. In this size range, droplet initiation could effectively compete with other mechanisms due to their large surface area. Their size was indeed similar to the corresponding latex particle size obtained after polymerization.     

Miniemulsion polymerization of styrene with chain transfer agent as cosurfactant

Miniemulsion polymerization of styrene with the chain transfer agent n-dodecyl mercaptan (DDM) used as cosurfactant was studied. Droplet size and shelf life for unpolymerized miniemulsions were measured and compared with those of equivalent macroemulsions. The miniemulsion monomer droplets with dodecyl mercaptan as cosurfactant were very stable. Shelf lives were from 17 h to 3 months. The kinetics of miniemulsion polymerization were studied. Unlike other miniemulsion systems where the cosurfactant does not act as a chain transfer agent, the polymerization rate falls with cosurfactant level because the chain transfer agent enhances radical desorption from the particles. The polymerization rate in all the miniemulsions was lower than that of the corresponding macroemulsions. Polymerized particles were larger than in the corresponding macroemulsions, but molecular weights were lower. Results indicate that DDM can serve as an effective cosurfactant as well as a chain transfer agent. The fact that the molecular weights are lower in the miniemulsion reactions indicates predominant droplet nucleation. © 1997 John Wiley & Sons, Inc.     

Miniemulsion copolymerization of styrene–methyl methacrylate

Miniemulsion copolymerization of 50 : 50 weight fraction of styrene–methyl methacrylate monomer, using hexadecane as the cosurfactant, was carried out in both unseeded and seeded polymerizations. Effects of the hexadecane concentration and the ultrasonification time on the conversion–time curves and particle size of the final latex were investigated for unseeded polymerization. The kinetic and particle size distribution results showed that an increase in hexadecane concentration and ultrasonification time cause faster polymerization rate and smaller particle size. The mechanism of mass transport from miniemulsion droplets to polymer particles was also investigated for seeded polymerization. For this purpose a monomer miniemulsion was mixed with a fraction of a previously prepared miniemulsion latex particles prior to initiation of polymerization, using residual oil-soluble initiator in the seed latex. The concentration of hexadecane and a water-insoluble inhibitor (2,5 di-tert-butyl hydroquinone) in the miniemulsions were the main variables. Seeded polymerizations were also carried out in the presence of miniemulsion droplets containing a water-insoluble inhibitor and water-soluble initiator. The inhibitor concentration and the agitation speed during the course of polymerization were the experimental variables. The kinetic and particle size results from these seeded experiments suggested that collision between miniemulsion droplets and polymer particles may play a major role in the transport of highly water-insoluble compounds.     

Kinetic and thermodynamic aspects of miniemulsion copolymerization

Miniemulsion polymerization involves initiation of polymerization in preformed stable monomer emulsion droplets with average droplet diameter of 50–500 nm. At the end of the polymerization, only a fraction of the initial number of monomer droplets become polymer particles. The emulsifier system used for the preparation of such emulsions comprises a mixture of ionic surfactant and a fatty alcohol or long chain alkane (termed cosurfactant). The cosurfactant is essential for the formation of stable emulsion droplets and in addition it plays an important role in the interparticle monomer transport. Kinetic results are presented on conventional emulsion and miniemulsion copolymerization of different pairs of monomers, showing the main differences for both processes. These differences were related to the particle formation mechanism and the influence of the cosurfactant in the miniemulsion process. A theoretical model was developed, based on mass balances and equilibrium thermodynamics, which was found to describe accurately the experimentally generated data on comonomer distribution during the course of the copolymerization process and the interdroplet mass transport process.     

Preparation of PEG-modified urethane acrylate emulsion and its emulsion polymerization

In order to improve stability and reduce droplet size, the PEG-modified urethane acrylates were synthesized by the reaction of polyethylene glycol (PEG) with residual isocyanate groups of urethane acrylate to incorporate hydrophilic groups into the molecular ends. The droplet sizes of the PEG-modified urethane acrylate emulsions were much smaller than those of unmodified urethane acrylate emulsions at the same surfactant composition, and the droplet sizes of these emulsions were significantly effected not by surfactant compositions and types, but by the reaction molar ratio of PEG, because the urethane acrylate containing polyoxyethylene groups as terminal groups aided the interfacial activity of surfactant molecules and acted as a polymeric surfactant. The actions of PEG-modified urethane acrylate were confirmed by the investigation of adsorption of urethane acrylate in a water/benzene interface.For polymerization of emulsions, the stability of emulsion in the process of emulsion polymerization was changed by the type of surfactant or initiator. In the case of emulsion polymerization with a water soluble initiator (K₂S₂O₈), the emulsions prepared using TWEEN 60 were broken in the process of polymerization. However, polymerization of these emulsions could be carried out using an oil soluble initiator (AIBN). The conversion of emulsion polymerization changed with the type of urethane acrylates, that is, the reaction molar ratio of PEG to 2-HEMA.     

Poly(ε-caprolactone) and cellulose ester hybrid nanoparticles via miniemulsion polymerization

Two types of hybrid acrylic nanoparticles based on biodegradable and biocompatible polymers, cellulose ester and poly(ε-caprolactone), were produced via miniemulsification through high-pressure homogenization. An efficient emulsification procedure was first devised to yield high-solids-content polymer–monomer waterborne miniemulsions, and the fundamental parameters governing the stability of these composite miniemulsions were assessed. In addition, strategies to control the droplet size were investigated upon varying several experimental parameters such as the interfacial tension between the organic and the aqueous phase, the organic phase viscosity and the nature/concentration of surfactant. A series of thermally initiated polymerizations were then performed to produce nanosized hybrid particles.     

Preparation of Stable Miniemulsions of Poly(2-ethyl hexyl acrylate-co-vinyl acetate)

Summary: This paper presents the results of the preparation of miniemulsions as well as the subsequent copolymerizations of 2-ethyl hexyl acrylate and vinyl acetate. The miniemulsions were prepared using a mixture of an anionic and a non-ionic surfactant. Sodium dodecyl sulfate (SDS) was used as the anionic surfactant and two non-ionic surfactants were studied: Triton X-405 and Disponil A3065. The miniemulsions prepared with a 90/10 mol.-% Disponil A3065 were able to reach a kinetically-stable state to yield latexes with 43 wt.-% solids content with a maximum change in the number of particles (N_p) with respect to the number of droplets (N_d) of ∼6%. A 2³ factorial design was then used to discern the influence of monomer, chain transfer agent and surfactant concentration on the droplet size distribution (DSD) and particle size distribution (PSD). Pressure-sensitive adhesive (PSA) properties were also examined.     

Evolution of droplet size distribution and composition in miniemulsions

A fundamental understanding of the formation, degradation and polymerization of miniemulsions has been hindered by difficulties in quantifying their monomer droplet size distribution (DSD). In this work, particle sizing techniques including capillary hydrodynamic fractionation, acoustic attenuation spectroscopy, surfactant titration, and microscopy were adapted to characterize miniemulsion DSDs. The key ingredient in miniemulsions is the costabilizer, a low water solubility compound that limits monomer diffusion from the smaller to larger droplets (Ostwald ripening). The DSD evolution of styrene miniemulsions employing hexadecane (HD) as costabilizer was characterized. With less costabilizer, droplets were initially smaller, but increased in average size with time, and their DSDs broadened. These changes were slowed with addition of extra surfactant after homogenization. After several days, the average droplet size increased to about 150 nm regardless of the amount of HD or surfactant used. The HD content of separated portions of centrifuged miniemulsions was measured and showed significant Ostwald ripening within minutes after preparation. The further evolution of the DSD is attributed primarily to droplet coalescence. Less composition change occurred with either higher HD content or post‐homogenization surfactant addition, both of which led to minimization of free energy, increasing stability. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014 , 52, 1529–1544     

氧化-还原低温引发苯乙烯/丙烯酸丁酯细乳液共聚合动力学与成核机理的研究

详细讨论了 [(NH4 ) 2 S2 O8/NaHSO3 ]氧化 还原引发体系引发苯乙烯 (St)丙烯酸丁酯 (BuA)体系的细乳液共聚合的动力学特征及其与成核机理的关系 .细乳液的聚合速率比相同条件下的常规乳液聚合速率低 ,引发期长 .随聚合温度、引发剂浓度、乳化剂浓度的增加 ,聚合速率增大 .共乳化剂正十六烷 (HDE)的浓度在一定范围内增大 ,反应的速率增大 ,然后再增加HDE ,反应速率下降 .建立动力学曲线数学模型 ,并深入讨论了细乳液的聚合动力学特征 ,与常规乳液所得结果相比较 ,探讨了细乳液的单体液滴成核机理 .     

Exploring the Limits of Emulsion Polymerization of Styrene for the Synthesis of Polymer Nanoparticles

Summary. Suspensions of polymer nanoparticles in water (latices) with average particle diameters between 20 and 80 nm were synthesized by batch emulsion polymerization of styrene using sodium dodecyl sulphate (SDS) as surfactant and potassium persulphate (KPS) as initiator. The influence of surfactant concentration, initiator concentration, monomer concentration, and reaction temperature on the final average particle diameters and size distributions of the latices were studied. The number of particles generated was proportional to the 0.56 power of the emulsifier concentration and to the 0.37 power of the initiator concentration in the whole concentration range which was observed. Furthermore, the final number of particles was dependant on the reaction temperature to the 2.06 power. With these correlations the average particle number as well as the average particle size could be estimated, and the results were in good agreement (±6%) with the experimental values. A reduction of the monomer/water ratio from 1:5 to 1:20 yielded smaller particle diameters, while leaving the particle number unaffected. The lower particle size limits for monomer ratios of 1:10 and 1:15 were estimated with diameters of about 18 and 16 nm.     

Exploring the Limits of Emulsion Polymerization of Styrene for the Synthesis of Polymer Nanoparticles

Suspensions of polymer nanoparticles in water (latices) with average particle diameters between 20 and 80 nm were synthesized by batch emulsion polymerization of styrene using sodium dodecyl sulphate (SDS) as surfactant and potassium persulphate (KPS) as initiator. The influence of surfactant concentration, initiator concentration, monomer concentration, and reaction temperature on the final average particle diameters and size distributions of the latices were studied. The number of particles generated was proportional to the 0.56 power of the emulsifier concentration and to the 0.37 power of the initiator concentration in the whole concentration range which was observed. Furthermore, the final number of particles was dependant on the reaction temperature to the 2.06 power. With these correlations the average particle number as well as the average particle size could be estimated, and the results were in good agreement (±6%) with the experimental values. A reduction of the monomer/water ratio from 1:5 to 1:20 yielded smaller particle diameters, while leaving the particle number unaffected. The lower particle size limits for monomer ratios of 1:10 and 1:15 were estimated with diameters of about 18 and 16 nm.     

A competitive particle nucleation mechanism in the polymerization of homogenized styrene emulsions

The effects of concentrations of surfactant (sodium lauryl sulfate [SLS]) and initiator (sodium persulfate [SPS]) on the polymerization of homogenized styrene emulsions, stabilized by SLS/lauryl methacrylate (LMA) or SLS/stearyl methacrylate (SMA), were studied. The rate of polymerization increases with increasing [SLS] or [SPS]. In addition to monomer droplet nucleation, the formation of particle nuclei in the aqueous phase (homogeneous nucleation) plays a crucial role in the polymerization kinetics. In comparison with the LMA containing polymerization system, monomer droplet nucleation becomes more important when the more hydrophobic SMA was used as the costabilizer. Furthermore, the degree of homogeneous nucleation increases with increasing [SPS].     

Nanoparticle formation by monomer-starved semibatch emulsion polymerization

Latexes with very small particle size are usually manufactured by microemulsion polymerization. This article explains the preparation of nanolatexes by monomer-starved nucleation in a conventional semibatch emulsion polymerization with a low surfactant/monomer ratio and with no need for a cosurfactant. The semibatch emulsion polymerization reactions started with an aqueous solution of a surfactant and a water soluble initiator. Monomer was added at a fixed rate. The size of particles decreased with decreasing rate of monomer addition. High solids content nanolatexes with particles as small as 25 nm in diameter were produced. Several monomers with different water solubilities were compared. The order of the number of particles in terms of the rate of monomer addition was found to depend on the type of monomer. Water soluble monomers produced more particles due to associated chain transfer to monomer and radical exit. The monodispersity of particles at the end of nucleation increased as the rate of monomer addition decreased. The technique seems to be preferable to microemulsion polymerization, which uses a high concentration of surfactant/cosurfactant and is limited to low monomer holdup.     

Double Emulsions Stabilized by Xanthan in the Absence of Hydrophilic Surfactant

The preparation of double water-in-oil-in-water (W/O/W) emulsions containing xanthan gum (XG) in the absence of hydrophilic surfactant was investigated. The emulsions were prepared by the two-step emulsification process. The stability of these systems was evaluated through the evaluation of physicochemical and rheological properties. Microscopic observations in combination with particle size analysis were also performed. The obtained results show that it is possible to prepare stable double emulsions with a single polysaccharide by using the indirect process. The stability depends on the viscosity of the continuous phase and hence the concentration of XG. The apparent viscosity of the emulsions increased with the increase of XG concentration. Particle size analysis shows that the droplet sizes are directly related to XG concentration.     

The emulsion polymerization of vinyl acetate. Factors controlling particle surface area and rate of polymerization

An emulsion polymerization system with uniform continuous addition of vinyl acetate monomer, Pluronic F68 surfactant, and persulfate initiator has been examined with variation of the surfactant concentration over a tenfold range. The particle surface area per unit weight of emulsion was found to vary directly as the surfactant/monomer ratio, as also did the emulsion viscosity. At constant polymer/emulsion weight the number of particles per unit emulsion weight varied directly as the cube of the surfactant concentration. It is shown that these relationships apply also to other monomers, such as styrene and methacrylate esters. The solubility of vinyl acetate in a range of Pluronic F68 aqueous solutions was determined, and it was shown that the rate of polymerization is dependent on the solubility of the monomer in the surfactant solution. It is concluded that when a water-soluble initiator is used, polymerization proceeds in the aqueous phase. The principal factors controlling the rate of polymerization in the emulsion polymerization of vinyl acetate are, consequently, the initiating system and the concentration of monomer in the aqueous phase. Solubilization characteristics indicate that the surfactant concentration will have a much greater effect on the less water-soluble monomers, such as styrene, than on the more soluble ones, such as vinyl acetate.