Particle nucleation in emulsion polymerization. II. Nucleation in emulsifier-free systems investigated by seed polymerization

Experiments with seeded polymerization in emulsifier-free systems were carried out with styrene in order to test the theory of particle nucleation presented in the first article in this series. The effect of amount, size, and surface charge density of the seed particles on the formation of new particles was investigated. An expression for the capture rate of oligomeric radicals from the water phase was evaluated in which the rate of capture was considered to be governed by the absorption of oligomers with chain length one less than the critical chain length for precipitation of the oligomer. Coagulation of primary particles was also included in the expression for the number of new particles obtained in the system. Limited coagulation of primary particles with already formed particles and with seed particles was found to play an important role in determining the final number of new particles found at the end of the runs.     

Particle nucleation in emulsion polymerization. IV. Nucleation in monomer droplets

Nucleation in finely dispersed monomer emulsions in competition with homogeneous and micellar mechanism was studied. The emulsions were prepared with a high-pressure homogenizer under varying homogenizing conditions and made stable by the addition of hexadecane to the monomer. Sodium dodecyl sulfate was used as emulsifer. The number of particles in latexes polymerized with potassium persulfate and benzoyl peroxide initiators was measured and plotted as a function of the free emulsifier concentrations. With persulfate initiator the particle number achieved a minimum in the transition region between droplet and water phase–micellar nucleation mechanisms. With very fine emulsions Smith–Ewart case II kinetics with n = 0.5 applied. The reaction rate, which differed from conventional emulsion polymerizations, decreased with time up to the point at which n began to increase. In these runs the particle size distribution became nearly monodisperse.     

Particle nucleation in emulsion polymerization. I. A theory for homogeneous nucleation

This paper is the first in a series intended to clarify the particle nucleation mechanisms in emulsion polymerization. The theory for particle nucleation by precipitation of oligomeric radicals from the water phase is discussed and a model based on the diffusion, propagation and termination steps is presented. The physical factors that influence the capture rate of oligomers in particles are discussed, and qualitative expressions for the electrostatic repulsion and reversible diffusion are derived. These factors are shown to be able to explain the relatively slow absorption rate of oligomers in particles and micelles. A kinetic model for simultaneous particle nucleation and limited flocculation is presented. Numerical integration of this model shows that the particle number goes through a maximum and that simultaneous nucleation and flocculation of primary particles may take place after Interval I in an emulsion polymerization is finished.     

Radiation-induced emulsion polymerization of ethylene. III. Emulsion polymerization with ammonium perfluorooctanoate as the emulsifier

Radiation-induced emulsion polymerization of ethylene with ammonium perfluoro-octanoate as an emulsifier was studied in order to elucidate the effect of the number of polymer particles. Owing to the stable structure of the emulsifier from a radical attack, no C? F bond was detected in the polyethylene as expected. The polyethylene produced was mostly gel containing a small amount of low molecular weight polyethylene. This may be attributable to chain transfer to the polyethylene. The effects of dose rate and of concentration of the emulsifier were determined without considering the chain-transfer reaction to the emulsifier. By considering the escape of the radical which is produced by chain transfer to the monomer from the polymer particle to the aqueous phase at the steady state, the following equation is derived: The experimental results could be explained by this equation, and the apparent rate constants were obtained.     

Preparation of multihollow polymer particles by seeded emulsion polymerization using seed particles with incorporated nonionic emulsifier

Emulsion polymerizations of styrene using poly(oxyethylene) nonylphenyl ether nonionic emulsifier were carried out at different emulsifier and initiator (potassium persulfate, KPS) concentrations to prepare polystyrene (PS) seed particles with incorporated nonionic emulsifier. Seeded emulsion polymerizations of styrene using the PS seed particles with different amounts of incorporated emulsifier were carried out to develop a novel method for the preparation of multihollow particles. When seed particles with a small amount of incorporated emulsifier were used, non-hollow spherical particles were prepared. However, multihollow particles were obtained in the case of seed particles with a large amount of incorporated emulsifier. Moreover, the higher the initiator concentration in the preparation of seed particles, the more effectively were hollow particles prepared. On the basis of the above results, a mechanism for the formation of multihollow structure was suggested.     

Particle formation in interval III of the emulsion polymerization of styrene with aerosol‐MA as an emulsifier

Particle nucleation in the seeded emulsion polymerization of styrene in the presence of Aerosol‐MA emulsifier micelles and in the absence of monomer droplets (interval III) was investigated. The seed particles were swollen with different amounts of the styrene monomer before the experiments. A larger number of polymer particles formed in interval III than in the corresponding seeded batch operation in the presence of monomer droplets. The increase in the number of particles could be attributed to the reduced rate of growth of new particles, which retarded the depletion of emulsifier micelles. The number of secondary particles initially increased with the initial polymer weight ratio in the seed particles (w_p0) but decreased at a higher range of w_p0, after reaching a maximum at w_p0 = 0.60, and eventually was reduced to zero. At high values of w_p0 (>0.75), polymerization occurred in the seed particles, whereas few or no new particles were formed despite the presence of micelles. The cessation of particle formation at high conversions was ascertained with a semibatch process in which the neat monomer feed was added to the reaction vessel containing the seed particles and emulsifier micelles. For w_p0 > 0.85, the emulsifier micelles were disintegrated to stabilize the seed particles with no secondary particle formation. The possible reasons for the cessation of particle formation at high w_p0 were examined. The size distribution of secondary particles showed a positive skewness in terms of volume because of the declining rate of growth for particles, together with a low rate of growth for small particles. The distribution breadth of new particles sharpened with increasing w_p0. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 1652–1663, 2002     

Kinetics and mechanism of emulsifier-free emulsion polymerization. III. Particle growth mechanism of seeded styrene/potassium persulfate system

For seeded emulsifier-free emulsion polymerizations of styrene/potassium persulfate (KPS)/water system using the three sizes of seeds: 1020, 1620, and 1923 Å, analysis on the data of conversion and MWD suggests a shell region polymerization mechanism for the particle growth period as the particle diameter is larger than about 1500–2000 Å. The shell region has thickness of about 100 Å. The occurrence of shell region polymerization is attributed to the higher average number of radicals per particle (n ? 2?7) for the large particle, causing the polymer radicals (with the sulfate ends anchoring on the particle surface) to be terminated by combination at lower MW. Thus, the radical ends have no chance to arrive at the core of the particles. As the smallest seed is used, the rate of polymerization is of zero order, the same as in the conventional emulsion polymerization. MW of the polymer produced in the cases, in which the shell region polymerization occurs, increases with conversion in the entire process, different from the conventional case in which the MW increases first up to about 60% conversion and then decreases. © 1992 John Wiley & Sons, Inc.     

Particle formation and coagulation in the seeded semibatch emulsion polymerization of butyl acrylate

Particle formation and coagulation in the seeded semibatch emulsion polymerization of butyl acrylate were studied under monomer‐starved conditions. To investigate the importance of the kinetics of the water phase in the nucleation process, the monomer feed rate was used as a variable to alter the monomer concentration in the aqueous phase. The emulsifier concentration in the feed was employed to alter the particle stability. Particle formation and coagulation were discussed in terms of critical surface coverage ratios. Particle coagulation occurred if the particle surface coverage dropped below θ_cr1 = 0.25 ± 0.05. The secondary nucleation occurred above a critical surface coverage of θ_cr2 = 0.55 ± 0.05. The number of particles remained approximately constant if the particle surface coverage was within θ_cr1 = 0.25 < θ < θ_cr2 = 0.55. This surface coverage band is equivalent to the surface tension band of 42.50 ± 5.0 dyne/cm that is required to avoid particle formation and coagulation in the course of polymerization. The kinetics of the water phase was shown to play an important role during homogeneous and micellar nucleations. For any fixed emulsifier concentration in the feed and above θ_cr2, the number of secondary particles increased with monomer concentration in the aqueous phase. Moreover, the presence of micelles in the reaction vessel is not the only perquisite for micellar nucleation to occur, a sufficient amount of monomer should be present in the aqueous phase to enhance the radical capture by partially monomer‐swollen micelles. The rate of polymerization increased with the surfactant concentration in the aqueous phase. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 3612–3630, 2000     

Preparation of multihollow polystyrene particles by seeded emulsion polymerization using seed particles with incorporated nonionic emulsifier: effect of temperature

Seeded emulsion polymerizations of styrene using polystyrene (PS) seed particles with incorporated nonionic emulsifier were carried out at 40 and 70 °C to investigate the influence of temperature during the polymerization process including the swelling step of the seed particles with monomer on the formation of multihollow PS particles. An increase in the temperature during the polymerization process caused an increase in the rate of coalescence (i.e., the degree of coalescence at any given time) of the small water domains in the inside. After the coalescence proceeded excessively, the water domains were eventually discharged from the particles to the medium, resulting in nonhollow particles. The results show that it is important for the preparation of the multihollow PS particles to control the coalescence of a lot of small water domains inside the seed particles with the incorporated nonionic emulsifier, and strongly support the formation mechanism previously proposed. Part CCCXX of the series “Studies on Suspension and Emulsion”.     

Kinetics and mechanisms of emulsion polymerization initiated by oil-soluble initiators. IV. Kinetic modeling of unseeded emulsion polymerization of styrene initiated by 2,2′-azobisisobutyronitrile

To explain the kinetic features of particle formation and growth in unseeded emulsion polymerization initiated by oil-soluble initiators, a mathematical kinetic model is proposed, based on the assumption that when initiator radicals or monomer radicals in the water phase enter monomer-solubilized emulsifier micelles, initiate polymerization, and propagate to a chain length which is long enough not to desorb from the micelles, the micelles are regarded to be transformed into polymer particles. It is demonstrated by comparing the experimental results obtained in the emulsion polymerization of styrene initiated by the oil-soluble initiator, 2,2'-azobisisobutyronitrile, with sodium lauryl sulfate as emulsifier that the proposed kinetic model satisfactorily explains the kinetic features such as the effects of initial emulsifier, initiator, and monomer concentrations on both the number of polymer particles produced and the monomer conversion versus time histories. © 1993 John Wiley & Sons, Inc.     

Gamma radiation-initiated polymerization of styrene at high pressure. III. Emulsion polymerization with anionic emulsifiers

Values calculated for the activation volume for chain propagation, ΔV, for the polymerization of styrene in emulsions under a variety of conditions agree closely with that previously obtained in pure styrene (ΔV = ?18.6 cm³ mol^?1). The rate of initiation of emulsion polymerization by radicals produced in the water phase was independent of pressure; therefore ΔV is zero. This differs from initiation in pure styrene which is slightly retarded by pressure (ΔV = 2.0 cm³ mol^?1). The activation energy for the reaction in emulsion, as in pure monomer, decreases slightly with pressure. Chain transfer to monomer occurs to a much greater extent in emulsions than in pure monomer under similar temperature and pressure conditions. Values for the dependence of the polymerization rate on the initiator (i.e., the irradiation dose rate) and emulsifier concentration are consistent with Smith–Ewart, Case II kinetics.     

Theory of colloid stability and particle nucleation kinetics in emulsion polymerization

The DLVO theory of colloid stability is applied to the uncatalyzed and catalyzed agglomeration of small primary particles formed in the earliest stages of emulsion polymerization in the manufacture of a carboxylic styrene/butadiene latex. The inverse 6th-power relationship between the stability ratio and the cation concentration revealed experimentally in an earlier work can be confirmed theoretically using variable Stern potentials. The Stern potential changes as parabolic function of log cation concentration. From the viewpoint of DLVO and Stern theory it is suggested that a spacer effect is crucial for the activity of the catalyst used.Presented at the 5th European and Interface Society Conference together with the 35th meeting of the Deutsche Kolloidgesellschaft on Trends in Colloid and Interface Science September 25–28, 1991, Maiz, FRG     

Determination of the latex particle size in emulsion polymerization of methyl methacrylate with low emulsifier concentrations

The mean size of the latex particles formed in emulsion polymerization of methyl methacrylate under definite conditions (water: monomer volume ratio 15: 1, 80°C, potassium persulfate concentration 0.07 wt %) decreases from 200 to 9–10 nm as the concentration of an ionic surfactant (anionic Disponil AES 60, SDS, cationic C₁₉H₄₂BrN) is increased from 0.0 to 1.0 wt %. The nonionic surfactants studied influence the size of the latex particles formed differently: with ALM-10, the particle size decreases from 200 to 150–190 nm, whereas with ALM-7 and ALM-2 it increases from 200 to 320 nm as the surfactant concentration is increased from 0.0 to 1.0 wt %. An increase in the concentration of F127 amphiphilic ternary block copolymer from 0.0 to 1.0 wt % leads to a monotonic decrease in the size of the poly(methyl methacrylate) latex particles formed from 200 to 53 nm.     

On the stability and properties of the polyacrylate/Na-MMT nanocomposite obtained by seeded emulsion polymerization

For high performance waterborne coatings usually polymer latexes with low emulsifier content are more preferred. Although polymer/clay nanocomposites offer improved properties, it is difficult to produce clay based nanocomposite latexes containing low emulsifier due to the stabilization problems especially caused by organoclays. Present study deals with the preparation of a tBA/BA/MAA ternary copolymer/clay nanocomposite containing 3 wt.% sodium montmorillonite (Na⁺-MMT) via seeded emulsion polymerization. Experimentally it was observed that even the usage of hydrophilic clay caused stabilization problem and a certain amount of emulsifier (>1 wt.%) was necessary to obtain stable latexes. In addition, the usage of a low molecular weight water soluble polymer as steric barrier was found to increase the stability of system. Obtained nanocomposite latex showed fine particle size diameter (127 nm) and very narrow size distribution (PDI = 0.06). The WAXD and TEM investigations indicated that a mostly exfoliated nanocomposite was obtained. Thermal analyses (DSC, DMTA and TGA) showed that there was no change at T_g of the copolymer while very high improvement was obtained for elastic modulus and a slight increase in thermal stability. According to the rheological measurements, the nanocomposite latex showed a higher low shear viscosity, a stronger shear thinning behavior and an improved physical stability in comparison to the reference latex.     

Particle nucleation in high solids batch miniemulsion polymerization stabilized with a polymeric surfactant

The nucleation of polymer particles in the miniemulsion polymerization of vinyl acetate/VeoVa10 (VAc/VeoVa10) stabilized with PVOH was studied. The effect of costabilizer type, PVOH concentration, and type (thermal water‐soluble and oil‐soluble; and redox water‐soluble) and concentration of initiator on the extent of droplet nucleation were studied. Droplet nucleation was maximized by improving miniemulsion stability and using efficient initiators. These high solids dispersions could not be obtained by using a conventional batch emulsion polymerization. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 6407–6415, 2008     

Phase transitions and microstructure of emulsion systems prepared with acylglycerols/zinc stearate emulsifier

The emulsification processes, during which acylglycerols/zinc stearate emulsifier, water, and oil phase formed ternary systems, such as water-in-oil (W/O) emulsions, oil-in-water (O/W) dispersions, and unstable oil-water mixtures, were investigated in order to characterize the progressive transformations of the dispersed systems. The type, structure, and phase transitions of the systems were found to be determined by temperature and water phase content. Crystallization of the emulsifier caused the destabilization and subsequent phase inversion of the emulsions studied, at a temperature of 60-61 degrees C. The observed destabilization was temporary and led, at lower temperature, to W/O emulsions, "O/W + O" systems, or O/W dispersions, depending on the water content. Simultaneous emulsification and cooling of 20-50 wt % water systems resulted in the formation of stable W/O emulsions that contained a number of large water droplets with dispersed oil globules inside them ("W/O + O/W/O"). In water-rich systems (60-80 wt % of water), crystallization of the emulsifier was found to influence the formation of crystalline vesicle structures that coexisted, in the external water phase, with globules of crystallized oil phase. Results of calorimetric, rheological, and light scattering experiments, for the O/W dispersions obtained, indicate the possible transition of a monostearoylglycerol-based alpha-crystalline gel phase to a coagel state, in these multicomponent systems.     

Polymeric and colloidal features of latex particles with surface amino groups obtained by semicontinuous seeded cationic emulsion polymerization*

Monodisperse latex particles with different amounts of surface amino and amidine groups were synthesized by means of a semicontinuous seeded cationic emulsion polymerization of styrene and a cationic monomer. High partial overall conversions for styrene and limited ones for the cationic monomer were achieved. A reliable method for the quantification of surface amidine and amino groups was developed. It was found that the amount of surface amidine groups provided by the cationic initiator was higher when the amount of cationic monomer added increased. The value for the partition coefficient of the cationic monomer indicated that this polymerizes with the same probability in the water phase as in the particle. The colloidal stability, in terms of critical coagulation concentration, shows that the latexes would be useful as polymeric supports in immunoassays. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 3878–3886, 2005     

Kinetics of emulsion copolymerization of methylmethacrylate and ethylacrylate: effect of type and concentration of initiator in unseeded polymerization system

The free radical emulsion copolymerization of methylmethacrylate (MMA) and ethylacrylate (EA) initiated by a water-soluble initiator (potassium persulphate, KPS) at 50 °C in the presence of anionic emulsifier above critical micelle concentration under constant stirring speed in an inert atmosphere is investigated. The effect of blend of KPS and oil-soluble initiators [KPS+2,2^′-azobisisobutyronitrile (AIBN)] is also examined. The order of the interval-II polymerization rate (R_p) is found to be 0.76±0.03 in KPS initiation alone and 0.72±0.04 in presence of fixed concentration of AIBN under similar experimental condition. On the other hand, interestingly, the rate of polymerization is found to be propotional to the 0.40th power of the AIBN concentration in presence of fixed concentration of KPS. The kinetic features of the present investigation indicate that probably the radical desorption is relatively facile and also the cage effect may be operative under high conversions (i.e. in polymer particles) in this MMA/EA emulsion copolymerization system. It is also found that the polydispersity index of polymer is being influenced by the type and concentration of initiators.     

Emulsifier-free emulsion polymerization of tetrafluoroethylene by radiation. III. Simultaneous formation of hydrofluoric acid

Simultaneous formation of hydrofluoric acid (HF) in the radiation-induced polymerization of tetrafluoroethylene (TFE) was investigated. HF concentration in PTFE latex was determined mainly by conductometric titration with 0.01 and 0.001N NaOH. The amount of HF formed is almost independent of agitation speed and the amount of n-hexadecane added and is maximal at ca. 70°C corresponding to the rate of polymerization. The rate of HF formation increases with the initial pressure of TFE monomer and dose rate and decreases with polymerization or TFE consumption. This fact suggests that HF is formed mainly by TFE reactions and not by the degradation of PTFE. The mechanism of HF formation in this reaction system in the absence of oxygen is shown in the following two schemes: scheme I is the reaction of TFE with primary radicals (OH·, H·, e) from the radiolysis of water; scheme II is the reaction of water with the species from the radiolysis of TFE. On the assumption that HF is formed only according to scheme I, the G value of HF formation G(HF)_calc can be calculated as 11.25. All observed G values G(HF)_obs are larger than G(HF)_calc. When the polymerization is carried out at 20 kg/cm² under various dose rates, G(HF)_obs increases with the dose rate. When the polymerization is carried out at 3.0 × 10⁴ rad/hr under various pressures, G(HF)_obs decreases with the decrease in pressure from 20 to 2 kg/cm² and is fairly close to G(HG)_calc at 2 kg/cm². This indicates that HF formation is due mainly to scheme II at high pressure (in the presence of enough TFE) and to scheme I as the pressure is lowered.     

Particle nucleation and monomer partitioning in styrene O/W microemulsion polymerization

Particle nucleation in the polymerization of styrene microemulsions was found to take place throughout the polymerization as indicated by measurements of the particle number as a function of conversion. A mechanism based on the nucleation in the microemulsion droplets was proposed to explain the experimental findings although homogeneous nucleation and coagulation during polymerization were not completely ruled out. A thermodynamic model was developed to simulate the partitioning of monomer in the different phases during polymerization. The model predicts that the oil cores of the microemulsion droplets were depleted early in the polymerization (4% conversion). Due to the high monomer/polymer swelling ratio of the polymer particles, most of the monomer resides in the polymer particles during polymerization. The termination of chain growth inside the polymer particles was attributed to the chain transfer reaction to monomer. The low n? (less than 0.5) of the microemulsion system was attributed to the fast exit of monomeric radicals.