Emulsion polymerization interval

Rgure 5.4. A schematic lepiesentation of typical polymerization rate as a function of monomer conversion profiles for (a) conventional emulsion polymerization (Interval II Smith-Ewart Case 2 kinetics), (b) miniemulsion polymerization, and (c) microemulsion polymerization. The distinct intenrals of the polymerization processes are also included in these plots. 

The progression of an ideal emulsion polymerization is considered in three different intervals after forming primary radicals and low-molecular weight oligomers within the water phase. In the first stage (Interval I), the polymerization progresses within the micelle structure. The oligomeric radicals react with the individual monomer molecules within the micelles to form short polymer chains with an ion radical on one end. This leads to the formation of a new phase (i.e., polymer latex particles swollen with the monomer) in the polymerization medium. 

Based on the Smith-Ewart theory, the number of latex particles formed and the rate of polymerization in Interval II is proportional with the 0,6 power of the emulsifier concentration. This relation was also observed experimentally for the emulsion polymerization of styrene by Bartholomeet al. [51], Dunn and Al-Shahib [52] demonstrated that when the concentrations of the different emulsifiers were selected so that the micellar concentrations were equal, the same number of particles having the same size could be obtained by the same polymerization rates in Interval II in the existence of different emulsifiers [52], The number of micelles formed initially in the polymerization medium increases with the increasing emulsifier concentration. This leads to an increase in the total amount of monomer solubilized by micelles. However, the number of emulsifier molecules in one micelle is constant for a certain type of emulsifier and does not change with the emulsifier concentration. The monomer is distributed into more micelles and thus, the... 

A variety of behaviors are observed for the polymerization rate versus conversion depending on the relative rates of initiation, propagation, and termination, which are in turn dependent on the monomer and reaction conditions (Fig. 4-2). Irrespective of the particular behavior observed, three intervals (I, II, III) can be discerned in all emulsion polymerizations based on the particle number N (the concentration of polymer particles in units of number of... 

The micelles are present at a concentration of about lO per ml of liquor and each micelle contains around 100 monomer molecules. In contrast, the number of monomer droplets is only about 10 ° per ml. Thus, despite the larger volume of monomer droplets, the micelles offer a very much larger surface area. A radical formed in the aqueous phase will thus encounter a monomer-filled micelle much more often than a monomer droplet. Therefore, the polymerization takes place practically only in the micelles and not in the monomer droplets. The monomer consumed in the micelles is replaced by diffusion from the monomer droplets through the aqueous phase. According to the theories of Harkins and of Smith and Ewart, the kinetic course of an emulsion polymerization is divided into three intervals At first some of the micelles increase rapidly in size as the polymerization advances and are transformed into so-called latex particles, containing both monomer and polymer. These are still very much smaller than the monomer droplets and have an initial diameter of about 20-40 pm, corresponding to about... 

It is usual to consider the course of emulsion polymerization to proceed through three intervals [16,17]. The particle number increases with time in Interval I, where latex particles are being formed, and then remains constant during Intervals II and II. The monomer concentration in particles is in equilibrium with a monomer saturated aqueous solution. Swelling is limited only by the opposite force of the particle surface/water tension. Hence, the concentration of monomer in the particles is usually taken as constant up to the point where free monomer droplets disappear. In Intervals I and II, the monomer concentration... 

Figure 1. Polydispersity index of the polymer produced in Interval II of an emulsion polymerization terminated solely by combination as a function of the average number of free radicals per particle...

The experimental results on the polymer produced in emulsion polymerizations published thus far are both confusing and contradictory. Several factors may be responsible for this first, many surfactants behave as chain transfer agents, which has often not been recognized second, measurements have often been made on samples that contain polymer from Intervals I, II and III, which leads to a significant increase in the polydispersity index becauseis sensitive to the presence of lower molecular weight species tRird, direct measurements of the MWD have only recently become possible with the advent of gel permeation chromatography. 

Nevertheless micelles are normally present during Interval I of an emulsion polymerization in which latex particles are nucleated. Micellar nucleation of latex particles is dominant for monomers which have only a low solubility in water (e.g. styrene). For such a monomer any effect of micellar catalysis is likely to be revealed by an increase in the number of latex particles formed which would also result in an increased rate of polymerization. The thermal emulsion polymerization cited above seem to be a prima facie case of micellar catalysis. The thermal emulsion polymerization of styrene is investigated further here. 

In Fig. 8 the calorimetric curve of a typical miniemulsion polymerization for 100-nm droplets consisting of styrene as monomer and hexadecane as hydrophobe with initiation from the water phase is shown. Three distinguished intervals can be identified throughout the course of miniemulsion polymerization. According to Harkins definition for emulsion polymerization [59-61], only intervals I and III are found in the miniemulsion process. Additionally, interval IV describes a pronounced gel effect, the occurrence of which depends on the particle size. Similarly to microemulsions and some emulsion polymerization recipes [62], there is no interval II of constant reaction rate. This points to the fact that diffusion of monomer is in no phase of the reaction the rate-determining step. 

The first interval is the interval of particle nucleation (interval I) and describes the process to reach an equilibrium radical concentration within every droplet formed during emulsification. The initiation process becomes more transparent when the rate of polymerization is transferred into the number of active radicals per particle n, which slowly increases to n 0.5. Therefore the start of the polymerization in each miniemulsion droplet is not simultaneous, so that the evolution of conversion in each droplet is different. Every miniemulsion droplet can be perceived as a separate nanoreactor, which does not interact with others. After having reached this averaged radical number, the polymerization kinetics is slowing down again and follows nicely an exponential kinetics as known for interval III in emulsion polymerization or for suspension polymer-... 

A mathematical model of emulsion polymerization as a whole would be too complicated. Therefore Smith and Ewart divided the process into three phases, and appropriately simplified the situation in each of these. The start of polymerization and the monomer-polymer transformation up to the disappearance of micelles were designated as phase I, the subsequent time interval until the complete consumption of monomer droplets as phase II, and the remaining part of polymerization as phase IIIt. 

It is clear from Eq. 1 that the monomer concentration in a polymer particle is one of the three key factors that control the particle growth rate, and accordingly, the rate of polymerization. In emulsion polymerization, the course of emulsion polymerization is usually divided into three stages, namely. Intervals I, II and III. In Intervals I and II of emulsion homopolymerization, the monomer concentration in the polymer particles is assumed to be approximately constant. In Interval III, it decreases with reaction time. Two methods are now used to predict the monomer concentration in the polymer particles in emulsion homopolymerization empirical and thermodynamic methods. 

By combining thermodynamically-based monomer partitioning relationships for saturation [170] and partial swelling [172] with mass balance equations, Noel et al. [174] proposed a model for saturation and a model for partial swelling that could predict the mole fraction of a specific monomer i in the polymer particles. They showed that the batch emulsion copolymerization behavior predicted by the models presented in this article agreed adequately with experimental results for MA-VAc and MA-Inden (Ind) systems. Karlsson et al. [176] studied the monomer swelling kinetics at 80 °C in Interval III of the seeded emulsion polymerization of isoprene with carboxylated PSt latex particles as the seeds. The authors measured the variation of the isoprene sorption rate into the seed polymer particles with the volume fraction of polymer in the latex particles, and discussed the sorption process of isoprene into the seed polymer particles in Interval III in detail from a thermodynamic point of view. 

In many emulsion polymerizations, the monomer/polymer ratio is kept constant during Interval II, and the accumulated MWD is approximately equal to the instantaneous distribution. Equation 84 shows that the Cm value can be determined from the slope of the InN(P) versus P plot. 

Although the kinetic behavior during Interval 111 was not considered in [311], P =0.5 is an important consideration when we look at the possibility of forming bimodal MWDs in emulsion polymerization that involves chain transfer to polymer. 

In contrast to emulsion polymerization, the reaction kinetics of microemulsion polymerization is characterized by two polymerization rate intervals the interval of constant rate characteristic of emulsion polymerization is missing [42,49,53], as shown in Fig. 2. Polymer particles are generated continuously during the reaction by both micellar and homogeneous mechanisms. As the solubility of the monomer in the continuous domain increases, homogeneous... 

Figure 23 Diagram of the three intervals of micro-emulsion polymerization, according to Candau [258]...

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