Monomer reservoirs

Figure C2.3.11 Key surfactant stmctures (not to scale) in emulsion polymerization micelles containing monomer and oligomer, growing polymer particle stabilized by surfactant and an emulsion droplet of monomer (reservoir) also coated with surfactant. Adapted from figure 4-1 in [67],...

Figure 1. Diagram of apparatus (M) monomer reservoir (F) flow meter (VG) vacuum gage (mercury manometer) (E) electrode (T) liquid nitrogen trap (P) mechanical pump (V,) needle valve (Vt) stop valve (Vs) pressure control valve (OSC) discharge frequency oscillator (AMP) amplifier (1MC) impedance matching circuit...

When the macromonomer is an amphiphilic polymer, its polymerization in the polar media is unusually rapid as a result of its organization into micelles. Under such conditions, the unsaturated groups are concentrated in the micelle they mostly form the hydrophobic core of aggregates (micelles). During the polymerization, the non-polymerizing micelles and/or the monomer saturated continuous phase act as a monomer reservoir. 

An investigation of the copolymer composition demonstrated the important effect of monomer transport on the copolymerization. The droplets in the macroemulsion act as monomer reservoirs. In this system, the effect of monomer transport will be predominant when an extremely water-insoluble comonomer, such as DOM, is used. In contrast with the macroemulsion system, the miniemulsion system tends to follow the integrated Mayo Lewis equation more closely, indicating less influence from mass transfer. 

Emulsion polymerisation in this case the monomer is emulsified in a nonsolvent (commonly water), usually in the presence of a surfactant. A water-soluble initiator is then added, whereupon particles of polymer form and grow in the aqueous medium as the monomer reservoir in the emulsified droplets is gradually used up. 

Before describing a qualitative picture of emulsion polymerization a note on monomer solubility and type of surface active agents is in order. Monomers for emulsion polymerization should be nearly insoluble in the dispersing medium but not completely insoluble. The solubility must be less than about 0.004 mol/L, as otherwise the aqueous phase will become a major locus of polymerization and the system will then not be typical emulsion polymerization. At the same time the monomer must be slightly soluble as this will allow the transport of monomer from the emulsified monomer reservoirs to the reaction loci (see later). 

A number of workers have suggested that emulsion polymerization may not occur homogeneously throughout a polymer particle but either at the particle surface [75] or within an outer monomer-rich shell surrounding an inner polymer-rich core [76]. The latter has been referred to as the shell or core-shell model. The latter model has been proposed to explobserved constant rate behavior up to about 60 percent conversion, which according to Eq. (6.232) requires [M] to be constant, and the considerable experimental evidence which indicates that emulsified monomer droplets (serving as monomer reservoirs) disappear at 25 to 30 percent conversion and the monomer concentration drops thereafter. 

The droplet surface barrier to radical entry might also result from partial diffu-sional degradation of monomer droplets (monomer reservoir) and decrease in the droplet size. The reduction in the droplet size due to Ostwald ripening increases the density of the oil-water interfacial layer formed by emulsifier and HD. This will later lead to the formation of a strong barrier to radical entry. 

It was found that the SDS/CA containing polymerization system shows a bi-modal PSD of latex particles, and the SDS/DMA containing system is characterized by a quite broad PSD. The fraction of small latex particles results from the initially generated highly monomer-swollen polymer particles, which serve as the monomer reservoir in the later stage of polymerization. On the contrary, the latex products obtained from both the SDS/SMA and SDS/HD containing systems show a relatively narrow PSD. These data further support the proposed competitive particle nucleation mechanism. 

Obviously, the ratio of monomer to initiator has a strong influence on the average size of the resulting polymer. The number of growing polymer chains, competing for the remaining monomer reservoir, increases with initial initiator concentration (Figure 6.8) [13]. 

At the beginning of an emulsion polymerization performed above the CMC, the free radicals generated in the aqueous phase promote the nucleation of particles by the homogeneous and micellar mechanisms explained previously. The fact that the surface area of all monomer droplets is by far much smaller than that of all the other colloidal species makes it unlikely that the radicals existing in the aqueous phase enter and polymerize into monomer droplets. Thus, the droplets play the role of monomer reservoirs. The diffusion of this component through the aqueous phase provides the monomer needed to replace that consumed by reaction and to swell the polymer produced in the particles. 

When a water-soluble initiator is added to a microemul-sion, polymer particles are nucleated mainly by the micellar mechanism. The role of the monomer-swollen micelles in microemulsion polymerization is not only to act as nucle-ation loci and surfactant reservoir but also as monomer reservoir. The fast nucleation rate leads to the initial increment of Rp. As the monomer is polymerized, its concentration in micelles diminishes and eventually monomer concentration within polymer particles decreases as well [205]. As a consequence, the nucleation and polymerization rates tend to decrease, explaining in this way the maximum in the Rp evolution curve experimentally observed. The final latex consists of surfactant-stabilized polymer particles that typically contain only polymer and empty micelles formed by excess surfactant. 

Stage n — At the end of stage I, the locus of further polymerization is shifted exclusively to the monomer-swoUen polymer particles since the micelles (the site of generation of new polymer particles) have aU disappeared. Polymerization proceeds homogeneously in the polymer particles by the maintenance of a constant monomer concentration within the particles through the diHusion of monomers from the monomer droplets, which, in eflfect, serve as monomer reservoirs. As the polymer particles increase in size, the size of the monomer droplets disappears. Since there is no new particle nucleation... 

Since monomer droplets have disappeared, a supply of monomers from the monomer reservoir (i.e., monomer droplets) is exhausted, hence rate of polymerization drops with depletion of monomers in latex particles. 

For both O/W and W/O systems, the amount of monomer is usually restricted to 5-10 wt% with respect to the overall mass, and that of surfactant(s) lies within the same range or even above. Nevertheless, there have been a few studies in which the formulation deviated from these conditions. For instance, surfactant concentrations of 2 wt V() were reported [56-58,69,124,125]. However, in this case the amount of monomer was also very low (< 2 wt%) so that the systems must be considered as micellar solutions rather than true microemulsions. Conversely, a 1994 study of Gan et al. [82] reported the polymerization of styrene up to 15 wt% using only about 1 wt V(> dodecyltrimethylammonium bromide surfactant (DTAB) in a Winsor I-like system. This system consists of a microemulsion (lower) phase topped off with pure styrene. The polymerization takes place in the microemulsion phase, while the styrene phase acts as a monomer reservoir. Such a polymerization process is novel, but it yields latices of large particle size ( 100 nm) that can be more easily obtained by conventional emulsion polymerization. 

Indirect methods for investigating polymerizations, which do not require isolation of the polymer, allow the polymerizations to be followed continuously. Dilatometry is particularly accurate. It measures the contraction of a polymerizing mixture. A dilatometer consists of precision tubing, 3 mm in diameter, to which a reaction vessel of 4-8 cm in volume is glass blown or joined. First, the initiator is added. The monomer is then distilled in from the monomer reservoir, preferably under nitrogen, and the dilatometer is placed in a thermostatted water bath. The yield u is determined from the observed volumes Vo at time zero and Vt at time /, of the monomer/polymer mixture and the partial specific volumes, Vmon of the monomer and Vpoi of the polymer ... 

Macro-emulsion polymerizations The conventional emulsion pol)rmerization is referred as macro-emulsion polymerization. In this chapter, all explanations included so far relate to macro-emulsion pol3zmerization. It is initially composed of a monomer emulsion of relatively large (1-100 pm) monomer droplets and significant free or micellar emulsifier. This emulsion is thermod3mamically unstable, but kinetically stable. Phase separation is rapid unless the system is well agitated. In macro-emulsion polymerizations, the nucleation takes place outside the monomer droplets which generally do not contribute to the particle nucleation due to their very small droplet surface area. The monomer droplets act as only monomer reservoirs which supply the monomers to the polymerization loci through the aqueous phase. 

For polymerisation in a normal emulsion, the hydrophobic monomer is dispersed in the aqueous phase, in which it is almost completely insoluble, with the help of a surfactant. On the whole it is then located in the form of reservoir droplets of diameter d 1-10 gm, and in surfactant micelles of diameter d 5-10 nm. A small fraction may be solubilised in the continuous aqueous phase. The initiator is generally present in the aqueous phase (see Fig. 6.5). Polymer particles are generated by two simultaneous processes. The first is free radical capture by micelles, called micellar nucleation. There are far fewer droplets than micelles and the total surface area of the micelles is extremely large. Consequently, radicals tend to penetrate micelles rather than monomer droplets. The latter act mainly as monomer reservoirs. The second of the processes mentioned above is the formation of oligomer radicals in the continuous phase, and is referred to as homogeneous nucleation. When radicals reach a certain size, they become insoluble and group together to form polymer particles similar to those formed by micellar nucleation. 

These factors alone may contribute significantly to variations in experimental results obtained in the polymerization of vinyl chloride. For example, at superatmospheric pressures, variation in the liquid volume of the monomer to the total free space of a closed reactor means a variation in the amount of gaseous monomer that can condense and/or diffuse into the polymer at some stage of the polymerization process. This vapor-phase monomer level may represent a monomer reservoir somewhat analogous to the monomer droplets postulated for the emulsion polymerization mechanism by Smith and Ewart. 

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