无皂乳液聚合中单分散粒子的形成过程

在少量双官能团水溶性共单体(磺化丁二酸-聚乙二醇-烯丙基缩水甘油醚酯)存在下进行MMA/BA无皂乳液聚合,用CoulterLS230激光粒径分析仪研究了乳胶粒子的成核机理和单分散粒子的形成过程.乳胶粒子的成粒过程属多步成粒机理:先均相成核,形成不稳定的初始粒子,然后凝聚成稳定乳胶粒,其粒径分布经历了先变宽后变窄的过程,这是聚合过程中成核-凝聚-增长共同作用,相互竞争的结果.它还导致聚合初期出现周期成核的现象.     

MMA/BA无皂乳液聚合机理研究--三阶段成粒机理

用Coulter LS230激光粒径分析仪研究MMA/BA无皂乳液共聚合中单分散粒子的成粒机理.根据理论和实验数据分析,其成核过程为均相成核机理.根据粒径分布和粒子数变化情况,把无皂乳液聚合过程分为3个阶段:第一阶段为成核-凝聚阶段,体系粒子数密度迅速增加,而粒径变化较小;第二阶段为成核-凝聚、增长-聚并共存阶段,当粒子数密度开始快速减少时,标志着第一阶段结束,第二阶段开始,当初级粒子开始消失时,标志着第二阶段结束;第三阶段为增长-聚并阶段.成核过程结束后,粒子迅速增长,较小粒子在增长过程中的优先聚并,导致粒径逐渐趋向均一,最终生成单分散性乳液.     

表面活性单体存在下的MMA／BA乳液共聚合（Ⅱ）：聚合动力？…

研究了表面活性单体「磺化－十二醇－烯丙基甘油－丁二酸酯钠盐（ＺＣ－Ｌ）」的用量对ＭＭＡ／ＢＡ／ＺＣ－Ｌ乳液聚合速率和粒径的影响,用Ｃｏｒｌｔｉｒ ＬＳ２３０型激光粒径分析仪测定聚合过程中乳液的粒径和粒径分布变化,并与ＭＭＡ／ＢＡ无皂乳液聚合及十二烷基苯磺酸钠存在下的ＭＭＡ／ＢＡ乳液聚合作了比较。「ＺＣ－Ｌ」〈ＣＭＣ时,成核机理为均相成核机理,乳胶粒需依靠粒子间的凝聚来提高表面电荷密度而稳定;「ＺＣ     

表面活性单体存在下的MMA/BA乳液共聚合(Ⅱ)聚合动力学及机理

研究了表面活性单体[磺化-十二醇-烯丙基甘油-丁二酸酯钠盐(ZC-L)]的用量对MMA/BA/ZC-L乳液聚合速率和粒径的影响,用CoulterLS230型激光粒径分析仪测定聚合过程中乳液的粒径和粒径分布变化,并与MMA/BA无皂乳液聚合及十二烷基苯磺酸钠存在下的MMA/BA乳液聚合作了比较.[ZC-L]CMC时,成核机理包括均相成核和胶束成核机理,生成的粒子因吸收体系中的表面活性单体而稳定存在.     

Particle Nucleation and Growth in the Emulsion Polymerization of Styrene: Effect of Monomer/Water Ratio and Electrolyte Concentration

This work is an extension of previous research results reported by our team (Colloid and Polymer Science 2013, 291: 2385-2398), where large scale and high solid content latexes of poly(n-butyl acrylate) were obtained with the particle coagulation method induced by the electrolyte. However, how to prepare controlled particle size distribution polymer latex has not been studied. Thus, in this study, the effect of the monomer/water ratios and electrolyte concentrations on particle formation and growth methods were studied by following the tracks of the evolutions of particle size, number and distribution as a function of reaction time or conversion. Experimental results showed that the length of time that particle nucleation occurred increased with increasing monomer charged for the systems without electrolyte. A point worthy of attention here is that homogeneous nucleation may occur at high monomer concentrations (30/70, 40/60). However, electrolyte added could be made the nucleation mechanism shift from micellar/homogeneous nucleation to micelle /coagulation nucleation. As a result, the final particle size distribution can be controlled by adding an appropriate electrolyte to regulate the nucleation mechanism. Spherical and uniformly sized particles could be obtained when electrolyte concentration is between 0.2 wt% and 0.4 wt% for water at the high monomer/water ratio (40/60). The effects of electrolyte concentration on nucleation mechanism mainly were expressed by decreasing the solubility of the monomer and interparticle potential, and then preventing homogeneous nucleation and enhancing particle coagulation.     

在18-冠-6存在下苯乙烯无乳化剂乳液聚合

本文研究了在苯乙烯不外加乳化剂的乳液聚合体系中,加入王冠醚18-冠-6对聚合反应及其产物的影响。发现18-冠-6能起到相转移催化作用,并提出两相粒子成核机理,对实验结果作出了合理的解释。     

A kinetic investigation of surfactant‐free emulsion polymerization of styrene using laponite clay platelets as stabilizers

We report the kinetics and mechanism of soap‐free emulsion polymerization of styrene using laponite platelets as stabilizers. The polymerization was initiated by potassium persulfate and the latex particles were stabilized by laponite platelets dispersed in water. Laponite adsorption on the polymer particles was enhanced by the addition of poly(ethylene glycol) monomethylether methacrylate (PEGMA). Particle nucleation can be described using the classical homogeneous nucleation mechanism followed by coagulation of unstable precursors. Oligomeric radicals formed in the water phase become insoluble and precipitate on the laponite surface leading to primary precursor particles composed of a few polymer chains and one or several clay platelets. Mature latex particles are then generated by coagulation and growth of the previously formed precursor particles. Both the nucleation and initial aggregation rates increased in the presence of PEGMA. Calorimetric monitoring of the polymerization allowed estimating the heat produced by the reaction and the monomer conversion. Hence, using the monomer material balance, the number of radicals in the polymer particles could be estimated precisely. The average number of radicals per particle, $ \bar n $ , was found to be high in the range 3–6. This result was attributed to strong attractive interactions between the growing radicals and the clay surface. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011.     

The mechanisms of latex particle formation and growth in the emulsion polymerization of styrene using the surfactant sodium dodecyl sulfate

A new method is presented that provides experimental information which is qualitatively and quantitatively sensitive to assumptions made as to the mechanisms of free radical entry and of latex particle formation in emulsion polymerization systems. The method consists of (1) obtaining (by electron microscopy) the full particle-size distributions (PSDs) at several different times soon after the cessation of latex particle nucleation, (2) using these PSDs to determine the volume dependences of the various rate coefficients governing particle growth by fitting the data to the appropriate evolution equations, and (3) employing these empirical rate coefficients to find that time dependence of the nucleation rate which fits the early-time PSD (again using the evolution equations). This method is quite sensitive to mechanistic assumptions: for example, one is able to determine whether or not the nucleation rate is an increasing or decreasing function of time. The technique is applied to a styrene nucleation system employing sodium dodecyl sulfate as surfactant at well above the critical micelle conventration. The data cannot be fitted even qualitatively by a simple one-step nucleation mechanis, whether it involes micellar entry or homogeneous nucleation. It is found, on the other hand, that the results can be accurately fitted by assuming that coagulation events between primary colloidal particles, perhaps formed by homogeneous nucleation, dominate both the nucleation process and the entry of free radicals into mature latex particles. In addition, the data indicate that the rate of free radical entry into the latex particles decreases with increasing particle size, at least for particles of unswollen radius less than ca. 40 nm.     

Crosslinked polystyrene latexes prepared with 12-(o-styryloxy)dodecyltrimethylammonium bromide

Crosslinked polystyrene latexes with high surface charge densities have been prepared with 1, 3, and 5 mol% divinylbenzene and 0.5–5.0 mol% of 12-(o-styryloxy)dodecyltrimethylammonium bromide ( 1 ). Within experimental error all of surfactant 1 was incorporated into the particles. Analysis of the latex particles by transmission electron microscopy showed nonspherical shapes, aggregates, and number average diameters ranging from 22 to 95 nm. The particle size decreases and the polydispersity increases with increasing amounts of surfactant 1 and of divinylbenzene in the monomer mixture. A mechanism is proposed for particle formation by precipitation of crosslinked polymer from monomer droplets to form primary particles, coagulation of the primary particles to colloidally stable aggregates, and growth by further polymerization.     

An investigation of the effect of sodium dodecyl sulfate on quasi-emulsifier-free emulsion polymerization for highly monodisperse polystyrene nanospheres

A quasi-emulsifier-free emulsion polymerization for the synthesis of highly monodisperse polystyrene (PS) microspheres with the diameter less than 100 nm has been developed, which is similar to the conventional emulsifier-free emulsion polymerization except for the addition of a small amount of sodium dodecyl sulfate (SDS) below its critical micelle concentration. To study the effect of SDS on the polymerization, we first explored the nucleation mechanism, and supposed that mixed micelles of SDS and surface active styrene oligomers may be formed and provide indispensable loci for the growth of PS primary particles. As the precursor particles grow, they become unstable and tend to adsorb the rest of SDS not participating in the formation of mixed micelles on their surfaces. Thereby, it reduces the coagulation tendency of the particles, and results in a significant increase of the particle number density which was found to be directly proportional to the square of the surfactant concentration, along with the increase of the polymerization rate and the decrease of the size of final PS particles. In addition, the presence of the surfactant will shorten the duration of the nucleation and, thus, lead to the highly uniform size of PS nanospheres.     

Mechanism of emulsion polymerization of styrene in soap-free systems

The formation and growth of monodisperse polystyrene latex particles in the absence of added surfactant has been studied by sampling polymerization reactions at different times and determining the surface and bulk properties of the latex. A large number of nuclei in excess of 5 × 10¹²/ml were generated during the first minute of reaction, but this fell due to coagulation until a constant number (10¹¹?10¹²/ml) was reached. The rate of polymerization per particle was then found to be proportional to the particle radius. Gel-permeation chromatography has shown that the initial particles consist mainly of material of MW 1000 with a small amount of polymer up to MW 10⁶, and the presence of this low molecular weight polymer, which in many cases can still be detected after 100% conversion, is taken as being indicative of particle formation via a micellization-type mechanism involving short-chain (MW 500) free-radical oligomers. M?_n values determined for the latex particles throughout the course of reactions show that the molecular weight increases to a maximum of about 10⁵ as the particles grow. The presence of anomalous regions within the particles has been confirmed by transmission electron microscopy, scanning electron microscopy, and gas adsorption studies. It has also been found possible to re-expose these regions within apparently homogeneous particles by stirring with styrene monomer; this is indicative of a molecular weight heterogeneity within the latex particles. The presence of sulfate, carboxyl, and hydroxyl groups upon the latex particle surfaces has been determined by conductometric titration.     

核/壳乳液聚合工艺中乳化剂的定量研究

以十二烷基硫酸钠(SDS)为乳化剂,采用多步种子乳液聚合方法制备了核/壳结构乳液,研究了乳化剂加入量以及加料速率对核壳乳液聚合的影响,并推导了核及壳乳液聚合阶段所需乳化剂量的计算公式.研究表明,当种子、核、壳乳液聚合阶段单体量分别为12g、50g和50g,种子乳液聚合阶段加入的乳化剂量为0.44g时,控制核、壳乳液聚合阶段乳化剂的加入量分别在0.64～2.07g及0.04～2.12g之间,且预乳化单体的滴加速度低于2.3g/min时,可以防止二次成核及新乳胶粒子的形成,制得粒径分布窄、核/壳结构明显的乳胶粒子.利用透射电镜(TEM)对所制备的核壳结构乳胶粒子的结构形态进行了验证,试验结果与理论预测结果一致.     

Synthesis of large-scale,narrowly dispersed,highly cross-linked,and spherical latex particles via one-step emulsion polymerization through particle coagulation

Particle coagulation technology is a facile approach to prepare large-scale and narrowly dispersed polymer particles. However, diverse shapes such as ellipsolid, snowman, dumbbell, and trimer among others were obtained if the cross-linker was directly added into the initial reaction mixtures due to the restriction of the highly cross-linking particle fusion process. In this study, we prepared sub-200?nm, narrowly dispersed, highly cross-linked, and spherical latex particles using particle coagulation technology by controlling the relation between the cross-linking net formation and particle coagulation. Depending on the addition time or feeding rate of the cross-linker (divinylbenzene, DVB), the particles with different sizes or shapes were obtained. The later the addition start time of DVB, the narrower the particle size distribution of the latex particles. Alternatively, the increase of the continuing feeding time could also be used to decrease the width of particle size distribution of the ultimate latex. In addition, narrowly dispersed and spherical latex particles also could be directly obtained by advancing the particle coagulation time using 2, 2′-Azobis (2-methylpropionamidine) dihydrochloride as a cationic initiator. Our study presents a new method that will further widen the fields of application of particle coagulation technology.     

Aggregation phenomena in the suspension polymerization of VCM

In the suspension polymerization of VCM, insoluble polymer particles are formed inside the monomer droplets. The growth and aggregation of these particles are responsible for important polymer properties, such as porosity. It is well established that the most characteristic polymer particles, the primary particles, are of a narrow distribution with a size (diameter) ranging from 0.10–0.20 m. This work studied the formation of primary particles based on the aggregation phenomena that take place inside a monomer droplet. This was done by formulating a population balance equation, which was based on the following considerations: a) polymerization occurs in both the monomer and the polymer phases; b) there is continuous formation of the basic particles in the monomer phase; c) the growth of the polymer particles occurs as a result of both polymerization in the polymer phase and aggregation of the particles; d) the colloidal properties of the particles that are responsible for the aggregation phenomena were considered to be the net result of attraction and repulsion energies.It was shown that for particles carrying a constant charge it was not possible to predict the formation of primary particles of size 0.10–0.20 m. The particle size distribution had a mode diameter equal to the diameter of the basic particles. Consequently, the particle charge was allowed to vary in a way proportional to the particle radius raised to a power coefficient. For values of the coefficient greater than zero, i. e., when the particle charge increased during polymerization, the aggregation of the basic particles was efficient enough to result in the formation of large primary particles.     

Large-scale and narrow dispersed latex formation in batch emulsion polymerization of styrene in methanol–water solution

This work is an extension of previous research results reported by our team (Colloid Polym Sci 291:2385–2389, 2013), where monodisperse, large-scale, and high-solid-content latexes of poly(n-butyl acrylate) were obtained with the particle coagulation method induced by the electrolyte. However, large-scale polystyrene latex particle is difficult to synthesize with this approach; moreover, demulsification phenomena easily take place especially in high solid content. In this article, a new approach to prepare large-scale polystyrene latex particle was proposed. Methanol was added to aqueous phases to decrease the interfacial tension between the polymer particle surface and continual phases, further decreasing interfacial free energy. Consequently, the surfactant molecules would loosely pack on the polymer particle surface, which is favored by particle coagulation. Experimental investigations showed that the final polystyrene particle scale only reaches to 93.5 nm when the methanol/water ratio is equal to 0:100, but the particle size attains 270 nm when the methanol/water ratio is equal to 30:70. These results indicated that polystyrene particle coagulation can be induced by methanol by varying the surfactant molecule adsorption on the particle surface. This investigation also provided a new simple approach to prepare large-scale, stable latex particles.     

Flocculation mechanism induced by cationic polymers investigated by light scattering

Three cationic polymers with molecular weights and charge densities of 3.0 x 10(5) g/mol and 10%, 1.1 x 10(5) g/mol and 40%, and 1.2 x 10(5) g/mol and 100% were chosen as flocculants to aggregate silica particles (90 nm), under various conditions, including change in polymer dosage, particle concentration, background electrolyte concentration, and shear rate. The size and structure of flocs produced were determined using the static light scattering technique. On the basis of measurements of polymer adsorption and its effect on the zeta potential and floc properties, it has been found that the polymer charge density plays an important role in determining the flocculation mechanism. Polymers with a 10% charge density facilitate bridging, 40% charged polymers bring about either a combination of charge neutralization and bridging or bridging, depending on the polymer dosage, and polymers with the charge density of 100% induce electrostatic patch flocculation mechanism at the optimum polymer dosage and below but bring about bridging mechanism at the polymer dosage approaching the adsorption plateau value. Bridging aggregation can readily be affected by the particle concentration, and an increase in particle concentration results in the formation of larger but looser aggregates, whereas electrostatic patch aggregation is independent of particle concentration. The addition of a background electrolyte aids in bridging aggregation while it is detrimental to electrostatic patch aggregation. It has also been found that the effect of shear rate on the mass fractal dimension depends on polymer charge density.     

Pickering emulsion polymerization kinetics of styrene: Comparison of bare and surface modified SiO2 nanoparticles

A step towards the understanding of some mechanistic events occurring in the styrene Pickering emulsions, using a SiO₂ dispersion, is presented. Polymerizations at 80°C with different levels of a water soluble initiator were performed. The emulsion polymer content was ca. 15% with conversions close to 90%. With conversion and particle size measurements, the particle density was estimated for bare and surface modified SiO₂ particles. Then, the average number of radicals per particle was inferred, yielding a pseudo-bulk type polymerization. It was found that bare SiO₂ nanoparticles do not participate in the nucleation mechanism; however, they, along with the initiator, promote an enhanced oligomer coagulation. On the other hand, the hexadecyltrimethylammonium bromide modified SiO₂ nanoparticles do participate in the nucleation and coagulation mechanisms, yielding more stable and smaller poorly-covered polymer particles. This approach allowed untangling some events such as: particle nucleation, radical entry to particles, particle density, coagulation and vitreous and Trommsdorff effects.     

Kinetics and mechanism of emulsifier-free emulsion polymerization: Styrene/surface active ionic comonomer system

The emulsifier-free emulsion polymerizations of styrene in the presence of the surface active comonomer, undecylenic isethionate sodium salt (at concentration below its critical micelle concentration), and of the initiator, potassium persulfate, indicate that the number of polymer particles and the rate of polymerization at steady state is dependent on 1-power of the comonomer concentration and 1/2-power of the initiator concentration. This result suggests a homogeneous nucleation mechanism by which particles are formed from coiled-up oligomeric radical chains originally dissolved in the aqueous phase. Size distribution of the particles is rather narrow and has a uniformity very close to one (ca. 1.02) after 30% conversion. Addition of salt such as sodium sulfate to increase the ionic strength in the aqueous phase results in a formation of micelles (which can grow to become polymer particles) in addition to the formation of polymer particles through the homogeneous nucleation mechanism. Variation of the ionic strength leads to a variation in the number of polymer particles due to a competition between these two nucleation mechanisms and gives a minimum of the number of polymer particles and a maximum of the average particle diameter.     

Poly(ethylene oxide) macromonomer-grafted polymer nanoparticles synthesised by emulsifier-free emulsion polymerisation

Ultrafine polymer nanoparticles based on poly(ethylene oxide) (PEO) macromonomer-grafted polystyrene (PS) have been synthesised by emulsifier-free emulsion polymerisation. In addition to the binary copolymerisation between PEO macromonomer and styrene, ternary copolymerisations were also conducted in the presence of a cationic monomer (2-(methacryloyloxy)ethyl) trimethylammonium chloride (MATMAC) as a second comonomer. The size and charge characteristics of fine nanoparticles were characterised using both photon correlation spectroscopy and transmission electron microscopy techniques as well as colloidal titration. It was found that after PEO chains (repeat unit 9 or higher) were incorporated into the PS latex, the particle size was significantly reduced owing to the steric effect contributed from grafted PEO chains. Ternary copolymerisation using MATMAC as comonomer further reduced the particle size, leading to nanoparticles as small as 60 nm. Increasing the MATMAC feed ratio gradually reduced the final size of the nanoparticle, owing to the enhancement in electrostatic stabilisation, whereas increasing the PEO macromonomer feed ratios led to slightly larger particles but significantly inhibited the agglomeration of primary particles. The formation mechanism of the nano- or microparticles with various sizes during polymerisation is discussed in terms of nucleation, agglomeration and adsorption of primary particles.     

Polymer mediated peptide immobilization onto amino-containing N-isopropylacrylamide-styrene core-shell particles

Monodisperse cationic core-shell latex particles have been prepared using a shot polymerization process, with N-(3-aminopropyl)-methacrylamide-hydrochloride (APMH) as the functional monomer. The final latexes were characterized with respect to final polymerization conversion, water soluble polymer formation, particle size and size distribution, surface charge density and electrokinetic properties. Then the covalent grafting of maleic anhydride-alt-methyl vinyl ether (MAMVE) copolymer onto aminated latex particles was investigated. The most efficient conditions to obtain derivatised particles with no alteration of the colloidal stability were to control both polymer amount/latex particles concentration ratio and the mixing method of the two species. The charge inversion of the hydrolysed MAMVE functionalized particles was demonstrated by measuring the electrophoretic mobility as a function of pH. Finally, the covalent binding approach was implemented with peptide-MAMVE conjugates, confirming the great potential of this promising methodology for the preparation of reactive latex particles bearing peptides.