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In tracing back the historical development of the technique of emulsion polymerization, it was first introduced on an industrial scale in 1932, and it was followed by its major development around the Second World War. In identifying the processes constituting up the term emulsion polymerization, it is indicated this term consists of diverse processes namely; conventional emulsion polymerization, inverse emulsion polymerization, miniemulsion polymerization, dispersion polymerization, and microemulsion polymerization. For defining this term, it is known as a terms referring to a compartmentalized polymerization reaction which exists in a wide number of reaction loci dispersed in a continuous external phase. This implies that it is conducted in heterogeneous systems, commonly with an aqueous phase and a non-aqueous phase where the monomer and polymer are usually found to be related to the latter phase. Furthermore, emulsion polymerization is defined as a commercial process which is widely utilized or made a use of for commercial purposes, and more specifically, it is used to produce a latex by polymerizing the monomer under free radical conditions in an aqueous medium. Concerning utilization of this term in the commercial area, currently, it is used as a process predominating the commercial polymerizations of products such as acrylamide, vinyl acetate, chloroprene, vinyl chloride, methacrylates, various acrylate copolymerizations, and copolymerizations of butadiene with styrene and acrylonitrile ( prins).
For its general applications, it is pointed out that emulsion polymerizations have been largely applied in diverse industrial areas such as producing synthetic rubbers, paper coatings, high-impact polymers, latex foam binders, latex paints, adhesives, barrier coating additives since they have been found to be characterized with several distinct advantages. The first advantage of using emulsion polymerizations in producing such products is that by using them, the perfect fluidness of the system can be available throughout the entire polymerization process, and the dissipation of the heat produced by the exothermic free radical polymerization into the aqueous phase can be readily achieved. Secondly, it is found that the greater the rate of polymerization can be, the higher the conversion and the higher the molecular weight of polymer obtained in emulsion polymerization are and even more than an equivalent bulk polymerization. In other words, the residual monomer problem can be reduced by the higher conversion, and because of the high weight of the molecular, it becomes easy for the chain-transfer agent to control the molecular weight of polymer. The third advantage is that the process of transporting high solids latex can be easily carried out since the continuous phase is water where the viscosity of emulsion polymerization is lower than bulk polymerization. Moreover, the safety and environmental hazards can be reduced by the water-based latex.
A classical system of emulsion includes an aqueous phase which is made up of commonly persulfates, K2S2O8 ) used as a solution of water soluble initiator and surfactant in water. The quantity of surfactant available during an emulsion polymerization is typically estimated almost above the critical micelle concentration (cmc) as the minimum concentration required by surfactant molecules for the process of forming micelle. By the time it is dissolved, assembly of the surfactant molecules to form micelles can occur. In addition to this aqueous phase, the monomer has a very limited solubility in water. Monomer droplets with a size relying on the stirring rate are made as a result of the dispersion of the largest portion of the monomer. Being dispersed into monomer droplets, they are made stable through surfactant molecules absorbed on their surfaces (pren). The size of such droplets estimated is large with diameters between 1-10µm. Thus, it is indicated that a typical emulsion polymerization usually has between 1017 - 1018 micelles per litre and 109 - 1011 monomer droplets per litre. The radicals available in the aqueous phase added to the monomer dissolved in the aqueous phase to as to form oligomeric radicals are generated from the dissociation of the initiator. In cases when a critically wide number of monomer units have been added to the oligomeric radicals, they are changed into hydrophobic and they are subjected to particle formation by one or more of three mechanisms namely; micellar, homogeneous, or droplet nucleation. The polymer particles are identified as the major sites of polymerizing an emulsion. Throughout the course of the reaction, the diffusion occurring across the aqueous phase from the monomer droplets to the particles provides the growing particles with monomer. During the proceeding of the polymerization, there will be a re-distribution of the surfactant molecules from the shrinking monomer droplets, and un-nucleated micelles to the growing polymer particles respectively. Once the reaction is complete, a conversion of all the monomers into polymers will have been done, and the sub-micron sized polymer particles which are dispersed in water and which are stabilized by surfactant will constitute up the resulting latex.
As previously stated that although there are three types of configuration, and three main mechanisms involved in particle mechanisms which may in theory exist concurrently, it is found that there is a typical domination of one mechanism over the others depending on the aqueous phase solubility of the monomer, the surfactant concentration, and the degree of subdivision in the monomer droplets. These three mechanisms as the basics for forming particles are described in further details below.
As displayed in Figure 1.4, the process of micellar nucleation is initiated by dissociation of the aqueous phase initiator to form radicals. Specifically, persulfate initiators such as KPS utilized or employed in emulsion polymerization are dissociated to form radical ions. Because of the wide charge on these initiator radicals, it is almost impossible for them to enter a micelle directly since the monomer inside a particle is very hydrophobic. Thus, such essential and primary radicals will be added to monomer that is dissolved in the aqueous phase until they result into forming a z-mer, which is identified as a short oligomeric radical that is surface active (26). Furthermore, these z-mer species can enter into a monomer-swollen micelle. As they are enclosed in a micelle where the monomer is more highly concentrated in comparison to that in the aqueous phase, there is a speedy propagation of the oligomer radicals as to produce a polymer particle. These polymer chains will continue propagating within this polymer particle until a formation of stable particle exists. Only 1 out of every 100-1000 micelles will be formed into a particle (27) where micelles which are not centered give up their surfactant and monomer to growing particles throughout the course of the polymerization. The time an adequate number of particles is available, radicals will only move to pre-exiting particles rather than nucleating new ones. All particle nucleation is believed to give up or vanish when the disappearance of the micelles takes place. This is because, in otherwise cases where micelles are present, it is believed there is a domination of micellar nucleation over these systems (surfactant concentrations are above the cmc), and similarly, this also takes place in systems in which the water solubility of monomer is low (monomer solubility < l5 mmol. L-1) (28).
In the same case of micellar nucleation, homogeneous nucleation usually starts or is initiated with the dissociation of the aqueous phase initiator for the purpose of forming radicals added to dissolved aqueous phase monomer to as to make formation of oligomeric radicals. In homogeneous nucleation, there is a continuity of monomer addition in the aqueous phase until the formation of oligomeric radicals formed, and then, there is no longer water-soluble (formation of a j-mer). In the case of styrene, the occurrence of such process takes place after four monomer units are added, and in the case of methyl methacrylate, it takes place after the addition of 65 monomer unib.2e It is also added that the precipitation of oligomeric radicals out of solution will occur , thus, stabilizing itself by adsorbing surfactant and forming a particle. This particle is possible to take on monomer generated from the aqueous phase to permit propagation to start further till a formation of stable particle happens. As an alternative to this, this particle can also unite with other growing particles for forming a stable particle. The overall process of homogenous nucleation is displayed in Figure 14. Homogeneous nucleation is expected to be prevalent in surfactant free emulsion polymerizations, in emulsion systems where the surfactant concentration is estimated to be below its cmc, or in systems having high water solubility monomers (monomer solubility > 170 mmol. L-1).28
The occurrence of droplet nucleation coincides with the entrance of aqueous phase radicals or surface-active oligomeric radicals into a monomer droplet and their propagation as to form particles. Despite the fact that this mechanism of particle formation is found to occur to a small event, it does not predominate emulsion polymerization, and hence, it is often ignored. However, this mechanism is proved to predominate both miniemulsion polymerization and microemulsion polymerization where there is an effective completion of the small-size droplets for radicals. These systems demand the utility of a co-surfactant as well as the main surfactant. In miniemulsion system, the availability of a low molar mass and concurrently low water solubility in the co-surfactant (e.g. hexadecane or cetyl alcohol) is a must. The occurrence of the co-surfactant in the microemulsion system is usually represented by a low molar mass alcohol such as pentanole or hexanol.
Progress of Polymerization: The three intervals of emulsion polymerization
The description depicting the first quantitative picture of what occurs during the course of an emulsion polymerization was offered by Harkins 3l and later followed by publication of his experimental verification of the original theory proposed by him was done by Smith and Ewart. He also stated that there are three distinct intervals (I, II, III) in all emulsion polymerizations which are based on the particle number N (the concentration of polymer particles in units of number of particles per liter) and the existence of a separate monomer phase (i.e., monomer droplets).
Interval I of emulsion polymerization is recognized as the particle nucleation stage, in which the process of forming particles is usually carried out by either micellar or homogeneous nucleation. It is also found that there is an increase in the number of particles in the emulsion system and the rate of polymerization throughout phase I, which is viewed as the shortest of the three phases. It is believed that the particle nucleation ceasing coincides with the weight fraction of polymer in the latex particles being ~0.35 (33). The disappearance of micelles indicates or marks the end of phase I.
In interval II of emulsion polymerization, propagation of the radical inside the particle is seen to be continuous. In this interval, monomer is also being provided to the growing particles by the monomer droplets, which function as reservoirs by providing the growing particles with monomer, and this continues till they disappear at approximately 30 - 40% conversion (27). It is indicated that the concentration of monomer in the particles is estimated around ~6 mol/L, which is below that of a monomer droplet whose monomer concentration is ~10 mol/L (34). The number of particles does not undergo a change, but it remains constant throughout interval II since particle formation is finished.
For Interval III of emulsion polymerization, it is found to start from the starting point in which the monomer droplets disappear, and it ends the time the reaction ends. However, the number of polymer particles does not change but remains constant during interval III. Inside the polymer particles, the radical chains react with monomer within the particles themselves, thus, propagating and making the monomer concentration within these particles decrease. As a result of the decrease in the amount of monomer in the system throughout interval III, the rate of polymerization is often seen to decrease. Sometimes however, there is an increase in the rate of polymerization seen to be experienced during interval III. While propagation continues, the weight fraction of the polymer within the particle increases, resulting into making the viscosity increase. In free radical polymerization, termination is generated from the reaction of a highly mobile short-chain radical species with a long-chain radical species of lower mobility (35). Therefore, the rate of termination is controlled by the rate of diffusion of the short-chain radical species through the reaction medium. As the viscosity of the medium increases, the mobility and the rate of diffusion of the short-chain radical species is reduced. This decreased rate of diffusion makes the rate coefficient for termination dramatically reduce, thus, leading into the Tromsdorff gel effect which is characterized by an increase in the polymerization rate (Rp α (kt)-1/2) (35).
For certain polymers, such as polystyrene, at very high conversions, the system is probable to become glassy and make the diffusion of monomer limited to the end of a growing chain. Due to the decrease in the rate coefficient for propagation and the lower monomer concentration, there is more and further reduction in the polymerization rate.
The polymerization is regarded to reach the ending point the time all the monomer has been consumed and converted to polymer.
Other Emulsion Polymerization Systems
Miniemulsion polymerization is found to include systems having monomer droplets in water and even with much smaller droplets than they are found in emulsion polymerization, about 50-500 nm compared to 1-100 mm in diameter [Antonietti and Landfester, 2002; Asua, 2002; Bechthold and Landfester, 2000; Landfester, 2001]. Due to the surfactant concentrations generally estimated below CMC, micelles are usually not present. The presence of water-insoluble costabilizers such as hexadecane and cetyl alcohol along with the surfactant is to stabilize the monomer droplets against diffusional degradation (coagulation) which is referred to as Ostwald ripening. The droplet size relies mainly not only on the amount of surfactant and costabilizer but also on the amount of energy used in the homogenization process. The final polymer particle and the monomer droplet have both almost the same or similar size. For the application of water-soluble and oil-soluble initiators, both have been used in miniemulsion polymerization. The usefulness of miniemulsion polymerizations is represented by the production of high-solids-content latexes.
Microemulsion polymerization is defined as an emulsion polymerization with very much smaller monomer droplets, about 10-100 nm compared to 1-100 mm. The presence of the micelles is attributed to the surfactant concentration which is above CMC. The final polymer particles generally have diameters of 10-50 nm. Although many of the characteristics of microemulsion polymerization are found to be in parallel to those of emulsion polymerization and the details are not exactly the same [Co et al., 2001; de Vries et al., 2001; Lopez et al., 2000; Medizabial et al., 2000]. Water-soluble initiators are commonly used, but there are many reports of microemulsion polymerization with oil-soluble initiators. The occurrence of nucleation in emulsion polymerization is almost exclusive to the early portion of the process (interval I). Its occurrence over a larger portion of the process in microemulsion polymerization is due to the large amount of surfactant present. Its extension is probable to be over most of the process. As a result of this, interval II with an approximately constant polymerization rate is not observed. Unlike emulsion polymerizations, only two intervals are observed in most microemulsion polymerizations. It is pointed out that the polymerization rate increases gradually with time, reaches a maximum, and then decreases.