Swollen latex particles

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. 

Figure 9 The schematical representation of dispersion polymerization process, (a) initially homogeneous dispersion medium (b) particle formation and stabilizer adsorption onto the nucleated macroradicals (c) capturing of radicals generated in the continuous medium by the forming particles and monomer diffusion to the forming particles (d) polymerization within the monomer swollen latex particles, (e) latex particle stabilized by steric stabilizer and graft copolymer molecules (f) list of symbols.

The free radicals produced from the fraction of initiator dissolved in the water phase are responsible for particle formation and growth in the emulsion polymerization of St initiated by AIBN. The free radicals produced in pairs in the polymer particles play almost no role in the polymerization inside the polymer particles because pairs of radicals produced within a volume as small as a monomer-swollen latex particle or a monomer-swollen micelle are very hkely to recombine. 

The swelling ratios were determined using the procedure of Vanderhoff et al. (9. Excess monomer was mixed with diluted latex and added surfactant. After swelling, the latex was mildly centrifuged to remove the excess monomer. Iso-octane was then used to extract monomer from the swollen latex particles. The concentration of monomer in the extracted solution was then determined using a UV detector (instrumentation Specialties Co. model 1840) by absorption spectra at 245nm. 

The behavior of the surfactant molecules in an emulsion polymerization is complex. The adsorption of the surfactant on the rapidly and continually growing surface of the monomer-swollen latex particles reduces their concentration in the aqueous phase, and also upsets the balance in equilibrium between the dissolved surfactant and the surfactant present in the inactivated micelles (those in which polymerization is not occurring), as shown in Figure 5.11. The point is quickly reached at which the surfactant concentration in the solution falls below its critical micelle concentration, CMC. When this occurs, the inactive micelles become unstable and disintegrate to restore the balance. In time all of the micelles disappear and the monomer droplets shrink in size. After a conversion of 10-20%... 

The foregoing discussion has clearly revealed that the structure of latex particles deriving from emulsion polymerization depends strongly on the mode in which the second monomer is introduced. A question intimately connected to seeded emulsion polymerization is related to possible inhomogeneities in swollen latex particles due to the wall-repulsion effect [99,100]. Emulsion polymerization can be used to obtain high molecular weight polymers whose radii of gyration may be of the order of the radius of the latex particles. If such a particle is swollen by a solvent, the polymer chains tend to avoid the surface of the particle because of their restricted conformations near a wall. Therefore the polymer concentration near an impenetrable surface is diminished. 

The experimental fact, that the monomer conversion takes place mainly inside the monomer swollen latex particles, is the justification for Equation 25.8 neglecting propagation in the continuous phase. Accordingly, the rate of emulsion polymerization can be approximated quite accurately by the rate of monomer conversion inside the polymer particles [12] ... 

The Smith and Ewart-Stockmayer-O Toole treatments [48-50] (see Chapter 4) that are widely used to calculate the average number of free radicals per particle (n) are based on the assumption that the various components of the monomer-swollen latex particles (e.g., monomer, polymer, free radicals, chain transfer agent, etc.) are uniformly distributed within the particle volume. A latex particle in emulsion homopolymerization of styrene involves uniform distribution of monomer and polymer within the particle volume except perhaps for a very thin layer near the particle surface. In the case of free radicals, this uniform distribution would only hold in a stochastic sense. However, as illustrated in Eq. (8.1), free radicals are not distributed uniformly in the latex particles when water-soluble initiators are used to initiate the free radical polymerization. The assumption of uniform distribution of free radicals in the latex particles would be valid only if the particles are very small or chain transfer reactions are the dominate mechanism for producing free radicals. If such a nonuniform free radical distribution hypothesis is accepted, the very basis of the Smith and Ewart-Stockmayer-O Toole methods might be questioned. Despite this potential problem, the Stockmayer-O Toole solutions for the average number of free radicals per particle have been used for kinetic studies of many emulsion polymerization systems. The theories seem to work reasonably well and have been tested extensively with monomers such as styrene. 

An emulsion polymerisation system comprises water, an initiator (usually water soluble), a water-insoluble monomer and a colloidal stabiliser, which may be added or maybe formed in situ. The main locus of polymerisation is within the monomer-swollen latex particles, which are either formed at the start of polymerisation or may be added initially (in which case one has a seeded emulsion polymerisation). The term emulsion polymerisation is a misnomer (arising for historical reasons the process was originally developed with the aim of polymerising emulsion droplets, although, in fact, this does not occur). The starting emulsion is not thermodynamically stable. An inverse emulsion polymerisation is one where the continuous phase is organic in combination with an aqueous discrete phase containing a water-soluble monomer (e.g. acrylamide). Two variants of emulsion polymerisation are... 

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