Time-Dependent Emulsion Polymerization

There are several situations described in the literature where the phenomenon of transience in emulsion polymerization must be considered. Some of these are reactor start-up or shutdown [19,20], reactor stability [21,22], and reactor controls [23,24] There are also a few applications in which the amount of emulsifier used is such that it gives a concentration less than the CMC. The particle sizes that result lie in the range 0.5-10 pm and are larger than those obtained fi om the usual emulsion polymerization described earlier. The product obtained in this condition is not a true emulsion from which polymer particles precipitate out when diluted with water. This is known as dispersion polymerization, to which the analysis developed earher is not applicable [25-27]. 

The time-dependent behavior of emulsion polymerization arises due to variation in monomer concentration, changes in the number of polymer particles Nt, or both. We have aheady observed that N, changes due to nucleation in stage I of emulsion polymerization and this normally ends at about 10-15% conversion. However, when the monomer-to-water ratio MIW) is high or the monomer is more than sparingly soluble, the constancy of N, cannot be assiuned up to conversions as large as 50%. If the monomer droplets are sufficiently small, they also become the loci of particle formation and, in such circumstances, the Smith-Ewart theory is inadequate to explain the experimental phenomena. We now present the outline of a mathematical model of emulsion polymerization that is 

The overall emulsion polymerization can be described in terms of reactions as follows. The initiator molecule (I2) is present in the aqueous phase, where it undergoes thermal deeomposition to give initiator radieal (I). These radicals are hydrophihe and eannot enter into hydrophobic monomer droplets (see Fig. 7.3). Consequently, they ean react only with monomer dissolved in water, giving the following  

These polymer radicals ( P,) in the aqueous phase can grow up to a certain critical length (say, ), after which they precipitate to form a primary particle. Here, a polymer particle is represented P p where the index j denotes the niunber of times it has been initiated. This means that P i represents the primary particle. Before reaching the critical length n, the polymer radicals P, can terminate as they do in the homogeneous polymerization discussed in Chapter 5. This means that the following reactions occur  

The polymer radicals can also enter into micelles (indicated by subscript or superscript c), monomer droplets (indicated by subscript or superscript d), or polymer particles. As soon as one of the former two happens, the micelle (MC) and the droplet (MD) become particles with one radical in them  

---

Adsorption behavior and the effect on colloid stability of water soluble polymers with a lower critical solution temperature(LCST) have been studied using polystyrene latices plus hydroxy propyl cellulose(HPC). Saturated adsorption(As) of HPC depended significantly on the adsorption temperature and the As obtained at the LCST was 1.5 times as large as the value at room temperature. The high As value obtained at the LCST remained for a long time at room temperature, and the dense adsorption layer formed on the latex particles showed strong protective action against salt and temperature. Furthermore, the dense adsorption layer of HPC on silica particles was very effective in the encapsulation process with polystyrene via emulsion polymerization in which the HPC-coated silica particles were used as seed. 

Dickinson, E., Matsumura, Y. (1991). Time-dependent polymerization of p-lactoglobulin through disulphide bonds at the oil-water interface in emulsions. International Journal of Biological Macromolecules, 13, 26-30. 

An organosol is the same mixture as described above, with the addition of solvent to reduce viscosity. These find their major applications in coatings. The solvent is evaporated before fusion of the film. Various pigments, colorants, stabilizers and fillers may be added, depending on the desired properties. Emulsion polymerization resins are generally employed because of their fast fusion rates. Coarser particle sized PVC resins would require extended time at the elevated temperature. 

The conversion-time curves appear to be very similar to the shape typical of emulsion polymerization, i.e., an S-shaped curve is attributed to the autoacceleration caused by the gel effect (Smith-Ewart 3 kinetics, n>>l). The rate of polymerization-conversion dependence is described by a curve with two rate maxima. The decrease in the rate after passing through the first maximum is ascribed to the decrease of the monomer concentration in particles. Particle nucleation ends between 40 and 60% conversion, beyond the second rate maximum. This is explained by the presence of coemulsifier which stabilizes the monomer droplets against diffusive degradation. 

Fig. 4. Dependence of monomer conversion (open symbols) and the rate of polymerization (closed symbols) in the emulsifier-free emulsion polymerization of PEO-VB macromonomers on reaction time and the PEO-VB type [85]. Recipe [PEO-VB] =0.045 mol dm-3, [AVA]=0.45xl0-3 mol dm"3,60 °C. In water Cr(EO)38-C7-VB (O, ), Cr-(EO)25-VB (A,A)...

Mayer et al. [358] investigated the performance of a PPC reactor in the continuous emulsion polymerization of St. They found that the number of polymer particles produced in the PPC reactor depended strongly on the residence time distribution (RTD) - in other words, on the pulsation conditions - and that it had a value between those recorded for the batch and the CSTR processes. 

Garden (1968) has solved Eq. (12) numerically for the case of negligihle desotption of radicals from the particles without assuming the steady state, stating that the Stockmayer solution for n is incorrect because there is no steady state in principle and because Eq. (12) includes the time-dependent parameter Vp. However, the results of numerical calculation by Garden cohxnde almost completely with those predicted by the Stockmayer solution for no radical desorption from the particles. This also supports the validity of applying the steady-state hypothesis to the solution for Eq.(12) under normal conditions for emulsion polymerization. 

Acres and Dalton (1963a) also studied the emulsion polymerization of methyl methacrylate initiated by Co y radiation using a recording dilatometer. Only the conversion-time curves were measured with constant dose rate, varying monomer concentration, and with constant monomer concentration at different dose rates. Except at the lowest monomer concentration a clear gel effect was observed, with linear rates up to that point. The linenr rates increased with increasing monomer concentration up to about 0.4 mol/liter and then leveled oif. The dependence of the rate, before the gel effect, on the dose rate was 0.4 and, unlike their findings with styrene, not dependent on the monomer concentration. Their results were consistent with those of Hummel ei al. that methyl methacrylate follows, with y radiation, the generally accepted Smith-Ewart Case 2 kinetics except for the marked gd effect. 

Ballantine (4) observed that the y-induced emulsion polymerization of styrene is about 100 times faster and yields higher molecular weights (up to 2 X 10 ) than the y-induced bulk polymerization. He explains the large difference in reaction rates by the high radical yield (G/ value) of water, as compared with the G/j value of styrene. An over-all activation energy of 3.7 kcal. per mole was calculated from the temperature dependence of the reaction. Allen et al. (1) prepared and grafted polystyrene and poly (vinyl acetate) dispersions under the influence of y-radiation. Mezhirova et al. (28) found a temperature-independent reaction rate of the y-induced emulsion polymerization of styrene. 

The course of the reaction mte/time function, measured by us, is exactly like the course of the same function in the case of the catalyzed emulsion polymerization of styrene. The small dependence of Ubf on the initiation rate in the period of zero order, which was observed by us, agrees well with classical conceptions The period of particle formation is over and the radicals formed now affect initiation and termination likewise. Therefore the initiation rate can not influence the reaction rate. (Actually the reaction rate increases somewhat, on strongly increasing the dose rate. This shows that the mean radical concentration, n, in the particles is slightly higher than 0.5. The termination reaction is already retarded.)... 

Methyl Acrylate (MA). The y-emulsion polymerization of MA was studied most intensively in our investigations. The dependence of the reaction rate/time function, and the maximum reaction rate, on composition of the mixture, dose rate, and temperature was studied. 

The duration of the polymerization process depends on the temperature employed and the degree of conversion desired and is, for instance, between 18 and 30 hr at 45-70°O. Emulsion polymerization recipes have also been developed which allow the reaction to proceed at temperatures as low as 0°C, and at much shorter reaction times than those cited before. This is achieved by the use of so-called redox, reduction-activation, systems. 

The Smith-Ewart kinetic theory of emulsion polymerization is simple and provides a rational and accurate description of the polymerization process for monomers such as styrene, butadiene, and isoprene, which have very limited solubility in water (less than 0.1%). However, there are a number of exceptions. For example, as we indicated earlier, large particles (> 0.1 to 0.5 cm diameter) may and can contain more than one growing chain simultaneously for appreciable lengths of time. Some initiation in, followed by polymer precipitation from the aqueous phase may occur for monomers with appreciable water solubility (1 to 10%), such as vinyl chloride. The characteristic dependence of polymerization rate on emulsifier concentration and hence N may be altered quantitatively by the absorption of emulsifier by these particles. Polymerization may actually be taking place near the outer surface of a growing particle due to chain transfer to the emulsifier. 

---