Monomer droplet

Surfactants provide temporary emulsion droplet stabilization of monomer droplets in tire two-phase reaction mixture obtained in emulsion polymerization. A cartoon of tliis process is given in figure C2.3.11. There we see tliat a reservoir of polymerizable monomer exists in a relatively large droplet (of tire order of tire size of tire wavelengtli of light or larger) kinetically stabilized by surfactant. 

In a suspension polymerisation monomer is suspended in water as 0.1—5-mm droplets, stabilised by protective coUoids or suspending agents. Polymerisation is initiated by a monomer-soluble initiator and takes place within the monomer droplets. The water serves as both the dispersion medium and a heat-transfer agent. Particle sise is controlled primarily by the rate of agitation and the concentration and type of suspending aids. The polymer is obtained as small beads about 0.1—5 mm in diameter, which are isolated by filtration or centrifugation. 

Suitable protective coUoids for the preparation of acryhc suspension polymers include ceUulose derivatives, polyacrylate salts, starch, poly(vinyl alcohol), gelatin, talc, clay, and clay derivatives (95). These materials are added to prevent the monomer droplets from coalescing during polymerisation (110). Thickeners such as glycerol, glycols, polyglycols, and inorganic salts ate also often added to improve the quahty of acryhc suspension polymers (95). 

Because the polymerization occurs totally within the monomer droplets without any substantial transfer of materials between individual droplets or between the droplets and the aqueous phase, the course of the polymerization is expected to be similar to bulk polymerization. Accounts of the quantitative aspects of the suspension polymerization of methyl methacrylate generally support this model (95,111,112). Developments in suspension polymerization, including acryUc suspension polymers, have been reviewed (113,114). 

Sta.g C I Pa.rtlcIeNucIea.tlon, At the start of a typical emulsion polymerization the reaction mass consists of an aqueous phase containing smaU amounts of soluble monomer, smaU spherical micelles, and much larger monomer droplets. The micelles are typicaUy 5—30-nm in diameter and are saturated with monomer emulsified by the surfactant. The monomer droplets are larger, 1,000—10,000-nm in diameter, and are also stabilized by the surfactant. 

Water-soluble initiator is added to the reaction mass, and radicals are generated which enter the micelles. Polymerization starts in the micelle, making it a growing polymer particle. As monomer within the particle converts to polymer, it is replenished by diffusion from the monomer droplets. The concentration of monomer in the particle remains as high as 5—7 molar. The growing polymer particles require more surfactant to remain stable, getting this from the uninitiated micelles. Stage I is complete once the micelles have disappeared, usually at or before 10% monomer conversion. 

Radicals generated from water-soluble initiator might not enter a micelle (14) because of differences in surface-charge density. It is postulated that radical entry is preceded by some polymerization of the monomer in the aqueous phase. The very short oligomer chains are less soluble in the aqueous phase and readily enter the micelles. Other theories exist to explain how water-soluble radicals enter micelles (15). The micelles are presumed to be the principal locus of particle nucleation (16) because of the large surface area of micelles relative to the monomer droplets. 

However, in the case of mini- and microemulsions, processing methods reduce the size of the monomer droplets close to the size of the micelle, leading to significant particle nucleation in the monomer droplets (17). Intense agitation, cosurfactant, and dilution are used to reduce monomer droplet size. Additives like cetyl alcohol are used to retard the diffusion of monomer from the droplets to the micelles, in order to further promote monomer droplet nucleation (18). The benefits of miniemulsions include faster reaction rates (19), improved shear stabiHty, and the control of particle size distributions to produce high soHds latices (20). 

During Stage I the number of polymer particles range from 10 to 10 per mL. As the particles grow they adsorb more emulsifier and eventually reduce the soap concentration below its critical micelle concentration (CMC). Once below the CMC, the micelles disappear and emulsifier is distributed between the growing polymer particles, monomer droplets, and aqueous phase. 

Emulsification is the process by which a hydrophobic monomer, such as styrene, is dispersed into micelles and monomer droplets. A measure of a surfactant s abiUty to solubilize a monomer is its critical micelle concentration (CMC). Below the CMC the surfactant is dissolved ia the aqueous phase and does not serve to solubilize monomer. At and above the CMC the surfactant forms spherical micelles, usually 50 to 200 soap molecules per micelle. Many... 

The completion stage is identified by the fact that all the monomer has diffused into the growing polymer particles (disappearance of the monomer droplet) and reaction rate drops off precipitously. Because the free radicals that now initiate polymerization in the monomer-swollen latex particle can more readily attack unsaturation of polymer chains, the onset of gel is also characteristic of this third stage. To maintain desirable physical properties of the polymer formed, emulsion SBR is usually terminated just before or at the onset of this stage. 

Suspension Polymerization. This method (10) might be considered as a number of bulk polymerizations carried out simultaneously in the monomer droplets with water acting as a heat-transfer medium. A monomer-soluble initiator, eg, a peroxide or azo compound, and a protective coUoid like poly(vinyl alcohol) or bentonite, are requited. After completion of the polymerization, the excess of monomer(s) is steam stripped, and the beads of polymer are collected and washed on a centrifiige or filter and dried on a vibrating screen or by means of an expeUer—extmder. 

Suspension polymerization of water-insoluble monomers (e.g., styrene and divinylbenzene) involves the formation of an oil droplet suspension of the monomer in water with direct conversions of individual monomer droplets into the corresponding polymer beads. Preparation of beaded polymers from water-soluble monomers (e.g., acrylamide) is similar, except that an aqueous solution of monomers is dispersed in oil to form a water-in-oil (w/o) droplet suspension. Subsequent polymerization of the monomer droplets produces the corresponding swollen hydrophilic polyacrylamide beads. These processes are often referred to as inverse suspension polymerization. 

Individual components in the formulation of the aqueous phase all contribute to the successful production of a GPC/SEC gel. The stabilizer acts as a protective coating to prevent the agglomeration of the monomer droplets. Polyvinyl alcohol, gelatin, polyacrylic acids, methylcellulose, and hydroxypro-... 

Several components of the organic phase contribute greatly to the character of the final product. The pore size of the gel is chiefly determined by the amount and type of the nonsolvent used. Dodecane, dodecanol, isoamyl alcohol, and odorless paint thinner have all been used successfully as nonsolvents for the polymerization of a GPC/SEC gel. Surfactants are also very important because they balance the surface tension and interfacial tension of the monomer droplets. They allow the initiator molecules to diffuse in and out of the droplets. For this reason a small amount of surfactant is crucial. Normally the amount of surfactant in the formula should be from 0.1 to 1.0 weight percent of the monomers, as large amounts tend to emulsify and produce particles less than 1 yam in size. 

Various novel applications in biotechnology, biomedical engineering, information industry, and microelectronics involve the use of polymeric microspheres with controlled size and surface properties [1-31. Traditionally, the polymer microspheres larger than 100 /urn with a certain size distribution have been produced by the suspension polymerization process, where the monomer droplets are broken into micron-size in the existence of a stabilizer and are subsequently polymerized within a continuous medium by using an oil-soluble initiator. Suspension polymerization is usually preferred for the production of polymeric particles in the size range of 50-1000 /Ltm. But, there is a wide size distribution in the product due to the inherent size distribution of the mechanical homogenization and due to the coalescence problem. The size distribution is measured with the standard deviation or the coefficient of variation (CV) and the suspension polymerization provides polymeric microspheres with CVs varying from 15-30%. 

The rate of an ideal emulsion polymerization is given by Eqn (4). In this expression [/] is the initiator concentration, [ ] is the emulsifier concentration, and [M] is the concentration of monomer within the forming latex particles. This value is constant for a long reaction period until all the monomer droplets disappear within the water phase. 

Suspension polymerizations are often regarded as "mini-bulk" polymerizations since ideally all reaction occurs w ithin individual monomer droplets. Initiators with high monomer and low water solubility are generally used in this application. The general chemistry, initiator efficiencies, and importance of side reactions are similar to that seen in homogeneous media. 

Microemulsion and miniemulsion polymerization differ from emulsion polymerization in that the particle sizes are smaller (10-30 and 30-100 nm respectively vs 50-300 inn)4" and there is no monomer droplet phase. All monomer is in solution or in the particle phase. Initiation takes place by the same process as conventional emulsion polymerization. 

Microemulsion and miniemulsion polymerization processes differ from emulsion polymerization in that the particle sizes are smaller (10-30 and 30-100 nm respectively vs 50-300 ran)77 and there is no discrete monomer droplet phase. All monomer is in solution or in the particle phase. Initiation usually takes place by the same process as conventional emulsion polymerization. As particle sizes reduce, the probability of particle entry is lowered and so is the probability of radical-radical termination. This knowledge has been used to advantage in designing living polymerizations based on reversible chain transfer (e.g. RAFT, Section 9.5.2)." 2... 

Successful NMP in emulsion requires use of conditions where there is no discrete monomer droplet phase and a mechanism to remove any excess nitroxide formed in the particle phase as a consequence of the persistent radical effect. Szkurhan and Georges"18 precipitated an acetone solution of a low molecular weight TEMPO-tcrminated PS into an aqueous solution of PVA to form emulsion particles. These were swollen with monomer and polymerized at 135 °C to yield very low dispersity PS and a stable latex. Nicolas et at.219 performed emulsion NMP of BA at 90 °C making use of the water-soluble alkoxyamine 110 or the corresponding sodium salt both of which are based on the open-chain nitroxide 89. They obtained PBA with narrow molecular weight distribution as a stable latex at a relatively high solids level (26%). A low dispersity PBA-WocA-PS was also prepared,... 

NMP in miniemulsion has been more successful. In miniemulsion polymerization nuclealion lakes place directly in the monomer droplets that become the polymer particles. Particle sizes are small (<100 nm). Most w ork has used TEMPO and high reaction temperatures (120-140 °C) with S or BA as monomer. 

Emulsion polymerization has proved more difficult. N " Many of the issues discussed under NMP (Section 9.3.6.6) also apply to ATRP in emulsion. The system is made more complex by both activation and deactivation steps being bimolecular. There is both an activator (Mtn) and a deactivator (ML 1) that may partition into the aqueous phase, although the deactivator is generally more water-soluble than the activator because of its higher oxidation state. Like NMP, successful emulsion ATRP requires conditions where there is no discrete monomer droplet phase and a mechanism to remove excess deactivator built up in the particle phase as a consequence of the persistent radical effect.210 214 Reverse ATRP (Section 9.4,1,2) with water soluble dialky 1 diazcncs is the preferred initiation method/87,28 ... 

When an aqueous phase radical enters the polymer particles it becomes a polymer phase radical, which reacts with a monomer molecule starting a propagating polymer chain. This chain may be stopped by chain transfer to monomer, by chain transfer to agent or it may terminate by coupling. Small radicals in the particle may also desorb from or reenter the particle. In a batch reactor. Interval I indicates the new particle formation period, Interval II particle growth with no new particles, and Interval III the absence of monomer droplets. 

Volume of Monomer and Particle Phases. Additional time dependent differential equations were written for the volumes of the particle and aqueous phases. The volume of the monomer phase was calculated from the total emulsion volume and the volumes of the aqueous and polymer phases. This procedure allowed the determination of whether monomer droplets are present (Interval II) or absent (Interval III). 

Elnulsifler Magg Balance. The overall emulsifier concentration in the system, Cgt. is constant however, it is distributed among the aqueous liiase (Cg ), the polymer particles and the monomer droplets intertaces (Cga) and the micellar aggregates (Cgm), according to the sinple balance ... 

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