Microemulsion polymerization of styrene in the presence of macroinimer

The kinetics of o/w electrostatically and sterically-stabilized microemulsion polymerization of styrene with and without macromonomeric azoinitiator (macroinimer; MIM) have been investigated. The microemulsion polymerization stabilized by the ionic emulsifier sodium dodecyl sulfate (SDS) or the non-ionic emulsifier Tween 20 (Tw 20) was initiated by ammonium peroxodisulfate (APS)/sodium thiosulfate (STS) redox system. The rate of polymerization vs. conversion curve shows the two non-stationary rate intervals. This behavior is a result of two opposing effects, the continuous particle nucleation and the decrease of monomer concentration at the reaction loci. The addition of MIM favors the additional particle nucleation. The sterically (Tw 20)-stabilized microemulsion polymerization is much faster than that of the electrostatically (SDS)-stabilized microemulsion polymerization. This was attributed to the higher Tw 20 concentration and increased solubilization of MIM and comonomer concentration in the polymer particles. The formation of initial large polymer particles is attributed to the intensive agglomeration polymer particles with monomer droplets. The continuous decrease in the average size is mainly attributed to the additional particle nucleation.     

乳液聚合粒子成核和粒子大小的影响因素

介绍了乳液聚合中胶束,均相和单体液滴三种不同的粒子成核机理。讨论了引发速率和乳化剂浓度以及不同聚合方法和各种条件对乳胶粒子大小的影响。     

Anionic ring-opening polymerization of octamethylcyclotetrasiloxane in emulsion above critical micelle concentration

Batch anionic ring-opening polymerization of octamethylcyclotetrasiloxane in emulsion using nonionic and cationic emulsifiers was studied. The concentration of emulsifiers was set above their critical micelle concentration. Effects of emulsifier concentration, nonionic/cationic emulsifier ratio and cationic emulsifier/initiator (KOH) ratio on the kinetics, average particle size and distribution and on the average molecular weight and distribution were investigated and discussed. At the beginning of the polymerization, empty micelles, active micelles (polymer particles) and monomer droplets co-exist in emulsion. The transport of monomer from monomer droplets toward empty micelles was confirmed by monomer droplets and empty micelles disappearance and by formation of smaller particles. The transport of monomer from monomer droplets toward polymer particles was not confirmed, since the average polymer particle size did not increase during polymerization. It was proposed, that at lower conversions, monomer diffuses from polymer particle interior to particle surface, while at higher conversions, the monomer diffuses from larger to smaller polymer particles. Emulsifier concentration, nonionic/cationic emulsifier ratio and cationic emulsifier/KOH ratio have an evident effect on the kinetics and on the average molecular weight, thus demonstrating that cationic emulsifier participates to the initiation reaction.     

Study of different types of monomer emulsion feedings to semibatch emulsion polymerization reactors

Semibatch emulsion polymerization processes with a monomer emulsion feed are of great importance in both academia and industry. Monomer emulsion feeds can be applied to semibatch reactors using either a stream of an emulsified monomer or two streams of a neat monomer feed and an aqueous solution of an emulsifier. The effect of the feeding policy on the rate of polymerization and on the secondary particle formation was studied for a seeded semibatch emulsion polymerization of styrene. When a single-stream monomer emulsion feed is applied to a semibatch process, the monomer-swollen micelles formed in the feed might become the locus of initiation upon entering the reaction vessel. Under the conditions of this study, the application of monomer emulsion feed in either one stream or two streams did not result in secondary particle formation. The incoming monomer-swollen micelles were disintegrated to supply emulsifier molecules for the stability of growing particles, before they can capture radicals and become polymer particles. The rate of polymerization was found to be independent of the way that the monomer emulsion feed is added. In the absence of nitrogen, the rate of polymerization decreased more appreciably for the monomer emulsion feed, due to the oxygen dissolved in the emulsified monomer. The number of particles, however, was not affected by the purging policy. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 82: 2472–2477, 2001     

Mechanism of particle formation in radical emulsion copolymerization of styrene with α-tert-butoxy-ω-vinylbenzyl-polyglycidol macromonomer

In the batch emulsion copolymerization of styrene and α-tert-butoxy-ω-vinylbenzyl-polyglycidol macromonomer, carried out at macromonomer concentrations exceeding the critical micelle concentration (c_mc), particles are formed by a two-step coagulative nucleation mechanism. This mechanism leaves its mark on morphology of particle interface, rate of polymerization and on molecular weight distribution of the obtained polymer. AFM studies revealed that the interface of particles is composed of objects with dimensions close to dimensions of the primary particles. Compartmentalization of styrene in the macromonomer micelles leads to the higher initial rate of styrene conversion than in the similar macromonomer free homopolymerization of styrene. The initial polymerization in the monomer-swollen macromonomer micelles, similar to the microemulsion polymerization, is responsible for the formation of the highest molecular weight component. In the mature particles there are two different polymerization loci: the interfacial layer and the core. This leads to bimodal molecular weight distribution of the formed polymer.     

Effect of stirring on the emulsion polymerization of styrene

The purpose of this paper is to clarify the effect of stirring on the course of emulsion polymerization of, for example, styrene. It establishes the existence of an optimum range of stirring speed and three important factors which must be considered in carrying out emulsion polymerization. (1) Stirring significantly affects the course of reaction in the presence of an imperfectly purified nitrogen atmosphere. Consequently, the number of polymer particles produced and the polymerization rate per particle will be affected. (2) At higher stirring speeds, polymer particles coagulate and coalesce. At lower stirring speeds, the reaction rate is controlled by the monomer transport rate from monomer droplets to the aqueous phase. (3) Stirring contributes to the reduction of the number of micelles because emulsifier molecules are adsorbed onto the surfaces of monomer droplets finely dispersed by the stirring. At low emulsifier concentrations near the critical micelle concentration, this effect cannot be neglected.     

Semibatch emulsion polymerization of butyl acrylate. II. Effects of emulsifier distribution

The effects of emulsifier distribution ratio between the initial charge and the feed on particle formation and kinetics of butyl acrylate emulsion polymerization, using sodium lauryl sulfate as emulsifier and potassium persulfate as initiator, were investigated. The number of particles increased with initial emulsifier concentration in the reactor charge. It was shown that traditional ranking, in terms of number of particles produced, of semibatch emulsion polymerization with monomer emulsion feed is not always justified and a semibatch emulsion polymerization can produce far more particles than a conventional batch emulsion polymerization. The number of polymer particles was found to be practically independent of the emulsifier distribution ratio between the charge and the feed for a high overall emulsifier concentration, while for a low overall emulsifier concentration, the number of particles increased with initial loading of the emulsifier. The polydispersity index (PDI) of the final latexes showed a minimum with emulsifier distribution. A bimodal particle size distribution, and a latex with a large PDI, was obtained when there was no emulsifier in the charge. As the initial emulsifier charge increased, a unimodal PSD with a smaller PDI was obtained. With higher proportions of emulsifier in the initial charge, the PDI rose again due to particle nucleation at monomer‐starved conditions, and a skewed unimodal PSD was obtained. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 79: 582–597, 2001     

Kinetics of emulsifier adsorption and nucleation of latex particles

Evidence suggesting that the ultimate latex particles formed in the emulsion polymerisation of styrene are formed by the coalescence of unstable primary particles appears incompatible with the usual assumption that latex particles are predominantly nucleated from monomer-swollen micelles in this system. However, this is not necessarily the case if the rate of emulsifier adsorption is not always sufficient to maintain a saturated monolayer of adsorbed emulsifier at the surface of growing latex particles so long as emulsifier micelles remain in the system. This appears to be the situation particularly when non-ionic emulsifiers are used. Even with ionic emulsifiers abnormally high emulsifier/monomer ratios are needed to prevent the coalescence of primary latex particles. Some very high molecular weight polymer (controlled by the transfer constant to monomer) would be expected in the initial stage of styrene emulsion polymerisation if the micellar nucleation of latex particles is dominant.     

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     

Semibatch anionic ring-opening polymerization of octamethylcyclotetrasiloxane in emulsion

The semibatch anionic ring-opening polymerization of octamethylcyclotetrasiloxane in emulsion was studied under the condition of no seed particles used in polymerization system. Concentration of emulsifier was set above critical micelle concentration. Effect of monomer feed rate on the chemical kinetics, average particle size and distribution and on the polymer molar mass and distribution was investigated. During monomer addition, polymerization rate was constant and controlled by the monomer feed rate. According to the proposed mechanism for anionic polymerization in emulsion, it was assumed that also the number of active polymerization sites and monomer concentration at the particle surface, where propagation occurs, remain constant. A continuous increase of average particle size and absence of extensive condensation reactions at high conversion suggested that the freshly added monomer diffuses toward hydrophobic polymer particles, where it is consumed in propagation reaction and/or accumulated in the particle core.     

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.     

Particle nucleation mechanism for the emulsion polymerization of styrene with a novel polyester emulsifier

The particle nucleation mechanism in emulsion polymerization of styrene with a novel polyester emulsifier, 5‐sulfoisophthalic acid dimethyl ester sodium salt‐modified tetracarboxylic acid‐terminated polyester (SMTAPE), was investigated. The consumption of SMTAPE micelles was monitored by the measurement of surface tension during the emulsion polymerization. Kinetic studies and emulsifier consumption clearly showed that a continuous nucleation mechanism without Smith–Ewart interval II was characteristic of this system. It was attributed to the high concentration of SMTAPE emulsifier in the polymerization, which led to a large surface area and a vast number of micelles around 10 nm in size that served as the major locus of particle nucleation. A broad particle size distribution was observed throughout the reaction, and the nucleation period lasted well into the reaction until the disappearance of the micelles or the disappearance of monomer droplets. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 82: 1061–1070, 2001     

Microemulsion Radical Polymerization of Alkyl Acrylates

The oil/water microemulsion polymerizations of alkyl (methyl (MA), ethyl (EA), butyl (BA), hexyl (HA) and 2-ethylhexyl (EHA)) acrylates initiated by a water-soluble (ammonium peroxodisulphate (APS)) initiator were investigated. The rate of polymerization versus conversion curve shows two inconstant intervals. The maximum rate of polymerization of MA, EA, BA and EHA is found to be proportional to the 0·53, 0·17, 0·46 and 0·59 powers of the APS concentration and to the first power of the monomer (MA, BA and EHA) concentration. Homogeneous (solution) radical polymerization is operative in the MA, BA and EHA systems. The strong rate dependence on the EA monomer concentration is assumed to be caused by both the polymerization within the micelle core and the gel effect. The particle size was observed to increase with conversion in the EA and BA runs and to decrease in the EHA and HA runs. The number of particles increased with conversion during the whole polymerization. The polymer particles grow by recruiting monomer and emulsifier from the free monomer-swollen micelles and dead particles. © 1997 SCI.     

乳液聚合成核阶段的模拟与分析

建立了乳液聚合成核阶段的Monte Carlo模型，并用计算机对一个体积为10^-17m^3的微型反应器中苯乙烯的乳液聚合进行了模拟。以计算机生成随机数作为自由基被胶束和乳胶粒捕获的几率，模拟了在微型反应器中每一个自由基的生成、被胶束或乳胶粒捕获的过程以及每一个乳胶粒的生成及增长过程。通过对每一个乳胶粒在增长过程中各参数的统计计算，研究了乳液聚合成核阶段诸参数(乳胶粒数目、乳胶粒直径与粒径分布、单体转化率、聚合反应速率等)与乳化剂浓度[S]及引发剂浓度[I]的关系。结果表明，苯乙烯的乳液聚合体系中乳胶粒数目与[S]^0.5996[I]^0.4016成正比：在成核阶段乳胶粒直径分布先变宽后变窄，乳液聚合过程中乳胶粒直径分布有自动变窄的趋势；成核阶段持续时间t12与[S]^0.60[I]^0.60成正比，成核阶段结束时的单体转化率X12与[S]^1.20[I]^0.20成正比。     

Modeling evidence in support of coagulative nucleation theory

An extensive lumped model was developed for emulsion polymerization. It incorporated all of the complex processes: aqueous‐phase radical balances for all radical species arising from initiator decomposition and from exit; determination of radical number inside the particles by balance among rates of radical entry into, exit from, and termination inside the particles; determination of the monomer concentration inside the particles and in the aqueous phase by a thermodynamic equation; and particle formation by micellar, homogeneous, and coagulative nucleation. Model validation was done for the system with styrene (monomer), potassium persulfate (initiator), and sodium dodeceyl sulfate (emulsifier) and for the variables, which included the duration of nucleation, conversion at the end of nucleation, and total number of particles formed. The validation process revealed that coagulation during nucleation needed to be included in the model, even for emulsifier concentrations above the critical micelle concentration. The model predictions were in good quantitative agreement with the experimental data. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011.     

Particle nucleation in emulsion copolymerization containing multifunctional monomers

Characteristic features of particle nucleation and growth in the emulsion copolymerization of styrene and multifunctional monomers such as ethylene glycol dimethacrylate and divinylbenzene were investigated. It was found that in these emulsion copolymerization systems the rate of polymerization per particle decreased drastically with an increase in the content of the multifunctional monomer in the initial monomer feed, and the number of polymer particles produced, on the other hand, increased corresponding to the decrease in the rate of polymerization per particle. The increase in the number of polymer particles produced was explained on the basis of Smith-Ewart theory, which predicts that the number of polymer particles produced would increase inversely proportional to the 0.4 power of the volumetric growth rate per particle, that is the rate of polymerization per particle during the interval of particle formation (Interval I). It was pointed out that the decrease in the rate of polymerization per particle in Interval I would be due mainly to a decrease in the monomer concentration in the polymer particles which was caused by crosslinking networks of the resultant polymer.     

Self‐assembly of pH‐responsive acrylate latex particles at emulsion droplets interface

The self‐assembly of pH‐responsive poly (methyl methacrylate‐co‐acrylic acid) latex particles at emulsion droplet interfaces was achieved. Raising pH increases the hydrophilicity of the latex particles in situ and the latex particle acts as an efficient particulate emulsifier self‐assembling at emulsion droplet interface at around pH 10–11 but exhibits no emulsifier activity at higher pH. This effect can be reversibly induced simply by varying the aqueous phase pH and thus the latex emulsifier can be reassembled. The effect factors, including the aqueous phase pH, the surface carboxyl content, ζ‐Potential of the latex particles and oil phase solvent have been investigated. Using monomer as oil phase, the latex particles could stabilize emulsion droplets during polymerization and cage‐like polymer microspheres with hollow core/porous shell structure were obtained after polymerization. The mechanism of the latex particles self‐assembly was discussed. The morphologies of emulsion and microspheres were characterized by optical microscopy, scanning electron microscopy, and transmission electron microscopy. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007     

核壳结构聚硅氧烷/丙烯酸酯复合乳液(Ⅰ)乳化剂对乳液聚合过程及粒径分布和粒子形态的影响

采用种子乳液半连续法合成了具有高有机硅含量的聚硅氧烷/丙烯酸酯核壳结构复合乳液,研究乳化剂的种类、复配比例及质量浓度对有机硅／丙烯酸酯壳核乳液性能与乳胶粒径、分布和结构的影响.结果表明：阴离子乳化剂十二烷基硫酸钠(SDS)、十二烷基磺酸钠(SDS-2)、十二烷基苯磺酸钠(SDBS)所合成的乳胶粒子粒径依次增大,SDS与非离子型乳化剂OP-10复配使用时,随OP-10质量分数的增加,聚合速率和转化率降低,化学稳定性增加,乳胶粒子粒径增大,分布变宽,确定了复合乳化剂的最佳配比.随复合乳化剂浓度的增加,聚合速率加快、转化率增加,乳胶粒子粒径减小而分布加宽.通过改变乳化剂加入方式可减小乳胶粒子的粒径分布.为减少壳层聚合物新粒子的产生,需严格控制乳化剂的浓度,使加入的壳层单体处于“饥饿”状态,在乳胶粒子表面富集、引发聚合,形成表层“过渡层”,最终形成核壳结构复合粒子.     

Experimental study of emulsion polymerization with crosslinking

Herein is reported the results of an extensive experimental investigation of the kinetics of emulsion polymerization as affected by crosslinking in the polymer particles. The model monomer system, methyl methacrylate (MMA) and ethylene glycol dimethacrylate (EGDMA), was chosen for this study due to its earlier comprehensive investigation in bulk polymerization. Standard recipes with sodium dodecylsulfate (SDS) as anionic emulsifier and potassium persulfate (KPS) as initiator were used for the batch emulsion polymerizations. Results, which clearly show the effect of crosslinking on the kinetics, are discussed in detail. These include swellability of polymer particles by monomer; polymer particle nucleation rates, below and above the critical micelle concentration (CMC); average number of radicals per particle; and gel-sol levels. It was found advantageous to use electron spin resonance (ESR) to follow radical concentrations during crosslinking in polymer particles. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 66: 935–957, 1997     

Simulation of particle size distribution in continuous emulsion polymerization styrene monomer

A detailed mathematical model for absolute particle-size distribution in continuous emulsion polymerization of styrene monomer is proposed on the basis of the modified population-balance theory. The computer simulation is then performed for styrene polymerization as an example of a nondesorption system. The effects of emulsifier and initiator concentrations as well as mean residence time on the absolute particle-size-distribution function, total concentration of polymer particles, conversion, average number and weight radii, average number of radicals per polymer particle, and polydispersity of radius are simulated by the model proposed.