乳液聚合中成核机理的研究进展

主要介绍了在乳液聚合反应中,各种成核过程的机理及其对乳液聚合反应和性能的影响;分析了胶束成核、均相成核、液滴成核的产生条件与影响因素;介绍了成核机理的最新研究进展.     

Kinetics of particle nucleation and growth in the emulsion polymerization of acrylic monomers

The rates of particle formation and growth during the earliest stages of the emulsion polymerization of several acrylic monomers have been studied as a function of surfactant concentration by means of the time-dependence of light scattering from the reaction mixtures. Investigated were methyl methacrylate, and methyl, ethyl and butyl acrylates at concentrations of SDS surfactant from 0 to several times the CMC. Under continuous photoinitiation the Rayleigh scattering intensities rose rapidly with slopes that increased with decreasing SDS concentration. The more water-soluble the monomer, the more slowly was the rate of increase in scattering intensity. Computer modeling, using HUFT theory, could be used to obtain values for the various parameters involved for MMA. The Fuchs stability factor, W, was found to be 1 for [SDS] = 0 and infinity for [SDS] = CMC or grester. The partition coefficient for J-mer oligomers between the aqueous and organic phases must rise exponentially with J. Electrostatic effects on radical capture by particles are considered to be negligible, but very important as they affect coagulation kinetics.     

Synthesis and nucleation mechanism of inverse emulsion polymerization of acrylamide by RAFT polymerization: A comparative study

Well-defined poly (acrylamide) is synthesized by RAFT inverse emulsion polymerization using hydrophilic and lipophilic initiators. The kinetic behavior observed for RAFT inverse emulsion polymerization is similar to that for RAFT inverse miniemulsion polymerization. The nucleation mechanism of inverse emulsion polymerization of acrylamide is firstly investigated by RAFT polymerization and verified by GPC and SEM measurements. Droplet nucleation is found to be the primary mechanism in the inverse emulsion polymerization of acrylamide. However, polymerization occurring in the continuous phase is not negligible when lipophilic initiator is used.     

w(MMA)对MMA乳液聚合成核的影响

以甲基丙烯酸甲酯(MMA)为单体,在c(乳化剂)=2mmol/L下制备乳液,研究不同w(MMA)对MMA乳液聚合成核的影响,并分析了不同w(MMA)对乳胶粒粒径的影响规律。实验结果表明:当w(MMA)≤10%,随着w(MMA)的增加转化率逐渐增大,乳胶粒的生成速率也逐渐增大;当w(MMA)≥10%时,转化率可以达到70%以上,w(MMA)对乳胶粒的生成速率的影响不大,多余的单体用于乳胶粒粒径的增长;在w(MMA)=10%时基本能够满足c(乳化剂)=2mmol/L时的成核要求。当5%≤w(MMA)≤30%,在乳化剂的自调节作用下,使得最终形成的乳胶粒数目相差不大。     

Crosslinking kinetics in emulsion polymerization

Network formation in free-radical polymerization is investigated both from experimental and theoretical points of view. In a bulk free-radical crosslinking copolymerization, the radical concentration increases significantly due to a decrease in the bimolecular termination rate. Some fraction of radicals is trapped during the course of network formation, and the reactivity of the radicals in liquid state is completely different from that in solid state (trapped radicals) in terms of both propagation and termination. In emulsion crosslinking polymerization, the crosslinking density tends to be fairly high even from the start of the polymerization. The crosslinking density development is substantially different from that in bulk and solution polymerizations, in which the crosslinking density increases with time, at least in the earlier stages of polymerization. Due to the elastic contribution of free energy change, the monomer concentration in the polymer particles is lower than for linear emulsion polymerizations as long as the monomer droplets exist, resulting in an enhanced tendency toward crosslinking reactions.     

Emulsification in batch emulsion polymerization

Dispersion of liquid–liquid systems is often applied in industrial processes such as extraction, suspension, and emulsion polymerization. The influence of emulsification of the monomer in the aqueous phase on the course and outcome of the batch emulsion polymerization of styrene has been studied. A visual criterion was applied for determining the lowest impeller speed for sufficient emulsification (N*). It appeared that in polymerization experiments under the same conditions, N* was the critical value above which no further increase in polymerization rate was observed. Using a turbine impeller instead of a pitched blade impeller as well as using a larger impeller diameter provides better emulsification at constant power input. The results indicate that scale‐up with constant impeller tip speed is most appropriate in case of a turbine impeller. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 74: 3225–3241, 1999     

Inifer surfactants in emulsion polymerization

Thiol-ended non-ionic surfactants, used in combination with t-butyl hydroperoxide are components of a redox system able to initiate the emulsion polymerization of styrene, as well as control the molecular weight by transfer. Most of the surfactants remain as side products in the water phase, while multimodal molecular weight distributions of polymer are observed depending on the structure of the surfactant, the conversion of the monomer and the process used for feeding the reactor. The maximum incorporation yield of these reactive surfactants in the polymer is around 40%.     

Robust nucleation in polymer-stabilized miniemulsion polymerization

The addition of a monomer soluble polymer to an emulsion was found to slow the effects of Ostwald ripening and impart diffusional stability to the droplets. Droplet nucleation was found to be the dominant nucleation mechanism in the polymerization of these polymer-stabilized miniemulsions (as distinguished from true miniemulsions). As a result these nucleations were more robust, and the polymerizations were less sensitive to variations in the recipe or contaminants levels. This was evident in the rates of polymerization and in the particle numbers. The miniemulsion polymerizations were subjected to changes in initiator concentration, a water-phase retarder, an oil-phase inhibitor, and agitation. Particle number was found to vary with each of these factors to the powers of 0.002, 0.02, 0.0031, and −0.026, respectively. The corresponding exponents for conventional emulsion were one to two orders of magnitude greater. These results demonstrate the potential of miniemulsion polymerization to greatly reduce the variability in particle number found in conventional emulsion polymerizations. © 1996 John Wiley & Sons, Inc.     

Silicone-stabilized nonaqueous emulsion polymerization

Stable dispersions are prepared by free-radical polymerization of methyl methacrylate in aliphatic hydrocarbons containing poly(dimethylsiloxane) modified with mercaptoalkyl side groups. The particles are pictured as being stabilized by a protective layer of solvated poly(dimethylsiloxane). Some important features of the polymerization are described.     

乳液聚合方法研究进展

乳液聚合方法具有各方面的优点和广泛的应用范围,因此,受到人们的广泛关注。本文介绍了乳液聚合的优缺点,并着重介绍了一些新的乳液聚合方法及其原理、特点、应用以及中外最新的一些研究成果。     

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.     

Particle nucleation rates in continuous emulsion polymerisation reactors

The design of a continuous, stirred emulsion polymerisation reactor requires a detailed characterisation of the particle nucleation rate. Here, consideration is given to systems which use micelle-forming surfactants and water-insoluble monomers. The surfactant micelles will be considered to participate in two competing rate processes; the nucleation of polymer particles and dissolution into the aqueous phase. When the reactor operates in a steady state it is possible to obtain a size distribution function for the particles. This distribution function is used in the development of a number of diffusion equations and conservation equations. By making various sets of assumptions concerning the nucleation process, these equations are then used to obtain expressions for the particle number. By comparing the different models with experimental results it can be seen that satisfactory predictions for the particle number can be obtained without assuming that the polymer particles are always saturated with surfactant. The models also show that the radical absorption processes are not controlled by diffusion in the aqueous phase. In some cases the particle number does not reach a steady state but oscillates with time. The nature of these oscillations is described by the solution of nonlinear differential equations. Boundary conditions for these equations depend on the reaction conditions.     

无皂乳液聚合的最新研究进展

简单阐述了无皂乳液聚合的定义及其常见的聚合方法,介绍了无皂乳液聚合方法在活性可控自由基聚合(CRP)领域中的应用,对其在原子转移自由基聚合(ATRP)领域和在可逆加成-断裂链转移聚合(RAFT)领域的应用机理进行阐述.简要分析了提高乳液稳定性的方法,最后结合无皂乳液聚合技术的应用进展对其发展趋势做出展望.     

双重乳液液滴在剪切流场中变形的数值模拟

制备高质量的大尺度聚α-甲基苯乙烯微球是激光惯性约束聚变靶丸的关键技术之一。研究了以旋转蒸发的流场为基础,简化为套筒旋转生成典型的剪切流场所生成外水相流场条件下油相/内水相复合液滴的运动、变形特性。结果表明:乳液液滴的形变主要受黏性剪切力和表面张力的作用。在相同条件下,乳液液滴内部的变形程度要小于液滴整体的变形程度。在相同的剪切力条件下,复合液滴毛细数Cawh随界面表面张力的减小而增大,液滴的变形程度随着Cawh数的增大而加剧。因此,表面张力是微球维持球形度的主要动力。当增加剪切率使得Cawh和复合液滴雷诺数Rewh达到0.22时,液滴发生断裂。     

Radiation-induced emulsion polymerization of tetrafluoroethylene

The radiation-induced emulsion polymerization of tetrafluoroethylene was carried out with the use of ammonium perfluorooctanoate as an emulsifier at an initial pressure of ca. 30–35 Kg/cm². The polymerization rate was shown to be proportinal to about the 0.8 power of the dose rate in the range of 2 × 10⁴ to 10⁵ R/hr and to be almost independent of emulsifier concentration. The molecular weight of the polymer lies in the range of 10⁴ to 10⁵, increases with reaction time at the initial stage, and decreases with emulsifier concentration, but is independent of the dose rate from 2 × 10⁴ to 6 × 10⁴ R/hr. If the emulsifier is not used, a polymer with a molecular weight as high as 1.8 × 10⁶ to 2 × 10⁷ is obtained. Apparently, the emulsifier and its radiolysis products act as chain transfer agents. Postirradiation polymerization was found to take place with the formation of products with increased molecular weight.     

有机硅氧烷 -丙烯酸酯乳液聚合研究进展

介绍了有机硅-丙烯酸酯乳液聚合物的3种制备方法及各种乳液的特点,对各种乳液的近期研究成果进行了总结。     

Computationally efficient solution of population balance models incorporating nucleation, growth and coagulation: application to emulsion polymerization

A computationally efficient solution technique is presented for population balance models accounting for nucleation, growth and coagulation (aggregation) (with extensions for breakage). In contrast to earlier techniques, this technique is not based on approximating the population balance equation, but is based on employing individual rates of nucleation, growth and coagulation to update the PSD in a hierarchical framework. The method is comprised of two steps. The first step is the calculation of the rates of nucleation, growth and coagulation by solving an appropriate system of equations. This information is then used in the second step to update the PSD. The method effectively decomposes the fast and the slow kinetics, thereby eliminating the stiffness in the solution. In solving the coagulation kernel, a semi-analytical solution strategy is adapted, which substantially reduces the computational requirement, but also ensures the consistency of properties such as the number and mass of particles.     

溶液聚合和乳液聚合的生产工艺流程

<正>根据合成条件(包括单体形状、介质物态)的不同,合成橡胶的聚合可分为本体聚合、气相聚合、乳液聚合及溶液聚合等四种。本体聚合虽具有所得聚合物纯度高,无需脱气、脱水、后处理等优点,但散热困难、聚合速度慢,对橡胶合成并不适用(但适用     

Free radical desorption in continuous emulsion polymerization

A mathematical model for continuous emulsion polymerization which accounts for the desorption of radicals from polymer particles is presented. The desorption factor has been incorporated into the Sato-Taniyama model and the resulting systems of equations solved iteratively. The effects of desorption on variables such as concentration of particles, polymerization rate, and average number of radicals per particle are illustrated. Fit of the model to experimental data is presented and discussed.